The bottom panel of the plot is a "dynamic spectrum." Note the hot spots outlined in black. They show that the ionosphere was ringing with periods of ~2 and ~8 minutes. Presumably, these "tones" are related to atmospheric pressure waves billowing up from the trembling Earth below.
The ionosphere is the stage upon which much of space weather plays out. Auroras, meteors, and noctilucent clouds all occur there. The "Ionosphere Natural Hazards Team" at JPL studies how Earth itself affects this stage via earthquakes, volcanoes and tsunamis. You can read their report about the Nepal earthquake here.