Role of the plasmapause in dictating the ground accessibility of ELF/VLF chorus

Abstract

[1] This study explores the manner in which the plasmapause is responsible for dictating which magnetospheric source regions of ELF/VLF chorus are able to propagate to and be received by midlatitude stations on the ground. First, we explore the effects of plasmapause extent on ground-based observations of chorus via a 3 month study of ground-based measurements of chorus at Palmer Station, Antarctica (L = 2.4, 50°S geomagnetic latitude), and data on the plasmapause extent from the IMAGE EUV instrument. It is found that chorus normalized occurrence peaks when the plasmapause is at L ∼ 2.6, somewhat higher than Palmer's L shell, and that this occurrence peak persists across a range of observed chorus frequencies. Next, reverse ray tracing is employed to evaluate the portion of the equatorial chorus source region, distributed in radial distance and wave normal, from which chorus is able to reach Palmer Station via propagation in a nonducted mode. The results of ray tracing are similar to those of observations, with a peak of expected occurrence when the plasmapause is at L ∼ 3. The exact location of the peak is frequency dependent. This supports the conclusion that the ability of chorus to propagate to low altitudes and the ground is a strong function of instantaneous plasmapause extent and that peak occurrence of chorus at a given ground station may occur when the L shell of the plasmapause is somewhat beyond that of the observing station. These results also suggest that chorus observed on the ground at midlatitude stations propagates predominantly in the nonducted mode.

Analysis of experimentally validated trans‐ionospheric attenuation estimates of VLF signals

“…Magnetospherically generated VLF emissions such as chorus and hiss are also extremely important in radiation belt dynamics, and in the absence of local measurements, ground‐based recordings of these waves can be used to estimate characteristics of the in situ distribution [ Horne et al ., ; Spasojevic and Inan , ; Golden et al ., ]. However, using ground‐based data to quantitatively estimate in situ wave power [e.g., Golden et al ., ] requires knowledge of how efficiently the VLF waves penetrate downward from the magnetosphere through the ionosphere and into the Earth‐ionosphere waveguide. Similarly, understanding trans‐ionospheric propagation is an important aspect of using ground‐based or space‐based whistler measurements to remotely sense plasmaspheric electron densities [e.g., Carpenter , ; Carpenter et al ., ; Lichtenberger et al ., ; Lichtenberger , ].…”

“…Despite the fact that quantitative estimates of trans‐ionospheric attenuation of VLF waves are important for many studies in space sciences, the preeminent reference for many of these studies [e.g., Abel and Thorne , ; Bortnik et al ., ; Kulkarni et al ., ; Starks et al ., ; Golden et al ., ] has been Figure 3‐35 of Helliwell []. The trans‐ionospheric absorption estimates of Helliwell [] were presented at the time with several known caveats, and recent in situ satellite observations [e.g., Starks et al ., ] have further questioned their validity.…”

Modeling the properties of plasmaspheric hiss: 2. Dependence on the plasma density distribution

“…The plasmapause, a sharp boundary separating the high density plasmasphere (>∼100 cm −3 ) and the low density trough region (<∼10 cm −3 ), has been recognized as a critical component in controlling the propagation characteristics and also the excitation of plasma waves in the magnetospheric medium. Examples of the effects include preferential excitation of EMIC waves along the plasmapause [ Horne and Thorne , 1993; Chen et al , 2009b, 2010], ground accessibility of chorus wave along the plasmapause [ Golden et al , 2010], equatorial accessibility of auroral kilometric radiation (AKR) controlled by the size of plasmapause [ Green et al , 1977; Xiao et al , 2007], azimuthal propagation of equatorially confined magnetosonic waves guided along the plasmapause [ Kasahara et al , 1994], and more relevant to the present study, the trapping of chorus waves inside the plasmasphere [ Bortnik et al , 2008].…”

Magnetospheric whistler mode ray tracing in a warm background plasma with finite electron and ion temperature

“…The basis of our numerical code is the VLF raytracer developed at Stanford University [Golden et al, 2010;Golden, 2011] which implements numerical ray tracing by solving the Haselgrove equations [Haselgrove, 1955;Kimura, 1966] at each time step. The background plasma properties such as density, magnetic field intensity, and temperature enter via the refractive index, , which is solved for from the dispersion relation and is modified by the finite temperature physics as described by Kulkarni et al [2015].…”

“…The effect of ions on whistler mode ray tracing was first studied by Kimura [1966]. Since the 1970s a very large body of magnetospheric whistler mode ray tracing work has been carried out [Inan and Bell, 1977;Huang and Goertz, 1983;Cairo and Lefeuvre, 1986;Bortnik et al, 2006aBortnik et al, , 2006bChum and Santolík, 2005;Gołkowski and Inan, 2008;Chen et al, 2009;Golden, 2011;Kulkarni et al, 2006Kulkarni et al, , 2015.…”