Substorm-origin sub-keV ring current ions: wedge-like structure ICS-9, Graz, ~7 Substorm : production of plasma Sub-keV ring current : fossil of substorm Question: source of sub-keV ring current? M. Yamauchi, R. Lundin, H. Nilsson (IRF-Kiruna), Y. Ebihara (Nagoya U.) I. Dandouras, H. Reme (CESR, Toulouse)

Sub-keV Ring Current? Traditional view: > 5 keV only, BUT It slowly moves eastward starting from early morning sector during substorms ( Plate 2, 3: Yamauchi and Lundin, 2006; Yamauchi et al., 2006b ) It decays as it moves (with decreasing characteristic energy) with few hour time scale ( Plate 2 ). Substantial numbers of them are formed in the early-morning sector rather than mid-night sector ( Plate 2 ). To invest these problems, we use * Cluster observation * Numerical mapping. Q1.The source population Q2.The exact relation to substorms Substorm produces various plasma components by energizing pre-exiting components or transporting plasma from one place to another. The wedge-like dispersed sub-keV ions in the ring current region (wedge-like structure: Plate 1: Yamauchi et al., 2006b) are one of them. We show recent few year’s progress on the wedge-like structure. m>2 H + cavity in sub-keV range 15 MLT

Plate 1: Sub-keV is not cavity Noon Early morning Late morning Eastward (electric) drift Westward (magnetic) drift only 1 keV also > 1 keV eastward drift cf. Viking 14 MLTpoleward Similar between Viking and Cluster: i.e., sub-keV ion reach Viking altitude.

Plate 2: MLT dist. (1) Total: Morning peak (2) > 1 keV: Morning only (3) Freja-Cluster difference: Mirror altitude? 6 MLT 9 MLT 12 MLT 15 MLT 18 MLT Time-lag (hours) Viking statistics (1) Moves eastward (2) Decrease in time Backward Superposed Epoch Analyses using 700 traversals. * 3h MLT bin & 3h window for time-lag * three categories: "clear structure", "marginal", and "quiet.

(Q1) Source location and drift speed (very quick)? (Q2) Relation between two (keV+wedge) components, i.e. source population? Seen at low-altitude (after substorm) day (12 MLT)Midnight (3 MLT) UT 00:30 01:00 01:30 02:00 Substorm onset difference Plate 3: It is a fossil of substorm activity.

Case study : event H+ O+ South North Substorm onset

Inter-SC time-of-flight ∆t < 13 min (100 eV/SC-3 & 10 keV/SC-4)  t < 30 min  drift = V E * t < km  started at 7~9 MLT. +0.1° 0° -0.1° 23:50UT: Yes 23:50 UT 23:40 UT 23:50 UT V(0.1 keV) ~ V E 10 keV0.1 keV 23:40 23:40UT: No H+H+ V(10 keV) = V E -V B << V E 9 MLT Substorm onset O+O+

Oxygen feature O+ structure is only at keV range (20 km/s ~ 50 km/s) 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+ PA O + = one direction H + = both direction

Source ? Observation = Bursty cold (0.1~0.2/cc) Isotropic Maxwellian (T=1 keV, N=0.5/cc) Back-trace to 8 Re

Re-construction by simulation both component together

Summary Wedge-like dispersed sub-keV ring current is a fossil of substorm activity carried by V ExB ≥ V B-drift. O+ source is different from H+ source. The source is cold ions, which is supplied from midnight to morning. A sample of morning source: event (next figure).

evening-midnight: 2000 nT morning: 2000 nT 0615:40 UT 0613:40 UT 0611:40 UT event Ground ∆B IMAGE/FUV Wide morning source of upward current (O+ escape) nearly simultaneously with substorm onset Yamauchi, Iyemori, Frey, & Henderson, 2006 JGR

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

Summary of event Case study from 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) minutes before.  Dispersion started and O+ is ejected from the morning (6~7 MLT).

Ground ∆B + auroral image evening- midnight morning 0611:40 UT 0613:40 UT 0615:40 UT 0617:40 UT IQA≈01 LT