Open AccessObservations and modeling of particle dispersion signatures at a hot flow anomaly
Author(s) -
Tjulin A.,
Lucek E. A.,
Dandouras I.
Publication year - 2009
Publication title -
journal of geophysical research: space physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/2009ja014065
Subject(s) - physics , bow shock (aerodynamics) , discontinuity (linguistics) , computational physics , foreshock , solar wind , beam (structure) , classification of discontinuities , shock wave , geophysics , mechanics , whistler , bow wave , magnetic field , optics , geology , aftershock , mathematical analysis , mathematics , quantum mechanics , seismology
We analyze a hot flow anomaly (HFA) event that the Cluster spacecraft encountered on 22 April 2008. HFAs are regions of hot, highly deflected ion flow near planetary bow shocks that are created from interactions between tangential discontinuities in the solar wind and the bow shock. This HFA event is studied in detail because it is flanked by suprathermal ion beams that contain signatures of particle dispersion and where the particles have a nongyrotropic distribution. The composition of the beam ions is similar to that of the solar wind, and it implies that the beams consist of reflected solar wind plasma. The particle pitch angles in the beam are found to evolve over time so that the pitch angle decreases closer to the discontinuity. A simple model of particle motion at a tangential discontinuity is developed, which successfully predicts many of the observed features of the particle beams. The model also shows that the best conditions for formation of similar beams occur when the field rotation in the tangential discontinuity corresponding to the HFA is between 25° and 40° in the case where the magnetic field magnitude is the same on both sides of the discontinuity. The particle beams are coincident with observations of “1‐Hz waves” that are frequently found in the foreshock region. On the leading side of the HFA event, these waves are shown to be generated by the beam, while on the trailing side, the properties of the waves indicate that they are whistler waves generated at the bow shock through electron dynamics and propagating upstream. The second mechanism is the standard one that has been used to explain most observations of the 1‐Hz waves in the literature.