Alfvén Waves and the Sun’s Corona
The corona, the low density outer layer of the sun, can only be viewed during a solar eclipse. It has a temperature of over a million degrees kelvin, while the layer beneath is much cooler. Currently, the cause of this is not well understood. Alfvén waves, which propagate on magnetic field lines, could explain this temperature differential. Theory predicts that Alfvén waves with frequencies 0.01-0.5 Hz can heat the corona, causing an observable oscillation in its brightness. Using Fast Fourier Transforms of two image series taken during the 2017 solar eclipse, we searched for periodicity in coronal brightness indicative of Alfvén wave heating.
Background
- The corona is the low density outer atmosphere of the sun, much hotter than the photosphere.
- There are several competing models to explain coronal heating.
- Most of the sun’s light comes from the photosphere, making the corona only observable during a solar eclipse.
Theory
- One model involves Alfvén waves, magnetic phenomena where energy in the core causes magnetic field lines to oscillate.
- The movement of the magnetic field heats the coronal plasma and causes variations in brightness.
- Waves with frequencies between 0.04 and 0.3 Hz can deposit their energy in the corona (see plot at right).
Soler et al. 2017
Data
- The images above were taken during the 2017 solar eclipse. In the lower left, the moon blocks the photosphere, revealing the faint corona above and to the right. The active region AR 2672 can be seen in the narrowband image.
- The broadband image is centered on 540 nm, showing light from the photosphere reflected by the corona. The narrowband image is centered on the 530.3 nm Fe XIV emission line, revealing light that originates in the corona. We obtained images with a frame rate of 2.36 Hz in the narrowband and 4 Hz in the broadband through 139 seconds of totality.
- Alfvén wave heating models predict intensity oscillations that would be present in the narrowband dataset but not in the broadband dataset.
Method
- We used a Fast Fourier Transform to determine the magnitude and frequency of brightness oscillations as a function of position throughout our datasets.
- The power spectrum for an active region of the corona in the narrowband data is shown above. Note the excess power present between 0.03 and 0.3 Hz.
Results
The images above show the fractional power present between 0.2 and 0.28 Hz as a function of position in the broadband and narrowband datasets. We found excess of power between 0.03 and 0.3 Hz in a localized section of the corona in the narrowband dataset; this excess was not found in the broadband dataset. The excess power was primarily found in the region of AR 2672. When the fractional power images are created for lower or higher frequencies, the structure is no longer visible. This section of the image has a coronal loop, so these data support the existence of Alfvén wave oscillations.
Acknowledgements
- This project was supported by the Emerging Scholars Program and the Physics & Astronomy Department.
- The Issac J Tressler Fund for Astronomy funded the expedition to record the eclipse.
- Background Image-https://www.flickr.com/photos/ramv/36692933241