From Observations to Science
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Imaging Electrons in the Void
The Sun's corona is a mixture of electrons and ions. Sunlight scatters off of electrons.
Interplanetary space is not empty. It is filled with protons and electrons and some heavier ions. These form a tenuous plasma and the electrons scatter sunlight, similar to how air molecules scatter light to form the Earth's bright blue sky. Thus, the brightness PUNCH observes indicates how many electrons lie along each line of sight, from the Earth through the inner heliosphere. The scattered sunlight from these electrons is brightest near the Sun, where you can see them directly during a solar eclipse.
Far from the Sun, the signal is still present — but much fainter than other sources of brightness, such as the dust cloud that orbits the Sun (“zodiacal light”) or the stars themselves. The PUNCH ground system estimates and removes this background light from each image, to reveal the light from electrons.
Eclipse image above: © 2008 Miloslav Druckmüller, Peter Aniol, Vojtech Rušin; Druckmüller webpage
Filling the Gaps
PUNCH stitches images from multiple cameras together
PUNCH stitches images from multiple cameras together in order to fill the gaps between existing coronal and heliospheric imagers. All four PUNCH cameras have overlapping fields of view, ensuring that no gaps remain in the final combined images. The result is a seamless image of the Young Solar Wind.
Subtracting the Deep Background
Signal is as little as 0.1% of star field
After images are photometrically calibrated and aligned, the NFI and three WFI images are merged into mosaics of positive-definite radial (RP) and tangential (TP) polarizer brightnesses. Background F corona (dust) and starfield are removed, resulting in Level 3 background-subtracted Brightness (B) and polarized Brightness (pB) mosaics of scattered sunlight from the electrons in the corona (“Thomson-scattered” light from the “K-corona”). Separation of background is precise to <3% of K corona (DeForest and Howard, 2015) despite the varying starfield and the much brighter dust in the inner solar system (the “F corona”).
Deblurring the Images
Moving averages improve resolution
All previous heliospheric imagers have been subject to motion blur. Typical CMEs and solar wind features move about a degree (the width of your thumb held at arm’s length) every hour. Since the features are faint on the sky, long exposures are required to produce clean images. This leads to motion blur (left).
PUNCH overcomes this problem by collecting a full polarized image set every four minutes. Far from the Sun, individual exposures are too faint to fully achieve all of the PUNCH science objectives. The PUNCH ground system averages the exposures together in a moving coordinate system (right), matched to the overall flow rate. This minimizes motion blur and improves image sharpness (spatial resolution).
Separating Features from the Diffuse Corona
Small feature 3D location
Image processing tools isolate compact features from diffuse background, permitting small-feature 3D location (DeForest et al. 2016). Tools like minsmooth background subtraction help isolate individual features in the solar wind, from the overall brightness of all the material along the line of sight. This, in turn, allows the science team to analyze features like the shifting interior structure of CMEs (pictured here).
Extracting 3D from Polarization
Scattering physics polarizes light depending on angle
Electrons scatter light in an interesting way. Light is an electromagnetic wave: ripples in the electric and magnetic fields that permeate space. When light passes an electron in space, the electric field ripples move the electron back and forth. The electron then “broadcasts” light just like the electrons shaking in a radio transmitter antenna broadcast radio waves.
The newly emitted light is polarized depending on the angle of scatter (called “χ” in the diagram above).
Punch uses polarization to measure 3d location of bright features
When PUNCH observes light from a feature in the corona or solar wind, it can measure how polarized that feature appears. It also measures the angle between the object and the Sun (“ε” on the diagram above). Polarimetry from PUNCH measures the angle χ directly. PUNCH-supplied analysis software automatically solves the triangle in the diagram, determining the location of each visible feature in three dimensions.
