R. H. Comfort

An empirical model of the earth's plasmasphere

Abstract We present here an empirical model of plasmaspheric low energy plasma consisting of H+. The model is developed from a data base derived from measurements taken by the Retarding Ion Mass Spectrometer on the Dynamics Explorer 1 satellite. An analytical expression that reproduces the density profiles for moderate geomagnetic activity is given and discussed. This expression reproduces the density fall off in the ionosphere as well as the sharp density decrease at the plasmaspause.

On the causes of the annual variation in the plasmaspheric electron density

This paper examines the relative importance of ionospheric and thermospheric densities and temperatures in producing the annual variation in the plasmaspheric electron density (Neq). In the Eastern American sector, whistler observations show a factor of 2–3 variation in Neq at low solar activity with a minimum in June and a maximum in December. The field line interhemispheric plasma (FLIP) model also shows an annual Neq variation in the American sector at solar minimum, but the model densities are about 30% higher. By using the ability of the FLIP model to be constrained by the measured hmF2, NmF2, and Te, we are able to show that, contrary to previous studies, it is the plasmaspheric thermal structure, and not ionospheric density that plays the key role in producing the annual variation in the model Neq at solar minimum. We find that the model annual Neq variations in the American and Australian sectors are out of phase by six months as would be expected from the opposite tilt of the Earths magnetic field. This study also shows that the plasmaspheric electron density is anticorrelated with magnetic and solar activity due to changes in neutral hydrogen density. Thermospheric neutral winds appear to play only a small role in the annual Neq variations because longitudinal variations in the measured hmF2 are small. However, magnetic storm induced variability in ionospheric density through changes in thermospheric winds and neutral densities do contribute to the day-to-day variability of Neq. At solar maximum in November 1989 the model densities are generally in good agreement with both whistler and satellite observation in both the American and New Zealand sectors. However, the standard model is unable to reproduce the relatively small annual variation in equatorial electron density that was observed from Dunedin, New Zealand at solar maximum in 1990. We show that the low measured densities at solar maximum may be explained by magnetic storm induced depletion of the plasmasphere by convection electric fields.

Thermal structure of the plasmasphere

Abstract The density structure of the plasmasphere has been a subject of interest and considerable research since the discovery of the plasmapause. For a number of reasons, plasmasphere thermal structure has been examined less thoroughly. Observations show that the thermal structure is closely tied to density structure and has significant impact on the composition of the plasma. In this report, we review observations of plasmasphere temperatures, displaying their variability in space and time and their relation to the plasmasphere density structure. Primary observations are from the Retarding Ion Mass Spectrometer on the Dynamics Explorer 1 satellite. Through comparisons with numerical simulations, we can begin to test our quantitative understanding of a number of factors which either influence or respond to this thermal structure. We review several studies which compare observations with numerical simulations of plasmaspheric properties, directing particular attention to energy sources and energy transport in the inner and outer plasmasphere. We find that photoelectrons, as currently understood, do not always provide sufficient energy to plasmaspheric plasma to produce the ion temperatures observed; additional possibilities are examined. In addition to what we appear to understand fairly well, there are several areas of deficiency, which we outline briefly.

Plasmasphere thermal structure as measured by ISEE-1 and DE-1

Abstract Characteristics of plasmaspheric ion thermal structure are presented from a statistical survey of low-energy of (

Plasmaphere and plasmapause region characteristics as measured by DE-1

Abstract Thermal ion composition measurements by the Retarding Ion Mass Spectrometer (RIMS) on Dynamics Explorer-1 have revealed new and intriguing features of the thermal ion distributions in the plasmasphere and plasmapause regions. Some of the interesting new findings include: the presence of intense fluxes of heated and equatorially-trapped light ions within the plasmapause region; the existence of a heavy ion (0+, 0++, N+) ‘torus’ or ‘shell’ in the outer plasmasphere; and the relatively stable nature of the He+/H+ concentration ratio (∼0.2–0.3) within the plasmasphere. The relatively short (∼7.5 hours) orbital period of DE-1 has also allowed improved observations on the formation of the new outer plasmasphere during the recovery of geomagnetic storms. Statistical studies of plasmaspheric density structure and boundaries are beginning to reveal a picture of their relation to other magnetospheric boundaries, such as the inner edge of the electron plasma sheet, and trends in the internal density structure of the plasmasphere.

Satellite observations of new particle and field signatures associated with SAR arc field lines at magnetospheric heights

Enhancements in thermal ion densities, an oxygen dominated ring current at energies below 17 key, and invariant latitude-limited bands of intense ELF hiss have been discovered on Stable Auroral Red (SAR) arc field lines at magnetospheric heights. These new signatures were revealed by an examination of 31 coordinated data sets taken simultaneously at magnetospheric and ionospheric heights by the DE-1 and -2 satellites during SAR arc traversals within the period September 1981 through April 1982. Data sets from DE-2, for the first time, provide information on the location of a SAR arc (determined by the F region electron temperature enhancement) during the nearly simultaneous passage of these field lines by DE-1 in the magnetosphere. These new high altitude signatures are examinedin the context of possible magnetospheric SAR arc energy source mechanisms.

Anomalous nighttime electron temperature events over Millstone Hill

This paper presents a statistical analysis of anomalous, short duration, nighttime, electron temperature enhancements (NETEs) in the ionosphere over Millstone Hill (42N, 71W). These temperature enhancements are characterized by a several hundred degree increase in the topside electron temperature with a typical duration of two to four hours. The NETEs can occur anytime at night but most frequently between eighteen and twenty hours local time. Occasionally, two NETEs may be observed in the same night. The maximum temperature reached is often comparable to the daytime temperature. NETEs are most frequent in the winter, but do occur in the spring and fall. There are rare cases of the NETE occurring in the summer. There does not appear to be a clear dependency on either solar or geomagnetic activity. The NETEs association with winter suggests a relationship with conjugate photoelectrons which, if it exists, is not simple. The observed durations are too short to be attributed solely to conjugate photoelectron heating.

A modified thermal conductivity for low density plasma magnetic flux tubes

In response to inconsistencies which have arisen in results from a hydrodynamic model in simulations of high ion temperatures (1-2 eV) observed in low density, outer plasmasphere flux tubes, we postulate a reduced thermal conductivity coefficient in which only particles in the loss cone of the quasi-collisionless plasma contribute to the thermal conduction. Other particles are assumed to magnetically mirror before they reach the topside ionosphere and therefore not to remove thermal energy from the plasmasphere. This concept is used to formulate a mathematically simple, but physically limiting model for a modified thermal conductivity coefficient. When this modified coefficient is employed in the hydrodynamic model in a case study, the inconsistencies between simulation results and observations are largely resolved. The high simulated ion temperatures are achieved with significantly less heat input, and result in substantially lower ion temperatures in the topside ionosphere. We suggest that this mechanism may be operative under the limited low density, refilling conditions in which high ion temperatures are observed.

Relationship of O+ field-aligned flows and densities to convection speed in the polar cap at 5000 km altitude

Abstract Measurements of thermal O+ ion number fluxes, densities, field-aligned velocities, and convection speeds from the Thermal Ion Dynamics Experiment (TIDE) on POLAR obtained near 5000 km altitude over the Southern hemisphere are examined. We find that the O+ parallel velocities, densities, and number fluxes are strongly related to the convection speeds. The polar cap densities decrease rapidly with convection speed, with a linear least square fit formula to bin averaged data giving the relationship log N O + =−0.33∗V conv +0.07 , with a correlation coefficient of r=−0.96 . The parallel bulk flow velocities are on average, slightly downward (0–2 km/s) for Vconv 2.5 km/s. We also find that the downward number flux is strongly related to convection speed by log Flux =−0.54V conv +5.14 , with a correlation coefficient of r=−0.98 . We interpret these relationships in terms of the Cleft Ion Fountain paradigm. The density decline with convection speed may result from increased spreading and resulting dilution from the restricted cleft source over the polar cap area with convection speed. The parallel velocities tend to be downward for low convection speeds because at such speeds, the ions fall earthward at shorter anti-sunward distances into the polar cap. At the higher convection speeds, the initially-upward flows are transported further into the polar cap and thus occupy a larger area of the polar cap.

Unsolved problems in plasmasphere refilling

The plasmasphere is a cold (~1 eV) plasma at middle to low magnetic latitudes surrounding the Earth. Its shape is dominated by Earths magnetic field and its cross-field motion is dominated by electric fields. It is a highly coupled part of the inner magnetosphere. Storm time conditions erode the outer plasmasphere, transporting that plasma into the dayside magnetosheath region, leaving behind a region of greatly reduced plasma density that will refill from ionospheric outflow. The processes involved in refilling remain incompletely understood. In this commentary, outstanding questions about plasmaspheric refilling are summarized in the context of recent publications.