The Jakimiec Track

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== Introduction ==
== Introduction ==
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== The (EM, T) diagram ==
== The (EM, T) diagram ==
-
We normally characterize the thermal spectrum of a microflare by its e''mission measure'' EM = n<sup>2</sup>V 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.
+
We normally characterize the thermal spectrum of a microflare by its ''emission measure'' EM = n<sup>2</sup>V 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.
 +
Here ''n'' and ''T'' are the plasma density and temperature respectively, and there are approximations involved in just saying this!
 +
Any good image of a solar flare shows it to be very complicated, so it is a gross simplification to imagine that it has these unique parameters; nevertheless to
 +
make progress in interpretation we often need to make this "isothermal" assumption.
 +
Of course for a stellar flare, or a very compact solar event, it is hard to see the complex structure, so that tends to ease the conscience a bit.
Thus a plot of the two parameters (EM, T) naturally provides diagnostic information about the evolution of the flaring plasma.
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 [http://en.wikipedia.org/wiki/Hertzsprung-Russell_diagram Hertzsprung-Russell diagram]- but for classes of flares, rather than classes of stars.
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 [http://en.wikipedia.org/wiki/Hertzsprung-Russell_diagram Hertzsprung-Russell diagram]- but for classes of flares, rather than classes of stars.
-
[[Image:95_markus_markup.jpg|500px|thumb|center|'''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.]]
+
[[Image:95_Markus_markup.jpg|500px|thumb|center|'''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.]]
 +
 
 +
The diagram above shows clearly that the (EM, T) variables do have a strong organization.
 +
Here we are seeing one point per flare, but in principle one can observe the motion of that point as a function of the time as the flare evolves - hence the analogy, perhaps, with the Hertzsprung-Russell diagram.
 +
Here though we can watch the evolution on time scales of minutes, rather than [http://en.wiktionary.org/wiki/gigayear Gy].
 +
The x's in this diagram show a sample of the RHESSI data analyzed by Marina Battaglia.
 +
The clouds of points may individually have correlations, as shown by the black lines, but different data sets seem to differ substantially.
 +
The RHESSI points in particular all look hotter (or with a smaller emission measure) than the bulk of the points in the original sample of Uri Feldman,
 +
which gave an excellent view of the correlation from the [http://en.wikipedia.org/wiki/Yohkoh Yohkoh} spacecraft.
 +
We go into the meaning of the brightly colored lines in the next section below.
   
   
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==References==
==References==
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* [http://adsabs.harvard.edu/abs/1988ApJ...330..474P Nanoflares and the solar X-ray corona] by E. N. Parker
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* [http://adsabs.harvard.edu/abs/2002ApJ...577..422S A Hertzsprung-Russell-like Diagram for Solar/Stellar Flares and Corona: Emission Measure versus Temperature Diagram] by K. Shibata and T. Yokoyama
* [http://adsabs.harvard.edu/abs/2002ApJ...577..422S A Hertzsprung-Russell-like Diagram for Solar/Stellar Flares and Corona: Emission Measure versus Temperature Diagram] by K. Shibata and T. Yokoyama

Revision as of 03:52, 1 March 2009


Nugget
Number: 95
1st Author: Hugh Hudson
2nd Author: Fabio Reale
Published: 2 March 2009
Next Nugget: The Jakimiec Track
Previous Nugget: Coronal Implosion
List all




Contents

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. Here n and T are the plasma density and temperature respectively, and there are approximations involved in just saying this! Any good image of a solar flare shows it to be very complicated, so it is a gross simplification to imagine that it has these unique parameters; nevertheless to make progress in interpretation we often need to make this "isothermal" assumption. Of course for a stellar flare, or a very compact solar event, it is hard to see the complex structure, so that tends to ease the conscience a bit. 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.

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.

The diagram above shows clearly that the (EM, T) variables do have a strong organization. Here we are seeing one point per flare, but in principle one can observe the motion of that point as a function of the time as the flare evolves - hence the analogy, perhaps, with the Hertzsprung-Russell diagram. Here though we can watch the evolution on time scales of minutes, rather than Gy. The x's in this diagram show a sample of the RHESSI data analyzed by Marina Battaglia. The clouds of points may individually have correlations, as shown by the black lines, but different data sets seem to differ substantially. The RHESSI points in particular all look hotter (or with a smaller emission measure) than the bulk of the points in the original sample of Uri Feldman, which gave an excellent view of the correlation from the [http://en.wikipedia.org/wiki/Yohkoh Yohkoh} spacecraft. We go into the meaning of the brightly colored lines in the next section below.


The Jakimiec Track (and other scaling laws)

Conclusion

References

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