The typical length of lightning channels can be estimated to be of the order of ℓ ≈ ⁠ 1 / 4 ⁠ λ = 8 km for R-strokes and ℓ ≈ ⁠ 1 / 2 ⁠ λ = 4 km for K-strokes. Often, a continuing current component flows between successive R-strokes. Its "pulse" time typically varies between about 10–150 ms, its electric current is of the order of J ≈ 100 A, corresponding to the numbers of Q ≈ 1–20 C, f ≈ 7–100 Hz and λ ≈ 3–40 Mm. Both R-strokes as well as K-strokes produce sferics seen as a coherent impulse waveform within a broadband receiver tuned between 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