Introduction

By “aurora” people usually mean the emission of light from the upper atmosphere, but in fact there are numerous related phenomena, each being a direct or indirect consequence of energetic particles entering the atmosphere from the magnetosphere. They include

1. (a) luminous aurora;

2. (b) radar aurora, by which is meant the reflection of radio signals from ionization in the auroral region;

3. (c) auroral radio absorption, the absorption of radio waves in the auroral ionization;

4. (d) auroral X-rays, which are generated by the incoming particles and may be detected on high-altitude balloons;

5. (e) magnetic disturbances, due to enhanced electric currents flowing in the auroral ionization, which may be detected by magnetometers;

6. (f) electromagnetic emissions in the very-low- and ultra-low-frequency bands, which are generated in the magnetosphere by wave–particle interactions (Section 2.5.6), and which then propagate to the ground where they may be detected with a radio receiver or a sensitive magnetometer.

Arising as they do from a common cause, the auroral phenomena display several common properties.

1. They all exhibit a general relationship with solar activity, though often there is no specific association with any obvious solar event. From the 1930s the term M region was used to signify a hypothetical and unseen solar region causing aurora and magnetic storms, and this served as a unifying hypothesis for some 40 years. It is now well appreciated, of course, that the unseen agent is the solar wind.

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