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The February 16, 1984 (0900 UT) solar flare was very energetic and produced a variety of emissions. The X-ray and gamma ray continuum measurement, made aboard the International Cometary Explorer (ICE) and the Pioneer Venus Orbiter (PVO), are briefly described.
The occurrence rate of solar flares exhibits a periodicity of about 152 days. The cause of the 152-day periodicity still remains a mystery. But answers to the following questions will enhance understanding of it. (1) Is the periodicity a local or a global phenomenon? (2) Is the periodicity due to the interaction of 'hotspots' rotating at different rates such that they align with one another once every 152-day period? (3) Is the periodicity due to the interaction of rotating features originating from g-mode oscillations? Here the authors report an analysis of 'major flares' observed with the Hard X-ray Burst Spectrometer (HXRBS) aboard SMM and conclude that the 152-day periodicity is a global phenomenon, and that the answers to questions (2) and (3) are negative.
Collaborative solar investigations by Tufts University and the Observatoire de Paris have resulted in simultaneous radio observations with the Very Large Array (VLA) and the Nancay Radioheliograph (NR), comparisons of this radio data with X-ray observations, and theoretical interpretations of the dominant radiation mechanisms during a weak impulsive solar flare observed on May 28, 1988. The VLA has mapped the flaring structures at time intervals of 3.3 s, showing that the preflash and flash-phase components of the impulsive emission originate in spatially separated sources. The 20.7 cm preflash source is ascribed to thermal gyroresonance emission from coronal loops with typical magnetic field strengths of up to 270 G; this emission is associated with heating and exhibits no detectable hard X-ray radiation above 30 keV. The flash-phase 20.7 cm source and the hard X-ray emission are attributed to nonthermal electrons in the coronal and chromospheric portions of a magnetic loop. The combination of imaging observations at 20.7 and 91.6 cm excludes emission from a confined hot plasma during the flash phase.
We describe the design of a balloon-borne Imaging X-ray Polarimeter for Solar flares (IX PS). This novel instrument, a Time Projection Chamber (TPC) for photoelectric polarimetry, will be capable of measuring polarization at the few percent level in the 20-50 keV energy range during an M- or X class flare, and will provide imaging information at the approx.10 arcsec level. The primary objective of such observations is to determine the directivity of nonthermal high-energy electrons producing solar hard X-rays, and hence to learn about the particle acceleration and energy release processes in solar flares. Secondary objectives include the separation of the thermal and nonthermal components of the flare X-ray emissions and the separation of photospheric albedo fluxes from direct emissions.
An experiment designed to measure energetic X-rays, electrons, protons and alpha particles from solar flares aboard OGO 5 is reported. A brief statement of the objectives of the experiment is followed by a description of the instrumentation including detector characteristics and associated electronics. The data handling system is then described and the operational history of the experiment is discussed. This is followed by a description of the format of the magnetic data tapes and a summary of the major computer processing programs. The measurements made with this experiment within the first year of its operation have led to several basic results regarding the role of non-thermal electrons in the physics of solar flares. These results are described in the papers listed in the bibliography given at the end of this report.
A composite numerical simulation model developed from a series of MHD models was used to compute the solar-flare-generated disturbances of physical parameters, such as density, temperature, velocity, and magnetic field from the solar surface (i.e., the photospheric level) to the earth's environment. It is shown that the disturbed earth's environment at high latitudes can be approximated by starting with the knowledge of the occurrence and the strength of a solar flare, then simulating the evolutionary consequences of the solar disturbance through interplanetary space up to and through the magnetosphere.
High-resolution (approximately 1 keV FWHM) spectral measurements from 13 to 300 keV of a solar flare hard X-ray burst observed on 1980 June 27 by a balloon-borne array of cooled germanium planar detectors are presented. At energies below about 35 keV, a new component of solar flare hard X-rays is identified. This component is characterized by an extremely steep spectrum which fits closely to that from a Maxwellian electron distribution with a maximum temperature of about 34 x 10 to the 6th K and an emission measure of 2.9 x 10 to the 48th per cu cm. This hot isothermal component appears at the peak of the normal power-law-like impulsive X-ray burst component, and it remains isothermal and dominates the X-ray emission below about 30 keV through the decay of the flare event.
Physical parameters relevant to the acceleration and propagation of energetic electrons during the impulsive phase of a solar flare are studied in hard X-ray, microwave and both type III and decimetric radio bursts associated with the December 4, 1978 solar flare. A one-to-one association between single type III bursts and hard X-ray peaks is established, along with a burst delay with respect to the peaks of 0.5 sec. The observed increase of the high frequency cutoff of the metric type III bursts during the impulsive phase is examined in terms of the decreasing altitude of the electron acceleration/injection region, increasing electron spectrum hardness, and decreasing acceleration time. A pulsating decimetric continuum was found to be present during and before the impulsive phase whose high frequency cutoff also systematically increased during the rise of the impulsive phase.
A review is presented regarding the current knowledge of the energetics of solar flares. Recent observations by the Solar Maximum Mission and by balloon-borne instrumentation indicate that the flare hard X-ray emission arises from nonthermal bremsstrahlung - the collisions of fast electrons into a cold ambient medium (Ee much greater than kT). Under this interpretation, most of the energy released for many flares is initially contained in the energetic electrons. These electrons can produce most of the observed flare phenomena via interactions with the solar atmosphere. In large flares a shock wave may result from explosive heating of the solar atmosphere by these electrons. This shock wave can accelerate nuclei to relativistic energies. It is argued that recent SMM observations of fast gamma-ray bursts are consistent with this picture of shock acceleration of nuclei.
Solar activity is a primary factor determining the state of the Earth’s space environment, geomagnetic and ionospheric disturbances, and radiation hazards. In the current state of knowledge, machine learning (ML) methods provide essential tools for processing data, investigating relationships among various physical properties and characteristics, uncovering hidden connections, and predicting hazardous solar events. The primary difficulty in developing and applying modern machine-learning tools in heliophysics is that the essential data are scattered among over a hundred data repositories developed by instrument teams of space missions and ground-based observatories. In addition, statistical and ML methods require long time series of homogeneous measurements. To facilitate ML-ready data preparation and access, we have developed an interactive database of solar flares integrating the most essential datasets (https://solarflare.njit.edu/). The database performs an initial data processing and is automatically updated. In addition, we are developing the Solar Energetic Particle Prediction Portal (SEP3, https://sun.njit.edu/SEP3), which hosts web applications that allow users to retrieve the database records. The Portal has a search page for browsing the events from the most widely used catalogs and a dedicated space to share the most recent achievements of the team. The interactive widget can display soft X-ray and proton flux time series from GOES satellites and the flare records. The data portal has been used to evaluate the forecasts of solar proton events and investigate machine-learning approaches to SEP prediction.
In a recent paper Gosling (1993) claims that solar flares are relatively unimportant for understanding the terrestrial consequences of solar activity, and argues that coronal mass ejections (CMEs) produce the most powerful terrestrial disturbances. This opinion conflicts with observation, as it is well known that CMEs and flares are closely associated, and we disagree with Gosling's insistence on a simplistic cause-and-effect description of the interrelated phenomena of a solar flare. In this brief response we present new Yohkoh data and review older results that demonstrate the close relationships among CMEs, flares, filament eruptions, and other forms of energy release such as particle acceleration.
Proton spectra, valid from non- to ultra-relativistic energies, resulting from stochastic Fermi acceleration in solar flares are calculated. These spectra were obtained by numerically solving the Fokker-Planck equation, in which the escape of the particles from the acceleration region is characterized by an energy-independent escape time. In addition to equilibrium spectra, time-dependent energy spectra showing the approach to equilibrium are also presented. These numerical equilibrium spectra are compared with previous results which were obtained either by Monte Carlo simulations or approximate analytical treatments. There are no analytic solutions valid in the transrelativistic regime, which is very important for the production of pions and neutrons in solar flares. The acceleration efficiency is related to physical parameters, in particular the energy density in either magnetosonic or Alfven waves, and a lower limit is placed on either of these energy densities from acceleration times implied by gamma-ray observations. Also discussed is the physical interpretation of the escape time.
We have developed a promising new technique for jointly analyzing BATSE hard X-ray observations of solar flares with simultaneous soft X-ray observations. The technique is based upon a model in which electric currents and associated electric fields are responsible for the respective heating and particle acceleration that occur in solar flares. A useful by-product of this technique is the strength and evolution of the coronal electric field. The latter permits one to derive important flare parameters such as the current density, the number of current filaments composing the loop, and ultimately the hard X-ray spectrum produced by the runaway electrons. We are continuing to explore the technique by applying it to additional flares for which we have joint BATSE/Yohkoh observations. A central assumption of our analysis is the constant of proportionality alpha relating the hard X-ray flux above 50 keV and the rate of electron acceleration. For a thick-target model of hard X-ray production, it can be shown that cv is in fact related to the spectral index and low-energy cutoff of precipitating electrons. The next step in our analysis is to place observational constraints on the latter parameters using the joint BATSE/Yohkoh data.
In 6 of the 7 instances where posteclipse brightening of Io has been reported by observers using blue filters, a major solar flare occurred within 10 degrees of the sub-Jovian longitude in the 100-day interval prior to observation. In none of the 18 instances where no posteclipse brightening was observed did such a flare occur. It is proposed that a phenomenon associated with a major solar flare causes an increase in the trapped particle flux at Io's orbit by an order of magnitude. The posteclipse brightening may be caused by thermoluminescence of Io's surface material upon emergence. Alternatively, it is possible that the increase in trapped particle flux would warm the surface, creating a temporary atmosphere which would precipitate during eclipse cooling and vaporize in the period of warming after reemergence.
Soft X-ray, Ca XIX, and H-alpha observations obtained for a set of four solar flares in the impulsive phase are analyzed. A blue asymmetry was observed in the coronal Ca XIX line during the soft-Xray rise phase in all of the events. A red asymmetry was observed simultaneously in chromospheric H-alpha at spatial locations associated with enhanced flare heating. It is shown that the impulsive phase momentum of upflowing soft X-ray plasma equalled that of the downflowing H-alpha plasma to within an order of magnitude. This supports the explosive chromospheric evaporation model of solar flares.
The free energy that is dissipated in a magnetic reconnection process of a solar flare, generally accompanied by a coronal mass ejection (CME), has been considered as the ultimate energy source of the global energy budget of solar flares in previous statistical studies. Here we explore the effects of the aerodynamic drag force on CMEs, which supplies additional energy from the slow solar wind to a CME event, besides the magnetic energy supply. For this purpose, we fit the analytical aerodynamic drag model of Cargill and Vršnak et al. to the height–time profiles r(t) of LASCO/SOHO data in 14,316 CME events observed during the first 8 yr (2010–2017) of the Solar Dynamics Observatory era (ensuring EUV coverage with AIA). Our main findings are (1) a mean solar wind speed of w = 472 ± 414 km s(exp −1), (2) a maximum drag-accelerated CME energy of E(drag) <~2 × 10(exp32) erg, (3) a maximum flare-accelerated CME energy of E(flare <~1.5 × 10(exp33) erg, (4) the ratio of the summed kinetic energies of all flare accelerated CMEs to the drag-accelerated CMEs amounts to a factor of 4, (5) the inclusion of the drag force slightly lowers the overall energy budget of CME kinetic energies in flares from ≈7% to ≈4%, and (6) the arrival times of CMEs at Earth can be predicted with an accuracy of ≈23%.
Results are reported for an observational and theoretical study of possible solar-flare effects on the zodiacal light. A seven-year search for flare-induced brightness variations in the light is described which discovered no significant changes in either the level of brightness from night to night or the shape of the main zodiacal-light cone at elongations greater than 25 deg from the sun. Several possible dynamical effects of solar flares on the zodiacal dust cloud are investigated theoretically, including increased ion drag from a passing shock wave, possible rotational bursting of dust particles as a result of ion impacts, and fluorescence of the dust particles. It is found that the kinetic energy carried by shock-front ions is not sufficient to cause the dust to fluoresce, that increased ion drag may be significant only at heliocentric distances less than 0.1 AU, and that particles larger than 10 microns cannot attain bursting speeds at the appropriate distances. The question is considered of whether changes in the brightness of the polarized component as a result of variations in electron number density can be observed from earth or the Helios solar probe.
Doschek et al. (1980) and Feldman et al. (1980) have discussed high-resolution soft X-ray spectra of solar flares. The present investigation is concerned with the characteristics of the impulsive events as observed in soft X rays and the cooling of the flare plasma in terms of energy loss processes. A class of intense, short-lived solar flares which appear to be characterized by very high densities at high temperatures is found. For the 1979 November 6 event, the cooling time is so rapid that the density obtained may be a lower limit. The main cooling mechanism for the flares is found to be radiative. Two of the events are associated with strong gamma-ray emission and both of these events are very unusual in their soft X-ray emission. The 1979 November 9 flare shows extremely broad X-ray emission lines during the rise phase, indicative of nonthermal velocities of the order of 300 km/s.