On the Annual Asymmetry of High‐Latitude Sporadic F

The polar F region ionosphere frequently exhibits sporadic variability (e.g., Meek, 1949, https://doi.org/10.1029/JZ054i004p00339; Hill, 1963, https://doi.org/10.1175/1520‐0469(1963)0202.0.CO;2). Recent satellite data analysis (Noja et al., 2013, https://doi.org/10.1002/rds.20033; Chartier et al., 2018, https://doi.org/10.1002/2017JA024811) showed that the high‐latitude F region ionosphere exhibits sporadic enhancements more frequently in January than in July in both the northern and southern hemispheres. The same pattern has been seen in statistics of the degradation and total loss of GPS service onboard low‐Earth orbit satellites (Xiong et al. 2018, https://doi.org/10.5194/angeo‐36‐679‐2018). Here, we confirm the existence of this annual pattern using ground GPS‐based images of TEC from the MIDAS algorithm. Images covering January and July 2014 confirm that the high‐latitude (>70 MLAT) F region exhibits a substantially larger range of values in January than in July in both the northern and southern hemispheres. The range of TEC values observed in the polar caps is 38–57 TECU (north‐south) in January versus 25–37 TECU in July. First‐principle modeling using SAMI3 reproduces this pattern, and indicates that it is caused by an asymmetry in plasma levels (30% higher in January than in July across both polar caps), as well as 17% longer O+ plasma lifetimes in northern hemisphere winter, compared to southern hemisphere winter. Plain Language Summary The polar F region ionosphere frequently exhibits sporadic variability. Satellite data analysis has indicated an annual pattern to this variability—there appears to be more sporadic F in January than in July in both hemispheres. The same pattern has been seen in statistics of GPS navigation system outages on low‐Earth orbit satellites. Here we confirm that pattern exists throughout the F region using GPS data collected at ground stations in the northern and southern high latitudes. A numerical model reproduces the observations accurately. In the model, the annual pattern of variability is caused by two factors: (1) 30% higher plasma levels in January than in July across both polar caps and (2) 17% longer plasma lifetimes in northern winter than in southern winter. Key Points The high‐latitude ionosphere is more variable in January than in July in both the northern and southern hemispheres First‐principle modeling reproduces the patterns observed using GPS‐derived TEC The pattern is caused by the annual