Coronal heating mechanisms

As a result of the large body of data available from solar and stellar coronae, our understanding of the mechanisms responsible for the heating of coronal plasmas to temperatures of the order of ～ 10⁸ K has changed. The solar corona is highly structured by magnetic fields and the acoustic shocks which, according to early theories, should have acted as the coronal energy source have not been observed. Einstein Observatory data show moreover that coronae are present in most regions of the H-R diagram. The observed relationship between X-ray luminosity and rotational velocity in dwarf stars from spectral types F to M again suggests an active role for the magnetic fields.The basic picture which is emerging is that coronae in stellar types from F to M are produced because of the interaction of the magnetic field with the convective velocity fields generated in the photosphere resulting in MHD waves or currents which dissipate in the corona. X-ray emission in early type stars cannot be explained with this mechanism and the models which have been proposed for these stars are not yet completely satisfactory.     

Dynamical magnetic reconnection in Parker''s coronal heating model

The excitation (flares, ejections, heating, …) of the corona can be understood in terms of the dynamics of the confectively driven magnetized plasma. In particular, anomalous ohmic heating may be a consequence of the formation and rapid dissipation of small-scale magnetic fields in the corona. We have performed numerical simulations of the loop heating model proposed by Parker (1972, 1994), and have studied its dynamics and global power balance in order to assess its viability as a coronal heating candidate, with promising results. Our results suggest the following view of the small-scale dynamics of coronal loops. First of all, photospheric granular motions quasi-statically twist the magnetic field of the corona in a random-walk fashion. In topologically closed structures, the perpendicular magnetic energy increases, causing magnetic shear to build up at the quasi-separatrices of the resulting close-packed magnetic flux tubes. At some point, the boundary driving causes this stressed configuration to cross the threshold of an ideal time-scale MHD instability (possibly magnetic coalescence or resistive tearing) or a point of nonequilibrium and the field lines pinch toward a small-scale sheared configuration. It then becomes energetically favorable for dynamic reconnection to occur, producing narrow current sheets and an Ohmic heating rate sufficient to balance the input Poynting flux.     

Magnetic fluxtube reconnection

We present the results of 3D numerical simulations of initially discrete magnetic fluxtubes interacting via magnetic reconnection. The initial topology consists of two orthogonal fluxtubes. Each fluxtube has a uniform twist, force-free magnetic field specified by the Gold-Hoyle model. The fluxtubes are then forced together by an initial flow configuration consisting of two superimposed stagnation point flows. We observe three distinct types of interaction, which depend on the twist and on the Lundquist numbers, between the fluxtubes. For low twist the fluxtubes experience an elastic collision. For a higher twist complete reconnection is observed. If the Lundquist numbers are raised fluxtube tunneling occurs.     

Collisionless reconnection in the magnetotail

Recent developments regarding collisionless reconnection in current sheets with a finite normal magnetic field component (B_z) are reviewed. In 2-D x, z configurations the ion tearing mode is stabilized by the electron compressibility. When the y dependence is included, cross-field current instabilities can be excited. Of these, the drift kink mode appears to be particularly important. 3-D electromagnetic particle simulations indicate that this mode can act as the precursor to the growth of tearing modes and subsequent reconnection.     

Coronal mass-ejection events

Nearly fifteen years have passed since the discovery of coronal mass ejection events from the solar atmosphere. Progress in the interpretation of the observational results has led to a body of knowledge concerning the geometrical and evolutionary properties, physical characteristics, and the association of this type of event with other forms of solar activity. Recent interpretive results taken from the large body of observational data now available are discussed below in some detail. A classification system based on kinetic properties of these events is presented.     

Magnetic reconnection in solar coronal loops

Simulations of the evolution of kink modes in line-tied coronal loops are presented which demonstrate the occurrence of magnetic reconnection in the non-linear stage of the instability. In loops which do not carry a net axial current (and are confined by a potential purely axial field) the reconnection is limited to the initial current-carrying channel and no overall loss of confinement is observed. In loops which carry a net current on the other hand, reconnection progressively involves field lines at greater and greater distances from the axis and even regions where the field was initially potential, leading to a total disruption of the magnetic field topology.     

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.     

Double null points and magnetic reconnection

2D reconnection is possible only in connection with the existence of a singularity in the magnetic field line topology, associated with a magnetic null point or a current sheet. Both of these provide an X-type structure of the magnetic field where fields of opposite polarity meet and reconnect. In 3D a similar topology is found in a null point pair, when the null points are connected by a separator line. The separator is defined as the intersection line of the two null-point fan planes. This paper reports on the topological evolution of this configuration with respect to different perturbations emerging from imposed boundary velocities, using a nonlinear numerical approach.     

Linear theory of steady X-point magnetic reconnection

Slow magnetic reconnection at a neutral X-point of a two-dimensional magnetic field is studied in an incompressible viscous resistive fluid. It is shown analytically that the combined effect of viscosity and resistivity resolves the current singularity appearing in both the ideal and resistive magnetohydrodynamic approximations at the X-point and along the separatrices when the flow is allowed to cross them. A previous attempt had retained a weak singularity at third order. A two-parameter family of exact solutions describing the structure of the flow and current density distribution is found for the corresponding basic equations.     

Ion signatures of reconnection at the magnetopause

For a rotational MHD discontinuity the bulk flow is Alfvénic in the de Hoffmann-Teller frame. Using AMPTE/IRM data, we present ion distribution functions during three crossings of the dayside low-latitude magnetopause. For these crossings a well defined de Hoffmann-Teller frame can be found, but the field-aligned bulk flow is always slower than the Alfvén speed. Nevertheless, we find signatures in the distribution functions that provide clear evidence for magnetic reconnection: solar wind ions reflected off the magnetopause, “D-shaped” solar wind ion distributions in the boundary layer, and counterstreaming of solar wind ions and ionospheric ions in the boundary layer.     

Imaging of reconnection processes in hard X-rays

The Hard X-ray Spectrometer aboard the SMM detected several events of energy release late in the development of two-ribbon flares. One such event, at 21:12 UT on 21 May, 1980 (～ 20 min after the flare onset and 15 min after the peak of the impulsive phase) is studied in detail. The site of new brightening first became visible in hard X-rays (> 22 keV) and only afterwards showed up at lower energies. It was clearly located high in the corona so that one can identify it with energy release at the tops of newly formed post-flare loops. Thus, if the Kopp and Pneuman model of the loop formation is adopted, we may have imaged here a reconnection process in the solar corona. An attempt is made to estimate physical parameters at the reconnection site.     

Theory of magnetic reconnection in three dimensions

Magnetic reconnection in traditional, two-dimensional theory can be closely associated with topological properties of the magnetic field, separatrices, neutral lines, or separators. Such well-defined topological properties may no longer exist, or become hopelessly complicated, in more general three-dimensional configurations, that otherwise behave physically quite similarly to the two-dimensional configurations. We discuss generalized definitions of reconnection applied to such situations and illustrate typical properties with simulations of three-dimensional reconnection in the magnetotail.     

Three-dimensional magnetic reconnection regimes: A review

The magnetic field in many astrophysical plasmas – such as the solar corona and Earth’s magnetosphere – has been shown to have a highly complex, three-dimensional structure. Recent advances in theory and computational simulations have shown that reconnection in these fields also has a three-dimensional nature, in contrast to the widely used two-dimensional (or 2.5-dimensional) models. Here we discuss the underlying theory of three-dimensional magnetic reconnection. We also review a selection of new models that illustrate the current state of the art, as well as highlighting the complexity of energy release processes mediated by reconnection in complicated three-dimensional magnetic fields.     

Coronal manifestations of solar variability

This contribution is mostly about phenomenology of what we see in space–time evolution of coronal green line brightness (CGLB), including its cyclic variations. Our own database (1943–2001) of the coronal Fe XIV 530.3 nm emission line intensities is used to display different aspects of the large-scale CGLB regularities and cyclic behaviour. Hemispheric asymmetry and relation of the longitudinal CGLB distribution to the rotational characteristics of the solar corona are particularly underlined.     

Coronal signatures of accelerated electrons

Whereas hard X-rays, microwaves and gamma-rays trace flare accelerated electrons and ions interacting in the lowcorona and the chromosphere, imaging and spectral radio observations in the decimetric-dekametric domain provide signatures of non thermal electrons in the middle and upper corona. These latter radio observations, combined with X-ray, EUV and optical measurements, contain unique information on the various circumstances of electron acceleration whether they are associated with flares or not. In this paper we outline the results of multiwavelength studies which provide: (i) information on the magnetic structure at various spatial scales into which flare accelerated electrons are injected/accelerated and (ii) evidence for various sites of electron acceleration outside flares which are located in the corona at altitudes ranging typically from 0.1 to 1 R above the photosphere.     

3-D reconnection related to new emerging flux

We present evidences that emergence of new flux in the lower atmosphere leads to magnetic reconnection of field lines. In a first phase the phenomenon is observed in the chromosphere by the formation of dark filaments (arch filament system) which are overlaid by bright loops visible in soft X-rays. Different types of event appear according to the magnetic field configuration and the amount of energy involved. 3-D modelling of the photospheric magnetic field provides a new tool for understanding reconnection in real configurations. The observed chromospheric and coronal loops are good diagnostics for the modelling. We document our statement by examples obtained during coordinated campaigns with the H Multichannel Subtractive Double Pass spectrographs-MSDP (Pic du Midi and Tenerife) and the Yohkoh instruments.     

Coronal mass ejections in the heliosphere

With the advent of the NASA STEREO mission, we are in a position to perform unique investigations of the evolution of coronal mass ejections (CMEs) as they propagate through the heliosphere, and thus can investigate the relationship between CMEs and their interplanetary counterparts, so-called interplanetary CMEs (ICMEs). ICME studies have been principally limited to single-point, in-situ observations; interpretation of the in-situ characteristics of ICMEs has been used to derive a range of ICME properties which we can now confirm or refute using the STEREO imaging data. This paper is a review of early STEREO CME observations and how they relate to our currently understanding of ICMEs based on in-situ observations. In that sense, it is a first glance at the applications of the new data-sets to this topic and provides pointers to more detailed analyses. We find good agreement with in-situ-based interpretations, but this in turn leads to an anomaly regarding the final stages of a CME event that we investigate briefly to identify directions for future study.     

Reevaluation of thermosphere heating by auroral electrons

This paper presents a new calculation of neutral gas heating by precipitating auroral electrons. It is found that the heating rate of the neutral gas is significantly lower than previous determinations below 200 km altitude. The neutral gas heating arises from the many exothermic chemical reactions that take place from the ions and excited species created by the energetic electrons. The calculations show that less than half the energy initially deposited ends up heating the neutral gases. The rest is radiated or lost in the dissociation of O₂ because the O atoms do not recombine in the thermosphere. This paper also presents a new way of calculating the heating rate per ionization that can be used for efficient determination of the overall neutral gas heating for global thermosphere models. The heating rates are relatively insensitive to the neutral atmosphere when plotted against pressure rather than altitude coordinates. At high altitudes, the heating rates are sensitive to the thermal electron density and long-lived species. The calculations were performed with the Field Line Interhemispheric Plasma (FLIP) model using a 2-stream auroral electron precipitation model. The heating rate calculations in this paper differ from previous heating rate calculations in the treatment of backscattered electrons to produce better agreement with observed flux spectra. This paper shows that more realistic model auroral electron spectra can be obtained by reflecting the up going flux back to the ionosphere at the upper boundary of the model. In this case, the neutral gas heating rates are 20%–25% higher than when the backscattered flux escapes from the ionosphere.     

Magnetic reconnection in solar flares and corresponding radio bursts

The 2D MHD model of the flare magnetic reconnection shows that a reconnection activity, changes of the magnetic field topology and generation of waves are connected. It is found that after the phase of a quasi-stationary reconnection in the extended current sheet above the flare arcade the tearing mode instability produces the plasmoids which then can interact and generate MHD waves. Results of particle-in-cell simulations of the tearing processes, which accelerate electrons, are mentioned. Then all these processes are discussed from the point of view of possible radio emissions. While shocks can contribute to the type II radio burst, the superthermal electrons trapped in plasmoids can generate so called drifting pulsating structures. Furthermore, regions with the MHD turbulence may manifest themselves as the lace or dm-spike bursts.