Abstract
Abstract. We present Van Allen Probe B observations of azimuthally limited, antisymmetric, poloidal Pc 4 electric and magnetic field pulsations in the pre-midnight sector of the magnetosphere from 05:40 to 06:00 UT on 1 May 2013. Oscillation periods were similar for the magnetic and electric fields and proton fluxes. The flux of energetic protons exhibited an energy-dependent response to the pulsations. Energetic proton variations were anticorrelated at medium and low energies. Although we attribute the pulsations to a drift-bounce resonance, we demonstrate that the energy-dependent response of the ion fluxes results from pulsation-associated velocities sweeping energy-dependent radial ion flux gradients back and forth past the spacecraft.
Highlights
Toroidal and poloidal Pc 4 and Pc 5 pulsations are common in both the dayside and nightside magnetosphere (e.g., Arthur and McPherron, 1981; Hughes et al, 1978; Anderson et al, 1990)
Toroidal pulsations have been attributed to a wide variety of mechanisms, including field line resonances triggered by abrupt changes and sudden impulses in the solar wind dynamic pressure (e.g., Zhang et al, 2010; Sarris et al, 2010), the Kelvin–Helmholtz instability (KHI) at the magnetopause (e.g., Pu and Kivelson, 1983; Fujita et al, 1996), and bursty reconnection and the generation of flux transfer events on the magnetopause (Gillis et al, 1987)
While we attribute the pulsation to a drift-bounce resonance, we demonstrate that the associated energy-dependent energetic ion flux variations result from pulsation-associated radial velocities periodically sweeping preexisting radial flux gradients back and forth across the spacecraft
Summary
Toroidal and poloidal Pc 4 and Pc 5 pulsations are common in both the dayside and nightside magnetosphere (e.g., Arthur and McPherron, 1981; Hughes et al, 1978; Anderson et al, 1990). Hughes et al (1979) studied poloidal Pc 4 pulsation and proton flux oscillations at geosynchronous altitudes in the midnight sector and found that proton fluxes in the lowenergy channels (< 2.1 keV) varied in quadrature with the magnetic field strength They explained the proton flux modulation by a radial drift motion produced by the azimuthal electric field of the polarized wave. Claudepierre et al (2013) presented NASA Van Allen probes observations of wave–particle interaction between ultralow-frequency (ULF) waves and energetic electrons They showed that the 3 min period flux modulations observed in the 20–500 keV channels were due to a localized drift resonance between 57 and 80 keV electrons and fundamental poloidal mode Pc 5 ULF waves observed near 06:00 MLT and L ∼ 6. While we attribute the pulsation to a drift-bounce resonance, we demonstrate that the associated energy-dependent energetic ion flux variations result from pulsation-associated radial velocities periodically sweeping preexisting radial flux gradients back and forth across the spacecraft
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