Upstream waves and field line resonances: simultaneous presence and alternation in Pc3 pulsation events

J Verõ,I Best,J C Miletits,L Holló,J Střeštik,M Vellante,J Szendrõi,B Zieger,H Lühr

doi:10.1007/s005850050577

Abstract

Based on a detailed study of Pc3 events at an array between L = 1.5 and 3 in Central Europe, the authors found quick changes between upstream waves (UW, i.e. pulsation directly driven by UW) and field line resonance (FLR, i.e. azimuthal oscillations of geomagnetic field lines). The alternation of the two types is especially characteristic (and the UW part stronger) if the interplanetary magnetic field (IMF) is highly variable. Events due to field line resonance may have a structure consisting of multiple lines with frequencies differing by about 10%, corresponding to neighbouring shells of field lines separated by about 100 km at the surface. This coincides with previous findings (about 10% at a meridional distance of 80 km). The frequency of the UW type is well correlated with the frequency of waves in the interplanetary medium. Additionally, there are signals of unidentified origin which also seem to be influenced by IMF.>Key words. Magnetosphere Physics Â· MHD waves and instabilities Â· Plasmasphere Â· Solar wind/magnetosphere interactions

Highlights

In the ®rst six months of 1991, the geomagnetic observatories Niemegk (NGK), Nagycenk (NCK) and L'Aquila (AQU), lying approximately along a meridian (L-values of the three stations at a height of 110 km are 2.25, 1.87 and 1.55, respectively), carried out a pulsation campaign
It will be shown that the occurrence frequency of such UW-impulses increases with increasing variability of IMF, and they may originate from the interplanetary medium, while beats are artefacts of the interference without any special source
Two with variable spectra were selected for a detailed study of the quick changes in the pulsation regime

Summary

Introduction

In the ®rst six months of 1991, the geomagnetic observatories Niemegk (NGK), Nagycenk (NCK) and L'Aquila (AQU), lying approximately along a meridian (L-values of the three stations at a height of 110 km are 2.25, 1.87 and 1.55, respectively), carried out a pulsation campaign. In a ®rst study of the collected material (VeroÄ et al 1995), the average hourly spectra obtained at each of the three stations were studied from the point of view of distinguishing the two types of pulsations and to ®nd characteristic parameters for them. The ®rst selection was subjectively made so that the sample should include all distinguishable pulsation types (period changing with L-value, constant period everywhere, intermittent activity, etc.). For these intervals high-resolution dynamic spectra were produced from the available records of all stations. The reliability of the power and dynamic spectra was checked by comparing spectra computed by dierent methods From these intervals, two with variable spectra were selected for a detailed study of the quick changes in the pulsation regime. The 0800±0900 UT, 22 May event is presented in some detail, as an event with several similar, and some dierent characteristics

Detailed study of the dynamic spectra

Dynamic spectra of the event with very high frequency resolution

Other intervals from the 1991 and 1984 arrays

Findings

Discussion