William W. L. Taylor

On the origin of whistler mode radiation in the plasmasphere

[1] The origin of whistler mode radiation in the plasmasphere is examined from 3 years of plasma wave observations from the Dynamics Explorer and the Imager for Magnetopauseto-Aurora Global Exploration spacecraft. These data are used to construct plasma wave intensity maps of whistler mode radiation in the plasmasphere. The highest average intensities of the radiation in the wave maps show source locations and/or sites of wave amplification. Each type of wave is classified on the basis of its magnetic latitude and longitude rather than any spectral feature. Equatorial electromagnetic (EM) emissions (30–330 Hz), plasmaspheric hiss (330 Hz to 3.3 kHz), chorus (2–6 kHz), and VLF transmitters (10–50 kHz) are the main types of waves that are clearly delineated in the plasma wave maps. Observations of the equatorial EM emissions show that the most intense region is on or near the magnetic equator in the afternoon sector and that during times of negative Bz (interplanetary magnetic field) the maximum intensity moves from L values of 3 to <2. These observations are consistent with the origin of this emission being particle-wave interactions in or near the magnetic equator. Plasmaspheric hiss shows high intensity at high latitudes and low altitudes (L shells from 2 to 4) and in the magnetic equator with L values from 2 to 3 in the early afternoon sector. The longitudinal distribution of the hiss intensity (excluding the enhancement at the equator) is similar to the distribution of lightning: stronger over continents than over the ocean, stronger in the summer than in the winter, and stronger on the dayside than on the nightside. These observations strongly support lightning as the dominant source for plasmaspheric hiss, which, through particle-wave interactions, maintains the slot region in the radiation belts. The enhancement of hiss at the magnetic equator is consistent with particle-wave interactions. The chorus emissions are most intense on the morningside as previously reported. At frequencies from 10 to 50 kHz, VLF transmitters dominate the spectrum. The maximum intensity of the VLF transmitters is in the late evening or early morning with enhancements all along L shells from 1.8 to 3.

The feasibility of radio sounding in the magnetosphere

A radio sounder outside the plasmasphere could provide nearly continuous remote density measurements of the magnetopause and plasmasphere, as well as other important density features elsewhere in this region. Using digital integration and tuned reception at frequencies from a few kilohertz to a few megahertz with 400-m to 500-m tip-to-tip dipole antennas and 10 W transmitter power, such a sounder would be capable of 10% density resolution and 500 to 1300 km spatial resolution in only a few minutes at distances of up to 4 RE. By providing such detailed observations of its principal density structures, such a sounder would then clearly revolutionize magnetospheric research.

Radio wave active Doppler imaging of space plasma structures: Arrival angle, wave polarization, and Faraday rotation measurements with the radio plasma imager

Radio sounding in the magnetosphere by the radio plasma imager on the IMAGE spacecraft will determine the dimensions and shape of the cavity between the magnetopause and the plasmapause. Omnidirectional transmission of pulsed radio signals results in echoes arriving from many directions. Quadrature sampling and Doppler analysis of the signals received on three orthogonal antennas will make it possible to determine the angles of arrival of the echoes, their polarization ellipses, and the Faraday rotation. Decomposition of the echo signals into the two characteristic waves is used to identify the O- and X-wave components.

Radio imaging of the magnetosphere

Radio sounding can be used to produce “images” of magnetospheric electron density distributions that could revolutionize research into the magnetosphere and its plasma content, especially when combined with other techniques. Based on more than a half-century heritage of ionospheric sounding combined with digital techniques, the magnetospheric radio sounder is yielding measurements that were once impossible to obtain. A magnetospheric radio sounder can provide unprecedented global magnetospheric information by providing quantitative electron density profiles simultaneously in different directions. From a sequence of these, a contour plot of the density structure in the orbital plane can be constructed, with some out of plane information as well.

The IMAGE mission and first observations from the radio plasma imager

The Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) mission is NASAs first satellite mission designed to image Earths magnetosphere using remote sensing techniques. Extreme and far ultraviolet (EUV, FUV), energetic neutral atom (ENA), and radio plasma (RPI) imagers are mapping the aurora, the helium ions of the plasmasphere, the plasma sheet and polar cusp, and the total plasma distribution from the ionosphere to the magnetopause. IMAGE was launched on 25 March 2000 into an elliptical polar orbit with an altitude of 7 R/sub E/ at apogee and 1000 km at perigee. First observations obtained by RPI in the sounding and relaxation mode are reported.