James L. Horwitz

Formation of density troughs embedded in the outer plasmasphere by subauroral ion drift events

A dynamic global core plasma model (DGCPM) is used to investigate the effects of subauroral ion drift (SAID) events on the formation of trough density profiles in the outer plasmasphere during periods of high magnetic activity. The DGCPM includes the influences of convection on the changing flux tube volumes, as well as daytime refilling and nighttime draining of plasma, to calculate the plasma tube contents and equatorial plasma density distribution versus time throughout the magnetosphere. SAIDs are regions of latitudinally narrow westward flow of plasma equatorward of the auroral zone. We present DGCPM results for various presumed SAID locations and durations relative to enhanced substorm convection onset and decay, to parametrically elicit the formation of plasmaspheric density trough structures resulting from SAID effects. It is found that imposing a SAID event in the dusk-evening sector for 30 min leads to the formation of a narrow (less than 1 RE near the equatorial plane) embedded plasma density trough in the dusk bulge region. The modeled plasmasphere density profiles with troughs generally resemble plasmasphere density profiles observed from DE 1 measurements.

Statistical relationships between high‐latitude ionospheric F region/topside upflows and their drivers: DE 2 observations

A statistical analysis is conducted on the relationships among high-latitude topside (850 - 950 km altitude) ionospheric plasma parameters and precipitating soft (≤1 keV) electron characteristics based on DE 2 satellite measurements from seven auroral zone passes. The parameters examined statistically for these relationships are 1137 independent samples of the field-aligned ion flow velocities, fluxes, Mach numbers, densities, ion and electron temperatures, and soft electron energy fluxes and average or characteristic energies. We find that both ion upward velocities and upward fluxes are well correlated with electron and ion temperatures. Least squares fits to the data averaged in restricted bins show the following correlation coefficients: Ion upward velocity with T e , correlation coefficient r= 0.97; with T i , r = 0.94; for ion upflux with T e , r = 0.97; with T i , r = 0.91. The somewhat higher correlations with T e than T i of both upflow velocities and upfluxes suggest the important role of enhanced ambipolar electric fields associated with enhanced T e , as heated by both direct collisions with the precipitating electrons as well as downward magnetospheric heat fluxes. The largest (≥10 10 ions cm -2 s -1 ) ion upfluxes are associated with ultrasoft electron precipitation having average energies of <80 eV. Significant anticorrelations of electron (r= -0.90) and ion (r= -0.89) temperatures with the average energies of the precipitating soft electrons suggest that for the same precipitation energy flux, the lowest-energy precipitating electrons are most effective in heating the topside thermal electrons. Finally, analysis of ion field-aligned flow Mach numbers shows that these Mach numbers were almost always less than 0.4 and are typically less than 0.2. Such Mach number measurements suggest that low-speed approximations in fluid transport models are usually valid for ≤1000 km altitude, even at high latitudes.

'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.

The polar cap environment of outflowing O(

Ion composition measurements by Dynamics Explorer 1 often show upward O+ beams at polar latitudes, with streaming energies of 1–20 eV or more. Here we utilize measurements of core (0–50 eV) and “energetic” (∼0–1 keV) ion composition, plasma waves, and auroral images from DE 1 and plasma ions and electrons from DE 2 to examine some of their properties in the context of the polar cap environment. It is found that two distinct populations of O+ beams are observed: “high-speed” (10–30 eV or higher streaming energies) and “low-speed” (generally <10-eV streaming energies). The “high-speed” polar beams show an “auroral” connection; i.e., they are observed on or near field lines threading auroral arcs seen in DE 1 images. The “low-speed” streams are on or near field lines threading the dark polar cap and may be convected from the cleft ion fountain. The low-speed streams are generally much more stable in energy and flux, while the high-speed streams tend to be bursty. In general, the streams are convecting antisunward, with velocities of 5–14 km/s in the orbital plane. We sought to obtain plasma density estimates from plasma wave measurements, through analysis of features of auroral hiss as well as upper hybrid emissions. Densities in the range 1–5 el/cm³ were indicated for one segment of a DE 1 polar cap pass; however, the measurements generally indicate little auroral hiss or upper hybrid emissions in the polar cap for the other cases considered here. Estimates of electrostatic potential drops above the DE 2 satellite have been made using energy-angle spectrograms of photoelectron data, under the assumption that the field lines of observation are “effectively” open. Potential drops often are in the 20- to 40-V range. At other times the potential falls below the ∼5-V instrument threshold, or there are insufficient photoelectron fluxes for estimation. These limited data suggest that the largest potential drops are just poleward of the cleft or near its poleward edge and there is a decline of the drop in the antisunward direction. No obvious correlation between the potential estimates and “nearby” O+ streaming energies is seen.

RING CURRENTS AND INTERNAL PLASMA SOURCES

The discovery of terrestrial O+ and other heavy ions in magnetospheric hot plasmas, combined with the association of energetic ionospheric outflows with geomagnetic activity, led to the conclusion that increasing geomagnetic activity is responsible for filling the magnetosphere with ionospheric plasma. Recently it has been discovered that a major source of ionospheric heavy ion plasma outflow is responsive to the earliest impact of coronal mass ejecta upon the dayside ionosphere. Thus a large increase in ionospheric outflows begins promptly during the initial phase of geomagnetic storms, and is already present during the main phase development of such storms. We hypothesize that enhancement of the internal source of plasma actually supports the transition from substorm enhancements of aurora to storm-time ring current development in the inner magnetosphere. Other planets known to have ring current-like plasmas also have substantial internal sources of plasma, notably Jupiter and Saturn. One planet having a small magnetosphere, but very little internal source of plasma, is Mercury. Observations suggest that Mercury has substorms, but are ambiguous with regard to the possibility of magnetic storms of the planet. The Messenger mission to Mercury should provide an interesting test of our hypothesis. Mercury should support at most a modest ring current if its internal plasma source is as small as is currently believed. If substantiated, this hypothesis would support a general conclusion that the magnetospheric inflationary response is a characteristic of magnetospheres with substantial internal plasma sources. We quantitatively define this hypothesis and pose it as a problem in comparative magnetospheres.

Statistical analysis of F region and topside ionospheric ion field‐aligned flows at high latitudes

A statistical examination is made of ionospheric ion field-aligned flow velocities, ion densities, and electron and ion temperatures as measured at high latitudes in the Northern Hemisphere with the Dynamics Explorer (DE) 2 spacecraft in the 300–1000 km altitude range. We explore the characteristic trends and morphology of these field-aligned ion flows in terms of their variations with invariant latitude, altitude, solar zenith angle, plasma temperatures, and geomagnetic activity. Enhancements in the electron and ion temperatures, field-aligned ion upflows, and ion densities are frequently observed above 600 km over the statistical cleft and auroral oval regions and over wide latitudes on the nightside. Intermittent, subsonic auroral upflows often reach bulk speeds of up to several hundreds meters per second. Upfluxes over the statistical auroral oval are generally in the range 108–109 ions cm−2 s−1, and occasionally up to 1010 ions cm−2 s−1. The nocturnal upflows have lower densities than those on the dayside. Statistical altitude profiles indicate that upflows accelerate above 450 km on the dayside, while detectable nightside upflows start in the lower altitudes. Upfluxes are more closely correlated to the electron temperatures than to the ion temperatures. The topside ion upflow fluxes generally are uncorrelated with Kp index. Downflows are more frequent over the statistical polar cap, especially above 600 km. The electron temperatures are usually 1000 K higher than the ion temperatures on the dayside; on the nightside the mean electron temperatures are comparable or occasionally even lower than those of the ions. The mean day-night temperature asymmetry is more than 1000 K for electrons and is smaller for the ions. Plasma temperatures generally increase with Kp index. Field-aligned ion flows of 0–100 m s−1 dominate the high-latitude F region (300–500 km altitude), and are mostly down-ward on the dayside but more consistently upward on the nightside. Density enhancements are frequently observed in the dayside statistical auroral region. Density depletions of about 30% are present in the nightside statistical auroral F region, and the depletions deepen with increasing magnetic activity.

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.

Refilling of the Earth's plasmasphere

The Earths ionosphere at mid-latitudes is produced chiefly by solar EUV-induced photo-ionization of the neutral atmosphere at altitudes of 90 to 300–400 kilometers. Such ionized gas, or plasma—entrained by the Earths (basically bipolar) magnetic field—flows upward along magnetic lines of force, until the plasma gas pressure is equalized along the entire line of force extending from the northern to the southern ionosphere. The plasma region above the ionosphere on such closed magnetic field lines is called the plasmasphere. In fact, the plasmasphere may be considered as an extension of the ionosphere, for there is no clear distinction between them. In the absence of plasma-removal processes, plasma densities may exceed 104 ions/cm3 even at high altitudes of 12,000 kilometers or more.

Astrophysical particle acceleration in geospace and beyond

Natural acceleration of charged particles occurs within a variety of physical contexts and through a range of physical processes. “Cosmic” charged particle acceleration can involve particle energies and energy changes from less than one electron Volt (eV),and on through kilovolt (keV) energies,up to 1018 and perhaps 1020 eV These acceleration processes can be in the forms of various types of electric fields directed parallel to the magnetic field lines, which are believed chiefly responsible for the energization of the auroral electrons which bombard the Earths upper atmospheres at high magnetic latitudes to create auroras. There are also “collisionless” shock layers in our interplanetary space and well beyond, to energize charged particles in solar and stellar winds. And there may be exotic—and somewhat speculative—processes such as the effect of intense radiation beams associated with non-thermal compact objects, or unipolar electric fields of huge magnitudes near a rotating and accreting black hole. These processes may accelerate particles to even 1020 eV energies. The propagation of such ultra-high energy particles in time and space may be severely modified by quantum gravity effects, which are no longer negligible in these extreme situations.

Sources, transport, energization, and loss of magnetospheric plasma

Sources, transport, energization, and loss of magnetospheric plasma was the theme of the third Huntsville Workshop on Magnetospheric Plasma Models, which was held in Guntersville, Ala., from October 4 to 8, 1992. Approximately ninety researchers attended the workshop, which was supported in part by a grant from the National Science Foundation. The first topical session summarized our knowledge of plasma distributions and set the stage for the later sessions. Dennis Gallagher reviewed the distributions of bulk parameters and reported on the new results of his empirical model of the plasmasphere. Ed Shelley traced the origins of the plasma sheet, concluding that medium-energy magnetospheric plasma is of between 5 and 50% ionospheric origin. Tony Lui addressed the distribution of energetic particles and noted that the observed pressure gradients account for the field-aligned currents in the inner magnetosphere. Dan Baker pointed out the glaring problem of how electrons gain relativistic energies during substorms.