Data modeling and assimilation studies with the MU radar

We report initial results of data modeling and assimilation studies for sev eral MU radar experiments. Various inputs to a one-dimensional ionospheric model are adjusted to provide agreement with observation and also to learn the sensitivity of the model to their variations. Certain observations are also used directly in the model to anchor or constrain its behavior. In par ticular, studies of the electron density from 100 to 500 km altitude in the ionosphere are carried out with the help of a theoretical model of O+, NO, O-2(+) and N-2(+) densities and MU radar observations of the power, ion-d rift and plasma-temperature profiles. Four typical cases are selected to st udy quantitatively the effects of the (A) perpendicular-north component of the plasma drift (15 December 1986), (B) atmospheric composition (7 October 1986), (C) solar EUV flux (2 August 1989) and (D) upper-boundary O+ densit y (5 October 1989) on the model N(m)F2, h(m)F2 and N-e profile, as well as on the neutral wind calculation from h(m)F2 and drift data. It is found tha t the measured Vertical ion drift explains quantitatively well the measured h(m)F2 (particularly at low solar activity) while the model gives a better match with the measured N-e when it uses the h(m)F2-based wind rather than the measured plasma drift. Different model values of the atmospheric O/N-2 ratio and EUV flux and different values of the upper-bound O+ density may modify not only N(m)F2 markedly but also h(m)F2: a lower O/N-2 ratio result s in higher h(m)F2; the EUVAC model gives higher h(m)F2 at high solar activ ity than does the EUV91 model, with a smaller upper-bound O+ density, h(m)F 2 is lower by day but little changed by night. We specifically note that th e meridional wind needed by the model to reproduce the observed h(m)F2 diff ered according to how well the model reproduced the observed N(m)F2. The un certainties in the MSIS86 and EUV model predictions are also discussed. It is found that if the MSIS and EUV91 models are used together. the model giv es an N(m)F2 higher than that measured at high solar activity. Thus the O/N -2 ratio needs to be reduced from the MSIS value if EUV91 is used. If EUVAC is used, no large modification is required. At equinox for low solar activ ity, modeling with either EUV model produces N(m)F2 values lower than those measured, and so the true O/N-2 ratio may be higher than that given by MSI S model. (C) 1999 Elsevier Science Ltd. All rights reserved.