Ionospheric signatures of cusp latitude Pc 3 pulsations

Abstract

We have compared search coil magnetometer, riometer, photometer, and ELF‐VLF receiver data obtained at South Pole Station and McMurdo, Antarctica (at invariant latitudes of −74° and −79°, respectively), during selected days in March and April 1986. Narrow‐band magnetic pulsations in the Pc 3 period range are observed simultaneously at both stations in the dayside sector during times of low interplanetary magnetic field (IMF) cone angle, but are considerably stronger at South Pole, which is located at a latitude near the nominal foot point of the dayside cusp/cleft region. Pulsations in auroral light at 427.8 nm wavelength are often observed with magnetic pulsations at South Pole, but such optical pulsations are not observed at McMurdo. We have occasionally observed Pc 3 period modulations of the ELF‐VLF signal at South Pole in the 0.5‐ to 1.0‐kHz and 1.0‐ to 2.0‐kHz frequency ranges as well. When Pc 3 pulsations are present, they exhibit nearly identical frequencies, proportional to the magnitude of the IMF, in magnetometer, photometer, and ELF‐VLF receiver signals at South Pole Station and in magnetometer signals at McMurdo. Signals from the 30‐MHz riometer at South Pole are modulated in concert with the magnetic and optical variations during periods of broadband pulsation activity, but no riometer variations are noted during periods of narrow‐band activity. Because riometers are sensitive to electrons of auroral energies (several keV and above), while the 427.8‐nm photometer is sensitive to precipitation with much lower energies, we interpret these observations as showing that precipitating magnetosheathlike electrons (with energies ≤ 1 keV) at nominal dayside cleft latitudes are at times modulated with frequencies similar to those of upstream waves. We suggest that these particles may play an important role, via modification of ionospheric currents and conductivities, in the transmission of upstream wave signals into the magnetosphere and in the generation of dayside high‐latitude Pc 3 pulsations.