The technique of Fabry-Perot CCD annular-summing spectroscopy, with particular emphasis on applications in aeronomy, is discussed. Parameter choices for optimizing performance by the use of a standard format CCD array are detailed. Spectral calibration methods, techniques for determining the ring pattern center, and effects imposed by limited radial resolution caused by superpixel size, variable by on-chip binning, are demonstrated. The technique is carefully evaluated experimentally relative to the conventional scanning Fabry-Perot that uses a photomultiplier detector. We evaluate three extreme examples typical of aeronomical spectroscopy using calculated signal-to-noise ratios. Predicted sensitivity gains of 10-30 are typical. Of the cases considered, the largest savings in integration time are estimated for the day sky thermospheric O(1)D case, in which the bright sky background dominates the CCD read noise. For profile measurements of faint night sky emission lines, such as exospheric hydrogen Balmer-α, long integration times are required to achieve useful signal-to-noise ratios. In such cases, CCD read noise is largely overcome. Predictions of a factor of 10-15 savings in integration time for night sky Balmer-α observations are supported by field tests. Bright, isolated night sky lines such as thermospheric O(1)D require shorter integration times, and more modest gains dependent on signal level are predicted. For such cases it appears from estimate results that the Fabry-Perot CCD annular-summing technique with a conventional rectangular format may be outperformed by a factor of 2-5 by special CCD formats or by unusual optical coupling configurations that reduce the importance of read noise, based on the ideal transmission for any additional optics used in these configurations.
Abstract. This paper reports high-accuracy measurements of geocoronal Balmer a line profiles and demonstrates that the profiles are well fit with a model which includes cascade excitation by solar Lyman series radiation from n > 3 in addition to the direct excitation of n = 3 by solar Lyman/3. The increase in the signal-to-noise of our data is made possible by the use of the Fabry-Perot annular summing technique implemented at our Fabry-Perot facility at the University of Wisconsin's Pine Bluff Observatory. The new sensitivity has allowed us to make a detailed examination of line profile asymmetries and to conclude that they are compatible with predictions that of the order of 10% of the geocoronal Balmer a emission is caused by the cascade process. Cascade excitation alters the observed profile because it produces Balmer a emission along fine structure paths yielding slightly shifted wavelengths not present in direct Lyman/3 excitation, which is the predominant excitation mechanism for geocoronal Balmer a. We discuss how fine structure excitation affects studies of non-Maxwellian exospheric hydrogen velocity distributions and effective temperatures through Balmer a line profile measurements. In a broader context, we consider how inclusion of the cascade excited emission in future radiation models can enhance their accuracy and their potential for assisting in the isolation in the data of shorter-term solar geophysical effects and longer timescale changes in exospheric hydrogen densities. gives a measure of the line intensity. The accurate extraction of geophysical parameters from this profile is crucially dependent on the appropriate choice of model fit to the data. Small errors in determining the line width, for example, can significantly alter exospheric effective temperature measurements since they are proportional to the square of the width of the line. In addition, low exospheric densities enable atoms to move on orbital trajectories and to escape the Earth's atmosphere, producing non-Maxwellian dynamical perturbations to the line profile [Chamberlain and Hunten, 1987]. In order to isolate dynamical perturbations to the line profile, it is important to account in detail for the excitation mechanisms for geocoronal Balmer a because the hydrogen line profile is excitation dependent. Finally, as shown by Meier [1995] and discussed more fully below, absolute and relative intensity errors of typically 10% are produced by exospheric Balmer a emission models that neglect cascade from hydrogen levels higher than n = 3 that are excited by solar Lyman radiation. This paper reports the most accurate measurements to date of geocoronal Balmer a line profiles and concludes that a detailed accounting of the cascade fine structure excitation is necessary for optimally fitting the profiles.Geocoronal Balmer a is excited primarily by solar Lyman/3. Most of the radiation that we detect originates in the sunlit portion of the geocorona above the Earth's shadow, and hence the changing height of the Earth's shadow provides a m...
Observations of the geocoronal Balmer α nightglow have been made from Wisconsin for more than a solar cycle with an internally consistent intensity reference to standard astronomical nebulae. These measurements were made with a double‐etalon, pressure‐scanned, 15‐cm aperture Fabry‐Perot interferometer. The resulting long time line data provides an opportunity to examine solar cycle influence on the mid‐latitude exosphere and to address accompanying questions concerning the degree to which the exosphere is locally static or changing. Our exospheric Balmer α absolute intensity measurements show no statistically significant variations throughout the solar cycle when the variation with viewing geometry is removed by normalizing the data to reference exospheric model predictions by Anderson et al. However, the relative intensity dependence on solar depression angle does show a solar cycle variation. This variation suggests a possible related variation in the exospheric hydrogen density profile, although other interpretations are also possible. The results suggest that additional well‐calibrated data taken over a longer time span could probe low‐amplitude variations over the solar cycle and test predictions of a slow monotonic increase in exospheric hydrogen arising from greenhouse gases.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.