Schematic representation of the solar plasma model. The surface S, located at the base of the transition region, acts as the interface between the fully collisional chromosphere (serving as a thermal reservoir) and the collisionless coronal plasma. Localized heating events, each occupying an area s_H, are shown in black. The coronal plasma above is embedded in a uniform magnetic field B (green), while particles are subject to a net external force g(m_p + m_e)/2 (black), which combines gravitational field and the Pannekoek–Rosseland electric field. The Cartesian reference frame (x, y, z) used throughout the paper is indicated in the top right.