Effects of different geomagnetic storm drivers on the ring current: CRCM results

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Date
2013-03Author
Cramer, W. Douglas
Turner, Niescja E.
Fok, M.C.
Buzulukova, N. Yu
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Show full item recordAbstract
The storm-time magnetic disturbance at the Earth’s equator, as commonly measured
by the Dst index, is induced by currents in the near-Earth magnetosphere. The ring
current is generally considered the most important contributor, but other magnetospheric
currents have also been found to have significant effects. Of the two main types of solar
geomagnetic storm drivers, Coronal Mass Ejections (CMEs) tend to have a much greater
impact on Dst than Corotating Interaction Regions (CIRs). Ring current models have
been found to underestimate Dst, particularly during storms driven by CIRs. One possible
explanation is that the models neglect to handle some aspect of ring current physics that
is particularly important for CIRs. This study uses the Comprehensive Ring Current
Model (CRCM) to estimate the ring current contribution to Dst for a selection of storms
of various strengths and different drivers (CMEs and CIRs) that have solar wind
parameters that fit a typical profile. The model boundary is set to 10 RE at the equator,
encompassing the entire ring current region. The magnetic field is held fixed, based on
average storm parameters, which limits our model results to the effects of convection and
plasma sheet density at the model boundary. Our model results generally show good
agreement with the size and timing of fluctuations in Dst, which indicates that convection
and boundary conditions play an important role in shaping Dst. We also find excellent
agreement with the magnitude of Dst for CME-driven storms. For CIR-driven storms,
however, the magnitude at the peak of the storm frequently deviates from actual Dst. In
general, we agree with the results of previous research that CIR-driven storms are more
underpredicted. However, this study includes some weaker CIR-driven storms for which
Dst is actually overpredicted. Overall, when examining the dependence of modeled Dst*
on actual Dst* at storm peak, we find that there is a statistically significant difference
between CME- and CIR-driven storms. We also find that approximately half of the total
ring current energy lies beyond an L-value of 6.6. However, this figure could be
overestimated due to the use of a static magnetic field, which limits radial transport.