Evolution of the Solar Wind Direction through the Heliosphere

<p>The solar wind non-radial velocity components observed beyond the Alfvén point are usually attributed to waves, the interaction of different streams, or other transient phenomena. However, Earth-orbiting spacecraft as well as monitors at L1 indicate systematic deviations of the wind velocity from the radial direction. Since these deviations are of the order of several degrees, the calibration of the instruments is often questioned. This paper investigates for the first time the evolution of non-radial components of the solar wind flow along the path from ≈ 0.17 to 10 AU. A comparison of observations at 1 AU with those closer to or farther from the Sun based on measurements of many spacecraft at different locations in the heliosphere (Parker Solar Probe, Helios 1 and 2, Wind, ACE, Spektr-R, ARTEMIS probes, MAVEN, Voyagers 1and 2) shows that (i) the average values of non-radial components are not zero and vary in a systematic manner with the distance from the Sun, (ii) their values significantly depend on the solar wind radial velocity, (iii) the deviation from radial direction well correlates with the cross-helicity, and (iv) the values of non-radial components peaks at 0.25 AU and gradually decreases toward zero in the outer heliosphere. Our results suggest that the difference in the propagation direction between the faster and slower winds is already established in the solar corona and is connected with the forces emitting solar wind plasma from the coronal magnetic field. The correlation with cross-helicity probably points to outward propagating Alfven waves generated in outer corona as the most probable source of observed deviations.</p>

Observations of the Solar Corona from Space

Space observations of the atmosphere of the Sun, obtained in half a century of dedicated space missions, provide a well established picture of the medium and large-scale solar corona, which is highly variable with the level of solar activity through a solar cycle and evolves with the long-term evolution of the magnetic cycles. In this review, we summarize the physical properties and dynamics of the medium and large-scale corona, consisting primarily of active regions, streamers and coronal holes; describe the dependence of coronal patterns on the magnetic field patterns changing through the solar cycle and the properties of the regions of open magnetic flux channeling the solar wind; the ubiquitous presence of fluctuations in the outer corona; the rotational properties of the large-scale corona; and the persistent hemispheric asymmetries in the emergence of magnetic fields and the distribution of the coronal emission.

Characteristics of solar wind suprathermal halo electrons

A suprathermal halo population of electrons is ubiquitous in space plasmas, as evidence of their departure from thermal equilibrium even in the absence of anisotropies. The origin, properties, and implications of this population, however, are poorly known. We provide a comprehensive description of solar wind halo electrons in the ecliptic, contrasting their evolutions with heliospheric distance in the slow and fast wind streams. At relatively low distances less than 1 AU, the halo parameters show an anticorrelation with the solar wind speed, but this contrast decreases with increasing distance and may switch to a positive correlation beyond 1 AU. A less monotonic evolution is characteristic of the high-speed winds, in which halo electrons and their properties (e.g., number densities, temperature, plasma beta) exhibit a progressive enhancement already distinguishable at about 0.5 AU. At this point, magnetic focusing of electron strahls becomes weaker and may be counterbalanced by the interactions of electrons with wave fluctuations. This evolution of halo electrons between 0.5 AU and 3.0 AU in the fast winds complements previous results well, indicating a substantial reduction of the strahl and suggesting that significant fractions of strahl electrons and energy may be redistributed to the halo population. On the other hand, properties of halo electrons at low distances in the outer corona suggest a subcoronal origin and a direct implication in the overheating of coronal plasma via velocity filtration.

The Solar Orbiter EUI instrument: The Extreme Ultraviolet Imager

Context. The Extreme Ultraviolet Imager (EUI) is part of the remote sensing instrument package of the ESA/NASA Solar Orbiter mission that will explore the inner heliosphere and observe the Sun from vantage points close to the Sun and out of the ecliptic. Solar Orbiter will advance the “connection science” between solar activity and the heliosphere. Aims. With EUI we aim to improve our understanding of the structure and dynamics of the solar atmosphere, globally as well as at high resolution, and from high solar latitude perspectives. Methods. The EUI consists of three telescopes, the Full Sun Imager and two High Resolution Imagers, which are optimised to image in Lyman-α and EUV (17.4 nm, 30.4 nm) to provide a coverage from chromosphere up to corona. The EUI is designed to cope with the strong constraints imposed by the Solar Orbiter mission characteristics. Limited telemetry availability is compensated by state-of-the-art image compression, onboard image processing, and event selection. The imposed power limitations and potentially harsh radiation environment necessitate the use of novel CMOS sensors. As the unobstructed field of view of the telescopes needs to protrude through the spacecraft’s heat shield, the apertures have been kept as small as possible, without compromising optical performance. This led to a systematic effort to optimise the throughput of every optical element and the reduction of noise levels in the sensor. Results. In this paper we review the design of the two elements of the EUI instrument: the Optical Bench System and the Common Electronic Box. Particular attention is also given to the onboard software, the intended operations, the ground software, and the foreseen data products. Conclusions. The EUI will bring unique science opportunities thanks to its specific design, its viewpoint, and to the planned synergies with the other Solar Orbiter instruments. In particular, we highlight science opportunities brought by the out-of-ecliptic vantage point of the solar poles, the high-resolution imaging of the high chromosphere and corona, and the connection to the outer corona as observed by coronagraphs.

Suprathermal electron strahls. From small scale modeling to heliospheric implications

<p>Recent advances in kinetic modeling reveal essential properties of the suprathermal populations opening perspectives for a realistic interpretation of their implications. Of particular importance are the suprathermal electron strahl (or beaming) populations, guided by the heliospheric magnetic field as kinetic-scale traces of the continuous solar outflows. We outline the main implications of the strahls by connecting their signatures in the velocity distributions with macroscopic properties of the solar wind, and processes conditioning their relaxation via coherent or non-coherent radiative emissions. The electron strahls may also help understanding major changes in the magnetic field topology in the outer corona, as shown by the most recent data from Solar Parker Probe, and during energetic (transient) events like coronal mass ejections, implying or not reconnection, but leading to strong interaction regions and shocks. </p>

Experimental Investigation of the Spatial and Temporal Evolution of the Tangential and Normal E-Field Components along the Stress Grading System of a Real Stator Bar

This paper presents results based on direct experimental measurements of tangential (Et) and normal (En) E-field components along the stress grading system (SGS) of a real stator bar (Roebel type) for different AC 60 Hz applied voltages. These measurements were made with a new electro-optic system allowing for the study of both spatial distributions of two E-field components along the bar and their temporal evolution at critical points. The results obtained allowed us to calculate the correlation between the distribution of En and Et along the SGS. In particular, it was demonstrated that the En distribution presents a characteristic minimum, which can be used to identify the zone of partial discharge inception. Moreover, it was possible to observe an enlargement of the Et component distribution followed by a saturation in magnitude with the applied voltage increase. Moreover, the results have demonstrated that the waveform of the En component is mostly affected by the SG material used, producing a greater distortion in its waveform than those obtained for the Et component. The more significant distortion was obtained at the end of the outer corona protection (OCP) material, corresponding to the first maximum of the En component and characterized by the appearance of a third harmonic of large amplitude.

Effect of the non-uniform solar chromospheric Lyα radiation on determining the coronal H I outflow velocity

We derived maps of the solar wind outflow velocity of coronal neutral hydrogen atoms at solar minimum in the altitude range 1.5–4.0 R⊙. We applied the Doppler dimming technique to coronagraphic observations in the UV H I Lyα line at 121.6 nm. The technique exploits the intensity reduction in the coronal line with increasing velocities of the outflowing plasma to determine the solar wind velocity by iterative modelling. The Lyα line intensity is sensitive to the wind outflow velocity and also depends on the physical properties of coronal particles and underlying chromospheric emission. Measurements of irradiance by the chromospheric Lyα radiation in the corona are required for a rigorous application of the Doppler dimming technique, but they are not provided by past and current instrumentations. A correlation function between the H I 121.6 nm and He II 30.4 nm line intensities was used to construct Carrington rotation maps of the non-uniform solar chromospheric Lyα radiation and thus to compute the Lyα line irradiance throughout the outer corona. Approximations concerning the temperature of the scattering H I atoms and exciting solar disc radiation were also adopted to significantly reduce the computational time and obtain a faster procedure for a quick-look data analysis of future coronagraphic observations. The effect of the chromospheric Lyα brightness distribution on the resulting H I outflow velocities was quantified. In particular, we found that the usual uniform-disc approximation systematically leads to an overestimated velocity in the polar and mid-latitude coronal regions up to a maximum of about 50−60 km s−1 closer to the Sun. This difference decreases at higher altitudes, where an increasingly larger chromospheric portion, including both brighter and darker disc features, contributes to illuminate the solar corona, and the non-uniform radiation condition progressively approaches the uniform-disc approximation.

Modeling and removal of optical ghosts in the PROBA-3/ASPIICS externally occulted solar coronagraph

Context. ASPIICS is a novel externally occulted solar coronagraph, which will be launched onboard the PROBA-3 mission of the European Space Agency. The external occulter will be placed on the first satellite ∼150 m ahead of the second satellite that will carry an optical instrument. For 6 h per orbit, the satellites will fly in a precise formation, constituting a giant externally occulted coronagraph. The large distance between the external occulter and the primary objective will allow observations of the white-light solar corona starting from extremely low heights ∼1.1 R⊙. Aims. We aim to analyze the possible influence of optical ghost images formed inside the telescope and to develop an algorithm for their removal. Methods. We implement the optical layout of ASPIICS in Zemax and study the ghost behavior in sequential and nonsequential regimes. We identify sources of the ghost contributions and analyze their geometrical behavior. Finally we develop a mathematical model and software to calculate ghost images for any given input image. Results. We show that ghost light can be important in the outer part of the field of view, where the coronal signal is weak, since the energy of bright inner corona is redistributed to the outer corona. However, the model allows for the ghost contribution to be removed. Due to the large distance between the external occulter and the primary objective, the primary objective does not produce a significant ghost. The use of the Lyot spot in ASPIICS is not necessary.