Solar wind analyzer - The solar orbiter milestone towards on-board intelligent decision making systems

The most important challenge underpinning the transition to next generation of space missions design is the discrepancy between the dramatic increases in observation rate and the marginal increase in downlink capacity, enforcing the shift from the traditional “acquire-compress-transmit” paradigm to highly efficient intelligent on-board processing of observations, minimizing downlink requirements while respecting the limitations in power and bandwidth resources. Solar Orbiter (SO), an ESA/NASA mission, is a milestone both in the purely technological and scientific sphere. SO is designed to study the connection between the Sun and the heliosphere, with particular interest to open issues such as the sources of solar wind streams and turbulence, the heliospheric variability, the origin of energetic particles and the solar dynamo. The selected science payload is required to support making the link between in-situ and remote sensing observations, and is composed of ten instruments or suites of instruments including spectrometers, imagers, wave and particle instruments – many the result of large international consortia. In particular, the plasma suite Solar Wind Analyzer (SWA) comprises: Proton-Alpha Sensor (PAS), Electron Analyzer System (EAS), Heavy Ion Sensor (HIS) together with the Data Processing Unit (DPU), and will provide high-resolution 3D velocity distribution function of ions and electrons, together with ion composition, necessary to infer the thermal state of solar wind and its source regions, identify structures such as shocks, CME's and other transients, and determine the link between particle dynamics and waves. SO will explore new distance and latitude regions that remain unexplored, even accounting for existing Helios and upcoming Parker Solar Probe observations. The technical challenges include heavy constraints such as the limited bandwidth available to SWA for downlink, so that the whole set of raw particle data collected cannot be transmitted back to ground. Data processing is thus used to evaluate concise scientific properties of the solar wind, particularly the moments of the particle velocity distribution functions (VDF), such that it is then acceptable to transmit the full VDF data only at low frequencies. Then processing is re-adopted on these distributions to meet the required (lossless) compression rates (2-8). Another step towards the aforementioned paradigm shift is represented by the SWA Book-Keeping Algorithm (BKA), which has been designed to ensure that the individual sensors remain within the allocated telemetry rate on an orbit-averaged basis. The philosophy of the SWA book-keeping scheme has since been applied to all instruments with ESOC’s Operations Team introducing the concept of Operations Telemetry Corridors (OTC) to finely tune the rate of telemetry generation by the instruments

Long term durability of protected silver coating for the mirrors of Ariel mission telescope

Ariel (Atmospheric Remote-sensing Infrared Exoplanet Large survey) is the fourth medium-size mission in ESA “Cosmic Vision” program. It is scheduled to launch in 2029. Ariel will conduct spectroscopic and photometric observations of a large sample of known exoplanets to survey their atmospheres with the transit method. Ariel is based on a 1 m class telescope designed for the visible and near infrared spectrum, but optimized specifically for spectroscopy in the waveband between 1.95 and 7.8 μm. Telescope and instruments will be operating in cryogenic conditions in the range 40-50 K. The telescope mirrors will be manufactured in aluminum 6061, with a protected silver coating deposited onto the optical surface to enhance reflectivity and prevent oxidation and corrosion. During the preliminary definition phase of the development work, leading to mission adoption, a silver coating with space heritage was selected and underwent a qualification process on disc-shaped samples of the mirrors substrate material. The samples were deposited through magnetron sputtering and then subjected to a battery of tests, including environmental durability tests, accelerated aging, cryogenic tests and mechanical resistance tests. Further to the qualification, the samples have been stored in cleanroom conditions and periodically re-examined and measured to detect any sign of coating degradation. The test program, still ongoing at the time of writing this article, consists of visual inspection with a high intensity lamp, spectral reflectance measurements and Atomic Force Microscopy (AFM) evaluation of nanometric surface features. The goal is to ensure stability of the optical performance, in terms of coating reflectance, during a time span comparable to the period that the actual mirrors of the telescope will spend in average cleanroom conditions. This study presents the interim results after three years of storage

Importance of energy and angular resolutions in top-hat electrostatic analysers for solar wind proton measurements

We use a numerical code which reproduces the angular/energy response of a typical top-hat electrostatic analyser starting from solar wind proton velocity distribution functions (VDFs) generated by numerical simulations. The simulations are based on the Hybrid Vlasov-Maxwell numerical algorithm which integrates the Vlasov equation for the ion distribution function, while the electrons are treated as a fluid. A virtual satellite launched through the simulation box measures the particle VDFs. Such VDFs are moved from the simulation Cartesian grid to energy-angular coordinates to mimic the response of a real sensor in the solar wind. Different energy-angular resolutions of the analyser are investigated in order to understand the influence of the phase-space resolution in existing and upcoming space missions, with regards to determining the key parameters of plasma dynamics

GIADA microbalance measurements on board Rosetta: submicrometer- to micrometer-sized dust particle flux in the coma of comet 67P/Churyumov-Gerasimenko

Context. From August 2014 to September 2016, Rosetta escorted comet 67P/Churyumov-Gerasimenko (67P) during its journey around the Sun. One of the aims of Rosetta was to characterize cometary activity and the consequent formation of dust flux structures in cometary comae. Aims: We characterize and quantify the submicrometer- to micrometer-sized dust flux that may be shaped in privileged directions within the coma of 67P inbound to and outbound from perihelion. Methods: The in situ dust-measuring instrument GIADA, part of the Rosetta/ESA payload, consisted of three subsystems, one of which was the Micro Balance Subsystem (MBS), composed of five quartz crystal microbalances. From May 2014 to September 2016, MBS measured the submicrometer- to micrometer-sized deposited dust mass every 5 min. Results: We characterized the submicrometer- to micrometer-sized dust mass flux in the coma of 67P. The anti-sunward and the radial direction are preferred, and the flux is higher in the anti-sunward direction. The measured cumulative dust mass in the anti-sunward direction is 2.38 ± 0.04 × 10-7 kg, and in the radial direction, it is 1.18 ± 0.02 × 10-7 kg. We explain the anti-sunward dust flux as the effect of nonuniform gas emission between the night- and dayside of the nucleus, which acts in combination with the solar radiation pressure. We compared the cumulated dust mass of particles ≤5 μm with particles ≥100 μm. The retrieved ratio of ≈2% implies a differential size distribution index of ≈-3.0, which confirms that particles of size ≥0.1 mm dominate the dust coma cross-section of 67P during the entire orbit. Conclusions: Submicrometer- to micrometer-sized dust mass flux measurements were made for the first time from the arising of cometary activity until its extinction. They indicate that these particles do not provide a substantial optical scattering in the coma of 67P with respect to the scattering caused by millimeter-sized particles. In addition, MBS data reveal that the measured dust flux is highly anisotropic: anti-sunward plus radial