The Global Ultraviolet Imager (GUVI) onboard the Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics (TIMED) satellite senses far ultraviolet emissions from O and N 2 in the thermosphere. Transformation of far ultraviolet radiances measured on the Earth limb into O, N 2 , and O 2 number densities and temperature quantifies these responses and demonstrates the value of simultaneous altitude and geographic information. Composition and temperature variations are available from 2002 to 2007. This paper documents the extraction of these data products from the limb emission rates. We present the characteristics of the GUVI limb observations, retrievals of thermospheric neutral composition and temperature from the forward model, and the dramatic changes of the thermosphere with the solar cycle and geomagnetic activity. We examine the solar extreme ultraviolet (EUV) irradiance magnitude and trends through comparison with simultaneous Solar Extreme EUV (SEE) measurements on TIMED and find the EUV irradiance inferred from GUVI averaged (2002-2007) 30% lower magnitude than SEE version 11 and varied less with solar activity. The smaller GUVI variability is not consistent with the view that lower solar EUV radiation during the past solar minimum is the cause of historically low thermospheric mass densities. Thermospheric O and N 2 densities are lower than the NRLMSISE-00 model, but O 2 is consistent. We list some lessons learned from the GUVI program along with several unresolved issues.

In the companion paper (Basu et al., this issue), a self-consistent transport-theoretic model for the combined electron-proton-hydrogen atom aurora was described. In this paper, numerical results based on the model are presented. This is done for the pure electron aurora, the pure proton-hydrogen atom aurora, and finally for the combined aurora. Adopting commonly used types of energy distributions for the incident particle (electron and proton) fluxes, we give numerical solutions for the precipitating electron, proton, and hydrogen atom differential number fluxes. Results are also given for ionization yields and emission yields of the following features: N• first negative group (3914 •), N 2 second positive group (3371 •), selected N 2 Lyman-Birge-Hopfield bands (1325 •, 1354 •, 1383 •, 1493 •, and all bands between 1700 and 1800 •), O I (1356 •), L• (1216 ]i), (4861 •), and H• (6563 •). The yield at 1493 • also contains a contribution from N I (1493 •), which in fact dominates LBH emission. A major new result of this study is that the secondary electron flux produced by the proton-hydrogen atom aurora is much softer than that produced by the electron aurora. This increased softness is due to the fact that (for energies of auroral interest) cross sections for secondary electron production by proton and hydrogen atom impact decrease exponentially with increasing secondary electron energy, whereas the cross sections for electron impact decrease as an inverse power law with increasing secondary energy. In our study of the pure electron aurora (no primary protons or hydrogen atoms present) and the pure protonhydrogen atom aurora (no primary electrons present), two important results obtain. First, certain emission features (for example, 3371 .&) are excited in completely different ways for the two kinds of aurora. Second, the "eV per electron-ion pair" as a function of the characteristic energy E 0 is nearly constant for the pure electron aurora, with a value of about 34, but varies by about 20% for the pure proton-hydrogen atom aurora. In our study of the combined electron-proton-hydrogen atom aurora, two additional results obtain. Since the proton-hydrogen atom contribution to the total incident energy flux in the midnight sector is, on the average, about 20 to 25% of that of the electrons, we find that when it is neglected, the ionization yield as well as the yields of many emission features will be underestimated, on the average, by about the same percentage. We also find that in the morning sector of the combined aurora, a double bump in the altitude profile of the E region electron density is possible for certain auroral conditions. INTRODUCTIONIn the companion paper [Basu et al., this issue], hereafter referred to as paper 1, we described a self-consistent transporttheoretic model for the combined electron-proton-hydrogen atom aurora. In this paper we present some numerical results based on this model. We do this for the pure electron aurora (section 2), the pure proton-hydrogen atom aurora (section 3), and fin...

The launch of the Defense Meteorological Satellite Program (DMSP) satellite F16 in 2003 provided the first opportunity to analyze extensive sets of high‐quality coincident auroral particle and FUV data obtained by the onboard sensors Special Sensor Ultraviolet Spectrographic Imager (SSUSI) and Special Sensor Auroral Particle Sensor (SSJ/5). Features of interest are Ly α (121.6 nm), Lyman‐Birge‐Hopfield short (LBHS, the SSUSI 140–150 nm channel), and Lyman‐Birge‐Hopfield long (LBHL, 165–180 nm). We report on comparisons of column emission rates (CERs) by deriving simulated SSUSI values using SSJ/5 electron and ion (treated as proton) spectra. Field‐line tracing is performed to determine the locations of coincidences. CERs are obtained by integrating the products of particle spectra and monoenergetic emission yields. A technique is reported for deriving these yields from nonmonoenergetic CERs obtained by our particle transport model. SSJ/5 ion spectra are extrapolated above 30 keV using a statistical representation based on Polar Orbiting Environmental Satellites particle data. Key quantities of interest are ratios of SSUSI to SSJ/5‐based CERs (S‐S ratios) and corresponding ratios of proton‐produced to total emission (unity for Ly α and from 0 to 1 for LBHS and LBHL). SSJ/5‐based CERs are used to derive the latter ratios. Median ratio values are determined in order to reduce the error budget to primarily calibration and model errors. The median LBH S‐S ratios increase by a factor of ∼2.5 from electron to proton aurora and support significantly higher proton LBH emission efficiencies (3 times the electron efficiencies) assuming reported calibration uncertainties. This calls for significant increases in proton and/or H‐atom LBH cross sections. In turn, FUV auroral remote‐sensing algorithms must explicitly address both electron and proton aurora.

[1] The NASA TIMED/GUVI experiment obtained unprecedented far ultraviolet images of thermospheric composition and temperature during the intense geomagnetic storm on 20-21 November 2003. Geographic maps of the atomic oxygen to molecular nitrogen column density ratio show severe depletions that extend to the equator near the peak of the storm. This ratio is a key indicator of how the thermospheric composition is disrupted at high latitudes and how the perturbed air moves globally as a result of dynamical forcing. For example, migrating regions of low oxygen-to-nitrogen air are invariably found to correlate with high thermospheric temperatures. As well, GUVI obtained altitudinallatitudinal (limb) images of temperature and composition, which show how the disturbances vary at different heights. The ASPEN thermospheric global circulation model was used to test our understanding of these remarkable images. The resulting simulations of thermospheric response show good agreement with GUVI data prior to the peak of the storm on 20 November. During the peak and recovery phases, serious discrepancies between data and model are seen. Although this initial attempt to model the storm is encouraging, much more detailed analysis is required, especially of the high-latitude inputs. The GUVI images demonstrate that far ultraviolet imaging is becoming a crucial component of space weather research and development.

[1] There is great interest in understanding how the thermosphere-ionosphere system responds to geomagnetic storms. New insights are possible using the new generation of fully coupled three-dimensional models, together with extensive ionospheric databases. The period of postsolar maximum geomagnetic storms in October and November 2003 were some of the largest storms ever recorded. In this paper, we explore how the thermosphere-ionosphere system responded to the onset of the 20 November 2003 geomagnetic storm, using the NCAR TIMEGCM. The model simulates dramatic changes in the thermospheric equatorward winds, O/N 2 , and corresponding ionospheric electron densities. The model is used as a framework to interpret an increase in the observed ionospheric total electron content, and F region electron density, in the European and North African sector, in terms of changes in the neutral gas. Corresponding compositional effects observed by the GUVI instrument on the TIMED satellite lend credence to the model results. We describe some of the important physical processes that will affect planning for the utilization of measurements from the Geospace investigations in NASA's Living With a Star Program. The study illustrates the value of measuring both the neutral and ionized gases, of obtaining quasi-global views from imaging instruments, and the synergy between satellite data, ground-based measurements, and models.