Quantifying and containing the curse of high resolution coronal imaging

Abstract : Future missions such as Solar Orbiter (SO), Inter- Helioprobe, 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 (80 km)2 at the Sun during perihelion. In order to guaran- tee 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). For example, if the inhomogeneities in the Quiet Sun emission prevail at higher resolution, one may hope to lo- cally have more photon counts than in the case of a uniform source. It is relevant to quantify how inhomogeneous the quiet corona will be for a pixel pitch that is about 20 times smaller than in the case of SoHO/EIT, and 5 times smaller than TRACE. 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 identifica- tion 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 proper- ties 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" im- ages, whereas the "low-resolution" coarsened images are re- binned so as to simulate a smaller angular resolution and/or a larger distance to the Sun. For a fixed difference in angu- lar resolution and in Spacecraft-Sun distance, we determine the proportion of pixels having a SNR preserved at high res- olution 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.
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  • HAL Id : hal-00808619, version 1

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Véronique Delouille, Pierre Chainais, Jean-François Hochedez. Quantifying and containing the curse of high resolution coronal imaging. Annales Geophysicae, European Geosciences Union, 2008, 26 (10), pp.3169-3184. 〈hal-00808619〉

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