Magnetospheric Comstellation And Tomography
Radio tomography imaging of the Earths magnetosphere is expected to play an important role in magnetospheric research in the coming decades, promising to extend magnetospheric research to a global level and to improve greatly our understanding of key magnetospheric processes. Once successfully applied to the Earth's magnetosphere, radio tomography imaging may be extended to planetary and solar missions.
The basic idea of radio tomography imaging is as follows (see Figure). There are sixteen satellites, ten of which are in a ~1.1 RE (geocentric) by ~14 RE polar (90o inclination) orbit and six in a ~1.6 RE by ~10 RE polar orbit that, in this particular example, focus on the central plasma sheet in the magnetotail. These particular orbits were chosen to optimize both radio tomography imaging and in situ observations.
The spacecraft transmit, one at a time, coherently-phased pairs of discrete radio frequency signals to be received by all other satellites (Panel a). Panel b displays a two-dimensional image of the plasma density in the GSM XZ plane (meridional cut) from a magneto-hydrodynamic (MHD) simulation. The color scale represents the plasma density. The two orbits, the instantaneous position of the spacecraft, and the ray paths between the spacecraft are superimposed on the density plot. The phase difference between the paired signals yields a very accurate measurement of the integrated electron density along the ray path, generally called ?total electron content? (TEC). There are 120 lines of sight (the 120 TEC measurements are redundant). The 120 TEC measurements then can be inverted to produce an image of plasma density in the region encompassed by the satellites. The lower panel displays reconstructed density images based solely on simulated TEC measurements along the ray paths. The inversion technique is similar to that used in computed tomography scans in medical science.
The entire tomographic cycle (or image) can be performed in ~12 s with each satellite transmitting, in turn, every ~0.75 s. Travel times of the signals are less than 0.3 s. The spatial resolution can be as fine as ~1/2 RE under the orbit configuration in the Figure. The images, combined with in situ observations of the plasma and electromagnetic fields, can be used to investigate processes that govern energy flow and plasma entry into the magnetosphere, and release energy during magnetic substorms. These are some of the most important processes in magnetospheric physics which include plasma entry via reconnection, diffusion, impulsive penetration, and magnetic substorms.
The initial results of the radio
tomography imaging project have been published in the Journal of Geophysical
Research in January 2000 by Prof. Robert Ergun.
of the Earth's magnetosphere with radio tomography, R.E. Ergun, D.E. Larson,
T. Phan, S. Bale, C.W. Carlson, I. Roth, and V. Angelopoulos, J. Geophys.
Res., 105, 361, 2000.
region reconstruction with 2 RE wave.
region reconstruction with 1/2 RE random variations.
Robert Ergun's home page.