V. Andretta

On-Orbit Degradation of Solar Instruments

We present the lessons learned about the degradation observed in several space solar missions, based on contributions at the Workshop about On-Orbit Degradation of Solar and Space Weather Instruments that took place at the Solar Terrestrial Centre of Excellence (Royal Observatory of Belgium) in Brussels on 3 May 2012. The aim of this workshop was to open discussions related to the degradation observed in Sun-observing instruments exposed to the effects of the space environment. This article summarizes the various lessons learned and offers recommendations to reduce or correct expected degradation with the goal of increasing the useful lifespan of future and ongoing space missions.

The EUV helium spectrum in the quiet Sun: A by-product of coronal emission?

In this paper we test one of the mechanisms proposed to explain the intensities and other observed properties of the solar helium spectrum, and in particular of its Extreme-Ultraviolet (EUV) resonance lines. The so-called Photoionisation- Recombination (P-R) mechanism involves photoionisation of helium atoms and ions by EUV coronal radiation, followed by recombination cascades. We present calibrated measurements of EUV flux obtained with the two CDS spectrometers on board SOHO, in quiescent solar regions. We were able to obtain an essentially complete estimate of the total photoionising flux in the wavelength range below 504 A (the photoionisation threshold for Hei), as well as simultaneous measurements with the same instruments of the intensities of the strongest EUV Hei and Heii lines: Hei584, Hei537, and Heii304. We find that there are not enough EUV ionising photons to account for the observed helium line intensities. More specifically, we conclude that Heii intensities cannot be explained by the P-R mechanism alone. Our results, however, leave open the possibility that the He i spectrum could be formed by the P-R mechanism, with the Heii304 line as a significant photoionisating source.

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.

Helium line formation and abundance in a solar active region

An observing campaign (SOHO JOP 139), coordinated between ground-based and Solar and Heliospheric Observatory (SOHO) instruments, has been planned to obtain simultaneous spectroheliograms of the same active region in several spectral lines. The chromospheric lines Ca II K, Hα, and Na I D, as well as He I 10830, 5876, 584, and He II 304 A lines have been observed. The EUV radiation in the range λ 1 × 104 K. This region, between the chromosphere and transition region, has been indicated as a good candidate for processes that might be responsible for strong variations of [He]. The set of our observables can still be well reproduced in both cases, changing the atmospheric structure mainly in the low transition region. This implies that, to choose between different values of [He], it is necessary to constrain the transition region with different observables, independent of the He lines.

The CaII infrared triplet as a stellar activity diagnostic - II. Test and calibration with high resolution observations

Aims. We report on our analysis of the high resolution spectra (R ≈ 86 000) of a sample of 42 late-type active stars (with measured logRspanning from ≈− 3t o≈−5) acquired with the Italian 3.6 m Telescopio Nazionale Galileo (TNG) using the SARG spectrome- ter in the 4960−10 110 A range. The high quality of the spectra and the good activity-level coverage allow us to measure two different chromospheric indicators that can be derived from the Ca ii infrared triplet (Ca ii IRT) lines: the residual equivalent width (EQW) and the chromospheric indicator RIRT. The aim of this work is determine and test the best way of deriving activity-level information and errors from the Ca ii IRT lines, in preparation of the GAIA Cornerstone mission by ESA, by which the Ca ii IRT spectral range will be spectroscopically observed for millions of stars. Methods. The RIRT index is calculated for each observed star as the difference between the calculated NLTE photospheric central intensity and the observed one. The residual EQW, ∆WIRT, is calculated as the area of the positive profile obtained as the difference between the calculated NLTE photospheric and the observed profiles. We correlate logRwith RIRT and the ∆WIRT. Results. This analysis indicates that Ca ii IRT lines are good chromospheric diagnostics. We find that both ∆WIRT and the RIRT quan- tities can be used as chromospheric indicators, although the former exhibits a tighter correlation with the logRindex. Furthermore, we find that the total chromospheric excess EQW in the Ca ii IRT is almost linearly correlated with the excess in the Ca ii H & K dou- blet, as estimated through the logRindex.

The EUV spectrum of the Sun: long-term variations in the SOHO CDS NIS spectral responsivities

We present SOHO Coronal Diagnostic Spectrometer (CDS) normal incidence, extreme-ultraviolet spectra of the Sun taken from the beginning of the mission in 1996 until now. We use various methods to study the performance of the instrument during such a long time span. Assuming that the basal chromospheric-transition region emission in the quiet parts of the Sun does not vary over the cycle, we find a slow decrease in the instrument sensitivity over time. We applied a correction to the NIS (Normal Incidence Spectrograph) data, using as a starting reference the NIS absolute calibration obtained from a comparison with a rocket flight in May 1997. We then obtained NIS full-Sun spectral irradiances from observations in 2008 and compared them with the EUV irradiances obtained from the rocket that flew on April 14, 2008 a prototype of the Solar Dynamics Observatory EVE instrument. Excellent agreement is found between the EUV irradiances from NIS and from the EVE-prototype, confirming the NIS radiometric calibration. The NIS instrument over 13 years has performed exceptionally well, with only a factor of about 2 decrease in responsivity for most wavelengths.

Solar magnetism eXplorer (SolmeX)

The magnetic field plays a pivotal role in many fields of Astrophysics. This is especially true for the physics of the solar atmosphere. Measuring the magnetic field in the upper solar atmosphere is crucial to understand the nature of the underlying physical processes that drive the violent dynamics of the solar corona—that can also affect life on Earth. SolmeX, a fully equipped solar space observatory for remote-sensing observations, will provide the first comprehensive measurements of the strength and direction of the magnetic field in the upper solar atmosphere. The mission consists of two spacecraft, one carrying the instruments, and another one in formation flight at a distance of about 200 m carrying the occulter to provide an artificial total solar eclipse. This will ensure high-quality coronagraphic observations above the solar limb. SolmeX integrates two spectro-polarimetric coronagraphs for off-limb observations, one in the EUV and one in the IR, and three instruments for observations on the disk. The latter comprises one imaging polarimeter in the EUV for coronal studies, a spectro-polarimeter in the EUV to investigate the low corona, and an imaging spectro-polarimeter in the UV for chromospheric studies. SOHO and other existing missions have investigated the emission of the upper atmosphere in detail (not considering polarization), and as this will be the case also for missions planned for the near future. Therefore it is timely that SolmeX provides the final piece of the observational quest by measuring the magnetic field in the upper atmosphere through polarimetric observations.

A method to estimate the effect of line blanketing in NLTE radiative transfer calculations

We present a method to estimate the contribution of line opacity to the total opacity as a function of wavelength. The estimated line-opacity function can then be used to simulate line-blanketing in NLTE radiative transfer calculations. Given a reference flux distribution (either observed or theoretical), our method allows to ob- tain a good estimate of the spectrum without the need for considering in detail all the millions of lines contributing to line blanketing. We applied the method to the spectra computed from a sample of photospheric models with eective temperatures Te = 4200, 5200 and 6200 K, logg =4 :0, 4.5, 5.0 and (A/H) = 0:0, 1:0, 2:0, taken from the NextGen database (Allard & Hauschildt 1995). The computed flux distributions agree quite well with the corresponding LTE line-blanketed NextGen fluxes when we introduce the estimated line-opacity contribution as a multiplicative factor of the continuum opacity in the radiative transfer calculations. In particular we discuss the importance of a correct estimate of the continuum flux, mainly in the UV, in the NLTE formation of the Ca ii H & K, the Ca ii InfraRed Triplet (IRT: = 8498, 8542, 8662), Na i D, Li i and K i resonance lines.

The EUV spectrum of the Sun: SOHO CDS NIS irradiances from 1998 until 2010

We present extreme-ultraviolet (EUV) irradiances of the Sun taken during the 1998‐2010 period from the Solar & Heliospheric Observatory (SOHO) Coronal Diagnostic Spectrometer (CDS) Normal Incidence Spectrograph (NIS). They were obtained from NIS full-Sun radiance observations, and represent the first set of EUV spectral observations spanning a solar cycle. We compare the CDS line irradiances with those obtained from rocket measurements, one that flew in May 1997 and one in April 2008, together wi th the Thermosphere Ionosphere Mesosphere Energetics Dynamics (TIMED) Solar EUV Experiment (SEE) EUV Grating Spectrograph (EGS) and various historical records. Excellent agreement (to within a relative 20%) is found in most cases, with a few notable exceptions. Lines formed in the transition region show very small changes with the solar cycle, with the exception of the helium lines. The irradiances of lines formed around 1 MK already change during the cycle by a factor∼ 5; for hotter lines (2.5 MK) the variability reaches factors of the order of 40. For lines formed around 1‐3 MK, and to a less extent, the helium lines, we find a good linea r correlation between CDS irradiances and the 10.7 cm radio flu x, although each line has a different coeffi cient. No correlation is found for the transition-region lines. Significant discrepancie s between the observed irradiances and those modelled is found. This confirms the importance in obtaining EUV spectral measurements of the solar irradiance.

POLAR investigation of the Sun—POLARIS

The POLAR Investigation of the Sun (POLARIS) mission uses a combination of a gravity assist and solar sail propulsion to place a spacecraft in a 0.48 AU circular orbit around the Sun with an inclination of 75° with respect to solar equator. This challenging orbit is made possible by the challenging development of solar sail propulsion. This first extended view of the high-latitude regions of the Sun will enable crucial observations not possible from the ecliptic viewpoint or from Solar Orbiter. While Solar Orbiter would give the first glimpse of the high latitude magnetic field and flows to probe the solar dynamo, it does not have sufficient viewing of the polar regions to achieve POLARIS’s primary objective: determining the relation between the magnetism and dynamics of the Sun’s polar regions and the solar cycle.