Fate of sub-keV ring current ions observed by Viking

Viking 20 years

Yamauchi and Lundin* Superposed epoch analyses

* Viking Ion data + AE (and Dst)

Eastward motion of sub-keV ions after substorms

http://www.copernicus.org/EGU/annales/24/1/355.htmYamauchi, M. and Lundin, R. (2006),Ann. Geophys., 355-366. SRef-ID: 1432-0576/ag/2006-24-355

XY0868 (EGU06-A-05964) 2006-4-6

Previous Works

Reversed(Type 2)

Ordinary (Type 1)

Both (island)

sub-keV ion precipitation @ subauroral region):

* Aureol 1 (400~2500 km): Sauvaud et al., 1981 00-06 MLT: increases some hours after substorms.

* DMSP F6/F7 (800 km):Newell and Meng, 1986 0830 MLT: correlated with Kp with some hours delay, and event may last a day.

* Viking (2~3 RE): Yamauchi et al., 1996a,b"Wedge-like dispersed structures"modulation by pc-5 pulsation

* Simulation: Ebihara et al., 2001drift model (ExB, grad|B|, and curvature)many hours after nightside injectiondispersion patterns + MLT dependence

* Freja/Viking/Cluster: Yamauchi et al., 200505-19 MLT: morning peakaltitude comparison: O+/H+ ratio change

* Others: Shelley et al., 1972; Chappel et al., 1982

Reversed (Type 2) Both

(island)

Viking Observation

Ebihara et al., 2001

Present Work

(1) simple statistics(2) case study(3) advanced statistics

Simulation indicates:

* Drift slowly eastward

* Originated from past substorm-related injections into the ring current region 5~20 hours before.

However, no solid data analyses has been done to confirm the dynamic part of the model.

Are they drifting? If so, velocity? from where?

Related to substorms? If so, time lag?

Simple AE correlation? misleadingAfternoon sector show negative correlation. Thus, one may not take direct correlation.

Simple Dst correlation? misleadingBefore all, only 58 out of 700 are during Dst < -30 nT. Magnetic storm activity is not the direct cause.

159 209 280 58 # traversals

80 198 192 159 77 # traversals

(2) Case study

It requires isolated substorm activity + consecutive traversals = rare.

Even the best case (860912) show superficial anti-correlation

(3) Backward Superposed Epoch Analyses* Probabilities with/without “wedge” signature at

various MLT in dayside is obtained for different time-lags from latest AE increase.

(a) Ideal AE profile gives 3 characteristic times(b) & (c) But we must fight against reality Hope statistics helps,

Why Viking? * AE index is available from all 12 stations. * Ideal orbit and instrumentation

Total only 700 sorted by LT & time-lags. * 3-hour MLT bins * 3-hour windows (running summation) for the

time-lag except for 0th and 1st hour * add all types of dispersions * divide into only three categories: "clear structure", "marginal", and "quiet.

Result : 1

Probabilities of observing the wedge-like structure after the end of AE activity. (cf. (2) in explanation)

Quiet probability corresponds to the last injection Lowest quiet probability start increase (=last wedge passing through) start to increase at later time-lag at larger MLT, it moves eastward, while the value itself increase eastward.

Time-lag (hours)

6 MLT

9 MLT

12 MLT

15 MLT

18 MLT

Result : 2

Probabilities of observing the wedge-like structure after the start of AE activity(cf (1) in explanation)

The peak probability is found at later time-lag at larger MLT, i.e., peak moves eastward, while the peak value of the probability decrease as the peak moves eastward.

6 MLT

9 MLT

12 MLT

15 MLT

18 MLT

Time-lag (hours)

Evacuation is seen (the probability is even lower than asymptotic one)

SummaryMLT 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%)

Ω = Ωcorot + Ωmodel·sin(LT) Ωmodel/Ωcorot Kp=2 Kp=3 Kp=4

L=4 0.08 0.13 0.25

L=6 0.25 0.4 0.8

Observed Ωmodel /Ωcorot from 6 MLT to 15 MLT > 0.5 :

This is faster than prediction.

1. The wedge-like structure drifts eastward (model is right!).

2. The structure is a fossil of substorm activity (model is right!).

3. Decay time is consistently several hours (model is right!).

4. However, it appears much earlier than prediction!

Conclusions : wedge-like structures1. The structure is related to the past AE activity but not directly to Dst

2. After hourly AE>400 nT, the majority of the structure reaches the noon, and nearly half of them reaches the early afternoon sector.

3. The structures in the evening sector most likely have traveled by eastward drift rather than directly from the nightside by westward drift.

4. The response at 6 MLT is nearly immediate after high AE activities. Source of wedge shifts or extends to the early morning, e.g., 4-5 MLT.

5. The drift speed for hourly AE>400 nT is somewhat faster than model prediction even taking into account of the morning-shift of source.

6. The decay time of several hours at all MLT is consist with the charge exchange life time.

7. Sub-keV ions are sometimes evacuated right after the onset of substorm or storm.

More statistics (∑3h)

Peak probabilities vs AE threshold

Optimum time-lag vs AE threshold

For different AE threshold values

Best threshold value values are 400 nT for start of activity and 300 nT for end of activity

Result of superposed epoch analyses (∑3h)From end of activity (cf (2) in explanation) From start of activity (cf (1) in explanation)

Time-lag (hours) Time-lag (hours)

6 MLT

9 MLT

12 MLT

15 MLT

18 MLT

Present Work

(1)case study

(2) statistics

Simulation indicates:

* Drift slowly eastward

* Originated from past substorm-related injections into the ring current region 5~20 hours before.

However,

No solid data analyses has been done to confirm the dynamic part of the model.

Are they drifting? If so, velocity?

Are they nightside origin?

Are they related to substorms?

If so, time lag?

(1) Case study : It requires isolated substorm activity + consecutive traversals, but even best case can be interpreted in many way.