Robert L. Richard

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.

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.

The population of the magnetosphere by solar winds ions when the interplanetary magnetic field is northward

We have examined some possible entry mechanisms of solar wind ions into the magnetosphere by calculating the trajectories of thousands of non-interacting ions in the magnetic and electric fields from a three dimensional global magneto-hydrodynamic (MHD) simulation of the magnetosphere and the magnetosheath, under northward interplanetary magnetic field (IMF) conditions. Particles, launched in the solar wind, entered the magnetosphere and formed the low latitude boundary layer (LLBL), plasma sheet and a region of trapped particles near the Earth. The densities and temperatures we obtained in these regions were realistic, with the exception of trapped particle densities. The dominant entry mechanism was convection into the magnetosphere on reconnecting field lines.

Substorm evolution as revealed by THEMIS satellites and a global MHD simulation

[1]xa0The major substorm that occurred on 1 March 2008 had excellent spacecraft coverage by the THEMIS spacecraft in the magnetotail, GOES 11, and GOES 12 at geosynchronous orbit and Geotail in the dayside magnetosheath. A global magnetohydrodynamic simulation of this substorm, driven by Wind solar wind observations, accurately reproduced the magnetospheric observations. The simulation revealed the complexity of magnetotail dynamics during the substorm, in particular, in the near-Earth plasma sheet. Reconnection began prior to the substorm on closed field lines and a flux rope formed there. Around substorm onset, the simulation exhibited flow vortices near the locations of THEMIS P3 and P4, in agreement with observations at P3 and P4. These vortices were associated with a duskside neutral line that formed early in the substorm. Six minutes later, another neutral line formed on the dawnside of the tail. These neutral lines then merged to form a single large reconnection region that extended across the tail and greatly expanded the flux rope. The least active part of the tail was the region around midnight. Strong flows were seen in the observations and in the simulation during the two intensifications of this substorm; in particular, tailward flows were seen at THEMIS P1 and P2. Reconnection on closed field lines, vortices in the near-Earth region, a channel of strong tailward flow, and enhanced precipitation into the ionosphere all contributed to substorm development.

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.

The response of the magnetotail to changes in the IMF orientation: The magnetotail's long memory

Abstract We have used a three dimensional global magnetohydrodynamic simulation of the interaction between the solar wind and the magnetosphere to investigate the changes which occur in the magnetotail when the interplanetary magnetic field (IMF) changes orientation. For IMF B y ≠ 0 the entire magnetotail twists when the IMF changes from southward to northward. The twisting at a given location on the tail magnetopause occurs shortly after the IMF change reaches that location. Inside the magnetosphere the response time is much longer. New field lines with open ends formed by the reconnection of tail lobe field at a near-Earth neutral line also twist as the field lines convect tailward. Initially formed with a southward B z they end up with a northward B z . The response of the tail to a change in the IMF is controlled by the penetration of the solar wind electric field into the magnetosphere.

Quasi-trapped ion and electron populations at Mercury

[1]xa0Mariner 10 and MESSENGER spacecraft observations have established that Mercury has an intrinsic magnetic field and magnetosphere. Following the March 2011 insertion of MESSENGER into orbit around Mercury, measurements show that ions and electrons with typical energies of about 1–10xa0keV form an equatorially centered distribution of plasma at 1.4 RM radial distance (where RMis Mercurys radius) around a substantial portion of the planet in local time from morning through night and into the afternoon sector. Coincident with the detection of plasma around Mercury, an observed drop in the total magnetic pressure is attributable to the ion and electron thermal pressure. Additionally, intense waves near the ion cyclotron frequency were observed at the same location as the quasi-trapped particle population, which are likely a result of anisotropic distributions created by the large loss cone (>30o) at these radial distances.

Global magnetohydrodynamic simulation of reconnection and turbulence in the plasma sheet

[1]xa0Plasma sheet turbulence is examined by using a global MHD simulation. The simulation used idealized purely southward interplanetary magnetic field (IMF) driving conditions to eliminate the effect of solar wind and IMF variations. The results were compared with spacecraft observations of turbulence by computing power spectral densities and probability distribution functions. The fluctuations in the simulation were found to have properties characteristic of turbulence. The MHD simulation exhibited nested vortices on multiple scales, with the largest scales associated with reconnection outflows and the diversion of high-speed flows in the near-Earth region. The importance of strong localized reconnection regions in the simulation for driving the largest scale fluctuations supports the idea it is the main process driving turbulence in the plasma sheet. Interplay between turbulence and the reconnection process is probably present. Scaling arguments show that the scale at which turbulence is dissipated is consistent with the resistivity in the model.

On the origin of the ion-electron temperature difference in the plasma sheet

The results of a study of proton and electron acceleration in the Earths magnetotail are presented. By following the trajectories of thousands of charged particles launched from mantle and tail lobe source regions, distribution functions are calculated at different locations in a model magnetotail. The magnetic field is based on the Tsyganenko [1989] model combined with a constant cross-tail convection electric field. Despite the simplicity of the model and the lack of self-consistent fields, a qualitative picture of the proton/electron plasma sheet emerges including an ion to electron temperature ratio Ti/Te ranging from about 4 to 6 in the magnetotail, in approximate agreement with plasma sheet observations. To explain this result, an analytic expression for Ti/Te is derived based on the particle motion of ions and electrons in a current sheet configuration with a varying normal magnetic field component. The derived expression depends on the ion to electron mass ratio to the one-third power (mi/me)1/3 and a factor that takes the local field gradient into account. Using numerical values from the Tsyganenko [1989] field model in the derived equation gives Ti/Te ∼ 5. Another result is that the heated electron distribution functions formed in the plasma sheet are not Maxwellian but instead have power law high-energy tails much like the so-called “kappa” distributions reported by Christon et al. [1989]. At the edge of the plasma sheet, the calculations show the electron plasma sheet boundary layer extends further towards the lobe than the ion plasma sheet boundary layer, also in agreement with observations [Takahashi and Hones, 1988]. Nonisotropic distribution functions form at different locations in the plasma sheet, including electron beams streaming along field lines just inside the separatrix in the deep magnetotail. Electron distributions that are highly skewed in velocity space are found very near the magnetic null point. The nonisotropic distributions suggest that plasma instabilities and wave-particle interactions could occur in those regions. That such a simple model should reproduce many of the features of the observed plasma sheet indicates that adiabatic and nonadiabatic single-particle motion play important roles in the quiet time magnetotail and suggests that ion and electron plasma sheet formation is a natural consequence of single-particle motion in an X line type magnetotail geometry.