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

Abstract

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.

Uncertainties in polarimetric 3D reconstructions of coronal mass ejections

“…When this emission is included (as for STEREO coronagraphs) Hα produces a strong spurious emission which, because it is not due to Thomson scattering by free electrons, provides unreliable results when the polarization ratio technique is applied to the prominence plasma often embedded in CME cores. At the same time, when the 3D position of the emitting prominence plasma can be determined via triangulation, a combination with the polarization ratio technique allows one to distinguish between the polarization due to Thomson scattering and Hα polarized emission, as recently shown by Dolei et al (2014).…”

Evolution of Magnetic Rayleigh–Taylor Instability into the Outer Solar Corona and Low Interplanetary Space

“…The tension force associated with the magnetic field suppresses the mixing of lighter and denser material at small scales thus plasma bubbles and fingers are evolved with higher growth rate (Stone et al 2007). Dolei et al (2014) describes a method to calculate the electron density in moving solar coronal structures using STEREO/COR-1 brightness images. They derive that electron density (n e , which can be estimated by solving the quadratic equation:…”

“…Energy release of the impacting prominence has also been observed by different EUV filters of SDO and STEREO (Gilbert et al 2013). The measurement of drag force acting on plasma fingers during the density evolution of returning plasma have also been estimated into intermediate corona using STEREO/COR-1 data (Dolei et al 2014).…”

“…Its upper northward segment shows spatio-temporal evolution of MRT instability in form of finger structures upto the outer corona and low inter-planetary space. Using method of Dolei et al(2014), It is estimated that the density in each bright finger is greater than corresponding dark region lying below of it in the surrounding intermediate corona. The instability is evolved due to wave perturbations that are parallel to the magnetic field at the density interface.…”

Visible light and ultraviolet observations of coronal structures: physical properties of an equatorial streamer and modelling of the F corona

“…Therefore, the technique can be used better in the case of STEREO coronagraphic observations. However, during investigations of eruptive events with STEREO it is necessary to take into account that the total brightness can be affected by the Hα emission of the chromospheric neutral hydrogen atoms raised into the corona, as discussed by Dolei et al (2014a). Actually, this approach will turn out to be very useful in the perspective of the METIS instrument, which will provide simultaneous total and polarized brightness images between 1.6 and 5.5 R and in the spectral range 580-640 nm.…”