Solar Phys (2017) 292:109
EARTH-AFFECTING SOLAR TRANSIENTS
Observation of an Extremely Large-Density
Heliospheric Plasma Sheet Compressed
by an Interplanetary Shock at 1 AU
· Kan Liou
· R.P. Lepping
· Simon Plunkett
· Dennis Socker
Received: 16 September 2016 / Accepted: 19 May 2017
© Springer Science+Business Media Dordrecht (outside the USA) 2017
Abstract At 11:46 UT on 9 September 2011, the Wind spacecraft encountered an inter-
planetary (IP) fast-forward shock. The shock was followed almost immediately by a short-
duration (∼ 35 minutes) extremely dense pulse (with a peak ∼ 94 cm
). The pulse induced
an extremely large positive impulse (SYM-H = 74 nT and Dst = 48 nT) on the ground.
A close examination of other in situ parameters from Wind shows that the density pulse was
associated with i) a spike in the plasma β (ratio of thermal to magnetic pressure), ii) mul-
tiple sign changes in the azimuthal component of the magnetic ﬁeld (B
), iii) a depressed
magnetic ﬁeld magnitude, iv) a small radial component of the magnetic ﬁeld, and v) a large
(> 90°) change in the suprathermal (∼ 255 eV) electron pitch angle across the density pulse.
We conclude that the density pulse is associated with the heliospheric plasma sheet (HPS).
The thickness of the HPS is estimated to be ∼ 8.2 × 10
km. The HPS density peak is
about ﬁve times the value of a medium-sized density peak inside the HPS (∼ 18 cm
1 AU. Our global three-dimensional magnetohydrodynamic simulation results (Wu et al. in
J. Geophys. Res. 212, 1839, 2016) suggest that the extremely large density pulse may be the
result of the compression of the HPS by an IP shock crossing or an interaction between an
interplanetary shock and a corotating interaction region.
Earth-affecting Solar Transients
Guest Editors: Jie Zhang, Xochitl Blanco-Cano, Nariaki Nitta, and Nandita Srivastava
S.T. Wu is deceased.
Naval Research Laboratory, Washington, DC, 20375, USA
Johns Hopkins Applied Physics Laboratory, Laurel, Maryland, 20723, USA
NASA/GSFC (Ret.), Greenbelt, Maryland, USA
IAASARS, Observatory of Athens, Athens, Greece
CSPAR, University of Alabama, Huntsville, Alabama, USA