Long‐term variations of the nighttime electron density enhancement during the ionospheric midlatitude summer

Abstract

[1] This study, for the first time, presented the long-term variations of Midlatitude Summer Nighttime Anomaly (MSNA) in the two hemispheres by using 66 ground-based ionosonde observations from 1957 to 2010. MSNA is characterized by the feature of higher nighttime electron density than daytime density in the midlatitude region during local summer months. Observations from 66 ionosonde stations were used to calculate the MSNA index which is defined by the difference between nighttime and noontime NmF2 values. The

Dusk-to-nighttime enhancement of mid-latitude <i>Nm</i>F2 in local summer: inter-hemispheric asymmetry and solar activity dependence

“…For example, the enhancement is more prominent at low solar activity level than at high solar activity level at Akita and Wakkanai, while it is more prominent at high solar activity level than at low solar activity level at Brisbane and Canberra, different from the conclusion of Chen et al (2012) that suggested a negative dependence of the MSNA index on solar activity. There is no obvious duskto-nighttime enhancement at low solar activity level at Brisbane and Canberra.…”

“…As for solar activity dependence of the occurrence of nighttime enhancement, the conclusions are somewhat discrepant when different ionospheric parameters were used (e.g., Chen et al, 2012;Horvath, 2006;Horvath and Essex, 2003;Jee et al, 2009;Liu et al, 2010). Chen et al (2012) measured nighttime enhancement of N mF2 with a mid-latitude summer nighttime anomaly (MSNA) index that quantifies the difference of N mF2 between nighttime and noontime to investigate the dependence of NmF2 nighttime enhancement on solar activity. They showed the MSNA index increases with solar activity decreasing and attributed it to the increment of neutral winds with solar activity decreasing (e.g., Kawamura et al, 2000).…”

“…A prominent nighttime enhancement phenomenon is the Weddell Sea Anomaly (e.g., Burns et al, 2008;He et al, 2009;Jee et al, 2009;Lin et al, 2009;Penndorf, 1965), which is characterized by higher electron density in nighttime than in daytime. Some mechanisms were proposed to explain nighttime enhancement of NmF2 (maximum electron density of the F2 layer), mainly including the combination of neutral wind effects and long-lasting sunlight at sunset as well as downward plasma flux (e.g., Bailey et al, 1991;Burns et al, 2008;Chen et al, 2012;He et al, 2009;Horvath and Essex, 2003;Le et al, 2014;Lin et al, 2009;Liu et al, 2010;Thampi et al, 2009). Nighttime equatorward wind uplifts the ionosphere to higher altitudes where recombination loss is weaker; then nighttime enhancement of N mF2 possibly occurs if sunlight causes enough photoionization.…”

The impact of FORMOSAT-5/AIP observations on the ionospheric space weather

“…At ionosphere mid-latitudes, the nighttime electron density enhancement has been observed and reported by previous studies (e.g., Bailey et al 1991;Farelo et al 2002;Dabas and Kersley 2003;Luan et al 2008;Park et al 2008;Chen et al 2011Chen et al , 2012Chen et al , 2013Lin et al 2011;Rajesh et al 2016). This density-enhanced ionospheric feature appeared more frequently during solar minimum conditions (Bailey et al 1991), and it is suggested to be caused by larger downward plasma flux from the plasmasphere during the nighttime (Mikhailov et al 2000;Farelo et al 2002;Dabas and Kersley 2003).…”

Statistics of Ionospheric Storms Using GPS TEC Measurements Between 2002 and 2014 in Jeju, Korea

“…Ionospheric variations normally responding to solar and geomagnetic activities show latitudinal, seasonal, and local time characteristics (Lekshmi et al 2011;Chen et al 2012;Yun et al 2013;Borries et al 2015). The GPS Total Electron Content (TEC) often increases or decreases in response to geomagnetic storms and solar activities as compared to that during periods of normal ionospheric states or their monthly median values.…”