ULF waves in the magnetosphere
MHD waves are an intrinsic signature of different processes taking place in the magnetosphere.
They are essentially of two types: the magnetosonic wave and the Alfven wave. They have different propagation properties: magnetosonic waves propagate in any direction and generate compression and rarefaction both of the magnetic field and plasma; Alfven waves propagate along the direction of the ambient magnetic field and produce magnetic perturbations transversal to the field lines. Compressive MHD waves, which propagate in the magnetosphere, can drive, through a resonance mechanism, standing oscillations of the geomagnetic field lines which behave as strings with the ends fixed in the ionosphere. A resonance occurs if the field line eigenfrequency matches the wave source frequency.
In the Earth’s magnetosphere these waves have frequencies ranging from approximately 1 mHz to 1 Hz (ULF pulsations). In particular, in the low frequency range (Pc5, 1-7 mHz) discrete frequency fluctuations have been interpreted in terms of global cavity modes, driven by the transmission of waves, already present in the solar wind, into the magnetosphere or by solar wind pressure pulses.
Higher frequency daytime pulsations (Pc3-4, 7-100 mHz) are currently believed to originate from upstream waves generated in the Earth’s foreshock region and transmitted in the magnetosphere.
Studies are mainly devoted to investigate: a) the origin of discrete frequencies spectra (cavity modes and/or solar wind direct drivers;) b) the entry mechanisms of upstream waves through the magnetosphere to the ground.
Recent papers:
• U. Villante, A. Del Corpo and P. Francia, Geomagnetic and solar wind fluctuations at discrete frequencies: a case study, J. Geophys. Res., 118, 218-231, doi: 10.1029/2012JA017971, 2013.
• P.Francia,M. Regi, M. De Lauretis, ULF fluctuations observed along the SEGMA array during very low solar wind density conditions, Planet. Space Sci., 81, 74-81, 2013.
• M. Regi, P. Francia, M. De Lauretis, K.H. Glassmeier, U. Villante, Coherent transmission of upstream waves to polar latitudes through magnetotail lobes, J. Geophys. Res., 118, 1–9, doi:10.1002/2012JA018472, 2013.
• M. Regi, M. De Lauretis and P. Francia, The occurrence of upstream waves in relation with the solar wind parameters: a statistical approach to estimate the size of the foreshock region, Planet. Space Sci., 90, 100-105, 2014.
