Abstract
Abstract. Compressional waves in the Earth's current sheet, driven by the high-speed plasma flows connected to substorms, are investigated using the Cluster magnetometer and plasma instrument. During the time that Cluster had its apogee in the magnetotail (July through October 2001), we have studied 5 events in detail. We find compressional waves in the 30–60mHz band, at a spectral power density that is dependent on when and where the event is observed. There is a difference of two orders of magnitude in power density between waves at substorm onset and waves during quiet times. Strong plasma flows are the driver of the wave power. The spacecraft location in the current sheet is also important for the spectral power density. Having four spacecraft available we can discern spatial from temporal variations. We have determined the propagation direction of the waves in the 30–60mHz band and found that in the Cluster rest frame they propagate in the same direction as the plasma flow at an angle 30° < ∆φ < 40° with respect to the plasma flow direction in the spacecraft' rest frame. Key words. Magnetospheric physics (magnetotail; MHD waves and instabilities; plasma sheet)
Highlights
Waves in the Earth’s magnetotail have been a subject of theoretical and experimental investigations since the late 1960s
The plasma sheet model proposed by Harris (1962) is mostly used to describe the magnetic field in the tail
1. at strong substorm onset/during the expansion phase; 2. onset/expansion phase the maximum power is 102≤P30−60 during late substorm phase and re-intensification; 3. at quieFtigure 1≤1 103 nT2/Hz, later we find 10≤P30−60≤102 nT2/Hz and times
Summary
Waves in the Earth’s magnetotail have been a subject of theoretical and experimental investigations since the late 1960s (see, e.g. Hruska and Hruskova, 1970). In this paper we will extend the investigation of Bauer et al (1995a, b) by making case studies of five events, using the magnetic field (Balogh et al, 2001) and plasma measurements (Reme et al, 2001) from the four Cluster spacecraft. The first two events that we will investigate occur during a substorm expansion phase, as defined by a sudden increase in the AE index They are the neutral sheet crossings on 17 July and 22 August 2001. The power in the left- and right-hand polarized waves is an order of magnitude smaller than that in the compressional waves (see Fig. 4a and Bauer et al, 1995a, b) and we will concentrate on the compressional waves This holds for all events discussed in this paper. We will not discuss these waves in this paper, but concentrate on signatures at higher frequencies
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