Many solar wind parameters vary systematically with increasing distance, and there are strong relationships amongst solar wind parameters and between the solar wind and Interplanetary Magnetic Field (IMF) particularly for the typical pattern of fast and slow solar wind referred to as the background solar wind. The fast wind is well associated with the solar coronal holes, which are dark regions on X-ray images of the Sun and corona. The more variable slow wind may emanate from streamers that are bright in the X-ray images and/or from the edges of coronal holes. Large “bubbles” that episodically erupt from the Sun are called Coronal Mass Ejections (CMEs). Many of the solar wind and IMF relationships found in the background wind do not hold for CMEs.
The solar wind speed has a long autocorrelation time making it the easiest of the solar wind parameters to forecast using prior values. The solar wind speed is also the easiest parameter to forecast using solar observations since the amount of bending or expansion factor in the solar magnetic field correlates well with the final solar wind speed measured at Earth. The fast wind is well associated with coronal holes, and the slow wind is thought to be associated with streamers and coronal hole boundaries. Owing to this source connection for the solar wind speed, many solar wind and IMF parameters correlate with the solar wind speed. The solar wind speed is also important because of systematic changes that occur as the fast and slow wind dynamically interact with increasing distance from the Sun. The Sun emits parcels of differing speed along the same radial line as different emitting regions on the Sun rotate around. A slow wind parcel will be compressed by a fast wind parcel emitted at a later time along the same radial line as the fast parcel catches up to the slower parcel. Similarly, a rarefaction or stretched low pressure region forms as a fast parcel runs away from a slower parcel emitted at a later time. Scalar quantities such as density, pressure, and field strength are systematically elevated (diminished) in compressions (rarefactions). By combining all of these relationships between solar wind and IMF parameters, radial profiles, and the Parker spiral direction, we can leverage the solar wind speed forecasts based on the solar observations to extend the forecast lead time to 3-4 days for all of the background solar wind and IMF properties.
We examine the radial profiles for the solar wind parameters using Helios, Ulysses, New Horizons, and Voyager observations. As fast and slow wind parcels dynamically interact with increasing distance from the Sun, the amplitudes of the alternating pattern of fast and slow solar wind speed structures decreases, but the solar rotation averaged speed is nearly constant. Eventually as the solar wind travels farther from the Sun, it encounters more and more interstellar material. By combining New Horizons solar wind observations with solar wind observations from ACE and STEREO, we determine how much the solar wind slows in the outer heliosphere as interstellar material is picked up by the solar wind. This decrease in speed is subtle and between 30 and 40 AU is only about a 6-7% decrease in speed.