7.1 Time-variable electromagnetic fields
Magnetic storms and substorms affect the electromagnetic field configuration in the inner magnetosphere
over time scales of minutes to hours and days. During substorms, the intense current sheet can stretch the
quasi-dipolar field lines at geostationary orbit to highly taillike ones, which together with the enhancing
electric field causes significant changes in particle drift paths and allows for penetration of the plasma sheet
plasma deep inside the inner magnetosphere. In the same way, the strong ring current causes
stretching of the magnetic field during magnetic storms, but in this case the changes occur
over time scales of days rather than hours. Substorm activity during magnetic storms causes
quasiperiodic stretching-dipolarization cycles embedded within the longer-term ring current-associated
changes.
There are a number of large-scale magnetospheric magnetic field models that give the field
configuration as a function of either solar wind conditions or magnetospheric activity parameters
(Tsyganenko, 1989, 1995
; Tsyganenko et al., 2003). While these models give the large-scale evolution of
the field quite well, the smaller-scale and shorter-term variations associated with substorms cannot be
accurately represented by these models (Ganushkina et al., 2004). Figure 19 shows results from
Ganushkina et al. (2005), who modeled the large-scale magnetic field evolution during a storm on
May 2 – 4, 1998 by fitting an empirical model with free parameters to observed magnetic field
values. The figure shows three snapshots of the current configuration at the equatorial plane as
well as in the noon-midnight meridian plane prior to the storm, during the storm main phase,
and during the recovery phase. Before the storm, the ring current is weak and the tail current
intensity varies in response to substorm activity. After the storm onset, the ring current in
the inner magnetosphere as well as the magnetotail current are strongly enhanced. During
the storm recovery, both current systems weaken, but the ring current decay time is much
longer than that of the tail current. On top of these large-scale field variations, there are rapid,
large-amplitude, and localized variations of the magnetic field as well in the inner magnetosphere as in the
magnetotail.
The rapidly varying magnetic field induces an electric field (∇× E = −∂B∕∂t) in the magnetosphere.
These induced electric fields are much larger than the large-scale, weak convection electric field imposed by
the solar wind flow past the magnetosphere. The small-scale fields can be very intense, include
high-frequency fluctuations, and be highly localized in space, which makes their characterization difficult.
The large-scale convection field can be given in a simple formulation parametrized by magnetospheric
activity or by solar wind parameters (see Section 4). Because of the associated difficulties, there are to date
only very few attempts to describe the time-varying, smaller-scale electric fields. However, Li et al. (1998)
and Sarris et al. (2002) have described the substorm-associated magnetic field dipolarization and
Earthward plasma flows in terms of Earthward-propagating, localized electric field pulses. By
computing the magnetic field changes from the electric field and adding those to a simple dipole
field and tracing particle drifts under the resulting electromagnetic fields, they were able to
reproduce the substorm-associated energetic electron signatures at geostationary orbit. The
role of electric fields in the ring current formation and acceleration is treated in more detail
below.
shows results from
Ganushkina et al. (2005
