SM14A-01 INVITED
The relationship between substorm onset locations and convection pattern features.
By design, the SuperDARN network provides multipoint observations of magnetospheric phenomena. It provides observations over the majority of the high-latitude region, with observations in the northern hemisphere from Finland westward through to Alaska, and at latitudes ranging from Wallops Island to the magnetic pole, with conjugate observations over a similar range in the southern hemisphere. In addition, because the network has been in operation for several years, it is now possible to examine magnetospheric phenomena statistically. This paper uses the SuperDARN database in conjunction with a list of substorm onsets determined from the IMAGE satellite [Frye et al., 2004] to examine the relationship between substorm onset locations and features of the convection pattern. Particular attention is devoted to the latitude of onset locations compared to the latitude of the near-midnight convection reversal boundary. Because of this focus, the set of observations examined is limited to cases where SuperDARN provided sufficient observations in the midnight sector to determine the latitude of the convection reversal.
SM14A-02
Ground Based optical observations of the March 23, 2007 substorm event from Alaska.
To monitor substorm associated auroras 20 THEMIS Ground Based Observatories (GBO-s), were deployed to provide near contiguous coverage over North America each with an all sky imagers (ASI) and a magnetometer. The network of imagers takes global scale image collages (mosaics) with 3 second cadence providing hitherto unprecedented spatial and temporal resolution. The GBO magnetometer data are sampled at 2 samples per second. The THEMIS GBOs provide a global perspective with unprecedented temporal and spatial resolution and sensitivity. The ASI network recorded substorm signatures of the 23rd of March 2007 substorm which was also the first substorm observed by the THEMIS satellite instruments. The GBO array was able to pinpoint the location and time of occurrence of the auroral break up associated with this substorm. To investigate the possible role of Alfven wave accelerated electrons in substorm aurora a special purpose camera was also constructed and fielded at Poker Flats, Alaska. This camera also observed the initial brightening of the onset of the 23rd of March substorm. The special purpose camera was designed to record band component ratios of the N2+ first negative band at 427.8 nm with the objective of estimating of the N2 rotational temperature of the emitting region. Since the rotational temperature varies substantially with altitude this method provides information regarding the altitude of the auroral emissions and thus about the energy of the precipitating particles. The ultimate goal of this instrument is to develop a ground based technique for discriminating soft, wave accelerated electron auroras of energy <2 keV from the conventional inverted V auroras of energy that is regularly > 4keV and to investigate whether initial onset arc brightening is caused by soft Alfven wave accelerated electrons. In dynamic conditions this technique has several advantages over other methods of estimating auroral electron energy based on comparisons of two different spectral features of differing color and quenching lifetime. On the 23rd of March the auroral break up arc was captured overhead with this instrument and it showed that the emission from the arc brightening had the characteristics of colder atmospheres namely that the aurora was produced by higher energy electrons. A mosaic presentation of the GBO all sky imager data from the GBO stations Kiana , Ft Yukon and Inuvik showed that the major onset brightening and break up occurred originally within the Kiana field of view and then it propagated mainly eastward to Ft Yukon and Poker Flats. Therefore the brightening seen at Poker Flats was not part of the original onset brightening. It is also noteworthy that the initial onset arc that brightened at Kiana appeared as a new feature located between two pre-existing arcs.
SM14A-03
Comparison of Auroral and Inner CPS in situ Measurements During an Expansive Phase Onset
On March 13, 2007, there was a sequence of auroral and contemporaneous magnetotail activity that occurred during a period when the THEMIS constellation was in the evening sector inner CPS. The satellites were magnetically conjugate to the auroral zone in western Canada. The viewing conditions over numerous All-Sky Imagers (ASIs) in Canada were excellent. Although the THEMIS satellites were still in commissioning phase, the fluxgate magnetometers (FGM) were operating on all five satellites during this event, as was the electrostatic analyzer instrument on THEMIS A. As well, three GOES satellites were collecting geosynchronous magnetic field data over Canada throughout the event. Preliminary analysis shows the following. There was an onset at roughly 0508 UT either at or earthward of the position of THEMIS D (just beyond geosynchronous distance), and then propagated outwards over THEMIS B, A, and E in turn. This appears as an outwardly propagating decrease in cross-tail current as inferred from FGM; however, the current sheet in the vicinity of THEMIS E was increasing in strength and/or thinning while the onset was already in progress closer to the Earth. ASI, meridian scanning photometer, and riometer data show that the onset occurs in the same meridian as the THEMIS spacecraft, and allow us to identify the ionospheric signatures of the dispersionless injection and the earthward edge of the ion plasma sheet. Our conclusions based on this preliminary study are as follows: 1) the 0508 UT onset occurred near geosynchronous orbit and was not preceded by a fast earthward flow; 2) growth phase continued further from the Earth on probes to the East of the onset meridian; 3) the optical onset, dispersionless injection (from riometers), and local decrease in cross tail current all happened at the same time; 4) inter-comparison of ground- based and in situ observations highlight the importance of the coordinated use of ground and satellite data for overcoming azimuthal and radial magnetic projection uncertainties.
SM14A-04
Coordinated THEMIS and ground-based magnetometer study of ULF wave excitation during a fast solar wind stream
We present results from a study of ULF waves observed during a conjunction between the THEMIS constellation and ground-based magnetometers of the combined CARISMA and THEMIS ground-based observatory (GBO) arrays during a fast solar wind stream in August 2007. Long wavetrain waves in the Pc4 and Pc5 bands were seen across the ground-based arrays during a conjugate overpass of the THEMIS constellation in the dusk magnetosphere over CARISMA on August 8th. The Pc4 waves, also seen on THEMIS, were very monochromatic with many characteristics reminiscent of giant pulsations (Pgs) except that they occurred in the dusk sector. The multiple satellite measurements, including supporting measurements from GOES satellites in this sector, allow characterization of the wavenumbers, spatial scale, and comparison of the widths of the waves on the ground and in space. The monochromatic nature of the Pc4 waves, and their dominant polarization in the D-component on the ground, suggests that they are poloidal Alfven waves perhaps driven by injected ions in the afternoon-side magnetosphere. We use the unique capabilities of the combined THEMIS-GOES constellation, and the extensive ground-based coverage, to address the issue of the source of wave excitation during this high speed stream. Attention is given to internal excitation by energetic ions spectra, and drivers at the dayside magnetopause.
SM14A-05
Multi-spacecraft THEMIS - Geotail observations of magnetosheath plasma penetration deep into the low-latitude dayside and nightside magnetosphere for equal northward and dawnward IMF
On June 3, 2007 the five THEMIS spacecraft traversed the dayside post-noon magnetosphere like pearls on a string in a near-equatorial orbit with a 15.4 RE apogee. The leading (probe B) and trailing (probe A) spacecraft were separated by ~~3 RE. While THEMIS B monitored the magnetosheath THEMIS A, C, D, and E were still located inside the magnetosphere and observed an extended layer of mixed cold magnetosheath and hot plasma sheet plasmas. The plasma temperature and density were intermediate between the magnetosheath and hot plasma sheet values. The electron pitch angle distributions in this cold and dense plasma sheet (CDPS) display perfectly balanced field-aligned counter-streaming electrons at all energies, indicating that this region is located on closed magnetic field lines. The CDPS was observed sequentially by the four THEMIS spacecraft, which allows the determination of the thickness and spatial evolution of this region. The dayside post-noon magnetopause appeared to be a stable boundary during this interval, with no evidence for Kelvin-Helmholtz waves. During the same interval Geotail observed CDPS in the dawn flank magnetotail, indicating that the magnetosheath plasma penetration was a global phenomenon. The CDPS observations were associated with an interval of equal northward and dawnward IMF. These observations may suggest that the capturing of the magnetosheath plasma by double-cusp reconnection occurs even with a significant IMF By component.
SM14A-06
New observations of electromagnetic waves in the boundary-layer from THEMIS
Since launch the THEMIS spacecraft have traversed the boundary layer of mixed magnetosheath/magnetospheric plasmas spanning the Earth's magnetopause multiple times. This is a region charaterised by a broad spectrum of electromagnetic fluctuations over a frequency range of mHz up to the local ion gyro-frequency. In this presentation we examine the properties of these waves with particular emphasis on mode conversion from surface or fast mode waves to kinetic Alfven waves on the Alfven speed gradients that exist across the magnetopause. We examine the subsequent particle acceleration processes that these waves may drive within the magnetosphere and provide estimates of the ability of these waves to transport magnetosheath plasmas across the magnetopause into the magnetosphere.
SM14A-07
Plasma jets and FTE Dayside Generation for Northward IMF on 8 June 2007: THEMIS Observations
Five-spacecraft THEMIS (TH) observations are presented for a 15.5 MLT equatorial magnetopause crossing on 8 June 2007 when the upstream IMF was predominantly northward with a negative IMF By component at Wind. During the 0650-0855 UT period on this day TH-B was the most tailward probe while TH-A was the most sunward probe. TH-E was closest to TH-A with a maximum separation of only 0.71 RE. The maximum TH-A to TH-B GSM separation was 1.85 RE. TH-B showed a clean magnetopause crossing into the magnetosphere as the magnetopause expanded over the probes while TH-A spent this 2-hour period within a boundary layer inside the magnetopause with frequent transitions between a magnetosheath-like and a magnetosphere-like plasma as previously seen by Cluster at high-latitudes for southward IMF [Wild et al., 2003]. TH-E observed similar activity for a shorter period of time. Many of the sheath-like transitions showed evidence of plasma jets at TH-A with enhanced speed in the tailward and/or duskward direction suggesting a subsolar component merging region. Some jets were related to frequent bipolar FTE signatures in the normal BN component with enhanced total pressure observed at their centers. The more common ±BN sequence suggests that TH-A observed tailward propagating FTEs on the sheath side of the magnetopause. We compare TH-E ExB velocities with the enhanced jet velocities observed by TH-A and discuss whether the jets observed within this boundary layer were caused by subsolar magnetopause reconnection. We also compare these low-latitude northward IMF observations with prior Cluster FTE observations at high-latitude for southward IMF.
SM14A-08
Observational Evidence for an Inside-Out Substorm Onset Scenario
We present observations which provide strong support for a substorm onset scenario in which a localized inner magnetospheric instability developed first and was later followed by the development of a Near Earth Neutral Line (NENL) farther down-tail. Specifically, we find that the onset began as a localized brightening of an intensified growth phase arc which developed as a periodic series of arc-aligned (i.e. azimuthally arrayed) bright spots. As the disturbance grew, it evolved into vortical structures that propagated poleward and eventually morphed into an east-west aligned arc system at the poleward edge of the auroral substorm bulge. The auroral intensification shows an exponential growth with an estimated e-folding time of around 188 seconds (linear growth rate, γ of 5.33× 10-3 s-1). During the initial breakup, no obvious distortions of auroral forms to the north were observed. However, during the expansion phase, intensifications of the poleward boundary of the expanding bulge were observed together with the equatorward ejection of auroral streamers into the bulge. A strong particle injection was observed at geosynchronous orbit, but was delayed by several minutes relative to onset. Ground magnetometer data also shows a two phase development of mid-latitude positive H-bays, with a quasi-linear increase in H between the onset and the injection. We conclude that this event provides strong evidence in favor of the so-called "inside-out" substorm onset scenario in which the near Earth region activates first followed at a later time by the formation of a near-to-mid tail substorm X-line. The ballooning instability is discussed as a likely mechanism for the initial onset.