The Alfven Speed above a Sunspot, and Gamma-rays

From RHESSI Wiki

(Difference between revisions)
Jump to: navigation, search
(tidied up, added last figure)
(polishing...)
Line 48: Line 48:
illustrate the distinct lack of a gamma-ray signature.
illustrate the distinct lack of a gamma-ray signature.
RHESSI has shown that gamma-ray flares are quite different from ordinary
RHESSI has shown that gamma-ray flares are quite different from ordinary
-
flares, i.e. that Kahler's "big flare syndrome" (where everything scales
+
flares, i.e. that the "big flare syndrome" (where everything scales
-
together) does not apply in this case.
+
together) does not apply in this case - here we see a big flare, with a sunquake as well, and yet no gamma rays.
For this flare, Figure 2 shows that there was negligible [http://en.wikipedia.org/wiki/Solar_surface_fusion 2.2 MeV gamma-ray emission line], RHESSI's most sensitive tool for this purpose, and  
For this flare, Figure 2 shows that there was negligible [http://en.wikipedia.org/wiki/Solar_surface_fusion 2.2 MeV gamma-ray emission line], RHESSI's most sensitive tool for this purpose, and  
that the hard X-ray spectrum was weak above 50 keV.
that the hard X-ray spectrum was weak above 50 keV.
-
[[File:149f2.png|thumb|center|400px|'''Figure 2''': Time series of counts at 50-100 keV (gold) and at 2200-2250 keV (red), the latter a crude indication of the presence or absence of gamma-rays. In this case, despite the X classification of the flare, there were no detectable gamma rays and only a weak 50-100 keV hard X-ray signature.
+
[[File:149f2.png|thumb|center|400px|'''Figure 2''': Time series of counts at 100 keV (blue), 200 keV (gold) and at 2200-2250 keV (red), the latter a crude indication of the presence or absence of gamma-rays. In this case, despite the X classification of the flare, there were no detectable gamma rays and only a weak hard X-ray signature with a relatively steep spectrum.
]]
]]
== Theory ==
== Theory ==
-
As the images show, in this flare - as with many of the most powerful  
+
As the image in Figure 1 shows, in this flare - as with many of the most powerful  
events - we are essentially looking deep into the core of the active
events - we are essentially looking deep into the core of the active
region.
region.
Line 87: Line 87:
altitude.
altitude.
-
[[File:149f3.png|thumb|center|600px|'''Figure 3''': Three panels showing, as  
+
[[File:149f3.png|thumb|center|700px|'''Figure 3''': Three panels showing, as  
a function of height above the center of a major sunspot, the magnetic field,
a function of height above the center of a major sunspot, the magnetic field,
the Alfvén speed, and the plasma beta.
the Alfvén speed, and the plasma beta.
Line 100: Line 100:
smaller as well.
smaller as well.
That means that the coronal field (intensity and scale) and Alfvén speed will  
That means that the coronal field (intensity and scale) and Alfvén speed will  
-
be correspondingly smaller.  
+
be correspondingly smaller in this particular event.
 +
A survey of a number of representative events will clearly be warranted, and we hope that [http://sprg.ssl.berkeley.edu/~tohban/wiki/index.php/Cycle_24_has_begun Cycle 24] will bring us events
 +
ten times as powerful.
== Speculation and Conclusion ==
== Speculation and Conclusion ==
Line 108: Line 110:
the theory of the processes in the corona just above them.
the theory of the processes in the corona just above them.
A relativistic Alfvén speed means that the [http://en.wikipedia.org/wiki/Flux Poynting flux] can rival  
A relativistic Alfvén speed means that the [http://en.wikipedia.org/wiki/Flux Poynting flux] can rival  
-
the motions of accelerated particles, for example.
+
the motions of accelerated particles, for example, in terms of the speed of energy transport.
Although the theory of these phenomena is still at its bare beginnings, it also seems likely that
Although the theory of these phenomena is still at its bare beginnings, it also seems likely that
such extreme values of Alfvén speed and plasma beta will also help to  
such extreme values of Alfvén speed and plasma beta will also help to  
Line 116: Line 118:
gamma rays to speak of?
gamma rays to speak of?
And yet, there was a
And yet, there was a
-
[http://sprg.ssl.berkeley.edu/~tohban/wiki/index.php/First_Sunquake_of_Solar_Cycle_24_Observed_by_Solar_Dynamics_Observatory sunquake].
+
[http://sprg.ssl.berkeley.edu/~tohban/wiki/index.php/First_Sunquake_of_Solar_Cycle_24_Observed_by_Solar_Dynamics_Observatory sunquake]; there have been suggestions that high-energy ions (the sources of flare gamma rays) might be implicated in sunquake
 +
activity.
Does the ion acceleration require a still stronger magnetic field?
Does the ion acceleration require a still stronger magnetic field?
-
In the Alfvénic  exhaust of
 
-
[http://sprg.ssl.berkeley.edu/~stephchow/cartoons/thepages/Duncan.html magnetic reconnection]
 
-
one might expect the bulk motion to be in the form of relativistic
 
-
particles, ie that the entire plasma would consist of high-energy particles
 
-
rather than the usually assumed [Maxwellian distributions].
 
In any case we now have marvelous tools to study these questions  
In any case we now have marvelous tools to study these questions  
observationally, and so we may expect some progress via studies of this
observationally, and so we may expect some progress via studies of this
and other flares with these tools; this Nugget and the  
and other flares with these tools; this Nugget and the  
-
[http://sprg.ssl.berkeley.edu/~tohban/wiki/index.php/First_Sunquake_of_Solar_Cycle_24_Observed_by_Solar_Dynamics_Observatory  sunquakes Nugget] on the same event have  
+
[http://sprg.ssl.berkeley.edu/~tohban/wiki/index.php/First_Sunquake_of_Solar_Cycle_24_Observed_by_Solar_Dynamics_Observatory  sunquakes Nugget] on the same event have only described parts of the rich data available now.
-
only begun the work needed.
+

Revision as of 14:27, 12 April 2011


Nugget
Number: 148
1st Author: H. Hudson
2nd Author: L. Fletcher
Published: 12 April 2011
Next Nugget: Decimetric Pulsations and Coronal X-ray Sources
Previous Nugget: Slow Magnetoacoustic Waves in Two-Ribbon Flares
List all



Contents

Introduction

It is common for powerful flares to occur actually within sunspots, with their H-alpha ribbons blithely crossing over these regions of very strong magnetic field. What does this association mean for flare theory, and what does it mean for the interesting differences RHESSI has discovered between flares with and without gamma-rays? We discuss these issues in the context of the X-class flare of February 15, 2011. This was only the second X-class flare of this solar maximum; it was not a gamma-ray event but it was the source of a clear sunquake.

Observations from Hinode and RHESSI

The flare was well-observed by the SOT white-light imager on Hinode, with 20-second cadence in three broad-band optical filters. This data set is quite remarkable because of the excellent resolution and brilliant image contrast achieved by the SOT telescope, the largest solar telescope ever put in space. Figure 1 shows an overlay of the patches of white-light emission extracted from the several images taken during the impulsive phase of the flare, which is when we ordinarily observe hard X-rays and gamma-rays.

Figure 1: The bright patches of the white-light flare emission, shown in different colors for the several different images at enhanced contrast. The background image is an unaltered view of the active region, which has well-developed spots. The white-light flare patches extend well into the umbrae of the spots.

At the same time as these white-light emissions, what was RHESSI seeing? Figure 2 offers a simplified view of the RHESSI counting rates, mainly to illustrate the distinct lack of a gamma-ray signature. RHESSI has shown that gamma-ray flares are quite different from ordinary flares, i.e. that the "big flare syndrome" (where everything scales together) does not apply in this case - here we see a big flare, with a sunquake as well, and yet no gamma rays. For this flare, Figure 2 shows that there was negligible 2.2 MeV gamma-ray emission line, RHESSI's most sensitive tool for this purpose, and that the hard X-ray spectrum was weak above 50 keV.

Figure 2: Time series of counts at 100 keV (blue), 200 keV (gold) and at 2200-2250 keV (red), the latter a crude indication of the presence or absence of gamma-rays. In this case, despite the X classification of the flare, there were no detectable gamma rays and only a weak hard X-ray signature with a relatively steep spectrum.

Theory

As the image in Figure 1 shows, in this flare - as with many of the most powerful events - we are essentially looking deep into the core of the active region. In these cases the many signatures of non-thermal effects in the lower solar atmosphere (hard X-rays, H-alpha, EUV, and intense white light) all implicate the strong magnetic fields to be found in the umbra and penumbra regions of major sunspots.

In this Nugget we remind readers that the coronal volume just above a sunspot is a particularly interesting and ill-understood place. Many research workers will express surprise at the strength of the coronal magnetic field above a big spot, about the Alfvén speed, and about the plasma beta as well - all are extreme, according to the best observational estimates. The Alfvén speed, if computed naively, exceeds the speed of light below a few Mm above a big spot! Figure 3 details some of these parameters for a conservatively chosen set of model conditions above a very large sunspot. One of the problems of the corona above a spot umbra is so dark and non-emissive that ordinary astronomical techniques do not work very well, and so in fact there are major observational uncertainties. The theory behind the extrapolations in Figure 3 is very simple and safe, though. For a spot like this we could expect an Alfvén speed of c/3 at 10,000 km altitude.

Figure 3: Three panels showing, as a function of height above the center of a major sunspot, the magnetic field, the Alfvén speed, and the plasma beta. The assumed parameters for the sunspot are a central field of 3500 G and an umbral radius of 10,000 km.

Is this estimation, which is based on Allen's Astrophysical Quantities, at all relevant to the particular flare we are discussing? Perhaps not directly, since the Hinode magnetographic observations suggest field intensities about a factor of three smaller, and the spatial scales smaller as well. That means that the coronal field (intensity and scale) and Alfvén speed will be correspondingly smaller in this particular event. A survey of a number of representative events will clearly be warranted, and we hope that Cycle 24 will bring us events ten times as powerful.

Speculation and Conclusion

This Nugget suggests that we begin to combine observations of the manifestations of major flares within sunspots, with new thinking about the theory of the processes in the corona just above them. A relativistic Alfvén speed means that the Poynting flux can rival the motions of accelerated particles, for example, in terms of the speed of energy transport. Although the theory of these phenomena is still at its bare beginnings, it also seems likely that such extreme values of Alfvén speed and plasma beta will also help to accelerate the particles that RHESSI detects through hard X-rays and gamma rays. But why, in the case of the flare we have illustrated here, were there no gamma rays to speak of? And yet, there was a sunquake; there have been suggestions that high-energy ions (the sources of flare gamma rays) might be implicated in sunquake activity. Does the ion acceleration require a still stronger magnetic field? In any case we now have marvelous tools to study these questions observationally, and so we may expect some progress via studies of this and other flares with these tools; this Nugget and the sunquakes Nugget on the same event have only described parts of the rich data available now.

Personal tools
Namespaces
Variants
Actions
Navigation
Toolbox