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Our Near-Earth Space

This text was published in BergensTidende June 6, 2005
Authors: Nikolai Østgaard and Arve Aksnes

The Sun is an enormous fusion reactor. In less than two seconds an amount of hydrogen corresponding to the mass of the Moon is being burned. This yields an energy production equal 100.000 million hydrogen bombs every second. It takes between 1 and 10 million years for the energy being produced in these nuclear reactions in the centre of the Sun to reach the surface of the star. However, at this point it takes only 8 minutes before the energy reaches the Earth as electromagnetic radiation. The primary radiation from the Sun is ultraviolet and visible light. In addition there is a continuously flow of charged particles (plasma). This plasma flow, known as the solar wind, is accompanied by the magnetic field between the Sun and the Earth, and has a typical speed of 400 km/s. This means it takes (typically) about 2-3 days for the solar wind to reach the Earth.

Figure 1: The interaction between the Solar wind and the Earth’s Magnetosphere (ESA/NASA SOHO).

A schematic of the interaction between the solar wind and the Earths magnetosphere is given in Fig.1. The two plasma domains are separated by a pink line, identifying the magnetopause. Usually, the solar wind plasma is unable to penetrate the magnetopause, as the Earth’s magnetic field configuration (blue lines) acts as a shield (like a roof on a house keeping people from being wet during rainfall). Therefore, the solar wind plasma will simply stream by the Earth system on the flanks, as illustrated by the white lines. Note that the planet Mars has lost its magnetic field, and therefore its solar wind shield. This may explain why most of the atmosphere on Mars is 'blown' away.

Figure 2: The day time auroral spot observed by the IMAGE satellite. The brown line illustrates a magnetic field line coupled to the Sun and Earth respectively (NASA).

Despite the Earths magnetic shield configuration, sometimes solar wind particles still manage to leak into the system. This may happen when the interplanetary magnetic field (IMF) (or the Sun’s magnetic field) is able to couple with the Earths magnetic field. This process, known as reconnection, causes solar wind plasma to stream into the Earths magnetosphere (we have a leak in the roof) . The charged particles from the solar wind will follow the magnetic field lines down towards the Earths atmosphere, causing the production of an auroral spot. In Bergen, scientists use images from the IMAGE satellite (Fig. 2) to study this day time aurora (Note that the day side of the Earth in Fig. 2 is at the upper left corner). This process we have described doesn’t just take place near the Earth, but on and between all magnetized stars and planets in the Universe.

Solar storms

The Sun is a bubbling gas sphere with pulsating activity. The solar activity varies in cycles of 11, 22 and 90 years, and this is revealed in the occurrence of phenomena like solar flares and coronal mass ejections (CME). Solar flares is the result of magnetic field coupling at the Sun’s surface, while CME is huge explosions below the surface. Strong CME events often take place just after a maximum has been reached in the 11-year solar activity cycle. This may cause severe geomagnetic disturbance in our near-Earth space (depending on whether the blowout on the Sun points towards the Earth). One of the largest solar storms ever detected took place in October, 2003. The solar wind had an enormous speed of 2000 km/s, which brought it to the Earth in only 19 hours! During such events, a significant amount of X-ray radiation is also being produced, as well as highly energetic particles. These are able to penetrate the Earths magnetic field, and may cause great damage on satellites and astronauts in space. Due to the Earths atmosphere, humans on the ground are well protected. During solar storms, the reconnection process is much more efficient, and large amounts of energy and plasma are being transferred from the interplanetary space to the Earths magnetosphere. This causes a massive strengtening of the electric currents flowing in our near-Earth space. One of these currents is encircling the Earths equator at an altitude of approximately 20.000-25.000 km and is known as the ring current. The group in Bergen has specialized to study variations in the ring current during geomagnetic storms. In 2003, scientists in Bergen discovered a connection between geomagnetic storms and the build-up of a belt of neutral and charged energetic particles about 800 km above equator.

Aurora

In addition to the day time auroral spot and the build-up of the ring current, reconnection also causes large amounts of energy to be transferred from the solar wind (when the roof is leaking) and stored in the Earths magnetosphere on the night side of the Earth. This will eventually lead to a very unstable situation (just like blowing a balloon to its breaking point). When the system 'breaks', a great amount of charged particles following the magnetic field lines will suddenly find themselves entering the Earths atmosphere and colliding with the neutrals, causing the production of aurora. Therefore, a dark night may suddenly display beautiful colors and a great variety of auroral shapes on the sky.

Birkeland-currents.

Figure 3: PIXIE-image of the X-ray aurora above the Northern Hemisphere. The colors indicate the intensity, with red representing the most intense X-ray aurora.

Understanding the flow of energy from the Sun to the magnetosphere and further into the upper atmosphere is another main research field for the research group in Bergen. We study how the large amount of charged particles entering our atmosphere during geomagnetic storms and substorms cause strong electric currents to flow in the auroral zone, as well as their impact on the atmospheric temperature and composition. For these studies, global imaging of atmospheric X-ray from the PIXIE camera on the NASA Polar satellite has shown to be most useful. Scientists from Bergen took part in the building of this X-ray camera, an instrument which has provided us with the opportunity to study for the first time the global effects of energetic particles in the near-Earth environment.

The Norwegian Kristian Birkeland was the first person to propose a realistic theory (in 1905) to explain the production of aurora. In his Terella experiment Birkeland showed how electrons being sent towards a magnetized sphere (the Earth) covered by fluorescent painting (the Earths atmosphere) produce two brightening circles around the magnetic poles. Birkeland’s auroral research was not applauded by his colleagues of that time, but the first satellite data in the 1960s confirmed his ideas. Today Birkeland is honored by being portrayed on the Norwegian 200 kroner bank note, and electrical currents along the magnetic field are usually referred to as Birkeland-current in the scientific community.

Figure 4: Reversed lightening far above the cloud cover (normal lightening occurs between the ground and the thunder cloud). We have red spirits, blue jets and elves. (T. Neubert, Science, 2003).

As we have described, the Sun is more than an energy source for the life on Earth. The sun also causes geomagnetic storms and it feeds substorms with energy. The Sun controls the space weather. We have already mentioned the 11-year solar activity cycle. A consequence of this is a variation in the amount of cosmic ray (from the centre of our Milky Way galaxy) entering the near-Earth space, as the super-energetic cosmic ray particles are affected by the Sun’s magnetic activity. Cosmic rays may be important for producing lightening between the top of thunder clouds and the upper atmosphere (Fig. 4), and have been given funny names like e.g. spirits, jets and elves. One of the most recent projects in Bergen involves the participation in the building of an X-ray detector (to be flown on the space shuttle), to study X-rays during these 'reversed' lightening phenomena.

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