Linear polarisation of comets observed with STEREO
Abstract
The Lyot coronographs on the twin STEREO spacecraft observe the vicinity of the Sun in the 650-750 nm wavelength range. As well as solar physics, they have been well-utilised for comet hunting. Unlike most similar instruments, STEREO coronographs have a polariser permanently mounted in their optical path, which means full polarimetric analysis can be performed on large datasets of comet passages. Although the integration time is short and the signal is usually dominated by solar activity, polarimetric analysis of bright comets and their tails can be performed. If the geometry of the cometary encounter is favourable, a wider range of phase angles - particularly those above 90 degrees - may be observed with solar-observing instruments than with other methods. We developed an improved method for polarimetric analysis of comets with known orbital elements observed with the STEREO spacecraft coronographs. The method reduces triplets of polarised images and calculates the degree of polarisation of the comet along its dust tail, with considerations of 3-D geometry, producing a phase angle curve. While most observed comets do not appear to survive the perihelion passage, a variety of polarisation properties has been observed, helping extend our knowledge of structure, origin and evolution of comets. The method has shown to be robust and may also be extended to other solar observatories (e.g. SOHO).
- Publication:
-
European Planetary Science Congress
- Pub Date:
- September 2018
- Bibcode:
- 2018EPSC...12.1016N