J. L. Horwitz

Plasmasphere refilling: Recent observations and modeling

The phenomenon of plasmasphere refilling, and general considerations of plasmasphere structure and plasmasphere-ionosphere coupling, have received increased attention in recent years in terms of both observational considerations and modeling investigations. New ideas arising from both observations and modeling efforts are revealing the refilling region to be rich in intriguing physical processes, including hot-cold plasma interactions through wave-particle effects and Coulomb collisions, electrostatic shocks, and differences in light and heavy ion behavior during refilling. “Global” observational and modeling studies of the plasmasphere structure are in the process of demonstrating the complex manner in which refilling and the spatial and temporal variations of convection interplay to produce complex structure in the plasmasphere; these studies are particularly timely as they could be extremely helpful for interpreting results from proposed imaging of plasmaspheric He+. This review will serve to highlight the observational and theoretical/modeling progress during the past few years as well as to introduce and place in context the particular contributions contained in this special section.

Centrifugal acceleration of the polar wind

The effect of parallel ion acceleration associated with convection was first applied to energization of test particle polar ions by Cladis (1986). However, this effect is typically neglected in “self-consistent” models of polar plasma outflow, apart from the fluid simulation by Swift [1990]. Here we include approximations for this acceleration, which we broadly characterize as centrifugal in nature, in our time-dependent, semikinetic model of polar plasma outflow and describe the effects on the bulk parameter profiles and distribution functions of H+ and O+. For meridional convection across the pole the approximate parallel force along a polar magnetic field line may be written as Fcent,pole = 1.5m(Ei/Bi)²(r²/ri³) where m is ion mass, r is geocentric distance; and Ei, Bi and ri refer to the electric and magnetic field magnitudes and geocentric distance at the ionosphere, respectively. For purely longitidinal convection along a constant L shell the parallel force is Fcent,long = Fcent,pole [1-(r/(riL)]3/2/[1-3r/(4riL)]5/2. For high latitudes the difference between these two cases is relatively unimportant below ∼5 RE. We find that the steady state O+ bulk velocities and parallel temperatures strongly increase and decrease, respectively, with convection strength. In particular, the bulk velocities increase from near 0 km s−1 at 4000 km altitude to ∼10 km s−1 at 5 RE geocentric distance for a 50-mV/m ionospheric convection electric field. However, the centrifugal effect on the steady O+ density profiles depends on the exobase ion and electron temperatures: for low-base temperatures (Ti = Te = 3000 K) the O+ density at high altitudes increases greatly with convection, while for higher base temperatures (Ti = 5000 K, Te = 9000 K), the high-altitude O+ density decreases somewhat as convection is enhanced. The centrifugal force further has a pronounced effect on the escaping O+ flux, especially for cool exobase conditions; as referenced to the 4000-km altitude, the steady state O+ flux increases from 105 ions cm−2 s−1 when the ionospheric convection field Ei = 0 mV/m to ∼107 ions cm−2 s−1 when Ei = 100 mV/m. The centrifugal effect also decreases the time scale for approach to steady-state. For example, in the plasma expansion for Ti = Te = 3000 K, the O+ density at 7 RE reaches only 10−7 of its final value ∼1.5 hours after expansion onset for Ei = 0. For meridional convection driven by Ei = 50 mV/m, the density at the same time after initial injection is 30-50% of its asymptotic level. The centrifugal acceleration described here is a possible explanation for the large (up to ∼10 km s−1 or more) O+ outflow velocities observed in the midaltitude polar magnetosphere with the Dynamics Explorer 1 and Akebono spacecraft.

A semikinetic model for early stage plasmasphere refilling: 1, Effects of Coulomb collisions

Recently we have modified our collisionless, time-dependent, kinetic plasma model (Wilson et al., 1990) to include the effects of Coulomb self-collisions in a theoretically rigorous fashion. The algorithm employed (Takizuka and Abe, 1977) faithfully mimics a Fokker-Planck operator and conserves both energy and momentum. This collisional kinetic model has been applied to the problem of baseline plasmasphere refilling of an initially depleted flux tube, neglecting the effects of wave-particle interactions. The companion report by Lin et al. (this issue) examines the effects of wave-particle interactions on refilling. We have performed refilling calculations for various flux tubes (L = 3, 4, 5, 6) and for different ionospheric plasma fluxes and temperatures. In each case considered, the same set of events occurs. Initially, two polar wind outflows develop from each hemisphere and set up counterstreaming beams. With time the vacant phase space region between these beams fills, primarily because of to collision-induced particle diffusion but also because of lowering ambipolax potential drops resulting from the increasing density in the plasmasphere. In contrast to all previous hydrodynamic approaches, we find no formation of shocks. The plasma first evolves an isotropic nearly Maxwellian velocity distribution in a region that starts near the ionosphere and moves outward toward the equator. We find that for reasonable topside ionospheric temperatures and fluxes the thermal plasma all along an L shell will become nearly isotropic in 6 to 30 hours (longer times for smaller fluxes or larger L shells), consistent with the observations of Horwitz et al. (1984).

Effects of frictional ion heating and soft‐electron precipitation on high‐latitude F‐region upflows

Observed ionospheric F-region upflows associated with convection-driven frictional ion heating and soft electron precipitation at high latitudes are modeled with a dynamic ionospheric fluid code. Precipitating soft (≤ 1 keV) auroral electrons are effective in rapidly enhancing the F-region ionization and electron temperature, which leads to a strong upward plasma expansion. It is shown, for example, that an electron flux of 1 erg cm−2s−1 with a characteristic energy of 150 eV can produce a 109cm−2s−1 O+ outflow at altitudes of 700–800 km. The more widely-recognized convection ion heating is indicated to be significant but somewhat smaller effects on the upflows. We have performed comparisons with published HILAT and DE-2 observations. Using a latitudinal distribution of ionospheric flux tubes with “inputs” of the observed average precipitating electron energies, energy fluxes and convection drift velocities, we find satisfactory agreement with latitudinal profiles of ion upflow velocities, densities, fluxes, and ion and electron temperatures. Therefore, we suggest that the combined effects of soft electron precipitation and frictional ion heating may be identified as the principal drivers of these upflows.

Systematic modeling of soft‐electron precipitation effects on high‐latitude F region and topside ionospheric upflows

An ionospheric plasma fluid transport model is used to investigate the effects of soft (<1 keV) electron precipitation on high-latitude F region/topside ionospheric O+ upflows. In this paper we present a systematic modeling study of ionospheric effects of varying soft-electron precipitation, focusing on the resulting upward O+ ion velocities and fluxes, as well as the elevated ion and electron temperatures, due to the precipitation. Recent satellite observations [Seo et al., 1997] suggest an inverse relationship between upward O+ fluxes and the characteristic energy of the precipitating electrons for the same energy flux level. The modeling results presented here show this inverse relationship explicitly. Our interpretation is that a declining characteristic energy at constant energy flux increases the number of precipitating electrons available to heat the thermal electrons, and thus enhances the thermal electron temperature and hence the ambipolar electric field for propelling the upward O+ flows. The modeled increase of the thermal electron temperature with enhanced auroral electron precipitation is also generally consistent with the Seo et al. [1997] topside ionospheric plasma measurements. In addition, the modeling results presented here illustrate characteristic temporal development responses, showing dramatic increases in velocity, Mach number, and flux values during the first 10–13 min after the precipitation is turned on. By ∼1 hour after the initiation of a soft-electron precipitation event the ion upward velocities and fluxes approach nearly stable, asymptotic values.

Achieving zero current for polar wind outflow on open flux tubes subjected to large photoelectron fluxes

In this study we investigate how the condition of zero current on open flux tubes with polar wind outflow, subjected to large photoelectron fluxes, can be achieved. We employ a steady state collisionless semikinetic model to determine the density profiles of O+, H+, thermal electrons and photoelectrons coming from the ionosphere along with H+, ions and electrons coming from the magnetosphere. The model solution attains a potential distribution which both satisfies the condition of charge neutrality and zero current. For the range of parameters considered in this study we find that a 45–60 volt discontinuous potential drop may develop to reflect most of the photoelectrons back toward the ionosphere. This develops because the downward flux of electrons from the magnetosphere to the ionosphere on typical open flux tubes (e.g. the polar rain) appears to be insufficient to balance the photoelectron flux from the ionosphere.

Modeling of F‐region ionospheric upflows observed by EISCAT

High-latitude ionospheric bulk parameter profiles measured with the European Incoherent Scatter Radar (EISCAT) system are modeled with a modified version of the Field Line Interhemispheric Plasma (FLIP) code. Three observed profiles of field-aligned ion velocities, densities, and ion and electron temperatures for the altitude range 200–800 km are modeled as influenced by the effects of soft (<1 keV) auroral electron precipitation, convection-driven frictional ion heating, and downward magnetospheric electron heat fluxes. Favorable matches between the observed and modeled profiles with reasonable precipitation, convection, and downward electron heat flux parameters suggest that soft electron precipitation, together with magnetospheric electron heat fluxes, are the primary drivers of the normal ionospheric F-region upflows observed at high latitudes.

Dynamic fluid-kinetic (DyFK) modeling of auroral plasma outflow driven by soft electron precipitation and transverse ion heating

We apply a recently developed dynamic fluid-kinetic (DyFK) model to simulate and investigate the effects of soft auroral electron precipitation and perpendicular ion heating by waves on the plasma outflow along auroral field lines. The DyFK model is constructed by coupling a fluid ionospheric model for the region from 120 to 800 km to a semikinetic treatment for topside through several RE altitude region. This approach, which is described in detail here, allows a partially self-consistent description of the plasma transport along high-latitude flux tubes where both low-altitude ionospheric heating and ionization production and loss as well as high-altitude energization and kinetic effects are incorporated and stressed. In the present work, we investigate the combined effects of the F region plasma production and electron heating by soft auroral electron precipitation and ion perpendicular wave heating at high altitudes, which produces ion conies. The auroral event simulated here involves 1.5 hours of moderate soft electron precipitation and relatively weak ion cyclotron waves along the magnetic field lines. The simulations reveal the F region electron heating and ionization by the soft electron precipitation, driving a topside O+ upflow of up to 109 cm−2 s−1 below 1000 km within 30 min after the electron precipitation is turned on. The enhanced O+ upflow plumes would be still gravitationally bound in the absence of further energization at higher altitudes. However, the synergistic effects of the increased upwelling ion supply driven by the precipitation and the wave-driven ion heating at higher altitudes combine to enhance O+ bulk outflow by an order of magnitude above the baseline polar wind level to a net outflow flux of 108 ions cm−2 s−1 with a density of 10 ions cm−3 and bulk velocity of 12 km s−1 at 3 RE altitude. Various O+ conic velocity distributions develop within 10 min after transverse heating is initiated, and their characteristic energies saturate at approximately 10 eV for the peak wave-induced heating rates of 10−14 ergs s−1 at 2 RE here. H+ is also affected by the increases of O+ due to H+–O+ collisional drag in the 1000 – 4000 km altitude transition region. H+ flow is much less affected by the wave heating because of the faster transit times through the high-altitude wave heating zone and the lower H+ perpendicular heating rates which were incorporated here. The H+ bulk flow consists of a flux of 108 ions cm−2 s−1, a density of 4 ions cm−3, and a velocity of 30 km s−1 at 3 RE altitude.

A semikinetic model for early stage plasmasphere refilling: 2, Effects of wave-particle interactions

We treat the early stages of plasmasphere refilling along an initially depleted L=4 magnetic flux tube through a semikinetic model. The companion paper by Wilson et al. (this issue) describes the plasma evolution for a “baseline” refilling situation in which Coulomb collisions play a central role. Here we focus on the effects of wave-particle interactions in which stochastic diffusion of ions in perpendicular velocity due to equatorially concentrated electromagnetic ion cyclotron waves plays a central role. We examine characteristic individual ion trajectories, as well as the evolution of bulk parameters and ion distribution functions when equal “polar wind” streams are injected at the northern and southern ionospheres. In the ion trajectories, it is found that relatively modest and realistic perpendicular electric field power levels lead to decreased mirror latitudes, substantial acceleration, and equatorial entrainment of these ions. After about 8 hours, significant general accumulation of plasma occurs all along the flux tube, with equatorial densities attaining levels of 15 ions/cm³, and equatorial parallel and perpendicular temperatures of around 2.4 and 60 eV, respectively. Incorporating an electron temperature distribution which is calculated as a fraction of the local ion temperature, and therefore also peaks at the magnetic equator, leads to an ambipolar electric field which acts to reflect incoming ions back toward the ionospheres. In this case, we find results strikingly similar to those of Olsen et al. (1987), wherein trapped 50 eV ion distributions are seen at the magnetic equator, while incoming and reflected, cool, 1–2 eV, field-aligned streams are seen on both sides of the equator. A substantial equatorial density depletion is also found, in good accord with the results of Olsen (this issue).

Effects of magnetospheric electrons on polar plasma outflow - A semikinetic model

A semikinetic model for the study of the effect of hot magnetospheric electrons on polar plasma outflow is developed. The model is based on a hybrid particle-in-cell approach which treats the ions (H+ and O+) as adiabatic, parallel-drifting gyro-centers injected as the upgoing portions of drifting bi-Maxwellian distributions at 1.6 RE, while the electrons are treated as a massless neutralizing fluid. As a first approach to understanding the effects of hot magnetospheric electrons on the outflow, we consider electron temperature profiles which increase from low temperatures at the ionospheric levels to high temperatures at high altitudes. The electric field is determined by both the electron temperature and its gradient. The electric field produced by the electron temperature alone generally accelerates ions outward while that associated with the electron temperature gradient increases the potential barrier and inhibits the outflow. For typical polar wind stream conditions, electron temperature gradients exceeding 3×104 K/RE cause reflection of much of the ion stream back downward toward the ionosphere. Under these circumstances the H+ outflow forms two counterstreaming beams at altitudes below the reflecting potential barrier and a cooler and faster transmitted beam at high altitudes. Above the potential barrier, the O+ density decreases by 7 orders of magnitude for a very large electron temperature gradient. For the case of an electron temperature profile established by thermal conduction the results show inhibition of polar plasma outflow very near the lower boundary, but continuous acceleration of the escaping ions along most of the flux tube. H+ shows a continuous decrease in net outward flux from 3×107 ions cm−2 s−1 when the electron temperature is isothermal at 4400 K to 1.5×107 ions cm−2 s−1 when the upper boundary temperature is increased to 1×106 K. On the other hand, the flux of O+ exhibits a rise and fall with upper boundary electron temperature with a peak of 1.1×107 ions cm−2 s−1 when the upper boundary electron temperature is approximately equal to 2×105 K.