Search for: All records

Abstract Although Strong Thermal Emission Velocity Enhancement (STEVE) and subauroral ion drifts (SAID) are often considered in the context of geomagnetically disturbed times, we found that STEVE and SAID can occur even during quiet times. Quiet‐time STEVE has the same properties as substorm‐time STEVE, including its purple/mauve color and occurrence near the equatorward boundary of the pre‐midnight auroral oval. Quiet‐time STEVE and SAID emerged during a non‐substorm auroral intensification at or near the poleward boundary of the auroral oval followed by a streamer. Quiet‐time STEVE only lasted a few minutes but can reappear multiple times, and its latitude was much higher than substorm‐time STEVE due to the contracted auroral oval. The THEMIS satellites in the plasma sheet detected dipolarization fronts and fast flows associated with the auroral intensification, indicating that the transient energy release in the magnetotail was the source of quiet‐time STEVE and SAID. Particle injection was weaker and electron temperature was lower than the events without quiet‐time STEVE. The plasmapause extended beyond the geosynchronous orbit, and the ring current and tail current were weak. The interplanetary magnetic field (IMF)B_zwas close to zero, while the IMFB_xwas dominant. We suggest that the small energy release in the quiet magnetosphere can significantly impact the flow and field‐aligned current system. 

An approach for creating continental-scale, multi-scale plasma convection maps in the nightside high-latitude ionosphere using the spherical elementary current systems technique has been developed and evaluated. The capability to reconstruct meso-scale flow channels improved dramatically, and the velocity errors were reduced by ∼30% compared to the spherical harmonic fitting method. Uncertainties of velocity vectors estimated by varying the model setup was also low. Convection maps for a substorm event revealed multiple flow channels in the polar cap, dominating the convection in the quiet time and early growth phase. The meso-scale flows extended toward the nightside auroral oval and had continuous flow channels over >20° of latitude, and the flow channels dynamically merged and bifurcated. The substorm onset occurred along one of the flow channels, and the azimuthal extent of the enhanced flows coincided with the initial width of the auroral breakup. During the expansion phase, the meso-scale flows repetitively crossed the oval poleward boundary, and some of them contributed to subauroral polarization streams enhancements. Increased flows extended duskward, along with the westward traveling surge. Then, flows near midnight weakened and evolved to the Harang flow shear. The meso-scale flow channels had significant (∼10%–40% on average) contributions to the total plasma transport. The meso-scale flows were highly variable on ∼10 min time scales and their individual maximum contributions reached upto 73%. These results demonstrate the capability of specifying realistic convection patterns, quantifying the contribution of meso-scale transport, and evaluating the relationship between meso-scale flows and localized auroral forms. 

Abstract Using high‐time‐resolution Super Dual Auroral Radar Network (SuperDARN) radar data, we investigated the dynamic evolution of ionospheric convection under various conditions, including an interplanetary magnetic field (IMF) southward turning, a substorm onset, quasi‐steady southward IMF, and Pc5 ultra‐low frequency (ULF) waves. The SuperDARN high‐resolution convection revealed that during an IMF southward turning, convection enhancements form narrow, transient flow channels in the polar cap rather than broad, uniform flows, with velocity peaks moving equatorward and crossing the nightside open‐closed boundary. These flow peaks are linked to poleward boundary intensifications. The convection response was nearly simultaneous across latitudes. During a substorm onset, enhanced equatorward flow across the polar cap boundary preceded auroral onset, suggesting that flow observations may be a more sensitive indicator of pre‐onset conditions than auroral data. Under quasi‐steady southward IMF, both aurora and plasma velocity show significant unsteady behavior, with flow channels moving equatorward beyond the extent of their corresponding auroral streamers. For Pc5 ULF waves, the data reveal fine‐scale velocity structures within larger poleward‐moving enhancements, which correspond to breaks in the ULF magnetic field. This study emphasizes that high‐time‐resolution convection observations are essential for accurately capturing the rapid evolution of ionospheric convection, which is often underestimated using conventional lower‐resolution methods. 

Abstract Solar flares are a rapid increase in solar irradiance, specifically in X‐ray and Extreme Ultraviolet spectra, which enhances the ionization in the dayside ionosphere and creates Sudden Ionospheric Disturbances (SIDs). SIDs are known to create space weather impacts on traveling high frequency (HF: 3–30 MHz) radio waves, by disrupting the communication channels. In this study, we examine ionospheric scatters at dawn terminator, which stems from a severe X9.3 flare on 6 September 2017 peaked at 12:02 UT, utilizing SuperDARN HF coherent scatter radars and Global Navigation Satellite System (GNSS) Total Electron Content (TEC) observations. Specifically, we are interested in the transients in the ionospheric electrodynamics at the sub‐auroral latitude near the terminator stemming from the flare effect. Observations suggest that flare‐induced density gradient likely favors the formation of gradient‐drift instability near the dawn terminator, leading to the irregularities observed by the SuperDARN radars with line‐of‐sight (LoS) Doppler velocity reaching nearly 300 m/s. The flare amplifies the eastward TEC gradient near the dawn terminator by approximately 2–3 times compared to a geomagnetically quiet and non‐flare day. The observed irregularities, attributed to flare‐driven instabilities, exhibit a velocity consistent with the equatorial return flow of ionospheric Hall convection. In contrast to prior studies indicating decreased cross‐polar‐cap potential and associated ionospheric convection flow, our findings show the flare is followed by an increase in localized electric field near the dawn terminator, as depicted in radar LoS velocity.