Abstract
Abstract. Day-time Pc 3–4 (~5–60 mHz) and night-time Pi 2 (~5–20 mHz) ULF waves propagating down through the ionosphere can cause oscillations in the Doppler shift of HF radio transmissions that are correlated with the magnetic pulsations recorded on the ground. In order to examine properties of these correlated signals, we conducted a joint HF Doppler/magnetometer experiment for two six-month intervals at a location near L = 1.8. The magnetic pulsations were best correlated with ionospheric oscillations from near the F region peak. The Doppler oscillations were in phase at two different altitudes, and their amplitude increased in proportion to the radio sounding frequency. The same results were obtained for the O- and X-mode radio signals. A surprising finding was a constant phase difference between the pulsations in the ionosphere and on the ground for all frequencies below the local field line resonance frequency, independent of season or local time. These observations have been compared with theoretical predictions of the amplitude and phase of ionospheric Doppler oscillations driven by downgoing Alfvén mode waves. Our results agree with these predictions at or very near the field line resonance frequency but not at other frequencies. We conclude that the majority of the observations, which are for pulsations below the resonant frequency, are associated with downgoing fast mode waves, and models of the wave-ionosphere interaction need to be modified accordingly.Key words. Ionosphere (ionosphere irregularities) · Magnetospheric physics (magnetosphere-ionosphere interactions) · Radio science (ionospheric physics)
Highlights
Geomagnetic pulsations are observed at the Earth's surface because magnetospheric hydromagnetic waves propagate down through the ionospheric plasma
The distribution of plasma within the ionosphere can be modi®ed by downgoing hm waves. This is demonstrated in experiments that monitor the Doppler shift of ionospherically reected HF CW radio transmissions
Several studies have demonstrated a correlation between ionospheric Doppler oscillations and geomagnetic pulsations, including Pc 3-4 (7 < f < 100 mHz) pulsations (Menk et al, 1983; Jarvis and Gough, 1988; Tedd et al, 1989; Menk, 1992); Pi 2 (7 < f < 22 mHz) pulsations (Lewis, 1967; Klostermeyer and RoÈ ttger, 1976; Watermann, 1987; Grant and Cole, 1992; Menk, 1992; Liu et al, 1991); and Pc 1 (0.1 < f < 5 Hz) pulsations (Aslin et al, 1991)
Summary
Geomagnetic pulsations are observed at the Earth's surface because magnetospheric hydromagnetic (hm) waves propagate down through the ionospheric plasma. Tedd et al, 1989; Al'perovich et al, 1991; Grant and Cole, 1992; Menk, 1992) This is mainly because of the diculty in obtaining a statistically signi®cant data set with sucient resolution to investigate the amplitude and phase of Pc 3-4 pulsations simultaneously on the ground and in the ionosphere. A surprising result was that for pulsation signals away from the local ®eld line resonance (FLR) frequency the phase of the ionospheric Doppler oscillations was found to consistently lead the pulsations on the ground by 30± 40°, independent of local time, season or magnetoionic mode. The results suggest that these Doppler oscillations may be driven by the electric ®eld of the downgoing fast mode wave
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