Acceleration of the Solar Wind

There are two types of solar wind flow--quasi-stationary and transient, and there are two types of quasi-stationary flow--fast and slow. Transient flows, from events called coronal mass ejections, can have any speed.

The fast, quasi-stationary solar wind originates in coronal holes and reaches its terminal speed within 10 solar radii of the Sun. In this fast wind, the speeds and temperatures of the heavy ions exceed that of the protons, while the protons are hotter than the electrons. The acceleration of the fast wind cannot be explained purely on the basis of heat conduction.

The anisotropies of the plasma distributions indicate that the magnetic moments of the ions and electrons are not conserved owing to wave-particle interactions generated by plasma instabilities.

Heliospheric Structure and Dynamics

The solar wind contains a heliospheric current sheet which separates regions of opposite magnetic polarity. The shape, locations, and tilt of the current sheet vary significantly through the solar cycle.

Interaction regions form where fast streams of solar wind overtake slower wind in its path. Particles can be accelerated to high energy in these interaction regions. These regions overtake and interact with each other with increasing distance from the Sun.

At solar-activity minimum, high-speed wind from the polar coronal holes fills the high-latitude heliosphere. The flow near the Sun is such that the radial component of the interplanetary magnetic field is independent of solar latitude.

The slow wind in the equatorial regions is filamentary and highly non-uniform in composition and plasma properties.

The properties of solar wind turbulence and waves evolve with distance from the Sun and differ in the high and low speed winds.

The solar wind flow remains supersonic to distances > 65 AU.

The solar wind modulates the flux of galactic cosmic rays and the dependence of that modulation on the phase of the solar cycle, on distance from the Sun (to 65 AU), and on latitude (at solar minimum, inside 5 AU) has been measured.

Interaction with the Interstellar Medium

The heliosphere is embedded in a region of the interstellar medium in which the velocity, density, and temperature of neutral hydrogen and helium are known but in which the properties of the plasma and magnetic field are unknown. There is a "wall" of neutral H at the "nose" of the heliosphere, and interstellar H is slowed in that region.

Radio emission comes from the direction of the nose of the heliosphere, and the frequency of that emission indicates it is generated in a region with a plasma density of 0.05 particles per cubic centimeter.

Interstellar gas is ionized when it enters the heliosphere to form a population of "pickup ions." The properties of pickup ions have been studied within 5 AU of the Sun, and the radial and solar-cycle variations of the "anomalous cosmic rays," believed to be accelerated out of the population of pickup ions, have been observed near the ecliptic to a distance of 65 AU.