A statistical study of the local time asymmetry of Pc 5 ULF wave characteristics observed at midlatitudes by SAMNET

A total of 129 Pc 5 events were identified in more than a year of mid-latitude magnetometer data from the United Kingdom Sub-Auroral Magnetometer Network (SAMNET). The horizontal polarization and azimuthal wavenumbers associated with these events were estimated, and their variations with magnetic local time were studied. A marked local time asymmetry was apparent in the Pc 5 wave characteristics with distinctly different polarization and azimuthal wave characteristics occurring on either side of noon. The morning sector observations of predominantly clockwise polarization and westward phase propagation suggest that the majority of the morning sector Pc 5 waves are the midlatitude signature of solar wind driven field line resonances occurring at higher latitudes. It is suggested that the most likely energy sources for these waves are the Kelvin-Helmholtz instability at the magnetopause boundary or cavity/waveguide modes prompted by impulsive variations in the solar wind. The large variation in the polarization azimuths (ellipse orientations) observed in the morning sector was interpreted as a result of the large ionospheric conductivity gradients that occur around and after dawn, which alter the ionospheric rotation of Alfven waves from 90°. It is suggested that the afternoon sector Pc 5 waves are the ground signature of compressional waves in the magnetosphere, mainly cavity/waveguide modes. There is little evidence of field line resonances in the afternoon sector. The possibly dominant role of cavity/waveguide modes in the magnetosphere is heightened by the observation of preferential frequencies in both the morning and afternoon sectors.