RHESSI Science Nuggets/Quiet Sun III
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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 axions, cosmic-ray electrons, albedo neutrons, 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 not to be very faint by non-solar astronomical standards. RHESSI's best observations are now completed, at least for the next few years, 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. We have not made any great discovery - no axions yet - but the RHESSI data have provided the best upper limits to date (Figure 1). Until focusing optics, such as those of [Nustar] or [FOXSI] become available, these are the best hard X-ray data.
(Figure from the Sylwester EOS article?)
This 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 thermal signatures (see next section).
But the Sun has a continuous level of [magnetic activity], culminating in sunspots, plage, flares, and CMEs. In particular [nanoflares] are an attractive scenario that might solve the [coronal heating] problem. In essence the nanoflare hypothesis involves episodic heating, perhaps resembling the action of true solar flares; to support a mean coronal temperature means 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. This kind of process therefore competes in an ill-understood manner with our search for faint hard X-ray sources not related to solar magnetic activity. It is also interesting in its own right and the subject of a great deal of literature (e.g. ref. ).
(Fig. 3 left from Hannah et al 2010)
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.
 Hannah et al
 Klimchuk review