Figure 1. Optical schematic of an f/1.4, CCD based, imaging spectrometer used to measure auroral spectra with high spatial and moderate spectral resolution.
Aurora is the light originating from the earthÕs upper atmosphere as a result of energetic electrons and protons colliding with atmospheric atoms and molecules. The electrons and protons originate in the earthÕs magnetosphere and are guided by the earthÕs magnetic field, where at high latitudes, the field lines bend to steer the particles into the atmosphere. As the particles encounter the atmosphere, the more energetic penetrate lower in altitude. The atmosphere has altitude distribution in the constituents (atoms and molecules) and consequently, the emissions associated with the aurora differ, depending on the altitude of emission.
A classical method for remote sensing of the characteristic energy and energy flux of auroral particles has been through the observation and modeling of emission of molecular Nitrogen (N2+) and atomic oxygyen (O). The N2+ 4278 Å to OI 6300 Å ratio has been a primary indicator of the characteristic energy of particles. More energetic electrons penetrate to lower altitudes (through more nitrogen) affecting the relative brightness to the higher altitude oxygen emission.
Experiments have been developed to view the auroral arcs from the slant path so that altitude distributions of emissions can be measured. An experiment had been operated on several occasions from a site at Godhavn, Greenland, which is located ~290 km NW of the NSF radar backscatter facility at Sondrestrom, Greenland. Sondrestrom is well instrumented with the radar, all-sky imager, and other optical diagnostics to characterize the aurora. Auroral arcs which form over the site at Sondrestrom can be viewed along the magnetic field line (along B), while simultaneously being viewed along the slantpath from Godhavn. Godhavn is nearly perpendicular to the magnetic latitude of Sondrestrom, a condition which favors arcs forming perpendicular to the slant path instrument so that altitude characterization can be accomplished.
CCD based imaging spectrometers have been developed to capture the entire visible spectrum over a large spatial field. This method has the advantage of measuring the emissions at all spatial regions viewed by a projected slit onto the atmosphere at the same time, eliminating temporal concerns for highly variable auroral conditions over the exposure times which have been typically 30 s. The imaging spectrometer optics and viewing geometry (from Godhavn) for these experiments is shown in Figures 1 and 2, respectively. The angular resolution for binned pixels has been such that the spatial resolution from a range of 300 km is 0.3 km in altitude.
Figure 1. Optical schematic of an f/1.4, CCD based, imaging spectrometer used to measure auroral spectra with high spatial and moderate spectral resolution.
Figure 2. The viewing geometry for slant path viewing of auroral arcs from Godhavn to Sondrestrom.
An image spectrum obtained from Godhavn is shown in Figure 3. Emissions are distributed in wavelength (horizontal axis) and altitude (vertical axis). Horizontal bars on the upper and lower panels indicate altitude in km for the arc which is known to be at the distance of Sondrestrom. The wavelength scale is indicated in Å. As the image contains 14 bits of dynamic range, two panels are displayed, showing the high bit (top) and low bit (bottom) scaling of this image. In order to process the measurements to a meaningful intensity unit, they are corrected for uniform field and instrument spectral response. During the observations, star occultations of the slit are used to accurately calibrate the spatial look direction.
Figure 3. An imaging spectrometer image shown at two dynamic range scales. The altitude scale corresponds to the distance of the arc which was in the magnetic zenith of Sondrestrom while being viewed from Godhavn. The horizontal scale is wavelength, in Å.
Figure 4 shows two spectra obtained at Sondrestrom, Greenland in March, 1996. An imaging spectrometer viewing along B at Sondrestrom was used to measure this spectrum. The observation on March 15, 1996 was of a thin, somewhat diffuse arc typical of soft precipitation (dashed plot in Fig. 3). Optical ratios using Figures 1 and 2 predict a characteristic energy of 2.1 keV with an energy flux of 18.9 ergs. The observation on March 23 was of a wide, more intense arc typical of a harder incident flux. Optical ratios for it predict a characteristic energy of 7.5 keV and a peak energy flux of 240 ergs. For these characteristic energies, the 'soft' arc penetrated to near 125 km and the 'hard' arc to near 105 km. The spectrograms are normalized with respect to the measured intensity of the N2+ 1N emission at 4708 Å in each case. Most of the features in the normalized spectrograms appear to be nearly identical, but a few stand out as being sensitive to the changed conditions. OI 8446 Å is clearly an emission which is spectrally clean and a distinctive discriminator for the two arcs with characteristically different energies.
Figure 4. An up-B spectrum of a 'soft' arc (dashed) and 'hard electron' arc (solid). The two spectra are normalized for energy flux (i.e. for the N2+ 4278 Å).
All Sky imagery is important for characterizing the spatial extent of the aurora and whether multiple arcs are present, especially important when analyzing for slant path information. Figure 5 is an All-sky image showing the March 15, 1996 thin arc whose up-B spectrum is plotted in Figure 4.
Figure 5. An all-sky image of 4278 Å emissions over Sondrestrom. The ÔboxÕ fiducial in the field marks the pointing position of the IBR radar antenna, which was taking data simultaneously with the optical measurements, characterizing the ionosphere. The altitude distribution of electrons and the electron and ion temperatures are measured and included in the modeling of arc emissions.
Photometers are another important part of the measurements. The time history of 3 auroral emissions are shown in Figure 6 for the period of the ÔbrightÕ event used in Figure 4. The characteristic energy and energy flux of auroral particles are plotted in Figures 7 and 8 respectively.
Figure 6. The time history of intensity for 4278 (blue), 5577 (green), and 6300 Å (red) as viewed up the magnetic field at Sondrestrom. The scale truncates the bright events near hour 1.2 UT.
Figure 7. A plot of Characteristic Energy vs. time as deduced from the intensities shown in Figure 6.
The altitude distributions of the emissions from the slant path measurements are a key to the absolute refinement of the characteristic energy and flux. In addition to the slant path data, radar, photometry, and all sky imagery are combined to provide a composite set of information for characterization of the geophysical conditions. Figure 8 is a plot showing data and a modeled energy and flux for an electron beam which emulates the measurements.
Figure 8. Same as Figure 7, except Energy flux vs. time.
The technique of using radar determined electron density profiles to estimate primary electron energy spectra has been described and used extensively. The value of defining the 'in arc' density and composition parameters versus altitude, coupled with optical data, provides a coherent set of parameters for modeling other geophysical phenomena. Our current studies involve the application of the combined optical and IBR data.
The current studies are being performed by G. Swenson, U.I, R. Rairden, Lockheed, and S. Solomon, U. of Colorado. The studies are sponsored by NSF.