LYRA, a solar UV radiometer on Proba2

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Abstract

LYRA is the solar UV radiometer that will embark in 2006 onboard Proba2, a technologically oriented ESA micro-mission. LYRA is designed and manufactured by a Belgian–Swiss–German consortium (ROB, PMOD/WRC, IMOMEC, CSL, MPS and BISA) with additional international collaborations. It will monitor the solar irradiance in four UV passbands. They have been chosen for their relevance to Solar Physics, Aeronomy and Space Weather: (1) the 115–125 nm Lyman-α channel, (2) the 200–220 nm Herzberg continuum range, (3) the Aluminium filter channel (17–70 nm) including He II at 30.4 nm and (4) the Zirconium filter channel (1–20 nm). The radiometric calibration will be traceable to synchrotron source standards (PTB and NIST). The stability will be monitored by onboard calibration sources (LEDs), which allow to distinguish between potential degradations of the detectors and filters. Additionally, a redundancy strategy maximizes the accuracy and the stability of the measurements. LYRA will benefit from wide bandgap detectors based on diamond: it will be the first space assessment of a pioneering UV detectors program. Diamond sensors make the instruments radiation-hard and solar-blind: their high bandgap energy makes them insensitive to visible light and, therefore, make dispensable visible light blocking filters, which seriously attenuate the desired ultraviolet signal. Their elimination augments the effective area and hence the signal-to-noise, therefore increasing the precision and the cadence. The SWAP EUV imaging telescope will operate next to LYRA on Proba2. Together, they will establish a high performance solar monitor for operational space weather nowcasting and research. LYRA demonstrates technologies important for future missions such as the ESA Solar Orbiter.

Introduction

The knowledge of the solar spectral irradiance is of large interest to Solar Physics, Aeronomy and to other fields of heliospheric or planetary research. The solar ultraviolet (UV) irradiance below 300 nm is the main source of the energy converted in the Earth’s atmosphere, controlling its thermal structure, dynamics and chemistry through photodissociation and photoionization. Of special interest for life and mankind is the balance of ozone formed by radiation below 240 nm in the stratosphere and mesosphere, but photodissociated above 200 nm in the stratosphere (e.g., Egorova et al., 2004, Rozanov et al., 2004). The VUV irradiance variability has significant effects on human technologies too, currently addressed in the frame of Space Weather studies.

The solar spectral irradiance is a function of both time and wavelength and one would ideally like to sample it with maximal temporal and spectral resolutions, together with the highest absolute accuracy, precision and time coverage. These quests, however, encounter physical limits, making them conflicting and imposing trade-offs. The strategy is thus to bridge the most reliable observations, with the best possible models (e.g., Kretzschmar et al., 2005, Lean et al., 2003, Warren et al., 2001, Woods, 2002).

Numerous experiments are monitoring the solar full-disk UV and X-ray irradiance. All of them differ in spectral coverage, time coverage, time cadence and nature of the instrument (spectrograph, photometer or imager). Some data characterizing these missions are summarized in Table 1. Full-disk spectrographs are used in UARS-SOLSTICE (Solar Stellar Irradiance Comparison Experiment; Rottman et al., 1993, Woods et al., 1993), in UARS-SUSIM (Solar Ultraviolet Spectral Irradiance Monitor, Brueckner et al., 1993, Vanhoosier et al., 1981), in SoHO-SEM (Solar EUV Monitor, Judge et al., 1998), in SORCE-SOLSTICE II, in SORCE-SIM (Spectral Irradiance Monitor) and in TIMED-SEE (Solar EUV Experiment, Woods et al., 1998). SEE includes XPS, a photometer system, for the short wavelength range, while photometer systems are or will be used in GOES-XRS (X-ray Sensor), GOES-EUVS (Viereck and Hanser, 2000) and in SNOE-SXP (Solar X-ray Photometer, Bailey et al., 2000).

In principle, more information can be extracted from spectrographs than from photometers such as LYRA, but under the sacrifice of cadence. Also, not all missions are designed for continuous data acquisition. For instance, measurements with TIMED-SEE are made for only 3 min per 97-min orbit during which the Sun moves per-chance into the field of view. This is of course not ideal for the study of phenomena that occur unexpectedly and vary in time, but even so, flares have been observed during those short periods (Woods et al., 2004). Generally, the required integration times are higher for spectrographs and time has to be spent on spectral scanning. It all unfavourably affects their time cadence. The SORCE-SOLSTICE experiment allows spectral scans to be restricted to Lyα and Mg II (280 nm), thereby achieving its highest cadence of ∼1 min. Lyα profile variations during flares have been detected that way (Wang et al., 2000, Woods et al., 2004). While spectral diagnostics are beyond its scope, LYRA has the advantage of quasi-continuous monitoring. Moreover, it offers the novelty of very high cadence observations, up to 100 Hz, of interest for the study of solar flares (Snow et al., 2004, Woods et al., 2003) and for the limb occultation technique (See Section 3.3). The maximum cadence of LYRA is higher than the one of SNOE-SXP, an instrument widely used for atmospheric studies and similar to LYRA except that it lacks longer wavelength channels at Lyα and at the Herzberg continuum. Also, SXP does not monitor the Sun in a continuous fashion. Continuous long-term time series of the EUV solar irradiance can bring insights into fundamental questions such as coronal heating (Greenhough et al., 2003), but here too, the higher the sampling rate, the lesser the bias of the statistics.

Full-disk imagers such as EIT (EUV Imaging Telescope) on SOHO, SPIRIT on CORONAS, SXT (Soft X-ray Telescope) on YOHKOH or SXI (Solar X-ray Imager) on the GOES series of satellites enable irradiance measurements with the additional (and actually primary) benefit of spatial resolution (Newmark et al., 2001). Sub-second cadence, however, cannot yet be achieved.

Future instruments measuring solar UV and X-ray irradiances are SolACES (Solar Auto-Calibrating EUV/UV Spectrophotometers), SOVIM (Solar Variability and Irradiance Monitor), SOLSPEC (Solar Spectrum Measurement Instrument) onboard the International Space Station (ISS) and EVE (EUV Variability Experiment) on SDO (Solar Dynamics Observatory). There is no guarantee though that there will be no gap in the future in terms of the time–wavelength coverage.

Of maximum benefit for astrophysical and atmospheric studies is the combination of data of complementary instruments. Spectrographs, imagers and photometers are all designed for their specific purposes and LYRA will add sub-second cadence capabilities to the ensemble of solar irradiance experiments.

This paper describes LYRA (the Large Yield RAdiometer), a solar VUV photometer and the preparation to exploitation of its observations. One purpose of the instrument is to demonstrate several technologies able to enhance vacuum ultraviolet measurements by increasing the overall effective area and the ability to maintain calibration. The former feature permits a better precision versus cadence trade-off, the latter, a higher accuracy. LYRA will benefit from diamond detectors: it will be the first space assessment of a pioneering UV detectors program (Hochedez et al., 2000, Hochedez et al., 2001, Hochedez et al., 2002, Hochedez et al., 2003a, Hochedez et al., 2003b, Schühle et al., 2004).

Section snippets

Instrument description

LYRA is part of the Proba2 (project for onboard autonomy) space mission of the European Space Agency (ESA), which aims at demonstrating the technologies used by its platform and payload. Proba2 is a follow-up of the successful Proba (now Probaf) in orbit since October 2001 (Teston et al., 1999). Proba2 includes major Belgian contributions. It is developed under an ESA General Support Technology Program (GSTP) contract by a consortium led by Verhaert Design and Development (Belgium). It will be

Science preparation

The span of LYRA science is large; a thorough review would be out of the scope of this paper. We present in this section some dedicated studies meant to feedback on the development decisions and to prepare the timely exploitation of our instrument.

Conclusions

The design, status and objectives of the LYRA instrument have been reviewed. The LYRA team keeps on preparing actively this mission to maximize its expected success in all regards.

Acknowledgements

LYRA is a project of the Centre Spatial de Liège, the Physikalisch-Meteorologisches Observatorium Davos and the Royal Observatory of Belgium funded by the Belgian Federal Science Policy Office (BELSPO) and by the Swiss Bundesamt für Bildung und Wissenschaft. LYRA receives development support from MPS. The LYRA team acknowledges the contribution of a very agreeable consortium. During the preparation of this paper, we have learnt the sudden death of Pierre Cugnon. He was the department head of

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