STEREO Images for the First Time Solar Material Passing by Earth

Taking advantage of the relative quiet solar conditions during this year's spring and summer, the STEREO B solar-orbiting spacecraft observed a succession of wavefronts of solar material passing by Earth, by using its Sun Earth Connection Coronal and Heliospheric Investigation instruments, or SECCHI for short. This is the first time when the Solar Terrestrial Relations Observatory spacecraft has been able to observe such large waves of solar material sweeping by Earth.

The existence of such density enhancements was previously acknowledged through measurements made by the situ spacecraft; however, this is the first time when scientists obtained actual images of this structure. By comparing the white-light images with the measurements of the plasma and magnetic field obtained by situ, scientists observed a perfect correlation between the appearance of these solar wind waves and the high-density regions, which rotate at the same time with the Sun. It is generally believed that these high-density regions form when the high speed wind, emitted from the 'dark' regions of the Sun observed in the corona, collides with the low-speed wind that forms in front of it.

By backtracking their trajectory towards the Sun, the scientists were able to view exactly where this wave of high-density material originated from. So far, it seems that the solar wind wave originates from the blobs of material continuously being ejected from the Sun's coronal streamers.

The two STEREO twin spacecrafts, STEREO A and STEREO B, have both been launched in October last year, with the mission to create a stereoscopic observation of the Sun from the Earth's orbit. Soon after launch into space, the two spacecrafts executed a series of maneuvers which were destined to put the A spacecraft into orbit closer to the Sun, moving ahead of the Earth in its orbit, and the second one, the B spacecraft, located farther from the Sun, slowly falling behind in Earth's orbit.

Both of them are equipped with SECCHI instruments, an extreme ultraviolet imager, two coronagraphs, and twoheliospheric imagers pointed at 13 respectively 53 degrees off to the sides.

Data extracted from the two spacecrafts will be discussed in this fall's American Geophysical Union, in San Francisco, in a talk initiated by Dr. Neil Sheeley, named "Secchi Observations of Mass Flows in the Inner Heliosphere".