Topside sounder proton‐cyclotron echo generation from a plasma memory process and electron Bernstein‐wave propagation

Abstract

Evidence is presented for the radiation of an electron Bernstein wave from a radio frequency pulse simulated by proton‐cyclotron motion. The protons are energized by an ionospheric topside sounder transmitter pulse and take part in a plasma memory process that simulates the original transmitter pulse. These echoes appear on topside ionograms as thin traces. At frequencies below the electron cyclotron frequency fH, the proton echo delay is a multiple of the proton‐cyclotron period tp. At frequencies above fH, there can be an additional delay to tp depending on two factors: 1) the antenna orientation and 2) the closeness of the proton echo frequency f to fH. A model is presented here in which protons passing within a few tens of centimeters of the antenna are energized and modulated by the RF pulse by an amount depending on the RF phase. At multiples of tp later, these protons return to almost the same field line they were on at the time of the RF pulse, although they may have moved some tens of meters parallel or antiparallel to the field line. At multiples of tp, ntp, the protons are thus located in a plane defined by the linear dipole antenna (73 m long tip to tip) at the time of the pulse and the direction of the Earth's magnetic field B. At ntp the protons produce a field which simulates the original field of the RF pulse, and, if wave propagation in the plasma is possible, a pulse is produced which propagates to the new position of the antenna which has moved in the plasma at the satellite velocity Vs. For f > fH, the perpendicularly propagating electron Bernstein waves are responsible for the propagation. No such Bernstein waves exist for f < fH, and proton echoes can be observed at these frequencies if Vs is approximately in the plane of the energized protons at ntp so that wave propagation is not necessary to produce a signal that appears as an echo. For the second factor above, as the wave frequency approaches the electron cyclotron frequency, the group velocity of the Bernstein waves approaches zero, and consequently the propagation time to the antenna increases. This change in group velocity explains the observed increase in tp, which can be as much as several percent.