The paper presents a solution to the two-century old problem of how solar activity influences biological objects on Earth. It gives a description of the modern state of the kT-problem, which for a long time has been the most difficult obstacle in the way of explaining solar activity effects. Based on recent advances in spin chemistry, magnetoplasticity physics, and physics of critical conditions, it is shown that a "molecular target" sensitive to weak electromagnetic fields and corresponding radio emissions of the Sun has spin dynamics in non-equilibrium and is near the lower critical point of dividing into layers. A way is proposed as to how solar activity can have an influence on Earth's molecular, including biological, processes through a "transparency window" of the Earth's atmosphere at the 80Mhz frequency.