# Ionospheric Science: An Example of the Importance of Diversity in Approaches to Scientific Research
## 2 Historical Background  
Modern ionospheric science has advanced greatly since the first suggestion of the existence of the ionosphere by [PERSON] in 1902 [8] and the pioneering work of key figures such as [PERSON] (e.g., see [9]) and [PERSON] (e.g., see [10]). It advanced greatly in the Mid-Twentieth Century through both theoretical advances such as [PERSON]'s work on dissociative recombination [11] and the wider understanding of the upper atmosphere that came from early satellite missions. Those advances continued through to the end of that century and into the new one, with many advances in understanding ionospheric storms and irregularities, and in particular much focus on the high-latitude ionosphere. As noted above, these scientific advances have always had a strong link with the radio applications community because of the practical importance of trans-ionospheric radio links whose propagation varies with the conditions in the ionosphere. This first came to prominence in the 1920s as high-frequency (HF) radio became the primary means of long-distance radio communications [12]. HF superseded the low-frequency systems (typically a few 10s of kHz) deployed earlier in the 20 th Century, e.g., the Franco-American Bordeaux-Lafayette system, which operated at 15 kHz [13]. HF provided much more bandwidth, thereby facilitating more sophisticated services, not the least of which was human voice rather than Morse code. However, it was quickly recognised that HF signals could be severely disturbed by space weather events such as magnetic storms [14; 15] and solar flares [16]. Thus, ionospheric science was from the outset a topic where there was a strong link between the applications community and wider research into the behaviour of the Sun and its impact on Earth. Many leading astronomers welcomed ionospheric observations as a complement to other methods of studying the Sun. For example, [PERSON], the leading solar astronomer at Greenwich Observatory in the UK [17; 18; 19; 20; 21], exploited several mid-latitude ionospheric effects that are indicators of solar flares: both HF blackout that occurs when solar X-rays enhance the D region and the magnetic crochets that occur when the solar Lyman-alpha enhances E region conductivity. Thus, from the 1930s onwards, ionospheric science was seen as an area where there was synergy between research and applications and that there was a need for operational capabilities that could monitor and forecast ionospheric conditions. For example, whilst telephone services between the UK and USA were first introduced in 1927 using low-frequency (60 kHz) links, they were quickly complemented in the following year by services using HF links [22]. However, these links were subject to disruption during intense geomagnetic storms such as that in January 1938 [23; 24]. Ionospheric studies became very important during the Second World War with all participants using HF as a key communications technique wherever and whenever land lines were not available. Thus, they continued ionospheric research together with operational monitoring, sometimes making great efforts to deploy monitoring instruments in strategic locations, e.g., the British deployment of ionosondes on the harsh and disputed islands of Svalbard in the High Arctic [25]. After that war, HF continued to be a mainstay of long-distance communications, but was gradually complemented by other systems. For example, the 1950s and 1960s saw extensive deployment of undersea coaxial cables capable of carrying multiple telephone circuits, thus removing a vulnerability to ionospheric effects (this technology was in turn superseded by the undersea optical fibre cables that now carry 99% of transoceanic telephone and Internet communications). Another example of technological progress has been the development of satellite communications from the 1960s onwards--both as a direct means of communications and as support for satellite applications such as navigation, remote sensing, etc.