Height profiles of the ionospheric electron density derived using space-based remote sensing of UV and X ray emissions and EISCAT radar data: A ground-truth experiment

摘要

In this study we have derived height profiles of the ionospheric electron density N e using remote sensing of UV and X-ray emissions from the Polar satellite and EISCAT radar data. The latter technique gives the most accurate determination of N e providing a means to ground-truthing the satellite imaging measurements. The UV-emission data are taken from the Ultraviolet Imager (UVI) on Polar, while the X-ray data are measured by the Polar Ionospheric X-ray Imaging Experiment (PIXIE). As UVI yields a far better resolution in time and space than PIXIE, our primary approach involves UVI and EISCAT data. For a substorm event occurring on 24 March 1998, we derive N e?UVI profiles valid in the E region above ～105 km. By comparing with simultaneous N e?EISCAT values, we find that the two techniques match fairly well in many cases. The altitude of maximum electron density is usually below 110 km. A few cases reveal N e?EISCAT maxima in the upper E region (130–150 km), indicating a very soft precipitating electron energy spectrum. During such conditions, we observe the largest discrepancies between the N e?UVI and N e?EISCAT profiles. This may reflect the difficulty of obtaining proper energy characteristics from UV emissions, when the mean electron energy is less than ～2 keV. A recalculation of these N e?UVI values has been performed, requiring that the altitudes of the N e?UVI maximum must match the altitudes of the N e?EISCAT maximum. The results reveal a much better agreement between the two data sets, suggesting that UVI is measuring about the same energy flux as EISCAT. Even though the modified N e?UVI values deviate strongly from the old N e?UVI profiles, the effects on the Pedersen conductance, Σ P , are insignificant. Also, we find that Σ P?UVI are within ±30% of Σ P?EISCAT for 15 of 18 cases, suggesting that remote sensing of UV-emissions provide a fairly reliable tool to monitor the Pedersen conductance. We have investigated a second approach by including PIXIE X-ray data to derive N e?UVI + PIXIE values valid in the whole E region and upper D region. Despite the coarse PIXIE resolution, we observe a fairly good match with the N e?EISCAT profiles. By calculating the Hall and Pedersen conductances, Σ H and Σ P , we find that the values derived from satellite imaging measurements are within ±25% of the EISCAT values for all four cases, supporting the space-based remote sensing technique to investigate the ionospheric electrodynamics. The results presented in this study suggest that the procedures developed to derive N e values from the satellite imaging measurements are reliable. We also find that the N e?UVI and N e?UVI+PIXIE values on average are slightly larger (5 and 13%) than the N e?EISCAT values. These discrepancies may be caused by the difference in resolution between the satellite remote sensing data and the radar data, as smoothing of discrete precipitation may result in an overestimation of N e .