Timing of changes in the solar wind energy input in relation to ionospheric response

We investigate four consecutive substorms using observations and global MHD simulation code GUMICS-4. The solar wind energy input is estimated by the epsilon parameter and evaluated from the simulation by direct integration of the energy transfer through the magnetopause. The ionospheric dissipation is estimated using the AE index and also evaluated by computing the Joule heating from both hemispheres in the simulation. Our results show that there is a clear and repeatable time delay between changes in the solar wind parameters and the energy input through the magnetopause. In the simulation, the Joule heating is quite directly driven by the energy input through the magnetopause, without any significant delays. The AE index shows two distinct responses: when the energy input through the magnetopause increases, the AE index responds with a delay; this can be interpreted as the growth phase. When the energy input decreases, the AE decreases simultaneously, without any significant delay. This indicates that after the onset, the energy input to the ionosphere is directly controlled by the energy input through the magnetopause. The results suggest that the recovery phase timing is determined by the solar wind parameters (with a delay associated with the magnetopause processes), while the onset timing is associated with internal magnetotail and/or magnetosphere-ionosphere coupling processes.