Magnetospheric chorus - Occurrence patterns and normalized frequency

Abstract New characteristics of VLF chorus in the outer magnetosphere are reported. The study is based on more than 400 hours of broadband (0.3–12.5 kHz) data collected by the Stanford University/Stanford Research Institute VLF experiment on OGO 3 during 1966–1967. Bandlimited emissions constitute the dominant form of whistler-mode radiation in the region 4⪝ L⪝ 10 . Magnetospheric chorus occurs mainly from 0300 to 1500 LT, at higher L at noon than at dawn, and moves to lower L during geomagnetic disturbance, in accord with ground observations of VLF chorus. Occurrence is moderate near the equator, lower near 15°, and maximum at high latitudes (far down the field lines). The centre frequency ƒ of the chorus band varies as L−3> and at low latitudes is closely related to the electron gyrofrequency on the dipole field line through the satellite. Based on the measured local gyrofrequency ƒ H , the normalized frequency distribution of chorus observed within 10° of the dipole equator shows two peaks, at ƒ ƒ H ≅ 0.53 and ƒ ƒ H ≅ 0.34 . This bimodal distribution is a persistent statistical feature of near equatorial chorus, independent of L, LT and Kp. However there is considerable variability in individual events, with chorus often observed above, below, and between these statistical peaks; in particular, it is not unusual for single emissions to cross ƒ ƒ H = 0.50 . When two bands are simultaneously present individual emission elements only rarely show one-to-one correlation between bands. For low Kp the median bandwidth of the upper band, gap and lower band are all ∼16% of their centre frequencies, independent of L; for higher Kp the bandwidth of the lower band increases. Bandwidth also increases with latitude beyond ~10°. Starting frequencies of narrowband emissions range throughout the band. The majority of the emissions rise in frequency at a rate between 0.2 and 2.0 kHz/sec; this rate increases with Kp and decreases with L. Falling tones are rarely observed at dipole latitudes ƒ ƒ H ≅ 0.5 , at which the curvature of the refractive index surface vanishes at zero wave normal angle. Near this frequency rays with initial wave normal angles between 0° and −20° are focused along the initial field line for thousands of km, enhancing the phase-bunching of incoming gyroresonant electrons. The upper peak in the bimodal normalized frequency distribution is attributed to this enhancement near the critical frequency, at latitudes of ~5°. Slightly below the critical frequency interference between modes with different ray velocities may contribute to the dip in the bimodal distribution. The lower peak may reflect a corresponding peak in the resonant electron distribution, or guiding in field-aligned density irregularities. The observations are consistent with gyroresonant generation of emissions near the equator, followed by spreading of the radiation over a range of L shells farther down the field lines.