Coronal heating problem -How deep does it go?

On the top of the list of problems in solar physics we find the coronal heating problem. The Corona is the outer plasma atmosphere of the Sun, with a temperature of 1-5 millions of degrees Kelvin, with coronal streamers as the hottest areas. While the photosphere, the visual surface of the Sun, has a temperature of only 5800 Kelvins, and can have sunspots with temperatures down to 4000 Kelvins. So how can heat energy from the 16 million degree hot fusion furnace in the solar core get through the photosphere and heat the corona? Usually just the outer layers of the Sun is investigated to find solutions to the Coronal heating problem, but may the problem run far deeper into the interior of the Sun? Recent observations might just indicate that:

1. The science of helioseismology interpret solar seismic waves and use them to investigate the solar interior. While the photosphere of the sun show difference in rotation and revolve faster around equator than the poles, helioseismology showed that 70 % of the Sun’s interior rotates as one spherical “solid” body beneath the tachocline. This is explained by a strong magnetic field which confines the interior plasma into a “solid” sphere. But helioseismology discovered another problem, inside the “solid” sphere they found no heat convectional flows, and in the layer above, which is even called the convection zone, they could not find any strong convectional flows either. This was a great surprise, as the leading theory on how the magnetic field of the Sun is made, by heat convection flows and magnetohydrodynamics, got in conflict with observations.

2. The helioseismological measurements have now become so good that we can map the solar “weather patterns” in the convection zone in great detail. The most accurate measurements show that the meridional weather patterns are double celled.

This makes the heliophysicists scratch their heads, as the weather pattern and its driving mechanism fits far better with cooling from beneath and heating from above. Experts are now wondering if there could be other driving mechanisms than heat driven convection.

The Sun’s meridional circulation is most likely mechanically driven and thermally braked, roughly opposite to the driving mechanism of the Hadley cell in the Earth’s atmosphere source

So now the convection zone is said to have convectional breaking, which is the opposite of current convection zone theory. But if the coronal heating problem runs deeper, there might still be heat driven convection we observe, as the weather cell convection might be driven by heating from above.

1. Helioseismology has studied differences in seismic wave speed inside the Sun to map different layers in the solar interior, and as we can see on the first picture, it shows contours of an inner and outer solar core, created by higher seismic velocity.

So now there are papers talking about a solar inner core and and a solar outer core, similar to what Earth have. On Earth we use seismic waves from Earthquakes to map the interior, but here seismic wave speed is mostly influenced by if the medium is in solid or liquid state. But the Sun is thought to consist of 90 % plasma, and the 16 million degree core is close to 100 % plasma, so it is just a coincidence that the picture show similarities with our planet.

We know that the Sun is powered by fusion, there is no doubt, as we can observe solar neutrinos from solar fusion reactions. But still there is hard to find any observations that the core heats the layers above. Still we think that 16 million degree heat energy is transferred all the way from the core and heats the corona to millions of degrees, we just don’t know how this heat energy is transferred. Often the outer layers of the Sun is just considered when we look at the Coronal heating problem, but may the root of the Coronal heating problem run all the way to the core?

• What is your question? – Rob Jeffries Mar 1 '17 at 18:01

So how can heat energy from the 16 million degree hot fusion furnace in the solar core get through the photosphere and heat the corona?

It doesn't. That is the problem. The coronal heating problem like walking away from a fire and instead of it getting cooler, you burn up.

The temperature gradient from the core to the photosphere is precisely what one would expect, i.e., hot-to-cold.

Usually just the outer layers of the Sun is investigated to find solutions to the Coronal heating problem, but may the problem run far deeper into the interior of the Sun?

No, not really. If you mean that the convection below the photosphere affects the magnetic fields that arc up into the corona, then perhaps, indirectly, yes through processes related to and caused by magnetic reconnection.

This makes the heliophysicists scratch their heads, as the weather pattern and its driving mechanism fits far better with cooling from beneath and heating from above. Experts are now wondering if there could be other driving mechanisms than heat driven convection.

No, this is not correct. The immense pressures experience within the core are responsible for the high temperatures, not the corona heating the core. If I follow your logic -- which seems to argue that heat cannot escape the core through convection thus it must be coming from somewhere else -- then how could heat from above get in to the core? It cannot be a one-way thing like that.

But the Sun is thought to consist of 90% plasma, and the 16 million degree core is close to 100% plasma, so it is just a coincidence that the picture show similarities with our planet.

This is likely driven by competing forces, radiation and pressure gradients. Below some depth one will win and above that threshold depth, the other will start to win. The constraints on convection are tied to viscosity, which depends upon the parameters of the fluid in question. A way to parameterize this is to examine the Reynolds number of the fluid to determine whether it "flows like honey" or can behave like a gas.

Still we think that 16 million degree heat energy is transferred all the way from the core and heats the corona to millions of degrees, we just don’t know how this heat energy is transferred.

No, we do not think the core heats the corona. It cannot be heating the corona because the photosphere is roughly three orders of magnitude cooler. This is why the conceptual dilemma arose in the first place. The solar corona is hotter than it should be were it only heated through thermodynamic processes (e.g., thermal conduction) from the photosphere.

Often the outer layers of the Sun is just considered when we look at the Coronal heating problem, but may the root of the Coronal heating problem run all the way to the core?

Then how would you transfer this energy through the comparatively cold photosphere and deposit it in the corona? Again, this is the crux of the dilemma.

The likely cause of the anomalously high temperatures in the corona are small-scale interactions between electromagnetic fields and the charged particles (e.g., wave heating). Even with ultra high resolution imaging telescopes, like on SDO, we cannot resolve features smaller than ~70 km on the sun. For comparison, Larmor radius of thermal electrons and protons in the corona (assuming $T_{e} \approx T_{p} \sim 10^{6} K$ and $B_{o} \sim 10^{-3} - 10^{-2} T$) are ~0.2--2.0 cm and ~0.1--1.0 m, respectively. Given that most plasma heating observed in the near-Earth environment involves sub-proton-scale waves and/or turbulence, it is not surprising that we cannot resolve the actual heating mechanisms in the solar corona.

Addressing the coronal heating problem is one of the science requirements of the upcoming Solar Probe Plus mission.

References

• Aschwanden, M. Physics of the Solar Corona: An Introduction with Problems and Solutions, Praxis Publishing Ltd, Chichester, UK, ISBN 3-540-30765-6, 2006.