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The heating mechanism at high densities during M-dwarf flares is poorly understood. Spectra of M-dwarf flares in the optical and near-ultraviolet wavelength regimes have revealed three continuum components during the impulsive phase: 1) an energetically dominant blackbody component with a color temperature of T ≈ 10 4 K in the blue-optical, 2) a smaller amount of Balmer continuum emission in the near-ultraviolet at λ ≤ 3 646  Å, and 3) an apparent pseudo-continuum of blended high-order Balmer lines between λ = 3 646  Å and λ ≈ 3 900  Å. These properties are not reproduced by models that employ a typical “solar-type” flare heating level of ≤ 10 11 erg cm − 2 s − 1 in nonthermal electrons, and therefore our understanding of these spectra is limited to a phenomenological three-component interpretation. We present a new 1D radiative-hydrodynamic model of an M-dwarf flare from precipitating nonthermal electrons with a high energy flux of 10 13 erg cm − 2 s − 1 . The simulation produces bright near-ultraviolet and optical continuum emission from a dense ( n > 10 15 cm − 3 ), hot ( T ≈ 12 000 – 13 500 K ) chromospheric condensation. For the first time, the observed color temperature and Balmer jump ratio are produced self-consistently in a radiative-hydrodynamic flare model. We find that a T ≈ 10 4 K blackbody-like continuum component and a low Balmer jump ratio result from optically thick Balmer ( ∞ → n = 2 ) and Paschen recombination ( ∞ → n = 3 ) radiation, and thus the properties of the flux spectrum are caused by blue ( λ ≈ 4 300  Å) light escaping over a larger physical depth range than by red ( λ ≈ 6 700  Å) and near-ultraviolet ( λ ≈ 3 500  Å) light. To model the near-ultraviolet pseudo-continuum previously attributed to overlapping Balmer lines, we include the extra Balmer continuum opacity from Landau–Zener transitions that result from merged, high-order energy levels of hydrogen in a dense, partially ionized atmosphere. This reveals a new diagnostic of ambient charge density in the densest regions of the atmosphere that are heated during dMe and solar flares.

We investigated how the rise rate and decay rate of solar flares affect the thermosphere and ionosphere responses to them. Model simulations and data analysis were conducted for two flares of similar magnitude (X6.2 and X5.4) that had the same location on the solar limb, but the X6.2 flare had longer rise and decay times. Simulated total electron content (TEC) enhancements from the X6.2 and X5.4 flares were ∼6 total electron content units (TECU) and ∼2 TECU, and the simulated neutral density enhancements were ∼15%–20% and ∼5%, respectively, in reasonable agreement with observations. Additional model simulations showed that for idealized flares with the same magnitude and location, the thermosphere and ionosphere responses changed significantly as a function of rise and decay rates. The “Neupert Effect,” which predicts that a faster flare rise rate leads to a larger EUV enhancement during the impulsive phase, caused a larger maximum ion production enhancement. In addition, model simulations showed that increased E × B plasma transport due to conductivity increases during the flares caused a significant equatorial anomaly feature in the electron density enhancement in the F region but a relatively weaker equatorial anomaly feature in TEC enhancement, owing to dominant contributions by photochemical production and loss processes. The latitude dependence of the thermosphere response correlated well with the solar zenith angle effect, whereas the latitude dependence of the ionosphere response was more complex, owing to plasma transport and the winter anomaly. Key Points Upper atmosphere responses vary greatly to flares with a same magnitude Plasma transport plays a significant role in ionosphere response to flares Solar zenith angle determines thermosphere response but not the ionosphere

Quasi-periodic pulsations (or QPPs) are periodic intensity variations in the flare emission that occur across all wavelength bands. In this article, we review the observational and modelling achievements since the previous review on this topic by Nakariakov and Melnikov ( Space Sci. Rev. 149 , 119, 2009 ). In recent years, it has become clear that QPPs are an inherent feature of solar flares because almost all flares exhibit QPPs. Moreover, it is now firmly established that QPPs often show multiple periods. We also review possible mechanisms for generating QPPs. Up to now, it has not been possible to conclusively identify the triggering mechanism or cause of QPPs. The lack of this identification currently hampers possible seismological inferences of flare plasma parameters. QPPs in stellar flares have been detected for a long time, and the high-quality data of the Kepler mission allows studying the QPP more systematically. However, it has not been conclusively shown whether the timescales of stellar QPPs are different or the same as those in solar flares.

Gas flaring has been identified as a major contributor to global warming and climate change. It is used either as a safety measure or as a means of disposal for technical or economic reasons. Over 250 toxins have been directly/indirectly associated with gas flaring and its associated emissions. Most of these toxins have been known to have significant inimical impacts on humans’ health, plant biodiversity, and the environment. With the recent rise in global energy insecurity, several EU countries have either returned to coal power generation or extended the lifetime of their coal-fired plants thereby increasing anthropogenic carbon emissions. This increase in carbon emission has necessitated the re-evaluate of gas flare practices vis-à-vis the environmental challenges and the financial potentials. This paper presents a holistic review of gas flaring, its types, composition, systems design, estimation methods, social and environmental challenges, the abatement measures, and the re-utilization strategies. It identified the potential to save a minimum of US$10.4 billion globally if more stringent gas flare abatement measures were pursued. Furthermore, the paper highlights the recent trends in flare gas re-utilization technologies such as the production of bioproducts which has been reported to hold a potential for an annual production of about 148 million bbl of biocrude and 67 million metrics of algae protein from 140 bcm of globally flared gas. Finally, it explored the possible way forward and stringent measures that can be pursued to disincentivize gas flare and also increase investments in gas processing technologies.

We present an empirical scenario of the energy release process during a solar limb flare on February 5, 2023. This event was observed by the Siberian Radioheliograph (SRH) within the 3 – 12 GHz range and the Advanced Space-based Solar Observatory / Hard X-ray Imager (ASO-S/HXI) within 10 – 300 keV range. The combination of these data allowed us to use information not only about the spectral features but also about the spatial evolution of the flare. The main source of the energy released was a small compact loop which was revealed in both the X-ray and microwave ranges. During the early phases of the flare evolution, a spectral analysis of microwave emission showed that thermal gyrosynchrotron emission turned to gyrosynchrotron emission of nonthermal electrons. This indicated the transition from the heating process to the acceleration processes. Spectral indices derived from hard X-ray and microwave data closely agree with each other and show the classical soft–hard–soft scenario of acceleration. The hardening of the average microwave spectrum at the end of the impulsive phase was caused by the contribution of jet emission to microwaves rather than by peculiarities of the acceleration processes.

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.

The extreme ultra-violet (EUV) portion of the solar spectrum contains a wealth of diagnostic tools for probing the lower solar atmosphere in response to an injection of energy, particularly during the impulsive phase of solar flares. These include temperature- and density-sensitive line ratios, Doppler-shifted emission lines, nonthermal broadening, abundance measurements, differential emission measure profiles, continuum temperatures and energetics, among others. In this article I review some of the recent advances that have been made using these techniques to infer physical properties of heated plasma at footpoint and ribbon locations during the initial stages of solar flares. I primarily focus on studies that have utilised spectroscopic EUV data from Hinode/EUV Imaging Spectrometer (EIS) and Solar Dynamics Observatory/EUV Variability Experiment (SDO/EVE), and I also provide some historical background and a summary of future spectroscopic instrumentation.

Solar flares are energetic events taking place in the Sun’s atmosphere, and their effects can greatly impact the environment of the surrounding planets. In particular, eruptive flares, as opposed to confined flares, launch coronal mass ejections into the interplanetary medium, and as such, are one of the main drivers of space weather. After briefly reviewing the main characteristics of solar flares, we summarise the processes that can account for the build-up and release of energy during their evolution. In particular, we focus on the development of recent 3D numerical simulations that explain many of the observed flare features. These simulations can also provide predictions of the dynamical evolution of coronal and photospheric magnetic field. Here we present a few observational examples that, together with numerical modelling, point to the underlying physical mechanisms of the eruptions.

Shortly after the seminal paper “Self-Organized Criticality: An explanation of 1/ f noise” by Bak et al. ( 1987 ), the idea has been applied to solar physics, in “Avalanches and the Distribution of Solar Flares” by Lu and Hamilton ( 1991 ). In the following years, an inspiring cross-fertilization from complexity theory to solar and astrophysics took place, where the SOC concept was initially applied to solar flares, stellar flares, and magnetospheric substorms, and later extended to the radiation belt, the heliosphere, lunar craters, the asteroid belt, the Saturn ring, pulsar glitches, soft X-ray repeaters, blazars, black-hole objects, cosmic rays, and boson clouds. The application of SOC concepts has been performed by numerical cellular automaton simulations, by analytical calculations of statistical (powerlaw-like) distributions based on physical scaling laws, and by observational tests of theoretically predicted size distributions and waiting time distributions. Attempts have been undertaken to import physical models into the numerical SOC toy models, such as the discretization of magneto-hydrodynamics (MHD) processes. The novel applications stimulated also vigorous debates about the discrimination between SOC models, SOC-like, and non-SOC processes, such as phase transitions, turbulence, random-walk diffusion, percolation, branching processes, network theory, chaos theory, fractality, multi-scale, and other complexity phenomena. We review SOC studies from the last 25 years and highlight new trends, open questions, and future challenges, as discussed during two recent ISSI workshops on this theme.

Quasi-periodic pulsations (QPPs) are found in solar flares of various magnetic morphologies, e.g. in two-ribbon or circular-ribbon flares, and the mechanisms of their generation are not yet clear. Here we present the first detailed analysis of QPPs (with a period P = 54 ± 13 seconds) found in the Ramaty High Energy Solar Spectroscopic Imager (RHESSI) observations of a relatively rare three-ribbon M1.1 class flare that occurred on 5 July 2012 (SOL2012-07-05T06:49). QPPs are manifested in the temporal profiles of temperature [ T ] and emission measure [ E M ] of “super-hot” ( T s ≈ 30  – 50 MK) plasma but are almost invisible in the profiles of “hot” ( T h ≈ 15  – 20 MK) plasma parameters when approximating X-ray spectra of the flare with the bremsstrahlung spectrum of a two-temperature thermal (Maxwellian) plasma. In addition, QPPs with a similar period are found in the temporal profiles of the flux and spectral index of nonthermal electrons if the observed X-ray spectra are approximated by a combination of the bremsstrahlung spectra of a single-temperature plasma and nonthermal electrons with a power-law energy distribution. QPPs are not well expressed in the X-ray flux according to RHESSI and GOES data, or in radio data. The QPPs are accompanied by apparent systematic movement of a single X-ray source at a low speed of 34 ± 21  km s −1 along the central flare ribbon over a narrow ( < 5  Mm) “tongue” of negative magnetic polarity, elongated ( ≈ 20  Mm) between two areas of positive polarity. The results of magnetic extrapolation in the nonlinear force-free field (NLFFF) approximation show that the X-ray source could move along curved and twisted field lines between two sheared flare arcades. It is worth noting that in the homologous three-ribbon M6.1 flare (SOL2012-07-05T11:39), which occurred in the same region about five hours later, the X-ray sources moved much less systematically and did not produce similar QPPs. We interpret the observed QPPs as a result of successive episodes of energy release in different spatial locations. In each pulsation, ≈(1 – 4) × 10 29  erg is released in the form of thermal energy of hot and super-hot plasmas (or accelerated electrons), which is comparable with the energy of a microflare. The total kinetic energy released during all QPPs is ≈(0.7 – 3.5) × 10 30  erg, which is about an order of magnitude less than the free magnetic energy ≈ 1.56 × 10 31  erg released in the flare region. We discuss possible propagating triggers of the quasi-periodic energy release (slow magnetoacoustic waves, asymmetric rise of curved/twisted field lines, flapping oscillations, and thermal instability in a reconnecting current sheet) and argue that the current state of available mechanisms and observations does not allow us to reach an unambiguous conclusion.