Enhancements of nighttime neutral and ion temperatures in the F region over Millstone Hill

Nighttime ion temperature in the F region is generally thought to be close to neutral temperature at a similar altitude. However, few studies have been conducted that explore the temporal variation between ion and neutral temperatures at night in the F region. In this paper, the similarities and differences of the nighttime ion and neutral temperatures in the F region over Millstone Hill (42.5 degrees N, 71.5 degrees W) were examined using the incoherent scatter radar and Fabry-Perot interferometer observations during the periods of 1-11 August 2011 and 12-23 January 2012, which represented summer and winter seasons, respectively. Our results showed that in winter, both neutral and ion temperatures exhibited the postmidnight enhancements, which occurred during 3-4 LT and had amplitudes of about 30-90 K. However, the situation was different during the August observation period when the postmidnight enhancement was only observed in the neutral temperatures so that the variation of the nighttime ion and neutral temperatures was almost out of phase. The causes for the postmidnight enhancements of neutral temperature in either winter or summer remain debatable. Our preliminary analysis indicates that the extension of midnight temperature maximum to the latitude of Millstone Hill is the preferred mechanism for the observed postmidnight enhancements of neutral temperature in winter and summer.