Reconnection during the December 10-11, 1996 substorm period

The first auroral auroral intensification of December 11 was observed by POLAR UVI in the eastern Greenland sector at 0019:11 UT, as shown by the image sequence below. It was a pseudobreakup since it was not followed by significant westward or poleward expansion. The expansion was mainly in the eastward direction. The ensuing period was characterized by a sequence of intensifications that could be defined as pseudobreakups since they did not develop significant poleward expansion.

0000 - 0046 UT

0049 - 0135 UT

0140 - 0226 UT

Sequence of LHBL images of the oval seen from above on December 11, 1996. White arc segments denote Sondrestrom Incoherent scatter radar scans along the local magnetic meridian at F-region altitude. Green lines denote the 0, 90, 180, and 270 Glong meridians.

The first intensification that eventually developed into a full breakup was observed at 0044:39 UT (K. Liou, personal communication.) The poleward expansion of the aurora initially was not accompanied by substantial westward expansion. Instead, the aurora intensified in the eastward
direction and did not advance past the local time sector defined by Sondrestrom's meridian.

Poleward expansion of the aurora continued without significant westward expansion until at least 0126 UT. Then there was a second auroral intensification at Sondrestrom's local time followed by an intensification and poleward expansion of an arc that encompassed a wide local time sector west of Sondrestrom, as can be seen in in the UVI image sequence. Poleward expansion reached a maximum latitude at 0155 UT (see Figure #.)

IMAGE observed an enhancement in the westward electrojet starting at 2330 UT on Dec 10, but it did not measure a breakup until 0040 UT on Dec 11. The breakup activity reached the western Greenland sector at ~0120 UT.

Convection and particle distribution evolution

Starting at 1700 UT on Dec 10, there was a 5-hour period of quasi-periodic earthward flows alternated with (weaker) tailward flows. At this time Geotail was fairly close to the neutral sheet, at X_GSM = -25.1, Y_GSM = 1.5, Z_GSM = -2.5 R_e. Onset of strong flows coincided with quasi-periodic LANL injections.

The first auroral intensification (0019:11 UT) coincided with the onset of tailward streaming observed by Geotail's EPIC instrument, as shown by the plots below. was preceded by duskward streaming between ~0000 and 0010 UT, when Geotail was at X = -27.1, Y = -2.5, Z = -3.1 R_e. Mapping Geotail's position to the ionosphere using a Tsyganenko 89 magnetic field model with Kp=3 places Geotail's footpoint at 68.1° N and 351.3° W at 0000 UT. This location lies approximately three hours local time east of Sondrestrom's location. High frequency magnetic fluctuations accompanied the onset of tailward streaming. Geotail's EPIC observed an enhancement of tailward flows at 0022 UT.

Ion streaming reversal occurred at ~0045-0047 UT, right after the second auroral intensification observed by UVI (0022 UT). Eventually, earthward and tailward flows were coexisting. Energetic electron flux enhancement started with the onset of earthward ion flow. A second intensification of earthward ion flow occurred at 0126 UT, coinciding with the intensification of the westwardmost arc seen by POLAR. The electron flux enhancement was mor intense this time and had a bi-directional character at ~0130 and ~0142 UT.

Loading and unloading of magnetic field in the northern magnetic lobe was measured by Interball Tail. The start of unloading was observed at 0045 UT, after the lobe had been subjected to loading since 2307 UT on December 10 (see Figure #.) The start of the unloading coincided with the onset of the first breakup observed by POLAR UVI. The rate of unloading varied with time. During the first 40 min. it was ### Wb/sec. Then, starting at 0125 UT it increased to ### Wb/sec. The increased unloading rate coincided with the onset and poleward expansion of further auroral activation observe POLAR UVI in the region west of the original activation.