Characteristics of neutral calcium and Ca+ near the mesopause, and their relationship with sporadic ion/electron layers at Arecibo

We present the first simultaneous observations of mesospheric neutral calcium, its ion, and electron density over Arecibo obtained on adjacent nights in December 2009. A good correlation is seen between Sporadic-E and ion layers while the neutrals have different characteristics. The measurements reveal that average abundances in electron densities and ions to be at a maximum on 9 December with values similar to 4.8 x 10(9) electrons cm(-2) and 3.4 x 10(7) ions cm(-2), while those of the neutrals to be low, of the order of 9.7 x 10(6) atoms cm-2. The neutral abundance maximizes with values of 1.8 x 10(7) atoms cm(-2) on 10 December, when both ions and electrons display weak layers with abundances almost 3 times lower than on the previous night. A significant correlation between the ions and neutrals indicates that sudden enhancements are caused by advection while a poor correlation is related to chemistry. This study reveals that the ion-neutral coupling is most efficient between 92 and 95 km. Also, the ion-to-neutral ratio is dependent on the strength of the ions or Sporadic-E and can exceed 3.0 during such events. We present evidence of neutralization of the ionized layers occurring on scales of a few tens of minutes for the first time. This agrees fairly well with the lifetimes of Ca+ against neutralization as predicted by recent ion-neutral reactions. Citation: Raizada, S., C. A. Tepley, N. Aponte, and E. Cabassa (2011), Characteristics of neutral calcium and Ca+ near the mesopause, and their relationship with sporadic ion/electron layers at Arecibo, Geophys. Res. Lett., 38, L09103, doi: 10.1029/2011GL047327.