Quantifying and containing the curse of high resolution coronal imaging

Future missions such as Solar Orbiter (SO), InterHelioprobe, or Solar Probe aim at approaching the Sun closer than ever before, with on board some high resolution imagers (HRI) having a subsecond cadence and a pixel area of about $(80km)^2$ at the Sun during perihelion. In order to guarantee their scientific success, it is necessary to evaluate if the photon counts available at these resolution and cadence will provide a sufficient signal-to-noise ratio (SNR). We perform a first step in this direction by analyzing and characterizing the spatial intermittency of Quiet Sun images thanks to a multifractal analysis. We identify the parameters that specify the scale-invariance behavior. This identification allows next to select a family of multifractal processes, namely the Compound Poisson Cascades, that can synthesize artificial images having some of the scale-invariance properties observed on the recorded images. The prevalence of self-similarity in Quiet Sun coronal images makes it relevant to study the ratio between the SNR present at SoHO/EIT images and in coarsened images. SoHO/EIT images thus play the role of 'high resolution' images, whereas the 'low-resolution' coarsened images are rebinned so as to simulate a smaller angular resolution and/or a larger distance to the Sun. For a fixed difference in angular resolution and in Spacecraft-Sun distance, we determine the proportion of pixels having a SNR preserved at high resolution given a particular increase in effective area. If scale-invariance continues to prevail at smaller scales, the conclusion reached with SoHO/EIT images can be transposed to the situation where the resolution is increased from SoHO/EIT to SO/HRI resolution at perihelion.Comment: 25 pages, 1 table, 7 figure

Segmentation d'Images solaires en Extrême Ultraviolet par une Approche Classification floue Multispectrale

L'étude de la variabilité de la couronne solaire et le suivi de régions caractéristiques à sa surface (régions actives, trous coronaux) sont d'une importance capitale en astrophysique et pour le développement de la météorologie de l'espace. Dans ce cadre, nous proposons un algorithme de segmentation multispectrale d'images du Soleil acquises en extrême ultraviolet, utilisant un algorithme de classification flou spatialement contraint. L'utilisation de la logique floue permet de prendre en compte les imprécisions et les incertitudes inhérentes à la définition des différentes régions d'intérêt dans l'image. La méthode est appliquée sur des images prises par le téléscope EIT du satellite SoHO, depuis janvier 1997 jusque mai 2005, couvrant ainsi presque l'intégralité d'un cycle solaire. Les résultats en terme de caractérisation géométrique et radiométrique des régions actives et des trous coronaux sont en accord avec d'autres observations menées par ailleurs. La méthode met de plus en évidence des périodes dans la série temporelle étudiée, reliées à des phénomènes de physique solaire connus

Amélioration virtuelle de la résolution d'images du Soleil par augmentation d'information invariante d'échelle

4 pagesNational audienceCurrent images of the quiet Sun from the spatial telescope EIT are such that 1 pixel = (1800km)2 whereas the smallest physical scales would be of about 100 m. The design of a high resolution spatial telescope where 1 pixel = (80 km)2 is planned. With a resolution 25 times finer, the images may be under-exposed or even useless. The point is to predict at best the quality of these images from the current observations. We exploit the scale invariance properties of images currently available to suggest a method to artificially improve (of a potentially infinite factor) the current images resolution by integrating details from a multifractal stochastic model. Quiet Sun images are magnified by a factor 32 while preserving the multiscale properties (spectrum, multiscaling) and assuring that reducing the magnified image gives the initial image back. We deduce from that an extrapolation of histograms of high resolution images allowing a prediction of the quality of images from a future high resolution telescope

Virtual Super Resolution of Scale Invariant Textured Images Using Multifractal Stochastic Processes

International audienceWe present a new method of magnification for textured images featuring scale invariance properties. This work is originally motivated by an application to astronomical images. One goal is to propose a method to quantitatively predict statistical and visual properties of images taken by a forthcoming higher resolution telescope from older images at lower resolution. This is done by performing a virtual super resolution using a family of scale invariant stochastic processes, namely compound Poisson cascades, and fractional integration. The procedure preserves the visual aspect as well as the statistical properties of the initial image. An augmentation of information is performed by locally adding random small scale details below the initial pixel size. This extrapolation procedure yields a potentially infinite number of magnified versions of an image. It allows for large magnification factors (virtually infinite) and is physically conservative: zooming out to the initial resolution yields the initial image back. The (virtually) super resolved images can be used to predict the quality of future observations as well as to develop and test compression or denoising techniques

PICARD payload thermal control system and general impact of the space environment on astronomical observations

International audiencePICARD is a spacecraft dedicated to the simultaneous measurement of the absolute total and spectral solar irradiance, the diameter, the solar shape, and to probing the Sun's interior by the helioseismology method. The mission has two scientific objectives, which are the study of the origin of the solar variability, and the study of the relations between the Sun and the Earth's climate. The spacecraft was successfully launched, on June 15, 2010 on a DNEPR-1 launcher. PICARD spacecraft uses the MYRIADE family platform, developed by CNES to use as much as possible common equipment units. This platform was designed for a total mass of about 130 kg at launch. This paper focuses on the design and testing of the TCS (Thermal Control System) and in-orbit performance of the payload, which mainly consists in two absolute radiometers measuring the total solar irradiance, a photometer measuring the spectral solar irradiance, a bolometer, and an imaging telescope to determine the solar diameter and asphericity. Thermal control of the payload is fundamental. The telescope of the PICARD mission is the most critical instrument. To provide a stable measurement of the solar diameter over three years duration of mission, telescope mechanical stability has to be excellent intrinsically, and thermally controlled. Current and future space telescope missions require ever-more dimensionally stable structures. The main scientific performance related difficulty was to ensure the thermal stability of the instruments. Space is a harsh environment for optics with many physical interactions leading to potentially severe degradation of optical performance. Thermal control surfaces, and payload optics are exposed to space environmental effects including contamination, atomic oxygen, ultraviolet radiation, and vacuum temperature cycling. Environmental effects on the performance of the payload will be discussed. Telescopes are placed on spacecraft to avoid the effects of the Earth atmosphere on astronomical observations (turbulence, extinction, ...). Atmospheric effects, however, may subsist when spacecraft are launched into low orbits, with mean altitudes of the order of 735 km

The Extreme Ultraviolet Imager (EUI) onboard the SOLAR ORBITER mission

peer reviewedSolar Orbiter will for the first time study the Sun with a full suite of in-situ and remote sensing instruments from inside 0.25 AU and will provide imaging and spectral observations of the Sun’s polar regions, from out of the ecliptic. This proximity to the Sun will also have the significant advantage that the spacecraft will fly in near synchronization with the Sun’s rotation, allowing observations of the solar surface and heliosphere to be studied from a near co-rotating vantage point for almost a complete solar rotation. The mission’s ambitious characteristics draw severe constraints on the design of these instruments. The scientific objectives of Solar Orbiter rely ubiquitously on the Extreme EUV Imager suite (EUI). The EUI instrument suite on board of Solar Orbiter is composed of two high resolution imagers (HRI), one at Lyman α and one dual band at the two 174 and 335 EUV passbands in the extreme UV, and one dual band full-sun imager (FSI) working alternatively at the two 174 and 304 EUV passbands. In all the units, the image is produced by a mirror-telescope, working in nearly normal incidence. The EUV reflectivity of the optical surfaces is obtained with specific EUV multilayered coatings, providing the spectral selection of the EUV units (1HRI and 1 FSI). The spectral selection is complemented with very thin filters rejecting the visible and IR radiation. Due to its orbit, EUI / Solar Orbiter will see 20 solar constants and an entrance baffle to limit the solar heat input into EUI is needed. The paper presents the scientific objectives of EUI and also covers the EUI instrument development plan which will require some trade-off between existing and promising technologies

Preliminary Results on Irradiance Measurements from Lyra and Swap

International audienceThe first and preliminary results of the photometry of Large Yield Radiometer (LYRA) and Sun Watcher using Active Pixel system detector and Image Processing (SWAP) onboard PROBA2 are presented in this paper. To study the day-to-day variations of LYRA irradiance, we have compared the LYRA irradiance values (observed Sun as a star) measured in Aluminum filter channel (171 Å-500 Å) with spatially resolved full-disk integrated intensity values measured with SWAP (174 Å) and Ca II K 1 Å index values (ground-based observations from NSO/Sac Peak) for the period from 01 April 2010 to 15 Mar 2011. We found that there is a good correlation between these parameters. This indicates that the spatial resolution of SWAP complements the high temporal resolution of LYRA. Hence SWAP can be considered as an additional radiometric channel. Also the K emission index is the integrated intensity (or flux) over a 1 Å band centered on the K line and is proportional to the total emission from the chromosphere; this comparison clearly explains that the LYRA irradiance variations are due to the various magnetic features, which are contributing significantly. In addition to this we have made an attempt to segregate coronal features from full-disk SWAP images. This will help to understand and determine the actual contribution of the individual coronal feature to LYRA irradiance variations

The effect of flares on total solar irradiance

Flares are powerful energy releases occurring in stellar atmospheres. Solar flares, the most intense energy bursts in the solar system, are however hardly noticeable in the total solar luminosity. Consequently, the total amount of energy they radiate 1) remains largely unknown and 2) has been overlooked as a potential contributor to variations in the Total Solar Irradiance (TSI), i.e. the total solar flux received at Earth. Here, we report on the detection of the flare signal in the TSI even for moderate flares. We find that the total energy radiated by flares exceeds the soft X-ray emission by two orders of magnitude, with an important contribution in the visible domain. These results have implications for the physics of flares and the variability of our star.Comment: accepted in Nature Physic

EIT Observations of the Extreme Ultraviolet Sun

The Extreme Ultraviolet Imaging Telescope (EIT) on board the SOHO spacecraft has been operational since 2 January 1996. EIT observes the Sun over a 45 x 45 arc min field of view in four emission line groups: Feix, x, Fexii, Fexv, and Heii. A post-launch determination of the instrument flatfield, the instrument scattering function, and the instrument aging were necessary for the reduction and analysis of the data. The observed structures and their evolution in each of the four EUV bandpasses are characteristic of the peak emission temperature of the line(s) chosen for that bandpass. Reports on the initial results of a variety of analysis projects demonstrate the range of investigations now underway: EIT provides new observations of the corona in the temperature range of 1 to 2 MK. Temperature studies of the large-scale coronal features extend previous coronagraph work with low-noise temperature maps. Temperatures of radial, extended, plume-like structures in both the polar coronal hole and in a low latitude decaying active region were found to be cooler than the surrounding material. Active region loops were investigated in detail and found to be isothermal for the low loops but hottest at the loop tops for the large loops

First Perihelion of EUI on the Solar Orbiter mission

Context. The Extreme Ultraviolet Imager (EUI), onboard Solar Orbiter consists of three telescopes: the two High Resolution Imagers in EUV (HRIEUV) and in Lyman-{\alpha} (HRILya), and the Full Sun Imager (FSI). Solar Orbiter/EUI started its Nominal Mission Phase on 2021 November 27. Aims. EUI images from the largest scales in the extended corona off limb, down to the smallest features at the base of the corona and chromosphere. EUI is therefore a key instrument for the connection science that is at the heart of the Solar Orbiter mission science goals. Methods. The highest resolution on the Sun is achieved when Solar Orbiter passes through the perihelion part of its orbit. On 2022 March 26, Solar Orbiter reached for the first time a distance to the Sun close to 0.3 au. No other coronal EUV imager has been this close to the Sun. Results. We review the EUI data sets obtained during the period 2022 March-April, when Solar Orbiter quickly moved from alignment with the Earth (2022 March 6), to perihelion (2022 March 26), to quadrature with the Earth (2022 March 29). We highlight the first observational results in these unique data sets and we report on the in-flight instrument performance. Conclusions. EUI has obtained the highest resolution images ever of the solar corona in the quiet Sun and polar coronal holes. Several active regions were imaged at unprecedented cadences and sequence durations. We identify in this paper a broad range of features that require deeper studies. Both FSI and HRIEUV operate at design specifications but HRILya suffered from performance issues near perihelion. We conclude emphasising the EUI open data policy and encouraging further detailed analysis of the events highlighted in this paper