Abstract
Abstract. A case study of medium-scale travelling ionospheric disturbances (TIDs) that are correlated with solar wind Alfvén waves is presented. The HF radar ground-scatter signatures of TIDs caused by atmospheric gravity waves with periods of 20-40min are traced to a source at high latitudes, namely pulsed ionospheric flows (PIFs) due to bursts in the convection electric field and/or the associated ionospheric current fluctuations inferred from ground magnetic field perturbations. The significance of PIFs and TIDs in the context of solar-terrestrial interaction is that Alfvénic fluctuations of the interplanetary magnetic field (IMF) observed in the solar wind plasma streaming from a coronal hole correlate with PIFs and TIDs. The link between the solar wind Alfvén waves and TIDs is corroborated by the ground magnetic field signatures of ionospheric current fluctuations that are associated with the IMF-By oscillations and TIDs. The observed PIFs and the associated negative-to-positive deflections of the ground magnetic field X component are interpreted as ionospheric signatures of magnetic reconnection pulsed by solar wind Alfvén waves at the dayside magnetopause. Although the clarity of the radar line-of-sight velocity data may have been affected by anomalous HF propagation due to intervening TIDs, the application of a pure state filtering technique to analyze the radar data time series reveals a one-to-one correspondence between PIFs, TIDs and solar wind Alfvén waves. The spectra of solar wind and ground magnetic field perturbations are similar to those of PIFs and TIDs. The ground-scatter signatures indicate TID wavelengths, phase velocities and travel times that are consistent with ray tracing, which shows a subset of possible gravity wave group paths that reach the F region from a source in the E region after the wave energy first travel downward to the upper mesosphere where the waves are reflected upward. The observed one-to-one correspondence between the convection electric field bursts and TIDs is consistent with the modeling results for large-scale TIDs by Millward et al. (1993a,b). The correlation with solar wind Alfvén waves points to very direct coupling of energy in the solar wind into the subauroral atmosphere.
Highlights
In his seminal paper on the existence of a new kind of wave, namely electromagnetic-hydrodynamic waves, Alfven (1942) foresaw their importance in solar physics
A one-to-one correspondence among solar wind Alfven waves, pulsed ionospheric flows (PIFs)/ionospheric current intensifications and travelling ionospheric disturbances (TIDs) points to a cause-effect relationship. It indicates that the observed wave fronts of atmospheric gravity waves (AGWs)/TIDs were produced by a series of quasi-periodic pulses in the convection electric field (PIFs) and/or associated ionospheric current rather than a continuous wave train generated by a single pulse
Solar wind Alfven waves coupling to the dayside magnetosphere are observed to pulse the ionospheric convection and associated current in the cusp footprint and dusk convection cell
Summary
In his seminal paper on the existence of a new kind of wave, namely electromagnetic-hydrodynamic waves, Alfven (1942) foresaw their importance in solar physics. It is shown that, through the coupling to the magnetosphere-ionosphere-atmosphere system, solar wind Alfven waves generate atmospheric gravity waves (AGWs) and their ionospheric signatures – travelling ionospheric disturbances (TIDs). HF radars were used to observe ionospherically generated mediumscale gravity waves, which may be Earth-reflected and produce trains of TIDs, many hundreds of kilometers equatorward of the original source region (Samson et al, 1989, 1990). These radars are a part of SuperDARN (Super Dual Auroral Radar Network) (Greenwald et al, 1995).
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