[Loops] FW: summaries on nanoflare debates in "coronalloopworkshops"/ and ... CHs

[Serge Koutchmy]

Dear All,
The debate about the reality of nano-flares reminds me an attempt we did 
in 1995 to understand the origin of the fast wind in CHs as a result of 
discrete events (jetlets). Sorry, this is not exactly loop physics.
We looked at the faintest Yohkoh SXR transient brightenings (TBs) inside 
the North CH at time of solar minimum and before the SoHO works appear. 
The threshold was typically 10^24 ergs in term of radiative output in 
SXR, as events were short in time (less than 3 min), over a background 
of virtually zero value, even when using long exposure times (up to 55 
s), times significantly larger than what Shimizu-san used in his 
classical PhD analysis of TBs. We thought that this is a way to avoid 
overlapping effects (events were single or 2 pixels events) and we 
avoided measurements done during the passage through the radiation belts 
to limit the influence of cosmic rays. Events were considered as real 
(solar) when at least 3 consecutive images show a definite signature. We 
got a lot of events in this 2 MK range (this was reported in an Astron. 
Astrophys. L. paper of the late Yohkoh epoch which is left un-noticed by 
almost all nano-flare afinados, but I attached it for your record- Thank 
you).

The lesson we got is that 2 MK small events (2 or more orders of 
magnitude fainter than what Shimizu-san analyzed) do exist and 
proliferate in CHs, and not just at the very surface of the Sun. The 
question “how much flux ?” seems indeed open. An individual event is 
associated with several consequences, not just a radiative signature at 
2 MK.

Is not correct to think that nano-flares can as well accelerate 
energetic particles (electrons…) in bi-directional flows and produce a 
signature (chromospheric “evaporation”) well outside the reconnection 
site? Looking at the faintest events seen in 304 inside a CH, with a 
partial frame sequence, gives even more “individual” events. The less is 
the energy range (temperature), the bigger is the number of events you 
see and probably the deeper you look at. And it is also true that the 
shaking of field lines (transverse waves, spicules, etc.) is starting 
well down the low corona and this should also affect loops but, in CHs, 
it is more obvious, although making reconnections in a dominant polarity 
open field could be a challenge.

This is a naïve interpretation of what is observed and I humbly and 
respectfully will read/evaluate with great interest all interpretations 
that you would propose in term of the network, quiet Sun, and or active 
regions coronal (transient) magnetic field that we cannot measure, 
including the case of multi-stranded loops.

Sorry for my broken English.
Serge

*

*

Arnold Benz a écrit :
> Dear David,
>
> Welcome back! Indeed you did leave the nanoflare business some time ago, 
> when we realized that we will never agree on power-law exponents because 
> as you described there is a subjective element in the definition of a 
> nanoflare. I fully agree with you, David, that power-law indices are 
> useless and that we must not extrapolate them.
> However, power-laws are not the only way to evaluate nanoflares.  In the 
> mean time we have tried to estimate the energy input by the observed 
> events. For this estimate, the distribution is not needed, just sum over 
> all events and pixels. The observed energy in the soft X-rays and EUV 
> events at peak flux is about 12% of the radiated output in the quiet sun 
> (Benz & Krucker 2002). This includes only observed events above 5 10^24 
> erg. What we measure, however, is the thermal energy at one instant of 
> the event. This thermal energy is not the flare energy, but the result 
> of precipitating particles heating the chromosphere. The particle 
> acceleration is not the flare either, but arguably the result of waves 
> that have been excited by the reconnection process (e.g. transit-time 
> damping). Thus the real difficulty is to estimate the total energy input 
> into the corona from the observed nanoflares.
>
> Regards,
> Arnold 
>
> ________________________________________________________________________
> Arnold Benz                                        
> Institute of Astronomy              email benz at astro.phys.ethz.ch     
> ETH Zurich,  HIT J 23.1              voice ++41-44-632 42 23 
> CH-8093 Zurich                           fax ++41-44-862 68 25
> Switzerland    web http://www.astro.phys.ethz.ch/staff/benz/benz.html 
> _________________________________________________________________________
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