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The high latitude ionospheric evolution of the May 10‐11, 2024, geomagnetic storm is investigated in terms of Total Electron Content and contextualized with Incoherent Scatter Radar and ionosonde observations. Substantial plasma lifting is observed within the initial Storm Enhanced Density plume with ionospheric peak heights increasing by 150–300 km, reaching levels of up to 630 km. Scintillation is observed within the cusp during the initial expansion phase of the storm, spreading across the auroral oval thereafter. Patch transport into the polar cap produces broad regions of scintillation that are rapidly cleared from the region after a strong Interplanetary Magnetic Field reversal at 2230UT. Strong heating and composition changes result in the complete absence of the F2‐layer on the eleventh, suffocating high latitude convection from dense plasma necessary for Tongue of Ionization and patch formation, ultimately resulting in a suppression of polar cap scintillation on the eleventh. Plain Language Summary The intense geomagnetic storm of May 2024 caused a plethora of different responses within the Earth's ionosphere. In the early phases of the storm, the auroral oval quickly expands to upper midlatitudes and induces strong variations in Global Navigation Satellite System (GNSS) phase measurements. Concurrently, midlatitude plasma is repeatedly lifted by 100–300 km on timescales of about an hour resulting in enhanced plasma densities. This intensified and lifted plasma is then drawn into the polar cap inducing variations in GNSS amplitude and phase. As the storm evolves, heating drives mixing of the thermosphere and causes an extreme depletion in ionospheric plasma. After 24 hr, despite severe geomagnetic conditions persisting, the depleted plasma environment results in only relatively weak plasma transport into the polar cap and significantly reduced impacts on GNSS. Key Points Plasma lifting during the storm caused midlatitude displacements of ionospheric peak height by as much as 300 km over the course of 1 hour Sporadic‐E is observed at the sub‐auroral convective boundary edge of the storm‐enhanced density with strong plasma drift shears present Severe depletion of electron density at mid and high latitudes significantly reduced the impact of subsequent geomagnetic activity on GNSS

The polar cap arc at 1500 MLT (15MLT‐PCA) has been considered as an auroral signature of the cusp's duskside boundary and been speculated to be caused by lobe reconnection. However, no observational evidence has been provided to support this speculation. Here we report a 15MLT‐PCA event occurred on 29 November 2017 using multi‐instrument observations. During the DMSP observed the 15MLT‐PCA, Cluster, with its footprints at the root of the 15MLT‐PCA, identified two FTEs in the southern hemisphere's lobe region, accompanied by an increase in electron and ion energy from hundreds of eVs to several keVs. AMPERE observed an increase in upward field‐aligned currents associated with the 15MLT‐PCA. SuperDARN observed a single cell convection with an enhancement of sunward plasma flow near the root of 15MLT‐PCA. We suggest that these observations provide the in‐situ observational evidence that the 15MLT‐PCA is generated by a lobe reconnection at the cusp's duskside boundary. Plain Language Summary Auroras primarily occur within an oval‐shaped region centered on Earth's magnetic poles. Under specific conditions, they can also occur within the polar cap, where auroral structures usually appear as arc‐like or spot‐like. Auroras exhibiting arc‐like structures in this region are collectively termed polar cap auroral arcs (PCA). One such common phenomenon is the 15 magnetic local time‐polar cap arc (15MLT‐PCA), typically observed near the 1500 MLT sector in the summer hemisphere, extending from the poleward boundary of the auroral oval to the pole. Additionally, its occurrence shows a strong dependence on the By component of the interplanetary magnetic field (IMF). It is widely accepted that the generation of north‐south asymmetric auroral structures in the polar cap results from reconnection between the IMF and Earth's magnetic field lines at high latitudes. Based on the observational characterization of 15MLT‐PCA, it is currently believed that this structure is associated with lobe reconnection. However, direct in situ observational evidence for this process is currently lacking. Therefore, in this study, we combined ionospheric and magnetospheric satellites to validate a possible magnetospheric process for 15MLT‐PCA. Key Points The observations show that the occurrence of 15MLT‐PCA was accompanied by FTEs on the dusk flank of the southern hemisphere's magnetopause The 15MLT‐PCA is accompanied by an enhancement of upward FACs and an increase in sunward plasma flow near its root We provided evidence that the 15MLT‐PCA results from lobe reconnection occurring at duskside boundary of the cusp

This paper reviews our current understanding of auroral features that appear poleward of the main auroral oval within the polar cap, especially those that are known as Sun-aligned arcs, transpolar arcs, or theta auroras. They tend to appear predominantly during periods of quiet geomagnetic activity or northwards directed interplanetary magnetic field (IMF). We also introduce polar rain aurora which has been considered as a phenomenon on open field lines. We describe the morphology of such auroras, their development and dynamics in response to solar wind-magnetosphere coupling processes, and the models that have been developed to explain them.

The SS-520-3 sounding rocket was launched on November 4th, 2021 as part of the Grand Challenge Initiative - Cusp from Ny-Ålesund, Svalbard. The rocket was launched into the cusp ionosphere during the main phase of a geomagnetic storm. In this study we utilize two low energy particle analyzers as well as a multi-needle Langmuir probe and an impedance probe as part of the rocket payload. This study aims to provide an overview of the flight conditions from a range of ground-based instruments and scintillation receivers. We were able to confirm that the rocket entered the cusp through the poleward edge at around of northern geographic latitude. Additionally, the rocket encountered polar cap patches (PCP), as well as a patch within the cusp (CP) and a newly-formed tongue of ionisation (TOI). Analysis of the density variations within different scale sizes show enhancements within meter-size and kilometer-size scales on the edges of PCP, within the CP and TOI. Overall, the enhancements within the variations on all sizes, as well as enhancements of the electron density were significantly higher within the CP and TOI in comparison to the PCP, though all structures were encountered at similar altitudes. The strongest enhancements were found on the poleward edge of the TOI, corresponding to strong fluctuations within the electron density. The TOI also had the largest enhancements within gradients of kilometer-size in comparison to meter-sizes. As the TOI is convecting with respect to the background plasma, the edges are susceptible to instabilities like the Kelvin-Helmholtz instability (KHI) and Gradient-Drift instability (GDI), giving rise to plasma density structures on several scale sizes.

Throat auroras and polar cap patches are common phenomena in the polar ionosphere resulting from magnetosphere‐ionosphere coupling. A campaign was organized, with all‐sky imagers at Yellow River Station, the European Incoherent Scatter Svalbard Radar, and coordinated low‐altitude spacecraft observations. During periods of radial interplanetary magnetic field (IMF), observations showed that, as poleward moving throat auroras faded around the polar cap boundary, they linked to poleward moving ionization patches. The throat auroras were produced by soft‐electron precipitation associated with dayside magnetic reconnection. The red line emission intensity of throat auroras was correlated with dayside reconnection events. Dense plasma from lower latitudes was transported poleward via enhanced convection in the throat auroras to form patches. This is a potentially new formation mechanism for patches associated with throat auroras and magnetic reconnection for radial IMF. Moreover, the patches move anti‐sunward due to the E × B drift. Plain Language Summary The polar ionosphere is a key region in the solar wind‐magnetosphere‐ionosphere coupling processes. The throat auroras and polar cap patches are important phenomenon in the polar ionosphere, and are associated with magnetopause dynamic processes. To clarify the relationship between throat auroras and patches, we organized a campaign of multiple instruments observations of Yellow River Station all‐sky imagers, European Incoherent Scatter Svalbard Radar, and low‐altitude spacecraft observations. We found that the throat auroras and patches were closely linked as they move poleward. The throat auroras were produced by soft‐electron precipitation associated with dayside magnetic reconnection for radial interplanetary magnetic field (IMF). We proposed a potentially new formation mechanism for patches from throat auroras under IMF Bx‐dominant that high density plasma from lower latitudes was transported poleward via enhanced convection in the throat auroras to form patches. Moreover, the patches moved anti‐sunward due to the E × B plasma drift. Key Points The formation of throat auroras and polar cap patches is studied with multiple instruments in space and on ground Both throat auroras and polar cap patches are closely linked as they move poleward The formation of throat auroras and patches is associated with magnetic reconnection for dominant radial interplanetary magnetic field

Type II solar radio bursts are signatures of the coronal shocks and, therefore, particle acceleration events in the solar atmosphere and interplanetary space. Type II bursts can serve as a proxy to provide early warnings of incoming solar storm disturbances, such as geomagnetic storms and radiation storms, which may further lead to ionospheric effects. In this article, we report the first observation of 32 type II bursts by measuring various plasma parameters that occurred between May 2021 and December 2022 in solar cycle 25. We further evaluated their accompanying space weather events in terms of ionospheric total electron content (TEC) enhancement using the rate of TEC index (ROTI). In this study, we find that at heliocentric distance ∼1–2 R⊙, the shock and the Alfvén speeds are in the range 504–1282 and 368–826 km−1, respectively. The Alfvén Mach number is of the order of 1.2≤MA≤1.8 at the above-mentioned heliocentric distance. In addition, the measured magnetic field strength is consistent with the earlier reports and follows a single power law B(r)=6.07r-3.96G. Based on the current analysis, it is found that 19 out of 32 type II bursts are associated with immediate space weather events in terms of radio blackouts and polar cap absorption events, making them strong indications of space weather disruption. The ROTI enhancements, which indicate ionospheric irregularities, strongly correlate with GOES X-ray flares, which are associated with the type II radio bursts recorded. The diurnal variability in ROTI is proportional to the strength of the associated flare class, and the corresponding longitudinal variation is attributed to the difference in longitude. This article demonstrates that since type II bursts are connected to space weather hazards, understanding various physical parameters of type II bursts helps to predict and forecast the space weather.

The paper summarizes the issues related to relationships between the PC index and magnetic disturbances: threshold level of the PC index required for the disturbances beginning, delay time in response of magnetic substorms and storms to the PC index growth, relation of PC index to magnetospheric field-aligned currents in course of substorm, different types of magnetic substorms (isolated, expanded, delayed, sawtooth) and magnetic storms (classic, pulsed and composite) and their relation to different regularities in the PC index alterations, linear dependence of the substorm and storm intensities on value of the preceding of PC index, special features of magnetic activity in the winter and summer polar caps, variations of PC index and magnetic disturbances in course of the 23/24 solar activity cycles. New aspects that have arisen due to the PC index application are concerned with the threshold-dependent mode of the substorm development and regular repeateness of sawtooth substorms occurring under conditions of steady powerful EKL field. The experimental results examined in the paper are indicative that the PC index serves as an indicator of the solar wind energy which comes in the magnetosphere and then realizes in the form of magnetosphere disturbances. This paper follows the review of Troshichev (Front Astron Space Sci 9:1069470, 2022), where the relationships between the solar wind electric field EKL and PC index have been examined.

The stratospheric polar vortex (SPV) weakening is linked to surface circulation changes. This study employs statistical analysis using reanalysis data to compare the anomalous SPV behaviour in the Northern (NH) and Southern (SH) hemispheres and its downward impacts on surface climate. The onset of annual SPV weakening occurs in mid-January and late September in the NH and SH hemispheres, respectively. Following the onset of SPV weakening, stratospheric polar cap height (PCH) anomalies were strongly correlated with tropospheric PCH anomalies. Significant cold anomalies were observed over Eurasia within 30 days after SPV weakening onset in the NH, whereas warming responses occurred in the SH 30–60 days after onset over Antarctica, except in the Antarctic Peninsula. These contrasting surface temperature responses to SPV weakening events in both hemispheres are the results of changes in the geopotential height in the troposphere, reminiscent of the change in geopotential height in the lower stratosphere, with a trough over Eurasia in the NH, and a higher height anomaly over East Antarctica in the SH. SPV changes have played a role in modulating surface climate via a downward influence on tropospheric circulation in recent decades. Even though they show a weakening trend in both hemispheres, SPV changes cannot fully explain long-term temperature trends. This is partially because SPV trends observed during the analysis period are relatively weak. This study enhances our understanding of the characteristics of the SPV coupled with troposphere circulation and can contribute to improved surface weather forecasting.

In this study, we investigate statistical features of polar cap north (PCN) and south (PCS) indices in response to various interplanetary conditions (interplanetary magnetic field [IMF] orientation in three‐dimensions) and terrestrial conditions (seasonal and magnetic local time [MLT] locations of the index stations). The concurrent PCN‐PCS pairs for 1998–2002 and 2004–2018 are divided based on their sign type (positive‐positive, negative‐negative, negative‐positive, and positive‐negative PCN‐PCS pairs) and time coverage (the times when both index stations are in the dawn/dusk MLT sector during northern summer/winter). Analyzing the IMF orientation dependence on the occurrence probabilities of concurrent indices and on the differences between the indices in various sign types for each time coverage reveals that the statistical features in PCN‐PCS pairs obtained in the dawn MLT sector can be largely explained by the effects of the three‐component IMF (related to the polar cap convection patterns) combined with season (related to the hemispheric asymmetry in solar illumination‐induced ionospheric conductance). However, those obtained in the dusk MLT sector are controlled dominantly by seasonal effects rather than IMF orientation effects. Our findings indicate that PCN‐PCS pair data provide local views about the solar wind‐magnetosphere‐ionosphere (SW‐M‐I) coupling system with different control efficiencies of IMF orientation and season depending on the MLT location of the stations. Therefore, introducing polar cap indices recorded simultaneously at various locations in both hemispheres and analyzing them are strongly required to infer global views of the coupled SW‐M‐I system in the open field regions with higher confidence.

We directly compare the relative GPS scintillation levels associated with regions of enhanced plasma irregularities called auroral arcs, polar cap patches, and auroral blobs that frequently occur in the polar ionosphere. On January 13, 2013 from Ny-Ålesund, several polar cap patches were observed to exit the polar cap into the auroral oval, and were then termed auroral blobs. This gave us an unprecedented opportunity to compare the relative scintillation levels associated with these three phenomena. The blobs were associated with the strongest phase scintillation (σϕ), followed by patches and arcs, with σϕ up to 0.6, 0.5, and 0.1 rad, respectively. Our observations indicate that most patches in the nightside polar cap have produced significant scintillations, but not all of them. Since the blobs are formed after patches merged into auroral regions, in space weather predictions of GPS scintillations, it will be important to enable predictions of patches exiting the polar cap.