Day-to-day variability and solar preconditioning of thermospheric temperature over Millstone Hill

We use a continuous 30-day incoherent scatter radar experiment at Millstone Hill in October 2002 to examine day-to-day thermospheric variability in exospheric temperature T_ex. Solar flux and magnetic activity influences as the main driving factors for day-to-day variability are investigated quantitatively. Solar ultraviolet flux levels are based on the TIMED/SEE space weather product, allowing for analysis of ultraviolet flux-T_ex correlation. T_ex is most sensitive to solar EUV flux with approximately a 2-day delay at wavelengths of 27-34-nm (including 30.4-nm). In particularly, a 20-60-h time delay occurs in T_ex response to EUV flux at 27-34-nm band, with shorter delays in the morning and longer delays in the afternoon and at night. The 1-∼-2-day delayed T_ex response to solar ultraviolet flux and associated thermospheric solar preconditioning ("memory") are most significant in the daily mean for the 27-34-nm band, in the diurnal and semidiurnal amplitudes for the soft X-ray flux at 0.1-7-nm, and in the diurnal amplitude for longer wavelengths. An empirical model driven only by EUV flux at 27-34-nm from 2-days in advance reproduces 90% of the observed variability in the T_ex daily mean. With a 2-day time delay, solar X-ray flux at 0.1-7-nm is correlated positively with T_ex diurnal amplitude and negatively with T_ex semidiurnal amplitude. Finally, magnetic activity control, as represented by the Dst index, is weaker during the day and stronger at night and is important for the semidiurnal amplitude but not important for the daily mean.