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= 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 subauroral region):
Aureol 1 (400~2500km, 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. Drift simulation (Ebihara et al., 2001) Reversed dispersion
Both dispersions
Start drifting from midnight sector from an impulsive source

6. Viking statistics 6 MLT 9 MLT (Yamauchi and Lundin, 2006) 12 MLT
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 Eastward (electric) drift
Westward (magnetic) 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
keV plasma forms at
Dispersive drift start at
scenario
night
No !
morning
(A)
(B) (C) (D)
(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
Þ Find events when the wedge is formed during a substorm.
Þ We found one case. Wedge is seen only at outbound.

13. Cluster case study H+ O+
South North

14. Relative S/C position: all at 9.0±0.1 MLT
+1° 0° -1° -2°
S/C-1
Yes No
23:40-24:00 UT
S/C-4
Yes 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.
Þ Difference between 23:40 UT (no low-energy and 23:50 UT 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 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 23:40 UT
0.1 23:40 UT
23:40 UT
10 23:50 UT
23:40 UT
0.1 23:50 UT
+0.1§ 0°
-0.1°

17. time-of-flight principal
0.1 keV
10 keV
V1 = VE-VB ~ 0.1 keV
V2 = VE-VB << 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 : keV)
= (E/B)*(q*R*B/3*W*g)
~ E [mV/m]/g
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) 23:40 UT, S/C-3  :50 UT, S/C-1
(b) 23:40 UT, S/C-3  10 23:53 UT, S/C-1
(c) :50 UT, S/C-1  10 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 < 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 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+ 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 scenario night No ! morning (A) (B) (C) (D)
keV plasma forms at
Dispersive drift start at
scenario
night
No !
morning
(A)
(B) (C) (D)
(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. General context (deduced from ENA image)
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
post-midnight preference
= strong E (could be ~10 mV/s)

24. Summary and conclusions
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.
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 MLT Minimum Quiet case After end of 300 nT activity
Asymptotic Quiet case
Maximum Clear case
After start of 400 nT activity
5~7
1~3h (0%)
8~9h (30%)
0~3h (85%)
8~10
2~3h (5%)
9~10h (50%)
2~4h (75%)
11~13
3~5h (10%)
10~11h (70%)
4~6h (70%)
14~16
4~6h (35%)
12~13h (80%)
6~7h (50%)
17~19
6~8h (50%)
14~16h (100%)
10h (25%)