Luc Damé

PICARD: Simulataneous measurements of the solar diameter, differential rotation, solar constant and their variations

Abstract PICARD is a CNES micro-satellite mission due for flight by the end of 2002, named after the name of a French astronomer who first observed with consistency the solar diameter changes during the Maunder minimum in the 16th century. It consists of two instruments measuring (i) the solar diameter and differential rotation, and (ii) the total solar irradiance. These quantities are fundamental for the understanding of the solar-Terrestrial relations, e.g. the influence of the Sun on the Earths climate, and of the internal structure of the Sun. The continuous — or nearly continuous — viewing of the Sun from an appropriate orbit, the 5 minutes sampling rate and the very low noise measurements, will allow g-modes detection and precise diameter measurements besides accurately establishing the relationship between irradiance and diameter changes. Providing an absolute measure of the solar diameter to 1 milliarcsecond, PICARD is the first step towards instruments capable of accurate and perennial measurements, for the centuries to come, of the solar-terrestrial influence. The objectives of the mission, instrument capabilities, observing modes and performances are described.

Temperature variability in X-ray bright points observed with Hinode/XRT

Aims. Our aim is to investigate the variability in the temperature as a function of time among a sample of coronal X-ray bright points (XBPs). Methods. We analysed a 7-hour (17:00 UT - 24:00 UT) long time sequence of soft X-ray images observed almost simultaneously in two filters (Ti poly and Al mesh) on April 14, 2007 with XRay Telescope (XRT) on-board the Hinode mission. We identifi ed and selected a sample of 14 XBPs and 2 background coronal regions for a detailed analysis. The light curves of XBPs have been derived using SolarSoft library in IDL. The temperature of XBPs was determined using the calibrated temperature response curves of the two filters by intensity ratio method. Results. We find that the XBPs show a high variability in their temperat ure and the average temperature ranges from 1.1 MK to 3.4 MK. The variations in the temperature are related to the different X-ray emission fluxes. These results suggest that XBPs of different temperatures may be present at the same height in the corona. It is evident from the results of time series that the heating rate of XBPs is highly variable on short time scales, and these variations provide a support for magnetic reconnection as the origin of XBPs.

SOLAR/SOLSPEC mission on ISS: In-flight performances for SSI measurements in the UV

The SOLar SPECtrum (SOLSPEC) experiment is part of the Solar Monitoring Observatory (SOLAR) payload, and has been externally mounted on the Columbus module of the International Space Station (ISS) since 2008. SOLAR/SOLSPEC combines three absolutely calibrated double monochromators with concave gratings for measuring the solar spectral irradiance (SSI) from 166 nm to 3088 nm. This physical quantity is a key input for studies of climatology, planetary atmospheres, and solar physics. Aims. A general description of the instrument is given, including in-flight operations and performance of the ultraviolet (UV) channel from 175 nm to 340 nm. Methods. We developed a range of processing and correction methods, which are described in detail. For example, methods for correcting thermal behavior effects, instrument linearity, and especially the accuracy of the wavelength and absolute radiometric scales have been validated by modeling the standard uncertainties. Results. The deliverable is a quiet Sun UV reference solar spectrum as measured by SOLAR/SOLSPEC during the minimum of solar activity prior to cycle 24 1 . Comparisons with other instruments measuring SSI are also presented.

White-light radiation from semi-empirical flare models

We show that some recently published semi-empirical models for solar flares predict a significant flux of visible continuum radiation, due to bound-free radiation from hydrogen atoms and H− ions in the chromospheric flare. The ratio of the emergent intensity in one flare model to that in the quiet Sun is more than 100% at the head of the Balmer continuum for a flare close to the limb, and 8% at disk centre. The predicted flare spectrum has a relatively strong Balmer jump. We compare the theoretical flare continuum with observations and find disagreement in several important respects. The main disagreements are: (1) the fact that few flares are observed to emit a white-light continuum, while the models suggest that they should do so; (2) the prediction of a strong Balmer jump, which is not observed in most white-light flares; and (3) the absence of a ‘blue continuum’ in the theoretical prediction. We conclude that observations of flare continua provide useful constraints on semi-empirical models, and that at present the models do not satisfy these constraints.

Segmentation of coronal features to understand the solar EUV and UV irradiance variability

Context. The study of solar irradiance variability is of great importance in heliophysics, the Earth’s climate, and space weather applications. These studies require careful identifying, tracking and monitoring of active regions (ARs), coronal holes (CHs), and the quiet Sun (QS). Aims. We studied the variability of solar irradiance for a period of two years (January 2011–December 2012) using the Large Yield Radiometer (LYRA), the Sun Watcher using APS and image Processing (SWAP) on board PROBA2, and the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO). Methods. We used the spatial possibilistic clustering algorithm (SPoCA) to identify and segment coronal features from the EUV observations of AIA. The AIA segmentation maps were then applied on SWAP images, and parameters such as the intensity, fractional area, and contribution of ARs/CHs/QS features were computed and compared with the full-disk integrated intensity and LYRA irradiance measurements. Results. We report the results obtained from SDO/AIA and PROBA2/SWAP images taken from January 2011 to December 2012 and compare the resulting integrated full-disk intensity with PROBA2/LYRA irradiance. We determine the contributions of the segmented features to EUV and UV irradiance variations. The variations of the parameters resulting from the segmentation, namely the area, integrated intensity, and relative contribution to the solar irradiance, are compared with LYRA irradiance. We find that the active regions have a great impact on the irradiance fluctuations. In the EUV passbands considered in this study, the QS is the greatest contributor to the solar irradiance, with up to 63% of total intensity values. Active regions, on the other hand, contribute to about 10%, and off-limb structures to about 24%. We also find that the area of the features is highly variable suggesting that their area has to be taken into account in irradiance models, in addition to their intensity variations. Conclusions. We successfully show that the feature extraction allows us to use EUV telescopes to measure irradiance fluctuations and to quantify the contribution of each part to the EUV spectral solar irradiance observed with a calibrated radiometer. This study also shows that SPoCA is viable, and that the segmentation of images can be a useful tool. We also provide the measurement correlation between SWAP and AIA during this analysis.

An innovative far UV telescope for space weather and solar variability studies (Conference Presentation)

SUAVE (Solar Ultraviolet Advanced Variability Experiment) is a far UV imaging solar telescope (Lyman Alpha, 121.6 nm, Herzberg continuum, 200-220 nm, etc.) of novel design for ultimate thermal stability and long lasting performances. SUAVE is a 80 mm Ritchey-Chretien off-axis telescope with SiC mirrors and no entrance window for long and uncompromised observations in the UV (no coatings of mirrors, flux limited to less than 2 solar constants on filters to avoid degradation), associated with an ultimate thermal control (no central obscuration resulting in limited thermal gradients and easier heat evacuation, focus control, stabilization). Design and anticipated performances will be detailed as well as the realization process under way. Tests on a representative breadboard will be performed in 2018 (CNES R&T). SUAVE is the main instrument of the SUITS/SWUSV (Solar Ultraviolet Influence on Troposphere/Stratosphere / Space Weather and Ultraviolet Solar Variability) microsatellite mission.

Think the way to measure the Earth Radiation Budget and the Total Solar Irradiance with a small satellites constellation

Within the past decade, satellites constellations have become possible and practical. One of the interest to use a satellites constellation is to measure the true Earth Radiation Imbalance, which is a crucial quantity for testing climate models and for predicting the future course of global warming. This measurement presents a high interest because the 2001-2010 decade has not shown the accelerating pace of global warming that most models predict, despite the fact that the greenhouse-gas radiative forcing continues to rise. All estimates (ocean heat content and top of atmosphere) show that over the past decade the Earth radiation imbalance ranges between 0.5 to 1W-2. Up to now, the Earth radiation imbalance has not been measured directly. The only way to measure the imbalance with sufficient accuracy is to measure both the incoming solar radiations (total solar irradiance) and the outgoing terrestrial radiations (top of atmosphere outgoing longwave radiations and shortwave radiations) onboard the same satellite, and ideally, with the same instrument. The incoming solar radiations and the outgoing terrestrial radiations are of nearly equal magnitude of the order of 340.5W-2. The objective is to measure these quantities over time by using differential Sun-Earth measurements (to counter calibration errors) with an accuracy better than 0.05Wm-2 at 1σ. It is also necessary to have redundant instruments to track aging in space in order to measure during a decade and to measure the global diurnal cycle with a dozen satellites. Solar irradiance and Earth Radiation Budget (SERB) is a potential first in orbit demonstration satellite. The SERB nano-satellite aims to measure on the same platform the different components of the Earth radiation budget and the total solar irradiance. Instrumental payloads (solar radiometer and Earth radiometers) can acquire the technical maturity for the future large missions (constellation that insure global measurement cover) by flying in a CubeSat. This paper is intended to demonstrate the ability to build a low-cost satellite with a high accuracy measurement in order to have constant flow of data from space.

Eight years of solar observations with PICARD

PICARD is a mission devoted to solar variability observation, which aims at perpetuating valuable historical time-series of the solar radius. PICARD contains a double program with in-space and on-ground measurements using Ritchey-Chrétien telescopes. The PICARD spacecraft was launched on June 15, 2010, commissioned in-flight in October of the same year, and was retired in April 2014. PICARD ground-based observatory is functional since May 2011 in the Plateau de Calern (France), and is still operational today. We shall give an overview of the PICARD instrumentation and the performances of the existing ground-based telescope. We will also present our current results about solar radius variations after eight years of solar observations.

Segmentation of photospheric magnetic elements corresponding to coronal features to understand the EUV and UV irradiance variability

Context. The magnetic field plays a dominant role in the solar irradiance variability. Determining the contribution of various magnetic features to this variability is important in the context of heliospheric studies and Sun-Earth connection. Aims. We studied the solar irradiance variability and its association with the underlying magnetic field for a period of five years (January 2011–January 2016). We used observations from the Large Yield Radiometer (LYRA), the Sun Watcher with Active Pixel System detector and Image Processing (SWAP) on board PROBA2, the Atmospheric Imaging Assembly (AIA), and the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO). Methods. The Spatial Possibilistic Clustering Algorithm (SPoCA) is applied to the extreme ultraviolet (EUV) observations obtained from the AIA to segregate coronal features by creating segmentation maps of active regions (ARs), coronal holes (CHs) and the quiet sun (QS). Further, these maps are applied to the full-disk SWAP intensity images and the full-disk (FD) HMI line-of-sight (LOS) magnetograms to isolate the SWAP coronal features and photospheric magnetic counterparts, respectively. We then computed full-disk and feature-wise averages of EUV intensity and line of sight (LOS) magnetic flux density over ARs/CHs/QS/FD. The variability in these quantities is compared with that of LYRA irradiance values. Results. Variations in the quantities resulting from the segmentation, namely the integrated intensity and the total magnetic flux density of ARs/CHs/QS/FD regions, are compared with the LYRA irradiance variations. We find that the EUV intensity over ARs/CHs/QS/FD is well correlated with the underlying magnetic field. In addition, variations in the full-disk integrated intensity and magnetic flux density values are correlated with the LYRA irradiance variations. Conclusions. Using the segmented coronal features observed in the EUV wavelengths as proxies to isolate the underlying magnetic structures is demonstrated in this study. Sophisticated feature identification and segmentation tools are important in providing more insights into the role of various magnetic features in both the short- and long-term changes in the solar irradiance.

The Space Weather & Ultraviolet Solar Variability Microsatellite Mission (SWUSV)

We present a summary of the scientific objectives, payload and mission profile of the Space Weather & Ultraviolet Solar Variability Microsatellite Mission (SWUSV) proposed to CNES and ESA (small mission).