Solar atmosphere
Key points
- Solar atmosphere has three principal layers (slide 2). The photosphere, which is quite thin, the source of most of the radiation. The state parameters (pressure, density) and opacity drop fast. It can be considered in the local thermodynamical equilibrium. The second is the chromosphere, which is not in LTE and is structured. That means that its thickness is not the same at all places. The third principal layer is the corona, completely driven by the structures of the magnetic field. It expands into the interplanetary space. Between the chromosphere and the corona, we find the transition region, where the temperature and density has a jump (the pressure is continuous due to the hydrostatic balance). Note that whereas the temperature decreases from the solar core towards the photosphere, it increases again in the chromosphere and farther into the corona. The so-called "coronal heating problem" belongs to one of the open issues of solar physics.
- The atmosphere is coupled mostly via the magnetic field. Slides 3 through 5 show it convincingly, as the figures of the same region were taken in different parts of the H-alpha spectral line, thereby performing the height scanning of the atmosphere. The appearance changes obviously.
- Photosphere is the source of mostly continuum radiation (via the negative hydrogen ion scattering) and also of may spectral lines of metals (such as iron, magnesium or calcium). We observe some of the activity phenomena there (sunspots, faculae). Slides 6 and 7.
- Chromosphere radiates in strong spectral lines, such as lines of hydrogen (resonant lines of hydrogen, Balmer and Paschen series mostly) and lines of calcium or so. Typically the chromosphere is being observed in the H-alpha line. The hydrogen lines are prominent because of the ionisation and recombination equilibria are similar to A-type stars (effective temperature of 10,000 K similarly to the temperature of the chromosphere), where also hydrogen lines are most prominent. The calcium line observations (slide 9) show a calcium network, where the emission is observed in the regions of weak magnetic-field concentrations at the borders of supergranules.
- Slide 10, the transition region. Discontinuity in the temperature and density. The pressure is continuous due to the hydrostatic balance.
- Slide 11. There were semiempirical models constructed for the solar atmosphere. Among the most often used, the set of VAL (Vernazza, Avrett, Loeser) must be named. These models somewhat represent the time-averaged properties of the given atmospheric structures (e.g., VAL-C is for the time-averaged quiet-Sun regions, VAL-B an average intranetwork, etc.). These models usually serve as reference models for further works.
- Slide 12, the prominences. The chromospheric plasma locked in the dips of the coronal magnetic loops. They form due to the evaporation of chromospheric plasma, which flows upwards along the magnetic loops due to the pressure gradient. When the plasma reaches the top of the loop, it may form (due to the gravity) a dip and stay there in the force-free state (the Lorentz force balances the gravity). This can also be computed using a simple model (hand-written notes).
- So the prominences are the chromospheric material in the corona. They split in between quiescent and active forms, where the time scales of the changes are different. The active prominences are usually connected with a stronger magnetic field in the holding loops. The prominences usually form above the PIL (polarity inversion line) in both the active and quiet-Sun regions. Two terms: prominence -- observed above the limb. Filament -- observed against the disc. Apart from the name, it is the same animal. Filaments are darker because the plasma in the cloud reprocesses the incoming photospheric radiation (mostly radial) also to other direction. Looking from above the filaments are missing this radiation scattered to other directions, hence they are dark. Looking from the side (above the limb), we see this scattered radiation, hence they are bright on top of the dark coronal background. Filaments form usually due to the shearing of the magnetic loop along the PIL (Slide 15). Due to the shearing and the validity of the frozen field approximation, some of the arcade loops change the shape to the "perpendicular" direction and form the magnetic flux rope. This rope is usually twisted (helical) due to the random motions of the photospheric plasma in the photosphere.
- Slide 16: There are many named chromospheric structures in both the quiet-Sun and active regions. They all have to do with the existence of the magnetic field, both weak and strong.
- Slides 17 through 20: Spicules. The chromospheric features present everywhere, they look like grass leaves. Two types, different by properties and likely also by origin. Type I -- shock waves induced likely by the waves in the photosphere. Type II -- shocks from the chromospheric reconnections.
- Slides 21 through 28: The atmospheric layers are strongly coupled via the magnetic field. For details check Wedemeyer-Boehm: 2009, Space Sci. Rev. 144, 317
- Slide 29, take-away points: Due to the coupling it is very difficult to understand and model the atmosphere. There were many successes recently but despite the effort, we are far from full understanding.