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The Large Yield Radiometer (LYRA) is an XUV–EUV–MUV (soft X-ray to mid-ultraviolet) solar radiometer onboard the European Space Agency Project for On-Board Autonomy 2 (PROBA2) mission, which was launched in November 2009. LYRA acquires solar-irradiance measurements at a high cadence (nominally 20 Hz) in four broad spectral channels, from soft X-ray to MUV, which have been chosen for their relevance to solar physics, space weather, and aeronomy. We briefly review the design of the instrument, give an overview of the data products distributed through the instrument website, and describe how the data are calibrated. We also briefly present a summary of the main fields of research currently under investigation by the LYRA consortium.

EIT waves are observed in EUV as bright fronts. Some of these bright fronts propagate across the solar disk. EIT waves are all associated with a flare and a CME and are commonly interpreted as fast-mode magnetosonic waves. Propagating EIT waves could also be the direct signature of the gradual opening of magnetic field lines during a CME. We quantitatively addressed this alternative interpretation. Using two independent 3D MHD codes, we performed nondimensional numerical simulations of a slowly rotating magnetic bipole, which progressively result in the formation of a twisted magnetic flux tube and its fast expansion, as during a CME. We analyse the origins, the development, and the observability in EUV of the narrow electric currents sheets that appear in the simulations. Both codes give similar results, which we confront with two well-known SOHO/EIT observations of propagating EIT waves (7 April and 12 May 1997), by scaling the vertical magnetic field components of the simulated bipole to the line of sight magnetic field observed by SOHO/MDI and the sign of helicity to the orientation of the soft X-ray sigmoids observed by Yohkoh /SXT. A large-scale and narrow current shell appears around the twisted flux tube in the dynamic phase of its expansion. This current shell is formed by the return currents of the system, which separate the twisted flux tube from the surrounding fields. It intensifies as the flux tube accelerates and it is co-spatial with weak plasma compression. The current density integrated over the altitude has the shape of an ellipse, which expands and rotates when viewed from above, reproducing the generic properties of propagating EIT waves. The timing, orientation, and location of bright and faint patches observed in the two EIT waves are remarkably well reproduced. We conjecture that propagating EIT waves are the observational signature of Joule heating in electric current shells, which separate expanding flux tubes from their surrounding fields during CMEs or plasma compression inside this current shell. We also conjecture that the bright edges of halo CMEs show the plasma compression in these current shells.

The equator-to-pole radius difference (Δ r = R eq − R pol ) is a fundamental property of our star, and understanding it will enrich future solar and stellar dynamical models. The solar oblateness (Δ ⊙ ) corresponds to the excess ratio of the equatorial solar radius ( R eq ) to the polar radius ( R pol ), which is of great interest for those working in relativity and different areas of solar physics. Δ r is known to be a rather small quantity, where a positive value of about 8 milli-arcseconds (mas) is suggested by previous measurements and predictions. The Picard space mission aimed to measure Δ r with a precision better than 0.5 mas. The Solar Diameter Imager and Surface Mapper (SODISM) onboard Picard was a Ritchey–Chrétien telescope that took images of the Sun at several wavelengths. The SODISM measurements of the solar shape were obtained during special roll maneuvers of the spacecraft by 30° steps. They have produced precise determinations of the solar oblateness at 782.2 nm. After correcting measurements for optical distortion and for instrument temperature trend, we found a solar equator-to-pole radius difference at 782.2 nm of 7.9±0.3 mas (5.7±0.2 km) at one  σ . This measurement has been repeated several times during the first year of the space-borne observations, and we have not observed any correlation between oblateness and total solar irradiance variations.

Knowledge of the Solar Diameter Imager and Surface Mapper (SODISM) plate scale is a fundamental parameter for obtaining the solar radius. We have determined the plate scale of the telescope on the ground and in flight onboard the Picard spacecraft. The results show significant differences; the main reason is that the conditions of observation are not the same. In addition, the space environment has an impact on the performance of a metrology instrument. Therefore, calibration in space and under the same conditions of observation is crucial. The transit of Venus allowed us to determine the plate scale of the SODISM telescope and hence the absolute value of the solar radius. The transit was observed from space by the Picard spacecraft on 5 – 6 June 2012. We exploited the data recorded by SODISM to determine the plate scale of the instrument, which depends on the characteristics of optical elements (mirrors, filters, or front window). The mean plate scale at 607.1 nm is found to be 1.0643 arcseconds pixel −1 with 3×10 −4  RMS. The solar radius at 607.1 nm from 1 AU is found to be equal to 959.86 arcseconds.

The Solar Diameter Imager and Surface Mapper (SODISM) onboard the Picard space mission provides wide-field images of the photosphere and chromosphere of the Sun in five narrow bandpasses centered at 215.0, 393.37, 535.7, 607.1, and 782.2 nm. The Picard spacecraft was successfully launched on 15 June 2010 into a Sun-synchronous dawn–dusk orbit. The Picard space mission represents a European asset in collecting solar observations useful to improve Earth climatic models. The scientific payload consists of the SODISM imager and of two radiometers, SOlar VAriability Picard (SOVAP) and PREcision MOnitor Sensor (PREMOS), which measure the Total Solar Irradiance (TSI) and part of the Solar Spectral Irradiance (SSI). The SODISM telescope continuously monitors solar activity from the middle ultraviolet to the near infrared spectral ranges and produces solar images that feed SSI reconstruction models. Further, SODISM probes the solar interior via a helioseismic analysis of the solar disc and limb images at 535.7 nm, and via astrometric investigations at the limb. The latter allows us to deduce the spectral dependence of the solar limb profile, and the asphericity of the Sun. Furthermore, SODISM data taken during the transit of Venus allow a determination of the absolute value of the solar diameter. This paper provides a detailed description of the SODISM instrument, including thermo-optical analysis, its different modes of observation, and its first performance in space.

We present a long-term assessment of the radiometric calibration and degradation of the Large Yield Radiometer (LYRA), which has been on orbit since 2009. LYRA is an ultraviolet (UV) solar radiometer and is the first space experiment using aboard a pioneering diamond detector technology. We show that LYRA has degraded after the commissioning phase but is still exploitable scientifically after almost 5 years on orbit thanks to its redundancy design and calibration strategy correcting for instrument degradation. We focus on the inflight detector’s calibration and show that diamond photodetectors have not degraded while silicon reference photodiodes that are even less exposed to the Sun show an increase of their dark current and a decrease of their photoresponse.

SECCHI-EUVI telescopes provide the first EUV images enabling a 3D reconstruction of solar coronal structures. We present a stereoscopic reconstruction method based on the Velociraptor algorithm, a multiscale optical-flow method that estimates displacement maps in sequences of EUV images. Following earlier calibration on sequences of SOHO-EIT data, we apply the algorithm to retrieve depth information from the two STEREO viewpoints using the SECCHI-EUVI telescope. We first establish a simple reconstruction formula that gives the radial distance to the centre of the Sun of a point identified both in EUVI-A and EUVI-B from the separation angle and the displacement map. We select pairs of images taken in the 30.4 nm passband of EUVI-A and EUVI-B, and apply a rigid transform from the EUVI-B image in order to set both images in the same frame of reference. The optical flow computation provides displacement maps from which we reconstruct a dense map of depths using the stereoscopic reconstruction formula. Finally, we discuss the estimation of the height of an erupting filament.

The PROBA2 Science Centre (P2SC) is a small-scale science operations centre supporting the Sun observation instruments onboard PROBA2: the EUV imager Sun Watcher using APS detectors and image Processing (SWAP) and Large-Yield Radiometer (LYRA). PROBA2 is one of ESA’s small, low-cost Projects for Onboard Autonomy (PROBA) and part of ESA’s In-Orbit Technology Demonstration Programme. The P2SC is hosted at the Royal Observatory of Belgium, co-located with both Principal Investigator teams. The P2SC tasks cover science planning, instrument commanding, instrument monitoring, data processing, support of outreach activities, and distribution of science data products. PROBA missions aim for a high degree of autonomy at mission and system level, including the science operations centre. The autonomy and flexibility of the P2SC is reached by a set of web-based interfaces allowing the operators as well as the instrument teams to monitor quasi-continuously the status of the operations, allowing a quick reaction to solar events. In addition, several new concepts are implemented at instrument, spacecraft, and ground-segment levels allowing a high degree of flexibility in the operations of the instruments. This article explains the key concepts of the P2SC, emphasising the automation and the flexibility achieved in the commanding as well as the data-processing chain.

Solar telescopes will never be able to resolve the smallest events at their intrinsic physical scales. Pixel signals recorded by SOHO/(CDS, EIT, SUMER), STEREO/SECCHI/ EUVI, TRACE, SDO/AIA, and even by the future Solar Orbiter EUI/HRI contain an inherent “spatial noise” since they represent an average of the solar signal present at subpixel scales. In this paper, we aim at investigating this spatial noise, and hopefully at extracting information from subpixel scales. Two paths are explored. We first combine a regularity analysis of a sequence of EIT images with an estimation of the relationship between mean and standard deviation, and we formulate a scenario for the evolution of the local signal-to-noise ratio (SNR) as the pixel size becomes smaller. Second, we use an elementary forward modeling to examine the relationship between nanoflare characteristics (such as area, duration, and intensity) and the global mean and standard deviation. We use theoretical distributions of nanoflare parameters as input to the forward model. A fine-grid image is generated as a random superposition of those pseudo-nanoflares. Coarser resolution images (simulating images acquired by a telescope) are obtained by rebinning and are used to compute the mean and standard deviation to be analyzed. Our results show that the local SNR decays more slowly in regions exhibiting irregularities than in smooth regions.

Full-disc full-resolution (FDFR) solar images obtained with the Extreme Ultraviolet Imaging Telescope (EIT) on board the Solar and Heliospheric Observatory (SOHO) were used to analyse the centre-to-limb function and latitudinal distribution of coronal bright points. The results obtained with the interactive and the automatic method, as well as for three subtypes of coronal bright points for the time period 4 June 1998 to 22 May 1999 are presented and compared. An indication of a two-component latitudinal distribution of coronal bright points was found. The central latitude of coronal bright points traced with the interactive method lies between 10 super() and 20 super(). This is closer to the equator than the average latitude of sunspots in the same period. Possible implications for the interpretation of the solar differential rotation are discussed. In the appendix, possible differences between the two solar hemispheres are analysed. More coronal bright points were present in the southern solar hemisphere than in the northern one. This asymmetry is statistically significant for the interactive method and not for the automatic method. The visibility function is symmetrical around the central meridian.