Time-Dependent Tomography Using STELab Data

The image depicting the heliosphere on July 14, 2000 at 2 UT, the model correlation with Advanced Composition Explorer (ACE) in-situ density near Earth and a 1.9 Mbyte *.avi video presentation that includes the image as one of the time sequence steps is a reconstruction 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 modeling technique operates on each day's data and maps day to day changes in the solar wind density content in three dimensions near Earth.

The IPS observations are least squares fit to a time-dependent kinematic model using time steps of one day. These short time steps restrict the data fits so that outward plasma motion alone is used to deconvolve the 3-D structures. The data for the video were obtained from Carrington Rotation 1965 (July 10 to August 4, 2000.

The spatial resolution is approximately 20 by 20 degrees in heliographic latitude and longitude and 0.25 AU (Earth to Sun) in solar distance. Higher density features are more yellow and more opaque. The Sun is depicted as a dot in the center of the image, 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 about 45 degrees west of the Sun-Earth line at a distance of 3 AU.

The video sequence presents each day's tomographic model result with a cadence of one-quarter day between frames over the time interval. Densities during the time interval from July 12 to July 22 correlate to in-situ densities from ACE near Earth with a CCF of ~0.9 (see figure). The major density structures directed toward Earth are associated with the halo CMEs that occurred on July 11 and July 14 observed by EIT and the LASCO coronagraphs on the SOHO spacecraft. The image and video sequence shows that the primary mass of the July 11 halo CME was well past 1 AU to the northeast of Earth when the high density associated with this event sequence arrived in Earth's vicinity early July 15.

An alternate 3.0 Mbyte mpeg video sequence of this event sequence is also available. This video sequence spans the time from midday 10 July until 29 July in quarter-day steps. Both this and the above video sequence removes a 1/r squared density gradient up to 1 AU, and outward from that distance, yet another 1/r density gradient which allows more structure (lower densities) to be observed in the inner heliosphere unobscured by the foreground. In both videos, the July 14 CME is well-observed moving outward past Earth. The tomographic sequence temporal and spatial filters are also adjusted to fit the in-situ density time and velocity sequence, and this gives a correlation of 0.77 over the whole Carrington time interval shown above. More information about the IPS data from STELab (and the data present here) can be found on the Web at: http://stesun5.stelab.nagoya-u.ac.jp/index-e.html.