Abstract
Due to the scarcity of UV–EUV observations of coronal mass ejections (CMEs) far from the Sun (i.e., at heliocentric distances larger than 1.5 Rsun) our understanding of the thermodynamic evolution of these solar phenomena is still very limited. This work focuses on the analysis of a slow CME observed at the same time and in the same coronal locations in visible light (VL) by the MLSO Mark IV polarimeter and in the UV Lyman-α by the SOHO UVCS spectrometer. The eruption was observed at two different heliocentric distances (1.6 and 1.9 Rsun), making this work a test case for possible future multi-slit observations of solar eruptions. The analysis of combined VL and UV data allows the determination of 2D maps of the plasma electron density and also the plasma electron temperature, thus allowing the quantification of the distribution of the thermal energy density. The results show that the higher temperatures in the CME front are due to simple adiabatic compression of pre-CME plasma, while the CME core has a higher temperature with respect to the surrounding CME void and front. Despite the expected adiabatic cooling, the CME core temperatures increased between 1.6 and 1.9 Rsun from 2.4 MK up to 3.2 MK, thus indicating the presence of plasma heating processes occurring during the CME expansion. The 2D distribution of thermal energy also shows a low level of symmetry with respect to the CME propagation axis, possibly related with the CME interaction with nearby coronal structures. This work demonstrates the potential of UV and VL data combination and also of possible future multi-slit spectroscopic observations of CMEs.
Highlights
For decades the origin and the early evolution of plasma embedded in coronal mass ejections (CMEs) have been studied via remote sensing observations [1,2]
This work demonstrates again the potential of UV and visible light (VL) data combination to determine the thermodynamic evolution of coronal mass ejections (CMEs)
Using unique observations acquired on 31 January 2000 by the UVCS spectrometers of the same CME at two different heliocentric distances (1.6 and 1.9 Rsun), it was possible to investigate the thermal evolution of the same CME structures, the CME front, void, and core
Summary
For decades the origin and the early evolution of plasma embedded in coronal mass ejections (CMEs) have been studied via remote sensing observations [1,2]. The CME early evolutions are typically studied with space- and ground-based coronagraphs such as the instruments (Mark IV, K-Cor, CoMP) at the Mauna Loa Solar Observatory (MLSO) [3], the LASCO coronagraphs on board SOHO [4,5], and the SECCHI COR1 and COR2 instruments on board STEREO [6,7] All these instruments measure the visible light (VL) emission, which is due to Thomson scattering of photospheric radiation by coronal electrons [8], and provides estimates of the local plasma column density and number density [9] (once the emission from the F-corona is removed [10]), as well as the projected expansion velocities by tracking specific features and applying image filtering analysis [11,12], or even the un-projected velocities by combining observations from different points of view [13,14]. After a description of the data (Secion 2), the results are presented (Secion 3) and discussed in the light of current and future space missions (Secion 4)
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