Minimization of the shadow patterns produced by periodic mesh grids in extreme ultraviolet telescopes

Bocchialini

Solomon

et al. 2014

A&A

We report on the detection (10 σ) of 917 events of long-period (3 to 16 hours) intensity pulsations in the 19.5 nm passband of the SOHO Extreme ultraviolet Imaging Telescope. The data set spans from January 1997 to July 2010, i.e the entire solar cycle 23 and the beginning of cycle 24. The events can last for up to six days and have relative amplitudes up to 100%. About half of the events (54%) are found to happen in active regions, and 50% of these have been visually associated with coronal loops. The remaining 46% are localized in the quiet Sun. We performed a comprehensive analysis of the possible instrumental artifacts and we conclude that the observed signal is of solar origin. We discuss several scenarios which could explain the main characteristics of the active region events. The long periods and the amplitudes observed rule out any explanation in terms of magnetohydrodynamic waves. Thermal nonequilibrium could produce the right periods, but it fails to explain all the observed properties of coronal loops and the spatial coherence of the events. We propose that moderate temporal variations of the heating term in the energy equation, so as to avoid a thermal nonequilibrium state, could be sufficient to explain those long-period intensity pulsations. The large number of detections suggests that these pulsations are common in active regions. This would imply that the measurement of their properties could provide new constraints on the heating mechanisms of coronal loops.

Section: Appendix A: Sources Of Spurious Frequenciesmentioning

confidence: 99%

Long-period intensity pulsations in the solar corona during activity cycle 23

Bocchialini

Solomon

et al. 2014

A&A

“…Indeed, several sometimes subtle effects can cause false positives in Fourier or wavelet analyses. For example, Appendix A of Auchère et al (2014) describes how the shadow cast on the detector by the mesh grid holding the focal filters in SOHO/EIT (Defise 1999;Auchère et al 2011) produces spurious frequencies in the range of those detected in coronal loops. This effect could not be found in identical processing of AIA images, which is explained by the much smaller amplitude of the mesh grid pattern in this latter instrument.…”

Section: Motivationmentioning

confidence: 99%

On the Fourier and Wavelet Analysis of Coronal Time Series

Froment

Bocchialini

et al. 2016

ApJ

156

Using Fourier and wavelet analysis, we critically re-assess the significance of our detection of periodic pulsations in coronal loops. We show that the proper identification of the frequency dependence and statistical properties of the different components of the power spectra provies a strong argument against the common practice of data detrending, which tends to produce spurious detections around the cut-off frequency of the filter. In addition, the white and red noise models built into the widely used wavelet code of Torrence & Compo cannot, in most cases, adequately represent the power spectra of coronal time series, thus also possibly causing false positives. Both effects suggest that several reports of periodic phenomena should be re-examined. The Torrence & Compo code nonetheless effectively computes rigorous confidence levels if provided with pertinent models of mean power spectra, and we describe the appropriate manner in which to call its core routines. We recall the meaning of the default confidence levels output from the code, and we propose new Monte-Carlo-derived levels that take into account the total number of degrees of freedom in the wavelet spectra. These improvements allow us to confirm that the power peaks that we detected have a very low probability of being caused by noise.

“…This implies that the supporting mesh grid of the focal filter (and to a lesser extent that of the front filter) can potentially produce a stronger modulation in the images than in other EUV imagers. However, as shown in Auchère et al (2011), the grid modulation is effectively suppressed if one grid period is equal to an integer multiple of the pupil footprint on the focal filter. The grid mesh is thus hexagonal, as the entrance pupil is, with a distance between hexagon centres of 0.4 mm, that is onethird of the beam footprint of 1.2 mm.…”

Section: Full Sun Imagermentioning

confidence: 99%

The Solar Orbiter EUI instrument: The Extreme Ultraviolet Imager

Rochus

Berghmans

et al. 2020

A&A

270

100

Context. The Extreme Ultraviolet Imager (EUI) is part of the remote sensing instrument package of the ESA/NASA Solar Orbiter mission that will explore the inner heliosphere and observe the Sun from vantage points close to the Sun and out of the ecliptic. Solar Orbiter will advance the “connection science” between solar activity and the heliosphere. Aims. With EUI we aim to improve our understanding of the structure and dynamics of the solar atmosphere, globally as well as at high resolution, and from high solar latitude perspectives. Methods. The EUI consists of three telescopes, the Full Sun Imager and two High Resolution Imagers, which are optimised to image in Lyman-α and EUV (17.4 nm, 30.4 nm) to provide a coverage from chromosphere up to corona. The EUI is designed to cope with the strong constraints imposed by the Solar Orbiter mission characteristics. Limited telemetry availability is compensated by state-of-the-art image compression, onboard image processing, and event selection. The imposed power limitations and potentially harsh radiation environment necessitate the use of novel CMOS sensors. As the unobstructed field of view of the telescopes needs to protrude through the spacecraft’s heat shield, the apertures have been kept as small as possible, without compromising optical performance. This led to a systematic effort to optimise the throughput of every optical element and the reduction of noise levels in the sensor. Results. In this paper we review the design of the two elements of the EUI instrument: the Optical Bench System and the Common Electronic Box. Particular attention is also given to the onboard software, the intended operations, the ground software, and the foreseen data products. Conclusions. The EUI will bring unique science opportunities thanks to its specific design, its viewpoint, and to the planned synergies with the other Solar Orbiter instruments. In particular, we highlight science opportunities brought by the out-of-ecliptic vantage point of the solar poles, the high-resolution imaging of the high chromosphere and corona, and the connection to the outer corona as observed by coronagraphs.