Time variation in exospheric sodium density on Mercury

doi:10.1016/j.pss.2006.10.010

Planetary and Space Science

Volume 55, Issue 11, September 2007, Pages 1509-1517

https://doi.org/10.1016/j.pss.2006.10.010 Get rights and content

Abstract

We conducted continuous spectroscopic observations of the Mercury's sodium exosphere with a 188 cm telescope and a high dispersion echelle spectrograph, for 1–6 h in the daytime on December 4, 13, 14, and 15, 2005. To correct the images of the sodium emission blurred by Earth's atmosphere, the observed distribution was deconvolved with the point spread function which was obtained using Hapke's surface reflection model and the observed surface reflection. The average column density of sodium atoms was $1 - 2 \times 10^{11} atoms / {cm}^{2}$ and significant diurnal changes were not observed. However, the sodium densities at low latitudes and high latitude changed during the observation and the rate of change in density at low latitude was higher than that at high latitude on December 14 and 15. Although the rates of suggested release processes are higher than the observed rate, the suggested release processes cannot explain the rapid change in density at low latitude. This may suggest the effect of transport of neutral atoms and the recycling of ions to the surface dominates the time variation in the spatial distribution of exospheric sodium atoms on Mercury.

Introduction

The Mariner 10 UVS (Ultraviolet Spectrometer) detected H, He, and O in Mercury's atmosphere (Broadfoot et al., 1974). Moreover, the presence of Na, K, and Ca were also detected in the ground-based observations (Potter and Morgan, 1985, Potter and Morgan, 1986; Bida et al., 2000). Mercury's atmosphere is extremely thin ( $n \sim 10^{5} atoms / {cm}^{3}$ , $P < 10^{- 12} bar$ ); therefore, the mean free path is greater than the scale height even near the surface. Hence, Mercury's atmosphere is often called a surface-bounded exosphere. Although the density of sodium observed is low, sodium was visible on several occasions in Mercury's exosphere due to its bright emission lines—Na D lines (589 nm).

The source processes considered responsible for producing Mercury's sodium exosphere are thermal desorption (Yakshinskiy et al., 2000, Leblanc and Johnson, 2003), photon-stimulated desorption (McGrath et al., 1986, Yakshinskiy and Madey, 1999), sputtering by impacting solar particles (Potter and Morgan, 1997; Killen et al., 2004), and meteoroid vaporization (Morgan et al., 1988, Cremonese et al., 2005). The column density of Na was estimated to be $10^{11} - 10^{12} atoms / {cm}^{2}$ ; moreover, McGrath et al. (1986) suggested that the dominant source processes are photon-stimulated desorption and thermal desorption. On the other hand, Potter and Morgan (1997) and Potter et al. (1999) detected concentration of the sodium atoms at high latitudes and hot components, which were attributed to solar wind sputtering. Potter et al. (1999) observed diurnal changes of column density and spatial distribution. Potter et al. (1999) suggested that coronal mass ejection (CME) events might explain these phenomena. Leblanc and Johnson (2003) also suggested that the diurnal changes were due to changes in sodium density in the surface layer caused by CME encounters that can induce diffusion from within the grains of new sodium atoms. However, it is difficult to know whether or not CME encounters occurred on Mercury at that time.

The lifetime of sodium atoms in the sunlit exosphere is dominated by photoionization. The Sun–Mercury distance changes by a factor of 1.52 because of the large eccentricity (0.206) of the planetary orbit; in addition, the photoionization lifetime ranges from 1.4 to 3.3 h. The transport dynamics of sodium atoms in the Mercury exosphere are dependent upon the acceleration due to gravity and the solar radiation pressure produced by the solar resonance scattering of sodium in the D lines. Solar radiation pressure drives the anti-sunward transport of the sodium atoms. The magnitude of the Doppler shift due to the relative velocity between Mercury and the Sun is an important factor in relation to radiation pressure. If $γ$ is the fraction of the solar continuum flux at the wavelength of the Na D line, the solar flux at the $D_{2}$ line becomes the weakest $(γ \sim 3 %)$ when the Sun–Mercury radial velocity is zero (at aphelion), and it becomes the strongest $(γ \sim 42 %)$ when the Mercury-Sun radial velocity is the largest $(\sim 10 km / s)$ . Potter and Morgan (1987) suggested an anti-correlation of sodium abundance with solar radiation pressure. This was explained by Smyth and Marconi (1995) as a result of the anti-sunward transport to the interplanetary space.

The timescale of density change should be less than one terrestrial day if the CME encounters or significant changes in solar wind parameters dominate the release rate of the sodium atoms. However, in the past, only daily observations of Mercury's atmosphere have been conducted. The aim of our work is to observe the time variation of the release rate and to discuss the release process of the sodium atoms.

Section snippets

Observation

We observed Mercury's sodium exosphere at the Okayama Astrophysical Observatory (OAO). The observations were carried out using a 188 cm telescope and a HIgh Dispersion Echelle Spectrograph (HIDES) on December 4, 13, 14, and 15 in 2005. Mercury's orbit had the largest elongation from the Sun on December 12 and the highest Mercury–Sun radial velocity on December 13. Table 1 shows the celestial conditions prevailing on Mercury during the observation.

The HIDES is installed on the coude focus of the

Data reduction

The standard corrections for bias offset, dark current subtractions, and flat fields were implemented with the Interactive Data Language (IDL) Astronomy User's Library (http://idlastro.gsfc.nasa.gov/). The spectrum of the Th–Ar lamp was used for wavelength calibration. The spectrum of the surface reflection and emission lines is obtained by subtracting the background from the observed spectrum. We calculated the spectra of the background using a frame of the sky with a fifth-order polynomial

Spatial distribution of Mercury's sodium

The time variation in the latitudinal distribution of sodium density is shown in Fig. 9. The average FWHM of the point spread function is approximately 3 arcsec, which corresponds to the apparent radius of Mercury. Although our deconvolution method is effective in correcting the change in the FWHM of the point spread function, an improvement in the spatial resolution should not be expected. Hence, the spatial resolution is lowered to half of the radius of Mercury per pixel in order to obtain a

Conclusions and future work

We conducted an observation of time variation in the intensity of Mercury's exospheric sodium emission on December 4 and December 13–15. The rate of change in the sodium density at low latitudes was quite different from that at high latitudes. On the other hand, the observed averaged density was stable during December 13–15. This suggests that the rate of change in the yield of the globally occurring source processes should be low and the timescale of the locally occurring source processes

Acknowledgment

The authors wish to thank Dr. H. Izumiura, M. Ohtsuka, and other staffs of OAO for supporting our observation. We also thank Dr. S. Okano, Dr. M. Kagitani, Mr. K. Aoi, and Mr. H. Sagawa for many helpful discussions. This research was supported in part by a Grant-in Aid for JSPS Fellows (1611325).

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Time variation in exospheric sodium density on Mercury

Abstract

Introduction

Section snippets

Observation

Data reduction

Spatial distribution of Mercury's sodium

Conclusions and future work

Acknowledgment

Planet. Space Sci.

Icarus

Icarus

Icarus

Icarus

Icarus

Icarus

Icarus

Icarus

Adv. Space Res.

Icarus

Icarus