Birmingham Solar Oscillations Network (BiSON)
The Birmingham Solar Oscillations Network (BiSON) is a global network of 6 ground-based remote helioseismology observatories that have been collecting high quality data continuously since 1976. The BiSON team works within the Sun Stars and Exoplanets Group at the University of Birmingham. BiSON is funded by the UK Science and Technology Facilities Council (STFC). The operation of the network is conducted from Birmingham, and the 6 remote observatories are strategically located around the planet to allow for continual solar data acquisition. They are located at:
- Las Campanas, Chile
- Mount Wilson, California
- Izaña, Tenerife
- Sutherland, South Africa
- Carnarvon, Western Australia
- Narrabri, New South Wales, Australia
By observing the “Sun as a star”, in Doppler velocity, BiSON provides exquisite data on its “whole-Sun” 5-minute oscillations. They probe the structure and rotation of the solar interior and energy-generating core, and provide a truly global seismic diagnostic of the solar cycle and the near-surface structural changes associated with it. This science requires continuous high-coverage, high-cadence observations. Results from BiSON have shed new light on the solar cycle, including intriguing ongoing changes in sub-surface structure that span multiple cycles; and added clarity to our picture of the rotation and structure of the deepest layers of the Sun.
All BiSON spectrometers use resonant scattering of light from the Fraunhofer line of potassium at 769.9 nm to measure the shift between the solar line and the same potassium resonance in atoms in a laboratory. We direct sunlight into a potassium vapour cell contained within a permanent, optically aligned, magnetic field. Due to the Zeeman effect this light is split into two components, a blue wing, and a red wing. Circularly polarized sunlight excites only one of the two Zeeman responses which we exploit to separate out these wings. The ratio between the blue and red wings is approximately a linear measure of the relative line-of-sight velocity of the solar surface. After removing the effects of orbital motion, spin, etc, we are left with a residual velocity which is dominated by the solar oscillation. As our instruments use the atomic properties of Potassium as the basis of measurement our data are very precise.
