Universe Today
Coronal loops, the elegant and bright structures threading through the solar surface and into the solar atmosphere, are key to understanding why the corona is so hot. Yes, it's the Sun, and yes, it's hot, but its atmosphere is
too
hot. The puzzle as to why the solar corona is hotter than the Sun's photosphere has kept solar physicists busy since the mid-twentieth century, but with the help of modern observatories and advanced theoretical models, we now have a pretty good idea what is causing this. So is the problem solved?
Not quite...
So why are solar physicists so interested in the solar corona anyway? To answer this, I'll pull up an excerpt from my first ever Universe Today article:
... - from " ", Universe Today, December 21st, 2007
This isn't only an academic curiosity. Space weather originates from the lower solar corona; understanding the mechanisms behind coronal heating has wide-ranging implications for predicting energetic (and damaging) solar flares and forecasting interplanetary conditions.
So, the coronal heating problem is an interesting issue and solar physicists are hot on the trail of the answer to why the corona is so hot. Magnetic coronal loops are central to this phenomenon; they are at the base of the solar atmosphere and experience rapid heating with a temperature gradient from tens of thousands of Kelvin (in the chromosphere) to tens of millions of Kelvin (in the corona) over a very short distance. The temperature gradient acts across a thin transition region (TR), which varies in thickness, but can be only a few hundreds of kilometers thick in places.
These bright loops of hot solar plasma may be easy to see, but there are many discrepancies between the observation of the corona and coronal theory. The mechanism(s) responsible for heating the loops have proven to be hard to pin down, particularly when trying to understand the dynamics of "intermediate temperature" (a.k.a. "warm") coronal loops with plasma heated to around one million Kelvin. We are getting closer to solving this puzzle which will aid space weather predictions from the Sun to the Earth, but we need to work out why the theory is not the same as what we are seeing.
Solar physicists have been divided on this topic for some time. Is coronal loop plasma heated by intermittent magnetic reconnection events throughout the length of a coronal loop? Or are they heated by some other steady heating very low in the corona? Or is it a bit of both?
I actually spent four years wrestling with this issue whilst working with the
Solar Group at the University of Wales, Aberystwyth
, but I was on the side of "steady heating". There are several possibilities when considering the mechanisms behind steady coronal heating, my particular area of study was Alfvén wave production and wave-particle interactions (shameless self-promotion... my 2006 thesis:
Quiescent Coronal Loops Heated By Turbulence
, just in case you have a spare, dull weekend ahead of you).
James Klimchuk from the Goddard Space Flight Center's Solar Physics Laboratory in Greenbelt, Md., takes a different opinion and favours the nanoflare, impulsive heating mechanism, but he is highly aware that other factors may come into play:
" " - James Klimchuk
Nanoflares are predicted to maintain warm coronal loops at their fairly steady 1 million Kelvin. We know the loops are this temperature as they emit radiation in the extreme ultraviolet (EUV) wavelengths, and a host of observatories have been built or sent into space with instruments sensitive to this wavelength. Space-based instruments such as the EUV Imaging Telescope (EIT; onboard the NASA/ESA
Solar and Heliospheric Observatory
), NASA's
Transition Region and Coronal Explorer
(
TRACE
), and the recently operational
Japanese Hinode mission
have all had their successes, but many coronal loop breakthroughs occurred after the launch of
TRACE
back in 1998. Nanoflares are very hard to observe directly as they occur over spatial scales so small, they cannot be resolved by the current instrumentation. However, we are close, and there is a trail of coronal evidence pointing to these energetic events.
" " - Klimchuk.
Slowly but surely, theoretical models and observation are coming together, and it seems that after 60 years of trying, solar physicists are close to understanding the heating mechanisms behind the corona. By looking at how nanoflares and other heating mechanisms may influence each other, it is very likely that more than one coronal heating mechanism is at play...
Aside:
Out of interest, nanoflares will occur at any altitude along the coronal loop. Although they may be called
nanoflares
, by Earth standards, they are huge explosions. Nanoflares release an energy of 10
24
-10
26
erg (that is 10
17
-10
19
Joules). This is the equivalent of approximately 1,600 to 160,000 Hiroshima-sized atomic bombs (with the explosive energy of 15 kilotonnes), so there is nothing
nano
about these coronal explosions! But on the comparison with the standard X-ray flares the Sun generates from time to time with a total energy of 6×10
25
Joules (over 100 billion atomic bombs), you can see how
nano
flares get their name...
Original source:
NASA