Proton cyclotron frequency phenomena in the topside ionosphere

Abstract

Proton cyclotron echoes and spurs are phenomena related to the proton cyclotron frequency discovered on topside sounder ionograms from Canadian Alouette satellites. The echoes and spurs appears on the ionograms at apparent ranges which lead to a frequency close to the proton cyclotron frequency; the frequency is obtained by taking the reciprocal of the time elapsed between the transmission of the sounder pulse and the reception of the signal at the satellite. Aloutte II and ISIS and II ionograms for about sixty satellite passes were scaled to study the charateristics of these phenomena. Generally, proton cyclotron echoes and spurs occured on the ionograms at frequencies below the electron plasma frequency f N , the echoes predominantly slightly above the electron cyclotron frequency f H and the spurs just below f N . They appeared most often when a harmonic of the electron cyclotron frequency nf H (n = 1, 2, 3, 4) was approximately equal to one of the other characteristic frequencies, that is: (1) nf H ≈ f N , (2) nf H ≈ f z S, the frequency of the Z wave at the heght of the satellite, and (3) nf H ≈ f T , the upper hybrid resonance frequency. Proton cyclotron echoes, spurs and protein cyclotron wave patterns have many features in common in addition to their fundamental relationship with the proton cyclotron frequency. The echoes and spurs are observed most often when when nf H overlaps one of the other characteristic frequencies, that is: nf H ≈ f N , nf H ≈ f z S, and nf H ≈ f T . The proton cyclotron wave pattern is observed under the first of the three conditions. It appears that the occurence of the phenomena is related to the plama conditions, the geographic location not being important in itself except that reflects different plasma conditions. Although proton cyclotron echoes and spurs were observed more often near the geomagnetic equator, consistent with the results of Matuura and Nishizaki,(8) they still observed at high latitudes even near the north geomagnetic pole. The echoes and spurs occur at frequencies below f N , the echoes predominantly slightly above f H and the spurs just below f N . Generally it is easy to distinguish between the two since usually they appear separately or, if together, often an echo would terminate and a spur begin at a slightly different apparent range. But it is not always easy since sometimes it appeared that a proton cyclotron echo and a spur formed a continuous trace, suggesting that perhaps they may be different manifestations of the same phenomenon. Work is continuing in an attemp to understand the origin of proton cyclotron echoes, spurs, and proton cyclotron wave patterns.