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Register a new userEffects of the chromospheric Ly{alpha} line profile shape on the determination of the solar wind HI outflow velocity using the Doppler dimming technique
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Added by
Giuseppe Emanuele Capuano
Publication date
2021
fields
Physics
and research's language is
English
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The determination of solar wind outflow velocity is fundamental in order to probe the mechanisms of wind acceleration in the corona. We aim to study, via the Doppler dimming technique, the effects that the chromospheric Ly{alpha} line profile shape causes on the determination of the outflow speed of coronal HI atoms. The Doppler dimming technique takes into account the decrease of coronal Ly{alpha} radiation in regions where HI atoms flow out in the solar wind. Starting from UV observations (UVCS/SOHO) of the coronal Ly{alpha} line and simultaneous measurements of pB (LASCO/SOHO and Mk3/MLSO), we studied the effect of the pumping chromospheric Ly{alpha} line profile through measurements from SOHO/SUMER, UVSP/SMM and LPSP/OSO-8, taken from representative on-disk regions and as a function of time during the solar activity cycle. In particular, we considered the effect of four chromospheric line parameters: line width, depth of the central reversal, asymmetry and distance of the peaks. We find that the range of variability of these parameters is of about 50% for the width, 69% for the depth of the central reversal, 35% for the asymmetry, and 50% for the distance of the peaks. Then, we find that the variability of the pumping Ly{alpha} profile affects the estimates of the coronal HI velocity by about 9-12%. Therefore, this uncertainty is smaller than other physical quantities uncertainties, and a constant in time and unique shape of the Ly{alpha} profile over the solar disk can be adopted in order to estimate the solar wind outflow velocity.
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Recent studies of interstellar neutral (ISN) hydrogen observed by the Interstellar Boundary Explorer (IBEX) suggested that the present understanding of the radiation pressure acting on hydrogen atoms in the heliosphere should be revised. There is a significant discrepancy between theoretical predictions of the ISN H signal using the currently used model of the solar Lyman-alpha profile by Tarnopolski et al. 2009 (TB09) and the signal due to ISN H observed by IBEX-Lo. We developed a new model of evolution of the solar Lyman-alpha profile that takes into account all available observations of the full-disk solar Lyman-alpha profiles from SUMER/SOHO, provided by Lemaire et al. 2015 (L15), covering practically the entire 23rd solar cycle. The model has three components that reproduce different features of the profile. The main shape of the emission line that is produced in the chromosphere is modeled by the kappa function; the central reversal due to absorption in the transition region is modeled by the Gauss function; the spectral background is represented by the linear function. The coefficients of all those components are linear functions of the line-integrated full-disk Lyman-alpha irradiance, which is the only free parameter of the model. The new model features potentially important differences in comparison with the model by TB09, which was based on a limited set of observations. This change in the understanding of radiation pressure, especially during low solar activity, may significantly affect the interstellar H and D distributions in the inner heliosphere and their derivative populations.
We present a modification of a model of solar cycle evolution of the solar Lyman-alpha line profile, along with a sensitivity study of interstellar neutral H hydrogen to uncertainties in radiation pressure level. The line profile model, originally developed by Kowalska-Leszczynska et al. 2018a, is parametrized by the composite solar Lyman-alpha flux, which recently was revised Machol et al. 2019. We present modified parameters of the previously-developed model of solar radiation pressure for neutral hydrogen and deuterium atoms in the heliosphere. The mathematical function used in the model, as well as the fitting procedure, remain unchanged. We show selected effects of the model modification on ISN H properties in the heliosphere and we discuss the sensitivity of these quantities to uncertainties in the calibration of the composite Lyman-alpha series.
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