Coronal Mass Ejections (CMEs) are the most violent manifestation of solar activity. They are also associated with a range of other solar phenomena, such as flares, EUV waves, dimming regions, etc. With the advancement of global 3-D numerical simulation tools and the new fleet of spacecraft (e.g., SoHO, STEREO, SDO, etc.) observing the Sun and the heliosphere, it becomes possible to combine numerical simulations with data analyses in order to gain new physical insight about the dynamical response of the solar atmosphere to solar eruptive events. This talks summarizes recent computational efforts aimed at better understanding the dynamics of CMEs and related phenomena in the solar corona.

We use our nonlinear force-free magnetic field calculations to show that, for a given boundary flux distribution, there may be an upper bound on the total magnetic helicity that force-free fields can contain. The accumulation of magnetic helicity in excess of this upper bound would initiate a non-equilibrium situation, resulting in a CME expulsion as a natural product of coronal evolution. We also show that, after a significant amount of total magnetic helicity has been accumulated, the field becomes open up and presents Parker-spiral-like structures around a current-sheet layer as the manifestation of magnetic helicity residing in the interplanetary space.

Post-CME Current Sheets (CS) are important consequences of post-CME magnetic reconnection. These structures appear in white light as radial, long-lived features observed co-aligned with the CME propagation angle, and are associated with strong brightenings in SOHO/UVCS spectra of hot lines, such as the Fe XVIII λ974A spectral line. This implies plasma temperatures around 5 MK, much larger than typical coronal temperatures. The real origin of this very high temperature plasma is still an open issue: it may originate for instance from turbulent reconnection occurring in the post-CME CS, or alternatively from Petschek reconnection occurring at the base of it, in correspondence of the location of the post-flare Hard X-Ray (HXR) source. For this study, we selected and analyzed an interesting limb event occurred on 2004, July 28. It is a fast CME associated with a C1 class flare at the west limb. The event has a good coverage from GOES/SXI, TRACE, SOHO/UVCS and EIT, and RHESSI. In particular, the UVCS slit was centered at a height of 1.8 solar radii with a central polar angle of 270°, quite perpendicular to the CS axis. A significant coronal emission - lasting for almost 26 hours - has been detected by UVCS in the Fe XVIII and Si XII λ499 A spectral lines. RHESSI data show a long lasting HXR source without non-thermal emission and the appearance of a new HXR source with the onset of Fe XVIII emission seen by UVCS. GOES/SXI movies show that this source is located approximately at the latitude where the FeXVIII is observed, slowly rising with time. Thermal energies in the HXR source and in the post-CME CS will be compared and a data interpretation will be provided.

During propagation, Magnetic Clouds (MC) interact with their environment and, in particular, may reconnect with the solar wind around it, eroding away part of its initial magnetic flux. We will present the analysis of such an interaction using combined, multi-point observations of the same MC flux rope observed by STEREO A, B, ACE, WIND and THEMIS on November 19-20, 2007. Observation of azimuthal magnetic flux imbalance inside a MC flux rope has been argued to stem from erosion due to magnetic reconnection at its front boundary. The present study adds to such analysis a large set of signatures expected from this erosion process. (1) Comparison of azimuthal flux imbalance for the same MC at widely separated points precludes the crossing of the MC leg as a source of bias in flux imbalance estimates. (2) The use of different methods, associated errors and parametric analyses show that only an unexpectedly large error in MC axis orientation could explain the azimuthal flux imbalance. (3) Reconnection signatures are observed at the MC front at all spacecraft, consistent with an on-going erosion process. (4) Signatures in suprathermal electrons suggest that the trailing part of the MC has a different large-scale magnetic topology, as expected. We will then present and quantify the influence of such an erosion process on geo-effectiveness.

This work focuses on the interaction of a Coronal Mass Ejection (CME) with the ambient solar corona, by studying the spatial and temporal evolution of the density oscillations observed by the SOHO/UV Coronagraph Spectrometer (UVCS) during a CME. The investigation is performed by applying a wavelet analysis to the HI Lyα 121.6 nm line intensity fluctuations observed with UVCS during the 2006 December 24 CME. Strong and coherent fluctuations, with a significant spatial periodicity of about 84 Mm ~ 0.12 solar radii, are shown to develop in about an hour along the front of the CME. The results seem to indicate the Rayleigh-Taylor (RT) instability, susceptible to the deceleration of the heavier fluid of the CME front into the lighter surrounding coronal plasma, as the likely mechanism underlying the generation of the observed plasma fluctuations. This is the first inference of the RT instability in the outer solar corona in UV; this interpretation is also supported by a linear magnetohydrodynamic analysis of the RT instability.

High spatial and temporal resolution METIS measurements of the white-light and UV/EUV solar corona, performed in quasi co-rotation with the Sun, will improve such studies, allowing a much more deep investigation of the interaction of CMEs with the surrounding coronal plasma. In particular, METIS measurements will give the unique opportunity to study the nonlinear temporal (and spatial) evolution of the RT instability during the expansion of the CME, thus possibly driving to constrain some observationally unknown parameters, such as the viscous and diffusion scales, and the surface tension, which controls the evolution of the RT instability.

Acceleration of solar energetic particles (SEPs) is largely thought to occur predominantly at two different locations; in the upper corona and interplanetary medium through stochastic acceleration at shocks driven by coronal mass ejections (CMEs) and in solar active regions through reconnection processes associated with solar flares. Unfortunately, from the vantage point of 1 AU it is not always simple to distinguish SEPs originating from the different sources as particle transport can affect the distribution of the particles as well as the composition and spectra observed in an SEP event. It is also possible that suprathermals created at a flare site contribute to the seed population accelerated by CME-driven shocks, further complicating the picture. With the launch of the twin STEREO spacecraft we have added a dimension to our observations with the ability to study SEP events simultaneously from significantly different longitudes. This has allowed investigations which probe some of the details of the SEP acceleration processes and test theories which strive to explain the variability in SEP event characteristics observed at 1 AU. With the launch of Solar Orbiter, we will gain yet another dimension and begin to examine radial dependences as well as make measurements much closer to the acceleration regions, limiting the effects due to particle transport. This talk will focus on our current understanding of SEP acceleration, the questions being addressed by the STEREO measurements and the questions we expect to address with Solar Orbiter observations.

Much work has been devoted to infer the origin of solar energetic particle (SEP) events through the analysis of statistical associations and correlations with their parent activity. In these works the importance of flares is most often measured by their peak soft X-ray flux, and that of coronal mass ejections (CMEs), by their speed projected onto the plane of the sky. In this contribution such statistical correlations are re-examined, using all SEP events associated with western solar flares of classes M and X in the 23rd activity cycle (1996-2006). Radio observations between the Sun and 1 AU are employed to see if flare-accelerated particles escape from the corona or not. Evidence for the confinement of flare-accelerated electrons in low coronal structures is indeed found in some major (X class) solar flares without SEP events. However, in the majority of SEP events type III bursts show that electrons escape to space since the early impulsive phase of the flare. Although we have no direct observation, we suggest that protons and ions from these flares would also escape and contribute to the SEP event detected at 1 AU. It is then shown that the peak SEP intensity, both for electrons and protons, correlates in an overall comparable way with the projected CME speed and with the peak soft X-ray flux. But the soft X-ray correlation is only clearly observed when the SEP are observed within the transient magnetic field configuration of an interplanetary CME (ICME). We argue that in the general case, where SEP are observed in the standard solar wind, an actually existing correlation is strongly blurred by the varying connection between the parent solar activity and the spacecraft measuring near 1 AU. This seems to be the case despite our restriction to western hemisphere events. It is concluded that the statistical interpretation of single-point SEP measurements near 1 AU suffers from our incomplete understanding of the particle propagation processes in the corona. Disentangling the contribution of flare acceleration within an active region and CME shock acceleration over larger spatial scales seems impossible with the data at hand. Going closer to the Sun with Solar Orbiter is expected to provide a clearer distinction of the SEP relationship with the parent flare and CME through the possibility to analyse SEP time profiles with much less smearing by interplanetary transport.