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 ∼4.8 × 10 9 electrons cm −2 and 3.4 × 10 7 ions cm −2 , while those of the neutrals to be low, of the order of 9.7 × 10 6 atoms cm −2 . The neutral abundance maximizes with values of 1.8 × 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.