Observations of solar wind plasma changes across the bow shock

Abstract

The Vela 3 satellites are in nearly circular orbits tilted at 60° to the ecliptic plane at geocentric distances of approximately 18.5 RE (earth radii). Thus the solar wind plasma can be sampled on both sides of the ‘bow shock,’ on both the dawn and dusk sides of the earth‐sun line, and at positions both in the ecliptic plane and far above and below it. An analysis of 13 crossings of the bow shock during July and August 1965, selected on both the dawn and dusk sides of the magnetosphere and with widely variable ecliptic latitudes show the solar wind bulk velocity to fall by ∼30% as the magnetosheath is entered. The density jumps by a factor of 2–5 typically, and the proton ‘temperature’ by a factor of ∼20. Multiple crossings of the shock front are common, and each crossing is easily recognized by the distinctly different characteristics of the solar wind plasma and magnetosheath plasma. For this time period typical average values of the bulk velocity and proton temperature near the shock front in the magnetosheath were 275 km/sec and 106°K. Thus the bulk velocity is larger than the mean thermal velocity, showing that most of the proton energy is associated with the ordered flow of the plasma around the magnetosphere.

The interplanetary medium and its interaction with the Earth's magnetosphere

“…The forms of the magnetopause and collisionless bow shock wave are not symmetrically located around the Earth-Sun line. This fact is confirmed by the data on plasma and magnetic field obtained by Vela 2, Vela 3 [49,226,227,234], OGO-1 [54], Explorer 33 [175]. The symmetry axis lies approximately 2 ~ to 4 ~ to the west.…”

“…The position of a bow shock wave is determined (in RE) by the expression [204] r s = 1.24 rco sec~ (l + sect0/(1 + secc~ cos(p), (7.8) where = arc sin (1~rams+), (7.9) rco is determined by (6.6), Mms+ = V/Vms+. For the subsolar point rso = 1.24 rco sec e. (7.10) For the subsolar region the magnetosheath thickness according to [213,224] The most complete investigations of the geomagnetic field, limited by the solar plasma, its boundaries and especially the magnetosheath and the collisionless bow shock wave have been performed by means of the spacecraft IMP-1 [45,46], IMP-2 [51], IMP-3 [202], OGO-1 [53,54,191], Vela 2 [49,226], Vela 3 [227], Explorer 33 [175]. The first magnetic measurements of the collisionless shock wave including the magnetopauze investigations and those inside the magnetosphere have been carried out on board IMP-1 [45,46].…”

“…Detailed studies of the mechanisms leading to the development of a collisionless bow shock wave [131,[235][236][237] with specific application to the solar wind interaction with the geomagnetic field are being conducted by a number of workers [238][239][240][241]. As to the collisionless bow shock wave it has to be characterized by a process that, together with the collisions produces the experimentally observed particle energy redistribution (the thermalized plasma) while the spacecraft are moving from the interplanetary medium into the magnetosheath [45,46,50,51,53,54,70,175,188,202,227,242,243]. Particle to particle interaction does not play an essential part in the energy redistribution in the collisionless plasma.…”

Direct observations of a flare related coronal and solar wind disturbance

“…Across the shock the solar wind speed increased from 290 to 580 km s -1, the proton density changed from about 3.6 to 11.0 cm -3, and the proton temperature increased from 7.1 • 104 to 7.4• 105K. These changes are considerably greater than those normally observed across interplanetary shocks (e.g., Gosling et aL, 1968;Hundhausen, 1972b) and are comparable to the changes observed across Earth's bow shock (e.g., Argo et al, 1967). From the mass continuity condition across the shock, the speed of the shock past the spacecraft was determined to be 722 km s -I *; the speed through the ambient solar wind was thus 432 km s -~.…”

Pioneer 6 plasma measurements in the magnetosheath

“…The density ratio across the bow shock has been observed on previous occasions to be lower than that predicted by gas-dynamic theory ( Argo et al, 1967), and this discrepancy remains unexplained.…”