Abstract
Abstract The Weddell Sea Anomaly (WSA) is defined when the nighttime plasma density is greater than the daytime density in the area near the Weddell Sea, more specifically in the region limited by 50° S–70° S in latitude and 225° E–315° E in longitude. A similar ionospheric anomaly is also observed near the Okhotsk Sea in the northern hemisphere, and such a feature was named as Okhotsk Sea Anomaly (OSA). The objective of this work is to infer possible physical causes of the WSA and OSA phenomena. To that end, we applied the principal component analysis (PCA) technique to the vertical total electron content (VTEC) from global International GNSS Service (IGS) in order to analyze the temporal and spatial variations of the ionosphere during noon and night in far-from-magnetic pole regions, during a 3-year period at high (2000–2002) and low (2006–2008) solar activity conditions. The first mode of PCA applied on VTEC scattering represents on average the 93 % of the total VTEC variability. Thus, the PCA expansions up to mode 1 resulted enough to show WSA and OSA during summer solstices in both solar activity conditions, as well as WSA during spring equinox during low solar activity. Besides, the analysis of the temporal variations of these first modes should provide the interpretation of a probable physical explanation to the observed anomalies. We conclude that the main contributors to the anomalies should be a combination of the same physical mechanisms that explain annual variation and semiannual anomaly in that regions located far from the magnetic poles.
Highlights
Having the greatest concentration of free electrons, the F2 region of the ionosphere is of huge interest in radio propagation
We show the existence of the Weddell Sea Anomaly and an equivalent feature in the northern hemisphere named as Okhotsk Sea Anomaly (OSA), taking into account their geomagnetic, seasonal, and solar activity dependences
In the “Results and discussion” section, we analyze the temporal and spatial behavior of vertical total electron content (VTEC) values at night and midday separately using Principal component analysis (PCA), and we present a discussion about the physical framework and the relative importance of the different excitation mechanisms
Summary
Having the greatest concentration of free electrons, the F2 region of the ionosphere is of huge interest in radio propagation. Meza et al Earth, Planets and Space (2015) 67:106 was rejected because the region near the Weddell Sea is located at mid-geomagnetic latitudes, which means high geographic latitudes Later, this phenomenon was associated with the longer daylight hours at these latitudes during summer and a combination of neutral winds blowing equatorward from day to night and from summer to winter (Dudeney and Piggott 1978). Liu and Yamamoto (2011) and Lin et al (2010) called this effect as “Midlatitude Summer Night Anomaly” (MSNA) The work by He et al (2009) on this anomaly based on FORMOSAT/ COSMIC data concludes that a possible cause for the NmF2 and the hmF2 increases could be due to the thermospheric wind effect. It states that photoionization is a first-order effect during the local summer and that the WSA arises from the evening enhancements induced by the winds and favored by the geometry of the magnetic field
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