Exposure of body to intense sun- A kind of austerity, which is practised by exposing one's naked body to intense sun-rays by sitting in front of sun in various (yogic) postures, on the sun-baked ground, in order to develop the power of Taijas Sarīra (fiery body) through appropriation of solar heat (energy).
...sūrābhimuhassa āyāvaṇabhūmīe āyāvemāṇassa viharittae...
(Bhaga 11.59)