Radio atmospheric

A radio atmospheric signal or sferic (sometimes also spelled 'spheric') is a broadband electromagnetic impulse that occurs as a result of natural atmospheric lightning discharges. Sferics may propagate from their lightning source without major attenuation in the Earth–ionosphere waveguide, and can be received thousands of kilometres from their source. On a time-domain plot, a sferic may appear as a single high-amplitude spike in the time-domain data. On a spectrogram, a sferic appears as a vertical stripe (reflecting its broadband and impulsive nature) that may extend from a few kHz to several tens of kHz, depending on atmospheric conditions. A radio atmospheric signal or sferic (sometimes also spelled 'spheric') is a broadband electromagnetic impulse that occurs as a result of natural atmospheric lightning discharges. Sferics may propagate from their lightning source without major attenuation in the Earth–ionosphere waveguide, and can be received thousands of kilometres from their source. On a time-domain plot, a sferic may appear as a single high-amplitude spike in the time-domain data. On a spectrogram, a sferic appears as a vertical stripe (reflecting its broadband and impulsive nature) that may extend from a few kHz to several tens of kHz, depending on atmospheric conditions. Sferics received from about 2,000 kilometres' distance or greater have their frequencies slightly offset in time, producing tweeks. When the electromagnetic energy from a sferic escapes the Earth-ionosphere waveguide and enters the magnetosphere, it becomes dispersed by the near-Earth plasma, forming a whistler signal. Because the source of the whistler is an impulse (i.e., the sferic), a whistler may be interpreted as the impulse response of the magnetosphere (for the conditions at that particular instant). A lightning channel with all its branches and its electric currents behaves like a huge antenna system from which electromagnetic waves of all frequencies are radiated. Beyond a distance where luminosity is visible and thunder can be heard (typically about 10 km), these electromagnetic impulses are the only sources of direct information about thunderstorm activity on the ground. Transients electric currents during return strokes (R strokes) or intracloud strokes (K strokes) are the main sources for the generation of impulse-type electromagnetic radiation known as sferics (sometimes called atmospherics). While this impulsive radiation dominates at frequencies less than about 100 kHz, (loosely called long waves), a continuous noise component becomes increasingly important at higher frequencies. The longwave electromagnetic propagation of sferics takes place within the Earth-ionosphere waveguide between the Earth's surface and the ionospheric D- and E- layers. Whistlers generated by lightning strokes can propagate into the magnetosphere along the geomagnetic lines of force. Finally, upper-atmospheric lightning or sprites, that occur at mesospheric altitudes, are short-lived electric breakdown phenomena, probably generated by giant lightning events on the ground. In a typical cloud-to-ground stroke (R stroke), negative electric charge (electrons) of the order of Q = 1 C stored within the lightning channel is lowered to the ground within a typical impulse time interval of τ = 100 μs. This corresponds to an average current flowing within the channel of the order of J = Q/τ = 10 kA. Maximum spectral energy is generated near frequencies of f = 1/τ = 10 kHz, or at wavelengths of λ = c/f = 30 km (c is the speed of light). In typical intracloud K strokes, positive electric charge of the order of C = 10 mC in the upper part of the channel and an equivalent amount of negative charge in its lower part neutralize within a typical time interval of τ = 25 μs. The corresponding values for average electric current, frequency and wavelength are J = 400 A, f = 40 kHz, and λ = 7.5 km. The energy of K strokes is in general two orders of magnitude weaker than the energy of R strokes. The typical length of lightning channels can be estimated to be of the order of L = λ/4 = 8 km for R strokes and L= λ/2 = 4 km for K strokes. Often, a continuing current component flows between successive R strokes. Its 'pulse' time typically varies between about 10 to 150 ms, its electric current is of the order of J = 100 A, corresponding to the numbers of Q = 1 to 20 C, f = 7 to 100 Hz and λ = 3 to 40 Mm. Both R strokes as well as K strokes produce sferics seen as a coherent impulse waveform within a broadband receiver tuned to 1–100 kHz. The electric field strength of the impulse increases to a maximum value within a few microseconds and then declines like a damped oscillator. The orientation of the field strength increase depends on whether it is a negative or a positive discharge