F. Zuccarello

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.

OBSERVATIONAL EVIDENCE OF TORUS INSTABILITY AS TRIGGER MECHANISM FOR CORONAL MASS EJECTIONS: THE 2011 AUGUST 4 FILAMENT ERUPTION

Solar filaments are magnetic structures often observed in the solar atmosphere and consist of plasma that is cooler and denser than their surroundings. They are visible for days—even weeks—which suggests that they are often in equilibrium with their environment before disappearing or erupting. Several eruption models have been proposed that aim to reveal what mechanism causes (or triggers) these solar eruptions. Validating these models through observations represents a fundamental step in our understanding of solar eruptions. We present an analysis of the observation of a filament eruption that agrees with the torus instability model. This model predicts that a magnetic flux rope embedded in an ambient field undergoes an eruption when the axis of the flux rope reaches a critical height that depends on the topology of the ambient field. We use the two vantage points of theSolar Dynamics Observatory (SDO) and the Solar TErrestrial RElations Observatory to reconstruct the three-dimensional shape of the filament, to follow its morphological evolution, and to determine its height just before eruption. The magnetograms acquired by SDO/Helioseismic and Magnetic Imager are used to infer the topology of the ambient field and to derive the critical height for the onset of the torus instability. Our analysis shows that the torus instability is the trigger of the eruption. We also find that some pre-eruptive processes, such as magnetic reconnection during the observed flares and flux cancellation at the neutral line, facilitated the eruption by bringing the filament to a region where the magnetic field was more vulnerable to the torus instability.

The X17.2 flare occurred in NOAA 10486: an example of filament destabilization caused by a domino effect

Context. It is now possible to distinguish between two main models describing the mechanisms responsible for eruptive flares : the standard model, which assumes that most of the energy is released, by magnetic reconnection, in the region hosting the core of a sheared magnetic field, and the breakout model, which assumes reconnection occurs at first in a magnetic arcade overlaying the eruptive features. Aims. We analyze the phenomena observed in NOAA 10486 before and during an X17.2 flare that occurred on 2003 October 28, to study the relationship between the pre-flare and flare phases and determine which model is the most suitable for interpreting this event. Methods. We performed an analysis of multiwavelength data set available for the event using radio data (0.8–4.5 GHz), images in the visible range (WL and Hα), EUV images (1600 and 195 A), and X-ray data, as well as MDI longitudinal magnetograms. We determined the temporal sequence of events occurring before and during the X17.2 flare and the magnetic field configuration in the linear force-free field approximation. Results. The active region was characterized by a multiple arcade configuration and the X17.2 flare was preceded, by ∼ 2h , by the partial eruption of one filament. This eruption caused reconnection at null points located in the low atmosphere and a decrease in magnetic tension in the coronal field lines overlaying other filaments present in the active region. As a consequence, these filaments were destabilized and the X17.2 flare occurred. Conclusions. The phenomena observed in NOAA 10486 before and during the X17.2 flare cannot be explained by a simple scenario such as the standard or breakout model, but instead in terms of a so-called domino effect, involving a sequence of destabilizing processes that triggered the flare.

AFS dynamic evolution during the emergence of an active region

Using data acquired during an observational campaign carried out at the THEMIS telescope in IPM mode, coordi- nated with other ground- and space-based instruments (IOACT, TRACE, EIT/SOHO, MDI/SOHO), we have analyzed the first evolutionary phases of a recurrent active region (NOAA 10050), in order to study the morphology and dynamics of its magnetic structures during their emergence and early development. The main result obtained from this analysis concerns the dynamic evolution of the arch filament system (AFS) crossing the polarity inversion line: the line of sight velocities determined from Doppler measurements confirm that the loops forming the AFS show an upward motion at their tops and a downward motion at their extremities, but also indicate that the upward motion decreases while the active region develops. Moreover, it has been found that, within the limits of the temporal cadence and spatial resolution of the instruments used, the first evidence of the active region formation is initially observed in the transition region and lower corona, and later on (i.e. after about 6 h) in the inner layers (chromosphere and photosphere). Another interesting result concerns the analysis of the magnetograms, indicating that the initial increase in the magnetic flux seems to be synchronous with the appearance od the active region appearance in the transition region and lower corona, and that the rate of increase of the magnetic flux during the formation of the active region is not constant, but is steeper at the beginning (i.e. during the first 150 h) than in the following period. All these results may indicate the presence of some mechanism that decelerates the magnetic flux emergence as more and more flux tubes rise towards higher atmospheric layers. Finally, we would like to stress the observed asymmetries between the preceding and the following sides of NOAA 10050: the p-side is more extented than the f-side, the p-side moves forward from the initial outbreak position much faster than the f-side recedes; the AFS f-side exhibits higher downflows than the p-side.

Observation of bipolar moving magnetic features streaming out from a naked spot

Context. Mechanisms responsible for active-region formation, evolution, and decay have been investigated by many authors and several common features have been identified. In particular, a key element in the dispersal of the magnetic field seems to be the presence of magnetic elements, called moving magnetic features (MMFs). Aims. We analyze the short-lived sunspot group NOAA 10977, which appeared on the solar disk between 2 and 8 December 2007, to study the details of its emergence and decay phases. Methods. We performed a multi wavelength analysis of the region using images at visible (G band and Hα) and near-IR (Ca ii) wavelengths acquired by both the IBIS instrument and SOT/HINODE, EUV images (17.1 nm) acquired by TRACE, and MDI and SOT magnetograms. Results. The observed region exhibits some peculiarities. During the emergence phase the formation of the f-pore was initially observed, while the p-polarity later formed a naked spot, i.e., a sunspot without a penumbra. We measured a moat flow around this spot, and observed some MMFs streaming out from it during the decay phase. The characteristics of these MMFs allowed us to classify them as type I (U-shaped) MMFs. They were also cospatial with sites of increased brightness both in the photosphere and the chromosphere. Conclusions. The presence of bipolar MMFs in a naked spot indicates that current interpretation of bipolar MMFs, as extensions of the penumbral filaments beyond the sunspot outer boundaries, should be revised, to take into account this observational evidence. We believe that our results provide new insights into improving models of sunspot evolution.

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 

Observational evidence of the primary role played by photospheric motions in magnetic helicity transport before a filament eruption

R_\odot

A solar eruption triggered by the interaction between two magnetic flux systems with opposite magnetic helicity

. 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.

Eruption of a helically twisted prominence

Many filament eruptions can be suitably described in the framework of the kink instability model, although it is not always easy to discriminate whether the helical flux rope writhes due to new emerging flux or to photospheric horizontal motions. In this paper we provide observational evidence of the important role which can be played by horizontal motions in filament instability and eruption. More precisely, we describe the analysis of the eruption of a reverse-S-shaped filament associated with a flare of class M6.3, that occurred on 15 June, 2001 in the active region NOAA 9502. Using TRACE 195 A images we studied the morphological evolution of the EUV filament channel. Using I minute cadence MDI full-disc longitudinal magnetograms we analyzed the magnetic evolution of the entire active region. The geometrical parameters of the EUV filament channel and the horizontal velocities in the areas corresponding to the filament footpoints were determined and agreed with the kink instability. Moreover, the analysis of MDI magnetograms showed that a sudden and strong increase in the magnetic helicity transport rate to the corona preceded and accompanied the filament eruption. During the same time interval, on the one hand the emergence of magnetic flux in both polarities became negligible, but on the other hand the velocity pattern at the filament ends showed horizontal, counterclockwise motions, which could make a significant contribution to the transformation, from twist to writhe, of the magnetic helicity accumulated along the filament before its eruption. This result seems to indicate that in this event the transport of magnetic helicity exceeding the limit for the kink instability is primarily due to photospheric motions, while the contribution from the emerging flux is negligible.

The age dependence of photospheric tracer rotation

Context. In recent years it has been stressed that the accumulation of magnetic helicity via emergence of new magnetic flux and/or shearing photospheric motions, can play an important role in the destabilization processes leading to eruptive phenomena occurring in the solar atmosphere. Aims. In this paper we want to highlight a specific aspect of the process of magnetic helicity accumulation, providing new observational evidences of the role played by the interaction of magnetic fields characterized by opposite magnetic helicity signs in triggering solar eruption. Methods. We used 171 u TRACE data to describe a filament eruption occurred on Nov 1st, 2001 in active region NOAA 9682 and MDI full disk line-of-sight magnetograms to measure the accumulation of magnetic helicity in corona before the event. We used the Local Correlation Tracking (LCT) and the Differential Affine Velocity Estimator (DAVE) techniques to determine the horizontal velocities and the methods proposed by Chae (2001) and Pariat et al. (2005) for the estimation of the magnetic helicity flux. Results. The chirality signatures of the filament involved in the eruption were ambiguous and the overlying arcade visible during the main phase of the event was characterized by a mixing of helicity signs. However, the measures of the magnetic helicity flux allowed us to deduce that the magnetic helicity was positive in the whole active region where the event took place, while it was negative near the magnetic inversion line where the filament footpoints were located. Conclusions. These results suggest that the filament eruption may be caused by magnetic reconnection between two magnetic field systems characterized by opposite signs of magnetic helicity. We also found that only the DAVE method allowed us to obtain the crucial information on the horizontal velocity field near the magnetic inversion line.