Solenoid emits electromagnetic radiation, why? A sinusoidal electric current is applied to the solenoid. The solenoid emits electromagnetic radiation, why?
 A: You may theoretically calculate it as magnetic dipole radiation. However, in physics, the "why" works in the opposite direction from what you assume. Classical electromagnetism allows you to calculate things like this because it is crafted to mathematically reproduce results from experiments.
A: An electric current is applied to the solenoid.
The coil radiates because the moving electrons are guided in a circle (spiral path) and any acceleration of the electrons - and movement in a circle is such - is accompanied by the conversion of kinetic energy into EM radiation.
A sinusoidal electric current is applied to a conducting wire.
The conduction radiates because the electrons are constantly being moved back and forth and these accelerations are accompanied in nature by a conversion of kinetic energy into EM radiation.
Why?
The process is more complex. In both described cases, a magnetic field also appears. Free electrons have an electric field and a magnetic field. Electrons with kinetic energy are deflected sideways in an external magnetic field and radiate. The deflection is associated with a spin. This spin is aligned parallel or anti-parallel to the intrinsic magnetic field of the subatomic particles. An established theory of how these properties lead to the emission of photons is not available.
