Spring 2009 Issue

VHF PROPAGATION

More New Space Weather Discoveries

By Tomas Hood, NW7US

Figure 1. Schematic of the Earth’s magnetosphere. The direction to the Sun is to the left. The IMF (interplanetary magnetic field), imbedded in the solar wind, impinges upon the magnetopause. If southward, as here, it connects to the Earth’s magnetic field at the X-line (shown as circle with X inside), resulting in a region of field lines connecting from the Earth to deep space. Plasma from the solar wind enters via the cusp, becomes trapped in the plasma sheets, and eventually precipitates to Earth or is lost
down the magnetotail. (Source: NASA)

In the Winter 2009 issue we began to explore some of the new scientific discoveries and efforts being made in space weather and radio propagation during solar Cycle 23’s approximately eleven years. Many new satellites and other research space craft have been launched and exciting research conducted that is leading to a deeper, better understanding of our Earth, the Sun, and the interaction between them. We looked specifically at one of the amazing discoveries made by the THEMIS (Time History of Events and Macroscale Interactions during Substorms) mission. This time, let’s continue to look at more of the new science being pursued by solar scientists.

The THEMIS Mission

THEMIS is a mission to investigate what causes auroras in the Earth’s atmosphere to dramatically change from slowly shimmering waves of light to wildly shifting streaks of color. Discovering what causes auroras to change will provide scientists with important details about how the planet’s magnetosphere works and the important Sun-Earth connection. During February 2007, NASA launched the five space craft that make up the THEMIS fleet. The University of California, Berkeley’s Space Sciences Laboratory managed the project development and is currently operating the THEMIS mission. Swales Aerospace, of Beltsville, Maryland, built the THEMIS satellites.

Geomagnetic Substorms

Substorms are atmospheric events visible in the Northern Hemisphere as a sudden brightening of the Northern Lights, or aurora borealis. These substorms are more accurately termed “geomagnetic substorms,” a description of the time-dependent build-up and release of magnetic energy in Earth’s magnetosphere.

We know that space is not a vacuum, at least in our solar system. The Sun’s atmosphere actually extends very far out from the Sun. Space in our system is filled with plasma, a low-density gas in which the individual atoms are charged. The temperature of the Sun’s atmosphere is so high that the Sun’s gravity cannot hold on to it. The plasma streams off the Sun in all directions at speeds of about 400 kilometers per second (about 1-million miles per hour). This is known as the “solar wind.”

The solar wind buffets the Earth’s magnetic field and can produce storms, or more properly substorms, in the Earth’s magnetosphere. Until this explosion was witnessed first-hand by THEMIS, however, scientists did not understand the full mechanisms of how substorms occurred.

The Earth has a magnetic field with a north and a south pole that is enclosed within a region surrounding the Earth called the “magnetosphere.” As the Earth rotates, its hot core generates strong electric currents that produce these magnetic fields, which reach 36,000 miles into space. The solar wind distorts the shape of the magnetosphere by compressing it at the front and causing a long tail to form on the side away from the Sun. This tail is called the magnetotail (figure 1), and on average is a million kilometers long.

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