Enthalpy flux cooling of the solar corona

The differential emission measure profile for quiet and flaring solar regions is considered, using a model in which the principal downflow of heat is due to the enthalpy of downward-flowing material, rather than conduction. It is found that the emission measure profile for quiet solar regions is matched well by a downward particle number flux which decreases with temperature. This would be expected if this particle flux is due to heated spicular material falling back onto the chromosphere. In flaring regions, however, a particle flux which increases with temperature is required to explain the steep emission measure profile. This could be a result of mass motions downward out of the flaring loops.     

Influence of interplanetary shocks on solar particle events

Energetic particles, ejected from the Sun during solar flare events, may encounter interplanetary plasma/field conditions, which deviate considerably from the quiet time values used normally to describe the particle propagation. This is due to the presence of a hydromagnetic shock, which is emitted from the Sun at the time of the explosion. In a theoretical blast wave model, which incorporates the interaction with plane polarized Alfvén waves, we have analysed the changes in different terms of the Fokker-Planck equation, which describes energetic particle propagation. In this treatment, the shock influence on energy changes and on the transport coefficients are discussed.     

Long-Term Variation of the Corona in Quiet Regions

Using Hinode EUV Imaging Spectrometer (EIS) spectra recorded daily at Sun center from the end of 2006 to early 2011, we studied the long-term evolution of the quiet corona. The light curves of the higher temperature emission lines exhibit larger variations in sync with the solar activity cycle while the cooler lines show reduced modulation. Our study shows that the high temperature component of the corona changes in quiet regions, even though the coronal electron density remains almost constant there. The results suggest that heat input to the quiet corona varies with the solar activity cycle.     

Partcle precipitation caused by a single whistler-mode wave injected into the magnetosphere

The resonant interaction of electrons with a coherent whistler-mode wave in the magnetosphere, and corresponding particle precipitation through the loss-cone, are considered. We show that, due to the inhomogeneity of the magnetic field, the phase untrapped resonant electrons play a basic role in the precipitation process. An effective change of their pitch-angles near the loss-cone is calculated and particle fluxes are estimated for quiet magnetospheric conditions (weak diffusion without the wave). It appears that observation of the precipitation caused by a single whistler-mode wave is within the scope of experimental possibilities. The duration of the precipitation process is of the order of the electron bounce period. It is also shown that precipitating current may produce an observable magnetospheric disturbance with a time characteristic of the order of the bounce period.     

Energy balance of the corona and the origin of quasi-stationary high-speed solar wind streams

The energy balance of open-field regions of the corona and solar wind and the influence of the flow geometry in the corona upon the density and temperature, are analyzed. It is found that the energy flux arriving at the corona is constant for the corona's open regions with different flow geometries. For the waves heating the corona and solar wind, the dependence of the absorption coefficient on the corona's plasma density is found to be within the range of distances r=1.05–1.5R . It is shown that the wave absorption is more dependent on electron density than the coronal emission. It is this difference that causes lower-density coronal holes to be colder than quiet regions. It is found that the additional energy flux necessary for providing energy balance of the corona and for producing solar wind is a flux of Alfvén waves, which can provide the energy needed for producing quasi-stationary high-speed solar wind streams. Theoretical models of coronal holes and the question of why the high-speed solar wind streams are precisely flowing out of coronal holes, are discussed.     

Chondrule collisions in shock waves

Abstract— Detailed numerical models have shown that solar nebula shock waves would be able to thermally process chondrules in a way that is consistent with experimental constraints. However, it has recently been argued that the high relative velocities that would be generated between chondrules of different sizes immediately behind the shock front would lead to energetic collisions that would destroy the chondrules as they were processed rather than preserving them for incorporation into meteorite parent bodies. Here the outcome of these collisions is quantitatively explored using a simple analytic expression for the viscous dissipation of collisional energy in a liquid layer. It is shown that molten chondrules can survive collisions at velocities as high as a few hundred meters per second. It is also shown that the thermal evolution of chondrules in a given shock wave varies with chondrule size, which may allow chondrules of different textures to form in a given shock wave. While experiments are needed to further constrain the parameters used in this work, these calculations show that the expected outcomes from collisions behind shock waves are consistent with what is observed in meteorites.     

Simulation of solar flare particle interaction with interplanetary shock waves

In order to study the propagation of solar cosmic rays in interplanetary space a computer program has been developed using a Monte-Carlo technique, which traces the histories of particles released impulsively at the Sun. The particle propagation model considers the adiabatic deceleration during the convective and diffusive transport of the particles, and the model of the interplanetary medium incorporates a radially expanding blast wave which exerts a sweeping action on the particles and accelerates them through the first-order Fermi process. It is shown that energetic storm particle events cannot be simulated by assuming a pure sweeping action of the interplanetary blast wave, but that energization of the particles while reflected at the shock can explain many observed features of such events.     

Determination and analysis of coronal hole radio spectra

Solar radio maps obtained by our group and others over a wide wavelength range (millimeter to meter) and over a considerable time span (1973–1978) have allowed us to compute the radio spectrum of an average coronal hole, i.e., the brightness temperature inside a coronal hole normalized by the brightness temperature of the quiet Sun outside the coronal hole measured at several different radio wavelengths. This radio spectrum can be used to obtain the changes of the quiet Sun atmosphere inside coronal holes and also as an additional check for coronal hole profiles obtained by other methods. Using a standard solar atmosphere and a computer program which included ray tracing, we have tried to reproduce the observed radio spectrum by computing brightness temperatures at many different wavelengths for a long series of modifications in the electron density, neutral particle density and temperature profiles of the standard solar atmosphere. This analysis indicates that inside an average coronal hole the following changes occur: the upper chromosphere expands by about 20% and its electron density and temperature decrease by about 10%. The transition zone experiences the largest change, expanding by a factor of about 6, its electron density decreases by a similar factor, and its temperature decreases by about 50%. Finally in the corona the electron density decreases by about 20% and the temperature by about 15%.     

Key Properties of Solar Chromospheric Line Formation Process

The distribution or wavelength-dependence of the formation regions of frequently used solar lines, Hα，Hβ，CaIIH and CaII8542,in quiet Sun,faint and bright flares is explored in the unpolarized case.We stress four aspects characterising the property of line formation process:1) width of line formation core;2) line formation region;3)influence of the temperature minimum region; and 4) wavelength ranges within which one can obtain pure chromospheric and photospheric filtergrams.It is shown that the above four aspects depend strongly on the atmospheric physical condition and the lines used. The formation regions of all the wavelength points within a line may be continuously distributed over one depth domain or discretely distributed because of no contribution coming from the temperature minimum region, an important domain in the solar atmosphere that determines the distribution pattern of escape photons. On the other hand, the formation region of one wavelength point may cover only one height range or spread over two domains which are separated again by the temperature minimum region. Different lines may form in different regions in the quiet Sun. However, these line formation regions become closer in solar flaring regions. Finally, though the stratification of line-of-sight velocity can alter the position of the line formation core within the line band and result in the asymmetry of the line formation core about the shifted line center, it can only lead to negligible changes in the line formation region or the line formation core width. All these results can be instructive to solar filtering observations.     

Charged particle acceleration in the front of the shock wave bounding supersonic solar wind

The process of cosmic ray acceleration in the front of the spherical shock wave bounding the supersonic solar wind is studied. On the basis of our analytical solution of the transport equation, the energy and spatial distributions of cosmic ray intensity and anisotropy are investigated. It is shown that the shape of accelerated particle spectrum is determined by the medium compressibility at the shock front and by cosmic ray modulation parameters.     

Solar wind heating beyond 1 AU

The effect of an interplanetary atomic hydrogen gas on solar wind proton, electron and α-particle temperatures beyond 1 AU is considered. It is shown that the proton temperature (and probably also the α-particle temperature) reaches a minimum between 2 AU and 4 AU, depending on values chosen for solar wind and interstellar gas parameters. Heating of the electron gas depends primarily on the thermal coupling of the protons and electrons. For strong coupling (whenT _p ≳T _e ), the electron temperature reaches a minimum between 4 AU and 8 AU, but for weak coupling (Coulomb collisions only), the electron temperature continues to decrease throughout the inner solar system. A spacecraft travelling to Jupiter should be able to observe the heating effect of the solar wind-interplanetary hydrogen interaction, and from such observations it may be possible of infer some properties of the interstellar neutral gas. Currently a National Research Council Resident Research Associate.     

Evolution of the Electron Distribution Function in the Whistler Wave Turbulence of the Solar Wind

The electron distribution functions from the solar corona to the solar wind are determined in this paper by considering the effects of the external forces, of Coulomb collisions and of the wave – particle resonant interactions in the plasma wave turbulence. The electrons are assumed to be interacting with right-handed polarized waves in the whistler regime. The acceleration of electrons in the solar wind seems to be mainly due to the electrostatic potential. Wave turbulence determines the electron pitch-angle diffusion and some characteristics of the velocity distribution function (VDF) such as suprathermal tails. The role of parallel whistlers can also be extended to small altitudes in the solar wind (the acceleration region of the outer corona), where they may explain the energization and the presence of suprathermal electrons.     

Equations of anisotropic hydrodynamics for electron component of the ionospheric plasma

In this paper we have derived a set of transport equations for thermal electron component of the ionospheric plasma in the presence of an anisotropy of the electron energy distribution. Expressions are calculated in a 16-moment approximation for the moments of integrals of elastic and inelastic collisions of thermal electrons with basic neutral ionospheric components. The obtained moments determine variations of the hydrodynamical parameters, such as macroscopic velocity, pressure tensor, viscosity tensor, heat fluxes in respective equations due to collisions. The results have been obtained for an arbitrary degree of electron temperature anisotropy.     

太阳风电子动力学不稳定性

电子是太阳风粒子中最为重要的组分之一,它可以通过多种机制对太阳风产生影响.太阳风中的电子通常具有温度各向异性和束流两种非热平衡分布特征,这些偏离热平衡分布的特征可以通过波粒相互作用激发电子不稳定性和等离子体波动,激发的等离子体波动又可以通过波粒相互作用调制太阳风粒子的分布,从而加热太阳风中的背景粒子.因此电子动力学不稳定性在太阳风的演化过程中扮演了极为重要的角色.详细介绍了太阳风中常见的电子动力学不稳定性,并基于等离子体动力论,详细介绍太阳风传播过程中所出现的各种不稳定性,尤其是在近日球层和太阳大气区域所出现的电子声热流不稳定性以及低混杂热流不稳定性,并分析其波粒相互作用机制,以便更加深入地研究太阳风传播过程中的电子分布函数演化.     

The effects of scattering on quiet Sun emission at frequencies less than 200 MHz

The problem of predicting the radio emission from the quiet Sun for meter and decameter waves may be formulated in terms of a standard radiative transfer problem with conservative scattering. One may therefore avoid the numerical complications involved in using a ray-tracing approach which incorporates a Monte-Carlo routine for representing scattering. The brightness and extent of the radio Sun are calculated for several values of the electron temperature and scattering parameter of Steinberg et al. (1971). It is concluded that the temperature and density model of Newkirk (1967) for solar maximum reasonably represents the observations. However, some observations appear to be inconsistent with scattering models and further observations are needed. It is shown (in Appendix B) that the standard ray-tracing technique incorporating a Monte-Carlo routine for the scattering may be replaced by a diffusion approximation.     

The Influence of Ion-Acoustic Turbulence on the Electron Acceleration in the Reconnecting Current Sheet

Through solving the single electron equation of motion and the Fokker-Planck equation including the terms of electric field strength and ion-acoustic turbulence, we study the influence of the ion-acoustic wave on the electron acceleration in turbulent reconnecting current sheets. It is shown that the ion-acoustic turbulence which causes plasma heating rather than particle acceleration should be considered. With typical parameter values, the acceleration time scale is around the order of 10^-6 s, the accelerated electrons may have approximately a power-law distribution in the energy range 20 ～100 keV and the spectral index is about 3～10, which is basically consistent with the observed hard X-ray spectra in solar flares.     

On an efficient shock wave generation mechanism in the quiet solar transition region

Two competing fundamental hypotheses are usually postulated in the solar coronal heating problem: heating by nanoflares and heating by waves. In the latter it is assumed that acoustic and magnetohydrodynamic disturbances whose amplitude grows as they propagate in a medium with a decreasing density come from the convection zone. The shock waves forming in the process heat up the corona. In this paper we draw attention to yet another very efficient shock wave generation process that can be realized under certain conditions typical for quiet regions on the Sun. In the approximation of stationary dissipative hydrodynamics we show that a shock wave can be generated in the quiet solar chromosphere–corona transition region by the fall of plasma from the corona into the chromosphere. This shock wave is directed upward, and its dissipation in the corona returns part of the kinetic energy of the falling plasma to the thermal energy of the corona. We discuss the prospects for developing a quantitative nonstationary model of the phenomenon.     

Effect of Coulomb collisions on the particle acceleration in collapsing magnetic traps

The problem of particle acceleration in collapsing magnetic traps in the solar corona has been solved by taking into account the particle scattering and braking in the high-temperature plasma of solar flares. The Coulomb collisions are shown to be weak in traps with lifetimes t _l < 10 s and strong for t _l > 100 s. In the approximation of strong collisions, collapsing magnetic traps are capable of confining up to 20% of the injected particles in the corona for a long time. In the collisionless approximation, this value exceeds 90%. The question about the observational manifestations of collisions is examined. For collision times comparable to t _l , the electron spectrumat energies above 10 keV is shown to be a double-power-law one. Such spectra were found by the RHESSI satellite in flares.     

Acceleration of the solar wind by whistler waves from coronal holes

We investigate the possibility of an additional acceleration of the high speed solar wind by whistler waves propagating outward from a coronal hole. We consider a stationary, spherically symmetric model and assume a radial wind flow as well as a radial magnetic field. The energy equation consists of (a) energy transfer of the electron beam which excites the whistler waves, and (b) energy transfer of the whistler waves described by conservation of wave action density. The momentum conservation equation includes the momentum transfer of two gases (a thermal gas and an electron beam). The variation of the temperature is described by a polytropic law. The variation of solar wind velocity with the radial distance is calculated for different values of energy density of the whistler waves. It is shown that the acceleration of high speed solar wind in the coronal hole due to the whistler waves is very important. We have calculated that the solar wind velocity at the earth's orbit is about equal to 670 km/sec (for wave energy density about 10^?4 erg cm^?3 at 1.1R⊙). It is in approximate agreement with the observed values.     

Neutral corpuscular energy flux by charge-transfer collisions in the vicinity of the sun

Charge-transfer collisions between solar-wind protons and neutral interstellar hydrogen in the vicinity of the sun have been considered. Due to the focusing effect of the sun's gravitational field interstellar particles entering the solar system in free flights produce a specific density distribution in the circumsolar space. On their way from the sun to the orbit of the earth solar protons will therefore generate fast neutrals by collisions with neutral hydrogen. Depending on the position at its orbit the earth will be hit by these fast neutrals which will come down directly into the thermosphere and will produce temperature and density increases. It is shown that the corpuscular energy flux connected with these fast neutrals will have a semi-annually varying profile along the earth's orbit. Interstellar particle densities of about 5 cm^–3 at infinity would produce energy fluxes of the order of 0.1 erg/cm² sec. Assuming a specific proper motion of interstellar matter surrounding the solar system we obtain a neutral corpuscular energy flux having nearly the same shape and phase as the wellknown semi-annual effect in atmospheric temperatures and densities. Collision-generated, fast neutrals reaching the earth could therefore possibly give an explanation of this effect.Mitteilungen der Astronomischen Institute Bonn, Nr. 102.