This paper reports on the statistical properties of the field-aligned particle acceleration region at high latitudes above the auroral region observed by the Akebono (EXOS-D) satellite. Akebono often observes downward accelerated electrons and upward accelerated ions at the same time, indicating that the satellite is inside the parallel acceleration region. We found three major regions of particle acceleration, two located in the dayside in the prenoon and postnoon sectors and one in the premidnight sector. In the dayside regions the average total field-aligned potential difference is about 600 V (800 V in the summer postnoon region), and the occurrence frequency of the particle acceleration events is larger in the summer. In the premidnight region the average total field aligned potential difference is about 4 kV, and the occurrence frequency of the particle acceleration events is two times larger in the winter. An important result in this study is showing the altitude profile of the parallel acceleration region. In the dayside regions, more than 80% of the ion acceleration is observed above 6000 km in the summer, and more than 70% of the electron acceleration is observed below 8000 km in the winter. In the premidnight region, 90% of the ion acceleration is above 6000 km and more than 90% of electron acceleration is below 6000 km in summer and winter, respectively. We discuss these seasonal changes of the acceleration altitude in terms of seasonal variations of the ambient plasma density at altitude of the acceleration region, with a lower density during the winter, causing acceleration at lower altitude. The importance of the density at the altitude of acceleration is supported by the fact that Akebono does not observe any change of the relationship between the density at the bottom of the potential drop and the potential difference with changing season and altitude. Also the occurrence frequency of auroral electron acceleration is likely to be controlled by the seasonal changes of the ambient plasma density on auroral field lines.