[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.

[1] The HAARP HF facility is used to modulate the components of the auroral electrojet that flow in the D-region of the ionosphere, creating ELF/VLF radiation which is then measured at a receiver co-located with the HAARP HF antenna. An HF heating model is coupled to a full wave plasma interaction FDTD code to determine the ELF/VLF response of the ionospheric plasma to the modulated HF stimulation. The predicted FDTD fields on the ground are found to be in remarkable agreement with those measured at a receiver co-located with HAARP. The FDTD code also predicts an upwardly propagating whistler mode that is tightly bound to the magnetic field lines. Citation: Payne,

[1] Magnetospheric chorus waves are a major driver of acceleration and loss in the Earth's outer electron radiation belt. The spectral extent of chorus is a key parameter in quantifying the global effect of chorus on energetic particle populations by determining the range of resonant electron energies. However, statistics of spectral properties are sparse, particularly in the off-equatorial magnetosphere. We use a database of chorus observations from the Polar spacecraft to generate statistics on the normalized chorus frequency (with the respect to the minimum field line gyrofrequency, Ω min ) as a function of magnetic local time (MLT) (0 < MLT < 24), L-shell (3 < L < 11), and magnetic latitude (|l|< 65 ). We find that, on average, the chorus spectrum peaks in the range of 0.1-0.4 Ω min , varying significantly with l, R 0 and MLT. The normalized chorus peak frequency is found to decrease with increasing R 0 , and decreases with increasing latitude below $ 25 . When fit to a Gaussian spectral model, lower band chorus is found to have a bandwidth < 0.1 Ω min , which is narrower than assumed in most diffusion models. Diffusion coefficients calculated using the University of California at Los Angeles (UCLA) Full Diffusion Code show that wave-particle interactions on the nightside are highly sensitive to both the peak frequency and bandwidth of chorus, yet on the dayside scattering is mostly sensitive to the peak frequency as a result of the wider latitudinal extent of the waves. We also find that fitting a Gaussian to the logarithm of the spectrum reduces fit errors by over 60%, indicating that inclusion of arbitrary spectral forms may improve the accuracy of wave models within radiation belt simulations.

[1] Recent observations by Starks et al. (2008) from multiple spacecraft suggest that the actual nighttime intensity of VLF transmitter signals in the radiation belts is approximately 20 dB below the level that is assumed in the model developed by Abel and Thorne (1998) and approximately 10 dB below model values during the day. In the present work, we discuss one experimentally established mechanism which might be responsible for some of this intensity discrepancy, linear mode coupling as electromagnetic whistler mode waves propagate through regions containing small-scale (2-100 m) magnetic field-aligned plasma density irregularities. The scattering process excites quasi-electrostatic whistler mode waves, which represents a power loss for the input waves. Although the distribution and amplitude of the irregularities is not well known at present, we construct plausible models in order to use numerical simulations to determine the characteristics of the mode coupling mechanism and the conditions under which the input VLF waves can lose significant power to the excited quasi-electrostatic whistler mode waves. For short propagation paths of approximately 15 km, the full-wave model predicts power losses ranging from −3 dB (25% probability) to −7 dB (2% probability). For longer propagation paths of approximately 150 km, the full-wave model predicts power losses ranging from −4 dB (25% probability) to over −10 dB (2% probability). We conclude that for the irregularity models investigated, the mode coupling mechanism can result in significant power loss for VLF electromagnetic whistler mode waves.Citation: Foust, F. R., U. S. Inan, T. Bell, and N. G. Lehtinen (2010), Quasi-electrostatic whistler mode wave excitation by linear scattering of EM whistler mode waves from magnetic field-aligned density irregularities,

[1] The effects of ground-based very low frequency (VLF) transmitters on the lower ionosphere are investigated. Controlled modulation experiments are performed with the 21.4 kHz, 424 kW VLF transmitter NPM in Lualualei, Hawaii, and physical effects of the NPM transmissions are studied with a subionospherically propagating VLF probe signal. Observed perturbations to the probe signal are consistent neither with expectations from transmitter-induced electron precipitation nor to off-path scattering from a concentrated heating region near the transmitter but rather appear to be the result of scattering from extended lateral heating of the ionosphere by the NPM transmitter. A large-scale computational modeling framework confirms theoretically that this form of ionospheric heating can account for the observed probe signal modulations, establishing that the lateral extent of ionospheric heating due to VLF transmitters is several thousand kilometers, significantly greater than previously recognized.