Topside-sounder resonances

The plasma resonances observed by topside sounder satellites appear as persistent ringing signals at the plasma frequency and at multiples of the electron-cyclotron and upper hybrid frequencies. They are attributed to slow, electrostatic waves which occur near these frequencies. Explanations have progressed from invoking stationary waves or waves moving with the satellite to invoking waves which are reflected back to the satellite by the vertical electron-density gradient. In addition to the principal resonances, plasma echoes, proton-period modulation, lower frequency resonant signals, and "floating" resonances are also observed.