Federico Landini

Morphology and density structure of post-CME current sheets

Context. Eruption of a coronal mass ejection (CME) drags and “opens” the coronal magnetic field, presumably leading to the formation of a large-scale current sheet and field relaxation by magnetic reconnection. Aims. We analyze the physical characteristics of ray-like coronal features formed in the aftermath of CMEs, to confirm whether interpreting this phenomenon in terms of a reconnecting current sheet is consistent with observations. Methods. The study focuses on measurements of the ray width, density excess, and coronal velocity field as a function of the radial distance. Results. The morphology of the rays implies that they are produced by Petschek-like reconnection in the large-scale current sheet formed in the wake of CME. The hypothesis is supported by the flow pattern, often showing outflows along the ray, and sometimes also inflows into the ray. The inferred inflow velocities range from 3 to 30 km s −1 , and are consistent with the narrow opening-angle of rays, which add up to a few degrees. The density of rays is an order of magnitude higher than in the ambient corona. The densityexcess measurements are compared with the results of the analytical model in which the Petschek-like reconnection geometry is applied to the vertical current sheet, taking into account the decrease in the external coronal density and magnetic field with height. Conclusions. The model results are consistent with the observations, revealing that the main cause of the density excess in rays is a transport of the dense plasma from lower to higher heights by the reconnection outflow.

METIS: a novel coronagraph design for the Solar Orbiter mission

METIS (Multi Element Telescope for Imaging and Spectroscopy) METIS, the “Multi Element Telescope for Imaging and Spectroscopy”, is a coronagraph selected by the European Space Agency to be part of the payload of the Solar Orbiter mission to be launched in 2017. The mission profile will bring the Solar Orbiter spacecraft as close to the Sun as 0.3 A.U., and up to 35° out-of-ecliptic providing a unique platform for helio-synchronous observations of the Sun and its polar regions. METIS coronagraph is designed for multi-wavelength imaging and spectroscopy of the solar corona. This presentation gives an overview of the innovative design elements of the METIS coronagraph. These elements include: i) multi-wavelength, reflecting Gregorian-telescope; ii) multilayer coating optimized for the extreme UV (30.4 nm, HeII Lyman-α) with a reflecting cap-layer for the UV (121.6 nm, HI Lyman-α) and visible-light (590-650); iii) inverse external-occulter scheme for reduced thermal load at spacecraft peri-helion; iv) EUV/UV spectrograph using the telescope primary mirror to feed a 1st and 4th-order spherical varied line-spaced (SVLS) grating placed on a section of the secondary mirror; v) liquid crystals electro-optic polarimeter for observations of the visible-light K-corona. The expected performances are also presented.

Multi Element Telescope for Imaging and Spectroscopy (METIS) coronagraph for the Solar Orbiter mission

METIS, the “Multi Element Telescope for Imaging and Spectroscopy”, is a coronagraph selected by the European Space Agency to be part of the payload of the Solar Orbiter mission to be launched in 2017. The unique profile of this mission will allow 1) a close approach to the Sun (up to 0.28 A.U.) thus leading to a significant improvement in spatial resolution; 2) quasi co-rotation with the Sun, resulting in observations that nearly freeze for several days the large-scale outer corona in the plane of the sky and 3) unprecedented out-of-ecliptic view of the solar corona. This paper describes the experiment concept and the observational tools required to achieve the science drivers of METIS. METIS will be capable of obtaining for the first time: • simultaneous imaging of the full corona in polarized visible-light (590-650 nm) and narrow-band ultraviolet HI Lyman α (121.6 nm); • monochromatic imaging of the full corona in the extreme ultraviolet He II Lyman α (30.4 nm); • spectrographic observations of the HI and He II Ly α in corona. These measurements will allow a complete characterization of the three most important plasma components of the corona and the solar wind, that is, electrons, hydrogen, and helium. This presentation gives an overview of the METIS imaging and spectroscopic observational capabilities to carry out such measurements.

Design status of ASPIICS, an externally occulted coronagraph for PROBA-3

The “sonic region” of the Sun corona remains extremely difficult to observe with spatial resolution and sensitivity sufficient to understand the fine scale phenomena that govern the quiescent solar corona, as well as phenomena that lead to coronal mass ejections (CMEs), which influence space weather. Improvement on this front requires eclipse-like conditions over long observation times. The space-borne coronagraphs flown so far provided a continuous coverage of the external parts of the corona but their over-occulting system did not permit to analyse the part of the white-light corona where the main coronal mass is concentrated. The proposed PROBA-3 Coronagraph System, also known as ASPIICS (Association of Spacecraft for Polarimetric and Imaging Investigation of the Corona of the Sun), with its novel design, will be the first space coronagraph to cover the range of radial distances between ~1.08 and 3 solar radii where the magnetic field plays a crucial role in the coronal dynamics, thus providing continuous observational conditions very close to those during a total solar eclipse. PROBA-3 is first a mission devoted to the in-orbit demonstration of precise formation flying techniques and technologies for future European missions, which will fly ASPIICS as primary payload. The instrument is distributed over two satellites flying in formation (approx. 150m apart) to form a giant coronagraph capable of producing a nearly perfect eclipse allowing observing the sun corona closer to the rim than ever before. The coronagraph instrument is developed by a large European consortium including about 20 partners from 7 countries under the auspices of the European Space Agency. This paper is reviewing the recent improvements and design updates of the ASPIICS instrument as it is stepping into the detailed design phase.

Improved stray light suppression performance for the solar orbiter/METIS inverted external occulter

The Solar Orbiter/METIS visible and UV coronagraph introduces the concept of occulter inversion in solar coronagraphy. Classical externally occulted coronagraphs usually have a disk in front of the telescope entrance pupil. According to the mission requirements, in order to reduce the amount of power entering the instrument and to limit the instrument dimensions, METIS is equipped with an inverted external occulter (IEO). The IEO consists of a circular aperture on the Solar Orbiter thermal shield that acts as coronagraph entrance pupil. A spherical mirror (M0), located ~800 mm behind the IEO, rejects back the disk-light through the IEO itself. A light-tight boom connects the IEO to the M0 through the thermal shield. In order to achieve high performance in stray light suppression, the IEO design needs optimization. Due to the novelty of the concept we can only use the heritage of past space-borne coronagraph occulters as a starting point to design a dedicated occulter optimization shape. A 1.5 years long, accurate test campaign has been carried out to evaluate the best optimization configuration for the IEO. Two prototypes were manufactured to take into account the impact of the boom geometry on the stray light suppression performance. Two optimization concepts were compared: the inverted cone (that derives from the conic optimization of classical occulting disks) and the serrated edge, of which several samples were manufactured, with different geometrical parameters, surface roughnesses and coatings. This work summarizes the activity we have been carrying on to define the flight specifications for the METIS occulter.

Stray-light analysis for the SCORE coronagraphs of HERSCHEL.

The HERSCHEL (Helium Resonance Scattering in the Corona and Heliosphere) suborbital mission scientific payload consists of two extreme ultraviolet (EUV) coronagraphs forming the SCORE (Sounding Coronagraph Experiment) and an EUV Sun disk imager. The mission will be of great importance for the investigation of solar wind dynamics by obtaining the first global image of the Sun (disk and corona) in the He ii30.4 nm line. The most stringent requirement for the optical design of a coronagraph is stray-light reduction. We summarize the stray-light analysis for the SCORE coronagraphs, which are characterized by an innovative optical design, optimized for stray-light reduction.

The solar orbiter METIS coronagraph data signal processing chain

METIS, the Multi Element Telescope for Imaging and Spectroscopy, is one of the instruments selected in 2009 by ESA to be part of the payload of the Solar Orbiter mission. The instrument design has been conceived to perform both multiband imaging and UV spectroscopy of the solar corona. The two sensors of the detecting system will produce images in visible light and in two narrow UV bands, at 121.6 and 30.4 nm. The instrument is constituted by several subunits that have to be properly controlled and synchronized in order to provide the expected performances. Moreover, the large amount of data collected by METIS has to be processed by the on board electronics to reduce the data volume to be delivered to ground by telemetry. These functionalities will be realized by a dedicated electronics, the Main Power and Processing Unit (MPPU). This paper will provide an overview of the METIS data handling system and the expected on board data processing.

Stray light analysis and optimization of the ASPIICS/PROBA-3 formation flying solar coronagraph

PROBA-3 is a technology mission devoted to the in-orbit demonstration of formation flying techniques and technologies. PROBA-3 will implement a giant coronagraph (called ASPIICS) that will both demonstrate and exploit the capabilities and performances of formation flying. ASPIICS is distributed on two spacecrafts separated by 150m, one hosting the external occulting disk and the other the optical part of the coronagraph. This part implements a three-mirror-anastigmat (TMA) telescope. Its pupil is placed about 800mm in front of the primary mirror, a solution allowing an efficient baffling and a high reduction of the stray light inside the instrument. A complete stray light analysis of the TMA has been carried out to design the baffles and to establish the required roughness of the mirrors. The analysis has been performed in two steps: first, by calculating the diffraction pattern behind the occulter due to an extended monochromatic source having the diameter of the Sun; second, by propagating this diffraction pattern, through all the telescope optical components, to the prime focal plane. The results obtained are described in this article.

Comparison of different algorithms and programming languages in the diffraction calculation for a coronagraph stray light analysis

In order to obtain an image of the solar corona, coronagraph optical design needs to be optimized with respect to stray light reduction. Despite the accurate optical design, some stray light is present on the focal plane in addition to the coronal signal. The stray light level has to be estimated in order to test the quality of the optical design. The stray light is given by scattering off the surfaces of the optical elements and by diffraction from the instrument apertures. In order to estimate the stray light level on the focal plane, a diffraction calculation is necessary. In this paper we describe the diffraction calculation for a coronagraph with an innovative stray light reduction design. For the same optical configuration we used two different algorithms, based on different approaches to Fresnel diffraction computation. By using the Fresnel-Kirchhoff scalar theory we developed an algorithm, and we used it to write codes in IDL (Interactive Data Language, by Research System Inc.), and C programming languages. By using the GLAD (General Laser Analysis and Design, by AOR) software, which diffraction algorithm is based on the principles of Fourier optics, we wrote a further code. In this paper we compare the results of the different codes and we discuss their efficiencies.

Evaluation of the stray light from the diffraction of METIS coronagraph external occulter

METIS (Multi Element Telescope for Imaging and Spectroscopy) is an externally occulted coronagraph part of the Solar Orbiter payload. METIS innovative occulting system, called inverted externally occulter (IEO), consists of a circular aperture, IEO, that acts also as the entrance pupil of the instrument, and a solar disk rejection mirror (M0), placed at the bottom end of the coronagraph boom. M0 reflects back through IEO the solar disk radiation, letting the coronal radiation enter the coronagraph telescope. Light diffracted by IEO enters the telescope and has to be minimized with a proper shape of the IEO edge. The paper describes the theoretical results of the diffraction analysis extended to the scattered light by the primary mirror of the telescope onto the primary focal plane. A summary of the entire stray light reduction capabilities of METIS is also given.