Carla Jacobs

THE ROLE OF STREAMERS IN THE DEFLECTION OF CORONAL MASS EJECTIONS: COMPARISON BETWEEN STEREO THREE-DIMENSIONAL RECONSTRUCTIONS AND NUMERICAL SIMULATIONS

On 2009 September 21, a filament eruption and the associated coronal mass ejection (CME) were observed by the Solar Terrestrial Relations Observatory (STEREO) spacecraft. The CME originated from the southern hemisphere and showed a deflection of about 15 ◦ toward the heliospheric current sheet (HCS) during the propagation in the COR1 field of view. The CME source region was near the central meridian, but no on-disk CME signatures could be seen from the Earth. The aim of this paper is to provide a physical explanation for the strong deflection of the CME observed on 2009 September 21. The two-sided view of the STEREO spacecraft allows us to reconstruct the three-dimensional travel path of the CME and the evolution of the CME source region. The observations are combined with a magnetohydrodynamic simulation, starting from a magneticfield configuration closely resembling the extrapolated potential field for that date. By applying localized shearing motions, a CME is initiated in the simulation, showing a similar non-radial evolution, structure, and velocity as the observed event. The CME gets deflected toward the current sheet of the larger northern helmet streamer due to an imbalance in the magnetic pressure and tension forces and finally gets into the streamer. This study shows that during solar minima, even CMEs originating from high latitude can be easily deflected toward the HCS, eventually resulting in geoeffective events. How rapidly they undergo this latitudinal migration depends on the strength of both the large-scale coronal magnetic field and the magnetic flux of the erupting filament.

THE INTERNAL STRUCTURE OF CORONAL MASS EJECTIONS: ARE ALL REGULAR MAGNETIC CLOUDS FLUX ROPES?

In this Letter, we investigate the internal structure of a coronal mass ejection (CME) and its dynamics by invoking a realistic initiation mechanism in a quadrupolar magnetic setting. The study comprises a compressible three-dimensional magnetohydrodynamics simulation. We use an idealized model of the solar corona, into which we superimpose a quadrupolar magnetic source region. By applying shearing motions resembling flux emergence at the solar boundary, the initial equilibrium field is energized and it eventually erupts, yielding a fast CME. The simulated CME shows the typical characteristics of a magnetic cloud (MC) as it propagates away from the Sun and interacts with a bimodal solar wind. However, no distinct flux rope structure is present in the associated interplanetary ejection. In our model, a series of reconnection events between the eruptive magnetic field and the ambient field results in the creation of significant writhe in the CMEs magnetic field, yielding the observed rotation of the magnetic field vector, characteristic of an MC. We demonstrate that the magnetic field lines of the CME may suffer discontinuous changes in their mapping on the solar surface, with footpoints subject to meandering over the course of the eruption due to magnetic reconnection. We argue that CMEs with internal magnetic structure such as that described here should also be considered while attempting to explain in situ observations of regular MCs at L1 and elsewhere in the heliosphere.

On the effect of the initial magnetic polarity and of the background wind on the evolution of CME shocks

The shocks and magnetic clouds caused by Coronal Mass Ejections (CMEs) in the solar corona and interplanetary (IP) space play an important role in the study of space weather. In the present paper, numerical simulations of some simple CME models were performed by means of a finite volume, explicit solver to advance the equations of ideal magnetohydrodynamics. The aim is to quantify here both the effect of the background wind model and of the initial polarity on the evolution of the IP CMEs and the corresponding shocks.
To simulate the CMEs, a high density-pressure plasma blob is superposed on different steady state solar wind models. The evolution of an initially non-magnetized plasma blob is compared with that of two magnetized ones (with both normal and inverse polarity) and the differences are analysed and quantified. Depending on the launch angle of the CME and the polarity of the initial flux rope, the velocity of the shock front and magnetic cloud is decreased or increased. Also the spread angle of the CME and the evolution path of the CME in the background solar wind is substantially different for the different CME models and the different wind models. A quantitative comparison of these simulations shows that these effects can be quite substantial and can clearly affect the geo-effectiveness and the arrival time of the events.

Morphology and density structure of post-CME current sheets

Context. Eruption of a coronal mass ejection (CME) drags and “opens” the coronal magnetic field, presumably leading to the formation of a large-scale current sheet and field relaxation by magnetic reconnection. Aims. We analyze the physical characteristics of ray-like coronal features formed in the aftermath of CMEs, to confirm whether interpreting this phenomenon in terms of a reconnecting current sheet is consistent with observations. Methods. The study focuses on measurements of the ray width, density excess, and coronal velocity field as a function of the radial distance. Results. The morphology of the rays implies that they are produced by Petschek-like reconnection in the large-scale current sheet formed in the wake of CME. The hypothesis is supported by the flow pattern, often showing outflows along the ray, and sometimes also inflows into the ray. The inferred inflow velocities range from 3 to 30 km s −1 , and are consistent with the narrow opening-angle of rays, which add up to a few degrees. The density of rays is an order of magnitude higher than in the ambient corona. The densityexcess measurements are compared with the results of the analytical model in which the Petschek-like reconnection geometry is applied to the vertical current sheet, taking into account the decrease in the external coronal density and magnetic field with height. Conclusions. The model results are consistent with the observations, revealing that the main cause of the density excess in rays is a transport of the dense plasma from lower to higher heights by the reconnection outflow.

Inverse and normal coronal mass ejections : evolution up to 1 AU

Simulations of Coronal Mass Ejections (CMEs) evolving in the interplanetary (IP) space from the Sun up to 1 AU are performed in the framework of ideal magnetohydrodynamics (MHD) by the means of a finite volume, explicit solver. The aim is to quantify the effect of the initiation parameters, such as the initial magnetic polarity, on the evolution and on the geo-effectiveness of CMEs. The CMEs are simulated by means of a very simple model: a high density and high pressure magnetized plasma blob is superposed on a background steady state solar wind model with an initial velocity and launch direction. The simulations show that the initial magnetic polarity substantially affects the IP evolution of the CMEs influencing the propagation velocity, the shape, the trajectory and even the geo-effectiveness. We also tried to reproduce the physical values (density, velocity, and magnetic field) observed by the ACE spacecraft after the halo CME event that occurred on April 4, 2000.

Simulation of a Breakout Coronal Mass Ejection in the Solar Wind

The initiation and evolution of coronal mass ejections (CMEs) is studied by means of the breakout model embedded in a 2.5D axisymmetric solar wind in the framework of numerical magnetohydrodynamics (MHD). The initial, steady equilibrium contains a pre-eruptive region consisting of three arcades with alternating magnetic flux polarity and with correspondingly three neutral lines on the photosphere. The magnetic tension of the overlying closed magnetic field of the helmet streamer keeps this structure in place. The most crucial part of the initial breakout topology is the existence of an X-point on the leading edge of the central arcade. By shearing part of this arcade, the reconnection with the overlying streamer field is turned on. The initial phase of the erupting arcade then closely follows the original breakout scenario. The breakout reconnection opens the overlying field in an energetically efficient way leading to an ever faster eruption. However, from a certain moment two new reconnections set in on the sides of the erupting central arcade and the breakout reconnection stops. The consequence of this change in reconnection location is twofold: (1) the lack of breakout reconnection so that the breakout plasmoid fails to become a fast CME; and (2) an eventual disconnection of the large helmet top resulting in a slow CME.

The effect of the solar wind on CME triggering by magnetic foot point shearing

Context. Photospheric motions and a sheared configuration of the magnetic field are often considered as precursors of violent solar phenomena such as flares and Coronal Mass Ejections (CMEs). Therefore, in many numerical CME initiation studies shearing of the magnetic foot points is used as a mechanism to make the magnetic field unstable and to trigger the CME event. Aims. From that point of view we decided to do a parameter study that investigates the effect of the different initiation parameters, in particular the effect of the shear flow velocity. Moreover, the simulations were performed on three different background solar wind models. In this way, both effects of the background wind and the initiation parameters on the CME evolution are quantified. Methods. The results are obtained by means of a finite volume, explicit solver to advance the equations of ideal magnetohydrodynamics. All simulations involve the same numerical grid, the same numerical technique and similar boundary conditions, so that the results can be compared in an unequivocal way. Results. The foot points of the magnetic field lines are sheared by introducing an extra longitudinal flow profile on the solar surface with a maximum velocity ranging from 3 km s -1 to 9 km s -1 . The temporal evolution of the magnetic energy, the velocity of the flux rope, and the magnetic helicity show a dependence on the maximum shear velocity as well as on the background wind model.

Modelling the initiation of coronal mass ejections: magnetic flux emergence versus shearing motions

Context. Coronal mass ejections (CMEs) are enormous expulsions of magnetic flux and plasma from the solar corona into the interplanetary space. These phenomena release a huge amount of energy. It is generally accepted that both photospheric motions and the emergence of new magnetic flux from below the photosphere can put stress on the system and eventually cause a loss of equilibrium resulting in an eruption. Aims. By means of numerical simulations we investigate both emergence of magnetic flux and shearing motions along the magnetic inversion line as possible driver mechanisms for CMEs. The pre-eruptive region consists of three arcades with alternating magnetic flux polarity, favouring the breakout mechanism. Methods. The equations of ideal magnetohydrodynamics (MHD) were advanced in time by using a finite volume approach and solved in spherical geometry. The simulation domain covers a meridional plane and reaches from the lower solar corona up to 30 

THE BRIGHTNESS OF DENSITY STRUCTURES AT LARGE SOLAR ELONGATION ANGLES : WHAT IS BEING OBSERVED BY STEREO SECCHI?

R_\odot

ON THE INTERNAL STRUCTURE OF THE MAGNETIC FIELD IN MAGNETIC CLOUDS AND INTERPLANETARY CORONAL MASS EJECTIONS: WRITHE VERSUS TWIST

. When we applied time-dependent boundary conditions at the inner boundary, the central arcade of the multiflux system expands, leading to the eventual eruption of the top of the helmet streamer. We compare the topological and dynamical evolution of the system when driven by the different boundary conditions. The available free magnetic energy and the possible role of magnetic helicity in the onset of the CME are investigated. Results. In our simulation setup, both driving mechanisms result in a slow CME. Independent of the driving mechanism, the overall evolution of the system is the same: the actual CME is the detatched helmet streamer. However, the evolution of the central arcade is different in the two cases. The central arcade eventually becomes a flux rope in the shearing case, whereas in the flux emergence case there is no formation of a flux rope. Furthermore, we conclude that magnetic helicity is not crucial to a solar eruption.