The Jakimiec Track

Nugget
Number:	95
1st Author:	Hugh Hudson
2nd Author:	Fabio Reale
Published:	2 March 2009
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Previous Nugget:	Coronal Implosion

Introduction

A great many tiny flares appear in the RHESSI data (and elsewhere). See the Nugget "A myriad of microflares" for an introduction, and stay tuned to the Nuggets pages for more interesting discoveries about them. The microflares (and the flares) occur in a power law distribution, which means that there is no well-defined average property (this is known "scale invariance"). The solar community (and many others) remain somewhat baffled by the deeper meaning of this fact, which is a bit difficult physically - normally one wants to have a clearly defined phenomenon to theorize about. That is difficult if there is no meaningful average. Another related concept is "stiction", a portmanteau word in the sense of Lewis Carroll. This one could be from "static friction" or maybe "sticking friction," but in any case its analog in the solar corona is Parker's nanoflare concept. The idea of nanoflares is that an apparently steady X-ray emission could just be the result of innumerably many tiny, tiny non-thermal events (the nanoflares) that blend together imperceptibly.

The (EM, T) diagram

We normally characterize the thermal spectrum of a microflare by its emission measure EM = n²V and its temperature T, because the X-ray flux S can be written as S = EM x f(T), where f(T) is a function of the temperature T that embodies most of the parameters that describe the emitting plasma and the specific detector that is making the observation. Thus a plot of the two parameters (EM, T) naturally provides diagnostic information about the evolution of the flaring plasma. Many researchers have noted that the (EM, T) diagram for flares has interesting properties, and the claim has even been made that its organizing power may rival that of the Hertzsprung-Russell diagram- but for classes of flares, rather than classes of stars.

File:95 markus markup.jpg

Figure 1: A recent compilation by Markus Aschwanden of observations of the (EM, T)-plane for the quiet Sun, solar microflares, solar flares, and stellar flares.