Time-Dependent Tomography Using
Helios 1 and 2 Photometer Data

The image depicting the heliosphere on April 05, 1977 at 00 UT, the model correlation with Helios 2 in-situ density at the spacecraft and a 1.9 Mbyte *.avi video presentation that includes the above 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 Helios photometer observations and UCSD interplanetary scintillation (IPS) velocity observations. This "time-dependent" tomographic modeling technique maps day to day changes in the solar wind density content in three dimensions over the heliosphere.

The Helios photometer Thomson-scattering brightness observations map density and 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 1653 (March 24 00 UT to April 20, 00 UT 1977. The spatial resolution is approximately 10 by 10 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 throughout the whole of the Carrington rotation time interval correlate to in-situ densities at the Helios 2 with a CCF of ~0.86 (see figure).

During Carrington Rotation 1653 Helios 1 moved from 0.52 AU 25 degrees east of the Sun-Earth line to 0.34 AU 88 degrees west. Helios 2 moved from 0.64 AU 12 degrees east to 0.30 AU 57 degrees west, and thus both spacecraft were near one another and near Earth. This is the same data set that was used provide input for the kinematic corotating model fit (see 3-D White-light Tomography.)

The tomographic model fit to brightness has a far more simple relationship with density than does IPS g-level with density (see "Time-Dependent" Cambridge/STELab Tomography) accessed in current Space Weather Forecasts. The time period shown in 1977 is during the ascending phase of solar activity cycle 21. There are several large Coronal Mass Ejections (CMEs) shown in the data, but most of the sustained activity (highlighted in the still image) can be shown to arise from above the northern hemisphere active region 701 throughout Carrington Rotation 1653.

The heliospheric data fit shows the extent that so-called corotating heliospheric regions maintain their integrity throughout a one-month time period. More information about these corotating features observed in the Helios photometer data can be found in: Jackson, B.V.: 1991, "Helios Spacecraft Photometer Observations of Elongated Corotating Structures in the Interplanetary Medium", J. Geophys. Res., 96, 11,307. More details of the tomography using the Helios Photometer data can be found in: Jackson, B.V. and P. Hick: 2000, "Three Dimensional Tomography of Heliospheric Features Using Global Thomson Scattering Data", Adv. Space Res., 25, No. 9, 1875.

An additional video sequence of during the May, 1979 activity maximum can be found at: Time-Dependent Tomography Using Helios 2 Photometer Data.