The minimum (ordinary) form of Ātāpanā (exposure of naked body to sun-rays by sitting on the sun-baked ground) - The Ātāpanā undertaken in the standing postures like Hastisuṇḍikā, Ekapādikā, and Samapādikā.
ūrdhvastliitasya jaghanya......ūrdhvasthānātāpanā'pi tridhā hastiśauṇḍikā ekapādikā samapādikā ceti.
(Aupa Vr Pa 75)
See - Utkṛṣṭa Ātāpanā.