Swarm Mass Density and Plasma Observations During the St. Patrick's Day Storm Event 2015 and its Global Numerical Modelling Challenges

The most severe geomagnetic storm in solar cycle 24 started with a sudden storm commencement (SSC) at 04:45 UT on St. Patrick's day March 17, 2015. It occurred without any significant precursor X- or M-type solar flares and appeared as a two-stage geomagnetic storm with a minimum SYM-H value of -233 nT. In the response to the storm commencement in the first activation, a short-term positive effect in the ionospheric vertical electron content (VTEC) occurred at low- and mid-latitudes on the dayside. The second phase commencing around 12:30 UT lasted longer and caused significant and complex storm-time changes around the globe with hemispherical different ionospheric storm reactions in different longitudinal ranges. Swarm-C observations of the neutral mass density variation along the orbital path as well as Langmuir probe plasma measurements of all three Swarm satellites and global TEC records during the storm interval are used for physical interpretations and modelling of the positive/negative storm scenario. At mid-latitudes, positive storm signatures were observed in the Northern Hemisphere (NH) of the European sector, whereas a large positive storm occurred in the Southern Hemisphere (SH) of the American sector. The negative storm phase was found to be strongest in the Asian sector, in particular in the NH, but developed globally on March 18 at the beginning of the recovery phase. These observations pose a challenge for the global numerical modelling of thermosphere-ionosphere storm processes as the storm, which occurred around spring equinox, obviously signify the existence of other impact factors than seasonal dependence for hemispheric asymmetries to occur. First numerical simulation trials using the Potsdam version of the Upper Atmosphere Model (UAM-P) are presented to explain these peculiar ionospheric storm processes.