The thermosphere is the layer of the Earth's atmosphere directly above the mesosphere. The exosphere is above that but is a minor layer of the atmosphere. Within this layer of the atmosphere, ultraviolet radiation causes photoionization / photodissociation of molecules, creating ions in the ionosphere. The radiative properties of UV rays cause an imbalance of positive and negative energy, creating ions. Taking its name from the Greek (pronounced thermos) meaning heat, the thermosphere begins about 85 kilometres (53 mi) above the Earth. At these high altitudes, the residual atmospheric gases sort into strata according to molecular mass (see turbosphere). Thermospheric temperatures increase with altitude due to absorption of highly energetic solar radiation. Temperatures are highly dependent on solar activity, and can rise to 2,000 C (3,630 F). Radiation causes the atmosphere particles in this layer to become electrically charged (see ionosphere), enabling radio waves to be refracted and thus be received beyond the horizon. In the exosphere, beginning at 500 to 1,000 kilometres (310 to 620 mi) above the Earth's surface, the atmosphere turns into space, although by the criteria set for the definition of the Karman line, the thermosphere itself is part of space.

The source of heating in the thermosphere is the absorption of? Okie... think carefully first, then make a decision:
To answer the question, consider the following: Thermospheric temperatures increase with altitude due to absorption of highly energetic solar radiation. So, the answer is highly energetic solar radiation.