During periods of sporadic flare activity, the Sun releases energy stored in the magnetic field into the plasma of the solar atmosphere. This is an extremely efficient process, with a large fraction of the magnetic energy going into plasma particles. The solar flares are accompanied by prompt electromagnetic emission virtually over the entire electromagnetic spectrum from gamma-rays down to radio frequencies. The Sun, through its activity, also plays a driving role in the Sun-Earth system that substantially influences geophysical space. Solar flare energetic particles from the Sun are detected in interplanetary space by in-situ measurements making them a vital component of the single Sun-Earth system. Although a qualitative picture is generally agreed upon, many processes solar flare processes are poorly understood. Specifically, the processes of acceleration and propagation of energetic particles interacting on various physical scales remain major challenges in solar physics and basic plasma physics. In the talk, I will review the current understanding of solar flare physics focusing on recent observational progress toward the understanding of energy release and particle acceleration in solar flares, which became possible due to the numerous spacecraft and ground-based observations. The unprecedented quality of the data in combination with novel data analyses techniques have revealed previously unknown details of solar flares, but have also brought new challenges. The diagnostics of radiation processes, particle transport, and acceleration, using both spectroscopic and imaging techniques will be discussed in the view of the planned investigations with ESA’s Solar Orbiter, which promises to open new era in the space-based observations and to improve the current understanding of solar flare processes, contributing to the predictions of disturbances of solar origin in the interplanetary medium.

In the classical solar flare model particle acceleration is thought to take place near or above the top of magnetic loops in the corona, with electrons being accelerated both downwards toward the dense chromosphere as well as upwards into interplanetary space. Hard X-ray images typically show the signature of the downward accelerated electrons as bright chromospheric footpoint emission. The outward moving beam is expected to produce hard X-rays, as well. However, due to the low coronal density, this emission will be much fainter than the chromospheric emission and thus is difficult to observe in the presence of bright footpoints with current X-ray imagers such as RHESSI. Thus far we rely on radio spectroscopy and imaging to gain information on the outward component. We explore the potential of STIX, in conjunction with a possible second X-ray imager or X-ray spectrograph to directly measure not only the downward component, but also the upward component of an accelerated electron beam. There are two scenarios for such stereoscopic observations. With its lower background compared with RHESSI, STIX should be able to image the outward component directly in larger limb occulted flares. In a second scenario, STIX will provide imaging spectroscopy of the chromospheric footpoints while another instrument observes the outward component. Combining such stereoscopic observations will give the first comprehensive direct X-ray study of flare accelerated electron beams and is crucial to answer questions such as the total number, and the relative number of upward and downward accelerated electrons.

One of the spectral signatures of the solar atmosphere that occurs often tens of minutes before a flare begins is that of an increase in the EUV, X-ray spectral line width (often termed the non-thermal velocity). There have been several explanations for this behaviour which include turbulence and Alfven wave propagation. Since Hinode was launched 6 years ago, there have now been a significant number of flares observed. In this paper we concentrate on 5 flares that have a GOES classification greater than M, where we have Hinode EUV Imaging Spectrometer data along with Solar Dynamics Observatory AIA and HMI data. In each case we travel the behaviour of the photosphere and how the atmosphere responds and aim to determine what causes the enhancement in the spectra line width.

Solar fares are very spectacular, and are associated with various phenomena. They, especially eruptive ones, affect the surrounding corona globally, although flares as themselves are rather compact. Coronal shocks or waves are one of such flare-related phenomena. Although Moreton waves and X-ray waves are well explained with MHD fast mode shocks propagating in the corona, there still remains a big problem on the nature of the waves, since they are very rare phenomena. On the other hand, EIT waves (or EUV waves) have been paid attention to as another phenomenon of coronal disturbances. However, the physical features (velocity, opening angle, and so on) are much different from those for Moreton waves and X-ray waves. Thanks to the recent developments on solar observations by STEREO, SDO, Hinode, and ground-based instruments, we have now detailed information on these coronal disturbances in the low corona. We review recent observations on H-alpha Moreton waves, EUV waves, X-ray waves, and their associations with plasma eruptions and CMEs. We also discuss the possible observations of coronal disturbances with Solar Orbiter.

Over the last 15 years continuous monitoring of the Sun provided by the space based observatories like SOHO gave us a new view of Coronal Mass Ejections (CMEs). In particular, data acquired by the SOHO/UV Coronagraph Spectrometer (UVCS) allowed us to study 3D expansion velocities of CME plasma, estimate the CME thermal energy content, characterize CME-driven shocks, study the evolution of post-CME current sheets and small scale eruptions (like narrow CMEs, polar jets, streamer puffs). At the same time, coronagraphic white light images acquired by space-based coronagraphs (like SOHO/LASCO and STEREO/COR) provided unique information on the CME kinematic, mass, density and (together with radio data) on CME-driven shocks, allowing the first statistical studies on these phenomena and their 3D reconstructions, thanks to multiple view-points now available. Nevertheless, many problems on CME origin, energetic and evolution are not yet solved. After a review of the main results derived so far on these topics, this presentation will focus on the new possibilities that will be offered by the Solar Orbiter mission and in particular by the METIS instrument, in coordination with other future missions/observatories like Solar Probe Plus.

In recent years there we have seen confirmation that the photospheric magnetic field undergoes abrupt and permanent changes in response to a solar flare (e.g., Wang et al. 2002, Sudol& Harvey 2005), provoking considerable debate about the interpretation of these changes. Hudson et al. (2008) explored ways in which the photospheric vector magnetic field can vary during a solar flare or a CME, postulating that the back reaction, or magnetic implosion, should result in the magnetic fields becoming more horizontal. However, as the flux rope erupts in a CME, its footpoints remain connected to the Sun, and consequently the magnetic field should stretch out towards the interplanetary space. At the same time, some of the field lines overlying the flux rope would be stretched up as well (e.g., Chen et a. 2002). The footpoints of these stretched field lines will be the sites of coronal dimmings. Here we explore whether or not the signature of the stretching-out of field lines involved in the above scenario is observable in the photospheric magnetic field of coronal dimming regions. Our analysis shows, for the first time, that there is an increase in the magnetic field strength at the onset of the dimming in the dominant polarity of the plage regions surrounding the AR, which persists during the dimming, recovering at the onset of the intensity recovery. Using geometric arguments we show that the increase in the field strength of the dominant polarity is consistent with a change in the inclination angle of the photospheric magnetic fields in the plage regions.