Solar abundances have been historically assumed to be representative of cosmic abundances. However, our knowledge of the solar abundance of helium, the second most abundant element, relies mainly on models1 and indirect measurements through helioseismic observations2, because actual measurements of helium in the solar atmosphere are very scarce. Helium cannot be directly measured in the photosphere because of its high first ionization potential, and measurements of its abundance in the inner corona have been sporadic3,4. In this Letter, we present simultaneous global images of the helium (out to a heliocentric distance of 3R⊙ (solar radii)) and hydrogen emission in the solar corona during the minimum of solar activity of cycle 23 and directly derive the helium abundance in the streamer region and surrounding corona (out to 2.2R⊙). The morphology of the He+ corona is markedly different from that of the H corona, owing to significant spatial variations in helium abundance. The observations show that the helium abundance is shaped according to and modulated by the structure of the large-scale coronal magnetic field and that helium is almost completely depleted in the equatorial regions during the quiet Sun. This measurement provides a trace back to the coronal source of the anomalously slow solar wind observed in the heliosphere at the Sun–Earth Lagrangian point L1 in 2009, during the exceptionally long-lasting minimum of solar activity cycle 23.
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The imaging data that support the plots within this paper and other findings of this study are available from the NASA Goddard Coordinated Data Analysis Workshop (CDAW) Data Center (https://cdaw.gsfc.nasa.gov/) or from the corresponding author on reasonable request. The data from Fig. 2 are provided as Source data files.
All relevant codes used during this study are available from the corresponding author on reasonable request. Source data are provided with this paper.
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The HERSCHEL suborbital investigation was funded by the NASA Heliophysics Living With a Star Program. The SCORE coronagraph (principal investigator, E.A.) was funded by the Ministero dell’Istruzione, dell’Università e della Ricerca (MIUR) under grant COFIN 2002, the Italian Space Agency (ASI) under grant ASI-I/015/07/0 and the Osservatorio Astrofisico di Torino of the Istituto Nazionale di Astrofisica (INAF). The HECOR coronagraph (principal investigator, F.A.) was developed at IAS and funded by Centre National d’Etudes Spatiales (CNES). The magnetic field extrapolations were performed using models that are part of the SolarModels facilities. SolarModels is supported by CNES. J.M.L. and D.W. were supported by NASA Heliophysics Supporting Research (NNH16AC39I), Heliophysics Grand Challenges (NNH17AE96I), Heliophysics Guest Investigators (80HQTR19T0029) and by basic research funds of the CNR. The HERSCHEL team would like to express their extreme gratitude to Dave Roberts, who, although he is no longer with us, continues to inspire by his example and dedication to the many missions he helped over the course of his long career. E.A. would also like to acknowledge A. Gherardi, L. Gori, G. Noci, E. Pace, D. Paganini, D. Gardiol, D. Loreggia, V. Da Deppo, M. G. Pelizzo, G. Naletto, P. Nicolosi and G. Tondello for their contributions to the development of SCORE.
The authors declare no competing interests.
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Moses, J.D., Antonucci, E., Newmark, J. et al. Global helium abundance measurements in the solar corona. Nat Astron 4, 1134–1139 (2020). https://doi.org/10.1038/s41550-020-1156-6