| 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) |