The magnetotail and substorms

Abstract

The tail plays a very active and important role in substorms. Magnetic flux eroded from the dayside magnelosphere is stored here. As more and more flux is transported to the magnetotail and stored, the boundary of the tail flares more, the field strength in the tail increases, and the currents strengthen and move closer to the Earth. Further, the plasma shcet thins and the magnetic flux crossing the neutral sheet lessens. At the onset of the expansion phase, the stored magnetic flux is returned from the tail and energy is deposited in the magnetosphere and ionosphere. During the expansion phase of isolated substorms, the flaring angle and the lobe field strength decrease, the plasma sheet thickens and more magnetic flux crosses the neutral sheet.In this review, we discuss the experimental evidence for these processes and present a phenomenological or qualitative model of the substorm sequence. In this model, the flux transport is driven by the merging of the magnetospheric and interplanetary magnetic fields. During the growth phase of substorms the merging rate on the dayside magnetosphere exceeds the reconnection rate in the neutral sheet. In order to remove the oversupply of magnetic flux in the tail, a neutral point forms in the near earth portion of the tail. If the new reconnection rate exceeds the dayside merging rate, then an isolated substorm results. However, a situation can occur in which dayside merging and tail rcconnection arc in equilibrium. The observed polar cap electric field and its correlation with the interplanetary magnetic field is found to be in accord with open magnetospheric models.

Observations of interactions between interplanetary and geomagnetic fields

“…The only direct observation of the actual inward motion, on March 27, 1968, was described by Aubry et al (1970). In this single event, which has been discussed extensively in the literature (e.g., Russell and McPherron, 1973b; OGO-5 crossed the magnetopause repeatedly in its inbound orbit. The just inside the original magnetopause position.…”

Dependence of the near‐Earth magnetotail magnetic field on storm and substorm activities

“…The increase and decrease in the magnetic field magnitude in the tail lobe at the expansion onset of substorms has been another key issue in substorm modeling [e.g., Caan et al, 1973; Russell and McPherron, 1973]. As will be shown later, the magnitude of the lobe magnetic field also correlates well with storm activity, implying that the lobe-field magnitude varies with a timescale of 8 -10 hours, as does the Dst index.…”

Microstructure of magnetic reconnection in Earth’s magnetotail

“…It has been proposed, in part to explain the transient nature of substorms, that reconnection in the magnetotail is not a steady process but rather Occurs impulsively when a magnetic X-line forms in the plasma sheet some 15 or 20 R e tailward of the earth [Russell and McPherron, 1973;Hones, 1977]. According to this model the reconnection process converts magnetic en6rgy stored in the magnetotail into particle kinetic energy, and this provides the energy source for a variety of substorm-related phenomena, including intensification of the auroral electrojet.…”

“…As in the pre-heating phase, the magnetic field is highly In summary, the observations presented here show that magnetic reconnection in the earth's magnetotail is a very complex and highly dynamic process. Much of the macroscopic behavior of the substorm plasma sheet can be understood on the basis of phenomenological and MHD models [Russell and McPherron, 1973;Hones, 1977], but many of the microstructural features reported here lie outside the scope of such models. In many respects, these novel magnetotail observations are similar to the laboratory observations of Stenzel et al [1983], which show that complicated interrelationships exist among magnetic reconnection, double layers, current disruptions, large inductive el$ctric fields, electron beams, and plasma waves.…”