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Study of Solar Eruptive Phenomena: Understand their Early Phases and Determine their Arrival Times to Earth

Final Report Summary - SEP (Study of Solar Eruptive Phenomena: Understand their Early Phases and Determine their Arrival Times to Earth)

The Solar Eruptive Phenomena (SEP) project, aims to enhance our understanding of the initial stages and the Sun-to-Earth propagation of Coronal Mass Ejections (CMEs). CMEs represent gigantic expulsions of plasma and frozen-in magnetic fields from the solar atmosphere, the corona, into the interplanetary medium. CMEs are a main driver of the variable Space Weather which impacts high-tech human activities and infrastructures in both space and Earth. More specifically SEP addresses the following: (1) understand the genesis of CMEs, (2) determine the Flare-CME-Coronal Waves relationships, and (3) determine accurate CME arrival times.

A major obstacle in our understanding of CMEs concerns their primordial magnetic structure, i.e. what is the magnetic structure when and before a CME is launched. While it is now widely accepted that most CMEs once in the outer corona and when they impact the Earth, posses a flux rope topology, i.e. coiled magnetic fields along the axis of a current channel, it is a matter of strong debate whether a flux-rope topology exists when a CME is born. Using a combination of imaging observations of hot plasmas (>10 MK) in the corona and magnetic field observations at the photospheric roots of CMEs, we concluded that flux-ropes are a common occurrence before and during CME onsets. In addition, we presented the first observations of a truly pre-existing flux-rope. This structure was formed during a confined (i.e. non-eruptive) solar flare. After almost 7 hours from its formation, the flux-rope erupted as a CME. We therefore conjectured that confined flares of all magnitudes could lead into the formation and the development of flux-ropes.

Moreover the role of the background magnetic field in the initiation of CMEs was studied. It is well-known that the tension of the overlying magnetic field lines is the dominant force opposing a CME to take place. When the magnetic field above the rising magnetic flux drops off relatively fast with height, then a CME could take place: otherwise the eruption is confined. Using magnetic field observations at the solar surface, i.e the photosphere, we calculated the coronal magnetic field above a solar active region which gave rise to several CMEs. The temporal evolution of the rate of the magnetic field decrease (decay-index) above this active region was calculated. The decay index evolution was not the prime factor leading to the observed CMEs, but it was rather the magnetic helicity injection.

Frequently and in tandem with eruptive flares large-scale wave disturbances are observed. These so-called EUV waves, sometimes cover the entire solar surface. The nature of the EUV waves is a matter of intense debate with both wave (fast-mode MHD) and non-wave interpretations (disk projection of expanding CME). The exact nature of EUV waves was addressed with a synthesis of the current observational and modeling information of this phenomenon. Understanding this phenomenon has significant implications for both understanding the early evolution of CMEs as well as for gauging coronal conditions. A hybrid picture invoking both wave and non-wave components was found to best reproduce the bulk of the observations recorded by modern instrumentation on-board various satellites (SOHO, TRACE, Hinode, STEREO, SDO). A period of strong lateral expansion that early CMEs undergo is the driver of EUV waves. The initially driven and then freely-propagating EUV wave drives several secondary phenomena along its path (e.g. loop and filament deflections and oscillations) while the erupting CME flux generates several non-wave phenomena like stationary dimmings. Estimates of the energetic content of EUV waves shows they rival the energy of small flares.

Analysis of coronal observations of eruptive solar phenomena in various domains of the spectrum (radio, EUV, white-light) showed that the rapid initial expansion of ambient magnetic structures, forming cavities, driven from below by erupting flux-ropes, is responsible for the generation of wave and shock phenomena in the inner corona. In addition, and further away in the outer corona, CME-driven shocks observed in the white-light connect both spatially and temporally with the sites of release and acceleration of geoeffective solar energetic particles.

We found that shocks formed around fast CMEs represent a crucial parameter in the description and modeling of their propagation in the interplanetary medium. Inclusion of shocked solar wind conditions upstream the propagating CMEs into the corresponding equation of motion leads to significant improvements in the prediction of their arrival times and speeds at Earth. In addition, rather excessive departures of CME shapes from sphericity are required in order to obtain significant changes in the anticipated arrival times and speeds of CMEs when they reach the Earth's space environment.