title: Observations of an extreme storm in interplanetary space caused by successive coronal mass ejections.
abstract: 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 h), 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.
authors.affiliation: State Key Laboratory of Space Weather, National Space Science Center, Chinese Academy of Sciences, Beijing 100190, China.
authors.forename: Ying D
authors.initials: YD
authors.lastname: Liu
authors.affiliation: Space Sciences Laboratory, University of California, Berkeley, California 94720, USA.
authors.forename: Janet G
authors.initials: JG
authors.lastname: Luhmann
authors.affiliation: 1] Université de Toulouse, UPS-OMP, IRAP, Toulouse 31400, France [2] CNRS, IRAP, 9 Av. colonel Roche, BP 44346, F-31028 Toulouse cedex 4, France.
authors.forename: Primož
authors.initials: P
authors.lastname: Kajdič
authors.affiliation: Department of Physics, University of Helsinki, Helsinki 00100, Finland.
authors.forename: Emilia K J
authors.initials: EK
authors.lastname: Kilpua
authors.affiliation: Space Science Center, University of New Hampshire, Durham, New Hampshire 03824, USA.
authors.forename: Noé
authors.initials: N
authors.lastname: Lugaz
authors.affiliation: Lockheed-Martin Solar and Astrophysics Laboratory, Palo Alto, California 94304, USA.
authors.forename: Nariaki V
authors.initials: NV
authors.lastname: Nitta
authors.affiliation: 1] Institute of Physics, University of Graz, Graz 8042, Austria [2] Space Research Institute, Austrian Academy of Sciences, Graz 8042, Austria.
authors.forename: Christian
authors.initials: C
authors.lastname: Möstl
authors.affiliation: 1] Université de Toulouse, UPS-OMP, IRAP, Toulouse 31400, France [2] CNRS, IRAP, 9 Av. colonel Roche, BP 44346, F-31028 Toulouse cedex 4, France.
authors.forename: Benoit
authors.initials: B
authors.lastname: Lavraud
authors.affiliation: Space Sciences Laboratory, University of California, Berkeley, California 94720, USA.
authors.forename: Stuart D
authors.initials: SD
authors.lastname: Bale
authors.affiliation: Space Science Center, University of New Hampshire, Durham, New Hampshire 03824, USA.
authors.forename: Charles J
authors.initials: CJ
authors.lastname: Farrugia
authors.affiliation: Space Science Center, University of New Hampshire, Durham, New Hampshire 03824, USA.
authors.forename: Antoinette B
authors.initials: AB
authors.lastname: Galvin