Previous studies have shown that solar flares can significantly affect Earth's ionosphere and induce ion upflow with a magnitude of ∼110 m/s in the topside ionosphere (∼570 km) at Millstone Hill ...(42.61°N, 71.48°W). We use simulations from the Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model (TIEGCM) and observations from Incoherent Scatter Radar (ISR) at Millstone Hill to reveal the mechanism of ionospheric ion upflow near the X9.3 flare peak (07:16 LT) on 6 September 2017. The ISR observed ionospheric upflow was captured by the TIEGCM in both magnitude and morphology. The term analysis of the F‐region ion continuity equation during the solar flare shows that the ambipolar diffusion enhancement is the main driver for the upflow in the topside ionosphere, while ion drifts caused by electric fields and neutral winds play a secondary role. Further decomposition of the ambipolar diffusive velocity illustrates that flare‐induced changes in the vertical plasma density gradient is responsible for ion upflow. The changes in the vertical plasma density gradient are mainly due to solar extreme ultraviolet (EUV, 15.5–79.8 nm) induced electron density and temperature enhancements at the F2‐region ionosphere with a minor and indirectly contribution from X‐ray (0–15.5 nm) and ultraviolet (UV, 79.8–102.7 nm).
Plain Language Summary
Solar flares with a sudden enhancement in solar X‐ray and extreme ultraviolet (EUV) radiation heat the upper atmosphere and increase electron density dramatically, leading to an expansion of Earth's upper atmosphere. Previous observations have shown an upward ion motion of ∼110 m/s at middle latitudes during solar flares; however, the cause of it remains largely speculative. In this paper, we provide a quantitative analysis of the ionospheric ion upflow during the 6 September 2017 X9.3 flare via model‐data comparison. During solar flares, X‐ray and EUV radiation are intensified at different degrees, thus producing different magnitudes of enhancement in ionospheric plasma density and temperature at different altitudes. Enhanced electron density and temperature at ionospheric F‐region due to solar EUV photoionization build up an additional upward pressure gradient force, pushing the plasmas upward.
Key Points
Flare‐induced ionospheric ion upflow at midlatitudes was simulated by a 3‐D ionosphere‐thermosphere coupled model
The midlatitude ion upflow was mainly due to changes in ambipolar diffusion during flares
The solar extreme ultraviolet (EUV) enhanced electron density and temperature were the primary driver of ambipolar diffusion velocity changes
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
The space hurricane is a three‐dimensional magnetic vortex structure with strong flow shears and electron precipitation in the polar cap. This study investigates for the first time how a space ...hurricane disturbs the polar thermosphere. During the formation and development of the space hurricane, the directional reversal of the horizontal neutral wind and the plasma convection will both be relocated from the poleward auroral oval boundary to the edge of the space hurricane, but the neutral wind responds slower compared to the plasma convection. Strong flow shears in the space hurricane causes enhanced Joule heating in the polar cap, which heats the thermosphere and triggers Atmospheric Gravity Waves (AGWs). Statistical results reveal that significant AGWs mainly are located on the dawnside of the space hurricane, suggesting that the space hurricane plays a significant role in ion‐neutral coupling and generation of polar cap AGWs.
Plain Language Summary
The space hurricane is a cyclonic aurora structure over the Earth's polar cap region under northward interplanetary magnetic field condition. It can transfer large amounts of energy and plasma into the polar ionosphere during otherwise extremely quiet geomagnetic times. Joule/Frictional heating due to ion‐neutral collisions plays a key role for the energy budget, resulting in non‐negligible atmospheric disturbances, which may impact the orbits of low‐altitude satellites through enhanced frictional drag. Using in situ observations from the Defense Meteorological Satellite Program satellites and the Gravity Field and Steady‐State Ocean Circulation Explorer satellite, we report the first evidence of a disturbed thermosphere triggered by a space hurricane. The neutral wind shows a slow variation in response to the space hurricane plasma convection. Significant wave‐like thermospheric density and vertical wind disturbances are observed around the space hurricane. Enhanced Joule heating from flow shears inside the space hurricane may be the generator. These findings show that the space hurricane plays a significant role for ion‐neutral coupling in the polar cap region.
Key Points
The space hurricane can relocate the reversal of the neutral wind from the auroral oval to the edge of the space hurricane
The space hurricane can heat the thermosphere and generate Atmospheric gravity waves (AGWs) that disturb the neutral density and vertical wind
The space hurricane can significantly enhance polar cap AGW activity in the dawn sector
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A quasi‐semidiurnal type pattern was observed earlier in the diurnal UT variation of the geomagnetic storms studied using mainly Kyoto Dst (disturbance storm‐time) index. However, the pattern has ...been argued as apparent due to uneven longitude distribution of the four Dst observatories. Unlike earlier studies, this paper investigates the diurnal UT variation of the storms automatically identified in six available indices including Kyoto Dst, USGS (United States Geological Survey) Dst, SymH (symmetric‐H), RC (ring current), Dcx (corrected extended Dst), and AER (Atmospheric and Environmental Research) in 50, 50, 36, 21, 5, and 7 years, respectively. The indices are derived using 4, 4, 12, 14, and 15 ground observatories (with maximum longitude separations of ∼120°, 120°, 70°, 110°, and 50°) and four DMSP (Defense Meteorology Satellite Program) satellites, respectively. The UT distribution of the storm intensity (minimum value of an index during the storm main phase) in all indices shows a striking quasi‐semidiurnal type variation with maxima around 06–08 UT and 21–23 UT and minima around 03–05 UT and 13–15 UT. Similar quasi‐semidiurnal variation is also observed in the computed values of the main energy input in the ring current. The variation correlates well with the variations of the dipole tilt angles μ and θ involved in the equinoctial hypothesis and Russell‐McPherron (RM) effect, respectively. These observations indicate that the quasi‐semidiurnal variation is real.
Plain Language Summary
Large disturbances in the geomagnetic field lasting form several hours to several days are known as geomagnetic storms. The variations of the occurrence and intensity of the storms with solar activity and season have been understood thanks to the works of a large number of scientists. The variation of the storms with the time‐of‐day studied using mainly the low latitude geomagnetic activity index Dst has shown a quasi‐semidiurnal pattern. The pattern, however, has been argued as apparent due to the uneven longitude distribution of the four magnetic observatories used for deriving Dst. The present study investigates the diurnal UT variation of the storms using six available indices. The results show similar striking quasi‐semidiurnal patterns in the UT distribution of the storm intensity in all indices and computed value of the main energy input in the ring current. The quasi‐semidiurnal pattern also correlates well with the angles μ and θ involved in the mechanisms of equinoctial hypothesis and RM effect. These observations indicate that the quasi‐semidiurnal variation is real.
Key Points
A striking quasi‐semidiurnal pattern is observed in the UT distribution of the geomagnetic storm intensity in six low latitude indices
Similar pattern exists in the UT variation of the computed value of the main energy input in the ring current
The quasi‐semidiurnal pattern correlates well with the angles μ and θ involved in the mechanisms of equinoctial hypothesis and RM effect
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In Earth's low atmosphere, hurricanes are destructive due to their great size, strong spiral winds with shears, and intense rain/precipitation. However, disturbances resembling hurricanes have not ...been detected in Earth's upper atmosphere. Here, we report a long-lasting space hurricane in the polar ionosphere and magnetosphere during low solar and otherwise low geomagnetic activity. This hurricane shows strong circular horizontal plasma flow with shears, a nearly zero-flow center, and a coincident cyclone-shaped aurora caused by strong electron precipitation associated with intense upward magnetic field-aligned currents. Near the center, precipitating electrons were substantially accelerated to ~10 keV. The hurricane imparted large energy and momentum deposition into the ionosphere despite otherwise extremely quiet conditions. The observations and simulations reveal that the space hurricane is generated by steady high-latitude lobe magnetic reconnection and current continuity during a several hour period of northward interplanetary magnetic field and very low solar wind density and speed.
Following substorm auroral onset, the active aurora region usually expands poleward toward the poleward auroral boundary. Such poleward expansion is often associated with a bulge region that expands ...westward and forms the westward travelling surge. In this study, we show all‐sky imager and Poker Flat Advanced Modular Incoherent Scatter Radar observations of two surge events to investigate the relationship between the surge and ionospheric flows that likely have polar cap origin. For both events, we observe auroral streamers, with an adjacent flow channel consisting of decreased density and low electron temperature plasma flowing equatorward. This flow channel appears to impinge and lead/feed surge formation, and to stay connected to the surge as it moves westward. Also, for both events, streamer observations indicate that, following initial surge development, similar flows led to explosive surge enhancements. The observation that the streamers are connected to the auroral polar boundary and that the flow channels consisted of low density, low electron temperature plasma suggests the possibility that the impinging plasma came from the polar cap. For both events, the altitude variations of F region plasma within the surges are related with aurora emission and the poleward/equatorward flow, and the surges develop strong auroral streamers that initiate along the poleward auroral boundary when contacted with the flow. These results suggest that the flow of polar cap origin, which maps to underlying processes in the magnetotail, may play a crucial role in auroral surges by feeding low entropy plasma into surge initiation and development, and also playing an important role in the dynamics within a surge.
Key Points
Flow channels of equatorward‐moving, low density plasma that likely originate from polar cap lead and keep impinging on the head of surge
The plasma adjacent to auroral streamers lead to the surge to explode rapidly westward and poleward and to be followed by strong streamers
During the development of the surge, the altitude of density varies with flow directions and auroral emissions
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At high latitudes, the Sporadic‐E layer (Es layer) is a common phenomenon but is still poorly understood due to sparse measurements and the difficulty of conventional mechanisms to operate. In this ...study, an interesting case of polar cap Es layer is first studied by using the twin incoherent scatter radars (northward‐looking face of Resolute Incoherent‐Scatter Radar and Resolute Incoherent‐Scatter Radar‐Canada), a Canadian Advanced Digital Ionosonde, and a Magnetometer, all at Resolute, Canada. From several electron density profiles of the twin radars, the horizontal scale of the polar cap Es layer is found to be greater than 350 km. Moreover, the polar cap Es layer is determined to be drifting from the bottom F region (>150 km) to the lower E region. Furthermore, a unique appearance of double polar cap Es layers is observed. As a result, these peculiar signatures inspire a newly proposed process that involves the combination of localized electric fields and gravity waves.
Plain Language Summary
The Sporadic‐E layer (Es layer) is a globally common phenomenon, which is comprehensively and unceasingly studied in decades by using the observation measurements and numerical simulations. The remarkable agreements on the characteristics and generation mechanisms of Es layer at middle latitudes have then been almost achieved. However, over the polar region, it is still poorly understood due to sparse measurements and the difficulty of conventional mechanisms to operate. In this study, an interesting case of polar cap Es layer are first reported by using the twin radars of northward‐looking face of Resolute Incoherent‐Scatter Radar and Resolute Incoherent‐Scatter Radar‐Canada, Canadian Advanced Digital Ionosonde, and Magnetometer simultaneously. Through comprehensively studies, the understanding of polar cap Es layer is clearly extended, not only on the characteristics but also on the mechanism processes. The polar cap Es layer is horizontally greater than 350 km; a new process on the generation mechanism is provoked including the functions of particle precipitation and gravity wave. As a consequence, we present a peculiar evidence for the first time to greatly enrich our knowledge on the polar cap Es layer, showing us a new insight on it.
Key Points
An amazing example of polar cap Es layer is first presented by the twin radars of northward‐looking face of Resolute Incoherent‐Scatter Radar and Resolute Incoherent‐Scatter Radar‐Canada at Resolute, Canada simultaneously
The morphology of the polar cap Es layer is comprehensively furthered, interpreting the horizontal scale size (>350 km) and double layers
To reasonably explain this phenomenon, a new process is hypothesized, consisting of localized electric field and gravity wave
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The geomagnetically induced current (GIC) produced during extreme geomagnetic storms can easily lead to large‐scale blackouts in China due to the increase in the scale of its electric power grid. A ...power grid's resilience is its capability to resist various natural hazards, withstand primary failures, and quickly resume normal operation. To avoid power grid damages, this study developed a resilient power grid, incorporating failure, power flow calculation and recovery models under a uniform induced geoelectric field. We chose a system's performance loss as the resilience evaluation indicator, which intuitively reflected a system's loss under GIC. In addition, the recovery model was optimized using a genetic algorithm, and two resilience improvement measures were proposed. The IEEE‐RTS‐79 system, consisting of 10 generators, 24 buses and 5 transformers, was chosen as an example to verify the feasibility of this study. The results show that the genetic algorithm and optimization measures effectively enhanced the system's resilience indicator and provided a reference for preventing system damages under GIC and quick recovery after possible failures.
Plain Language Summary
Extreme space weather produces large geomagnetic field disturbances; these disturbances propagate down to Earth, inducing secondary geomagnetically induced currents (GICs) in earthed electric power grids. GICs increase transformers' reactive power loss and raise their temperatures; this affects a power system's normal operation, and in extreme cases causes major blackouts. Therefore, it is vital to study power grids' resilience under GIC events. In this study, we calculated the GIC level using a uniform geoelectrical conductivity model. Based on this, we proposed a resilient power grid model consisting of the failure model, power flow calculation model and recovery model, and evaluated optimization strategies. Our results show that the proposed model can improve a power system's recovery performance after possible failure due to GICs and effectively avoid GIC‐related large‐scale blackouts.
Key Points
We proposed a resilience assessment method for transmission systems under extreme space weather
We calculated a resilience indicator and identified multiple effective methods to improve a system’s resilience
We chose the IEEE‐RTS‐79 system as an example to verify the effectiveness of a genetic algorithm and optimization measures
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Simultaneous observations from Defense Meteorological Satellite Program, Swarm, Resolute Bay all‐sky imagers, GPS Total Electron Content and Super Dual Auroral Radar Network, are used to investigate ...the evolution and key characteristics of the Tongue of Ionization (TOI) being restructured into a polar cap patch. Six satellites crossed the TOI of patch as it moved from the dayside to the nightside. It was initially hot, then a mix of both cold and hot, and finally it became a cold patch. This suggests that cold patch is not only a result of solar extreme ultraviolet radiation, but may also develop when a hot patch cools down. Soft‐electron precipitation and flow shears both contribute to the TOI restructuring and the appearance of polar cap patch. The plasma density of patch at ∼500 km was at least 4 times higher than at ∼800 km. The plasma density enhancement gradually decreased as the patch evolved due to decreased production and transport of cold nightside low‐density plasma. Moreover, the duskward motion of the patch was influenced by changes in the ionospheric convection pattern.
Plain Language Summary
The appearance of high‐density plasma is a common phenomenon in the polar ionosphere. High‐density polar cap patches usually form near the dayside sunlit region, and then move from the dayside to the nightside with the ionospheric flow. In the paper, we use multiple instruments including six satellites and ground‐based observations to carefully investigate an event that started out on the dayside. It was initially a hot tongue of ionization (TOI) of high density and high electron temperature, followed by a mix of hot and cold (high vs. low electron temperature), before it finally became a cold patch. Soft‐electron precipitation and flow shears both influenced the dayside high‐density TOI plasma restructuring and the appearance of polar cap patch. The enhanced plasma density gradually weakened as the plasma migrated toward the nightside due to decreased production and transport of cold nightside low‐density plasma. Moreover, the duskward motion of the patch was influenced by changes in the ionospheric convection pattern.
Key Points
Cold dense plasma in the polar cap can appear either via transport of solar extreme ultraviolet plasma or after a hot patch cools down
Soft‐electron precipitation and flow shears both influence the Tongue of Ionization restructuring and the appearance of polar cap patches
The enhanced plasma density is reduced due to decreased production and transport of cold nightside low‐density plasma
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
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