Applying ionospheric plasma resonance scaling laws to interpret active and passive 
planetary magnetospheric radio spectra

Robert F. Benson (1), Vladimir A. Osherovich (2), Joseph Fainberg (1), 
James L. Green (1), Bodo Reinisch (3)

(1) NASA/Goddard Space Flight Center, Code 692
Greenbelt, 20771, USA
Tel: +01-301-286-4037, Fax: +01-301-286-1683, Email: u2rfb@lepvax.gsfc.nasa.gov

(2) Emergent/Goddard Space Flight Center
Tel: +01-301-286-3649, Fax: +01-301-286-1683, Email: vladimir@urap.gsfc.nasa.gov

(3) Environmental, Earth, and Atmospheric Sciences Department, Center for
Atmospheric Research, University of Massachusetts Lowell
Lowell, 01854, USA
Tel: +01-978-934-4903, Fax: +01-978-459-7915, Email: bodo_reinisch@uml.edu

This paper addresses a controversy concerning the application of scaling laws,
developed to describe ionospheric topside-sounder-stimulated plasma resonances, to
the interpretation of active and passive radio spectra in planetary magnetospheres.
 At issue is the application of scaling laws developed by Osherovich and Benson
[JGR, 96, 19331, 1991] to describe a sequence of these resonances, designated as Dn
resonances, to the interpretation of (1) plasma resonances stimulated by the
Ulysses Unified Radio and Plasma Wave (URAP) relaxation sounder in Jupiter's Io
plasma torus [Osherovich et al., JGR, 98, 18751, 1993] and (2) banded
magnetospheric radio emissions observed by the Active Magnetospheric Particle and
Tracer Explorer/Ion Release Module (AMPTE/IRM) [Benson et al., JGR, 106, 13,179,
2001a]. In addition to the Dn resonances, which are observed at frequencies below
the electron plasma frequency fpe and between the harmonics of the electron
gyrofrequency fce, the above interpretations involved the sounder-stimulated Qn
resonances at frequencies above fpe and between nfce. Benson et al. [Radio Sci.,
32, 1127, 1997] presented arguments to support the claim of Osherovich et al.
[1993] that the URAP Io plasma torus spectra did not include nfce resonances. Le
Sager et al. [JGR, 103, 26667, 1998] presented an alternative interpretation of the
URAP sounder spectra based on Doppler-shifted nfce resonances rather than Dn
resonances. Canu [RS, 36, 171, 2001a] presented GEOS 1 and 2 and ISEE 1
magnetospheric relaxation sounder data to counter the claims of Benson et al.
[1997] and to support the interpretation of Le Sager et al. [1998]. The Canu
[2001a] work has led to a further exchange [Benson et al., RS, 36 1649, 2001b;
Canu, RS, 36, 1645, 2001b]. A point stressed in both Canu [2001a] and Canu [2001b]
is that the Dn resonances are not observed by the GEOS 1 and 2 and ISEE 1
relaxation sounders in the magnetosphere. In addition, Canu [2001b] calls into
question the Dn interpretation of the AMPTE/IRM banded emissions by Benson et al.
[2001a].

A resolution of this controversy is important for three reasons: (1) to test the
claim of Osherovich [JGR, 94, 5530, 1989] that the Dn resonant frequencies
represent a signature of a fundamental new mode of plasma oscillation, namely, to
eigenmodes of cylindrical-electromagnetic plasma oscillations where the energy
densities of the electric and magnetic fields are in balance, (2) to obtain the
proper interpretation of, and possible relationship between, the sounder-stimulated
plasma resonances and banded magnetospheric emissions (also known as (n+1/2)fce
emissions) and (3) to provide a reliable determination of the magnetospheric
electron density from (2) above.

Here we will demonstrate that the Radio Plasma Imager (RPI) on the Imager for
Magnetopause-to-Aurora Global Exploration (IMAGE) satellite stimulates Dn
resonances in the magnetosphere when fpe/fce > 2. We will also demonstrate the
importance of using all three of the RPI mutually orthogonal dipole receiving
antennas for identifying them and that they can correspond to natural emissions.
The Dn identification is based on the scaling laws discussed earlier and on the
accurate (within a few percent) determination of fpe and fce from other resonant
and wave cutoff features.

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Presented at the 27th General Assembly of International Union of Radio 
Science (URSI), Maastricht, The Netherlands, August 17-24, 2002.