Comparison of the He⁺ layer observed over Arecibo during solar maximum and solar minimum with CTIP model results

Abstract

[1] Light ion concentration observations from the Arecibo incoherent scatter radar have shown regions of He + layering in the topside ionosphere in the postmidnight hours. The He + layering phenomenon has been observed during both solar maximum and solar minimum conditions. During solar maximum the layer is well defined, giving maximum relative abundances of over 50%. The observational results also show regions of He + dominance in the topside ionosphere during solar maximum in the height range 750-1200 km. During solar minimum the magnitude of the layer is less, giving a maximum relative abundance of only $20%; however, the layer is still a well-defined feature. The coupled thermosphere-ionosphere-plasmasphere (CTIP) model is shown to model accurately the He + layer and the regions of He + dominance, provided that the modeled temperature profiles are normalized to the observed values. This study serves to further highlight the importance of helium ions in the topside ionosphere, particularly during solar maximum nighttime conditions.

Observations of He+ concentrations in the topside ionosphere under equinox conditions near solar maximum: Morningside morphology

“…As a result, He + tends to concentrate in a layer at and above the O + ‐H + transition region throughout the night. Wilford et al [2003] have found that the CTIP model required modification to accurately produce He + concentrations that agreed with observational data taken at Arecibo. They found He + constituted up to 57% of the postmidnight ion composition under solar maximum conditions.…”

“…While the data taken at Arecibo [ Erickson and Swartz , 1994; Wilford et al , 2003; Gonzalez et al , 2004] showed higher values of the He + fraction, it should be noted that those measurements were taken throughout a night over an altitude range while at a fixed latitude and longitude. The data presented here are fixed in altitude and local time, while covering all latitudes and longitudes.…”

“…Investigations using the incoherent scatter radar at Arecibo, Puerto Rico, have revealed the formation of a nighttime He + layer. The percentage of He + ranged from 10 to 20% under solar minimum conditions at equinox [ Gonzalez and Sulzer , 1996] and 30% under solar maximum conditions in the winter [ Erickson and Swartz , 1994] to as much as 57% at equinox under solar maximum conditions [ Wilford et al , 2003]. Gonzalez et al [2004] examined additional Arecibo data and found examples of the He + fraction reaching up to 60–65% of the total ion concentration under solar maximum conditions at equinox.…”

“…Ambipolar electric fields tend to hold He + above the O + level, preventing it from diffusing downward at night to altitudes where it can be lost through reactions with N 2 [ Murphy et al , 1979]. Wilford et al [2003] found that the formation of a nighttime He + layer above Arecibo is correlated to the location of the O + ‐H + transition height. During the day, the O + ‐H + transition height is broad, and the He + level formed overnight expands in altitude, making He + a minor species.…”

Derivation of neutral oxygen density under charge exchange in the midlatitude topside ionosphere

“…The transition height, h t , between the O + dominated ionosphere and the H + dominated protonosphere, collapses following sunset as seen in Figure A1c. This collapse is associated with the formation of a He + layer near h t ( Figure A1d), which forms under ambipolar diffusion and attains a maximum peak density near 0400 LT [González and Sulzer, 1996;Wilford et al, 2003;González et al, 2004].…”