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Distinguishing Alfvén waves from quasi‐static field structures associated with the discrete aurora: Sounding rocket and HILAT satellite measurements
Author(s) -
Knudsen D. J.,
Kelley M. C.,
Earle G. D.,
Vickrey J. F.,
Boehm M.
Publication year - 1990
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/gl017i007p00921
Subject(s) - physics , sounding rocket , ionosphere , geophysics , magnetosphere , rocket (weapon) , computational physics , magnetic field , depth sounding , electric field , geology , aerospace engineering , astronomy , oceanography , engineering , quantum mechanics
We present and analyze sounding rocket and HILAT satellite measurements of the low frequency (< 1 Hz) electric and magnetic fields δ E and δ B perpendicular to the Earth's magnetic field B 0 in the auroral oval. By examining the time‐domain field data it is often difficult to distinguish temporal fluctuations from static structures which are Doppler shifted to a non‐zero frequency in the spacecraft frame. However, we show that such a distinction can be made by constructing the impedance function Z(f) = μ 0 |δ E ( f )/δ B ( f )|. Using Z(f) we find agreement with the static field interpretation below about 0.1 Hz in the spacecraft frame, i.e. Z(f) = Σ p −1 where Σ p is the height‐integrated Pedersen conductivity of the ionosphere. Above 0.1 Hz we find Z(f) > Σ p −1 , which we argue to be due to the presence of Alfvén waves incident from the magnetosphere and reflecting from the lower ionosphere, forming a standing wave pattern. These waves may represent an electromagnetic coupling mechanism between the auroral acceleration region and the ionosphere.