Presentation on theme: "Image: water flow around a rock in the Colorado River on a UCLA Outdoor Adventures canoe trip. Flow is from the left and the backflow region behind the."— Presentation transcript:
Image: water flow around a rock in the Colorado River on a UCLA Outdoor Adventures canoe trip. Flow is from the left and the backflow region behind the rock is like a ‘magnetosphere’. The separatrix is typified by vortices and called an “eddy line”
This talk will focus on the post-midnight sector, using THEMIS and ARTEMIS data The most interesting events appear to be associated with substorm onset Ps 6 “pulsations” and the associated optical form of omega bands are observed in the morning sector We study the hypothesis that their initiation or cessation is near the time of expansive phase onset This often appears to be the case, but a clear relation to enhanced flow speeds, speculated to lead to these associations, is not necessarily verified Although KHI has been of interest during northward IMF, presumably since reconnection dominates during soutward IMF, recent work shows KHI may lead to local reconnection
1. Omega Bands 2. Ps 6 “Pulsations” 3. Physical Origin and Substorm Phase 4. December Event 5. Triggering of Instability 6. Relation to Substorm Onset
Omega bands can be quite striking as optical features on the scale of all-sky imager footprints (ca. 800 km wavelength). Dec from Gakona AK. “Omega” E S
How do these striking auroral forms arise?
GEOPHYSICAL RESEARCH LETTERS, VOL. 11, NO. 3, PAGES 271–274, 1984 Can substorm expansive phase effects and low frequency Pc magnetic pulsations be attributed to the same source mechanism? Gordon Rostoker John C. Samson Akasofu et al. in 1971 had suggested that omega bands were associated with the boundary of the plasma sheet and lobe. Rostoker and Samson were probably right about omega bands mapping to near the magnetopause…
Averaged background velocity field – 90 km wide, 2 m/s flow 10m drifter – instantaneous x leads to v July T
Flow energy goes into the vortices (non-magnetized case). ‘Wind on water’ instability in that the more the interface bulges, the more it gets pushed on to grow the instability. Stabilized by surface tension. John Styers/Antonius Otto/Katariina Nyryri UofA Fairbanks
Southwood (1979) stability analysis for thin shear layer with B KHI growth is favored by high shear, stabilized by B parallel to k. Note resemblence of right hand term to k · V A. Low V A is destabilizing. A thick ( d ) shear layer favors growth at about its thickness. Roughly kd ~1. Manuel & Samson (JGR, 1993) studied a realistic LLBL and get k ~0.8 R E -1. This gives λ ~6 R E far downtail. Ps 6 maps closer and at ionosphere has about λ ~800 km. The phase velocity is the density-weighted mean of the two flows: For this reason, Rostoker and Samson invoked sunward flow to explain sunward motion of Ps 6.
Gakona Ft Simpson Ft Smith Yellowkn.
Ps 6 can feature magnetic perturbations of order 100s of nT, i.e. of the same amplitude as substorms, but usually in only the eastward and vertical magnetic components. Ps6 can stop (as in this case) or start (many cases) abruptly, perhaps (Saito 1976, Connors et al. 2003) associated with substorm onsets. It appears that detectable Ps 6 are associated with high average levels of activity, and the THEMIS era has not featured many events. Y (eastward)Z(downward)
At ground level, Ps 6 can be explained by filamentation (KHI) and drift at the local convection velocity (sunward), allowing Pedersen system currents to be detected on the ground. The field- aligned currents (FAC) map into space relatively near Earth.
The most prominent Ps 6 signal in the event was at Yellowknife. It appears to start with a minor intensification about 10:40 UT It appears to end with a substorm onset in the midnight sector about 11:50 UT The relative phase of Y (eastward) and Z (downward) perturbations indicates a motion of the causative (equivalent) current systems toward the east (sunward in the morning sector). The X (northward) perturbation indicates disturbed conditions, although association with recovery phase, which has become common in the literature, may be an artifact of use of single station measurements.
12 UT Dec
THEMIS C (YKC) actually on magnetopause due to large SW V Y THEMIS D,E (FSMI,FSIM) in morning sector magnetosphere THEMIS B in magnetosheath (not on magnetopause) Note absence of IMF triggers Southward turning 10:30
THC, near the magnetopause, began traverals of it near the time that Ps 6 were observed at YKC In-situ measurements allow calculating the Alfven speed: low V A (low |B|) is destabilizing (Miura & Pritchett 1982) Associated with the southward turning of IMF was a reduction of |B| (green) and of Alfven speed B (V A ) pattern propagated to THB, but it remained in magnetosheath Confirms proposed mapping of Rostoker and Samson (1984) of Ps 6 to magnetopause Next Ps 6 initiationSouthward turning
These morning sector inner magnetosphere spacecraft show what resembles flapping at a period similar to YKC Ps 6 THD delayed from THE, dawnward propagation Next Ps 6 initiation Next Ps 6 initiation (FSMI, FSIM) corresponds to |B| decrease in magnetosheath No change seen at conjugate satellites Substorm onset corresponds to change in pulsations – decline at YKC, start at Gakona, end of flapping BUT little change at FSMI, FSIM
Alaskan subauroral negative Y (SHU,SIT) gives an SCW position W of Alaska. Siberian subauroral positive Y (IRT, NVS) backs this up. Moshiri Pi 2 (not shown) consistent giving onset time of 11:48 UT Only auroral zone station showing this onset is Tixie with approximate onset time of 11:45. KIAN AK has delayed response. At onset time, Ps 6 ceased at YKC
Moderately fast flow arrives shortly before substorm onset, traveling from the center toward the magnetopuase (dawnward) Larger flows (200 km/s) and larger flapping signatures in post onset time after Ps 6 turns off at conjugate ground stations
Although the “post-onset flapping” signatures seem similar in magnetic signature to the earlier flapping, they are accompanied by extreme low temperature signatures: Plasmapause KHI?
J Z downward toward equatorial plane at Z=3 R E, corresponds to Region 2 upward FAC at ionosphere, consistent with simple model. Near flow reversal to sunward flow. Important in growth rate calculation OpenGGCM with solar wind input (B XIMF =-5 nT)
Note color scale change
Appears to be dominated by Region 1 downward FAC in Ps 6 region: moving sunward
Kelvin-Helmholtz waves at the Earth’s magnetopause: Multiscale development and associated reconnection By Hasegawa et al. JGR 2009 Under northward IMF, found evidence of outflow jet in between bifurcated current layer (but B M -20 nT)
If reconnection is inferred for northward IMF, is it not more likely under southward IMF like in this event? Are large Z velocities possibly evidence of jetting? It is known that omega bands are associated with high energy (30 keV) precipitation and also with pulsating aurora. Although it is possible the pulsating aurora originates in a relaxation oscillator near the ionosphere, it is worth further investigation to see whether reconnection is involved in omega bands under southward IMF.
Dec 12, 2007 was a minor storm period in the first half of the UT day (Dst=-30, Kp 3, AL to -600 nT) THEMIS in early deployment configuration GOES 10 showed –no- signatures GOES 11 in midnight sector Geotail in nearby solar wind
Step-like variation in solar wind B Speed near-constant at high value Bz highly correlated with Vz 9-10 UT features growth phase followed by onset at time of northward turning Geotail data courtesy T. Nagai/Y. Saito Gillam Meridian Scanning Photometer, inferred current, IMF Bz
With a minute time delay, an auroral zone negative X bay is seen in the morning sector In the west (near-midnight) there is a +Y signature and in the east (near noon) sector a –Y signature during growth phase These are the signatures of a three dimensional current system very similar to the substorm current wedge but centered on dawn At onset, midnight sector station FTY has an abrupt drop in X, others die off; near-midnight subauroral now show –Y of downward FAC
Courtesy James Weygand
At onset, THB and THC went to south lobe TOTAL FIELD at THB went near zero The x-line was near THB
Possibly a reconnection jet is seen at THC (scale change) and with large VY at THD and THE Recall, these signatures are quite far in morning sector THEMIS AE does not indicate correct time of onset THBTHCTHDTHE
Careful analysis of ground magnetic data is needed to time and understand any event THEMIS AE is good but cannot replace careful analyis Driven system growth phase electrojet resembles substorm current wedge but in morning sector Evidence of neutral line at -13 Re in morning sector
THC is closer to Earth and the KHI goes by it first Expected in distant tail that KHI propagates anti-sunward There were several other alignments with KHI this winter
Government of Canada – CRC Program, NSERC (support), NRCan, CSA (data) CCMC (modeling) Kate Ramer, UCLA (data analysis tools) University of Alaska Geophysical Institute NASA contract NAS Google Earth CDAWeb, SSCWeb O. Troshichev and STELAB (magnetic data) Y. Nishimura, V. Angelopoulos, R. L. Mcpherron, R. J. Walker, C. T. Russell, M. G. Kivelson, K.-H. Glassmeier, E. Donovan, H. Frey, J. Manuel
Note color scale change
THEMIS observations support the presence of unusual velocities and densities in the regions to which the few observed conjugate Ps 6 events map. Returning to Dec event…extreme signatures are seen, delayed at THEMIS D from THEMIS E…