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Bieler A.,University of Michigan | Bieler A.,University of Bern | Altwegg K.,University of Bern | Balsiger H.,University of Bern | And 22 more authors.
Astronomy and Astrophysics | Year: 2015

67P/Churyumov-Gerasimenko (67P) is a Jupiter-family comet and the object of investigation of the European Space Agency mission Rosetta. This report presents the first full 3D simulation results of 67P's neutral gas coma. In this study we include results from a direct simulation Monte Carlo method, a hydrodynamic code, and a purely geometric calculation which computes the total illuminated surface area on the nucleus. All models include the triangulated 3D shape model of 67P as well as realistic illumination and shadowing conditions. The basic concept is the assumption that these illumination conditions on the nucleus are the main driver for the gas activity of the comet. As a consequence, the total production rate of 67P varies as a function of solar insolation. The best agreement between the model and the data is achieved when gas fluxes on the night side are in the range of 7% to 10% of the maximum flux, accounting for contributions from the most volatile components. To validate the output of our numerical simulations we compare the results of all three models to in situ gas number density measurements from the ROSINA COPS instrument. We are able to reproduce the overall features of these local neutral number density measurements of ROSINA COPS for the time period between early August 2014 and January 1 2015 with all three models. Some details in the measurements are not reproduced and warrant further investigation and refinement of the models. However, the overall assumption that illumination conditions on the nucleus are at least an important driver of the gas activity is validated by the models. According to our simulation results we find the total production rate of 67P to be constant between August and November 2014 with a value of about 1 × 1026 molecules s-1. © ESO, 2015. Source

Kariyappa R.,Indian Institute of Astrophysics | Kariyappa R.,Harvard - Smithsonian Center for Astrophysics | Deluca E.E.,Harvard - Smithsonian Center for Astrophysics | Saar S.H.,Harvard - Smithsonian Center for Astrophysics | And 4 more authors.
Astronomy and Astrophysics | Year: 2011

Aims. We investigate the variability in temperature as a function of time among a sample of coronal X-ray bright points (XBPs). Methods. We analysed a 7-h (17:00-24:00 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 X-ray telescope (XRT) onboard the Hinode mission. We identified and selected 14 XBPs for a detailed analysis. The light curves of XBPs were derived using the SolarSoft library in IDL. The temperature of XBPs was determined using the calibrated temperature response curves of the two filters by means of the intensity ratio method. Results. We find that the XBPs show a high variability in their temperature and that the average temperature ranges from 1.1 MK to 3.4 MK. The variations in temperature are often correlated with changes in average X-ray emission. It is evident from the results of time series that the XBP heating rate can be highly variable on short timescales, suggesting that it has a reconnection origin. © 2010 ESO. Source

Kumara S.T.,Indian Institute of Astrophysics | Kumara S.T.,Bangalore University | Kariyappa R.,Indian Institute of Astrophysics | Zender J.J.,European Space Agency | And 10 more authors.
Astronomy and Astrophysics | Year: 2013

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. © ESO, 2013. Source

Irbah A.,LATMOS Laboratoire Atmospheres | Meftah M.,LATMOS Laboratoire Atmospheres | Hauchecorne A.,LATMOS Laboratoire Atmospheres | Dame L.,LATMOS Laboratoire Atmospheres | And 2 more authors.
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2014

The Earth's atmosphere introduces a spatial frequency filtering in the object images recorded with ground-based instruments. A solution is to observe with telescopes onboard satellites to avoid atmospheric effects and to obtain diffraction limited images. However, similar atmosphere problems encountered with ground-based instruments may subsist in space when we observe the Sun since thermal gradients at the front of the instrument affect the observations. We present in this paper some simulations showing how solar images recorded in a telescope focal plane are directly impacted by thermal gradients in its pupil plane. We then compare the results with real solar images recorded with the PICARD mission in space. © 2014 SPIE. Source

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