Dependence of ground-based Pc5 ULF wave power on F10.7 solar radio flux and solar cycle phase Kyle R. Murphy n , Ian R. Mann, I. Jonathan Rae, David K. Milling University of Alberta, Department of Physics, 11322–89 Ave, Edmonton Alberta, Canada T6G 2G7 article info Article history: Received 1 April 2010 Received in revised form 20 December 2010 Accepted 23 February 2011 Available online 8 March 2011 Keywords: Pc5 ULF wave power Solar cycle F10.7 Field line resonances abstract Utilising fifteen (1990–2005) years of ground-based magnetometer data from four magnetometer stations, we characterise the statistical dependence of the Pc5 ULF wave power spectra on variations in F10.7 solar radio flux and on solar cycle phase. We show that the median Pc5 ULF wave power spectra can be characterised as a power-law with a localised Gaussian centred at a specific frequency superimposed on the power-law spectrum. Further, we demonstrate that the location of the Gaussian in frequency systematically varies with both solar cycle phase and F10.7 and is more pronounced during high-speed solar wind intervals. We postulate that the localised power spectrum enhancement (or Gaussian) is a manifestation of the local eigenfrequency of field line resonances in the Earth’s magnetosphere and that the variation in the location of the Gaussian occurs as a result of increased ionospheric outflow during periods of enhanced F10.7 and active solar activity. & 2011 Elsevier Ltd. All rights reserved. 1. Introduction Ultra low frequency (ULF) waves in the Pc5 wave band (150–600 s period) are thought to play a pivotal role in energising the magnetospheric cavity in a variety of ways (see e.g., the review by Wright and Mann, 2006). These waves can play an important role in a number of magnetospheric processes including, but not limited to, energy transport across the magnetopause (e.g., Mann et al., 2002, 1999; Miura, 1987; Rae et al., 2005; Southwood, 1974), ohmic energy dissipation in the ionosphere (e.g., Rae et al., 2008), the energisation and transport of relativistic electrons (e.g., Degeling et al., 2007; Elkington et al., 1999, 2003; Hudson et al., 2001; Mann et al., 2004; Mathie and Mann, 2000), the formation of auroral arcs (Samson et al., 1996), the modulation of electron precipitation (Rae et al., 2007), and the dynamics of the ring current (e.g., Ozeke and Mann, 2001, 2008). In this study, we present results of a statistical analysis of the Pc5 wave power spectra during varying solar conditions, specifically to examine their relationship to solar cycle and F10.7 solar radio flux. The characterisation of the Pc5 wave spectra has become increasingly important especially for ULF radial diffusion models which use Pc5 wave power as in input parameter for determining the electric and magnetic field diffusion coefficients (Brizard and Chan, 2004; Elkington et al., 2003; Mathie and Mann, 2001 and references therein). Historically, the characterisation of Pc5 ULF wave power has typically concentrated on the dependence of summed or integrated Pc5 wave power as a function of solar wind speed. Engebretson et al. (1998) utilised 6 months of high-latitude ground- based magnetometer data from two magnetometer stations, one from the International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometer array and the other from Magnetometer Array for Cusp and Cleft Studies (MACCS), and showed that the integrated horizontal (sum of the power in the geographic north– south and east–west components) Pc5 wave power exhibited a clear power-law dependence on solar wind velocity. In particular, Engebretson et al. (1998) demonstrated that the integrated Pc5 wave power could be modelled by a power-law of the form Av sw n , where v sw is the velocity of the solar wind and the constants n and A are individually determined for each magnetometer. Mathie and Mann (2001) used 3 years of data from six IMAGE magnetometer stations to characterise the summed H-component (north–south) Pc5 power as a function of latitude (L-shell) and solar wind speed. Similar to Engebretson et al. (1998), Mathie and Mann (2001) found that the summed Pc5 wave power is highly correlated with solar wind speed and can be characterised by a power-law of the form Av sw n with exponents surprisingly being very close to the L-shell of the station ranging between 3.50 and 7.98 across this L-shell range, this relationship being verified for 3 years of data from 1995 to 1997. Additionally, Mathie and Mann (2001) showed that the summed H-component Pc5 wave power is correlated with the latitudinal location of individual magnetometer stations. More specifically these authors showed that the summed Pc5 wave power decreased exponentially with decreasing L. Pahud et al. (2009) conducted one of the largest statistical studies of integrated Pc5 band wave power. These authors utilised over 15 years of ground-based magnetometer data from the Canadian Auroral Network for the Open Program Unified Study (CANOPUS (Rostoker et al., 1995), now the Canadian Array for Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jastp Journal of Atmospheric and Solar-Terrestrial Physics 1364-6826/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jastp.2011.02.018 n Corresponding author. Tel.: þ1 780 9142539. E-mail address: kmurphy@ualberta.ca (K.R. Murphy). Journal of Atmospheric and Solar-Terrestrial Physics 73 (2011) 1500–1510