Understanding the Sources of the Mid-latitude Storm Positive Phase: A Case Study of 16 July 2017

Geomagnetic storms impact the mid-latitude ionosphere through an initial positive phase followed by a much longer negative phase as the ionosphere recovers, with the positive and negative phases representing respectively an increase and a decrease in the F-region density. Are today's observations sufficiently detailed to allow for a study of the mechanisms that generate the positive storm phase? The search for an answer leads to additional questions about the larger system of solar-magnetosphere-ionosphere couplings.

In this case study we examine the moderate geomagnetic storm of 16 July 2017. While this storm achieved only a moderate intensity, with Kp topping out at a value of 6+ and Dst reaching down to a value of about -60, a peculiar and perhaps very important characteristic of this storm is that it was preceded by an unbroken period of very quiet conditions lasting about four days. We employ three databases to study the ionosphere during the storm period: Global GPS TEC observations provide the mid-latitude F-region dynamics at a cadence of 5 minutes. Three DMSP satellites provide a suite of in situ measurements of the ionosphere and of the magnetospheric drivers. Additionally, mid-latitude ionosondes provide observations of the F-region's ion's peak density and height. Together, these three data sets yield a detailed picture of the ionosphere's positive phase response to the storm.

We systematically address individual mechanisms that may contribute to the dayside stormtime ionospheric density increase, and evaluate their effects within the context of the ionospheric observations and the storm profile. The TDIM ionospheric model is used to simulate individual storm mechanisms, with the primary storm drivers being the penetration electric field, magnetospheric convection, vertical ExB transport, frictional heating, field-aligned plasma fluxes (following 4 very quiet days), and, as the storm progresses, the neutral winds and neutral heating leading to composition changes in the thermosphere.