Magnetic measurements in the Earth’s magnetosphere and magnetosheath: Mariner 5

Abstract

Magnetic field measurements near earth, obtained by a vector helium magnetometer on board Mariner 5, indicate that inside the magnetosphere there are waves with modest amplitudes (∼2 to 3γ) and periods near 5 minutes that have components both transverse and parallel to the ambient field. The distant magnetospheric field involves a simultaneous compression and twisting of field lines similar to those observed previously on earth satellites. Some features of the distorted field, such as an apparent reversal in field gradient approaching the magnetopause and nearly equal field magnitudes in the magnetosphere and magnetosheath, show poor qualitative agreement with a simple Chapman‐Ferraro model and imply either changes with time or a complicated structure. The absence of a significant field component normal to the magnetopause, however, is consistent with a closed magnetosphere. Multiple magnetopause crossings that imply a quasi‐sinusoidal surface motion with a peak to peak amplitude of ∼⅓ earth radii, a period of ∼2 min, and a peak velocity of ∼50 km sec−1 are observed. In the magnetosheath, adjacent to the magnetopause, a train of non‐sinusoidal waves having the appearance of Alfvén waves are observed with amplitudes of ∼5γ and periods of ∼2 min, oscillating essentially in a plane parallel to the magnetopause. A region of highly disturbed irregular fields with ΔB/B∼1 when B ≈ 30γ is observed for ∼1 hour in the magnetosheath well inside the shock and is presumably associated with changing conditions in the solar wind. The directions normal to the magnetopause and shock, derived from the magnetic fields adjacent to those boundaries, agree well with the corresponding directions based on the average shapes of the two surfaces.

Identification of interplanetary tangential and rotational discontinuities

“…The field magnitude is likely to be reduced, but not go to 0, if the fields are oriented at other than 180 ø . Similar local reductions in magnitude have been observed within the earth's magnetopause current layer [Smith and Davis, 1970]. The several methods undoubtedly all have their uses.…”

Structure of the magnetopause

“…These measurements were made under conditions that are thought to favor magnetic merging, namely relatively high magnetic activity and a strong southward component of the interplanetary magnetic field. Another probable identification of a tangential discontinuity has been reported by Smith and Davis [1970]. Aubry et al [1971] have discussed a further series of multiple erossings of the magnetopause that occurred during the 2-hour period mentioned above, when the magnetopause was displaced nonuniformly 2 R• toward the earth.…”

“…Near the dawn meridian there is often very little difference in field magnitude on the two sides of the magnetopause [Heppner et al, 1967]. Sometimes there is no clear discontinuity in either field magnitude or direction near the noon meridian [Fairfield and Ness, 1967], and on an isolated occasion there was evidence for an apparent reversal in field gradient (lB[ increasing rather than decreasing with increasing geocentri½ distance) on approaching the midafternoon magnetopause from the interior of the magnetosphere [Smith and Davis, 1970]. The absence of a distinct magnetopause in the magnetic field data invariably corresponds to a diffuse magnetopause characterized by slowly changing currents in the plasma data [Fairfield and Ness, 1967].…”

“…The spatial amplitudes of the motion vary from about 0.2 to 2.2 R•, and the motion is sometimes periodic. The characteristic times of the motion usually vary from 2 to 15 minutes [Anderson et al, 1968;Smith and Davis, 1970], but both longer periods Kaufmann and Konradi, 1969] and shorter periods [Aubry et al, 1971] are sometimes observed. On at least one occasion the dayside magnetopause oscillations were found to be consistent with ripples on the magnetopause, possibly produced by the Kelvin-Helmholtz instability, which drift toward the geomagnetic tail with a velocity of the same order of magnitude as the plasma flow velocity in the magnetosheath [Aubry et al, 1971].…”

Interplanetary fields and plasmas

“…The mapping of bow shock locations and the study of the structure of the magnetosheath was continued, with magnetic field observations by , Behannon and Fairfield [1969], , Fairfield and Ness [1961], Heppner et al [1967], and Smith and Davis [1970] and plasma observations by Binsack [ 1968], Burlaga and Ogilvie [1968], Gosling et al [1967a], Howe [1970], Hundhausen et al [1969], Olbert [1968], Strong [1968], and Wolfe and McKibbin [1968a, b]. An advance in technique was the use of simultaneous observations from two or more satellites by Binsack and Vasyliunas [1968], Fairfield [1968], and Greenstadt et al [19686, 1970a].…”