arXiv subjects

Mariana Cécere

Publications and source records attributed to Mariana Cécere.

7 recordsLinked to original sources

arXiv2026

Nonlinear steepening of a fast magnetoacoustic wave in the vicinity of a coronal magnetic null point

The interaction of a fast magnetoacoustic wave with a magnetic null point is studied in the context of the sympathetic flare phenomenon. Attention is paid to steepening the wave caused by the finite-amplitude effects in a non-uniform plasma environment. The null point is modelled by a potential magnetic configuration without a guiding field. The equilibrium plasma density and temperature are taken to be constant. The fast wave is excited by an impulsive point source outside the distance at which the local Alfvén and sound speeds are equal to each other. The incoming fast wave approaches the null point along the bisector of the magnetic configuration, i.e., across the local field. The fast-speed non-uniformity around the null point causes the refraction of the incident fast wave. However, the segment of the incoming wave, which approaches the null point is locally plane. The decrease in the fast speed towards the null point amplifies the nonlinear deformation of the incoming wave. Hence, the fast wave can become subject to nonlinear dissipation at a distance from the null point and not reach it.

astro-ph.SR↗

arXiv2023

Quasi-periodic pulsations in solar flares: a key diagnostic of energy release on the Sun

Solar flares are among the most powerful and disruptive events in our solar system, however the physical mechanisms driving and transporting this energetic release are not fully understood. An important signature associated with flare energy release is highly variable emission on timescales of sub-seconds to minutes which often exhibit oscillatory behaviour, features collectively known as quasi-periodic pulsations (QPPs). To fully identify the driving mechanism of QPPs, exploit their potential as a diagnostic tool, and incorporate them into our understanding of solar and stellar flares, new observational capabilities and initiatives are required. There is a clear community need for flare-focused, rapid cadence, high resolution, multi-wavelength imaging of the Sun, with high enough sensitivity and dynamic range to observe small fluctuations in intensity in the presence of a large overall intensity. Furthermore, multidisciplinary funding and initiatives are required to narrow the gap between numerical models and observations. QPPs are direct signatures of the physics occurring in flare magnetic reconnection and energy release sites and hence are critical to include in a unified flare model. Despite significant modelling and theoretical work, no single mechanism or model can fully explain the presence of QPPs in flares. Moreover, it is also likely that QPPs fall into different categories that are produced by different mechanisms. At present we have insufficient information to observationally distinguish between mechanisms. The motivation to understand QPPs is strengthened by the geo-effectiveness of flares on the Earth's ionosphere, and by the fact that stellar flares exhibit similar QPP signatures. QPPs present a golden opportunity to better understand flare physics and exploit the solar-stellary analogy, benefiting both astrophysics, heliophysics, and the solar-terrestrial connection.

astro-ph.IM↗

arXiv2022

Estimating the coronal supra-arcade downflows radio emission: from centimetre through submillimetre wavelengths

Supra-arcade downflows (SADs) are infrequent, wiggly opaque structures observed to descend through the solar corona, mostly in EUV and soft X-ray frequencies. From their physical characteristics, SADs have been interpreted as voided (subdense) bubbles and are related to magnetic reconnection processes during long-term erupting flares. In this work we use numerical MHD simulations to compute flux density maps, which are convolved with telescope beams to synthesise images with the aim to assess the expected SADs emission at radio wavelengths and propose observing strategies, including the instruments that can be used. We assume that the emission is thermal bremsstrahlung from a fully ionised plasma without any appreciable gyroresonance contribution since magnetic fields are of the order of ~10 G. We find that SADs emission should be optically thin in the frequency [10-1000] GHz range, and the spatially integrated flux should be larger than 1 Jy. We conclude, therefore, that observing SADs in radio frequencies between [0.5-1000] GHz is feasible with present instrumentation. Moreover, since the emission is for the most part optically thin, the flux density is proportional to temperature, density and line-of-sight depth, and when combined with EUV and soft X-ray images, may allow a better density and temperature determination of SADs.

astro-ph.SR↗

arXiv2021

An MHD Study of Large-Amplitude Oscillations in Solar Filaments

Quiescent filaments are usually affected by internal and/or external perturbations triggering oscillations of different kinds. In particular, external large-scale coronal waves can perturb remote quiescent filaments leading to large-amplitude oscillations. Observational reports have indicated that the activation time of oscillations coincides with the passage of a large-scale coronal wavefront through the filament, although the disturbing wave is not always easily detected. Aiming to contribute to understand how -- and to what extent -- coronal waves are able to excite filament oscillations, here we modelled with 2.5 MHD simulations a filament floating in a gravitationally stratified corona disturbed by a coronal shock wave. This simplified scenario results in a two-coupled oscillation pattern of the filament which is damped in a few cycles, enabling a detailed analysis. A parametric study was accomplished varying parameters of the scenario such as height, size and mass of the filament. An oscillatory analysis reveals a general tendency where periods of oscillations, amplitudes and damping times increase with height, whereas filaments of larger radius exhibit shorter periods and smaller amplitudes. The calculation of forces exerted on the filament shows that the main restoring force is the magnetic tension.

astro-ph.SR↗

arXiv2021

Polarity relevance in flux rope deflections triggered by coronal holes

Many observations show that coronal holes (CHs) deviate coronal mass ejections (CMEs) away from them. However, there are some peculiar events reported where the opposite occurs. To contribute to a space weather forecast efforts, in relation to the prediction of CME trajectories, we study the interaction between flux ropes (FRs) and CHs through numerical simulations. We perform 2.5D numerical simulations where FRs and CHs interact with different relative polarity configurations. We also reconstruct the trajectory and magnetic environment of a peculiar event occurred on 30 April 2012. The numerical simulations indicate that at low coronal levels, depending on the relative magnetic field polarity between the FR and the CH, the deflection will be attractive, i.e. the FR moves towards the CH (for anti-aligned polarities) or repulsive, i.e. the FR moves away to the CH (for aligned polarities). This is likely due to the formation of vanishing magnetic field regions or null points, located between the FR and the CH or, at the other side of the FR, respectively. The analysed observational event shows a double-deflection, first departing from the radial direction by approaching the CH and then moving away from it suggesting that the trajectory could result from a magnetic configuration with an anti-aligned polarity. We numerically reproduce the double deflection of the observed event, providing support to this conjecture.

astro-ph.SR↗

arXiv2020

Influence of coronal holes on CME deflections: numerical study

The understanding of the causes that produce the deflection of coronal mass ejections (CMEs) is essential for the space weather forecast. In this article, we study the effects on CMEs trajectories produced by the different properties of a coronal hole close to the ejection area. For this analysis, we perform numerical simulations of the ideal magnetohydrodynamics equations that emulate the early rising of the CME in presence of a coronal hole. We find that, the stronger the magnetic field and the wider the coronal hole area, the larger the CME deflection. This effect is reduced when the coronal hole moves away from the ejection region. To characterize this behavior, we propose a dimensionless parameter that depends on the coronal hole properties and properly quantifies the deflection. Also, we show that the presence of the coronal hole near a CME magnetic structure produces a minimum magnetic energy region which is responsible for the deflection. Thus, we find a relationship between the coronal hole properties, the location of this region and the CME deflection.

astro-ph.SR↗

arXiv2015

A Study of Radio Polarization in Protostellar Jets

Synchrotron radiation is commonly observed in connection with shocks of different velocities, ranging from relativistic shocks associated with active galactic nuclei, gamma-ray bursts or microquasars to weakly- or non-relativistic flows as those observed in supernova remnants. Recent observations of synchrotron emission in protostellar jets are important not only because they extend the range over which the acceleration process works, but also because they allow to determine the jet and/or interstellar magnetic field structure, thus giving insights on the jet ejection and collimation mechanisms. In this paper, we compute for the first time polarized (synchrotron) and non polarized (thermal X-ray) synthetic emission maps from axisymmetrical simulations of magnetized protostellar jets. We consider models with different jet velocities and variability, as well as toroidal or helical magnetic field. Our simulations show that variable, low-density jets with velocities $\sim$ 1000 km s$^{-1}$ and $\sim$ 10 times lighter than the environment can produce internal knots with significant synchrotron emission, and thermal X-rays in the shocked region of the leading bow shock moving in a dense medium. While models with a purely toroidal magnetic field show a very large degree of polarization, models with helical magnetic field show lower values and a decrease of the degree of polarization, in agreement with observations of protostellar jets.

astro-ph.SR↗