Space Borne Radio Sounding of the Topside Ionosphere and
Magnetosphere

B. W. Reinisch, D. M. Haines, University of Massachusetts Lowell
R. R. Benson, S. F. Fung, J. L. Green,NASA Goddard Space Flight Center
W. W. L. Taylor, Raytheon STX/GSFC.

Ninth International Ionospheric Effects Symposium, IES-99,
Alexandria, Virginia, May 4-6, 1999.


Two new space borne instruments are being developed to remotely
sense plasma distributions in the magnetosphere and the topside
ionosphere. The first instrument is the TOPside Automated Doppler
Sounder (TOPADS) which is being developed for the Ukrainian
WARNING mission scheduled for launch in 2001, and may find
application in the U.S. NPOES program. At a high inclination
circular orbit (600 km altitude), TOPADS will measure the topside
vertical electron density profiles every 75 km. Three orthogonal
20 m tip-to-tip dipole antennas will be used for reception, one of
them for transmission. Operating as a HF radar, TOPADS will be
providing for the first time topside plasma velocities by tracking
the motions of plasma irregularities. Automatic processing of the
ionogram data will provide real time electron density profiles
that will be made freely available as a space weather diagnostics
tool.

The second instrument is the Radio Plasma Imager (RPI) that is
being built for the IMAGE satellite mission, 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 and will provide real time
data for space weather assessment. These techniques are similar to
the ones used by the Digisonde Portable Sounder (DPS) on the
ground. RPI transmits radio frequency pulses omni-directionally
and receives any reflected echoes on three orthogonal antennas..
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. With the three orthoganal
antennas, RPI will be able to automatically identify and measure
the wave polarization. 

To determine the electron densities N over the range of plasma
conditions in the magnetosphere, RPI will operate between 3 kHz
and 3 MHz, corresponding to N = 0.1 to 105 cm-3. 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 1%. 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
N1/2, 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. 

The first instrument is the Radio Plasma Imager. RPI is a low
frequency sounder designed to sweep from 3 kHz to 3 MHz which will
be part of NASA's IMAGE mission to be launched in January 2000.
While in the magnetospheric cavity (7 Re altitude), RPI will
receive echoes from the magnetopause and the plasmasphere and will
measure the direct response of the magnetosphere's configuration
to changes in the solar wind. With three orthogonal dipole
antennas (two 500 m tip-to-tip antennas in the spin plane used for
transmission and reception, one 20 m antenna along the spin axis
for reception only) the angle of arrival of returning echoes can
be determined with high accuracy. Similar to the groundbased
Digisondes, RPI will operate like a low-frequency radar system
measuring the location (range and angle-of-arrival), plasma
density, and motion of the reflecting plasma structures.