The relationship between the duration of solar X-ray flares and the mass of associated coronal ejections

The statistical relationship between the parameters of X-ray flares and coronal mass ejections on the Sun that are associated with these flares, is considered. It is shown that short X-ray flares are characterized on average by a lower mass ejection in the outer layers of the corona and interplanetary space as compared to high-energy long-duration events.     

Statistical relationship between sunspots and major flares

In this study, the statistical relationship between sunspots and major flares observed in the descending branch of the 20th and in the ascending branch of the 21st solar cycle is evaluated. It is found that the sunspots which produced these major flares are of the type Dki or Eki with magnetic class D and the largest magnetic field strength between 1600 and 2500 G.     

Similarities and differences between magnetospheric substorms and solar flares

Magnetospheric substorms and solar flares seem to follow a pattern where a sudden transition from a slow passive evolution to a fast active evolution occurs. This concept which has proven useful for constructing a theory of the onset of magnetospheric substorms is tentatively applied to current sheets which may be relevant to flares.     

Correlation between solar microwave bursts and hard X-ray flares

We compared the microwave bursts with short timescale fine structure observed at 2.84 GHZ at Beijing Astronomical Observatory with the hard X-ry bursts (HXB) observed by the YOHKOH satellite during the period 1991 Oct–1992 Dec, and found that of the 20 microwave events, 12 had HXB counterparts. For the typical event of 1992-06-07, we analyzed the common quasi-period oscillations on the order of 10² s and calculated the parameters of the source region, together with a brief discussion.     

Stellar and solar flares

Some ideas are developed concerning solar flares which have been presented earlier by the author (Schatzman, 1966a). Emphasis is laid on the problem of energy transport; from the energy supply to the region of the optical flare, on the storage of low energy cosmic ray particles in a magnetic bottle before the beginning of the optical flare, and the mechanism which triggers both the optical flare, and the production of high-energy cosmic rays. The relation between solar and stellar flares is considered.Lecture given at Goddard Space Flight Center, November 4, 1966.     

Relation between VLF phase deviations and solar X-ray fluxes during solar flares

Sudden phase anomalies (SPA's) observed in the phase of GBR 16 kHz VLF signals during the years 1977 to 1983 have been analysed in the light of their associated solar X-ray fluxes in the 0.5–4 Å and 1–8 Å bands. An attempt has been made to investigate the solar zenith angle () dependence of the integrated solar X-ray flux for producing SPA's. It is deduced from the observations for < 81° that the phase deviation increases linearly as a whole with increasing solar X-ray fluxes in these two bands. The threshold X-ray flux needed to produce a detectable SPA effect has been estimated to be 1.6 × 10^–4 ergcm^–2 s^–1 and 1.8 × 10^–3 ergcm^–2 s^–1 in the 0.5–4 Å and 1–8 Å bands, respectively. For both bands the average cross section for all atmospheric constituents at a height of 70 km is almost equal to the absorption cross section for the 3 Å X-ray emission.     

The relation between hard X-ray and transition-region line emission in solar flares

Observational evidence suggests that both the hard X-ray and ultraviolet emission from the impulsive phase of flares result from an electron beam. We present the results of model calculations that are consistent with this theory. The impulsive phase is envisioned as occurring in many small magnetically confined loops, each of which maintains an electron beam for only a few seconds. This model successfully matches several observed aspects of the impulsive phase. The corona is heated to less than 2 × 10⁶ K, maximum enhanced emission occurs in lines formed near 10⁵ K, and there is only slight enhancement between 10⁵ and 2 × 10⁶ K. The slope of the observed relationship between hard X-ray and Ov 1371 Å emission is also matched, but the relative emission is not. The calculations indicate that UV emission lines formed below a temperature of about 10⁵ K will arise predominantly from the chromospheric region heated by the electron beam to transition region temperatures. Emission lines formed at higher temperatures will be produced in the transition region. This should be detectable in density-sensitive line ratios. To account successfully for the impulsive UV emission, the peak temperature in the impulsively heated loops must remain below about 2 × 10⁶ K. Thus our model implies that the impulsive heating takes place in different loops from the hotter gradual phase emission.     

The magnetic nature of solar flares

The main challenge for the theory of solar eruptions has been to understand two basic aspects of large flares. These are the cause of the flare itself and the nature of the morphological features which form during its evolution. Such features include separating ribbons of H emission joined by a rising arcade of soft x-ray loops, with hard x-ray emission at their summits and at their feet. Two major advances in our understanding of the theory of solar flares have recently occurred. The first is the realisation that a magnetohydrodynamic (MHD) catastrophe is probably responsible for the basic eruption and the second is that the eruption is likely to drive a reconnection process in the field lines stretched out by the eruption. The reconnection is responsible for the ribbons and the set of rising soft x-ray loops, and such a process is well supported by numerical experiments and detailed observations from the Japanese satellite Yohkoh. Magnetic energy conversion by reconnection in two dimensions is relatively well understood, but in three dimensions we are only starting to understand the complexity of the magnetic topology and the MHD dynamics which are involved. How the dynamics lead to particle acceleration is even less well understood. Particle acceleration in flares may in principle occur in a variety of ways, such as stochastic acceleration by MHD turbulence, acceleration by direct electric fields at the reconnection site, or diffusive shock acceleration at the different kinds of MHD shock waves that are produced during the flare. However, which of these processes is most important for producing the energetic particles that strike the solar surface remains a mystery. Received 2 January 2001 / Published online 17 July 2001     

The initial development of solar flares

The extent to which the early phases of solar-flare development can be accounted for by a simple high-temperature chromospheric explosion model is investigated without involving a particular energy source. A model is developed in which it is shown that a point explosion in the lower chromosphere can be treated as a virtually instantaneous release of energy throughout a volume of radius R 100 km, which subsequently expands as a classical hydrodynamic blast wave in which R~ t ( < 2/3). This model is in substantial agreement with areal growth-rate observations of disk flares. An explanation for the fact that limb-flare observations can imply > 2/3 is suggested by considering the effects of the large atmospheric density gradient in the lower chromosphere on an upward travelling shock wave.     

The importance of solar white-light flares

The basic results of white-light flare (WLF) photometric and spectrographic observations are reviewed. WLFs represent the most extreme density conditions in solar optical flares and are similar to stellar flares in many respects. It is shown that WLFs originate in the low chromosphere and upper photosphere, and that their huge radiative losses remain difficult to explain within the context of known mechanisms of energy transport.Operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation. Partial support for the National Solar Observatory is provided by the USAF under a Memorandum of Understanding with the NSF.     

The explosive phase of solar flares

The explosive phase of a flare can be defined by a simple photometric measurement of H film records of the flare development. Using the quantitative definition, improved correlations are found between the start of the explosive phase and the start of 10.7 cm radio bursts and Sudden Frequency Deviations compared to earlier correlations of the same data using visual estimates of the start of the explosive phase. Explosive development may be confined to only part of a flare.     

Long-duration solar and stellar flares

According to one of the most popular classifications, solar flares may be assigned either to the category of small short-lived events, or to the category of large, long-duration two-ribbon (2-R) flares. Even if such abroad division oversimplifies the flare phenomenon, our knowledge of the characteristics of stellar flares is so poor, that it is worthwhile to investigate the possibility of adopting this classification scheme for stellar flares as well. In particular we will analyze Einstein observations of a long duration flare on EQ Peg to establish whether it might be considered as a stellar analogy of 2-R solar events. To this end we apply to EQ Peg data a reconnection model, developed originally for solar 2-R flares, and conclude that stellar observations are consistent with model predictions, although additional information is required to identify uniquely the physical parameters of the flare region. Application of the model to integrated observations of a 2-R solar flare, for which high spatial resolution data are also available, shows, however, that future integrated observations may allow us to solve the ambiguities of the model and use it as a diagnostic tool for a better understanding of stellar flares.     

The optical continuum of solar and stellar flares

A further development of the Kostyuk-Pikelner's model is presented. The response of the chromosphere heated by non-thermal electrons of the power-law energy spectrum has been studied on the basis of the numerical solution of the one-dimensional time-dependent equations of gravitational gas dynamics. The ionization and energy loss for the emissions in the Lyman and Balmer lines have been determined separately for the optically thin and thick L-line layers. Due to the initial heating, a higher-pressure region is formed. From this region, disturbances propagate upwards (a shock wave with a velocity of more than 1000 km s^-1) and downwards. A temperature jump propagates downwards, and a shock is formed in front of the thermal wave. During a period of several seconds after the beginning of this process, the temperature jump intensifies the downward shock wave and the large radiative loss gives rise to the high density jump ( ₂/ ₁ 100). The numerical solution has been analyzed in detail for the case heating of the ionized and neutral plasma, and a value of this heating is close to the upper limit of the admissible values. In this case, the condensation located between the temperature jump and the shock wave front, may emit in the observed optical continuum.In their essential features, the gas dynamic processes during the flares in red dwarf atmospheres are the same as those in the solar atmosphere. However, the high atmospheric densities, smaller height scale in red dwarf atmospheres, and greater energy of this processes in stellar flares, give rise, in practice, to the regular generation of optical continuum. The photometric parameters of a source with n 0¹⁵ cm^-3, T 9000 K, and _z 10 km are in a good agreement with observations.     

The relevance of solar flares to astrophysics

The physical mechanisms associated with solar flares are reviewed. The relevance of flare mechanisms to other astrophysical phenomena is discussed. In this context, specific models of quasars and radio galaxies, Sco X-1 and gamma-ray bursts are examined.     

The reason for magnetospheric substorms and solar flares

It has been proposed that magnetospheric substorms and solar flares are a result of the same mechanism. In our view this mechanism is connected with the escape, or attempted escape, of energized plasma from a region of closed magnetic field lines bounded by a magnetic bottle. In the case of the Earth, it must be plasma that is able to maintain a discrete auroral arc, and we propose that the cross-tail current connected to the arc is filamentary in nature to provide the field-aligned current sheet above the arc. A localized meander of such an intense current filament could be caused by a tearing instability in the neutral sheet. Such a meander will cause an inductive electric field opposing the current change everywhere. In trying to reduce the component of the induction electric field parallel to the magnetic field lines, the plasma must enhance the transverse or cross-tail component; this action leads to eruptive behavior, in agreement with tearing theories. This enhanced induction electric field will cause a discharge along the magnetic neutral line at the apex of the magnetic arches, constituting an impulsive acceleration of all charged particles originally near the neutral line. The products of this phase then undergo betatron acceleration for a second phase. This discharge eventually reduces the electric field along the neutral line, and thereafter the enclosed magnetic flux through the neutral line remains nearly constant. The result is a plasmoid that has definite identity; its buoyancy leads to its escape. The auroral breakup (and solar flare) is the complex plasma response to the changing electromagnetic field.     

The physical relationship between flares and surges observed in the extreme ultraviolet

A search was made for EUV surges among the EUV flares recorded by the Harvard spectroheliometer on ATM. Out of a large set of partial observations of such flares, a subset of 24 complete events was chosen. More than 24 associated surges were found, many of them multiple events. The flare-surge correlation is therefore considerably higher in the EUV than in H, presumably because EUV surges generally appear in emission, and in high contrast compared to H. In over 70% of the cases, the surges were found to grow out of the flare structure. Making reasonable assumptions, it was possible to infer the magnitude of the gas pressure gradient from the flare core into the surge by using the EUV intensity gradient. The inferred pressure gradient appears sufficient to drive the surge, although higher resolution observations will be required to corroborate this, and rule out the importance of magnetic Lorentz force.     

The solar albedo of hard X-ray flares

The calculations of Compton backscattering from the solar surface of flare X-rays performed by Tomblin (1972) are extended to higher energies. It is shown that the effect is even more pronounced in the 40 keV region and that it can lead to substantial corrections to the observed X-ray spectra.     

Current limitation and solar flares

A flare model based on force-free currents in the solar atmosphere is considered. The energy of the flare is supposed to be stored as magnetic energy in the current system. If the current density exceeds a certain critical limit an over-voltage may arise in the circuit which will give rise to a rapid release of the stored energy. At the end of the paper some results yielded by the model are compared with observational evidence of flares.     

Magnetospheric substorms and solar flares

Assuming that basic plasma processes associated with magnetospheric substorms and solar flares are similar and thus assuming also that a flare ribbon is produced by the impact of field-aligned current-carrying electrons on the chromosphere, a chain of processes leading to solar flares is considered, including the dynamo process in the photospheric level in the vicinity of bipolar sunspots, the formation of a sheet current in the lower coronal level, the interruption of the sheet current, the subsequent diversion of it to the chromosphere, the development of a potential drop along magnetic field lines, the acceleration of current-carrying electrons and their impact on the chromosphere, producing a pair of flare ribbons.     

The role of eruption in solar flares

This article focuses on two problems involved in the development of models of solar flares. The first concerns the mechanism responsible for eruptions, such as erupting filaments or coronal mass ejections, that are sometimes involved in the flare process. The concept of loss of equilibrium is considered and it is argued that the concept typically arises in thought-experiments that do not represent acceptable physical behavior of the solar atmosphere. It is proposed instead that such eruptions are probably caused by an instability of a plasma configuration. The instability may be purely MHD, or it may combine both MHD and resistive processes. The second problem concerns the mechanism of energy release of the impulsive (or gradual) phase. It is proposed that this phase of flares may be due to current interruption, as was originally proposed by Alfvén and Carlqvist. However, in order for this process to be viable, it seems necessary to change one's ideas about the heating and structure of the corona in ways that are outlined briefly.