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The long-recognized association of brighter aurora with more deeply penetrating, and hence more energetic, electrons is examined. Using the Knight (1973) relation between the magnetic-field-aligned current density and potential drop (derived from the theory of single-particle motion in the presence of a magnetic-field-aligned electric field), an approximate expression relating the energy flux of the precipitating electrons over discrete aurora and the mean particle energy is derived. This expression is used in conjunction with an auroral optical excitation and emission model to specify the dependence of the red/blue ratio of auroral optical emissions on the brightness of the aurora. It is shown that the quantitative predictions of the discrete auroral theory are in accord with observations of the aurora.

OH nightglow emission from the (6,2) band were observed during February 12-14, 1986, at Sondre Stromfjord, Greenland. The data were analyzed using time series analysis techniques to determine {eta} = ({Delta}I/{anti I})/({Delta}T/{anti T}), where {Delta}I and {Delta}T represent correlated fluctuations from the mean OH intensity ({anti I}) and temperature ({anti T}) in various frequency bands. For correlated fluctuations due to measurement error, {vert bar}{eta}{vert bar} 1.5. The 4-hour (2-hour) period wave has an {vert bar}{eta}{vert bar} equal to 3.1 {plus minus} 1.1 (1.6 {plus minus} 0.5) and a phase angle, between the intensity and temperature components, of 5{degree} {plus minus} 23{degree} (40{degree} {plus minus} 20{degree}). These results are consistent with a recent theoretical model for the passage of AGWs through an OH emission layer. Agreement is closest for the 4-hour (2-hour) period wave whenmore » the layer is peaked at 87 km (83 km) and the O scale height is {minus}2.0 ({minus}4.0km).« less

We conclude from a study of the production and loss of O(¹D) in auroras that the “traditional” sources, direct electron inpact excitation of atomic oxygen and dissociative recombination of molecular oxygen ions, can account for most of the O I 6300-A emission rate. In a specific application of the model to the comprehensive observation of an auroral event by Sharp et al. (1979), we show that there is no compelling need for the reaction N(²D) + O2 → NO + O(¹D). We also present a study of the sensitivity of the red line emission rate to a wide variety of input conditions.

We examine the problem of monitoring composition and the behavior of precipitating electron spectra in auroras using N2+ 4278 A (blue), O I 6300 A (red), O I 7774 A (narrow; e+O), and O I 7774 A (broad; e+O2) as observed from the ground. Calculated column emission rates for these features as well as those of narrow O I 8446 A and broad O I 8446 A are presented as functions of the hardness of the incident electron spectrum and the concentrations of O and O2. Selected ratios of these rates such as narrow/broad (for 7774 A) and red/blue are also presented. Incident spectra are characterized by Maxwellian energy distributions with characteristic energies ranging from 0.1 to 8 keV. Composition is modeled using a Jacchia (1977) model where scaling factors are applied to the O and O2 number densities. Scaling factors for O range from 0.1 to 1, and factors of 1 and 1.5 are considered for O2. As a group, the above rates and ratios show considerable variation over the just described parameter ranges, making them attractive for monitoring incident electron energy flux, its mean energy, and the concentrations of O and O2 relative to N2. The red line is a key feature of the above group which is sensitive to the low-energy portion (subkilovolt) of the incident electron spectrum. Since this can vary considerably from one aurora to another having the same approximate mean energy, it becomes an added parameter to be considered within any algorithm using the red line. Paper 2 (Meier et al., this issue) discusses this problem as part of a detailed investigation of the red line. In the current paper, one particular representation of a low-energy component is considered for making the red line calculations. A final subject of this paper is the use of temperatures deduced from measurements of rotational line distributions and atomic line Doppler widths to infer mean energies of precipitating electrons. Calculated effective temperatures versus hardness of the incident electron spectrum are presented and discussed in the context of more precise techniques for relating measurements of rotational line distributions and Doppler widths to electron spectral hardness.

Several possible analogs of cosmic dust have been produced by a variety of techniques. Extinction and emission spectra of these grain samples reveal that hydrated silicates, in particular those which are Mg-rich, produce infrared spectra that bear strong resemblances to the spectra of a variety of astronomical dust grains ranging from those found within the solar system to those found in dense, cold, molecular clouds. An identification of hydrated silicates as the origin of the 6.0 and 6.8 micron features is proposed as part of a model which is consistent with all the spectra available to date on both cosmic dust and the laboratory samples studied. 34 references.

Ground-based optical observations have been made of the N{sup +}{sub 2} (4278 A), O I (6300 A), O I (7774 A), and O I (8446 A) auroral emissions. Data were collected on 12 nights from 1984 to 1986 at observatories in the auroral zone and were analyzed using the Strickland et al. (this issue) aurroal electron impact excitation model results. It was found that during quiet geomagnetic conditions these data were consistent with O densities, at altitudes from 110 to 200 km, less than one-half the densities given in the Jacchia 1977 or MSIS 1983 models. During highly disturbed periods such as during the great February 1986 storm, where {ital Ap} reached 202, the O density decreased an additional factor of 8. During the same storm the O{sub 2} density increased by less than a factor of 2 from its quiet value. These data also imply that the decrease in the O density is larger at higher altitudes during disturbed periods, in qualitative agreement with the heating model of Hays et al. (1973). {copyright} American Geophysical Union 1989

Temporal variations in the absolute intensity (I) and the rotational temperature (T) of the polar winter airglow OH(8, 3) bands were observed over Longyearbyen, Spitsbergen (78.2°N), in December 1984. The high latitude of the observatory places it in a continuous polar night around the winter solstice period, thereby permitting 24-hour optical measurements. Spectral analyses of the I and T time series reveal a dominant semidiurnal component and a wave with a period of 3.7 hours. Other waves with shorter periods may be present for part of the total observing period, but the signal-to-noise ratios of their amplitudes averaged over a 24-hour interval are too low to permit definitive identification. The ratio η of the correlated Fourier components of (I − Ī)/Ī to that of (an overbar signifies time average) has amplitude 3.2 ± 0.9 and 4.3 ± 1.3 for the 12- and 3.7-hour waves, respectively, while the phase ϕ of η for these periods is 6° ± 16° and −12° ± 12°. The value of η for the 3.7-hour wave can be compared with the theoretical prediction of η for gravity wave modulation of the OH emission. For an emission height of 87 km and an atomic oxygen scale height ℋ(O) of −2 km, theory gives |η| = 4.2 and ϕ = 0° for the 3.7-hour wave, in good agreement with the observation. The theoretical interpretation of η at the 12-hour period is complicated by the likelihood that the observed semidiurnal oscillation is not a simple tide. For tidal modulation of the OH nightglow, theory gives |η| between 2.6 and 0.9 and ϕ between 0° and 10° for high-order semidiurnal modes and for the same values of emission height and ℋ(O) as above.

A CEDAR campaign organized to observe Sun-aligned arcs from Sondre Stromfjord, Greenland, obtained a coordinated satellite- and ground-based observation of the development and subsequent zenithal passage of a Sun-aligned transpolar arc on February 26, 1987. A broad array of ground-based optical instruments plus the Sondre Stromfjord incoherent scatter radar characterized the optical and ionospheric parameters of the arc as it passed through the geomagnetic zenith. Optical instrumentation obtained detailed intensity measurements in the geomagnetic zenith for several auroral emissions and recorded the spatial and temporal evolution of the arc. During the transit of the arc the incoherent scatter radar was pointed in the geomagnetic zenith direction, providing electron density height profiles. The DMSP F6 satellite imaged a segment of the Sun-aligned arc and measured the precipitating flux along its path at the same time that ground-based measurements were being taken. All-sky images in N2+ 1 NG and O I (6300 A) emissions reveal the progression of the arc from an F region feature to an E region feature and its resulting stability for the following 20 min as it drifted out of the zenith. The maximum emission intensity of O I (6300 A) within the arc was 1.1 kR. The observed λ6300/λ4278 spectral ratio was 1.3. The altitude of peak electron density changed from 170 to 250 km preceding the zenithal transit of the arc and then shifted to 120 km as the brightest structure moved into the radar field of view. Model calculations based on the observed electron density height profile implied a peak energy of 3.5 keV for the precipitating electrons with a total electron energy flux of 2.1 ergs/cm² s assuming a Gaussian incident electron distribution with a low-energy tail included to match the observed emissions. The low Balmer Hα emission intensity implied a proton energy flux of less than 0.04 erg/cm² s. Matching model results with high-resolution auroral O I (7772 A) measurements implies scaling the MSIS model atomic O densities by 0.4 for this event. Optical spectral ratios predicted by the model were near observed values. Our results show that this event is best described by a soft electron precipitation with a very low upper limit imposed on the proton contribution.

The extinction of Herschel 36 was measured and found to be peculiar in the same sense as that observed in Orion. Following the treatment of Mathis and Wallenhorst, this can be explained by the presence of large silicate and graphite grains than are normally found in the interstellar medium. Correcting the stellar flux for foreground extinction results in a residual extinction curve for the associated dust cloud, with an unusually small normalized extinction (less than 1.0) at 1500 A. This low UV extinction may be due to the effects of scattering by the dust cloud material.