Stars like our Sun are made of ionised gas known as plasma. In fact, space is dominated by plasma – space scientists (astrophysicists) believe that about 99% of matter in the universe is plasma.

The Sun

Astrophysicists classify the Sun as a star of average size, temperature and brightness. It consists mainly of hydrogen (71% by mass) and helium (27% by mass). Due to the very high temperatures found within the Sun, these elements exist not in the gaseous state but as plasma.

The vast amounts of energy emitted by the Sun come from thermonuclear fusion reactions occurring within the core that convert hydrogen nuclei into helium nuclei. It has been calculated that the energy output of the Sun is 3.86 x 1023 kJ per second.

Solar wind

The outer atmosphere of the Sun is known as the corona. Temperatures within this region are extremely high, giving some of the charged particles present sufficient energy to escape from the strong gravitational pull of the Sun. This stream of charged particles emanating from the Sun in all directions at speeds of about 400 km/s is called the solar wind. It is rapidly moving plasma that pushes out to the edge of the solar system

The solar wind is not uniform. Although it is always directed away from the Sun, its intensity and speed are dependent on the activity of the Sun. For example, major solar eruptions known as coronal mass ejections (CMEs) can increase the plasma density and speed of the solar wind. This can impact on the Earth’s magnetic field, causing increased auroral activity and, in extreme cases, geomagnetic storms that can disrupt communication systems and cause power surges on electrical transmission grids.

Ionosphere

In the uppermost region of the Earth’s atmosphere, the intense incoming solar radiation causes the ionisation of gaseous atoms. Ionisation is the process in which neutral atoms or molecules gain or lose electrons to become electrically charged. This creates a mix of neutral atoms, electrons and positive ions called plasma.

The term ‘ionosphere’ is used to describe this region of near-Earth space that extends mostly within the altitude range of 85-600 km. On descending through the ionosphere, the more energetic solar radiation is absorbed, resulting in a drop-off in the degree of ionisation. In addition, the ratio of plasma/gas mix changes until, at an altitude of about 50 km, only gas exists.

Various regions within the ionosphere based on electron density have been described. These vary from daytime to night-time and play a key role in absorbing harmful radiation from the Sun and outer space. In addition, the ionosphere influences radio wave propagation. It can be used to ‘bounce’ certain types of radio wave signals down to the ground, allowing for communication over very large distances.

Auroras

An aurora is a luminous glow in the E-region of the ionosphere seen mainly in high latitudes close to the poles. They are visual reminders of the solar wind – the more intense the solar wind, the more spectacular the aurora’s display of coloured lights.

Auroras are caused by high-energy charged particles from the solar wind becoming trapped in the Earth’s magnetic field. As these particles spiral back and forth along the magnetic field lines, they come down into the ionosphere near the North and South Magnetic Poles where the magnetic field lines disappear into the body of the Earth.

As these high-energy charged particles collide with oxygen and nitrogen atoms in the ionosphere, they excite them to higher energy levels. On returning to their normal resting levels, the atoms emit the energy gained in the form of visible light. It is this light in shades of green and red that we see in the aurora. In the northern hemisphere, we see the Aurora Borealis, and in the southern hemisphere, we see the Aurora Australis.

Useful links

More information about solar missions.
www.windows2universe.org/sun/mission.html

Find out more about the SOHO mission.
http://sohowww.nascom.nasa.gov/about/docs/SOHO_Fact_Sheet.pdf

Excellent link to video on the involvement of the Earth’s magnetic field in the production of auroras. (Scroll to the bottom of the page.)
http://sohowww.nascom.nasa.gov/classroom/for_students.html

Frequently asked questions about auroras and answers produced by the Geophysical Institute of the University of Alaska Fairbanks.
www.gi.alaska.edu/AuroraForecast/faq

Read about the March 1989 geomagnetic storm that caused the collapse of Quebec’s electricity transmission system.
www.nasa.gov/topics/earth/features/sun_darkness.html

    Published 29 April 2014