Abstract
Abstract. In this work we perform a statistical analysis of the ionospheric echo response observed by six radars of the SuperDARN network in the Northern Hemisphere, over 236 Sudden Impulses (SI) of solar wind dynamic pressure events (from 1997 through 2000). For that purpose, we make use of MRS, the Mean Rate of Scattering, as a function of time during the SI event. We classify the events in sudden increases (I events, 144 cases) and decreases (D events, 92 cases) of the solar wind dynamic pressure. Moreover, we make use of the AE index to define two distinct conditions of the ionosphere under which each event may take place: Quiet and Disturbed. Regarding Quiet conditions, for both I and D events, we find that MRS displays an increase related to the SI time. On the contrary, for Disturbed conditions, D events display an increase in MRS, while I events show a clear dip. The similarity of response for I and D events under Quiet conditions is briefly discussed, but the smaller number of D events does not allow one to further analyse them. As for the I events, a latitudinal analysis shows that the MRS increase for Quiet conditions is seen both at low latitudes (60°–70° Λ) and at high latitudes (70°–80° Λ); for Disturbed Is the MRS decrease is stronger at high latitudes. We suggest that the MRS increase for Quiet Is can be due to two different mechanisms: 1) a soft electron precipitation induced by Field Line Resonances (FLR) or loss cone instability at lower latitudes; 2) an enlargement of the cusp at higher latitudes, which in turn may induce enhanced particle precipitation. For what concerns Disturbed Is, the MRS decrease can be produced by a higher energy electron precipitation (>1 keV), that enhances the electron density in the E and D regions. This provokes a strong absorbtion of the radio waves in the D region and a higher refraction in the E region, leading to a decrease in MRS, especially at higher latitudes. For I events a further classification has been made on the basis of IMF orientation: this suggests that the effects summarized above are due to the SI itself. Keywords. Ionosphere (Ionospheric irregularities) – Magnetospheric physics (Magnetosphere-ionosphere interactions; Solar wind-magnetosphere interactions)
Highlights
Sudden Impulses of solar wind dynamic pressure (SI) have long been known to trigger a global response of the magnetosphere-ionosphere system
We can conclude that the mean level of geomagnetic activity is surely influenced by the IMF orientation, but our results indicate that the solar wind dynamic pressure itself leads to clear effects on the Mean Rate of Scattering (MRS), in both situations of northward and southward Bz
In this work 236 SI events were examined to investigate their effects on SuperDARN HF radar echoes
Summary
Sudden Impulses of solar wind dynamic pressure (SI) have long been known to trigger a global response of the magnetosphere-ionosphere system. SIs are sudden variations of the solar wind dynamic pressure associated with corotating and travelling solar wind shocks and tangential discontinuities, whose effects on the magnetosphere extend from hours to days Coupling to the ionosphere requires the generation of a Field-Aligned Current (FAC) system that is carried by a field-guided Alfven mode, leading to the formation of characteristic vortex-like structures, whose signatures are well identified in ground magnetometers Following Araki (1994), both positive variations (sudden compressions of the magnetopause), and negative variations (sudden relaxations of the magnetopause) of the solar wind dynamic pressure, should act on the magnetosphere-ionosphere system in the same way, but for the FAC direction and the vortex circulation sense, which should be opposite in the two cases. Takeuchi et al (2000) made a case study of a negative sudden impulse, and found a good agreement with Araki’s picture. Thorolfsson et al (2001) reported about two SIs, one positive and one negative, with a few hours separation
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