D. D. Wallis

Inverted‐V events simultaneously observed with the Freja satellite and from the ground

The paper reports data received from the Freja satellite during two passes over broad auroral arc systems or inverted-V events above Gillam/Manitoba when special wide-angle CCD cameras were operated at this location in addition to the CANOPUS network. Detailed comparisons of the visible structures with modulations of the primary electron fluxes are performed. Motions of this fine structures are interpreted in terms of high-altitude electric fields shielded from the lower ionosphere. Simultaneous readings of current density, accelerating voltage and energy flux, the latter determined both from particle and auroral brightness measurements, are found to be internally consistent. We calculate from these data the effective resistance encountered by the electric currents and find agreement with the kinetic theory of the mirror impedance, if we allow for substantial variations in density and energy of the source electrons in the magnetosphere.

Observations of traveling Pc5 waves and their relation to the magnetic cloud event of January 1997

Measurements of ULF waves in the Pc5 frequency range are presented and discussed. The waves were observed during the magnetic cloud event of January 1997 by instruments on the Polar satellite and ground instrumentation. These large-amplitude waves are best interpreted as traveling shear Alfven waves rather than in the usual standing-wave scenario. Characterization of the associated complex particle environment shows that the waves were largely confined to the plasma trough. Energetic protons modulated by the wave are shown to have caused modulated proton aurora. It is argued that the waves were caused by the interaction of a magnetic hole with the magnetosphere.

Freja and ground-based analysis of inverted-V events

During two campaigns, ground-based auroral observations were performed in coordination with Freja. The high temporal and spatial resolution of the satellite instruments as well as the real-time recording with a stereoscopic camera system from the ground enabled detailed comparison of small- and large-scale optical phenomena with particle and field data measured by the satellite. Three passes of the satellite over inverted-V auroral arcs and over precipitation regions with strong field-aligned electron spectra are investigated. Brightness modulations within auroral arcs coincide with modulations of primary electron fluxes. The dynamics of small-scale structures within arcs as well as the proper motion of arcs are analyzed and compared with electric fields measured by the satellite and with BARS radar measurements. Energy fluxes independently determined from the ground and from the satellite are used to calculate the field-aligned conductance. The results agree with predictions of the kinetic theory of the mirror force, if we allow for variations of the density and thermal energy of the electrons in the source region of the magnetosphere. Detailed comparison of electron spectra and electric and magnetic field perturbations provide evidence of different acceleration mechanisms for the electrons, electrostatic acceleration inside inverted-Vs, and wave acceleration in transient regions.

Studies of auroral arcs using Freja satellite and ground-based data

During two Freja passes over auroral arc systems special wide-angle CCD cameras were operated from the ground. Comparisons of the optical observations with satellite-measured plasma parameters are performed. Motions of fine structures of the arcs are interpreted in terms of high-altitude electric fields shielded from the lower ionosphere. Independent measurements of current density, accelerating voltage and energy flux are evaluated and show good agreement.

Lower hybrid ion heating cavities in the auroral ionosphere

Summary form only given. Lower hybrid cavities (LHCs) are localized, density-depleted regions of enhanced VLF wave amplitude found within regions of VLF hiss. They can cause ion heating to the level of a few eV in the direction transverse to the geomagnetic field B0. Statistical studies indicate that the cavities are cylindrical or ellipsoidal in shape, are aligned with B/sub 0/, and have diameters of order 20-50 m. There are many outstanding questions surrounding the formation of LHCs, including the origin of their density depletion, their B/sub 0/ aligned length, and the detailed relation between wave fields, plasma density and ion acceleration. The GEODESIC sounding rocket, launched in early 2000, brings some new information to bear on these problems. Over 100 LHCs were encountered near the leading edge of an auroral substorm expansion. A Suprathermal Ion Imager (SII) newly developed for the flight provides 2D ion distribution function images from 0-20 eV with a time resolution of 11 ms, sufficient to resolve core and tail ion distributions at the edges and interiors of individual LHCs cavities. The measurements show tail heating to 1-2 eV superimposed on an unaffected core distribution. Magnetic field observations on GEODESIC indicate a significant enhancement of 0-10 kHz magnetic field fluctuations inside LHCs, in contrast to all previous attempts to detect such fields.