Proton-concentrated plasma sheath The model presented is a synthesis of the theory put forth by previous authors. An RF pulse is applied across either the X or Y 500 m sounding antenna on IMAGE, disturbing the ambient plasma. During the positive half cycle of the pulse, a group of electrons are attracted to the antenna. During the negative half cycle, though, the group of protons that migrate to the antenna cannot be as large as the preceding group of electrons because protons have much lower mobility. This asymmetrical movement of charge enables the antenna to attain a negative potential. Explore. fce+ echo fce+ echoes occur close to the electron gyrofrequency, seen as a resonance spike on the plasmagram (which plots virtual range from the satellite against sounding frequency). They have a distinctive curvature and occur at multiples of the proton gyroperiod. Because they propagate in a theoretical frequency stopband, they were considered anomalous. Since then, they have been linked to the general family of proton gyroharmonic (PGH) events. Explore. Whistler echo Whistler echoes are a form of PGH phenomena that occur when the IMAGE satellite is at low altitudes. They are seen predominantly around frequencies of 9 to 12 kHz in the whistler-mode of propagation. Even though Whistler echoes occur in a frequency range that allows propagation, their presence was previously unanticipated. Appearing as horizontal lines of higher amplitude on plasmagrams, WM echoes are characterized by their constant delay times (range) that correspond to multiples of the proton gyroperiod. Explore. Z echo Z echoes bridges the gap between fce+ and Whistler echoes. They occur on records after fce+ echoes are prevalent and before Whistler echoes become dominant. With spectra shifted beyond the Z-mode cutoff, they show the highest frequencies of all known PGH phenomena. Their flat curvatures and low multiplicities make them an interesting comparison case relative to the other echoes. Explore. fce+ resonance Resonances appear as continous vertical streaks on the high frequency spike of the electron gyrofrequency spikes. They occur in the same region as mid-altitude echoes, though the two phenomena are not correlated. The small delay times of the resonances again point to localized waves. Explore. Links Visit IMAGE website Visit Stanford VLF website