Heavy ion density enhancements in the outer plasmasphere

Abstract

A region of density enhancements of thermal heavy ions (O+, O++, and N+) has been observed on numerous occasions by the retarding ion mass spectrometer on the Dynamics Explorer 1 satellite. Outer plasmasphere densities of heavy ions are often observed to be up to 2 orders of magnitude higher than equatorward densities within an orbital pass. This phenomenon is almost always observed in the region of the plasmasphere just inside the plasmapause and has been seen at all local times. A statistical study of these heavy ion density enhancements, covering almost 600 passes of DE 1 through the plasmasphere, shows that O+ and O++ enhancements occur over about 64% of the observed passes, with the highest frequency of occurrence being found in the late evening and morning regions. O+ enhancements tend to be seen more frequently in the morning, while O++ enhancements are more likely to be observed in the evening. O+ appears to have a higher outer plasmasphere density enhancement than O++ in all local time regions. Evidence showing that the L shell of the heavy ion density enhancement peaks is dependent upon Dst is presented. Finally, mechanisms for the creation of the heavy ion density enhancements are briefly discussed, including the “geomagnetic mass spectrometer” and plasmapause‐associated electron heating of the topside ionosphere.

Circulation of Heavy Ions and Their Dynamical Effects in the Magnetosphere: Recent Observations and Models

“…The O + density is significantly below He + but highly variable (Horwitz et al 1990). A narrow region just inside the plasmapause, termed the "O + torus", can have O + densities two orders of magnitude higher than further in Roberts et al (1987). The location of the plasmapause changes with geomagnetic activity (Kp index) and plumes of dense plasma from the plasmasphere are observed in the dusk sector, moving to the magnetopause.…”

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

“…Chappell [1982] ions in the outer plasmasphere. This "oxygen torus" has been identified in subsequent studies using the DE-1/RIMS data [Horwitz et al, 1984[Horwitz et al, , 1986Roberts et al, 1987;Comfort et al, 1988]. Horwitz et al [1984Horwitz et al [ , 1986 showed that the O + and O 2+ densities become comparable to or exceeds the H + density at L = 3-4.…”

“…Horwitz et al [1984Horwitz et al [ , 1986 showed that the O + and O 2+ densities become comparable to or exceeds the H + density at L = 3-4. A statistical analysis of the oxygen torus (identified by O + and O 2+ ions) was made by Roberts et al [1987], who found that (1) the oxygen torus is observed just inside the plasmasphere at all local time with higher occurrence frequency in the late evening and morning sectors, (2) it is identified on ∼50% of orbits, (3) its occurrence frequency does not depend on the Kp index and is still fairly high even in the low Kp case (Kp = 0-2+), and (4) a density peak of the oxygen torus generally shifts toward lower L shell with increasing geomagnetic activity. Comfort et al [1988] also performed a statistical study and showed an enhancement of O + and O 2+ ions in the outer plasmasphere (L = 3-5).…”

The Earth: Plasma Sources, Losses, and Transport Processes

“…From Sandel et al (2003) events, originating from deeper in the plasmasphere and having an outward expansion velocity towards higher L-shells. Chappell (1982) and Roberts et al (1987), using the retarding potential ion mass spectrometer on board the Dynamics Explorer 1 satellite, also reported heavy ion observations in the region of the plasmasphere just inside the plasmapause. These observations allowed the separation of O + , O ++ , N + and N ++ ions, all of which were observed in the plasmasphere.…”