First light observations of the solar wind in the outer corona with the Metis coronagraph

Abstract

In this work, we present an investigation of the wind in the solar corona that has been initiated by observations of the resonantly scattered ultraviolet emission of the coronal plasma obtained with UVCS-SOHO, designed to measure the wind outflow speed by applying Doppler dimming diagnostics. Metis on Solar Orbiter complements the UVCS spectroscopic observations that were performed during solar activity cycle 23 by simultaneously imaging the polarized visible light and the H I Lyman-α corona in order to obtain high spatial and temporal resolution maps of the outward velocity of the continuously expanding solar atmosphere. The Metis observations, taken on May 15, 2020, provide the first H I Lyman-α images of the extended corona and the first instantaneous map of the speed of the coronal plasma outflows during the minimum of solar activity and allow us to identify the layer where the slow wind flow is observed. The polarized visible light (580–640 nm) and the ultraviolet H I Lyα (121.6 nm) coronal emissions, obtained with the two Metis channels, were combined in order to measure the dimming of the UV emission relative to a static corona. This effect is caused by the outward motion of the coronal plasma along the direction of incidence of the chromospheric photons on the coronal neutral hydrogen. The plasma outflow velocity was then derived as a function of the measured Doppler dimming. The static corona UV emission was simulated on the basis of the plasma electron density inferred from the polarized visible light. This study leads to the identification, in the velocity maps of the solar corona, of the high-density layer about ±10° wide, centered on the extension of a quiet equatorial streamer present at the east limb – the coronal origin of the heliospheric current sheet – where the slowest wind flows at about 160 ± 18 km s−1 from 4 R⊙ to 6 R⊙. Beyond the boundaries of the high-density layer, the wind velocity rapidly increases, marking the transition between slow and fast wind in the corona.

Possible Evidence for Shear-driven Kelvin–Helmholtz Instability along the Boundary of Fast and Slow Solar Wind in the Corona

“…During the minimum activity year 2020, the Metis coronagraph (Antonucci et al 2020b) on board Solar Orbiter (Müller et al 2020) has recently explored the hydrogen component of the solar wind within ±10°from the equatorial current sheet. This has been found to flow at about 95−185 km s −1 between 3.5 and 6.3 R e (Romoli et al 2021;Telloni et al 2021). In summary, the open magnetic field lines, filling the entire outer corona beyond a few solar radii, originate primarily from polar holes with an expansion factor varying with heliolatitude: the field divergence, lowest in the core, the source of the fast wind, rapidly increases toward the borders of the polar holes and peaks close to the hole−streamer interface, the source of the slow wind.…”

Tomography of the Solar Corona with the Metis Coronagraph I: Predictive Simulations with Visible Light Images

“…This will in principle allow 3D mapping of the Lyman-α Doppler-dimming factor. Those results could then be inverted to compute 3D maps of the solar-wind velocity (Bemporad et al, 2021;Romoli et al, 2021). This project will be the focus of a subsequent effort.…”

The eruption of 22 April 2021 as observed by SolarOrbiter, STEREO and Earth bound instruments

“…All images were taken with a 4×4 pixel binning, corresponding to a spatial scale in the plane of the sky (at 0.87 au from the Sun) of ∼ 25400 km/pixel for the VL channel and ∼ 50000 km/pixel for the UV channel. The acquired data were processed and calibrated following the procedure described in Romoli et al (2021) and subsequently updated as described in Andretta et al (2021).…”