Similar to filaments in appearance, arch filament systems have a different structure and connect opposite polarities of newly emerging flux regions. Dr. Sergio J. González Manrique, from the Astronomical Institute of the Slovak Academy of Science (Slovakia), explains more about them and how the European Solar Telescope will help better understand their formation.
Credit: S.J. González Manrique, C.Kuckein, M. Collados et al., 2018, A&A, 617, A55.
Compared to the photosphere, the chromosphere is very inhomogeneous, with several features that can be observed: spicules, dynamic fibrils, mottles, filaments and arch filament systems (AFSs), among others.
For this post we should distinguish between AFSs and filaments. Both look similar on the chromosphere as long dark loops. However, their structure - which depends on the magnetic field configuration - is different. Filaments are formed by cool dense plasma and are located between opposite polarities along the so-called polarity inversion line. In contrast, AFSs cross the polarity inversion line, connecting the two opposite polarities of newly emerging flux regions.
It was A. Bruzek who described these system of fibrils for the first time and called them AFS. Their lengths are comparable to the size of supergranular network cells (around 20,000 - 30,000 kilometers). Typically, the width of a single AFS loop is only a few thousand kilometers. The height of the arches varies between 5,000 and 15,000 kilometers with a lifetime of about 30 minutes. However, some individual loops of AFS have heights of 25,000 kilometers (reaching even the solar corona) and lengths near 20,000-40,000 kilometers.
Generally, the shapes of AFSs do not vary for hours. However, significant changes can occur during episodes of flux emergence. AFSs typically vanish around three days after their formation.
Scientists studying the evolution and dynamics of AFSs reported upflows in the center of the arches (called loop tops) and downflows at the footpoints (the legs of the loops). The downflows reach velocities of 20-90 km/s (which are supersonic in the chromosphere) near the two footpoints of the AFS, whereas the loop tops rise with speeds of 1.5-20 km/s.
The movie below shows an example of the dynamics of AFSs. The upper panel displays maps of the He I line core intensity recorded with the GREGOR Infrared Spectrograph (GRIS). The arch filaments connect two pores with opposite polarities. The bottom panel depicts the He I Doppler shifts of the AFS. Blue and red colors represent up- and downflows, respectively. The time series of GRIS data covers about one hour and reveals persistent chromospheric downflows near the footpoints of the AFS (big patches in red).
Even though it was not easy to infer the vector magnetic field of these features with the instruments, tools and spectral lines available in the past, some studies reported magnetic field strengths between 390–1200 G in the footpoints and 50 G in the loop tops.
The formation and decay of AFSs are still poorly understood. However, these processes can give us information on how the plasma evolve through the different layers of the solar atmosphere and, more importantly, how the plasma is heated by AFSs. Thanks to its multi-wavelength capabilities and high cadence observations, EST will provide a unique opportunity to study flows and vector magnetic fields in AFSs. With this information we expect to be able to validate/refute current AFS models and provide new constraints for AFS formation theory.
More information: González Manrique et al., 2018, A&A, 617, A55