Geoscience Reference
In-Depth Information
ENA SIGNALS COMING FROM THE
HELIOSPHERIC BOUNDARIES
K. C. HSIEH
Department of Physics, University of Arizona
Tucson, AZ 85721, USA
hsieh@physics.arizona.edu
Being the only article in this volume concerning energetic neutral atoms (ENA)
coming from the heliospheric boundaries, this will be a mini-review, beginning
with a brief introduction to the subject, and then highlighting the current
status on observation and modeling in this field. We conclude with a view for
future investigations.
1. Introduction
Our Sun moves inside a local interstellar cloud (LIC).
1
Pressure balance
between the radially expanding supersonic solar wind (SW) and the imping-
ing LIC plasma creates boundary layers that separate the plasmas, each
carrying its own imbedded magnetic fields.
2
The region of supersonic SW
is like a bubble bounded by the termination shock (TS), at which SW
becomes subsonic and the magnetic field more intense. The subsonic solar
plasma fills the heliosheath (HS), the region bounded by TS on the inside
and the heliopause (HP) on the outside, separating the solar plasma from
the LIC plasma. Sun's motion relative to LIC compresses HS in the apex or
upwind direction, i.e., the direction of Sun's relative motion, and stretches
HS into a tail in the anti-apex or downwind direction. The subsonic solar
plasma in HS is convected from the apex to the tail region, while the LIC
plasma flows outside HP in the same direction. Away from the apex of
HP, the LIC plasma may form a bow wave or bow shock, depending on
the ratio of Sun's speed relative to the local interstellar medium (LISM)
to the speed of sound therein. We note that terms “LIC” and “LISM” are
used interchangeably in the literature. The regions and boundaries of the
heliosphere described above are shown schematically in Fig. 1.
Unlike the supersonic SW in the bubble and the subsonic solar plasma
in the HS, which are predominantly charged particles, LIC plasma has a sig-
nificant neutral component.
3
Unaffected by magnetic field, neutral atoms of
367
