Paolo Pagano

Magnetohydrodynamic simulations of the ejection of a magnetic flux rope

Context. Coronal mass ejections (CME’s) are one of the most violent phenomena found on the Sun. One model to explain their occurrence is the flux rope ejection model. In this model, magnetic flux ropes form slowly over time periods of days to weeks. They then lose equilibrium and are ejected from the solar corona over a few hours. The contrasting time scales of formation and ejection pose a serious problem for numerical simulations. Aims. We simulate the whole life span of a flux rope from slow formation to rapid ejection and investigate whether magnetic flux ropes formed from a continuous magnetic field distribution, during a quasi-static evolution, can erupt to produce a CME. Methods. To model the full life span of magnetic flux ropes we couple two models. The global non-linear force-free field (GNLFFF) evolution model is used to follow the quasi-static formation of a flux rope. The MHD code ARMVAC is used to simulate the production of a CME through the loss of equilibrium and ejection of this flux rope. Results. We show that the two distinct models may be successfully coupled and that the flux rope is ejected out of our simulation box, where the outer boundary is placed at 2.5 R . The plasma expelled during the flux rope ejection travels outward at a speed of 100 km s−1, which is consistent with the observed speed of CMEs in the low corona. Conclusions. Our work shows that flux ropes formed in the GNLFFF can lead to the ejection of a mass loaded magnetic flux rope in full MHD simulations. Coupling the two distinct models opens up a new avenue of research to investigate phenomena where different phases of their evolution occur on drastically different time scales.

Simulating AIA observations of a flux rope ejection

Context. Coronal mass ejections (CMEs) are the most violent phenomena observed on the Sun. Currently, extreme ultraviolet (EUV) images from the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamic Observatory (SDO) are providing new insights into the early phase of CME evolution. In particular, observations now show the ejection of magnetic flux ropes from the solar corona and how they evolve into CMEs. While this is the case, these observations are difficult to interpret in terms of basic physical mechanisms and quantities. To fully understand CMEs we need to compare equivalent quantities derived from both observations and theoretical models. This will aid in bridging the gap between observations and models. Aims. To this end, we aim to produce synthesised AIA observations from simulations of a flux rope ejection. To carry this out we include the role of thermal conduction and radiative losses, both of which are important for determining the temperature distribution of the solar corona during a CME. Methods. We perform a simulation where a flux rope is ejected from the solar corona. From the density and temperature of the plasma in the simulation we synthesise AIA observations. The emission is then integrated along the line of sight using the instrumental response function of AIA. Results. We sythesise observations of AIA in the channels at 304 A, 171 A, 335 A, and 94 A. The synthesised observations show a number of features similar to actual observations and in particular reproduce the general development of CMEs in the low corona as observed by AIA. In particular we reproduce an erupting and expanding arcade in the 304 A and 171 A channels with a high density core. Conclusions. The ejection of a flux rope reproduces many of the features found in the AIA observations. This work is therefore a step forward in bridging the gap between observations and models, and can lead to more direct interpretations of EUV observations in terms of flux rope ejections. We plan to improve the model in future studies in order to perform a more quantitative comparison.

Effect of gravitational stratification on the propagation of a CME

Our aim is to study the role of gravitational stratification on the propagation of CMEs. In particular, we assess how it influences the speed and shape of CMEs and under what conditions the flux rope ejection becomes a CME or when it is quenched. We ran a set of MHD simulations that adopt an eruptive initial magnetic configuration that has already been shown to be suitable for a flux rope ejection. We varied the temperature of the backgroud corona and the intensity of the initial magnetic field to tune the gravitational stratification and the amount of ejected magnetic flux. We used an automatic technique to track the expansion and the propagation of the magnetic flux rope in the MHD simulations. From the analysis of the parameter space, we evaluate the role of gravitational stratification on the CME speed and expansion. Our study shows that gravitational stratification plays a significant role in determining whether the flux rope ejection will turn into a full CME or whether the magnetic flux rope will stop in the corona. The CME speed is affected by the background corona where it travels faster when the corona is colder and when the initial magnetic field is more intense. The fastest CME we reproduce in our parameter space travels at 850 km/s. Moreover, the background gravitational stratification plays a role in the side expansion of the CME, and we find that when the background temperature is higher, the resulting shape of the CME is flattened more. Our study shows that although the initiation mechanisms of the CME are purely magnetic, the background coronal plasma plays a key role in the CME propagation, and full MHD models should be applied when one focusses especially on the production of a CME from a flux rope ejection.

Contribution of mode-coupling and phase-mixing of Alfvén waves to coronal heating

Context. Phase-mixing of Alfven waves in the solar corona has been identified as one possible candidate to explain coronal heating. While this scenario is supported by observations of ubiquitous oscillations in the corona carrying sufficient wave energy and by theoretical models that have described the concentration of energy in small-scale structures, it is still unclear whether this wave energy can be converted into thermal energy in order to maintain the million-degree hot solar corona.Aims. The aim of this work is to assess how much energy can be converted into thermal energy by a phase-mixing process triggered by the propagation of Alfvenic waves in a cylindric coronal structure, such as a coronal loop, and to estimate the impact of this conversion on the coronal heating and thermal structure of the solar corona.Methods. To this end, we ran 3D MHD simulations of a magnetised cylinder where the Alfven speed varies through a boundary shell, and a footpoint driver is set to trigger kink modes that mode couple to torsional Alfven modes in the boundary shell. These Alfven waves are expected to phase-mix, and the system allows us to study the subsequent thermal energy deposition. We ran a reference simulation to explain the main process and then we varied the simulation parameters, such as the size of the boundary shell, its structure, and the persistence of the driver.Results. When we take high values of magnetic resistivity and strong footpoint drivers into consideration, we find that i) phase-mixing leads to a temperature increase of the order of 105 K or less, depending on the structure of the boundary shell; ii) this energy is able to balance the radiative losses only in the localised region involved in the heating; and iii) we can determine the influence of the boundary layer and the persistence of the driver on the thermal structure of the system.Conclusions. Our conclusion is that as a result of the extreme physical parameters we adopted and the moderate impact on the heating of the system, it is unlikely that phase-mixing can contribute on a global scale to the heating of the solar corona.

Uncertainties in polarimetric 3D reconstructions of coronal mass ejections

This work is aimed at quantifying the uncertainties in the 3D reconstruction of the location of coronal mass ejections (CMEs) obtained with the polarization ratio technique. The method takes advantage of the different distributions along the line of sight (LOS) of total (tB) and polarized (pB) brightnesses to estimate the average location of the emitting plasma. To this end, we assumed two simple electron density distributions along the LOS (a constant density and Gaussian density profiles) for a plasma blob and synthesized the expected tB and pB for different distances

Future capabilities of CME polarimetric 3D reconstructions with the METIS instrument: A numerical test

z

In Situ Generation of Transverse Magnetohydrodynamic Waves from Colliding Flows in the Solar Corona

of the blob from the plane of the sky (POS) and different projected altitudes

Contribution of phase-mixing of Alfvén waves to coronal heating in multi-harmonic loop oscillations

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Measuring the electron temperatures of coronal mass ejections with future space-based multi-channel coronagraphs: a numerical test

. Reconstructed locations of the blob along the LOS were thus compared with the real ones, allowing a precise determination of uncertainties in the method. Independently of the analytical density profile, when the blob is centered at a small distance from the POS (i.e. for limb CMEs) the distance from the POS starts to be significantly overestimated. Polarization ratio technique provides the LOS position of the center of mass of what we call folded density distribution, given by reflecting and summing in front of the POS the fraction of density profile located behind that plane. On the other hand, when the blob is far from the POS, but with very small projected altitudes (i.e. for halo CMEs,

Origin and Ion Charge State Evolution of Solar Wind Transients during 4 – 7 August 2011

\rho < 1.4