Transcript Fate of sub-keV ring current ions observed by Viking

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.htm
Yamauchi, 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
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., 2001
drift model (ExB, grad|B|, and curvature)
many hours after nightside injection
dispersion patterns + MLT dependence
* Freja/Viking/Cluster: Yamauchi et al., 2005
05-19 MLT: morning peak
altitude comparison: O+/H+ ratio change
* Others: Shelley et al., 1972; Chappel et al., 1982
Ordinary
(Type 1)
Both
(island)
Reversed
(Type 2)
Viking Observation
Reversed
(Type 2)
Both
(island)
Ebihara et al., 2001
Present Work
Simulation indicates:
Simple AE correlation?  misleading
Afternoon sector show negative correlation.
Thus, one may not take direct correlation.
* Drift slowly eastward
Are they drifting? If so,
velocity? from where?
Related to substorms?
If so, time lag?
(1) simple statistics
(2) case study
(3) advanced statistics
% traversals
However, no solid data
analyses has been done to
confirm the dynamic part of
the model.
75
80
198
192
159
77 # traversals
with clear structure (>oo)
50
25
0
hourly
AE<50
50 ~ 100 100 ~ 200 200 ~ 400
400 ~
[nT]
Simple Dst correlation?  misleading
Before all, only 58 out of 700 are during Dst < -30 nT.
Magnetic storm activity is not the direct cause.
159
209
280
58 # traversals
100
% traversals
* Originated from past
substorm-related injections
into the ring current region
5~20 hours before.
100
75
with clear structure (>oo)
50
25
0
Dst > 0
0 ~ -10
-10 ~ -30
< -30
[nT]
(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 wedgelike 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.
6 MLT
9 MLT
12 MLT
15 MLT
18 MLT
Time-lag (hours)
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 timelag at larger MLT, i.e.,
peak moves
eastward, while the
peak value of the
probability decrease
as the peak moves
eastward.
Evacuation is seen (the
probability is even lower
than asymptotic one)
6 MLT
9 MLT
12 MLT
15 MLT
18 MLT
Time-lag (hours)
Summary
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%)
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!
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.
Conclusions : wedge-like structures
1. 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.
100
100
80
80
60
40
20
0
6
8
0
4
6
6
0
8
10
2
4
6
8
10
40
0
2
4
6
8
10
peak / clear structure
20
20
% traversals
100 200 300 400 500
Peak probabilities vs AE threshold
20
40
0
100
100
80
80
hours from "end" of activity
0
60
40
12
2
Best threshold value values are 400 nT for
start of activity and 300 nT for end of activity
60
0
4
4
AE threshold (nT) for "end"
60
20
6
AE threshould (nT) for "end"
80
10
8
20
80
40
2
100 200 300 400 500
80
60
0
0
100
AE(hr)<500nT
8
8
100
0
6
6
100
20
4
4
timing / quiet traversal
8
2
0
40
2
2
40
10
AE(hr)<400nT
8
0
60
80
60
0
4
80
0
6
18 M LT
0
100
20
4
6
100
40
2
15 M LT
0
60
0
20
8
% traversals
2
AE(hr)<300nT
0
12 M LT
80
20
timing / clear structure
40
80
40
Optimum time-lag vs AE threshold
9 M LT
100
60
6 M LT
60
100
% traversals
4
% traversals
2
AE(hr)<200nT
0
quiet traversals
0
2
4
6
8
10 12
0
100 200 300 400 500
AE threshold (nT) for "end"
60
40
20
0
0
2
4
6
8
10 12
hours from "end" of activity
minimum / quiet traversal
peak probability (%)
% traversals
AE(hr)<100nT
with clear structure
More statistics (∑3h)
hours from "end"
For different AE threshold values
100
80
60
40
20
0
100 200 300 400 500
AE threshold (nT) for "end"
Result of superposed epoch analyses (∑3h)
From end of activity (cf (2) in explanation)
From start of activity (cf (1) in explanation)
6 MLT
9 MLT
12 MLT
15 MLT
18 MLT
Time-lag (hours)
Time-lag (hours)
Present Work (1) Case study : It requires isolated substorm activity
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
(2) statistics
+ consecutive traversals, but even best case can be interpreted
in many way.