Whistlers

The Earth and its dipole field, seen from the side: the played key's field line, the lines of every C for scale, and the plasmasphere, shaded, out to the plasmapause. A stroke at one foot sends its crack up the line as a spread of frequencies, coloured from the nose (the fastest, bright) down to the low ones (slow, amber), which you can watch fall behind; at the far foot each echo turns back. Chorus sparks at the equator.

Each whistler is a falling curve that starts at its nose, the note, with a short rising branch above it that stops at the duct's cutoff, half the gyrofrequency. Its echoes fall more slowly, and the crack's tweeks hook down onto the waveguide's cutoff near 1.8 kHz.

When each frequency arrives after the stroke, for the first hop and its echoes: the curve the spectrogram draws. The nose is where it turns; Eckersley's law, a delay of D/√f (dashed), fits the low frequencies and misses the nose, which comes from the gyrofrequency along the line. The dotted line is the duct's cutoff, half the gyrofrequency on the equator.

Carpenter and Anderson's equatorial density against L: the plasmasphere, the plasmapause's fall, and the trough beyond. The keys sit along the top at the lines whose nose they are, the played one marked.