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\"The countdown is on. In less than forty eight hours, Earth will be hit by a major solar storm. At first, it is thought to be a Coronal Mass Ejection (CME) that will severely damage our world's electronic infrastructure. A crisis to be certain, but one that can be survived--until something far more frightening is discovered by NASA's solar observation teams. It is not just a CME. The sun is about to let loose with a solar explosion of such intensity it will result in an ELE, an 'Extinction Level Event.' A final hour might be approaching that could see the near extinction of all life on earth. How will humanity react to such news? How would you react?\"-- Provided by publisher.

While the isolated effects of solar flares on low‐latitude ionospheric electrodynamics have been well documented, the coupled system response of the equatorial electrojet (EEJ), auroral electrojet (AEJ), field‐aligned currents (FACs), and asymmetric ring current (ASY‐H) remains poorly understood. This study statistically analyzes 1,657 X/M‐class flares (2001–2017) to quantify rapid electrodynamic changes across current systems. Our results indicate (a) flare intensity‐dependent enhancements in eastward EEJ, suppressed equatorial ionospheric vertical drift (Vz), and increased ASY‐H; (b) negligible flare influence on AEJ; and (c) R2 FACs intensification in the dusk sector, linking ionospheric dynamics to asymmetric ring current perturbations. These observations reveal transient electrodynamic coupling within the geospace associated with flares, independent of solar wind forcing, advancing understanding of flare‐driven ionosphere‐magnetosphere interactions.

Solar cycle 23 witnessed the most complete set of observations of coronal mass ejections (CMEs) associated with the Ground Level Enhancement (GLE) events. We present an overview of the observed properties of the GLEs and those of the two associated phenomena, viz., flares and CMEs, both being potential sources of particle acceleration. Although we do not find a striking correlation between the GLE intensity and the parameters of flares and CMEs, the solar eruptions are very intense involving X-class flares and extreme CME speeds (average approx. 2000 km/s). An M7.1 flare and a 1200 km/s CME are the weakest events in the list of 16 GLE events. Most (80 %) of the CMEs are full halos with the three non-halos having widths in the range 167 to 212 degrees. The active regions in which the GLE events originate are generally large: 1290 msh (median 1010 msh) compared to 934 msh (median: 790 msh) for SEP-producing active regions. For accurate estimation of the CME height at the time of metric type II onset and GLE particle release, we estimated the initial acceleration of the CMEs using flare and CME observations. The initial acceleration of GLE-associated CMEs is much larger (by a factor of 2) than that of ordinary CMEs (2.3 km/sq s vs. 1 km/sq s). We confirmed the initial acceleration for two events for which CME measurements are available in the inner corona. The GLE particle release is delayed with respect to the onset of all electromagnetic signatures of the eruptions: type II bursts, low frequency type III bursts, soft X-ray flares and CMEs. The presence of metric type II radio bursts some 17 min (median: 16 min; range: 3 to 48 min) before the GLE onset indicates shock formation well before the particle release. The release of GLE particles occurs when the CMEs reach an average height of approx 3.09 R(sub s) (median: 3.18 R (sub s) ; range: 1.71 to 4.01 R (sub s) ) for well-connected events (source longitude in the range W20–W90). For poorly connected events, the average CME height at GLE particle release is ∼66 % larger (mean: 5.18 R (sub s) ; median: 4.61 R (sub s) ; range: 2.75–8.49 R (sub s) ). The longitudinal dependence is consistent with shock accelerations because the shocks from poorly connected events need to expand more to cross the field lines connecting to an Earth observer. On the other hand, the CME height at metric type II burst onset has no longitudinal dependence because electromagnetic signals do not require magnetic connectivity to the observer. For several events, the GLE particle release is very close to the time of first appearance of the CME in the coronagraphic field of view, so we independently confirmed the CME height at particle release. The CME height at metric type II burst onset is in the narrow range 1.29 to 1.8 R(sub s), with mean and median values of 1.53 and 1.47 R(sub s). The CME heights at metric type II burst onset and GLE particle release correspond to the minimum and maximum in the Alfven speed profile. The increase in CME speed between these two heights suggests an increase in Alfvenic Mach number from 2 to 3. The CME heights at GLE particle release are in good agreement with those obtained from the velocity dispersion analysis, including the source longitude dependence. We also discuss the implications of the delay of GLE particle release with respect to complex type III bursts by approx 18 min (median: 16 in; range: 2 to 44 min) for the flare acceleration mechanism. A similar analysis is also performed on the delay of particle release relative to the hard X-ray emission.

Solar flares significantly impact the conditions of the Earth’s ionosphere. In particular, the sudden increase in X-ray flux during a flare penetrates down to the lowest-lying D-region and dominates ionization at these altitudes ( ≈ 60  – 100 km). Measurements of very low frequency (VLF: 3 – 30 kHz) radio waves that reflect at D-region altitudes provide a unique remote-sensing probe to investigate the D-region response to solar-flare emissions. Here, using a combination of VLF amplitude measurements at 24 kHz together with X-ray observations from the Geostationary Operational Environment Satellite (GOES) X-ray sensor, we present a large-scale statistical study of 334 solar-flare events and their impacts on the D-region over the past solar cycle. Focusing on both GOES broadband X-ray channels, we investigate how the flare peak fluxes and position on the solar disk dictate an ionospheric response and extend this to investigate the characteristic time delay between incident X-ray flux and the D-region response. We show that the VLF amplitude linearly correlates with both the 1 – 8 Å and 0.5 – 4 Å channels, with correlation coefficients of 0.80 and 0.79, respectively. For the two X-class flares in our sample, however, there appears to be a turnover in the linear relationship, similar to previous works. Unlike higher altitude ionospheric regions for which the location of the flare on the solar disk affects the ionospheric response, we find that the D-region response to solar flares does not depend on the flare location. By comparing the time delays between the peak X-ray fluxes in both GOES channels and VLF amplitudes, we find that there is an important difference between the D-region response and the X-ray spectral band. We also demonstrate for several flare events that show a negative time delay, the peak VLF amplitude matches with the impulsive 25 – 50 keV hard X-ray fluxes measured by the Ramaty High Energy Solar Spectroscopic Imager (RHESSI). These results highlight the importance of performing full spectral analysis when studying the ionospheric responses to solar flares.

Solar flares commonly have a “hot onset precursor event” (HOPE), detectable from soft X-ray observations. To detect this requires subtraction of pre-flare fluxes from the non-flaring Sun prior to the event, fitting an isothermal emission model to the flare excess fluxes by comparing the GOES passbands at 1 – 8 Å and 0.5 – 4 Å, and plotting the timewise evolution of the flare emission in a diagram of temperature vs. emission measure. The HOPE then appears as an initial “horizontal branch” in this diagram. It precedes the nonthermal impulsive phase of the flare and thus the flare peak in soft X-rays as well. We use this property to define a “flare anticipation index” (FAI), which can serve as an alert for observational programs aimed at solar flares based on near-real-time soft X-ray observations. This FAI gives lead times of a few minutes and produces very few false positive alerts, even for flare brightenings that are too weak to merit NOAA classification.

Solar activity can significantly influence the performance and reliability of both space-borne and ground-based technological systems on Earth. This phenomenon, known as space weather, can impact modern aviation operations through complex and highly interconnected mechanisms, presenting a challenging area of study. Until now, no systematic investigation has explored the influence of space weather on flight cancellations outside of specific polar routes. By analyzing ~ 5 million flight departure records from 5 hub airports in China between 2015 and 2019, a period spanning the declining phase and subsequent minimum of Solar Cycle 24, we provide the first evidence of a systematic increase in flight cancellations during space weather events. Our comparative analysis reveals that the average cancellation rate during space weather events, specifically Solar Flares, Interplanetary Coronal Mass Ejections, and Solar Proton Events, was 96.96% higher than during quiet periods. Counter-intuitively, Stream Interaction Regions showed no increase and even a slight decrease in cancellation rates. Additionally, investigations on geomagnetic-ionospheric disturbances reveal a quasi-linear relation between the rate of flight cancellations and the magnitude of perturbations. Such increased flight cancellations persist even when accounting for other factors, such as the seasonal effects. These findings imply that disruptions in communication and navigation systems during space weather events can significantly impact modern aviation operations, leading to more frequent flight cancellations. The superposed epoch analysis uncovers a well-defined temporal response curve of cancellation rates to independent space weather events. The correlations observed suggest that prolonged flight delays induced by space weather may be another important driver of the increased cancellation rates. This research expands the conventional understanding of space weather’s impact on aviation and it implies the importance of integrating space weather as a systematic factor in flight operations and planning, which may also help improve aviation efficiency and safety.

Solar eruptive activities, mainly including solar flares, coronal mass ejections (CME), and solar proton events (SPE), have an important impact on space weather and our technosphere. The short-term solar eruptive activity prediction is an active field of research in the space weather prediction. Numerical, statistical, and machine learning methods are proposed to build prediction models of the solar eruptive activities. With the development of space-based and ground-based facilities, a large amount of observational data of the Sun is accumulated, and data-driven prediction models of solar eruptive activities have made a significant progress. In this review, we briefly introduce the machine learning algorithms applied in solar eruptive activity prediction, summarize the prediction modeling process, overview the progress made in the field of solar eruptive activity prediction model, and look forward to the possible directions in the future.

Solar flare effects (Sfe) are rapid variations in the Earth’s magnetic field and are related to the enhancement of the amount of radiation produced during Solar flare events. They mainly appear in the Earth’s sunlit hemisphere at the same time as the flare observation and have a crochet-like shape. Much progress has been made since Carrington’s first observations in 1859 which are considered to represent the first direct evidence of the connection between the Sun and the Earth’s environment but there is still much to discover. In this paper, we review state-of-the-art developments and the advances made in the knowledge concerning Sfe phenomena while also looking at the challenges that lie ahead. First, we offer a historical approach with a comprehensive description that allows for a better understanding of the main characteristics of Sfe. This frames specific topics like the puzzling reversed-Sfe or the nighttime Sfe. The role played by the Service of Rapid Magnetic variations (SRMV) is also assessed, followed by a discussion of the main current limiting factors in the process of detection and proposed ways to overcome challenges such as by creating an automatic detection method. The paper clarifies some aspects related to the geo-effectiveness of the solar flares producing magnetic disturbances. The importance of the global modelling studies covering critical aspects needed to understand this Sun–Earth system is assessed. Also, we provide an overview of the temporal evolution of the electric currents producing Sfe. The importance of key subjects such as the dynamic aspects of Sfe is developed in another section. Finally, estimations of the size of large flares using ionospheric and magnetic data are reviewed as well as the prospects of these large flare events putting technological systems in danger.

The chromosphere is a highly dynamic outer plasma layer of the Sun. Its physical processes accounting for the variability are poorly understood. We reconstructed the solar chromospheric flare index (SFI) to study the solar chromospheric variability from 1937 to 2020. The new SFI database is a composite record of the Astronomical Institute Ondřejov Observatory of the Czech Academy of Sciences from 1937 – 1976 and the records of the Kandilli Observatory of Istanbul, Turkey from 1977 – 2020. The SFI records are available in daily, monthly, and yearly resolutions. We carried out the time-frequency analyses of the new 84-year long SFI records using the wavelet transform. We report the periodicities of 21.88 (Hale cycle), 10.94 (Schwabe cycle), 5.2 (quasi-quinquennial cycle), 3.5, 1.7, 1, 0.41 (or 149.7 days, Rieger cycle), 0.17 (62.1 days), 0.07 (25.9 days, solar rotational modulation) years. All these periodicities seem always present and persistent throughout the observational interval. Thus, we suggest that there is no reason to assume these solar periodicities are absent from other solar cycles. Time variations of the amplitude of each oscillation or periodicity were also studied using the inverse wavelet transform. We found that for the SFI the most active flare cycles over the record were Cycles 17, 19, and 21, while Cycles 20, 22, 23, and 24 were the weakest ones with Cycle 18 was intermediate in flare activity. This shows several differences to the equivalent relationships for solar activity implied by sunspot number records. Furthermore, this confirms that solar activity trends and variability in the chromosphere as captured by SFI are not necessarily the same as those of the Sun’s photosphere, as implied by the sunspot number activity records, for instance. We have also introduced a new signal/noise wavelet coherence metric to analyze two different chromospheric indices available (i.e. the SFI and the disk-integrated chromospheric Ca  ii  K activity indices) and to quantify the differences and similarities of the oscillations within the solar chromosphere. Our findings suggest the importance of carrying out additional co-analyses with other solar activity records to find physical inter-relations and connections between the different solar layers from the photosphere, the chromosphere to the corona.