We estimate the global impact of storms on the global structure and dynamics of the nightside plasma sheet (PS) from observations by the NASA mission THEMIS. We focus on an intense storm occurring in December 2015 triggered by interplanetary coronal mass ejections (ICMEs). It starts with a storm sudden commencement (SSC) phase (SYM-H~+50nT) followed by a growth phase (SYM-H~-188nT at the minimum) and then a long recovery phase. We investigate THEMIS observations when the spacecraft were located in the midnight sector of the PS at distances typically between 8 and 13RE. It is found that the PS has been globally compressed up to a value of about~>4nPa during the SSC and main phases, i.e. 8 times larger than its value during the quiet phase before the event. This compression occurs during periods of high dynamic pressure in the ICME (20nPa) about one order of magnitude larger than its value in the pristine solar wind. We infer a global increase of the lobe magnetic field from 30nT to 100nT, confirmed by THEMIS data just outside the PS. During the SSC and main phases, the PS is found thinner by a factor of 2 relative to its thickness at quiet times, while the Tsyganenko T96 magnetic field model shows very stretched magnetic field lines from inner magnetospheric regions toward the nightside. During the recovery phase, whereas the interplanetary pressure has dropped off, the PS tends to gradually recover its quiet phase characteristics (pressure, thickness, magnetic configuration, etc) during a long recovery phase.

The interaction of a solar wind discontinuity with the backstreaming particles of the Earth’s ion foreshock can generate hot, tenuous plasma transients such as foreshock bubbles (FB) and hot flow anomalies (HFA). These transients are known to have strong effects on the magnetosphere, distorting the magnetopause (MP), either locally during HFAs or globally during FBs. However, previous studies on the global impact of FBs have not been able to determine whether the response stems directly from the transverse scale size of the phenomenon or its fast motion over the magnetosphere. Here we present the observation of an FB and its impact on the magnetosphere from different spacecraft scattered over the dayside magnetosphere. We are able to constrain the size of the transverse scale of an FB from direct observations to be about 10 $R_\mathrm{E}$. We further suggest that a combination of this scale and the motion of the FB over the MP is responsible for the previously reported global response of the dayside magnetosphere.