Magnetospheric field‐line resonances: Ground‐based observations and modeling

Abstract

[1] We present theory and ground-based observations of field-line resonances (FLRs) excited in Earth's magnetosphere. Three FLR observations are reported, which correspond to large-scale standing shear Alfvén wave (SAW) oscillations on nightside field lines extending from premidnight to close to dawn. The eigenfrequencies for these events are modeled using a nonorthogonal covariant-contravariant analysis of the ideal magnetohydrodynamic (MHD) equations. This allows us to use a general field-line topology, an example of which is computed using the Tsyganenko 1996 magnetic field model. We show that field-line stretching, along with assumptions regarding the distribution of density along field lines, is sufficient to explain the observed FLR frequencies. Then, we consider dispersive effects operating at the level of the electron inertial scale near the ionosphere or the ion-acoustic gyroradius in the vicinity of the equatorial plane. Specifically, we estimate the spatial saturation widths and phase mixing timescales based on a simple model of dispersive SAWs. By considering a new model for ionospheric Pedersen conductivity modification by field-aligned currents in SAWs and numerical (finite element model) solutions to the reduced MHD equations, we model specific FLR observations in an approximate axisymmetric field topology. We discuss the interplay of linear and nonlinear phase mixing, along with gradients in dispersion and time-dependent losses due to Pedersen conductivity enhancements.

Alfvén Waves and Their Roles in the Dynamics of the Earth’s Magnetotail: A Review

“…Ground-based observations provide evidence, supported by numerical work, that latitudinally narrow FLR in the 1-4 mHz band, Sect. 2.1, cause these auroral arcs (e.g., Samson et al 1991aSamson et al , 1991bSamson et al , 1996Samson et al , 2003Xu et al 1993;Trondsen et al 1997;Rankin et al 1999aRankin et al , 1999bRankin et al , 2000Rankin et al , 2005Milan et al 2001;. Direct observations of FLR in the CPS and their association with auroral structures are however very sparse.…”

“…Ample ground-based evidence of nightside FLR activity in the 1-7 mHz band (i.e., the Pc5 band) on field lines that map from the CPS is available (e.g., Samson et al 1991aSamson et al , 1991bSamson et al , 1992aSamson et al , 1992bSamson et al , 1996Ruohoniemi et al 1991;Ziesolleck and McDiarmid 1994;Rankin et al 1999aRankin et al , 1999bRankin et al , 2000Rankin et al , 2005. However, few studies have correlated ground-based observations with low-altitude spacecraft (<4000 km) observations (e.g., Lotko et al 1998;and Samson et al 2003).…”

Polarization properties of standing shear Alfvén waves in non-axisymmetric background magnetic fields

“…We note that in the particular case of compressed dipole background field, we have explicit expressions for the fieldaligned coordinate system as functions of the usual spherical coordinates. This greatly simplifies our calculations and makes many numerical steps described in the previous papers (Rankin et al, 2005(Rankin et al, , 2006Kabin et al, 2007) unnecessary. The availability of analytical expressions for the gradients of the coordinates also allows us to minimize numerical errors.…”

The interaction of ultra-low-frequency pc3-5 waves with charged particles in Earth’s magnetosphere

“…Self-consistent electric and magnetic fields of standing mode ULF waves can be calculated using the approach described in Rankin et al (1999Rankin et al ( , 2005. A monochromatic driver with harmonic dependence exp i xt À m/ ð Þ ½ is added to the wave equations to excite poloidal mode ULF waves with a radial (azimuthal) magnetic (electric) field in the equatorial magnetosphere.…”