Data of three all sky cameras in Kiruna and Tjautjas (Sweden) were used for estimation of altitude of pulsating arc like forms and investigation of so called internal modulation (2 3 Hz) during the ON phase of a few second pulsations. It is found that for two closely spaced arcs, internal modulation took place only in the lowest arc. Based on this finding , an explanation is proposed due to the pulsations of anomalous resistance in the field-aligned current associated with a pulsating structure appart to the traditional mechanism of particle scattering by VLF waves.