Sundrops

It's always a little surprising to find out that phenomena that are (figuratively speaking) right in our own neighborhood are still a mystery.

One example is the temperature of the solar corona.  We aren't usually aware of the solar corona -- its eerie pinkish luminescence is ordinarily lost in the much brighter radiance of the solar photosphere.  But it becomes visible during a total solar eclipse:

The total solar eclipse of 21 August 2017, photographed by Giuseppe Donatiello [Image is in the Public Domain]

The core of the Sun is estimated to have a temperature of about 15,700,000 K; that heat energy reaches the surface (largely through convection) and then is lost to space.  The outer layer, or photosphere -- the part we can see from Earth on a sunny day -- is around 5,800 K, which is still pretty hot.  But the wispy corona that surrounds the sun is around 5,000,000 K.

But how can that be?  Since, presumably, it's obtaining its heat from the photosphere, how can it be hotter than its own heat source?  Doesn't that break the Second Law of Thermodynamics, which says (amongst other things) that heat energy only flows from hotter objects to cooler ones?

Well, one thing to keep in mind -- not that it solves the mystery, or anything, but at least to get the facts straight -- is that temperature and heat energy are not the same thing, although they are clearly related.  Temperature is a measure of the kinetic energy of molecules, and that depends on more than their heat energy content, but factors like what the material is made of and how densely packed it is.  When I explained this to my students, I used the example of a pot of water heated to boiling (100 C) and an oven heated to 100 C (212 F).  Now imagine putting one hand in the pot of water and the other in the oven for five seconds.

Wouldn't be the same, would it?  Water holds a great deal more heat energy than air does -- at the same temperature.

So the five million Kelvin temperature of the corona is a measure of how fast the molecules are moving.  But still -- something is giving them that much kinetic energy.  So how's it all work?

Now, a new study from the National Solar Observatory has provided one piece of the puzzle -- but in the process, raised more questions.

It appears that packets of extremely hot material are being launched from the surface of the Sun.  When they get away from the turbulent photosphere, the pressure drops, and these "heat bombs" explode, releasing their energy into the corona.  The cooled plasma then recondenses and falls back into the Sun as "coronal raindrops."

Raindrops twenty kilometers wide.

So at least part of the answer is that this launching of plasma from the surface is acting as a heat energy transporter.  But how this process sustains the coronal temperature at a (much) higher value than the surface of the Sun below it is still mysterious, as is the connection between coronal rain and larger-scale phenomena like sunspots, solar prominences, and coronal mass ejections (including the scarily enormous Miyake events).

Like the best science, this study suggests an explanation for some facets of the phenomenon, but leaves a great deal of room for further study.  And points out the fact that we still have many mysteries left to ponder, including about our closest star, something we see every clear day.  Even the familiar can lead us into deep waters fast -- if you ask the right questions.

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