Oxygen temperature anisotropy and solar wind heating above coronal holes out to 5 R $_\odot$

Spadaro

Ventura

2016

A&A

Context. Synergistic visible light and ultraviolet coronagraphic observations are essential to investigate the link of the Sun to the inner heliosphere through the study of the dynamic properties of the solar wind. Aims. We perform spectroscopic mapping of the outer solar corona to constitute a statistically significant database of neutral hydrogen coronal temperatures, which is suitable for overcoming the lack of spectrometric information in observations performed by coronagraphs that are solely equipped for visible light and ultraviolet imaging; these include the forthcoming Metis instrument on board Solar Orbiter. Methods. We systematically analysed neutral hydrogen Lyα line data that was obtained by UVCS/SOHO observations of the extended solar corona relevant to a lot of polar, mid-latitude and equatorial structures at different phases of solar activity, and collected far longer than a whole solar cycle (1996−2012). Results. We created a database consisting in both the neutral hydrogen temperature components, which are perpendicular and parallel to the radially symmetric coronal magnetic field lines, as a function of the heliocentric distance and polar angle and for different phases of the solar activity cycle. We validated the reliability of the constituted neutral hydrogen temperature database, investigating a new set of UVCS Lyα data with the Doppler dimming technique. The solar wind outflow velocities obtained by adopting both the neutral hydrogen temperature distribution directly derived from the observed Lyα profiles and those taken from our database well agree within the uncertainties.

Section: H I Temperature Anisotropymentioning

confidence: 97%

Mapping the coronal hydrogen temperature in view of the forthcoming coronagraph observations by Solar Orbiter

Spadaro

Ventura

2016

A&A

“…It is worth noting that T HI contains both the thermal component and that due to plasma flows and waves, and is generally assumed isotropic, although it can be anisotropic in coronal structures, such as coronal holes (see e.g. Dolei et al 2016;Telloni et al 2007). The last parameter u w is the outflow velocity of the scattering H I atom in P due to the solar wind.…”

Section: H I Lyα Line Radiation In the Solar Coronamentioning

confidence: 99%

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

Susino

Sasso

et al. 2018

A&A

Self Cite

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.

“…Heavier ions such as O VI exhibit spectral line widths increasing up to 100 MK at 3 R (Cranmer et al 1999a;Antonucci et al 2000a,b;Akinari 2008). At higher altitudes, the oxygen kinetic temperature flattens with fluctuations around its maximum value of about 100 MK (Antonucci 2006;Antonucci et al 2012;Telloni et al 2007).…”

Section: Introductionmentioning

confidence: 99%

“…The considerably high temperatures found in coronal holes for O VI ions have been explained as the signature of preferential plasma heating and acceleration via the absorption of Alfvén waves at ion cyclotron frequency (Cranmer et al 1999b;Hu et al 1999;Telloni et al 2007). Understanding the trend observed for the H I Lyα line width beyond 3 R requires further investigation with the prospect of obtaining accurate analyses of UVCS/SOHO observations.…”

Section: Introductionmentioning

confidence: 99%

Investigating the behaviour of neutral hydrogen Lyα spectral line width in polar coronal holes at solar minimum

Spadaro

Susino

et al. 2017

A&A

Self Cite

We investigate the behaviour of the H I Lyα spectral line widths measured by UVCS/SOHO in polar coronal holes at minimum of solar magnetic activity. The line widths are reported to significantly increase up to 3 R , while above 3 R there is observational evidence of either nearly constant or slightly decreasing values. We adopt empirical models of polar coronal holes at solar activity minimum reported in the literature and calculate the characteristic timescales relevant to different processes coupling neutral hydrogen atoms and protons, which are heated and accelerated in the outflowing plasma. This analysis leads us to believe that the progressive decoupling of the two sets of particles below 10 R , caused by the decrease of the plasma density due to the rapid expansion of the wind, cannot explain the behaviour of the Lyα line profile observed in polar coronal holes. We also synthesise the intensity and profile of the Lyα line as a function of heliocentric distance from the coronal hole models, adopting H I densities computed in non-equilibrium ionisation with the aim of satisfactorily reproducing the UVCS Lyα observations reported in the literature. Our analysis shows that the coronal Lyα emission decreases with heliocentric distance, down to values below the interplanetary Lyα emission, owing to the decrease of the plasma density and to non-equilibrium ionisation effects in the expanding plasma. This can lead to the predominance of the interplanetary emission, which is characterised by H I velocity distributions corresponding to temperatures about one order of magnitude lower than the coronal temperatures, and to the narrowing of the resulting coronal profile at higher heliocentric distances. This scenario can be a plausible explanation for the behaviour of the Lyα line profile with height observed in polar coronal holes at solar activity minimum.