HSS/CIR Driven Positive Ionospheric Storm at Mid-latitudes: Insights from Millstone Hill Radar and GNSS TEC measurements

During the declining phase of the solar cycle, geomagnetic storms, primarily driven by high-speed solar wind streams (HSSs) and associated co-rotation interaction regions (CIRs), become prominent. One of the major effects of these storms are the F region electron density perturbations, usually referred to as ionospheric storms. This study focuses on a positive ionospheric storm, characterized by an increase in electron density at mid-latitudes (40°- 60° MLAT) and observed during a moderate yet prolonged HSS/CIR-driven geomagnetic storm with a SYM-H minimum of -65 nT. The storm commenced on 14 March 2016 at 17:20 UT with a strong storm sudden commencement (SSC) and lasted until 21 March. This study uses global navigation satellite system (GNSS) total electron content (TEC) data for a global perspective of electron density variations and Millstone Hill incoherent scatter radar (52° MLAT, MLT=UT-4.6) data to provide local measurements of plasma parameters during the positive storm.

In the global analysis of the TEC variations during the storm, a 6-h long strong positive ionospheric storm (TEC increase up to 50 %) at the mid-latitudes was observed in the day and dusk sectors, whereas a depletion in TEC (negative storm) prevailed at the high latitudes. The positive ionospheric storm initiated during the SSC and subsequently intensified with the onset of the main phase. The local electron density data from the Millstone Hill incoherent scatter radar showed an enhancement throughout the local evening MLTs. An uplift in the peak height together with an increased line-of-sight upward ion velocity was observed simultaneously as the traveling ionospheric disturbances (TIDs) reached Millstone Hill from the north-east direction with a phase velocity of 760 m/s. When the plasma is uplifted to greater altitudes in the F region, the recombination rate becomes slower and electron density may be enhanced. The TIDs were plausibly triggered by the Joule heating at high latitudes during the main phase of the geomagnetic storm. After the initial uplift, the peak height of electron density at Millstone Hill descended but electron densities were further enhanced. We will discuss the possible mechanisms including transportation of oxygen-rich air from high to mid latitudes when interpreting the measurements.