G. R. Wilson

'Self-consistent' production of ion conics on return current region auroral field lines - A time-dependent, semi-kinetic model

We describe initial results from a time-dependent, semi-kinetic model of plasma outflow incorporating wave-particle interactions along current-carrying auroral field lines. Electrostatic waves are generated by the current driven ion cyclotron instability (CDICI), causing perpendicular velocity diffusion of ions plus electron heating via anomalous resistivity when and where the relative drift between electrons and ions exceeds certain critical velocities. Using the local bulk parameters we calculate these critical velocities, and so are able to self-consistently switch on and off the heating of the various particle species. Due to the dependence of these critical velocities on the bulk parameters of the species the heating effects exhibit quite complex spatial and temporal variations. A wide range of ion distribution functions are observed in these simulations, including conics with energies of a few electron volts and ‘ring’ distributions. The rings are seen to be a natural result of transverse heating and velocity filter effects and do not require coherent acceleration processes. We also observe the formation of a density depletion in hydrogen and enhanced oxygen densities at high altitudes.

Dynamics of the H+ and O+ polar wind in the transition region as influenced by ionospheric convection and electron heating

We have conducted a set of systematic generalized semikinetic simulations to study the polar H+/O+ upflows in the ionosphere and transition region as influenced by varying convection. Effects of both frictional ion heating and centrifugal acceleration are included. We find that in regions where the convection electric field is strong (Ei ≥ 100 mV/m) the steady state polar wind may be characterized as primarily a centrifugally accelerated O+ outflow together with an ambipolar H+ outflow, as a minor component, up to 4 RE geocentric distance. Owing to the increase in the O+ upward flow speed, the increase in the O+ density, and the decrease in the H+ flow speed, H+–O+ collisions are important to extended altitudes during enhanced convection periods. The exobase (defined here as the altitude where the O+ scale height is equal to the mean free path of an H+ ion with a speed three thermal speeds larger than the H+ bulk speed) shifts from 1900 km for Ei = 0 mV/m to 3000 km for Ei = 100 mV/m. For the range of convection electric fields considered here (Ei = 0 mV/m to Ei = 100 mV/m), we identify an upper and a lower transition region which coincide roughly with the region of downward and upward H+ heat flux, respectively. A set of relationships between ion parallel speeds and normalized collisional mean free paths was found which are associated with the maximum upward and downward heat flux, regardless of the value of Ei, for steady state conditions. We find that the heated and centrifugally accelerated O+ ions can obtain upward bulk velocities of 5 km/s above 3 RE geocentric distance for Ei ≥ 80 mV/m. These ions exert a large downward drag on the H+ ions which stretches out the tail on the lower velocity side of the distribution creating large downward heat fluxes. These effects may explain features of the large downward heat fluxes observed in the H+ distributions to large altitude by the retarding ion mass spectrometer (RIMS) instrument on DE 1. We have also considered impulsive events, consisting of pulses of cleft associated enhanced convection and elevated electron temperatures, followed by convection across the polar cap. These result in O+ ions falling back into the ionosphere on the dayside and nightside [e.g., Horwitz and Lockwood, 1985]. Downward speeds of 1–2 km/s are seen up to several thousand kilometers altitude which is consistent with DE 1/RIMS observations as presented by Chandler[1995].

Equatorial heating and hemispheric decoupling effects on inner magnetospheric core plasma evolution

We have extended our previous semikinetic study of early stage plasmasphere refilling with perpendicular ion heating (Lin et al., 1992) by removing the restriction that the northern and southern boundaries are identical and incorporating a generalized transport description for the electrons (e.g., Brown et al., 1991). This allows investigation of the effects of electron heating and a more realistic calculation of electric fields produced by ion and electron temperature anisotropies. The combination of perpendicular ion heating and parallel electron heating leads to an equatorial electrostatic potential peak, which tends to shield and decouple ion flows in the northern and southern hemispheres. Unequal ionospheric upflows in the northern and southern hemispheres lead to the development of distinctly asymmetric densities and other bulk parameters. At t= 5 hour after the initiation of refilling with different source densities (Nnorth = 100 cm−3, Nsouth = 50 cm−3), the maximum potential drops of the northern and southern hemispheres are 0.6 and 1.3 V, respectively. At this time the minimum ion densities are 11 and 7 cm−3 for the northern and southern hemispheres. DE 1 observations of asymmetric density profiles by Olsen (1992) may be consistent with these predictions. Termination of particle heating causes the reduction of equatorial potential and allows interhemispheric coupling. When the inflows from the ionospheres are reduced (as may occur after sunset), decreases in plasma density near the ionospheric regions are observed while the heated trapped ion population at the equator persists.

Synergistic effects of hot plasma-driven potentials and wave-driven ion heating on auroral ionospheric plasma transport

Transverse acceleration by waves and parallel acceleration by field-aligned electric fields are important processes in the transport of ionospheric ions along auroral field lines. In order to study the transport of ionospheric plasma in this environment we have developed a generalized semikinetic model which combines the tracking of ionospheric ion gyrocenters with a generalized fluid treatment of ionospheric electrons. Large-scale upward and downward directed electric fields are generated within the model by introducing magnetospheric plasma whose components have differing temperature anisotropies. We study the effects of such potentials when combined with the effect of ion heating by a distribution of waves along the flux tube. We find that the combination of wave heating and an upward electric field results in an order of magnitude increase in O + outflow (compared to a case with an upward electric field and no wave heating). Under these conditions we observe the formation of bimodal conics. When a downward electric field is added to a case with wave heating, the energy gained by the ions from the waves increases by a factor of 2 or 3 (over the scenario with wave heating and no hot plasma-driven electric field) owing to their slower transit of the heating region. Typically, the velocity distributions under these conditions are toroids and counterstreaming conics. We also find that the upflowing, dense, heated ionospheric plasma acts to reduce the potential set up by the anisotropies in the magnetospheric components.

Plasma expansion and evolution of density perturbations in the polar wind: Comparison of semikinetic and transport models

Comparisons are made between transport and semikinetic models in a study of the time evolution of plasma density perturbations in the polar wind. The situations modeled include plasma expansion into a low-density region and time evolution of localized density enhancements and cavities. The results show that the semikinetic model generally yields smoother profiles in density, drift velocity, and ion temperature than the transport model, principally because of ion velocity dispersion. While shocks frequently develop in the results of the transport model, they do not occur in the semikinetic results. In addition, in the semikinetic results, two ion streams, or double-humped distributions, frequently develop. In the transport model results the bulk parameters, at a given time, often have a one-to-one correspondence in the locations of their local minima or maxima. This is a consequence of the coupling of the fluid equations. There is, however, no such relationship among the moments produced by the semikinetic model where the local moment maxima and minima are often shifted in altitude. In general, incorporation of enhanced heat fluxes in the transport model leads to somewhat improved agreement with the semikinetic results.

Semikinetic modeling of plasma flow on outer plasmaspheric field lines

Abstract We have developed a large scale, plasma transport model which treats ions as particles and electrons as a massless neutralizing fluid. In using this model to study plasmaspheric flows we have included the effects of ion Coulomb collisions, pitch angle scattering, and wave-induced perpendicular heating. We have also include electron heating. Among the results we find: (1) Coulomb collisions are sufficient for plasmasphere refilling within observed time scales, (2) Counterstreaming beams can last up to 30 hours for an L = 4.5 flux tube, provided there is not much wave-particle heating or scattering occurring, (3) At early times, pitch angle scattering and perpendicular heating acting together are much more effective than either acting alone in producing plasma accumulation, (4) Heating power levels as low as 10−13 V2m−2Hz−1 can result in equatorial densities of 10 cm−3 in 12 hours for an L = 4 flux tube, and (5) Equatorial electron heating can create electric potential barriers which will tend to isolate equatorially trapped ions and decouple interhemispheric flow, provided the electron heat flow is not too large. Results from cases with perpendicular ion heating and parallel electron heating are similar to what is seen in the DE RIMS ion observations.

Collisional to collisionless ion outflow at the ionosphere-magnetosphere interface

Abstract Simple calculations show that ions in the ionophere are collision dominated while ions in low density regions of the magnetosphere are collisionless. For the free outflow of light ions such as H + and He + , this collisional-collisionless transition occurs in the region where the ions are accelerated by the charge separation electric field resulting from the steep gradient in the O + density. In this case, specific distortions of the light ion velocity distributions are produced that significantly alter ion temperatures and heat flows. The types of nonMaxwellian velocity distributions seen are distributions with downward suprathermal tails, inverted bowl shaped distributions, and highly skewed distributions with large parallel or perpendicular heat flows. In addition, distributions with multiple peaks or one peak and an extended plateau also develop under some conditions. These various distributions are the result of relative flow between species, the v −4 dependence of the Coulomb collision cross section, and altitude distributed sources of the ions. This paper presents results of studies of the evolution of the ion velocity distribution in the transition region done with a collisional semikinetic model combining the effects of large scale forces and field-aligned transport with the effects of ion-neutral and ion-ion collisions. Comparison of these results with data from the RIMS instrument on the DE 1 spacecraft suggest that these distributions have been seen and that additional processes, such as downward electric fields, may frequently be present on polar cap field lines.