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Physics of coronal mass ejection, flares and associated waves

Coronal mass ejections (CMEs) and flares are the most energetic events in our solar system. CMEs are magnetized plasma clouds ejected from the Sun with speed of hundreds to thousands of kilometers per second, disrupting the coronal magnetic structure and the steady solar wind flow. Flares refer to the impulsive enhancement of the solar radiation over a wide range of energies resulting from the heated plasma and accelerated particles. CMEs and flares often show up as two aspects of a single event, both being driven by magnetic instabilities and reconnection of coronal field lines. Magnetic reconnection is a fundamental process in magnetized plasmas, by which the field connectivity is reconfigured and part of the magnetic energy is impulsively released to heat and accelerate the plasma.

During the impulsive acceleration phase of fast CMEs, which tends to occur low in the corona, the CME body strongly expands. This impulsive expansion is accompanied by a compression of the ambient plasma which may steepen into a large-amplitude or shock wave. Such waves are well observed in EUV image sequences, propagating over large ranges in the solar corona. The strongest events also show up as propagating disturbances in Hα filtergrams of the more inert chromosphere. In association with CMEs and waves we also often observe so-called "coronal dimmings". Dimmings appear as regions of reduced intensity resulting from mass evacuation and density depletion behind the erupting CME.

In our research group, we try to understand the physics behind flares and CMEs through imaging and spectroscopic observations from a variety of space-based (SDO, RHESSI, IRIS, Hinode, STEREO, etc.) and ground-based observatories, such as Kanzelhöhe Observatory of the University of Graz. The scientific questions we address include: How is energy released in magnetic reconnection and transformed? What is the relationship of the CME dynamics and the energy release in the flare? How is the initial CME dynamics related to the initiation of coronal waves?

The group is also involved in software development for the Spectrometer Telescope for Imaging X-rays (STIX) onboard ESA's Solar Orbiter mission, which was successfully launched on 2020 February 9 on an Atlas V 411 rocket from Cape Canaveral, Florida, to study the physics of the Sun and the inner heliosphere in an elliptical orbit of the spacecraft with a perihelion as close as 0.28 AU. The aims of STIX are to study high-energy electrons accelerated in solar flares and their propagation into interplanetary space. STIX uses 32 pairs of grids placed before coarsely pixelated Cadmium telluride detectors, sensitive to energies in the range 4 to 150 keV, to recover the spatial distribution from the modulation of the incoming X-ray flux depending on its directivity.

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