Dayglow: Neutral Composition, Solar EUV, and Ionization

Remote sensing has an important role to play in monitoring the states of the global thermosphere and ionosphere that vary, and in some situations, significantly with changing solar and geomagnetic activity. Beyond scientific interest in their variabilities are practical concerns related to satellite orbits (changes due to satellite surface interactions with neutral densities) and signals (ionospheric effects on ground-to-ground, radar, and trans-ionospheric signals). CPI has taken a leading role in developing remote sensing algorithms for extracting thermospheric and ionospheric information from satellite observations of the dayglow. The focus of the work has necessarily been at far ultraviolet (FUV) wavelengths, where, in contrast to longer wavelengths, observed emission while viewing the Earth's disk is only from thermospheric/ionospheric regions. The opportunity to develop remote sensing techniques has come from DMSP support through APL and NRL (specific to the SSUSI and SSULI sensors) and NASA/NSF support (specific to the TIMED/GUVI sensor and to a basic research program addressing thermospheric/ionospheric space weather effects. The needed first-principles modeling of FUV emission characteristics has come from CPI's AURIC model. The algorithms developed under the above programs address the following physical quantities:

O/N₂ - column density ratio referenced to a fixed N₂ column density

QEUV - integrated measure of the solar EUV flux below 45 nm

NmF2 - electron density at the peak of the F2 layer

HmF2 - height at which NmF2 occurs

NDPs - neutral density profiles of N₂, O2, and O

The algorithms developed for the SSUSI sensor provide this full set of quantities from disk and limb observations. For GUVI, the quantities of interest are the first two and the last. Under NASA sponsorship, a variation on the SSUSI O/N₂ algorithm has been developed for addressing DE 1 data (see above space weather link). The SSULI sensor, like SSUSI, extracts NDPs from limb data using an NRL algorithm. CPI provided a key component to this algorithm in the form of parameterized g-factors for rapid determination of volume emission rates [Strickland et al., J. Geophys. Res., 102, 14,485, 1997].

The paper by Strickland et al. [J. Geophys. Res., 100, 12,217, 1995] describes in detail how O/N₂ and QEUV are extracted from SSUSI and GUVI disk observations. The paper by Strickland et al. [J. Geophys. Res., 104, 4251, 1999] addresses O/N₂ as extracted from DE 1 data. NmF2 and HmF2 are more indirectly related to the observations than O/N₂ and QEUV. Their estimates come from knowledge of O/N₂ and QEUV along with climatologies of meridional winds and electric field induced drifts (the latter being relevant at low latitudes). With regard to CPI's NDP algorithm for SSUSI, a database of model limb profiles is used to fit the measured limb profiles in order to extract the N₂, O₂, and O NDPs from the data.

CPI's work under space weather research illustrates the importance of O/N₂. Not only does satellite FUV imagery allow for its characterization on a global scale, but also provides a pointer to negative ionospheric storm regions that are now known to be coincident with regions of reduced O/N₂ [Strickland et al., J. Geophys. Res., 106, 21,049, 2001; J. Geophys. Res., 106, 30,291, 2001 ].