Dear all,
the following paper has recently been accepted by ApJ Letters. You can
find a copy of the paper and relevant movies at
www.lmsal.com/~ptesta/hic_moss/
thanks,
Paola
title:
Observing coronal nanoflares in active region moss
authors:
P. Testa, B. De Pontieu, J. Martinez-Sykora, E. DeLuca, V. Hansteen, J.
Cirtain, A. Winebarger, L. Golub, K. Kobayashi, K. Korreck, S. Kuzin, R.
Walsh, C. DeForest, A. Title, M.Weber
abstract:
The High-resolution Coronal Imager (Hi-C) has provided Fe XII 193A
images of the upper transition region moss at an unprecedented spatial
(~0.3-0.4 arcsec) and temporal (5.5s) resolution.
The Hi-C observations show in some moss regions variability on
timescales down to ~15s, significantly shorter than the minute scale
variability typically found in previous observations of moss, therefore
challenging the conclusion of moss being heated in a mostly steady
manner. These rapid variability moss regions are located at the
footpoints of bright hot coronal loops observed by SDO/AIA in the 94A
channel, and by Hinode/XRT.
The configuration of these loops is highly dynamic, and suggestive of
slipping reconnection.
We interpret these events as signatures of heating events associated
with reconnection occurring in the overlying hot coronal loops, i.e.,
coronal nanoflares.
We estimate the order of magnitude of the energy in these events to be
of at least a few $10^{23}$~erg, also supporting the nanoflare scenario.
These Hi-C observations suggest that future observations at comparable
high spatial and temporal resolution, with more extensive temperature
coverage are required to determine the exact characteristics of the
heating mechanism(s).
5
7
cooling time
by Klimchuk, James A. (GSFC-6710)
23 Aug '13
23 Aug '13
I was asked to define the cooling time, t_cool, in my last e-mail. Sorry, I should have done this. It is the thermal energy density divided by the energy loss rate:
t_cool = (3/2) P / R_loss ,
where P is the pressure. When thermal conduction dominates the cooling, as is the case early in a nanoflare event, the loss rate is approximately
R_loss = (2/7) kappa_0 T^(7/2) / L^2 ,
where T is the peak temperature in the strand, and L is the distance between the location of the peak and the chromosphere (typically the loop half length). Generally speaking, temperature decreases according to
T(t) = T_0 exp(-t / t_cool) .
I assumed this in getting the percentage temperature variations in the original e-mail.
Cheers,
Jim
********************************************************************************
James A. Klimchuk
NASA Goddard Space Flight Center
Solar Physics Lab, Code 671
Bldg. 21, Rm. 158
Greenbelt, MD 20771
USA
Phone: 1-301-286-9060
Fax: 1-301-286-7194
E-mail: James.A.Klimchuk(a)nasa.gov<mailto:James.A.Klimchuk@nasa.gov>
Homepage: http://science.gsfc.nasa.gov/sed/index.cfm?fuseAction=people.jumpBio&&iPhon…
No endorsement by NASA is implied for any correspondence related to my official role in professional organizations.
********************************************************************************
Dear Loops Friends,
Given all the recent work on the subject, I think it would be useful to establish some definitions regarding the repetition frequency of nanoflares (on a given strand). The current bimodal classification of high frequency versus low frequency is no longer adequate. We can all agree that when the repetition time, t_rep, is much longer than or much shorter than the cooling time, t_cool, then the frequency can be straightforwardly classified as low or high, respectively. But what if t_rep and t_cool are not greatly different? Recent studies seem to fall within this intermediate regime (e.g., results presented at the SPD meeting by Inaki and by Amy's student Scott Ripperda, and results presented at ISSI by Peter). I fear it is misleading to classify nanoflares as low or high frequency if t_rep is only marginally different from t_cool. I therefore suggest the following classification scheme:
High frequency: t_rep < 0.5 * t_cool
Intermediate frequency: 0.5 * t_cool < t_rep < 2 * t_cool
Low frequency: t_rep > 2 * t_cool
In these ranges, the temperature variation in the strand will be approximately:
High frequency: deltaT < 39%
Intermediate frequency: 39% < deltaT < 86%
Low frequency: deltaT > 86%
I would be interested in your thoughts on this suggestion.
Thanks,
Jim
********************************************************************************
James A. Klimchuk
NASA Goddard Space Flight Center
Solar Physics Lab, Code 671
Bldg. 21, Rm. 158
Greenbelt, MD 20771
USA
Phone: 1-301-286-9060
Fax: 1-301-286-7194
E-mail: James.A.Klimchuk(a)nasa.gov<mailto:James.A.Klimchuk@nasa.gov>
Homepage: http://science.gsfc.nasa.gov/sed/index.cfm?fuseAction=people.jumpBio&&iPhon…
No endorsement by NASA is implied for any correspondence related to my official role in professional organizations.
********************************************************************************