Abstract
Abstract. We present a multi-instrument study of the ionospheric response to a northward turning of the IMF. The observations were made in the near-noon (11:00 MLT) sector on Svalbard (at 75° MLAT). The data set includes auroral observations, ionospheric flows obtained from the EISCAT and CUTLASS radars, the spectral width of the HF radar backscatter, particle precipitation and plasma flow data from the DMSP F13 satellite, and Pc1 frequency band pulsations observed by induction magnetometers. Careful collocation of all the observations has been made with the HF radar backscatter located by a ray-tracing procedure utilizing the elevation angle of arrival of the signals and an ionospheric plasma density profile. Prior to IMF turning northward, three auroral arcs were observed at the poleward boundary of the closed llbl, inside the llbl, and in the equatorward part of the llbl, respectively. The northward IMF turning was accompanied by enhanced HF radar returns with a broad Doppler spectrum collocated with the arcs. The auroral arcs shifted poleward whereas the backscatter region moved in the opposite direction, which is consistent, respectively, with reconnection beyond the cusp and the capturing of magnetosheath plasma during northward IMF. Locally, magnetic noise enhancement in the Pc1 frequency band occurred simultaneously with the anomalous radar backscatter, and the absence of such signals at more remote magnetic observatories indicates a local generation of the Pc1 turbulence, which is collocated with the radar backscatter. Finally, we discuss possible interpretation errors which may be caused by limited observational data.
Highlights
The entrance into the dayside magnetosphere of new plasma, the characteristics of which are different from those of the ambient (“old”) plasma, plays an important role in the magnetosphere, since it may lead to plasma instabilities
We present a multi-instrument study of the ionospheric response to an isolated positive excursion in the interplanetary magnetic field (IMF) Bz component
The data set includes auroral observations (ASC TV camera), convection patterns obtained from UHF European incoherent scatter (EISCAT) and HF CUTLASS radars, spectral width of the HF radar backscatter, particle precipitation and plasma flow data from the DMSP F13 satellite, Pc1 pulsations observed by induction magnetometers, and solar wind parameters measured by the Geotail satellite just in front of the magnetosphere
Summary
The entrance into the dayside magnetosphere of new plasma, the characteristics of which are different from those of the ambient (“old”) plasma, plays an important role in the magnetosphere, since it may lead to plasma instabilities. The penetration of solar wind plasma into the magnetosphere is possible by a number of different mechanisms It could occur through the development of MHD instabilities (Farrugia et al, 1998; Safargaleev and Lyatsky, 1991). Moen et al, 2001), whereas during intervals of positive (northward) Bz, green auroras were related to the cusp poleward boundary (Sandholt et al, 2001) In spite of this progress, the very limited available sets of coordinated satellite/radar/optical observations have far not allowed a comprehensive analysis of the magnetospheric response to IMF Bz northward turning. We show that the sequence of radar, optical and magnetic events accompanying the positive Bz excursion can be interpreted as indications of magnetosheath plasma captured into the dayside magnetosphere. A further purpose of this paper is to extend the set of IMF transition events investigated by documenting the effects occurring in the high-latitude ionosphere during a “pure magnetic” event (i.e. one with no changes in the solar wind pressure)
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