THEMIS Na exosphere observations of Mercury and their correlation with in-situ magnetic field measurements by MESSENGER
Introduction
The sodium bright doublet emission at 5890–5896 Å, thanks to its good visibility also obtained from ground-based observations, is broadly used to study the exosphere of Mercury (e.g. Potter and Morgan, 1997; Sprague et al., 1997; Killen et al., 2001; Leblanc et al., 2008, Leblanc et al., 2009, Leblanc and Johnson, 2010, Leblanc et al., 2013). Earth-based observations were performed for 30 years, since the discovery of Na component in 1985 (Potter and Morgan, 1985) and provided a large dataset of images in which recurrent patterns are observed, as well as a variable intensity. Earth-based observations of Mercury often take advantage of solar telescopes that allow observations during daytime. Hence, valuable series of data for many hours per day are now collected and can be used to study the variability of the exosphere of Mercury through its sodium component.
The study of the exosphere of Mercury and its dynamics is important to understand the processes that generates the planet's tenuous (collisionless) atmosphere given strong bombardment of the surface by solar wind plasma and micrometeoroids, as well as the relatively strong IMF at such a close distance from the Sun. A proper understanding of the exosphere relates with the inter-relations among all the parts of Mercury's environment (for a review, see Milillo et al., 2005). Earth-based observations of the Na exosphere often exhibit a two-peak pattern. These peaks are usually located at mid/high latitudes in both hemispheres of Mercury and they can often differ in intensity and extent, e.g. one peak is more intense or wider than the other. Asymmetries between dawn and dusk have also been hypothesized (Potter et al., 2006, Schleicher et al., 2004, Sprague et al., 1997). Moreover, the major overall variability observed from ground is seasonal, meaning that it is linked to the True Anomaly Angle (TAA) (e.g. Schmidt et al. 2010; Leblanc and Johnson, 2010; Cassidy et al., 2014) and is interpreted as a composite effect of the planet's distance from the Sun, radiation pressure, Doppler shift on resonant scattering efficiency and Na deposition and migration over the surface (day/night anisotropies). More recently, in-situ measurements from MASCS on-board MESSENGER spacecraft, proved that Na exhibits a clear year to year recurrence (Cassidy et al., 2014); nevertheless, no other variability could be distinguished. It should be noted, however, that the MASCS observations are restricted to the dayside equatorial region and that the global exosphere configuration can be observed only by ground based telescopes. In fact, when removing the known yearly variations from the images of Na emission obtained by THEMIS, a solar telescope with good spatial resolution, variability in intensity and different emission features becomes evident also on shorter time-scales, i.e. 1 h (Leblanc et al., 2009, Mangano et al., 2013).
The morphology of the two-peaks of Na intensity is believed to be related to the interaction of the solar wind particles with the intrinsic magnetic field that drives them into preferred regions of the surface (i.e., the footprints of the magnetic cusps). Some debate exists about the processes acting to produce such peculiar features. In fact, the ion-sputtering process alone is not expected to be able to release enough Na to account for the intensity of the observed peaks at mid- to high-latitude position (Mura et al., 2009). Leblanc and Johnson (2003) modeled the efficiency of the various release processes during different orbital phases, suggesting that the exosphere generation is the result of the complex relationship between surface and external environment. Also Mura et al. (2009) suggested a synergy of more than a single process to account for observational evidence of the two peaks features: the plasma ions impacting the surface at the cusp footprints loosen the Na atoms in the crystalline structure of rocks and regolith; the subsequent desorption (induced both from the action of temperature and photons) results in the final release of Na into the exosphere.
Thanks to the relevant dataset collected over decades of observations, statistical studies on the Na exosphere configurations are possible. Such a statistical analysis was performed by Potter et al. (2006) using a dataset of 7 years (1997–2003); they analyzed the Na exospheric asymmetries observed on the disk of Mercury in both longitudinal and latitudinal directions. They found a dawn/dusk asymmetry with statistically higher terminator-to-limb ratios when dawn is in view. Their analysis suggests that the south peak is more frequent when dawn hemisphere is observed. They did not find any correlation between peaks and TAA and no clear predominance of one hemisphere to the other.
Similarly, Leblanc and Johnson (2010) using the global intensity data collected by a huge set of ground-based observations and the Mercury Exosphere Global Circulation Model (MEGCMS), investigated the processes responsible for the Na exosphere modulation along the Mercury orbit. They concluded that the dominant active process varies during Mercury's year but it is mainly consistent with the Photon Stimulated Desorption process, apart from the portion of the orbit when TAA is 25–40°, where the Thermal Desorption prevails. The temperatures observed by MESSENGER/MASCS are in agreement with a PSD distribution (Cassidy et al., 2014).
Nevertheless the role of the solar wind entry inside Mercury's magnetosphere after the reconnections between the IMF and the internal magnetic field is evidently linked to the spatial distribution of Na exosphere (Killen et al., 2001). So that, a detailed study of Na emission pattern correlated to the IMF configurations is the way to solve the Na puzzling case. Thanks to the MESSENGER spacecraft (Solomon et al., 2007), we now have the possibility to directly compare ground-based data to in-situ IMF measurements.
In the present paper we use a dataset of 5 years (2009–2013) of observations obtained at the THEMIS solar telescope in Tenerife to make a statistical analysis of recurrent patterns. In addition, we cross-check a subset of data (years 2011–2013) with the magnetic data of IMF coming from in-situ measurements of the MAG sensor on-board MESSENGER spacecraft to try to give new insights in the connection between recurrent Na exospheric patterns and the IMF configuration. In Section 2 the sodium exospheric observations and the magnetic field dataset are described and analyzed. In Section 3 we discuss the statistical analysis, and in Section 4 we draw our conclusions.
Section snippets
Datasets
Two different sets of data are used: Earth-based spectroscopic images of Na D2 emission of the exosphere (5890 Å), and in-situ measurements of the IMF as obtained from MAG, the magnetometer on-board MESSENGER spacecraft (Solomon et al., 2007).
Earth-based data are obtained by using the THEMIS solar telescope (López Ariste et al., 2000) located in Tenerife (Canaries, Spain). THEMIS telescope has a 0.9 m primary mirror and a 15.04 m focal length and it can be used during the daylight to image Mercury
Discussion
As the tiny magnetosphere of Mercury is known to be tightly connected to the IMF, we made an effort to check if and how the recurrent Na exospheric emission patterns can be affected by the IMF variability. Due to limitations affecting the Fast Imaging Plasma Spectrometer (FIPS) on board of MESSENGER, the solar wind speed and density are not routinely available and can be reconstructed only in particular cases (see e.g., Gershman et al., 2013; Raines et al., 2013). Because of this, our present
Conclusions
We performed a statistical analysis of the Na emission from the exosphere of Mercury, based on ground based observations taken by the THEMIS telescope, and spanning over a time period of 5 years (2009–2013). We categorized the exospheric Na emission into 8 different recurrent patters and studied their occurrence, also as a function of the in-situ IMF, as measured by the MAG instrument on-board the MESSENGER spacecraft. The results of this study can be summarized as follows:
- 1.
by considering the
Acknowledgments
The authors would like to thank all the THEMIS staff in Tenerife (Canary Islands, Spain) for their fruitful help throughout the years of observation campaign. This work was supported by the ASI-SERENA Contract no. I/081/09/0 “SERENA: scientific activity”. Planetary Data System (PDS) and Planetary Plasma Interactions Node (PPI) are acknowledged for on-line data distribution.
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