Confirmation of quasi-perpendicular shock reformation in two-dimensional hybrid simulations

Details Confirmation of quasi-perpendicular shock reformation in two-dimensional hybrid simulations Confirmation of quasi-perpendicular shock reformation in two-dimensional hybrid simulations

Mar 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009georl..3605103y&link_type=abstract

Geophysical Research Letters, Volume 36, Issue 5, CiteID L05103

Physics

Plasma Physics

6

Space Plasma Physics: Shock Waves (4455), Space Plasma Physics: Kinetic Waves And Instabilities

Scientific paper

Shock reformation involves regions of a shock undergoing periodic collapse and redevelopment on a time scale close to the ion cyclotron period. Reformation is often observed in one-dimensional (1-D) hybrid and particle in cell (PIC) simulations of quasi-perpendicular collisionless shocks provided the Alfvén Mach number M A and ion plasma beta β i are sufficiently high and low, respectively. Initial 2-D PIC simulations showed some evidence for shock reformation, with ion reflection providing the main energy dissipation mechanism, while recent spacecraft observations showed a reforming shock with large amplitude whistler waves in the foot region. While recent spacecraft observations showed an case with reforming shock crossing with whistler waves dominated in the foot region. However, recent 2-D hybrid and PIC simulations suggest that reformation does not occur in exactly perpendicular 2-D shocks. This paper re-examines shock reformation in quasi-perpendicular shocks using 1-D and 2-D hybrid simulations. We find that 2-D quasi-perpendicular shocks ($\theta$ bn = 85°) indeed undergo cyclic reformation providing M A and β i are high and low enough, respectively. For low M A <= 4, 2-D quasi-perpendicular shocks are found to be quasi-stationary, despite 1-D simulations predicting reformation, confirming and extending recent work for perpendicular 2-D shocks. The dynamics of reformation are quite different in 2-D than in 1-D: in 2-D large amplitude whistler waves grow in the shock foot, have amplitudes of order the downstream magnetic field, and affect the reformation. The whistlers have almost zero phase speeds in the shock frame and oblique wave vectors with respect to the upstream magnetic field. The predicted reformation period increases in 2-D compared with 1-D and increases nonlinearly as M A decreases towards the reformation threshold.

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