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Full text of "NASA Technical Reports Server (NTRS) 19980028486: The Ambient and Perturbed Solar Wind: From the Sun to 1 AU"

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NASA-CR-2 04771 

/ / / 



Principal Investigator: 

Dr. R.S. Steinolfson, Staler Scientist 
Aurora Science, Inc. 

4502 Centerview Drive, Suite 215 

San Antonio, TX 78228 
(210) 670-1787 
Fax: (210) 670-0962 

Period Covered: April 1, 1994 - March 31, 1997 


The overall objective of the proposed research was to use numerical solutions of the 
magnetohydrodynamic (MHD) equations along with comparisons of the computed results with 
observations to study the following topics: (1) ambient solar wind solutions that extend from the 
solar surface to 1 astronomical unit (AU), contain closed magnetic structures near the Sun, and 
are consistent with observed values; (2) magnetic and plasma structures in coronal mass ejections 
(CMEs) as they propagate to the interplanetary medium; (3) relation of MHD shocks to CMEs in 
the interplanetary medium; (4) interaction of MHD shocks with structures (such as other shocks, 
corotating interaction regions, current sheets) in the interplanetary plasma; and (5) simulations of 
observed interplanetary structures. A planned close collaboration with data analysts served to 
make the model more relevant to the data. The outcome of this research program is an improved 
understanding of the physical processes occurring in solar-generated disturbances in the 
interplanetary medium between the Sun and 1 AU. 


Two separate papers reporting the research performed with support from this grant are in 


During the first year of the proposed investigation, MHD codes developed by the PI were 
used to generate solutions for the ambient solar wind that contain closed magnetic structures 
(coronal streamers) near the surface of the Sun and for which the computed values of physical 
quantities are consistent with available observations from the Sun to 1 AU. Driving mechanisms 
(e.g., magnetic flux emergence, photospheric shear) at the coronal base of the closed magnetic 
stuctures were then used to generate CMEs, and their evolution was followed numerically through 
the interplanetary medium. Specific studies completed during the first year include shock-shock 
interactions, shock conversions, and initiation of an effort to determine the three-dimensional 
configurations of CMEs as they propagate to the interplanetary medium. 

Additional necessary physics were then incorporated into the solar wind model. This topic 
covers the inclusion and evaluation of the physics needed to generate a physically consistent 
ambient solar wind and interplanetary disturbance. It is a nontrivial problem to represent the 
initial solar wind and the driver mechanisms realistically and requires more than just developing 
basic 2-D and 3-D MHD codes, which are already available for this study. A large part of the 
work on this task involved using the numerical relaxation procedure to find ambient solar wind 
solutions, for selected models of the thermodynamics, that are consistent with available 
observations from near the Sun to 1 AU. The initial magnetic field configuration was also 
computed self-consistently as part of this work. A large portion of this work was completed with 
2-D models since this part of the effort serves mainly as a test bed for the geometrically more 
appropriate (and CPU-intensive) 3-D studies. One of the primary results of this study was to 
demonstrate that a realistic ambient solar wind can only be generated within the confines of this 
fluid model if either the temperature or the polytropic index are certain specified functions of 

radial distance. The temperature must decrease with radius and the polytropic index must be near 
one near the Sun and increase to approximately 5/3 near 1 AU. When either the temperature or 
the poly tropic function are given as a function of radius, the other quantity is determined from the 
relaxation procedure used to compute the ambient solar wind. Observed ambient solar wind 
variations with latitude can also be mimicked with given temperature or polytropic index 
variations. Although this may seem to be a fairly arbitrary procedure, the necessity for this 
procedure perhaps says something about the fundamental physics of the solar wind or may 
demonstrate a basic limitation in a single-fluid model. 

Once the ambient solar wind has been determined in 2-D and 3-D models, the next step 
involved development of multi-dimensional MHD simulations of interplanetary disturbances. This 
simulation study offers the opportunity to make real progress toward a more complete 
understanding of the solar/interplanetary connection. For instance, although MHD shocks are an 
important part of most interplanetary disturbances (CMEs) generated by dynamic events near the 
solar surface, the relation of the shock to the driver mechanism remains, for the most part, 
unclear, other than the fact that the shock wave (if one is formed) would be expected to precede 
the interplanetary signature of the driver. In earlier work I studied in detail the relation of shocks 
to CMEs observed by coronagraphs in the near-Sun corona and found that only near the center of 
the CME does shock compression contribute to the observed image. The shocks at the flanks of 
the CME are often too weak for the density compression to be observed in coronagraphs. The 
interplanetary consequences of such composite CME (the bright image identified in coronagraphs) 
and shock structures had yet to be determined. 

Both 2-D and 3-D simulations were used to study the evolution of solar eruptions as they 
propagated out into the solar wind near the Earth’s orbit. The primary factors that determine the 
signature of a solar eruption near 1 AU are the initial magnetic structure in which the eruption 
occurs and the nature of the driving mechanism. For instance, simulations have shown for years 
that the essentially instantaneous release of a large amount of energy near the solar surface in an 
essentially open magnetic field will result is a coronal structure that expands in all directions with 
a strong shock at the leading edge. There is not a whole lot of observational evidence to support 
that the majority of solar eruptions occur in this manner. The hypothesis that solar eruptions occur 
as a result of the build-up of stressed magnetic fields in a closed magnetic structure and the release 
of that energy when the magnetic field can no longer be contained near the Sun and expand 
outward. Indeed, the present simulations have demonstrated the significant difference in the 
eruption signature near 1 AU when the driver is more consistent with observations. The coronal 
disturbance produced by the more realistic driver mechanism is also more in agreement with 
observations taken near 1 AU. The actual thermodynamic conditions and velocity of the ambient 
solar wind have only a secondary effect of the coronal disturbance near 1 AU. The magnetic 
structure and the driver mechanism near the Sun are the most important factors. 

Other work that required 3-D MHD codes was also initiated, but using less sophisticated 
drivers than magnetic drivers, that involved the study of the interaction of structures in the solar 
wind, e.g., the interaction of MHD shocks with corotating interaction regions and with current 
sheets. Moreover, despite the emphasis in this work on 3-D simulations, one of the studies where 
the essential physics could be studied in 2-D involved investigations of the interaction of MHD 

shocks and the possible conversion of MHD shocks from one type to another. With respect to this 
latter topic, the concave upward shape or the CME leading edge seen in coronagraphs suggests 
that either slow or intermediate MHD shocks form within about 6 R. of. the solar surface, as 
demonstrated in a series of papers by Steinolfson and Hundhausen. Yet reported observations of 
either slow or intermediate shocks in the interplanetary medium are relatively rare. The obvious 
question, then, involves the possible conversion of these shocks to fast shocks as they move from 
the low-beta corona near the Sun to the higher-beta interplanetary medium. The present study has 
shown the due to the changing magnetic structure in the ambient solar wind, both slow and 
intermediate shocks formed near the Sun are converted to fast shocks near 1 AU. Only under very 
limited conditions (primarily a radial ambient magnetic field) will the slow and intermediate shocks 
survive out to 1 AU.