Solar Wind Sources and Flow Structure Over the 1995-2000
Period
N.A.Lotova, V.N.Obridko
IZMIRAN, Troitsk, Moscow Region, 142190 Russia
tel.: (7-095)3340902, (7-095)3340282 / fax: (7-095)3340124
e-mail: [email protected] / [email protected]
K.V.Vladimirsky
Lebedev Physical Institute, Moscow, 117924 Russia
tel.: (7-095)9382251 / fax: (7-095)1326274
e-mail: [email protected]
Abstract. Evolution of the large-scale stream structure of the solar wind flow is studied in the main acceleration zone at 10 to 40
solar radii from the Sun. Three independent sets of the experimental data were used: observations of the radio wave scattering
using the large radio telescopes of the Lebedev Physical Institute, white solar corona images obtained with the SOHO spacecraft,
and solar magnetic field strength computed from J.Wilcox Solar Observatory data. The positions of the transonic region of the
solar wind flow derived from the radio astronomical observations data were used as a parameter reflecting the intensity of the
solar wind acceleration process. Correlation studies of these data with the magnetic field strength in the solar corona permit us to
reveal several different types of the solar wind streams. The 1995--2000 data show important changes in the solar corona
magnetic fields and corresponding changes of the solar wind flow.
Transonic area studies are very important because it is
an erea of basic solar wind acceleration [Bird and
Edenhofer, 1990]. An insight into the mechanism of
the solar wind streams formation was obtained from
correlation plots of Rin values combined with the
magnetic field intensity |BR| at the source area of the
solar wind, at the start of the acceleration process
(Figure 1a-d). As can be seen from Fig. 1, correlation
relations Rin, |BR| fall into several branches on each
diagram. Different types of the solar wind streams
correspond to each of these branches, differing in the
source conditions and in the acceleration progress. The
nature of these differences was revealed by the more
detailed study of both magnetic field and flow
structures, by calculations giving the magnetic field
topology, and by direct flow observations with the
SOHO coronographs. Investigation of the stream
structure of the flow seems to be one of the most
important problems in studies of the real mechanisms
of the supersonic solar wind formation. It is enough to
remind that the existence of an analogous structure of
the flow in nonconducting media is totally impossible;
INTRODUCTION
Evolution of the solar wind sources and flow stream
structure is studied on the basis of regular radio
scattering observations with compact natural sources
[Lotova et al., 1985; 1995; 1998; 2000a,b; 2002a,b].
Large radio telescopes of the Lebedev Physical
Institute, Pushchino, were used (DCR-1000 at 103
MHz and RT-22 at 22.2 GHz). Apparent movement of
the sources permits us to obtain in lasting about a
month series of daily observations a radial dependence
of the scattering. Characteristic, repeating shape of this
dependence reveals an area of increased scattering,
which is identified as a transition, transonic region
[Lotova et al., 1985]. Internal boundary of this area Rin
is used as a natural characteristic of the solar wind
acceleration process intensity: an increased level of
acceleration corresponds to a closer to the Sun location
of the transonic region.
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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interaction of a submerged jet with the surrounding
liquid is characterized there by a conic structure of the
flow with an opening angle of 25° [Landau and
Lifshitz, Hydrodynamics]. In near-solar plasmas,
streams of magnetized plasmas with an opening angle
of about 5° or less can be observed. The cause for such
a violent difference are the well-known effects of
frozen-in fields, but the details of these processes are
very complicated and not yet studied up to now.
pairs correspond to the same streamline of the flow.
The second form is topology of the magnetic field in
the neighbourhood of the same points, open type with
field lines going into the space, or closed with loopshape field lines. The difference between these two
types is very important one concerning the frozen-in
field phenomena.
Solar magnetic fields are doubtless decisive factor in
formation of the solar wind streams. Nevertheless,
optical observations of the white solar corona
structure, INTERNET data obtained with LASCO C3
and LASCO C2 coronographs of the SOHO
spacecraft, were of invaluable help in understanding
the nature of some types of the solar wind streams. In
comparisons of the single-momented optical
observations with the Rin data obtained in long series
of the radio astronomical observations the temporal
and space coordinates were matched with the moments
of the Rin determinations, which ensures that both
radio and optical data belong to the same stream of the
solar wind.
SOURCE AREA DATA
Solar magnetic fields were calculated in a narrow area
R ~ (1-2.5)Rs disregarding possible variations
introduced by the medium. Parameters of the solar
wind plasma in this area, density of free electrons ≤108
cm-3 [Guhathakurta and Sitter, 1999], support this
approach. Real hard point is incompleteness of the
initial data. J.Wilcox Solar Observatory results for R =
Rs, solar surface, were used. These Zeeman
measurements give a modulus, not a vector of the
field, some interpolation methods were used to
overcome this trouble and deficient density of the
BASIC RESULTS
measuring data [Hoeksema et al., 1982,1983; Obridko
and Shelting, 1992, 1999; Lotova et al., 2002]. The
results of calculations were presented in two forms.
The |BR| used in Fig. 1 is the radial component of the
field vector at the specified points of the sphere R =
2.5 Rs; angular coordinates determined so that Rin, |BR|
The use of three independent diagnostic methods
permitted us to isolate four characteristic types of the
solar wind streams differing in the source conditions
and in the progress of acceleration, two high-speed and
two low-speed ones. Repeatedly presented in
observations are high-speed streams designated in Fig.
111
Guhathakurta M. and Sitter E. (1999): in Proc. 9th Int.
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(Eds.), New York, p. 79, 1999.
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Lotova N.A., Blums D.F., and Vladimirskii K.V.
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Lotova N.A., Vladimirskii K.V., and Obridko V.N.
(2000a): Phys. Chem Earth ©, Vol. 25, p.
121, 2000a.
Lotova N.A., Obridko V.N., and Vladimirskii K.V.
(2000b): Astron. Astrophys., Vol. 357, p.
1051, 2000b.
Lotova N.A., Obridko V.N., and Vladimirskii K.V.
(2002a): Astron Zh. (in Russian) (in press).
Lotova N.A., Obridko V.N., Bird M.K., and Janardhan
P. (2002b): Solar Phys. (in press).
Obridko V.N. and Shelting B.D. (1992): Solar Phys.,
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Obridko V.N. and Shelting B.D. (1999): Solar Phys.,
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1 with triangles. They are characterized by weak or
moderate field strength and open field structure in the
source area. In the white corona structure this group of
streams originates in local coronal holes or in side
lobes of the streamers.
Quite different type of high-speed streams was
observed in [Lotova et al., 200b] (diamonds in Fig. 1).
They start from polar coronal holes, from strong open
magnetic field and are only observable at the minima
of solar activity, when polar coronal holes reach
medium solar latitudes.
The filled circles in Fig. 1 designate low-speed
streams. This group corresponds to the closed looplike structure of the magnetic field lines and to the
main body of the streamers.
The slowest type of the streams, which corresponds to
the farthest from the Sun position of the transonic area,
is designated in Fig. 1 by open circles. For this type of
the streams, a mixed type of the field-line structure is
characteristic, in which the field lines going into the
space alternate with the loop-like lines. In the white
corona structure, wide areas of intense, structureless
luminosity are observed.
On the whole, the correlation plots in Fig. 1
characterize evolution of the process of the solar wind
stream formation during the first half of the solar
activity cycle no. 23. The completeness of the results
is to a great extent due to availability of the SOHO
data for these years. The year-to-year differences in
the flow structure are well pronounced. In the year of
solar activity minimum, 1995, the high-speed streams
prevail due to the open configuration of the magnetic
field. In 2000, at the maximum of solar activity, on the
contrary, low-speed streams dominate. The dominating
role of the magnetic field structure is clearly
pronounced. One can see that the field topology, rather
than strength, is here the crucial factor.
ACKNOWLEDGEMENTS
The authors are extremely grateful to the J.Wilcox
Observatory staff for INTERNET solar magnetic field
data and to the SOHO staff for the INTERNET white
corona data.
REFERENCES
Bird M.K. and Edenhofer P. (1990): in Physics of the
Inner Heliosphere, R.Schwenn, E.Marsch
(Eds.), Springer, Vol. 1, p. 80, 1990.
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