SM52A-01 INVITED TI: Understanding the substorm-related plasma injections depends on understanding the variety of the magnetic field dipolarizations, including such different phenomena as global reconfigurations, effects of the substorm current wedge (SCW), as well as meso/small scale features, associated with inward-propagated sharp fronts and structures. We briefly discuss recent progress made possible owing to excellent opportunities provided by new Themis spacecraft and ground observations combined with new models and monitoring tools. Among the topics covered are: a new SCW magnetospheric model and its initial tests; spatial relationship between the dipolarizations and SCW; earthward propagation of the sharp dipolarization front and its features; identification and properties of auroral signatures of the magnetic field dipolarization region. We also briefly discuss such related issues as manifestations of the substorm onset in the inner region, including the pre- dipolarization stage and what was previously called the Explosive Growth Phase.
SM52A-02
Deviations from the Frozen-In Condition during Substorm Dipolarizations
THEMIS electric and magnetic field measurements are combined with ion flows determined from the plasma and energetic particle instruments to investigate deviations from the "ion frozen-in" condition (E + VixB = 0) during geomagnetic substorms. It is found that the vast majority of ion flows during inner-magnetospheric dipolarizations can be classified as "frozen-in" within measurement uncertainties. In an unanticipated result, the largest deviations from frozen-in are found at the end of the substorm growth phase as the plasmasheet thins, rather than during the dipolarization process. Plasmasheet thinning is coincident with an increased current density which manifests itself as an increasing ion flow in the duskward direction. This duskward flow is the result of the ion pressure gradient perpendicular to the current sheet and has no associated electric field. Applying these measurements to the generalized Ohms Law, it appear that the non-zero "VixB" term that we measure during growth phase is just that portion of the Hall term, JxB/ne, which results from ion motion in the cross tail current. Deviations from the ion frozen-in condition are also found for brief periods during the dipolarization process, however these deviations are generally associated with ion pressure gradients, and therefore likely due to similar diamagnetic currents. These results suggest that the current disruption model, which requires significant breaking of the frozen-in condition by terms other than those resulting from diamagnetic effects, is inconsistent with the observations.
SM52A-03
The Microphysics of the Dipolarization/Jet Braking Region in the Near-Earth Magnetotail: Cluster Multi-Point Observations
The dipolarization/jet braking region in the near-Earth magnetotail is a key region associated with magnetic field reconfiguration and electromagnetic energy conversion. While these processes affect large volumes of space, some of the important physics occurs at scales comparable with the ion scale and below. It is therefore important to study this region at different spatial scales. Here we present Cluster measurements of electromagnetic fields and particles during a few dipolarizations (Bz enhancement) on October 27, 2007 when Cluster crossed the plasma sheet around X~-10 RE. The spacecraft separation ranged from ~40 km to ~10000 km thus allowing simultaneous observations at scales from electron to fluid. The observations indicate that turbulent regions where earthward fast flows brake are associated with dipolarizations. Current filaments with strong DC electric fields, waves and energetic particles are observed within such turbulent regions. We analyze in detail several current filaments and discuss their role for local energy conversion and acceleration of energetic particles. We discuss the possible role of small-scale processes within the turbulent dipolarization region for large-scale substorm features.
SM52A-04
The Sources and Dynamics of Plasma Sheet Ions during a Substorm
On March 1, 2008 four THEMIS spacecraft were aligned radially in the near-Earth plasma sheet during a major magnetospheric substorm. The outer probes P1 and P2 located at 23 RE and 16RE, observed evidence of reconnection prior to substorm onset as well as inverse dispersion in which the flux of low energy ions increases first. Shortly after onset the inner probes P3 and P4 located at 9.3RE and 8RE respectively observed large increases in both thermal and energetic ions. The energetic ions reached energies as large as 500 keV. To explain the observations at both the outer and inner probes we have carried out a simulation study in which we first ran a global magnetohydrodynamic (MHD) simulation of the magnetosphere and then launched both solar wind and ionospheric ions into the electric and magnetic fields obtained from the MHD simulation. The solar wind source included both plasma mantle ions and low latitude boundary layer (LLBL) ions. In the MHD simulation like the observations the substorm was initiated by reconnection in the region between 15-20RE which was then followed by dipolarization in the near Earth tail. Results from the particle calculations show evidence of inverse dispersion near the outer probes and large ion acceleration in the near earth tail. The particle simulations reproduce the observed energy flux spectrograms and number density with such fidelity that we have used them to locate the sources of the ions observed by the THEMIS spacecraft. Surprisingly during this substorm the major source of the ions observed by THEMIS is the LLBL and not the plasma mantle. In addition the ionosphere supplied a small but not negligible component of the plasma sheet population. The very energetic ions observed by P3 and P4 were accelerated by non-adiabatic motion in the induced and convection electric fields.
SM52A-05 INVITED
The Alfvenic Aurora and Substorm Onset
Observations of auroral particles indicate that in many cases the auroral energy distribution is broad in energy and strongly field-aligned in pitch angle. These observations have been interpreted in terms of time dependent acceleration of electrons in fluctuating electric fields associated with kinetic Alfvén waves trapped in the ionospheric Alfvén resonator, leading them to be termed "Alfvénic aurora." Observations from FAST and IMAGE indicate that the auroral arc that brightens during substorm onset is predominantly Alfvénic, suggesting that it is a transitional phase to the quasi-static aurora. This interpretation is also supported by measurements from THEMIS, which have found cases in which the auroral arc brightens before the establishment of a quasi-static substorm current wedge. These waves may also be associated with Pi1 pulsations observed at substorm onset; however, the waves observed on the ground must have large perpendicular wavelength while the kinetic Alfvén waves that can accelerate electrons have small wavelength, indicating that a transfer of energy across scales must take place. The implication of these theoretical considerations for the physics of current generation and the timing of substorm processes will be discussed.
SM52A-06
A comparison of ground and satellite observations of Pi1B pulsations
Ground-based observations of Pi1B pulsations at substorm onset have been reported by several researchers, using data from various ground-based platforms. Some recent evidence indicates that these pulsations may drive Alfvenic aurora at onset, providing, at least in part, a source for the initial brightening. Arnoldy et al. [1998] showed that these pulsations can also be observed at geosynchronous orbit at substorm onset, with such observations having a reasonable correlation to ground observations. These authors noted, however, that specific onset times depend strongly on differences in MLT between the ground stations and the satellite. Still, the occurrence of Pi1B pulsations at geosynchronous orbit implies that they originate near or beyond this region, perhaps providing a means to couple energy from deeper in the tail to the ionosphere at onset. In order to better understand the apparent relationship between ground-based and geosynchronous observations of Pi1B pulsations, we have investigated a number of events having good conjunctions in detail. In this presentation, we show the results of this study and discuss the resulting implications. Arnoldy, R. L., J. L. Posch, M. J. Engebretson, H. Fukunishi and H. J. Singer, Pi1 magnetic pulsations in space and at high latitudes on the ground, JGR, 103, A10, 23581-23592, doi:10.1029/98JA01917
SM52A-07
Constraining the substorm problem using ULF wave techniques
Using ground-based magnetometers from CARISMA and THEMIS and in-situ magnetic observations by THEMIS and GOES, we present the results obtained from an objective wavelet-based technique to determine the first onset of ULF wave activity during expansion phase onset on the ground and in space. We validate ground-based ULF timing against the large-scale IMAGE FUV and smaller-scale THEMIS ASI auroral observations. We find clear, coherent and repeatable characteristics of these ULF waves on the ground indicating a localized onset epicentre that provides a clear and strong constraint on the location in time and space of expansion phase onset. Specifically, we show that the onset of long-period Pi1/short-period Pi2 ULF waves commence at an epicentre in the ionosphere which is co-located in time and space with the development of spatially-localised, latitudinally narrow small-scale undulations on a faint isolated arc several degrees equatorward of the pre- existing discrete auroral arcs. These optical undulations have a periodicity in the same Pi1 frequency band as the magnetic perturbations. During this activity, the pre-onset poleward discrete arc system remains spatially and temporally distinct from, and quasi-stable and unaffected by, the rapid dynamics of the new more equatorward auroral activity during the first the 2-3 minutes following onset. These optical and magnetic manifestations of expansion phase onset initiation may represent a characteristic ionospheric signature of a near-Earth plasmasheet instability. An alternate scenario is that reconnection in a severely stretched geometry produces these signatures, but in that case the stability and magnetospheric location of the more poleward arc system mapping to the plasmasheet must be explained. Regardless, the combination of high cadence and spatial resolution magnetic and optical measurements such as those outlined here provide a remarkably tight constraint on the mechanisms responsible for the initiation of substorm onset We outline the characteristics of ULF pulsations in both the Pi1 and Pi2 bands in the nightside ionosphere and magnetosphere during substorms. We describe the use of these techniques in creating a substorm onset database during the THEMIS era for use by the scientific community. Finally, we detail the development of a Canadian AE calculation that will be routinely available at the Canadian Space Sciences Data Portal (www.cssdp.ca)