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Abstract
In this study, midlatitude summer nighttime anomalies (MSNAs) are analyzed via observations and tidal/planetary wave features using measurements from the Formosat-3/Constellation Observing System for Meteorology, Ionosphere, and Climate (F3C) for 2007, a year with low solar activity, and 2014, a year with high solar activity. The total ionospheric electron content, ECion, an integrated quantity derived from F3C measurements, was used to compare the observational data. The ECion values were derived from accurate radio-occultation-retrieved electron density profiles without assuming spherical symmetry and from a model that separated the ground total electron content into the plasmaspheric and the ionospheric electron content contributions. An analysis of the ECion data set confirmed that MSNAs were present in three different regions of the world for the months surrounding the local summer solstice during both 2007 and 2014. In the southern hemisphere, the so-called Weddell Sea Anomaly showed a maximum increase in ECion, measured as the difference between nighttime and midday values, that was more than three times that in the northern MSNAs. For each individual MSNA, the corresponding maximum increases in electron content were similar between the two years analyzed, so they were not significantly affected by solar activity. Then, linear least-square fit to the frequency–wave number basis functions was used to derive the tidal and planetary wave components contributing to MSNAs. The main component that appears to produce the Weddell Sea Anomaly is D0, followed by SPW1, DW2, and DE1, in this order, which make secondary but still relevant contributions. The presence of MSNAs in the northern hemisphere was clearly supported by the migrating tide SW2 in combination with DE1. SW2 also supported an early morning MSNA being observed in the northern hemisphere. The main tidal and planetary wave signatures producing the MSNAs did not significantly differ between 2007 and 2014.
Highlights
IntroductionThe term midlatitude summer nighttime anomaly (MSNA) refers to the electron density (ED) or total electron content (TEC) in Earth’s ionosphere being greater during the night than during noon, which is a phase reversal compared with the usual diurnal cycle of the ionospheric ED or the TEC
We investigated the increase in summer nighttime ionospheric electron content (EC) at midlatitudes from observations and tidal and planetary wave (T/PW) features, comparing two years of extreme solar activity: 2007 and 2014
A new quantitative variable was used: the EC from the ionosphere calculated after excluding the plasmaspheric contribution to the total electron content (TEC)
Summary
The term midlatitude summer nighttime anomaly (MSNA) refers to the electron density (ED) or total electron content (TEC) in Earth’s ionosphere being greater during the night than during noon, which is a phase reversal compared with the usual diurnal cycle of the ionospheric ED or the TEC. MSNAs are known to occur in three regions [1–4], with all of them located within the middle magnetic latitudes. The first, discovered in the 1950s using ionosonde measurements [5], is commonly known as the Weddell Sea. Anomaly (WSA), and is observed in the southern hemisphere around geographic longitude. The two other regions, observed in the northern hemisphere, are identified as MSNAs [1–3], with no unified terminology to describe them.
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