Erratum: The Spectroscopic Detectability of Argon in the Lunar Atmosphere

Details Erratum: The Spectroscopic Detectability of Argon in the Lunar Atmosphere Erratum: The Spectroscopic Detectability of Argon in the Lunar Atmosphere

Feb 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999apj...512l..77p&link_type=abstract

The Astrophysical Journal, Volume 512, Issue 1, pp. L77-L77.

Astronomy and Astrophysics

Astrophysics

1

Scientific paper

In the Letter "The Spectroscopic Detectability of Argon in the Lunar Atmosphere" by J. Wm. Parker, S. A. Stern, G. R. Gladstone, and J. M. Shull (ApJ, 509, L61 [1998]), there were errors in the calculation of the g-factors of argon in emission (section 2.2) that somewhat change our conclusions regarding the possibility of lunar argon having been detected by ORFEUS (B. Flynn, ApJ, 500, L71 [1998]). In our calculations of the appropriate g-factors, we had modeled the energy distribution of the solar wind electrons with a multicomponent Maxwellian distribution. There was an error in the code that normalized the fit of the model to the data. We now find that a kappa distribution is a better fit to the data, particularly the high-energy (power-law) tail. These changes affect only the g-factors due to solar wind electron impact; the radiation g-factors are unchanged from our previous values. The resulting g-factors due to solar wind are comparable to those for radiation; this in itself is an interesting result. For the epoch of the ORFEUS observations, the values for the total g-factors (radiation+solar wind) are about (8.9+/-0.1)x10^-8 s-1 for the Ar I lambda1048 line and (5.1+/-0.7)x10^-8 s-1 for the Ar I 1067 line (the values we had used in our Letter were 6.0x10^-8 and 2.6x10^-8 s-1, respectively). We also corrected our curve-of-growth (COG) radiative transfer calculation to better account for the two g-factors: because the collision cross section is significantly smaller than the cross section for resonance scattering, the COG due to collisions is optically thin at higher surface densities than is the COG due to fluorescence. We assumed an electron-argon collision cross section of 10-15 cm2. In Figure 2 (below), we present a corrected version of Figure 2 from our Letter. It shows that for the case of argon accommodated to expected daytime surface temperatures of ~400 K, the argon surface density implied by the 2.2 R line brightness around 1048 Angstroms claimed to be detected in the ORFEUS data would be n_Ar ~ 2.5x10^6 cm-3. This value is no longer in strong conflict with the in situ measurements by Apollo, but it is close to the upper limit (for even a slightly lower temperature, <~ 300 K, the total density implied by the ORFEUS data would be too high to be permissible). Further, for n_Ar=2.5x10^6 cm-3, the Ar I lambda1067 line would have a brightness of 1.7 R, which is below the 3 sigma detection threshold in the ORFEUS data and consistent with the nondetection of that line reported in Flynn (1998). We emphasize that this correction does not affect any of our conclusions about the case of the detectability of argon in absorption, e.g., our finding that the lower limit established by the in situ lunar argon measurements implies that any absorption-line measurement of argon in the lower, dayside lunar atmosphere requires analysis in the optically thick regime.

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