Dartmouth Events

Abstract: Over the last two decades, maps of GPS total electron content (TEC) have
improved our understanding of the large perturbations in ionospheric
electron density which occur during geomagnetic storms. However, previous
regional and global studies of ionospheric storms have performed only a
limited separation of storm time, local time, longitudinal, and seasonal
effects. Using 13 years of GPS TEC data, we present a complete statistical
characterization of the ionospheric response to geomagnetic storms for
midlatitudes in the North American sector where dense ground receiver
coverage is available. The rapid onset of a positive phase is observed
across much of the dayside and evening ionosphere followed by a
longer-lasting negative phase across all latitudes and local times. Our
results show clear seasonal variations in the storm time TEC, such that
summer events tend to be dominated by the negative storm response while
winter events exhibit a stronger initial positive phase with minimal
negative storm effects. We find no discernable difference between spring
and fall equinox events with both being equivalent to the average storm
time response across all seasons. We also identify a prominent magnetic
declination effect such that stronger dayside positive storm effects are
observed in regions of negative declination (i.e., eastern North America).
On the nightside, asymmetries in the TEC response are observed near the
auroral oval and midlatitude trough which may be attributed to
thermospheric zonal winds pushing plasma upward/downward along field lines
of opposite declination.