Solar studies

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.

For Astronomers

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Solar Flares

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.