Large-Scale Magnetotail Dynamics During Continued External Forcing.|
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INTRODUCTION TO STUDY OF DECEMBER 22-23, 1996 EVENT: INTERBALL Multi-Satellite and Multi-Instrument Observations
Prepared by Yu.I.Yermolaev(1), L.M.Zelenyi(1), V.A.Sergeev(2), V.N.Lutsenko(1), R.A.Kovrazhkin(1), Yu.I.Galperin(1), A.A.Petrukovich(1) et al.
(1) Space Research Institute, Moscow, Russia
(2) St.Petersburg University, St.Peterburg, Russia
The Main Objectives
Ground-based and POLAR UVI data show that the magnetosphere was disturbed during observed period from 12:00 on December 22 to 08:00 UT on December 23, 1996. Fig.1 (a,b) presents the keograms from POLAR UVI (courtesy by K.Liou and POLAR UVI team) showing the auroral dynamics in coordinates "Latitude - UT" at two meridians, 22:48 MLT (top), and 02:30 MLT (bottom), i.e. at the meridians close to the footpoints of Interball/Tail Probe and Geotail, correspondingly. In addition to two large-scale substorm expansions starting at ~13 and ~16:30 UT there was a number of smaller scale activations. Detail analysis of ground station magnetograms confirms continuous magnetic activity throughout this interval, see Fig.2 where SYM-H and PCn indexes illustrate a behaviour of ring current and polar cap convection, respectively. These data allowed us to identify four substorms marked by figures from 1 to 4 and presented in Table 1.
Interplanetary conditions (dynamic pressure P, density N, velocity V, magnitude B and three components of IMF) are presented on bottom panels of Fig.2. This time interval is characterized by low, nearly constant pressure and continuous southward IMF. First substorm is likely to be initiated by IMF Bz changing from -5 to +1 nT at 12:10 UT, and second substorm could be due the pressure jump recorded at 15:50 UT. There was no changes in the interplanetary space which might initiate third and fourth substorms. Table 1 consists of substorm time intervals and times of interplanetary condition changes.
LANL geosynchronous electron data (see Fig.3) and ion data (not shown) show some flux increases during the substorms identified as well as some bursts in between.
Schematic view of available data and the most interesting features observed on different spacecraft (INTERBALL/Tail Probe, INTERBALL/Auroral Probe, GEOTAIL, LANL serie, POLAR) during this time interval are presented in Fig.4. Conventional signs and symbols are explained on the right side of panels and the features are explained in details below.
Bottom panels of Fig.5 show trajectory of Tail Probe (in GSM coordinate system) which was located at X ~ -20, Y ~ -10 and Z ~ -2 RE in the plasma sheet region. Measurements of magnetic field magnitude and 3 components and ion energy spectrogram (measurements of CORALL channel oriented perpendicular to X-axis - satellite spin axis) confirm that during the most part of interval the Tail Probe was in the plasma sheet (near neutral sheet) and only after 03 UT on December 23, 1996 the satellite began to move to south lobe. Substorm periods were characterized by short (several minutes) intervals of PS plasma disappearance (1st, 3rd and 4th substorms) or by change in parameters of PS plasma (2nd substorm).
Several unusual phenomena were observed during and between substorms. Fig.6 presents ion energy spectra measured by CORALL and DOK-2 instruments during short ion burst at 13:25:00-13:25:30 in 1st substorm interval. Before and after this burst the satellite was outside PS (in north lobes). Tailward ion spectra measured by CORALL under angle 66 degrees relative to X-axis (black curve) and DOK-2 under angle 62 degrees (red curve) demonstrate a common smooth spectrum with temperature ~ 10 KeV. The CORALL channel, which is the nearest to the Earthward direction and is oriented under 138 degrees relative to the Sun (blue curve), showed that ion flux in this direction was higher in energy range > 10 KeV than tailward flux. More high ion flux was observed by DOK-2 channel measured Earthward flux (ping curve). Thus, the flux difference between Earthward and tailward ions with E ~ 10-30 KeV was ~ 2 order of magnitude and Earthward ions had a large and narrow peak with E ~ 200 KeV.
During and between substorms there was a large number of plasma bursts (Bursty Bulk Flow) and/or magnetic field disturbances (Magnetic Flux Rope) in the plasma sheet. Fig.7 presents CORALL ion energy spectrograms for 3 channels (CORALL 1 and 5 are the nearest to Earthward and tailward directions, respectively) and magnitude and 3 components of magnetic field. In time intervals of 15:50-16:00 and 16:10-16:20 UT we can see two events mentioned above: plasma were more dense and more anisotropic than in usual PS, and fluctuations in magnetic field components were observed.
Distribution function calculated on the basis of CORALL measurements for time interval of 15:54:30-15:56:30 are presented in Fig.8. Top panel shows ion energy spectrogram, medium panel - projection of distribution on the plane perpendicular to spin axis, bottom panel - cylindrical projection relative to spin axis. We can see that plasma has large components of velocity as Earthward as perpendicular to this direction. Our calculations show that plasma moved in GSM frame with Vx ~ 700, Vy ~ 600 and Vz ~ 100 km/s (Fig.9 presents the direction of plasma motion relative to the average position of magnetopause).
Hodograph presented in Fig.10 shows that magnetic field change has more complicated type than usually observed in magnetic flux rope, e.g. on the whole in the Y-Z plane the vector of magnetic field went around point with coordinates By ~ -3 and Bz ~ 3 and radius ~ 2 nT, but it had 2 short trips (distances were 12 and 7 nT, respectively) in -By and -Bz directions.
At the beginning of 4th substorm the Tail Probe observed a tailward plasmoid. Fig.11 presents ion energy spectrogram (top panel) for time interval 02:01:20-02:03:20 on December 23, 1996, and two projections of ion distribution for the same time interval: in plane perpendicular to spin axis (middle panel) and in plane contained the spin axes (bottom panel). Duration of plasmoid was near 30 sec.(about 1/4 spin period). So, at the beginning of spin the CORALL observed the PS plasma during ~ 30 sec and then the plasmoid was observed. PS ions disappeared at ~ 02:02:20 UT and then plasma with lower energy (PSBL) was observed. Taking into account that CORALL does not measure ions in cones near spin axis, distribution on bottom panel shows that plasmoid moved tailward.
Orbits of INTERBALL/Tail Probe and GEOTAIL for December 22-23, 1996, are shown in Fig.12 in GSM frame. Both satellites have approximately the same X ~ -20 and Z ~ -3 RE, but the Y difference is about 20 RE, i.e. they were located in different flanks of magnetic tail.
Fig.13 presents a 8-hour example of ion and electron energy spectrograms measured on December 22, 1996 on GEOTAIL. Initially GEOTAIL was in the PS. The beginning of 1st substorm at ~ 12:50 UT coincides with disturbances in the spectrograms. At 13:20 UT and latter we see more strong plasma disturbances and PS plasma disappearances (similar to Tail Probe observations).
Fig.14 allows us to compare INTERBALL/Tail Probe (solid line) and GEOTAIL (diamonds) key parameters: total pressure, 3 components of magnetic field, Vx velocity component, beta-parameter and a set of solar wind parameters. Taking into account of differences in measurement methods on both satellites, it is possible to get significant information about temporal and spatial scales of events. Pressure increases in 1st panel at ~ 12:00-13:00 and 15:30-16:30 UT indicate the growth phases of 1st and 2nd substorms.
As have been shown by the diagram in Fig.4, there was a good data set obtained on INTERBALL/Auroral Probe in the auroral zone and polar cap during 2nd, 3rd and 4th substorms. Fig.15 (a,b,c) present ion, O+ and electron energy spectrograms measured with ION instrument during time intervals at 16:00-17:00 and 17:00-18:00 UT on December 22, 1996, respectively. In auroral zone which began at ~ 16:38 UT the instrument observed ion injections with the time-of-flight energy dispersion. Estimates shows that these ion fluxes probably originate at the distance ~ 15-20 RE from the observation point. Simultaneously with the injections the upward O+ fluxes with energy up 10 KeV were observed.
If we study the difference between model and measured magnetic field along Auroral Probe trajectory (see Fig.16) we can find that the magnetic field discrepancies are observed simultaneously with ion injections at ~ 16:40 UT on December,1996. It means that during ion injections there were currents which could disturb global magnetic field.
During time interval of 16:30 -17:00 FAST moved lower than Auroral Probe and allows us to compare data of both satellites obtained at different heights. It should be noted that auroral data monitored with 4 Finnish All-Sky-Cameras are also available at http://www.geo.fmi/InterballWorkshop/ASC/.
Basic features of studied period
Ground and Solar Wind Observations.
Magnetotail Observations.
Auroral Zone Observations.
Preliminary conclusions and possible tasks for investigations.
Interval D. December 22-23, 1997
Large-Scale Magnetotail Dynamics During Continued External Forcing.
Subjects of Study (Responsible: Yuri Yermolaev)
Multipoint Study of Bursty Bulk Flows (BBF).
Subjects of Study (Responsible: Anatoly Petrukovich)
Event Study of Impulsive Phenomena During Poleward Auroral Expansion
Subjects of Study (Responsible: Victor Sergeev and Jean-Andre Sauvaud)
We are very grateful for providing data
POLAR
WIND
LANL
Ground-based Data
INTERBALL/Tail Probe
INTERBALL/Auroral Probe
GEOTALL
FAST - C.Carlson.
We thank .A.G.Yahnin, S.P.Savin, B.Nikutowski, and D.Milling for helpful discussion of data.