SUBSTORM EVENT ON NOVEMBER 13, 1996, 17 - 22 UT

Prepared by N.L.Borodkova (1), A.G.Yahnin(2), L.M.Zelenyi(1), Yu.I.Galperin(1), V.N.Lutsenko(1), J.Hanasz(3), R.A.Kovrazhkin(1), J.-A.Sauvaud(4), V.A.Sergeev(5)

Data for event of November 13, 1996 have been obtained from several Scandinavian/Kola and Siberia ground stations, from INTERBALL-1 making the measurements in the tail lobes and INTERBALL-2 in the auroral region, from GEOTAIL located in the plasma sheet/tail lobe regions, from the geosynchronous LANL satellites, and from WIND spacecraft at 36 RE upstream in the solar wind. Figure1 shows the location of the spacecraft in the X-Z GSM plane. Both satellites are located at the same GSM coordinate Y=-3 RE. Magnetopause position is drawn in accordance with Shue model for the solar wind conditions of P = 3.5 nPa and Bz = -12 nT. Field lines correspond to Tsyganenko96 model for the moment 18:00 UT and pass through the INTERBALL-1 position

Magnetic field and solar wind pressure measurements from 12 to 24 UT on November 13, 1996 onboard WIND spacecraft are shown in Figure 2. WIND being located at ~36 RE upstream in the solar wind monitored interplanetary conditions. It is clearly seen that there were two increases of the solar wind pressure at ~12.50 UT and at ~17.00 UT as well as the strong decrease at ~17.40 UT. But nevertheless, solar wind dynamic pressure remained to be very high, two times more that its average value. Magnetic field components were very fluctuating. Bz component changed the sign and value from ~+5 nT to -10 nT at 17.40 UT and remained strongly southward during more then two hours. At 18.20 UT magnetic field rotated from southward-duskward to southward-dawnward direction.

Magnetometer networks at auroral latitudes (IMAGE, chain 210, some Russian Arctic stations) observed disturbances of the magnetic field during the time interval 17.30 - 22 UT on November 13, 1996. Some magnetic stations (e.g. Norilsk) show slow increase of electrojet (substorm growth phase signature) starting at ~17.30 UT. Magnetometer measurements from Amderma, Norilsk, Dixon and Cape Cheluskin (some of them are displayed in Figure 3) show a sharp, negative deflection in the H component trace starting at ~18.25-18.30 UT. Pi2 pulsations recorded in Irkutsk and Lovozero indicate the onset and intensifications of expansion phase at 18.28, 18.30, 18.36, 18.39, 18.54 UT. The new (and strong) substorm activation were recorded at and after ~19 UT mainly by IMAGE. At this time (19 UT) the auroral breakup and very sharp enhancement of the riometer absorption have been registered by auroral TV camera in Kilpisjarvi, Finland and by Finnish riometer network, respectively.

It is interesting to mention that the weak substorm-like signature (weak Pi2 burst) was observed in Irkutsk at ~17.50 UT. At that time some magnetic variations were detected on the ground. It can be attributed to the weak substorm triggered by the solar wind pressure pulse or to magnetic effect of the pulse itself. Several other data sets (magnetic field data from Interball, the LEP data from Geotail, particle data from LANL satellite) show some changes just at that time as well.

In Figures 4 and 5 we present energetic electron measurements in 9 energy channels covering the range 0.05 - 1.5 MeV from two geosynchronous satellites LANL-080 and LANL-084. Decrease of particle fluxes at ~17.25-17.30 UT marks the start of the growth phase. The flux intensities restored for a short time at 17.50 UT at both satellite locations (substorm injection or effect of the magnetospheric magnetic field compression?). Satellite 1991-080 located at ~23.30 MLT detected strong injection at around 18.32 UT.

Another injection (better seen onboard 1991-084 at ~01.30 MLT) started at 18.39 UT. New strong dispersionless injection seen at midnight just after 19.00 UT. Simultaneously, injection with dispersion has been detected at 02 MLT. This means (in accordance with ground-based data) that active region was shifted to the west.

We present observations by four experiments on Interball-1: ELECTRON, CORALL, DOK-2 and MIF-M on November 13, 1996 at 17 - 22 UT. Figure 6 shows from top to bottom GSM B-x, B-y, B-z components, magnetic field value, and energy-time spectra of electrons and ions. The measurements have been done in the northern tail lobes. Magnetic field value increased in the time interval 17.20 - 17.50 UT from 27 to 33 nT, likely, as a response to the solar wind pressure enhancement. Dropping of the solar wind pressure at 17.40 UT causes the slight magnetic field decrease from 33 to 29 nT at 17.50. ELECTRON measurements reveal sunward flow during that time. At 18.35 - 18.45 UT a burst of plasma was observed in the tail lobes. Bottom panels of Figure 6 show appearance of the electrons with energy 80 - 630 eV and ions with energy 0.5 - 10 keV. Appearance of this plasma coincides with the diamagnetic dropout in magnetic field. Figure 7 shows electron and ion measurements with better time resolution. DOK-2 energetic particles detector (Figure 8) recorded the increase of electron fluxes with energies 21 - 95 keV (top panel of Figure 8) and proton fluxes with energies 21 - 133 keV (two middle panels of Figure 8) at the same time. Bottom panel of Figure 8 shows the ratio of sunward proton fluxes to tailward proton fluxes. Measurements indicate strong tailward flow at 18.38 - 18.45 UT. We consider this flow along with magnetic field data as an evidence for the tailward moving plasmoid. Observations described above correlate with the assumption about substorm starting at around 18.30 UT as determined from ground-based and geosynchronous orbit data. At that time the substorm activity occupied the MLT sector of INTERBALL-1 observations.

The character of the plasma flow changed when plasmoid moved tailward. From 18.45 UT CORALL and DOK-2 experiments began to observe tailward ion flows with low energies of 50-300 eV, while ELECTRON experiment began to detect sunward-tailward counterstreaming electron flows with average energy of about 125 eV.At 19.02 UT magnetic field value began gradually decrease from 30 nT, and at 19.30 UT it reached 21 nT. This means that decrease of pressure in the lobes and current sheet disruption were observed at that time (see also GEOTAIL data).

Data from other ion experiment PROMICS presented in Figure 9 show, from top to bottom, tailward oxygen O+ flow, tailward, perpendicular to the spin axis, and sunward flows of protons . PROMICS T1 (0.001 - 1 keV) also observed low energy part of ions distribution at 18.35-18.45 UT. After that time PROMICS T1 began to register tailward flowing ions. At 20.10-20.30 UT PROMICS recorded increasing of tailward protons with average energy about 100 eV. Finally, flux of tailward oxygen O+ was observed at 19.40 - 20.50 UT.

INTERBALL-1 plasma observations imply that mantle-like plasma was observed at 18.45 - 20.50 UT. There are two other points in favour of the plasma mantle interpretation. First, during all the time IMF Bz component was strongly southward oriented. This could lead to the extension of the mantle to the plasma sheet. And second, decrease of pressure in the tail lobes observed at that time means the decrease of the magnetotail diameter, or distance between plasma sheet and magnetopause (note, that solar wind pressure increased at that time).

During this event GEOTAIL was located in the southern plasma sheet with GSM coordinates (-22, +3, -2 RE). Electrons and ions energy-time spectra are presented in Figure 10. Some density increase and temperature decrease both in ions and electrons were observed after 17.45 UT, close to the time of substorm-like (or pressure pulse) signatures described above. GEOTAIL passed to the tail lobes at ~ 18.35 UT and observed signatures of tailward moving plasmoid at 18.45 UT (short encounter with the tailward moving plasma sheet plasma).

There were short-time plasma sheet observations at 19.40, 20.00 UT, but finally GEOTAIL returned in the plasma sheet only at 20.20 UT.

It should be emphasized that simultaneously two satellites INTERBALL-1 and GEOTAIL observed tailward-propagating plasmoid being located, respectively, in the northern and southern lobes.

On November 13, 1996 from 18 to 23 UT INTERBALL-2 moved from the evening to the morning sector (from 22 to 5.30 MLT). For this event DOK-2 and IMAP-2 measurements onboard INTERBALL-2 are available starting from 18.30 UT. Partially POLRAD, ION and HYPERBOLOID experiments data overlap at the time interval under study.

Figure 11 shows energetic proton and electron fluxes measured by DOK-2. Interball-2 crossed the auroral zone from lower to higher latitudes, and at ~19 UT left the region of the intense precipitation of energetic particles (region of active auroras?). It seems that the auroral bulge propagating to the north overtook the satellite between 19.16 and 19.19 UT.

At ~19.18:30 UT very unusual ion energetic spectra were registered by DOK-2: they exhibited the existence of monochromatic beams of ions with energy of some 100 and 200 keV.

Next, Figure 12 presents the data from POLRAD experiment. POLRAD experiment was switch on at 19 UT and observed AKR emission, which is likely related to the activation started before 19 UT. The closest approach of the spacecraft to the AKR source region was at 19.10 UT. VLF was seen in POLRAD spectra in the low frequency range below 20 kHz. Sudden enhancement of VLF was observed at 19.12 UT.

Finally, at 20.40 UT ION experiment was switch on and observed high fluxes of protons and O+ ions of 80-18000 eV in the magnetosphere. The data from ION experiment are presented in Figure 13.

1. The considered substorm occurred under variable solar wind conditions. The solar wind pressure was enhanced, and even more strong pressure pulse was detected in the beginning of the growth phase. The pressure pulse was associated with specific variation of tail lobe magnetic field, variation in plasma sheet plasma parameters, and in some signatures of the substorm-like phenomenon at the ground.
2. The main substorm developed under negative Bz conditions and consisted of several intensifications.
3. The first onset occurred in the MLT sector that enveloped the positions of INTERBALL-1 and GEOTAIL. The satellites located at both sides of the plasma sheet observed plasma sheet thinning and tailward propagating plasmoid signatures.
4. The only signature of another substorm intensification that occurred out of the MLT sector of INTERBALL-1 and GEOTAIL was decrease of the tail lobe magnetic field (global disruption of current sheet) seen simultaneously in the different tail lobes.
5. During the current disruption stage the appearance of low-energy ion (including O+) tailward flows and bidirectional electron fluxes in the tail is, likely, a manifestation of the plasma mantle approaching the satellite due to the decrease of pressure in the tail lobes.
6. During the pass of INTERBALL-2 through the region of substorm-related auroral bulge the intensive AKR emissions were registered as well as the unusual spectra of precipitating energetic ions.
7. Large abundance of O+ ions in the magnetosphere is magnetic storm related feature. However, the measured Dst didn't increase to the reasonable for the magnetic storm values. At that time strong solar wind pressure led to the increase of the dayside magnetopause current which masked ring current value. Corrected ring current contribute amounts to ~ -50 nT.

It is pleasure to thank the following collegues for the providing of data:

• WIND

• GEOTAIL

• INTERBALL-TAIL

• INTERBALL-AURORAL

• LANL

• SAMNET

• GROUND DATA