An analytic model of toroidal half-wave oscillations: Implication on plasma density estimates

• Published in: Journal of geophysical research. Space physics 2015-06, Vol.120 (6), p.4164-4180
• Main Authors: Bulusu, Jayashree, Sinha, A. K., Vichare, Geeta
• Format: Article
• Language: English
• Subjects: Boundary conditions; Computation; Density; Estimates; Exact solutions; field line oscillations; geomagnetic pulsations; Ion density (concentration); Ionosphere; Magnetic fields; Magnetospheres; Mathematical analysis; Mathematical models; Oscillations; Plasma; Plasma density; plasma density modeling; Power law; ULF waves
• ISSN: 2169-9380; 2169-9402
• DOI: 10.1002/2014JA020797

The developed analytic model for toroidal oscillations under infinitely conducting ionosphere (“Rigid‐end”) has been extended to “Free‐end” case when the conjugate ionospheres are infinitely resistive. The present direct analytic model (DAM) is the only analytic model that provides the field line structures of electric and magnetic field oscillations associated with the “Free‐end” toroidal wave for generalized plasma distribution characterized by the power law ρ = ρo(ro/r)m, where m is the density index and r is the geocentric distance to the position of interest on the field line. This is important because different regions in the magnetosphere are characterized by different m. Significant improvement over standard WKB solution and an excellent agreement with the numerical exact solution (NES) affirms validity and advancement of DAM. In addition, we estimate the equatorial ion number density (assuming H+ atom as the only species) using DAM, NES, and standard WKB for Rigid‐end as well as Free‐end case and illustrate their respective implications in computing ion number density. It is seen that WKB method overestimates the equatorial ion density under Rigid‐end condition and underestimates the same under Free‐end condition. The density estimates through DAM are far more accurate than those computed through WKB. The earlier analytic estimates of ion number density were restricted to m = 6, whereas DAM can account for generalized m while reproducing the density for m = 6 as envisaged by earlier models. Key Points Analytic solution for Free‐end modes Importance of boundary condition on plasma density estimation Improvement over WKB method