Anatomy of a solar filament
posted: October 10, 2013

As filaments and filament eruptions continue to determine the current solar activity, it may be an idea to get a bit more acquainted.

Solar filaments are clouds of ionized gas above the solar surface squeezed between magnetic regions of opposite polarity. Being cooler and denser than the plasma underneath and their surroundings, these magnetic borderlines appear as dark lines when seen on the solar disk using special filters. One such a filter is the Hydrogen-alpha (H-alpha) line in the red part of the solar spectrum. It shows the cool inner atmosphere of the Sun. Filaments can appear in e.g. sunspot regions, where they are called active region filaments. They can also appear completely isolated on the solar disk, and then they are called quiescent filaments. These can become very long and can last for one or more solar rotations.

Since systematic H-alpha observations started over a century ago, the filament structure and its constituting parts have all been given names. As such, a filament consists of a spine, barbs, legs and veins (see annotated sketch above). Here, "spine" is the name for the overall, long, very thin, darkish filament feature. It can be several tens of thousands kilometers high and hundreds of thousands kilometers long, yet only a few thousands of kilometers wide. Therefore in modeling, it's often called a "slab". Due to perspective effects, the filaments seem actually quite a bit wider than they actually are.

On many occasions, one can see filamentary pieces sticking out of the spine. These are the "barbs", very similar in outlook to the barbs on a harpoon. Barbs are a topic of scientific research. Indeed, the direction of the barbs as seen from the positive magnetic field that borders the filament, gives an indication of the magnetic helicity, which in turns may help in determining the magnetic orientation of the coronal mass ejection if such a filament would erupt. A large majority of the filaments on the northern solar hemisphere is directed to the right ("dextral"), whereas on the southern hemisphere, most of them are directed to the left ("sinistral"). Amazingly, this "rule" remains valid over all the solar cycles, and thus does not depend on e.g. the polar magnetic field reversal.

The outer ends of the filament are called "legs", and they can terminate in a single or in multiple points. Often, magnetic instabilities near one of the legs may cause (part of) the filament to erupt.

High resolution images of filaments have shown that the spine, barbs and legs of a filament actually all consist of many filamentary threads (also called "veins"). These thin threads are considered as the fundamental structures of solar filaments. The H-alpha picture underneath shows these threads making up a filament. It was taken by Prof. Oddbjørn Engvold (University of Oslo) with the Swedish solar telescope (SST) on La Palma. For a sense of the scale of these features, an image of the Earth has been added.

Credits - Data and imagery for the movie clips were taken from the GONG H-alpha network and the Swedish 1m Solar Telescope.