The persistence of the NWA effect during the low solar activity period 2007–2009

Abstract

The ionospheric Nighttime Winter Anomaly (NWA) was first reported more than three decades ago based on total electron content (TEC) and vertical sounding data. The aim of this paper is to provide further evidence that the NWA effect is a persistent feature in the Northern Hemisphere at the American and in the Southern Hemisphere at the Asian longitude sector under low solar activity conditions. The analysis of ground‐based GPS derived TEC and peak electron density data from radio occultation measurements on Formosat‐3/COSMIC satellites confirms and further supports the findings published in earlier NWA papers. So it has been confirmed and further specified that the NWA appears at longitude sectors where the displacement between the geomagnetic and the geographic equator maximizes. Here NWA peaks at around 40°–50° geomagnetic midlatitude supporting the idea that wind‐induced plasma uplifting in the conjugated summer hemisphere is the main driving force for the accumulation of ionospheric plasma in the topside ionosphere and plasmasphere. In parallel, the midsummer nighttime anomaly (MSNA) is caused at the local ionosphere. Simultaneously, interhemispheric coupling causes severe downward plasma fluxes in the conjugated winter hemisphere during night causing the NWA at low solar activity. With increasing solar activity, the downward plasma fluxes lose their impact due to the much stronger increasing background ionization that masks the NWA. It is assumed that MSNA and related special anomalies such as the Weddell Sea Anomaly and the Okhotsk Sea Anomaly are closely related to the NWA via enhanced wind‐induced uplifting of the ionosphere.

Nighttime Enhancements in the Midlatitude Ionosphere and Their Relation to the Plasmasphere

“…The CHAMP and DMSP observations confirm several midlatitude features in the ionosphere that have been mentioned in recent literature. During solstice, midlatitude enhancements are observed in the winter hemisphere (Figure 1), likely corresponding to the NWA feature (see Jakowski et al, 2015, and the references therein). Another recent study used N m F 2 measurements from the COSMIC constellation to find that the enhancements maximize at the magnetic latitudes of about 40 ∘ -50 ∘ in the winter hemisphere (Chen et al, 2015).…”

The importance of neutral hydrogen for the maintenance of the midlatitude winter nighttime ionosphere: Evidence from IS observations at Kharkiv, Ukraine, and field line interhemispheric plasma model simulations

“…Higher neutral hydrogen densities are also supported by recent measurements of the H‐alpha airglow [ Nossal et al ., ]. The FLIP plasmaspheric H + density in the equatorial plane (~3000 cm −3 ) calculated using doubled NRLMSISE‐00 H density is in good agreement with the results of whistler measurements conducted for L = 2 at Tihany (46.9°N, 17.9°E, 41.2° ILAT) during the October–February of 1970–1974 when F 10.7 index was mostly ~ 80–110 [ Singh et al ., , ]. It is likely that the H density plays a crucial role in the development of other types of nighttime N m F 2 enhancements such as the Nighttime Winter Anomaly [ Jakowski and Förster , ; Jakowski et al ., ]. The disappearance of the enhancements with increased solar activity may be explained by the decrease of the plasmaspheric fluxes due to the reduction in the H density.…”

“…12. It is likely that the H density plays a crucial role in the development of other types of nighttime N m F 2 enhancements such as the Nighttime Winter Anomaly [Jakowski and Förster, 1995;Jakowski et al, 2015]. The disappearance of the enhancements with increased solar activity may be explained by the decrease of the plasmaspheric fluxes due to the reduction in the H density.…”

“…Their simulation showed that nighttime enhancements of N m F 2 can occur when the plasmasphere becomes sufficiently full. Numerous experimental studies support the importance of the plasmaspheric flows for the formation of the nighttime electron density enhancements [e.g., Bertin and Papet‐Lepine , ; Davies et al ., ; Standley and Williams , ; Williams and Jain , ; Bailey et al ., ; Jakowski and Förster , ; Farelo et al ., ; Dabas and Kersley , ; Jakowski et al ., ; Chen et al ., ].…”

“…A number of mathematical modeling studies of the coupling between the F region and the plasmasphere were published over the past 40 years [e.g., Moffett and Murphy, 1973;Banks, 1974, 1975;Marubashi and Grebowsky, 1976;Bailey et al, 1978Bailey et al, , 1979Marubashi, 1979;Murphy et al, 1979Murphy et al, , 1980Quegan et al, 1982;Nagmoosh and Murphy, 1983;Schunk, 1988;Mikhailov et al, 2000;Richards et al, 2000;Pavlov and Pavlova, 2005;Jee et al, 2005]. Williams and Jain, 1984;Bailey et al, 1991;Jakowski and Förster, 1995;Farelo et al, 2002;Dabas and Kersley, 2003;Jakowski et al, 2015;Chen et al, 2015].…”