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

TL;DR

This study compares the evolution of the white-light corona during Solar Cycle 25's minimum and ascending phases with past cycles, revealing a steeper increase in activity and persistent CME activity, indicating a more active solar corona than predicted.

Contribution

It provides the first detailed comparison of coronal radiance and CME properties during SC 25 with previous cycles, highlighting unexpected activity levels and hemispheric asymmetries.

Findings

Steeper increase in coronal radiance and CME occurrence during SC 25's ascending phase.

Larger CME occurrence rate during the SC 25 minimum compared to previous minima.

Persistent large apparent width of CMEs and halo CMEs indicating ongoing heliospheric pressure.

Abstract

We 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 SC. 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 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 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 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. Phasing the descending branches of SC 23 and 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 one in agreement with several predictions and the current evolution of the hemispheric sunspot numbers. 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 the perspective of a corona more active than anticipated are extremely promising for the forthcoming observations by Solar Orbiter and Parker Solar Probe.