Determination of the Substorm Initiation Region From a Major Conjunction Interval of THEMIS Satellites A T Y Lui, V Angelopoulos, S B Mende, O LeContel, H Frey, E Donovan, D G Sibeck, W Liu, H U Auster, X Li, M Nose, and M O Fillingim Outline Conjunction of THEMIS and GOES 11 &12 satellites Auroral activity (AU/AL indices and GBO observations) THEMIS observations Time history of substorm disturbances (including GOES 11&12) Summary and Conclusions

Conjunction of satellites Major conjunction of THEMIS satellites along midnight axis on 2008 Jan 29, 07-09 UT. GOES 11 & 12 bracket the MLT of THEMIS alignment.

AU/AL indices During Conjunction During the conjunction interval, there were one isolated substorm and another substorm with multiple intensifications. The three time indicators correspond closely to onsets of substorm activity seen by THEMIS satellites.

Satellite Projections AE stations are shown in blue, THEMIS GBO stations in green, and THEMIS satellite projections along magnetic field lines in red. The projected satellite locations are well embedded within these stations.

Auroral Activity at Fort Smith (FSMI) First auroral substorm onset at ~0714 UT starting slightly to the east - note the relatively undisturbed arc (no breakup activity) poleward of the onset arc. Second substorm onset at ~0742 UT starting slightly to the west - note undisturbed arc poleward of onset arc. Third substorm activity at ~0832 UT from the west of the station.

Auroral Activity at Fort Simpson (FSIM) • Fort Simpson (FSIM) is west of FSMI. • Substorm activity seen was similar to that at FSMI, i.e., arc brightening followed by poleward expansion at ~0714 UT and ~0742 UT and with relatively undisturbed poleward arc. • Auroral substorm activity at the poleward boundary starting at ~0831 UT.

Keograms From 4 GBO Stations • From top to bottom are keograms from FSMI, FSIM, Inuvik (INUV), and Fort Yukon (FYKN). • INUV & FYKN are west of and at higher latitudes than the former two stations. • Onset at ~0742 UT reached INUV & FYKN with poleward expansion at ~0813 UT. •Another activation at ~0828 UT at the poleward boundary for INUV & FYKN. Double oval: retreating equatorward at FSIM but expanding poleward at INUV & FYKN 0742 0828

Movie From GBO Stations

THEMIS B (P1) • No dipolarization seen by P1 at X ~ –30 Re. • Negative Bz but no tailward plasma flows seen near the first substorm onset. • P1 exited the plasma sheet later with negative Bz and earthward plasma flows, implying plasma sheet thinning after (not before) the substorm expansion onset in the mid-tail region. • It remained in the tail lobe for the rest of the interval.

THEMIS C (P2) • Two dipolarizations seen by P2 at X ~ –18 Re, negative Bz and no tailward plasma flows for first substorm onset. • Negligible earthward plasma flow seen prior to the 2nd dipolarization. • Strong earthward plasma flows (~500 km/s) seen prior to the third dipolarization.

THEMIS D (P3) • Three dipolarizations seen by P3 at X ~ –11 Re with time corresponding well to ground auroral onsets of activity. • Weak earthward plasma flows (<150 km/s) seen prior to the first dipolarization. • Negligible earthward plasma flows (<50 km/s) prior to the second and third dipolarizations.

THEMIS E (P4) • Three dipolarizations seen by P4 at X ~ –11 Re, similar to P3 closeby. However, there are big differences between P3 & P4 in temporal profiles of Vx and Bz during dipolarizations. • Weak earthward plasma flows (<100 km/s) seen prior to the first dipolarization. • Tailward plasma flows prior to 2nd dipolarization. • Weak earthward plasma flows (<200 km/s) prior to the 3rd dipolarization.

THEMIS A (P5) • Two dipolarizations seen by P5 at X~ –8 Re, similar to P2 at X~ –18 Re. • Weak earthward plasma flows (~100 km/s) seen prior to the 2nd dipolarization. • Very weak earthward plasma flows (<50 km/s) seen prior to the 3rd dipolarization.

Observations from GOES 11 & 12 satellites • Dipolarization most prominent for first substorm onset. • GOES 12 at post-midnight observed a larger Bz increase than GOES 11 at pre-midnight, suggesting first onset was located in the post-midnight.

THEMIS Vix and Bz For 0714 UT, temporal sequence: D/E => B For 0742 UT, temporal sequence: A => D/E => C For 0828 UT, temporal sequence: C => E => D => A

Time History of Events For first auroral activation (onset of an isolated substorm), onset location was between P2 and P3/P4 (X = –11 to –18 Re). It was closer to P3/P4 than P2 because dipolarization at P3/P4 was simultaneous with auroral breakup whereas that at P2 was delayed substantially. For the second auroral activation (onset of a longer duration substorm), onset location was between P5 and P3/P4 (X = –8 to –11 Re). For the third auroral activation (onset of substorm intensification, not a new substorm expansion onset), the activity onset location was beyond P1 (X< –30 Re).

No Magnetic Reconnection on Open Field Lines For first and second auroral activations (onsets of two substorms), there was a relatively undisturbed arc poleward of the onset arc. The substorm activity did not reach the oval’s poleward boundary. These two optical features from GBO observations indicate the absence of magnetic reconnection on open field lines. Even though negative Bz (|Bz|<<|Bx| => southward dipping) was seen by P2 at X= –18 Re, there was no associated tailward plasma flow, no tailward streaming of energetic electrons, and no signature of the quadruple By perturbation. P1 at X= –30 Re observed southward dipping and earthward plasma flow later. When P1 exited the plasma sheet after substorm onset, it did not detect tailward streaming of energetic electrons. These observed features from THEMIS satellites indicate there was no signature of magnetic reconnection at the plasma sheet boundary, consistent with the presence of an undisturbed auroral arc at the poleward boundary of the auroral oval. Therefore, there was no magnetic reconnection on open field lines at the two substorm onsets.

Summary and Conclusions THEMIS major conjunction: covering from X= –5 to –30 Re. Three auroral activations during this interval - first two are substorm onsets and the third one is a substorm intensification. Not all dipolarizations are preceded by significant earthward plasma flows, suggesting earthward plasma flows are not a necessary condition for dipolarization. For the first substorm onset, negative Bz occurs at X = –18 Re (P2) before at X= –30 Re (P1), suggesting thinning starts earthward of P2 and then propagates to P1 (tailward going thinning wave?). The first substorm is initiated in the inner plasma sheet (X = –11 to –18 Re; closer to –11 Re than –18 Re) and does not involve magnetic reconnection on open field lines. The second substorm is initiated in the inner plasma sheet (X = –8 to –11 Re) but subsequently involves substorm disturbance initiated beyond X= –30 Re for a substorm intensification. These two substorms fit the substorm synthesis model.