HR: 1340h
AN: SM23A-0465 [Abstracts]
TI: Interpretation of Vector Electric Field Measurements by RPI Using Three Dipole Antennas on
IMAGE
AU: * Sonwalkar, V S
EM: ffvss@uaf.edu
AF: University of Alaska Fairbanks, 306 Tanana Drive, Duckering 203, P.O.Box 755915, Fairbanks, AK
99775-5915
United States
AU: Li, J
EM: fsjl3@uaf.edu
AF: University of Alaska Fairbanks, 306 Tanana Drive, Duckering 203, P.O.Box 755915, Fairbanks, AK
99775-5915
United States
AU: Proddaturi, R
EM: ftrkp@uaf.edu
AF: University of Alaska Fairbanks, 306 Tanana Drive, Duckering 203, P.O.Box 755915, Fairbanks, AK
99775-5915
United States
AU: Venkatasubramanian, A
EM: ftav@uaf.edu
AF: University of Alaska Fairbanks, 306 Tanana Drive, Duckering 203, P.O.Box 755915, Fairbanks, AK
99775-5915
United States
AU: Carpenter, D L
EM: dlc@nova.stanford.edu
AF: Stanford University, STAR Laboratory, Packard Building, Stanford, CA 94305-9515
United States
AU: Benson, R
EM: robert.f.benson@nasa.gov
AF: NASA Goddard Space Flight Center, Code 692, Greenbelt, MD 20771
United States
AU: Reinisch, B
EM: bodo_reinisch@uml.edu
AF: University of Massachusetts, Center for Atmospheric Research, 600 Suffolk Street, Lowell, MA 01845
United States
AB:
RPI is a multi-mode instrument using three orthogonal
thin-wire antennas - two long (nominal length of 500-m tip-to-tip)
dipoles in the spin plane (X and Y antenna) and one short (nominal
length of 20-m tip-to-tip) dipole along the spin axis (Z antenna).
The X antenna is used for transmission and all three antennas are
used for reception to measure three components of electric field.
Unfortunately, on October 3, 2000, one of the X-axis monopoles was
partially severed, apparently by a micrometeorite, reducing the
dipole length to 340 m. On September 18, 2001 an unknown section
of the Y antenna was lost. Therefore, it is difficult to estimate
three components of wave electric field. Measurement of three
components of electric field is important to determine wave normal
directions of the received echoes and to estimate the radiated and
received powers. We have used known source locations, relations
for polarization and refractive index as a function of wave normal
direction, and antenna orientations in order to interpret the
amplitudes of electric field components as measured by three
antennas. The RPI transmission frequency ranges from 3 kHz to 3
MHz, which allows multiple modes of propagation including
whistler, Z, and free space modes. We analyzed two kinds of
signals: 1) whistler mode waves from the ground transmitter
observed at the IMAGE satellite, 2) discrete Z mode echoes
propagating within the Z mode cavities. In both cases the wave
normal direction is presumed to be known - vertical in the case of
ground transmitter signal and parallel to the geomagnetic field in
the case of discrete Z mode echoes. Ground transmitter signals in
the frequency range 10-30 kHz are commonly observed when IMAGE is
in the northern hemisphere. In one case, observed on October 2,
2002, NAA signal at 24 kHz was observed on three antennas.
Voltages induced across X and Y antenna terminals were
consistently $\sim$30-40 dB higher than those measured across the
Z antenna. These observations can be explained by taking into
account the differences in the three antenna lengths, their
orientation, and the vertical wave normal direction. Discrete Z
mode echoes result from reflections at the Z mode lower cutoff
frequency ($f_{z}$) when waves propagate in both upward and
downward directions from the satellite. The expected polarization
for these echoes based on parallel propagation is circular. The
measured polarization showed significant deviations from circular
polarization. We show that multipath propagation in Z mode
cavities can explain these deviations from circular polarization.
A further evidence of multipath propagation is provided by
multiple time delays at each frequency for the discrete Z mode
traces.
DE: 2409 Current systems (2708)
DE: 2471 Plasma waves and instabilities
DE: 2487 Wave propagation (6934)
DE: 2772 Plasma waves and instabilities
SC: SPA-Magnetospheric Physics [SM]
MN: 2004 AGU Fall Meeting