A. V. Streltsov

Whistler propagation in ionospheric density ducts: Simulations and DEMETER observations

[1]xa0On 16 October 2009, the Detection of Electromagnetic Emissions Transmitted from Earthquake Regions (DEMETER) satellite observed VLF whistler wave activity coincident with an ionospheric heating experiment conducted at HAARP. At the same time, density measurements by DEMETER indicate the presence of multiple field-aligned enhancements. Using an electron MHD model, we show that the distribution of VLF power observed by DEMETER is consistent with the propagation of whistlers from the heating region inside the observed density enhancements. We also discuss other interesting features of this event, including coupling of the lower hybrid and whistler modes, whistler trapping in artificial density ducts, and the interference of whistlers waves from two adjacent ducts.

Production of small-scale Alfvén waves by ionospheric depletion, nonlinear magnetosphere-ionosphere coupling and phase mixing

The authors acknowledge the International Space Science Institute (Switzerland) for funding the program that inspired this work. AJBR is grateful to the Royal Commission for the Exhibition of 1851 for present support and acknowledges an STFC studentship that funded part of this work.

Whistler interactions with density gradients in the magnetosphere

Naturally occurring density variations of a few to tens of percent from the background are a ubiquitous feature of the magnetospheric plasma. Very-low frequency (VLF) whistler mode waves (whistlers), which have wavelengths in the middle of this range, are particularly affected by the presence of these density variations. At sub-wavelength scales, these variations are responsible for localized enhancements of wave magnetic and/or electric fields and mode conversion to electrostatic lower hybrid waves. At scales much larger than the wavelength, these variations can be responsible for guiding whistlers along the ambient magnetic field. Recently, large-amplitude whistlers have been observed in the Earths radiation belts. Unlike the more frequently observed whistler-mode chorus, which propagates primarily parallel to the background magnetic field, these waves were found to propagate at highly oblique angles.

Propagation of whistler mode waves through the ionosphere

[1]xa0We present results from numerical studies of whistler mode wave propagation in the Earths ionosphere when artificially created plasma ducts are present. Using realistic density profiles from the SAMI2 ionospheric code, we solve the two-dimensional electron magnetohydrodynamics equations to study the trans-ionospheric propagation of artificially generated whistler waves at HAARP latitudes (L = 4.9). Both ducted and non-ducted propagation is considered, but only ducted whistlers are able to propagate without a significant reduction in wave amplitude. The conditions necessary for the trapping of waves in both high- and low-density ducts are discussed with particular attention paid to the practical accessibility of these parameter regimes.

ULF waves and discrete aurora

[1]xa0We present results from the numerical study of ULF waves generated by the ionospheric feedback instability in density cavities. The goal of the study is to explain several spectral features of ULF waves detected on the ground in close vicinity of intense discrete auroral arcs. These features include (1) localization of the waves packages across the ambient magnetic field, (2) variation of the wave frequency in relatively small amplitude waves, and (3) presence of several discrete harmonics in the spectrum of the relatively large amplitude waves. Time-dependent, two-dimensional simulations based on the two-fluid MHD model performed in the dipole, axisymmetrical geometry of the ambient magnetic field with realistic parameters of the plasma density in the ionosphere and the magnetosphere demonstrate that the ionospheric feedback instability inside the density cavity indeed provides a good, quantitative explanation of these features of ULF waves observed at high latitudes during substorm onsets.

Whistler interaction with field‐aligned density irregularities in the ionosphere: Refraction, diffraction, and interference

Field-aligned density irregularities (FAI) with kilometer-scale sizes transverse to the background magnetic field are a common feature in the ionosphere at all latitudes and local times. In this paper, we investigate the effect of these irregularities on the transionospheric propagation of very low frequency whistler waves and develop a quantitative description of FAI-related effects on whistler propagation through the lower ionosphere. Using an electron magnetohydrodynamics simulation, we provide two applications of our model. First, we show that the presence of kilometer-scale FAI in the ionosphere can reduce the power observed in the equatorial magnetosphere by more than 10 dB in some cases. Second, we demonstrate that multiple FAIs can act as a discrete lens for whistlers, providing a possible means for increasing wave power in artificial whistler ducting experiments.

Van Allen Probes observations of structured whistler mode activity and coincident electron Landau acceleration inside a remnant plasmaspheric plume

We present observations from the Van Allen Probes spacecraft that identify a region of intense whistler mode activity within a large density enhancement outside of the plasmasphere. We speculate that this density enhancement is part of a remnant plasmaspheric plume, with the observed wave being driven by a weakly anisotropic electron injection that drifted into the plume and became nonlinearly unstable to whistler emission. Particle measurements indicate that a significant fraction of thermal (<100xa0eV) electrons within the plume were subject to Landau acceleration by these waves, an effect that is naturally explained by whistler emission within a gradient and high-density ducting inside a density enhancement.

Ionospheric Alfvén resonator and aurora: Modeling of MICA observations

We present results from a numerical study of small-scale, intense magnetic field-aligned currents observed in the vicinity of the discrete auroral arc by the Magnetosphere-Ionosphere Coupling in the Alfven Resonator (MICA) sounding rocket launched from Poker Flat, Alaska, on 19 February 2012. The goal of the MICA project was to investigate the hypothesis that such currents can be produced inside the ionospheric Alfven resonator by the ionospheric feedback instability (IFI) driven by the system of large-scale magnetic field-aligned currents interacting with the ionosphere. The trajectory of the MICA rocket crossed two discrete auroral arcs and detected packages of intense, small-scale currents at the edges of these arcs, in the most favorable location for the development of the ionospheric feedback instability, predicted by the IFI theory. Simulations of the reduced MHD model derived in the dipole magnetic field geometry with realistic background parameters confirm that IFI indeed generates small-scale ULF waves inside the IAR with frequency, scale-size and amplitude showing a good, quantitative agreement with the observations. The comparison between numerical results and observations was performed by “flying a virtual MICA rocket through the computational domain, and this comparison shows that, for example, the waves generated in the numerical model have frequencies in the range from 0.30 to 0.45 Hz, and the waves detected by the MICA rocket have frequencies in the range from 0.18 to 0.50 Hz.

Magnetospheric resonances at low and middle latitudes

We present results from a numerical study of structure and dynamics of dispersive Alfven waves in the near-earth magnetosphere containing proton radiation belt (near L = 1.5 dipole magnetic shell). The interest in this problem is motivated by numerous observations of magnetic oscillations with frequencies in the range of 0.1-4.0 Hz detected on the ground at low and middle latitudes. In a number of studies these oscillations interpreted as shear Alfven waves standing inside the so-called ionopspheric Alfven resonator (IAR). We present results from two-dimensional, time dependent simulations of the reduced two-fluid MHD model performed in the dipole magnetic field geometry with the realistic parameters of the magnetospheric plasma. These simulations show that these pulsations can be produced by the fundamental mode of the global field line resonator, spanning the entire magnetic field line in the low or middle magnetosphere. Simulations also show that even the waves with the highest considered frequencies (2.44 Hz) are not trapped inside the ionospheric resonator. Therefore, if these waves will be generated by some ionospheric source, then they can reach the equatorial magnetosphere and interact with energetic protons in the proton radiation belt.