The Earth's magnetic tail

Extensive measurements of the magnetic field of the earth at distances greater than approximately 7Re (earth radii) have been performed by the Imp 1 satellite. These magnetic field measurements began on November 27, 1963, and ended on May 30, 1964. During this six-month interval the apogee-earth-sun angle in solar ecliptic coordinates decreased from 336° to 156°. The apogee of the satellite was 31.7Re, and the range of the magnetometers was between 0.25 and 300γ. This paper is concerned principally with the topology of the magnetic field within the magnetosphere and the position of both its boundary and the detached collisionless bow shock wave. The geomagnetic field is observed to trail out far behind the earth in the antisolar direction, thus forming a magnetic tail. Magnetic field strengths of approximately 10 to 30 γ are observed out to satellite apogee. The diameter of the magnetosphere at a distance of 30Re behind the earth is found to be approximately 40Re. The direction of the field is parallel to the earth-sun line and in the antisolar direction below the solar magnetospheric equatorial plane and in the solar direction above this plane. A neutral surface separating antisolar directed fields in the southern hemisphere from solar directed fields in the northern hemisphere has been detected over a large area. This experimental result suggests the development of quantitative theories explaining the aurora, gegenschein, day-night asymmetry, and formation of the radiation belts. On the basis of a preliminary review of the data, it appears that the geomagnetic field trails out far behind the earth following the flow field of the solar plasma to a distance far beyond the orbit of the moon. No termination of the magnetic tail is detected or suggested by the data. Thus the earth can be compared to the nucleus of a comet, the radiation belts and co-rotating magnetosphere being the coma and the magnetic tail being the cometary tail.