The collapse of the midnight ionosphere and behavior of meridional neutral winds at Townsville over a full solar cycle

Abstract

This paper investigates the causes of the sudden descent (midnight collapse) of the ionosphere at Townsville, Australia, during the equinox periods of years between 1970 and 1980. The collapse of hmF2 at midnight is found to occur on 89% of the 330 equinox nights that are investigated, and the mean magnitude of the midnight collapse is 84 km in the March equinox periods and 99 km in the September equinox periods. Observations of hmF2 are used to determine equivalent meridional neutral winds using a first principles physics model. Harmonic analysis of these derived winds reveals the existence of significant diurnal (24 h), semidiurnal (12 h), and terdiurnal (8 h) tidal components. The contribution of wind harmonics to the midnight collapse is determined by band‐pass filtering the winds to only allow certain tides and then modeling their effect on hmF2 near midnight. The results indicate that the diurnal, semidiurnal, and terdiurnal components of the meridional neutral wind all play a significant role at various times, but the effect of the 6 h wind component is minimal. The spectral analysis also reveals that the terdiurnal wind component becomes dominant during solar maximum. Electric fields do not appear to be responsible for the midnight hmF2 collapse because it is seldom seen at the near‐conjugate station of Akita, Japan.

MENTAT: A New Wind Model for Earth's Thermosphere

“…The HWM14 wind has a 24‐hr tide that has half the power of the MENTAT 24‐hr tide, and the other components are very weak. These findings are consistent with previous research in which a complex interaction of the first three tidal components are the primary drivers for the midnight collapse (Crary & Forbes, ; Dandenault & Richards, ).…”

“…Journal of Geophysical Research: Space Physics research in which a complex interaction of the first three tidal components are the primary drivers for the midnight collapse (Crary & Forbes, 1986;Dandenault & Richards, 2015).…”

“…Day-to-day variations of the ionosphere may also result from waves from the lower atmosphere. For example, the semidiurnal tide has a strong signature in the ionosphere at low latitudes (Crary & Forbes, 1986;Dandenault & Richards, 2015;Richards & Wilkinson, 1998). Immel et al (2006) proposed that the observed 1,000-km scale longitudinal variation in ionospheric densities may vary with the strength of atmospheric tides that are driven by weather in the tropics.…”

Ionospheric Pre‐Sunrise Uplift: Comparison of Sanya Incoherent Scatter Radar Observations and Numerical Simulations

“…However, its night-to-night scale variability and the detailed mechanism were not involved in this study. Several other studies show similar phenomena but for the middle-latitude or equatorial area (e.g., Dandenault & Richards, 2015;Gong et al, 2012;Nicolls et al, 2006), which is not the focus region of our study. Furthermore, we did not find any similar investigations from other ISR measurement in low geomagnetic latitude region, such as Jicamarca (e.g., Milla et al, 2013) and ALTAIR ISR.…”

“…Before 02:00, the vertical E × B was downward with an amplitude of ∼20 m/s, which lowered the ionosphere. Although the wind tended to lift the ionosphere during this time interval, the combined net effect (b) of diffusion, E × B drift, and neutral wind lowered the ionosphere, which resulted in the so‐called post‐midnight ionospheric collapse (Dandenault & Richards, 2015; Gong et al., 2012). However, after ∼02:00, the downward E × B drift began to weaken and even reversed to become upward at ∼03:30.…”