The Radio Plasma Imager

Bodo W. Reinisch and the RPI Team, Center for Atmospheric Research,
  University of Massachusetts, Lowell, MA 01854

The New Millennium Magnetosphere: Integrating Imaging, Discrete
  Observations, and Global Simulations, Sixth Huntsville Modeling Workshop,
  Guntersville, Alabama, 26-30 October 1998.


The Radio Plasma Imager (RPI) instrument is being built for the IMAGE satellite
mission which is scheduled for launch in January 2000. IMAGE will have an
elliptical polar orbit with an altitude range from 1000 km to 7 RE. The
scientific objectives of the RPI observations include the detection of plasma
influx into the magnetosphere during magnetic substorms and storms, and the
assessment of the response of the magnetopause and plasmasphere to variations
of the solar wind. Unlike the wave plasma instruments on WIND and POLAR, RPI
uses active Doppler radar techniques for the remote sensing of plasma
structures. These techniques are similar to the ones used by the Digisonde
Portable Sounder (DPS), a modern groundbased ionosonde. RPI transmits radio
frequency pulses omni-directionally and receives any reflected echoes on three
orthogonal antennas. Echo reflections occur at plasma structures whose surface
normals are parallel to the wave normals of the incident waves, and where
N = 0.0124 f**2, where f is the sounding frequency in kHz and N is the number
of electrons per cm**3. Since the transmitted signals generally contain both
characteristic polarizations, i.e. the ionic or ordinary mode, and the
electronic or extraordinary mode, there will generally be two echoes from a
given plasma structure: the ordinary echo reflected at the density level
described above, and the extraordinary echo reflected at a density given by
N(x) = 0.0124 f (f - fc) where fc is the cyclotron frequency.

To determine N over the range of plasma conditions in the magnetosphere, RPI
operates between 3 kHz and 3 MHz, corresponding to N = 0.1 to 105 cm-3. The
frequency steps can be as small as 100 Hz. The largest distance from which
echoes can be detected is a function of the transmitter power, the antenna
size and the noise environment. RPI uses 10 Watt transmitters to drive two
orthogonal 500 m dipole antennas. The transmit antennas are tuned under
computer control to the sounding frequencies. The dominant noise sources are
Type III solar noise bursts, auroral kilometric radiation (AKR) and the
non-thermal continuum. Cosmic noise and receiver noise plays a minor role. RPI
applies pulse compression techniques and spectral coherent integration to
enhance the signal to noise ratios. The level of the noise floor will vary
with time and spacecraft location and we expect to see echoes from more than
10 RE during favorable conditions and 5 RE or less during disturbed conditions.
The range resolution was chosen as 240 km, setting the pulse width to 3.2 ms
and the receiver bandwidth to 300 Hz.

Radio imaging is required to identify the locations of the reflecting plasma
structures. Echoes with different ranges and angles-of-arrival will return from
the magnetopause, the cusp region, and the plasmasphere. RPI determines the
angle-of-arrival of the echoes by sampling the wave field at two times that are
one quarter of a cycle apart (quadrature samples). By making simultaneous
measurements at the three orthogonal antennas, each 3-antenna sample defines
the instantaneous E vector, and the vector product of the two quadrature
vectors defines the orientation of the plane of polarization of the arriving
wave, and therefore the angle-of-arrival. The angular accuracy with respect to
the antenna coordinates is limited by the signal-to-noise ratio (SNR). For a
voltage SNR of 100 the accuracy is 1o. The polarization ellipse and its
orientation relative to the orientation of the transmitted wave can also be
calculated from the measurements making it possible to determine the reflection
coefficients and the Faraday rotation. The electron density profiles of the
reflecting regions can be calculated from the echo ranges R'(f). The measured
echo delay time t defines the "virtual" echo range given by R' = 0.5 c*t where
c is the free space speed of light. The group velocity of the pulsed signal is
close to c in the magnetospheric cavity when the frequency f is much larger
than the plasma frequency fp = 9 N**(1/2), but approaches zero at the
reflection point. Profile inversion algorithms have been tested on simulated
data and have been found to accurately reproduce the input electron density
profile.

Within the plasmasphere and at low altitude over the southern polar region, RPI
may at times operate in the whistler mode, transmitting at frequencies below
the local electron gyrofrequency to study wave-particle interactions,
plasmasphere fine structure, and ionosphere signal penetration. In the passive
"Thermal Noise Mode" RPI will measure the bulk electron density and temperature
of the local plasma, and possibly the ion bulk speed.