Bow shock associated waves observed in the far upstream interplanetary medium

Abstract

Fifty orbits of Explorer 34 data have been used to study 0.01–0.05 Hz transverse waves in the interplanetary medium region between the bow shock and the spacecraft apogee of 34 RE. It is concluded that the waves are associated with the earth's bow shock since they only occur when projection of the interplanetary field observed at the spacecraft intersects the shock. The waves are observed 18.5% of the time when a total of 134 days of interplanetary data is considered, but more than 90% of the time when the field has the proper orientation with respect to the bow shock. On the basis of this result it is suggested that these waves with 20–100 second periods are a permanent feature of the solar wind‐earth interaction. The transverse component of the waves is typically several gammas in amplitude in 4–8 gamma fields. The disturbance vector in the XY plane generally exhibits the same sense of rotation in a coordinate system where the field is oriented along the positive z axis. Attenuation of wave amplitudes with distance from the bow shock is estimated to be only a factor of 2 when the spacecraft is 15 RE from the bow shock. The absence of waves at particular field orientations, even though the field line intersects the shock, is interpreted as a propagation effect. This observation is the basis for calculations that yield an average velocity in the plasma frame of 2.7 ± 0.4 times the solar wind velocity. Whistler propagation and local generation by two‐stream instability are discussed as alternate theoretical explanations for the presence of the waves. It is suggested that the data favor the latter mechanism.

Tangential discontinuities in the solar wind

“…The magnetic field experiments on Explorers 33, 34 and 35 have been described respectively by Behannon (1967), Fairfield. (1969 and Ness et al (1967).…”

Bow Shock and magnetosheath waves at Mercury

“…Whe^ this angle is greater than 40U the transition is complex and extends over a region many thou iarids of kilometers thick in which irregular large amplitude waves occur (Greenstadt et al, 1970) . In the latter case, low frequency waves are frequently observed to persist many 10's of R E (Earth Radii) into the upstream region (Fairfield, 1969) . Such waves typically ;lave periods 20-60 sec, polarizations that are generally left-handed, ai,d…”

“…Such Naves are thought to be generated in the upstream region by particles emanating from the shock (Fairfield, 1969;Barnes, 1970;and Fredricks, 1975).…”

Influence of thermal anisotropy on best-fit estimates of shock normals

“…If actually present, such influences could markedly change the shock parameters from one interplanetary observation point to the other and yet have a negligible effect on the shock front's ability to remain plane on the scale of interest here. However, the condition for expecting these influences to be present, i.e., that the interplanetary magnetic field line (pre-shock field in this case) passing through either spacecraft position be connected to the earth's bow shock (Fairfield, 1969), does not hold in this case. Therefore, these differences are more likely related to true interplanetary conditions at that time rather than to phenomena associated with the earth.…”