13 research outputs found
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.Comment: 33 pages, 7 figures; accepted in Astrophys.
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.Comment: 8 pages, 5 figures, proceedings of SPIE Astronomical Telescopes +
Instrumentation 2012 Conference 8446 (1-5 July 2012
Magnetic fields and plasma heating in the Sun's atmosphere
We use the first publically available data from the Daniel K. Inouye Solar
Telescope (DKIST) to track magnetic connections from the solar photosphere into
the corona. We scrutinize relationships between chromospheric magnetism and
bright chromospheric, transition region and coronal plasmas. In June 2022, the
Visible Spectro-Polarimeter (ViSP) instrument targeted unipolar network within
a decaying active region. ViSP acquired rastered scans with longitudinal Zeeman
sensitivities of 0.25 Mx/cm2 (Fe I 630.2 nm) and 0.5 Mx/cm2 (Ca II 854.2 nm).
ViSP was operated in a "low" resolution mode (0.214" slit width, spectral
resolution R ~ 70,000) to produce polarization maps over a common area of 105"
x 50". Data from SDO and IRIS are combined to ask: Why is only a fraction of
emerging flux filled with heated plasma? What is the elemental nature of the
plasmas? No correlations were found between heated plasma and properties of
chromospheric magnetic fields derived from the WFA, on scales below
supergranules. Processes hidden from our observations control plasma heating.
While improved magnetic measurements are needed, these data indicate that "the
corona is a self-regulating forced system" (Einaudi et al. 2021). Heating
depends on the state of the corona, not simply on boundary conditions. Heating
models based upon identifiable bipolar fields, including cool loops, tectonics
and observable magnetic reconnection, are refuted for these regions with
unipolar chromospheric magnetic fields
Design of a Full-Stokes Polarimeter for VLT/X-shooter
X-shooter is one of the most popular instruments at the VLT, offering
instantaneous spectroscopy from 300 to 2500 nm. We present the design of a
single polarimetric unit at the polarization-free Cassegrain focus that serves
all three spectrograph arms of X-shooter. It consists of a calcite Savart plate
as a polarizing beam-splitter and a rotatable crystal retarder stack as a
"polychromatic modulator". Since even "superachromatic" wave plates have a
wavelength range that is too limited for X-shooter, this novel modulator is
designed to offer close-to-optimal polarimetric efficiencies for all Stokes
parameters at all wavelengths. We analyze the modulator design in terms of its
polarimetric performance, its temperature sensitivity, and its polarized
fringes. Furthermore, we present the optical design of the polarimetric unit.
The X-shooter polarimeter will furnish a myriad of science cases: from
measuring stellar magnetic fields (e.g., Ap stars, white dwarfs, massive stars)
to determining asymmetric structures around young stars and in supernova
explosions.Comment: Proc. SPIE 8446-7
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".Comment: 11 pages, 5 figures, accepted for publication to ApJ Letters
(February 2023
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.Comment: White paper submitted to the Decadal Survey for Solar and Space
Physics (Heliophysics) 2024-2033; 6 pages, 1 figur