Nuclear processes and accelerated particles in solar flares

Nuclear processes and particle acceleration in solar flares are reviewed. The theory of gamma-ray and neutron production is discussed and results of calculations are compared to gamma-ray, neutron, and charged-particle observations from solar flares. The implications of these comparisons on particle energy spectra, total numbers, anisotropies, electron-to-proton ratios, as well as on acceleration mechanisms and the interaction site, are presented. The information on elemental and isotopic abundances derived from gamma-ray observations is compared to abundances obtained from escaping accelerated particles and other sources.NAS/NRC Resident Research Associate.     

Observational support of reconnection in solar flares

We present a detailed analysis of the magnetic topology of flaring active region. TheH kernels are found to be located at the intersection of the separatrices with the chromosphere when the shear, deduced from the fibrils or/and transverse magnetic field direction, is taken into account. We show that the kernels are magnetically connected by field lines passing close to the separator. We confirm, for other flares, previous studies which show that photospheric current concentrations are located at the borders of flare ribbons. Moreover we found two photospheric current concentrations of opposite sign, linked in the corona by field lines which follow separatrices. These give evidence that magnetic energy is released by reconnection processes in solar flares.     

Interplanetary shock waves generated by solar flares

Recent observational and theoretical studies of interplanetary shock waves associated with solar flares are reviewed. An attempt is made to outline the framework for the genesis, life and demise of these shocks. Thus, suggestions are made regarding their birth within the flare generation process, MHD wave propagation through the chromosphere and inner corona, and maturity to fully-developed coronal shock waves. Their subsequent propagation into the ambient interplanetary medium and disturbing effects within the solar wind are discussed within the context of theoretical and phenomenological models. The latter — based essentially on observations — are useful for a limited interpretation of shock geometric and kinematic characteristics. The former — upon which ultimate physical understanding depends — are used for clarification and classification of the shocks and their consequences within the solar wind. Classification of limiting cases of blast-produced shocks (as in an explosion) or longer lasting ejecta (or piston-driven shocks) will hopefully be combined with the study of the flare process itself.The theoretical approach, in spite of its contribution to clarification of various concepts, contains some fundamental limitations and requires further study. Numerical simulations, for example, depend upon a non-unique set of multi-parameter initial conditions at or near the Sun. Additionally, the subtle but important influence of magnetic fields upon energy transport processes within the solar wind has not been considered in the numerical simulation approach. Similarity solutions are limited to geometrical symmetries and have not exploited their potential beyond the special cases of the blast and the constant-velocity, piston-driven shock waves. These continuum fluid studies will probably require augmentation or even replacement by plasma kinetic theory in special situations when observations indicate the presence of anomalous transport processes. Presently, for example, efforts are directed toward identification of detailed shock structures (as in the case of Earth's bow shock) and of the disturbed solar wind (such as the piston).Further progress is expected with extensive in situ and remote monitoring of the solar wind over a wide range of heliographic radii, longitudes and latitudes.This paper is a revised and updated version of an invited review originally presented at the IUGG XV General Assembly, Moscow, U.S.S.R., 2–14 August 1971.     

Observational studies of gamma-rays and neutrons from solar flares

Gamma-ray observations from HINOTORI satellite and possible neutron observations from the Tokyo neutron monitor are reviewed. Time histories of gamma-ray and X-ray emissions for both typical impulsive and gradual flares are discussed in connection with the particle acceleration time. The gamma-ray spectral hardening observed around 400 keV is explained from superimposition of two different electron bremsstrahlung spectra. Proton-energy spectra derived from the gamma-ray observations are compared with the solar energetic particle spectra in interplanetary space. The weak correlation between the gamma-ray fluence and the proton flux is discussed in connection with the particle trapping and escaping in the flare region. The limb darkening of the 2.22 MeV line resulting from neutron-proton capture is interpreted in terms of the attenuation by the Compton scattering in the photosphere. Possible solar neutron events recorded by the Tokyo neutron monitor are presented and the correlation between the gamma-ray fluence and the neutron fluence are described.     

Particle acceleration in solar flares and some related phenomena

Observational features concerning solar energetic particles are compactly reviewed with some emphasis on the spectra and time histories. Velocity dependent characteristics in the energy spectra are pointed out, and compared to the results of the interplanetary shocks. A shock drift acceleration is introduced in order to interpret the observational features, especially a very fast acceleration to MeV energies within an order of second. There is a strong evidence of the shock drift acceleration in the interplanetary shocks. When some conditions are satisfied in the corona, only one or several encounters of particles with a near perpendicular shock accelerates protons to gamma-ray emitting energies (> 10 MeV). Pre-acceleration is inevitable for any kind of acceleration mechanisms in solar flares. To fulfill the requirements from the abundance ratios between various species of accelerated ions, pre-acceleration to the same velocities before the injection into a main acceleration process turns out to be absolutely necessary.     

Energy build-up and release mechanisms in solar and auroral flares

Flare phenomena in the solar atmosphere and in the terrestrial magnetosphere exhibit many similarities. The mechanical energy of enhanced photospheric motion is converted and stored in the form of magnetic potential energy in sunspot fields, which is analogous to the case of the growth phase of magnetospheric substorms. The energy release during the explosive phase is initiated by a sudden collapse in the magnetic field topology and the X-type magnetic neutral point is created in the corona. Subsequent electrical discharge takes place in the form of an intense electrojet current flowing in the base of the chromosphere at the altitude where the Cowling conductivity is a maximum. It is suggested that the acceleration of particles by field-aligned electric fields and the Ohmic heating in the chromosphere result in major features of solar flares.This article also appears inSolar Physics 40 (1975) 217–226. By way of exception this paper is reproduced here for the sake of completeness.     

Space-system timekeeping in the presence of solar flares

It has become increasingly clear that the enhanced space radiation environment associated with solar activity can play havoc with satellite systems. An important special case of this problem concerns precise timekeeping among orbiting communications satellites, where the loss of synchronization can mean the loss of communications. Here, we discuss one satellite system's solution to this problem, where radiation-sensitive spacecraft crystal oscillators are "slaved" to radiation-insensitive spacecraft atomic clocks.     

Variations of the magnetic fields in large solar flares

We present preliminary results from high resolution observations obtained with the Michelson Doppler Imager (MDI) instrument on the SOHO of two large solar flares of 14 July 2000 and 24 November 2000. We show that rapid variations of the line-of-sight magnetic field occured on a time scale of a few minutes during the flare explosions. The reversibility/irreversibility of the magnetic field of both active regions is a very good tool for understanding how the magnetic energy is released in these flares. The observed sharp increase of the magnetic energy density at the time of maximum of the solar flare could involve an unknown component which deposited supplementary energy into the system. This revised version was published online in August 2006 with corrections to the Cover Date.     

Auroral spectroscopy

Recent advances in the techniques of auroral spectroscopy are discussed. Present knowledge of the spectrum is reviewed including a collection of semi-empirical band intensities for the observed band systems. Variations in the spectrum in different colour classes and types of aurora are reviewed together with an account of recent developments in the theories of the excitation mechanism of auroral emissions.     

The analysis of energy release in solar flares based on X-ray observations

Extended review of selected papers which deal with the problem of flare heating in solar coronal loops is presented. Discussed methods of the analysis of flare heating based on the X-ray observations have been worked out using the Palermo-Harvard hydrodynamic code. The case is presented when the assumption of the uniform heating across the loop is made. The existence of multiple elementary heating episodes is postulated as well. Next the possibility of the non-uniform heating across the loop is assumed and its manifestation in the X-ray observations is investigated. The application of proposed methods of the analysis to the observations of solar flares in X-rays is presented.     

New Perspectives on Auroral Morphology

The visual aurora takes on a variety of forms. Aurora has a tendency to appear first as very thin, highly structured forms. Over time, these tend to diffuse creating much thicker forms. It is suggested that the extreme variety of auroral forms can be understood in terms of one acceleration mechanism to produce a narrow, field-aligned beam and another process that scatters electrons into trapped orbits. The scattering is due to beam- plasma interactions that generate waves on the upper-hybrid resonance curve. These waves are effective in scattering electrons from parallel to perpendicular directions. The diffuse forms are therefore caused by precipitation of quasi-trapped electrons that have drifted from the field lines on which they were accelerated. Electrons scattered into trapped orbits may also constitute the seed population for the electron radiation belts. It is also suggested that the electron beams are accelerated by inertial Alfven waves that propagate current filaments from the turbulent region in the near-Earth plasma sheet to the auroral zone ionosphere. Electrons can be accelerated by becoming trapped in inertial Alfven waves whose phase velocity increases as they propagate toward the Earth. Specific numerical simulations that could give substance to these suggestions are proposed.     

Auroral arc formation: Kinetic and MHD effects

Recent observations by the Dynamics Explorer satellites indicate that auroral field lines are not equipotentials. The field-aligned conductivity is the same for up and downgoing currents and of the order of several 10^–8 to several 10^{–9 –1}m^–2. This is not in agreement with models based on the resistance provided by the magnetic mirror force alone. It is suggested that the resistivity is due to damping of kinetic shear-Alfven wave packets. This damping may be nonlinear in nature or Landau damping. We briefly review the properties of kinetic Alfven waves.     

Stellar flares

Radio and X-ray observations of stellar flares provide the most direct probes of energy relaase particle acceleration, and energy transport on stars other than the Sun. In this review, the observational basis for our understanding of the flare phenomenon on other stars is briefly described and outstanding interpretive and theoretical issues are discussed. I shall confine my attention to objects which are solar-like, to the extent that they possess deep convective envelopes and display activity which is presumed to be magnetic in origin. These include pre-main sequence objects, classical flare stars, and close binaries. Future directions are briefly discussed.     

Auroral investigations by means of rockets

A survey of rocket experiments undertaken to study auroral zone events includes summary information about instrumentation and results in the field of energetic electrons and protons, of charged particle densities, of optical observations, of magnetic and electric fields, of bremsstrahlung X-rays, of thermal electrons, and of production rates. Other auroral investigations except those involving rockets have been largely ignored.     

Solar Wind Control of the Magnetospheric and Auroral Dynamics

A dependence of the polar cap magnetic flux on the interplanetary magnetic field and on the solar wind dynamic pressure is studied. The model calculations of the polar cap and auroral oval magnetic fluxes at the ionospheric level are presented. The obtained functions are based on the paraboloid magnetospheric model calculations. The scaling law for the polar cap diameter changing for different subsolar distances is demonstrated. Quiet conditions are used to compare theoretical results with the UV images of the Earth’s polar region obtained onboard the Polar and IMAGE spacecrafts. The model calculations enable finding not only the average polar cap magnetic flux but also the extreme values of the polar cap and auroral oval magnetic fluxes. These values can be attained in the course of the severe magnetic storm. Spectacular aurora often can be seen at midlatitude during severe magnetic storm. In particularly, the Bastille Day storm of July 15–16, 2000, was a severe magnetic storm when auroral displays were reported at midlatitudes. Enhancement of global magnetospheric current systems (ring current and tail current) and corresponding reconstruction of the magnetospheric structure is a reason for the equatorward displacement of the auroral zone. But at the start of the studied event the contracted polar cap and auroral oval were observed. In this case, the sudden solar wind pressure pulse was associated with a simultaneous northward IMF turning. Such IMF and solar wind pressure behavior is a cause of the observed aurora dynamics.     

Chapter 5 - Statistics and Mapping of Auroral Features

Space Science Reviews -