Magnetic field measurements in space

The magnetosphere boundary has been penetrated in several places, conflicting evidence about the ring current location has been found, and the field exterior to the boundary has revealed some unexpected features. Pronouncements about the structure of the geomagnetic and interplanetary magnetic fields are still based on scanty evidence but the experimental basis of such estimates is more adequate than in 1958. The boundary between the geomagnetic field and the interplanetary medium has been found, by Explorer XII, to be located at approximately 10 R E on the sunlit side of the earth near the equator. It has been observed to fluctuate between 8 and 12 R E during August, September and October of 1961. During several days in March, 1961, the boundary, on the dark side of the earth, was penetrated repeatedly by Explorer X on an outbound pass near 135° from the earth-sun line. Several interpretations are possible; the most reasonable one at present is that the boundary was fluctuating in this period, placing the satellite alternately inside the geomagnetic field and outside in a region of turbulent magnetic fields and plasma flow. A region of turbulent magnetic fields was also observed by Pioneer I, Pioneer V, and Explorer XII between 10 and 15 R E on the sunlit side of the earth. Pioneer V observed also a steady field 2 to 5 gammas in magnitude beyond 20 R E. It appears that there exists a region of turbulent magnetic fields between the geomagnetic field boundary near 10 R E, and another boundary, located near 14–15 R E near the earth-sun line. This second boundary was seen only by Pioneer I and Pioneer V; Explorer XII and Explorer X apparently did not reach it. This boundary has been tentatively identified as a shock front in the flow of solar plasma about the magnetosphere (see Figure 5).41, 42 The geomagnetic field inside the boundary is relatively quiet. An abrupt transition in the magnitude of fluctuations occurs at the boundary surface. The ratio of fluctuation amplitude, ΔB, to average field, B, decreases from 1 to 0.1 on a passage through the boundary on 13 September 1961.43 The boundary is not unstable in the solar wind but fluctuations in solar wind pressure do cause changes in boundary location.42,43 The ring current location appears to be above 1.4 R E and below 5 R E on the basis of Pioneer I, Vanguard III, and Explorer XII data. Lunik I and II records indicate that it is located between 3 and 4 R E. Explorer VI data indicates that it must be at distances greater than 4 R E on the dark side of the earth. Some variation in altitude of a ring current with time appears likely, but the bulk of present evidence limits a possible ring current to a distance of 3 to 5 R E. The interplanetary field during quiet times is of the order of 2 to 5 gammas. The direction indicated for this field, with a significant component perpendicular to the earth-sun line, is puzzling in view of solar cosmic ray transit times. Solar disturbances with resultant plasma flow past the satellite produce increases in the field magnitude. Field increases at the satellite are sometimes correlated with disturbances observed at the earth. Further investigations are needed to map the magnetosphere and boundary more completely, to investigate the postulated shock front and the turbulent region inside, to refute or confirm the ring current theory, and to measure the interplanetary field direction and magnitude more completely. Theoretical studies are needed to support these experiments and to suggest new avenues of investigations. Particularly needed are theoretical investigations of collisionless shock fronts in plasma flow and of characteristics of the flow between the shock front and the obstacle.