Radar, satellite, and modelling studies of the low-latitude protonosphere

A study of the variations in the composition of the topside ionosphere at low and equatorial latitudes is presented. Observations of (H{dollar}sp+{dollar}) /{dollar}nsb{lcub}e{rcub}{dollar} are used to study the behavior of the transition altitude over Arecibo during summer solstice for both solar minimum and solar maximum conditions. The solar maximum observations are complemented with DE-2 RPA data from passes over Arecibo to estimate the concentrations of helium ions in the topside at that location. The transition altitude over Arecibo is found to be very dependent on solar activity level. For summer solar maximum conditions, the transition altitude varies anywhere from 2000 to 2500 km in altitude during the day (depending on solar flux) down to 1000 km at night. The DE-2 composition data show that during solar maximum conditions, helium ions are an important fraction of the total ionospheric plasma near 900 km over Arecibo with typical concentrations of 1-3 {dollar}times{dollar} 10{dollar}sp4{dollar} ions cm{dollar}sp{lcub}-3{rcub}.{dollar}; Observations from the BIMS and RPA instruments aboard the AE-E spacecraft are used to study the O{dollar}sp+,{dollar} H{dollar}sp+,{dollar} and He{dollar}sp+{dollar} concentrations in the equatorial topside ionosphere during solar minimum conditions. For these conditions He{dollar}sp+{dollar} is found to be a minor species and essentially negligible at all times. The transition altitude is found to be near 800 km during the day and below 600 km during the night for all seasons.; Finally, we present simulations using the University of Sheffield coupled ionosphere thermosphere theoretical model. The results of these simulations are used to try to reproduce our observations and also to study the effect of equatorial E {dollar}times{dollar} B drifts on the transition altitude. The model reproduces the major diurnal features in qualitative manner; it does especially well with our equatorial solar minimum observations. However, more work is needed to achieve quantitative agreement with the Arecibo solar maximum conditions results and with the DE2 He{dollar}sp+{dollar} measurements. For solar maximum conditions, the simulations predict peak helium ion concentrations in the topside over Arecibo for equinox with the lowest concentrations occurring during summer solstice.