Nomenclature
A type of electromagnetic signal propagating in the Earth–ionosphere waveguide, known as a radio atmospheric signal or sferic, may escape the ionosphere and propagate outward into the magnetosphere. The signal is now prone to bounce-mode propagation, reflecting back and forth on opposite sides of the planet until totally attenuated. To clarify which part of this hop pattern the signal is in, it is specified by a number, indicating the portion of the bounce path it is currently on. On its first upward path, it is known as a 0+. After passing the geomagnetic equator, it is referred to as a 1-. The + or - sign indicates either upward or downward propagation, respectively. The numeral represents the half-bounce currently in progress. The reflected signal is redesignated 1+, until passing the geomagnetic equator again; then it is called 2-, and so on.
Read more about this topic: Whistler (radio)