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This work studies the kinematics of the leading edge and the core of 6 Coronal Mass Ejections (CMEs) in the combined field of view of Sun Watcher using Active Pixel System detector and Image Processing (SWAP) on-board PRoject for On-Board Autonomy (PROBA-2) and the ground-based K-Cor coronagraph of the Mauna Loa Solar Observatory (MLSO). We report, for the first time, on the existence of a critical height h$_\mathrm{c}$, which marks the onset of velocity dispersion inside the CME. This height for the studied events lies between 1.4-1.8 R$_{\odot}$, in the inner corona. We find the critical heights to be relatively higher for gradual CMEs, as compared to impulsive ones, indicating that the early initiation of these two classes might be different physically. We find several interesting imprints of the velocity dispersion on CME kinematics. The critical height is strongly correlated with the flux-rope minor radius and the mass of the CME. Also, the magnitude of velocity dispersion shows a reasonable positive correlation with the above two parameters. We believe these results will advance our understanding of CME initiation mechanisms and will help provide improved constraints to CME initiation models., Comment: 16 Pages, 9 Figures, Accepted for publication in The Astrophysical Journal Letters

On 2022 February 15, an impressive filament eruption was observed off the solar eastern limb from three remote-sensing viewpoints, namely Earth, STEREO-A, and Solar Orbiter. In addition to representing the most-distant observed filament at extreme ultraviolet wavelengths -- captured by Solar Orbiter's field of view extending to above 6 $R_{\odot}$ -- this event was also associated with the release of a fast ($\sim$2200 km$\cdot$s$^{-1}$) coronal mass ejection (CME) that was directed towards BepiColombo and Parker Solar Probe. These two probes were separated by 2$^{\circ}$ in latitude, 4$^{\circ}$ in longitude, and 0.03 au in radial distance around the time of the CME-driven shock arrival in situ. The relative proximity of the two probes to each other and to the Sun ($\sim$0.35 au) allows us to study the mesoscale structure of CMEs at Mercury's orbit for the first time. We analyse similarities and differences in the main CME-related structures measured at the two locations, namely the interplanetary shock, the sheath region, and the magnetic ejecta. We find that, despite the separation between the two spacecraft being well within the typical uncertainties associated with determination of CME geometric parameters from remote-sensing observations, the two sets of in-situ measurements display some profound differences that make understanding of the overall 3D CME structure particularly challenging. Finally, we discuss our findings within the context of space weather at Mercury's distances and in terms of the need to investigate solar transients via spacecraft constellations with small separations, which has been gaining significant attention during recent years., Comment: 31 pages, 13 figures, 5 tables, accepted for publication in The Astrophysical Journal

Magnetic flux ropes are a key component of coronal mass ejections, forming the core of these eruptive phenomena. However, determining whether a flux rope is present prior to eruption onset and, if so, the rope's handedness and the number of turns that any helical field lines make is difficult without magnetic field modelling or in-situ detection of the flux rope. We present two distinct observations of plasma flows along a filament channel on 4 and 5 September 2022 made using the \textit{Solar Orbiter} spacecraft. Each plasma flow exhibited helical motions in a right-handed sense as the plasma moved from the source active region across the solar disk to the quiet Sun, suggesting that the magnetic configuration of the filament channel contains a flux rope with positive chirality and at least one turn. The length and velocity of the plasma flow increased from the first to the second observation, suggesting evolution of the flux rope, with the flux rope subsequently erupting within $\sim$5~hours of the second plasma flow. The erupting flux rope then passed over the \textit{Parker Solar Probe} spacecraft during its Encounter 13, enabling \textit{in-situ} diagnostics of the structure. Although complex and consistent with the flux rope erupting from underneath the heliospheric current sheet, the \textit{in-situ} measurements support the inference of a right-handed flux rope from remote-sensing observations. These observations provide a unique insight into the eruption and evolution of a magnetic flux rope near the Sun., Comment: 19 pages, 11 figures, accepted for publication in The Astrophysical Journal

(Shortened version) Current efforts in space weather forecasting of CMEs have been focused on predicting their arrival time and magnetic structure. To make predictions, methods have been developed to derive the true CME speed, size and position, among others. Difficulties in determining input parameters for CME forecasting arise from the lack of direct measurements of the coronal magnetic fields and uncertainties in estimating the CME 3D geometric and kinematic parameters. White-light coronagraph images are usually employed by a variety of CME reconstruction techniques. We explore how subjectivity affects the 3D CME parameters that are obtained from the GCS reconstruction technique. We have designed two different synthetic scenarios where the ``true'' geometric parameters are known in order to quantify such uncertainties for the first time. We explore this as follows: 1) Using the ray-tracing option from the GCS model software, and 2) Using 3D MHD simulation data from the MAS code. Our experiment includes different viewing configurations using single and multiple viewpoints. CME reconstructions using a single viewpoint had the largest errors and error ranges for both synthetic GCS and simulated MHD white-light data. Increasing the number of viewpoints to two, the errors decreased by about 4$^\circ$ in latitude, 22$^\circ$ in longitude, 14$^\circ$ in tilt, and 10$^\circ$ in half-angle, pointing towards a need for at least two viewpoints. We found the following CME parameter error bars as a starting point for quantifying the minimum error in CME parameters from white-light reconstructions: $\Delta\theta$ (latitude)=${6^\circ}$, $\Delta\phi$ (longitude)=${11^\circ}$, $\Delta\gamma$ (tilt)=${25^\circ}$, $\Delta\alpha$ (half-angle)=${10^\circ}$, $\Delta h$ (height)=$0.6 R_{\odot}$, and $\Delta\kappa$ (ratio)=$0.1$.

The Flux Rope in 3D (FRi3D, Isavnin, 2016), a coronal mass ejection (CME) model with global three-dimensional (3D) geometry, has been implemented in the space weather forecasting tool EUHFORIA (Pomoell and Poedts, 2018). By incorporating this advanced flux rope model in EUHFORIA, we aim to improve the modelling of CME flank encounters and, most importantly, the magnetic field predictions at Earth. After using synthetic events to showcase FRi3D's capabilities of modelling CME flanks, we optimize the model to run robust simulations of real events and test its predictive capabilities. We perform observation-based modelling of the halo CME event that erupted on 12 July 2012. The geometrical input parameters are constrained using the forward modelling tool included in FRi3D with additional flux rope geometry flexibilities as compared to the pre-existing models. The magnetic field input parameters are derived using the differential evolution algorithm to fit FRi3D parameters to the in situ data at 1 AU. An observation-based approach to constrain the density of CMEs is adopted, in order to achieve a better estimation of mass corresponding to the FRi3D geometry. The CME is evolved in EUHFORIA's heliospheric domain and a comparison of FRi3D's predictive performance with the previously implemented spheromak CME in EUHFORIA is presented. For this event, FRi3D improves the modelling of the total magnetic field magnitude and Bz at Earth by ~30% and ~70%, respectively. Moreover, we compute the expected geoeffectiveness of the storm at Earth using an empirical Dst model and find that the FRi3D model improves the predictions of minimum Dst by ~20% as compared to the spheromak CME model. Finally, we discuss the limitations of the current implementation of FRi3D in EUHFORIA and propose possible improvements., Comment: 22 pages, 14 figures

In this work we performed a polarimetric study of a fast and wide coronal mass ejection (CME) observed on 12 July 2012 by the COR1 and COR2 instruments onboard Solar TErrestrial RElations Observatory (STEREO) mission. The CME source region was an X1.4 flare located at approximately S15W01 on the solar disk as observed from the Earth's perspective. The position of the CME as derived from the 3D Graduated Cylindrical Shell (GCS) reconstruction method was at around S18W00 at 2.5 solar radii and S07W00 at 5.7 solar radii, meaning that the CME was deflected towards the Equator while propagating outward in the corona. The projected speed of the leading edge of the CME also evolved from around 200 km s$^{-1}$ in the lower corona to around 1000 km s$^{-1}$ in the COR2 field of view. The degree of polarisation of the CME is around 65 % but it can go as high as 80 % in some CME regions. The CME showed deviation of the polarisation angle from the tangential in the range of 10$^\circ$ - 15$^\circ$ (or more). Our analysis showed that this is mostly due to the fact that the sequence of three polarised images from where the polarised parameters are derived is not taken simultaneously, but at a difference of few seconds in time. In this interval of time, the CME is moving by at least two pixels in the FOV of the instruments and this displacement results in uncertainties in the polarisation parameters (degree of polarisation, polarisation angle, etc.). We propose some steps forward to improve the derivation of the polarisation. This study is important for analysing the future data from instruments with polarisation capabilities.

Aims. Our goal is to thoroughly analyse the dynamics of single and multiple solar eruptions, as well as a stealth ejecta. The data were obtained through self-consistent numerical simulations performed in a previous study. We also assess the effect of a different background solar wind on the propagation of these ejecta to Earth. Methods. We calculated all the components of the forces contributing to the evolution of the numerically modelled consecutive coronal mass ejections (CMEs) obtained with the 2.5D magnetohydrodynamics (MHD) module of the code MPI-AMRVAC. We analysed the thermal and magnetic pressure gradients and the magnetic tension dictating the formation of several flux ropes in different locations in the aftermath of the eruptions. These three components were tracked in the equatorial plane during the propagation of the CMEs to Earth. Their interaction with other CMEs and with the background solar wind was also studied. Results. We explain the formation of the stealth ejecta and the plasma blobs (or plasmoids) occurring in the aftermath of solar eruptions. We also address the faster eruption of a CME in one case with a different background wind, even when the same triggering boundary motions were applied, and attribute this to the slightly different magnetic configuration and the large neighbouring arcade. The thermal pressure gradient revealed a shock in front of these slow eruptions, formed during their propagation to 1 AU. The double-peaked magnetic pressure gradient indicates that the triggering method affects the structure of the CMEs and that a part of the adjacent streamer is ejected along with the CME.

Context. Observations reveal that shocks can be driven by fast coronal mass ejections (CMEs) and play essential roles in particle accelerations. A critical ratio, $\delta$, derived from a shock standoff distance normalized by the radius of curvature (ROC) of a CME, allows us to estimate shock and ambient coronal parameters. However, true ROCs of CMEs are difficult to measure due to observed projection effects. Aims. We investigate the formation mechanism of a shock driven by an aspherical CME without evident lateral expansion. Through three-dimensional (3D) reconstructions without a priori assumptions of the object morphology, we estimate two principal ROCs of the CME surface and demonstrate how the difference between two principal ROCs of the CME affects the estimate of the coronal physical parameters. Methods. The CME was observed by the Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI) instruments and the Large Angle and Spectrometric Coronagraph (LASCO). We used the mask-fitting method to obtain the irregular 3D shape of the CME and reconstructed the shock surface using the bow-shock model. Through smoothings with fifth-order polynomial functions and Monte Carlo simulations, we calculated the ROCs at the CME nose. Results. We find that (1) the maximal ROC is 2-4 times the minimal ROC of the CME. A significant difference between the CME ROCs implies that the assumption of one ROC of an aspherical CME could cause over-/under- estimations of the shock and coronal parameters. (2) The shock nose obeys the bow-shock formation mechanism, considering the constant standoff distance and the similar speed between the shock and CME around the nose. (3) With a more precise $\delta$ calculated via 3D ROCs in space, we derive corona parameters at high latitudes of about -50$^{\circ}$, including the Alfv{\'e}n speed and the coronal magnetic field strength., Comment: 11 pages, 7 figures, 1 table, accepted by A&A

Aims: Our goal is to propagate multiple eruptions - obtained through numerical simulations performed in a previous study - to 1 AU and to analyse the effects of different background solar winds on their dynamics and structure at Earth. We also aim to improve the understanding of why some consecutive eruptions do not result in the expected geoeffectiveness, and how a secondary coronal mass ejection (CME) can affect the configuration of the preceding one. Methods: Using the 2.5D magnetohydrodynamics (MHD) package of the code MPI-AMRVAC, we numerically modeled consecutive CMEs inserted in two different solar winds by imposing shearing motions onto the inner boundary. The initial magnetic configuration depicts a triple arcade structure shifted southward, and embedded into a bimodal solar wind. We compared our simulated signatures with those of a multiple CME event in Sept 2009 using data from spacecraft around Mercury and Earth. We computed and analysed the Dst index for all the simulations performed. Results: The observed event fits well at 1 AU with two of our simulations, one with a stealth CME and the other without. This highlights the difficulty of attempting to use in situ observations to distinguish whether or not the second eruption was stealthy, because of the processes the flux ropes undergo during their propagation in the interplanetary space. We simulate the CMEs propagated in two different solar winds, one slow and another faster one. Only in the first case, plasma blobs arise in the trail of eruptions. Interestingly, the Dst computation results in a reduced geoeffectiveness in the case of consecutive CMEs when the flux ropes arrive with a leading positive Bz. When the Bz component is reversed, the geoeffectiveness increases, meaning that the magnetic reconnections with the trailing blobs and eruptions strongly affect the impact of the arriving interplanetary CME., Comment: 15 pages, 13 figures

Geomagnetic storms are an important aspect of space weather and can result in significant impacts on space- and ground-based assets. The majority of strong storms are associated with the passage of interplanetary coronal mass ejections (ICMEs) in the near-Earth environment. In many cases, these ICMEs can be traced back unambiguously to a specific coronal mass ejection (CME) and solar activity on the frontside of the Sun. Hence, predicting the arrival of ICMEs at Earth from routine observations of CMEs and solar activity currently makes a major contribution to the forecasting of geomagnetic storms. However, it is clear that some ICMEs, which may also cause enhanced geomagnetic activity, cannot be traced back to an observed CME, or, if the CME is identified, its origin may be elusive or ambiguous in coronal images. Such CMEs have been termed "stealth CMEs." In this review, we focus on these "problem" geomagnetic storms in the sense that the solar/CME precursors are enigmatic and stealthy. We start by reviewing evidence for stealth CMEs discussed in past studies. We then identify several moderate to strong geomagnetic storms (minimum Dst < -50 nT) in solar cycle 24 for which the related solar sources and/or CMEs are unclear and apparently stealthy. We discuss the solar and in situ circumstances of these events and identify several scenarios that may account for their elusive solar signatures. These range from observational limitations (e.g., a coronagraph near Earth may not detect an incoming CME if it is diffuse and not wide enough) to the possibility that there is a class of mass ejections from the Sun that have only weak or hard-to-observe coronal signatures. In particular, some of these sources are only clearly revealed by considering the evolution of coronal structures over longer time intervals than is usually considered. We also review a variety of numerical modelling approaches..., Comment: 60 pages, 25 figures, 5 tables. Accepted for publication in Space Science Reviews

Eruptions of coronal mass ejections (CMEs) from the Sun are usually associated with a number of signatures that can be identified in solar disc imagery. However, there are cases in which a CME that is well observed in coronagraph data is missing a clear low-coronal counterpart. These events have received attention during recent years, mainly as a result of the increased availability of multi-point observations, and are now known as 'stealth CMEs'. In this work, we analyse examples of stealth CMEs featuring various levels of ambiguity. All the selected case studies produced a large-scale CME detected by coronagraphs and were observed from at least one secondary viewpoint, enabling a priori knowledge of their approximate source region. To each event, we apply several image processing and geometric techniques with the aim to evaluate whether such methods can provide additional information compared to the study of "normal" intensity images. We are able to identify at least weak eruptive signatures for all events upon careful investigation of remote-sensing data, noting that differently processed images may be needed to properly interpret and analyse elusive observations. We also find that the effectiveness of geometric techniques strongly depends on the CME propagation direction with respect to the observers and the relative spacecraft separation. Being able to observe and therefore forecast stealth CMEs is of great importance in the context of space weather, since such events are occasionally the solar counterparts of so-called 'problem geomagnetic storms'., Comment: 26 pages, 8 figures, 1 table, accepted for publication in Frontiers in Astronomy and Space Sciences

Coronal mass ejections (CMEs) and solar energetic particles (SEPs) are two phenomena that can cause severe space weather effects throughout the heliosphere. The evolution of CMEs, especially in terms of their magnetic structure, and the configuration of the interplanetary magnetic field (IMF) that influences the transport of SEPs are currently areas of active research. These two aspects are not necessarily independent of each other, especially during solar maximum when multiple eruptive events can occur close in time. Accordingly, we present the analysis of a CME that erupted on 2012 May 11 (SOL2012-05-11) and an SEP event following an eruption that took place on 2012 May 17 (SOL2012-05-17). After observing the May 11 CME using remote-sensing data from three viewpoints, we evaluate its propagation through interplanetary space using several models. Then, we analyse in-situ measurements from five predicted impact locations (Venus, Earth, the Spitzer Space Telescope, the Mars Science Laboratory en route to Mars, and Mars) in order to search for CME signatures. We find that all in-situ locations detect signatures of an SEP event, which we trace back to the May 17 eruption. These findings suggest that the May 11 CME provided a direct magnetic connectivity for the efficient transport of SEPs. We discuss the space weather implications of CME evolution, regarding in particular its magnetic structure, and CME-driven IMF preconditioning that facilitates SEP transport. Finally, this work remarks the importance of using data from multiple spacecraft, even those that do not include space weather research as their primary objective., Comment: 50 pages, 14 figures, 2 tables, accepted for publication in Space Weather

Solar eruptions such as Coronal Mass Ejections (CMEs) observed in the inner solar corona (up to 4 R$_{\odot}$) show acceleration profiles which appear as parabolic ridges in height-time plots. Inspired by the white-light automated detection algorithms, Computer Aided CME Tracking System (CACTus) and Solar Eruptive Events Detection System (SEEDS), we employ the parabolic Hough Transform for the first time to automatically detect off-disk solar eruptions {from height-time plots. Due to the limited availability of white-light observations in the inner corona, we use Extreme UltraViolet (EUV) images of the Sun. In this paper we present a new algorithm, CME Identification in Inner Solar Corona (CIISCO), which is based on Fourier motion filtering and the parabolic Hough transform, and demonstrate its implementation using EUV observations taken from {\it Atmospheric Imaging Assembly} (AIA) on-board the Solar Dynamics Observatory (SDO), {\it Extreme Ultra Violet Imager} (EUVI) on-board the STEREO-A and B satellites, and {\it Sun Watcher using Active Pixel System detector and Image Processing} (SWAP) Imager on-board PROBA2. We show that CIISCO is able to identify any {off-disk} outward moving feature in EUV images. The use of automated detection algorithms, like CIISCO, can potentially be used to provide early warnings of CMEs if an EUV telescope is located at $\pm$ 90$^{\circ}$ from the Sun-Earth line, providing CME characteristics and kinematics close to the Sun. This paper also presents the limitations of this algorithm and the prospects for future improvement., Comment: Accepted for publication in Solar Physics; 27 pages, 10 figures, 1 table

Coronal waves are large-scale disturbances often driven by coronal mass ejections (CMEs). We investigate a spectacular wave event on 7 March 2012, which is associated with an X5.4 flare (SOL2012-03-07). By using a running center-median (RCM) filtering method for the detection of temporal variations in extreme ultraviolet (EUV) images, we enhance the EUV disturbance observed by the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) and the Sun Watcher using Active Pixel System detector and Image Processing (SWAP) onboard the PRoject for Onboard Autonomy 2 (PROBA2). In coronagraph images, a halo front is observed to be the upper counterpart of the EUV disturbance. Based on the EUV and coronagraph images observed from three different perspectives, we have made three-dimensional (3D) reconstructions of the wave surfaces using a new mask-fitting method. The reconstructions are compared with those obtained from forward-fitting methods. We show that the mask fitting method can reflect the inhomogeneous coronal medium by capturing the concave shape of the shock wave front. Subsequently, we trace the developing concave structure and derive the deprojected wave kinematics. The speed of the 3D-wave nose increases from a low value below a few hundred $\mathrm{km\,s^{-1}}$ to a maximum value of about 3800 $\mathrm{km\,s^{-1}}$, and then slowly decreases afterwards. The concave structure starts to decelerate earlier and has significantly lower speeds than those of the wave nose. We also find that the 3D-wave in the extended corona has a much higher speed than the speed of EUV disturbances across the solar disk., Comment: Solar Physics: Topical Collection "Ten Years of Solar Observations with PROBA2", accepted

We use The Sun Watcher with Active Pixel System detector and Image Processing (SWAP) imager onboard the Project for Onboard Autonomy 2 (PROBA2) mission to study the evolution of large-scale EUV structures in the solar corona observed throughout Solar Cycle 24 (from 2010 to 2019). We discuss the evolution of the on-disk coronal features and at different heights above the solar surface based on EUV intensity changes. We also look at the evolution of the corona in equatorial and polar regions and compare them at different phases of the solar cycle, as well as with sunspot number evolution and with the PROBA2/Lyman-Alpha Radiometer (LYRA) signal. The main results are as follows: The three time series (SWAP on-disk average brightness, sunspot number and LYRA irradiance) are very well correlated, with correlation coefficients around 0.9. The average rotation rate of bright features at latitudes of +15, 0, and -15 degrees was around 15 degree/day throughout the period studied. A secondary peak in EUV averaged intensity at the Poles was observed on the descending phase of SC24. These peaks (at North and South poles respectively) seem to be associated with the start of the development of the (polar) coronal holes. Large-scale off-limb structures were visible from around March 2010 to around March 2016, meaning that they were absent at the minimum phase of solar activity. A fan at the North pole persisted for more than 11 Carrington rotations (February 2014 to March 2015), and it could be seen up to altitudes of 1.6 Rs.

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., Comment: 20 pages, 10 figures, 3 tables, accepted for publication in The Astrophysical Journal

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.

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., Comment: Accepted for publication in Astrophysical Journal

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., Comment: 11 pages, 9 figures, to appear in The Astrophysical Journal

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., Comment: 29 pages, 29 (colour) figures, 2 tables

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., Comment: 3 figures

Coronal fans (see Figure 1) are bright observational structures that extend to large distances above the solar surface and can easily be seen in EUV (174 angstrom) above the limb. They have a very long lifetime and can live up to several Carrington rotations (CR), remaining relatively stationary for many months. Note that they are not off-limb manifestation of similarly-named active region fans. The solar conditions required to create coronal fans are not well understood. The goal of this research was to find as many associations as possible of coronal fans with other solar features and to gain a better understanding of these structures. Therefore, we analyzed many fans and created an overview of their properties. We present the results of this statistical analysis and also a case study on the longest living fan.