AbstractObservationally, electron cyclotron harmonic (ECH) waves are often terminated at the outer boundary of the plasmasphere boundary layer (PBL, i.e., plasmapause). Physics of this empirical relation is not well established. In this study, two categories of ECH waves are shown by their different behaviors near PBL. For Category I, all bands of ECH waves terminate at PBL because the density ratio (nh/nc) between hot and cold electrons decreases dramatically across PBL. For Category II, ECH waves, especially the lower harmonic bands, can be excited deeper inside the plasmasphere because nh/nc gradually decreases across PBL. A statistical study using Van Allen Probes observation is performed for these two categories. We find that the two categories of ECH waves are preferred to occur at nightside and dawnside. The two categories of ECH waves may be separated by the wave intensity outside the PBL or nh/nc with the threshold on the order of 10−10–10−9 (V/m)2 and 10−2, respectively.