Ionosphere

The ionosphere is that part of the Earth’s atmosphere (see figure 3.9) ranging from a height of about 70km up to 1000km. The properties of this part of the atmosphere are determined by the fact that the gas atoms and molecules are ionised. The ionosphere as a whole is still electrically neutral, but ionisation enables the flow of electric current. Ionisation in the ionosphere is mainly caused by solar radiation and the interaction of the solar wind with the Earth’s magnetic field (magnetosphere). It is therefore dependent on location, local time of day, season and on the current level of solar activity, which changes with the 11-year sunspot cycle [AR02].

The ionosphere is divided into different regions, or layers, based on the plasma density that prevails in that layer. These layers have different properties. For example, the lower layers (D, E) reflect radio waves of relatively low frequency (1–10MHz) and enable HF (high frequency) communication over large distances [Bar02]. The higher F layer (sometimes divided into F1 and F2 layers) reflects radio waves of higher frequencies. The exact properties and heights of the different layers of the ionosphere at any given time can be probed with instruments such as an ionosonde, an example is the EISCAT Dynasonde at Tromsø [Dav96].

CHAPTER 3. RIOMETERS 36

3.4.2 Riometer Observations

Riometer observations are unique in that they provide continuous information about the lowest region of the ionosphere, the D layer. Due to their wide field of view and continuous coverage, (imaging) riometers provide information on the spatial extent and lifetimes of precipitation fea- tures that complement the spot measurements taken by radar and in-situ. Riometer observations provide the spatial context within which to interpret radar data and reveal the dynamics of pre- cipitation regions [CHHW97]. Other instruments such as radars or ionosonde are more sensitive to higher layers. Riometer observations extend the total observable region downwards [Hon01]. Riometers are therefore often used together with other instruments, enabling the derivation of the entire height profile of geophysical events. Such events are usually solar-driven and en- able the study of the coupling process between the solar wind, the interplanetary magnetic field (IMF) and the Earth’s atmosphere, with riometers observing the ‘footprint’ of these events in the ionosphere.

3.4.3 Ionospheric Processes

Man-made modifications of the ionosphere, so-called ‘heater’ or ‘artificial aurora’ experiments increase ionisation in small parts of the ionosphere using strong transmitters in the range of several MW. Again, the results can be observed with riometers (and other instruments such as optical cameras), and the obtained data allows insight into wave-plasma interactions and chemical processes in the ionosphere.

3.5

Radio Stars

The bright radio sources in the sky stand out considerably from the cosmic noise background. This causes effects like scintillation (rapid variations in apparent brightness of a distant object when viewed through a medium such as the atmosphere or ionosphere, caused by refraction due to small-scale variations in the medium density [Ric77]), which are not always wanted. In any case, it is important to know what strong radio sources there are and where they come from. This section gives a brief description.

Figure 3.10 shows some of the strong radio stars in the sky. We find, that at the frequency of interest (the operating frequency of most riometers, namely around 38MHz — a protected frequency band), the strongest radio sources in the northern hemisphere are Cassiopeia A and

magnitude weaker at these frequencies, therefore the quiet sun is not usually visible in riometer data.

For details about the spectra of Cas A, Cyg A and other radio sources, see for example [BGPW77, KPW69].

The term ‘radio star’ is somewhat misleading and is merely used for historical reasons. Sources of strong radiation are not necessarily associated with stars. The first ‘radio star’ was discovered by J. S. Hey after the Second World War [Jen66, p. 52] in the constellation of Cygnus. Soon after that, John Bolton discovered a smaller radio source in the constellation of Taurus, the position of which coincided with the so called Crab Nebula. Finally, the strongest radio source, Cassiopeia A, was discovered by Martin Ryle in 1947 [Jen66, p. 54].

3.5.1 Cassiopeia A

Cassiopeia A is a supernova remnant within our own galaxy. From observations of the motion of individual diffuse filaments in the Cassiopeia nebulosity, it can be deduced that the initial supernova explosion leading to the creation of Cassiopeia A must have happened about 320 years ago [Jen66, p. 56].

Cassiopeia A is becoming weaker over the years, recent studies show a clear decay in power. This decay might just about be spotted in recorded IRIS measurements, though no effort to do this has been undertaken as of yet.

3.5.2 Cygnus A

Cygnus A, the second brightest radio source in the sky, is an extragalactic radio source, situated at a distance of about 550,000,000 light years from the Earth [Jen66, p. 77]. Cygnus A is a so-called binary source: it consists of two centres of emission. This fact was first discovered by R. C. Jennison in 1950 [Jen66, p. 55].