Emmanuel Chané

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.

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.

High-latitude circulation in giant planet magnetospheres

We follow-up the proposal by Cowley et al. (2004) that the plasma circulation in the magnetospheres of the giant planets is a combination of two cycles or circulation systems. The Vasyliunas cycle transports heavymaterial ionized deep within themagnetosphere eventually to loss in themagnetotail. The second cycle is driven by magnetic reconnection between the planetary and the solar wind magnetic fields (the Dungey cycle) and is found on flux tubes poleward of those of the Vasyliunas cycle. We examine features of the Dungey system, particularly what occurs out of the equatorial plane. The Dungey cycle requires reconnection on the dayside, and we suggest that at the giant planets the dayside reconnection occurs preferentially in the morning sector. Second, we suggest that most of the solar wind material that enters through reconnection on to open flux tubes on the dayside never gets trapped on closed field lines but makes less than one circuit of the planet and exits down tail. In its passage to the nightside, the streaming ex-solar wind material is accelerated centrifugally by the planetary rotation primarily along the field; thus, in the tail it will appear very like a planetary wind. The escaping wind will be found on the edges of the tail plasma sheet, and reports of light ion streams in the tail are likely due to this source. The paper concludes with a discussion of high-latitude circulation in the absence of reconnection between the solar wind and planetary field.

Transient small‐scale structure in the main auroral emission at Jupiter

The main auroral emission at Jupiter results from the ionosphere-magnetosphere coupling current system associated with the corotation breakdown of iogenic plasma in the current sheet. The morphology and brightness of the main auroral emission are generally suggested to be stable during time intervals of the order of an hour. Here we reveal a transient small-scale structure in the main emission that is characterized by a localized brightness enhancement close to noon local time. The evolution of this small-scale structure is investigated in both hemispheres on the basis of far UV images obtained with the Hubble Space Telescope between 1997 and 2007. Our observations indicate that the transient feature vary within a few tens of minutes. As one plausible explanation based on Galileo observations, we suggest that the localized enhancement of the field-aligned currents associated with the transient structure is due to the shear induced by intermittent inward plasma flow near noon in the equatorial plane.

Stagnation of Saturn's auroral emission at noon

Auroral emissions serve as a powerful tool to investigate the magnetospheric processes at Saturn. Solar wind and internally driven processes largely control Saturns auroral morphology. The main auroral emission at Saturn is suggested to be connected with the magnetosphere-solar wind interaction, through the flow shear related to rotational dynamics. Dawn auroral enhancements are associated with intense field-aligned currents generated by hot tenuous plasma carried toward the planet in fast moving flux tubes as they return from tail reconnection site to the dayside. In this work we demonstrate, based on Cassini auroral observations, that the main auroral emission at Saturn, as it rotates from midnight to dusk via noon, occasionally stagnates near noon over a couple of hours. In half of the sequences examined, the auroral emission is blocked close to noon, while in three out of four cases, the blockage of the auroral emission is accompanied with signatures of dayside reconnection. We discuss some possible interpretations of the auroral “blockage” near noon. According to the first one, it could be related to local time variations of the flow shear close to noon. Auroral local time variations are also suggested to be initiated by radial transport process. Alternatively, the auroral blockage at noon could be associated with a plasma circulation theory, according to which tenuously populated closed flux tubes as they return from the nightside to the morning sector experience a blockage in the equatorial plane and they cannot rotate beyond noon.

Numerical simulations of the solar corona and Coronal Mass Ejections

Numerical simulations of Coronal Mass Ejections (CMEs) can provide a deeper insight in the structure and propagation of these impressive solar events. In this work, we present our latest results of numerical simulations of the initial evolution of a fast CME. For this purpose, the equations of ideal MagnetoHydroDynamics (MHD) have been solved on a three-dimensional (3D) mesh by means of an explicit, finite volume solver, where the simulation domain ranges from the lower solar corona up to 30Re. In order to simulate the propagation of a CME throughout the heliosphere, a magnetic flux rope is superposed on top of a stationary background solar (MHD) wind with extra density added to the flux rope. The flux rope is launched by giving it an extra initial velocity in order to get a fast CME forming a 3D shock wave. The magnetic field inside the initial flux rope is described in terms of Bessel functions and possesses a high amount of twist.

Simulating CME Initiation and Evolution: State-of-the-art

A review is given of some recent results on CME initiation and evolution simulations obtained at the Centre for Plasma Astrophysics (CPA, K.U.Leuven) on the background of the international developments in this very dynamic field