Research needed to improve solar flare and particle forecasting.

We briefly review the status of our physical understanding of energy buildup and release in transient active phenomena on the Sun. Such understanding is necessary in order to improve our capabilities to predict such events and their effects in interplanetary space and near-Earth environment. We then discuss the research that we consider is needed for such improvement.     

Numerical study of solar flare particle propagation in the heliosphere

Numerical solutions are presented for the propagation of solar cosmic rays interplanetary space, including the effects of pitch-angle scattering and adiabatic focusing. The intensity-time profiles can be well fitted by a simple radial spatial diffusion equation with scattering mean-free path λ_fit. For low-rigidity particles the radial mean-free path so obtained is significantly larger than the mean-free path calculated from the scattering coefficient due to the inapplicability of the diffusive approximation early in the event. The well-known discrepency between λ_fit and the theoretical predictions may be resolved by these calculations.     

Long lasting energetic particle injection from a weak flare

A relatively weak solar cosmic ray event registered at the Earth orbit following the flare of December 17, 1976 is discussed. The main feature of the event is the existence of a prolonged unusually high proton and electron anisotropy; even at the end of the decay phase of the flare the motion of the particles were mainly directed away from the Sun. The durations of proton and electron anisotropies were different. If prolonged particle injection is neglected the value of the anisotropy considerable exceeds all diffusive estimates. Time-intensity and anisotropy profiles of electrons and protons are fitted by a diffusive model including prolonged particle injection at the Sun. The best agreement with the data is obtained if the duration of injection equals about 20 and 7 hours for protons and electrons, respectively.     

Information on particle acceleration and transport derived from solar flare spectropolarimetry

The hydrogen Hα line has been found to be linearly polarized at some locations and times during a June 15th 2001 flare observed with THEMIS. This flare was accompanied by radio pulses and hard X-ray emission. Linear polarization is below the noise level in the flare kernels. However, it is present at the edges of these kernels, in the line center and near wings where the polarization degree exceeds 4%. The directions of polarization are not random but close within ±15° to the tangential and radial directions. This polarization can be due either to electron beams and their associated return currents or to electron and proton beams.     

Reconnection and fast particle production in tokamak and solar plasmas

Detailed in situ studies of magnetic reconnection and particle acceleration, which play a crucial role in the release and redistribution of energy in solar flares, can be performed in tokamak plasmas under conditions resembling those of the flaring solar corona. Recent measurements and modelling of fast particle production during reconnection events in the Mega-Amp Spherical Tokamak (MAST) are described. Specifically, observations in this device of electron acceleration during edge localised modes, and of both ion and electron acceleration during merging-compression plasma start-up, are presented, and possible implications of these studies for particle acceleration in flares are discussed. The results from MAST lend weight to the conjecture that large numbers of ions are accelerated to sub-MeV energies in flares.     

The 1980 April 12 flare and transient: Report on progress in interpretation

A multidisciplinary study of this solar-interplanetary event is summarized by two main points: this flare was an incident in a process that began days before the flare, and continued after the flare; and the chain of events can be interpreted most simply in terms of energy input over scales of time and space that are large compared to the flare seen in the light of Hα. In support of these points, 5 aspects of the flare are described here: (1) hours before the flare, slow changes in coronal structure were associated with radio continuum emission, suggesting large-scale magnetic-field changes and the presence of energetic electrons; (2) long-lived X-ray loops require sustained energy input for at least an hour after the flare start; (3) interplanetary disturbance near earth is probably related to this limb flare, although the (expected) absence of a shock makes identification uncertain; (4) the coronal mass ejection overlay decaying magnetic field; (5) speed derived from frequency drift of the type II radio burst in the low corona, and from the travel time of the disturbance to 1 a.u., are about twice as great as the observed speed of the coronal mass ejection and of the disturbed solar-wind speed.     

Origin of coronal and interplanetary shock and particle acceleration of a flare/CME event

By using radio data from ground-based telescopes (from 270 MHz to 25 MHz), and from the Radio and Plasma Wave experiment (WAVES) on board the WIND spacecraft (1–14 MHz and several kHz-11 MHz), as well as FY -2 satellite data, the origin of coronal and interplanetary shock and particle acceleration of the 14 July 2000 flare/CME event (the Bastille day event) have been studied. Main conclusions are as follows: (1) We investigate the causal relationship between metric type 11 bursts observed by the digital IZMIRAN radio spectrograph and type II radio emissions in the frequency range from 1–14 MHz and several kHz-11 MHz observed by the WAVES/WIND. The analysis indicate that the fast CME is the origin of both coronal and interplanetary shocks. (2)According to the time profiles of Hard X-ray, and energetic particles (include proton, ³He, and ⁴He) from FY-2 satellite, it is obvious that the Bastille day event is the event, in which both impulsive and gradual phenomena occur. The energetic particles accelerated not only in flare but also in CME.     

Modeling the time-intensity profile of solar flare generated particle fluxes in the inner heliosphere.

It is possible to model the time-intensity profile of solar particles expected in space after the occurrence of a significant solar flare on the sun. After the particles are accelerated in the flare process, if conditions are favorable, they may be released into the solar corona and then into space. The heliolongitudinal gradients observed in the inner heliosphere are extremely variable, reflecting the major magnetic structures in the solar corona which extend into space. These magnetic structures control the particle gradients in the inner heliosphere. The most extensive solar particle measurements are those observed by earth-orbiting spacecraft, and forecast and prediction procedures are best for the position of the earth. There is no consensus of how to extend the earth-based models to other locations in space. Local interplanetary conditions and structures exert considerable influence on the time-intensity profiles observed. The interplanetary shock may either reduce or enhance the particle intensity observed at a specific point in space and the observed effects are very dependent on energy.     

Solar flare particle injection spectra and spectra of flux maxima at the point of observation

Numerical models of impulsive solar flare particle events usually assume the radial diffusion coefficient to be independent of energy per nucleon, T, although the observations indicate a T^0.5 dependence (constant mean free path). The assumption of a constant diffusion coefficient results in a preservation of a power law injection spectrum at all radial distances throughout the event. We investigate the effect of an energy dependent diffusion coefficient on the spectrum of flux maxima at a fixed point in interplanetary space. This spectrum is harder than that of initial differential number densities close to the sun. Furthermore, the spectrum hardens with increasing radial distance which seems to be at variance with observations.     

Review on latest progress on supergiant fast X-ray transients and future direction

In the recent years, the discovery of a new class of Galactic transients with fast and bright flaring X-ray activity, the supergiant fast X-ray transients, has completely changed our view and comprehension of massive X-ray binaries. These objects display X-ray outbursts which are difficult to be explained in the framework of standard theories for the accretion of matter onto compact objects, and could represent a dominant population of X-ray binaries. I will review their main observational properties (neutron star magnetic field, orbital and spin period, long term behavior, duty cycle, quiescence and outburst emission), which pose serious problems to the main mechanisms recently proposed to explain their X-ray behavior. I will discuss both present results and future perspectives with the next generation of X-ray telescopes.     

Activity in the homologous flare site

The evolution of a site where homologous flares occured on June 8, 1980 is analysed by using observations both in the photosphere and in the chromosphere. The homology is discussed through space, energy and dynamical aspects. The criteria are used in order to propose the definition of a coefficient of homology.     

Large-scale reconnection in a large flare

Some specific features of the large-scale magnetic reconnection in large solar flares are briefly reviewed. In particular, the large-scale structure and dynamics of the 3B/X5.7 flare on 14 July 2000 are interpreted in terms of the topological model. The role of the betatron effect in collapsing magnetic traps, that are created by reconnection in the solar corona, is considered. We discuss some possibilities to observe the collapsing trap acceleration in solar flares.     

Evolution of flare ribbons and energyrelease

We examined the relation between the evolutions of the H flare ribbons and the released magnetic energiesat a solar flare which occurred on 2001 April 10. This is the first study to evaluate the released energy quantitatively, based on the magnetic reconnection model, and by using the data obtained with the multi wavelength observation. We measured the, photospheric magnetic field strengths and the separation speeds of the fronts of the H flare ribbon, and compared them the nonthermal behaviors observed in HXRs and microwaves. Those nonthermal radiation sources tell us when and where large energy releases occur. Then, by using the photospheric and chromospheric features, we estimated the released magnetic energy at the flare. The estimated energy release rates at the H kernels associated with the HXR sources are locally large enough to explain the difference between the spatial distribution the H kernels and the HXR sources. Their temporal evolution of the energy release rates also shows peaks corresponding to HXR bursts.     

The multi-wavelength study of the effect of energetic particle beams on the chromospheric emission in the 25th July 2004 solar flare

We present the multi-wavelength study of short-term variations of Hα line emission located in multiple kernels on the both sides from magnetic neutral line in the 25th July 2004 solar flare observed by VTT (Tenerife). The HXR and Hα emission in the kernels 1 and 3 is close spatially and temporally while in kernels 4 and 7 there is only delayed Hα emission observed tens seconds after HXR in the kernels 1 and 3. The locations of Hα kernels 1, 3, 4 and 7 are on the opposite sides from the magnetic neutral line. The temporal variations of Hα emission in kernels 1 and 3 coincide within 5 s with the HXR photon emission. The latter is found to have double power law photon spectra, which were corrected to a single power law with the turning point technique accounting for Ohmic losses and collisions. The Hα emission is fit by full non-LTE simulations in an atmosphere heated by an electron beam with the parameters derived from the HXR emission. The combination of radiative, thermal and non-thermal mechanisms of excitation and ionization of hydrogen atoms is considered. The temporal evolution of simulated Hα emission in the kernel 3 fits rather well the two observed intensity increases: the first at the flare onset (13:38:39–13:39:30 UT) caused by pure non-thermal excitation by beam electrons and the second one appearing after 13:40:00 UT because of a hydrodynamic heating. The observed close temporal correlation or delay of Hα emission with HXR emission points out to the precipitation either of electron (kernels 1 and 3) or protons (4 and 7).     

Solar EUV flux variation and flare brightenings

On 2010 February 8, the Extreme ultraviolet (EUV) flux variation in 195 Å and flare brightening has been examined in different sizes of active regions by using SOHO/EIT, MDI and Hα$\alpha$ observational data. These three active regions represent a large active region with a sunspot group, a moderate active region without a sunspot and a small region with weak plage in Hα$\alpha$ band respectively. Our study shows that the main full disk EUV flux comes from active regions, especially from large active regions. The sudden increases of EUV flux are corresponding to the EUV flare brightenings. For the large active region, the local EUV 195 Å flux peaks are well correlated to that of the GOES X-ray flux. The EUV 195 Å flux peaking time of M-class flares delay GOES X-ray flux a few minutes. For the moderate active region, the local EUV 195 Å flux is not well correlated to GOES X-ray flux. The EUV 195 Å flare brightenings in the moderate active region appeared in the duration of sudden increase of its own local EUV flux. For the small active region, the local EUV 195  Å flux varied almost independently of the GOES X-ray flux. Our study suggests that for an active region its local EUV 195 Å flux is more closely correlated to the EUV flare brightening than the full disk GOES X-ray flux.     

Similarity in corona and flare production mechanisms in sun and stars

In this paper we establish a similarity in the various phenomena seen in outer atmosphere of Sun and stars. We show that the chromospheric networks, coronal loops and flares can be looked upon as manifestations of the same physical process varying only in their energy content. We then discuss the betratron mechanism as a possible source of this activity.     

Magnetic reconnection and energy release in a long-duration stellar flare

A dynamical model of magnetic reconnection in solar two-ribbon flares is applied to EXOSAT observations of a long-decay flare from the star EQ Peg. We show that the model is able of reproducing correctly the energy release rate and temporal evolution of the decay phase of the observed flare. We conclude that the flare was the stellar counterpart of solar two-ribbon flares and we derive the physical parameters of the emitting region.     

On soft electron beams in solar active and flare region

On the basis of the experimental data obtained from the high resolution X-ray spectra for solar flares and active regions the Suprathermal electron model (SEM) was proposed. This model suggests the existance of the multitemperature structure of the solar plasma emitting Fe and Ca X-rays and the presence of additional electrons with low energies E ? 10 keV and small densities ～ 1–5% relative to the thermal component.     

X-ray and gamma-ray observations of a white-light flare

HEAO-1 observed hard radiations (X- and gamma-rays) from a major solar flare on 11 July 1978. The observations showed gamma-ray line and continuum emission extending to the highest energy observed. The lines are identified with the 2.2 MeV line of deuterium formation and the 4.4 MeV line of inelastic scattering on ¹²C, both previously observed in the flares of August 1972 [1]. The 11 July flare was identified as a white-light flare by observations at Debrecen [2]. It thus provides the first opportunity for a detailed examination of white-light flare theories that depend upon proton heating of the photosphere. The line strength over a four-minute integration at 2.2 MeV was 1.00 ± 0.29 ph(cm² sec)⁻¹, and the gamma-ray emission (excluding the 2.2 MeV line which was appreciably delayed) lagged by less than 20 sec approximately after the hard X-ray and microwave fluxes. We conclude that the “second-stage” acceleration of high-energy solar particles must commence promptly after the impulsive phase.     

Coronal fast wave trains of the decimetric type IV radio event observed during the decay phase of the June 6, 2000 flare

The 22 min long decimetric type IV radio event observed during the decay phase of the June 6, 2000 flare simultaneously by the Brazilian Solar Spectroscope (BSS) and the Ond?ejov radiospectrograph in frequency range 1200–4500 MHz has been analyzed. We have found that the characteristic periods of about 60 s belong to the long-period spectral component of the fast wave trains with a tadpole pattern in their wavelet power spectra. We have detected these trains in the whole frequency range 1200–4500 MHz. The behavior of individual wave trains at lower frequencies is different from that at higher frequencies. These individual wave trains have some common as well as different properties. In this paper, we focus on two examples of wave trains in a loop segment and the main statistical parameters in their wavelet power and global spectra are studied and discussed.