High-energy X-rays and γ-rays from solar flares were discovered just over fifty years ago. Since that time, the standard for the interpretation of spatially integrated flare X-ray spectra at energies above several tens of keV has been the collisional thick-target model. After the launch of the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) in early 2002, X-ray spectra and images have been of sufficient quality to allow a greater focus on the energetic electrons responsible for the X-ray emission, including their origin and their interactions with the flare plasma and magnetic field. The result has been new insights into the flaring process, as well as more quantitative models for both electron acceleration and propagation, and for the flare environment with which the electrons interact. In this article we review our current understanding of electron acceleration, energy loss, and propagation in flares. Implications of these new results for the collisional thick-target model, for general flare models, and for future flare studies are discussed.

Abstract.We compare the coronal transient wave phenomenon discovered by SOHO extreme ultraviolet observations ("EIT waves") with the associated radio signature of a coronal shock wave (type II burst). 90% of the type II bursts are associated with an EIT wave. On average, the speed derived from the radio burst is about three times larger than the EIT wave speed. Within the sample, there is no correlation between the speeds of both tracers of a coronal disturbance. Under very general assumptions we conclude that both wave phenomena can be different signatures of the same fast magnetosonic disturbance.

Abstract. At the Sun, shock waves are produced either by flares and/or by coronal mass ejections (CMEs) and are regarded as the source of solar energetic particle events. They can be able to generate solar type II radio bursts. The non-radial propagation of a disturbance is considered away from an active region through the corona into the interplanetary space by evaluating the spatial behaviour of the Alfvén speed. The magnetic field of an active region is modelled by a magnetic dipole superimposed on that of the quiet Sun. Such a magnetic field structure leads to a local minimum of the Alfvén speed in the range 1.2-1.8 solar radii in the corona as well as a maximum of 740 km s −1 at a distance of 3.8 solar radii. The occurrence of such local extrema has important consequences for the formation and development of shock waves in the corona and the near-Sun interplanetary space and their ability to accelerate particles. It leads to a temporal delay of the onset of solar energetic particle events with respect to both the initial energy release (flare) and the onset of the solar type II radio burst.

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Abstract. Type II radio bursts recorded in the metric wavelength range are excited by MHD shocks traveling through the solar corona. They often expose the fundamental and harmonic emission band, both frequently being split in two parallel lanes that show a similar frequency drift and intensity behaviour. Our previous paper showed that band-splitting of such characteristics is a consequence of the plasma emission from the upstream and downstream shock regions. Consequently, the split can be used to evaluate the density jump at the shock front and to estimate the shock Mach number, which in combination with the shock speed inferred from the frequency drift provides an estimate of the Alfvén velocity and the magnetic field in the ambient plasma. In this paper such a procedure is applied to 18 metric type II bursts with the fundamental band starting frequencies up to 270 MHz. The obtained values show a minimum of the Alfvén velocity at the heliocentric distance R ≈ 2 amounting to v A ≈ 400-500 km s −1 . It then increases achieving a local maximum of v A ≈ 450-700 km s −1 at R ≈ 2.5. The implications regarding the process of formation and decay of MHD shocks in the corona are discussed. The coronal magnetic field in the range 1.3 < R < 3 decreases as R −3 to R −4 , or H −1.5 to H −2 if expressed as a function of the height. The results are compared with other estimates of the coronal magnetic field in the range 1 < R < 10. Combined data show that below H < 0.3 the magnetic field is dominated by active region fields, whereas above H = 1 it becomes radial, behaving roughly as B = 2 × R −2 with a plausible value of B ≈ 5 nT at 1 a.u.