Time-Dependent Tomography

The depicting the heliosphere on July 7, 1994 at 0 UT and a 1.4 Mbyte *.avi video presentation that includes this as one of the images is a view of heliospheric plasma density (extrapolated to heights as great as 3.0 times the distance of the Earth from the Sun) derived from a combination of interplanetary scintillation (IPS) intensity-level and velocity observations. This "time-dependent" tomographic technique operates on each day's data and maps day to day changes in the solar wind density content in three dimensions near Earth. Unlike our previous tomographic programs that assume corotation and outward plasma motion and fit the observations to the model, the time steps of one day used in this modeling restrict data fits so that outward plasma motion alone is used to deconvolve the 3-D structures.

The data for the video were obtained from June 7, 1994 to September 23, 1994 (Carrington Rotations 1884-1886). The spatial resolution is approximately 10 by 10 degrees in heliographic latitude and longitude and 0.25 AU (Earth to Sun) solar distance. The plasma density is normalized to the value of the distance of the Sun from Earth by the removal of a 1/r squared distance dependence. Higher density features are more yellow. The Sun is depicted as a dot in the center of the figure, and the Earth, a blue dot on its orbit around the Sun. The view is from 30 degrees above the ecliptic plane from a position at 3.0 AU opposite the Earth. The video sequence presents each day's tomographic model result at 0 UT and at 12 UT with a cadence of one-half day between frames over the three-month interval. Clearly observed in the video are the larger transient and corotating structures present in the heliosphere at this time.