The third hop whistler anomaly: possible evidence of df/dt-dependent wave amplification
On occasion the third hop of an echoing magnetospheric whistler exceeds the first hop in intensity over some low range of frequencies. Case studies show that this anomalous behavior can occur over periods ranging from tens of minutes to hours in duration. Most of the five cases studied involved coupling of waves from one ducted magnetospheric path to another (at the point of ionospheric reflection) rather than repeated propagation on the same path. The coupling tended to be from lower to higher latitudes. In two of the five cases, it occurred from paths inside the plasmapause to paths outside. The anomaly is interpreted as the result of df/dt-dependent amplification of weak whistler signals by a gyroresonant wave-particle interaction in the magnetosphere. This amplification is facilitated by interpath coupling. Through this process the waves are either able to retain a degree of coherence not achieved by other signals in the same whistler and/or are coupled into a region of relatively large interacting particle fluxes. The anomaly appears to provide a way of looking with some degree of isolation at the factors of spectral purity, df/dt, and magnetospheric location (with respect to particle activity) that may affect the amplitication of much larger classes of whistlers.