Quasi-coherent chorus properties: 1. Implications for wave-particle interactions

Abstract

A study of dayside ELF/VLF electromagnetic (EM) waves from L* = 2 to 9 and magnetic local time (MLT) from 09 to 15 was conducted using plasma wave data from the Polar spacecraft. EM waves were detected from L* = 4 to 9 from 09 to 12 MLT with a decrease in the afternoon sector (12 to 15 MLT). Some of the chorus was clearly related to generation by substorm injected ∼5 to 100 keV electrons drifting from the midnight sector to the local noon sector. However, dayside chorus also showed two solar wind ram pressure dependences: increased (above average) pressures and unusually low pressures. Possible chorus generation mechanisms are discussed. Chorus detected by Polar away from the magnetic equator generation region (∼25° to 55° magnetic latitude) was substantially different than chorus detected in previous studies within the ∼0° to 10° generation region. (1) Two separate bands of chorus were often detected simultaneously: a higher-frequency downgoing (toward the Earth) band of waves and a lower-frequency upcoming band. (2) The downgoing waves are ∼2 orders of magnitude more intense (∼10−2 nT2) than simultaneously detected lower-frequency upcoming waves (∼10−4 nT2). (3) Chorus, when viewed as a Fourier spectrum, appears as a band of semicoherent hiss. (4) A scenario and schematic is presented to explain these observations: chorus is presumed to be generated at the equator at large L*, propagate downward toward Earth and inward across L* shells, and then refract back up to the spacecraft location. (5) The waves detected at Polar latitudes did not possess the temporal structure or the coherency of the ∼10 to 100 ms duration equatorial chorus subelements, although full single cycles with right-hand, circularly polarized structures were identified. This quasi-coherent EM turbulence may be formed by wave dispersive effects. The longer the wave path length, the greater is the reduction in coherency. (6) This feature of chorus has significant consequences for off-equatorial wave-particle interactions. For example, the microburst mechanism of Lakhina et al. (2010) that can account for rapid pitch angle diffusion of ∼5 to 100 keV electrons in the chorus generation region will not work for off-equatorial scattering of relativistic electrons because of the lack of chorus coherence there. (7) Some comments about semicoherent chorus (hiss) in the outer magnetosphere are made as challenges to theorists in the field.