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The State of the White-Light Corona over the Minimum and Ascending Phases of Solar Cycle 25 - Comparison with Past Cycles

AuthorLamy, Philippe; Gilardy, Hugo;
KeywordsParker Data Used; Corona; K-corona; Activity; Astrophysics - Solar and Stellar Astrophysics
AbstractWe report on the state of the corona over the minimum and ascending phases of Solar Cycle (SC) 25 on the basis of the temporal evolutions of its radiance and of the properties of coronal mass ejections (CMEs), as determined from white-light observations performed by the SOHO/LASCO-C2 coronagraph. These evolutions are further compared with those determined during the past two Solar Cycles using the same methods. The integrated radiance of the K-corona and the occurrence rate of CMEs closely track the indices/proxies of solar activity, prominently the total magnetic field for the radiance and the 10.7 cm radio flux for the CMEs, all undergoing a steep increase during the ascending phase of SC 25. This increase is much steeper than anticipated on the basis of the predicted quasi-similarity between SC 25 and SC 24, and is confirmed by the recent evolution of the sunspot number. The radiance reached the same base level during the minima of SC 24 and SC 25, but the latitudinal extent of the streamer belt differed, being flatter during the latter minimum and in fact more similar to that of the minimum of SC 23. Synchronizing the descending branches of SC 23 and SC 24 led to a duration of SC 24 of 11.0 years, similar to that given by the sunspot number. In contrast, the base level of the occurrence rate of CMEs during the minimum of SC 25 was significantly larger than during the two previous minima. The southern hemisphere is conspicuously more active than the northern, in agreement with several predictions and the current evolution of the hemispheric sunspot numbers. In particular, the occurrence rate of the subset of CMEs with known mass, their mass rate, and the number of CMEs with speeds larger than 350 km s$^\ensuremath-1$ in the southern hemisphere exceeds by far the respective values in the northern hemisphere. The mean apparent width of CMEs and the number of halo CMEs remains at relatively large, constant levels throughout the early phase of SC 25, implying the persistence of weak total pressure in the heliosphere. These results, and particularly the perspective of a corona being more active than anticipated, are extremely promising for the forthcoming observations by both Solar Orbiter and Parker Solar Probe.
Year of Publication2022
Number of Pages140
Date Publishedoct