Global distributions of presunrise ion heating were discovered by the Ionospheric Plasma and Electrodynamics Instrument onboard the ROCSAT-1 satellite in the low- to midlatitude topside ionosphere. The most significant presunrise ion heating was clearly found in the 0400-0500 LT sector and asymmetrically located in the South Pacific and North Atlantic region during the June and December soltices, respectively. Local time variations of ion temperature from 0000 to 0600 LT indicate that the temperature of the presunrise heating increases gradually after its early onset. Despite the late onset of the sunrise heating, the temperature increases rapidly and goes beyond that of the presunrise heating after 0430 LT. To identify possible heating sources, electron temperature observed by the HINOTORI satellite is compared and found similar patterns to the distributions of the presunrise ion heating. With the aid of the SAMI2 model of the ionosphere, these distributions and the local time variations of the temperature at 600 km altitude can be reproduced. The presunrise plasma heating is caused by photoelectrons streaming along field lines from low altitudes of the sunlit magnetic conjugate ionosphere. The early onset of the presunrise heating is one of the main factors to maintain the plasma temperature higher before 0430 LT. In addition, abundant amounts of light ions act as proxies in the processes of the presunrise heating to indirectly transfer thermal energy from electrons to O(+) and to speed up the increase of O+ temperature. Asymmetrical hemispheric temperature distributions of the presunrise heating during the solstices are the result mainly of the geomagnetic dipole offset from the center of the Earth.
