Longitudinal variations of nighttime electron auroral precipitation in both the northern and southern hemispheres from TIMED/GUVI

Using 6 years of TIMED/GUVI auroral observations in both hemispheres, we have studied the longitudinal variations of auroral precipitation during the magnetic nighttime period of 2100-0300 MLT. There was a strong seasonal dependence of the longitudinal variations of the aurora: 1) During solstices and for both hemispheres, auroral precipitation peaked between MLON 210° and 360°E in June and between MLON 120° and 300°E in December. 2) In the equinoxes, the auroral longitudinal pattern was generally similar to that in local summer in each hemisphere, except that, in the northern hemisphere, the maximum precipitation was usually located in more westward longitudes in equinox than in summer. 3) The ratios between the maximum and the minimum of the precipitation energy flux along longitudes varied between 1.3-1.9, which were similar to those in previous studies. Similar longitudinal features of auroral precipitation energy flux can be derived from POLAR UVI observations. These features of the auroral longitudinal patterns did not change much from Kp=1 to Kp=4 conditions. Our analysis shows that there was a significant negative correlation (coefficient |r|=~0.4-0.8) between the peak auroral precipitation intensity and the solar EUV-produced ionospheric conductivity of the same hemisphere (in summer and equinox), or of the conjugate hemisphere (in winter). These results indicate the important effects of the solar EUV-produced ionospheric conductivity, which has significant longitudinal variations, on the longitudinal patterns of the aurora at magnetic nighttime. Our results also indicate that the hemispheric differences in the conjugate magnetic field strengths, although it appears not to be a major factor, contributes to the longitudinal variations of the aurora.