Abstract
[1] Under geomagnetically quiet conditions, the daytime midlatitude ionosphere is mainly influenced by solar radiation: typically, electron densities in the ionosphere peak around solar noon. Previous observations from the Millstone Hill incoherent scatter radar (ISR) have evidenced the presence of evening electron densities higher than daytime densities during the summer. The recent development of midlatitude Super Dual Auroral Radar Network (SuperDARN) radars over North America and Japan has revealed an evening enhancement in ground backscatter during the summer. SuperDARN observations are compared to data from the Millstone Hill ISR, confirming a direct relation between the observed evening enhancements in electron densities and ground backscatter. Statistics over a year of data from the Blackstone radar show that the enhancement occurs during sunset for a few hours from April to September. The evening enhancement observed by both SuperDARN and the Millstone Hill ISR is shown to be related to recent satellite observations reporting an enhancement in electron densities over a wide range of longitudes in the Northern Hemisphere midlatitude sector during summer time. Finally, global results from the International Reference Ionosphere (IRI) and the horizontal wind model (HWM07) are presented in relation with previously published experimental results and proposed mechanisms of the evening enhancement, namely, thermospheric horizontal winds and geomagnetic field configuration. It is shown that the IRI captures the features of the evening enhancement as observed by SuperDARN radars and satellites.
Highlights
[2] Under geomagnetically quiet conditions, the daytime midlatitude ionosphere is mainly influenced by solar radiation, tides, gravity waves and thermospheric neutral winds
The first noticeable but not surprising difference is the time extent of the ground scatter being longer in summer (11:00 to 04:00 UT) than in winter (12:00 to 23:00 UT): this is due to longer Sun exposure during the summer, resulting in longer lasting high electron densities
A third difference is the stronger daytime returned power in the winter: this is explained by both the absence of a lower E region that would otherwise absorb and deflect part of the power reaching the F region and the winter anomaly which results in higher F region densities in winter [Rishbeth and Garriott, 1969, p. 179–181]
Summary
[2] Under geomagnetically quiet conditions, the daytime midlatitude ionosphere is mainly influenced by solar radiation, tides, gravity waves and thermospheric neutral winds. Midlatitude ionospheric conditions being generally less active, during the daytime the predominant mode of backscatter involves reflection from the ground after the HF signals are redirected downward by refraction This feature of HF propagation is used in over‐ the‐horizon HF communication and radar surveillance [e.g., Hughes et al, 2002]. [7] In this study we report that during geomagnetically quiet summer days, the daily ground scatter distribution observed at midlatitude SuperDARN radars does not follow the expected diurnal pattern. [9] This body of work was motivated by the identification of a peculiar and recurring feature in midlatitude SuperDARN observations: an unexpected enhancement in ground backscatter around sunset The relation between this feature and the previously reported summer evening anomaly (SEA) will be demonstrated here. On the basis of this relation, we provide the first comparison with global empirical models such as the International Reference Ionosphere (IRI) and the horizontal wind model (HWM)
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