GUVI Science Objectives:
Composition, Energetics, and Dynamics
GUVI provides information on the following:
- Obtain accurate quantitative global maps of thermospheric composition, including N2, O2, O, and H and temperature in the lower thermosphere (LT) region using N2(LBH), OI(130.4), OI(135.6), and HI(121.6) emission features.
- Investigate the development and evolution of lower thermospheric-ionosphere composition changes induced by geomagnetic activity and storms.
- Generate quantitative maps of the total electron content and height of the peak electron density in the tropical nightside ionosphere.
Ideally, we would like to produce a global database of the neutral and ion composition and temperatures as a function of altitude and location in the LTI. GUVI will approach this ideal by providing routinely 3-dimensional (latitude, longitude, and altitude) pictures of the neutral density, O, H, N2, and O2 concentrations and exospheric temperature in the dayside thermosphere, and O+ altitude profiles on the night side. It will also provide a frequently updated EUV solar flux scalar on each scan of the sunlit limb.
Atmospheric composition and temperature respond strongly to varying conditions in the upper atmosphere. For example, O and N2 change considerably following a magnetic storm. Large changes in atomic oxygen have been seen optically from various spacecraft as well as in mass spectrometer data. Over the past decade, studies have demonstrated that spectroscopic observations from space can provide information about the compositional state of the upper atmosphere. Meng and Huffman have extended the power of spectroscopy by demonstrating the concept of FUV monochromatic imaging.
GUVI provides information on the following:
- Auroral oval size and location variation.
- The particle energy inputs to the high latitude region from electron and proton precipitation.
- The height integrated Pedersen and Hall conductivities needed to specify the Joule heat inputs at high latitudes.
- A measure of the integrated solar EUV flux shortward of 40 nm.
The analysis of auroral images at FUV wavelengths and the results of detailed modeling for the interaction of precipitating electrons with the upper atmosphere have demonstrated the immense contribution to be made by remotely sensing auroral optical emissions. The Energy flux and characteristic energy of precipitating electrons can be determined through the auroral emissions at selected FUV wavelengths. Hence, in combination with well developed models, FUV images of the aurora at several wavelengths provide the rates of ionization and energy input due to charged particle precipitation as a function of latitude, longitude,, and altitude. The ionization profiles obtained as part of the data analysis procedures are valuable in determining the height integrated Hall and Pedersen conductivities. They are required for improved estimates of the rate of energy input due to Joule heating.
These analyses require information on particle type, flux and energy spectrum. the relative brightness of the various emission bands is indicative of the type and energy spectrum of the incoming particles. For electrons, increasing the spectral hardness leads to energy deposition deeper in the atmosphere, causing greater attenuation of the emission as observed from space. since the atmosphere opacity varies with wavelength, the emission spectrum of N2 can be used to deduce the altitude of energy deposition and consequently the average energy of the precipitating electrons. We refer to the particle energy spectra by their average energy E0 (in keV) and energy flux Q (in ergs cm-2s-1).
The analysis, in principle, provides a measure of the atmospheric O/N2 ration in auroral regions.
Our analysis will greatly improve upon the work to extract the energy flux, characteristic energy and conductance from the DE auroral imager data. The earlier effort was hampered by by the lack of spectral resolution in the DE imager which responded almost exclusively to atomic oxygen emissions.
GUVI will provide information on the following:
- The tidal and planetary wave effects on the concentration of atmospheric gases.
- Testing the predictive capabilities of thermosphere general circulation models (TGCM) to impulsive thermospheric energy and momentum inputs.
- Input to constrain tidal models and TGCMs.
The GUVI payload will measure structures in the airglow which are intimately related to dynamical forcing by upward and horizontally propagating waves. The measurements will focus on planetary scale structures both day and night, especially the compositional and temperature responses resulting from dynamical phenomena. In addition, compositional responses due to impulsive geomagnetic events should be observable.
From the GUVI data, two-dimensional maps of O/N2 can be constructed. These maps will provide a look at the spatial structure of large scale waves as seen in the signature of the composition. Careful analysis will be done to estimate problems associated with under sampling of the tides or aliasing of certain zonal waves. In regions where successive orbits provide good temporal resolution, additional information can be obtained regarding periodicities of these large-scale waves. Care will need to be taken as observations will be limited to dayside, thereby limiting the ability to observe diurnal variations. Altitude profiles on the limb of changes in composition will give a further look at tide and planetary wave amplitude and phase structures.
Atmospheric gravity waves can be studied with the composition maps and changes provided the gravity wave produces a significant change in intensity of the airglow. From these images and other measures of geomagnetic activity, and examination of gravity waves excited in-situ can be done. It is expected that the waves will be observed as banded structures from which spatial dimensions can be determined.
The major contributions towards dynamics will be done in conjunction with additional observations from the other TIMED instruments as well as ground-based instrumentation. For large scale, long period tides and planetary waves, GUVI data will be used to record tidal and planetary wave structures in the form of compositional maps. Especially important are the compositional changes which result from tidal or high latitude forcing. These will provide a comprehensive climatology of relative amplitudes and variability.