Variation of g-values of major species with heliocentric velocity

 

The discrete photoemission properties of atomic and molecular species stimulated by solar radiation are an important tool for quantitative work with remote sensing experiments. In an optically thin atmosphere, the total column amount of a given species is given in terms of a solar-forced g-value, defined as an emission probability per atom (photon s⁻¹atom⁻¹). For an optically thin gas and a measured emission brightness (4πI), the column abundance N is obtained through the relation 4πI= gN. Discrete emission lines and Fraunhofer features in the solar spectrum are responsible for a strong dependence of the g-values on Doppler velocity for many species. Because of Mercury’s eccentric orbit, some g-values can vary by over an order of magnitude during the orbital period. In addition, g-values are dependent on the instantaneous heliocentric radial velocity, which varies as an atom is ejected from the surface and moves through gravitational and radiation pressure accelerations. Thus knowledge of how the g-values vary is critical to interpretation of spectroscopic data. In previous work, g-values were calculated for 12 species that were to be targeted by MESSENGER. The species of interest included sodium, potassium, and calcium, which had been observed in Mercury’s exosphere through ground-based observations pre-MESSENGER, and hydrogen and oxygen, measured (or potentially only an upper limit in the case of O) by the UV spectrometer experiment on the Mariner 10 spacecraft. In addition, sulfur, magnesium, carbon, Ca⁺ and Mg⁺ are observable by Bepi-Colombo as well as MESSENGER. Helium EUV emission at 584 Å, not within the MESSENGER UVVS wavelength range, will be observable with the Bepi-Colombo UV spectrometer. Of particular importance for Mercury is the dependence of the g-values on the bulk motion of the gas relative to the Sun. This heliocentric relative velocity is often quite large due to the high initial velocity of the ejected atoms and the subsequent radiation pressure acceleration. 

• Motivation

Mercury’s orbit is highly elliptical, having an ellipticity of 0.2. This results in a variation of the heliospheric relative velocity varying by ± 10 km/s. The g-values published by Killen et al. (2009) were therefore calculated for Doppler shifts of this magnitude. However, it has become apparent that the heliocentric relative velocity of atoms in Mercury’s exosphere varies considerably more than the velocity at rest with respect to the planet, both due to the initial ejection velocity and due to radiation pressure, that is especially strong for Na. The velocity of Ca atoms is also extreme, due to as yet unknown processes, but possibly due to dissociation of a Ca-bearing molecule. We have therefore extended the g-values to ±50 km/s relative to their at-rest values. The g-values have been scaled using the solar spectrum originally used by Killen et al. (2009) (e.g. Hall and Anderson, 1991) and available in the appendix to that paper. In addition, for the Mg line at 285.296 nm, we used the TSIS-1 solar spectrum available from the LISIRD website (https://lasp.colorado.edu/lisird/). In March 2022, the TSIS-1 HSRS was recommended as the new solar irradiance reference spectrum by the Committee on Earth Observation Satellites (CEOS) Working Group on Calibration and Validation (WGCV). TSIS-1 HSRS is developed by applying a modified spectral ratio method to normalize very high spectral resolution solar line data to the absolute irradiance scale of the TSIS-1 Spectral Irradiance Monitor (SIM) and the cubesat Compact SIM (CSIM). The spectral resolution of this spectrum is 0.25 nm. The high spectral resolution solar line data from the Air Force Geophysical Laboratory ultraviolet solar irradiance balloon observations, used in the Killen et al. (2009) work, have a spectral resolution of 0.015 nm. The g-values calculated herein at high resolution must be convolved to the spectral resolution of the instrument used for the observations to be analyzed. The UVVS spectrometer on MESSENGER was a scanning grating monochromator that covered the wavelength range 1150 - 6100 Å with an average 6 Å spectral resolution (McClintock & Lanton 2007). The Bepi Colombo UV spectrometer, PHEBUS, is a double spectrometer for the Extreme Ultraviolet range (550 - 1550 Å) and the Far Ultraviolet range (1450 - 3150 Å) using two micro-channel plate (MCP) detectors with spectral resolution of 10 Å for the EUV range and 15 Å for the FUV range (Chassefiere et al. 2008).

We show plots of extended g-values for Na (D1), K (D1 & D2), Ca, and Mg. The Mg g-values are given for both the Hall and Anderson (1991) solar spectrum and for the TSIS-1 spectrum for comparison of high and low-resolution results.

Figure 1. The g-value for Na (D1) at 589.756 nm in vacuum continues to increase as the Doppler shift increases. The D2 line g-values similarly increase beyond a velocity of ±10 km/s to ± 50 km/s. Therefore the column abundance for high velocity atoms will be over-estimated using the formerly published g-values. This is especially important for estimates of escape.

 

Figure 2. G-values for the Ca 422.7 nm line calculated to ±50 km/s. In the case of the Ca line, the g-value continues to increase for heliocentric relative velocity > 25 km/s, but decreases for heliocentric relative velocities < -35 km/s. As for Na, the Ca column abundance anti-sunward of Mercury will be overestimated using the previously published g-values.

Figure 3. The extended g-values for the K (D2) line at 404.53 nm (green) and K (D1) 404.84 nm (vacuum wavelengths) at 0.352 AU are quite complex owing to the underlying solar spectrum. Care must be taken to calculate the column abundance at the instantaneous heliocentric relative velocity of the atom.

 

Figure 4. The g-value for the Mg 285.296 nm line is also quite complex. This figure shows the g-value calculated using the high resolution of the Hall and Anderson (1991) solar spectrum at a spectral resolution of 15 mÅ.

 

Figure 5. The g-value for the Mg 285.296 nm line using the low resolution TSIS-1 spectrum at 0.25 Å resolution shows little high frequency variation, as expected.