The Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission investigates the coupling between the solar wind and Earth's magnetosphere using five identical spinning spacecraft. Each spacecraft rotates at a predefined rate to stabilize it and carries a suite of instruments, including a fluxgate magnetometer (FGM) that measures the three-dimensional magnetic field. Since May 24, 2024, the FGM on board THEMIS E has no longer recorded the magnetic field component along the spin axis, while the two components approximately within the spin plane remain fully available.</p> <p>We present a method to reconstruct the missing magnetic field component using data from these two remaining components together with the spacecraft spin, yielding full three-component magnetic field vectors at spin-period resolution. The approach was validated using original THEMIS E data from 2017, when all three sensor components were available. The width of the residual distributions between reconstructed and original measurements is approximately 4 nT in the low-field magnetosphere, 12 nT in the high-field magnetosphere, 10 nT near magnetopause crossings, 14 nT in the magnetosheath, 6 nT in the quasi-parallel solar wind, and 3 nT in the quasi-perpendicular solar wind.</p> <p>These results demonstrate that on a spinning spacecraft, two components that are almost within but slightly out of the spin plane are sufficient to reconstruct three-dimensional magnetic field vectors at spin-period resolution, allowing continued magnetospheric observations despite the loss of one sensor component.