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Abstract. During minor to moderate geomagnetic storms, caused by corotatinginteraction regions (CIRs) at the leading edge of high-speed streams (HSSs), solar windAlfvén waves modulated the magnetic reconnection at the daysidemagnetopause. The Resolute Bay Incoherent Scatter Radars (RISR-C andRISR-N), measuring plasma parameters in the cusp and polar cap, observedionospheric signatures of flux transfer events (FTEs) that resulted in theformation of polar cap patches. The patches were observed as they moved over the RISR, and the Canadian High-Arctic Ionospheric Network (CHAIN)ionosondes and GPS receivers. The coupling process modulated the ionospheric convection and the intensity of ionospheric currents, including the auroral electrojets. The horizontal equivalent ionospheric currents (EICs) are estimated from ground-based magnetometer data using an inversion technique. Pulses of ionospheric currents that are a source of Joule heating in the lower thermosphere launched atmospheric gravity waves, causing travelingionospheric disturbances (TIDs) that propagated equatorward. The TIDs wereobserved in the SuperDual Auroral Radar Network (SuperDARN) high-frequency (HF) radar groundscatter and the detrended total electron content (TEC) measured by globallydistributed Global Navigation Satellite System (GNSS) receivers. 

Abstract. The high-latitude atmosphere is a dynamic region with processes that respond to forcing from the Sun, magnetosphere, neutral atmosphere, andionosphere. Historically, the dominance of magnetosphere–ionosphere interactions has motivated upper atmospheric studies to use magneticcoordinates when examining magnetosphere–ionosphere–thermosphere coupling processes. However, there are significant differences between thedominant interactions within the polar cap, auroral oval, and equatorward of the auroral oval. Organising data relative to these boundaries hasbeen shown to improve climatological and statistical studies, but the process of doing so is complicated by the shifting nature of the auroral ovaland the difficulty in measuring its poleward and equatorward boundaries. This study presents a new set of open–closed magnetic field line boundaries (OCBs) obtained from Active Magnetosphere and Planetary ElectrodynamicsResponse Experiment (AMPERE) magnetic perturbation data. AMPERE observations of field-aligned currents (FACs) are used to determine the location ofthe boundary between the Region 1 (R1) and Region 2 (R2) FAC systems. This current boundary is thought to typically lie a few degrees equatorwardof the OCB, making it a good candidate for obtaining OCB locations. The AMPERE R1–R2 boundaries are compared to the Defense MeteorologicalSatellite Program Special Sensor J (DMSP SSJ) electron energy flux boundaries to test this hypothesis and determine the best estimate of thesystematic offset between the R1–R2 boundary and the OCB as a function of magnetic local time. These calibrated boundaries, as well as OCBsobtained from the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) observations, are validated using simultaneous observations of theconvection reversal boundary measured by DMSP. The validation shows that the OCBs from IMAGE and AMPERE may be used together in statisticalstudies, providing the basis of a long-term data set that can be used to separate observations originating inside and outside of the polar cap. 

This study provides first storm time observations of the westward‐propagating medium‐scale traveling ionospheric disturbances (MSTIDs), particularly, associated with characteristic subauroral storm time features, storm‐enhanced density (SED), subauroral polarization stream (SAPS), and enhanced thermospheric westward winds over the continental US. In the four recent (2017–2019) geomagnetic storm cases examined in this study (i.e., 2018‐08‐25/26, 2017‐09‐07/08, 2017‐05‐27/28, and 2016‐02‐02/03 with minimum SYM‐H index −206, −146, −142, and −58 nT, respectively), MSTIDs were observed from dusk‐to‐midnight local times predominately during the intervals of interplanetary magnetic field (IMF) Bz stably southward. Multiple wavefronts of the TIDs were elongated NW‐SE, 2°–3° longitude apart, and southwestward propagated at a range of zonal phase speeds between 100 and 300 m/s. These TIDs initiated in the northeastern US and intensified or developed in the central US with either the coincident SED structure (especially the SED basis region) or concurrent small electron density patches adjacent to the SED. Observations also indicate coincident intense storm time electric fields associated with the magnetosphere–ionosphere–thermosphere coupling electrodynamics at subauroral latitudes (such as SAPS) as well as enhanced thermospheric westward winds. We speculate that these electric fields trigger plasma instability (with large growth rates) and MSTIDs. These electrified MSTIDs propagated westward along with the background westward ion flow which resulted from the disturbance westward wind dynamo and/or SAPS.

 

The occurrence of St. Patrick's Day (17 March) geomagnetic storms during two different years (2013 and 2015) with similar solar flux levels but varying storm intensity provided an opportunity to compare and contrast the responses of the ionosphere‐thermosphere (IT) system to different levels of geomagnetic activity. The evolution of positive ionospheric storms at the southern polar stations Bharati (76.6°S MLAT) and Davis (76.2°S MLAT) and its causative connection to the solar wind driving mechanisms during these storms has been investigated in this paper. During the main phase of both the storms, significant enhancements in TEC and phase scintillation were observed in the magnetic noon/ midnight period at Bharati and Davis. The TEC in the midnight sector on 17 March 2015 was significantly higher compared to that on 17 March 2013, in line with the storm intensity. The TEC enhancements during both the storm events are associated with the formation of the storm‐enhanced densities (SEDs)/tongue of ionization (TOI). The strong and sustained magnetopause erosion led to the prevalence of stronger storm time electric fields (prompt penetration electric field (PPEF)/subauroral polarization streams (SAPS)) for long duration on 17 March 2015. This combined with the action of neutral winds at midlatitudes favored the formation of higher plasma densities in the regions of SED formation on this day. The same was weaker during the 17 March 2013 storm due to the fast fluctuating nature of interplanetary magnetic field (IMF)B_z. This study shows that the duration and extent of magnetopause erosion play an important role in the spatiotemporal evolution of the plasma density distribution in the high‐midlatitude ionosphere.