Whistler Mode Waves In Magnetosphere Research Articles

Whistler Mode Quasiperiodic Emissions: Contrasting Van Allen Probes and DEMETER Occurrence Rates

AbstractQuasiperiodic emissions are magnetospheric whistler mode waves at frequencies between about 0.5 and 4 kHz which exhibit a nearly periodic time modulation of the wave intensity. We use large data sets of events observed by the Van Allen Probes in the equatorial region at larger radial distances and by the low‐altitude DEMETER spacecraft. While Van Allen Probes observe the events at all local times and longitudes, DEMETER observations are limited nearly exclusively to the daytime and significantly less frequent at the longitudes of the South Atlantic Anomaly. Further, while the events observed by Van Allen Probes are smoothly distributed over seasons with only mild maxima in spring/autumn, DEMETER occurrence rate has a single pronounced minimum in July. The apparent inconsistency is explained by considering a nondipolar Earth's magnetic field and significant background wave intensities which in these cases prevent the quasiperiodic events from being identified in DEMETER data.

Using Eccentricity to Locate Ionospheric Exit Points of Magnetospheric Whistler Mode Waves

Whistler mode waves are a type of electromagnetic plasma wave and play a dominant role in the energy dynamics of the near-earth space environment and associated space weather processes. Ground-based observations of these waves are important for numerical model validation, space weather monitoring, and discovery science. Unfortunately, ground-based observations are often challenging to interpret since information about traversal through the ionosphere and distance propagated in the earth–ionosphere waveguide is rarely available. A new approach to identifying the ionospheric exit points of the magnetospheric whistler mode emissions is presented, which takes into account the observed wave polarization. The method relies on the initial circular polarization of the ducted magnetospheric emissions which is shown to convert to linear polarization after propagation in the earth–ionosphere waveguide at a predictable rate. Finite-difference time domain modeling of observed eccentricity provides a metric for determining distance from observer to ionospheric exit point. The method is shown to produce good agreement with observations of magnetospheric chorus emissions, artificially triggered emissions, and lightning-induced whistlers.

Self‐consistent formation of a 0.5 cyclotron frequency gap in magnetospheric whistler mode waves

AbstractDecades of in situ observations of whistler mode waves in Earth's magnetosphere reveal that there is frequently a gap in the spectral power at around half the local electron gyrofrequency. Recent theoretical and kinetic simulation studies have suggested that the gap arises due to the presence of temperature anisotropy in both a “warm” and a “hot” electron population, leading to two separate (and independent) regions of wave growth in frequency space. We present two‐dimensional kinetic plasma simulations using the powerful EPOCH (Extendable PIC Open Collaboration) particle‐in‐cell code that offer an alternative explanation. After an initial linear‐growth period, our simulations show self‐consistent formation of a gap feature. In most cases this arises where linear theory predicts the maximum growth rate and is associated with subtle local structuring of the hot electron distributions. This feature persists in multiple simulations with varying hot electron parameters. We discuss these results in the context of in situ observations of both waves and electron distribution functions and argue that the rapid reorganization of electron distributions in a small, but key, region of phase space during the growth of whistler mode waves naturally results in the spectral gap often observed at half the electron gyrofrequency.

Study of whistler mode waves for ring distribution function in Saturn’s magnetosphere

In present paper whistler mode waves in magnetosphere of Saturn have been investigated. Linear properties of ring velocity distribution function are used to derive dispersion relation for whistler mode waves in ambient magnetic field of Saturn in extended plasma sheet of planet’s magnetosphere. The derivation for perturbed distribution function, dispersion relation and growth rate have been determined by using the method of characteristic and kinetic approach. Analytical expressions for growth rate and real frequency of whistlers propagating parallel to magnetic field direction are attained. The analysis shows that temperature anisotropy, increases in number density and energy density increases the growth rate of whistler waves along with significant shift in wave number. It has been shown that whistler mode waves have grown due to loss of perpendicular kinetic energy of ring electrons. Calculations have been performed at two radial distances in Saturn’s magnetosphere. The results are of importance in analyzing observed VLF emissions over wide spectrum of frequency range in Saturnian magnetosphere. The analytical model developed can also be used to study various types of instabilities in planetary magnetospheres.

Whistler–electron interactions in the magnetosphere: new results and novel approaches

Attention is focused here on the quasilinear and nonlinear physics of cyclotron interactions between magnetospheric whistler mode waves and energetic electrons on dipolar geomagnetic flux tubes. These interactions can lead to the generation of noise-like emissions or phase-coherent discrete signals in the frequency-time domain. In the magnetosphere noise-like emissions called hiss are accompanied by a smooth electron precipitation pattern. Examples of discrete emissions are ELF/VLF chorus or VLF emissions triggered by whistlers from lightning or by radio transmitters on the ground. The rapid temporal variations of these signals are associated with fine structure of the distribution function of the radiation belt electrons, such as a transient step-like deformation or a well-organized beam, which are prepared by initial noise-like emissions or by a quasimonochromatic whistler–wave packet, respectively. These cause the properties of the electrons, which may be observed on a satellite, to evolve rapidly in time and on relatively short spatial scales. Bursts of precipitating electrons occur, and can contribute significantly to depleting the radiation belts. Recent results on improvements in the theoretical understanding of such processes and on new observations of magnetospheric electrons and whistler-mode waves are presented.

Quasilongitudinal approximation for whistler-mode waves in the magnetospheric plasma

The range of applicability of an improved quasilongitudinal approximation for whistler-mode waves in the equatorial magnetosphere (4 ≲ L ≲ 6.6) is specified based on the direct comparison between numerical solutions of the hot electromagnetic dispersion equation with the corresponding analytical quasilongitudinal solutions. It is pointed out that this approximation can be used at frequencies ω less than but not close to the electron gyrofrequency Ω (ω ≲ 0.6 Ω) and wave normal angles θ less than but not close to the resonance cone angle θ R. At ω = 0.8 Ω the analytical results deviate considerably from numerical ones due to the strong damping of the waves, and so the quasilongitudinal solution becomes no longer valid.