Geocoronal Hydrogen Emission Variation Over Two Solar Cycles

Abstract

Ground-based hydrogen Balmer-α observations from Northern midlatitudes span multiple solar cycles, facilitating investigation of decadal scale variations, including natural variability in the hydrogen response to solar geophysical changes. Here we present a reanalysis of ground-based hydrogen emission observations from the early 1990s and their comparison with observations obtained in [2000][2001] in the context of the extended Northern Hemisphere midlatitude geocoronal hydrogen emission data set. This work suggests an increase in hydrogen emission intensity between the solar-maximum period of 1990-1991 (Solar Cycle 22) and the near-solar-maximum period of 2000-2001 (Solar Cycle 23), with the caveat that this is a limited data set and that there are calibration uncertainties discussed in this paper. Solar activity was higher during the earlier solar maximum period. Thus, the apparent increase in intensity is counter to previous observations from midlatitudes in which the observed intensity increases with higher solar activity. This increase was also not seen in comparison of intensities from three solar minima periods. Further, the apparent increase in intensity is also likely of larger magnitude than model simulations would predict due to increases in methane and carbon dioxide. We will discuss the reanalysis and recalibration of the 1990-1991 observations using current analysis approaches and the interpretation of these observations in the context of observations and modeling of hydrogen variation over different time scales. The detailed review of the calibration procedures has also provided insights to guide design of future observations. Plain Language Summary Atomic hydrogen is a key constituent of the thermosphere and exosphere, the uppermost region of the Earth's atmosphere. Hydrogen in this region is a by-product of molecules at lower altitudes that contain hydrogen such as water vapor and methane, two of the greenhouse gases most important to the energy balance of the Earth. The 11-year solar cycle is a major source of natural variability in the upper atmosphere. Here we present observations of geocoronal hydrogen emissions (~400 km and above) taken by ground-based Fabry-Perot instruments at Wisconsin and at the Kitt Peak, Observatory in Arizona. These observations span two solar cycles and suggest an increase in the geocoronal hydrogen emission intensity between the 1990-1991 solar maximum (Solar Cycle 22) and the 2000-2001 near-solar-maximum (Solar Cycle 23) periods, with the caveat that this is a limited data set and there are calibration uncertainties. Solar activity was higher during the earlier solar maximum period. The apparent increase in intensity is counter to previous observations from midlatitudes in which the observed intensity increases with higher solar activity. Further, the apparent increase in intensity is also likely of larger magnitude than model simulations would predict due to increases in methane and carbon dioxide.

Solar Cycle, Seasonal, and Dawn‐To‐Dusk Variations of the Hydrogen in the Upper Thermosphere

“…From the top panels, we see that the absolute intensity values do not increase linearly with the solar F 10.7 flux. This is because these intensities depend not only on the solar flux but also on the geocoronal H density, which decreases with increasing solar activity (e.g., Nossal et al, 1993Nossal et al, , 2019Qin & Waldrop, 2016). Moreover, the shapes of the intensity profiles observed near dawn and near dusk show evident differences, indicating dawn-dusk asymmetry of the H density distribution.…”

“…Since the 1980s, ground‐based observations of the Balmer‐α (656.3 nm) emission has been used to monitor the long‐term variations of the exospheric hydrogen (e.g., Nossal et al., 1993, 2004, 2008, 2019). In the last two decades, the geocoronal Lyman‐α (121.6 nm) emission has also been routinely observed by the Global Ultraviolet Imager (GUVI) onboard NASA's Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite (e.g., Joshi et al., 2019; Paxton et al., 2017; Qin et al., 2017; Qin & Waldrop, 2016) and by the Two Wide‐Angle Imaging Neutral‐Atom Spectrometers (TWINS) missions (e.g., Cucho‐Padin & Waldrop, 2018, 2019, 2020; Zoennchen et al., 2010, 2013, 2015).…”

The Far Ultra-violet Background

“…The Hα brightness at large shadow heights (see, Figure 5) was found to be 1.5 R at solar minimum and twice that at solar maximum (Nossal et al 2008(Nossal et al , 2019. The corresponding twophoton brightness, using the formulation discussed in Section 3, is 85 CU which, when summed with other sources of two-photon emission (Sections 4, 6), would exceed the level of the offset emission of 120-180 CU.…”

“…Typically, WHAM, during dusk and dawn, undertook scans as a function of zenith angle for aeronomical studies (see, for example, Nossal et al 2008Nossal et al , 2019. It is traditional to report these scans (Figure 5) as a function of "shadow height" (see Figure 6 for definition and explanation).…”

“…So, we will make the simplifying assumption that Hα is solely powered by Lyβ. Typically the Hα surface brightness measured at zenith in the middle of the night 8 is 1.5 R at solar minimum and twice that at solar maximum (Nossal et al 2008(Nossal et al , 2019. Following the emission of an Hα photon the hydrogen atom finds itself in the 2s level.…”