VLF emissions during magnetic storms and their association with 40‐keV electrons

Spectrograms of very low frequency radio noise recorded by University of Iowa satellite Injun 3 at invariant latitudes greater than 50°N are used to determine the behavior of VLF emissions during magnetic storms. Variations in the wide‐band intensity of VLF emissions from L = 3 to L = 8 are studied for the period from April 28 to August 28, 1963, by means of the automatic gain control levels of the satellite VLF receiver. During a sudden‐commencement magnetic storm the VLF emission called polar chorus characteristically appears at the onset of the storm, may increase in upper frequency extent to ∼5 kHz, may change from spike to burst structure (normal chorus), occurs over the greatest area on the fourth day of the storm, subsequently fades into low‐frequency spike‐structure polar chorus again, and eventually disappears into the ELF hiss band generally present. Chorus occurrence shows symmetry about the 9h 00m–21h 00m, magnetic local time meridian with a large maximum in magnetic morning and a smaller maximum in magnetic evening. Daily regions of occurrence are shown for the duration of a prototypical storm. Contour maps of wide‐band VLF field strength as a function of shell parameter L and universal time are presented for May through August 1963. A correlation with Dst is observed. The wide‐band VLF noise intensity rises from the background noise level at the onset of a magnetic storm and peaks during the early recovery phase. The Kennel‐Petschek limiting flux hypothesis is investigated by looking for VLF noise on the appropriate L shells when equatorial ≥40‐kev electron fluxes reach predicted limiting values. The VLF noise levels predicted in the equatorial plane by Kennel and Petschek are not observed at the altitude of the Injun 3 satellite (less than 2785 km).