The Earth's ionosphere is that part of the high altitude atmosphere, starting at about 90 km, which is strongly ionised. Electrons are stripped off the gas molecules, resulting in ions, by the ultra-violet radiation (1 -10 x 10-8 m) of the Sun as well as incident X-rays. The mix of positively and negatively charged ions, negative electrons and neutral gas is called a plasma, which is the most common state of the universe. The most abundant gas molecules are molecular oxygen (O2) and nitrogen (N2) below 200 km, atomic oxygen (O) above 200 km, and hydrogen (H) and Helium (He) above 600 km altitude. Although less than 1% of the upper atmosphere becomes ionised the charged particles make the gas electrically conducting, which completely changes its characteristics. The ionosphere can carry electrical currents as well as reflect, deflect and scatter radio waves. It received its name from Sir Robert Watson-Watt in only 1926, although Carl Friedrich Gauss had already speculated about its existence in 1839.
Figure 1: A typical electron density altitude profile, the most important ions, and the various ionospheric layers.
The ionosphere is divided into different layers according to the electron density present, which is always equal to the ion density. Figure 1 shows a schematic of the different nomenclature, a typical electron density profile and which ions dominate with altitude. Sir Edward Appleton named the (E)lectrical-Layer in 1927 after Guglielmo Marconi showed, during communication experiments in 1901, that radio waves between Europe and America had to be bouncing off an electrically conducting layer at around 100 - 150 km altitude. Subsequently, other layers were discovered which simply received the names D- and F-layers. The D-layer is particularly complicated with more than 50 chemical reactions present. The F-layer electron density can exceed 1012 m-3, especially at high latitudes where high energy electrons originating from the sun can impact onto the upper atmosphere causing additional ionisation. The structure of the ionosphere is a balance between solar production and the destructive processes of recombination. The electron density undergoes a strong daily variation, especially at sunrise and sunset, as well as annual fluctuations. The sun has an 11-year activity cycle, which is witnessed by the number of sunspots on its surface. Figure 2 shows the number of sunspots from 1994 until the present (blue line) and the prediction for the next 7 years (red line). The next maximum is in 2001 where additional solar X-ray radiation will increase the level of ionisation of the ionosphere, thereby changing its radio wave propagation characteristics.
Figure 2: Number of sunspots versus time (blue line). The red line is the prediction for the remaining part of the 11-year cycle.
The aurora has long fascinated mankind (See Figure 3). This natural optical phenomenon was often interpreted as warnings from God in the 16th and 17th centuries or as signals of impending disaster and war (See Figure 4). Although early Greek and Roman philosophers such as Aristoteles and Seneca theorised on the phenomenon of aurora, it was only in the 18th century that any serious interpretation was attempted. Many explanations revolved around the reflection of sunlight by ice crystals, clouds or atmospheric gases. Only after the spectral measurements of Angström in 1867 did it become clear that the aurora was caused by optical emissions of the atmospheric gases themselves.
Figure 3: The colourful aurora. Top panel: A common green aurora with some clouds and sunset in the background. Bottom panel: The human eye's poor colour sensitivity make the red and blue colours difficult to see. The bright spots are stars.
What causes the aurora? Besides the heat and light emitted by the sun, large quantities of charged particles blow off the sun's surface to form the interplanetary solar wind. The solar wind consists mainly of protons and electrons moving at ultra-sonic speeds of 400 - 800 km/s (more than a million miles per hour). The Earth is surrounded by its magnetic field in space, called the magnetosphere, which forms a barrier to the solar wind. Without the solar wind, the magnetic field lines of the Earth would be symmetric and similar to those of a bar magnet. However, the solar wind pressure strongly compresses the magnetosphere on the dayside and draws it out into an extremely long tail on the nightside of the Earth (See Figure 5). Since the charged particles of the solar wind can not cross the Earth's magnetic field lines, they flow around the magnetosphere similar to water around a rock in a river. This forms a standing shock wave in space upstream of the Earth, called the bow shock, much like the boom of an aircraft breaking the sound barrier (See Figure 5). The magnetospheric tail flaps in space much like a flag in the wind.
Figure 5: A schematic showing the solar wind, bow shock, planet Earth, the magnetosphere and the plasmasheet. The inset illustrates how electrons spiralling down magnetic field lines may energise the atmospheric gases to emit light.
Through complicated and not completely understood processes, electrons out of the solar wind are able to diffuse into magnetospheric tail and form a reservoir called the plasma sheet. The magnetosphere and the solar wind form an enormous electrical dynamo where large and complicated electrical currents flow. One component of these currents is carried by the electrons in the plasma sheet which can descend along magnetic field lines along spiral paths (See Figure 5). Should these electrons descend down to altitudes of 100 - 300 km, they may collide with the atmospheric gas causing it to glow. This, very basically, is the cause of aurora. Note that the plasma sheet is only connected to magnetic field lines from the Earth's polar regions on the nightside, which explains why the aurora is mostly seen only at high latitudes at night. However, dayside aurora also exist because solar wind particles have direct access to the Earth's atmosphere via the cusp regions, which are formed by the divide between magnetic field lines bent towards and away from the sun (See Figure 5).
Figure 6: Satellite images of the auroral oval. Left panel: A Dynamics Explorer image from 21000 km altitude. The bright crescent is the sunlit dayside. Right panel: A Viking image from 6000 km altitude taken at ultraviolet wavelengths. The Earth's magnetic pole is in the center of the ovals.
Since the magnetosphere is 3 dimensional, the regions of frequent aurorae form an oval centered about the magnetic poles of the Earth (See Figure 6). During periods of high solar activity, especially around solar maximum, massive explosions in the sun's atmosphere result in Coronal Mass Ejections (CMEs). The energy released (~1013 kg with speeds of 500 - 2000 km/s) is so great that the earth's magnetosphere becomes sufficiently deformed that the plasma sheet becomes connected to lower latitudes. Hence, the auroral oval expands to such an extent that aurorae can even be witnessed over the UK. Such events have negative consequences for satellites and communication links.The colours of the aurora depend on the chemical composition of the atmosphere with altitude and the energy of the precipitating electrons. Higher energies mean a deeper penetration of the atmosphere with a corresponding change in composition. The typical colours of the aurora are green (557.7 nm) and red (630 nm) from atomic oxygen (O) and blue (391.4 and 427.8 nm) from molecular nitrogen (N2) (See Figure 3). Many other emissions also occur outside the visible wavelengths. Auroral structures take on many shapes and movements. These result from the changing spatial arrangement of the precipitating electrons coming out of the plasma sheet. By studying the various aspects of the aurora, much information can be gleened about space plasma processes occurring out in the magnetosphere.