Cascade excitation in the geocoronal hydrogen Balmer α line

Abstract

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...

Geocoronal hydrogen studies using Fabry–Perot interferometers, part 1: Instrumentation, observations, and analysis

“…The Balmer a column emission rate (also referred to as intensities in the following) is dependent upon the hydrogen density profile, the solar Lyman b line center excitation flux, and the observational viewing geometry. A portion of the signal is also due to multiple scattering of Lyman b into the Earth's shadow (see e.g., Anderson et al, 1987;Bishop et al, 2001) and from cascade contributions from hydrogen excitation by higher order solar Lyman series lines (Meier, 1995;Nossal et al, 1998). Typical zenith Balmer a intensities are in the range of 2-15 Rayleighs.…”

“…This paper will discuss the technique of FabryPerot spectroscopy as applied to ground-based geocoronal hydrogen Balmer a observations, including strategies for obtaining data of sufficient precision and scope to address the scientific areas of focus outlined above. Observations by the Wisconsin group and collaborators (Reynolds et al, 1973;Yelle and Roesler, 1985;Shih et al, 1985;Harlander and Roesler, 1989;Nossal et al, 1993Nossal et al, , 1997Nossal et al, , 1998Nossal et al, , 2001Nossal et al, , 2004Coakley et al, 1996;Bishop et al, 2001Bishop et al, , 2004Mierkiewicz, 2002) will be used to illustrate capabilities and challenges associated with using Fabry-Perot spectroscopy as a means to explore the hydrogen geocorona.…”

Hα observations with DEFPOS

“…Light is collected simultaneously in all spectral elements. Annular summing spectroscopy makes use of the property of the FabryPerot interference pattern that equal area annuli correspond to equal spectral intervals (Reynolds et al 1990;Coakley et al 1996;Nossal et al 1998;Mierkiewicz et al 2006). Using this technique whereby the pixel intensities are integrated within concentric equal area annuli, the reduced CCD image of the Fabry-Pérot interference pattern is converted to a spectral profile.…”

Intensity calibration of DEFPOS data

“…The ring on the CCD image represents the spectrum within 4 beam. After the annular summing procedure [2,14], this ring pattern was converted into one dimensional Hα spectrum to determine the physical parameters of the selected region. For all DEFPOS data, in general, we have used a curve fitting program to fit Gaussians to the observed data from the sky [19,20].…”