Daytime Mesosphere and Lower Thermosphere Neutral Winds and their
relationship with lower E region layers over Arecibo
Abstract
The formation of layers at mid-latitudes has been related to neutral
winds activity at altitudes below 130km in the Mesosphere and Lower
Thermosphere (MLT). Recent SAMI3 simulations by Krall et al. (2020) of
ionospheric metallic layers at Arecibo suggest that forces induced by
the meridional winds cause low altitude layers near 100 km. However, the
classic mechanism, originally proposed by Whitehead (1961), correctly
states that zonal wind shear has a bigger effect than meridional wind
shear in the lower E region. Haldoupis and Shalimov (2021), referring to
observations of ionosonde-based sporadic E statistics and radio
occultation sporadic E measurements using low Earth orbiting satellites,
support the idea that zonal winds dominate layer formation at these
altitudes, apparently disputing the findings of Krall et al. (2020).
Perhaps the latest technique to continuously measure mid-latitude MLT
daytime neutral winds was developed by Hysell et al. (2014). That
technique used a unique configuration of the Arecibo radar dual-beam.
Unfortunately, since Arecibo lost the capability of the dual-beam in
2017 (when one antenna was destroyed by Hurricane Maria), there are only
few valuable data sets that can help elucidate the origin of the lower
altitude layers at Arecibo. We present Arecibo neutral wind data
correlated with lower altitude layers. While not disputing current
theory, we find that, near 100 km, meridional neutral wind shear can be
much stronger than zonal wind shear when a layer is present, with the
meridional shear correctly positioned to support the layer. We also
present a complete analysis of the vertical ion drift, including
declination, where the meridional winds become more important and with a
reversed mechanism for altitudes below 115km for Arecibo conditions.
References: Haldoupis et al. (2021),
https://doi.org/10.1016/j.jastp.2021.105537 Hysell et al. (2014),
http://doi.org/10.1002/2013JA019621 Krall et al. (2020),
https://doi.org/10.1029/2019JA027297