The NASA Time History of Events and Macroscale Interactions during Substorms (THEMIS) project is intended to investigate magnetospheric substorm phenomena, which are the manifestations of a basic instability of the magnetosphere and a dominant mechanism of plasma transport and explosive energy release. The major controversy in substorm science is the uncertainty as to whether the instability is initiated near the Earth, or in the more distant >20 Re magnetic tail. THEMIS will discriminate between the two possibilities by using five in-situ satellites and ground-based all-sky imagers and magnetometers, and inferring the propagation direction by timing the observation of the substorm initiation at multiple locations in the magnetosphere. An array of stations, consisting of 20 all-sky imagers (ASIs) and 30-plus magnetometers, has been developed and deployed in the North American continent, from Alaska to Labrador, for the broad coverage of the nightside magnetosphere. Each ground-based observatory (GBO) contains a white light imager that takes auroral images at a 3-second repetition rate ("cadence") and a magnetometer that records the 3 axis variation of the magnetic field at 2 Hz frequency. The stations return compressed images, "thumbnails," to two central databases: one located at UC Berkeley and the other at the University of Calgary, Canada. The full images are recorded at each station on hard drives, and these devices are physically returned to the two data centers for data copying. 358 S.B. Mende et al.morphology changes until the arc breaks up. The breakup was timed to the nearest frame (<3 s) and located to the nearest latitude degree at about ±3 o E in longitude. The data also showed that a similar breakup occurred in Alaska ∼10 minutes later, highlighting the need for an array to distinguish prime onset.

We report observations by Mars Global Surveyor (MGS) of thousands of peaked electron energy spectra similar to terrestrial auroral electrons. They are observed on the Martian nightside, near strong crustal magnetic sources. The spectra have peak energies ranging from 100 eV – 2.5 keV, and fluxes near the peak are 10–10000 times higher than typical nightside spectra. They occur on magnetic field lines that connect the shocked solar wind to crustal magnetic fields, and on adjacent closed field lines. Their detection is directly controlled by the solar wind, suggesting that magnetic reconnection is required for their observation. We calculate that the most energetic distributions could produce atmospheric emission with intensity comparable to that recently reported from the Mars Express (MEX) spacecraft. Half of the most energetic examples occur during the passage of space weather events past Mars, suggesting that a disturbed plasma environment is favorable for electron acceleration along magnetic field lines.

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[1] The night side ionosphere of Mars is known to be highly variable: essentially nonexistent in certain geographic locations, while occasionally nearly as strong as the photoionization-produced dayside ionosphere in others. The factors controlling its structure include thermospheric densities, temperatures and winds, day-night plasma transport, plasma temperatures, current systems, solar particle events, crustal magnetic fields, and electron precipitation, none of which are adequately understood at present. Using a kinetic Monte Carlo approach called Mars Monte Carlo Electron Transport (MarMCET), we model the dynamics of precipitating solar wind electrons on the nightside ionosphere of Mars to study the effects of these last two factors on ionospheric density and structure. We calculate ionization rate profiles and, using simple assumptions concerning atmospheric chemistry, also calculate electron density profiles, total electron content, and equivalent ionosphere slab thickness. We present the first model investigation of the coupled effects of crustal magnetic field gradients and precipitating electron pitch angle distributions (PADs). Including such effects, particularly in cases of nonisotropic PADs, is found to be essential in accurately predicting ionization rate and electron density profiles: peak ionization rates can vary by a factor of 20 or more when these effects are included.

[1] On March 18, 2002, under northward interplanetary magnetic field (IMF) and high ($15 nPa) solar wind dynamic pressure conditions, Cluster observed reconnection signatures and the passage of an X-line at the large ($175°) magnetic-shear high-latitude magnetopause (MP). The observations are consistent with the occurrence of a reconnection site tailward of the cusp and in the vicinity of the spacecraft. At the same time IMAGE observed a bright spot poleward of the dayside auroral oval resulting from precipitating protons into the atmosphere. The intensity of the proton spot is consistent with the energy flux contained in the plasma jets observed by Cluster. Using the Tsyganenko-01 magnetic field model with enhanced solar wind pressure, the Cluster MP location is mapped to the vicinity of the IMAGE proton spot. Mapping the auroral spot out to the MP implies an X-line of at least 3.6 R E in y GSM . In addition to confirming the reconnection source of the dayside auroral proton spot, the Cluster observations also reveal sub-Alfvénic flows and a plasma depletion layer in the magnetosheath next to the MP, in a region where gas dynamic models predict super-Alfvénic flows.

[1] FAST wave and particle observations on the nightside polar cap boundary indicate the operation of the ionospheric Alfven resonator (IAR). Large impulsive electric and magnetic field deviations on the boundary between the auroral oval and the polar cap close to magnetic midnight are correlated with accelerated electrons and excite semi periodic oscillations with a frequency of $0.5 Hz. Linear one-dimensional simulations of the Alfven resonator including parallel electric fields due to electron inertial effects, the ionospheric feedback instability and statistically determined altitude dependent density and composition profiles in a dipole geomagnetic field yield waveforms and electron energy spectra qualitatively similar to observations. However, from comparison with a case study example observed above a sunlit ionosphere, the observed electron energies (which exceed 10 keV) suggest that the observed wave carries a parallel electric field larger than possible from electron inertial effects in the linear approximation particularly if this acceleration occurs at altitudes within the ionospheric Alfven resonator.

[1] We report in situ observations from the Cluster and FAST spacecraft showing the deposition of energy into the auroral ionosphere from broadband ULF waves in the cusp and low-latitude boundary layer. A comparison of the wave Poynting flux with particle energy and flux at both satellites indicates that energy transfer from the broadband waves to the plasma occurs through field-aligned electron acceleration, transverse ion acceleration, and Joule heating. These processes are shown to result in precipitating electron fluxes sufficient to drive bright aurora and cause outflows of energized electrons and O + ions from the ionosphere into the low-latitude boundary layer. By solving an eigenmode equation for Alfvén waves in the observed plasma environment, it is shown that the broadband waves observed at Cluster and FAST are dispersive Alfvén waves. It is demonstrated that these waves have wavelengths perpendicular to the geomagnetic field extending from significant fractions of an L shell down to ion gyroradii and electron inertial lengths and wave frequencies in the plasma frame from 1 mHz up to 50 mHz. These waves are shown to have wavelengths along the geomagnetic field of the order of the field line length between the ionosphere and the equatorial plane and become field line resonances (FLRs) when on closed field lines. It is shown that the inclusion of nonlinear and/or nonlocal kinetic effects is required in the description of these waves to account for accelerated particles observed. On the basis of the wave polarization and spectral properties observed from Cluster and FAST it is speculated that these waves are generated through the mode conversion of surface Alfvén waves driven by tailward flows in the low-latitude boundary layer.Citation: Chaston, C. C., et al. (2005), Energy deposition by Alfvén waves into the dayside auroral oval: Cluster and FAST observations,