Context. Due to its physical nature, the solar corona exhibits large spatial variations of intensity that make it difficult to simultaneously visualize the features present at all levels and scales. Many general-purpose and specialized filters have been proposed to enhance coronal images. However, most of them require the ad hoc tweaking of parameters to produce subjectively good results. Aims. Our aim was to develop a general purpose image enhancement technique that would produce equally good results, but based on an objective criterion. Methods. The underlying principle of the method is the equalization, or whitening, of power in the à trous wavelet spectrum of the input image at all scales and locations. An edge-avoiding modification of the à trous transform that uses bilateral weighting by the local variance in the wavelet planes is used to suppress the undesirable halos otherwise produced by discontinuities in the data. Results. Results are presented for a variety of extreme ultraviolet (EUV) and white light images of the solar corona. The proposed filter produces sharp and contrasted output, without requiring the manual adjustment of parameters. Furthermore, the built-in denoising scheme prevents the explosion of high-frequency noise typical of other enhancement methods, without smoothing statistically significant small-scale features. The standard version of the algorithm is about two times faster than the widely used multiscale Gaussian normalization (MGN). The bilateral version is slower, but provides significantly better results in the presence of spikes or edges. Comparisons with other methods suggest that the whitening principle may correspond to the subjective criterion of most users when adjusting free parameters.

The solar prolateness (also known as Ovalisation, a french origin name) of the extended dynamical chromosphere is established from measurements performed above 2 Mm heights during the years of solar minimum, using the Hα, Ca II K and HeII 304 line emissions from both ground-based and space-based observations. Coronal X-EUV emissions usually penetrate deep enough into the chromosphere to completely mask this effect on transition region lines and produce the so-called coronal hole effect. However, cool lines like Hα and Ca II lines, do NOT show this Coronal Hole (CH) effect. Coronal lines and HeI (D3; 1083 nm) do show CHs but do not show the prolateness effect. We first briefly review different methods which can potentially be used to measure the prolateness. Further we note the similarity of the geometric behaviour of the prolateness and its variation along the solar cycle compared to the behaviour of the fast solar wind. It suggests the same origin possibly related to the emergence of the small scale network and internetwork magnetic field towards the corona and small scale magnetic reconnections. A simple geometric model was proposed to explain the effect of the prolateness of the solar chromosphere by considering that the specific dynamical part of the solar atmosphere above the 2 Mm level, being a mixture of up and down moving jets of chromospheric matter with the coronal plasma between them, is responsible for the solar prolateness (Filippov and Koutchmy, 2000). We however note that polar regions are also showing different types of activity in the low corona, including small prominence eruptions seen e.g. in Hα and linear jets seen in SXR and EUV as well as in W-L (eclipses). Some kind of dynamical dissipation of the newly emerged magnetic field is needed. More systematic measurements should be done to build a more complete, possibly 3D, picture to explain the extended in the horizontal direction lifting effect of a large part of the polar chromosphere.

The Extreme Ultraviolet Imager (EUI) onboard Solar Orbiter ob-served an eruption through both of its channels (17.4/30.4 nm) of its Full Sun Imager, on 22 April 2021. At the time, the spacecraft was at 0.87 au from the Sun,and 98◦ east of the Sun-Earth line. The eruption was slightly back-sided, emerging close to the southwest limb, starting at 04:24 UT, with the source located at S20W103 from the Solar Orbiter perspective. The Solar Orbiter coronagraph, Metis, observed the CME at 06:05 UT. The Spectrometer/Telescope for Imaging X-rays (STIX) on Solar Orbiter sampled the associated X-ray flare, which was partially occulted. This allowed the characterization of both the thermal plasmaand any potential contribution of non-thermal electrons in the tenuous coronal source. The X-ray source location is compared to the extreme ultraviolet coronal structures seen by EUI, and it is established that STIX imager only sees the top part of the flaring loops, while most of the flare – in particular the non-thermal foot points – remain occulted. From the Earth’s perspective the eruption source region was observed at S20W05 (close to disk centre), the Atmospheric Imaging Assembly (AIA), onboard the Solar Dynamics Observatory (SDO) and the Sun Watcher using Active Pixel System detector and Image Processing (SWAP), onboard the PRoject for OnboardAutonomy (PROBA2) observed dimmings and an associated large-scale coronal wave starting around 04:07 UT. The Extreme Ultraviolet Imager (EUVI) on the Solar-TErrestial RElations Observatory (STEREO-A), located 53◦ east of the Sun-Earth line at the time, observed similar signatures of an eruption starting around 04:17 UT, on-disk at S20W50. The corresponding CME was observedas a partial halo CME shortly after (∼06:00 UT) by the C2 Large Angle and Spectrometric Coronagraph (LASCO), from the Earth perspective onboard the SOlar and Heliospheric Observatory (SOHO), and by the STEREO-A/COR2 coronagraph as a clear structured CME around 05:23 UT. The corresponding ICME arrived at Earth on 24 April 2021, it was driving ashock and created minor geomagnetic storm conditions. We simulate the CMEwith the 3D MHD heliospheric model EUHFORIA. We provide an analysis ofthe eruption as observed by these various instruments from different vantage points. The combination of data from Solar Orbiter as well as other space-based assets with numerical modeling clearly showcases the scientific potential for the science phase of Solar Orbiter, and the unique observations available.