Source Location of the Wedge-like Dispersed (sub-keV) Ring Current in the Morning Sector

During a Substorm

Cluster workshop, September 2006, Ivalo, Finland.

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

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.

poleward

Viking

e-

ion+

INV 57° INV 59°

Cluster also detects (cf. Viking)

Previous Observationssub-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., 2005O+ at low-altitude / H+ at high-altitude

* Viking (2~3 RE , dayside): Yamauchi and Lundin, 2006Fossil of substorm (AE) activities, but not storm (Dst) .morning source?

others: Shelley et al., 1972; Chappel et al., 1982

Drift simulation

Reversed dispersion

Both dispersions

Ebihara et al., 2001

Start drifting from midnight sector from an impulsive source

(Ebihara et al., 2001)

Viking statistics

6 MLT

9 MLT

12 MLT

15 MLT

18 MLT

Time-lag (hours)

(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

Viking Summary

It is a fossil of substorm activity (model is right!).

However, it appears much earlier than prediction!

Now, back to Cluster observationsSimilar structure between Viking and Cluster:the trapped ions mostly reach Viking altitude.

westward drift

eastward drift

eastward drift

H+

Viking14 MLT

Cluster11 MLT

poleward

Cluster observation

only < 1 keV also > 1 keV also > 1 keVNoon Early morning Late morning

Eastward (electric) drift Westward (magnetic) drift

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 keVLimited to < 1 keV

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?

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

Find events when the wedge is formed during a substorm. We found one case. Wedge is seen only at outbound.

keV plasma forms at Dispersive drift start at scenarionight night No !night morning (A)morning morning (B) (C) (D)

Cluster case study

H+

O+

South North

+1° 0° -1° -2°

No

Yes

Yes

?

No

No 2001-10-21

S/C-4

S/C-1

S/C-3

Relative S/C position: all at 9.0±0.1 MLT

23:40-24:00 UT

Observation summaryS/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. Difference 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 |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)

Time-of-flight / velocity filter

0.1 keV @ 23:40 UT

0°+0.1§ -0.1°

S/C-1,4 @ 23:40 UT

0.1 keV @ 23:50 UT

10 keV @ 23:40 UT

10 keV @ 23:50 UTS/C-3 @ 23:40 UT

Radial component is only from ExB drift (energy independent) but not magnetic drift

time-of-flight principal

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

V1 = VE-VB ~ VE @ 0.1 keV

V2 = VE-VB << VE @ 10 keV

t

t+∆t

0.1 keV

10 keV

q: the chargeR ~ 4 RE: geocentric distanceW: 10 keV is the ion energyg: ~ 1, 0.9, 0.7 for 90°, 40°, 0° PA

Observed E and pitch angle

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

1~3 mV/m

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.

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

O+ trajectory (30 min trajectory)

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Pitch angle: 0°, 45°, 90°, 135°, 180°

Tsyganenko T89 B-model & Weimer 2001 E-modelBackward trace of 0.1 keV O+ (reference point S/C-3)

Again, morning source

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.

keV plasma forms at Dispersive drift start at scenarionight night No !night morning (A)morning morning (B) (C) (D)

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)

Summary and conclusionsCase 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.

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

MLT Minimum Quiet case After end of 300 nT activity

Asymptotic Quiet case After end of 300 nT activity

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%)

Viking statistics