JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115, A00F12, doi:10.1029/2009JA014579, 2010 Click Here for Full Article

Locations of chorus emissions observed by the Polar Plasma Wave Instrument K. Sigsbee,1 J. D. Menietti,1 O. Santolík,2,3 and J. S. Pickett1 Received 18 June 2009; revised 20 November 2009; accepted 17 December 2009; published 8 June 2010.

[1] We performed a statistical study of the locations of chorus emissions observed by the Polar spacecraft’s Plasma Wave Instrument (PWI) from March 1996 to September 1997, near the minimum of solar cycles 22/23. We examined how the occurrence of chorus emissions in the Polar PWI data set depends upon magnetic local time, magnetic latitude, L shell, and L*. The Polar PWI observed chorus most often over a range of magnetic local times extending from about 2100 MLT around to the dawn flank and into the dayside magnetosphere near 1500 MLT. Chorus was least likely to be observed near the dusk flank. On the dayside, near noon, the region in which Polar observed chorus extended to larger radial distances and higher latitudes than at other local times. Away from noon, the regions in which chorus occurred were more restricted in both radial and latitudinal extent. We found that for high‐latitude chorus near local noon, L* provides a more reasonable mapping to the equatorial plane than the standard L shell. Chorus was observed slightly more often when the magnitude of the solar wind magnetic field BSW was greater than 5 nT than it was for smaller interplanetary magnetic field strengths. We also found that near solar minimum, chorus is twice as likely to be observed when the solar wind speed is greater than 450 km/s than it is when the solar wind speed is less than 450 km/s. Citation: Sigsbee, K., J. D. Menietti, O. Santolík, and J. S. Pickett (2010), Locations of chorus emissions observed by the Polar Plasma Wave Instrument, J. Geophys. Res., 115, A00F12, doi:10.1029/2009JA014579.

1. Introduction Meredith et al., 2001, 2002; Summers et al., 2004]. Chorus emissions are thought to be generated through nonlinear [2] Whistler mode chorus emissions are electromagnetic processes involving an electron‐cyclotron resonance between waves at frequencies between about one tenth of the equa- whistler mode waves and energetic electrons in the outer torial cyclotron frequency, f /10, up to near f , which typi- ce ce radiation belt [Helliwell, 1967; Nunn et al., 1997]. These cally covers a frequency range from a few hundred Hz to waves have also been associated with solar wind pressure several kHz [see Burtis and Helliwell, 1969; Tsurutani and pulses [Lauben et al., 1998] and they may play an important Smith, 1974; Burtis and Helliwell, 1976]. When viewed on role in geomagnetic storms [Santolík et al., 2004; Li et al., timescales of minutes or hours, chorus typically appears in 2007]. Chorus emissions may be able to accelerate elec- time‐frequency spectrograms as narrow band emissions that trons in the outer radiation belts to energies greater than sometimes have a gap near f /2 that divides these waves into a ce 1 MeV through a stochastic process occurring in the low lower and an upper band. When viewed on timescales of a few density region outside the plasmapause [Horne and Thorne, seconds, a time‐frequency spectrogram of chorus emissions 1998; Summers et al., 2002; Horne et al., 2003, 2005a, reveals discrete chorus elements, which correspond to individual 2005b]. Recent simulation results have suggested that rela- whistler mode wave packets. Chorus elements typically tivistic turning acceleration due to nonlinear particle consist of rising tones, but falling tones and elements with dynamics could also accelerate electrons from a few hundred more complicated structures have also been observed [Helliwell, keV up to a few MeV through a resonant wave trapping 1965; Burtis and Helliwell, 1976; Lauben et al., 2002]. process [Omura et al., 2007]. High‐energy electron micro- [3] Substorm electron injections can excite intense whis- bursts are thought to be associated with chorus [e.g., Oliven tler mode chorus emissions near the geomagnetic equator, and Gurnett, 1968; O’Brien et al., 2003] and may be outside the plasmapause [Tsurutani and Smith, 1974, 1977; related to auroral X rays observed on balloon experiments [Anderson and Milton, 1964]. Other research has suggested 1Department of Physics and Astronomy, University of Iowa, Iowa City, that the scattering of electrons by chorus emissions may Iowa, USA. contribute to particle precipitation in the diffuse aurora [Inan 2Institute of Atmospheric Physics, Academy of Sciences of the Czech Republic, Prague, Czech Republic. et al., 1992; Ni et al., 2008; Meredith et al., 2009]. 3Faculty of Mathematics and Physics, Charles University, Prague, [4] Although chorus emissions are thought to be gener- Czech Republic. ated in the equatorial plane, chorus‐like electromagnetic emissions can be found at higher latitudes. Some studies Copyright 2010 by the American Geophysical Union. have suggested that the whistler mode waves observed at 0148‐0227/10/2009JA014579

A00F12 1of17 A00F12 SIGSBEE ET AL.: LOCATIONS OF POLAR PWI CHORUS EMISSIONS A00F12 high latitudes are chorus generated near the magnetic (PWI) [Gurnett et al., 1995], which used three orthogonal equator that has propagated to high latitudes [e.g., Bortnik et electric dipole antennas provided by the Electric Field al., 2007]. Under certain conditions, it appears that these Instrument (EFI) [Harvey et al., 1995], two in the spin plane waves can be reflected back toward the equator after prop- and one aligned along the spacecraft spin axis. Magnetic agating to high latitudes [Chum and Santolík, 2005; Santolík fields were measured using the PWI’s magnetic loop et al., 2006]. There has also been evidence that some of the antenna and triaxial magnetic search coil antenna. The Polar off‐equatorial whistler mode emissions may be generated in PWI used five receiver systems to process signals from minimum B pockets at high latitudes on the dayside, near these antennas: a digital wideband receiver (WBR), a high‐ the cusp [Tsurutani and Smith, 1977; Pickett et al., 2001; frequency waveform receiver (HFWR), a low‐frequency Vaivads et al., 2007; Tsurutani et al., 2009]. waveform receiver (LFWR), two multichannel analyzers [5] In this paper, we will explore how the occurrence (MCA), and a pair of sweep frequency receivers (SFR). A probability of chorus emissions in the Polar Plasma Wave key feature of the Polar plasma wave instrument was the Instrument (PWI) data set depends upon magnetic local time, ability to make simultaneous measurements from six magnetic latitude, L shell, and L*. We will also discuss how the orthogonal electric and magnetic field sensors. This feature chorus occurrence probability depends upon the Kp and Dst permitted the analysis of the chorus wave vectors [LeDocq geomagnetic indices, as well as upstream solar wind conditions. et al., 1998; Sigsbee et al., 2008]. The WBR was capable [6] The Polar spacecraft was launched on 24 February of using up to 8‐bit resolution and sample rates as high as 1996 into a highly elliptical orbit, initially with apogee at 249 k samples/s, providing sufficient time and frequency ∼9 Earth radii (RE) in the Northern Hemsiphere, perigee resolution for viewing individual chorus elements. These at 1.8 RE in the Southern Hemisphere, and an approximately features of the Polar PWI are essential for identifying chorus 90° inclination [Acuña et al., 1995]. The Polar PWI made events and analyzing the properties of the chorus emissions. observations of plasma waves in the Earth’s magnetosphere The Polar PWI provided plasma wave data from 26 March between frequencies of 0.1 Hz and 800 kHz from March 1996 to 16 September 1997 (or 540 days). 1996 to September 1997. During 1996 and 1997, as the [9] In our data analysis, we were careful to avoid known Polar spacecraft traveled from high latitudes in the Northern difficulties with the PWI data set. During intervals of Hemisphere down to the equatorial plane, it often skimmed roughly 1 h or less centered on the lowest L shell values of along the boundary of the plasmapause, cutting through the the Polar orbit (twice per orbit), almost all PWI data taken Van Allen radiation belts and entering the plasmasphere at with the electric and loop antennas are contaminated with low altitudes near the equator. After reaching perigee in the interference and crosstalk. This contamination resulted from Southern Hemisphere, Polar encountered similar regions as electric field antenna preamplifier oscillations in regions of it traveled back to high latitudes in the Northern Hemi- high plasma density and interference from the EFI. Polar sphere. The 18 h period of Polar’s orbit provided roughly was generally inside the plasmasphere, where chorus is not two passes through the radiation belts every day. likely to be observed, when these oscillations occurred so [7] The extended coverage at high latitudes provided by eliminating these intervals should not adversely affect our Polar is especially important near local noon, where struc- statistics. These oscillations can be clearly seen in the SFR tured, chorus‐like emissions can often extend into the electric field spectrograms at, or slightly below, the upper vicinity of the cusp. These high‐latitude dayside emissions hybrid resonance frequency and its harmonics. The PWI can feature the classic rising elements of equatorial chorus, electric field measurements taken during the approximate 5– but we have also found falling tones, elements with hook‐ 10 min periods near the end of lengthy eclipses are also like structures, combinations of different spectral forms, and considered uninterpretable due to abnormal behavior of the possible triggered emissions [see, e.g., Helliwell, 1967, EFI electric antennas, so we did not include data from p. 206] in the Polar PWI data set at high latitudes. periods when Polar was eclipsed. The behavior during Regardless of their original source, these waves propagate in eclipses may have been caused by a combination of the the whistler mode and may interact with electrons in a preamplifier oscillations and the effects of spacecraft similar manner to chorus generated near the equator. Our charging [e.g., Laakso, 2002] on the antennas when they are study of chorus using the Polar PWI data set provides a in shadow. We also avoided intervals when the Polar Plasma different perspective on chorus emissions from earlier Source Instrument (PSI) was operating, as interactions with studies using satellites such as CRRES [Meredith et al., PSI caused enhanced background noise levels and interfer- 2002], which were not polar orbiting and unable to ence below 40 kHz. observe high‐latitude chorus events near noon. The studies [10] To insure that only chorus emissions were included in of chorus using CRRES data [e.g., Meredith et al., 2002, our study, we identified intervals when chorus emissions 2003] were also performed using data taken near the max- were observed by visually inspecting the PWI SFR spec- imum of solar cycle 22, while the Polar PWI data were trograms from 26 March 1996 to 16 September 1997. The available near the minimum of solar cycle 22/23. Our study PWI SFR experiment obtained both electric and magnetic is therefore complementary to past statistical studies using field measurements over the frequency range 24 Hz to data from CRRES and other satellites. 808 kHz. For logarithmically spaced frequencies, the PWI SFR took about 33 s to sweep through the frequency range 2. Data Sets and Models in which chorus is typically observed (200 Hz to 12.5 kHz). [11] Many previous studies of chorus and electron 2.1. Polar Plasma Wave Instrument acceleration have used fixed frequency ranges for the lower [8] The suite of 12 scientific instruments on board the ( fce/10 < f < fce/2) and upper chorus emission bands ( fce/2 < Polar satellite included the Polar Plasma Wave Instrument f < fce) with amplitude thresholds to automatically distin-

2of17 A00F12 SIGSBEE ET AL.: LOCATIONS OF POLAR PWI CHORUS EMISSIONS A00F12 guish chorus from other waves [e.g., Meredith et al., 2002; may not be the best parameter to use in this study due to the Li et al., 2009; Pokhotelov et al., 2008]. Chorus is believed high latitudes reached by Polar and the compression of the to be generated near the equatorial plane, so at the high Earth’s magnetic field on the dayside. A parameter that may latitudes reached by Polar it may not be appropriate to use more accurately reflect the structure of the magnetospheric the local electron cyclotron frequency to distinguish these magnetic fields and behavior of the trapped particles in the waves from other emissions. While it is possible to project radiation belts is L*[Roederer, 1970]. The definition of this the local cyclotron frequency down to the equator using a parameter is dipole field or other magnetic field model, we preferred to 2k visually screen the PWI SFR data for evidence of chorus as L* ¼ 0 ð2Þ there are uncertainties in the field line mapping. When ÈRE chorus was observed by the PWI SFR, we recorded the where k0 is the Earth’s dipole moment, RE is the radius of lower frequency limit of these emissions. We will discuss F the behavior of the lower chorus frequency limit and the Earth, and is the magnetic flux enclosed by a drift uncertainties in determining the equatorial electron cyclo- shell [Roederer, 1970]. In a dipole magnetic field L* is the tron frequency further in section 3. distance from the center of the Earth to the equatorial point of a given field line, in units of Earth radii. All pitch angles [12] When available, data from the PWI WBR were also have the same L* for a given point in space. To determine used to help distinguish chorus emissions from other types ‐ of waves. Polar data do not always show a clear, distinct L*, we used the ONERA DESP library version 4.2 (available plasmapause, particularly during times when the spacecraft at http://craterre.onecert.fr/support/user_guide.html) and the is skimming along this boundary. This can make it difficult Tsyganenko 1989c magnetic field model [Tsyganenko, to determine whether or not the waves observed by the SFR 1989] with real Kp values downloaded from the World are chorus or plasmaspheric hiss. Like chorus, plasma- Data Center for Geomagnetism in Kyoto, Japan. The Tsy- spheric hiss is also a whistler mode emission, but hiss tends ganenko 1989c magnetic field model was selected because to be more broadband, has less fine structure than chorus, the only geomagnetic parameter required as an input to this and may be involved in the loss of radiation belt electrons model was the Kp index, which is available throughout the [e.g., Li et al., 2007; Santolík et al., 2001; Meredith et al., entire time period of interest. Later magnetic field models 2006]. Recent studies suggest that in some regions, chorus require additional inputs which were often unavailable may leak into the plasmasphere and evolve into hiss [Chum during 1996 and 1997. and Santolík, 2005; Bortnik et al., 2008, 2009]. In addition, structureless, hiss‐like emissions within the typical chorus 2.3. Geomagnetic Indices and Solar Wind Parameters frequency range are often observed outside of the plasma- [14] The auroral electrojet (AE) index may be relevant to sphere. Close to midnight, the highly elliptical nature of the physical processes studied in this paper and has been Polar’s orbit often caused the spacecraft to leave the plas- used in earlier chorus studies [e.g., Meredith et al., 2001, masphere at high latitudes and enter directly into the auroral 2002; Li et al., 2009]. Unfortunately, the AE indices have zone. While chorus is expected to be observed just outside not been calculated for 1996 by the World Data Center for the plasmasphere near the equator, the waves observed by Geomagnetism and only the quick‐look AE indices, which Polar near the plasmapause on the nightside often appeared are intended just for space weather monitoring purposes, are to be generated locally in the auroral zone. Close to noon, available in 1997. Because AE was not available throughout chorus emissions can propagate to high latitudes and the entire time period of interest, we instead attempted to become confused with whistler mode and magnetosonic look for correlations between chorus occurrence rates and waves generated locally in the cusp region. These features of the Kp and Dst indices. The planetary Kp index is an activity the waves observed in the inner magnetosphere sometimes level rating from 0 to 9 for 3‐h intervals based upon ground make it difficult to identify chorus from the SFR alone, so magnetometer data. The Dst indices are derived from a we consulted the high‐resolution WBR data to confirm the network of near‐equatorial geomagnetic observatories that presence of discrete chorus elements whenever possible. As measures the low‐latitude horizontal magnetic variations the actual frequencies of chorus emissions may vary due to the globally symmetrical equatorial electrojet, also somewhat and the lower frequency of chorus is not always known as the ring current. The equatorial ring current causes exactly fce/10, our approach has the advantage that it sepa- a global depression in the H‐component of the magnetic rates chorus from other types of waves found in the inner field during the main phase of geomagnetic storms. The magnetosphere. final Dst values, in units of nT, are available for all of the time intervals of interest in this paper. 2.2. Magnetic Field Mapping and L* Calculation [15] We also attempted to explore possible relations between chorus and upstream solar wind conditions using [13] The standard dipole L shell the OMNI 2 data set [see King and Papitashvili, 2005] R provided by the NASA Goddard Space Flight Center Space L ¼ ð1Þ cos2 Ã Physics Data Facility (SPDF). The OMNI 2 data set con- tains multispacecraft solar wind magnetic field and plasma where L is the dipole magnetic latitude and R is the radial data. It includes data from the ISEE 3, Wind, and ACE distance measured from the center of the Earth, is often used spacecraft, which are often located about an hour upstream to map spacecraft locations along the magnetic field to of the magnetosphere, as the solar wind flows. OMNI 2 also determine the radial distance away from the Earth at which a includes data from 15 geocentric spacecraft located closer to given field line crosses the equatorial plane. However, L Earth, such as IMP 8. In constructing the OMNI 2 data set,

3of17 A00F12 SIGSBEE ET AL.: LOCATIONS OF POLAR PWI CHORUS EMISSIONS A00F12 the SPDF took into consideration studies of solar wind latitudes of −5° to 5° degrees had f/fce ratios within the upper structures [e.g., Richardson and Paularena, 1998; Weimer band range. Most of the chorus observations which had et al., 2002] and time‐shifted the data to Earth, when 0.5 ≤ f/fce ≤ 1.0 occurred at latitudes above 20°, and many of appropriate. To merge upstream ISEE 3, Wind, and ACE these observations occurred near noon and midnight. These data into OMNI 2, it was assumed that solar wind variation observations could indicate times when only upper band phase fronts are planar, normal to the ecliptic plane, and chorus was observed. However, this appears unlikely, as intersect the ecliptic plane along a line exactly halfway inspection of the Polar WBR measurements showed that the between the ideal Parker spiral and a normal to the Earth‐ upper band tends to be bursty, often appearing and dis- Sun line (the Y GSE axis). The spacecraft locations and the appearing intermittently throughout the same radiation belt measured solar wind flow speeds in the data sets being pass, frequently embedded in structureless, hiss‐like emis- shifted were used with the above assumptions to compute sions. Another possibility is that there could have been an the appropriate time delays. Data from the three upstream off‐equatorial source for some of these waves [e.g., spacecraft were time‐shifted to Earth at 1–5 min resolution Tsurutani et al., 2009], but there may also have been pro- and then average values were computed using the data blems with the field line mappings to the equator at high points whose shifted time tags fell within a given hour. For latitudes near noon and midnight due to the structure of the example, the first OMNI 2 value of the day is the average of magnetosphere in these regions. the data points with shifted time tags between 0000 and [18] We suspect that there were errors in the equatorial 0100 UT. During the time periods of interest in 1996–1997, cyclotron frequency calculated from the model for the the OMNI 2 solar wind parameters consisted mainly of data remaining 4% of our chorus observations (17 h). For about from either Wind or IMP 8. When multiple data sources are 15 h of data, spread over the entire range of magnetic lati- available, OMNI 2 gives inclusion priority to the more time‐ tudes and local times covered by Polar, the estimated f/fce continuous data source. Although IMP 8 was closer to Earth, ratio was less than 0.1. However, the average f/fce value for Wind data were given priority for inclusion in OMNI 2 these data points was 0.08, which is quite close to the typical when they were available because Wind data had fewer gaps lower limit of the chorus frequency range. The calculation of than IMP 8 data. The merged OMNI 2 data set gives the the equatorial cyclotron frequency may have been slightly computed hourly averages at Earth of parameters such as the inaccurate for these times, but it is also possible that these GSM BZ component of the solar wind magnetic field, solar values simply represent occasional outliers of the typical wind dynamic pressure, and solar wind bulk speed, all of chorus frequency range. For about 2 h of data, the calcu- which are useful in assessing coupling between the solar lation of the equatorial cyclotron frequency failed because wind and magnetosphere. the estimated f/fce ratio was greater than 1. These data points were located at magnetic latitudes greater than 24°, mainly near noon and midnight. Problems with the field line mapping 3. Chorus Location Statistics to the equatorial plane might be expected in these regions, [16] Nearly 13,000 h of PWI data were recorded between owing to the dayside compression of the magnetosphere and 26 March 1996 and 16 September 1997. Out of the nearly the stretching of the magnetotail on the nightside. 8602 h of data recorded between magnetic latitudes of −70° [19] On the basis of our event selection using the observed and 70°, about 407 h of data had chorus emissions. The lower frequency limit f of the chorus emissions obtained highest southern magnetic latitude at which chorus was from visual inspection of the SFR spectrograms and the observed was −53°, but there were very few cases of chorus above examination of f/fce, the chorus location statistics we (about 50 min) in the Southern Hemisphere at latitudes will present in this paper represent the occurrence proba- higher than −30°. These cases represent only about 0.2% of bility of lower band chorus, although the upper band may the chorus emissions observed by the Polar PWI during have been present in some cases. To establish the character 1996–1997. During 1996–1997, Polar’s perigee was located of Polar’s orbit and the regions it samples, we will first in the Southern Hemisphere, so Polar generally entered the examine the orbit of the Polar spacecraft and the occurrence plasmasphere before it crossed the magnetic equator as it rate of chorus observed by the Polar PWI as a function of traveled from the Northern Hemisphere to Southern Hemi- ZSM and radial distance in the plane of the magnetic equator. sphere. The maximum magnetic latitude at which chorus In solar magnetospheric (SM) coordinates, the Z axis is was observed in the Northern Hemisphere was 63°. aligned along the Earth’s magnetic dipole axis and gives the [17] The manner in which we selected chorus events distance above the plane of the magnetic equator. In later allowed us to examine the ratio f/fce using the observed sections, we will explore the regions in which Polar lower frequency limit f of the chorus emissions obtained observed chorus using polar coordinate maps of R versus from visual inspection of the SFR spectrograms and the MLT, L versus MLT, and L* versus MLT to gain a better electron cyclotron frequency fce calculated using the equa- understanding of the field line mappings in the regions torial magnetic field from the T89 magnetic field model. For sampled by Polar along its orbit and how chorus may be 93% of our chorus observations (378 h), the ratio of the related to particle motions within the large‐scale magnetic lower chorus frequency limit to the equatorial electron field structure of the regions sampled by Polar. cyclotron frequency fell within the typical lower band 3.1. ZSM Position and Radial Distance chorus range 0.1 ≤ f/fce ≤ 0.5, as expected. For about 3% of our chorus observations (12 h), the ratio of the lower chorus [20] Figure 1 shows the percentage of time that chorus frequency limit to the equatorial cyclotron frequency fell was observed by the Polar spacecraft from March 1996 to within the typical upper band chorus range 0.5 ≤ f/fce ≤ 1.0. September 1997 and the number of minutes spent by Polar None of the chorus observations made between magnetic between ±70° magnetic latitude in four local time sectors

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Figure 1

5of17 A00F12 SIGSBEE ET AL.: LOCATIONS OF POLAR PWI CHORUS EMISSIONS A00F12 using SM coordinates in units of Earth radii (RE). Following the distribution was peaked around 1.5 to 2.5 RE above the the example of Meredith et al. [2002], we plotted the chorus equatorial plane. occurrence rate using Polar’s ZSM position and radial distance [24] Figure 1c shows the percentage of time chorus was R′ in the plane of the magnetic equator. In SM coordinates, observed in the noon sector (0900 to 1500 MLT) and the Z axis is aligned along the Earth’s magnetic dipole number of minutes spent by Polar in the same 0.5 RE bins of axis, the X and Y axes lie in the plane of the magnetic equator, ZSM and R′ used in Figure 1a. We have about 200 h of and the Y axis is oriented perpendicular to the Earth‐Sun line chorus observations between 0900 and 1500 MLT, which with the positive direction pointing toward dusk. represents about 49% of our chorus observations. Polar [21] Figure 1a (top) shows the percentage of time chorus spent 2173 h between 0900 and 1500 MLT, so chorus was was observed in the midnight sector (2100 to 0300 MLT) observed about 9% of the time Polar spent in this sector. for 0.5 RE bins of ZSM and R′. Figure 1a (bottom) shows the Chorus was observed most often in this local time sector. As number of minutes spent by the Polar spacecraft in the same in the midnight and dawn sectors, Polar spends a great deal 0.5 RE bins of ZSM and R′ in the midnight sector. The white of time more than 4 RE above the equatorial plane. How- areas in Figure 1 and all subsequent figures indicate regions ever, in the noon sector, chorus was observed up to 7 RE where no data were available. To facilitate comparisons above the equatorial plane. This is consistent with past between figures, the color scales in Figure 1 and all sub- studies showing that the extent of the region in which chorus sequent figures start from 0% or 0 min of data. The black is observed on the dayside is much larger [Bortnik et al., and purple colored areas therefore represent very small, but 2007; Sigsbee et al., 2008]. Although chorus was observed nonzero numbers of minutes or very small, but nonzero for ZSM locations up to 7 RE in this local time sector, the percentages of time when chorus was observed. Following distribution of chorus observations still had a peak for ZSM < the example of Meredith et al. [2003], we have overplotted 4RE, close to the equatorial plane. black lines to indicate dipole field lines for L shells [25] Figure 1d shows the percentage of time chorus was (equation (1)) L =2,4,6,and8RE. The diagonal lines observed in the dusk sector (1500 to 2100 MLT) and indicate lines of constant magnetic latitude for ±10°, 20°, number of minutes spent by Polar in the same 0.5 RE bins 30°, 40°, 50°, 60°, 70°, and 80°. of ZSM and R′ as before. We have about 39 h of chorus [22] Our event database included 73 h of chorus observa- observations between 1500 and 2100 MLT, which represents tions between 2100 and 0300 MLT, which represents about 10% of our chorus observations. Polar spent about 2206 h in 18% of our chorus observations. Polar spent about 2242 h between 1500 and 2100 MLT, so chorus was only observed between 2100 and 0300 MLT, so chorus was observed about 2% of the time Polar spent in this sector. Chorus was about 3% of the time Polar spent in this sector. Although observed least often in this local time sector. As in the the Polar spacecraft spends a great deal of the time in the midnight, dawn, and noon sectors, Polar spends a great deal midnight sector more than 4 RE above the plane of the of the time more than 4 RE above the equatorial plane. The magnetic equator, chorus was most often observed less than distribution of ZSM locations where chorus was observed in 2RE above the equatorial plane in this local time sector. As the dusk sector is surprisingly similar to the distribution of Figure 1a illustrates, the orbit of Polar during 1996–1997 the locations where chorus was observed near dawn. was such that the spacecraft was located at small radial [26] The local time distribution of chorus presented in this distances from Earth in the Southern Hemisphere. Because paper may be different from what many readers may believe Polar was located so close to Earth, it was often deep inside the expected distribution of chorus occurrence to be. In an the plasmasphere during perigee passes through the Southern early paper on ground‐based whistler mode wave observa- Hemisphere. As a result, we do not have many chorus tions [Storey, 1953], the term “dawn chorus” was used to observations for ZSM < 0 in any of the four local time sectors. describe the type of discrete emissions discussed in our [23] Figure 1b shows the percentage of time chorus was paper. Although Storey [1953] reported that chorus emis- observed in the dawn sector (0300 to 0900 MLT) and sions varied in strength daily, with a maximum near 0600, number of minutes spent by Polar in 0.5 RE bins of ZSM and later satellite data studies [e.g., Russell et al., 1969; Dunckel R′ in the same format as Figure 1a. Polar made about 95 h of and Helliwell, 1969; Tsurutani and Smith, 1977; Meredith et chorus observations between 0300 and 0900 MLT, which al., 2009] have shown that chorus can be observed at all represents about 23% of our chorus observations. Polar magnetic local times, including near dusk. Although we spent about 1981 h between 0300 and 0900 MLT, so chorus found a similar spatial distribution of chorus observations was observed about 5% of the time Polar spent in this sector. near dusk and dawn, we can see from Figure 1d that the As in the midnight sector, Polar spends a great deal of its percentage of time when chorus was observed in the dusk time in this sector more than 4 RE above the equatorial sector is much less than in the dawn sector. We will discuss plane, but chorus was mainly observed close to the equa- the local time variation in chorus occurrence further in torial plane. However, in the dawn sector, some chorus was sections 3.2 and 3.3. observed up to 5 RE above the equatorial plane, even though

Figure 1. The percentage of time that chorus was observed by the Polar Plasma Wave Instrument (PWI) and the number of minutes spent by the Polar spacecraft from March 1996 to September 1997 in four local time sectors. (a) At the top is shown the percentage of time chorus was observed in the midnight sector for 0.5 RE bins of the ZSM location and the radial distance R′ in the plane of the magnetic equator. At the bottom is shown the number of minutes spent by the spacecraft in the same bins of ZSM and R′. Black lines indicate constant magnetic latitude and L shells. (b) Same format as Figure 1a for the dawn sector. (c) Same format as Figure 1a for the noon sector. (c) Same format as Figure 1a for the dusk sector.

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dayside outer zone chorus in minimum B pockets located at ∼30°–40° above the magnetic equator. These pockets are created by the solar wind compression of the dayside magnetosphere and the field magnitudes inside them may be less than at the equator. The character of the waves observed by Polar in this region is often slightly different from those observed near the equator. The elements often do not have the typical rising tone structure of chorus. Both rising and falling tones, as well as elements with a hook‐like structure can be found here, often embedded in a background of structureless, hiss‐like emissions. However, these waves do appear to be whistler mode [Santolík et al., 2006] so they may still be of dynamical importance to the behavior of high‐energy electrons in this region. 3.2. Variation With R, Magnetic Local Time, and Magnetic Latitude [29] Figure 2 (top) shows a polar coordinate plot of the overall percentage of time chorus was observed by Polar PWI as a function of radial distance R in Earth radii (RE) and magnetic local time. Figure 2 (bottom) shows the orbital coverage by Polar during the entire time interval when Polar PWI data were available (about 540 days) in polar coordinates as a function of radial distance R in RE and magnetic local time. The data have been organized into bins of 0.5 RE in radial distance and 1 h in magnetic local time for magnetic latitudes between −70° and 70°. Note that in Figure 2, R is the total radial distance of Polar from the center of the Earth, not the radial distance in the plane of the magnetic equator. The minimum radial distance at which chorus Figure 2. (top) Percentage of time chorus was observed by was observed was about 2.2 RE, and the maximum radial Polar PWI as a function of radial distance R in Earth radii distance at which chorus was observed was about 7.8 RE. (RE) and magnetic local time in bins of 0.5 RE in radial dis- [30] The plane of the Polar spacecraft’s orbit underwent a tance and 1 h in magnetic local time for magnetic latitudes virtual rotation through approximately 24 h of MLT in − between 70° and 70°. (bottom) Orbital coverage by Polar 1 year, owing to the orbit of the Earth around the Sun. Polar during the entire time interval when Polar PWI data were PWI data were only available for a little over 17 months available. (1.417 years), so the spacecraft covered a region of about 10 h of MLT twice and the other 14 h of MLT only once [27] The difference in the extent of the regions above the during the time interval we considered. This partially explains equatorial plane where chorus was observed in the four local the slight bias in orbital coverage toward the duskside of time sectors is due to the differences in the structure of the the magnetosphere from 12 noon to midnight visible in magnetosphere in the midnight, dawn, and noon local time Figure 2. However, the apsidal precession rate of Polar’s sectors. On the nightside, near midnight, the Polar spacecraft orbit also was a factor. The bias in orbital coverage toward often encountered the auroral zone shortly after leaving the the duskside of the magnetosphere is much more apparent at plasmasphere. The waves observed by Polar on the night- radial distances greater than 6 RE because the apogee of the side tend to be dominated by a variety of waves from the Polar spacecraft’s orbit moved toward the equator at a rate auroral zone, but chorus is still observed occasionally on the of about 15° per year [Acuña et al., 1995]. Because Polar’s nightside. The small area in which chorus was observed by apogee was no longer located directly over the north pole, Polar on the nightside is due both to the structure of the the spacecraft spent noticeably more time at larger radial magnetosphere in this local time sector and the behavior of distances (>6 RE) on the duskside than on the dawnside. Polar’s orbit. [31] In spite of the slight orbital bias of the Polar space- [28] On the dayside, waves showing chorus‐like element craft toward the duskside of the magnetosphere, Figure 2 structures are often observed heading into the region near shows a typical distribution of chorus observations [e.g., the cusp at high latitudes. It is not always clear whether or Tsurutani and Smith, 1977; Meredith et al., 2003]. The not these waves are chorus emissions that have propagated distribution of chorus observed by the Polar spacecraft is from an equatorial source or if the source region is located at peaked in a region starting from about 21 h magnetic local higher latitudes. Some of the chorus‐like emissions observed time, extending around the dawn flank in the direction of by the Polar spacecraft on the dayside at high latitudes may increasing local times to 15 h magnetic local time on the have a source near the magnetic equator [Menietti et al., dayside. Chorus was observed most often on the day side 2009]. Other studies [e.g., Tsurutani and Smith, 1977; from about 9 h magnetic local time to 13 h magnetic local Pickett et al., 2001; Vaivads et al., 2007; Tsurutani et al., time. In the magnetic local time region between 9 and 13 h, 2009] indicate there may be a high‐latitude source for chorus was also observed at greater radial distances than it

7of17 A00F12 SIGSBEE ET AL.: LOCATIONS OF POLAR PWI CHORUS EMISSIONS A00F12 was at other magnetic local times. This is in part due to the outward spiral from low L values on the nightside to higher large number of high‐latitude dayside chorus events L values on the dayside, similar to what was reported by observed by Polar. Burtis and Helliwell [1976]. [32] Figure 3 shows polar coordinate plots of the distribu- [34] While previous studies have used L to examine the tion of chorus observed by Polar PWI in four different chorus occurrence rate, a better parameter to use is L* magnetic latitude ranges as a function of 0.5 RE bins in radial (equation (2)), which attempts to model the drift shells of the distanceand1hbinsinmagneticlocaltime.Figures3a trapped particles in the magnetosphere. The value of L*we and 3b show that the magnetic local time distribution of used was computed using a magnetic field model that takes chorus from −15° to 15° magnetic latitude and 15° to 30° into account the compression of the magnetosphere on the magnetic latitude reflects the overall distribution of chorus day side, as we discussed in section 2.2. Figure 4b (top) shown in Figure 2. However, in Figure 3c, we see that shows a polar coordinate plot of the overall percentage of between 30° and 45° magnetic latitude, the distribution of time chorus was observed by Polar PWI as a function of L* chorus is beginning to shift more toward the dayside mag- and MLT. Figure 4b (bottom) shows the orbital coverage by netosphere. In Figure 3d, we see that between 45° and 70° Polar during the entire time interval when Polar PWI data magnetic latitude, chorus is only observed in a narrow were available in polar coordinates as a function of L* and region between 6 h magnetic local time to 15 h magnetic magnetic local time. As before, the data have been orga- local time. This appears to be due to both the nature of nized into bins of 0.5 RE in L* and 1 h in magnetic local Polar’s orbit and the structure of the magnetosphere, as time for magnetic latitudes between −70° and 70°. The discussed in the previous section. These high‐latitude day- outward spiral of the region of most frequent chorus side chorus events represent only a small percentage of the occurrence from the nightside to the dayside [Burtis and chorus emissions observed by Polar. However, as Figure 3 Helliwell, 1976] is more readily visible in Figure 4b than illustrates, chorus is not observed exclusively in the region it was in Figure 4a, thanks to the improved mapping pro- immediately surrounding the equatorial plane. The waves vided by L*. observed by Polar away from the equatorial plane may [35] Figure 5 shows polar coordinate plots of the per- simply be chorus generated near the magnetic equator that centage of time chorus was observed by Polar PWI and the has propagated to higher magnetic latitudes. However, these orbital coverage by Polar as a function of L* and magnetic waves could also have been generated by other means, such local time for the same magnetic latitude ranges used in as anisotropic electrons that have drifted from the midnight Figure 3. As before, the data have been organized into bins sector to dayside minimum B pockets [Tsurutani et al., of 0.5 RE in L* and 1 h in magnetic local time. The amount 2009] or by the effects of solar wind pressure fluctuations of data shown for magnetic latitudes between 45° and 70° in on an existing population of energetic electrons in the dayside Figure 5d is greatly reduced from the amount of data shown outer zone [Lauben et al., 1998; Tsurutani et al., 2009]. in Figure 3d for the same magnetic latitude range. L* could not always be calculated in the region near noon at high L L 3.3. Dependence Upon , *, Magnetic Local Time, latitudes because the drift shells were not closed. Although and Magnetic Latitude Polar spent 4255 h between 45° and 70° magnetic latitude, [33] Several past studies of chorus have examined the L* could only be calculated for about 36 h of data in this chorus occurrence rate as a function of L and MLT [Russell latitude range. Chorus was observed for about 21 h between et al., 1969; Burtis and Helliwell, 1976; Tsurutani and 45 and 70° latitude, but L* could only be calculated for Smith, 1977; Li et al., 2009]. Figure 4a (top) shows a about 3.5 h. In spite of the greatly reduced amount of data polar coordinate plot of the overall percentage of time shown for high latitudes, Figures 4b and 5 clearly illustrate chorus was observed by Polar PWI as a function of L and that the L* parameter maps Polar’s location on the dayside MLT. Figure 4a (bottom) shows the orbital coverage by at high latitudes to locations in the equatorial plane at radial Polar during the entire time interval when Polar PWI data distances much closer to Earth than the standard L shell. were available in polar coordinates as a function of L [36] Although we used a different approach to selecting (equation (1)) and magnetic local time. The data have been chorus events to that of Li et al. [2009] and other previously organized into bins of 0.5 RE in L and 1 h in magnetic local published studies, our results for the chorus occurrence time for magnetic latitudes between −70° and 70°. As one probability were similar. Figure 5 shows that the high‐latitude can see from Figure 4a, the standard L shell mapping to the chorus occurs mainly on the dayside, in agreement with radial distance away from the Earth in the equatorial plane other recent studies [Li et al., 2009; Tsurutani et al., 2009]. may not be accurate when Polar is located at high latitudes We also agree with the finding of Li et al. [2009] that near noon. Because the standard L shell does not take into nightside chorus is generally observed at lower latitudes account the compression of the magnetic field in the dayside than on the dayside. The distribution of chorus with L* and magnetosphere, when Polar is located at high latitudes near MLT observed by Polar in the midlatitude range from 15° to noon the L shell values are unrealistically high. However, in 30° magnetic latitude is similar to that found by Li et al. spite of the problems with the standard L shell mapping at [2009] for the distribution of chorus observed by THEMIS high latitudes, the distribution of Polar PWI chorus with L and MLT for 10° to 25° magnetic latitude. observations as a function of L and MLT is similar to that of other studies. As Figure 4a shows, the chorus occurrence rate 4. Dependence Upon Geomagnetic Indices from Polar PWI data has a peak near noon on the dayside in agreement with earlier results [Burtis and Helliwell, 1976; and Upstream Parameters Tsurutani and Smith, 1977; Li et al., 2009]. The region of the [37] The AE index is not available for 1996, so attempting most frequent chorus occurrence in Figure 4 features an to examine the dependence of chorus occurrence probabil-

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Figure 3

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Figure 4. (a) At the top is shown the percentage of time chorus was observed by Polar PWI as a function of 0.5 RE bins of L and 1 h bins in magnetic local time for magnetic latitudes between −70° and 70°. At the bottom is shown the orbital coverage by Polar during the entire time interval when Polar PWI data were available using the same bins in L and magnetic local time. (b) The same format as Figure 4a, only now the data are plotted in 0.5 RE bins of L* and 1 h bins in magnetic local time. ities on AE would require us to abandon half of our data set about 18% of these observations occurred for Dst > 0 nT. As and would leave us with too few events to obtain statistically a result of the very low activity and limited range of Dst significant results. We instead attempted to examine the during the times when chorus was observed by Polar, we chorus locations and occurrence probability as a function of were also unable to obtain statistically meaningful correla- the Kp and Dst indices. Unfortunately, we found that most tions with the Dst index. Like Li et al. [2009], we found that of the chorus events in the Polar data set occurred during chorus does occur during relatively quiet times, even though Kp < 3 so we were unable to obtain statistically meaningful many recent studies have focused upon chorus during geo- correlations with this geomagnetic index for a variety of magnetic storms. The Polar data set includes a high number activity levels. As Kp is a 3‐h index, this parameter may also of dayside chorus observations during quiet times at high L* be inappropriate for our data set. values, which is consistent with the idea proposed by Li et [38] We then attempted to look for correlations with the al. [2009] that enhanced electron anisotropies in this Dst index. The largest negative value of Dst reached during region provide favorable conditions for chorus generation a time interval when chorus was observed by Polar was only even during low geomagnetic activity. −70 nT. Only about 1/3 of the Polar chorus observations [39] We used the OMNI 2 data set to examine how the occurred for Dst < −20 nT. About 2/3 of the Polar chorus locations and occurrence rate of chorus emissions might observations occurred during times when Dst ≥−20 nT and depend upon upstream solar wind parameters. Figure 6a

Figure 3. (a) At the top is shown the percentage of time chorus was observed by Polar PWI as a function of 0.5 RE bins of radial distance and 1 h bins in magnetic local time for magnetic latitudes between −15° and 15°. At the bottom is shown the orbital coverage by Polar during the entire time interval when Polar PWI data were available for magnetic latitudes between −15° and 15° using the same bins in R and magnetic local time. (b) The same format as Figure 3a for magnetic latitudes between 15° and 30°, (c) for magnetic latitudes between 30° and 45°, and (d) for magnetic latitudes between 45° and 70°.

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Figure 5

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Figure 6. (a) At the top is shown the percentage of time chorus was observed by Polar PWI as a function of radial distance R in Earth radii (RE) and magnetic local time for bins of 0.5 RE in R and 1 h in magnetic local time for magnetic latitudes between −70° and 70° during intervals when 250 km/s < VSW < 450 km/s. At the bottom is shown the orbital coverage by Polar during the entire time interval when Polar PWI data were available and when 250 km/s < VSW < 450 km/s. (b) The same as Figure 6a for intervals when 450 km/s < VSW < 750 km/s.

(top) shows the percentage of time chorus was observed for Figures 6a and 6b, it does appear that chorus tends to be hourly averaged solar wind bulk speeds VSW between 250 observed significantly (1.5–2 times) more often for higher and 450 km/s as a function of radial distance R in RE and solar wind speeds. magnetic local time. Figure 6a (bottom) shows the orbital [40] To investigate the possible association of chorus with coverage by Polar as a function of radial distance R in RE electron microbursts, we compared Figure 6 with the results and magnetic local time for intervals when the hourly of O’Brien et al. [2003]. O’Brien et al. examined the occur- averaged solar wind speed was between 250 and 450 km/s. rence of low‐altitude MeV electron microbursts observed The data have been organized into bins of 0.5 RE in radial by the SAMPEX satellite from 1996 to 2001 as a function of distance and 1 h in magnetic local time for magnetic lati- L, MLT, and solar wind speed. Analysis shown by O’Brien tudes between −70° and 70°. Figure 6b shows the percent- et al. [2003] indicated that >1 MeV microbursts occur over a age of time chorus was observed and the orbital coverage for wider range of MLT for lower solar wind speeds than they solar wind speeds between 450 and 750 km/s in the same do for higher solar wind speeds. For high solar wind speeds format as Figure 6a. Although the overall distribution of (500 < VSW < 600 km/s), the microburst occurrence fre- chorus observations does not appear to differ much between quency appeared to be strongly peaked near 30% between

Figure 5. (a) At the top is shown the percentage of time chorus was observed by Polar PWI as a function of 0.5 RE bins of L* and 1 h bins in magnetic local time for magnetic latitudes between −15° and 15°. At the bottom is shown the orbital coverage by Polar during the entire time interval when Polar PWI data were available for magnetic latitudes between −15° and 15° using the same bins in L* and magnetic local time. (b) The same format as Figure 5a for magnetic lati- tudes between 15° and 30°, (c) for magnetic latitudes between 30° and 45°, and (d) for magnetic latitudes between 45° and 70°.

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0600 and 1200 MLT, and 4 < L <7RE. For lower solar wind which may not appear in an hourly averaged solar wind speeds (VSW < 400 km/s), microbursts appeared to be found magnetic field data set. between 4 < L <7RE at nearly all MLT, but had a peak [43] We found a better correlation between chorus observa- occurrence of only about 3% on the nightside. O’Brien et al. tions and the magnitude BSW of the solar wind magnetic [2003] found that electron acceleration at low L shells was field. Figure 7a (top) shows the percentage of time chorus associated with ULF wave activity and MeV microbursts and was observed for hourly averaged solar wind BSW <5nT assumed that it was therefore also associated with chorus as a function of radial distance R in RE and magnetic local activity. However, they did not have access to in situ plasma time. Figure 7a (bottom) shows the orbital coverage by Polar wave observations, so they could not confirm this. For low as a function of radial distance R in RE and magnetic local solar wind speeds, we found that chorus occurred between time for intervals when BSW < 5 nT. Figure 7b shows the 20% and 30% of the time Polar spent on the dayside, which is percentage of time chorus was observed and the orbital an order of magnitude higher than the peak microburst coverage for solar wind BSW > 5 nT in the same format as occurrence frequency found by O’Brien et al. [2003] for Figure 7a. As shown in Figure 7, we were slightly more similar solar wind speeds. For high solar wind speeds, we likely to observe chorus in the Polar PWI data set when found a peak occurrence frequency of around 40%, which is BSW > 5 nT than for times when BSW < 5 nT. The effects of once again higher than the peak microburst occurrence fre- increasing solar wind magnetic field strength on the chorus quency found by O’Brien et al. [2003] for similar solar wind occurrence probability seem to be more pronounced on the speeds. For both high and low solar wind speeds, the chorus dayside between radial distances of 4 and 6 RE than at other occurrence rate was peaked on the dayside in the Polar data magnetic local times. It is possible that conditions are more set. This result does not agree very well with the MLT dis- favorable for the creation of minimum B pockets in which tribution of MeV microbursts for high and low solar wind waves may be generated for BSW > 5 nT. However, this may speeds found by O’Brien et al. [2003]. However, this is also indicate more efficient coupling in general between the consistent with an early study of chorus and > 40 keV elec- processes responsible for dayside chorus and upstream trons from the Injun 3 satellite [Oliven and Gurnett, 1968] conditions during times when the interplanetary magnetic which showed that microbursts are always accompanied by field is more intense. chorus, but not all chorus observations are associated with microbursts. Oliven and Gurnett showed an MLT distribution of chorus emissions from Injun 3 that appears quite similar to 5. Discussion ours. They found that the distribution of microbursts fell [44] Other recent studies have examined the average within the region of maximum chorus occurrence but that the chorus amplitudes as a function of L and magnetic local region of microburst occurrence was more restricted in local time. While we cannot compare our results directly to these time. studies, we can compare the shape of the regions in which [41] We also attempted to examine how chorus occurrence these studies found the largest chorus amplitudes to the rates and locations might depend upon the solar wind shape of the regions in which we found the highest chorus dynamic pressure. In past studies of Polar data [Lauben et al., occurrence probability. A survey of chorus observations 1998] there has been evidence for a correlation between from the Cluster STAFF‐SA experiment found that the solar wind pressure pulses and dayside chorus generation. largest lower‐band chorus amplitudes occurred on the day- However, we found little statistical variation with dynamic side and that intense chorus was observed at larger radial pressure in the Polar data set. It appears likely that while the distances on the dayside than on the nightside [Pokhotelov solar wind speed may be more or less steady on the time et al., 2008]. We found that dayside chorus occurs quite scales of the hourly OMNI 2 data set, density perturbations often and can reach larger L values than nightside chorus, associated with pressure pulses are short‐lived [see, e.g., which compares favorably with their result. We also found Fox et al., 1998] and tend to be lost in the hourly averages. that the chorus occurrence probability is greatest near dawn As a result, most of the Polar chorus events are associated and on the dayside and smallest near dusk. However, the with a range of average dynamic pressures in the OMNI 2 distribution of the peak amplitude regions for lower band data set that has too little variation to obtain any meaningful chorus from Pokhotelov et al. [2008] is much more sym- statistical results. metrical with magnetic local time than our chorus occur- [42] The next upstream solar wind parameter we exam- rence distribution. The data shown by Pokhotelov et al. ined using the OMNI 2 data set was the direction of the solar [2008] indicate that the largest amplitude chorus is found wind magnetic field BZ GSM component. We had hoped in the postnoon region, while we generally found the largest that we might see some kind of correlation with the solar chorus occurrence rates right around noon or in the prenoon wind magnetic field direction and the occurrence of the sector. The smaller frequency range of the STAFF‐SA high‐latitude events on the dayside. The distribution of instrument, differences in the orbit of Cluster and Polar, the chorus observation locations did appear to shift slightly usage of L instead of L*, and the usage of fixed frequency toward the postnoon and dusk regions for BZ GSM > 0, but ranges and amplitude thresholds to identify chorus by the number of Polar chorus observations in the dusk region Pokhotelov et al. [2008] all are factors in the difference is still quite low, even for these BZ GSM > 0 conditions. The between our results and theirs. shift toward dusk for BZ GSM > 0 is probably not a sta- [45] On the other hand, the distributions of the average tistically significant result, so we do not show it here. Also, chorus amplitudes from THEMIS [Li et al., 2009] and much of the geomagnetic activity during solar minimum CRRES [Meredith et al., 2001; Bortnik et al., 2007] show appears to be associated with oscillating BZ GSM magnetic the same kind of dawn‐dusk asymmetry in the regions of field directions in the solar wind [Tsurutani et al., 2006] large wave amplitudes that we see in our chorus occurrence

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Figure 7. (a) At the top is shown the percentage of time chorus was observed by Polar PWI as a function of radial distance R in Earth radii (RE) and magnetic local time for bins of 0.5 RE in R and 1 h in magnetic local time for magnetic latitudes between −70° and 70° during intervals when BSW < 5 nT. At the bottom is shown the orbital coverage by Polar during the entire time interval when Polar PWI data were available and when BSW < 5 nT. (b) The same as Figure 7a for intervals when BSW > 5 nT. rates. This is quite interesting, as the CRRES data were the lifetime of the Polar PWI due to the low levels of activity taken during solar maximum, and the Polar PWI data are and small range of variations in these indices. Even though from solar minimum. However, the regions of the largest chorus generation has been associated with pressure pulses chorus electric field amplitudes from CRRES appear to be [Lauben et al., 1998] attempts to find correlations with the much broader on the nightside than the region on the solar wind dynamic pressure were unsuccessful. It is pos- nightside in which we found the highest chorus occurrence sible that the time scales of the variations in solar wind rates. Once again, differences in the orbits of Polar and dynamic pressure that directly affect chorus are much CRRES, the characteristics of the instruments used, our shorter than the hourly averages provided by the OMNI 2 event selection criteria, and the usage of L instead of L* can data set. probably explain these differences. The difference in the [47] The direction of the solar wind magnetic field directly geomagnetic activity level during the lifetimes of CRRES influences the global convection electric field imposed upon and the Polar PWI also may have played a role in the dif- the magnetosphere and controls energy input from the solar ferences between our results. wind into the magnetosphere. Although chorus has been [46] Many recent studies have focused upon chorus as a associated with substorms [Tsurutani and Smith, 1974; storm‐time phenomenon and the possible role of chorus in Meredith et al., 2001, 2002] our attempts to find correlations the acceleration of radiation belt electrons [e.g., Horne and between chorus occurrence and southward solar wind BZ Thorne, 1998; Summers et al., 2002; Santolík et al., 2004]. GSM were not very successful. Our results indicate that the Our results and those of Li et al. [2009] show that chorus direction of the solar wind magnetic field may only have a can also occur during time periods of relatively low to weak or indirect effect on the physical processes and particle moderate geomagnetic activity, consistent with past work populations involved in chorus generation. A complicated indicating the association of chorus with substorms [e.g., sequence of events must take place before solar wind energy Tsurutani and Smith, 1974]. We had difficulties finding transported into the magnetosphere during periods of correlations with the geomagnetic indices available during southward interplanetary magnetic field can be released in

14 of 17 A00F12 SIGSBEE ET AL.: LOCATIONS OF POLAR PWI CHORUS EMISSIONS A00F12 the form of a substorm electron injection. The unpredict- may have been related to the impact of the CME shock wave ability of the time scales for substorms and other magne- upon the magnetosphere. The two CIR/high speed solar tospheric processes may be the main reason we were unable wind events featured more gradual increases in the electron to establish a direct relationship between the solar wind fluxes over a period of several days. The data from these magnetic field direction and chorus occurrence probability events indicate that the role played by resonant interactions in the Polar PWI data set. On the other hand, the data shown with chorus in accelerating electrons may depend on the in Figure 7 indicate that the strength of the solar wind upstream solar wind conditions driving the storm. Our magnetic field may have some influence on the chorus results from Polar support previous studies [e.g., Summers et occurrence probability. al., 2004; Lyons et al., 2005] which have suggested that [48] When we examined the dependence of chorus during solar minimum, events associated with CIRs and occurrence on OMNI 2 upstream parameters, we found the high‐speed solar wind have conditions that are highly best correlation between the chorus occurrence probability favorable for chorus generation and possibly for the accel- throughout the magnetosphere and the solar wind speed. eration of radiation belt electrons by stochastic processes The character of the solar wind is distinctly different between over a period of several days. the maximum and minimum of the solar cycle. Near solar [51] Although our past comparisons between Polar chorus maximum, coronal mass ejections (CMEs) occur frequently observations and CEPPAD data during selected case studies and are the cause of strong geomagnetic storms. During the suggest a relationship between chorus and high‐energy declining phase of the solar cycle, high‐speed solar wind electrons, the chorus occurrence distributions presented here from coronal holes and corotating interaction regions (CIRs) did not agree very well with the statistics of MeV micro- are the main causes of geomagnetic activity (see review by bursts shown by O’Brien et al. [2003]. We found that chorus Tsurutani et al. [2006] and references therein). CIRs often was about twice as likely to be observed by Polar when the have oscillating BZ GSM magnetic field components, and as solar wind speed is larger and that there is a similar distri- a result they rarely produce geomagnetic storms with Dst < bution of chorus in MLT for both ranges of solar wind −100 nT. Storms associated with CIRs appear to have speeds shown in Figure 6. O’Brien et al. [2003] found that extended recovery phases which can last up to a solar rotation microbursts were about 10 times more likely to occur for (27 days) due to Alfvén waves within the high‐speed stream. high solar wind speeds than for low solar wind speeds and These Alfvénic solar wind structures have been associated that the MLT distribution of events depended upon the solar with high‐intensity, long‐duration continuous AE activity wind speed. One possible reason why our results do not (HILDCAA) events [Tsurutani and Gonzalez, 1987]. Recent correspond very well to the distribution of MeV microburst work indicates that chorus may play a role in accelerating occurrence in the work of O’Brien et al. [2003] is that they radiation belt electrons during HILDCAA events [Summers used SAMPEX data from 1996 to 2001, so a variety of solar et al., 2004; Lyons et al., 2005]. wind conditions are included, not just those near solar [49] The results presented in this paper and our past work minimum. The orbits of Polar and SAMPEX may also play support the idea that moderate, but prolonged geomagnetic a role. However, Oliven and Gurnett [1968] believed that activity associated with CIRs and high‐speed solar wind the difference was because electron motions are more streams can establish conditions that are quite favorable for restricted by the Earth’s magnetic field than the propagation chorus generation. Although several well‐studied storms of chorus emissions. Recent work has explored the whistler associated with CMEs occurred during the lifetime of the mode wave frequencies that should be resonant with elec- Polar PWI, much of the PWI data set appears to consist of trons of various energies [Lorentzen et al.,2001;Chum et activity associated with high‐speed solar wind and CIRs, as al., 2007]. According to Lorentzen et al. [2001], MeV is typical for solar minimum [Watari and Watanabe, 1998]. electrons will only resonate with waves at frequencies less The statistical results obtained in this paper are consistent than 200 Hz near the equator, and interactions with waves at with the results from our earlier work with Polar PWI data. frequencies of 1–2 kHz will only take place at magnetic In the work of Sigsbee et al. [2008], we investigated the role latitudes near 30 degrees. Our chorus observations include of whistler mode chorus in accelerating outer radiation belt waves with a wide range of frequencies observed at many electrons during four moderate geomagnetic storms when different latitudes. When these factors are considered, it is Polar PWI data were available. The storm time periods we not very surprising that the chorus occurrence rates shown in examined included two storms associated with coronal mass Figure 6 are very different from the MeV electron micro- ejections (CMEs), the well‐studied 10–13 January 1997 burst occurrence rates presented by O’Brien et al. [2003]. International Solar Terrestrial Physics (ISTP) event [e.g., Fox et al., 1998; Lauben et al., 1998; Li et al., 1998; Selesnick and Blake, 1998; Thomsen et al., 1998] and the 12–15 May 1997 6. Conclusions event [Baker et al., 1998; Brueckner et al., 1998; Thompson et al., 1998]. We compared these two storms with two [52] The overall distribution of chorus observations made geomagnetically active periods that appeared to be associ- by the Polar spacecraft is similar to the distribution found in ated with CIRs and high speed solar wind streams. past studies. However, we found that near noon on the [50] During all four events studied by Sigsbee et al. dayside, chorus can extend to much higher latitudes and [2008], the Polar Comprehensive Energetic Particle and larger radial distances than it does in other local time sectors. Pitch Angle Distribution (CEPPAD) experiment observed Comparison of different mapping techniques to the equa- increases in the fluxes of energetic electrons (0.8 MeV < E < torial plane indicated that the L* parameter may provide 6.4 MeV). The two events associated with CMEs featured a more realistic and physically meaningful mappings, partic- ‐ sudden increase in the electron fluxes above 0.8 MeV that ularly for the high latitude chorus events on the dayside.

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[53] We were not able to find any statistically meaningful (CMEs): March 1996 through June 1997, Geophys.Res.Lett., 25(15), changes in the spatial distribution or occurrence probability 3019–3022, doi:10.1029/98GL00704. Burtis, W. J., and R. A. Helliwell (1969), Banded chorus‐A new type of of Polar PWI chorus observations as a function of Dst and VLF radiation observed in the magnetosphere by OGO 1 and OGO 3, Kp, but we did see a dependence upon the solar wind drivers J. Geophys. Res., 74(11), 3002–3010, doi:10.1029/JA074i011p03002. of geomagnetic activity. Chorus was slightly more likely to Burtis, W. J., and R. A. Helliwell (1976), Magnetospheric chorus: Occur- – be observed when the magnitude of the interplanetary rence patterns and normalized frequency, Planet. Space Sci., 24, 1007 1024, doi:10.1016/0032-0633(76)90119-7. magnetic field was larger. We found that chorus was about Chum, J., and O. Santolík (2005), Propagation of whistler mode chorus to twice as likely to be observed when the solar wind speed low altitudes: Divergent ray trajectories and ground accessibility, Ann. was greater than 450 km/s than it was for lower speeds, Geophys., 23(12), 3727–3738. Chum, J., O. Santolík, A. W. Breneman, C. A. Kletzing, D. A. Gurnett, and which is consistent with the idea that much of the geo- J. S. Pickett (2007), Chorus source properties that produce time shifts and magnetic activity during solar minimum is associated with frequency range differences observed on different Cluster spacecraft, high‐speed solar wind streams and CIRs. This agrees with J. Geophys. Res., 112, A06206, doi:10.1029/2006JA012061. Dunckel, N., and R. A. Helliwell (1969), Whistler‐mode emissions on the our past work [Sigsbee et al., 2008], which indicated that OGO 1 satellite, J. Geophys. 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