Low polarised emission from the core of coronal mass ejections

Pagano

2015

A&A

Aims. The aim of this work is to quantify the uncertainties in the three-dimensional (3D) reconstruction of the location of coronal mass ejections (CMEs) obtained with the so-called polarization ratio technique. The method takes advantage of the different distributions along the line of sight of total (tB) and polarized (pB) brightnesses emitted by Thomson scattering to estimate the average location of the emitting plasma. This is particularly important to correctly identify of CME propagation angles and unprojected velocities, thus allowing better capabilities for space weather forecastings. Methods. To this end, we assumed two simple electron density distributions along the line of sight (a constant density and Gaussian density profiles) for a plasma blob and synthesized the expected tB and pB for different distances z of the blob from the plane of the sky and different projected altitudes ρ. Reconstructed locations of the blob along the line of sight were thus compared with the real ones, allowing a precise determination of uncertainties in the method. Results. Results show that, independently of the analytical density profile, when the blob is centered at a small distance from the plane of the sky (i.e. for limb CMEs) the distance from the plane of the sky starts to be significantly overestimated. Polarization ratio technique provides the line-of-sight position of the center of mass of what we call folded density distribution, given by reflecting and summing in front of the plane of the sky the fraction of density profile located behind that plane. On the other hand, when the blob is far from the plane of the sky, but with very small projected altitudes (i.e. for halo CMEs, ρ < 1.4 R ), the inferred distance from that plane is significantly underestimated. Better determination of the real blob position along the line of sight is given for intermediate locations, and in particular when the blob is centered at an angle of 20 • from the plane of the sky. Conclusions. These result have important consequences not only for future 3D reconstruction of CMEs with polarization ratio technique, but also for the design of future coronagraphs aimed at providing a continuous monitoring of halo-CMEs for space weather prediction purposes.

Section: Discussionmentioning

confidence: 99%

Uncertainties in polarimetric 3D reconstructions of coronal mass ejections

Pagano

2015

A&A

“…As also pointed out by Mierla et al (2011), an equation similar to (1) can be applied when dealing with the polarized components of the white-light, since the polarized emission in COR1 images, integrated along a LOS through the optically thin chromospheric plasma ejected into the corona, is due to the same three contributions. Hence we have that pB = pB Hα + pB Th + pB Th (13) and, in a similar manner to the calculation of the Thomson scattering total brightness tB Th , we can calculate the polarized brightness pB Th as:…”

Section: Estimate Of Hα Polarized Emission From the Blobsmentioning

confidence: 99%

“…Results are in good agreement for instance with Wiehr & Bianda (2003) and Leroy et al (1984), who measured the linear polarization of Hα radiation from prominences and found it varies within a few percent. Moreover, by the examination of a CME on August 31, 2007, Mierla et al (2011 obtained a Hα contribution to the total brightness emission more than 88% with a very low polarization, less than 5%.…”

Section: Estimate Of Hα Polarized Emission From the Blobsmentioning

confidence: 99%

“…The Mierla et al (2011) approach was to combine results from the tie pointing and Polarization Ratio (Moran & Davila 2004) techniques in order to justify the inconsistent position of some plasma "horns" in the 3D reconstruction of the event they studied; they ascribed this anomaly to the presence of the low polarized Hα component.…”

Section: Estimate Of Hα Polarized Emission From the Blobsmentioning

confidence: 99%

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Measurements with STEREO/COR1 data of drag forces acting on small-scale blobs falling in the intermediate corona

Dolei

Spadaro

2014

A&A

In this work we study the kinematics of three small-scale (0.01 R ) blobs of chromospheric plasma falling back to the Sun after the huge eruptive event of June 7, 2011. From a study of 3D trajectories of blobs made with the Solar TErrestrial RElations Observatory (STEREO) data, we demonstrate the existence of a significant drag force acting on the blobs and calculate two drag coefficients, in the radial and tangential directions. The resulting drag coefficients C D are between 0 and 5, comparable in the two directions, making the drag force only a factor of 0.45-0.75 smaller than the gravitational force. To obtain a correct determination of electron densities in the blobs, we also demonstrate how, by combining measurements of total and polarized brightness, the Hα contribution to the white-light emission observed by the COR1 telescopes can be estimated. This component is significant for chromospheric plasma, being between 95 and 98% of the total white-light emission. Moreover, we demonstrate that the COR1 data can be employed even to estimate the Hα polarized component, which turns out to be in the order of a few percent of Hα total emission from the blobs. If the drag forces acting on small-scale blobs reported here are similar to those that play a role during the CME propagation, our results suggest that the magnetic drag should be considered even in the CME initiation modelling.

“…Moran et al (2010) have already compared the results of the polarimetric technique with the triangulation technique using STEREO spacecraft observations. In addition, Mierla et al (2011) performed an analysis of the limitations of the polarization technique from an observational point of view. More recently, Dai et al (2014) analysed in detail the ambiguities that arise in the application of this technique and suggested a method for correctly reproducing the CME morphology.…”

Section: Introductionmentioning

confidence: 99%

Future capabilities of CME polarimetric 3D reconstructions with the METIS instrument: A numerical test

Pagano

Mackay

2015

A&A

Context. Understanding the 3D structure of coronal mass ejections (CMEs) is crucial for understanding the nature and origin of solar eruptions. However, owing to the optical thinness of the solar corona we can only observe the line of sight integrated emission. As a consequence the resulting projection effects hide the true 3D structure of CMEs. To derive information on the 3D structure of CMEs from white-light (total and polarized brightness) images, the polarization ratio technique is widely used. The soon-to-be-launched METIS coronagraph on board Solar Orbiter will use this technique to produce new polarimetric images. Aims. This work considers the application of the polarization ratio technique to synthetic CME observations from METIS. In particular we determine the accuracy at which the position of the centre of mass, direction and speed of propagation, and the column density of the CME can be determined along the line of sight. Methods. We perform a 3D MHD simulation of a flux rope ejection where a CME is produced. From the simulation we (i) synthesize the corresponding METIS white-light (total and polarized brightness) images and (ii) apply the polarization ratio technique to these synthesized images and compare the results with the known density distribution from the MHD simulation. In addition, we use recent results that consider how the position of a single blob of plasma is measured depending on its projected position in the plane of the sky. From this we can interpret the results of the polarization ratio technique and give an estimation of the error associated with derived parameters. Results. We find that the polarization ratio technique reproduces with high accuracy the position of the centre of mass along the line of sight. However, some errors are inherently associated with this determination. The polarization ratio technique also allows information to be derived on the real 3D direction of propagation of the CME. The determination of this is of fundamental importance for future space weather forecasting. In addition, we find that the column density derived from white-light images is accurate and we propose an improved technique where the combined use of the polarization ratio technique and white-light images minimizes the error in the estimation of column densities. Moreover, by applying the comparison to a set of snapshots of the simulation we can also assess the errors related to the trajectory and the expansion of the CME. Conclusions. Our method allows us to thoroughly test the performance of the polarization ratio technique and allows a determination of the errors associated with it, which means that it can be used to quantify the results from the analysis of the forthcoming METIS observations in white light (total and polarized brightness). Finally, we describe a satellite observing configuration relative to the Earth that can allow the technique to be efficiently used for space weather predictions.