Transmission of Alfen waves through the earth's bow shock: Theory and observation

Journal Article · Wed Feb 01 04:00:00 EST 1978 · J. Geophys. Res.; (United States)

DOI:https://doi.org/10.1029/JA083iA02p00643· OSTI ID:5087431

Hassam, A B

Alfven waves, propagating along the magnetic field, are considered to be interacting with strong fast MHD shocks. The wave-associated magnetic field perturbation is taken to lie in the magnetic field-shock normal plane. The interaction perturbs the shock surface and gives rise to transmitted waves consisting of a fast magnetoacoustic wave, forward and backward slow magnetoacoustic waves, and an entropy wave. To zeroth order in inverse upstream Mach numbers the wave vectors of the latter three waves are collinear and bend toward the normal, while the fast wave bends away from the normal, propagating parallel to the shock surface for incident angles of 61/sup 0/.To the same order, for incident angles of less than 61/sup 0/ and for ..gamma..=5/3, the magnetic field amplitude associated with one of the slow waves is roughly the same size as the incident amplitude, while that associated with the other slow wave is amplified 3 times.Magnetic field and plasma data from Explorer 35, taken in the vicinity of 14 bow shock crossings, are analyzed. On spacecraft time scales of 12-20 min the data suggest the presence of hydromagnetic waves on both sides of all the shocks; the presence of predominantly Alfvenic fluctuations is demonstrated in the upstream regions of atleast six of the crossings. Magnetic field fluctuations with frequencies of >1/100 Hz are found to be amplified across the shock by factors ranging from 0.9 through 2.5. It is suggested that the discrepancy in the observed and theoretical field amplifications is due to strong damping of any transmitted magnetoacoustic modes with relatively little damping of any transmitted Alfven waves (all of which may be excited for a more general incidence).

• Research Organization: Department of Physics and Center for Space Research, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
• OSTI ID: 5087431
• Journal Information: J. Geophys. Res.; (United States), Journal Name: J. Geophys. Res.; (United States) Vol. 83:A2; ISSN JGREA
• Country of Publication: United States
• Language: English

Related Subjects

640203* -- Atmospheric Physics-- Magnetospheric Phenomena-- (-1987)
71 CLASSICAL AND QUANTUM MECHANICS
GENERAL PHYSICS
ALFVEN WAVES
EXPLORER SATELLITES
FLUCTUATIONS
HYDROMAGNETIC WAVES
MACH NUMBER
MAGNETOACOUSTIC WAVES
MAGNETOSHEATH
SATELLITES
SHOCK WAVES
VARIATIONS
VELOCITY
WAVE PROPAGATION