Replacing the polarizer wheel with a polarization camera to increase the temporal resolution and reduce the overall complexity of a solar coronagraph

Abstract

"Replacing the polarizer wheel with a polarization camera to increase the temporal resolution and reduce the overall complexity of a solar coronagraph," J. Astron. Telesc. Instrum. Syst. 3(1), 014001 (2017), doi: 10.1117/1.JATIS.3.1.014001. Abstract. Experiments that require linearly polarized brightness measurements, traditionally have obtained three successive images through a linear polarizer that is rotated through three well-defined angles and the images are combined to get the linearly polarized brightness. This technique requires a mechanism to hold the linear polarizer in place and to precisely turn it through the three angles. Obviously, the temporal resolution is lost in such a scenario, since the three images that are used to derive the linearly polarized brightness are taken at three different times. Specifically, in a dynamic corona that is in constant reshaping of its structures, the linearly polarized brightness image produced in this manner may not yield true values all around the corona. In this regard, with the advent of the polarization camera, the linearly polarized brightness can be measured from a single image. This also eliminates the need for a linear polarizer and the associated rotator mechanisms and can contribute toward lower weight, size, power requirements, overall risk of the instrument, and most importantly, increase the temporal resolution. We evaluate the capabilities of a selected polarization camera and how these capabilities could be tested in a ground experiment conducted in conjunction with a total solar eclipse. The ground experiment requires the measurement of the linearly polarized brightness, also known as K-corona, in a corona that also contains unpolarized brightness, known as F-corona, in order to measure three important physical properties pertaining to coronal electrons, namely, the electron density, electron temperature, and the electron speed.

Graphical evidence for the solar coronal structure during the Maunder minimum: comparative study of the total eclipse drawings in 1706 and 1715

“…Note that beyond about 2.5 R , depending on conditions (see discussion below), the intensity of the F-corona (due to dust) exceeds that of the K-corona (due to electrons) (e.g., Fig. 1 of Reginald et al, 2017). As the lights seen at larger extents reported do not show streamers, they are almost certainly due to the F-corona and not the K-corona.…”

“…One is to consider this as the extension of F-corona contrasted with the eclipse sky. Figure 1 of Reginald et al (2017) adopted from Phillips (1992) shows that the F corona starts to be brighter than the modern K corona at about~2.5 R and is brighter than the "eclipse sky" up to a distance of~4 R from solar centre. This interpretation has some difficulty on its asymmetric extension but shows more consistency with other textual reports and Eimmart's eclipse drawing.…”

Electron temperature maps of the low solar corona: ISCORE results from the total solar eclipse of 1 August 2008 in China

“…It is the short duration of just 120 s of totality that deprived us of the opportunity to make additional K‐coronal intensity measurements through two additional filters centered at 3987.0 Å and 4233.0 Å for speed measurements and to turn a linear polarizer through three angles for each filter to remove the unpolarized F‐coronal intensity from the images. The lack of a linear polarizer was compensated by restricting our measurements close to the solar limb where the K‐coronal intensity overwhelms the F‐coronal intensity by an order of magnitude as seen Figure 1 in Reginald et al []. One option of using all four filters in the ISCORE instrument for simultaneous and global measurements of both electron temperature and speed in the low solar corona close to the solar limb is by splitting the incoming beam four ways to pass through the four filters and then simultaneously imaging using four cameras that are cross calibrated.…”

“…A detailed description of a polarization camera and a planned ground observation using the ISCORE instrument coupled to a polarization camera during the total solar eclipse of 21 August 2017 is presented in Reginald et al []. In the instrument configuration described in Reginald et al [] we plan to map the electron density using the white‐light K‐coronal images taken through an open filter, electron temperature using the K‐coronal images taken through two filters centered at 4100.0 Å and 3850.0 Å and electron radial speed using K‐coronal images taken through two filters centered at 4233.0 Å and 3987.0 Å up to a coronal height of 1.5 R ⊙ from the Sun center. With these data we will also be able to map the degree of polarization and angle of polarization in white‐light for the four color filters centered at 3850.0, 3987.0, 4100.0, and 4233.0 Å.…”

The Balloon-Borne Investigation of Temperature and Speed of Electrons in the Corona (BITSE): Mission Description and Preliminary Results

“…The BITSE mission is the culmination of several technological activities in coronagraphy undertaken at NASA's Goddard Space Flight Center (NASA/GSFC) over the past several years (Gopalswamy and Gong, 2018): (i) A breadboard version of a single-stage, externally occulted coronagraph known as the Goddard Miniature Coronagraph (GMC) was built and tested in the laboratory and in a vacuum tank and (ii) a commercial polarization camera was tested during the August 21, 2017, total solar eclipse in the United States, that obtained both polarization and total brightness images in four narrow band filters (Reginald et al, 2017;Gopalswamy et al, 2018;Cho et al, 2020). The polarization camera allows us to image the corona at different polarization positions simultaneously, instead of sequentially observing at different polarization positions using a polarization wheel.…”