arXiv subjects

A. G. de Wijn

Publications and source records attributed to A. G. de Wijn.

At least 19 recordsLinked to original sources

arXiv2025

The making of robust and highly performing imaging spectropolarimeters for large solar telescopes

We discuss the requirements, concepts, simulations, implementation, and calibration of two dual Fabry-Perot based imaging spectropolarimeters, CRISP and CHROMIS, at the Swedish 1-meter Solar Telescope, and CRISP2 that is under construction. These instruments use a combination of a high-resolution and a low-resolution etalon together with an order-sorting prefilter to define the bandpass. The overall design is made robust and stable by tailoring the low-resolution etalon reflectivity to accommodate expected cavity errors from both etalons, and by using a compact optical design that eliminates the need for folding mirrors. By using a telecentric design based on lenses rather than mirrors, image degradation by the FPI system is negligible, as shown in a previous publication, and the throughput of the system is maximised. Initial alignment, and maintaining that alignment over time, is greatly simplified. The telecentric design allows full calibration and/or modelling of essential system parameters to be carried out without interfering with the optical setup. We also discuss briefly the polarimeters developed for CRISP and CHROMIS. The high performance of CRISP and CHROMIS has been demonstrated in an earlier publication through measurements of the granulation contrast and comparisons with similar measurements simultaneously made through broadband continuum filters. Here, we focus on the aspects of the design that are central to enabling high performance and robustness, but also discuss the calibration and processing of the data, and use a few examples of processed data to demonstrate the achievable image and data quality. We put forward a proposal for a similar conceptual design for the European Solar Telescope and conclude by discussing potential problems of the proposed approach to designs of this type. Some aspects of these FPI systems may be of interest also outside the solar community.

astro-ph.IM↗

arXiv2023

DKIST unveils the serpentine topology of quiet Sun magnetism in the photosphere

We present the first quiet Sun spectropolarimetric observations obtained with the Visible SpectroPolarimeter (ViSP) at the $4-$m Daniel K. Inouye Solar Telescope (DKIST). We recorded observations in a wavelength range that includes the magnetically sensitive Fe I $6301.5/6302.5$ $Å$ doublet. With an estimated spatial resolution of 0.08'', this represents the highest spatial resolution full-vector spectropolarimetric observations ever obtained of the quiet Sun. We identified $53$ small-scale magnetic elements, including $47$ magnetic loops and $4$ unipolar magnetic patches, with linear and circular polarisation detected in all of them. Of particular interest is a magnetic element in which the polarity of the magnetic vector appears to change three times in only $400$ km and which has linear polarisation signals throughout. We find complex Stokes $V$ profiles at the polarity inversion lines of magnetic loops and discover degenerate solutions, as we are unable to conclusively determine whether these arise due to gradients in the atmospheric parameters or smearing of opposite polarity signals. We analyse a granule which notably has linear and circular polarisation signals throughout, providing an opportunity to explore its magnetic properties. On this small scale we see the magnetic field strength range from $25$ G at the granular boundary to $2$ kG in the intergranular lane (IGL), and sanity check the values with the weak and strong field approximations. A value of $2$ kG in the IGL is among the highest measurements ever recorded for the internetwork.

astro-ph.SR↗

arXiv2023

Derivation of Instrument Requirements for Polarimetry using Mg, Fe, and Mn lines between 250 and 290 nm

Judge et al. (2021) recently argued that a region of the solar spectrum in the near-UV between about 250 and 290 nm is optimal for studying magnetism in the solar chromosphere due to an abundance of Mg II, Fe II, and Fe I lines that sample various heights in the solar atmosphere. In this paper we derive requirements for spectropolarimetric instruments to observe these lines. We derive a relationship between the desired sensitivity to magnetic field and the signal-to-noise of the measurement from the weak-field approximation of the Zeeman effect. We find that many lines will exhibit observable polarization signals for both longitudinal and transverse magnetic field with reasonable amplitudes.

astro-ph.SR↗

arXiv2023

Ground-based Synoptic Studies of the Sun

Ground-based synoptic solar observations provide critical contextual data used to model the large-scale state of the heliosphere. The next decade will see a combination of ground-based telescopes and space missions that will study our Sun's atmosphere microscopic processes with unprecedented detail. This white paper describes contextual observations from a ground-based network needed to fully exploit this new knowledge of the underlying physics that leads to the magnetic linkages between the heliosphere and the Sun. This combination of a better understanding of small-scale processes and the appropriate global context will enable a physics-based approach to Space Weather comparable to Terrestrial Weather forecasting.

astro-ph.IM↗

arXiv2022

Helio2024 Science White Paper: ngGONG -- Future Ground-based Facilities for Research in Heliophysics and Space Weather Operational Forecast

Long-term synoptic observations of the Sun are critical for advancing our understanding of Sun as an astrophysical object, understanding the solar irradiance and its role in solar-terrestrial climate, for developing predictive capabilities of solar eruptive phenomena and their impact on our home planet, and heliosphere in general, and as a data provider for the operational space weather forecast. We advocate for the development of a ground-based network of instruments provisionally called ngGONG to maintain critical observing capabilities for synoptic research in solar physics and for the operational space weather forecast.

astro-ph.IM↗

arXiv2022

The Visible Spectro-Polarimeter of the Daniel K. Inouye Solar Telescope

The Daniel K. Inouye Solar Telescope (DKIST) Visible Spectro-Polarimeter (ViSP) is a traditional slit-scanning spectrograph, with the ability to observe solar regions up to a $120\times78~\mathrm{arcsec}^2$ area. The design implements dual-beam polarimetry, a polychromatic polarization modulator, a high-dispersion echelle grating, and three spectral channels that can be automatically positioned. A defining feature of the instrument is its capability to tune anywhere within the 380-900~nm range of the solar spectrum, allowing for a virtually infinite number of combinations of three wavelengths to be observed simultaneously. This enables the ViSP user to pursue well-established spectro-polarimetric studies of the magnetic structure and plasma dynamics of the solar atmosphere, as well as completely novel investigations of the solar spectrum. Within the suite of first-generation instruments at the DKIST, ViSP is the only wavelength-versatile spectro-polarimeter available to the scientific community. It was specifically designed to be a discovery instrument, for the exploration of new spectroscopic and polarimetric diagnostics, and to test improved models of polarized line formation, through high spatial-, spectral-, and temporal-resolution observations of the Sun's polarized spectrum. In this instrument article, we describe the science requirements and design drivers of ViSP, and we present preliminary science data collected during the commissioning of the instrument.

astro-ph.IM↗

arXiv2021

Design and Performance Analysis of a Highly Efficient Polychromatic Full-Stokes Polarization Modulator for the CRISP Imaging Spectrometer

We present the design and performance of a polychromatic polarization modulator for the CRisp Imaging SpectroPolarimeter (CRISP) Fabry-Perot tunable narrow-band imaging spectropolarimer at the Swedish 1-m Solar Telescope (SST). We discuss the design process in depth, compare two possible modulator designs through a tolerance analysis, and investigate thermal sensitivity of the selected design. The trade-offs and procedures described in this paper are generally applicable in the development of broadband polarization modulators. The modulator was built and has been operational since 2015. Its measured performance is close to optimal between 500 and 900~nm, and differences between the design and as-built modulator are largely understood. We show some example data, and briefly review scientific work that used data from SST/CRISP and this modulator.

astro-ph.IM↗

arXiv2018

Characterization of Cameras for the COSMO K-Coronagraph

Digital image sensors are ubiquitous in astronomical instrumentation and it is well known that they suffer from issues that must be corrected for data to be scientifically useful. I present discussion on errors resulting from digitization and characterization of nonlinearity and ADC errors of the PhotonFocus MV-D1024E cameras selected for the K-Coronagraph of the Coronal Solar Magnetism Observatory. I derive an analytic expression for quantization errors. The MV-D1024E camera has adequate bit depth for which quantization error is not an issue. I show that this is not the case for all cameras, particularly those with deep wells and low read noise. The impact of nonlinearity and ADC errors on science observations of the K-Coronagraph is analyzed using a simplified telescope model. Errors caused by the camera ADCs result in systematic errors in the measurement of the polarimetric signal of several times $10^{-9}~B_\odot$, which is about an order of magnitude above the desired sensitivity. I demonstrate a method for post-facto data correction using a lookup table and derive parameters from camera characterization measurements that were made with a lab setup. Nonlinearity is traditionally addressed with a global correction. I show through analysis of calibration data that for the MV-D1024E this correction leaves residual systematic errors after dark and gain correction of up to 1% of the signal. I demonstrate that a pixel-wise correction of nonlinearity reduces the errors to below 0.1%. These corrections are necessary for the K-Coronagraph data products to meet the science requirements. They have been implemented in the instrument data acquisition system and data reduction pipeline. While no other instruments besides the K-Coronagraph or cameras besides the MV-D1024E are discussed here, the results are illustrative for all instruments and cameras.

astro-ph.IM↗

arXiv2017

First detection of sign-reversed linear polarization from the forbidden [O I] 630.03 nm line

We report on the detection of linear polarization of the forbidden [O i] 630.03 nm spectral line. The observations were carried out in the broader context of the determination of the solar oxygen abundance, an important problem in astrophysics that still remains unresolved. We obtained spectro-polarimetric data of the forbidden [O i] line at 630.03 nm as well as other neighboring permitted lines with the Solar Optical Telescope of the Hinode satellite. A novel averaging technique was used, yielding very high signal-to-noise ratios in excess of $10^5$. We confirm that the linear polarization is sign-reversed compared to permitted lines as a result of the line being dominated by a magnetic dipole transition. Our observations open a new window for solar oxygen abundance studies, offering an alternative method to disentangle the Ni i blend from the [O i] line at 630.03 nm that has the advantage of simple LTE formation physics.

astro-ph.SR↗

arXiv2014

On the Instrument Profile of Slit Spectrographs

We derive an analytic expression for the instrument profile of a slit spectrograph, also known as the line spread function. While this problem is not new, our treatment relies on the operatorial approach to the description of diffractive optical systems, which provides a general framework for the analysis of the performance of slit spectrographs under different illumination conditions. Based on our results, we propose an approximation to the spectral resolution of slit spectrographs, taking into account diffraction effects and sampling by the detector, which improves upon the often adopted approximation based on the root-sumsquare of the individual contributions from the slit, the grating, and the detector pixel.

physics.optics↗

arXiv2012

Probable identification of the on-disk counterpart of spicules in Hinode Ca II H observations

I present a study of high-resolution time series of Ca II H images and Fe I 630.15 nm spectra taken with the Solar Optical Telescope on the Hinode spacecraft. There is excellent correspondence between the Ca II H and Fe I line core intensity, except tenuous emission around the network field concentrations in the former that is absent in the latter. Analysis of on-disk observations and a comparison with limb observations suggests that this "network haze" corresponds to spicules, and likely to type-II spicules in particular. They are known to appear in emission in on-disk broadband Ca II H diagnostics and the network haze is strongest in those areas where features similar to type-II spicules are produced in simulations.

astro-ph.SR↗

arXiv2012

Measuring Magnetic Fields in the Solar Atmosphere

Since the discovery by Hale in the early 1900s that sunspots harbor strong magnetic field, magnetism has become increasingly important in our understanding of processes on the Sun and in the Heliosphere. Many current and planned instruments are capable of diagnosing magnetic field in the solar atmosphere. Photospheric magnetometry is now well-established. However, many challenges remain. For instance, the diagnosis of magnetic field in the chromosphere and corona is difficult, and interpretation of measurements is harder still. As a result only very few measurements have been made so far, yet it is clear that if we are to understand the outer solar atmosphere we must study the magnetic field. I will review the history of solar magnetic field measurements, describe and discuss the three types of magnetometry, and close with an outlook on the future.

astro-ph.SR↗

arXiv2011

Quiet-Sun imaging asymmetries in NaI D1 compared with other strong Fraunhofer lines

Imaging spectroscopy of the solar atmosphere using the NaI D1 line yields marked asymmetry between the blue and red line wings: sampling a quiet-Sun area in the blue wing displays reversed granulation, whereas sampling in the red wing displays normal granulation. The MgI b2 line of comparable strength does not show this asymmetry, nor does the stronger CaII 8542 line. We demonstrate the phenomenon with near-simultaneous spectral images in NaI D1, MgI b2, and CaII 8542 from the Swedish 1-m Solar Telescope. We then explain it with line-formation insights from classical 1D modeling and with a 3D magnetohydrodynamical simulation combined with NLTE spectral line synthesis that permits detailed comparison with the observations in a common format. The cause of the imaging asymmetry is the combination of correlations between intensity and Dopplershift modulation in granular overshoot and the sensitivity to these of the steep profile flanks of the NaI D1 line. The MgI b2 line has similar core formation but much wider wings due to larger opacity buildup and damping in the photosphere. Both lines obtain marked core asymmetry from photospheric shocks in or near strong magnetic concentrations, less from higher-up internetwork shocks that produce similar asymmetry in the spatially averaged CaII 8542 profile.

astro-ph.SR↗

arXiv2010

Observations of solar scattering polarization at high spatial resolution

The weak, turbulent magnetic fields that supposedly permeate most of the solar photosphere are difficult to observe, because the Zeeman effect is virtually blind to them. The Hanle effect, acting on the scattering polarization in suitable lines, can in principle be used as a diagnostic for these fields. However, the prediction that the majority of the weak, turbulent field resides in intergranular lanes also poses significant challenges to scattering polarization observations because high spatial resolution is usually difficult to attain. We aim to measure the difference in scattering polarization between granules and intergranules. We present the respective center-to-limb variations, which may serve as input for future models. We perform full Stokes filter polarimetry at different solar limb positions with the CN band filter of the Hinode-SOT Broadband Filter Imager, which represents the first scattering polarization observations with sufficient spatial resolution to discern the granulation. Hinode-SOT offers unprecedented spatial resolution in combination with high polarimetric sensitivity. The CN band is known to have a significant scattering polarization signal, and is sensitive to the Hanle effect. We extend the instrumental polarization calibration routine to the observing wavelength, and correct for various systematic effects. The scattering polarization for granules (i.e., regions brighter than the median intensity of non-magnetic pixels) is significantly larger than for intergranules. We derive that the intergranules (i.e., the remaining non-magnetic pixels) exhibit (9.8 \pm 3.0)% less scattering polarization for 0.2<u<0.3, although systematic effects cannot be completely excluded. These observations constrain MHD models in combination with (polarized) radiative transfer in terms of CN band line formation, radiation anisotropy, and magnetic fields.

astro-ph.SR↗

arXiv2009

On the propagation of p-modes into the solar chromosphere

We employ tomographic observations of a small region of plage to study the propagation of waves from the solar photosphere to the chromosphere using a Fourier phase-difference analysis. Our results show the expected vertical propagation for waves with periods of 3 minutes. Waves with 5-minute periods, i.e., above the acoustic cut-off period, are found to propagate only at the periphery of the plage, and only in the direction in which the field can be reasonably expected to expand. We conclude that field inclination is critically important in the leakage of p-mode oscillations from the photosphere into the chromosphere.

astro-ph.SR↗

arXiv2009

Statistics of convective collapse events in the photosphere and chromosphere observed with the Hinode SOT

Aims. Convective collapse, a theoretically predicted process that intensifies existing weak magnetic fields in the solar atmosphere, was first directly observed in a single event by Nagata et al. (2008) using the high resolution Solar Optical Telescope (SOT) of the Hinode satellite. Using the same space telescope, we performed a statistical analysis of convective collapse events. Methods. Our data sets consist of high resolution time series of polarimetric spectral scans of two iron lines formed in the lower photosphere and filter images in Mg I b2 and Ca II H, spectral lines that are formed in the high photosphere and the lower chromosphere, respectively. We were thus able to study the implication of convective collapse events on the high photospheric and the chromospheric layers. The physical parameters from the full Stokes profiles were obtained with the MERLIN Milne-Eddington inversion code. Results. We found that in all cases, the event was accompanied by a continuum bright point and nearly always by a brightening in the Ca II H images. The magnesium dopplergram exhibits a strong downflow in about three quarters of the events that took place within the field of view of the magnesium dopplergram. For each of the 49 events we determined the duration, maximum photospheric downflow, field strength increase and size. We found event durations of about 10 minutes, magnetic element radii of about 0.43 arcsec and 0.35 arcsec, before and after the event, respectively, and field strengths of up to 1.65 kG.

astro-ph.SR↗

arXiv2009

Hinode's SP and G-band co-alignment

We analyze the co-alignment between Hinode's BFI-Gband images and simultaneous SP maps with the aim of characterizing the general off-sets between them and the second order non-linear effects in SP's slit scanning mechanism. We provide calibration functions and parameters to correct for the nominal pixel scales and positioning.

astro-ph.SR↗

arXiv2008

Small-scale solar magnetic fields

As we resolve ever smaller structures in the solar atmosphere, it has become clear that magnetism is an important component of those small structures. Small-scale magnetism holds the key to many poorly understood facets of solar magnetism on all scales, such as the existence of a local dynamo, chromospheric heating, and flux emergence, to name a few. Here, we review our knowledge of small-scale photospheric fields, with particular emphasis on quiet-sun field, and discuss the implications of several results obtained recently using new instruments, as well as future prospects in this field of research.

astro-ph↗