Aurora: Particle Precipitation, Ionization, and Neutral Composition

There is considerable scientific and operational interest in the ionization that occurs within the auroral oval (due to precipitation by energetic electrons, protons, and hydrogen atoms with electrons, on average, being the dominant energy source), especially during intense magnetospheric substorms. Auroral ionization 1) plays a role in Joule heating that can lead to significant thermospheric disturbances and 2) effects signals reflecting from or passing through auroral regions. There is an important role to be played by satellites that carry far ultraviolet (FUV) imaging instruments capable of observing sizable portions of the oval, in particular, instruments capable of simultaneous measurements over key wavelength intervals within the FUV spectral region. The region is nominally defined to be between 110 and 180 nm and is dominated by emission from atomic hydrogen (the HI 121.6 nm line arising from a combination of geocoronal and proton/H-atom auroral emission), atomic oxygen (the OI 130.4 nm and OI 135.6 nm multiplets), and molecular nitrogen (the Lyman-Birge-Hopfield (LBH) band system distributed throughout the FUV region). Due to atmospheric absorption effects, the spectrum of auroral LBH emission seen against the Earth's disk from satellite altitudes varies in brightness and shape with changes in the intensity and hardness of the precipitation. Such variations offer the opportunity to remotely sense the gross characteristics of the precipitation and in turn ionization characteristics. With knowledge of the precipitation, measurements of the brightness of either of the above O features relative to LBH further allows for remotely sensing O/N₂, the column abundance of O relative to N₂ within the emitting region. The concept for remotely sensing O/N₂ is the same as from dayglow observations, with the common requirement that the altitude distribution of emission be known.

CPI's auroral remote sensing activities have and continue to address algorithms directed to interpretation of ground-based, rocket, and satellite data. The basis of the work is the first-principles model B3C (Boltzmann 3-Constituent) that performs auroral electron, proton, and H-atom transport with derived quantities being energetic particle distributions, volume excitation and ionization rates, electron density profiles, column emission rates, and spectral radiances from extreme UV to near IR wavelengths. The paper by Strickland et al. [Deducing Composition and Incident Electron Spectra from Ground-Based Auroral Optical Measurements: Theory and Model Results, J. Geophys. Res., 94, 13527, 1989] provided the theoretical basis for a series of successful investigations by J. Hecht (Aerospace Corp) and colleagues utilizing photometer data from ground-sites in Alaska (documented in various JGR papers with Hecht as leading author). Emphasis has been on the dynamic nature of O/N₂ in aurorally heated regions. Collaboration with Hecht is continuing.

With regard to rocket data, B3C has been employed most recently to analyze altitude profiles of column emission rates obtained by a set of filtered photometers as part of the ARIA II rocket experiment conducted by the Aerospace Corp. J. Hecht was the principle investigator for the photometer measurements. A significant finding published by Strickland et al. [Thermospheric disturbance recorded by photometers on-board the ARIA II rocket, J. Geophys. Res., 105, 2461, 2000] was a strong horizontal gradient in the column density ratio of O to N₂ above 200 km (not to be confused with O/N₂ as defined above) that is a signature of structured heating within the auroral oval. The finding was made possible by zenith-viewing of the photometer system in combination with horizontal motion when the rocket was near apogee. Altitude profiles (corresponding to rocket ascent) of measured and calculated column emission rates along with displays showing the composition gradient may be seen in the above referenced paper.

Most of CPI's auroral remote sensing work has been directed to observations by the satellite sensors DMSP/SSUSI and TIMED/GUVI, both built by APL. The basis of the techniques for inferring data products from measurements by these sensors comes from auroral modeling work published by Strickland et al. [J. Geophys. Res., 88, 8051, 1983 and J. Geophys. Res., 98, 21533, 1993]. New algorithm design and development will be needed for the future NPOESS FUV imaging sensor (AURORA to be built by APL) that will utilize CPI personnel and experience gained through the SSUSI and GUVI work. This will continue a long working relationship with APL that began with CPI support to various satellite sensor projects prior to SSUSI. Auroral data products from SSUSI and GUVI observations utilizing CPI algorithms include the electron precipitation parameters E_o (characteristic energy in keV) and Q (energy flux in mW m^-2), O/N₂, HmE (altitude of E-layer peak), and NmE (electron density at peak). Images of the first three of these products from GUVI auroral data along with the data themselves may be seen under TIMED/GUVI. The reader is also directed to DMSP/SSUSI Cal/Val for our latest SSUSI-related work. A significant finding from that work is the higher efficiency for auroral FUV production by proton compared to electron aurora. This calls for a significant revision to the current algorithms in which explicit attention must be given to proton aurora (presently, the auroral FUV emission recorded by SSUSI is assumed to arise from electron impact excitation).