Cluster workshop, September 2006, Ivalo, Finland .

1. Source Location of the Wedge-like Dispersed (sub-keV) Ring Current in the Morning Sector During a Substorm M. Yamauchi (IRF-Kiruna), P.C. Brandt, Y. Ebihara, H. Nilsson, R. Lundin, I. Dandouras, H. Rème, C. Vallat, P.-A. Lindqvist, A. Balogh, and P.W. Daly Cluster workshop, September 2006, Ivalo, Finland.

2. What is "wedge-like dispersion" ? Sub-keV trapped ions with wedge-like energy-latitude dispersion are seen almost all satellites at around L=4-6. They are drifting trapped ions from nightside. e- ion+ Viking INV 57° INV 59° poleward

3. Cluster also detects (cf. Viking)

4. Previous Observations • sub-keV ion precipitation @ subauroral region): • Aureol 1 (400~2500km, 00-06 MLT): Sauvaud et al., 1980 • increases after substorms. • * DMSP F6/F7 (800 km, 0830 MLT):Newell & Meng, 1986 • correlated with Kp (some hrs delay), event may last a day. • * Viking (2~3 RE , dayside): Yamauchi et al., 1996a,b • "Wedge-like dispersed structures", • modulation by pc-5 pulsation. • * Freja/Viking/Cluster (dayside) Yamauchi et al., 2005 • O+ at low-altitude / H+ at high-altitude • * Viking (2~3 RE , dayside): Yamauchi and Lundin, 2006 • Fossil of substorm (AE) activities, but not storm (Dst) . • morning source? • others: Shelley et al., 1972; Chappel et al., 1982

5. (Ebihara et al., 2001) Drift simulation Ebihara et al., 2001 Reversed dispersion Both dispersions Start drifting from midnight sector from an impulsive source

6. 6 MLT 9 MLT 12 MLT 15 MLT 18 MLT Viking statistics (Yamauchi and Lundin, 2006) Further study using the backward superposed epoch analyses over 700 traversals on the wedge observation vs. time-lag from nearest high AE activities. (1) Moves eastward (2) Decrease in time Time-lag (hours)

7. Viking Summary It is a fossil of substorm activity (model is right!). However, it appears much earlier than prediction!

8. Now, back to Cluster observations Similar structure between Viking and Cluster: the trapped ions mostly reach Viking altitude. eastward drift Viking 14 MLT poleward westward drift eastward drift H+ Cluster 11 MLT

9. Cluster observation Westward (magnetic) drift Eastward (electric) drift Noon Early morning Late morning only < 1 keV also > 1 keV also > 1 keV

10. MLT dependence:Morning peak at all altitude Good agreement among Cluster, Viking, Freja and simulation (mirror altitude may explain the difference in altitude.) But Morning Peak but not post-midnight Extended to > 1 keV Limited to < 1 keV

11. Cluster wedge = Viking wedge≈ fossil of substorm activity Question is: Why does it appears much earlier than prediction? Why Morning Peak but not post-midnight? Drift velocity? Starting (source plasma) location?

12. We have several possibilities (A) Strong E-field push ions quickly. (B) Scattering of <10 keV ions to lower energy (C) Precipitating energetic ions sputter ionospheric ions. (D) Unknown local energization process. We need to identify the source location from case study ÞFind events when the wedge is formed during a substorm. ÞWe found one case. Wedge is seen only at outbound.

13. Cluster case study H+ O+ South North

14. +1° 0° -1° -2° S/C-1 2001-10-21 Yes No 23:40-24:00 UT Yes S/C-4 No S/C-3 ? No Relative S/C position: all at 9.0±0.1 MLT

15. Observation summary S/C-1 (23:45 UT), S/C-4 (23:50 UT), and S/C-3 (23:40 UT) passed through the same magnetic flux tube at 9 MLT (L≈4). Butterfly-trapped distribution: bouncing between hemispheres. ÞDifference between 23:40 UT (no low-energy signature@south) and 23:50 UT (wedge@north) in the same flux tube means an temporal variation. Eastward ExB drift (VE) = energy independent, MLT dependent) Westward Ñ|B|+curvature drift (VB) = energy dependent VE >> VB at low energy (<100 eV) VE ~ VB at high energy (value depends on E-field strength, at 20 keV in the present case because the last-coming ions are 10-20 keV in the dispersion curve)

16. Time-of-flight / velocity filter Radial component is only from ExB drift (energy independent) but not magnetic drift 10 keV @ 23:40 UT 0.1 keV @ 23:40 UT S/C-1,4 @ 23:40 UT 10 keV @ 23:50 UT S/C-3 @ 23:40 UT 0.1 keV @ 23:50 UT +0.1§ 0° -0.1°

17. 0.1 keV 10 keV time-of-flight principal V1 = VE-VB ~ VE @ 0.1 keV V2 = VE-VB << VE @ 10 keV t t+∆t For observation near equatorial plane: V1*t = V2*(t+∆t) or (t+∆t)/∆t = V1/(V1-V2) ~ VE/VB(note : VB@10 keV) = (E/B)*(q*R*B/3*W*g) ~ E [mV/m]/g or t = ∆t*E [mV/m]/g - ∆t q: the charge R ~ 4 RE: geocentric distance W: 10 keV is the ion energy g: ~ 1, 0.9, 0.7 for 90°, 40°, 0° PA

18. Observed E and pitch angle 1~3 mV/m 40°~90° PA g=0.9~1.0 t ≤ (1.1*E[mV/s] - 1) * ∆t E=1~3 mV/mt = 0.1~2.3*∆t & VE= 3~10 km/s

19. From t = 0.1~2.3*∆t & VE= 3~10 km/s (a) Nothing @ 23:40 UT, S/C-3  0.1 keV @ 23:50 UT, S/C-1 (b) Nothing @ 23:40 UT, S/C-3  10 keV @ 23:53 UT, S/C-1 (c) 0.1 keV @ 23:50 UT, S/C-1  10 keV @ 23:53 UT, S/C-1 From (b) which is temporal change (cf. (a)) ∆t < 13 min  t < 30 min before 23:40 UT drift distance = VE * t < 20000 km dispersion started at 7~9 MLT. Start dispersion 20~30 min before (i.e., 23:20~23:30 UT). From (c) when assuming temporal change (cf. (b)) ∆t ~ 3 min  t = 0.5~8 min before 23:50 UT drift distance = VE * t = 100~5000 km dispersion started at 8~9 MLT. This cannot be true (c) is spatial structure.

20. Other method: Oxygen feature O+ wedge is only at 0.05-0.3 keV range (20 km/s ~ 50 km/s). The 0.05 keV O+ takes 20~30 min to travel from the ionosphere to the Cluster location along B in best case. Therefore, O+ should not have mirror-bounced, and this is confirmed from nearly uni-direction pitch angle. Again, 20~30 min elapsed time (with morning source) ! H+ O+ O+ PA

21. O+ trajectory (30 min trajectory) Tsyganenko T89 B-model & Weimer 2001 E-model Backward trace of 0.1 keV O+ (reference point S/C-3) Pitch angle: 0°, 45°, 90°, 135°, 180° Again, morning source

22. Three possibilities (A) Strong E-field push ions quickly. (B) Scattering of <10 keV ions to lower energy (C) Precipitating energetic ions sputter ionospheric ions. (D) Unknown local energization process.

23. ENA image indicates (1) strong E (2) No energetic H+ in the late morning sector (3) qualitative difference between O+ and H+ Strong E  scenario (A) in the Table: Ions < 10 keV could have convected to the morning sector quickly without forming the dispersion. No energetic H+  difficult for scenario (B) in the Table unless electron is important H+ - O+ difference  At least O+ wedge can be formed local General context (deduced from ENA image) post-midnight preference = strong E (could be ~10 mV/s)

24. Summary and conclusions Case study from 2001-10-21 event (9 MLT) shows that * The "wedge" suddenly appeared in the magnetic flux tube in which no signature was recognized 10 minutes before. * The dispersion is formed within 3 Re distance from the spacecraft within 30 minutes before the observation. * Observed oxygen ions of the "wedge" were not mirrored, i.e., they directly came from northern ionosphere (ionospheric source) 20-30 minutes before. The dispersion might start in the morning for a substantial numbers of the wedge-like structure. Future task : understand the source of the wedge-like structure for both O+ and H+. This final target is still far away.

25. Backward superposed epoch analyses * Probabilities with/without “signature” for different time-lags from latest AE increase. Total only 700 sorted by LT & time-lags. To increase statistics: * 3-hour MLT bins * 3-hour windows (running sum) for the time-lag * Intensity is divide into only three categories: "clear structure", "marginal", and "quiet. Ideal AE history In reality In reality

26. Viking statistics