Oxygen torus in the deep inner magnetosphere and its contribution to recurrent process of O⁺-rich ring current formation

Abstract

[1] Using the magnetic field and plasma wave data obtained by the Combined Release and Radiation Effects Satellite (CRRES), we search for enhancements of O + ion density in the deep inner magnetosphere known as "the oxygen torus". We examine 4 events on the dayside in which toroidal standing Alfvén waves appear clearly. From the frequency of the toroidal waves, the magnetospheric local mass density (r L ) is estimated by solving the MHD wave equation for realistic models of the magnetic field and the field line mass distribution. We also estimate the local electron number density (n eL ) from the plasma wave spectrograms by identifying narrow-band emission at the upper-hybrid resonance frequency. Assuming the quasi-neutral condition of plasma, we infer the local average ion mass (M L ) by r L /n eL . It is found that M L is approximately 3 amu in the plasma trough, while it shows an enhancement of >7 amu at L ∼ 4.5-6.5 that is close to the plasmapause at L ∼ 3.5-6.0. This indicates an existence of the oxygen torus in the vicinity of the plasmapause. The oxygen torus is found preferentially during the storm recovery phase. We interpret that these features of the oxygen torus (i.e., close relations with the plasmapause and the storm recovery phase) reflect its generation mechanism; that is, the ionospheric temperature is enhanced by heat conduction from high altitudes in the limited L range where the plasmasphere, because of its inflation during the recovery phase, encounters the ring current, and then the ionosphere has a larger scale height and supplies O + ions to the inner magnetosphere, resulting in the oxygen torus. We also discuss the contribution of the oxygen torus to the formation of the O + -rich ring current. It is proposed that the O + -rich ring current is formed in a recurrent process, in which the oxygen torus, the plasmasphere, and the ring current interact with each other.

Pc2-3 geomagnetic pulsations on the ground, in the ionosphere, and in the magnetosphere: MM100, CHAMP, and THEMIS observations

“…The distribution of particle and plasma parameters near the plasmapause is rather complicated due to the combination of steep plasma gradients at the plasmapause and the oxygen torus (Nose et al, 2011). This may lead to different dependencies of the Pc1-2 frequency on the concentration of heavy ions (Klimushkin et al, 2006;Mikhailova, 2014). However, O + ions are observed in the outer magnetosphere during magnetic storms (Yu and Ridley, 2013), although we have studied a quiet interval in SeptemberOctober 2007.…”

“…Pc2-3 frequencies are close to the oxygen cyclotron frequencies in the outer magnetosphere. The distribution of particle and plasma parameters near the plasmapause is rather complicated due to the combination of steep plasma gradients at the plasmapause and the oxygen torus (Nose et al, 2011). This may lead to different dependencies of the Pc1-2 frequency on the concentration of heavy ions (Klimushkin et al, 2006;Mikhailova, 2014).…”

“…MHD wave transmission through and reflection from a thin ionosphere has been studied theoretically in numerous papers beginning with (Nishida, 1964;Hughes and Southwood, 1976). After this initial period, the problem has also been studied for more general geometries of the background magnetic field (see, e.g., Leonovich and Mazur, 1991;Alperovich and Fedorov, 1993;Sciffer and Waters, 2002;Sciffer et al, 2004Sciffer et al, , 2005.…”

Estimating the effects of ionospheric plasma on solar wind/magnetosphere coupling via mass loading of dayside reconnection: Ion‐plasma‐sheet oxygen, plasmaspheric drainage plumes, and the plasma cloak

“…MPA cannot discern the ion species and the data analysis is made assuming all ions are protons (e.g., Figure , top). Ion‐composition measurements of the “torus of oxygen” [ Chappell , ] just outside the plasmasphere have found fractional oxygen number densities f O ~ 0.5 [ Horwitz et al ., ; Grew et al ., ; Nose et al ., ] (see also Fraser et al . [] and Dent et al .…”

Energization of O+ ions in the Earth's inner magnetosphere and the effects on ring current buildup: A review of previous observations and possible mechanisms

“…Previous in situ ion measurements have indicated the enhancements of cold O + ions adjacent to the plasmapause [e.g., Chappell , ; Horwitz et al ., , ; Roberts et al ., ; Comfort et al ., ]. The so‐called oxygen torus was confirmed with indirect methods that can estimate ion mass (from mass density determined from the frequency of the toroidal standing Alfvén waves and electron density determined from the upper hybrid resonance frequency) [ Denton et al ., ; Takahashi et al ., ; Nosé et al ., ]. It is possible that this O + ‐rich cold population could be accelerated up to >10 keV or even >100 keV to make a significant contribution to O + pressure in the inner magnetosphere, as proposed by Nosé et al .…”