This paper presents a quantitative assessment of high‐latitude energy input and its partitioning in the polar cap by synthesizing various space and ground‐based observations during the 17 January 2005 geomagnetic storm. It was found that Joule heating is the primary form of magnetospheric energy input, especially during active times when the hemispheric‐integrated Joule heating can be an order of magnitude larger than the hemispheric‐integrated auroral power. Most of magnetospheric energy is dissipated in the auroral zone rather than in the polar cap. On average, only about 22–25% of the total hemispheric energy input is dissipated into the polar cap region bordered by the convection reversal boundary (CRB) and the poleward auroral flux boundary (FXB). The impact of high‐latitude energy input was also investigated to unveil the causal relationship between Joule heating and the formation of polar cap mass density anomalies. Our numerical simulation demonstrated that thermosphere dynamics readily redistributes composition, temperature, and mass through upwelling and atmospheric gravity waves. The polar cap mass density anomalies observed by the CHAMP satellite during the storm were largely a result of large‐scale atmospheric gravity waves. Therefore, an increase in local thermospheric mass density does not necessarily mean there is direct energy input. Key Points The polar cap is not the primary location of high‐latitude energy input Thermospheric mass density anomalies do not always coincide with direct local energy dissipation Thermosphere dynamics redistributes mass globally through gradient‐wave adjustment