[Loops] Two new time lag analysis papers

[VIALL-KEPKO, NICHOLEEN M. (GSFC-6710)]

Dear Loop Enthusiasts,

We would like to draw your attention to two papers recently accepted to ApJ and posted to arxiv. Please let me know if you have any comments or questions!

Patterns of Activity in a Global Model of a Solar Active Region
http://adsabs.harvard.edu/abs/2016ApJ...821...63B
http://arxiv.org/abs/1603.06670
S. J. Bradshaw & N. M. Viall

Abstract: In this work we investigate the global activity patterns predicted from a model active region heated by distributions of nanoflares that have a range of frequencies. What differs is the average frequency of the distributions. The activity patterns are manifested in time lag maps of narrow-band instrument channel pairs. We combine hydrodynamic and forward modeling codes with a magnetic field extrapolation to create a model active region and apply the time lag method to synthetic observations. Our aim is not to reproduce a particular set of observations in detail, but to recover some typical properties and patterns observed in active regions. Our key findings are the following. (1) Cooling dominates the time lag signature and the time lags between the channel pairs are generally consistent with observed values. (2) Shorter coronal loops in the core cool more quickly than longer loops at the periphery. (3) All channel pairs show zero time lag when the line of sight passes through coronal loop footpoints. (4) There is strong evidence that plasma must be re-energized on a timescale comparable to the cooling timescale to reproduce the observed coronal activity, but it is likely that a relatively broad spectrum of heating frequencies are operating across active regions. (5) Due to their highly dynamic nature, we find nanoflare trains produce zero time lags along entire flux tubes in our model active region that are seen between the same channel pairs in observed active regions.


Signatures of Steady Heating in Time Lag Analysis of Coronal Emission
http://arxiv.org/abs/1607.02008
Nicholeen M. Viall & James A. Klimchuk

Abstract: Among the many ways of investigating coronal heating, the time lag method of Viall & Klimchuk (2012) is becoming increasingly prevalent as an analysis technique complementary to those traditionally used. The time lag method cross correlates light curves at a given spatial location obtained in spectral bands that sample different temperature plasmas. It has been used most extensively with data from the Atmospheric Imaging Assembly on the Solar Dynamics Observatory. We have previously applied the time lag method to entire active regions and surrounding quiet Sun and create maps of the results (Viall & Klimchuk 2012; Viall & Klimchuk 2015). We find that the majority of time lags are consistent with the cooling of coronal plasma that has been impulsively heated. Additionally, a significant fraction of the map area has a time lag of zero. This does not indicate a lack of variability. Rather, strong variability must be present, and it must occur in phase in the different channels. We have shown previously that these zero time lags are consistent with the transition region response to coronal nanoflares (Viall & Klimchuk 2015; Bradshaw & Viall 2016), but other explanations are possible. A common misconception is that the zero time lag indicates steady emission resulting from steady heating. Using simulated and observed light curves, we demonstrate here that highly correlated light curves at zero time lag are not compatible with equilibrium solutions. Such light curves can only be created by evolution.


Best Regards,

Nicki

***************************
Nicholeen M. Viall
Research Astrophysicist
Solar Physics Laboratory
Code 671
NASA Goddard Space Flight Center
Greenbelt, MD 20771
V: 301-286-4054
F: 301-286-7194
Nicholeen.M.Viall at nasa.gov
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mithra.physics.montana.edu/pipermail/loops/attachments/20160714/9e8aea2c/attachment.html>