How the Inner Magnetosphere Works
 
R. A. Wolf


This tutorial will attempt to explain how the basic elements of the Earth's
inner magnetosphere, particularly the plasma sheet, ring current, and
plasmasphere, are driven by magnetospheric convection but interact with each
other and with the underlying ionosphere. The discussion will start with a
mid-1970's picture of the system, which included a fundamentally correct picture
of plasmasphere dynamics as well as a basic understanding of how the inner edge
of the plasma sheet tends to shield the region earthward of it from the
convection electric field.

We will then move on to discussion of several topics for which our understanding
is still evolving: (1) The dynamics of the magnetospherically generated electric
fields that violate the shielding and penetrate deep into the inner
magnetosphere and the low- and mid-latitude ionosphere; these penetration
electric fields apparently play an important role in disrupting the structure of
the ionosphere during major storms; (2) Polarization jets, SAIDs, and SAPS,
which are regions of intense electric field just equatorward of the diffuse
aurora; only in the last few years have we begun to appreciate how these intense
subauroral electric fields affect ionospheric dynamics in major storms; (3) 
Injection of the storm-time ring current; the basic picture that emerged in the
early 1970's had to be modified when it was discovered a few years later that
much of the ring current was O+ rather than H+, as had always been assumed; much
more recently, energetic-neutral-atom images of the ring current ion
distribution from the IMAGE spacecraft have led to additional fundamental
reconsiderations; (4) The dynamics of the plasmasphere, including features that
were known and understood in the 1970's as well as phenomena that were
discovered in the last few years from the first EUV images. In all of these
cases, an account will be given of how theory and modeling sometimes led the
observations, but frequently lagged behind.

An attempt will be made to summarize the present state of understanding of these
issues in terms of large-scale quantitative modeling. We are just getting to the
point where we can do simulations that solve a reasonably complete set of basic
physical equations for the large-scale dynamics of the system. These models all
have significant numerical problems, but the most difficult challenge facing
them lies in the need to capture the effects of small-scale plasma processes,
which violate the basic large-scale equations but nonetheless play central roles
in overall system dynamics. Examples of phenomena in which small-scale plasma
processes affect the large-scale system include magnetic reconnection and
current interruption in the inner plasma sheet, field-aligned potential drops
and the acceleration of auroral electrons, energization of upflowing ions, and
electron and ion pitch-angle scattering. Physical understanding of these
small-scale processes is generally only partial, and we are struggling to find
ways to parameterize that understanding in terms that the large-scale codes can
effectively utilize.

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Global Aspects of Magnetosphere-Ionosphere Coupling, 2006 Yosemite Workshop, 
Yosemite National Park, CA, USA, 7-10 February 2006