A statistical study of plasmawaves and energetic particles in the outer magnetosphere

The Earth magnetosphere contains energetic particles undergoing specific motions around Earth’s magnetic field, and interacting with a variety of waves. The dynamics of energetic particles are often described in terms of three kinds of adiabatic invariants. Energetic electrons are often unstable to the whistler-mode chorus waves, and ions, to the electromagnetic ion cyclotron (EMIC) instability. These waves play an important role in the dynamics of the magnetosphere by energizing electrons to form a radiation belt, extracting energy from the hot, anisotropic ions and causing pitch angle scattering of energetic ions and relativistic electrons into the loss cone. EMIC waves correspond to the highest frequency waves in the ultra-low frequency (ULF) spectral regime, and field line resonances at the lower frequency may serve as diagnostics for the plasma distribution in the magnetosphere. This dissertation investigates (1) a rapid, efficient way of specifying particle’s adiabatic motion in the magnetosphere, (2) source of the whistler-mode chorus waves, (3) physical properties and coherent spatial dimensions of the EMIC waves and (4) a diagnostic use of the toroidal mode Alfvén waves on the plasma density distribution in the Earth magnetosphere. The studies presented in this dissertation have significantly been benefited from the comprehensive data obtained by several space missions, including the Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft, Cluster mission, the Geostationary Operational Environment Satellites (GOES), Los Alamos National Laboratory (LANL) satellites, the Polar spacecraft and the Active Magnetospheric Particle Tracer Explorers (AMPTE)/Charge Composition Explorer (CCE), and from ground-based Automatic Geophysical Observatories (AGO). The main findings and achievements in this dissertation are as follows: (1) A method of rapidly and efficiently computing the magnetic drift invariant (L*) was developed. This new method is not only fast enough for near real-time calculation of L*, enabling spacecraft tracking in this coordinates, but scalable to a large number of L* values that are often required for inter-comparison between simulation results and observations. (2) The relationship between the electron injection and the chorus waves was studied from the simultaneous observations of a substorm event on 23 March 2007 made in space and on ground. Timing analysis and a test particle simulation indicated that the electrons injected during the substorm could form a pitch-angle distribution suitable for the whistler-mode instability when they arrive near the dawn-side magnetopause. (3) The EMIC waves are found to occur ubiquitously throughout the outer magnetosphere and their properties distribute asymmetrically in local time. The asymmetry in the wave properties seems to be correlated with the electron density distribution and ion temperature anisotropy, as supported by a linear EMIC instability model. (4) The size of coherent activity of the EMIC waves was estimated using the multi-spacecraft observations made by the THEMIS spacecraft and cross correlation analysis. It is found that the characteristic dimension in the direction transverse to the local magnetic field is 2–3 times the local EMIC wavelength. (5) The global distribution of the equatorial mass density was derived from the toroidal mode standing Alfvén waves in an unprecedented spatial scale. The equatorial mass density is distributed asymmetrically with a bulge at the dusk sector and the magnitude falls logarithmically with increasing radial distance. It is confirmed that the variation in the derived mass density is only weakly related to the geomagnetic activity, but has strong correlation with the solar activity. The major contribution of this dissertation is the extension of the scope of previous understanding of various plasma wave properties and energetic particle dynamics in the inner magnetosphere to outer magnetosphere by new, in-depth analyses of the data from the THEMIS, GOES and AMPTE/CCE missions

Linear Instability and Saturation Characteristics of Magnetosonic Waves along the Magnetic Field Line

Equatorial noise, also known magnetosonic waves (MSWs), are one of the frequently observed plasma waves in Earth’s inner magnetosphere. Observations have shown that wave amplitudes maximize at the magnetic equator with a narrow extent in their latitudinal distribution. It has been understood that waves are generated from an equatorial source region and confined within a few degrees magnetic latitude. The present study investigates whether the MSW instability and saturation amplitudes maximize at the equator, given an energetic proton ring-like distribution derived from an observed wave event, and using linear instability analysis and particle-in-cell simulations with the plasma conditions at different latitudes along the dipole magnetic field line. The results show that waves initially grow fastest (i.e., with the largest growth rate) at high latitude (20°–25°), but consistent with observations, their saturation amplitudes maximize within ±10° latitude. On the other hand, the slope of the saturation amplitudes versus latitude revealed in the present study is not as steep as what the previous statistical observation results suggest. This may be indicative of some other factors not considered in the present analyses at play, such as background magnetic field and plasma inhomogeneities and the propagation effect

Field line distribution of mass density at geostationary orbit

The distribution of mass density along the field lines affects the ratios of toroidal (azimuthally oscillating) Alfv\u27{e}n frequencies, and given the ratios of these frequencies we can get information about that distribution. Here we assume the commonly used power law form for the field line distribution, rho_{m} = rho_{m,eq} ( L R_{E} /R )^alpha, where rho_{m,eq} is the value of the mass density rho_{m} at the magnetic equator, L is the L shell, R_{E} is the Earth\u27s radius, R is the geocentric distance to a point on the field line, and alpha is the power law coefficient. Positive values of alpha indicate that rho_{m} increases away from the magnetic equator, zero value indicates that rho_{m} is constant along the magnetic field line, and negative alpha indicates that there is a local peak in rho_{m} at the magnetic equator. Using 12 years of observations of toroidal Alfven frequencies by the Geostationary Operational Environmental Satellites (GOES), we study the typical dependence of inferred values of alpha on the magnetic local time (MLT), the phase of the solar cycle as specified by the F10.7 extreme ultraviolet solar flux, and geomagnetic activity as specified by the auroral electrojet (AE) index. Over the mostly dayside range of the observations, we find that alpha decreases with respect to increasing MLT and F10.7, but increases with respect to increasing AE. We develop a formula that depends on all three parameters, alpha_{3Dmodel} = 2.2 + 1.3 cos(MLT 15 degrees) + 0.0026 {AE} cos((MLT-0.8) 15 degrees) + 2.1 10^{-5} {AE} {F10.7} - 0.010 {F10.7},\r\nthat models the binned values of alpha within a standard deviation of 0.3. While we do not yet have a complete theoretical understanding of why alpha should depend on these parameters in such a way, we do make some observations and speculations about the causes. At least part of the dependence is related to that of rho_{m,eq}; higher alpha, corresponding to steeper variation with respect to MLAT, occurs when rho_{m,eq} is lower

Van Allen Probes Observations of Second Harmonic Poloidal Standing Alfvén Waves

Long-lasting second-harmonic poloidal standing Alfvén waves (P2 waves) were observed by the twin Van Allen Probes (Radiation Belt Storm Probes, or RBSP) spacecraft in the noon sector of the plasmasphere, when the spacecraft were close to the magnetic equator and had a small azimuthal separation. Oscillations of proton fluxes at the wave frequency (∼10 mHz) were also observed in the energy (W) range 50–300 keV. Using the unique RBSP orbital configuration, we determined the phase delay of magnetic field perturbations between the spacecraft with a 2nπ ambiguity. We then used finite gyroradius effects seen in the proton flux oscillations to remove the ambiguity and found that the waves were propagating westward with an azimuthal wave number (m) of ∼−200. The phase of the proton flux oscillations relative to the radial component of the wave magnetic field progresses with W, crossing 0 (northward moving protons) or 180° (southward moving protons) at W ∼ 120 keV. This feature is explained by drift-bounce resonance (mωd ∼ ωb) of ∼120 keV protons with the waves, where ωd and ωb are the proton drift and bounce frequencies. At lower energies, the proton phase space density ( ) exhibits a bump-on-tail structure with occurring in the 1–10 keV energy range. This is unstable and can excite P2 waves through bounce resonance (ω ∼ ωb), where ω is the wave frequency

Study of EMIC wave excitation using direct ion measurements

With data from Van Allen Probes, we investigate electromagnetic ion cyclotron (EMIC) wave excitation using simultaneously observed ion distributions. Strong He band waves occurred while the spacecraft was moving through an enhanced density region. We extract from helium, oxygen, proton, and electron mass spectrometer measurement the velocity distributions of warm heavy ions as well as anisotropic energetic protons that drive wave growth through the ion cyclotron instability. Fitting the measured ion fluxes to multiple sinm-type distribution functions, we find that the observed ions make up about 15% of the total ions, but about 85% of them are still missing. By making legitimate estimates of the unseen cold (below ∼2 eV) ion composition from cutoff frequencies suggested by the observed wave spectrum, a series of linear instability analyses and hybrid simulations are carried out. The simulated waves generally vary as predicted by linear theory. They are more sensitive to the cold O+ concentration than the cold He+ concentration. Increasing the cold O+ concentration weakens the He band waves but enhances the O band waves. Finally, the exact cold ion composition is suggested to be in a range when the simulated wave spectrum best matches the observed one

Dataset for "Equatorial evolution of the fast magnetosonic mode in the source region: Observation-simulation comparison of the preferential propagation direction"

<p>Includes 2D PIC simulation results presented in the paper.</p