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Interball observations of multiple flux transfer events
ARTICLE IN PRESS
Journal of Atmospheric and Solar-Terrestrial Physics 70 (2008) 391–398 www.elsevier.com/locate/jastp
Interball observations of multiple flux transfer events G.I. Korotovaa,b,, D.G. Sibeckc, T. Rosenbergb, V. Petrova, V. Styazhkina a
IZMIRAN, Moscow Region, Troitsk 142190, Russian Federation b IPST, University of Maryland, College Park, MD 20742, USA c Code 674, NASA/GSFC, Greenbelt, MD 20723, USA Accepted 27 August 2007 Available online 4 October 2007
Abstract We present results from a statistical study of multiple flux transfer events (FTEs) observed by the Interball-1 spacecraft during 1995–1999. We identified 21 days on which Interball-1 observed 10 or more FTEs. In a database of 807 FTEs, 319 can be identified as multiple FTEs with recurrence times ranging between 50 s and 20 min. Multiple events occur at both equatorial and high latitudes but are most frequently seen on Interball-1 passes that graze the flanks of the magnetosphere. The peak-to-peak amplitudes of multiple FTE Bn signatures vary from 3 to 20 nT but typically lie in the range from 3 to 9 nT, less than amplitudes typical for the FTE database as a whole. The durations of individual FTEs increase with increasing inter-arrival times during sequences of multiple FTEs. Multiple FTEs show a marked tendency to occur for southward IMF orientations, but no preference for high or low solar wind dynamic pressures or velocity. Since the conditions favoring the occurrence of multiple FTEs are similar to those for FTEs as a whole, we conclude that sequences of multiple FTEs must be the norm during periods of southward IMF orientation. r 2007 Elsevier Ltd. All rights reserved. Keywords: Magnetic reconnection; Magnetopause; Solar wind; Flux transfer events
1. Introduction The transfer of solar wind energy and momentum into the magnetosphere is one of the main topics in magnetospheric physics. Under some interpretations, bursty reconnection may represent the dominant mode of solar wind–magnetosphere interaction (Lockwood et al., 1995). Events exhibiting bipolar magnetic field signatures in the direction normal to the nominal magnetopause (i.e. bipolar Bn), transient enhancements in the total magnetic Corresponding author. IPST, University of Maryland,
College Park, MD 20742, USA. Tel.: +1 301 405 4835; fax: +1 301 405 4874. E-mail address: [email protected] (G.I. Korotova). 1364-6826/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jastp.2007.08.055
field strength, rotations in the magnetic field orientation, accelerated plasma flows, and streaming energetic particles are common in the vicinity of the magnetopause, where they are termed flux transfer events or FTEs and interpreted in terms of magnetic reconnection (Russell and Elphic, 1978). There has been a large number of case and statistical studies of FTEs but many questions about the mechanisms of generation of FTEs and their dynamics have not been solved. Studies of FTE amplitudes, durations, and interarrival times provide important information concerning the dynamics of reconnection on the magnetopause. Questions have arisen as to whether reconnection occurs quasi-periodically or simply randomly and whether there is a relationship
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between event amplitude and recurrence time. It is not clear whether solar wind conditions determine whether events occur in sequences or in isolation. Most previous studies have employed ISEE-1/2 observations and there is a need to see whether the results obtained for this data set hold true at higher latitudes, greater distances downstream, and other phases of the solar cycle. We present a new study of Interball-1 observations to address these questions. 2. Background Rijnbeek et al. (1984) surveyed ISEE-1/2 magnetometer observations of FTEs with amplitudes X10 nT and durations X1 min observed within 30 min of magnetopause crossings. They restricted themselves to an analysis of the magnetopause crossings occurring between 0600 and 1800 local time in GSM coordinates. They found that, on average, during periods of southward magnetosheath magnetic field FTEs repeated at intervals of 7–8.2 min. Typical durations were 1–2 min. Kawano and Russell (1996) developed an automatic program to identify FTEs and surveyed 9 years of ISEE-1 observations. Their program searched for isolated bipolar signatures in Bn having amplitudes X3 nT, durations from 1 to 40 min, and locations within the range of GSM local times from 0400 to 2000 MLT. They found the median peak-to-peak magnitude for the bipolar signature to be 14 nT and the median peak-to-peak duration to be 36 s. Lockwood and Wild (1993) investigated the repetition period for ISEE-1/2 FTEs. They examined 1-h intervals about each magnetopause crossing. The FTEs were defined using the criterion employed by Rijnbeek et al. (1984) but the minimum duration was lowered to 30 s. The mean value for the recurrence time was 8 min, very similar to that found by Rijnbeek et al. (1984). However, the distribution was highly skewed, with the lower decile being just 1.5 min and the upper decile being 18.5 min. The mode of the distribution was 3 min. There was no significant peak in the distribution at the average value of 8 min, suggesting that FTEs are not really quasiperiodic, but rather part of a wider spectrum of reconnection rate variations. Kuo et al. (1995) surveyed ISEE-1 FTEs observed by ISEE-1 with peak-to-peak amplitudes X10 nT and durations X30 s. They reported that average separation times range from 8.6 to 10.5 min with a median of 8 min. They concluded that some intrinsic
property of the magnetospheric system controls the occurrence of FTEs rather than solar wind parameters. Only a few researchers have surveyed sequences of FTEs in other data sets. Neudegg et al. (2000) removed events with inter-arrival times greater than 20 min on the grounds that these FTEs might not be part of the same reconnection sequence and obtained average FTE separation times of 8.8 min in the Equator-S database. About 35% of the interarrival times were less than 5 min. Cluster offers an opportunity to study FTE characteristics at high latitudes as well. Wang et al. (2005) presented results from a statistical study of Cluster FTE observations. They did not state any quantitative FTE identification thresholds. Following Neudegg et al. (2000), they also removed events separated more than 20 min and reported an average FTE separation time of 7.09 min (median of 5.62 min), a mean peak-to-peak magnitude of 25.36 nT (median of 22.09 nT), and an average FTE Bn peak-to-peak time of 25.79 s (median of 20.07 s). Again, 35% of inter-arrival times were less than 5 min. Elphic (1990) proposed that FTEs occur quasiperiodically because the process generating them exhibits intrinsic time scales associated with the growth and decay of reconnection. If so, there should be a relationship between the energy released in an FTE and the time required for this energy to build up. Elphic took the duration of FTEs as an indication of their energy content and the interarrival time as an indication of the time required for energy to accumulate. His survey of AMPTE UKS/ IRM and ISEE-1/2 FTEs indicated that larger FTEs tend to be separated by longer inter-arrival times. 3. Interball-1 orbital characteristics and data sets Interball-1 was launched on August 3, 1995 into an elliptical orbit with an apogee of 31.4RE, inclination 62.81, and period 92 h. The spacecraft encountered the high-latitude northern dayside magnetopause at GSM Z 410RE and the magnetotail magnetopause at distances up to 20RE downstream from Earth. Observations from the two fluxgate magnetometers, MIF-M and FM-3I, on the spacecraft have been intercalibrated and averaged to produce the merged data set with the 6 s time resolution that we employ for this study. FTEs are most readily identified on the basis of their bipolar signatures normal to the nominal
ARTICLE IN PRESS G.I. Korotova et al. / Journal of Atmospheric and Solar-Terrestrial Physics 70 (2008) 391–398
magnetopause. It is therefore helpful to plot the Interball-1 observations in boundary normal (LMN) coordinates (Russell and Elphic, 1978), where N points outward along the local model normal determined from the Roelof and Sibeck (1993) model magnetopause for nominal solar wind conditions (solar wind dynamic pressure ¼ 2 nPa, IMF Bz ¼ 0), L lies in the plane of the magnetopause and points northward, while M lies in the plane of the magnetopause and points dawnward (M ¼ N L). 4. Results Fig. 1 presents Interball-1 magnetometer observations in LMN coordinates for the interval from 13:30 to 14:00 UT on July 19, 1998. The spacecraft was in the southern pre-noon high-latitude magnetosheath at GSM (x, y, z) ¼ (6.5, 10.6, 9.7) RE where it observed southward (Bl o0), then northward (Bl 40), and again southward (Bl o0) and dawnward (Bm 40) magnetosheath magnetic field. During this 30-min interval, Interball-1 observed seven FTEs marked by bipolar (,+) signatures in the Bn component at 1331, 1336, 1350, 1352, 1355, 1357, and 1358 UT. According to our criteria, the events are classical ‘reverse’ FTEs marked by enhanced magnetic field strengths. We employ conservative criteria to identify FTEs based upon the magnetic fields alone (Sibeck et al., 2005). Each FTE event must exhibit a peak-to-peak
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amplitude of at least 3 nT, with a minimum duration of 10 s. Using these criteria, we have identified a total of 807 Interball-1 events from 1995 to 1999 in our database, where sequences of events like that seen in Fig. 1 were common. From our database of FTEs we identified 21 days on which 10 or more events occurred. We labeled events quasi-periodic or multiple when the time between successive events was 20 min or less, and identified 319 multiple events in sequences from 3 to 13 events. Fig. 2a–c presents the locations of multiple FTEs observed by Interball-1 in the X–Y, X–Z, and Y–Z GSM planes. They were observed over the full range of latitudes (60.81–60.21 GSM) and longitudes covered by Interball-1. Many FTEs occurred on the flanks of the magnetosphere. The number of magnetospheric FTEs is less than one-fourth that in magnetosheath, indicating that FTEs are far more prominent in the magnetosheath than in the magnetosphere. Magnetosheath and magnetosphere FTEs are part of the same physical phenomenon and the preponderance of magnetosheath FTEs is consistent with an interpretation in terms of FTEs that extend further into the magnetosheath than into the magnetosphere (Ding et al., 1991). As illustrated in Fig. 3a, Interball-1 FTEs (807) typically exhibit amplitudes from 3 to 20 nT. Although multiple FTEs exhibit amplitudes in the same range (with an average amplitude of 10.4 nT) a greater proportion of them exhibits lower
Bm (nT)
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10 20 0 30 20 13:30
13:35
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13:45 13:50 13:55 14:00 UT (X,Y, Z) GSM = (3.5, -10.3, 9.7) Re
Fig. 1. Interball-1 magnetometer observations in LMN coordinates for the interval from 13:30 to 14:00 UT on July 19, 1998. The numbers identify a sequence of seven FTEs. Arrows illustrate our definition of duration and inter-arrival time.
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20 ZGSM (Re)
YGSM (Re)
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20
30
-30
30
XGSM (Re)
-30 XGSM (Re)
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Fig. 2. Locations of multiple FTEs observed by Interball-1 in the X–Y, X–Z and Y–Z GSM plane. Events can be observed throughout the full range of locations surveyed by Interball-1.
Fig. 3. Occurrence histogram for the amplitudes of (a) all (807) and (b) only 319 multiple FTEs in our database.
amplitudes (3–9 nT, Fig. 3b), primarily due to the presence of many small amplitude events on the magnetotail flanks.
We now consider FTE durations and inter-arrival times for the subset of our multiple FTEs on 21 days. As illustrated in Fig. 1, FTE duration is defined as the time between the extrema of the bipolar Bn signature, while the inter-arrival time is the time elapsed since the last FTE. Fig. 4a shows the occurrence histogram for multiple FTE durations. The durations of multiple FTEs vary from 10 to 90 s with a peak near 25 s. Fig. 4b shows the occurrence histogram for multiple FTE inter-arrival times. As we impose a maximum inter-arrival time of 20 min, there are only 110 events in this plot. The recurrence times vary from 50 to 840 s with a peak inter-arrival time near 165 s. This is close to the mode value of 3 min obtained by Lockwood and Wild (1993). Average duration and inter-arrival time are 36 and 258 s, respectively. Fig. 5a and b presents the distributions of multiple event amplitudes versus duration and inter-arrival time. We find no marked tendency for
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Fig. 5. Distributions of amplitudes versus (a) durations, (b) interarrival times of multiple FTEs. Black bars show the mean values for each bin.
Fig. 4. (a, b) Occurrence histograms for the durations and interarrival times of multiple FTEs. Black bars show the mean values of the amplitude for each bin.
the amplitudes of multiple FTEs to increase with either parameter, in agreement with the results reported by Wang et al. (2005). To check the suggestion of Elphic (1990) that longer accumulation times lead to larger FTEs, we plotted durations of multiple FTEs versus interarrival times. Fig. 6a confirms that longer interarrival times separate FTEs with larger durations. Although we have found no tendency for event amplitude (as measured by the size of the Bn signature) to depend on inter-arrival time, Fig. 6b
shows that the parameter ‘amplitude duration’ does depend on inter-arrival time. This parameter tends to increase with increasing inter-arrival time; consequently it may be used as a measure of ‘FTE size’. We wish to determine whether special solar wind conditions favor multiple FTEs. Simultaneous solar wind observations were available for 137 multiple FTEs. We lagged high-time resolution IMF observations in GSM coordinates by the IMP-8 (King, 1982) and Geotail (Kokubun et al., 1994) spacecraft to Earth. When observations from both Geotail and IMP 8 were available, we used observations from the spacecraft nearest to the Sun–Earth line. For simplicity, we calculated arrival times under an assumption that the solar wind discontinuities lay perpendicular to the Sun–Earth
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Fig. 6. Distributions of (a) durations and (b) durations amplitudes for multiple FTEs as a function of inter-arrival times. Black bars show the mean values for each bin.
line and were advected anti-sunward with the solar wind velocity. Events occurring for jIMF Bzjo1 nT were assigned to the IMF Bz ¼ 0 category. Fig. 7a–c shows normalized multiple FTE occurrence patterns as a function of solar wind pressure and velocity and the orientation of the interplanetary magnetic field. Most multiple FTEs occurred for solar wind velocities and pressures in the range from 300 to 500 km s1 and from 1 to 3 nPa, respectively. For comparison, shaded bars indicate the normalized distributions of the same solar wind parameters from 1996 to 1998. The normalized distributions for the solar wind pressure and velocity prevailing at the times of multiple FTEs observations greatly resemble those for the entire period from 1996 to 1998. This indicates that multiple FTEs occur for typical solar wind conditions. Fig. 7c demonstrates that, just as in the case
Fig. 7. Normalized multiple FTE occurrence patterns as a function of (a) SW pressure, (b) SW velocity, and (c) IMF Bz. For comparison, shaded bars indicate the normalized distributions of the same solar wind parameters from 1996 to 1998.
of previous studies, multiple FTEs tend to occur for southward IMF orientations. Here 44% of our multiple FTEs occurred for IMF Bz in the range from 0 to 2 nT. The tendency for multiple FTEs to occur for southward IMF orientation is emphasized by comparison with the normalized distribution of IMF Bz from 1996 to 1998. 5. Conclusions Surveying FTEs observed by Interball from 1995 to 1999, we identified 319 multiple events on 21 days
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when 10 or more FTEs occurred. We sought to determine whether the occurrence pattern of multiple FTEs resembles that for all FTEs. The characteristics of the multiple FTEs observed by Interball are similar to those previously reported for the events occurring at the dayside magnetopause. The number of magnetosheath FTEs greatly exceeds those seen in the magnetosphere. Multiple events can be seen over a wide range of latitudes and longitudes but are most often seen on skimming orbits along the flanks of the magnetosphere. Multiple events typical amplitudes vary from 3 to 20 nT, but in comparison with the distribution of amplitudes for all FTEs in our database, multiple FTEs have smaller amplitudes on average. This could indicate that isolated events attain greater amplitudes than those in sequences, but as discussed earlier we think it more likely that this result is due to an orbital bias. Orbital bias enables Interball to observe multiple FTEs almost exclusively on the flanks, where their amplitudes are smaller. The durations of multiple FTEs vary from 10 to 90 s. The inter-arrival times vary from 50 to 840 s. The mode for the distribution of inter-arrival times is about 3 min. The average amplitude, duration, and inter-arrival times of multiple events are 10.4 nT, 36 and 258 s, respectively. While the amplitudes and durations of our events resemble those reported in previous studies, the inter-arrival times are less than those obtained in most papers. However, this is consistent with the lower-amplitude criteria we impose and an interpretation that smaller-amplitude events occur more frequently than larger-amplitude events. There is no marked tendency for event amplitude to increase with the duration or inter-arrival time, but the duration of multiple FTEs increases with increasing inter-arrival time. Apparently, the flux removed from the magnetopause remains relatively constant with either rapid sequences of flux removal by small FTEs or more prolonged intervals between larger FTEs each removing large amounts of flux. We studied solar wind conditions favoring multiple FTEs, using IMP-8 and Geotail as solar wind monitors. As in previous studies, the events show a marked tendency to occur for southward IMF orientations. The distributions of solar wind pressure and velocity of solar wind for multiple FTEs observations and for the period from 1996 to 1998 are very similar, indicating
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that no special conditions are needed to generate multiple FTEs. Since the conditions favoring multiple events do not differ from those favoring the occurrence of all events, the corollary of this conclusion is that multiple events may almost invariably be present on the magnetopause for typical solar wind conditions and southward IMF orientations. Acknowledgments We thank the SPDF and NSSDC at GSFC for supplying the Geotail, IMP-8 magnetic field observations. Work at GSFC was supported by NASA’s Guest Investigator Program, while work by G.I.K. at the University of Maryland was supported by a grant from NASA/GSFC.
References Ding, D.Q., Lee, L.C., Ma, Z.W., 1991. Different FTE signatures generated by the bursty single X line reconnection and the multiple X line reconnection at the dayside magnetopause. Journal of Geophysical Research 96, 57–66. Elphic, R.C., 1990. Observations of flux transfer events: are FTEs flux ropes, islands, or surface waves? In: Russell, C.T., Lee, L.C. (Eds.), Physics of Magnetic Flux Ropes, Geophysical Monograph Series, vol. 58. AGU, Washington, DC, pp. 455–471. Kawano, H., Russell, C.T., 1996. Survey of flux transfer events observed with the ISEE 1 spacecraft: rotational polarity and the source region. Journal of Geophysical Research 101, 27299–27308. King, J.H., 1982. Availabiity of IMP-7 and IMP-8 data for the IMS period. In: Russell, C.T., Southwood, D.J. (Eds.), The IMS Source Book. AGU, Washington, DC, pp. 10–20. Kokubun, S., Yamamoto, T., Acuna, M., Hayashi, K., Shiokawa, K., Kawano, H., 1994. The Geotail magnetic field experiment. Journal of Geomagnetism and Geoelectricity 46, 7–21. Kuo, H., Russell, C.T., Le, G., 1995. Statistical studies of flux transfer events. Journal of Geophysical Research 100, 3513–3519. Lockwood, M., Wild, M.N., 1993. On the quasi-periodic nature of magnetopause flux transfer events. Journal of Geophysical Research 98 (A4), 5935–5940. Lockwood, M., Cowley, S.W.H., Smith, M.F., Rijnbeek, R.P., Elphic, R.C., 1995. The contribution of flux transfer events to convection. Geophysical Research Letters 22, 1185–1188. Neudegg, D.A., et al., 2000. The UV aurora and ionospheric flows during flux transfer events. Annales Geoophysicae 18, 416–435. Sibeck, D.G., Korotova, G.I., Petrov, V., Styazhkin, V., Rosenberg, T.J., 2005. Flux transfer events on the high-latitude
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magnetopause: Interball-1 observations. Annales Geophysicae 23, 1–11. Rijnbeek, R.P., Cowley, S.W.H., Southwood, D.J., Russell, C.T., 1984. A survey of dayside fluxtransfer events observed by ISEE 1 and 2 magnetometer. Journal of Geophysical Research 89, 786–800. Roelof, E., Sibeck, D.G., 1993. Magnetopause shape as a bivariant function of interplanetary magnetic field Bz and
solar wind dynamic pressure. Journal of Geophysical Research 98, 21421–21450. Russell, C.T., Elphic, R.C., 1978. Initial ISEE magnetometer results: magnetopause observations. Space Science Reviews 22, 681–715. Wang, Y.L., Elphic, R.C., Lavraud, B., et al., 2005. Initial results of high-latitude magnetopause and low-latitude flank flux transfer events from 3 years of Cluster observations. Journal of Geophysical Research 110, A11221.
Journal of Atmospheric and Solar-Terrestrial Physics 70 (2008) 391–398 www.elsevier.com/locate/jastp
Interball observations of multiple flux transfer events G.I. Korotovaa,b,, D.G. Sibeckc, T. Rosenbergb, V. Petrova, V. Styazhkina a
IZMIRAN, Moscow Region, Troitsk 142190, Russian Federation b IPST, University of Maryland, College Park, MD 20742, USA c Code 674, NASA/GSFC, Greenbelt, MD 20723, USA Accepted 27 August 2007 Available online 4 October 2007
Abstract We present results from a statistical study of multiple flux transfer events (FTEs) observed by the Interball-1 spacecraft during 1995–1999. We identified 21 days on which Interball-1 observed 10 or more FTEs. In a database of 807 FTEs, 319 can be identified as multiple FTEs with recurrence times ranging between 50 s and 20 min. Multiple events occur at both equatorial and high latitudes but are most frequently seen on Interball-1 passes that graze the flanks of the magnetosphere. The peak-to-peak amplitudes of multiple FTE Bn signatures vary from 3 to 20 nT but typically lie in the range from 3 to 9 nT, less than amplitudes typical for the FTE database as a whole. The durations of individual FTEs increase with increasing inter-arrival times during sequences of multiple FTEs. Multiple FTEs show a marked tendency to occur for southward IMF orientations, but no preference for high or low solar wind dynamic pressures or velocity. Since the conditions favoring the occurrence of multiple FTEs are similar to those for FTEs as a whole, we conclude that sequences of multiple FTEs must be the norm during periods of southward IMF orientation. r 2007 Elsevier Ltd. All rights reserved. Keywords: Magnetic reconnection; Magnetopause; Solar wind; Flux transfer events
1. Introduction The transfer of solar wind energy and momentum into the magnetosphere is one of the main topics in magnetospheric physics. Under some interpretations, bursty reconnection may represent the dominant mode of solar wind–magnetosphere interaction (Lockwood et al., 1995). Events exhibiting bipolar magnetic field signatures in the direction normal to the nominal magnetopause (i.e. bipolar Bn), transient enhancements in the total magnetic Corresponding author. IPST, University of Maryland,
College Park, MD 20742, USA. Tel.: +1 301 405 4835; fax: +1 301 405 4874. E-mail address: [email protected] (G.I. Korotova). 1364-6826/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jastp.2007.08.055
field strength, rotations in the magnetic field orientation, accelerated plasma flows, and streaming energetic particles are common in the vicinity of the magnetopause, where they are termed flux transfer events or FTEs and interpreted in terms of magnetic reconnection (Russell and Elphic, 1978). There has been a large number of case and statistical studies of FTEs but many questions about the mechanisms of generation of FTEs and their dynamics have not been solved. Studies of FTE amplitudes, durations, and interarrival times provide important information concerning the dynamics of reconnection on the magnetopause. Questions have arisen as to whether reconnection occurs quasi-periodically or simply randomly and whether there is a relationship
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between event amplitude and recurrence time. It is not clear whether solar wind conditions determine whether events occur in sequences or in isolation. Most previous studies have employed ISEE-1/2 observations and there is a need to see whether the results obtained for this data set hold true at higher latitudes, greater distances downstream, and other phases of the solar cycle. We present a new study of Interball-1 observations to address these questions. 2. Background Rijnbeek et al. (1984) surveyed ISEE-1/2 magnetometer observations of FTEs with amplitudes X10 nT and durations X1 min observed within 30 min of magnetopause crossings. They restricted themselves to an analysis of the magnetopause crossings occurring between 0600 and 1800 local time in GSM coordinates. They found that, on average, during periods of southward magnetosheath magnetic field FTEs repeated at intervals of 7–8.2 min. Typical durations were 1–2 min. Kawano and Russell (1996) developed an automatic program to identify FTEs and surveyed 9 years of ISEE-1 observations. Their program searched for isolated bipolar signatures in Bn having amplitudes X3 nT, durations from 1 to 40 min, and locations within the range of GSM local times from 0400 to 2000 MLT. They found the median peak-to-peak magnitude for the bipolar signature to be 14 nT and the median peak-to-peak duration to be 36 s. Lockwood and Wild (1993) investigated the repetition period for ISEE-1/2 FTEs. They examined 1-h intervals about each magnetopause crossing. The FTEs were defined using the criterion employed by Rijnbeek et al. (1984) but the minimum duration was lowered to 30 s. The mean value for the recurrence time was 8 min, very similar to that found by Rijnbeek et al. (1984). However, the distribution was highly skewed, with the lower decile being just 1.5 min and the upper decile being 18.5 min. The mode of the distribution was 3 min. There was no significant peak in the distribution at the average value of 8 min, suggesting that FTEs are not really quasiperiodic, but rather part of a wider spectrum of reconnection rate variations. Kuo et al. (1995) surveyed ISEE-1 FTEs observed by ISEE-1 with peak-to-peak amplitudes X10 nT and durations X30 s. They reported that average separation times range from 8.6 to 10.5 min with a median of 8 min. They concluded that some intrinsic
property of the magnetospheric system controls the occurrence of FTEs rather than solar wind parameters. Only a few researchers have surveyed sequences of FTEs in other data sets. Neudegg et al. (2000) removed events with inter-arrival times greater than 20 min on the grounds that these FTEs might not be part of the same reconnection sequence and obtained average FTE separation times of 8.8 min in the Equator-S database. About 35% of the interarrival times were less than 5 min. Cluster offers an opportunity to study FTE characteristics at high latitudes as well. Wang et al. (2005) presented results from a statistical study of Cluster FTE observations. They did not state any quantitative FTE identification thresholds. Following Neudegg et al. (2000), they also removed events separated more than 20 min and reported an average FTE separation time of 7.09 min (median of 5.62 min), a mean peak-to-peak magnitude of 25.36 nT (median of 22.09 nT), and an average FTE Bn peak-to-peak time of 25.79 s (median of 20.07 s). Again, 35% of inter-arrival times were less than 5 min. Elphic (1990) proposed that FTEs occur quasiperiodically because the process generating them exhibits intrinsic time scales associated with the growth and decay of reconnection. If so, there should be a relationship between the energy released in an FTE and the time required for this energy to build up. Elphic took the duration of FTEs as an indication of their energy content and the interarrival time as an indication of the time required for energy to accumulate. His survey of AMPTE UKS/ IRM and ISEE-1/2 FTEs indicated that larger FTEs tend to be separated by longer inter-arrival times. 3. Interball-1 orbital characteristics and data sets Interball-1 was launched on August 3, 1995 into an elliptical orbit with an apogee of 31.4RE, inclination 62.81, and period 92 h. The spacecraft encountered the high-latitude northern dayside magnetopause at GSM Z 410RE and the magnetotail magnetopause at distances up to 20RE downstream from Earth. Observations from the two fluxgate magnetometers, MIF-M and FM-3I, on the spacecraft have been intercalibrated and averaged to produce the merged data set with the 6 s time resolution that we employ for this study. FTEs are most readily identified on the basis of their bipolar signatures normal to the nominal
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magnetopause. It is therefore helpful to plot the Interball-1 observations in boundary normal (LMN) coordinates (Russell and Elphic, 1978), where N points outward along the local model normal determined from the Roelof and Sibeck (1993) model magnetopause for nominal solar wind conditions (solar wind dynamic pressure ¼ 2 nPa, IMF Bz ¼ 0), L lies in the plane of the magnetopause and points northward, while M lies in the plane of the magnetopause and points dawnward (M ¼ N L). 4. Results Fig. 1 presents Interball-1 magnetometer observations in LMN coordinates for the interval from 13:30 to 14:00 UT on July 19, 1998. The spacecraft was in the southern pre-noon high-latitude magnetosheath at GSM (x, y, z) ¼ (6.5, 10.6, 9.7) RE where it observed southward (Bl o0), then northward (Bl 40), and again southward (Bl o0) and dawnward (Bm 40) magnetosheath magnetic field. During this 30-min interval, Interball-1 observed seven FTEs marked by bipolar (,+) signatures in the Bn component at 1331, 1336, 1350, 1352, 1355, 1357, and 1358 UT. According to our criteria, the events are classical ‘reverse’ FTEs marked by enhanced magnetic field strengths. We employ conservative criteria to identify FTEs based upon the magnetic fields alone (Sibeck et al., 2005). Each FTE event must exhibit a peak-to-peak
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amplitude of at least 3 nT, with a minimum duration of 10 s. Using these criteria, we have identified a total of 807 Interball-1 events from 1995 to 1999 in our database, where sequences of events like that seen in Fig. 1 were common. From our database of FTEs we identified 21 days on which 10 or more events occurred. We labeled events quasi-periodic or multiple when the time between successive events was 20 min or less, and identified 319 multiple events in sequences from 3 to 13 events. Fig. 2a–c presents the locations of multiple FTEs observed by Interball-1 in the X–Y, X–Z, and Y–Z GSM planes. They were observed over the full range of latitudes (60.81–60.21 GSM) and longitudes covered by Interball-1. Many FTEs occurred on the flanks of the magnetosphere. The number of magnetospheric FTEs is less than one-fourth that in magnetosheath, indicating that FTEs are far more prominent in the magnetosheath than in the magnetosphere. Magnetosheath and magnetosphere FTEs are part of the same physical phenomenon and the preponderance of magnetosheath FTEs is consistent with an interpretation in terms of FTEs that extend further into the magnetosheath than into the magnetosphere (Ding et al., 1991). As illustrated in Fig. 3a, Interball-1 FTEs (807) typically exhibit amplitudes from 3 to 20 nT. Although multiple FTEs exhibit amplitudes in the same range (with an average amplitude of 10.4 nT) a greater proportion of them exhibits lower
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Fig. 1. Interball-1 magnetometer observations in LMN coordinates for the interval from 13:30 to 14:00 UT on July 19, 1998. The numbers identify a sequence of seven FTEs. Arrows illustrate our definition of duration and inter-arrival time.
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20 ZGSM (Re)
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Fig. 2. Locations of multiple FTEs observed by Interball-1 in the X–Y, X–Z and Y–Z GSM plane. Events can be observed throughout the full range of locations surveyed by Interball-1.
Fig. 3. Occurrence histogram for the amplitudes of (a) all (807) and (b) only 319 multiple FTEs in our database.
amplitudes (3–9 nT, Fig. 3b), primarily due to the presence of many small amplitude events on the magnetotail flanks.
We now consider FTE durations and inter-arrival times for the subset of our multiple FTEs on 21 days. As illustrated in Fig. 1, FTE duration is defined as the time between the extrema of the bipolar Bn signature, while the inter-arrival time is the time elapsed since the last FTE. Fig. 4a shows the occurrence histogram for multiple FTE durations. The durations of multiple FTEs vary from 10 to 90 s with a peak near 25 s. Fig. 4b shows the occurrence histogram for multiple FTE inter-arrival times. As we impose a maximum inter-arrival time of 20 min, there are only 110 events in this plot. The recurrence times vary from 50 to 840 s with a peak inter-arrival time near 165 s. This is close to the mode value of 3 min obtained by Lockwood and Wild (1993). Average duration and inter-arrival time are 36 and 258 s, respectively. Fig. 5a and b presents the distributions of multiple event amplitudes versus duration and inter-arrival time. We find no marked tendency for
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Fig. 5. Distributions of amplitudes versus (a) durations, (b) interarrival times of multiple FTEs. Black bars show the mean values for each bin.
Fig. 4. (a, b) Occurrence histograms for the durations and interarrival times of multiple FTEs. Black bars show the mean values of the amplitude for each bin.
the amplitudes of multiple FTEs to increase with either parameter, in agreement with the results reported by Wang et al. (2005). To check the suggestion of Elphic (1990) that longer accumulation times lead to larger FTEs, we plotted durations of multiple FTEs versus interarrival times. Fig. 6a confirms that longer interarrival times separate FTEs with larger durations. Although we have found no tendency for event amplitude (as measured by the size of the Bn signature) to depend on inter-arrival time, Fig. 6b
shows that the parameter ‘amplitude duration’ does depend on inter-arrival time. This parameter tends to increase with increasing inter-arrival time; consequently it may be used as a measure of ‘FTE size’. We wish to determine whether special solar wind conditions favor multiple FTEs. Simultaneous solar wind observations were available for 137 multiple FTEs. We lagged high-time resolution IMF observations in GSM coordinates by the IMP-8 (King, 1982) and Geotail (Kokubun et al., 1994) spacecraft to Earth. When observations from both Geotail and IMP 8 were available, we used observations from the spacecraft nearest to the Sun–Earth line. For simplicity, we calculated arrival times under an assumption that the solar wind discontinuities lay perpendicular to the Sun–Earth
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Fig. 6. Distributions of (a) durations and (b) durations amplitudes for multiple FTEs as a function of inter-arrival times. Black bars show the mean values for each bin.
line and were advected anti-sunward with the solar wind velocity. Events occurring for jIMF Bzjo1 nT were assigned to the IMF Bz ¼ 0 category. Fig. 7a–c shows normalized multiple FTE occurrence patterns as a function of solar wind pressure and velocity and the orientation of the interplanetary magnetic field. Most multiple FTEs occurred for solar wind velocities and pressures in the range from 300 to 500 km s1 and from 1 to 3 nPa, respectively. For comparison, shaded bars indicate the normalized distributions of the same solar wind parameters from 1996 to 1998. The normalized distributions for the solar wind pressure and velocity prevailing at the times of multiple FTEs observations greatly resemble those for the entire period from 1996 to 1998. This indicates that multiple FTEs occur for typical solar wind conditions. Fig. 7c demonstrates that, just as in the case
Fig. 7. Normalized multiple FTE occurrence patterns as a function of (a) SW pressure, (b) SW velocity, and (c) IMF Bz. For comparison, shaded bars indicate the normalized distributions of the same solar wind parameters from 1996 to 1998.
of previous studies, multiple FTEs tend to occur for southward IMF orientations. Here 44% of our multiple FTEs occurred for IMF Bz in the range from 0 to 2 nT. The tendency for multiple FTEs to occur for southward IMF orientation is emphasized by comparison with the normalized distribution of IMF Bz from 1996 to 1998. 5. Conclusions Surveying FTEs observed by Interball from 1995 to 1999, we identified 319 multiple events on 21 days
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when 10 or more FTEs occurred. We sought to determine whether the occurrence pattern of multiple FTEs resembles that for all FTEs. The characteristics of the multiple FTEs observed by Interball are similar to those previously reported for the events occurring at the dayside magnetopause. The number of magnetosheath FTEs greatly exceeds those seen in the magnetosphere. Multiple events can be seen over a wide range of latitudes and longitudes but are most often seen on skimming orbits along the flanks of the magnetosphere. Multiple events typical amplitudes vary from 3 to 20 nT, but in comparison with the distribution of amplitudes for all FTEs in our database, multiple FTEs have smaller amplitudes on average. This could indicate that isolated events attain greater amplitudes than those in sequences, but as discussed earlier we think it more likely that this result is due to an orbital bias. Orbital bias enables Interball to observe multiple FTEs almost exclusively on the flanks, where their amplitudes are smaller. The durations of multiple FTEs vary from 10 to 90 s. The inter-arrival times vary from 50 to 840 s. The mode for the distribution of inter-arrival times is about 3 min. The average amplitude, duration, and inter-arrival times of multiple events are 10.4 nT, 36 and 258 s, respectively. While the amplitudes and durations of our events resemble those reported in previous studies, the inter-arrival times are less than those obtained in most papers. However, this is consistent with the lower-amplitude criteria we impose and an interpretation that smaller-amplitude events occur more frequently than larger-amplitude events. There is no marked tendency for event amplitude to increase with the duration or inter-arrival time, but the duration of multiple FTEs increases with increasing inter-arrival time. Apparently, the flux removed from the magnetopause remains relatively constant with either rapid sequences of flux removal by small FTEs or more prolonged intervals between larger FTEs each removing large amounts of flux. We studied solar wind conditions favoring multiple FTEs, using IMP-8 and Geotail as solar wind monitors. As in previous studies, the events show a marked tendency to occur for southward IMF orientations. The distributions of solar wind pressure and velocity of solar wind for multiple FTEs observations and for the period from 1996 to 1998 are very similar, indicating
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that no special conditions are needed to generate multiple FTEs. Since the conditions favoring multiple events do not differ from those favoring the occurrence of all events, the corollary of this conclusion is that multiple events may almost invariably be present on the magnetopause for typical solar wind conditions and southward IMF orientations. Acknowledgments We thank the SPDF and NSSDC at GSFC for supplying the Geotail, IMP-8 magnetic field observations. Work at GSFC was supported by NASA’s Guest Investigator Program, while work by G.I.K. at the University of Maryland was supported by a grant from NASA/GSFC.
References Ding, D.Q., Lee, L.C., Ma, Z.W., 1991. Different FTE signatures generated by the bursty single X line reconnection and the multiple X line reconnection at the dayside magnetopause. Journal of Geophysical Research 96, 57–66. Elphic, R.C., 1990. Observations of flux transfer events: are FTEs flux ropes, islands, or surface waves? In: Russell, C.T., Lee, L.C. (Eds.), Physics of Magnetic Flux Ropes, Geophysical Monograph Series, vol. 58. AGU, Washington, DC, pp. 455–471. Kawano, H., Russell, C.T., 1996. Survey of flux transfer events observed with the ISEE 1 spacecraft: rotational polarity and the source region. Journal of Geophysical Research 101, 27299–27308. King, J.H., 1982. Availabiity of IMP-7 and IMP-8 data for the IMS period. In: Russell, C.T., Southwood, D.J. (Eds.), The IMS Source Book. AGU, Washington, DC, pp. 10–20. Kokubun, S., Yamamoto, T., Acuna, M., Hayashi, K., Shiokawa, K., Kawano, H., 1994. The Geotail magnetic field experiment. Journal of Geomagnetism and Geoelectricity 46, 7–21. Kuo, H., Russell, C.T., Le, G., 1995. Statistical studies of flux transfer events. Journal of Geophysical Research 100, 3513–3519. Lockwood, M., Wild, M.N., 1993. On the quasi-periodic nature of magnetopause flux transfer events. Journal of Geophysical Research 98 (A4), 5935–5940. Lockwood, M., Cowley, S.W.H., Smith, M.F., Rijnbeek, R.P., Elphic, R.C., 1995. The contribution of flux transfer events to convection. Geophysical Research Letters 22, 1185–1188. Neudegg, D.A., et al., 2000. The UV aurora and ionospheric flows during flux transfer events. Annales Geoophysicae 18, 416–435. Sibeck, D.G., Korotova, G.I., Petrov, V., Styazhkin, V., Rosenberg, T.J., 2005. Flux transfer events on the high-latitude
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magnetopause: Interball-1 observations. Annales Geophysicae 23, 1–11. Rijnbeek, R.P., Cowley, S.W.H., Southwood, D.J., Russell, C.T., 1984. A survey of dayside fluxtransfer events observed by ISEE 1 and 2 magnetometer. Journal of Geophysical Research 89, 786–800. Roelof, E., Sibeck, D.G., 1993. Magnetopause shape as a bivariant function of interplanetary magnetic field Bz and
solar wind dynamic pressure. Journal of Geophysical Research 98, 21421–21450. Russell, C.T., Elphic, R.C., 1978. Initial ISEE magnetometer results: magnetopause observations. Space Science Reviews 22, 681–715. Wang, Y.L., Elphic, R.C., Lavraud, B., et al., 2005. Initial results of high-latitude magnetopause and low-latitude flank flux transfer events from 3 years of Cluster observations. Journal of Geophysical Research 110, A11221.