UV core dimming in coronal streamer belt and the projection effects

During solar minimum activity, the coronal structure is dominated by a tilted streamer belt, associated with the sources of the slow solar wind. It is known that some UV coronal spectral observations show a quite evident core dimming in heavy ions emission in quiescent streamers. In this paper, our purpose is to investigate this phenomenon by comparing observed and simulated UV coronal ion spectral line intensities. First, we computed the emissivities and the intensities of HI Lyα and OVI spectral lines starting from the physical parameters of a time-dependent 3D three-fluid MHD model of the coronal streamer belt. The model is applied to a tilted dipole (10°) solar minimum magnetic structure. Next, we compared the results obtained from the model in the extended corona (from 1.5 to 4 R⊙) to the UV spectroscopic data from the Ultraviolet Coronagraph Spectrometer (UVCS) onboard SOHO during the minimum of solar activity (1996). We investigate the line-of-sight integration and projection effects in the UV spectroscopic observations, disentangled by the 3D multifluid model. The results demonstrate that the core dimming in heavy ions is produced by the physical processes included in the model (i.e., combination of the effects of heavy ion gravitational settling, and energy exchange of the preferentially heated heavy ions through the interaction with electrons and protons) but it is visible only in some cases where the magnetic structure is simple, such as a (tilted) dipole.

Three-dimensional multi-fluid model of a coronal streamer belt with a tilted magnetic dipole

Observations of streamers in extreme ultraviolet (EUV) emission with SOHO/UVCS show dramatic differences in line profiles and latitudinal variations in heavy ion emission compared to hydrogen Ly-α emission. In order to use ion emission observations of streamers as the diagnostics of the slow solar wind properties, an adequate model of a streamer including heavy ions is required. We extended a previous 2.5-D multi-species magnetohydrodynamics (MHD) model of a coronal streamer to 3-D spherical geometry, and in the first approach we consider a tilted dipole configuration of the solar magnetic field. The aim of the present study is to test the 3-D results by comparing to previous 2.5-D model result for a 3-D case with moderate departure from azimuthal symmetry. The model includes O5+ ions with preferential empirical heating and allows for calculation of their density, velocity and temperature in coronal streamers. We present the first results of our 3-D multi-fluid model showing the parameters of protons, electrons and heavy ions (O5+) at the steady-state solar corona with a tilted steamer belt. We find that the 3-D results are in qualitative agreement with our previous 2.5-D model, and show longitudinal variation in the variables in accordance with the tilted streamer belt structure. Properties of heavy coronal ions obtained from the 3-D model together with EUV spectroscopic observations of streamers will help understanding the 3-D structures of streamers reducing line-of-sight integration ambiguities and identifying the sources of the slow solar wind in the lower corona. This leads to improved understanding of the physics of the slow solar wind.

On the influence of CMEs on the global 3-D coronal electron density

In order to analize the influence of a Coronal Mass Ejection (CME) on the coronal streamer belt, we made 3-D reconstructions of the electron density in the corona at heliospheric distances from 1.5 to 4 R⊙ for periods before and after a CME occured. The reconstructions were performed using a tomography technique. We studied two CME cases: (i) a slow CME on 1 June 2008; (ii) two fast CMEs on 31 December 2007 and 2 January 2008. For the first case of slow CME, it was found: (i) the potential magnetic field configuration in the CME initiation region before the CME does not agree with the coronal density structure while after the CME the agreement between the field and density is much better. This could be manifistation of that that the field was non-potential before the CME and after the CME the field relaxes towards a more potential state. (ii) It was shown that the dimming caused by the slow CME is not due to rotation of the corona and a line-of-sight (LOS) effect but a streamer blow out effect took place.