222 research outputs found
Low polarized emission from the core of coronal mass ejections
In white-light coronagraph images, cool prominence material is sometimes
observed as bright patches in the core of coronal mass ejections (CMEs). If, as
generally assumed, this emission is caused by Thomson-scattered light from the
solar surface, it should be strongly polarised tangentially to the solar limb.
However, the observations of a CME made with the SECCHI/STEREO coronagraphs on
31 August 2007 show that the emission from these bright core patches is
exceptionally low polarised. We used the polarisation ratio method of Moran and
Davila (2004) to localise the barycentre of the CME cloud. By analysing the
data from both STEREO spacecraft we could resolve the plane-of-the-sky
ambiguity this method usually suffers from. Stereoscopic triangulation was used
to independently localise the low-polarisation patch relative to the cloud. We
demonstrated for the first time that the bright core material is located close
to the centre of the CME cloud. We show that the major part of the CME core
emission, more than 85% in our case, is H radiation and only a small
fraction is Thomson-scattered light. Recent calculations also imply that the
plasma density in the patch is 8 10 cm or more compared to 2.6
10 cm for the Thomson-scattering CME environment surrounding the
core material.Comment: 5 pages, 3 figure
Comparisons of CME morphological characteristics derived from five 3D reconstruction methods
We compare different methods to reconstruct the three-dimensional (3D) CME
morphology. The explored methods include geometric localisation, mask fitting,
forward modeling, polarisation ratio and local correlation tracking plus
triangulation. The five methods are applied to the same CME event, which
occurred on August 7 2010. Their corresponding results are presented and
compared, especially in their propagation direction and spatial extent in 3D.
We find that mask fitting and geometric localisation method produce consistent
results. Reconstructions including three-view observations are more precise
than reconstructions done with only two views. Compared to the forward modeling
method, in which a-priori shape of the CME geometry is assumed, mask fitting
has more flexibility. Polarisation ratio method makes use of the Thomson
scattering geometry. We find spatially the 3D CME derived from mask fitting
lies mostly in the overlap region obtained with the polarisation method from
COR2 A and B. In addition, mask fitting can help resolve the front/back
ambiguity inherent in the polarisation ratio method. However, local correlation
tracking plus triangulation did not show a consistent result with the other
four methods. For reconstructions of a diffuse CME, when the separation angle
between STEREO A and B is large, finding two corresponding points in a STEREO
image pair becomes very difficult. Excluding the local correlation tracking
method, the latitude of the CME's centre of gravity derived from the other
methods deviates within one degree and longitude differs within 19 degrees.Comment: to appear in Solar Physic
Observation-based modelling of magnetised coronal mass ejections with EUHFORIA
Context. Coronal mass ejections (CMEs) are the primary source of strong space weather disturbances at Earth. Their geo-effectiveness 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. Aims. We study two well-observed Earth-directed CMEs using the EUropean Heliospheric FORecasting Information Asset (EUH-FORIA) model, testing for the first time the predictive capabilities of a linear force-free spheromak CME model initialised using parameters derived from remote-sensing observations. Methods. Using observation-based CME input parameters, we performed magnetohydrodynamic simulations of the events with EU-HFORIA, using the cone and spheromak CME models. Results. Simulations show that spheromak CMEs propagate faster than cone CMEs when initialised with the same kinematic parameters. We interpret these differences as the result of different Lorentz forces acting within cone and spheromak CMEs, which lead to different CME expansions in the heliosphere. Such discrepancies can be mitigated by initialising spheromak CMEs with a reduced speed corresponding to the radial speed only. Results at Earth provide evidence that the spheromak model improves the predictions of B (B-z) by up to 12-60 (22-40) percentage points compared to a cone model. Considering virtual spacecraft located within +/- 10 degrees around Earth, B (Bz) predictions reach 45-70% (58-78%) of the observed peak values. The spheromak model shows inaccurate predictions of the magnetic field parameters at Earth for CMEs propagating away from the Sun-Earth line. Conclusions. The spheromak model successfully predicts the CME properties and arrival time in the case of strictly Earth-directed events, while 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 located around it.Peer reviewe
Using A Kernel P System to Solve The 3-Col Problem
The newly introduced Kernel P systems offer an unitary and
elegant way of integrating established features of existing P system variants
with new elements with potential value for formal modelling. This
paper presents a case study illustrating the expressive power and efficiency
of kernel P systems on the 3-Col problem. The use of model
checking (in particular of Spin) for formal verification of kernel P systems
is also discussed and illustrated in this case.Ministerio de Ciencia e Innovación TIN2009–13192Junta de AndalucÃa P08–TIC–0420
Space Weather Monitor at the L5 Point: A Case Study of a CME Observed with STEREO B
An important location for future space weather monitoring is the Lagrange point 5 (L5) of the Sun-Earth system. We test the performance of L5 for space weather monitoring using STEREO B observations of an Earth-directed coronal mass ejection (CME), seen as a partial halo by SOHO at L1. STEREO B (located close to L5) continuously tracked the CME. By using these data in combination with methods to calculate the CME arrival time at the Earth (extrapolation, drag-based model, and a magnetohydrodynamic model), we demonstrate that the estimation of the CME arrival time can be drastically improved by adding L5 data. Based on the L1 data alone, one could predict that the CME would arrive at the Earth. Using only the L5 data, one would not expect an arrival, as the estimations of the CME 3-D configuration is uncertain. The combination of L1 and L5 data leads to an ambiguous prediction of the CME arrival due to low CME brightness in L1 data. To obtain an unambiguous prediction, one needs its 3-D configuration, from observing the CME material close to the plane of the sky from at least two viewpoints (in this case L5 and, coincidentally, L4). This event demonstrates that L1 observations may be better to determine CME arrival, but L5 observations are superior for constraining arrival time. In this work, the advantages and caveats of using data from a space weather monitor at L5 for predicting interplanetary propagation of CMEs are discussed and demonstrated in a direct case study
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
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