  Space weather refers to dynamic conditions on the Sun and in the space
environment of the Earth, which are often driven by solar eruptions and their
subsequent interplanetary disturbances. It has been unclear how an extreme
space weather storm forms and how severe it can be. Here we report and
investigate an extreme event with multi-point remote-sensing and in-situ
observations. The formation of the extreme storm showed striking novel
features. We suggest that the in-transit interaction between two closely
launched coronal mass ejections resulted in the extreme enhancement of the
ejecta magnetic field observed near 1 AU at STEREO A. The fast transit to
STEREO A (in only 18.6 hours), or the unusually weak deceleration of the event,
was caused by the preconditioning of the upstream solar wind by an earlier
solar eruption. These results provide a new view crucial to solar physics and
space weather as to how an extreme space weather event can arise from a
combination of solar eruptions.
