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Sub-keV Ring Current Ions: Source, Transport, and O+/H+ difference M. Yamauchi, R. Lundin, H. Nilsson, S. Arvelius (IRF-Kiruna), Y. Ebihara (NIPR), and Cluster-CIS team westward drift eastward drift "Wedge-like dispersion" Sub-keV trapped ions are seen almost all satellites at around L=4-6. They are wedge-like energy-latitude dispersed as shown in both Viking data (mid-altitude) and Cluster data (equatorial plane). They are trapped ions drifting eastward, i.e., the ExB drift (including corotation) is stronger than the magnetic drift ( B and curvature) at this energy range. We show (1) Viking statistics (2) Cluster event studies. H+
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Previous Works sub-keV ion precipitation @ subauroral region): Aureol 1 (400~2500km): Sauvaud et al., 1980 00-06 MLT: increases after substorms. * DMSP F6/F7 (800 km):Newell & Meng, 1986 0830 MLT: correlated with Kp with some hours delay, and event may last a day. * Viking (2~3 R E ): Yamauchi et al., 1996a,b "Wedge-like dispersed structures" modulation by pc-5 pulsation. * Simulation: Ebihara et al., 2001 drift model (ExB, |B|, and curvature) many hours after nightside injection dispersion patterns + MLT dependence. * Freja/Viking/Cluster: Yamauchi et al., 2005 morning peak O+ at low-altitude / H+ at high-altitude * others: Shelley et al., 1972; Chappel et al., 1982 Reversed dispersion Both dispersions Ebihara et al., 2001 extend >1keV limited < 1keV Cluster (2001-2003)
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(1) Viking backward superposed epoch analyses Probabilities of observing the wedge-like structure after the start of AE activity. Probability is calculated from numbers of traversals with/without the structure for each 3-hr bin (3-hr running sum) for each 3-hr MLT bin. The peak probability 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)
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Viking Summary MLTMinimum 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 5~71~3h (0%)8~9h (30%)0~3h (85%) 8~102~3h (5%)9~10h (50%)2~4h (75%) 11~133~5h (10%)10~11h (70%)4~6h (70%) 14~164~6h (35%)12~13h (80%)6~7h (50%) 17~196~8h (50%)14~16h (100%)10h (25%) The wedge-like structure drifts eastward, and is a fossil of substorm activity (model is right!). Decay time is several hours (charge exchange model is right!). However, it appears much earlier than prediction, suggesting that a substantial amount of "wedge" might be formed in the morning sector during substorms. We need to identify the source location from event study.
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(2) Cluster We have several possibilities (A) Strong electric field push ions quickly. (B) Scattering of <10 keV ions (C) Energetic ions precipitate and sputter ionospheric ions into the space. (D) Unknown local energization process. Need to find events when the wedge is formed during a substorm. We found one case. Wedge is seen only at outbound. Case study! Ion sourcedispersionscenario night No ! nightmorning(A) morning (B) (C) (D) H+ O+
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+1° 0°-1°-2° No Yes ? No 2001-10-21 23:40-24:00 UT Yes No S/C-4 S/C-1 S/C-3 Relative S/C position: all at 9.0±0.1 MLT H+ O+ H+ O+ H+ O+
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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). Wedge-like dispersion at 23:50 UT. No low-energy signature at 23:40 UT. Butterfly-trapped distribution Bounce inside the geomagnetic bottle. Difference between 23:40 UT and 23:50 UT in the same flux tube means an temporal variation although observation is made in the opposite hemisphere. VE: eastward ExB drift speed = energy independent, MLT dependent VB: westward magnetic ( |B|+curvature) drift velocity = energy dependent VE >> VB at low energy (<100 eV) and VE ~ VB at high energy (value depends on E-field strength). From dispersion curve, the last- coming ions are 10-20 keV. Therefore, VE ~ VB at 20 keV in the present case. 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 for observation near equatorial plane, where E and B are the field strengths, q is the charge, R = 4 RE is the geocentric distance, W = 10 keV is the ion energy, and g ~ 1, 0.9 &0.7 for 90°, 40° & 0° pitch angles V1 = VE-VB ~ VE @ 0.1 keV V2 = VE-VB << VE @ 10 keV V1 ~ VE V2 ~ VE-VB t t+∆t 0.1 keV 10 keV time-of-flight principal
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Dispersion analysis Pitch angle of the "wedge" is about 40~90° (g=0.9~1.0) t ≤ (1.1*E[mV/s] - 1) * ∆t Electric field is 1~3 mV/m for half an hour t = 0.1~2.3*∆t & VE = 3~10 km/s (a)0.1 keV @ 23:50 UT, S/C-1 Nothing @ 23:40 UT, S/C-3 : temporal chance (b) 10 keV @ 23:53 UT, S/C-1 Nothing @ 23:40 UT, S/C-3 : temporal change (c) 0.1 keV @ 23:50 UT, S/C-1 10 keV @ 23:53 UT, S/C-1 : temporal or spatial Combination of (a)+(b) : it is temporal change ∆t < 13 min t < 30 min before 23:40 UT drift distance = VE * t < 20000 km dispersion started at 7~9 MLT. Combination of (b)+(c) : if temporal ∆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. 1~3 mV/m On the other hand, we observed O+ "wedge" at 0.05-0.3 keV (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. From this: (1) Source timing is about 20~30 min before, i.e., at 23:20~23:30 UT. (2) The combination (b)+(c) cannot be true, i.e., the observed dispersion is mostly the spatial structure. (3) O+ pitch angle is uni-direction, i.e., should not have been mirror-bounced, endorsing point (1).
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O+ source ≠ H+ source O+ motion ≠ H+ motion The 2001-10-21 event showed a clear O+/H+ difference inside the wedge, with H+ bounce-averaged feature (with butterfly pitch-angle distribution), whereas O+ is not bounce-averaged. Statistically the wedge-like structure is O+ rich at low-altitudes (Freja) whereas it is H+ rich as high-altitudes (Cluster). These fact suggests that O+ source could be different from H+ source. We found couple of good Cluster examples that endorse this idea correlated & anti-correlated H+ O+ H+/O+ differences Correlation part means that H+ and O+ has the same bounce-average drift motion. Then, how can we understand the anti-correlation part just 15 minutes later?
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Summary and conclusions (1) The dispersion might start in the morning for a substantial numbers of the wedge-like structure. This is suggested by the local time distribution, superposed epoch analyses, and a case study on the 2001-10-21 event (source <30 min, <3 RE distance). (2) Pitch-angle distribution, particularly for O+, suggest ionospheric source (consistent with morning source). (3) In addition to the altitude dependence of the O+/H+ ratio, O+ are sometimes behaving in a different way from H+. * non-bounce-average feature (2001-10-21 event). * correlation and anti-correlation in a single traversal. ??? 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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END
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What is "wedge-like dispersion" / What is the problem? Sub-keV trapped ions seen almost all satellites at around L=4-6. They are wedge- like energy-latitude dispersed as shown in both Viking data (mid-altitude) and Cluster data (equatorial plane). See Poster XY0868 by Yamauchi and Lundin in this session. Data analyses and simulation confirmed that they are drifting trapped ions (I.e., Cluster should see the same phenomena as Viking). Past analyses raised two main questions. (1) Past identification of "wedge" observed by Cluster was in-appropriate because the resultant distribution does not agree with the simulation or other satellites. We need to refine the criterion. (2) Past statistics suggests that the source can be in the morning sector during substorms. We need to identify the source location from event study. westward drift eastward drift
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New criterion and statistics Past criterion: completely isolated from > 5 keV ring current. Only (a) is identified as "wedge" but not (b) or (c) New criterion: isolated from > 5 keV component at the wedge location, as long as wedge is extended from sub-keV. All of (a), (b), and (c) are identified as "wedge" New statistics shows morning peak, which is consistent with the other statistics and simulation. (a) (b)+(c)
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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.
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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
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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.
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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
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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) 6 MLT 9 MLT 12 MLT 15 MLT 18 MLT
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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 R E ): 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 Reversed (Type 2) Both (island) Viking Observation Ebihara et al., 2001
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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? misleading Afternoon sector show negative correlation. Thus, one may not take direct correlation. 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 80198 192 159 77 # traversals
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(2) Case study It requires isolated substorm activity + consecutive traversals = rare. Even the best case (860912) show superficial anti- correlation
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(1) 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 three characteristic times (1)(2)(3). (b) & (c) We must fight against reality Hope statistics helps. Total only 700 traversals, sorted by * 3-hour MLT bins * 3-hour windows (running summation) for the time-lag * add all types of dispersions
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Case study We have several possibilities (A) Strong electric field push ions quickly. (B) Scattering of <10 keV ions (C) Energetic ions precipitate and sputter ionospheric ions into the space. (D) Unknown local energization process. Need to find events when the wedge is formed during substorms. We found one case. Wedge is seen only at outbound. Ion sourcedispersionscenario night No ! nightmorning(A) morning (B) (C) (D)
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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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