Strongly Southward IMF Substorms, Dynamic Pressure Disturbances, and Null Events

L.R. Lyons, D. Lee, C. Wang, S.B. Mende, and H.U. Frey


Solar wind discontinuities can lead to important large-scale disturbances that
significantly affect the space environment, including energetic particle fluxes,
the aurora, and magnetospheric and ionospheric current systems. Understanding
what discontinuity characteristics lead to what kind of disturbance is thus
critical for disturbance prediction and understanding.  Global auroral images
from the wideband imaging camera (WIC) on the IMAGE spacecraft show striking new
information on this relationship. Two well-studied types of discontinuity driven
disturbance are: substorms resulting from northward interplanetary magnetic
field (IMF) turnings and dynamic pressure (P) disturbances that result from
enhancements of solar wind dynamic pressure. During typical substorms, auroral
activity initiates near the equatorward boundary of the auroral oval within a
~1-2 hr MLT sector within the Harang electric-field reversal region and then
expands to cover a few hours in MLT. Typical P disturbances show rapid global
enhancement of auroral emissions as well as a significant poleward motion of the
poleward boundary of the aurora, but enhancement related to the Harang reversal
is not evident. The WIC images show that, during periods of strongly southward
IMF, substorms expand to a significantly broader MLT range than do typical
substorms, and that, in addition to a global auroral enhancement, P disturbances
exhibit a substorm-like auroral enhancement within the Harang reversal that
extends over a broad MLT range. These observations show that, for strongly
southward IMF, both IMF and P changes cause Harang region activation. Because of
this, it is reasonable to expect that IMF and P interplay effects may be
important for solar wind discontinuities having both a significant IMF change
and a significant P change. The  WIC images show that such interplay effects can
indeed be important. In particularly, discontinuities having a significant IMF
northward turning and a significant decrease in P or having a significant
increase in P and a significant southward turning of the IMF are found to not
lead to a substorm-like aurora disturbance within the Harang reversal region. We
refer to such events as "null events," since the IMF northward turning or P
increase for each would, by themselves, be expected to cause a large substorm
disturbance, but the effects of these appear to be nullified by the simultaneous
change in the other quantity.

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Presentation, Fall A.G.U. Meeting, San Francisco, CA, 13-17 December 2004