arXiv subjects

Marilena Mierla

Publications and source records attributed to Marilena Mierla.

24 records · Page 2Linked to original sources

arXiv2019

Multipoint study of successive coronal mass ejections driving moderate disturbances at 1 AU

We analyse in this work the propagation and geoeffectiveness of four successive coronal mass ejections (CMEs) that erupted from the Sun during 21--23 May 2013 and that were detected in interplanetary space by the Wind and/or STEREO-A spacecraft. All these CMEs featured critical aspects for understanding so-called "problem space weather storms" at Earth. In the first three events a limb CMEs resulted in moderately geoeffective in-situ structures at their target location in terms of the disturbance storm time (Dst) index (either measured or estimated). The fourth CME, which also caused a moderate geomagnetic response, erupted from close to the disc centre as seen from Earth, but it was not visible in coronagraph images from the spacecraft along the Sun--Earth line and appeared narrow and faint from off-angle viewpoints. Making the correct connection between CMEs at the Sun and their in-situ counterparts is often difficult for problem storms. We investigate these four CMEs using multiwavelength and multipoint remote-sensing observations (extreme ultraviolet, white light, and radio), aided by 3D heliospheric modelling, in order to follow their propagation in the corona and in interplanetary space and to assess their impact at 1 AU. Finally, we emphasise the difficulties in forecasting moderate space weather effects provoked by problematic and ambiguous events and the importance of multispacecraft data for observing and modelling problem storms.

astro-ph.SR↗

arXiv2019

Observation-based modelling of magnetised Coronal Mass Ejections with EUHFORIA

Coronal Mass Ejections (CMEs) are the primary source of strong space weather disturbances at Earth. Their geoeffectiveness is largely determined by their dynamic pressure and internal magnetic fields, for which reliable predictions at Earth are not possible with traditional cone CME models. We study two Earth-directed CMEs using the EUropean Heliospheric FORecasting Information Asset (EUHFORIA) model, testing the predictive capabilities of a linear force-free spheromak CME model initialised using parameters derived from remote-sensing observations. Using observation-based CME input parameters, we perform MHD simulations of the events with the cone and spheromak CME models. Results show that spheromak CMEs propagate faster than cone CMEs when initialised with the same kinematic parameters. We interpret these differences as due to different Lorentz forces acting within cone and spheromak CMEs, leading to different CME expansions. Such discrepancies can be mitigated by initialising spheromak CMEs with a reduced speed corresponding to the radial speed only. Results at Earth evidence that the spheromak model improves the predictions of B(Bz) up to 12-60(22-40) percentage points compared to a cone model. Considering virtual spacecraft located around Earth, B(Bz) predictions reach 45-70%(58-78%) of the observed peak values. The spheromak model predicts inaccurate magnetic field parameters at Earth for CMEs propagating away from the Sun-Earth line, while it successfully predicts the CME properties and arrival time in the case of strictly Earth-directed events. Modelling CMEs propagating away from the Sun-Earth line requires extra care due to limitations related to the assumed spherical shape. The spatial variability of modelling results and the typical uncertainties in the reconstructed CME direction advocate the need to consider predictions at Earth and at virtual spacecraft around it.

astro-ph.SR↗

arXiv2019

Evolution of Coronal Cavity from Quiescent to Eruptive Phase in Association with Coronal Mass Ejection

We present the evolution of a coronal cavity encompassing its quiescent and eruptive phases in the lower corona. Using the multi-vantage point observations from the SDO/AIA, STEREO SECCHI/EUVI and PROBA2/SWAP EUV imagers, we capture the sequence of quasi-static equilibria of the quiescent cavity which exhibited a slow rise and expansion phase during its passage on the solar disc from 2010 May 30 to 2010 June 13. By comparing the decay-index profiles of the cavity system during the different stages of its quiescent and pre-eruptive phases we find that the decay-index value at the cavity centroid height can be used as a good indicator to predict the cavity eruption in the context of torus instability. Combining the observations of SWAP and LASCO C2/C3 we show the evolution of the EUV cavity into the white-light cavity as a three-part structure of the associated CME observed to erupt on 2010 June 13. By applying successive geometrical fits to the cavity morphology we find that the cavity exhibited non self-similar expansion in the lower corona, below 2.2 +/- 0.2 Rs, which points to the spatial scale for the radius of source surface where the coronal magnetic field lines are believed to become radial. Furthermore, the kinematic study of the erupting cavity captures both the "impulsive" and "residual" phases of acceleration along with a strong deflection of the cavity at 1.3 Rs. We also discuss the role of driving forces behind the dynamics of the morphological and kinematic evolution of the cavity.

astro-ph.SR↗

arXiv2014

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 -- and 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 SDO and STEREO 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/HMI 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.

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arXiv2012

The SWAP EUV Imaging Telescope. Part II: In-flight Performance and Calibration

The Sun Watcher with Active Pixel System detector and Image Processing (SWAP) telescope was launched on 2 November 2009 onboard the ESA PROBA2 technological mission and has acquired images of the solar corona every one - two minutes for more than two years. The most important technological developments included in SWAP are a radiation-resistant CMOS-APS detector and a novel onboard data-prioritization scheme. Although such detectors have been used previously in space, they have never been used for long-term scientific observations on orbit. Thus SWAP requires a careful calibration to guarantee the science return of the instrument. Since launch we have regularly monitored the evolution of SWAP detector response in-flight to characterize both its performance and degradation over the course of the mission. These measurements are also used to reduce detector noise in calibrated images (by subtracting dark-current). Since accurate measurements of detector dark-current require large telescope off-points, we have also monitored straylight levels in the instrument to ensure that these calibration measurements are not contaminated by residual signal from the Sun. Here we present the results of these tests, and examine the variation of instrumental response and noise as a function of both time and temperature throughout the mission.

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arXiv2010

SWAP-SECCHI Observations of a Mass-Loading Type Solar Eruption

We present a three-dimensional reconstruction of an eruption that occurred on 3 April 2010 using observations from SWAP onboard PROBA2 and SECCHI onboard STEREO. The event unfolded in two parts: an initial flow of cooler material confined to low in the corona, followed by a flux rope eruption higher in the corona. We conclude that mass off-loading from the first part triggered a rise, and, subsequently, catastrophic loss of equilibrium of the flux rope.

astro-ph.SR↗