Variations of topside ionospheric scale heights over Millstone Hill during the 30-day incoherent scatter radar experiment

Abstract. A 30-day incoherent scatter radar (ISR) experiment was conducted at Millstone Hill (288.5° E, 42.6° N) from 4 October to 4 November 2002. The altitude profiles of electron density N_e, ion and electron temperature (T_i and T_e), and line-of-sight velocity during this experiment were processed to deduce the topside plasma scale height H_p, vertical scale height VSH, Chapman scale height H_m, ion velocity, and the relative altitude gradient of plasma temperature (dT_p/dh)/T_p, as well as the F₂ layer electron density (N_mF₂) and height (h_mF₂). These data are analyzed to explore the variations of the ionosphere over Millstone Hill under geomagnetically quiet and disturbed conditions. Results show that ionospheric parameters generally follow their median behavior under geomagnetically quiet conditions, while the main feature of the scale heights, as well as other parameters, deviated significantly from their median behaviors under disturbed conditions. The enhanced variability of ionospheric scale heights during the storm-times suggests that the geomagnetic activity has a major impact on the behavior of ionospheric scale heights, as well as the shape of the topside electron density profiles. Over Millstone Hill, the diurnal behaviors of the median VSH and H_m are very similar to each other and are not so tightly correlated with that of the plasma scale height H_p or the plasma temperature. The present study confirms the sensitivity of the ionospheric scale heights over Millstone Hill to thermal structure and dynamics. The values of VSH/H_p tend to decrease as (dT_p/dh)/T_p becomes larger or the dynamic processes become enhanced.