Configuration of the geomagnetic tail during substorms

Abstract

Imp 4 vector magnetic field measurements and the geomagnetic AE index are used to study the configuration of the geomagnetic tail within 34 RE and its variations during substorms. The existence of a depressed field magnitude region corresponding to the average position of the plasma sheet is confirmed and the average solar magnetospheric component of the field is found to be northward and independent of distance from the expected position of the neutral sheet for |Z8m≲8 RE. The Z8m component measured some distance from the equatorial plane is a measure of flux crossing the neutral sheet and can be studied as a function of time during substorms. Prior to and during the early phases of a substorm the tail field magnitude increases and the northward field component decreases as the plasma sheet becomes thin. Later in the substorm the field magnitude decreases and the northward field component increases as the tail field relaxes to a more dipolar state. The greater number of field lines crossing the neutral sheet after the substorm is in agreement with the predictions of reconnection theories. Enhanced Z8m components after substorms and during the rare occurrences of very quiet intervals suggest that many more field lines close near the earth and fewer go into the gomagnetic tail at these times.

Observations of interactions between interplanetary and geomagnetic fields

“…lijima (1972) and McPherron (1973) have also employed the technique of keying on the temporally-isolated low-latitude onset time to confirm and quantify previous results of Fairfield and Ness (1970) and Aubry and McPherron (1971) concerning pre-and post-onset variations of the tail-lobe magnetic energy density. The results of McPherron (1973), which are similar to.those of lijima (1972) are shown in Figure 2.…”

On the relationship between the growth and expansion phases of substorms and magnetospheric convection

“…As is well known (Cummings, at al, 1968;Fairfield and Ness 1970;Coleman and Mc pherron, 1970;Camidge and Rostoker, 1970;Russell at at, 1971) and as is illustrated in Figure 2, the magnetic field in the equatorial plane at -6R e often becomes tail-like during an -1 hour period before the expansion phase and becomes dipole-like during the expansion phase. These facts may be interpreted as evidence of enhanced convection during the growth phase since such convection brings the tail-like magnetic field geometry closer to the earth.…”

Magnetotail changes in relation to the solar wind magnetic field and magnetospheric substorms

“…The indirect dependence of the tail characteristics on the solar wind through the substorm process has been shown by many authors (Heppner et al, 1967, Lazarus et al, 1968, Hones et al, 1968, Hones 1969, Akasofu et al, 1970, Hones et al, 1970, Fairfield and Ness 1970, Brody and Holzer 1970, Meng and Anderson 1970, Camidge Lnd Rostoker 1970. We have shown in this paper that the tail characteristics depend also directly on the interplanetary field and that the tail at 30 Re reacts immediately to the changes in the motion electric field 22 (VxB) associated with changes in the solar wind magnetic field orientation.…”

“…Magnetic field observations between 10 Re and 40 Re behind the earth have shown two main kinds of variation of the tail magnetic field namely, slow increase or decrease of the field at a rate of some gammas per hour, and rapid increase or decrease of the field at a rate which can be as high as several gammas per minute (Anderson and Ness 1966, Heppner et al, 1967, Fairfield and Ness 1970, Camidge and Rostoker 1970, Brody and Holzer 1970. The typical substorm signature in the tail is a slow increase of the field followed by a rapid decrease.…”