Substorms: Multi-Spacecraft Observation and Global MHD Modeling
Observational and numerical modeling evidence demonstrates that substorms are a global, coherent set of processes within the magnetosphere-ionosphere system. This supports the view that substorms are a configurational instability of the coupled region since the entire magnetosphere changes during the expansion phase onset. The magnetosphere progresses through a specific sequence of energy-loading and stress- developing states until the entire system suddenly reconfigures. Present-day MHD models are seen to produce substorm-like global instabilities even if they do not treat fully the extremely thin current sheets that play an essential role in the near-tail dynamics prior to substorm onset. Continuing research concerning the substorm onset location and mechanisms addresses important questions of when and exactly how the substorm expansion develops. In this paper we focus on the largely unsolved issues associated with the recovery phase of substorms wherein the system returns toward the ground state. We present MHD simulations of several specific events that have been observed with the Cluster, THEMIS, Polar, and various geostationary orbit (operational) spacecraft.
The Interrelationship of auroral onset, dipolarization, fast flow, and injection
Using a variety of ground-based instruments/techniques we are now able to quantify the location of certain magnetospheric boundaries and phenomena. These include: (1) the equatorward boundary of the proton aurora (428nm), which is believed to represent the inner edge of a strong pitch angle scattering region determined by the magnetic topology. Across this boundary the magnetic field changes from a tail-like to dipolar-like, (2) The poleward edge of the 630nm (redline) aurora which is believed to correspond to the ionospheric footprint of the open-closed field line boundary, and (3) sudden rises in riometer absorption that have been connected to the onset of dispersionless injection at a given location. Using the THEMIS-ASI array, the NORSTAR Multispectral ASI array, the CGSM photometers and riometers we place the auroral breakup and onset of dispersionless injection relative to these magnetospheric locations. In this paper, we combine information provided by these ground-based arrays with the THEMIS satellite data in an effort to elucidate the relationship between aurora breakup, dispersionless injection, dipolarization and magnetospheric fast flow.
Nightside Ionospheric Electrodynamics Associated with Substorms: Multi Instrument Observations
Poker Flat Incoherent Scatter Radar (PFISR), as a next-generation ISR with phased-array antenna system, can measure multiple ionospheric parameters with unprecedented temporal resolution and thus enables us to study rapidly evolving ionospheric processes, such as those associated with substorms. In this paper, we present observations from PFISR and complimentary ground-based magnetometers and THEMIS all-sky imaging array for substorm events and discuss the evolution of the nightside ionospheric convection as well as the current closing system, with particular emphasis on those near the Harang reversal region. Distinct but persistent features are found west of, within, and east of the onset region. By synthesizing observations from these three categories, as well as those from the SuperDARN radars and the IMAGE satellite, a 2-D picture of the evolution of ionospheric substorm electrodynamics has been established, which reveals features of an important relationship between the Region 2 and the substorm current systems. We believe these new observations can contribute to a better understanding of the magnetosphere-ionosphere coupling process during substorms and shed light on the underlying magnetospheric dynamics.
What Do Simultaneous, Conjugate Observations of Substorm Time Scales Tell Us About Magnetosphere-Ionosphere Coupling?
Previous observations have shown that auroral activity and intense auroral
emission are more common when the ionosphere is in darkness and are suppressed
when the ionosphere is in daylight. This suggests that the ionospheric
conductivity plays an important role governing how magnetospheric energy is
transferred to the ionosphere during substorms. More recent analyses have
indicated that the recovery time scales of substorms occurring in the winter
and near equinox (when the nighttime auroral zone was in darkness) were
roughly twice as long as the recovery time scales for substorms occurring in
the summer (when the nighttime auroral region was sunlit). These results
strongly suggest that auroral substorms in the northern and southern
hemispheres develop differently during solstice conditions with substorms
lasting longer in the dark (winter) hemisphere than in the sunlit (summer)
hemisphere. This also implies that more energy is deposited by electron
precipitation in the winter hemisphere than in the summer one during
substorms. Therefore, the ionosphere itself may dictate how much energy it
will accept from the magnetosphere during substorms rather than this being an
externally imposed quantity. What is lacking, however, is a detailed
investigation of how individual substorms develop in the conjugate
hemispheres. Here, we extend earlier work by analyzing the recovery time
scales for substorms observed in the conjugate hemsiphere simultaneously by
two orbiting global auroral imagers: Polar UVI and IMAGE FUV. The results
presented here will lead to new insights into the role of the ionosphere in
the transport of energy during substorms.
Ground-based and in-situ timing of substorm expansion phase onset: Locating the initiation region and determining the timescale of magnetosphere-ionosphere coupling
Despite the characterisation of the auroral substorm more than 40 years ago, controversy still surrounds the processes triggering substorm onset initiation. 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. Furthermore, we show that the onset of ULF wave activity in space occurs contemporaneously with the onset of ULF wave activity on the ground, suggesting that magnetosphere-ionosphere coupling may occur remarkably fast during the onset process, perhaps by means of energetic electron precipitation that have been accelerated via shear Alfvén waves. Furthermore, 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)
Wave Propagation and Substorm Timing: Beyond Ideal MHD
THEMIS observations have indicated that wave propagation by ideal MHD waves may not be fast enough to account for the timing of events during substorms. The question then arises if non-MHD wave modes or particle propagation could convey information through the tail during substorms. Kinetic Alfven waves have a faster group velocity than MHD Alfven waves, but also suffer wave damping. Whistler mode waves can travel much faster than the Alfven speed for parallel propagation, but not for perpendicular propagation. In addition, observations of whistler mode waves at sufficient amplitudes to carry significant amounts of energy have not been observed to our knowledge. Of course, electrons can travel at speeds much larger than MHD wave speeds, but must overcome the mirror force to reach the inner magnetosphere to produce the aurora. The physics of each of these modes of transport will be investigated to assess the roles each may play in the substorm process.
Global Alfvenic Interaction and Substorm Onset
We suggest that substorm onset is the result of Alfvenic interactions in the global current system including the tail and magnetopause current sheets as well as the auroral field-aligned current system. During the growth phase, Alfvenic interaction between the solar wind and magnetosphere occurs in multiple localized regions throughout the magnetopause current sheets and stresses the tail current sheet, leaving it susceptible to further dynamical processes that often involve the generation of MHD waves and wave mode conversion. The decrease of momentum transfer from the solar wind into the magnetosphere due to changes in solar wind parameters leads to a force imbalance in the whole magnetotail which may cause plasma flows and excite fast mode waves. These waves interact with the stressed current sheet and cause the breakdown of the frozen-in condition and the perturbations of fields and flows in multiple localized regions throughout the tail current sheet. During these processes and the further reconfiguration of the plasma sheet, Alfven waves carrying field aligned currents can be generated which lead to the subsequent substorm auroral development in the global M-I coupling system seen in the expansion phase. Unlike the CD or NENL (Rx) models, where substorm onset is assumed to be the result of a simple causal chain of events, this alternative Alfvenic interaction scenario suggests the substorm onset results from coupled dynamical processes in a driven system that may follow a more complicated temporal sequence. Recent results of a statistical study of timing sequences of substorms using THEMIS observations, are consistent, to some extent, with the suggested Alfvénic interactions in the global current system.