SPREAD ECHOES FROM THE PLASMAPAUSE REGION AND MAIN PLASMASPHERE RECEIVED
DURING HIGH ALTITUDE RADIO SOUNDING BY THE RPI INSTRUMENT ON THE IMAGE
SPACECRAFT

**D. L. Carpenter^(1)^, M. A. Spasojević^(2)^, T. F. Bell^(3)^, U. S.
Inan^(4)^, B. W. Reinisch^(5)^, I. A. Galkin^(6)^, R. F. Benson^(7)^, J.
L. Green^(8)^, S. F. Fung^(9)^, S. A. Boardsen^(10)^**

***^(1)^Space, Telecommunications and Radioscience Laboratory, Stanford
University,*** ***STAR laboratory, Stanford University, Stanford, CA
94305-9515; E-mail <dlc@nova.stanford.edu>***

***^(2)^As (1) above, but E-mail <mssm@nova.stanford.edu>***

***^(3)^As (1) above, but E-mail <bell@nova.stanford.edu>***

***^(4)^As (1) above, but E-mail <inan@nova.stanford.edu>***

#  ^(5)^Center for Atmospheric Research, University of Massachusetts Lowell; E-mail <bodo_reinisch@uml.edu>

***^(6)^As (5) above, but E-mail <ivan_galkin@uml.edu>***

***^(7)^A NASA Goddard Space Flight Center, Greenbelt, MD;
E-mail<u2rfb@lepvax.gsfc.nasa.gov>***

***^(8)^As (7) above, but E-mail <green@bolero.gsfc.nasa.gov>***

***^(9)^As (7) above, but E-mail <fung@ndadsb-f.gsfc.nasa.gov>***

***^(10)^EMERGENT Information Technologies, Inc., Largo, MD; E-mail
<boardsen@xfiles.gsfc.nasa.gov>***

## ABSTRACT

### Since April, 2000, the Radio Plasma Imager (RPI) instrument on the IMAGE satellite has been sounding the Earth's plasma environment from a 1200 km -- 8R~E~ polar orbit. Repeated observations of echo range spreading suggest that electron density irregularities are commonly "embedded" in the plasmasphere "surface," and that the middle and outer plasmasphere are regularly permeated by geomagnetic-field aligned irregularities with cross B scale sizes ranging from 200 m to \> 10 km and electron density within \< 10 % of background. Antecedents for this work exist in reports from topside sounders and whistler receivers, and in theory of plasmaspheric instabilities.

## SPREAD ECHOES FROM THE PLASMASPHERE DETECTED BY RPI

Since May, 2000, the Radio Plasma Imager (RPI) on 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 \[1\]. 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 \[2\]. 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 ≈ 200 m to \> 10 km and electron
density within \< 10 % of background. For echo turning points within the
plasmapause region, the spreading appears to be partially attributable
to a longitudinal distribution of irregularities along the plasmapause
"surface." That the irregularities are field-aligned is suggested by the
efficiency with which RPI, in addition to the spread "direct" echoes
noted, excites other echoes that are guided, sometimes into both
hemispheres, along the geomagnetic field, and which commonly return with
minimal range spreading \[3\]. Although the occasional existence of the
irregularities in question is supported by evidence from other radio
experiments such as topside sounders \[4,5\] and whistler-mode
instruments \[6,7\], and is also supported (somewhat less directly) by
in situ measurements on satellites such as CRRES \[8\], ISEE,
synchronous satellites \[9\], and CLUSTER \[10\], the apparently new
aspect of the RPI data is the regularity with which the spreading is
observed, even during periods of deep magnetic quieting. Z mode echoes
are frequently well defined in RPI plasmagrams, and may contain both
discrete and widely spread components. Their potential as diagnostic
tools complementary to the free space modes is being explored. In future
studies that may involve full wave treatments and ray tracing in density
models that include field aligned irregularities, it is hoped to learn
about the specific ways in which irregularities act to produce the
observed range spreading effects. Once progress is made in that area,
high altitude radio sounding may provide a means of studying the scale
sizes and occurrence properties of plasmasphere structures that are
otherwise not easily susceptible to measurement. The RPI data should add
impetus to an already existing body of theoretical work on mechanisms
that may naturally give rise to irregular structures within the
plasmasphere and plasmapause regions \[11,12,13\], such as interchange
and quasi-interchange instabilities, shear flow instabilities, and the
perturbing action of electric fields with various spatial scales.

\[1\] Reinisch, B. W., et al., "The Radio Plasma Imager investigation on
the IMAGE spacecraft," *Space Science Reviews*, IMAGE special issue,
vol. 91, pp. 319-359, February 2000.

\[2\] Carpenter, D. L., et al., "Small-scale field-aligned plasmaspheric
density structures inferred from RPI on IMAGE," *J. Geophys. Res.,* vol.
107, in press, 2002.

\[3\] Fung, S. F., et al., "Observations of Magnetospheric Plasmas by
the Radio Plasma Imager (RPI) on the IMAGE Mission," to be published in
*Adv. Space Res*., 2002.

\[4\] Muldrew, D. B., "Radio propagation along magnetic-field-aligned
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\[5\] Calvert , W., and J. M. Warnock, "Ionospheric irregularities
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\[6\] Smith, R. L., "Propagation characteristics of whistlers trapped in
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\[7\] Angerami, J. J., "Whistler duct properties deduced from VLF
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\[8\] LeDocq, M. J., D. A. Gurnett, and R. R. Anderson, "Electron number
density fluctuations near the plasmapause observed by the CRRES
spacecraft," *J. Geophys. Res.*, vol. 99, p. 23661, 1994.

\[9\] Moldwin, M. B., M. F. Thomsen, S. J. Bame, and D. McComas, "The
fine-scale structure of the outer plasmasphere," *J. Geophys. Res*., v.
100, p. 8021, 1995.

\[10\] Décréau., P. M., et al., "Outer plasmaspheric structures,
topology and dynamics: view from the Whisper instrument on Cluster,"
presented at the American Geophysical Union Fall meeting, paper no.
2273, 2001.

\[11\] Richmond, A. D., "Self induced motions of thermal plasma in the
magnetosphere and the stabiity of the plasmapause," *Radio Sci*., vol.
8, pp.1019-1027, 1973.

\[12\] Lemaire, J., and K. I. Gringauz, *The Earth\'s Plasmasphere*,
Cambridge University Press, 1998.

\[13\] Ferrière, K. M., C. Zimmer, and M. Blanc, "Quasi-interchange
modes and interchange instability in rotating magnetospheres," *J.
Geophys. Res*., vol.106, p. 327, 2001.