Studying the Sun with Daksha
The Daksha satellite configuration and orbit is such that there are four ME packages on each satellite that continuously observe the Sun. This gives a very high total effective area of 2400 cm² for solar studies in the 20-200 keV range: the highest to date.
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Solar flares are sudden releases of energy in the solar atmosphere leading to emission across the entire electromagnetic spectrum and often release of ener- getic particles into the interplanetary medium. According to the standard flare model, the underlying mechanism powering the flares is magnetic reconnection that leads to acceleration of particles into non-thermal distributions and also heating of the plasma to temperatures often exceeding 10 MK (Benz, 2017). While the standard flare model picture explains the observations in a broader context, several details such as the acceleration mechanism are still not well understood. As the accelerated electrons emit in hard X-rays by non-thermal bremsstrahlung, observations of the hard X-ray spectrum provide the most direct diagnostics of the non-thermal electron population (Krucker et al., 2008). By modeling the observed hard X-ray spectrum, the distribution of the non- thermal electron population as well as quantitative estimates of their total energy content can be obtained. Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI, Lin et al., 2002) that observed the Sun in hard X-rays for 16 years until 2018 provided wealth of information on particle acceleration in solar flares with its broad band spectroscopic and imaging observations. RHESSI could observe non-thermal emission up to few tens of keV for flares down to GOES B-class intensities (Hannah et al., 2008); however, it was not possible to extend this to lower intensity flares.
Daksha + Sun
Daksha, with its Sunward ME packages, will provide measurements of hard X-ray spectra of solar flares in 20–200 keV energy band. With 4 ME packages in the Sunward direction, Daksha will have about an order of magnitude larger effective area than that of RHESSI in this energy range. With the added ad- vantage of simultaneous background measurements from other faces, Daksha is expected to have much better sensitivity than RHESSI for solar flare spec- tra. Simultaneous observations of flares by Daksha with instruments at other vantage points such as the Spectrometer/Telescope for Imaging X-rays (STIX, Krucker et al., 2020) on Solar Orbiter also provides the opportunity to probe hard X-ray directivity.