Radio plasma imager simulations and measurements

The Radio Plasma Imager (RPI) will be the first-of-its kind instrument designed to use radio wave sounding techniques to perform repetitive remote sensing measurements of electron number density (N _e) structures and the dynamics of the magnetosphere and plasmasphere. RPI will fly on the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) mission to be launched early in the year 2000. The design of the RPI is based on recent advances in radio transmitter and receiver design and modern digital processing techniques perfected for ground-based ionospheric sounding over the last two decades. Free-space electromagnetic waves transmitted by the RPI located in the low-density magnetospheric cavity will be reflected at distant plasma cutoffs. The location and characteristics of the plasma at those remote reflection points can then be derived from measurements of the echo amplitude, phase, delay time, frequency, polarization, Doppler shift, and echo direction. The 500 m tip-to-tip X and Y (spin plane) antennas and 20 m Z axis antenna on RPI will be used to measures echoes coming from distances of several R _E. RPI will operate at frequencies between 3 kHz to 3 MHz and will provide quantitative N _e values from 10^–1 to 10⁵ cm^–3. Ray tracing calculations, combined with specific radio imager instrument characteristics, enables simulations of RPI measurements. These simulations have been performed throughout an IMAGE orbit and under different model magnetospheric conditions. They dramatically show that radio sounding can be used quite successfully to measure a wealth of magnetospheric phenomena such as magnetopause boundary motions and plasmapause dynamics. The radio imaging technique will provide a truly exciting opportunity to study global magnetospheric dynamics in a way that was never before possible.     

The Radio Plasma Imager investigation on the IMAGE spacecraft

Radio plasma imaging uses total reflection of electromagnetic waves from plasmas whose plasma frequencies equal the radio sounding frequency and whose electron density gradients are parallel to the wave normals. The Radio Plasma Imager (RPI) has two orthogonal 500-m long dipole antennas in the spin plane for near omni-directional transmission. The third antenna is a 20-m dipole along the spin axis. Echoes from the magnetopause, plasmasphere and cusp will be received with the three orthogonal antennas, allowing the determination of their angle-of-arrival. Thus it will be possible to create image fragments of the reflecting density structures. The instrument can execute a large variety of programmable measuring options at frequencies between 3 kHz and 3 MHz. Tuning of the transmit antennas provides optimum power transfer from the 10 W transmitter to the antennas. The instrument can operate in three active sounding modes: (1) remote sounding to probe magnetospheric boundaries, (2) local (relaxation) sounding to probe the local plasma frequency and scalar magnetic field, and (3) whistler stimulation sounding. In addition, there is a passive mode to record natural emissions, and to determine the local electron density, the scalar magnetic field, and temperature by using a thermal noise spectroscopy technique.     

Comet Giacobini-Zinner diagnosis from radio measurements

The physics of using a radioastronomy receiver as an in-situ detector of plasma, and in some cases of molecules and dust grains is reviewed, and applied to ICE encounter with comet Giacobini-Zinner. In the comet's plasma tail, the receiver recorded mostly quasi-thermal plasma noise. The spectroscopy of that noise yields the density and temperature of the main (cold) electron population, and parameters of hot electrons. The absence of grain detection yields a quantitative upper limit on grain mass or flux. An additionnal diagnosis is provided by partial occultations of both the radio galactic noise and the terrestrial kilometric radiation. Implications for comparison with earth-based measurements are indicated.     

Solar Wind Electron Observations Near Solar Maximum at High Latitudes From Thermal Noise Spectroscopy

The Ulysses spacecraft is reaching high heliolatitudes during the approach to solar maximum. We show preliminary in situ electron observations from the URAP experiment, using thermal noise spectroscopy. This method is especially suited to measure accurately the electron density and thermal temperature. The data acquired in the period June–September 2000 are compared to those obtained at similar heliolatitudes near solar activity minimum and in the ecliptic plane near both solar maximum and minimum. This revised version was published online in August 2006 with corrections to the Cover Date.