Observations of Volcanic Lightning During the 2009 Eruption of Redoubt Volcano

VOLGEO-04820; No of Pages 21 Journal of Volcanology and Geothermal Research xxx (2012) xxx–xxx

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Journal of Volcanology and Geothermal Research

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Observations of volcanic lightning during the 2009 eruption of Redoubt Volcano

Sonja A. Behnke a,⁎, Ronald J. Thomas a, Stephen R. McNutt b, David J. Schneider c, Paul R. Krehbiel a, William Rison a, Harald E. Edens a a Langmuir Laboratory, New Mexico Tech, 801 Leroy Place, Socorro, NM 87801 USA b Alaska Volcano Observatory, University of Alaska Fairbanks, 903 Koyukuk, Fairbanks, AK 99775 USA c Alaska Volcano Observatory, United States Geological Survey, 4200 University Drive, Anchorage, AK 99508 USA article info abstract

Article history: Observations of volcanic lightning during the eruption of Redoubt Volcano in March and April 2009 were Received 1 June 2011 made with the Lightning Mapping Array. During the eruption twenty-three distinct episodes of volcanic Accepted 21 December 2011 lightning were observed. Electrical activity occurred as either a volcanic lightning storm with up to thousands Available online xxxx of lightning discharges or as a weak electrical event with only a handful of lightning discharges. During the volcanic lightning storms we observed two phases of electrical activity: the explosive phase and the plume Keywords: – Volcanic lightning phase. The explosive phase consisted of very small discharges (on the order of 10 100 m) occurring directly Redoubt above the vent while an explosive eruption was ongoing, whereas the plume phase was comprised of dis- Explosive Volcanism charges occurring throughout the plume subsequent to the explosive eruption. The area of discharges during VHF Lightning Mapping the explosive phase ranged from less than 1 km2 to 50 km2 or more. The electrical activity at the beginning of the plume phase was dominated by small discharges. Over time the horizontal extent of the flashes increased, with the largest flashes occurring at the end of the plume phase. The distribution of the horizontal size of the discharges over the lifetime of the storm indicate that the charge structure of the plume evolved from a complex and ‘clumpy’ structure to a more simple horizontally stratified structure. Plume height was shown to be a key factor in the quantity of lightning in a storm. The volcanic lightning storms occurred in plumes with column heights greater than 10 km. The tall plumes may contribute to the efficiency of charge generation through ice collisions by providing strong updrafts from the large thermal energy input from the eruption. © 2011 Elsevier B.V. All rights reserved.

1. Introduction The LMA was previously used to study lightning during the 2006 eruption of Augustine Volcano, also in Alaska, which characterized Lightning is an inherent feature of many types of volcanic erup- the styles and types of electrical activity that occurred as a result of tions (McNutt and Williams, 2010), but it is not well understood the eruption (Thomas et al., 2007; Thomas et al., 2010). One of the how the characteristics of an eruption (e.g., plume height, tempera- main results of the Augustine Volcano study was that lightning oc- ture, magma composition, ash or gas content) influence the resulting curred in two distinct phases, an explosive phase and a plume electrical activity. The relationships between lightning and eruptive phase. The explosive phase described electrical activity observed during activity have recently been explored by some researchers (Hoblitt, the actual explosive event, while the plume phase described lightning 1994; McNutt and Davis, 2000; Bennett et al., 2010) using instru- that occurred in the plume subsequent to the explosive eruption. Addi- ments that primarily detect the signals from cloud-to-ground light- tionally, lightning during the Augustine Volcano eruption was classified ning, while others (Thomas et al., 2010) have used methods that into three categories: vent discharges (small, 10–100 m, high rate dis- look at all the electrical activity in a plume including cloud-to- charges close to the vent), near-vent lightning (medium sized, 1–7km ground and intracloud lightning. Observations of so-called ‘total light- discharges extending from the volcano into the eruption cloud) and ning’ are possible with the Lightning Mapping Array (LMA) (Rison et plume lightning (large sized, 10 km or more, branched discharges al., 1999; Thomas et al., 2004), which locates sources of VHF radiation similar to intracloud (IC) lightning observed in thunderstorms). that are produced from lightning as it propagates. The LMA effectively More recently, the LMA was used to study lightning during the provides a map of all of the electrical activity in a plume, making de- eruption of Redoubt Volcano, located in south-central Alaska tailed studies of volcanic lightning and its relationships to eruptive (Fig. 1). Redoubt Volcano underwent a series of explosive eruptions activity possible. in late March and early April of 2009, most of which produced sub- stantial electrical activity. The major events occurred between 23 March and 4 April, 2009, though the full period of explosive activity ⁎ Corresponding author. extended through April 5. Peak plume heights for each explosive E-mail address: [email protected] (S.A. Behnke). event ranged from 5 to 19 km, and the entire eruption was classified

the eastern side of the volcano that extended approximately 600 m Alaska above the vent. This prevented any electrical signals that occurred below and behind the ridge from reaching the stations. 61.5 Area of The LMA uses time-of-arrival methods (Rison et al., 1999) to lo- figure cate sources of VHF radiation (typically 63 MHz; in this deployment Anchorage 63 and 57 MHz were used) that are produced during the develop- 61.0 ment of lightning channels. In successive short time windows (10 or 80 μs, depending on the operating mode), the peak amplitude and arrival time of the received power is recorded if the signal exceeds a Nikiski μ Redoubt local noise threshold. The LMA was operating in 10 s mode during 60.5 K-Beach the Redoubt Volcano deployment. Each station uses timing from a GPS receiver to measure the arrival times with an accuracy of Latitude Clam Gulch 40–50 ns. Each electrical signal recorded by the LMA represents a small piece of a discharge, and hundreds or thousands of signals 60.0 Ninilchik may be recorded for one lightning discharge, depending on the size of a discharge. A minimum of four stations is needed to determine the three dimensional location and time of the source event. 59.5 Cook Inlet An example of the data recorded by an LMA station is shown in Fig. 2. In each of these three plots, the colors indicate the relative den- N sity of points, purple being least dense and red most dense. Panel (a) 50 km shows the peak power received in each 10 μs time window. The bands 59.0 of background radiation (the steady purple sources between rough- -154 -153 -152 -151 -150 -149 ly −80 and −70 dBm and the greenish sources below −80 dBm) Longitude throughout the plot are caused by local noise sources such as electron- ics, motors, or transformers. A discrete vertical line on a plot of this time Fig. 1. Map of the locations of the lightning mapping stations (squares) and Redoubt scale is most likely caused by a lightning discharge, but can also be pro- Volcano (triangle). duced by a strong, discrete noise source. Panel (b) shows how often the signal was above the power threshold in each time window (referred to as a VEI 3 event (Schaefer, 2011). A 4-station LMA was deployed on as the number of points above threshold), and the time between each the east side of Cook Inlet about two months before the eruption to recorded signal is shown in panel (c). The maximum value for the num- study the electrical activity of the volcano. During the two week period ber of points above threshold depends on the size of the time window of explosive activity, the ash plumes from Redoubt Volcano produced that each signal is recorded in. If the time window is 10 μs, the maxi- prolific lightning, which was recorded by the LMA, though visual obser- mum is 250 and if the time window is 80 μs, the maximum is 2000. vations were scarce due to inclement weather in the area. To accurately locate the three dimensional position of a lightning The research presented in this article expands on the Augustine source, the LMA stations must be sufficiently separated from each Volcano results and shows the relationship between the eruptive other so that the signal from a source arrives at each station at signifi- and electrical activity at Redoubt Volcano in an effort to understand cantly different times (Thomas et al., 2004). The four stations, Nikiski, what lightning can tell us about a volcanic eruption. We present an K-Beach, Clam Gulch and Ninilchik, were all approximately equidistant overview of the electrical activity that occurred during the eruptive from Redoubt Volcano due to the limitations on available station sites, events and compare it to the previous observations from Augustine with distances ranging from 77 to 81 km. In this situation, variations Volcano. The trends observed in the progression of flash rate and in the altitude of a source would not result in a large enough change flash area over time are shown and the types of lightning discharges in the arrival time differences between pairs of stations. For example, that occurred throughout the eruption column are discussed. Finally, the difference in arrival time between the Clam Gulch and K-Beach sta- the electrical activity is compared to the peak plume heights and the tions will be nearly the same for a source located 5 km above Redoubt seismic and acoustic signals from each explosive event. Volcano as for one 10 km above, making it impossible to determine the altitude of the source. Due to this limitation, the x (east–west) and 2. Data and methods y(north–south) positions of the sources were determined assuming a fixed altitude of 5 km. Fig. 3 shows the predicted absolute errors in This study used lightning data from the New Mexico Tech Light- the x and y locations for test data at 5 km altitude using 40 ns timing ning Mapping Array, seismic, acoustic, and radar data from the Alaska noise in the test data. The errors were determined from the covariance Volcano Observatory, and measurements of cloud-to-ground light- matrix resulting from solving the non-linear system of equations de- ning from the Alaska BLM and the World Wide Lightning Location scribing the arrival time of a source for the parameters x, y, and t Network. (time) assuming a fixed z (Thomas et al., 2004). The errors in the x and y position were less than 200 m within a 50 km radius of Redoubt 2.1. The Lightning Mapping Array Volcano. Some of the located data were processed using only the arrival Four LMA stations were installed on the Kenai Peninsula, approxi- times from 3 stations because there were only 3 stations operating mately 80 km east of Redoubt Volcano, between 28 and 31 January during these particular times due to unexpected power issues. The 2009. Station sites were selected based on access to AC power and rest of the locations were determined using arrival times from all 4 broadband internet, on having a line-of-sight view to Redoubt Volcano, stations. This difference is noted throughout the article where and on being a radio frequency (RF) quiet location. The internet, line of pertinent. sight, and noise constraints greatly limited the number of desirable sites, so the authors made use of the best locations available. A map of 2.2. Cloud-to-ground lightning the station locations is shown in Fig. 1. Although each station had a line-of-sight view to Redoubt Volcano, the actual vent of the volcano Locations and times of cloud-to-ground lightning were obtained was blocked from view at the stations because there was a ridge on from the World Wide Lightning Location Network (WWLLN)

-50

-60

-70 Power (dBm) -80

07:15:00 07:15:49 07:16:39 07:17:29 07:18:19 07:19:09 07:19:59 Number of Points Above Threshold (b) 250 200 150 100 50 Number of Points 0 07:15:00 07:15:49 07:16:39 07:17:29 07:18:19 07:19:09 07:19:59 (c) Time Between VHF Sources 10-1 10-2

10-3

10-4 Time (s) 10-5 10-6 07:15:00 07:15:49 07:16:39 07:17:29 07:18:19 07:19:09 07:19:59

23 March, 2009 Time UTC (hr:min:sec)

Fig. 2. Example of unprocessed (raw) lightning mapping data recorded by a single station. Panel (a) shows the peak power in units of decibel milliwatts (dBm) recorded during each 10 μs window, panel (b) shows the number of points that were above the power threshold in each time window, and panel (c) shows the time between the recorded peaks. The apparent step and shift in colors from purple to green in panel (c) near 07:19 is due to the station's automatic adjustment in the minimum power threshold. The colors in each plot indicate the relative density of points, red being the densest and purple the least.

(Dowden et al., 2002; Rodger et al., 2005) and the Alaska BLM (T. a WWLLN detection efﬁciency of 26% for cloud-to-ground lightning Weatherby, pers. comm.). The WWLLN detects very low frequency and 10% for intracloud lightning. The Alaska BLM operates a low fre- (VLF) radiation from lightning and uses time of group arrival quency time of arrival (TOA) lightning location network manufac- (TOGA) methods to locate the sources. Rodger et al. (2005) reported tured by Vaisala. The detection efﬁciency of this network is

x (east-west) position error (m) y (north-south) position error (m)

0 500 1000 1500 2000 0 500 1000 1500 2000

200 200

100 100

0 0

-100 -100 y (north-south) position (km) y (north-south) position (km)

-200 -200

-200 -100 0 100 200 -200 -100 0 100 200 x (east-west) position (km) x (east-west) position (km)

Fig. 3. Standard deviation of the x and y positions of synthetic data at a ﬁxed altitude of 5 km. The locations of the stations are marked with white squares and the location of Re- doubt Volcano is marked with a white triangle.

2.3. Infrasound data Seismic waveforms, signal durations and reduced displacements

(Dr) from the eruption were obtained from the Redoubt Volcano seis- This article incorporates data of infrasound waveforms, acoustic mic network (McNutt et al., this issue). The instruments used were pressures and signal durations for the eruptive events. The acoustic Mark Products L4-C short period (period of 1 s) seismometers. Unless signal is used primarily as an indicator of when and for how long an otherwise noted, all of the data presented here are from the RDT sta- explosive event occurred. Infrasound data were obtained from a tion, which was approximately 21 km from the vent. All plots of the Chaparral Model 2.5 microphone installed at the DRF station approx- waveforms take into account a time delay of 5 s for the signals to imately 12 km north–northeast of Redoubt Volcano's summit reach the seismometer. Seismic durations were determined in a sim- (McNutt et al., this issue). A high gain and a low gain channel were ilar manner as the infrasound durations. recorded during the eruption. All plots of the waveforms presented Reduced displacement is a calculation that provides a normalized here are from the high gain channel and take into account the ap- metric for the amplitude of ground motion and has been related to proximate 38 second time delay for the signals to reach the station. ash column height (McNutt, 1994). Essentially, reduced displacement Signal durations (Table 1) were measured from the time when the is the product of waveform amplitude and distance to the source. The signal amplitude increased to twice as large as the noise background calculation assumes surface wave propagation and takes instrumental to the time when the signal returned to the noise background level. magniﬁcation and geometrical spreading into account.

Table 1 Observations of lightning during the explosive events of the 2009 eruption of Redoubt Volcano.

a Date Time Event Acoustic Acoustic Seismic Dr Plume Lightning LMA Regular Peak Small Peak Single- Peak Cloud- number duration pressure duration height duration sources ﬂashes rate discharges rate source rate to-ground (mm/ (UTC) (cm2) (# located) discharges discharges dd) (min) (Pa) (min) (km) (min) (#/ (#/ (#/ (BLM/ min) min) min) WWLLN)

03/23 06:35:54 1 26 25 – 5 5.5 2.4 558 4 1 0 0 0 0 0/0 03/23 07:02:30 2 3 151 8 13.3 14.0 20.6 48,653 573 84 527 121 5089 1664 0/5 03/23 08:14:43 3 13 38 14 8.6 14.6 29.9 116,929 1536 161 2447 276 21,792 2383 3/8 03/23 09:39:30 4 8 70 9 74 13.4 39.1 230,939 1975 170 1318 128 28,422 2519 79/172 03/23 09:48:54 12 90 30 56 13.7 n/ab n/ab n/ab n/ab n/ab n/ab n/ab n/ab n/ab 03/23 10:53:31 2 12 7 13 – 0 0 0 0 0 0 0 0 0/0 03/23 12:30:59 5 16 173 22 43 18.3 53.4 266,382 2739 233 4981 563 63,814 5392 11/24 03/23 12:59:32 1 14 2 45 – n/ab n/ab n/ab n/ab n/ab n/ab n/ab n/ab n/ab 03/24 03:41:13 6 12 76 17 16 16.2 34.7 908,235c n/ad n/ad n/ad n/ad n/ad n/ad 3/7 03/26 16:35:07 7 1 7 4 3 8.2 2 153 2 1 0 0 0 0 2/1 03/26 17:24:52 8 7 100 14 12 18.9 34.4 194,758 1967 175 4086 575 55,128 7186 13/1 03/27 07:47:49 9 15 31 21 12 12.5 39.9 26,550 229 35 213 55 4671 2010 18/14 03/27 08:29:00 10 4 54 9 12 14.9 38.7 165,927 1341 129 1594 252 20,714 3404 155/106 03/27 08:42:43 3 8 7 6.7 – n/ab n/ab n/ab n/ab n/ab n/ab n/ab n/ab n/ab 03/27 16:12:18 0.6 4.1 0.9 1.1 – 0 0 0 0 0 0 0 0 0/0 03/27 16:39:36 11 4 83 10 22 15.5 38.6 466,833c n/ad n/ad n/ad n/ad n/ad n/ad 41/12 03/28 01:35:21 12 2 146 9 19 14.6 30.3 70,011 534 72 501 126 5319 1535 13/3 03/28 03:24:56 13 3 138 4 8.1 15.2 34.8 116,826 930 125 1059 231 19,939 5795 53/15 03/28 07:20:17 14 2 78 2 26 12.5 41.1 62,174 462 74 537 138 6326 1449 14/32 03/28 09:20:02 15 2 59 2 3.6 14.6 42.8 54,476 445 65 430 110 5130 1367 28/39 03/28 10:00:15 b1106 13– 0 0 0 0 0 0 0 0 0/0 03/28 16:11:19 0.5 2 0.5 1.2 – 0 0 0 0 0 0 0 0 0/0 03/28 21:40:38 16 2 28 12 5.7 5.2 0 0 0 0 0 0 0 0 0/0 03/28 23:09:37 3 5.9 1 2.3 – 0 0 0 0 0 0 0 0 0/0 03/28 23:30:03 17 3 67 37 30 12.5 40 90,145 1013 142 900 173 8246 1830 12/0 03/29 03:24:09 18 4 49 44 5.2 14.6 23.2 63,234 629 98 577 108 5600 1504 6/7 03/29 19:19:45 ––2 6.8 6.1 7.9 1214 8 2 0 0 0 0 0/0 03/30 07:11:33 ––3 4.3 6.1 4.4 1149 9 4 0 0 0 0 0/0 03/30 17:43:46 ––47– 0 0 0 0 0 0 0 0 0/0 03/30 18:50:36 ––5 0.6 6.1 b1 181 10 00 00/0 03/31 07:31:28 6 1.3 7 3.8 6.1 6.2 524 6 2 0 0 0 0 0/1 03/31 07:41:54 0.3 0.25 0.5 2.4 – n/ae n/ae n/ae n/ae n/ae n/ae n/ae n/ae 0/0 04/01 00:07:00 b1 1.9 10 1.7 4.6 3.8 921 5 2 0 0 0 0 0/0 04/01 03:46:29 0.5 0.4 0.5 3.5 – 0 0 0 0 0 0 0 0 0/0 04/04 13:57:43 19 19 38 19 10.7 15.2 69.8 618,009 6841 316 10,376 497 130,635 6548 40/30 04/04 14:16:26 12 88 56 15.3 15.2 n/ab n/ab n/ab n/ab n/ab n/ab n/ab n/ab n/ab 04/05 18:36:51 1 3.7 1.5 7.7 – 0 0 0 0 0 0 0 0 0/0

a Time of explosive event is the arrival time at the DFR station. There was an approximate travel time of 38 s between the vent of the volcano and the station. b A volcanic lightning storm was already occurring due to the previous event when this explosive eruption occurred. All lightning statistics are included in the entry for the previous event. c LMA source locations were determined using 3 station data because only 3 stations were operating at the time. The total number of located LMA sources are not comparable to events processed with 4 station data. d Statistics on numbers and rates of discharges were excluded because the locations were determined with 3 station data. e A weak electrical event was already occurring due to the previous event when this explosive eruption occurred. All lightning statistics are included in the entry for the previous event.

2.5. Eruption column heights and radar plume. The physical types of discharges are then described according to classifications previously defined by (Thomas et al., 2010). Finally, Radar observations of the plumes during the eruption were the overall amount of electrical activity from each event is compared obtained from the FAA NEXRAD radar in Anchorage, AK and the to several parameters of the eruptive activity, which has implications USGS-operated EEC Minimax 250C Doppler radar in Kenai, AK. The for the electrification processes of the plume. USGS radar operates in C-band at 250 W (Schneider and Hoblitt, this issue). Peak column heights for each eruptive event were determined by comparing observations from NEXRAD and the USGS 3.1. Typical examples of electrical activity radar and selecting the observation that gave the higher altitude. Each column height measurement corresponds to the center of the We observed 23 distinct episodes of electrical activity associated beam. Because the radar beam must be at least 50% full to give a re- with explosive activity, as described in Table 1. During larger explo- turn, each measurement represents the minimum peak altitude of sive events, like event 2 on 23 March at 07:02 UTC, we observed what the column. The uncertainty in the column height measurements we describe as a volcanic lightning storm. These storms had dura- are +1.3 km (for NEXRAD) and +2.3 km (for USGS). The USGS tions between 20 and 70 min and commonly had more than 1000 radar was also used to determine the column height soon after the lightning flashes. Sixteen distinct volcanic lightning storms were ob- onset of the explosive eruption on 28 March at 0324 UTC. served during the 2009 eruption. During smaller events, such as the event on 1 April at 00:07 UTC, we saw either no lightning or just a 2.6. Data from the 2006 eruption of Augustine Volcano few lightning discharges over spans of less than 10 min. We refer to the events that had only a few discharges as weak electrical The Redoubt Volcano lightning observations are compared to the events. Weak electrical events were observed subsequent to 7 of previous observations of lightning from the 2006 eruption of Augus- the smaller explosive events. In the following sections we present tine Volcano, located in south-central Alaska. The LMA data from typical examples of a volcanic lightning storm and a weak electrical the Augustine Volcano eruption presented here is from the two- event and compare both types of electrical activity to previous obser- station array that was located on the Kenai Peninsula in late January vations of volcanic lightning from the 2006 eruption of Augustine 2006 (Thomas et al., 2010). The LMA was operating in 80 μs mode Volcano. during this deployment. Infrasound data are from the AUE station This article uses many terms to describe the phases of eruptive on Augustine Volcano and seismic data are from the Oil Point station and electrical activity and the types of discharges that occurred, as (OPT) (McNutt et al., 2010). summarized in Fig. 4. The temporal sequence of the phases is shown along with the temporal variation of the typical rates of the 3. Results and discussion various categories of electrical activity. Black and white gradient bars illustrate the frequency of each type of discharge, black being Here we present our observations of lightning during the eruption most frequent and white being least frequent. The electrical activity of Redoubt Volcano. We first present typical examples of the electrical is categorized in two different groups. The first group describes the activity that occurred during distinct explosive events and compare it three physical types of discharges observed and the second describes to the electrical activity observed during previous studies of Augus- the three classifications of discharges used by the flash algorithm (see tine Volcano. We then show how the lightning during an explosive Appendix A). The flash algorithm classified discharges based on the event evolved and what that implies for the charge structure of the number of lightning sources in each discharge; this classification is

Summary of Volcanic Lightning Terminology and Evolution of Lightning Phases Time Explanation: More frequent - Less frequent Volcanic Explosion

Explosive Phase Plume Formation Stable Plume

Plume Phase

Vent Discharges

Near-vent Lightning Physical Types of

Electrical Discharges Plume Lightning

Single-source Discharges

Small Discharges Sources)

Regular Flashes Categories of Lightning Flashes (Grouped LMA

Fig. 4. Summary of the terminology used to describe phases of volcanic and electrical activity, the physical types of lightning discharges observed during explosive volcanic eruptions, and categories of lightning ﬂashes as grouped using a ﬂash algorithm. The black and white gradient bars indicate when each type of discharge was typically observed, black being most frequent and white least frequent. The phrase ‘stable plume’ refers to a fully developed plume that is no longer convectively active.

(a) Peak VHF Power -40 -50 -60 -70 Power (dBm) -80

07:00:00 07:05:00 07:10:00 07:15:00 07:20:00 07:25:00 07:30:00 (b) Number of Points Above Threshold 250 200 150 100

50 Number of Points of Number 0 07:00:00 07:05:00 07:10:00 07:15:00 07:20:00 07:25:00 07:30:00 Time Between VHF Sources (c) -1 10 -2 10 -3 10 -4 10 Time (s) -5 10 -6 10 07:00:00 07:05:00 07:10:00 07:15:00 07:20:00 07:25:00 07:30:00 (d) Acoustic Activity 20

Pressure (Pa) -10

-20 07:00:00 07:05:00 07:10:00 07:15:00 07:20:00 07:25:00 07:30:00 (e) Seismic Activity 3000 2000

1000

Velocity (nm/s) -1000 -2000 07:00:00 07:05:00 07:10:00 07:15:00 07:20:00 07:25:00 07:30:00 23 Mar, 2009 Time UTC (hr:min:sec)

Fig. 5. Unprocessed (raw) LMA, acoustic and seismic data from explosive event 2, 23 March at 07:02 UTC. Panels (a), (b), and (c) show the peak received power, number of points above threshold and the time between recorded peaks from the Clam Gulch LMA station. The steps and shifts in coloring visible between 07:19 and 07:26 in panel (c) are caused by changes in the minimum power threshold. The colors indicate the relative density of sources, red being the densest and purple the least. Panels (d) and (e) show the acoustic signal from the DFR station and the seismic signal from the RDT station. Both signals have been time-shifted to account for the travel time delay.

40 of the average calculation for each segment. The signal from segment 1 is referred to as a ‘continual RF’ signal due to the relatively high and N steady average source rate (this is not meant to imply that the electrical activity that produced this signal was radiating continuously), and the 20 spiky nature of the source rate for segments 2, 3, and 4 leads us to call these types of signals ‘discrete electrical activity.’ It is also important to point out that the continual RF signal in this 0 example began while the volcano was actively erupting and producing ash, as determined from the infrasound observations. The continual RF signal tapered off and ended at the same time that the acoustic signal began to return back to the background level. Similarly, in all other -20 storms the continual RF signal was also observed during the actual eruptive event. North-South distance (km) -40 Cook Inlet 3.1.2. Comparison of the volcanic storm with the 2006 eruption of Augustine Volcano Previous observations of lightning from a relatively large explosive event during the 2006 eruption of Augustine Volcano, shown in -60 Fig. 9, revealed that electrical activity occurred in two separate -20 0 20 40 60 80 phases: the explosive phase and the plume phase (Thomas et al., East-West distance (km) 2007; Thomas et al., 2010). The initial continual RF signal was referred to as the explosive phase because it occurred concurrently Fig. 6. Located LMA data from the 23 March 07:02 UTC event (explosive event 2), with the explosive eruption. The discharges that gave rise to this sig- shown in plan position. The colors indicate the progression of time, blue being earlier nal were inferred to have occurred directly above Augustine Volcano times and red later times. The activity at the beginning of the volcanic lightning at low altitude. This is because the explosive phase discharges were storm (blue and green sources) is mostly covered up by the later activity (red sources), and covers a larger area than what is visible in the figure. Redoubt Volcano is located at only observed by the station that had a direct line-of-sight view to the coordinates (0,0) (black cross), station locations are marked with green squares. the vent (the other station was situated inland and could not see the volcano but could see above it). The set of discrete discharges that occurred subsequent to the continual RF signal was referred to as the plume phase because the discharges occurred progressively and red sources) was not anchored to the vent and the storm as a whole over and downwind of the volcano in the drifting plume. had moved off the vent. The explosive and plume phases observed at Augustine Volcano There are two distinct features in the peak received power shown were also observed during the volcanic lightning storms at Redoubt in panel (a) of Fig. 5: the strong and continuous signal at the begin- Volcano, which allowed us to make a more concrete description of ning, and the discrete electrical signals that followed. To illustrate the two phases. The continual RF signal shown in Fig. 7b, character- the differences of these two features, four 30-second segments of ized by the relatively high average source rate with lower variability time were selected from the peak power for comparison. The 4 seg- (8.5 [std dev 7.4] sources per 10 ms) is a typical example of an explo- ments of the peak power are shown on expanded scales in Fig. 7. Seg- sive phase signal. The discrete activity shown in Fig. 7c–e, character- ment 1 is from the initial period of strong continuous activity, ized by a lower average source rate but with higher variability segments 2 and 3 are from periods of high rate discrete activity, and (averages between 2.0 [std dev 10.4] and 3.9 [std dev 12.0] sources segment 4 is from a period of low rate discrete activity at the end of per 10 ms) are typical examples of a plume phase signal. Similar pat- the storm. The signal from segment 1 is clearly more continuous terns in the source rates for continual and discrete activity were also than segments 2–4, which have obvious spikes in activity separated observed during the eruption of Augustine Volcano, shown in Fig. 10. by periods of less activity. To further demonstrate these differences During continual RF activity at Augustine Volcano (05:31:25 to we examined the rate of recorded sources for each of the four seg- 05:31:55 UTC on 28 January, 2006), the average number of sources ments. In order to fairly compare the source rates from the 4 seg- per 10 ms was 34.8 (std dev 9.0), significantly larger than what was ments to Augustine Volcano, the Redoubt Volcano data were observed at Redoubt Volcano. The average number of sources per decimated from 10 μs mode (maximum of one recorded source per 10 ms during low rate (05:42:10 to 05:42:40) discrete activity at Au- 10 μs) to 80 μs mode (maximum of one recorded source per 80 μs), gustine Volcano was 3.3 (std dev 17.8), similar to what was observed which was used during the Augustine Volcano experiment. The at Redoubt Volcano (there was no corresponding time of high rate peak power data for both Redoubt Volcano and Augustine Volcano discrete activity at Augustine Volcano). The key difference between were then normalized to be the power at a distance of 100 km from continual and discrete signals is the standard deviation, which each volcano (the VHF data shown in Figs. 7 and 9 do not have this shows the variability in the source rate. The continual RF is character- normalization applied, however.). An artificial power threshold was ized by source rates that do not vary over relatively long time periods then applied to the data to eliminate most of the local noise. Only (30 s, for example), while the discrete signals are characterized by sources with a power greater than or equal to −78 dBm were included high variability in the source rates over relatively long time periods. in the source rate plots and average rate calculation. Fig. 8 shows the In the rest of this article the explosive and plume phase terminology number of recorded sources in successive 10 ms intervals during the will be used to describe the time periods when the peak VHF rates time period of each segment. The average number of sources per met similar standards. 10 ms over each 30 second time period for segments 1 through 4 The Augustine Volcano results showed that the two phases of elec- were 8.5 (std dev 7.4), 3.7 (std dev 11.0), 3.9 (std dev 12.0), and 2.0 trical activity were caused by separate electrification mechanisms (std dev 10.4), respectively. In the 80 μsmodethemaximumnumber (Thomas et al., 2007; Thomas et al., 2010). The explosive phase dis- ofsourcesin10msis125.AsshowninFig. 8, the source rate for seg- charges occurred concurrently with the explosive eruption, therefore ment 1 did not vary greatly over time compared to the rates for seg- it is believed that these discharges came about because the ejecta ments 2, 3 and 4, which had spikes in the source rate associated with from the volcano was charged during the eruption process. Charge discrete lightning flashes. This is also reflected in the standard deviation generation by the fracturing of magma, known as fractoemission

(a) Peak VHF Power -40 -50 -60 Segment 2 Segment 4 -70 Segment 1 Segment 3 Power (dBm) -80

07:00:00 07:05:00 07:10:00 07:15:00 07:20:00 07:25:00 07:30:00

(b) Peak VHF Power -- Segment 1

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(e) Peak VHF Power -- Segment 4 -50

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07:14:00 07:14:05 07:14:10 07:14:15 07:14:20 07:14:25 07:14:30

23 March, 2009 Time UTC (hr:min:sec)

Fig. 7. Selected segments of time of the peak VHF power from the 23 March 07:02 event plotted on expanded time scales. Each segment is 30 s in duration. The size of the points in each panel is relative and has no signiﬁcance.

(James et al., 2000), is thought to be the dominant charge mechanism processes in the plume were taking place in order to give rise to the for silica-rich plumes (James et al., 2008), though boiling of surface plume lightning. These processes could simply involve the gravita- water (such as ice or snow) may contribute to electrification as well tional separation of the already-charged ash or could be a result of (James et al., 2008). In addition to charge generation through rapid water-based charging mechanisms that are responsible for thunder- boiling, ice and snow covering the volcano could potentially increase storm lightning (e.g., Williams, 1985). Water-based charging mecha- the electrification by adding a phreatomagmatic component, which nisms rely on collisions between differing types of ice particles (e.g., would create a finer grained ash and therefore provide more charge graupel and ice crystals), therefore ambient atmospheric conditions carriers. As for the plume phase lightning, the period of inactivity be- play a role in water charging mechanisms (plumes must be cold tween the explosive and plume phases suggests that electrification enough to create ice). Further discussion on plume charging

(a) LMA Source Rate -- Segment 1 120 100 80 60 40 20

Sources per 10 ms 0 07:04:00 07:04:10 07:04:20 07:04:30 (b) LMA Source Rate -- Segment 2 120 100 80 60 40 20

Sources per 10 ms 0 07:06:00 07:06:10 07:06:20 07:06:30 (c) LMA Source Rate -- Segment 3 120 100 80 60 40 20

Sources per 10 ms 0 07:10:10 07:10:20 07:10:30 07:10:40 (d) LMA Source Rate -- Segment 4 120 100 80 60 40 20

Sources per 10 ms 0 07:14:00 07:14:10 07:14:20 07:14:30 Time (UTC)

Fig. 8. Number of LMA sources per 10 ms for each of the 4 power segments shown in Fig. 7. mechanisms during the Redoubt Volcano eruption will be presented lightning regardless of the charging mechanism, since a more ener- in Section 3.5. getic eruption would produce both more ash and larger column Though the explosive and plume phases occurred at both Redoubt heights, which would influence both ash and water-based charging Volcano and Augustine Volcano, there were differences in these mechanisms. Furthermore, the atmospheric conditions during the Re- phases among the two eruptions. First, we had predicted that the ex- doubt Volcano and Augustine Volcano eruptions were similar, and plosive phase signal at Redoubt Volcano would be as strong or stron- probably did not affect electrification. Finally, the explosive phase ger than Augustine Volcano because the eruption was bigger and lightning at Augustine Volcano started concurrently with the explo- more energetic, however the average source rates observed during sive event, while at Redoubt Volcano there were delays ranging the explosive phase at Redoubt Volcano were lower than Augustine from 30 to 40 s to several minutes between the eruption onset and Volcano. Redoubt Volcano differs from Augustine Volcano in that it the beginning of the explosive phase. had more snow and ice cover (Trabant and Hawkins, 1997), but we The reason for the observed delay between the start of the erup- would expect this to increase electrification, not decrease it. This sug- tion and the start of the explosive phase lightning at Redoubt Volcano gests that much of the continual RF activity was being blocked from is not clear, but it is very likely that the eastern ridge on Redoubt Vol- view at the stations by the eastern ridge, and that the intensity of cano is responsible for the delay. The delay between the start of the this activity decreased quickly with altitude. Secondly, the plume eruption and the start of the explosive phase lightning at Redoubt phase signal from the Redoubt Volcano observations often over- Volcano was mostly likely caused by the electrical activity being lapped with the explosive phase signal, unlike Augustine Volcano, blocked by Redoubt Volcano's eastern ridge. Due to the difference in where a lull of several minutes was observed between the two the source rates of the continual RF activity between Redoubt Volcano phases. The electrical activity at Redoubt Volcano was even so intense and Augustine Volcano, we are fairly certain that there was electrical at times that the two phases were difficult to distinguish from each activity occurring behind the ridge that the stations couldn't see. But other. We assume this is also because the Redoubt Volcano eruption because of the eastern ridge on Redoubt Volcano, we do not know was overall more energetic than the Augustine Volcano eruption. when the electrical activity actually began. If we assume that the elec- We expect a more energetic eruption to produce more plume phase trical activity began right at the onset of the eruption as was observed

Peak VHF Power

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AUE BDF AUE -3

Pressure (Pa) -7 -11 Infrasound, high gain Infrasound, (b) 110 70 30 0 AUE BDL AUE

Pressure (Pa) -30 Infrasound, low gain low Infrasound, 05:30:00.000 05:32:00.000 05:34:00.000 05:36:00.000 05:38:00.000 05:40:00.000 05:42:00.000 05:44:00.000 2006028 2006028 2006028 2006028 2006028 2006028 2006028 2006028 3200 2800 (c) 2400 2000 1600 1200 800 400 0 -400

OPT EHZ -800 Seismic Activity

Velocity (nm/sec) Velocity -1200 -1600 -2000 -2400 -2800 -3200

Fig. 9. Unprocessed (raw) LMA, acoustic and seismic data from the explosive eruption on 28 January 2006 05:31 UTC of Augustine Volcano. Panel (a) shows the peak VHF power received by the station located in Homer. Panels (b) and (c) show the acoustic signal from the AUE station and seismic signal from the OPT station. Both signals have been time- shifted to account for the travel time delay. (a) LMA Source Rate -- Continual RF 120 100 80 60 40 20 Sources per 10 ms 0 05:31:25 05:31:35 05:31:45 05:31:55 (b) LMA Source Rate -- Low Frequency Discrete 120 100 80 60 40 20

Sources per 10 ms 0 05:42:10 05:42:20 05:42:30 05:42:40 Time (UTC)

Fig. 10. Number of LMA sources per 10 ms for 30 second periods of continual RF and low rate discrete electrical activity during the eruption of Augustine Volcano.

(a) Peak VHF Power -40

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00:10:00 00:11:40 00:13:20 00:14500 00:16:40 00:18:20 00:20:00

(b) Acoustic Activity 3 2 1 0 -1 Pressure (Pa) -2 -3 00:10:00 00:11:40 00:13:20 00:15:00 00:16:40 00:18:20 00:20:00 (c) Seismic Activity 150 100

Velocity (nm/s) -50 -100 00:10:00 00:11:40 00:13:20 00:15:00 00:16:40 00:18:20 00:20:00 01 Apr, 2009 Time UTC (hr:min:sec)

Fig. 11. Unprocessed (raw) LMA, acoustic and seismic data from the 1 April 00:07 UTC explosive event. Panel (a) shows the peak received power from the Clam Gulch station. The colors indicate the relative density of sources, red being the densest and purple the least. Arrows in panel (a) point to the discharges. Panels (b) and (c) show the acoustic signal from the DFR station and the seismic signal from the RDT station. Both signals have been time-shifted to account for the travel time delay.

with when and where in the plume the discharges occurred. In this 10 section we present examples of how the rates of each discharge cate- N gory and the sizes of the discharges change over time. 3.2.1. Rates and sizes of lightning flashes 0 The electrical activity during the explosive phase of a volcanic lightning storm was dominated by single-source discharges and small discharges, while plume phase activity was marked by an increase in the rate of regular flashes. Fig. 13 demonstrates how the -10 rate of total located LMA sources, regular flashes, small discharges, and single-source discharges varied over the lifetime of the storm resulting from the explosive eruption on 23 March at 07:02 UTC. The explosive eruption began at approximately 07:01:52 (this is the -20

North-South distance (km) estimated origin time, determined by subtracting the approximate travel time from the arrival times in Table 1) and stopped around 07:05:00, according to the infrasound data. Explosive phase electrical Cook Inlet activity began at 07:02:50, about 1 min after the onset of the explo- -30 sion. The explosive phase activity consisted of mostly small discharges and single-source discharges. Explosive phase activity -10 0 10 20 30 peaked between 07:04 and 07:05 with rates of approximately 120 East-West distance (km)

Fig. 12. Located LMA data from the 1 April 00:07 event, shown in plan position. The (a) Located LMA Source Rate colors indicate the progression of time, blue being earlier times and red later times. Re- 6000 doubt Volcano is located at the coordinates (0,0) (black cross). 5000 4000 3000 2000 1000 signal, but it is also possible that no continual RF activity occurred. The 0 Sources per min source altitudes would make it easier to determine if any of the dis- 07:00:00 07:06:00 07:12:00 07:18:00 07:24:00 charges were confined to the plume and thus qualified as plume Time (UTC) phase lightning. However, discrete discharges observed during several (b) Regular Flash Rate of the other weak electrical events, (e.g., 29 March 19:19 UTC) did 100 80 occur away from Redoubt Volcano (up to 12 km away) and are consid- 60 ered to be plume phase discharges caused by electrification mecha- 40 nisms in the plume. 20 0 3.2. Evolution of lightning discharges during volcanic lightning storms Flashes per min 07:00:00 07:06:00 07:12:00 07:18:00 07:24:00 Time (UTC) Each lightning signal recorded by the LMA is from the formation or (c) Small Discharge Rate 140 extension of a lightning channel, and hundreds of thousands of sig- 120 nals may be recorded during a single storm. Each lightning flash is 100 80 made up of a group of physically joined channels, extending out 60 from an initiation site and branching in different directions. The located 40 20 signals, often referred to in this article as sources or located LMA 0 Flashes per min sources, outline the flash, showing its size and extent. Each source rep- 07:00:00 07:06:00 07:12:00 07:18:00 07:24:00 Time (UTC) resents a piece of a discharge that is on the order of 100 m in length, but (d) may vary from a few tens of meters to several hundred meters. From the Single-Source Discharge Rate individual measurements there is no way of knowing how many flashes 2000 there were or which sources corresponded to which flash. By looking at 1500 the source locations in time it is often easy to see the groups of sources 1000 that make up a flash. To automate this, a flash algorithm was used to 500 sort the located sources that were close in time and space into individ- 0 ual flashes, which allowed us to study flash characteristics in a storm Sources per min 07:00:00 07:06:00 07:12:00 07:18:00 07:24:00 (e.g., flash area and flash rate). The algorithm used on this data set is a Time (UTC) modification of a previously existing algorithm and is described in (e) Acoustic Activity Appendix A. The previous algorithm worked well on most meteorologi- 20 cal thunderstorms, but did not perform well on the Redoubt Volcano 10 lightning because of the high data rate and unique features of volcanic 0 lightning. -10

The located lightning signals recorded during Redoubt Volcano's Pressure (Pa) -20 volcanic lightning storms were grouped into three categories of dis- 07:00:00 07:06:00 07:12:00 07:18:00 07:24:00 charges: regular ﬂashes (groups of 10 or more sources), small dis- Time (UTC) charges (groups of less than 10 sources) and single-source Fig. 13. The rate of total located LMA sources (panel a), regular ﬂashes (panel b), small discharges. These categories only describe the number of sources in discharges (panel c), and single-source discharges (panel d), and the acoustic activity the discharges. Section 3.4 describes the physical types of the dis- (panel e) during the volcanic lightning storm on 23 March at 07:02 UTC. The acoustic charges, which are determined by combining the number of sources signal has been time-shifted to account for the travel time delay.

10 regular ﬂashes/min. The total number of located sources (and the Sources per min 03:23:00 03:29:45 03:36:30 03:43:15 03:50:00 Time (UTC) total number of recorded sources) during the explosive phase of this storm was much less than the explosive phase of the 23 March (e) Acoustic Activity 07:02 storm, which demonstrates how the strength (peak source 20 rate and duration of sources) of the explosive phase varied between 10 explosive events. This correlates well with the difference in the 0 peak acoustic pressure between each explosive event; the peak pres- -10

sure was 151 Pa during the 23 March 07:02 event and only 49 Pa dur- Pressure (Pa) -20 ing the 29 March 03:24 event. The rates of regular flashes, small 03:23:00 03:29:45 03:36:30 03:43:15 03:50:00 discharges and single-source discharges dramatically increased at Time (UTC) 03:28, which coincided with the approximate end of the explosive ac- Fig. 14. The rate of total located LMA sources (panel a), regular flashes (panel b), small tivity and the beginning of the plume phase. discharges (panel c), and single-source discharges (panel d), and the acoustic activity A third example of the evolution of lightning rates comes from the (panel e) during the volcanic lightning storm on 29 March at 03:24 UTC. The acoustic 23 March 08:14 volcanic lightning storm and is an example of a storm signal has been time-shifted to account for the travel time delay. with a long duration explosive event. In this case, shown in Fig. 15,we still saw higher rates of small discharges and single source discharges discharges. At this time the area of regular flashes began to increase than regular flashes early on in the explosive phase, but regular flash significantly and reached a peak of 11 km2 between 03:37 and rates increased before the explosive eruption had ended. This was 03:38. The regular flash rate remained more or less constant through- typical behavior for long duration explosive events and was due to out the rest of the storm and very few single-source discharges and the plume phase lightning overlapping with the explosive phase. small discharges were observed after 03:37. The trend of increasing Once an explosive eruption had ended and the lightning transi- discharge area at the same time as decreasing discharge rate is com- tioned to being predominantly plume phase activity, we typically ob- mon to the plume phase of all the volcanic lightning storms, with served decreasing discharge rates concurrent with increasing maximum flash areas for all storms ranging between 10 and 50 km2. discharge area over the course of the rest of the storm. During the 29 March 03:24 storm plume phase activity was highest between 3.2.2. Plume charge structure 03:29 and 03:35, as shown by Fig. 14. During this period the peak Without the altitudes of the sources, we weren't able to infer the rates were approximately 100/min for regular flashes, 110/min for details of the plumes' charge structures, but the sizes and types of dis- small discharges and roughly 1500/min for single-source discharges. charges that occurred in the plume phases do reveal that the charge While the rates were at their peak, the average area of the regular structure evolved from an initially chaotic structure consisting of flashes was relatively small (2–3km2), as shown in Fig. 16. Moreover, groups of charge spread throughout the plume to a horizontally strat- all of the discharges that occurred during the time of peak discharge ified structure similar to thunderstorms (for background on thunder- rates were relatively small due to the fact that small discharges and storm lightning and charge structure, see MacGorman and Rust, single-source discharges are small discharges by definition. Between 1998). During the 29 March 03:24 storm, high rates of located LMA 03:36 and 03:37 the flash rates declined to 20/min for regular flashes, sources were observed between 03:29 and 03:35. In this period, the 5/min for small discharges and less than 50/min for single-source regular flash rate reached its peak and the average area of the regular

100 2 50 20 0 Flashes per min 08:14:00 08:22:15 08:30:30 08:38:45 08:47:00 10 area (km Time (UTC) 0 (c) Small Discharge Rate 03:23:00 03:29:45 03:36:30 03:43:15 03:50:00 300 Time (UTC) 250 200 (c) Average Flash Area 150 15 )

100 2 50 10 0 Flashes per min 08:14:00 08:22:15 08:30:30 08:38:45 08:47:00 5 Time (UTC) area (km (d) Single-Source Discharge Rate 0 2500 03:23:00 03:29:45 03:36:30 03:43:15 03:50:00 2000 Time (UTC) 1500 1000 Fig. 16. The rate and average area per minute of regular flashes during the volcanic 500 lightning storm on 29 March at 03:24 UTC. The bars in panel (b) indicate the minimum fl fl 0 and maximum ash area in each minute. Panel (c) shows the ash area without the

Sources per min 08:14:00 08:22:15 08:30:30 08:38:45 08:47:00 bars and is on a slightly expanded scale. The apparent gap in the data near the end of Time (UTC) each panel is because no lightning was detected at that time. (e) Acoustic Activity 20 charge layers had developed over time. The large ﬂashes at the end 10 of the storms signaled that charge layers had become very extensive. 0 Our results imply that, regardless of the mechanism of charge gen- -10 eration, gravitational settling of charged particles leads to a stratiﬁed

Pressure (Pa) -20 charge structure, similar to previous studies (Lane and Gilbert, 1992; 08:14:00 08:22:15 08:30:30 08:38:45 08:47:00 Miura et al., 2002) of the gross charge structure of small volcanic Time (UTC) plumes from Sakurajima volcano in Japan, which revealed dipole

Fig. 15. The rate of total located LMA sources (panel a), regular flashes (panel b), small and tripole charge structures similar to the gross charge structure of discharges (panel c), and single-source discharges (panel d), and the acoustic activity thunderstorms (Williams, 1989). Lane and Gilbert (1992) and Miura (panel e) during the volcanic lightning storm on 23 March at 08:14 UTC. The acoustic et al. (2002) measured the potential gradient beneath ash plumes at signal has been time-shifted to account for the travel time delay. Sakurajima and determined that gravitational separation was responsible for separating oppositely charged particles that were charged during the eruption process. The Redoubt Volcano and Sakurajima flashes was no more than 3 km2, as shown in Fig. 16. At the same measurements differ in terms of column height and charge genera- time, the rates of small discharges and single-source discharges tion mechanism, however; Redoubt Volcano's plumes were much were declining but were still occurring at relatively high rates. The higher and ice was likely responsible for much of the charge genera- small average area of regular flashes and the prevalence of large tion. Despite these differences, both Sakurajima results are consistent amounts of single-source discharges and small discharges at this with the Redoubt Volcano and Augustine Volcano (Thomas et al., early stage of the plume phase were a sign that the charge structure 2010) results that stratified layers develop over time due to gravita- in the plume was very disorganized, clumpy and was probably con- tional separation. stantly overturning in convective eddies at the beginning of a storm. Similar observations of complex charge structures have been ob- 3.3. Altitudes of discharges during the explosive phase served in supercell thunderstorms (Krehbiel et al., 2000; Wiens et al., 2005; Weiss et al., 2008). The convective turmoil in the plume Even though we weren't able to locate the altitudes of the light- would prevent stratification of the large charge layers that are needed ning discharges from the time-of-arrival processing methods, we for extensive intracloud lightning flashes commonly observed in have been able to compare our locations to radar data and put a thunderstorms (Coleman et al., 2003). As the rate of regular flashes, range on the altitudes of some of the discharges that occurred during small discharges and single-source discharges declined during the the explosive phase. 29 March 03:24 storm, as shown in Fig. 14, the average flash area in- In almost every volcanic lightning storm, we observed single- creased, shown in Fig. 16. The increase in the area of regular flashes source discharges directly over the vent of Redoubt Volcano while and decline of small discharges and single-source discharges over the explosive eruption, and thus the explosive phase, was going on. time indicated that convective motions had subsided and stratified Oftentimes single-source discharges were the only types of

Thomas et al. (2010) identiﬁed three types of discharges that oc- (a) LMA Source Rate curred during the 2006 eruption of Augustine Volcano: vent dis- 20 charges, near-vent lightning, and plume lightning. Vent discharges 15 were small (on the order of 10–100 m), high rate discharges occurring directly above the vent in the eruption column. These discharges 10 were responsible for the continual RF signal of the explosive phase. 5 Near-vent lightning referred to discharges that were presumed to ex- Sources per sec 0 tend upward from the volcano into the eruption column, and could be 03:22:00 03:23:45 03:25:30 03:27:15 03:29:00 1 to 7 km in length and were also explosive phase discharges. Plume Time (UTC) lightning described the larger lightning discharges (10 km or more) (b) Single-Source Discharge Rate that occurred in the plume above and away from the volcano, and 20 were thought to be similar to intracloud lightning common to 15 thunderstorms. Vent discharges, near-vent lightning, and plume lightning were 10 also observed at Redoubt Volcano. The single-source discharges dis- 5 cussed in Section 3.3 were identiﬁed as vent discharges because

Sources per sec 0 they occurred during explosive events and were at low altitude. 03:22:00 03:23:45 03:25:30 03:27:15 03:29:00 Small discharges that occurred during the explosive eruption were ei- Time (UTC) ther larger vent discharges or small near-vent lightning. The single- (c) Acoustic Activity source discharges and small discharges that occurred after the explo- 20 sive eruption in the plume phase were likely occurring throughout 10 the height of the eruption column, and thus qualified as small plume lightning and were the result of plume-based charge mecha- 0 nisms. The majority of discharges classified as regular flashes during -10 the plume phase also qualified as plume lightning. The LMA observa- Pressure (Pa) tions indicate that plume lightning often had large horizontal extents, -20 03:22:00 03:23:45 03:25:30 03:27:15 03:29:00 similar to intracloud lightning, though photographs (B. Higmann, Time (UTC) pers. comm.) of plume lightning early in the storms show discharges with large vertical extents as well. The explosive phase lightning ob- Fig. 17. Total located LMA source rate, single-source discharge rate, and acoustic activ- served during the weak electrical events was near-vent lightning, and ity from the volcanic lightning storm on 28 March at 03:24 UTC. The acoustic signal has the few plume phase discharges observed were plume lightning. been time-shifted to account for the travel time delay. The scales on the x and y axis fi have been expanded to emphasize the electrical activity that occurred during explosive In addition to these three classi cations of discharges, cloud-to- event 13. ground lightning was detected by the WWLLN and BLM networks.

– 78.0 lightning. In Figs. 20 22, the data given with the red diamonds are the measurements (column height, acoustic pressure, etc.) for the first ex- 73.0 N plosive event. Additionally, there are two large explosive events (24 68.0 March 03:40 and 27 March 16:39 UTC) that were excluded from 10 km 63.0 Figs. 20–22. They were excluded because only 3 of the 4 stations were 58.0 operating during these events due to power issues and the quantity of 53.0 located LMA sources for 3-station locations cannot be fairly compared to the 4-station locations. 48.0 43.0 3.5.1. Eruption column heights 38.0 Cook Inlet Comparison of the peak column height to the total number of LMA 33.0 sources located for each explosive event showed that the volcanic 28.0 lightning storms occurred in plumes with peak column heights greater 23.0 than 10 km. The relationship between column height and total lightning is shown in Fig. 20. All of the volcanic lightning storms had more 18.0 than 10,000 located LMA sources and occurred in eruption columns 13.0 with peak altitudes greater than 10 km. Among these events we gener- 10.0 ally saw that the higher in altitude the column reached, the more light- -10.0 ning there was. All of the weak electrical events had less than 10,000 -31.5 03:25:53 - 03:26:02 located sources and occurred in eruption columns with peak altitude dBz Column Height: 4 km less than 10 km. Additionally, there were several small events that had no lightning, but a column height measurement was only available for one of these events, which was also less than 10 km. Fig. 19. Plot of the corrected reflectivity for the 2.5 degree elevation angle scan. This scan took place between approximately 03:25:53 and 03:26:02 UTC on 28 March 2009. 3.5.2. Acoustic activity Analysis of the acoustic and lightning data revealed that all of the The cloud-to-ground lightning was observed during the plume phase volcanic lightning storms occurred when the peak pressure was and would likely have been classified by the flash algorithm as regular greater than 30 Pa, as shown in Fig. 21. Events without lightning flashes. Because we don't know the altitudes of the located LMA and weak electrical events all occurred when the peak pressure was sources we can't make the distinction between plume lightning and less than 30 Pa. Among the events with peak pressure greater than cloud-to-ground lightning with the flash algorithm. 30 Pa, there was no correlation between the peak pressure and the total number of located LMA sources. All types of activity (no light- 3.5. Comparison of electrical and eruptive activity ning, weak electrical events, and volcanic lightning storms) were observed when the acoustic signal durations were less than 5 min. With In this section we compare the total amount of electrical activity a few exceptions, events with durations longer than 5 min all pro- to several measures of the eruptive activity, including peak column duced a volcanic lightning storm. There were two events with dura- heights, peak acoustic pressures and durations, seismic durations, tions longer than 5 min that instead produced a weak electrical and reduced displacement (Table 1). The total number of located LMA sources is used as a measure of the total amount of electrical ac- – Acoustic Duration vs Total LMA Sources tivity in a storm. In Figs. 20 22, the symbols referring to 4 of the 30 events are marked in red. These 4 events produced volcanic lightning storms and during each of these storms another explosive event oc- 25 curred while vigorous electrical activity was already occurring. 20 ‘ ’ fi These subsequent, or secondary, events are identi ed in Table 1.In 15 two cases (23 March 09:48 and 4 April 13:57) the second event was large enough to have contributed to the lightning production, but be- 10 cause a volcanic lightning storm was already occurring, we can't sep- Duration (min) 5 arate the lightning data between the two explosive events. In the two 0 other cases (23 March 12:30 and 27 March 08:28) the second explo- 0 100 102 104 106 108 fl sive event was small, and may have had very little or no in uence on the Number of located LMA sources Peak Pressure vs Total LMA Sources Peak Column Height vs Total LMA Sources 200 20 150 15 100 10 50

5 Pressure (Pa) Altitude (km) 0 0 0 100 102 104 106 108 0 2 4 6 8 0 10 10 10 10 10 Number of located LMA sources Number of located LMA sources Fig. 21. Acoustic duration and peak pressure versus total located LMA sources for dis- Fig. 20. Peak column height versus total number of located LMA sources for distinct ex- tinct explosive events during the 2009 Redoubt Volcano eruption. The red diamonds plosive events during the 2009 Redoubt Volcano eruption. The red diamonds indicate indicate events that had secondary explosive events while a volcanic lightning storm events that had secondary explosive events while a volcanic lightning storm was still was still ongoing following the ﬁrst event. The black diamonds indicate events with a ongoing following the ﬁrst event. ‘pulsed’ waveform (see text).

Seismic Duration vs Total LMA Sources plume charging methods were previously explored by Thomas et al. 50 (2010). If water-based charging mechanisms are dominant, factors 40 important for ice production and collisions should be observed to in- ﬂuence the total amount of lightning in a volcanic lightning storm. If 30 ash charging mechanisms are dominant, there should be a relationship between the properties and quantity of erupted ash and the 20 total amount of lightning.

Duration (min) 10 The above results show that column height had the strongest correlation to lightning production, while there was a weak association 0 between acoustic activity and lightning and an even weaker link be- 0 100 102 104 106 108 tween seismic activity and lightning. The relationships between Number of located LMA sources acoustic activity and lightning suggest a connection between the gas Reduced Displacement vs Total LMA Sources released and lightning, but further study is needed to understand 80 this connection. The amount and temporal distribution of gas re- 60 leased during an eruption could affect the explosivity of an eruption; ) 2 higher explosivity results in ﬁner ash and thus more charge carriers, 40 which should result in an increase in overall electrical activity. In (cm r

D terms of the seismic activity, reduced displacement has been correlat- 20 ed to volcano explosivity index (VEI) and ash column height (McNutt, 1994), however some of the largest sources of uncertainty in these re- 0 0 100 102 104 106 108 lationships came from eruptions with column heights between 10 Number of located LMA sources and 20 km and with VEI of 3, which is the range consistent with the Redoubt Volcano observations. The correlation between column Fig. 22. Seismic duration and reduced displacement vs total located LMA sources for height and lightning is the most revealing, and supports the idea distinct explosive events during the 2009 Redoubt Volcano eruption. The red diamonds that volcanic thunderstorms become electrified in a similar manner indicate events that had secondary explosive events while a volcanic lightning storm was still ongoing following the first event. to meteorological thunderstorms, via ice-charging mechanisms (Williams and McNutt, 2005; James et al., 2008). There are two factors related to column height that could contribute event. One of those was 6 min long but had a peak pressure of only to ice-charging mechanisms: the ambient atmospheric temperature 1.3 Pa (31 March 07:31). The other event was 26 min long, but the and the thermal input from the eruption. First, the temperature in the signal pulsed over that time period (it did not keep a high amplitude troposphere generally decreases with altitude, so higher column alti- over the whole duration), and reached a peak amplitude of only tudes would favor ice production. Laboratory results have shown that 25 Pa. Because of the pulsating nature of the signal, we have marked ice production on silicic particles occurs when the ambient temperature this event (23 March 06:35; event 1, which produced very little ash) is between −10 and −20 °C (Durant et al., 2008). National Weather and a similar event with a pulsed waveform (27 March 07:47) with a Service atmospheric soundings from Anchorage (retrievable from: black diamond in Fig. 21. http://weather.uwyo.edu/upperair/sounding.html) indicate that during the two week eruption the atmosphere was typically −20 °C or colder 3.5.3. Seismic activity at altitudes of 3.5 km and above, and was at least −50 °C above alti- Comparison of the total amount of lightning to the reduced dis- tudes of 10 km. Modeling of eruption columns (see Fig. 4.4 of Sparks placement and seismic durations for each explosive event demon- et al., 1997) indicate that plume temperatures initially decline rapidly strated that events with either long duration (greater than 12 min) with increasing altitude, and then decrease more gradually until reach- or large reduced displacements (greater than 13 cm2) produced vol- ing atmospheric temperature at the level of neutral buoyancy. As given canic lightning storms, though storms also occurred as a result of by the Sparks et al. (1997) model for a typical eruption column, a plume events with shorter durations and smaller reduced displacements. with peak column height of 12 km has a roughly 10–15 degree temper- Fig. 22 shows the relationship between the duration of the seismic ature difference at 9 km altitude, 3 km below the top of the column. signals and reduced displacement for each explosive event and the Given ambient atmospheric temperatures of −50 °C in Alaska at total number of located LMA sources. All types of activity (no light- 9 km, the corresponding modeled plume temperature would be be- ning, weak electrical events, and volcanic lightning storms) were ob- tween −35 and −40 °C, which is cold enough for ice production. The served when the seismic durations were less than 12 min. Events smaller, 6–8 km plumes observed during the Redoubt Volcano eruption with durations longer than 12 min produced volcanic lightning would have temperatures of −35 °C (for 6 km) and −45 °C (for 8 km) storms. Similarly, all types of activity were observed during events at the level of neutral buoyancy, and we expect that freezing conditions with reduced displacements less than 13 cm2, while events with re- would also be present within the column at altitudes 1 or 2 km below duced displacements greater than 13 cm2 produced volcanic light- the top of the column. Second, peak column height has been related ning storms. Among the events with either long duration or large to the amount of thermal energy input during an eruption (Sparks reduced displacement there is no correlation between the duration et al., 1997), meaning that higher altitude columns should have more or reduced displacement and the total number of located LMA thermal energy, which should, in turn, create faster updrafts (column sources. height is also affected by the stratification of the atmosphere and the height of the tropopause; the soundings also show that there were diur- 3.5.4. The relationship between eruptive activity and plume electrification nal and day-to-day variations in the height of the tropopause, which A significant unknown in volcanic lightning research is the degree was less than 10 km). In a regular thunderstorm, strong updrafts aid to which plume electrification processes are controlled by water- electrification by the condensation of water for hydrometeor growth based charging mechanisms or by the separation of charged ash. Pre- and through an increase in collisions between graupel and ice crystals vious research has shown that volcanic plumes contain an abundant by keeping graupel particles aloft in a cloud (MacGorman and Rust, amount of water (Sparks et al., 1997), which is in excess of thunder- 1998). storms (Williams and McNutt, 2005), and charged ash is believed to The ambient temperature during the eruption of Redoubt Volcano be responsible for at least the explosive phase lightning. The possible was certainly cold enough to support ice production, even at altitudes

Please cite this article as: Behnke, S.A., et al., Observations of volcanic lightning during the 2009 eruption of Redoubt Volcano, J. Volcanol. Geotherm. Res. (2012), doi:10.1016/j.jvolgeores.2011.12.010 18 S.A. Behnke et al. / Journal of Volcanology and Geothermal Research xxx (2012) xxx–xxx between 3.5 and 10 km, therefore it is possible that the difference be- activity observed during the 2006 eruption of Augustine Volcano. tween the weak electrical events or the events with no lightning and The explosive and plume phases of electrical activity first observed the volcanic lightning storms was the variation in updraft strength. at Augustine Volcano were also observed during Redoubt Volcano's Even after ice creation is thermodynamically possible in a plume, suf- volcanic lightning storms. Similar to Augustine Volcano, explosive ficient collisions must occur between particles of different size in phase activity at Redoubt Volcano consisted of vent discharges and ordertochargetheparticles,whichthenmustbesufficiently separated near-vent lightning that were identified using the flash algorithm as from each other to create strong electric fields. In the weak electrical single-source discharges and small discharges. Plume phase activity events the updrafts may not have been strong enough or did not last at Redoubt Volcano was much more intense than what was observed long enough to create and separate a large amount of charged particles. at Augustine Volcano (the explosive eruptions of Redoubt Volcano Because there is little distinction between the weak events and the were also much larger than Augustine Volcano); a few hundred dis- events with no lightning in terms of the seismic and acoustic activity charges were observed during the plume phase of an explosive or the column height (though there is only one plume height measure- event at Augustine Volcano, while thousands of discharges were ob- ment for the no-lightning events), there is not a clear indication of why served during the plume phases of the volcanic lightning storms at some of the small plumes did not become electrified enough to produce Redoubt Volcano. Plume lightning at Augustine Volcano consisted of lightning. The ambient atmospheric conditions do not seem to have thunderstorm-like discharges that were 10 km or more in length, contributed to the production or non-production of lightning in the while plume lightning at Redoubt Volcano varied greatly in size and small plumes since the −20 °C isotherm varied between only 2.6 and included the roughly 100-meter single-source discharges and exten- 3.7 km between 23 March and 5 April (with the exception of one sive regular flashes up to 50 km2 in area and 20 km or more in length. spike to 4.7 km on 26 March). The Redoubt Volcano results support the Augustine Volcano find- If charged ash was affecting the electrification, we would expect to ings that two separate electrification mechanisms were responsible see a relationship between the amount of ash and the total amount of for the two observed regimes of electrical activity, namely an lightning from each explosive event. Though the ash deposits from explosion-based charging mechanism and a plume-based charging the eruption were measured (Wallace et al., this issue), distinct mea- mechanism. Despite the observed delay in the onset of the continual surements for every single explosive event were not possible because RF (explosive phase) signal during the explosive eruptions at Redoubt several of the explosive events happened in quick succession and left Volcano, the signal was still correlated with the explosive activity and a composite layer of ash. Without distinct measurements for each in- was likely also caused by an explosion based charging mechanism dividual event, we can't compare the amount of lightning in each vol- such as fractoemission and the boiling of water. The difference in canic lightning storm to the total amount of ash. But even if we could, the relative strength of the continual RF signal between Redoubt Vol- the role that ash played in electrification would still be unclear be- cano (Redoubt Volcano being less strong) and Augustine Volcano was cause ash could contribute to electrification in multiple ways. First, probably due to the eastern ridge of Redoubt Volcano blocking the because the ash was already charged, it could have added to the elec- view of the activity. Additionally, even though there was no lull ob- trification of the plume simply through gravitational separation of ash served between the explosive and plume phases at Redoubt Volcano, of opposite polarities. But ash and ice were not necessarily separate the time between the onset of the eruption and the onset of the particles in the plume. Ash can act as condensation and/or ice nuclei, plume phase is sufficiently long to account for the time needed for so the charged ash could have become ice particles, which would water-based or ash-based electrification of the plume phase. This dif- have then acquired a greater or lesser charge through collisions ference and the difference in the amount of plume lightning between with other ice particles. Therefore, ash and water-based electrifica- Augustine Volcano and Redoubt Volcano is most likely due to Re- tion mechanisms may have been working together in the plume, doubt Volcano's explosive events being much larger and more ener- and aren't necessarily separable from each other. getic. Background atmospheric conditions can play a significant role A similar correlation between column height and lightning was in ice creation and thus electrification, but the size of the explosive observed by Bennett et al. (2010) during the eruption of Eyjafjallajö- events is the dominant factor here. The observations of lightning kull, however due to differences in the types of lightning measured from Redoubt Volcano support the findings from Augustine Volcano and the types of eruptions between Redoubt Volcano and Eyjafjallajö- that the explosive phase was the result of charged ash electrified by kull, a detailed comparison between both sets of results is not appro- eruptive mechanisms and that the plume phase was the result of sub- priate. Bennett et al. (2010) compared lightning rates detected by the sequent and likely water-based electrification processes in the plume. UK Met Office ATDnet (a long-range lightning location network that Our lightning observations show that the charge structure of the detects return strokes from cloud-to-ground lightning with a peak plume evolved over the duration of a storm from an initially disorga- current greater than 3 kA) and found that during times when the nized and clumpy structure to a horizontally stratified structure similar plume produced lightning, there was a linear correlation between to what is observed in meteorological thunderstorms. Lightning during column height and the number of detected lightning strokes (there the early stages of the plume phase consisted of high rates of single- were conflicting times when elevated column heights did not pro- source discharges, small discharges, and high rates of regular flashes duce lightning and Bennett et al. (2010) noted that ambient atmo- that were on average no larger than 3 km2. This indicates that the spheric conditions could have played a role in this). Cloud-to- charge structure was initially chaotic and most likely consisted of ground lightning is, however, not necessarily the best measure of clumps of positive and negative charge strewn throughout the initially total electrical activity. In the Redoubt Volcano data, for example, turbulent plume. Over time, discharge rates decreased and the area of there is not a linear relationship between the number of regular regular flashes increased, indicating that turbulence was subsiding flashes in a storm and the number of cloud-to-ground discharges and stratified charge layers similar to those in thunderstorms were detected by either the WWLLN or BLM networks. Nevertheless, in developing. the larger picture these results are further evidence that factors related The overall size of a volcanic lightning storm, as measured by the to plume height are playing a role in lightning production in volcanic total number of located LMA sources, was most strongly influenced plumes. by the peak height of the eruption column. The volcanic lightning storms occurred in plumes with peak column heights greater than 4. Conclusions 10 km and the weak electrical events and events with no lightning occurred in plumes with peak column heights less than 10 km. It is The electrical activity observed during the explosive events of the possible that the thermal energy responsible for the eruption columns 2009 Redoubt Volcano eruption was very similar to the electrical greater than 10 km created strong updrafts, providing similar

(a) 5 4 3 2 y (km) 1 0 -1 09:23:58 09:24:04 09:24:10 09:24:16 09:24:22 (b) Time (UTC) 8 6 4 2

y (km) 0 -2 -4 09:28:18 09:28:24 09:28:30 09:28:36 09:28:42 Time (UTC)

Fig. 23. Two examples of located LMA data from 28 March, plotted as the y (north–south) position vs time. Panel (a) shows 20 s of data during a period of relatively high-rate activity, while panel (b) shows 20 s of data during a period of relatively low-rate activity. The colors indicate groups of points that were identified as flashes or small discharges by the flash algorithm. The black points are sources identified as single-source discharges.

(a) 8 6 Flash detail 4 shown below

y (km) 2

0 -2 09:30:40.00 09:30:45.00 09:30:50.00 09:30:55.00 09:31:00.00 (b) Time (UTC) 8 6

y (km ) 2

0 -2 09:30:54.80 09:30:54.95 09:30:55.10 09:30:55.25 09:30:55.40 Time (UTC)

Fig. 24. An example of relatively large intracloud lightning on 28 March, plotted as the y (north–south) position vs time. Panel (b) shows one of the large discharges from panel (a) on an expanded time scale. pass and 2500 m and 150 ms for the second pass. These parameters James, M.R., Lane, S.J., Gilbert, J.S., 2000. Volcanic plume electrification — experimental investigation of fracture-charging mechanism. Journal of Geophysical Research are more restrictive than those reported by MacGorman et al. 105, 16641–16649. (2008), who used a space and time separation of 3000 m and James, M.R., Wilson, L., Lane, S.J., Gilbert, J.S., Mather, T.A., Harrison, R.G., Martin, R.S., 500 ms for thunderstorm studies. 2008. Electrical charging of volcanic plumes. Space Science Reviews 137, 399–418. fi fl Krehbiel, P.R., Thomas, R.J., Rison, W., Hamlin, T., Harlin, Davis, M., 2000. GPS based After the algorithm classi es the ashes, the area of each dis- mapping system reveals lightning inside storms. 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