Solar Wind Streams and Their Interactions
A. Lazarus 1, J. Kasper 1, A. Szabo 2, K. Ogilvie 2
1
MIT Center for Space Research, Cambridge, MA;
2
NASA/GSFC, Greenbelt, MD
Abstract. We present observations of solar wind parameters and magnetic fields from the Wind spacecraft during 1995. We
concentrate on periods of interaction between fast and slow streams. We look at speeds, flow angles, alpha particle
concentrations, and specific entropy to identify stream interfaces.
well-known parameters, but some puzzles remain:
OBSERVATIONS
Figure 1 shows 62 days of observations from the
Wind spacecraft which include at least three stream
interfaces (indicated by “SI”). The time periods
around each SI (A, B, and C) are expanded in Figures
2, 3, and 4. The three interfaces were initially chosen
on the basis of the return of the flow angles to zero
degrees.
Note the following well-known features related to
the interaction between two streams of solar wind:
•
An increase in density before the arrival of
the higher-speed wind.
•
The increase in thermal speed after the
interface.
•
The flow deflections: (slower wind coming
first from the East and then from the West)
as would be expected from a faster stream
colliding with slower wind; the N/S flow
angle change is less well understood.
•
A sudden increase in entropy per proton
(T/N1/2 ) after the interface [1].
1.
Though the East/West deflection of the streams
is expected from co-rotating structures, what
causes the North/South deflection? Is it due to
streams at different latitude?
2.
Although an increase in entropy is expected
from the interaction between streams, our
definition of entropy may be too limited in that
it does not take into account the obvious
compression of the magnetic field and
associated fluctuations in the region of the
interface. The sharp increase in entropy, as
defined, is consistent with the typical lower
density and higher temperature for higher speed
wind. Neugebauer et al. [2] report a decrease in
specific entropy before the interface; we do not
observe such a decrease.
The change in alpha particle content and relation of
the SI to magnetic field direction relative to the Parker
spiral are clearly visible in the detailed plots.
SUMMARY AND QUESTIONS
Identification of interfaces is accomplished using
CP679, Solar Wind Ten: Proceedings of the Tenth International Solar Wind Conference,
edited by M. Velli, R. Bruno, and F. Malara
© 2003 American Institute of Physics 0-7354-0148-9/03/$20.00
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FIGURE 1. Solar wind parameters for 61 days in 1995.
Stream interfaces are indicated by “SI”.
FIGURE 3. A detailed view of interface “B”. In this case,
there is no magnetic sector boundary.
FIGURE 2. A detailed view of interface “A”. Note the
increase in the magnitude of the field surrounding the “SI”.
FIGURE 4. A detailed view of interface “C”. The major
change in flow direction is in the North/South direction.
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ACKNOWLEDGEMENTS
This work was supported in part by NASA/Goddard
under Grant NAG-10915. We appreciate the hard
work of the organizers of the Solar Wind 10
Conference.
REFERENCES
1.
Siscoe, G and D. Intriligator, Geophys. Res. Lett.,
20, 2267-2270, 1993.
2.
Neugebauer, M., P.C. Liewer, E.J. Smith,
R.M. Skoug, and T.H. Zurbuchen, J. Geophys.
Res., in press, 2002.
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