Is it permissible to sit with one's back muscles and head in the shade and the rest of the body in the sun for treatment, given that there is a prophetic prohibition against sitting between the sun and the shade?

Sitting between the shade and the sun is forbidden, and the Prophet, peace and blessings be upon him, described it as "the sitting place of Satan." However, the prohibition is understood by scholars to be one of dislike (makrooh), especially since the Prophet, peace and blessings be upon him, sat between the shade and the sun on another occasion. The dislike is removed in cases of necessity, so there is no harm in sitting in this manner for medicinal purposes.