Quiet-Sun hydrogen Lyman-α line profile derived from SOHO/SUMER solar-disk observations

Context. The solar radiation in the Lyman-α spectral line of hydrogen plays a significant role in the illumination of chromospheric and coronal structures, such as prominences, spicules, chromospheric fibrils, cores of coronal mass ejections, and solar wind. Moreover, it is important for the investigation of the heliosphere, Earth’s ionosphere, and the atmospheres of planets, moons, and comets. Aims. We derive a reference quiet-Sun Lyman-α spectral profile that is representative of the Lyman-α radiation from the solar disk during a minimum of solar activity. This profile can serve as an incident radiation boundary condition for the radiative transfer modelling of chromospheric and coronal structures. Because the solar radiation in the Lyman lines is not constant over time but varies significantly with the solar cycle, we provide a method for the adaptation of the incident radiation Lyman line profiles (Lyman-α and higher lines) to a specific date. Moreover, we analyse how the change in the incident radiation influences the synthetic spectra produced by the radiative transfer modelling. Methods. We used SOHO/SUMER Lyman-α raster scans obtained without the use of the attenuator in various quiet-Sun regions on the solar disk. The observations were performed on three consecutive days (June 24, 25, and 26, 2008) during a period of minimum solar activity. The reference Lyman-α profile was obtained as a spatial average over eight available raster scans. To take into account the Lyman-α variation with the solar cycle, we used the LISIRD composite Lyman-α index. To estimate the influence of the change in the incident radiation in the Lyman lines on the results of radiative transfer models, we used a 2D prominence fine structure model. Results. We present the reference quiet-Sun Lyman-α profile and a table of coefficients describing the variation of the Lyman lines with the solar cycle throughout the lifetime of SOHO. The analysis of the influence of the change in the incident radiation shows that the synthetic spectra are strongly affected by the modification of the incident radiation boundary condition. The most pronounced impact is on the central and integrated intensities of the Lyman lines. There, the change in the synthetic spectra can often have the same amplitude as the change in the incident radiation itself. The impact on the specific intensities in the peaks of reversed Lyman-line profiles is smaller but still significant. The hydrogen Hα line can also be considerably affected, despite the fact that the Hα radiation from the solar disk does not vary with the solar cycle.

Determining the dynamics and magnetic fields in He I 10830 Å during a solar filament eruption

Aims. We investigate the dynamics and magnetic properties of the plasma, including the line-of-sight velocity (LOS) and optical depth, as well as the vertical and horizontal magnetic fields, belonging to an erupted solar filament. Methods. The filament eruption was observed with the GREGOR Infrared Spectrograph at the 1.5-meter GREGOR telescope on July 3, 2016. We acquired three consecutive full-Stokes slit-spectropolarimetric scans in the He I 10830 Å spectral range. The Stokes I profiles were classified using the machine learning k-means algorithm and then inverted with different initial conditions using the HAZEL code. Results. The erupting-filament material presents the following physical conditions: (1) ubiquitous upward motions with peak LOS velocities of ∼73 km s−1; (2) predominant large horizontal components of the magnetic field, on average, in the range of 173−254 G, whereas the vertical components of the fields are much lower, on average between 39 and 58 G; (3) optical depths in the range of 0.7−1.1. The average azimuth orientation of the field lines between two consecutive raster scans (<2.5 min) remained constant. Conclusions. The analyzed filament eruption belongs to the fast rising phase, with total velocities of about 124 km s−1. The orientation of the magnetic field lines does not change from one raster scan to the other, indicating that the untwisting phase has not yet started. The untwisting appears to start about 15 min after the beginning of the filament eruption.

Evolution of supersonic downflows in a sunspot

Context. Supersonic downflows have been observed in transition region spectra above numerous sunspots, however, little research has been conducted to date into how persistent these signatures are within sunspots on timescales longer than a few hours. Aims. We aim to analyse the lead sunspot of AR 12526 to infer the properties and evolution of supersonic downflows occurring within it using high-spatial and spectral resolution data. Methods. We analysed 16 large, dense raster scans sampled by the Interface Region Imaging Spectrograph. These rasters tracked the lead sunspot of AR 12526 across the solar disc at discrete times between 27 March 2016 and 2 April 2016, providing spectral profiles from the Si IV, O IV, Mg II, and C II lines. Additionally, we studied one sit-and-stare observation acquired on 1 April 2016 centred on the sunspot to analyse the evolution of supersonic downflows on shorter timescales. Results. Supersonic downflows are variable within this sunspot both in terms of spatial structuring and velocities. Thirteen of the 16 raster scans display some evidence of supersonic downflows in the Si IV 1394 Å line co-spatial to a sustained bright structure detected in the 1400 Å slit-jaw imaging channel; a peak velocity of 112 km s−1 is recorded on 29 March 2016. Evidence for supersonic downflows in the O IV 1401 Å line is found in 14 of these rasters; the spatial structuring in this line often differs from that inferred from the Si IV 1394 Å line. Only one example of a supersonic downflow is detected in the C II 1335 Å line and no downflows are found in the Mg II 2796 Å lines at these locations. In the sit-and-stare observations, no dual flow is initially detected, however, a supersonic downflow develops after approximately 60 min. This downflow accelerates from 73 km s−1 to close to 80 km s−1 in both the Si IV 1394 Å and O IV 1401 Å lines over the course of 20 min before the end of the observation. Conclusions. Supersonic downflows are found in the Si IV 1394 Å line in 13 of the 16 rasters studied in this work. The morphology of these downflows evolved over the course of both hours and days and was often different in the Si IV 1394 Å and O IV 1401 Å lines. These events were found co-spatial to a bright region in the core of the Si IV 1394 Å line, which appeared to form at the footpoints of coronal fan loops. Our results indicate that one raster is not enough to conclusively draw inferences about the properties of supersonic downflows within a sunspot during its lifetime.

The complex analysis of X-ray mesh scans for macromolecular crystallography

In macromolecular crystallography, mesh (raster) scans are carried out either as part of X-ray-based crystal-centring routines or to identify positions on the sample holder from which diffraction images can be collected. Here, the methods used inMeshBest, software which automatically analyses diffraction images collected during a mesh scan and produces a two-dimensional crystal map showing estimates of the dimensions, centre positions and diffraction qualities of each crystal contained in the mesh area, are presented. Sample regions producing diffraction images resulting from the superposition of more than one crystal are also distinguished from regions with single-crystal diffraction. The applicability of the method is demonstrated using several cases.

Antenna Pattern Measurements of Weather Radars Using the Sun and a Point Source

The sun is known to be a good target for weather radar calibration. In this paper high-resolution raster scans of the sun at high elevations will be used to derive the antenna pattern of weather radar, without being affected by beam propagation effects and reflections close to the earth’s surface. It is shown that this pattern matches well to pattern measurements using a point source. Hence, a good estimation of the real antenna pattern can be derived using the sun.Furthermore, formulas to extract undistorted antenna patterns from the sun, even at high-elevation angles, are derived. The signal processing required to achieve high sensitivity for the antenna pattern measurements will be described. Important parts of the antenna pattern—for example, sidelobes—become visible when using long integration times.The polarimetric receiver channel cross-correlation coefficient is proposed as a figure of merit of the cross-polar isolation of the antenna and hence the cross-polar pattern. The results are also compared to point source measurements. This illustrates how an unpolarized signal source like the sun can be used to derive polarimetric variables.

A look-up table based approach to characterize crystal twinning for synchrotron X-ray Laue microdiffraction scans

An automated method has been developed to characterize the type and spatial distribution of twinning in crystal orientation maps from synchrotron X-ray Laue microdiffraction results. The method relies on a look-up table approach. Taking into account the twin axis and twin plane for plausible rotation and reflection twins, respectively, and the point group symmetry operations for a specific crystal, a look-up table listing crystal-specific rotation angle–axis pairs, which reveal the orientation relationship between the twin and the parent lattice, is generated. By comparing these theoretical twin–parent orientation relationships in the look-up table with the measured misorientations, twin boundaries are mapped automatically from Laue microdiffraction raster scans with thousands of data points. Taking advantage of the high orientation resolution of the Laue microdiffraction method, this automated approach is also applicable to differentiating twinning elements among multiple twinning modes in any crystal system.

Validation of Point Spread Functions of CERES Radiometers by the Use of Lunar Observations

The Clouds and the Earth’s Radiant Energy System (CERES) scanning radiometers have been operating to make raster scans of the moon on a quarterly basis to validate the point response function for the three channels of flight models 1–4 aboard the Terra and Aqua spacecraft. Instrument pointing accuracy was verified by this method to 0.2° for the total channel of FM-3. The point response functions were computed from the lunar observations and were found to be nominal with the exception of the FM-2 window channel, which was found to have a region of high sensitivity. This anomaly is attributed to a delamination of the detector flake from the heat sink in that region. The influence of this anomaly is accounted for by the in-flight calibration and has no adverse effect on the application of the data.