Sporadic Sodium Layers and the Average Vertical Distribution of Atmospheric Sodium

We have used atmospheric sodium profiles obtained at São José do Campos over the past 30 years to study the influence of sporadic layers on the background sodium distribution. All the profiles for the time period 19 to 22 hours local time were individually classified as either normal or sporadic. Profiles were considered sporadic if they contained sharp increases above the background profile by more than a factor of 2, occurring on the topside of the layer, with a width of less than 10 km. For each night's data the averages of the normal and sporadic profiles were stored separately. A comparison of the average profiles for those days on which both normal and sporadic profiles were observed showed no significant difference between them. This indicates that, on average, there is no additional sodium in the sporadic profiles. As a consistency check, the same analysis was applied to the data divided into two subsets consisting of alternate days. We found the same result for the two subsets. This analysis suggests that at least part of the topside of the sodium layer originates in a source that also gives rise to sporadic layers, which would occur when the source itself becomes layered. The most likely source is sodium ions, which would be layered by the tide and/or gravity wave driven wind-shear mechanism believed to be responsible for some types of ionospheric sporadic-E.