title: Mercury's magnetosphere after MESSENGER's first flyby.
abstract: Observations by MESSENGER show that Mercury's magnetosphere is immersed in a comet-like cloud of planetary ions. The most abundant, Na+, is broadly distributed but exhibits flux maxima in the magnetosheath, where the local plasma flow speed is high, and near the spacecraft's closest approach, where atmospheric density should peak. The magnetic field showed reconnection signatures in the form of flux transfer events, azimuthal rotations consistent with Kelvin-Helmholtz waves along the magnetopause, and extensive ultralow-frequency wave activity. Two outbound current sheet boundaries were observed, across which the magnetic field decreased in a manner suggestive of a double magnetopause. The separation of these current layers, comparable to the gyro-radius of a Na+ pickup ion entering the magnetosphere after being accelerated in the magnetosheath, may indicate a planetary ion boundary layer.
authors.affiliation: Heliophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA. james.a.slavin@nasa.gov
authors.forename: James A
authors.initials: JA
authors.lastname: Slavin
authors.forename: Mario H
authors.initials: MH
authors.lastname: Acuña
authors.forename: Brian J
authors.initials: BJ
authors.lastname: Anderson
authors.forename: Daniel N
authors.initials: DN
authors.lastname: Baker
authors.forename: Mehdi
authors.initials: M
authors.lastname: Benna
authors.forename: George
authors.initials: G
authors.lastname: Gloeckler
authors.forename: Robert E
authors.initials: RE
authors.lastname: Gold
authors.forename: George C
authors.initials: GC
authors.lastname: Ho
authors.forename: Rosemary M
authors.initials: RM
authors.lastname: Killen
authors.forename: Haje
authors.initials: H
authors.lastname: Korth
authors.forename: Stamatios M
authors.initials: SM
authors.lastname: Krimigis
authors.forename: Ralph L
authors.initials: RL
authors.lastname: McNutt
authors.forename: Larry R
authors.initials: LR
authors.lastname: Nittler
authors.forename: Jim M
authors.initials: JM
authors.lastname: Raines
authors.forename: David
authors.initials: D
authors.lastname: Schriver
authors.forename: Sean C
authors.initials: SC
authors.lastname: Solomon
authors.forename: Richard D
authors.initials: RD
authors.lastname: Starr
authors.forename: Pavel
authors.initials: P
authors.lastname: Trávnícek
authors.forename: Thomas H
authors.initials: TH
authors.lastname: Zurbuchen