2007/02/27 by I. G. Hannah, Iain G. Hannah, G. J Hurford +4 · 1 citation
Physics and Astronomy · #Astrophysics and Cosmic Phenomena #Dark Matter and Cosmic Phenomena #Solar and Space Plasma Dynamics #astro-ph
paper · pdf · doi:10.1086/516750
published as Astrophys.J.659:L77-L80,2007 · 4 pages, 3 figures, Accepted by ApJ letters
arxiv created 2007/02/27 · openalex publication_date 2007/03/09 · arxiv updated 2009/12/01 · openalex created_date 2016/06/24 · openalex updated_date 2026/07/31
We present the first results using the Reuven Ramaty High-Energy Solar Spectroscopic Imager ( RHESSI ) to observe solar X-ray emission not associated with active regions, sunspots, or flares (the quiet Sun). Using a newly developed chopping technique (fan-beam modulation) during seven periods of offpointing between 2005 June and 2006 October, we obtained upper limits over 3-200 keV for the quietest times when the GOES 12 1-8 Å flux fell below 10 -8 W m -2 . These values are smaller than previous limits in the 17-120 keV range and extend them to both lower and higher energies. The limit in 3-6 keV is consistent with a coronal temperature ≤6 MK. For quiet-Sun periods when the GOES 12 1-8 Å background flux was between 10 -8 and 10 -7 W m -2 , the RHESSI 3-6 keV flux correlates to this as a power law, with an index of 1.08 ± 0.13. The power-law correlation for microflares has a steeper index of 1.29 ± 0.06. We also discuss the possibility of observing quiet-Sun X-rays due to solar axions and use the RHESSI quiet-Sun limits to estimate the axion-to-photon coupling constant for two different axion emission scenarios.