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Published byBaldric Hensley Modified over 6 years ago
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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.
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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
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Cluster also detects (cf. Viking)
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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
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Drift simulation (Ebihara et al., 2001) Reversed dispersion
Both dispersions Start drifting from midnight sector from an impulsive source
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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)
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Viking Summary It is a fossil of substorm activity (model is right!).
However, it appears much earlier than prediction!
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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
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Cluster observation Eastward (electric) drift
Westward (magnetic) drift Noon Early morning Late morning only < 1 keV also > 1 keV also > 1 keV
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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
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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?
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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.
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Cluster case study H+ O+ South North
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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
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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)
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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°
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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
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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
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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.
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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
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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
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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.
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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)
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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.
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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
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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%)
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