Associated Research Papers

AIP_Sokolov_2008.pdf - Igor V. Sokolov and Ilia I. Roussev. MHD turbulence model for global simulations of the solar wind and SEP acceleration

ApJ_595_L57.pdf - Roussev et al., A Three Dimensional Model of the Solar Wind Incorporating Solar Magnetogram Observations

ApJL_654_L163.pdf - Cohen et al., A Semiempirical Magnetohydrodynamical Model of the Solar Wind

Research

Solar Wind Origin & Structure

The Sun's hot outer atmosphere, the solar corona, is constantly being accelerated into space. This escaping plasma (a soup of ionized particles) travels from the sun and permeates the entire solar system. This wind consists primarily of electrons and protons, but alpha particles (He nuclei) and other ionized species are also present. The magnetospheres of planets and the ionic tails of comets are affected by the charged particles and weak magnetic field within the solar wind.

On Earth, interaction with the solar wind is observed most commonly as aurora. When the solar wind reaches the Earth, the Earth's magnetic field channels the charged particles towards the north and south poles. They strike atoms and molecules in the Earth's atmosphere, exciting them temporarily. The atoms and molecules eventually return to their ground states, emitting light and creating an Aurora Borealis. The solar wind, when stronger, may also cause geomagnetic storms, knocking out satellites & power grids and creating auroras far beyond the Earth's polar regions.


PHOTO:Edward Kennedy/ U.S. Naval Research Laboratory

Observations show that the solar corona is heated to very high temperatures, but the mechanism for this dramatic heating is still unknown. Two types of theories are currently being investigated: 1) Heating and acceleration are caused by waves below the photosphere. 2) The corona is heated by transient events in the solar atmosphere such as magnetic reconnection. Due to this heating, the corona is constantly expanding outwards. Magnetic fields on the sun act to hold on to the charged particles in the corona, however, if the fast-moving particles manage to reach a distance of 0.5-1.0 AU (the distance of the Sun to the Earth), then the particles are able to escape into the rest of the solar system, creating the solar wind.

There are two types of solar wind - fast (~800 km/s at the Earth) and slow (~400 km/s at the Earth). The fast solar wind is observed over coronal holes, low density regions in the corona, while the slow wind is associated with coronal streamers which straddle regions of different magnetic polarity. Furthermore, the Sun's magnetic field expands with the solar wind, in a sense "frozen" into the wind as it travels outwards. Because the Sun is rotating, this results in the magnetic field taking on a somewhat spiral structure when looking down on the Sun's poles. Additionally, the poles of the magnetic field are not perfectly aligned with the rotational axis causing the magnetic field to "wobble" in space creating something like an outward traveling wave (see figure).





Models of the solar wind must take all of these characteristics into account - fast & slow winds, varying magnetic fields, a source of heating in the corona, transient events in the corona, and how it all changes as the wind expands out into the solar system. One of the goals of C2H2 is to create better MHD models of the solar wind.

Director of C2H2, Ilia Roussev, worked on a simulation that strived to reproduce the global structure of the heliosphere under realistic conditions. While most studies were able to reproduce the global structure of the outer solar corona, they could not model the bimodality of the solar wind without adding empirical sources. Normally, in a fully ionized plasma such as the solar wind, there are almost no internal degrees of freedom. In this model, however, the energy stored in waves and turbulent fluctuations close to the Sun were considered to be "turbulent" degrees of freedom. This allowed for the calculation of a kinetic temperature and a separate temperature associated with the "internal" degrees of freedom. This turbulent motions near the Sun are also considered to be an additional source of stored energy . Calculating the temperature in a static intial state and imposing a temperature variation across the solar surface depending on radial magnetic field strength, the bimodal structure of the solar wind was reproduced. Testing the code for a non-tilting, rotating magnetic dipole, the fast and slow winds were also appropriate correlated with coronal holes and streamers (respectively).

A second modeling attempt by a team including Ilia Roussev attempted to provide a quantitatively accurate model - a model that attempted to provide a reliable and quantitative agreement with the observed values of the solar wind beyond simply calculating the overall shape of the model. The biggest problem, which was addressed at a level in the first model, is that the energy of an electron and proton at 1AU is much greater than their energy would be in the corona (as calculated from the temperatures observed in the corona). A solar model needs to explain how that energy is input into the particles in the solar wind. This second modeling attempt also successfully reproduced the bimodality of the solar wind - fast wind at high latitudes and slow wind at low latitudes. The overall structure of electron density above the solar surface was also accurately reproduced when compared with Mauna Loa Mark III K-coronameter measurments (see figure below).



Because the solar wind impacts the Earth everyday, and sometimes in extraordinary ways, astronomers at C2H2 continue to work on creating better models to describe and explain the characteristics of the solar wind.