We present new and previously unreported in-situ observations of Hertz frequency multi-harmonic mode field line resonances detected by the Electric Field and Waves (EFW) instrument on-board the NASA Van Allen Probes during low-L perigee passes. Spectral analysis of the spin plane electric field data reveals the waves in numerous perigee passes, in sequential passes of Probes A and B, and with harmonic frequency structures from ~0.5 to 3.5 Hz which vary with L-shell, altitude, and from day to day. Comparing the observations to wave models using plasma mass density values along the field line given by empirical power laws and from the International Reference Ionosphere model (IRI), we conclude the waves are standing Alfvén field line resonances, and that only odd-mode harmonics are excited. The model eigenfrequencies are strongly controlled by the density close to the apex of the field line, suggesting a new diagnostic for equatorial ionospheric density dynamics.

We report observational evidence for a class of coherent magnetic dipolarization structures that are long lived and radially extensive. The reported dipolarization structures, a subset of general dipolarizations, typically remain coherent over 20-30 min in real time, 3-6 hours in MLT, and 10-20 Re in radial distance. Arrays of more than three spacecraft in non-collinear geometry are used to determine the propagation vector, including both the normal speed and direction, of such dipolarizations in the equatorial plane. The determined azimuthal propagation is ~3 deg/min, which corresponds to ~50 km/s at 6.6 Re. This speed is consistent with those obtained from two azimuthally separated spacecraft in previous works. Further analysis suggests that these azimuthally propagating dipolarizations (APDs) are often finger-like in shape, ranging from 5 to 20 Re in length and several Re in width. The reported APD may accompany the earthward flow channel and dipolarizing flux bundle (DFB).