Hot prominence detected in the core of a coronal mass ejection: Analysis of SOHO/UVCS Lαand SOHO/LASCO visible-light observations

Abstract

Context. The paper deals with the physics of erupting prominences in the core of coronal mass ejections (CME). Aims. We determine the physical parameters of an erupting prominence embedded in the core of a CME using SOHO/UVCS hydrogen L α and L β lines and SOHO/LASCO visible light observations. In particular we analyze the CME event observed on August 2, 2000. We develop the non-LTE (NLTE; i.e. considering departures from the local thermodynamic equilibrium – LTE) spectral diagnostics based on L α and visible light observations. Methods. Our method is based on 1D NLTE modeling of eruptive prominences and takes into account the effect of large flow velocities, which reach up to 300 km s -1 for the studied event (the so-called Doppler dimming). The NLTE radiative-transfer method can be used for both optically thin and thick prominence structures. We combine spectroscopic UVCS observations of an erupting prominence in the core of a CME with visible light images from LASCO-C2 in order to derive the geometrical parameters like projected thickness and velocity, together with the effective temperature and column density of electrons. These are then used to constrain our NLTE radiative transfer modeling which provides the kinetic temperature, microturbulent velocity, gas pressure, ionization degree, the line opacities, and the prominence effective thickness (geometrical filling factor). Results. Analysis was made for 69 observational points (spatial pixels) inside the whole erupting prominence. Roughly one-half of them show a non-negligible L α optical thickness for flow velocity 300 km s -1 and about one-third for flow velocity 150 km s -1 . All pixels with L α τ 0 ≤ 0.3 have been considered for further analysis, which is presented in the form of statistical distributions (histograms) of various physical quantities such as the kinetic temperature, gas pressure, and electron density for two representative flow velocities (150 and 300 km s -1 ) and non-zero microturbulence. For two pixels co-temporal LASCO visible-light data are also available, which further constrains the diagnostics of the electron density and effective thickness. Detailed NLTE modeling is presented for various sets of input parameters. Conclusions. The studied CME event shows that the erupting prominence expands to large volumes, meaning that it is a low-pressure structure with low electron densities and high temperatures. This analysis provides a basis for future diagnostics using the METIS coronagraph on board the Solar Orbiter mission.