5 research outputs found
Numerical simulations of shear-induced consecutive coronal mass ejections
Methods: Stealth CMEs represent a particular class of solar eruptions that
are clearly distinguished in coronagraph observations, but they don't have a
clear source signature. A particular type of stealth CMEs occurs in the
trailing current sheet of a previous ejection, therefore, we used the 2.5D MHD
package of the code MPI-AMRVAC to numerically simulate consecutive CMEs by
imposing shearing motions onto the inner boundary. The initial magnetic
configuration consists of a triple arcade structure embedded into a bimodal
solar wind, and the sheared polarity inversion line is found in the southern
loop system. The mesh was continuously adapted through a refinement method that
applies to current carrying structures. We then compared the obtained eruptions
with the observed directions of propagation of an initial multiple coronal mass
ejection (MCME) event that occurred in September 2009. We further analysed the
simulated ejections by tracking the centre of their flux ropes in latitude and
their total speed. Radial Poynting flux computation was employed as well to
follow the evolution of electromagnetic energy introduced into the system.
Results: Changes within 1\% in the shearing speed result in three different
scenarios for the second CME, although the preceding eruption seems
insusceptible to such small variations. Depending on the applied shearing
speed, we thus obtain a failed eruption, a stealth, or a CME driven by the
imposed shear, as the second ejection. The dynamics of all eruptions are
compared with the observed directions of propagation of an MCME event and a
good correlation is achieved. The Poynting flux analysis reveals the temporal
variation of the important steps of eruptions. For the first time, a stealth
CME is simulated in the aftermath of a first eruption, through changes in the
applied shearing speed.Comment: 11 pages, 12 figures, to be published in "Astronomy & Astrophysics",
and the associated movies will also be available on the journal's websit
Effect of the solar wind density on the evolution of normal and inverse coronal mass ejections
Context. The evolution of magnetised coronal mass ejections (CMEs) and their interaction with the background solar wind leading to deflection, deformation, and erosion is still largely unclear as there is very little observational data available. Even so, this evolution is very important for the geo-effectiveness of CMEs.
Aims. We investigate the evolution of both normal and inverse CMEs ejected at different initial velocities, and observe the effect of the background wind density and their magnetic polarity on their evolution up to 1 AU.
Methods. We performed 2.5D (axisymmetric) simulations by solving the magnetohydrodynamic equations on a radially stretched grid, employing a block-based adaptive mesh refinement scheme based on a density threshold to achieve high resolution following the evolution of the magnetic clouds and the leading bow shocks. All the simulations discussed in the present paper were performed using the same initial grid and numerical methods.
Results. The polarity of the internal magnetic field of the CME has a substantial effect on its propagation velocity and on its deformation and erosion during its evolution towards Earth. We quantified the effects of the polarity of the internal magnetic field of the CMEs and of the density of the background solar wind on the arrival times of the shock front and the magnetic cloud. We determined the positions and propagation velocities of the magnetic clouds and thus also the stand-off distance of the leading shock fronts (i.e. the thickness of the magnetic sheath region) and the deformation and erosion of the magnetic clouds during their evolution from the Sun to the Earth. Inverse CMEs were found to be faster than normal CMEs ejected in the same initial conditions, but with smaller stand-off distances. They also have a higher magnetic cloud length, opening angle, and mass. Synthetic satellite time series showed that the shock magnitude is not affected by the polarity of the CME. However, the density peak of the magnetic cloud is dependent on the polarity and, in case of inverse CMEs, also on the background wind density. The magnitude of the z-component of the magnetic field was not influenced by either the polarity or the wind density.status: publishe
Ultrahigh-resolution model of a breakout CME embedded in the solar wind
Aims. We investigate the effect of a background solar wind on breakout coronal mass ejections, in particular, the effect on the different current sheets and the flux rope formation process.
Methods. We obtained numerical simulation results by solving the magnetohydrodynamics equations on a 2.5D (axisymmetric) stretched grid. Ultrahigh spatial resolution is obtained by applying a solution adaptive mesh refinement scheme by increasing the grid resolution in regions of high electrical current, that is, by focussing on the maximum resolution of the current sheets that are forming. All simulations were performed using the same initial base grid and numerical schemes; we only varied the refinement level.
Results. A background wind that causes a surrounding helmet streamer has been proven to have a substantial effect on the current sheets that are forming and thus on the dynamics and topology of the breakout release process. Two distinct ejections occur: first, the top of the helmet streamer detaches, and then the central arcade is pinched off behind the top of the helmet streamer. This is different from the breakout scenario that does not take the solar wind into account, where only the central arcade is involved in the eruption. In the new ultrahigh-resolution simulations, small-scale structures are formed in the lateral current sheets, which later merge with the helmet streamer or reconnect with the solar surface. We find that magnetic reconnections that occur at the lateral breakout current sheets deliver the major kinetic energy contribution to the eruption and not the reconnection at the so-called flare current sheet, as was seen in the case without background solar wind
Comparing the heliospheric cataloging, analysis, and techniques service (HELCATS) manual and automatic catalogues of coronal mass ejections using solar terrestrial relations observatory/heliospheric Imager (STEREO/HI) Data
We present the results of a comparative study between automatic and manually compiled coronal mass ejection (CME) catalogues based on observations from the Heliospheric Imagers (HIs) onboard NASAâs Solar Terrestrial Relations Observatory (STEREO) spacecraft. Using the Computer Aided CME Tracking software (CACTus), CMEs are identified in HI data using an automatic feature-detection algorithm, while the Heliospheric Imagers Catalogue (HICAT) includes CMEs that are detected by visual inspection of HI images. Both catalogues were compiled as part of the EU FP7 Heliospheric Cataloguing, Analysis and Techniques Service (HELCATS) project (www.helcats-fp7.eu). We compare observational parameters of the CMEs from CACTus to those listed in HICAT, such as CME frequency, position angle (PA), and PA-width. We also compare CACTus-derived speeds to speeds derived from applying geometric modelling to the majority of the HICAT CMEs, the results of which are listed in the HELCATS Heliospheric Imagers Geometric Catalogue (HIGeoCAT). We find that both CACTus and HICAT catalogues contain a similar number of events when we exclude events narrower than 20â, which are not included in the HICAT catalogue but are found to be identified by CACTus. PA-distributions are strongly peaked around 90â and 270â, with a slightly larger CME frequency northwards of the equatorial plane (particularly for the STEREO-A versions of both catalogues). The CME PA-widths in both HICAT and CACTus catalogues peak at approximately 60â. Manually derived speeds from HIGeoCAT and automatically derived speeds by CACTus correlate well for values lower than 1000 kmâsâ1, in particular when CMEs are propagating close to the plane of the sky
Comparing the Heliospheric Cataloging, Analysis, and Techniques Service (HELCATS) Manual and Automatic Catalogues of Coronal Mass Ejections Using Solar Terrestrial Relations Observatory/Heliospheric Imager (STEREO/HI) Data
We present the results of a comparative study between automatic and
manually-compiled coronal mass ejection (CME) catalogues based on observations
from the Heliospheric Imagers (HIs) on-board NASA's Solar Terrestrial
Relations Observatory (STEREO) spacecraft. Using CACTus, CMEs are identified in HI data using an automatic feature detection algorithm, while the HICAT
catalogue includes CMEs that are detected by visual inspection of HI images.
Both catalogues were compiled as part of the EU FP7 HELCATS (Heliospheric
Cataloguing, Analysis and Techniques Service) projects (http://www.helcats-fp7.
eu/). We compare observational parameters of the CMEs from CACTus to those
listed in HICAT, such as CME frequency, position angle (PA) and PA width. We
also compare CACTus-derived speeds to speeds derived from applying geometric
modelling to the majority of the HICAT CMEs, the results of which are listed
in the HELCATS HIGeoCAT catalogue. We find that both CACTus and HICAT
catalogues contain a similar number of events when we exclude events narrower
than 20 degrees, which are not included in the HICAT catalogue but found to be identified by CACTus. PA distributions are strongly peaked around 90 degrees and 270 degress, with
a slightly larger CME frequency northwards of the equatorial plane (particularly
for the STEREO-A versions of both catalogues). The CME PA widths in both
HICAT and CACTus catalogues peak at approximately 60 degrees. Manual speeds from
HIGeoCAT and automatically derived speeds by CACTus correlate well for values
lower than 1000km/s, in particular when CMEs are propagating close to the
plane of the sky.status: publishe