Introduction to Longwave Monitoring Earth's ionosphere reacts to ultraviolet, X-ray, gamma-ray and other types of cosmic rays. Longwave radio propagation depends on the lower regions of the ionosphere called E- and D-Layer. These are in fact not sharp bordered layers. So we call them better as "regions" of plasma. Night time radio propagation happens via the ionosphere at heights above 100 km. During the day solar radiaton generates the D-Region at 60-90 km. If the ionisation of this D-Region is weak it mainly absorbes long radio waves and signals appear attenuated. Around periods of high solar activity the ionisation peaks to levels where the radio waves are bent back to earth. Due to this refraction effect longwave radio signals can appear stronger. Unfortunately the ionosphere is not homogeneous and some amount of ground waves are present coming in directly from the transmitter. What the antenna collects is a mix of waves radiated from a single source over multiple pathes and the resulting signal is a product of interference of these multiple waves fed into the receiver's front-end. Interference can be constructive or destructive and therefore an interpretation of the net result is ambiguous. Nevertheless sudden changes of ionisation can be observed quite well via simultaneous fluctuations of signal magnitude. Detecting those events is the primary purpose of such a longwave radio monitor . Solar radiation is not constant and sometimes outbursts of X-rays occur from Solar Flares. During such a flare the D-Region immediately changed to higher levels of ionisation and the reflection heights decline (example). These events are called Sudden Ionospheric Disturbances abbreviated as SID. Other sources of radiation which may cause such SIDs are Thunderstorms, powerful Gamma Ray Bursts or X-ray flashes from outer space. But such events are very rare. Solar eclipses show remarkable effects as well (example). Thus, in strong sense, a longwave monitor like this is not a "radio telescope". It is an observatory for ionizing radiation with low angular resolution but extremly high bandwidth of energy. The Kiel Longwave Monitor records the magnitude of HBG - a time service transmitter in Switzerland at 75 KHz - (Receiver 1) and the VLF/LF-region from 10 to 96 KHz (Receiver 2) round the clock hunting for such SID events.

Receiver 1: High Resolution, Single Channel 75 KHz B/W 100 Hz at 10 samples/sec. >> Real-Time Chart and Archive << >> Outline Waterfall-Charts of HBG << Receiver 2: Low Resolution, Broadband 10-96 KHz Software Defined Receiver, 1 sample/sec. >> Interactive Spectrograms << >> Current Spectrum << >> List of Signals (15-100KHz) << >> Block Diagram << >> Azimuth Pattern << SID-Related Links: AAVSO, The American Association of Variable Star Observers Lionel Loudet's Ionosphere Monitor (France) Stanford Solar Center (USA) Paul Nicholson's SID-Monitor (Receiver 2 based upon this) Moore Observatory VLF Monitor (USA) Today's Space Weather, Space Environment Center (USA) SID-GRB@home (USA) Detection of a Gamma Ray Burst by Longwave Radio 2010-02-02: Receiver 2 "Current Spectrum" back on-line Receiver 2 "Interactive Spectrograms" off-line