J. A. Davies

FAST observations of ULF waves injected into the magnetosphere by means of modulated RF heating of the auroral electrojet

Results are reported from an experiment in which the HF high power facility at Tromso was utilised to inject artificial ULF waves into the magnetosphere by means of modulated heating of the auroral electrojet. Local electric field oscillations associated with the artificially stimulated ULF waves were detected on board the FAST spacecraft, at an altitude of 2550 km. In addition, a modulated downward flux of electrons was also detected. The artificially excited waves, together with these energised downward electrons, were observed in a narrow region only a few tens of km across the geomagnetic field, which mapped down the geomagnetic field line to the heated volume in the ionosphere. Furthermore, the downward flux exhibited energy dispersion in a manner that was consistent with the artificially excited waves having followed the geomagnetic field line out beyond the spacecraft, where they appear to have stimulated electron precipitation back down the field line.

DAYSIDE ION FRICTIONAL HEATING - EISCAT OBSERVATIONS AND COMPARISON WITH MODEL RESULTS

Abstract An extended period of intense dayside ion frictional heating was observed on 3 April 1992 with the UHF radar of the European Incoherent Scatter (EISCAT) facility during a run of the common program CP-1-J. The elevated F -region ion temperature resulted from an enhancement in the ion drift velocity, principally in the field-orthogonal zonal direction, which steadily increased, in a westward direction, to a value in excess of 2 km s −1 during a 4 h interval commencing at approximately 10 UT (12 MLT). The maximum enhancement in the field-parallel ion temperature measured at 300 km altitude exceeded 700 K. The electron concentration in the F -region was substantially depleted during the interval of ion frictional heating, which further resulted in an increase in the F -region electron temperature as, at the local time of the event, the plasma was solar illuminated. A zonal E ∧ B velocity signature modelling that observed over EISCAT during the aforementioned interval was imposed on the Sheffield University plasmasphere and ionosphere model (SUPIM), previously used to study the effects of sub-auroral ion drifts (SAID). The model yields densities, temperatures and field-aligned velocities for the six major ion species and for the electron, and has recently been modified to account for O + temperature anisotropy. The plasma parameters modelled at F -region altitudes were compared with those measured by the EISCAT radar and, although they tended to exhibit the same general trends, the model results predicted substantially larger field-parallel ion temperature enhancements than were observed. However, the inclusion of a time-dependent calculated neutral wind, caused by ion drag, reduced the modelled field-parallel ion temperature to the values measured by EISCAT without severely affecting the other parameters. During extended periods of high ion flow, particularly on the dayside, an enhanced neutral wind becomes highly significant in determining the extent of frictional heating of the ion population.

Heating of the high-latitude ionospheric plasma by electric fields

Abstract Electric fields in the high-latitude ionosphere, perpendicular to the magnetic field, drive plasma flow in the F-region at speeds which regularly exceed 2000 m s−1. Relative motion between the ions and the neutral atmosphere heats the ion population through collisions with neutral particles. Ion frictional heating constitutes a systematic heating effect on the F-region ions in the presence of ionospheric electric fields. In the E-region, however, the ions are coupled to the neutrals by frequent collisions and, since the electrons are still magnetised, these electric fields set up currents. E-region electric currents are responsible for the generation of instabilities which turbulently heat the electrons. This paper reports a statistical study of both ion frictional heating and electron turbulent heating based on around 900 hours of common programme observations by the European incoherent scatter (EISCAT) UHF radar. Although limited to a far narrower altitude regime, electron turbulent heating is demonstrated to constitute as systematic an effect as ion frictional heating.

Nightside ion frictional heating: atomic and molecular ion temperature anisotropy and ion composition changes

An ion frictional heating event was observed by the EISCAT facility at 22:00 UT on 5th September (1989). The Sheffield University plasmasphere and ionosphere model has been used to investigate this event. A closed subauroral tube of plasma is considered in the model which has been developed to include temperature anisotropy in the NO+ as well as in the O+ ion populations. During the event the O+ temperature distribution becomes more anisotropic than the NO+. This means that in the F region the temperature of the NO+ ions parallel to the magnetic field is substantially greater than the temperature of the O+ ions parallel to the magnetic field. The model predicts that the ion composition in the F region becomes more molecular during the event. At an altitude of 300 km the composition changes from almost 100% O+ ions to 70% NO+ ions. To account for the change in ion composition which is not considered in the standard EISCAT analysis the EISCAT data were reanalysed using the ion composition predicted by the model. To compare with the measured ion temperature an average ion temperature was calculated from the model O+ and NO+ distributions. An additional heat source was introduced to enhance the model electron temperatures. There was then reasonable agreement between the modelled and measured parameters. Similar observational and model results were obtained for the ion frictional heating event observed at 21:00 UT on 9th May (1982).

A nightside ion-neutral frictional heating event: Ion composition and O+ and NO+ temperature anisotropy

Abstract An ion frictional heating event was observed by the EISCAT facility at 22:00 UT on 5 Sept 1989. The Sheffield University plasmasphere and ionosphere model has been used to investigate this event. During the event the O + temperature distribution becomes more anisotropic than the NO + . This means that in the F region the temperature of the NO + ions parallel to the magnetic field is substantially greater than the temperature of the O + ions parallel to the magnetic field. To compare with the measured ion temperature an average ion temperature was calculated from the O + and NO + distributions. The model predicts that the ion composition in the F region changes during the event, becoming more molecular. The measured ion temperatures were scaled by the mean molecular mass predicted by the model to account for the change in composition which is not considered in the analysis of EISCAT data.