Solar cycle variations of geocoronal Balmer α emission

Abstract

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.

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

“…Mid-latitude geocoronal hydrogen Balmer a observations by Wisconsin observers now span 1977 to the present with some gaps in the data record (Reynolds et al, 1973;Yelle and Roesler, 1985;Shih et al, 1985;Nossal et al, 1993Nossal et al, , 1997Nossal et al, , 1998Nossal et al, , 2001Nossal et al, , 2004Bishop et al, 2001Bishop et al, , 2004Mierkiewicz, 2002). The majority of these observations were made from Wisconsin, with additional observations from Haleakala (HI) (Harlander and Roesler, 1989;Bishop et al, 2001), and Kitt Peak (AZ) (Nossal et al, , 2004.…”

“…If confined to a single element detector, such as a photomultiplier tube (PMT), the interferometer must be scanned through the desired spectral interval, sequentially sampling one spectral resolution element dl at a time. Although there are a number of ways to accomplish this (e.g., mechanical, tilt, or pressure scanning) the method most commonly used in geocoronal Balmer a studies involves pressure scanning (see e.g., Atreya et al, 1975;Meriwether et al, 1980;Shih et al, 1985;Yelle and Roesler, 1985;Harlander and Roesler, 1989;Kerr et al, 1986;Kerr et al, 2001a, b;Nossal et al, 1993).…”

“…Earlier observations, compiled in Nossal et al (1993), made use of a scanning photo-multiplier detection system, while later observations make use of the CCD camera for annular summing detection (see e.g., Coakley et al, 1996). Each instrument was coupled to a siderostat or situated next to another Fabry-Perot that was coupled to a siderostat.…”

“…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.…”

Geocoronal hydrogen studies using Fabry–Perot interferometers, part 2: Long-term observations

“…Ground-based Fabry-Perot interferometer observations of the hydrogen Balmer a column emission intensity now span more than two decades (Atreya et al, 1975;Meriwether et al, 1980;Yelle and Roesler, 1985;Shih et al, 1985;Kerr et al, 2001a,b;He et al, 1993;Nossal et al, 1993Nossal et al, , 1998Nossal et al, , 2004Bishop et al, 2001;Mierkiewicz, 2002, and references therein).…”