A statistical correlation of Pc5 pulsations and solar wind pressure oscillations P.T.I. Eriksson a, * , A.D.M. Walker b , J.A.E. Stephenson b a Alfve ´n Laboratory, Department of Plasma Physics, Royal Institute of Technology, KTH 10044 Stockholm, Sweden b School of Pure and Applied Physics, University of KwaZulu-Natal, Durban 4041, KwaZulu-Natal, South Africa Received 28 July 2004; received in revised form 26 May 2005; accepted 16 August 2005 Abstract The SHARE high frequency (HF) radar in Antarctica is used to compare ionospheric plasma flow oscillations in the Pc5 frequency range with low-frequency oscillations in the solar wind pressure measured by the ACE spacecraft. Ten different days in 2000 and 2001 are analysed with respect to different frequencies and geomagnetic latitudes. Both data sets are bandpass filtered and a complex demodula- tion technique is used to calculate the correlation in each band. On a number of occasions the wave packet structure of the Pc5 pulsations is in good or excellent agreement with the wave packet structure of the solar wind pressure oscillations. This strongly suggests that the oscillations were directly driven by the solar wind. Particularly good correlation is found in the frequency band 0.8–1.2 mHz. Pulsations in this frequency range are hard to reconcile with the magnetospheric cavity mode model. We conclude that, at least on some occasions, Pc5 pulsations may be directly driven and the magnetosphere cavity/waveguide then assumes a more passive role. Ó 2005 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: ULF waves; Pc5 pulsations; Solar wind waves; Magnetospheric waveguide 1. Introduction Rapid variations in the geomagnetic field are called magnetic pulsations. These pulsations can either be contin- uous, they are then called Pc pulsations, or irregular, called Pi pulsations. A classification based on frequency for geo- magnetic pulsations is given by Jacobs et al. (1964). The first observations of these pulsations were made on the ground by Stewart (1861), but it was not until 1954 that Dungey suggested that mangnetohydrodynamic (MHD) waves in the magnetosphere were the source of the mea- sured fluctuations in the magnetic field. The MHD waves propagate along the magnetic field lines and are reflected at the ionosphere due to its high conductivity. The waves can only satisfy the boundary conditions at the ionosphere for certain frequencies. This gives a requirement on the wavelength of the wave. For a given latitude (L-shell) the eigenfrequencies are given by x Z dS v A ¼ np ð1Þ where n is an integer and the integration is performed along the field line. Transverse waves with these frequencies are then standing waves. Here, we will direct our attention to the Pc5 pulsations. This type of geomagnetic pulsations have been observed (Samson et al., 1991; Ruohoniemi et al., 1991; Mathie et al., 1999) to occur at certain favoured frequencies: 1.3, 1.9, 2.6 and 3.3 mHz. The source (or sources) of the Pc5 pulsations is still a matter of debate. The first theory (Southwood, 1974; Chen and Hasegawa, 1974) was that a Kelvin–Helmholtz instability at the magnetopause, driven by the streaming magnetosheath plasma, produces surface waves. These surface waves create compressions in the magnetosphere and thus compressional (fast mode) waves. The compressional waves can propagate across the geo- magnetic field lines in the magnetosphere and excite shear Alfve ´n waves at the location where the wave frequency matches the eigenfrequency of the local magnetic field line 0273-1177/$30 Ó 2005 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2005.08.023 * Corresponding author. Tel.: +46 8 7907694. E-mail address: tommy.eriksson@alfvenlab.kth.se (P.T.I. Eriksson). www.elsevier.com/locate/asr Advances in Space Research 38 (2006) 1763–1771