Stellar flares

Coronal mass ejections: a driver of severe space weather

Recently, many superflares on solar-type stars have been discovered as white-light flares (WLFs). The statistical study found a correlation between their energies (E) and durations (τ):, similar to those of solar hard/soft X-ray flares,. This indicates a universal mechanism of energy release on solar and stellar flares, i.e., magnetic reconnection. We here carried out statistical research on 50 solar WLFs observed withSolar Dynamics Observatory/HMI and examined the correlation between the energies and durations. As a result, theE–τrelation on solar WLFs () is quite similar to that on stellar superflares (). However, the durations of stellar superflares are one order of magnitude shorter than those expected from solar WLFs. We present the following two interpretations for the discrepancy: (1) in solar flares, the cooling timescale of WLFs may be longer than the reconnection one, and the decay time of solar WLFs can be elongated by the cooling effect; (2) the distribution can be understood by applying a scaling law () derived from the magnetic reconnection theory. In the latter case, the observed superflares are expected to have 2–4 times stronger magnetic field strength than solar flares.

Light curves of thermal and non-thermal emission associated with solar flares usually contain statistically significant quasi-periodic pulsations (QPP). Typical periods range from a fraction of a second to several tens of minutes. Physical mechanisms responsible for QPP can be attributed to three main, while non-exclusive, groups: modulation of the emitting plasma parameters and kinematics of non-thermal electrons by coronal magnetohydrodynamic oscillations, spontaneous periodic magnetic reconnection, and magnetic reconnection periodically induced by magnetohydrodynamic oscillations. Similar QPP are also detected in stellar flares, including those hosted by Sun-like stars. Recently, a number of stellar flare QPP events were detected in white light with Kepler. In several cases, the characteristic time signatures of stellar flare QPP are very similar to those detected in solar flares, suggesting that they may be created by the same mechanisms. This report summarises the invited review talk given by V.M. Nakariakov at the workshop “Superflares on solar-type stars and solar flares, and their impacts on exoplanets and the Earth” at Kyoto University in March 2016.

Intense solar flares originated in sun spots produce high energy particles (protons, $\alpha$) well observable by satellites and ground-based detectors. The flare onset produces signals in different energy bands (radio, X, gamma and neutrons). The most powerful solar flares as the ones occurred on 23 February 1956, 29 September 1989 and the more recent on October 28th, and the 2nd, 4th, 13th of November 2003 released in sharp times the largest flare energies (${E}_{FL} \simeq {10}^{31}\div {10}^{32} erg). The high energy solar flare protons scatter within the solar corona and they must be source of a prompt neutrino burst through the production of charged pions. Later on, solar flare particles hitting the atmosphere may marginally increase the atmospheric neutrino flux. The prompt solar neutrino flare may be detected in the largest underground $\nu$ detectors. Our estimate for the October - November 2003 solar flares gives a number of events above the unity. The electron/muon $\nu$ signals and spectra may reflect the neutrino flavour mixing. A surprising tau appearance may occur for a hard {E}_nu_mu}} \to {E}_{{\nu}_{\tau}}\simeq> 4 GeV$) flare spectra.

Solar flares and geomagnetic storms of September 2017: Their impacts on the TEC over 75°E longitude sector

Solar white-light flares (WLFs) are solar flares exhibiting enhanced emission in the optical continuum. They are critical for understanding energy release and transport mechanisms in solar flares and for conducting comparative studies with stellar WLFs. However, the scarcity of accurately and reliably measured optical continuum light curves for solar WLFs significantly hampers related studies. Based on the optimized solar WLF identification method, we construct a dataset of optical continuum light curves for 70 solar WLFs using 6173 {\AA} continuum intensity images from the Solar Dynamics Observatory. Moreover, for each solar WLF event, we also provide the location of the white-light emission enhancement signals and key parameters including bolometric energies and durations derived from both the traditional fixed-temperature blackbody model and the refined variable-temperature blackbody model. This dataset will serve as a valuable resource for future statistical investigations of solar WLFs and for comparative studies between solar and stellar flares.

The effect of solar features on geospheric conditions leading to geomagnetic storms (GMSs) with planetary index,A P ≥ 20 and the range of horizontal component of the Earth’s magnetic fieldH such that 250γ <H < 400γ has been investigated using interplanetary magnetic field (IMF), solar wind plasma (SWP) and solar geophysical data (SGD) during the period 1978–99. Statistically, it is observed that maximum number of GMSs have occurred during the maximum solar activity years of 21st and 22nd solar cycles. A peculiar result has been observed during the years 1982, 1994 when sunspot numbers (SSNs) decrease very rapidly while numbers of GMSs increase. No distinct association between yearly occurrence of disturbed days and SSNs is observed. Maximum number of disturbed days have occurred during spring and rainy seasons showing a seasonal variation of disturbed days. No significant correlation between magnitude (intensity) of GMSs and importance ofH α , X-ray solar flares has been observed. Maximum number of GMSs is associated with solar flares of lower importance, i.e., SF during the period 1978-93.H α , X-ray solar flares occurred within lower helio-latitudes, i.e., (0–30)°N to (0–30)°S are associated with GMSs. NoH α , X-ray solar flares have occurred beyond 40°N or 40°S in association with GMSs. In helio-latitude range (10–40)°N to (10–40)°S, the 89.5% concentration of active prominences and disappearing filaments (APDFs) are associated with GMSs. Maximum number of GMSs are associated with solar flares. Coronal mass ejections (CMEs) are related with eruptive prominences, solar flares, type IV radio burst and they occur at low helio-latitude. It is observed that CMEs related GMS events are not always associated with high speed solar wind streams (HSSWSs). In many individual events, the travel time between the explosion on the Sun and maximum activity lies between 58 and 118 h causing GMSs at the Earth.

The GOES X1 flare SOL2014-10-25T17:08:00 was a three-ribbon solar flare observed with the Interface Region Imaging Spectrograph (IRIS) in the near-UV (NUV) and far-UV. One of the flare ribbons crossed a sunspot umbra, producing a dramatic, ∼1000% increase in the NUV continuum radiation. We comprehensively analyze the UV spectral data of the umbral flare brightenings, which provide new challenges for radiative−hydrodynamic modeling of the chromospheric velocity field and the white-light continuum radiation. The emission line profiles in the umbral flare brightenings exhibit redshifts and profile asymmetries, but these are significantly smaller than in another, well-studied X-class solar flare. We present a ratio of the NUV continuum intensity to the Fe ii λ2814.45 intensity. This continuum-to-line ratio is a new spectral diagnostic of significant heating at high column mass (log m/[g cm−2] &gt; −2) during solar flares because the continuum and emission line radiation originate from relatively similar temperatures but moderately different optical depths. The full spectral readout of these IRIS data also allow for a comprehensive survey of the flaring NUV landscape: in addition to many lines of Fe ii and Cr ii, we identify a new solar flare emission line, He i λ2829.91 (as previously identified in laboratory and early-type stellar spectra). The Fermi/GBM hard X-ray data provide inputs to radiative−hydrodynamic models (which will be presented in Paper II) in order to better understand the large continuum-to-line ratios, the origin of the white-light continuum radiation, and the role of electron beam heating in the low atmosphere.

The study of stellar flares has increased with new observations from CoRoT, Kepler, and TESS satellites, revealing the broad-band visible emission from these events. Typically, stellar flares have been modelled as 104 K blackbody plasma to obtain estimates of their total energy. In the Sun, white-light flares (WLFs) are much fainter than their stellar counterparts, and normally can only be detected via spatially resolved observations. Identifying the radiation mechanism for the formation of the visible spectrum from solar and stellar flares is crucial to understand the energy transfer processes during these events, but spectral data for WLFs are relatively rare, and insufficient to remove the ambiguity of their origin: photospheric blackbody radiation and/or Paschen continuum from hydrogen recombination in the chromosphere. We employed an analytical solution for the recombination continuum of hydrogen instead of the typically assumed 104 K blackbody spectrum to study the energy of stellar flares and infer their fractional area coverage. We investigated 37 events from Kepler-411 and five events from Kepler-396, using both radiation mechanisms. We find that estimates for the total flare energy from the H recombination spectrum are about an order of magnitude lower than the values obtained from the blackbody radiation. Given the known energy transfer processes in flares, we argue that the former is a physically more plausible model than the latter to explain the origin of the broad-band optical emission from flares.

This study investigates the ionospheric response over the East Africa region during solar flares and a geomagnetic storm from 8–15 May 2024. During this period, 12 X-class solar flares and one extreme geomagnetic storm occurred, causing pronounced variability in total electron content (TEC). X-ray flux measurements from the Extreme Ultraviolet and X-ray Irradiance Sensors (EXIS) on board the Geostationary Operational Environmental Satellites (GOES) are analyzed to detect solar flares. TEC derived from four GNSS receiver stations and the IRI-2020 model, O/N2 ratio maps, and an ionospheric electric fields model are used to identify ionospheric variation owing to the space weather events. The X-class flares prior to May 11 produced immediate TEC enhancements of up to +15 TECU relative to quiet day levels, consistent with sudden ionospheric disturbances. In contrast, the geomagnetic storm on May 10–11 induced both positive and negative storm phases, with TEC deviations ranging from –31.46 to +33.13 TECU. During the main phase of the geomagnetic storm, at the ADIS station, TEC increased by +10.8 TECU and then decreased by –8 TECU. In the recovery phase, it increased to +31 TECU. At the DJIG station during the main phase, TEC decreased by –16.1 TECU, followed by a significant positive enhancement reaching +30.5 TECU on May 12th. Similarly, the MAL2 station recorded a minimum negative TEC deviation of –16.6 TECU during the main phase, with a notable maximum positive deviation of +33.03 TECU also occurring on May 12th. For the MBAR station, the main storm phase on May 10th showed a minimum negative TEC deviation of –15.94 TECU, and a maximum positive deviation of +33.13 TECU was observed on May 12th. We have used correlation coefficients ( $$r$$ ), Percentage Root-Mean Square Error (PRMSE) and root mean square errors (RMSE) to examine the variation of the IRI-2020 TEC from the GPS TEC during the storm. The results show that the model performed best at the ADIS station, with the highest $$r$$ (0.93) and the lowest RMSE (13.33) and PRMSE (28.31%). These study enhance our understanding of solar flare and geomagnetic storm impacts in equatorial and low latitude regions, which is crucial for improving space weather forecasting and mitigating risks.

Solar white-light flares (WLFs) are defined as those flares which are visible in optical continuum. Up to now, only less than 100 WLFs have been reported. WLFs are of great importance because they present the most extreme conditions in solar flares and provide a severe challenge to energy transport mechanisms and atmospheric models.

A study of Solar Flares and Geomagnetic Storms Impact on Total Electron Content Over High-Latitude Region During July- November 2021: The Case of Tromso Station

Solar flares are known to produce fast Corona Mass Ejections (CMEs) that can lead to the occurrence of different classes of geomagnetic storms. Severe geomagnetic storms can generate disturbances in the magnetosphere and the ionosphere that can affect communication channels; by disrupting Satellite and navigation systems, such as GPS, Galileo, Compass and GLONASS. During intense Solar flares, enhancement in the ionospheric electron density usually occurs, leading to the absorption of the High Frequency (HF) signals by the ionosphere. Enhancement in the Very Low Frequency (VLF) radio waves (3 – 30 kHz) usually takes place during solar flares. This phenomenon is called Sudden Ionospheric Disturbance (SID). These SIDs serves as an opportunity for the tracking of solar flares using VLF. In this study, the diurnal variation of the VLF signals transmitted from six locations selected from USA, Australia and Japan were used to monitor SIDs. The signals were received using the 0-50 kHz frequency receiver (Super SID Monitor) installed at the Kebbi State University of Science and Technology (KSUST), Aliero, Nigeria (latitude: 12.31°N and Longitude: 4.50°E). The diurnal variation of the VLF signals alongside some magnetic indices (Dst, kp, and ap), solar wind speed and density as well as the solar flux index (f10.7) for the month of February, 2020 was investigated. Results from this study reveal that; the VLF amplitudes appeared to be stronger when the lowest level of the geomagnetic activity was recorded across all stations on the quietest day of the month. During this day, the intensity of the signals received vary across the stations, ranging from 2*10⁴ to 4*10⁷dB. During the disturbed period, decrease in the Disturbance Storm Time (Dst) index was observed to have two minimum excursion with values of -31 and -33 nT, thus indicating a weak geomagnetic storm (-30<Dst>-50) event. Consequently a gradual increase in the solar wind speed with a peak value of 520 km/s, significant decrease in the VLF amplitude ranging from 50 – 7*10⁵dB was observed during the weak geomagnetic storm, on 19 February, 2020. It is also evident from this study that the intensity/strength of the VLF signal and its pattern of propagation are greatly affected by the geomagnetic storm. In spite of the changes in the VLF amplitude observed, there was no trace of solar flares during the weak geomagnetic storm. This therefore suggests that not all classes of geomagnetic storms are connected to solar flares.

This study examines the ionospheric Total Electron Content (TEC) responses to a solar flare on October 28, 2021, and a geomagnetic storm on November 4, 2021, across low, middle, and high latitude regions. We utilized GPS-TEC data from the University NAVSTAR Consortium’s dual-frequency GPS devices at the IFR1, IISC, YIBL, YKRO, KERG, and SVTL stations. While the solar flare on October 28, 2021, triggered the geomagnetic storm on November 4, 2021, our analysis revealed notable TEC changes during the latter event. TEC fluctuations were observed across all stations during the geomagnetic storm, with significant disruptions and variable depletion rates. However, distinct TEC variations were noted at KERG and YIBL stations before the storm, likely due to the preceding solar flare. Continuous wavelet analysis (CWT) showed higher periodicity during the storm compared to the flare, proving CWT to be an effective tool for analyzing TEC variability by revealing periodicity fluctuations at all stations. In conclusion, we found that both solar flares and geomagnetic storms can cause significant positive TEC changes.

High and mid latitude and near subsolar point ionospheric and thermospheric responses to the solar flares and geomagnetic storms during low solar activity periods of 2017 and 2020