At its lowest level geomagnetic activity is due to magnetic reconnection and the
opposite senses for 
convection of plasma for northward and southward 
.
This is illustrated in Figure 15.13, following on from Section 14.4.
Figure 15.13: Magnetic topologies and convection velocities for Earth's
magnetosphere under northwards and southwards solar wind magnetic field conditions.
Dashed lines show the locations of the bow shock and magnetopause. Solid lines show
illustrative magnetic field lines in the solar wind, magnetosheath, and magnetosphere either
before or well after reconnection. Dotted lines show magnetic field lines fairly soon
after reconnection.
When the solar wind magnetic field is northward it can be seen that magnetic reconnection occurs at greater magnetic latitudes than the cusp and that one of the reconnected field lines adds to the closed field region sunward of the magnetopause while the other is entirely connected to the solar wind with no connection to Earth. Under these conditions, then, the total flux of closed magnetic field lines sunward of Earth is increased, the tail flux is decreased, the magnetopause is enhanced, plasma in the sunward portion of the inner magnetosphere convects Earthward, while plasma convects out of the tail. The situation is entirely reversed when the magnetic field is southward. Under these conditions closed field lines are lost from the dayside and transported to the tail, many more terrestrial field lines are magnetically open to the solar wind, the sunward magnetopause is eroded, the tail plasma flows Earthward and toward the center of the plasmasheet, and the tail magnetic field increases. Put another way, under southward conditions, energy is loaded into the tail, the ring current, and the cross-tail current sheet and the cusp and magnetosphere as a whole is much better magnetically connected to the solar wind.
Geomagnetic activity can arguably be separated into auroral magnetic activity and
magnetic substorms. Auroral magnetic activity involves the enhanced auroral light displays,
currents, and magnetic perturbations associated with times when favorable magnetic coupling
causes enhanced plasma flows down the cusp field lines into the auroral regions. Magnetic substorms
occur, however, when (1) the tail has been loaded with excess energy
during a period with southwards and the solar wind turns northward, or (2) the tail is
loaded into such a high energy state that even with continued southward driving it must relax.
Magnetic substorms occur when remains southwards for extended periods; they involve
convection of tail plasma Earthward to cause the ring current to grow with time, continual
auroral activity due to plasma transport along cusp field lines and from the plasma sheet
boundary layer, the thinning of the plasmasheet, the formation of a second magnetic reconnection
site about 
downtail from Earth, the injection of energetic particles (and their
subsequent acceleration by the dawn-to-dusk electric field as the particles drift in the ring current)
from the near-Earth reconnection site, and the ejection of the tail plasma as a ``plasmoid''
which convects tailward with the solar wind. Figure 15.14 (and Figures 15.4 and 15.9)
Figure 15.14: Definitions of the main phases of a magnetic substorm in terms of two
auroral activity indices AU and AL [McPherron, 1995].
illustrates the main phases of a substorm: the ``growth'' and ``expansion'' phases involve the initial slow growth and then faster intensification of the ring current and the main auroral current systems, while the ``recovery'' phase involves the slow decay of the ring current and the auroral current systems and the disappearance downtail of the plasmoid in the absence of further driving. The near-Earth reconnection site is believed to become active just at the spike of the substorm. Figure 15.5 illustrates the ejection of a plasmoid.
Figure 15.15: The main stages in the ejection of a plasmoid during a magnetic substorm.
[Hones, 1984; Cravens, 1997].
Many details of the above physical model are unclear and are the subject of active international research now, as they have been for over 100 years. Details of some of the popular models can be found in McPherron's [1995] article and Cravens' [1997] book.
For those of you interested, how about looking up today's space weather at
http://www.sec.noaa.gov/today.html .