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

[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 (2002a, 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.

[1]  Paul D. Craven,et al.  Global Core Plasma Model , 2000 .

[2]  B. J. Fraser,et al.  A technique to investigate plasma mass density in the topside ionosphere using ULF waves , 1999 .

[3]  J. Matthew,et al.  The variation of geomagnetic micropulsation periods with latitude and the plasmapause , 1971 .

[4]  M. Gokhberg,et al.  Restoration of the meridional structure of geomagnetic pulsation fields from gradient measurements , 1989 .

[5]  M. Seto,et al.  Using two ground stations to identify magnetospheric field line eigenfrequency as a continuous function of ground latitude , 2002 .

[6]  J. Samson,et al.  Variation of plasmatrough density derived from magnetospheric field line resonances , 1996 .

[7]  R. Denton,et al.  Latitudinal Density Dependence of Magnetic Field Lines Inferred from Polar Plasma Wave Data , 2001 .

[8]  J. Weygand,et al.  An automated method for the detection of field line resonance frequencies using ground magnetometer techniques , 2003 .

[9]  M. Kivelson,et al.  Alfven wave resonances in a realistic magnetospheric magnetic field geometry , 1981 .

[10]  R. McPherron,et al.  Harmonic structure of Pc 3–4 pulsations , 1982 .

[11]  S. Hattingh,et al.  Pc 3 pulsation eigenperiod determination at low latitudes , 1987 .

[12]  V. Troitskaya,et al.  Geomagnetic micropulsations and diagnostics of the magnetosphere , 1967 .

[13]  Nikolai A. Tsyganenko,et al.  A model of the near magnetosphere with a dawn-dusk asymmetry 2. Parameterization and fitting to observations: A NEW MAGNETOSPHERE MAGNETIC FIELD MODEL, 2 , 2002 .

[14]  J. L. Green,et al.  Plasma density distribution along the magnetospheric field: RPI observations from IMAGE , 2001 .

[15]  Phase structure of low-latitude Pc3 - 4 pulsations , 1991 .

[16]  F. Menk,et al.  A coordinated ground‐based and IMAGE satellite study of quiet‐time plasmaspheric density profiles , 2003 .

[17]  S. M. Krylov,et al.  High resolution method of direct measurement of the magnetic field lines' eigen frequencies , 1985 .

[18]  N. Tsyganenko,et al.  A model of the near magnetosphere with a dawn-dusk asymmetry 1. Mathematical structure , 2002 .

[19]  Robert Rankin,et al.  Dispersive shear Alfvén waves on model Tsyganenko magnetic field lines , 2001 .

[20]  Barry J. Fraser,et al.  Monitoring spatial and temporal variations in the dayside plasmasphere using geomagnetic field line resonances , 1999 .

[21]  J. L. Green,et al.  A plasmaspheric mass density model and constraints on its heavy ion concentration , 2005 .

[22]  R. Denton,et al.  Frequencies of standing Alfvén wave harmonics and their implication for plasma mass distribution along geomagnetic field lines: Statistical analysis of CRRES data , 2004 .

[23]  N. Tsyganenko,et al.  Modeling the dynamics of the inner magnetosphere during strong geomagnetic storms , 2005 .

[24]  Justin C. Kasper,et al.  Storm‐time distortion of the inner magnetosphere: How severe can it get? , 2003 .

[25]  V. Tikhonchuk,et al.  Field line resonances in a stretched magnetotail: CANOPUS optical and magnetometer observations , 2002 .

[26]  Colin L. Waters,et al.  The temporal variation of the frequency of high latitude field line resonances , 1995 .

[27]  P. Coleman,et al.  Standing Alfvén waves in the magnetosphere , 1969 .

[28]  James A. Slavin,et al.  Heavy ion mass loading of the geomagnetic field near the plasmapause and ULF wave implications , 2005 .

[29]  R. Denton,et al.  Determining the mass density along magnetic field lines from toroidal eigenfrequencies , 2000 .