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Observations of ion cyclotron harmonic structure in type 1 to 4 VHF auroral echoes
Author(s) -
Prikryl P.,
André D.,
Sofko G. J.,
Koehler J. A.,
McKibben M. J.
Publication year - 1988
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/gl015i006p00557
Subject(s) - physics , cyclotron , doppler effect , spectral line , plasma , ion , instability , harmonic , computational physics , radar , atomic physics , geophysics , astrophysics , astronomy , nuclear physics , telecommunications , quantum mechanics , computer science , mechanics
The understanding of E region plasma instabilities is crucial for the interpretation of the enormous stream of data from large ongoing projects like STARE and SABRE in Europe, and BARS in Canada, in which measurements of radar scatter from E region plasma waves are used to infer the electric fields, hydromagnetic waves, and field‐aligned currents in the E region. Auroral echoes measured by 50 MHz CW Doppler radars strongly indicate that the electrostatic ion cyclotron (EIC) mechanism may be more important in auroral irregularity formation than was previously suspected. Doppler spectra taken over 64 ms intervals are highly structured, showing as many as seven or eight discrete frequency components whose frequency separation is often approximately the cyclotron frequency of one of the principal ions in the E region. In this note, we show that when examples of each of the four standard spectral types (1 to 4), whose classification is based upon time‐averaging over several seconds, are examined with the finer time resolution of 64 ms (i.e. a resolution of the same order as the lifetime of the individual plasma irregularities), all four types appear to be composites of many harmonically structured bursts as described above. Since the spectral components in individual bursts may occur at frequencies that are not exact multiples of the ion gyrofrequency, time averaging over many bursts usually smears out the harmonic content, resulting in broader spectral peaks. The resulting smoothed spectra then appear to be caused by instabilities other than the EIC instability.