The 'hiss' phenomenon on Short Waves.
In a recent letter to Nature  November 3, 1945, pp 534-535 Sir Edward Appleton gives some new information about a phenomenon which a few years ago interested, not to say mystified, many radio men. This is the hissing sound which was occasionally heard when receiving on the short wavelengths, more particularly around the time of the last sunspot maximum.
Sir Edward remarks that both Reber and Southworth, working on wavelengths of the centimetre order, have recently succeeded in detecting and measuring radiations coming from the sun. Jansky, however, using the longer wavelength of 14.6 metres, was unable to detect any solar radiation, although his apparatus did detect that coming from the vicinity of the Milky Way. But the hissing phenomenon, states Sir Edward, furnishes evidence which suggests that, during periods of high solar activity, the sun occasionally radiates energy in the radio spectrum of an intensity greatly in excess of that noticed by Reber and Southworth.
Reports describing the hearing of a hissing sound when receiving in the range 10-40 MHz were sent to Sir Edward by D W Heightman and other amateurs, and from these he concluded that the noise was due to electromagnetic radiation coming from active areas on the sun. The noise was only heard during daylight, and often preceded one of the sudden short-wave fadeouts which take place at the same time as the occurrence of a solar flare, and which are known to be due to a marked increase in the D layer ionisation, caused by an outburst of ultra-violet light from the active solar area. Radio sky-waves are then completely absorbed within the D layer, and short-wave communication across the day lit hemisphere of the earth is stopped. It is natural, therefore, to associate the hissing noise heard before the fadeout with the same solar area.
But, once the increase in D layer attenuation has occurred the noise is no longer heard, at least in the band 10-30 MHz, for the solar radiation which causes the noise is itself attenuated within the layer. However, on the extremely short waves this attenuation would be much less, and Sir Edward would expect them to be able to make the single journey through the layer with only slightly diminished strength, and so they ought to be detectable not only before, but also throughout the fadeout.
Sir Edward goes on to show that ordinary solar radiation is of insufficient intensity to be detectable on the type of receiving aerial used in ordinary short-wave reception. When extremely short waves are used, however, highly directional aerial systems, such as parabolic mirrors, can be used, and in order to detect this ordinary radiation, it is necessary for the aerial to have a power gain of the order of 104, relative to a half-wave dipole. Considering the low power gains of the aerials used in actual reception of the hissing noise Sir Edward concludes that, during the periods when it was observed, the intensity of the radiation from the active area must have been about 104 times that of the ordinary radiation from the whole disc.
Now that sensitive micro-wave receivers are available, and observing that the sun's activity is increasing towards a maximum, it should, in the future, be possible to obtain some more detailed information about these radiations, both before, and during the course of sudden fadeouts.