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Particles in Space--From the Earth or the Sun?

Rick Chappell
Science and Research Communication Program
Vanderbilt University
August 2001

Space is not empty. The space environment around the Earth is filled with a variety of electrically charged particles. We are trying to understand where these particles come from--the Earth or the Sun. By measuring their composition using satellite instruments, we can gain information about their place of origin. This work will help us understand how the changing Sun continuously influences the Earth's environment.

Space is not empty. The space environment around the Earth is filled with a variety of electrically charged particles that may have come from the Earth itself or from the Sun. If we measure the composition of these particles, we can get an idea of their place of origin and through this we can better understand how the Sun controls the Earth's environment.

At large distances from the Earth, most of the particles are observed to be very energetic. Their high energies originally led researchers to conclude that they came from the Sun, which has a very hot outer atmosphere that streams outward past the planets of the solar system. This energetic particle stream, called the solar wind, is made up primarily of electrons and electrically charged hydrogen and helium atoms.

When we look at the energetic particles in the magnetosphere, the region of space around the Earth that is dominated by the Earth's magnetic field, we do indeed find particles like those in the solar wind. But we also observe other electrically charged atoms such as oxygen and nitrogen, which are found dominantly in the Earth's upper atmosphere. Where then do the different particles come from and how are they energized and trapped in the Earth's magnetosphere?

Using satellites that orbit in the magnetosphere, we can measure the composition of the different charged particles as well as their energies and movements. Instruments called ion mass spectrometers can sample the particles and separate them according to their mass, thereby giving their relative abundance. This information will give us insight into their place of origin and the drifts and accelerations that they undergo as they move outward from the Earth or inward from the solar wind.

Contrary to earlier scientific thought, we may find that the Earth itself is the source of most if not all of these particles! Starting in the Earth's upper atmosphere, these terrestrial particles can move upward and become energized by magnetospheric electric fields that are caused by the rapid flow of the solar wind past the Earth. Thus, the Earth may supply the particles, while the Sun supplies the energy to accelerate them.

Studying the origin of the electrically charged particles in the Earth's magnetosphere gives us detailed information about how the continuously varying Sun controls the Earth's space environment. This insight helps us forecast space environmental changes such as terrestrial magnetic storms that can interrupt satellite communications and disrupt the transmission of electrical power in many nations of the world.