Computing magnetospheric mass density from field line resonances in a realistic magnetic field geometry

Abstract

[1] Ultra-low-frequency (ULF) field line resonances can be used to infer the mass density along magnetospheric magnetic field lines. By specifying how mass density is distributed along the magnetic field (usually a power law as a function of distance from the Earth) and a dipole magnetic field geometry, the MHD standing wave equation can be analytically solved and mass density inferred from observed field line eigenfrequencies. However, the geometry of the Earth's magnetic field can deviate significantly from a dipole, even at relatively low L shells and on the dayside magnetosphere. This study investigates the importance of including a realistic magnetic field geometry when computing plasma mass density from observed field line eigenfrequencies. A generalized version of the toroidal mode MHD standing wave equation is solved using the Tsyganenko (2002aTsyganenko ( , 2002b) empirical magnetic field model (T01). The results are compared to those found using a dipole. We find that assuming a dipole magnetic field geometry results in an overestimation of mass density. The overestimation is larger for more disturbed levels of geomagnetic activity. Our results have important implications for the inference of heavy ions in the magnetosphere. Namely, an increase in heavy ion concentration as a result of enhanced geomagnetic activity will be exaggerated unless the proper magnetic field geometry is taken into account when calculating mass density from field line eigenfrequencies.

Comparison of equatorial plasma mass densities deduced from field line resonances observed at ground for dipole and IGRF models

“…At latitudes higher than those examined in this paper (L > 4), we expect from previous studies [Warner and Orr, 1979;Singer et al, 1981;Berube et al, 2006] that the effects of the external sources can become more relevant than those investigated here. In particular, Berube et al [2006], using the T01 magnetic field model [Tsyganenko, 2002], found that these effects are significant even at relatively low L-shells (L ≅ 3) for disturbed magnetospheric conditions (Dst <~À50 nT).…”

“…In particular, Berube et al [2006], using the T01 magnetic field model [Tsyganenko, 2002], found that these effects are significant even at relatively low L-shells (L ≅ 3) for disturbed magnetospheric conditions (Dst <~À50 nT). On the other hand, for average solar wind/magnetospheric conditions, a preliminary analysis conducted by Vellante et al [2013] shows that mass densities computed using the IGRF model result to be only moderately overestimated with respect to those computed using the T01 model (less than 10% for L ≤ 3, and at most 20% at L = 4).…”

“…A more exact method for computing field line eigenfrequencies was derived by Singer et al [1981]. The Singer et al [1981] method was then used by considering more and more sophisticated models of the magnetospheric field: Olson-Pftizer model [Walker, 1979;Singer et al, 1981], T87 model [Tsyganenko, 1987;Waters et al, 1995], T89 model [Tsyganenko, 1989;Takahashi et al, 2002], T96 model [Tsyganenko, 1995;Takahashi et al, 2004Takahashi et al, , 2006, T01 model [Tsyganenko, 2002;Berube et al, 2006;Kale et al, 2009], and TS05 model [Tsyganenko and Sitnov, 2005;Takahashi et al, 2010].…”

Remote sensing the plasmasphere, plasmapause, plumes and other features using ground-based magnetometers

“…Limitations of the FLR technique include the choice of fieldaligned density and magnetic field models (Berube et al 2006;Takahashi et al 2006;Vellante & Föster 2006), experimental uncertainty in determining resonant frequencies, and the lack of FLR observations at night.…”

The Rest of the Solar System

“…Singer et al (1981) investigated the structure of Alfvén waves in a realistic (nondipolar) field and provided tools for understanding how ULF wave frequencies depend on the structure of the background magnetic field. The work was formulated so that it can be applied to any field model, and is being used today in work relating resonant frequencies to mass content of plasmaspheric flux tubes (Berube et al 2006).…”