<P> In 1912, the U.S. Congress imposed the Radio Act of 1912 on amateur radio operators, limiting their operations to frequencies above 1.5 MHz (wavelength 200 meters or smaller). The government thought those frequencies were useless . This led to the discovery of HF radio propagation via the ionosphere in 1923 . </P> <P> In 1926, Scottish physicist Robert Watson - Watt introduced the term ionosphere in a letter published only in 1969 in Nature: </P> <P> We have in quite recent years seen the universal adoption of the term' stratosphere'...and...the companion term' troposphere'...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 . </P> <P> In the early 1930s, test transmissions of Radio Luxembourg inadvertently provided evidence of the first radio modification of the ionosphere; HAARP ran a series of experiments in 2017 using the eponymous Luxembourg Effect . </P>

What type of wave is reflected by particles in the ionosphere