Abstract
Abstract. In the first part of this study we present two case studies of pulsations that, with the help of ground-based data, are identified as field line resonances (FLRs). These pulsations occurred at frequencies which belong to a set of frequencies that has been suggested to be preferred in the terrestrial magnetosphere (CMS frequencies). We go on to show that for both events there is a significant signature at the same frequency in the time series of the compressional magnetic field observed by the conjugate Cluster satellites. We interpret these as signatures of the compressional mode driving the FLRs. In the second part we present a statistical study including one year's worth of Cluster magnetic field data. For each orbit between May 2004 and June 2005 we identified a three hour interval during which the satellites were located on closed magnetic field lines. We identified peaks in the spectrum between 1.0 and 15.0 mHz of the compressional, poloidal and toroidal components of the magnetic field. We use this database of spectral peaks observed on closed magnetic field lines to investigate whether peaks occur at a preferred set of frequencies which would be indicative for the Earth's magnetosphere behaving like a cavity/waveguide. We find no consistent preference for all CMS frequencies in our dataset, however we do find a preference for certain higher frequencies suggesting that higher harmonics of the cavity/waveguide are a persistent feature of the inner magnetosphere, and are detected by the Cluster spacecraft. This result could be explained by the polar orbit of the Cluster satellites.
Highlights
Since ground-based magnetometers were deployed in many regions of the world it is well known that ultra-low frequency (ULF) waves are a ubiquitous feature of the Earth’s magnetosphere
We have presented two events where pulsation activity was observed in the compressional component by the Cluster satellites simultaneously to field line resonances (FLRs) activity on the ground
It was shown that our algorithm detected peaks at the FLR frequency in the spectra of the compressional magnetic field component measured by Cluster
Summary
Since ground-based magnetometers were deployed in many regions of the world it is well known that ultra-low frequency (ULF) waves are a ubiquitous feature of the Earth’s magnetosphere. Using ground-based magnetometer and HighFrequency (HF) radar data, they found that FLRs tended to occur at frequencies of 1.3, 1.9, 2.6 and 3.4 mHz, largely independent of the local time of the observation (Samson et al, 1991; Fenrich et al, 1995; Provan and Yeoman, 1997; Mathie et al, 1999; Plaschke et al, 2009) This set of frequencies is known as “CMS frequencies” – cavity mode model of Samson et al (1991). Using the cavity model by Kivelson and Southwood (1985), the predicted eigenfrequencies of the proposed global standing compressional modes were estimated to lie slightly above the CMS frequencies observed on the ground This inconsistency prompted Samson et al (1992) to propose that the magnetosphere has to be treated like a waveguide rather than a cavity, allowing azimuthal propagation of the global compressional modes. None of the above cited works is able to explain why pulsations at a certain frequency are more prominent in the solar wind
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