Radio sounding the Earth's plasmasphere and excitation of the whistler
and Z modes by the Radio Plasma Imager (RPI) instrument on the IMAGE
satellite

D. L. Carpenter, M. A. Spasojević, T. F. Bell, and U. S. Inan, Space,
Telecommunications, and Radioscience Laboratory, Stanford University

V. S. Sonwalkar, Dept. of Electrical Engineering, University of Alaska
Fairbanks

B. W. Reinisch and I. A. Galkin, Center for Atmospheric Research,
University of Massachusetts Lowell

R. F. Benson, J. L. Green, and S. F. Fung, NASA Goddard Space Flight
Center, Greenbelt, MD

W. W. L. Taylor, Raytheon ITSS, GSFC, Greenbelt, MD

S. A. Boardsen, EMERGENT Information Technologies, Inc., Largo, MD

Abstract

Since May, 2000, the Radio Plasma Imager (RPI) on Imager for
Magnetopause-to-Aurora Global Exploration (IMAGE) has been sounding the
Earth\'s plasmasphere from various points along the satellite\'s polar
orbit, with apogee ≈ 8 R~E~ geocentric distance and perigee near 1200 km
altitude. RPI also measures local plasma parameters along the IMAGE
orbit through passive measurements of natural wave activity. A principal
new result is that both the plasmapause region and the main plasmasphere
tend to be "rough" targets at sounding frequencies ranging from about 50
kHz to 1 MHz. Echoes that return from directions generally Earthward or
transverse to the geomagnetic field are usually not the discrete traces
on range-versus-frequency records (plasmagrams) that ray tracing
simulations in smooth magnetospheric density models predict. Instead,
they exhibit various amounts of spreading, from ≈ 0.5 to 2 R~E~ in
virtual range (range assuming free-space speed of light propagation).
For echo turning points within the main plasmasphere, the range
spreading is attributed to scattering from, partial reflection from, and
partial-path propagation along geomagnetic-field -aligned electron
density irregularities with cross-field scale sizes ranging from about
200 m to over 10 km and electron density within ≈10 % of background. For
turning points within the plasmapause region, the spreading appears to
be partially attributable to a longitudinal distribution of
irregularities along the plasmapause "surface." When RPI operates in the
inner plasmasphere and at moderate to low altitudes over the polar
regions, pulses emitted at the low end of its 3 kHz to 3 MHz sounding
frequency range can propagate in the whistler mode or in the Z mode.
During soundings with both 51.2 ms pulses and 3.2 ms pulses, whistler
mode echoes have been observed both in discrete, lightning-whistler-like
forms, and in diffuse forms indicative of mode coupling at the
boundaries of density irregularities. Both of these types of echoes have
potential for diagnostics of plasma distributions and structures.

1\. Introduction

Radio sounding of the topside ionosphere has been a highly successful
means of investigating the physics of the ionosphere and of the
innermost plasmasphere \[e.g., Jackson et al., 1980\]. Radio scientists,
including those in the ISIS program, have long envisioned extension of
sounding to altitudes well beyond 3000 km altitude, the approximate
limit of the Alouette 2 and ISIS 1 orbits \[e. g., Franklin and Maclean,
1969\], but only in the year 2000, some 30 years after the principal
Alouette/ISIS work, have such aspirations been realized in the form of
RPI, the Radio Plasma Imager on the Imager for Magnetopause-to-Aurora
Global Exploration (IMAGE) satellite \[e.g., Reinisch et al., 2000\].
This instrument was designed to operate between 3 kHz and 3 MHz so as to
sound the full range of electron densities expected to exist between the
Earth's magnetosheath and topside ionosphere. An initial feasibility
study was performed by Calvert et al. \[1995\], who, among other topics,
considered the problem of sounding the plasmapause and plasmasphere from
ranges varying from 2 to 4 R~E~ . They concluded that with a sounder
transmitter power of 10W and tuned reception, an echo signal-to-noise
ratio greater than unity could be achieved in the frequency range 30 kHz
to 300 kHz, which corresponds to reflections from near the plasmapause
region to points deep within the plasmasphere. Substantial improvements
in S/N were expected upon application of digital integration.

Two experimental objectives of IMAGE and of RPI are of special interest
in this report. One is sounding of the Earth's plasmasphere as part of
the overall IMAGE mission to obtain global perspectives on interactions
among the Earth's main plasma regions. The other is to explore the use
of RPI as a source of whistler-mode signals.

2\. Instrument description

RPI is a multi-mode instrument \[Reinisch et al., 2000\] in which
sounding and listening frequencies, range detection, pulse
characteristics and repetition rate are adjustable parameters over a
wide range of values. The instrument covers the frequency range from 3
kHz to 3 MHz with a receiver bandwidth of 300 Hz. There are three
orthogonal thin-wire antennas, two 500-m tip-to-tip dipoles in the spin
plane (X and Y) and a 20-m tip-to-tip dipole along the spin axis (Z).
The long dipoles are used for transmission, and all three antennas are
used for reception. The nominal radiated power from RPI, variable (in
terms of free space mode excitation) from 0.1 mW at low frequencies to
10 W per dipole at 200 kHz, was reduced by 3 dB on May 8, 2000 when the
power supply for the Y axis transmitter failed. A further reduction
occurred on October 3, 2000, when one of the X axis monopoles was
partially severed, apparently by a micrometeorite. In spite of these
difficulties, excellent data have continued to be acquired.

Having been designed as a sounder, RPI is far from ideal for
whistler-mode applications, for example, being limited for the most part
to pulses of 125 ms duration or less and to a receiver bandwidth of 300
Hz. However, there are compensating factors, such as the availability of
a 500-m dipole antenna for transmission (now reduced to ≈ 340 m) and
crossed 500-m antennas (one ≈ 340 m) and the 20 m Z axis antenna for
reception.

3\. Experimental results

3.1. Sounding the plasmasphere

RPI operations both beyond and within the plasmasphere have yielded a
number of new results, several of which have recently been reported
\[e.g., Carpenter et al., 2002\]. Evidence has been found that the
plasmasphere is a "rough" target for sounding, both in terms of the
plasmapause region and in terms of the main plasmasphere at altitudes
above ≈5000 km. This result is in sharp contrast to prelaunch
theoretical expectations, based upon "smooth" models of the electron
density distribution \[e.g., Green et al., 2000\]. The effect is
primarily manifested in terms of spreading of echoes in range. For
echoes from the plasmapause region, spreading is such that echo ranges
are up to ≈ 0.5 R~E~ longer than the expected range to the nearest
reflection point. Such effects may be due to a longitudinal distribution
of irregularities on the plasmasphere "surface."

Within the main plasmasphere, RPI echoes appear both at ranges shorter
than those expected in a smooth medium and at longer ranges \[Carpenter
et al., 2002\]. To at least some extent they appear as counterparts to
well established effects in ionospheric sounding data \[e.g., Muldrew,
1969; James, 1989\]. This persistent range spreading is believed to be
caused by interactions with geomagnetic-field-aligned electron density
structures, including scattering from irregularities with cross-field
dimensions roughly one half the signal wave length as well as refraction
into and partial propagation along ducts with cross-field scales of
several signal wave lengths. The RPI data, supported by evidence from
other radio experiments such as topside sounders \[e.g., Muldrew, 1963,
1969; Loftus et al., 1966\] and whistler-mode instruments \[e.g., Smith,
1961; Helliwell, 1965; Strangeways and Rycroft, 1980; Hamar et al.,
1990; Strangeways, 1991\], suggest that the plasmasphere is regularly
permeated by small scale field-aligned irregularities with cross **B**
scale sizes ranging from ≈ 200 m to over 10 km and density within \< 10
% of background. The regular presence of field aligned irregularities
under both quiet and disturbed conditions, is supported by the
appearance on many RPI plasmagrams of discrete echoes that propagate on
geomagnetic field aligned paths \[e.g., Reinisch et al., 2001a, 2001b;
Fung et al., 2002, rather than on the "direct" paths associated with
substantial range spreading. The appearance on plasmagrams of these
magnetospheric field-line echoes is similar to the signature of ducted
echoes in the topside ionosphere \[e.g. Muldrew, 1963; Dyson and Benson,
1978\]. Although the specific mechanisms of irregularity formation are
not yet known, there exists a substantial body of theoretical work on
mechanisms that might explain the appearance of irregularities both
within and beyond the plasmasphere \[e.g., Gold, 1959; Richmond, 1973;
Lemaire and Gringauz, 1998; Ferrière et al., 1999, 2001\]. In
experimental work Carpenter et al. \[2002\] have suggested that some
structures may develop due to instabilities at the edges of known large
scale density features such as the plasmapause, density cavities in the
plasmasphere, and equatorial ionization depletions \[e.g., Sales et al.,
1996\].

It remains to establish the more important physical mechanisms that give
rise to the scattering. Since the irregularities involved are part of
the coupled magnetosphere-ionosphere system, it is possible that some of
them may be of importance in controlling the propagation and
distribution in space of the naturally occurring and man made
whistler-mode waves that are known to interact strongly with electrons
in the Earth\'s radiation belts \[e.g., Kennel and Petschek, 1966; Imhof
et al., 1983\].

3.2. Excitation of whistler mode and Z mode echoes

> 3.2.1. Background

The operation of a whistler-mode transmitter in the magnetosphere has
been an unrealized goal of space scientists for many years. In the late
1970s a NASA study group was formed to consider possible active wave
experiments from the Shuttle \[e.g., Fredricks et al., 1978; Dyson,
1978; Inan et al., 1981\]. This group and its successor developed plans
for a whistler-mode/Z mode wave injection mission called WISP, to
include a transmitter on the Shuttle and signal detection on a
sub-satellite positioned variously with respect to the orbiter. The
proposed experiments would include verification of basic dispersion
relations, investigation of coupling between wave modes, and study of
wave-particle energy and momentum exchange \[Fredricks et al., 1978\].
The project was selected for a Shuttle flight in the mid 1980s but was
cancelled after the Challenger accident in 1986.

Once IMAGE moved into the development phases, its broad potential for
whistler mode probing became clear, and a paper discussing this
potential was prepared prior to launch by Sonwalkar et al. \[2001\]. It
was pointed out that in major parts of the plasmasphere, the
whistler-mode refractive index is high, of order 10, and the long RPI
antennas are approximately a half wavelength in extent and can have a
radiation efficiency at whistler-mode frequencies of as much as 10 %
\[e.g., Sonwalkar et al., 2001\], much higher than when the same
antennas are used for transmitting in the free-space modes. For the
latter conditions the efficiency of the antennas drops steeply with
decreasing frequency below 100 kHz from a maximum near 200\--300 kHz
\[Reinisch et al., 2000\].

> 3.2.2. Observations

Discrete whistler mode echoes of 3.2 ms and 51.2 ms sounding pulses have
been observed on RPI during regular sounding operations at altitudes
generally less than 7000 km in the plasmasphere and over southern polar
regions. They are interpreted as having returned to IMAGE after
reflection from the region of steep electron density gradients at the
bottom of the ionosphere \[see Sonwalkar et al., 2001\]. On
frequency-versus-time records, the echoes were whistler-like in form,
and in the case of soundings with 3.2 ms pulses at ≈ 1500 km altitude
within the plasmasphere, included evidence of a frequency of minimum
delay or nose frequency near 300 kHz. Poleward of the plasmasphere,
where the plasma frequency was usually lower than the gyrofrequency,
whistler-like forms extending to 500 kHz were detected. They exhibited
upper limits near the local plasma frequency and roughly constant delays
with frequency above 200 kHz, the lower frequency limit of many of the
records.

Diffuse whistler mode echoes have also been observed, and are
interpreted as evidence of a previously reported process involving
linear mode coupling between electromagnetic whistler mode signals
transmitted from the ground and quasi-electrostatic modes excited at the
boundaries of density irregularities \[e.g., Bell and Ngo, 1988; 1989\].

Z mode echoes are observed on essentially all RPI soundings at medium
and low altitudes. They often exhibit discrete forms, which should be
capable of use as integral measures of the plasma regions near the
satellite when conditions are such that the free space mode echoes are
not practical for use. Soundings in the Z mode offer exceptional
opportunities to track key plasma parameters such as the plasma
frequency or upper hybrid frequency along lower-altitude IMAGE orbital
segments where passive recording methods are more difficult to apply.

4\. Concluding remarks.

Direct echoes from the plasmapause and plasmasphere received by the RPI
on IMAGE are not discrete in form, as ray tracings in a smooth medium
would predict. Instead they exhibit substantial range spreading, which
has been interpreted as the result of interactions with widely
distributed and persistent geomagnetic-field-aligned irregularities.
Within the plasmasphere the irregularities are found to have cross **B**
scale sizes ranging from ≈ 200 m to over 10 km and density within \< 10
% of background. Such irregularities are probably not detectable by most
in situ density probes (although the WHISPER instrument on CLUSTER does
appear to have the required capability \[e.g., Décréau et al., 2001\]).

RPI has produced an abundance of Z mode echo phenomena as well as many
examples of whistler mode echoes when RPI is at altitudes below a few
thousand km. The whistler mode reflections are believed to occur at the
steep refractive index gradients at the bottom of the ionosphere.

Most of the whistler and Z mode activity found thus far has been
incidental to sounding experiments designed for the free space modes.
However, those sounding experiments have brought to light potential
applications of the whistler mode, including remote density measurements
in the lower altitude polar regions, study of whistler and Z mode
propagation to satellites of opportunity, investigation of linear mode
coupling at the boundaries of density irregularities, and study of the
conditions of ionosphere signal penetration in the whistler mode.

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