First limits on the 3-200 keV X-ray spectrum of the quiet Sun using RHESSI

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 June 2005 to October 2006, we obtained upper limits over 3-200 keV for the quietest times when the GOES12 1-8A flux fell below

10^{-8}

Wm

^{-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

\leq 6

MK. For quiet Sun periods when the GOES12 1-8A background flux was between

10^{-8}

Wm

^{-2}

and

10^{-7}

Wm

^{-2}

, the RHESSI 3-6 keV flux correlates to this as a power-law, with an index of

1.08 \pm 0.13

. The power-law correlation for microflares has a steeper index of

1.29 \pm 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.

SUN X-RAYS
SUN GAMMA-RAYS
SUN ACTIVITY
SUN CORONA
ELEMENTARY PARTICLES

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