A T Y Lui Outline  Themis: What does it stand for?  Plasma parameters to check for arrival of substorm disturbance: Plasma bulk flow Magnetic field elevation angle Is there another plausible interpretation on Themis first substorm observations?

 THEMIS is an acronym for Time History of Events and Macroscale Interactions during Substorms  THEMIS is also the Greek goddess of justice, wisdom, and good council. The scales she is holding symbolizes impartiality. THEMIS © Chad Awalt

XY-Projections of THEMIS satellites

Precise Locations of THEMIS satellites On 2007 March 23 at 1115 UT, their locations were (T96 mapping): THEMIS A (X-,Y-, Z-GSM): –44326, 58981, –3576 km (65.7, 19.8) THEMIS B (X-,Y-, Z-GSM): –44221, 57273, –3642 km ( ) THEMIS C (X-,Y-, Z-GSM): –39912, 35638, –4005 km ( ) THEMIS D (X-,Y-, Z-GSM): –44159, 56073, –3689 km ( ) THEMIS E (X-,Y-, Z-GSM): –43424, 72336, –2843 km (66.5, 19.3) The time history of events would be: Eastward spreading: E => A => B => D => C (based on Y-coord) Westward spreading: C => D => B => A => E (based on Y-coord) Outside-in spreading: A => B => D => E => C (based on X-coord) Inside-out spreading: C => E => D => B => A (based on X-coord) Sequence is valid even for projected locations (westward:C D B/A E)

Some comments on determining the time history of substorm activity Since substorm activity is very dynamic, it is prudent to bear in mind that substorm disturbances in plasma parameters may not show nearly similar shape and/or magnitude if the separation in time/distance of detection is relatively long. Also, the indication of substorm disturbance is better judged by comparing the variation of the parameter value after the perceived arrival of substorm disturbance with that in the preceding time interval before the disturbance.

A B C D E The profiles for A, B, and D are quite similar, so let us look at this sequence first. It is definitely D => B => A. This means that the time history is inconsistent with eastward or outside-in spreading of substorm disturbance. Eastward spreading: E => A => B => D => C Westward spreading: C => D => B => A => E Outside-in spreading: A => B => D => E => C Inside-out spreading: C => E => D => B => A

A B C D E What left are westward and inside- out spreading. Now let us look at E and A. The sequence seems to be E => A. If E activity can be related to A activity, then the time history indicates inside-out spreading rather than westward spreading. C may be too far to relate activity with the others. If not, it is still consistent with inside- out spreading. Eastward spreading: E => A => B => D => C Westward spreading: C => D => B => A => E Outside-in spreading: A => B => D => E => C Inside-out spreading: C => E => D => B => A

A B C D E The profiles for A, B, and D are quite similar, so let us look at this sequence first. It is definitely D => B => A. This means that the time history is inconsistent with eastward or outside-in spreading of substorm disturbance. Eastward spreading: E => A => B => D => C Westward spreading: C => D => B => A => E Outside-in spreading: A => B => D => E => C Inside-out spreading: C => E => D => B => A ?

Eastward spreading: E => A => B => D => C Westward spreading: C => D => B => A => E Outside-in spreading: A => B => D => E => C Inside-out spreading: C => E => D => B => A A B C D E  at E & A are nearly the same ? What left are westward and inside- out spreading. Now let us look at E and A. The sequence seems to be E => A. If E activity can be related to A activity, then the time history indicates inside-out spreading rather than westward spreading. C may be too far to relate activity with the others. If not, it is still consistent with inside- out spreading.

Spreading of substorm disturbance in the current disruption model In the current disruption model for substorms, a current-driven plasma instability disrupting the cross-tail current can instigate development of the same instability in adjacent regions of the tail, both tailward and dawnward/duskward sides of the current disruption site. The spreading of substorm activity is quite complicated. Thus, relating convection and speed of various plasma waves to the averaged speed in the spreading of substorm activity for this model is not meaningful. Lui et al., AG, 25, , 2007

Summary I have examined two main plasma parameters (V x and  B ) to determine the time history of the substorm disturbance among the Themis satellites for the substorm that started at ~1115 UT on 2007 March 23. Taking the full array of observations into consideration, I think the time history can be most easily interpreted in terms of the inside-out scenario for the time sequence of substorm development at different locations in the magnetotail. It is at least another plausible interpretation.