Periodic behaviour of coronal mass ejections, eruptive events, and solar activity proxies during solar cycles 23 and 24

We report on the parallel analysis of the periodic behaviour of coronal mass ejections (CMEs) based on 21 years [1996 -- 2016] of observations with the SOHO/LASCO--C2 coronagraph, solar flares, prominences, and several proxies of solar activity. We consider values of the rates globally and whenever possible, distinguish solar hemispheres and solar cycles 23 and 24. Periodicities are investigated using both frequency (periodogram) and time-frequency (wavelet) analysis. We find that these different processes, in addition to following the $\approx$11-year Solar Cycle, exhibit diverse statistically significant oscillations with properties common to all solar, coronal, and heliospheric processes: variable periodicity, intermittence, asymmetric development in the northern and southern solar hemispheres, and largest amplitudes during the maximum phase of solar cycles, being more pronounced during solar cycle 23 than the weaker cycle 24. However, our analysis reveals an extremely complex and diverse situation. For instance, there exists very limited commonality for periods of less than one year. The few exceptions are the periods of 3.1--3.2 months found in the global occurrence rates of CMEs and in the sunspot area (SSA) and those of 5.9--6.1 months found in the northern hemisphere. Mid-range periods of $\approx$1 and $\approx$2 years are more wide spread among the studied processes, but exhibit a very distinct behaviour with the first one being present only in the northern hemisphere and the second one only in the southern hemisphere. These periodic behaviours likely results from the complexity of the underlying physical processes, prominently the emergence of magnetic flux.Comment: 33 pages, 15 figures, 2 table

Restoration of the K and F Components of the Solar Corona from LASCO-C2 Images over 24 Years [1996 – 2019]

International audienceWe present a photometrically accurate restoration of the K- and F-coronae from white-light images obtained over 24 years [1996 – 2019] by the Large-Angle Spectrometric COronagraph (LASCO-C2) onboard the Solar and Heliospheric Observatory (SOHO). The procedure starts with the data set of unpolarized images of 512 × 512 pixels produced by the polarimetric analysis of the routine C2 polarization sequences (Lamy et al., Solar Phys. 295, 89, 2020) in which the F-corona, the instrumental stray light, and possible remnants of the K-corona due to the imperfect polarimetric separation are entangled. Disentangling these components requires a complex procedure organized in three stages, each composed of several steps. Stage 1 establishes the distinct variations of the radiance of these components with the Sun–SOHO distance, and generate a new data set of median images calculated for each Carrington rotation. Stage 2 achieves the restoration of a set of 36 stray-light images that account for the temporal variation of the stray-light pattern, in particular those associated with the periodic roll maneuvers of SOHO, which started in 2003. Stage 3 achieves the restoration of the F-corona, and a time series of daily images is generated. Combining these images with the set of stray-light images allowed us to process the whole set of routine LASCO-C2 images of 1024 × 1024 pixels (approximately 626,000 images) and to produce calibrated, high-resolution images of the K-corona. The two sets of images of the K-corona, that produced by polarimetric separation of 512 × 512 pixels images and that presently produced by subtraction, are in excellent photometric agreement. We extend our past conclusions that the temporal variation of the integrated radiance of the K-corona tracks the solar activity over Solar Cycles 23 and 24, and that it is highly correlated with the temporal variation of the total magnetic field. The behaviors of the integrated radiance during the last few years of the declining phases of Solar Cycles 23 and 24 are remarkably similar, reaching the same base level and leading to a duration of 11.0 years for the latter cycle, in agreement with that derived from sunspots

Coronal Photopolarimetry with the LASCO-C2 Coronagraph over 24 Years [1996 – 2019]

International audienceWe present an in-depth characterization of the polarimetric channel of the Large-Angle Spectrometric COronagraph/LASCO-C2 onboard the Solar and Heliospheric Observatory (SOHO). The polarimetric analysis of the white-light images makes use of polarized sequences composed of three images obtained though three polarizers oriented at +60∘, 0∘, and −60∘, complemented by a neighboring unpolarized image, and relies on the formalism of Mueller. The Mueller matrix characterizing the C2 instrument was obtained through extensive ground-based calibrations of the optical components and global laboratory tests. Additional critical corrections were derived from in-flight tests relying prominently on roll sequences of SOHO, on the basis of consistency criteria (e.g. the “tangential” direction of polarization), and from several applications, notably the time-dependent tomographic reconstruction of the coronal electron density. This took several years of effort, but resulted in the quasi-uninterrupted photopolarimetric analysis of the corona over two complete Solar Cycles 23 and 24, and the comparison with a variety of eclipse data obtained at different phases of these cycles. Our final results encompass the characterization of the polarization of the white-light corona, of its polarized radiance, of the two-dimensional electron density, and of the K-corona. The agreement with the eclipse data is excellent except for slight discrepancies affecting the innermost part of the C2 field of view. The present work leaves the two unpolarized components, F-corona and stray light, entangled and their complex separation will be dealt with in a follow-on article