Kavli Affiliate: Tadayuki Takahashi
| First 5 Authors: Juan Camilo Buitrago-Casas, Lindsay Glesener, Steven Christe, Säm Krucker, Juliana Vievering
| Summary:
Solar nanoflares are small eruptive events releasing magnetic energy in the
quiet corona. If nanoflares follow the same physics as their larger
counterparts, they should emit hard X-rays (HXRs) but with a rather faint
intensity. A copious and continuous presence of nanoflares would deliver
enormous amounts of energy into the solar corona, possibly accounting for its
high temperatures. To date, there has not been any direct observation of such
sustained and persistent HXRs from the quiescent Sun. However, Hannah et al. in
2010 constrained the quiet Sun HXR emission using almost 12 days of quiescent
solar-off-pointing observations by RHESSI. These observations set upper limits
at $3.4times 10^{-2}$ photons$^{-1}$ s$^{-1}$ cm$^{-2}$ keV$^{-1}$ and
$9.5times 10^{-4}$ photons$^{-1}$ s$^{-1}$ cm$^{-2}$ keV$^{-1}$ for the 3-6
keV and 6-12 keV energy ranges, respectively. Observing feeble HXRs is
challenging because it demands high sensitivity and dynamic range instruments
in HXRs. The Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket
experiment excels in these two attributes. Particularly, FOXSI completed its
third successful flight (FOXSI-3) on September 7th, 2018. During FOXSI-3’s
flight, the Sun exhibited a fairly quiet configuration, displaying only one
aged non-flaring active region. Using the entire $sim$6.5 minutes of FOXSI-3
data, we constrained the quiet Sun emission in HXRs. We found $2sigma$ upper
limits in the order of $sim 10^{-3}$ photons$^{-1}$ s$^{-1}$ cm$^{-2}$
keV$^{-1}$ for the 5-10 keV energy range. FOXSI-3’s upper limit is consistent
with what was reported by Hannah et al., 2010, but FOXSI-3 achieved this result
using $sim$1/2640 less time than RHESSI. A possible future spacecraft using
FOXSI’s concept would allow enough observation time to constrain the current
HXR quiet Sun limits further or perhaps even make direct detections.
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