Quiet Sun III

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	Now we have the definitive RHESSI measurements in hand (reference A), in the		Now we have the definitive RHESSI measurements in hand (reference A), in the
	sense that the Sun has now passed (with difficulty) a [quite unusual] [solar		sense that the Sun has now passed (with difficulty) a [quite unusual] [solar
-	minimum], and RHESSI data acquisition for faint hard X-ray emissions	+	minimum], and RHESSI data acquisition for faint exotic hard X-ray emissions
	has come to an end.		has come to an end.
	Reference to the earlier Nuggets cited will bring up unusual subjects such as		Reference to the earlier Nuggets cited will bring up unusual subjects such as
-	axions, cosmic-ray electrons, albedo neutrons, and the diffuse cosmic X-ray	+	cosmic-ray electrons, albedo neutrons, axions, and the diffuse cosmic X-ray
	background.		background.
	Since RHESSI only obtains upper limits, we cannot say very much about these		Since RHESSI only obtains upper limits, we cannot say very much about these
-	interesting things, except that they are predicted to be very faint, though not	+	interesting things, except that they are predicted to be very faint,
-	by astronomical standards.	+	though not by astronomical standards.
-	RHESSI's best observations are now completed because of competition from increasing [solar activity] (see the discussion of "nanoflares" below).	+	RHESSI's best observations are now completed because of competition from
		+	increasing [solar activity] (see the discussion of "nanoflares" below).
	(Fig. 2 from Hannah et al 2010, upgraded with Edwards & McCracken,		(Fig. 2 from Hannah et al 2010, upgraded with Edwards & McCracken,
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	In Reference A we describe the full observing program for faint global		In Reference A we describe the full observing program for faint global
	hard X-ray emission from the Sun.		hard X-ray emission from the Sun.
-	We have not made any great discovery - no axions - but	+	It has resulted in better upper limits rather than in a positive
-	the RHESSI data have provided the best upper limits to date (Figure 1).	+	detection (see Figure 1).
-	Until focusing optics, such as those of [Nustar] or [FOXSI] become available,	+	Not only does RHESSI not detect any of the more exotic possible
-	these are the best hard X-ray data.	+	sources; these limits also exclude low levels of "normal" magnetic
-		+	activity.
-	(Figure from the Sylwester EOS article?)	+	We discuss this latter result the "Nanoflares" section below.
-	This is the final statement from RHESSI on this topic owing to the	+	Until focusing optics, such as those of [NuSTAR] or [FOXSI] become available,
		+	these are the best hard X-ray data.
		+	Figure 1 is the final statement from RHESSI on this topic owing to the
	increase of [solar activity].		increase of [solar activity].
	Other recent instruments (e.g., [SphinX]) or [MESSENGER] may improve		Other recent instruments (e.g., [SphinX]) or [MESSENGER] may improve
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	As discussed below high spectral resolution may be very important in		As discussed below high spectral resolution may be very important in
	this domain, as well as good background rejection, to distinguish		this domain, as well as good background rejection, to distinguish
-	exotic sources from the expected thermal signatures (see next section).	+	exotic sources from the expected magnetic signatures (see next section).
	== Nanoflares ==		== Nanoflares ==
-	But the Sun has a continuous level of [magnetic activity], culminating in	+	The Sun has a continuous but variable level of [magnetic activity], seen for
-	sunspots, plage, flares, and CMEs.	+	example as [plage], [flares], and [CMEs].
-	In particular [nanoflares] are an attractive scenario that might	+	There is a hot [corona], brightest in [sunspot] regions, and in
-	solve the [coronal heating] problem.	+	a well-known theory [Parker] suggested that "nanoflares" - tiny
		+	non-thermal energy releases, undetectable individually but very numerous,
		+	could merge together and explain the heating needed.
	In essence the nanoflare hypothesis involves episodic heating, perhaps		In essence the nanoflare hypothesis involves episodic heating, perhaps
	resembling the action of true solar flares; to support a mean coronal		resembling the action of true solar flares; to support a mean coronal
-	temperature means that impulsive nanoflare heating would produce	+	temperature means [mathematically] that impulsive nanoflare heating
-	temperatures higher than the mean.	+	would produce temperatures higher than the mean.
	In other words, the instantaneous temperature distribution of the		In other words, the instantaneous temperature distribution of the
	corona would be broader than for a steady heating process that only		corona would be broader than for a steady heating process that only
	has to attain the mean coronal temperature.		has to attain the mean coronal temperature.
-	This kind of process therefore competes in an ill-understood manner	+	If the nanoflares were physically similar to ordinary flares, we
-	with our search for faint hard X-ray sources not related to solar	+	would also expect bursts of [hard X-ray bremsstrahlung].
-	magnetic activity.	+	This component of quiet-Sun emission should be detectable by an
-	It is also interesting in its own right and the subject of a great deal	+	appropriately sensitive measurement.
-	of literature (e.g. ref. [2]).	+	See reference [B] for fuller background.
	(Fig. 3 left from Hannah et al 2010)		(Fig. 3 left from Hannah et al 2010)
		+
		+	In Figure 2 we show recent observations from the [SphinX] experiment.
		+	It makes sensitive soft X-ray observations with spectral resolution
		+	somewhat better than RHESSI's, and greatly improves on the standard
		+	[GOES] soft X-ray data.
		+	The point of this figure is that the data show periods of quiet coronal
		+	emission, with no flares; even during the presence of active regions,
		+	there are apparently steady emission levels.
		+	This absence of flare-like variation suggests that the physics of
		+	nanoflares, if they exist, is quite different from that of flares.
		+	The real test will be in the spectroscopy, ie in the search for
		+	significant broadening of the temperature distribution away from
		+	the mean..
		+
		+	(Figure from the Sylwester EOS article)
	== Conclusions ==		== Conclusions ==
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	the quiet Sun, including several possible mechanisms.		the quiet Sun, including several possible mechanisms.
	RHESSI cannot improve on these limits for another several years,		RHESSI cannot improve on these limits for another several years,
-	and there are no plans now for new and more sensitive dedicated hard X-ray	+	and there are no plans now for new and more sensitive hard X-ray
	solar instrumentation (but we look forward to [FOXSI] and [NuSTAR]).		solar instrumentation (but we look forward to [FOXSI] and [NuSTAR]).
	These limits may stand as definitive ones for some time to come.		These limits may stand as definitive ones for some time to come.
	In the meanwhile it is important to study the 1-10 keV range with		In the meanwhile it is important to study the 1-10 keV range with
	available data.		available data.
		+	Because the thermal spectra are steep, spectroscopic observations
		+	are important.
	== References ==		== References ==
-	[1] Hannah et al	+	[A] ["Constraining the hard X-ray properties of the quiet sun with new
		+	RHESSI observations"]
-	[2] Klimchuk review	+	[B] ["On solving the coronal heating problem"]

---

Revision as of 17:12, 27 September 2010

Nugget
Number:	136
1st Author:	Iain Hannah
2nd Author:	Hugh Hudson
Published:	2010 September 27
Next Nugget:	TBD
Previous Nugget:	X-ray Limb
List all

Contents 1 Introduction 2 The new data 3 Nanoflares 4 Conclusions 5 References

Introduction

Earlier Nuggets, starting in 2005, have explored various facets of high-energy radiation from the quiet Sun: 1, 2, 3, 4. Now we have the definitive RHESSI measurements in hand (reference A), in the sense that the Sun has now passed (with difficulty) a [quite unusual] [solar minimum], and RHESSI data acquisition for faint exotic hard X-ray emissions has come to an end.

Reference to the earlier Nuggets cited will bring up unusual subjects such as cosmic-ray electrons, albedo neutrons, axions, and the diffuse cosmic X-ray background. Since RHESSI only obtains upper limits, we cannot say very much about these interesting things, except that they are predicted to be very faint, though not by astronomical standards. RHESSI's best observations are now completed because of competition from increasing [solar activity] (see the discussion of "nanoflares" below).

(Fig. 2 from Hannah et al 2010, upgraded with Edwards & McCracken,
RESIK, SphinX, and maybe XRT data if I can interpret them)

The new data

In Reference A we describe the full observing program for faint global hard X-ray emission from the Sun. It has resulted in better upper limits rather than in a positive detection (see Figure 1). Not only does RHESSI not detect any of the more exotic possible sources; these limits also exclude low levels of "normal" magnetic activity. We discuss this latter result the "Nanoflares" section below.

Until focusing optics, such as those of [NuSTAR] or [FOXSI] become available, these are the best hard X-ray data. Figure 1 is the final statement from RHESSI on this topic owing to the increase of [solar activity]. Other recent instruments (e.g., [SphinX]) or [MESSENGER] may improve on these limits, especially at lower energies, and we encourage the analysis of such data on the quiet Sun. As discussed below high spectral resolution may be very important in this domain, as well as good background rejection, to distinguish exotic sources from the expected magnetic signatures (see next section).

Nanoflares

The Sun has a continuous but variable level of [magnetic activity], seen for example as [plage], [flares], and [CMEs]. There is a hot [corona], brightest in [sunspot] regions, and in a well-known theory [Parker] suggested that "nanoflares" - tiny non-thermal energy releases, undetectable individually but very numerous, could merge together and explain the heating needed. In essence the nanoflare hypothesis involves episodic heating, perhaps resembling the action of true solar flares; to support a mean coronal temperature means [mathematically] that impulsive nanoflare heating would produce temperatures higher than the mean. In other words, the instantaneous temperature distribution of the corona would be broader than for a steady heating process that only has to attain the mean coronal temperature. If the nanoflares were physically similar to ordinary flares, we would also expect bursts of [hard X-ray bremsstrahlung]. This component of quiet-Sun emission should be detectable by an appropriately sensitive measurement. See reference [B] for fuller background.

(Fig. 3 left from Hannah et al 2010)

In Figure 2 we show recent observations from the [SphinX] experiment. It makes sensitive soft X-ray observations with spectral resolution somewhat better than RHESSI's, and greatly improves on the standard [GOES] soft X-ray data. The point of this figure is that the data show periods of quiet coronal emission, with no flares; even during the presence of active regions, there are apparently steady emission levels. This absence of flare-like variation suggests that the physics of nanoflares, if they exist, is quite different from that of flares. The real test will be in the spectroscopy, ie in the search for significant broadening of the temperature distribution away from the mean..

(Figure from the Sylwester EOS article)

Conclusions

This Nugget reports the best upper limits on hard X-rays from the quiet Sun, including several possible mechanisms. RHESSI cannot improve on these limits for another several years, and there are no plans now for new and more sensitive hard X-ray solar instrumentation (but we look forward to [FOXSI] and [NuSTAR]). These limits may stand as definitive ones for some time to come. In the meanwhile it is important to study the 1-10 keV range with available data. Because the thermal spectra are steep, spectroscopic observations are important.

References

[A] ["Constraining the hard X-ray properties of the quiet sun with new RHESSI observations"]

[B] ["On solving the coronal heating problem"]