Science history — The Ionosphere

The ionosphere

The conductive layer surrounding our planet was first given the name “ionosphere” in scientific literature in 1929, but the suggestion of that name dates back to 1926. As told by G. W. Gardiner, a letter addressed to the secretary and Radio Research Board was discovered during renovations to the Radio and Space Research Station in Slough, Buckinghamshire. The letter, written by Sir Robert Alexander Watson-Watt, suggests the new term:

We have in quite recent years seen the universal adoption of the term ‘stratosphere’ in lieu of a previously well established misnomer ‘isothermal layer’, and the adoption of the companion term ‘troposphere’ for the ‘convective layer’.

The term ‘ionosphere’, for the region in which the main characteristic is large scale ionisation with considerable mean free paths, appears appropriate as an addition to this series. The objection that ionisation occurs throughout the atmosphere is no more adequate against the proposed term that [sic] is the fact that stratification occurs locally in the troposphere, the systematic name should be characteristic of the main ‘grand scale’ phenomena without reference to minor and local phenomena.

Gardiner, G. Origin of the Term Ionosphere. Nature 224, 1096 (1969).

So now we have a name for this amazingly complex ball of plasma hovering over our heads. Similar to metallic materials, the free electrons in the ionosphere give rise to its conductive nature. The conductive nature of the ionosphere enables to interact with electromagnetic radiation.

Furthermore, it turns out that higher frequencies tend to interact with the ionosphere less than lower frequencies. This means that the higher-frequency radiation used in television and FM radio escape the ionosphere and begin their endless through space. Alternatively, lower frequency radiation (AM radio for example) will never escape the ionosphere. I suppose if intelligent life does exist beyond Earth and they are interested in decoding our electromagnetic waste, they would benefit from having an inclination towards music.

Ionosphere layers

Today, we recognize three layers of the ionosphere. We call these the D, E, and F layers…but where did this convention come from? The genesis of this naming convention can be found in Sir Edward Victor Appleton’s 1930 journal article:

Let us call the region of ionisation which normally reflects long waves, and which normally deflects the primary, downcoming waves on broadcasting wave-lengths, the E region. Hypothetical reflection regions above and below the E region may be termed the F and D regions respectively.

Appleton, E. V. “On Some Measurements of the Equivalent Height of the Atmospheric Ionised Layer.” 

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Interestingly, it was his discovery of the F layer — also know as the Appleton Layer — that would secure Appleton’s 1947 Nobel Prize in Physics for “his investigations of the physics of the upper atmosphere especially for the discovery of the so-called Appleton layer”. A “C” layer (below the D layer) \cite{rakshit36} and “G” layer (above the \begin{math}F_2\end{math} layer) \cite{maeda37} have also been proposed, but have never been adopted.

Today, ionosphere layer separation is based on electron density distribution as a function of altitude. The constantly changing parameters of each layer only allow a window to be placed on the approximate altitude and thickness for each layer. Typical accepted values for the D region altitude and thickness are 60 km and 30 km, respectively. The E region rises from 90 km to 150 km above the surface, and is often the lowest layer encountered in ionospheric measurements. From the top of the E region, the F region vaults an additional 350 km towards space.

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