Proton temperature properties in the solar wind

Proton temperature in the solar wind between 0.68 and 0.98 a.u. is investigated from the point of view of heat generation in the interplanetary space based on the solar wind data from the Japanese interplanetary spacecraft Suisei. The heliocentric radial gradient of proton temperature tends to decrease in high-speed regions owing to heat generation accompanied by a velocity increase of more than 50 km s-1 per hour. Temperature measurements in the region with decreasing or nearly constant flow velocity reveal cooling which is still slower than the adiabatic case. The remnant difference suggests the existence of another heating mechanism for protons, especially in a low-speed stream. (author)