Dear all,
you might be interested in the following paper that has just been published on Science: "Evidence of non-thermal particles in coronal loops heated impulsively by nanoflares" It can be downloaded from astro-ph: http://arxiv.org/abs/1410.6130 or directly from the Science pages: http://www.sciencemag.org/content/346/6207/1255724 The abstract is below.
cheers, Paola
Abstract: The physical processes causing energy exchange between the Sun's hot corona and its cool lower atmosphere remain poorly understood. The chromosphere and transition region (TR) form an interface region between the surface and the corona that is highly sensitive to the coronal heating mechanism. High-resolution observations with the Interface Region Imaging Spectrograph (IRIS) reveal rapid variability (~20 to 60 seconds) of intensity and velocity on small spatial scales (?500 kilometers) at the footpoints of hot and dynamic coronal loops. The observations are consistent with numerical simulations of heating by beams of nonthermal electrons, which are generated in small impulsive (?30 seconds) heating events called "coronal nanoflares." The accelerated electrons deposit a sizable fraction of their energy (?10^25 erg) in the chromosphere and TR. Our analysis provides tight constraints on the properties of such electron beams and new diagnostics for their presence in the nonflaring corona.
If the sun releases 10^25 erg in 30s in the chromosphere, as needed by these data, cannot this be just local magnetic heating? A "chromospheric flare"?
Consider magnetic energy density
E= B^2/ 8pi.
In plage let B be say 300G, then E=3e3. Then for total energy of 10^25 erg we need 3e21 cm3 and d^3=V gives length d of 10^7 cm, or 0.1 Mm. If I use rho= 1e-10 g for mid chromosphere, then the alfven speed v_a is 80 kms and d/v_a is 1.2 sec. Fast.
So I am really puzzled why you appeal to beams at all. There is certainly nothing in the iris data that I can see could ever be used to give direct evidence for e- beams... all I can glean from these data is that there is a sudden release of energy under the place where Si IV is formed.
So I am very puzzled...
Philip Judge, Scientist, HAO, NCAR 3037759863
On Oct 30, 2014 2:49 PM, "Paola Testa" ptesta@cfa.harvard.edu wrote:
Dear all,
you might be interested in the following paper that has just been published on Science: "Evidence of non-thermal particles in coronal loops heated impulsively by nanoflares" It can be downloaded from astro-ph: http://arxiv.org/abs/1410.6130 or directly from the Science pages: http://www.sciencemag.org/content/346/6207/1255724 The abstract is below.
cheers, Paola
Abstract: The physical processes causing energy exchange between the Sun’s hot corona and its cool lower atmosphere remain poorly understood. The chromosphere and transition region (TR) form an interface region between the surface and the corona that is highly sensitive to the coronal heating mechanism. High-resolution observations with the Interface Region Imaging Spectrograph (IRIS) reveal rapid variability (~20 to 60 seconds) of intensity and velocity on small spatial scales (≲500 kilometers) at the footpoints of hot and dynamic coronal loops. The observations are consistent with numerical simulations of heating by beams of nonthermal electrons, which are generated in small impulsive (≲30 seconds) heating events called “coronal nanoflares.” The accelerated electrons deposit a sizable fraction of their energy (≲10^25 erg) in the chromosphere and TR. Our analysis provides tight constraints on the properties of such electron beams and new diagnostics for their presence in the nonflaring corona.
Loops mailing list Loops@solar.physics.montana.edu https://mithra.physics.montana.edu/mailman/listinfo/loops
Hi Phil,
we have addressed this point in the supplementary material of the paper, giving several reasons why we think it's highly unlikely, and the non-thermal electrons are a much more natural explanation.
thanks, Paola
On 10/30/14, 9:54 PM, Philip Judge wrote:
If the sun releases 10^25 erg in 30s in the chromosphere, as needed by these data, cannot this be just local magnetic heating? A "chromospheric flare"?
Consider magnetic energy density
E= B^2/ 8pi.
In plage let B be say 300G, then E=3e3. Then for total energy of 10^25 erg we need 3e21 cm3 and d^3=V gives length d of 10^7 cm, or 0.1 Mm. If I use rho= 1e-10 g for mid chromosphere, then the alfven speed v_a is 80 kms and d/v_a is 1.2 sec. Fast.
So I am really puzzled why you appeal to beams at all. There is certainly nothing in the iris data that I can see could ever be used to give direct evidence for e- beams... all I can glean from these data is that there is a sudden release of energy under the place where Si IV is formed.
So I am very puzzled...
Philip Judge, Scientist, HAO, NCAR 3037759863
On Oct 30, 2014 2:49 PM, "Paola Testa" <ptesta@cfa.harvard.edu mailto:ptesta@cfa.harvard.edu> wrote:
Dear all, you might be interested in the following paper that has just been published on Science: "Evidence of non-thermal particles in coronal loops heated impulsively by nanoflares" It can be downloaded from astro-ph: http://arxiv.org/abs/1410.6130 or directly from the Science pages: http://www.sciencemag.org/content/346/6207/1255724 The abstract is below. cheers, Paola Abstract: The physical processes causing energy exchange between the Sun’s hot corona and its cool lower atmosphere remain poorly understood. The chromosphere and transition region (TR) form an interface region between the surface and the corona that is highly sensitive to the coronal heating mechanism. High-resolution observations with the Interface Region Imaging Spectrograph (IRIS) reveal rapid variability (~20 to 60 seconds) of intensity and velocity on small spatial scales (≲500 kilometers) at the footpoints of hot and dynamic coronal loops. The observations are consistent with numerical simulations of heating by beams of nonthermal electrons, which are generated in small impulsive (≲30 seconds) heating events called “coronal nanoflares.” The accelerated electrons deposit a sizable fraction of their energy (≲10^25 erg) in the chromosphere and TR. Our analysis provides tight constraints on the properties of such electron beams and new diagnostics for their presence in the nonflaring corona. _______________________________________________ Loops mailing list Loops@solar.physics.montana.edu <mailto:Loops@solar.physics.montana.edu> https://mithra.physics.montana.edu/mailman/listinfo/loops
Dear Paola
thanks for pointing me to the supplementary stuff. It is a pity this was not made central to the paper, a casualty of choosing Science Magazine as a vehicle.
please understand my comments below are driven by the provocative nature of your conclusion and my interest in understanding basic physical processes in the Sun's atmosphere.
OK so I have read this through and finally found your explanation, here are my reactions, your words are in quotes:
1. "Chromospheric reconnection could in principle provide an alternative explanation for the observed chromospheric and TR variability, but we find that the observations support the hypothesis of beam heating. " The moss brightenings clearly occur at conjugate footpoints of hot loops undergoing heating, and there is a clear correlation between the coronal and chromospheric/TR emission, naturally explained by beam heating."
since "moss" (=phenomenology) is believed to be the hot transition region heated by conduction it should be impossible to get moss at just one footpoint. Hence footpoint emission at both footpoints says nothing other than conduction dominates. A clear correlation between corona and TR is always expected when conduction is important. The correlation between chromosphere and corona says something else.
2. "The Si IV brightenings are strong and occur throughout the region of the hot loop footpoints; if they were caused by chromospheric nanoflares, the reconnection and energy release would have to happen in all these locations consistently at a specific height appropriate to yield strong (and blueshifted) Si IV emission (i.e., if they occurred over a range of heights, some of them would happen too deep and would not produce any Si IV increase). Beam heating naturally explains the spatial and temporal coherence of various brightenings throughout the field-of-view, especially since the deposition height of electron beams (through the thick-target mechanism) naturally occurs at the height of the IRIS observations."
But so does reconnection in a stratified atmosphere- V_A the Alfven speed varies with a scale height of 2H where H is the density scale height, 120 km or so. So, reconnection (~ V_A) will always occur fastest in the least dense upper reaches of the chromosphere for a given magnetic field. (Another example is radiative heating from above which reaches only to tau=1 or so).
3. "Finally, given that moss variability is observed only at time when the overlying coronal loops are heated, if qchromospheric nanoflares were the source of the observed variability, the correlation with the coronal emission would have to be explained. iris_moss_rev1 "
This is the same as the point 1. above. So this is I think your major point.
Now I am very puzzled because there is a huge literature talking of spicules that generate heating events into the corona which is precisely what would be needed to explain your points 1. and 3.
My conclusion: 1. science should be about refuting hypotheses not supporting them. We already have a "surfeit of support for hypotheses" in solar physics owing to the non-unique interpretations that are possible, examples are given above. 2. Your data reveal just one essential observation to believe your hypothesis, but it is very far removed from a direct indicator of beam physics, and 3. Your data can be interpreted in a reconnection-driven spicule that has been advoctaed for very forcefully by some.
So I remain extremely puzzled and unconvinced. No doubt those advocating both for this process and spicules/reconnection can perform some Houdini-like "rescuing of the phenomena", but I must say this is all a very funny business.
Phil
loops@solar.physics.montana.edu