Exploring Sources of Magnetospheric Plasma Using BATS-R-US
After decades of research, there is still no true consensus concerning the source or entry mechanism for plasma sheet and ring current particles in the magnetosphere. This work addresses the long standing problem by examining the source of magnetospheric plasma predicted by the multispecies version of the BATS-R-US MHD code. Two Hydrogen species, one ionospheric in origin and one solar wind in origin, are followed as they progress through the system during idealized simulations of various solar wind drivers. It is found that the magnetosphere has two modes of convection and plasma entry that depend on the solar wind conditions. During southward IMF, convection is reconnection-driven and the dominant source of plasma is ionospheric particles entering near the tail reconnection point. During northward IMF conditions, viscous interactions along the flanks of the magnetosphere drive convection and deliver large amounts of solar wind plasma to the system. These results are examined further by performing real-event simulations and making data-model comparisons using LANL Geosynchronous MPA particle data.
Spectral Characteristics of Ions in the Earth's Plasmasheet: Statistical Results from Cluster CIS and RAPID
We present a statistical study of the spectral characteristics of protons in the Earth's plasma sheet for various geomagnetic disturbance levels. The study is based on more than 2000 hours of data combined from the Cluster RAPID and CIS instruments obtained during the tail season (July-October). Whereas the lower energy range considered is typically well reproduced by Maxwellian energy spectra, the higher energies often deviate significantly from such a distribution, and is better represented by a κ distribution. The actual spectral slope depends on various magnetospheric driver parameters. During disturbed conditions, a significant hardening of the spectra is observed. Periods with high solar wind pressure also characterized by a much harder spectra. Unlike the electron spectra, we do not see any local time dependence in the proton spectra.
Multi-satellite Investigations of the Cusps Under Various Pitch Angle Sorting Algorithms
The magnetospheric cusps are important conduits between the magnetosheath and ionospheric plasma populations, however, their dynamic nature and large spatial size make them difficult regions of study. Previous work by Keith and Stubbs [Adv. Space Res., Vol. 41, No. 10, 2008] to identify conjunctions in the cusps between the Cluster and DMSP satellite missions have resulted in the two data sets currently under investigation. The limited field of view of the SSJ/4 instruments on the DMSP spacecraft (F-6 through F-15) means that the low-altitude data is always of downgoing (precipitating) particles. In contrast, the 3D nature of the particle data sets of Cluster, together with their much higher altitude, require accurate pitch angle sorting of the particle spectra as an important aspect of the data analysis. Multiple pitch angle sorting algorithms, both those including bulk velocity and those ignoring it, will be subjected to similar analysis, noting the qualitative and quantitative differences and the effects it may have on the conclusions that may be reached. This should help determine the sensitivity of these data to the sophistication of the sorting method used and the robustness of the analysis when being compared to low-altitude data.
Particle Energization on Flux Tubes Threading the Auroral Ionososphere - a Proposed Polar Orbiting Satellite Mission
Observations from the NASA POLAR and FAST missions reveal that Alfven waves are intimately associated with electron and ion particle acceleration in Earth's magnetosphere. Data from POLAR shows intense geomagnetic field-aligned wave Poynting flux near and within the plasma sheet tail lobe boundary. The corresponding UVI auroral imaging provides strong, but nevertheless circumstantial evidence that the associated electron acceleration powers auroral emissions above the ionosphere. To explain the data, we present results of modeling that agree with observations of Alfven wave activity, and describe the characteristics of the resulting wave-particle interactions along the entire extent of a magnetic flux tube. Model results using the Vlasov-Maxwell equations reveal detailed characteristics of the electron distribution functions observed by POLAR, while two-fluid modeling of the resulting ion dynamics is shown to agree with observations of up-flowing ion beams. We argue that missions such as FAST and POLAR reveal the need for more detailed observations at high altitude, on the order of 4-5Re. Such an opportunity is presented by the decision by Canada to launch two Polar Communications Weather (PCW) satellites that sample the relevant region of Geospace within which acceleration by Alfven waves is optimal. We discuss how a well-instrumented PCW mission would be used to demonstrate closure on the nature of Alfven wave-induced particle acceleration.
Effects of Heavy Ions on Magnetopause Transport
Recent ion composition measurements near the magnetopause have shown that heavy ionospheric ions can dominate the mass density as much as 30 percent of the time. Magnetopause transport processes, such as reconnection, Kelvin-Helmholtz instability, and kinetic-scale Alfvenic fluctuations, can all be significantly affected by the presence of heavy ions. Heavy ions modify the onset and growth of the tearing mode as well as reduce the steady state reconnection rate by lowering the Alfven speed. Increased mass density reduces the effect of magnetic tension and therefore lowers the Kelvin-Helmholtz instability threshold and increases the growth rate. The presence of heavy ions can also increase the efficiency of mode conversion of compressional Pc3 waves to transverse, field-aligned Alfven modes with small-scale structure perpendicular the the magnetopause. We show that nonlinear heating and transport associated with mode converted waves will preferentially affect the heavy ions. Because heavy ions can significantly influence physical processes at the magnetopause associated with mass, momentum, and energy transport; we discuss how they could be used as a tool to probe those physical processes responsible for the transport.
Heavy Ion Effects on the Linear Polarized Pc 1-2 Waves
Field-aligned eigenmodes can be excited by mode conversion of compressional waves propagating across magnetospheric field lines. For low frequencies, the mode conversion occurs at the Alfven resonance, but mode conversion may also occur for higher frequencies at the ion-ion hybrid (IIH) resonance in multi ion plasmas. Because the mode-converted waves at the IIH resonance are electromagnetic and have linear polarization, it has been suggested as a mechanism to explain linearly polarized Pc 1-2 waves at Earth magnetosphere. These waves propagate along the magnetic field and set up field-aligned eigenmodes localized between the Buchsbaum resonances. The mode converted waves can also excite global Alfven resonances that stand between the ionospheres. The aim of this study is to determine the dependence of the eigenmodes of the mode-converted wave at the IIH resonance on the concentration of heavy ions in Earth magnetosphere. To achieve this goal, we examine the eigenfrequencies of IIH resonances using the WKB method. For this study, we use an axisymmetric dipole magnetic field model and empirical electron density model between 3 and 9 Earth radii L shell. For different heavy ion concentration ratio, the eigenfrequencies and their maximum latitudinal extent are presented. The results show that the linear polarized waves are localized near the magnetic equator (less than 10 degree) and the frequency increases as the heavy ion concentration ratio increases. We also compare the results with satellite observations of Pc 1-2 waves.
Survey of Magnetic Pulsation at Geosynchronous Orbits
Geomagnetic pulsations are considered one of the modes of energy and momentum transfer between the solar wind and magnetosphere, and different plasma regions within the magnetosphere. These ultra low frequency (ULF) electromagnetic waves are usually identified using ground based magnetometer measurements and are categorized based on frequency. In this study we have used eleven years of GOES satellite geosynchronous magnetic field measurements to identify and characterize 1214 magnetic pulsation events in the Pc4 and Pc5 bands. Local time of occurrence and frequency were recorded for each pulsation, the latter using time-frequency spectral analysis. The local time distribution for pulsation occurrence showed a broad dayside peak and the seasonal distribution indicated high pulsation occurrence in the winter months compared to summer months. Pulsations also occurred over a wide range of frequencies, however discrete frequency bands were observed in some individual events. These observations and their implications will be discussed.
THEMIS observations of magnetospheric ELF emissions, ULF Pc5 waves and their auroral features
Narrow-banded ELF/VLF emissions at about (n+1/2)fce, where fce is the electron gyrofrequency, are commonly observed in the inner magnetosphere. They are generally believed to originate from an electron-cyclotron harmonic instability due to an interaction between a "cold" (<100 eV) electron plasma and a "hot" (>1 keV) electron population. The hot component provides the local source of free energy for the instability growth, while the cold electrons facilitate the propagation of the instability and control its spatial amplification. Such ELF/VLF emissions are widely considered as a main candidate of driving the pitch-angle scattering and in turn the precipitation of 1-10 keV electrons. In this presentation we report two events of strong ELF emissions near 3/2fce band at L∼10 in the postmidnight equatorial plasma sheet from recent THEMIS observations. Both events occurred during the late substorm expansion phase and/or recovery phase, and the probes were located at a couple of MLT hours east of the onset meridian determined from the ground auroral observations. In one event the 3/2 fce emissions appeared in forms of discrete bursts modulated by a large-amplitude ULF Pc5 waves with period 5-10 minutes. The Pc5 waves are identified as an eastward-propagating poloidal/compressional waves, and non-FLR in nature. In the other event the 3/2 fce emissions appeared as a somewhat continuous band lasting a few tens of minutes. On a inspection of the electron distribution functions for both events we found that during the event intervals there was a coexistence of cold electron population(∼10 eV), ambient plasma sheet electrons (1-2 keV), and enhanced energetic electrons (>10 keV) presumably drifted from the injection region of the substorm, consistent with the theoretical expectations. Conjugate auroral observations from THEMIS all-sky imagers, CGSM multi-spectral imagers, CGSM meridian scanning photometers, and several higher time resolution white light imagers, reveal distinct features such as eastward-drifting auroral patches and pulsating auroras with period of few seconds during the event intervals. We attempt to relate those drifting patches and pulsating auroras to the in-situ observed ELF emission and its resulting pitch-angle scattering process.