Descending layer variability over Arecibo

Abstract

Descending layers of ionization over Arecibo exhibit very diverse behavior from night to night that does not appear to be strongly correlated to geomagnetic activity, solar forcing, or average semidiurnal tidal winds. On some nights, three or more distinct layers are observed to form near 170 km over timescales of ∼2 hours. Rather than descending smoothly over periods of several hours, these layers stall, abruptly disappear, or even reverse direction in the midst of their descent. The time scales for their disappearance are examined and compared to loss rates arising from diffusion and recombination. Diffusion alone is found to be too slow to account for the observations, but recombination is fast enough provided that the convergent wind shear that forms the layer is relatively weak coincident with their disappearance. The continuity equation is solved in conjunction with a time sequence of radar profiles to estimate the vertical drift and horizontal neutral wind consistent with the observed behavior. The resultant wind field is northward, has an average speed of ∼80 m s−1, and varies significantly near the altitude where the layers are observed. These inferred winds are consistent with the presence of the observed layers, and their magnitudes as obtained from the classical continuity and momentum equations are reasonable for this altitude range.

The intermediate layers and associated tidal motions observed by a digisonde in the equatorial anomaly region

“…However, if the layer motion is considered, the increase/decrease of densities is more meaningful. During the single-peak months, the layer descent and associated density increase demonstrate that the descending convergent null could capture a large number of molecular and metallic ions (Osterman et al, 1994(Osterman et al, , 1995Carter and Forbes, 1999;Earle et al, 2000). Moreover, the morning descents in the doublepeak months may also share the same cause.…”

“…Mathews et al, 1993;Szuszczewicz et al, 1995;Earle et al, 2000). ISR observations can easily detect the layers at night, when background E-region density is low, as is the signal-to-noise ratio (Earle et al, 2000). The layer data (descent rate, formation time, and final descent altitude) can be obtained from the ISR, except for wind and composition data (Shen et al, 1976;Mathews et al, 1993;Earle et al, 2000).…”

“…Mathews et al, 1993;Szuszczewicz et al, 1995;Earle et al, 2000). ISR observations can easily detect the layers at night, when background E-region density is low, as is the signal-to-noise ratio (Earle et al, 2000).…”

“…ISR observations can easily detect the layers at night, when background E-region density is low, as is the signal-to-noise ratio (Earle et al, 2000). The layer data (descent rate, formation time, and final descent altitude) can be obtained from the ISR, except for wind and composition data (Shen et al, 1976;Mathews et al, 1993;Earle et al, 2000). However, during the daytime the layers are less prominent in radar power profiles due to the higher background plasma density in the E-F valley region (Earle et al, 2000).…”

“…− 10 10 el/m 3 form in the daytime and nighttime, respectively (Wilkinson et al, 1992;Mathews et al, 1993;Szuszczewicz et al, 1995;Earle et al, 2000).…”