Abstract
Abstract. We have examined several cases of magnetosphere compression by solar wind pressure pulses using a set of instruments located in the noon sector of auroral zone. We have found that the increase in riometric absorption (sudden commencement absorption, SCA) occurred simultaneously with the beginning of negative or positive magnetic variations and broadband enhancement of magnetic activity in the frequency range above 0.1 Hz. Since magnetic variations were observed before the step-like increase of magnetic field at equatorial station (main impulse, MI), the negative declinations resembled the so-called preliminary impulse, PI. In this paper a mechanism for the generation of PI is introduced whereby PI's generation is linked to SCA – associated precipitation and the local enhancement of ionospheric conductivity leading to the reconstruction of the ionospheric current system prior to MI. Calculation showed that PI polarity depends on orientation of the background electric field and location of the observation point relative to ionospheric irregularity. For one case of direct measurements of electric field in the place where the ionospheric irregularity was present, the sign of calculated disturbance corresponded to the observed one. High-resolution measurements on IRIS facility and meridional chain of the induction magnetometers are utilized for the accurate timing of the impact of solar wind irregularity on the magnetopause.
Highlights
The geomagnetic sudden commencement (SC) is a very global phenomenon
We have examined several cases of magnetosphere compression by solar wind pressure pulses using a set of instruments in the around-noon sector
We have found that the increase in the riometric absorption occurred simultaneously with the beginning of positive or negative magnetic variation in auroral zone and broadband enhancement of magnetic activity in the frequency range above 0.1 Hz
Summary
The geomagnetic sudden commencement (SC) is a very global phenomenon. Due to clear onset and well identified source, which is the sudden increase in the solar wind dynamic pressure at interplanetary shocks and discontinuities, the SC-associated events may serve as a diagnostic tool for the magnetosphere and continue to attract scientific interest. Araki, 1994), pulsations of Pc 5 and Pc 1 frequency ranges at higher latitudes (e.g., Saito and Matsushita, 1967; Tepley and Wentworth, 1962), the enhancement of ionospheric absorption (e.g., Ranta and Ranta, 1990) and the intensification of electron and proton auroras (e.g., Brittnacher et al, 2000; Hubert et al, 2003) These effects and inter-relations between them are studied in the present paper. In this study we perform a comparative analysis of magnetic and riometric absorption disturbances caused by SC with the aim to test the hypothesis by Safargaleev and Maltsev (1987) on the ionospheric origin of preliminary impulse and SC-associated Alfven oscillations at high latitudes. In the light of our findings, we make recommendations on the use of SC-excited Pc 1 pulsations in the tasks of magnetospheric physics
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