JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. A12, 1440, doi:10.1029/2001JA009154, 2002
April 2000 magnetic storm: Solar wind driver and magnetospheric response K. Emilia J. Huttunen,1 Hannu E. J. Koskinen,1,2 Tuija I. Pulkkinen,2 Antti Pulkkinen,2 Minna Palmroth,2 E. Geoffrey D. Reeves,3 and Howard J. Singer4 Received 2 November 2001; revised 4 June 2002; accepted 26 June 2002; published 13 December 2002.
[1] On 4 April 2000, a coronal mass ejection (CME) took place close to the western limb of the Sun. The shock front of the CME hit the Earth’s magnetosphere on 6 April. A strong interplanetary southward BZ event in the sheath region caused a magnetic storm that was the second strongest in the year 2000 if quantified by the peak of the Dst index. We have analyzed this sequence of events using observations of several spacecraft in the solar wind and at geostationary orbit as well as recordings from more than 80 magnetometer stations at latitudes higher than 40 N. In the sheath region behind the shock, the interplanetary magnetic field had an intense and long-sustained southward magnetic field orientation, and the solar wind magnetic pressure was very large, which compressed the dayside magnetopause inside geostationary orbit for a period of more than 6 hours. We conclude that it was the fluctuating but strongly southward field accompanied by the high pressure that allowed for the exceptionally strong driving of magnetospheric activity. During the main phase of the storm, the magnetosphere and ionosphere were in highly perturbed states, with several activations all around the auroral region. Detailed analysis shows that many of these activations were not substorms, in the sense that they were not associated with poleward and westward electrojet/auroral enhancement or geostationary particle injections, but were directly driven perturbations due to variations in the solar wind features. In fact, it was found that the development of the entire storm was quite independent of substorm activations and injections. Instead, the ring current development was driven by the strong convection enhancements. During the storm, the geomagnetically induced currents were strongly enhanced during several periods. While some activations were associated with substorm onsets or electrojet enhancements, others were caused by extremely localized and short-lived electrojet activations. INDEX TERMS: 2788 Magnetospheric Physics: Storms and substorms; 2111 Interplanetary Physics: Ejecta, driver gases, and magnetic clouds; 2463 Ionosphere: Plasma convection; 2139 Interplanetary Physics: Interplanetary shocks; 1515 Geomagnetism and Paleomagnetism: Geomagnetic induction Citation: Huttunen, K. E. J., H. E. J. Koskinen, T. I. Pulkkinen, A. Pulkkinen, M. Palmroth, E. G. D. Reeves, and H. J. Singer, April 2000 magnetic storm: Solar wind driver and magnetospheric response, J. Geophys. Res., 107(A12), 1440, doi:10.1029/2001JA009154, 2002.
1. Introduction www-istp.gsfc.nasa.gov/istp/events/2000july14/)). In this study we analyze a large storm during which the second [2] The year 2000 provided several intense and interesting lowest Dst value during the year 2000, 288 nT (with solar–terrestrial events. The preliminary (no pressure cor- pressure correction 314 nT), was reached. The storm rection) Dst index reached levels below 200 nT three times, occurred on 6–7 April 2000, when magnificent auroral of which one exceeded 300 nT. In particular, the so-called displays were seen in Central Europe as the main phase of Bastille Day storm, initiated by a coronal mass ejection the storm took place in the time sector 18–01 UT. (CME) on 14 July, has received considerable interest (for [3] It is now well established that CMEs lie behind preliminary information, see the ISTP Web page (http:// practically all large magnetospheric storms [Tsurutani et al., 1988; Gosling et al., 1991; Richardson et al., 2001]. 1Department of Physical Sciences, Theoretical Physics Division, However, there is a rich variety in the geoefficiency of University of Helsinki, Helsinki, Finland. various features associated with interplanetary manifesta- 2Geophysical Research, Finnish Meteorological Institute, Helsinki, tions of CMEs (ICMEs) and it can make a large difference Finland. 3 whether the magnetosphere is hit by a slow ICME without Los Alamos National Laboratory, Los Alamos, New Mexico, USA. an interplanetary shock, by an ICME with shock, or by the 4NOAA Space Environment Center, Boulder, Colorado, USA. shock only when the ICME ejecta does not intercept the Copyright 2002 by the American Geophysical Union. magnetosphere [Gosling et al., 1991; Huttunen et al., 0148-0227/02/2001JA009154$09.00 2002].
SMP 15 - 1 SMP 15 - 2 HUTTUNEN ET AL.: APRIL 2000 MAGNETIC STORM
[4] Finally, the orientation of the interplanetary magnetic field (IMF) plays a decisive role in the energy transfer from the solar wind to the magnetosphere and only events with prolonged and sufficiently strong southward IMF can drive intense magnetospheric storms. For example, Gon- zalez and Tsurutani [1987] determined that IMF Z com- ponent less than 10 nT lasting more than 3 hours can be used as a predictor for the Dst index reaching values below 100 nT. [5] The storm on 6–7 April 2000, was an example of a magnetospheric storm driven by an intense southward IMF in the sheath region between the shock and the ICME. As discussed below, early on 7 April there were signatures of CME ejecta, but the observations suggest that the spacecraft hit only the flanks of the ejecta. In this paper we perform a thorough analysis of the solar wind driver of the storm based on three spacecraft (SOHO, ACE, and WIND) in the upstream solar wind, and one (GEOTAIL) in the dayside magnetosheath. Furthermore, the geostationary GOES-8 and GOES-10 satellites crossed the dayside magnetopause during the event, which allows a reliable confirmation of the magnetopause location determined using the empirical magnetopause model of Shue et al. [1998]. Figure 1. Position of SOHO, ACE, WIND, and GEO- [6] We examine the magnetospheric response to the solar TAIL satellites in 6 and 7 April projected on the ecliptic wind driver using observations in the magnetosphere and in plane. The satellite locations at the time of the shock the polar ionosphere. This includes a careful analysis of the observation are indicated by filled circles. major substorm-like activations using data from a large number of magnetometer stations all around the high- latitude northern auroral zone. Furthermore, we discuss 1989-046, 1991-080, and 1994-084, to study the dynamics the large geomagnetically induced current (GIC) observed of the inner magnetosphere. in Finland during the storm main phase. A better under- [8] To determine the level of magnetic activity at the standing of the detailed connection between the features of Earth we have used the Kp and Dst indices from the World ICMEs and their geomagnetic consequences is essential in Data Center C2 in Kyoto. The Dst index aims to measure developing reliable warning and forecasting systems for the strength of the equatorial ring current, whereas Kp is a operators of ground-based energy distribution systems. more global storm indicator often having significant con- tribution from the high-latitude auroral electrojets [e.g., 2. Data and Methods Gonzalez et al., 1994]. We have also utilized 77 ground- based magnetometers all around the high-latitude auroral [7] We have used observations from several spacecraft, zone in order to study the global ionospheric response and both inside and outside the magnetosphere. Three space- to identify substorm signatures during the storm period. craft, SOHO, ACE, and WIND, were monitoring the solar wind upstream of the Earth. ACE and SOHO were both 3. Storm Sequence From the Sun to the Earth located near the L1 point more than 200 RE from the Earth, and WIND was located, early on 6 April, around 64 RE from 3.1. Solar Observations the Earth, and was traveling toward the Earth. Solar wind [9] The LASCO [Brueckner et al., 1995] C2 coronagraph data from ACE, due to its relatively steady position, have onboard SOHO first observed a halo CME on 4 April 2000 been used to analyze features of the intense southward at 1632 UT. This CME caused a strong magnetic storm at magnetic field structure after the shock that caused the the Earth 2 days later. The initial observations were made major magnetic storm. The WIND data, in turn, due to after a 90 min data gap, when the leading edge had already the closer location of the spacecraft to the Earth, have been left the C2 field of view. The coronagraph images show that used to connect the disturbances in the solar wind to most of the CME material was centered over the west limb. magnetic disturbances in the Earth’s magnetosphere and The plane-of-the-sky speed was reported to be 1188 km/s ionosphere and to estimate the energy input into the (SOHO/LASCO CME catalog (http://cdaweb.gsfc.nasa. magnetosphere as well as the magnetopause location. GEO- gov)). This CME event was associated also with other types TAIL made observations in the magnetosheath, and trav- of solar activity: GOES-8 satellite recorded a C9.7 class ersed from the morning side magnetosphere through the flare in the NOAA active region 8933 (N18W58) at 1524 subsolar magnetopause to the duskside magnetosphere dur- UT. Also H-alpha images from Holloman AFB, New ing the early phases of the storm. Positions of these four Mexico, showed a disappearance of a large filament pre- spacecraft projected on the ecliptic plane on 6–7 April 2000 ceding the CME onset. The filament was located near the are shown in Figure 1. Their locations at the time of the northwest limb (N25W55) and it extended vertically along shock arrival are marked by filled circles. We have also used the surface of the Sun. The filament activation started at data from five geostationary satellites, GOES-8, GOES-10, 1441 UT and continued until 1535 UT. HUTTUNEN ET AL.: APRIL 2000 MAGNETIC STORM SMP 15 - 3
Figure 2. Solar wind parameters and geomagnetic indices for a 2 day interval from 6 to 8 April 2000 measured by ACE. The panels from top to bottom show the magnetic field intensity (a) and magnetic field components in the GSE coordinate system (b–d), the ion speed (e), density (f ), and temperature (g), and the alpha-to-proton ratio and hourly average O+7/O+6 ratio (h), proton beta (i), and dynamic pressure ( j). The dashed horizontal line in panel g presents the temperature expected for normal solar wind expansion and the solid horizontal lines in panels i and j show limit values for CME-associated solar wind plasma.
[10] The first signature of this solar event near 1 AU was field components in the GSE coordinate system (b–d), solar observed soon after the flare when a solar particle event was wind speed (e), density (f ) and temperature (g) as well as recorded. For example, GOES-8 measured the increase in the solar wind alpha to proton ratio and the O+7/O+6 ratio >10 MeV protons starting around 1600 UT on 4 April 2000. divided by 10 (h). We have also calculated the proton beta Continuing acceleration of particles at the interplanetary (i) and dynamic pressure (j). The shock is clearly identifi- shock maintained the intensities at an enhanced level until able as a distinct jump in the solar wind parameters at 1604 the shock arrival at 1 AU on 6 April. UT. Before the shock, the solar wind was flowing at about 380 km/s; at the shock the solar wind speed increased 3.2. Solar Wind Observations suddenly by about 200 to almost 600 km/s. Simultaneously 3 [11] The strong interplanetary shock was first observed the ion density increased from 7 to about 22 cm , and the by the CELIAS instrument onboard SOHO at 1601 UT on 6 magnetic field intensity from 8 to 28 nT. The upstream April. Shortly after this, the shock was recorded by ACE at Alfve´n speed was 64 km/s and the downstream Alfve´n 1604 UT and at 1632:28 UT by WIND. Finally it was speed 128 km/s. The distance between ACE and WIND in detected at 1641:30 UT by GEOTAIL located in the the X-direction was 171 RE and the time difference between magnetosheath. Figure 2 shows the ACE measurements the shock observations was about 28.5 min. If the shock for a 2 day period from 6 to 8 April. Shown in Figure 2 propagation is assumed to have been in the X-direction, this are the solar wind magnetic field intensity (a), magnetic would yield an average shock speed of 638 km/s. SMP 15 - 4 HUTTUNEN ET AL.: APRIL 2000 MAGNETIC STORM
Figure 2. (continued)
[12] The direction of the magnetic field displayed irreg- The chosen shock normal direction gives the shock prop- ular behavior both upstream and downstream of the shock. agation speed 635 km/s along the shock normal which is Also velocity downstream of the shock had irregular behav- consistent with the value calculated above and the Alfve´nic ior. Thus, the shock normal direction (nsh) could not be Mach number 5.1. The shock normal direction is consistent determined very accurately. We have applied different with the CME originating from northern solar hemisphere, techniques [e.g., Tsurutani and Lin, 1985] for different but the shock longitudinal angle is not that large as would upstream and downstream intervals using data from WIND be expected from the western source of the CME. From and ACE spacecraft. The velocity coplanarity theorem WIND measurements we obtain the shock normal direction could not be used for ACE data as there was about a 10 (f, q) = (169 , 48 ) using magnetic coplanarity theorem min data gap in ACE solar wind measurement upstream of and (f, q) = (170 , 44 ) using velocity coplanarity u d u d u the shock. Depending on the method used and selected theorem (nshk(B B ) Â ((V V ) Â B )). These intervals quite different shock normal orientations are values agree substantially well with the direction obtained found. We have applied the magnetic coplanarity theorem from ACE but they give too slow shock velocity (about 560 u d u d nshk(B B ) Â (B Â B ) where u denotes upstream km/s). Besides of the irregular behavior of the magnetic values and d downstream values of the shock for ACE data, field and velocity, the magnetic coplanarity theorem is not which was available with the highest cadence. This yields reliable for near-perpendicular shocks. The shock normal shock normal orientation ( 0.85, 0.13, 0.52) in solar determination could be improved by solving the nonlinear ecliptic coordinates or the longitudinal angle f =171 least squares Rankine–Hugoniot equations [e.g., Vin˜as and and latitudinal angle q = 31 . The shock angle qBn, i.e., Scudder, 1986], but as the solar wind data needed for this the angle between the shock normal and the upstream method was available only from WIND with low cadence magnetic field direction was 85 defining the shock as and the shock orientation is not the main concern of our quasi-perpendicular. The shock speed can be determined paper this is not discussed further. The transfer time of first from conservation of the mass flux. Along the shock normal C2 CME observation to the shock observation at ACE was u u d d u d it is obtained from Vsh = nsh Á ((r V r V )/(r r )). approximately 47.5 hours. However, it should be noted that HUTTUNEN ET AL.: APRIL 2000 MAGNETIC STORM SMP 15 - 5 coronagraph images are associated with the driver gas not In this case, just after the shock, proton beta was over 0.5 and the shock front. it dropped to about 0.2 at the time of the BZ decrease. Later, [13] The IMF Z-component upstream of the shock was the beta values rose again due to the increased particle slightly negative, decreasing after the shock to values below pressure. 10 nT. About 1 hour after the shock, the magnetic field [16] The duration of the high-B structure after the shock turned strongly southward (BZ < 20 nT) and the solar was 16 hours giving a radial extent of 0.22 AU using the wind density and temperature decreased. BZ remained average speed of 570 km/s. Following the high-B structure, below 20 nT for about 7 hours and turned abruptly to Figure 2 shows a period of fast flow with low magnetic field northward orientation before 24 UT on 6 April 2000. After intensity (about 5 nT) starting around 0830 UT on 7 April the shock the magnetic field intensity was high, over 30 nT, 2000. This flow was not a corotating stream [Smith and but its direction fluctuated rapidly. The IMF X-component Wolf, 1976] as the temperature was very low. Many sig- was mainly positive and the Y-component mainly negative natures of the ICME-related plasma [e.g., Gosling, 1990] but they both showed a complex behavior, especially BY were detected a few hours before and during the fast flow. which had long amplitude fluctuations. Slow, monotonic On 7 April there was a discontinuity at 0609 UT when the rotation of the magnetic field, which is the signature of an longitude angle rotated about 170 and the latitude angle ICME exhibiting flux rope topology, commonly a called about 80 . Solar wind plasma parameters and magnetic field magnetic cloud [Burlaga, 1988], was not detected. Instead, intensity did not change in magnitude and the magnetic BZ jumped rapidly from south to north. field normal to the discontinuity boundary had a nonzero [14] The solar wind speed at the edge of the high-B value suggesting a rotational discontinuity. Just after the structure was 570 km/s, and the speed profile stayed discontinuity, the magnetic field became smoother and there relatively flat. The solar wind density (Figure 2f) was was an increase above the CME-related plasma limit in the +7 +6 clearly enhanced with respect to the ambient value through- ratio of na/np (>0.08) as well as in the O /O ratio (>1.0). out the high-B structure. Initially, the density was between However, magnetic field strength decreased within 2 hours 3 10 and 20 cm , but about 1 hour before the BZ northward to as low as 5 nT. At 08 UT on 7 April plasma beta dropped turning the density increased and had two larger enhance- to very low values (<0.1) and coincidentally the temperature ments reaching almost 50 cm 3. Solar wind temperature was much lower than would be expected during such high was also unusually high with a maximum value of about solar wind speed plasma [Richardson and Cane, 1995]. 3 Â 105 K. The dashed line in Figure 2g shows the expected Also WIND 3D plasma summary plots (not shown) showed temperature (Tex) from the correlation of the solar wind a counterstreaming suprathermal electron event starting speed and proton temperature for normal solar wind expan- around 08 UT on 7 April. Based on the magnetic field sion [Richardson and Cane, 1995]. Unusually low temper- observations and other ICME signatures described above, atures are characteristic of ICMEs, and the proton we suggest that the boundary between the sheath region temperature (Tp) is often significantly depressed relative to plasma and the ICME plasma was around 06 UT on 7 April. the expected temperature (Tp/Tex 0.5) [Richardson and Thus, the ICME arrived about 12 hours after the shock. The Cane, 1995]. In this case the measured proton temperature transit time for the ICME from the first C2 observation is well exceeded the expected temperature. Unusually high 61.5 hours, giving the average transit speed of 668 km/s. ratios of ionized helium number density to proton number [17] In conclusion, the geoeffective structure after the density (na/np > 0.08) have been shown to be present in shock (1604 UT on 6 April to 06 UT on 7 April 2000), many ICME events before, during, or after the ICME that is our main interest, was certainly not expanding and [Borrini et al., 1982]. Together with the na/np ratio in fails to fulfill the criteria for an ICME. The rapidly varying Figure 2h is plotted the hourly average of the solar wind magnetic field direction, high temperature and density are O+7/O+6 density ratio divided by 10. Within slow-speed rather characteristic of the sheath region [Tsurutani et al., solar wind the O+7/O+6 ratio has high variability which 1988], i.e., the piled-up solar wind between the shock and distinguishes different sources of slow solar wind [Zurbu- the CME ejecta. After the sheath region, starting around 06 chen et al., 2000] and a high ratio of O+7/O+6 has been UT on 7 April, there were many ICME signatures present related to ICMEs having magnetic cloud topology [Henke et but it was not evident where the boundary of ICME plasma al., 1998]. The interval of the high helium abundance and was exactly located. The identification of an ICME is not enhanced O+7/O+6 ratio did not occur until 06 UT on 7 unambiguous as a single ICME seldom exhibits all ICME April. Counterstreaming suprathermal electrons are some of signatures and different features do not necessarily occur at the most commonly used ICME signatures [e.g., Gosling, the same time and during the whole passage of the ICME 1990; Zwickl et al., 1983] interpreted to be indicative of a [e.g., Gosling, 1990; Zwickl et al., 1983]. For this case the closed magnetic field configuration with field lines either identification was further complicated as the spacecraft (and attached to the Sun at both ends or forming closed loops. the Earth) most likely hit the ejecta far from its center where During the high-B structure, there was no evidence of the magnetic field and solar wind signatures are not so counterstreaming electrons in WIND 3D plasma summary distinct as near the center of the ICME. plots (not shown). [18] Magnetic clouds are in many cases associated with [15] The solar wind dynamic pressure (Figure 2i) was 2 nPa disappearing filaments and the orientation of magnetic before the shock and increased to 10 nPa. The pressure clouds axes are found to generally coincide with the increased further with time and reached values as high as orientation of the filament axis [Bothmer and Schwenn, 25 nPa. The proton beta is shown in Figure 2j with the solid 1994; Marubashi, 1997] (i.e., filaments that are elongated line marking the value 0.1. Usually beta is less than 0.1 in the horizontally along the surface of the Sun produce magnetic ICME-associated solar wind plasma [Burlaga et al., 1981]. clouds that have their axes lying in the ecliptic plane and SMP 15 - 6 HUTTUNEN ET AL.: APRIL 2000 MAGNETIC STORM
Figure 3. Magnetic field Z-component in GSM coordinates (a), solar wind speed (b), dynamic pressure from WIND data (c), and calculated subsolar distance to the magnetopause (d). WIND data have been shifted 12 min before and 8 min after the shock arrival at 1640 UT (indicated by the vertical line). The overlapping points have been omitted.
vertical filaments produce CMEs that have their axis section 3.3 measurements from geostationary orbit (GOES- perpendicular to the ecliptic plane). Enhanced O+7/O+6 8 and GOES-10) showed that the magnetosphere was more and na/np ratio, low beta and temperature as well as compressed at the dawnside. Based on the observations counterstreaming electrons are characteristic to magnetic described above we suggest the scenario where the mag- clouds [e.g., Zwickl et al., 1983; Gosling, 1990; Henke et netic cloud, highly inclined relative to ecliptic, swept the al., 1998]. Furthermore as the CME on 4 April 2000 was Earth’s magnetosphere from above and dawnside. Also the associated with the filament disappearance it is likely that Z-component of the velocity had large negative values the structure spacecraft encountered on 7 April 2000 at (down to 120 km/s in GSE coordinate system) down- about 6 UT was a part of the ICME with magnetic cloud stream of the shock consistent with the direction of the structure. As the spacecraft hit only the flanks of this plasma flow bending around the ejecta having the above magnetic cloud, we cannot determine its orientation from mentioned orientation. magnetic field data. However, Ha-images of the associated filament show that it was vertically elongated along the 3.3. Arrival at Earth surface of the Sun, from which we can infer that the axis [19] Figure 3 shows the IMF Z-component, the solar wind of the magnetic cloud likely had high inclination relative speed and the dynamic pressure as measured by WIND and to the ecliptic plane. It should be noted that the shock the subsolar distance to the magnetopause as computed surface is not a planar structure but curved around the from the Shue et al. [1998] model. As discussed in the ejecta and thus in this case the axis of the magnetic cloud previous section, the shock was detected at the location of should have had a large positive inclination relative to WIND at 1632 UT and in the magnetosheath by GEOTAIL ecliptic to explain the obtained large negative latitudinal at 1640 UT. This is consistent with the propagation time of angle of the shock normal. Furthermore, as discussed in 8 min of the shock calculated from WIND observations. HUTTUNEN ET AL.: APRIL 2000 MAGNETIC STORM SMP 15 - 7
Note that the structures in front of the shock arrived at a [25] Within the 1 min accuracy of magnetometer data lower speed with a transit time of 12 min. In Figure 3 we both in space and on ground one gets a consistent picture have shifted the data before the shock by 12 min and after of the arrival of the interplanetary shock effects at the the shock by 8 min to represent the plasma conditions at the magnetopause. By 1641 UT the change in the dayside dayside magnetopause. The overlapping data points were magnetopause current system was observable all around omitted as physically they were hidden inside the shock the globe. structure. [20] The empirical model by Shue et al. [1998] gives the 3.4. Magnetospheric Activity magnetopause location as a function of IMF BZ and the solar 3.4.1. Global Activity and Energy Input wind dynamic pressure. We have used the WIND measure- [26] In order to estimate the total energy input to the ments and to calculate the dynamic pressure from proton magnetosphere we have calculated the epsilon parameter density we have assumed the alpha-to-proton ratio to be 4%. using measurements from the WIND satellite. Figure 6a Before the shock arrival, the subsolar magnetopause was repeats the IMF BZ as measured by WIND and shifted 8 min close to its quiet-day position around 11 RE. The magneto- to the magnetopause (here we have not used the longer pause started to move inward before the shock arrival. This delay before the shock as it is not relevant for the analysis). 7 2 2 was caused by both the decrease of BZ and the density Figures 6b and 6c present the -parameter ( =10VB l0 increase in the shock foot region. The magnetopause was sin4(q/2), using SI units) which is a function of the solar compressed close to the geostationary orbit at about 1644 UT. wind velocity V and the IMF intensity B and direction (tan q The magnetosphere remained in a very compressed state until = BY/BZ)[Akasofu, 1979, 1981]. l0 =7RE is an empirical the early hours of 7 April. The closest subsolar standoff parameter determined to fit the average energy input to the distance was about 5.5 RE at 2351 UT. average estimated output. The large variations in the mag- [21] In Figure 4 we compare geostationary magnetic field nitudes are the reason for showing the e parameter both in observations to the magnetopause position as predicted by logarithmic and linear scales. As the IMF was southward at the Shue et al. [1998] model. The upper two panels show the shock arrival, the energy input to the magnetosphere the magnetic field observations from GOES-10 and the increased immediately after the shock passage to above calculated distance to the magnetopause in the direction of 5 Â 1012 W, although the largest energy input started only the spacecraft (R, thick line) and to the subsolar point (R0, around 1750 UT when the IMF turned strongly southward. thin line). The two lower panels show the same parameters The maximum value of epsilon, 1.3 Â 1013 W, was reached at for GOES-8. Both GOES-8 and GOES-10 observed the 2311 UT. The interplanetary BZ reached its minimum value shock hitting the magnetopause at 1640 UT as a rapid jump 33 nT (in the GSM coordinate system) at about 2312 UT. in the magnetic field intensity. At this time both spacecraft The IMF BZ stayed significantly southward (<