The spatial distribution of hydrogen ions at topside ionosphere in local daytime

Using DEMETER and DMSP satellite data, the spatial distribution of hydrogen ion (H+) in local daytime has been compared and analyzed. At 840 km of DMSP height, the seasonal variations of H+ density is basically symmetric, with similar density values near to the magnetic equator at northern hemisphere in December and at southern hemisphere in June. But at DEMETER satellite height, the peak H+ density shows obvious enhancement at northern hemisphere in December solstice, while with approximate small values at both hemispheres in June. This spatial distribution feature is totally different with other ions such as O+ and He+ at the topside ionosphere. And also it influences the transition height in topside ionosphere, with lower transition height over northern hemisphere in December season at 10 20°N than those at equator and over southern hemisphere in June season. The solstitial asymmetry index (AI) of H+ at 670 km altitude gives the significant December season enhancement over northern hemisphere, which is typically reversed with electron density (Ne) and ion density (Ni) with large numbers over southern hemisphere in December season. Finally combining with the distribution of H atoms and neutral wind velocity in upper atmosphere, the forming mechanism and asymmetry feature of peak H+ density is discussed. It is illustrated that the upwelling movement at equatorial area and northward neutral wind play important roles in H+ peak drift in December season at DEMETER satellite altitude below the transition height where H+ is not the main composition in the local daytime in solar minimum years. Article history: Received 28 March 2016 Revised 26 April 2017 Accepted 30 June 2017

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