Substorm-origin sub-keV ring
current ions: wedge-like
structure
M. Yamauchi, R. Lundin, H. Nilsson (IRF-Kiruna),
Y. Ebihara (Nagoya U.)
I. Dandouras, H. Reme (CESR, Toulouse)
Substorm : production of plasma
Sub-keV ring current : fossil of substorm
Question: source of sub-keV ring current?
ICS-9, Graz, 2008-5-3~7
Sub-keV Ring
Current?
Traditional view: > 5
keV only, BUT
m>2
H+
cavity in sub-keV range
15 MLT
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.
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
Q1.The source population
* Cluster observation
Q2.The exact relation to substorms * Numerical mapping.
Plate 1: Sub-keV is not cavity
Eastward (electric) drift Westward (magnetic) drift
Noon
only < 1 keV
cf. Viking 14 MLT
eastward drift
Early morning
Late morning
also > 1 keV
also > 1 keV
poleward
Similar between Viking
and Cluster:
i.e., sub-keV ion
reach Viking altitude.
Plate 2: MLT dist.
6 MLT
9 MLT
(1) Total: Morning peak
(2) > 1 keV: Morning only
(3) Freja-Cluster difference:
Mirror altitude?
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.
12 MLT
15 MLT
18 MLT
Time-lag (hours)
UT 00:30
01:00
day (12 MLT)
01:30
difference
02:00
Midnight (3 MLT)
Substorm onset
Plate 3: It is a fossil
of substorm activity.
(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)
Case study : 2001-10-21 event
Substorm onset
H+
O+
South
North
Inter-SC time-of-flight
V(10 keV) = VE-VB << VE
10 keV
H+
Substorm
onset
23:40 UT
+0.1°
0°
-0.1°
SC-3 @ 23:40UT: No
23:40 UT
9 MLT
SC-1,4 @ 23:50UT: Yes
23:50 UT
V(0.1 keV) ~ VE
O+
0.1 keV
23:50 UT
∆t < 13 min
(100 eV/SC-3 & 10 keV/SC-4)
⇒ t < 30 min
⇒ drift = VE * t < 20000 km
⇒ started at 7~9 MLT.
Oxygen feature
H+ = both direction
O+ structure is only at 0.05-0.3
keV range (20 km/s ~ 50 km/s).
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 unidirection pitch angle.
Again, 20~30 min elapsed
time (with morning source) !
H+
PA
O+
PA
O+ = one direction
Source ?
Observation =
Back-trace to 8 Re
Bursty cold (0.1~0.2/cc)
Back-trace to 8 Re
Isotropic Maxwellian
(T=1 keV, N=0.5/cc)
Re-construction by simulation
both component together
Summary
Wedge-like dispersed sub-keV ring
current is a fossil of substorm activity
carried by VExB ≥ VB-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: 2003-1029 event (next figure).
IMAGE/FUV
0611:40 UT
Ground ∆B
2003-10-29 event
evening-midnight:
2000 nT
0613:40 UT
0615:40 UT
Wide morning source
of upward current
(O+ escape) nearly
simultaneously with
substorm onset
morning: 2000 nT
Yamauchi, Iyemori, Frey,
& Henderson, 2006 JGR