The proposed observations are in support of the upcoming launch of NASAfs FOXSI (Focusing Optics X-ray Solar Imager) sounding rocket. FOXSI will provide hard X-ray imaging observations in the 5 – 15 keV range at a 50 times improved sensitivity compared to RHESSI. This will allow us to investigate the low-level hard X-ray emission that is predicted in some coronal heating scenarios.
The current launch data is confirmed for November 2; the exact launch time is TBC in the next weeks. During the 6-minute FOXSI flight, two different targets will be observed. 1) The first minute will be spent on observing a non-flaring active region (the FOXSI field-of-few is 10x10 arcmin). This will ensure that we will get enough counts to test if the hard X-ray focusing optic works properly. Additionally, it will also allow us to study non-flaring hard X-ray emissions from active regions. 2) During the remaining 5 minutes, FOXSI will point to the quiet Sun and polar region to search for hard X-ray emissions from so-called enetwork flaresf (e.g. Krucker et al. 1996).
Coronal heating is one of the main unsolved problems in Heliophysics. Low-level energy releases at high occurrence rates (so-called nanoflares) are one of the proposed mechanisms to heat the corona (e.g. Hannah et al. 2012). While large flares are known to be an insufficient energy input to heat the solar corona, low-level events are a hot topic discussed in many studies providing different, often controversial results. For active region, Schmelz et al 2009 reported the existence a hot (~8 MK) component, while Warren et al. 2011 does not observe such a signature. While EUV and soft X-ray observations are providing a good estimate of the several-million-degree plasma, hard X-ray observations are uniquely suited to observe hot plasma (typically above ~6 MK). Additionally, non-thermal bremsstrahlung emissions can be efficiently detected. The science objectives for the two different points of FOXSI are as following:
For the active region observations, we will be able to search for a non-flaring hot component (a component such as reported in Schmelz et al. would produce a clear signal in the FOXSI data), or at least find a new upper limit. We ask for supporting observations from EIS and XRT to characterize the few-million-degree plasma.
For the quiet Sun, FOXSI will be able to search for a non-thermal signature of network flares. The absence of such a signature would clearly show that the quiet Sun network flares do not accelerate electrons in the same way as regular flares. We ask for supporting observations from EIS and XRT to observe the thermal emissions associated with network flares.
Furthermore, we ask for SOT observations to provide magnetic context observations. In particular for the quiet Sun it is important to see if the observed hard X-ray events are indeed associated with the strongest magnetic fields.