Pitch angle scattering of diffuse auroral electrons by whistler mode waves

Abstract

Resonant electron-whistler interactions in the plasma sheet are investigated as possible explanations of the nearly isotropic fluxes of low-energy electrons observed above the diffuse aurora. Whistler mode waves, propagating near the resonance cone with frequencies near or larger than half the equatorial electron cyclotron frequency, can interact with low-energy plasma sheet electrons. A Hamiltonian formulation is developed for test particles interacting with the coherent chorus emission spectra. We consider the second-order resonance condition which requires that inhomogeneities in the Earth's magnetic field be compensated by a finite bandwidth of wave frequencies to maintain resonance for extended distances along field lines. These second-order interactions are very efficient in scattering the electrons toward the atmospheric loss cone. Numerical calculations are presented for the magnetic shell L -5.5 for wave amplitudes of .-• 10 -6 V/m, using different frequency and magnetospheric conditions. 1. Introduction The pitch angle scattering of energetic electrons by whistler waves in the the Earth's radiation belts is a long-standing research problem [Lyons and Williams, 1984, and references therein]. Whistler waves are responsible for the precipitation of electrons in both the plasmasphere and the plasma sheet [Bell, 1984]. As electrons scatter toward smaller pitch angles, they give up small quantities of energy, amplifying the waves to the point where the interaction becomes self-sustaining. The limit for stably trapped particle fluxes was first investigated by Kennel and Petschek [1966], and later in self-consistent quasi-linear diffusion models by Bespalov and Tvakhtengerts [1986], Villaldn et al. [1989], and Villa16n and Burke [1991]. Past studies have considered whistler waves for which the ratio between the wave and the electron cyclotron frequencies is 0•/•e << 1. In this case, only electrons whose energies are larger than or of the order of the magnetic energy per particle Ec, may interact with the waves. Normalized to the electron rest energy, Ec = (•e/0•p) 2. Here, •e and 0•p are the electron cyclotron and the plasma frequencies, respectively. As pointed out by Johnstone el al. [1993], in the outer plasma sheet the threshold energy for resonant interactions is Paper number 95JA01161. 0148-0227/95/95JA-01161 $05.00 the possibility of resonant interactions of whistler mode waves with electrons with energies well below 10 keV. For these interactions to take place, the wave frequency must be close to The diffuse aurora is formed by nearly isotropic fluxes of electrons, mostly with energies of <10 keV, that precipitate from the plasma sheet [Johnstone, 1983; Inan et al., 1992]. A number of studies have attempted to explain the diffuse aurora by the interaction of the electrons with electrostatic electron cyclotron harmonic (ECH) waves [Swift, 1981]. However, it does not that the amplitudes of ECH waves are large enough to account for the electron precipitation [Belmont et al., 1983; Roeder and Koons, 1989]. ...

A study of fine structure of diffuse aurora with ALIS-FAST measurements

“…These are electron interaction with electromagnetic whistler mode waves (Kennel and Petscheck, 1966;Bespalov and Trakhtengerts, 1986;Johnstone et al, 1993;Villalón and Burke, 1995) and interaction with electrostatic electron cyclotron harmonic (ECH) waves (Kennel et al, 1970;Lyons, 1974;Horne and Thorne, 2000). While the interaction of whistler mode waves with high energy electrons (E>10 keV) was proved theoretically and experimentally, it was not clear that these waves would effectively interact with electrons at energies less than 10 keV that are typical of the diffuse auroral zone.…”

“…There are two main candidates. The first is that the particle precipitation is a result of pitch angle diffusion due to interaction of particles with electrostatic electron cyclotron harmonic (ECH) waves (Kennel and Petscheck, 1966;Bespalov and Trakhtengerts, 1986;Johnstone et al, 1993;Villalón and Burke, 1995). The alternative theory is electron scattering by whistler mode waves (Kennel et al, 1970;Lyons, 1974;Horne and Thorne, 2000).…”

Fine-scale dynamics of black auroras obtained from simultaneous imaging and particle observations with the Reimei satellite

“…However, Inan et al [1992] reported, on the basis of modeling work, that upper band (UB) whistler mode waves could scatter electrons in a range of energies from 1 to 10 keV into the loss cone. In addition, Villalón and Burke [1995] suggested that plasma sheet electrons scattered into the loss cone by whistler mode waves would produce a diffuse aurora. Our data clearly show that electron flux with energies greater than 2 keV is deficient in regions corresponding to black auroras, while electron flux corresponding to the surrounding diffuse aurora with energies below 2 keV shows no change.…”

“…In the downward electron data, we identify two types of large-scale electron precipitations: one type includes plasma sheet electrons that precipitate continuously from the beginning to the end, mostly in the keVenergy range, except for some deficiencies or dropouts, and the other represents low-energy (∼100-2000 eV) inverted V electrons from 08:15:17 to 08:15:29 UT. The former plasma sheet electron precipitation is not shaped like an inverted V, indicating that it was probably precipitated by pitch angle scattering [e.g., Johnstone et al, 1993;Villalón and Burke, 1995]. It seems that the low-energy inverted V electrons rarely contribute to the E region auroral emissions measured by MAC, since the E region aurora is generally produced by keV-range electrons.…”

Bounce-averaged Fokker-Planck diffusion equation in non-dipolar magnetic fields with applications to the Dungey magnetosphere

“…Resonant wave-particle interactions play an essential role in understanding the dynamics of radiation belt energetic electrons (e.g., Lyons et al, 1972;Summers et al, 1998Summers et al, , 2007aSummers et al, , 2009O'Brien et al, 2003;Albert, 2004Albert, , 2005Albert, , 2008Horne et al, 2005a, b;Meredith et al, 2003Meredith et al, , 2006Meredith et al, , 2007Meredith et al, , 2009aMiyoshi et al, 2006;Li et al, 2007;Shprits et al, 2008aShprits et al, , b, 2009bShprits et al, , 2011Thorne et al, , 2007Zong et al, 2009;Thorne, 2010;Xiao et al, 2009aXiao et al, , 2010b and plasma sheet source electrons (e.g., Inan et al, 1992;Johnstone et al, 1993;Villalón and Burke, 1995;Horne and Thorne, 2000;Horne et al, 2003;Ni et al, 2008Ni et al, , 2011aSu et al, 2009;Thorne et al, 2010;Tao et al, 2011b). To demonstrate the gyroresonant diffusion processes of magnetospheric electrons due to various plasma waves, quasi-linear theory has been well established assuming that the particle distribution function averaged over space changes slowly on time scales associated with the motion of the waves (Kennel and Engelmann, 1966).…”