T. E. Sarris

Onset of nonadiabatic particle motion in the near‐Earth magnetotail

The onset of nonadiabatic proton motion is studied using direct integration of the Lorentz force equation of motion in the T89c magnetic field model with no electric field. Irreversible changes in the magnetic moment μ occur on traversals of the equator and give the gyrophase dependence predicted by Birmingham [1984]. Birminghams expression δB and the semiemipirical centrifugal impulse model of Delcourt et al. [1996] δCIM2 nave linear regression coefficients with Δμ/μ of 0.99 and 0.95, respectively, for Δμ/μ ≤ 1. By contrast, e = 1/κ2, where κ is the kappa parameter, has a linear regression coefficient with Δμ/μ of only 0.5. To reliably estimate the onset of nonadiabatic behavior, one must therefore use δB or δCIM2 rather than κ. Using isocontours of constant δB we map the regions of nonadiabatic ion motion. For a given energy the transition to nonadiabatic motion occurs over a radial distance of ∼2 RE on the nightside and is closest to the Earth at midnight. At midnight the nonadiabatic regime for protons extends inward to ∼11 RE (∼7.5 RE) for 1 keV and to ∼6 RE (∼4.5 RE) for 1 MeV with the Kp = 0 (Kp = 6) model. For O+ the nonadiabatic regime is 1.5 to 2 RE closer to the Earth than for protons. Drift trajectory calculations and analytical estimates show that particles drifting through regions with δB > 0.01 suffer net Δμ ∼ μ. The net Δμ is extremely sensitive to initial gyrophase and it is shown that for δB 50 keV, whose adiabatic drift paths are closed in the presence of a convection electric field. The implications of nonadiabatic effects for ring current modeling based on Liouvilles theorem apply equally well in the zero and finite electric field cases.

Evolution of the dispersionless injection boundary associated with substorms

One manifestation of energetic particle accelera- tion during magnetospheric substorms is the sudden appear- ance of particle injections into the inner magnetosphere, of- ten observed near geosynchronous orbit. Injections that show simultaneous flux increases in all energy ranges of a detector are called dispersionless injections, and are most often ob- served in a narrow region around local midnight. In these events it is assumed that the satellite is located close to or in- side the region where acceleration and/or transport processes are taking place, called the injection region. We present a study of the location, extent and temporal evolution of the injection region, based on simulation results of a model of the expansion of the electric and magnetic fields associated with a substorm. The model simulates the fields during a substorm onset with an electric field and consistent magnetic field pulse that propagates towards the Earth with a decreas- ing speed. Our simulation shows that the dispersionless in- jection boundary can be considered coincident with the lead- ing edge of the pulse field, which transports particles toward the Earth across a certain range of local time. Under the same model field, the dispersionless injection boundary shifts east- ward for electrons and westward for protons, consistent with the observation results deduced from statistical analysis of multiple spacecraft measurements.

Ion dynamics and tail current intensification prior to dipolarization: The June 1, 1985, event

The present study investigates the tail current intensification prior to dipolarization, focusing on the dynamics of energetic ions. The event selected for this study, which was initially reported by Lui et al. [1988], occurred on June 1, 1985. The uniqueness of this event is that the AMPTE/CCE spacecraft remained close to the neutral sheet until the local magnetic field was dipolarized. Energetic (30-200 keV) ion fluxes were larger when the telescope was looking dawnward than when it was looking in the opposite direction. Until several tens of seconds before the (local) onset of dipolarization, this dawn-dusk flux anisotropy was more manifest for higher-energy ions, suggesting that the anisotropy was caused by the radial gradient of the energetic particle density. That is, the tail current was driven by radial pressure gradient. However, just prior to the onset, the flux level and the dawn-dusk anisotropy of the lowest-energy (31-43 keV) channel increased significantly, causing a further intensification of the tail current. The feature was less obvious in higher-energy channels. This energy dependence is better explained in terms of ion motion across the magnetic field. The velocity of this motion is a function of the ion energy and is estimated at 4% of the velocity corresponding to a given energy. It is likely that the associated enhancement of the tail current density was comparable to the enhancement during the entire growth phase. A positive feedback process between the unmagnetization of ions and the thinning of the current sheet is proposed for explaining this explosive intensification of the tail current. It is also found that electron pressure anisotropy did not contribute to the tail current intensification in the present event. The present result strongly suggests that the ion kinetics is important in the tail current intensification prior to the local onset of tail current disruption.

Observations and analysis of Alfvén wave phase mixing in the Earth's magnetosphere

(1) Signatures of Alfven wave phase mixing in the Earths magnetosphere, observed as polarization rotation of a transverse, Pc5 magnetospheric pulsation, are presented and compared to theory. The polarization rotation occurred during a rare event of a dayside narrowband ULF magnetospheric pulsation that lasted for 5 consecutive days, from 24 to 30 November 1997; details of this event were reported by Sarris et al. (2009) through observations at geosynchronous orbit and on the ground. In this paper we investigate the polarization signatures of the pulsation by performing a detailed analysis of its transverse components as observed through hodogram plots. Density measurements from one of the Los Alamos National Laboratory (LANL) spacecraft which had its L shells closest to GOES-8 are used to calculate field line resonance frequencies at geosynchronous orbit; these frequency calculations show good agreement with the observed pulsations but also have a local time offset. For an instance of an observed polarization rotation we estimate the observed poloidal lifetime of the pulsation by the time taken for the poloidal and toroidal amplitudes to become equal, which we compare with the theoretical approximation to the poloidal lifetime, as calculated in a box model magnetosphere by Mann and Wright (1995). Density measurements from different LANL spacecraft at geosynchronous orbit and their varying L shells as derived from their varying local times are used to estimate a local gradient in the local Alfven speed, which is then used in the calculation of the predicted poloidal lifetime. This is the first time that such polarization rotations are directly observed and compared with theoretical predictions.

On the relationship between electron flux oscillations and ULF wave‐driven radial transport

The objective of this study is to investigate the relationship between the levels of electron flux oscillations and radial diffusion for different Phase Space Density (PSD) gradients, through observation and particle tracing simulations under the effect of model Ultra Low Frequency (ULF) fluctuations. This investigation aims to demonstrate that electron flux oscillation is associated with and could be used as an indicator of ongoing radial diffusion. To this direction, flux oscillations are observed through the Van Allen Probes’ MagEIS energetic particle detector; subsequently, flux oscillations are produced in a particle tracing model that simulates radial diffusion by using model magnetic and electric field fluctuations that are approximating measured magnetic and electric field fluctuations as recorded by the Van Allen Probes’ EMFISIS and EFW instruments, respectively. The flux oscillation amplitudes are then correlated with Phase Space Density gradients in the magnetosphere and with the ongoing radial diffusion process.

On the Initial Enhancement of Energetic Electrons and the Innermost Plasmapause Locations: CME‐Driven Storm Periods

Using Van Allen Probes’ observations and established plasmapause location (Lpp) models, we investigate the relationship between the location of the initial enhancement (IE) of energetic electrons and the innermost (among all magnetic local time sectors) Lpp over five intense storm periods. Our study reveals that the IE events for ~30-keV to ~2-MeV electrons always occurred outside of the innermost Lpp. On average, the inner extent of the IE events (LIE) for <800-keV electrons was closer to the innermost Lpp when compared to the LIE for >800-keV electrons that was found consistently at ~1.5 RE outside of the innermost Lpp. The IE of tens of kiloelectron volts electrons was observed before the IE of hundreds of kiloelectron volt electrons, and the IE of >800-keV electrons was observed on average 12.6 ± 2.3 hr after the occurrence of the earliest IE event. In addition, we report an overall electron (~30 keV to ~2 MeV) flux increase outside the plasmasphere during the selected storm periods, in contrast to the little change of energy spectrum evolution inside the plasmasphere; this demonstrates the important role of the plasmasphere in shaping energetic electron dynamics. Our investigation of the LIE-Lpp relationship also provides insights into the underlying physical processes responsible for the dynamics of ~30-keV to ~2-MeV electrons.

Determining the Location of the Dispersionless Injection Boundary During Substorms

Particle acceleration in the inner magnetosphere during magnetospheric substorms is usually manifested with the appearance of particle injections, i.e. with sudden enhancements in the fluxes of energetic electrons and ions. Injections that show simultaneous flux increases in all energy ranges of a detector are called “dispersionless injections”; whether particle injections will appear to be dispersionless depends on the location of the spacecraft relative to the injection region. How particles of different energies can be transported to inside geosynchronous orbit without dispersion and where these particles come from are long standing issues in magnetospheric physics. We present a modeling study of the extent and temporal evolution of the region where particle acceleration and Dispersionless injections occur. The study is based on simulation results from a model of the expansion of the electric and magnetic fields associated with a substorm. The model that we use is able to reproduce simultaneous particl...

Simulating the Effects of ULF Waves on Energetic Electron Populations

A model of magnetic and electric perturbations in the ULF regime is used to simulate magnetospheric variability. The model is based on the assumption that compressional oscillations propagate from the magnetopause into the inner magnetosphere, and are reflected at an inner boundary. Electrons are traced under the effect of the modeled ULF fluctuations and the radial diffusion coefficient is calculated through the electrons’ radial displacement. The numerically calculated diffusion coefficient is compared to theoretical estimates, and is found to show a similar L‐dependence.