Solar Atmosphere

The instrumentation at the TURM Observatory is optimized for the observation of the sun, i.e., the apparent surface of the sun – the photosphere – and the atmospheric layers above – the chromosphere. In contrast to many faint and fuzzy night-time objects, the sun offers a huge intensity across the full visible spectrum, so much so that reducing it to technically acceptable levels requires special efforts. The radiation energy captured by a small telescope is enough to melt or burn any material that comes close to the focal point, e.g., a camera chip or your eye!

WARNING: Never look at the sun with telescopes or binoculars without special protective filters. Never point your camera at the sun without filters. Never look directly at the sun with the unaided eye. Permanent damage is guaranteed!

Having enough signal allows us to be very selective regarding the wavelength window we want to observe, while still working with exposure times in the millisecond regime. Such super-short exposures essentially freeze the atmospheric turbulence (seeing) and we can apply a “lucky imaging” strategy, i.e., we capture several hundred images within a time window of a few seconds and then select only the best ones for post-processing.

White-Light / Continuum

The most common and the technically simplest type of solar observations are “white light” observations. Essentially one only needs filters to attenuate the intensity over all wavelengths by several orders of magnitude. Additional wavelength-selective filters can be used to enhance the contrast. The main features of the photosphere observed in white-light are:

  • Sun Spots: The most obvious feature on the solar surface are sun spots – they were already recorded by Galileo Galilei with his first telescopes. They result from magnetic-field loops that penetrate the surface and lead to a locally reduced temperature. The number of sun spots is a proxy for solar activity and varies in an 11-year cycle.
  • Faculae: There are also brighter regions on the surface, indicating higher temperatures, the so-called faculae. They are also connected to the magnetic field structure of the photosphere and appear as a web-like structure surrounding sun spots. Sometimes they appear before the birth of a new sun spot group or after its disappearance.
  • Granulation: The quiet surface exhibits a grainy structure with a typical angular size of ~1″ corresponding to ~1000 km on the solar surface. They are to top ends of convection columns in the photosphere with hot gas streaming upwards in the central bright part and cooled-down gas dropping back down in the outer part.

We use a small apochromatic refractor (TS 80/500) in combination with a Baader Herschel Wedge and a Solar Continuum filter for white-light observations. The Herschel Wedge reduces the total intensity by 95% and the continuum filter passes the wavelength window of about 10 nm width around a central wavelength of 540 nm. Together with a fast, high-resolution monochrome camera (QHYIII 178) we obtain full-disk images, with a resolution of 1″ per pixel, which leads to a diameter of about 1800 pixels for the solar disk.

Calcium-K Line

Using the narrow band-pass filters for observations allows us to tune into a specific transition line of a specific atomic species. The intensity of such an emission or absorption line depends on many parameters of the local environment on the sun, such as temperature, densities, and magnetic-field strength. There are many different transitions that reveal interesting details about the solar environment, but many of them are beyond the wavelength regime accessible from the earth’s surface (because of atmospheric absorption).

One of the important transition lines that can be observed from earth is the K-line of singly-ionized Calcium (CaII) atoms at 393.4 nm. This transition happens in the lower chromosphere, up to about 1000 km above the photosphere. The brightness of the Ca-K line is strongly influenced by the local magnetic field – with moderate magnetic fields the absorption is reduced, thus, brighter features indicate stronger magnetic fields. The exception are sun spots which feature very strong magnetic fields and appear dark again. Some of the prominent Ca-K features are:

  • Chromospheric Network & Supergranulation:  A network of bright lines separating darker irregularly shaped cells of about 30000 km size, the supergranulation cells. They result from a large-scale convective horizontal flow, where material flows outwards from the centre and downward flow has been observed at the edges. The brighter edges are produced by magnetic field lines that are concentrated there by the fluid motions in the supergranules.
  • Plages & Faculae: The magnetic structure that produces sun spots also affects the lower chromosphere. Due to its magnetic-field sensitivity the Ca-K line emphasizes active regions around sun spots. Prominent bright regions, the plages (beaches), appear around sun spots. The counterpart of white-light faculae also appears in Ca-K light as chromospheric faculae.

For Ca-K observations we also use a small apochromatic refractor (a second TS 80/500) with a Lunt Calcium-K B1200 Module, which transmits light at 393.4 nm with a bandwidth of <0.24 nm. In combination with the same fast camera as for the white-light we get images at exactly the same scale, which facilitates direct comparisons.

Hydrogen-α Line

The queen of atomic transition lines in the astrophysical context is the Ballmer-α line in neutral Hydrogen atoms, the H-α for short. Since the sun is made from 75% of Hydrogen, the emission and absorption processes through the Ballmer-α line are abundant play a big role in understanding the structure and dynamics at the solar surface and beyond. What makes the H-α observations so exciting are active regions where matter is ejected into the chromosphere. Some of the prominent H-α features are:

  • Filaments: Dense clouds of gas suspended above the photosphere by magnetic-field loops. In front of the bright photosphere they appear as dark absorption structures, forming curtain-like streaks (filaments) of substantial length. They remain in a quiet or quiescent state for days or weeks, slowly following the change of the magnetic field loops that suspend them.
  • Prominences: The same thing as filaments, only viewed from the side at the edge of the solar disk. They appear as bright emission structures in nice contrast to the dark backdrop of space. Many different morphologies can be distinguished.
  • Spicules: Small bright spikes that extend from the solar limb. Formed by plasma jets shooting up from the photosphere, they only live for 15 minutes or so. On the disk they appear as a dark hairy structure. About 1% of the solar surface is covered by spicules.
  • Coronal Mass Ejections: Rapid reconfigurations of the magnetic field can cause an ejection of matter from filaments/prominences into space – an absolutely spectacular sight. This highly dynamical process is the reason for geomagnetic storms on earth.

We use two different telescope systems for H-α observations. A dedicated Lunt HA60 Telescope with pressure tuned etalon plus an second tilt-tuned etalon DS60. This double-stacked configuration reaches a pass bandwidth below 0.05 nm or 0.5 Å. This scope has the same focal length as the other full-disk setups, thus, we also get full-disk H-α images with the same size and resolution as the white-light and Ca-K channels.

For higher resolution pictures in H-α light we use a longer focal-length setup with a larger APO refractor (TS 130/860) and an additional telecentric Barlow lens, giving an effective focal length of 3400 mm. The Barlow is connected to a research-grade temperature-tuned Daystar Quantum H-α filter with 0.05 nm or 0.5 Å bandwidth followed by a super-fast camera, which provides 0.35″ per pixel resolution. The filter can be detuned in steps on 0.01 nm (computer controlled) to emphasize different components of the H-α emission, which might be shifted in wavelength due to the Doppler effect.