Satellite remote sensing of thermospheric O/N2 and Solar EUV: 2. Data analysis

Abstract

The companion paper by Strickland et al. (this issue) describes a technique for deriving QEUV and O/N2 from disk observations of OI 135.6‐nm and N2 LBH dayglow emission. QEUV refers to the integrated solar EUV energy flux below 45 nm and is derived from knowledge of the 135.6/LBH ratio or O/N2 in conjunction with either the 135.6 nm or LBH intensity. O/N2 refers to the ratio of the atomic oxygen (O) column density to the molecular nitrogen (N2) column density at a given value of the N2 column density. Strickland et al. show that the least uncertainty in derived values O/N2 occurs in the vicinity of an N2 depth of 1017 cm−2. The O/N2 values presented in this paper are referenced to this depth. While QEUV is obtained from the intensity of either 135.6 nm or LBH, O/N2 is obtained from the intensity ratio designated by 135.6/LBH. The technique has been used to derive O/N2 and QEUV values from nadir‐viewing far ultraviolet dayglow data obtained by the auroral and ionospheric remote sensor instrument on board the Polar BEAR satellite. Data are considered from single passes on July 15, 16, and 21, 1987, spanning a latitude range from 25° to 55°N. The 3‐hour ap index was between 20 and 30 for the first two passes and only about 5 for the third pass. The 135.6‐nm and LBH signals were obtained from spectra recorded at a resolution of 3.6 nm from which a background was subtracted followed by integration over the intervals from 134.5 to 139.0 nm and 155.0 to 170.0 nm. Uncertainties were assigned to the signals and their ratios that took into account statistical uncertainties in the true signal and in the subtracted background signal. Profiles of derived O/N2 with uncertainties reflecting the data uncertainties are shown along with mass spectrometer/incoherent scatter (MSIS) O/N2 profiles over 25°–55°N. More structure is seen in the profiles on July 15 and 16, which are more disturbed days than July 21. In all cases, O/N2 increases from high to low latitudes. MSIS also shows such an increase but is much less structured. The results are shown to agree qualitatively with O/N2 values obtained from a general circulation model. Derived QEUV values are in the range from 1.1 to 1.4 ergs cm−2 s−1 and are in good agreement with values derived from the Hinteregger spectrum for the F10.7 values appropriate to the observations. The results illustrate the potential of the technique for monitoring thermospheric dynamics through latitudinal and temporal variations in O/N2 which are signatures of ascending and descending motion in the thermosphere.