Abstract Large ionospheric disturbances were observed over Europe during the extreme geomagnetic storm on 10 May 2024. This study analyzes GNSS‐derived total electron content (TEC) and all‐sky imager data to investigate these disturbances. Pronounced TEC enhancements, irregularities, and strong gradients were confined to the auroral region above the mid‐latitudes. These anomalies evolved coherently with the equatorward expansion of the optical aurora. Large‐scale TEC disturbances were found to be moving at mid‐latitudes with an equatorward velocity of 726 m/s and an amplitude of up to 7 TECU. These characteristics were indicative of the auroral expansion rather than of large‐scale traveling ionospheric disturbances (LSTIDs), which exhibited weaker amplitudes near the auroral boundary. This finding contrasts with the interpretation of some previous studies on GNSS observations. This study demonstrates the visualization of the auroral oval at mid‐latitudes using TEC and TEC‐derived parameters.

We analyzed fragmented auroral-like emissions (FAEs) and picket fence structures observed in northern Scandinavia during a magnetic storm on 1 January 2025. The analysis is based on ground-based high-sensitivity optical observations and in-situ measurements from the Swarm satellites. While FAEs and picket fences have previously been reported in the polar cap and subauroral region, respectively, this study reports simultaneous occurrences of both phenomena in auroral latitudes, near the poleward edge of the oval. Ground-based camera observations revealed that some FAEs exhibited orientations closely aligned with the modeled local magnetic field in the image plane and appeared simultaneously at multiple longitudinally separated locations. Furthermore, the FAEs appeared to follow the motion of red auroras, suggesting that the background electric field structure and spatial gradients in the electron density may influence their formation. Consistent with previous studies, the generation of FAEs is considered to be due to local acceleration of electrons in the ionosphere rather than electron precipitation from the magnetosphere. While we could not clearly identify the generation mechanisms, the morphological diversity observed in this event suggests that multiple plasma instabilities may be involved in the generation of both FAEs and picket fence structures.

Abstract Heavy ions originating from the ionosphere, such as molecular nitrogen ions ( $${N}_{2}^{+}$$ N 2 + ), have been observed in the magnetosphere during periods of geomagnetically active conditions. However, the mechanism that transfers heavy ions into the magnetosphere is not fully understood. Measurement of $${N}_{2}^{+}$$ N 2 + upflow in the Earth’s upper atmosphere would contribute to the understanding of the mechanism, although such measurements have not been done yet. In this study, we report a new attempt to measure the Doppler shift of auroral $${N}_{2}^{+}$$ N 2 + first negative band (1NG) emissions around 427.8 nm through a Fabry–Perot interferometer (FPI) in Norway. The CCD detector position was adjusted to the focusing point of the 427.8-nm emission, because it was originally located at the focusing point of the 632.8-nm calibration laser. An intense 427.8-nm emission was observed at dawn on 29th September 2024, probably due to resonant scattering of sunlight by high-altitude $${N}_{2}^{+}$$ N 2 + ions. Downward velocities of 100–300 m/s with a 90% confidence interval of $$\pm$$ ± 50–120 m/s were observed with a time resolution of 4 min 20 s at 02:00–03:00 UT (03:00–04:00 LT). Based on the comparison with the auroral Hall current estimated by ground geomagnetic variations, we consider that the observed ion velocities are significantly influenced by the perpendicular ion velocity due to ExB plasma drift. We also noted that systematic errors of the measured Doppler velocity may occur due to difference of $${N}_{2}^{+}$$ N 2 + temperatures at two directions in sky scanning, which varies the ratio of $${N}_{2}^{+}$$ N 2 + (1NG) band emission lines that form the observed interference fringes. Based on model calculations, we estimated that 5% temperature difference in the two observation points possibly makes systematic errors of 5–10 m/s for 400 K and that of 8–13 m/s for a rotational temperature of 1000 K. In summary, we succeeded in detecting the Doppler shift even from band emission observations, but the current observations are strongly affected by variations in the electric field perpendicular to the magnetic field and N₂⁺ temperature. This problem can be solved by simply observing only a single field-aligned measurement using a frequency-stabilized laser at wavelengths near 427.8 nm. Graphical Abstract

The May 2024 superstorm, as the most intense geomagnetic storm since 2003, caused a variety of disturbances in the magnetosphere-ionosphere-thermosphere system. This study investigates the long-lasting electron density depletion in the polar region and the underlying ionosphere-thermosphere coupling, based on a comprehensive set of observations from ground and space. Initially, a significant amount of solar wind energy was dissipated at high latitudes, and we estimate that the hemispheric Joule heating reached 1.25 TW by using a newly developed method that utilizes SuperDARN, SuperMAG, and AMPERE data. This intense heating increased the ion temperature by 500-1,200 K in the polar region, as detected by the EISCAT Svalbard radar (ESR). Furthermore, Joule heating caused significant upwelling of the polar thermosphere, evidenced by 300%-480% increase in neutral mass density and a substantial depletion in up to 50%, as observed by several low-Earth-orbit satellites. Both the increase in ion temperature and the change in neutral composition are crucial factors in accelerating the F-region recombination process. Consequently, the transition altitude of molecular to oxygen ions increased dramatically from 200 to 380 km, as detected by the ESR radar. The ultimate consequence was a severe depletion in electron density in the polar F-region ionosphere, reaching 70%-85% on 11 May, which gradually recovered over the next two days. Our analysis underscores the importance of simultaneous, multi-instrument observations for a comprehensive understanding of the coupling chain during extreme geomagnetic disturbances.

This study investigated whether the frequency spectra of cosmic noise absorption (CNA), observed by the spectral riometer in Kilpisjärvi, Finland, can indicate the hardness of the energy spectrum of precipitating particles. CNA data have traditionally been used to evaluate electron density enhancements in the mesosphere. When observed at multiple frequencies, CNA can be approximated as 𝐶𝑁𝐴 ∝ 𝑓-n, where 𝑓 is the radio frequency of cosmic noise and 𝑛 is referred to as spectral index. Specifically, a smaller 𝑛 corresponds to electron enhancement at lower altitudes, and consequently a harder particle energy spectrum. In this study, variation in spectral indices was analyzed for three natural events, i.e., substorms, solar flares, and solar proton events, which were identified using geomagnetic data and satellite-based proton and X-ray observations. The CNA spectra were fitted with a one-minute resolution to ensure statistical reliability. The frequency distributions of the spectral index during these event types were centered around 2.0. For substorms, the spectral index in the morning sector was smaller than those in the evening and night sectors, consistent with previous studies, which found that harder electron precipitation tends to occur at that time. The smallest index also suggested that high-energy electrons caused electron density enhancement below 59 km altitude. During solar proton events, variations in proton flux hardness observed by the satellite correlated well with changes in the CNA spectral index. The results demonstrated that the spectral index derived from CNA is a valid indicator of particle spectral hardness in both substorms and solar proton events. This study presents the first observational evidence that spectral riometers can retrieve ionization information below 70 km altitude, a region previously difficult to monitor continuously.

Strong Thermal Emission Velocity Enhancement (STEVE) is a latitudinally narrow, purple-band emission observed at subauroral latitudes. Stable Auroral Red (SAR) arcs characterized by major red emission, and red/green arcs with both red and green emissions also occur at subauroral latitudes. Characteristics of magnetospheric source plasma and electromagnetic fields of these three types of arcs have not been fully understood because of the limited conjugate observations between magnetosphere and the ground. In this study, we report 11 conjugate observations (2 STEVEs, 7 SAR arcs, and 2 red/green arcs), using all-sky images obtained at seven ground stations over more than four years from January 2017 to April 2021 and magnetospheric satellites (Arase and Van Allen Probes). We found that, in the inner magnetosphere, the source region of STEVEs and red/green arcs were located outside the plasmasphere, and that of the SAR arc was in the region of spatial overlap between the plasmasphere and ring current region. Electromagnetic waves at frequencies below 1 Hz were observed for STEVEs and red/green arcs. SuperDARN radar data showed a strong westward plasma flow in the ionosphere, especially during STEVE events, whereas the plasma flows associated with SAR arcs and red/green arcs were generally weaker and variable. The STEVE and SAR arc can appear simultaneously at slightly different latitudes and STEVEs and red/green arcs can transform into SAR arcs. These first comprehensive ground-satellite measurements of three types of subauroral-latitude auroras increase our understanding on similarlity, differences, and coupling of these auroras in the ionosphere and the magnetosphere.

In the polar middle and upper atmosphere, nitric oxide (NO) is produced in large amounts by both solar EUV and X-ray radiation and energetic particle precipitation, and its chemical loss is driven by photodissociation. As a result, polar atmospheric NO has a clear seasonal variability and a solar cycle dependency which have been measured by satellite-based instruments. On shorter timescales, NO response to magnetospheric electron precipitation has been shown to take place on a day-to-day basis. Despite recent studies using observations and simulations, it remains challenging to understand NO daily distribution in the mesosphere-lower thermosphere during geomagnetic storms and to separate contributions of electron forcing and atmospheric chemistry and dynamics. This is due to the uncertainties existing in the available electron flux observations, differences in representation of NO chemistry in models, and differences between NO observations from satellite instruments. In this paper, we use mesospheric-lower-thermospheric NO column density data measured with a millimeter-wave spectroscopic radiometer at the Syowa station in Antarctica. In the period 2012-2017, we study both the long-term and short-term variability of NO. Comparisons are made with results from the Whole Atmosphere Community Climate Model to understand the shortcomings of current electron forcing in models and how the representation of the NO variability can be improved in simulations. We find that, qualitatively, the simulated year-to-year and day-to-day variability of NO is in agreement with the observations. On the other hand, there is up to a factor of 2 underestimation of the NO column density in wintertime. Also, the model captures only 27 % of the range of observed daily NO values. The observed day-to-day variability has a good correlation with three different geomagnetic indices, indicating the importance of electron forcing in atmospheric NO production. Using electron flux measurements from the Arase satellite, we demonstrate their potential in atmospheric research. Our results call for improved representation of electron forcing in simulations to capture the observed day-to-day variability.

Observational data sets for the high latitude middle atmosphere are key to understand the dynamics over those latitudes and the coupling between the lower and middle atmosphere. Utilizing long‐term data sets from an all‐sky imager at Tromsø, Norway (69.6°N, 19.2°E), the characteristics of 18 mesospheric frontal events in the Arctic winter mesosphere from 2011 to 2015 were studied. These frontal events exhibit horizontal extensions exceeding 500 km and were characterized by a sharp leading front, sometimes followed by a quasi‐monochromatic wave train or a turbulent region. A subset of these frontal gravity wave events has been identified in the past as “bores.” While there have been numerous previous reports from low‐ and mid‐latitude sites, and also from southern high latitudes, there have been a few from northern high latitudes. This study focuses on the frontal events in the northern high latitudes and provides new insights into the characteristics of these events. Their horizontal wavelengths primarily ranged from 20 to 40 km, and they exhibited phase speeds in the range 30–80 m/s. Most events were observed before local midnight. No clear link between these events and auroral activity was found. The majority of fronts were found propagating in the north‐west direction, which might be due to the wind filtering effects.

AbstractSolar cycles 24–25 were quiet until a geomagnetic storm with a Sym‐H index of −170 nT occurred in late March 2023. On March 23–24, a Fabry‐Perot interferometer (FPI; 630 nm) in Tromsø, Norway, recorded the highest thermospheric wind speed of over 500 m/s since 2009. Comparisons with magnetometer readings in Scandinavia showed that a large amount of electromagnetic energy was transferred to the ionosphere‐thermosphere system. Total electron content maps suggested an enlarged auroral oval and revealed that the FPI observed winds near the polar cap instead of inside the oval for a long period during the storm main phase. The FPI wind had a strong equatorward component during the storm, likely because of the powerful anti‐sunward ionospheric plasma flow in the polar cap. The positive Y‐component of the IMF for 6 days before the storm caused a successive westward component of the FPI‐measured wind during the storm main phase. On March 24, the first day of the storm recovery phase, thermospheric wind disturbed and the ionospheric density decreased significantly at high latitudes. This density depression lasted for several days, and a large amount of electromagnetic energy during the storm modified the thermospheric dynamics and ionospheric plasma density.

We report an Arase-all sky imager (ASI) conjugate event in which the pulsating aurora (PsA) has a one-to-one correspondence with chorus bursts. Wavelet analysis displayed three peaks at similar to 0.3 Hz, 4 Hz, and >10 Hz, corresponding to the main pulsation, internal modulation, and fast modulation, respectively. These correspond to the old terms of similar to 5-15 s pulsations, chorus risers/elements and subelements/subpackets, respectively. Electron "microbursts" correspond to the 4-Hz peak. The internal and fast modulations are further verified by the analysis based on fast Fourier transform analyses. Moreover, the spatial distributions of the Fourier spectral amplitude show that the internal and fast modulations are well-structured within auroral patches. The above results indicate a paradigm shift away from quasilinear theory which implicitly assumes diffuse wave generation. The three time-scale modulations are consistent with coherent chorus which has been theoretically argued to lead to pitch angle transport three orders of magnitude faster. Plain Language Summary Pulsating aurora exhibit irregular patches of brightness with quasiperiodic on-off transitions (similar to 2-20 s). More rapid modulations, such as internal modulation (similar to 3-4 Hz) or fast modulation (>10 Hz), have been detected within the pulsation "on" time. However, due to the measurement limitations, the simultaneous observation of three time-scale modulations has never been reported. In this study, we analyze the conjugate observations of the pulsating aurora (PsA) and chorus recorded by the ground-based Arase-all sky imager and the Arase satellite, which demonstrates the coexistence of three time-scale modulations in the PsA. The spatial distributions of Fourier spectral amplitude show that the internal and fast modulations are well-structured within the aurora patches. The three time scales of chorus modulation have been previously called chorus 5-15 s pulsations, risers/elements and subelements/subpackets corresponding to the three aurora peaks. This study verifies the existence of internal and fast modulations in PsA, implying an extremely rapid electron loss mechanism. Quasilinear theory cannot explain any of the three time-scale modulations. The discovery that chorus is coherent and will lead to pitch angle transport 1,000 times faster than diffuse waves is consistent with the fast modulations shown in this paper. A new theory of chorus generation is needed to update that of quasilinear theory.

Pc1 geomagnetic pulsations within the frequency range of 0.2-5 Hz, the ground signatures of electromagnetic ion cyclotron (EMIC) waves, are usually observed to progress westward. A plausible explanation is that their westward-excursion speed is defined by the drift velocity of the source particles in the geomagnetic dipole field. In this study, this relationship is examined through conjugate observations from ground-based and spaceborne instruments. The ground-based observations, including Pc1 pulsations and auroral emissions, consistently demonstrate a westward-excursion speed of similar to 4-5 MLT per hour, which agrees with the time delay between EMIC wave observations at two equatorial spacecraft. Observations of the wave source particles, however, indicate an inconsistency between the slow westward excursion speed and the high energy of the wave source protons. This apparent inconsistency is reconciled by the trapping motion of energetic particles within magnetic dips, which is supported by the simultaneous flux enhancements of protons across a broad energy range between similar to 50 and similar to 100 keV. These results improve our understanding of the westward excursion of Pc1 pulsations, and also underscore the need for caution when directly linking their westward-excursion speed to the energies of the source particles. Ground-based magnetometers often observe geomagnetic field oscillations at various frequencies, among which are Pc1 pulsations within the frequency range of 0.2-5 Hz. It is commonly believed that these pulsations correspond to electromagnetic ion cyclotron (EMIC) waves in Earth's magnetosphere, which are excited by anisotropic distributions of energetic ions. The Pc1/EMIC waves are usually observed to progress westward, and the excursion speed is thought to be determined by the drift velocity of source ions in the geomagnetic dipole field. This relationship is examined in this study based on multi-point observations of ground-based magnetic pulsations, auroral emissions, EMIC waves near the equatorial magnetosphere, and energetic particles in space. Ground-based magnetometers show the westward excursion of Pc1 pulsations at a speed of similar to 4-5 magnetic local time per hour, consistent with proton auroral emissions and equatorial spacecraft observations. However, this speed does not match the drift speed of observed source protons in the geomagnetic dipole field. We suggest that this apparent inconsistency can be reconciled after considering the revision of particles' drift motion in the presence of concomitant, localized field depressions in the magnetosphere. This explanation, supported by spacecraft observations, sheds new light on the intricate interactions between geomagnetic disturbances and the behavior of energetic particles. We observe the westward excursion of ground Pc1 pulsations at similar to 4-5 MLT/hr, consistent with auroral and multi-spacecraft observations The excursion speed appears to be lower than the drift speed of the anisotropic source protons in the unperturbed geomagnetic field The apparent inconsistency can be reconciled after considering the trapped motion of particles in localized magnetic dips

Enhancements in electron density in the D-region ionosphere attributed to the precipitation of high-energy electrons, have previously been inferred from increases in cosmic radio noise absorption (CNA) using ground-based riometers. However, there have been few studies of CNA observations at multi-point stations distributed in longitudes. Thus, the spatio-temporal development of the global distribution of CNA is not well understood. In this study, we investigated the longitudinal extent of CNA using simultaneous riometer observations at six stations at subauroral latitudes in Canada, Alaska, Russia, and Iceland. These stations are located encircling the earth at similar to 60(degrees) north magnetic latitudes. We have conducted simultaneous observations of CNA at these stations since October 2017. Here we focus on seven substorms during a geomagnetic storm 25-28 August 2018 and study the spatio-temporal development of the global distribution of CNA during these substorms. For all seven substorms, some stations observed CNA enhancements after the substorm onsets. In five cases, the CNA enhancements started around midnight and expanded eastward. The other two cases show westward and anti-sunward development of CNA. The eastward expansion of CNA indicates the eastward drift of high-energy electrons, which is the source of the CNA, due to gradient and curvature drift in the geomagnetic field. The westward expansion of CNA may correspond to westward expansion of the substorm injection region due to dawn-to-dusk electric fields. These results indicate that spatio-temporal development of CNA at subauroral latitudes corresponds to high energy electron drift in the inner magnetosphere.

The effect of storms driven by solar wind high-speed streams (HSSs) on the high-latitude ionosphere is inadequately understood. We study the ionospheric F-region during a moderate magnetic storm on 14 March 2016 using the EISCAT Troms & oslash; and Svalbard radar latitude scans. AMPERE field-aligned current (FAC) measurements are also utilized. Long-duration 5-day electron density depletions (20%-80%) are the dominant feature outside of precipitation-dominated midnight and morning sectors. Depletions are found in two major regions. In the afternoon to evening sector (12-21 magnetic local time, MLT) the depleted region is 10 degrees-18 degrees magnetic latitude (MLAT) in width, with the largest latitudinal extent 62 degrees-80 degrees MLAT in the afternoon. The second region is in the morning to pre-noon sector (04-10 MLT), where the depletion region occurs at 72 degrees-80 degrees MLAT within the auroral oval and extends to the polar cap. Using EISCAT ion temperature and ion velocity data, we show that local ion-frictional heating is observed roughly in 50% of the depleted regions with ion temperature increase by 200 K or more. For the rest of the depletions, we suggest that the mechanism is composition changes due to ion-neutral frictional heating transported by neutral winds. Even though depleted F-regions may occur within any of the large-scale FAC regions or outside of them, the downward FAC regions (R2 in the afternoon and evening, R0 in the afternoon, and R1 in the morning) are favored, suggesting that downward currents carried by upward moving ionospheric electrons may provide a small additional effect for depletion.

Pulsating Aurora (PsA) is one of the major classes of diffuse aurora associated with precipitation of a few to a few tens of keV electrons from the magnetosphere. Recent studies suggested that, during PsA, more energetic (i.e., sub-relativistic/relativistic) electrons precipitate into the ionosphere at the same time. Those electrons are considered to be scattered at the higher latitude part of the magnetosphere by whistler-mode chorus waves propagating away from the magnetic equator. However, there have been no actual cases of simultaneous observations of precipitating electrons causing PsA (PsA electrons) and chorus waves propagating toward higher latitudes; thus, we still do not quite well understand under what conditions PsA electrons become harder and precipitate to lower altitudes. To address this question, we have investigated an extended interval of PsA on 12 January 2021, during which simultaneous observations with the Arase satellite, ground-based all-sky imagers and the European Incoherent SCATter (EISCAT) radar were conducted. We found that, when the PsA shape became patchy, the PsA electron energy increased and Arase detected intense chorus waves at magnetic latitudes above 20 degrees, indicating the propagation of chorus waves up to higher latitudes along the field line. A direct comparison between the irregularities of the magnetospheric electron density and the emission intensity of PsA patches at the footprint of the satellite suggests that the PsA morphology and the energy of PsA electrons are determined by the presence of "magnetospheric density ducts," which allow chorus waves to travel to higher latitudes and thereby precipitate more energetic electrons.

Two interacting high-speed solar wind streams (HSSs) and associated stream interaction regions (SIR) caused a moderate geomagnetic storm during 14-20 March 2016. The spatio-temporal evolution of the total electron content (TEC) during the storm is studied by using Global Navigation Satellite System (GNSS) data. The moderate storm caused significant and long-lasting changes on TEC within the polar cap (70 degrees ${}<^>{\circ}$-90 degrees ${}<^>{\circ}$ MLAT), at auroral and sub-auroral latitudes (60 degrees ${}<^>{\circ}$-70 degrees ${}<^>{\circ}$ MLAT), and at mid-latitudes (40 degrees ${}<^>{\circ}$-60 degrees ${}<^>{\circ}$ MLAT). A 25%-50% depletion in TEC was observed for six days in the day, dusk and dawn sectors in the polar cap region and in the day and dusk sectors at the auroral and sub-auroral latitudes. Sub-auroral polarization streams observed by the Defense Meteorological Satellite Program satellite contributed to the sub-auroral dusk TEC decreases. At mid-latitudes, TEC depletion was observed in all local time sectors 21 hr after the storm onset. It is suggested that ion-neutral frictional heating causes the TEC depletions, which is further supported by the observed spatial correlation between TEC depletions and & sum; $\sum $O/N2 decreases at mid-latitudes observed by TIMED/GUVI. The storm induced a prolonged positive phase at mid-latitudes lasting 9 hr. In the polar cap, enhancements of TEC up to 200% were caused by polar cap patches. TEC increases were the dominant feature in the night and morning sectors within the auroral oval because of particle precipitation and resulted up to regionally averaged 6 TECU (200%) increases.

Electron density enhancements in the ionospheric D-region due to the precipitation of high-energy electrons (>30 keV) have been measured as increases in cosmic radio noise absorption (CNA) using ground-based riometers. CNA has been studied since the 1960s. However, there have been few studies of the spatiotemporal development of CNA at multi-point ground stations distributed in longitude at subauroral latitudes, where plasma particles with a wide energy range are intermingled. In this study, we analyzed the longitudinal development of CNA steep increases using simultaneous riometer observations at six stations at subauroral latitudes in Canada, Alaska, Russia, and Iceland over 3 years from 2017 to 2020. The results revealed that the occurrence rate of steep increases in CNA was highest at midnight at 22-08 magnetic local time (MLT), and lowest near dusk at 17-21 MLT. We also showed statistically that the CNA steep increases expanded eastward on the dawn side and westward on the dusk side. The CNA expansion velocity was slightly faster than the results of previous studies in the auroral zone. Correlation and superposed epoch analyses of CNA with solar wind and geomagnetic parameters revealed that CNA intensity was dependent on the Interplanetary Magnetic Field Bz, Interplanetary Electric Field Ey, SYM-H index, and SME index. These results indicate that the CNA at subauroral latitudes is closely related to solar wind and geomagnetic activities, and its propagation characteristics correspond to the dynamics of high energy electrons in the inner magnetosphere.

Medium-scale traveling ionospheric disturbances (MSTIDs) are one of the ionospheric plasma density structures and are observable through 630-nm airglow images. Previous studies using airglow images at Tromso (69.6 degrees N, 19.2 degrees E; magnetic latitude: 66.7 degrees N), Norway, reported high-latitude MSTIDs (here we call them as polar-type MSTIDs) whose propagation direction changes associated with auroral brightening and magnetic field disturbances. However, there has been little statistical analysis on the connection of MSTIDs occurring at high and middle latitudes. In this study, we statistically analyzed the MSTIDs observed by an airglow imager at Nyrola (62.3 degrees N, 25.5 degrees E; magnetic latitude: 59.4 degrees N), Finland, which is located similar to 7 degrees south of Tromso, corresponding to subauroral latitudes. The period analyzed was from 23 January 2017, to 30 September 2021. We found 11 cases of MSTIDs during this period. Eight cases were found to be the polar-type MSTIDs whose motion changes associated with auroral brightening and magnetic field disturbances. We found that 9 cases of MSTID show the low-latitude boundary at 61 degrees +/- 2 degrees N for geographic latitude and 58 degrees +/- 2 degrees N for magnetic latitude, indicating disconnection between high- and mid-latitude MSTIDs. We also derived occurrence probability, velocity, wavelength, period, wave front direction, and propagation direction of these MSTIDs. The occurrence probability of MSTIDs at Nyrola is 1.9%, which is much lower than those at high (Tromsoe, more than 50%) and middle (Japan, similar to 30%) latitudes. We discuss these MSTID characteristics at subauroral latitudes based on possible difference of generation mechanisms of nighttime MSTIDs at high and middle latitudes. Medium-scale traveling ionospheric disturbances (MSTIDs) are one of the ionospheric plasma density structures and are observable through 630-nm airglow images. Previous studies using airglow images at Tromso (69.6 degrees N, 19.2 degrees E; magnetic latitude: 66.7 degrees N), Norway, reported high-latitude MSTIDs (here we call them as polar-type MSTIDs) whose propagation direction changes associated with auroral brightening and magnetic field disturbances. However, there has been little statistical analysis on the connection of MSTIDs occurring at high and middle latitudes. In this study, we analyzed 11 MSTID cases observed by an airglow imager at Nyrola (62.3 degrees N, 25.5 degrees E; magnetic latitude: 59.4 degrees N), Finland, which is located similar to 7 degrees south of Tromso, corresponding to subauroral latitudes. We found that 9 cases of MSTID show the low-latitude boundary at 61 degrees +/- 2 degrees N for geographic latitude and 58 degrees +/- 2 degrees N for magnetic latitude, indicating disconnection between high- and mid-latitude MSTIDs. We also derived occurrence probability, velocity, wavelength, period, wave front direction, and propagation direction of these MSTIDs. We statistically analyzed medium-scale traveling ionospheric disturbances (MSTIDs) observed at subauroral latitudes at Nyrola, Finland Most MSTIDs are the polar-type MSTIDs whose motion changes associated with auroral brightening and/or magnetic field disturbances The low-latitude boundary of these MSTIDs was at similar to 61 degrees N latitude, indicating disconnection between high- and mid-latitude MSTIDs

This study focuses on the poorly known effect of polar cap patches (PCPs) on the ion-neutral coupling in the F-region. The PCPs were identified by total electron content measurements from the Global Navigation Satellite System (GNSS) and the ionospheric parameters from the Defense Meteorological Satellite Program spacecraft. The EISCAT incoherent scatter radars on Svalbard and at Troms & oslash;, Norway observed that PCPs entered the nightside auroral oval from the polar cap and became plasma blobs. The ionospheric convection further transported the plasma blobs to the duskside. Simultaneously, long-lasting strong upper thermospheric winds were detected in the duskside auroral oval by a Fabry-Perot Interferometer (FPI) at Troms & oslash; and in the polar cap by the Gravity Recovery and Climate Experiment satellite. Using EISCAT ion velocities and plasma parameters as well as FPI winds, the ion drag acting on neutrals and the time constant for the ion drag could be estimated. Due to the arrival of PCPs/blobs and the accompanied increase in the F-region electron densities, the ion drag is enhanced between about 220 and 500 km altitudes. At the F peak altitudes near 300 km, the median ion drag acceleration affecting neutrals more than doubled and the associated median e-folding time decreased from 4.4 to 2 hr. The strong neutral wind was found to be driven primarily by the ion drag force due to large-scale ionospheric convection. Our results provide a new insight into ionosphere-thermosphere coupling in the presence of PCPs/blobs. This study investigates how the evolution of the polar cap patches (PCPs) affects the upper layer of the Earth's atmosphere, termed the thermosphere. PCPs are dense patches of charged particles that move from the dayside to the nightside of the high-latitude ionosphere through the polar cap region. Using the measurements by multiple ground-based instruments and satellites, this study found that PCPs can enhance the formation of strong upper thermospheric winds. The winds are primarily driven by the ion drag force due to the interactions between charged particles and neutral gases. The results show that because of the arrival of PCPs, which increase the F-region electron densities in the auroral oval, the ion drag acceleration acting on the neutrals can more than double and the related time constant of the ion drag can be halved. Transportation of polar cap patches (PCPs) and their development in the nightside auroral oval was observed by multiple instruments Very strong, long-lasting westward upper thermospheric wind in the duskside oval was associated with large-scale ionospheric convection High electron density produced by PCPs increases the ion drag force that drives the upper thermospheric wind

Protons of tens of keV can be resonantly scattered by electromagnetic ion cyclotron (EMIC) waves excited in the magnetosphere, resulting in proton precipitation down to the upper atmosphere. In this study, we report for the first time the ionospheric height‐dependent ionization in response to EMIC‐associated isolated proton aurora (IPA) using simultaneous space‐borne and ground‐based measurements. On 06 March 2019, the Polar Orbiting Environmental Satellites observed significant proton precipitation in the dusk sector (MLT ∼ 19), while ground‐based magnetometers detected a clear signature of EMIC waves. Meanwhile, the conjugated all sky imager captured an IPA and the nearby Poker Flat incoherent scatter radar (PFISR) showed enhanced electron density in the E region, suggesting a potential consequence of the EMIC wave‐driven proton precipitation. The Global Airglow model simulations confirmed the dominant impact of proton precipitation on the ionosphere and agreed well with PFISR observations. This study confirmed physical links from the magnetosphere to the ionosphere through EMIC‐driven proton precipitation.