Using plasma wave emissions to measure plasma density and to deduce plasma
structure and dynamics

Roger R. Anderson (1,2), Isamu Nagano (2), Satoshi Yagitani (2), 
Hiroshi Matsumoto (3), Kozo Hashimoto (3), Hirotsugu Kojima (3), 
Hironobu Takano (4), Donald L. Carpenter (5), James L. Green (6), and 
John C. Foster (7)

(1) The University of Iowa, Department of Physics and Astronomy, Iowa City, IA
(2) Department of Information and Systems Engineering, Faculty of Engineering,
    Kanazawa University, Kodatsuno, Kanazawa, Ishikawa, JAPAN
(3) Radio Science Center for Space and Atmosphere, Kyoto University, Gokanosho,
    Uji, Kyoto, JAPAN
(4) Toyama Prefectural University, Kosugi, Toyama, JAPAN
(5) Stanford University, STARLab, Stanford, CA
(6) NASA Goddard Space Flight Center, Code 630, Greenbelt, MD
(7) MIT Haystack Observatory, Westford, MA


Both in situ and remote Observations from the GEOTAIL and POLAR Plasma Wave
Instruments (PWI) of emissions related to the electron plasma frequency have
enabled us to measure the electron plasma density in the magnetosphere and deduce
its structure and dynamics. On one pass in January 1993, GEOTAIL went through the
plasmasphere during orbit changing maneuvers. In addition to intense plasmaspheric
hiss seen inside the plasmasphere and chorus seen outside, the PWI also detected
upper hybrid resonance (UHR) emissions that clearly showed the structure and
irregularities of the plasmasphere including ducts and an increase in the density
just before the plasmapause. Similar UHR observations were made by POLAR PWI in
1996 and 1997 during its perigee passes. Remote measurements, although more
challenging to interpret, have provided many interesting observations. Enhanced
escaping terrestrial continuum radiation is believed to be due to injected
electrons from the plasmasheet during substorms drifting around the earth eastward
and impinging on the plasmapause and magnetopause where steep density gradients
occur. Electrostatic emissions at the local electron plasma frequency near
harmonics of the electron cyclotron frequency are mode converted into
electromagnetic radiation that propagates out unless or until it encounters a
region denser than where it was generated. Kilometric continuum radiation in the
100 kHz to 800 kHz frequency range (first identified in the GEOTAIL PWI data) is
believed to be due to a similar 1 source mechanism but from deeper within the
plasmasphere. These emissions contain much narrowband filamentary and fine
structure. Occasionally narrowband filaments lasting many hours have been
observed. Several instances of the escaping terrestrial continuum radiation
evolving into the kilometric continuum radiation have been observed. By comparing
the appearance of these emissions from two or more spacecraft we are able to
deduce remotely the structure of the plasmasphere and magnetosphere. Some of the
structure observed in both these emissions can be explained by the bulges, ducts,
and irregularities found in the plasmasphere. Comparisons of wave data from
GEOTAIL and wave and EUV imaging data from IMAGE indicate that some kilometric
continuum events have their source in density notches deep within the
plasmasphere. Some of these notches have been observed by the IMAGE/EUV to last
for several days as had previously been surmised from ISEE and CRRES PWI number
density measurements. Ground-based Millstone Hill radar observations and GPS
propagation data show equatorial anomalies during extreme geomagnetic storm
conditions where the low mid latitude (near L=2) total electron content (TEC) are
greatly enhanced while deep TEC depletions are observed at the magnetic equator.
In each case studied so far, these equatorial anomaly events were always
coincident with enhanced episodes of kilometric continuum radiation.

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Presented at the 35th COSPAR Scientific Assembly, Paris, France, July 18-25, 2004.