Relation between magnetotail variations and auroral activities during substorms



The interaction between the solar wind and the earth’s magnetosphere results in the transport of magnetic flux into the magnetotail and to avoid a continued buildup in the tail, there is a return convection of magnetic flux from the magnetotail into the night-side dipole-like region and from there to the day-side. Since there is energetic plasma with this magnetic flux, hence electric currents exist that disturb the magnetic intensity in the earth’s surface ( substorms ) and particularly by interaction with the earth’s ionosphere producing auroral activities .
We have compared magnetotail variations with auroral activities during 3 substorms using GEOTAIL , Polar UVI and 4-Cluster spacecraft data . In the substorm event on 15 December 1996 , auroral breakups and intensifications were highly correlated with fast plasma flows, with the variations in the north-south magnetic field and with the total pressure in the magnetotail. GEOTAIL was located around X~ -21, and several fast tailward flows were observed in the early expansion phase with the southward magnetic field and the total pressure enhancement, associated with plasmoids. These flows were observed simultaneously with or within 1min of auroral breakups or pseudobreakups. In the late expansion or recovery phase, some fast earthward flows were observed with small auroral intensifications. More investigations imply that the total pressure in the magnetotail significantly decreases during auroral breakups or poleward expansion of the auroral bulge. The duration of the expansion and the maximum size of the auroral bulge are closely correlated with the duration and amount of total pressure decrease in the magnetotail, respectively. These results also imply that the substorms are the response of magnetosphere to solar wind and its frozen-in magnetic field. Also in the substorm event on September 2002 investigation of Cluster data shows that direction reverse of fast plasma flow is highly correlated with total pressure variations in the magnetotail by magnetic disturbs.
Review GEOTAIL and Polar UVI data for December 1996 and March 1997 substorms shows high correlation between changes in rapid plasma flux and magnetic field north- south and changes in the magnetotail's total pressure. Fluxes begin towards the tail and then they come back. This implies that events of this activity that retreat near the Earth’s neutral line are periodic.
The results show that communication between the auroral activity domain and substorms is dependent on energy dissipation in the magnetotail.
The data by 4- Cluster and earth's magnetograms for the September 2002 substorm also shows that the magnetic energy is stored during substorms and released when fast fluxes electron tubes are reversed . These results can be used to describe the substorms in following stages:
- The presence of a strong south component of interplanetary magnetic field (IMF) and increase in the magnetic reconnection and transmission magnetic flux into the magnetotail.
- Transfer pressure of magnetic flux from the tail to night- side and restriction of the magnetosphere.
- Creation of the new structure for magnetic flux in tail.
- Increase in the pressure of magnetic flux in side lobes and thinning the plasmasheet and formation of the tail like magnetic field lines.
- Thinning the plasmasheet sufficiently for broken (MHD) magnetohydrodynamic conditions and beginning magnetic reconnection again in places near the earth's neutral lines.
- Injection of energy and plasma in southward of the tail and re-coming toward the night-side of the earth and creating substorm's auroral activities.