Mapping the solar wind HI outflow velocity in the inner heliosphere by coronagraphic ultraviolet and visible-light observations

Abstract

We investigated the capability of mapping the solar wind outflow velocity of neutral hydrogen atoms by using synergistic visible-light and ultraviolet observations. We used polarised brightness images acquired by the LASCO/SOHO and Mk3/MLSO coronagraphs, and synoptic Lyα line observations of the UVCS/SOHO spectrometer to obtain daily maps of solar wind H I outflow velocity between 1.5 and 4.0 R⊙ on the SOHO plane of the sky during a complete solar rotation (from 1997 June 1 to 1997 June 28). The 28-days data sequence allows us to construct coronal off-limb Carrington maps of the resulting velocities at different heliocentric distances to investigate the space and time evolution of the outflowing solar plasma. In addition, we performed a parameter space exploration in order to study the dependence of the derived outflow velocities on the physical quantities characterising the Lyα emitting process in the corona. Our results are important in anticipation of the future science with the Metis instrument, selected to be part of the Solar Orbiter scientific payload. It was conceived to carry out near-sun coronagraphy, performing for the first time simultaneous imaging in polarised visible-light and ultraviolet H I Lyα line, so providing an unprecedented view of the solar wind acceleration region in the inner corona.

Metis: the Solar Orbiter visible light and ultraviolet coronal imager

“…For example, Gibson et al (1999) and Vasquez et al (2003) provide such profiles for an equatorial streamer and polar regions, respectively. These profiles appear to better reproduce the electron temperatures inferred by several observations below 2 R , and their reliability at higher altitudes is supported by several studies, as already discussed by Dolei et al (2018). The kinetic temperature of the coronal hydrogen atoms T k,n can be provided by the extended database that was constructed by Dolei et al (2016) from the analysis of a very large set of UVCS spectrometric data, complemented by the results of the temperature of protons obtained by the in situ instruments.…”

Models and data analysis tools for the Solar Orbiter mission

“…Assuming that heavy ions have the same temperature as the main plasma constituents and some typical coronal abundances, the brightness of the corona in EUV predicted by 3D models can be compared with EUV images from STEREO and SDO. Techniques that exploit images and spectroscopy have also been put in place to infer the distribution of coronal plasma temperature and speed (see section 4.3) (e.g., Dolei et al 2018) that can also be compared with coronal models. In this respect, white-light images are crucial to determine if the position and brightness of streamers and pseudo-streamers are correctly reproduced.…”

“…The analysis of UVCS daily Lyα synoptic data, in combination with electron densities derived from the white-light RS observations, has allowed to derive H I outflow speed maps (see e.g. Dolei et al 2018Dolei et al , 2019. One of the methods based on the synergy between UV and WL observations is the Doppler dimming technique (Hyder & Lites 1970;Noci et al 1987;Withbroe et al 1982).…”

“…The best match between computed and observed Lyα intensity provides an estimate of the solar wind H I outflow velocity. Figure 9 shows an example of 2D outflow velocity map, in the range of heliocentric distances between 1.5 and 4.0 R , obtained via Doppler dimming technique (Dolei et al 2018). The speed values radially increase with altitude up to about 150-200 km s −1 in the equatorial regions and 400 km s −1 in the polar regions.…”

“…Once 2D maps of the electron density (hence mass density ρ) and outflow speed distribution have been derived on the plane of the sky (as shown by Dolei et al 2018), these maps can be further analysed to provide very important information on the solar wind acceleration and coronal heating energy deposition processes. Following the analysis formerly performed by ; ; Withbroe (1988), and most recently by Bemporad (2017), the first step corresponds to write the momentum equation for the solar wind dynamic, by considering a single fluid, stationary, and spherically symmetric solar wind flow, and by adding an unknown external force f ext responsible for the solar wind acceleration:…”