arXiv subjects

Alfred G. de Wijn

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

12 recordsLinked to original sources

arXiv2026

Optimal polarization modulation and calibration schemes

We review the algebraic definition of the efficiency of a polarization modulation scheme, which is commonly adopted for solar and stellar spectro-polarimetry applications, and generalize it to allow distinct states of the modulation cycle to have arbitrary throughput and different photon-noise statistics for each state. Such a generalization becomes necessary to model and optimize the polarimetric efficiency of instruments implementing spatial polarization modulation schemes, where different optical paths are assigned to different polarization analysis states, which may be characterized by different throughput values. The proposed algebraic extension also proves essential for introducing a workable concept of the efficiency of a polarization calibration scheme, which can then be used to create a merit function for the optimization of calibration sequences, which take into account the specific characteristics of the polarimetric instrument and of its calibration optics.

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arXiv2026

Image instabilities and polarization cross-talk

We expand on our previous study of the impact of atmospheric seeing on polarization cross-talk, and show how the formalism that was developed in that work can be applied to treat the case of spatial modulators of polarization. Beside formally demonstrating how the problem of cross-talk is fully eliminated in such devices, we also gain insight on the meaning of polarimetric noise of temporal modulation schemes in the limit of very high modulation frequency. We also describe the problem of spectrograph instabilities, and how the spectral gradients that are naturally associated with a line spectrum feed into the problem of polarimetric errors induced by mechanical vibrations, thermal drifts, and pointing jitter. Finally, we show how this formalism can be used to estimate the contribution of polarization cross-talk to the errors on the elements of the 4$\times$4 Stokes response matrix, for the purpose of producing realistic error budgets for polarimetric instrumentation.

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arXiv2023

Small Platforms, High Return: The Need to Enhance Investment in Small Satellites for Focused Science, Career Development, and Improved Equity

In the next decade, there is an opportunity for very high return on investment of relatively small budgets by elevating the priority of smallsat funding in heliophysics. We've learned in the past decade that these missions perform exceptionally well by traditional metrics, e.g., papers/year/\$M (Spence et al. 2022 -- arXiv:2206.02968). It is also well established that there is a "leaky pipeline" resulting in too little diversity in leadership positions (see the National Academies Report at https://www.nationalacademies.org/our-work/increasing-diversity-in-the-leadership-of-competed-space-missions). Prioritizing smallsat funding would significantly increase the number of opportunities for new leaders to learn -- a crucial patch for the pipeline and an essential phase of career development. At present, however, there are far more proposers than the available funding can support, leading to selection ratios that can be as low as 6% -- in the bottom 0.5th percentile of selection ratios across the history of ROSES. Prioritizing SmallSat funding and substantially increasing that selection ratio are the fundamental recommendations being made by this white paper.

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arXiv2023

First Observation of Chromospheric Waves in a Sunspot by DKIST/ViSP: The Anatomy of an Umbral Flash

The Visible Spectro-Polarimeter (ViSP) of the NSF Daniel K. Inouye Solar Telescope (DKIST) collected its Science Verification data on May 7-8, 2021. The instrument observed multiple layers of a sunspot atmosphere simultaneously, in passbands of Ca-II 397 nm (H-line), Fe-I 630 nm, and Ca-II 854 nm, scanning the region with a spatial sampling of 0.041" and average temporal cadence of 7.76 seconds, for a 38.8 minute duration. The slit moves southward across the plane-of-the-sky at 3.83 km/s. The spectropolarimetric scans exhibit prominent oscillatory 'ridge' structures which lie nearly perpendicular to the direction of slit motion (north to south). These ridges are visible in maps of line intensity, central wavelength, line width, and both linear and circular polarizations. Contemporaneous Atmospheric Imaging Assembly observations indicate these ridges are purely temporal in character and likely attributed to the familiar chromospheric 3-minute umbral oscillations. We observe in detail a steady umbral flash near the center of the sunspot umbra. Although bad seeing limited the spatial resolution, the unique high signal-to-noise enable us to estimate the shock Mach numbers (= 2), propagation speeds (= 9 km/s), and their impact on longitudinal magnetic field (delta B = 50 G), gas pressure, and temperature (delta T/T = 0.1) of the subshocks over 30 seconds. We also find evidence for rarefaction waves situated between neighboring wave-train shocks. The Ca-II 854 nm line width is steady throughout the umbral flash except for a sharp 1.5 km/s dip immediately before, and comparable spike immediately after, the passage of the shock front. This zig-zag in line width is centered on the subshock and extends over 0.4".

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arXiv2019

Astro2020 Science White Paper: Synoptic Studies of the Sun as a Key to Understanding Stellar Astrospheres

Ground-based solar observations provide key contextual data (i.e., the 'big picture') to produce a complete description of the only astrosphere we can study in situ: our Sun's heliosphere. The next decade will see the beginning of operations of the Daniel K. Inouye Solar Telescope (DKIST). DKIST will join NASA's Parker Solar Probe and the NASA/ESA Solar Orbital mission, which together will study our Sun's atmosphere with unprecedented detail. This white paper outlines the current paradigm for ground-based solar synoptic observations, and indicates those areas that will benefit from focused attention.

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arXiv2012

Analysis of Seeing-Induced Polarization Cross-Talk and Modulation Scheme Performance

We analyze the generation of polarization cross-talk in Stokes polarimeters by atmospheric seeing, and its effects on the noise statistics of spectropolarimetric measurements for both single-beam and dual-beam instruments. We investigate the time evolution of seeing-induced correlations between different states of one modulation cycle, and compare the response to these correlations of two popular polarization modulation schemes in a dual-beam system. Extension of the formalism to encompass an arbitrary number of modulation cycles enables us to compare our results with earlier work. Even though we discuss examples pertinent to solar physics, the general treatment of the subject and its fundamental results might be useful to a wider community.

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arXiv2012

The chromosphere and prominence magnetometer

The Chromosphere and Prominence Magnetometer (ChroMag) is conceived with the goal of quantifying the intertwined dynamics and magnetism of the solar chromosphere and in prominences through imaging spectro-polarimetry of the full solar disk. The picture of chromospheric magnetism and dynamics is rapidly developing, and a pressing need exists for breakthrough observations of chromospheric vector magnetic field measurements at the true lower boundary of the heliospheric system. ChroMag will provide measurements that will enable scientists to study and better understand the energetics of the solar atmosphere, how prominences are formed, how energy is stored in the magnetic field structure of the atmosphere and how it is released during space weather events like flares and coronal mass ejections. An integral part of the ChroMag program is a commitment to develop and provide community access to the "inversion" tools necessary for the difficult interpretation of the measurements and derive the magneto-hydrodynamic parameters of the plasma. Measurements of an instrument like ChroMag provide critical physical context for the Solar Dynamics Observatory (SDO) and Interface Region Imaging Spectrograph (IRIS) as well as ground-based observatories such as the future Advanced Technology Solar Telescope (ATST).

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arXiv2012

Preliminary design of the Visible Spectro-Polarimeter for the Advanced Technology Solar Telescope

The Visible Spectro-Polarimeter (ViSP) is one of the first light instruments for the Advanced Technology Solar Telescope (ATST). It is an echelle spectrograph designed to measure three different regions of the solar spectrum in three separate focal planes simultaneously between 380 and 900 nm. It will use the polarimetric capabilities of the ATST to measure the full Stokes parameters across the line profiles. By measuring the polarization in magnetically sensitive spectral lines the magnetic field vector as a function of height in the solar atmosphere can be obtained, along with the associated variation of the thermodynamic properties. The ViSP will have a spatial resolution of 0.04 arcsec over a 2 arcmin field of view (at 600 nm). The minimum spectral resolving power for all the focal planes is 180,000. The spectrograph supports up to 4 diffraction gratings and is fully automated to allow for rapid reconfiguration.

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arXiv2012

Stray light and polarimetry considerations for the COSMO K-Coronagraph

The COSMO K-Coronagraph is scheduled to replace the aging Mk4 K-Coronameter at the Mauna Loa Solar Observatory of the National Center for Atmospheric Research in 2013. We present briefly the science objectives and derived requirements, and the optical design. We single out two topics for more in-depth discussion: stray light, and performance of the camera and polarimeter.

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arXiv2010

Wavelength-Diverse Polarization Modulators for Stokes Polarimetry

Information about the three-dimensional structure of solar magnetic fields is encoded in the polarized spectra of solar radiation by a host of physical processes. To extract this information, solar spectra must be obtained in a variety of magnetically sensitive spectral lines at high spatial, spectral, and temporal resolution with high precision. The need to observe many different spectral lines drives the development of Stokes polarimeters with a high degree of wavelength diversity. We present a new paradigm for the design of polarization modulators that operate over a wide wavelength range with near optimal polarimetric efficiency and are directly applicable to the next generation of multi-line Stokes polarimeters. These modulators are not achromatic in the usual sense because their polarimetric properties vary with wavelength, but they do so in an optimal way. Thus we refer to these modulators as polychromatic. We present here the theory behind polychromatic modulators, illustrate the concept with design examples, and present the performance properties of a prototype polychromatic modulator.

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arXiv2009

Interactions between reversed granulation, p-modes, and magnetism?

We investigate features that are observed in Ca ii H sequences from Hinode in places where reversed granulation seems to interact with p-modes. These features appear ubiquitously in the quiet sun. They are co-spatial with reversed granulation, and display similar general properties, but have sharper edges and show fast brightness changes. They also appear predominantly above wide intergranular lanes, indicating a potential connection with magnetism. We report on the appearance and dynamics of these features using high-resolution, high-cadence observations from Hinode, and we discuss their possible origin.

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arXiv2009

On the relationship between magnetic field and mesogranulation

We investigate the relation between Trees of Fragmenting Granules (TFGs) and the locations of concentrated magnetic flux in internetwork areas. The former have previously been identified with mesogranulation. While a relationship has been suggested to exist between these features, no direct evidence has yet been provided. We present some preliminary results that show that concentrated magnetic flux indeed collects on the borders of TFGs.

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