Abstract
Abstract. In this paper, climatological features of the polar F2-region electron density (Ne) are investigated by means of statistical analysis using long-term observations from the European Incoherent Scatter UHF radar (called EISCAT in the following) and the EISCAT Svalbard radar (ESR) during periods of quiet to moderate geomagnetic activity. Field-aligned measurements by the EISCAT and ESR radars operating in CP-1 and CP-2 modes are used in this study, covering the years 1988–1999 for EISCAT and 1999–2003 for ESR. The data are sorted by season (equinox, summer and winter) and solar cycle phase (maximum, minimum, rising and falling). Some novel and interesting results are presented as follows: (1) The well-known winter anomaly is evident during the solar maximum at EISCAT, but it dies out at the latitude of the ESR; (2) The daytime peaks of Ne at EISCAT for all seasons during solar maximum lag about 1–2 h behind those at ESR, with altitudes about 10–30 km lower. (3) In addition to the daytime peak, it is revealed that there is another peak just before magnetic midnight at ESR around solar maximum, especially in winter and at equinox. The day-time ionization peak around magnetic noon observed by ESR can be attributed to soft particle precipitation in the cusp region, whereas the pre-midnight Ne maximum seems likely to be closely related to substorm events which frequently break out during that time sector, in particular for the winter case. (4) Semiannual variations are found at EISCAT during solar minimum and the falling phase of the solar cycle; at the rising phase, however, the EISCAT observations show no obvious seasonal variations.
Highlights
The Earth’s ionosphere is formed by the ionization of the atmosphere, owing to the solar EUV and X-ray radiations and, mainly for the high-latitudes, by energetic particles impinging on the atmosphere from the magnetosphere
(4) Semiannual variations are found at EISCAT during solar minimum and the falling phase of the solar cycle; at the rising phase, the EISCAT observations show no obvious seasonal variations
Electron densities measured during the daytime at the EISCAT site are generally much higher than the corresponding EISCAT Svalbard radar (ESR) measurements, especially in winter, and the daytime Ne peak at EISCAT lags behind that at ESR by about 1–2 h
Summary
The Earth’s ionosphere is formed by the ionization of the atmosphere, owing to the solar EUV and X-ray radiations and, mainly for the high-latitudes, by energetic particles impinging on the atmosphere from the magnetosphere. The ionosphere is coupled closely with both the magnetosphere and the thermosphere in which the ionosphere is immersed These lead the ionosphere to vary significantly with solar cycle, season, local time, and geomagnetic activity. Using ionosonde data, Rishbeth et al (2000b) studied the semiannual and annual variations of the ionospheric F2-peak at middle and low latitudes. They found that HmF2 has a well-defined relationship with solar activity. Zou et al (2000) numerically investigated the annual, seasonal and semiannual variation of the F2-layer electron density under geomagnetically quiet conditions, and claimed that the noon NmF2 is closely related to the ambient atomic/molecular concentration ratio. Zou et al (2000) numerically investigated the annual, seasonal and semiannual variation of the F2-layer electron density under geomagnetically quiet conditions, and claimed that the noon NmF2 is closely related to the ambient atomic/molecular concentration ratio. Rishbeth et al (2000a) highlighted the role of global-scale thermospheric motions, which produce the composition changes, in the Published by Copernicus Publications on behalf of the European Geosciences Union
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