Vahe Peroomian

Consequences of magnetotail ion dynamics

The trajectories of a large ensemble of particles are calculated in a modified Tsyganenko magnetic field model with a uniform cross-tail electric field. The model magnetotail can be divided into several distinct dynamical regimes of ion motion. Near Earth, where the field lines are dipolar the adiabatic formalism is adequate. In the mid-tail and distant tail, guiding-center theory breaks down and must be replaced by a quasi-adiabatic formalism. There is an important transition region between the adiabatic and quasi-adiabatic regions where ion trajectories become more complicated and no simple analytical description holds. This wall region is characterized by rapid ion acceleration and a major loss of particles to the dusk flank. The moments of the ion distribution function are constructed from the ion trajectories, including density, temperature, and pressure in the x-z and x-y planes. In the noon-midnight meridian plane, parameters are relatively constant except near the Earth, while the x-y plots show strong gradients across the magnetotail. Magnetotail plasma convects earthward, drifts toward dusk, and is squeezed out of the tail in the near-Earth region. A thin current sheet forms in the quasi-adiabatic region, and the pressure tensor has significant off-diagonal terms at its edges. These terms are the result of quasi-adiabatic ion trajectories which lead to azimuthally asymmetric distribution functions capable of maintaining approximate stress balance across the current sheet. Simplified analytical descriptions provide further physical insight into ion dynamics that are observed.

Dispersed ion structures at the poleward edge of the auroral oval: Low‐altitude observations and numerical modeling

We have compared the AUREOL 3 (A3) observations of auroral ion precipitation, particularly ion beams, with the results from the global kinetic model of magnetotail plasma of Ashour-Abdalla et al. (1993). We have identified 101 energetic 2-20 keV H+ velocity dispersed precipitating ion structures (VDIS) with fluxes above 10−3 ergs. cm−2. s−1 in the A3 record between the end of 1981 and mid-1984. These beams display a systematic increase in energy with increasing latitude and were observed in a narrow region within less than 1 deg in latitude of the polar cap boundary. The VDIS are the most distinctive feature in the auroral zone of the plasma sheet boundary layer. We report first on a statistical analysis of the possible relationships between magnetic activity or substorm phase and the VDIS properties. The VDIS are found on 15-18% of the A3 orbits. In general their frequency of occurrence is not correlated with activity. However in the 2200-0200 MLT sector, the probability of observing more energetic VDIS increases for larger values of the AE index. Our particle simulations of the precipitating ions have been extended by using a series of modified versions of the Tsyganenko (1989) magnetic field model and by varying the cross-magnetosphere electric field. In the simulations, plasma from a mantle source is subject to strong nonlinear acceleration, forming beams which flow along the PSBL. Only 3 to 4% of these beams precipitate into the ionosphere to form the VDIS while the majority return to the equatorial plane after mirroring and form the thermalized central plasma sheet. The final energy and the dispersion of the beams in the model depend on the amplitude of the cross-tail electric field. Two unusual observations of low-energy (< 5 keV) O+ VDIS, shifted by 4°-5° in invariant latitude equatorward of H+ VDIS are analyzed in detail. The sparsity of such O+ events and the absence of the changes in the flux and frequency of occurrence indicate a solar wind origin for the plasma. Finally, large-scale kinetic modeling, even with its simplifications and assumptions (e.g., static magnetic field, solar wind source), reproduces low-altitude auroral ion features fairly well; it may therefore be presented as an appropriate framework into which data on energization and transport of the hot plasma, obtained in the equatorial plane, could be inserted in the near future.

The formation of the wall region: Consequences in the near Earth magnetotail

This paper discusses important new findings obtained from global kinetic simulations of magnetotail plasma. A region of strongly non-adiabatic ion acceleration (known as the [open quotes]wall[close quotes] region) exists in the near Earth tail and demarcates two very different regimes of ion motion: Adiabatic and quasiadiabatic. A strong enhancement of the cross-tail current occurs on the tailward side of the wall. The authors comparison of numerical and adiabatic pressure profiles indicates that non-adiabatic processes operating in this region may contribute significantly to a pressure balance relief in the course of quasisteady magnetospheric convection. 23 refs., 4 figs.

Proton velocity distributions in the magnetotail: Theory and observations

The structure of a plasma sheet ion distribution is modeled by using the large-scale kinetic approach in which the trajectories of a distribution of plasma mantle particles are followed through the magnetotail. In the plasma sheet boundary layer, beamlets that originate from localized regions in the current sheet maintain their identities even after multiple interactions with the current sheet. Because of their origin, beamlet distribution functions appear to be phase bunched and azimuthally asymmetric, even far from the current sheet. Structures in the distribution of ions manifest themselves in both configuration space and velocity space. At the outer edge of the plasma sheet the particles experiencing the greatest acceleration in the distant magnetotail form a structure with multiple peaks in velocity space. When earthward and tailward beamlets are found in the same region, there is an increase in density and a decrease in bulk velocity. At the outer edge of the plasma sheet and thus near the boundary layer, counterstreaming distributions are observed. In the plasma sheet, two populations are found: beamlets with large drift velocity and a chaotic counterpart which has been thermalized by multiple crossings of the current sheet. In the vicinity of the earthward edge of the plasma sheet, the two populations merge to become an isotropic distribution function. A detailed comparison of these results with observations from the plasma instrument (PLS) on the Galileo spacecraft during the Earth 1 flyby is presented. The principal features of the observed and computed distributions are in substantial qualitative agreement.

Ion sources and acceleration mechanisms inferred from local distribution functions

This study investigates the sources of the ions making up the complex and nonisotropic H + velocity distribution functions observed by the Geotail spacecraft on May 23, 1995, in the near-Earth magnetotail region and recently reported by Frank et al. [1996]. A distribution function observed by Geotail at ∼10 R E downtail is used as input for the large scale kinetic (LSK) technique to follow the trajectories of approximately 90,000 H + ions backward in time. Time-dependent magnetic and electric fields are taken from a global magnetohydrodynamic (MHD) simulation of the magnetosphere and its interactions with appropriate solar wind and IMF conditions. The ion population described by the Geotail distribution function was found to consist of a mixture of particles originating from three distinct sources: the ionosphere, the low latitude boundary layer (LLBL), and the high latitude plasma mantle. Ionospheric particles had direct access along field lines to Geotail, and LLBL ions convected adiabatically to the Geotail location. Plasma mantle ions, on the other hand, exhibited two distinct types of behavior. Most near-Earth mantle ions reached Geotail on adiabatic orbits, while distant mantle ions interacted with the current sheet tailward of Geotail and had mostly nonadiabatic orbits. Ions from the ionosphere, the LLBL, and the near-Earth mantle were directly responsible for the well-separated, low energy structures easily discernible in the observed and modeled distribution functions. Distant mantle ions formed the higher energy portion of the Geotail distribution. Thus, we have been successful in extracting useful information about particle sources, their relative contribution to the measured distribution and the acceleration processes that affected particle transport during this time.

The mosaic structure of plasma bulk flows in the Earth's magnetotail

Moments of plasma distributions observed in the magnetotail vary with different time scales. In this paper we attempt to explain the observed variability on intermediate timescales of ∼10–20 min that result from the simultaneous energization and spatial structuring of solar wind plasma in the distant magnetotail. These processes stimulate the formation of a system of spatially disjointed, highly accelerated filaments (beamlets) in the tail. We use the results from large-scale kinetic modeling of magnetotail formation from a plasma mantle source to calculate moments of ion distribution functions throughout the tail. Statistical restrictions related to the limited number of particles in our system naturally reduce the spatial resolution of our results, but we show that our model is valid on intermediate spatial scales Δx × ΔZ ∼ 1 RE × 1000 km. For these spatial scales the resulting pattern, which resembles a mosaic, appears to be quite variable. The complexity of the pattern is related to the spatial interference between beamlets accelerated at various locations within the distant tail which mirror in the strong near-Earth magnetic field. Global motion of the magnetotail results in the displacement of spacecraft with respect to this mosaic pattern and can produce variations in all of the moments (especially the x-component of the bulk velocity) on intermediate timescales. The results obtained enable us to view the magnetotail plasma as consisting of two different populations: a tail ward-Earthward system of highly accelerated beamlets interfering with each other, and an energized quasithermal population which gradually builds as the Earth is approached. In the near-Earth tail, these populations merge into a hot quasi-isotropic ion population typical of the near-Earth plasma sheet. The transformation of plasma sheet boundary layer (PSBL) beam energy into central plasma sheet (CPS) quasi-thermal energy occurs in the absence of collisions or noise. This paper also clarifies the relationship between the global scale where an MHD description might be appropriate and the lower intermediate scales where MHD fails and large-scale kinetic theory should be used.

Population of the near‐Earth magnetotail from the auroral zone

This paper reports the development and performance of a large scale kinetic simulation using a three-dimensional model of the terrestrial electric and magnetic fields in an effort to reach a better understanding of the ionospheric contribution to the near-Earth (x < 10 R E ) region during quiet and slightly disturbed times. The simulation employed the Tsyganenko [1989] magnetic field model and an electric field derived from the Heppner and Maynard [1987] ionospheric potentials. For the conditions considered in this study (southward interplanetary magnetic field (IMF), φ XT = 20 - 40 kV), it was found that the cleft ion fountain plays a relatively minor role in supplying particles to the near-Earth region. The ionospheric contribution to the near-Earth proton population is significant during quiet times with the bulk of the O + ions in the near-Earth region coming from the auroral zone upwelling region. However, the plasma mantle becomes the dominant hot proton source during more active times. Using the nightside auroral zone as a source, we launched distributions of H + , He + , and O + ions and calculated densities, pressures, and other bulk parameters in the near-Earth plasma sheet and partial ring current. Because of the static nature of the model, ionospheric ions had very limited access to the trapped ring current, but the ions formed a reservoir of energetic particles just outside this region that in theory could act as a source for the ring current during more active times. The residence time of ions in the model is too short for charge exchange losses to become significant, and the principal loss mechanism is through the dusk flank of the magnetopause, with precipitation into the ionosphere playing a minor role.

Localized reconnection and substorm onset on Dec. 22, 1996

This study uses observations from the Wind, Geotail and Interball spacecraft together with global MHD simulation to investigate the onset of a substorm on Dec. 22, 1996. At 1240 UT, during the growth phase, a small localized neutral line formed in the dawn sector at x ∼ −10 RE and initially extended less than 3 RE in azimuth. The formation of this neutral line was associated with the total Poynting flux being focused in the region close to the neutral line. During the growth phase the neutral line expanded in azimuth and moved tailward. At the onset of the expansion phase lobe field lines began to reconnect, and a second small, localized neutral line formed in the dusk magnetotail at ∼1256 UT. Lobe reconnection at this neutral line corresponded to a second intensification of the substorm at ∼1316 UT.

Modeling Magnetospheric Sources

We have used global magnetohydrodynamic, simulations of the interaction between the solar wind and magnetosphere together with single particle trajectory calculations to investigate the sources of plasma entering the magnetosphere. In all of our calculations solar wind plasma primarily enters the magnetosphere when the field line on which it is convecting reconnects. When the interplanetary magnetic field has a northward component the reconnection is in the polar cusp region. In the simulations plasma in the low latitude boundary layer (LLBL) can be on either open or closed field lines. Open field lines occur when the high latitude reconnection occurs in only one cusp. In the MHD calculations the ionosphere does not contribute significantly to the LLBL for northward IMF. The particle trajectory calculations show that ions preferentially enter in the cusp region where they can be accelerated by non-adiabatic motion across the high latitude electric field. For southward IMF in the MHD simulations the plasma in the middle and inner magnetosphere comes from the inner (ionospheric) boundary of the simulation. Solar wind plasma on open field lines is confined to high latitudes and exits the tailward boundary of the simulation without reaching the plasma sheet. The LLBL is populated by both ionospheric and solar wind plasma. When the particle trajectories are included solar wind ions can enter the middle magnetosphere. We have used both the MHD simulations and the particle calculations to estimate source rates for the magnetosphere which are consistent with those inferred from observations.

Source Distributions of Substorm Ions Observed in the Near-Earth Magnetotail

This study employs Geotail plasma observations and numerical modeling to determine sources of the ions observed in the near-Earth magnetotail near midnight during a substorm. The growth phase has the low-latitude boundary layer as its most important source of ions at Geotail, but during the expansion phase the plasma mantle is dominant. The mantle distribution shows evidence of two distinct entry mechanisms: entry through a high-latitude reconnection region resulting in an accelerated component, and entry through open field lines traditionally identified with the mantle source. The two entry mechanisms are separated in time, with the high-latitude reconnection region disappearing prior to substorm onset.