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We examined evolution of Global Positioning System (GPS) scintillation during a substorm in the nightside high latitude ionosphere, using 1‐s phase and amplitude scintillation indices from the Canadian High Arctic Ionospheric Network (CHAIN) network. The traditional 1‐min scintillation indices showed that the phase scintillation was dominant, while the amplitude scintillation was weak. However, the 1‐s amplitude scintillation occurred more often in association with major auroral structures (polar cap arc, growth phase arc, onset arc, poleward expanding arc, poleward boundary intensification, and diffuse aurora) that were detected by the THEMIS all‐sky imagers (ASIs). The 1‐min index missed much of the amplitude fluctuations because they only lasted ∼10 s near a local peak or at the gradients of the auroral structures. The 1‐s phase scintillation was concurrent with the amplitude scintillation but was much weaker than the 1‐min phase scintillation. The frequency spectral analysis showed that the spectral power above ∼1 Hz was diffractive and below ∼1 Hz was refractive. We suggest that the amplitude scintillation in the high‐latitude ionosphere is much more common than previously considered, and that a short time window of the order of 1 s should be used to detect the scintillation. The 1‐min phase scintillation index is largely influenced by refractive effects due to total electron content (TEC) variations, and the spectral power below ∼1 Hz should be removed to identify diffractive scintillation.

We utilized citizen scientist photographs of subauroral emissions in the upper atmosphere and identified a repeatable sequence of proton aurora and subauroral red (SAR) arc during substorms. The sequence started with a pair of green diffuse emissions and a red arc that drifted equatorward during the substorm expansion phase. Simultaneous spectrograph and satellite observations showed that they were subauroral proton aurora, where ion precipitation created secondary electrons that illuminated aurora in green and red colors. The ray structures in the red arc also indicated existence of low‐energy electron precipitation. The green diffuse aurora then decayed but the red arc (SAR arc) continued to move equatorward during the substorm recovery phase. This sequence suggests that the SAR arc was first generated by secondary electrons associated with ion precipitation and may then transition to heat flux or Joule heating. Proton aurora provides observational evidence that ion injection to the inner magnetosphere is the energy source for the initiation of the SAR arc.

Trans‐ionospheric high frequency (HF: 3–30 MHz) signals experience strong attenuation following a solar flare‐driven sudden ionospheric disturbance (SID). Solar flare‐driven HF absorption, referred to as short‐wave fadeout, is a well‐known impact of SIDs, but the initial Doppler frequency shift phenomena, also known as “Doppler flash” in the traveling radio wave is not well understood. This study seeks to advance our understanding of the initial impacts of solar flare‐driven SID using a physics‐based whole atmosphere model for a specific solar flare event. First, we demonstrate that the Doppler flash phenomenon observed by Super Dual Auroral Radar Network (SuperDARN) radars can be successfully reproduced using first‐principles based modeling. The output from the simulation is validated against SuperDARN line‐of‐sight Doppler velocity measurements. We then examine which region of the ionosphere, D, E, or F, makes the largest contribution to the Doppler flash. We also consider the relative contribution of change in refractive index through the ionospheric layers versus lowered reflection height. We find: (a) the model is able to reproduce radar observations with an root‐median‐squared‐error and a mean percentage error (δ) of 3.72 m/s and 0.67%, respectively; (b) the F‐region is the most significant contributor to the total Doppler flash (∼48%), 30% of which is contributed by the change in F‐region's refractive index, while the other ∼18% is due to change in ray reflection height. Our analysis shows lowering of the F‐region's ray reflection point is a secondary driver compared to the change in refractive index.

Using Defense Meteorological Satellite Program (DMSP) and National Oceanic and Atmospheric Administration (NOAA) satellite observations and ground‐based observations by the THEMIS all‐sky imagers (ASIs) and SuperDARN radars, we determine how the equatorward boundary locations of ring current ions and plasma sheet electrons at pre‐midnight relate to occurrence of strong thermal emission velocity enhancement (STEVE) and intense subauroral ion drifts (SAID) during substorms. We found that the STEVE events are associated with a sharper gradient of electron precipitating flux, lower precipitating ion flux, and a narrower (<1°) latitudinal gap between the equatorward boundaries of trapped ring current ions and precipitating plasma sheet electrons and narrower region‐2 field‐aligned currents (FACs) than for the non‐STEVE events. The narrow gap of the particle boundaries contains intense SAID, higher upflow velocity, lower trough density, and slightly higher electron temperature than those for the non‐STEVE events. The non‐STEVE substorms have much wider gaps between the trapped ions and precipitating electrons, and subauroral polarization streams (SAPS) do not show intense SAID. These results indicate that subauroral flows and downward FACs for the STEVE events can only flow within the latitudinally narrow subauroral low‐conductance region between the ion and electron boundaries, resulting in intense SAID and heating. During the non‐STEVE events, the SAPS flows can flow in the latitudinally wide region without forming intense SAID.

We present high‐resolution Resolute Bay Incoherent Scatter Radar (RISR) measurements in the cusp region during an IMF southward turning. The simultaneous RISR‐N and RISR‐C operation provided 3‐D observations of the dayside polar region, and offered an opportunity to identify the cusp dynamics and polar cap patch formation. Associated with the IMF southward turning, the F‐region density and temperature increased in the cusp, and the increase was particularly evident in the topside ionosphere. The high‐density plasma drifted into the polar cap by an enhanced poleward convection, and became a polar cap patch. The patch plasma was initially dominated by density originating in the cusp, and then later the subauroral ionospheric plasma also contributed to the density enhancement. Weak upflows were present but their contribution within the RISR altitude range was minor. We suggest that the patch source region switches due to dynamic variations of the cusp precipitation and convection from lower latitudes. RISR also detected a flow vortex embedded in the large‐scale convection, which is likely a poleward moving auroral form (PMAF) signature. Joule heating peaked in the cusp E and lower F‐regions. The F‐region Pedersen conductivity increased more than the Hall conductivity, and the high conductivity region extended poleward associated with the patch density enhancement. A 1‐D cusp simulation reproduced the density and temperature enhancements by soft electron precipitation, indicating the importance of soft electron precipitation for the cusp dynamics and the initial part of the patch formation.

Using the University Navstar Consortium (UNAVCO) Global Positioning System (GPS) receiver network in North America, we present 2‐D distributions of GPS radio signal scintillation in the mid‐latitude ionosphere during the 7–8 September 2017 storm. The mid‐latitude ionosphere showed a variety of density structures such as the storm enhanced density (SED) base and plume, main trough, secondary plume, and secondary trough during the storm main and early recovery phases. Enhanced phase and amplitude scintillation indices were observed at the density gradients of those structures. SuperDARN radar echoes were also enhanced at the density gradients. The collocation of the scintillation and HF radar echoes indicates that density irregularities developed across a wide range of wavelengths (tens of meters to tens of kilometers) in the mid‐latitude density structures. The density gradients and irregularities were also detected by Swarm and DMSP as in‐situ density structures that disturbed the GPS signals. The irregularities were a substantial fraction (∼10%–50%) of the background density. The density irregularity had a power law spectrum with slope of ∼ −1.8, suggesting that gradient drift instability (GDI) contributed to turbulence formation. Both high‐latitude and low‐latitude processes likely contributed to forming the mid‐latitude density structures, and the mid‐latitude scintillation occurred at the interface of high‐latitude and low‐latitude forcing.

Evolution of large‐scale and fine‐scale plasmaspheric plume density structures was examined using space‐ground coordinated observations of a plume during the 7–8 September 2015 storm. The large‐scale plasmaspheric plume density at Van Allen Probes A was roughly proportional to the total electron content (TEC) along the satellite footprint, indicating that TEC distribution represents the large‐scale plume density distribution in the magnetosphere. The plasmaspheric plume contained fine‐scale density structures and subauroral polarization streams (SAPS) velocity fluctuations. High‐resolution TEC data support the interpretation that the fine‐scale plume structures were blobs with ∼300 km size and ∼500–800 m/s in the ionosphere (∼3,000 km size and ∼5–8 km/s speed in the magnetosphere), emerging at the plume base and drifting to the plume. The short‐baseline Global Navigation Satellite System receivers detected smaller‐scale (∼10 km in the ionosphere, ∼100 km in the magnetosphere) TEC gradients and their sunward drift. Fine‐scale density structures were associated with enhanced phase scintillation index. Velocity fluctuations were found to be spatial structures of fine‐scale SAPS flows that drifted sunward with density irregularities down to ∼10 s of meter‐scale. Fine‐scale density structures followed a power law with a slope of ∼−5/3, and smaller‐scale density structures developed slower than the larger‐scale structures. We suggest that turbulent SAPS flows created fine‐scale density structures and their cascading to smaller scales. We also found that the plume fine‐scale density structures were associated with whistler‐mode intensity modulation, and localized electron precipitation in the plume. Structured precipitation in the plume may contribute to ionospheric heating, SAPS velocity reduction, and conductance enhancements.

We examined the source region of dayside large‐scale traveling ionospheric disturbances (LSTIDs) and their relation to cusp energy input. Aurora and total electron content (TEC) observations show that LSTIDs propagate equatorward away from the cusp and demonstrate the cusp region as the source region. Enhanced energy input to the cusp initiated by interplanetary magnetic field (IMF) southward turning triggers the LSTIDs, and each LSTID oscillation is correlated with a TEC enhancement in the dayside oval with tens of minutes periodicity. Equatorward‐propagating LSTIDs are likely gravity waves caused by repetitive heating in the cusp. The cusp source can explain the high LSTID occurrence on the dayside during geomagnetically active times. Poleward‐propagating ΔTEC patterns in the polar cap propagate nearly at the convection speed. While they have similar ΔTEC signatures to gravity wave‐driven LSTIDs, they are suggested to be weak polar cap patches quasiperiodically drifting from the cusp into the polar cap via dayside reconnection.

The Sun was remarkably active during the first week of September 2017 producing numerous solar flares, solar radiation storms, and coronal mass ejections. This activity caused disruption to terrestrial high‐frequency (HF, 3–30 MHz) radio communication channels including observations with the Super Dual Auroral Radar Network (SuperDARN) HF radars. In this paper, we analyze the response of SuperDARN groundscatter observations and decreases in background sky noise level in response to multiple solar flares occurring in quick succession and co‐occurring with solar energetic protons and auroral activity. We estimate the attenuation in HF signal strength using an approach similar to riometry and find that the radars exhibit a nonlinear response to compound solar flare events. Additionally, we find that the three different space weather drivers have varying degrees of influence on the HF signal properties at different latitudes. Our study demonstrates that in addition to monitoring high‐latitude convection, SuperDARN observations can be used to study the spatiotemporal evolution of disruption to HF communication during extreme space weather conditions.

The formation of polar cap density enhancements, such as tongues‐of‐ionization (TOIs), are often attributed to enhanced dayside reconnection and convection due to solar wind changes. However, ionospheric poleward moving density enhancements can also form in the absence of changes in the solar wind. This study examines how TOI and patch events that are not triggered by solar wind changes relate to magnetospheric processes, specifically substorms. Based on total electron content and Super Dual Auroral Radar Network (SuperDARN) observations, we find substorms that occur at the same time as TOIs are associated with sudden enhancements in dayside poleward flows during the substorm expansion phase. Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) observations also show enhanced field‐aligned currents (FACs) that extend into the dayside ionosphere during this period. We suggest that the global enhancement of FACs and convection during these substorms are the drivers of these TOIs by enhancing dayside convection and transporting high‐density lower‐latitude plasma into the polar cap. However, we also find that not all substorms are coincident with polar cap density enhancements. A superposed epoch study showed that the AL index for TOIs during substorms is not particularly stronger than substorms without TOIs, but epoch studies of AMPERE observations do show events with TOIs to have a higher total FAC on both the dayside and nightside. Our results show the importance of TOI formation during substorms when solar wind drivers are absent, and the importance of considering substorms in the global current system. This work also shows the need to incorporate substorms into models of high‐latitude global convection and currents.