The highly polarized state of light scattered from the corona has been known since 1871. In 1905 Schwartzchild suggested that the polarization was produced from Thompson scattering of photons by free electrons trapped along magnetic field lines. This also led to the prediction that the polarization should have radial symmetry which was confirmed by Ney et. al. in 1959 through a series of photographs. The 4D PolarCam Micropolarizer Camera allows real-time data on polarization to be collected which can add additional information about the transient nature of the magnetic fields.
For the August 21st 2017 eclipse we outfitted a 2 megapixel PolarCam to record the eclipse and dubbed it the “CoronaCam.” A telescope with 400mm focal length was selected to fill approximately 1/2 of the 7mm sensor array with the image of the moon/sun/corona (~0.5 degree FOV) while also allowing it stay within the field of view during the 2 minutes transit time (also ~0.5 degree FOV) without the need to reposition the telescope. A refractive telescope with the correct focal length and 70mm aperture was selected and was preferred over a reflective design owing to the alteration of polarization that can occur when reflecting from metallic mirrors at an angle.
Data was collected in Corvallis Oregon at a rate of 10 frames per second (Figure 1). Exposure time was continuously monitored and adjusted to keep most pixels just below saturation. Figure 2 shows the evolution of the partial eclipse. A false temperature color scale is used to enhance contrast of the mostly uniform image. Sun spots are visible as dim areas. Bright spots correspond to sensor defects in the uncorrected data.
Figure 2 shows a single image taken near the middle of totality processed in three different ways: the total flux received (So), Degree of linear polarization (DoLP) and angle of linear polarization (AoLP). The radial symmetry was confirmed. Corrections for offset and orientation have not been applied. Data will be further analyzed in the coming weeks.