Reconnection during the February 8-9, 1997 substorm period

Intermittent geomagnetic activity was observed during the entire period, which was characterized by an interplanetary magnetic field that alternated its orientation between Bz=+5 nT and Bz=-5 nT. We will concentrate our analysis on the 0000-0400 UT interval of February 9, where there is POLAR UVI and VIS coverage and Sondrestrom convection measurements throughout. Auroral activity detected by POLAR's UVI and VIS imagers during the first substorm period extended over 8 hours local time. By contrast, the second substorm period was concentrated over a smaller sector around western Greenland. Multiple auroral intensifications ocurred between 00 and 0050 UT on February 9. UVI shows intensifications at 0004, 0013, 0025, 0031, and 0040 UT. None led to a full breakup. The first intensification that evolved into a full breakup started at 0050 UT. (Get a postscript image here.)

      Interball Tail was in the northern dusk magnetosheath (-4.6, 19.6, 19.5) and IMP 8 was in the southern morning lobe (-27.2, 5.3, -18.9). Wind was in the solar wind at X_GSM=200 R_e, Y_GSM=10 R_e, and Z_GSM=-15 R_e. Thus, given an average solar wind bulk speed of 550 km/s, there was a travel time of 40 min between Wind and the dayside magnetopause.
 

Convection and particle distribution evolution

    The enhancement progressed poleward and was detected in the full range of latitudes sampled by the radar scan centered at 0030 UT.

    The enhanced earthward convection was embedded in the 0015-0035 UT period where Geotail's Low Energy Particle (LEP) detector observed a net earthward ion flow, as shown by the moment calculations. Phase space density ion measurements from LEP, however, reveal that the ion plasma population was composed of counterstreaming components with the earthward population having higher average energy than the tailward population.
Furthermore, the plasma "beta" parameter was of order 0.25. The bidirectionality of the ion flows in combination with the observed value of beta suggest that Geotail was in the plasma sheet boundary layer and that it observed ions ejected from the distant neutral line and ions bouncing back from their ionospheric mirror points. Furthermore, the simultaneity of the PSBL flows and the ionospheric convection enhancement indicate that distant tail reconnection started at 0015 UT in the Geotail meridian. Ionospheric convection was very weak at all latitudes covered by the radar scan in the period between ~0130 and ~0200 UT.
 

Convection calculated from a combination of elevation scans	Convection calculated from single elevation scans

 At this time, IMP 8 detected the start of a monotonic decrease in magnetic flux. It lasted at least until IMP 8 crossed the high-latitude magnetopause at 0205 UT. The decrease in the magnetic field strength could have been due to either one of three effects acting alternatively or cooperatively: i) the excursion of the spacecraft into the plasma mantle due to flapping of the tail; ii) a decrease in the lobe field strength due to magnetotail magnetic flux unloading; iii) a change in the solar wind flow into a more northward orientation that made IMP 8 travel into the plasma mantle. The excursion into the weaker field occurring in a monotonic fashion and for a period lasting much longer than the flapping period seems to rule out the first possibility. The second mechanism would be consistent with the occurrence of clearly indicates that even if IMP 8 entered the mantle it was because the magnetic flux in the tail was decreasing. Therefore, simultaneous ionospheric convection enhancement and tail lobe unloading suggest that lobe reconnection occurred at that time.

     A period of tailward flow was observed by LEP starting at 00 47 UT. Geotail was located at X_GSM=-29.1, Y_GSM=5.8, Z_GSM=0.1 Re at 00 UT on February 9. Geotail's location at that time places the spacecraft's footpoint a half hour local time eastward of the magnetic meridian where the auroral intensification was most intense. Although initially mostly magnetic field-aligned, the convective component became apparent later and continued until ~0125 UT, as shown by the moment calculations.
 

Ion Fv at 0025:05 UT

Ion Fv at 0050:06 UT

Ion Fv at 0057:22 UT

    The intensification of the polewardmost arc coincided with a period of significant decrease in the equatorward convection, between ~0135 and ~0205 UT. At the end of the low convection period, STF measured a new intensification in the equatorward convection that coincided with the intensification of an arc that was equatorward of the polar cap boundary.
Ion and electron space distribution measurements by Geotail for the period preceding the first auroral intensification.
Energetic electron injection was apparent in LANL 1990-095, whose footpoint is very close to Narssarssuarq (see Plate #), at 0104 UT. It lasted only until 0130 UT and was non-negligible only in the two lowest energy channels, namely 50-75 and 75-105 keV. Therefore, particle injection lagged the expansion onset by at least 14 min.Notice that the electron flux enhancement was not preceded by a flux depletion that characterizes most growth phase intervals.