1. Stefano Orsini 1, Valeria Mangano 1, Alessandro Mura 1, Francois Leblanc 2, Anna Milillo 1 1 IFSI-INAF Roma, Italy 2 CNRS/IPSL Paris, France Stefano Orsini 1, Valeria Mangano 1, Alessandro Mura 1, Francois Leblanc 2, Anna Milillo 1 1 IFSI-INAF Roma, Italy 2 CNRS/IPSL Paris, France SERENA-HEWG meeting, Visby, Sweden, Aug 24-27, 2010, ORSINI et al.

2. Outline It is well known that solar wind particles and photons, meteoritic impacts and interplanetary magnetic field have an influence on the morphology, content and time evolution of the Hermean exosphere. By using a wide set of data taken during three years of Mercury sodium observation from the THEMIS solar telescope in the Canary Islands, we intend to analyze the variability of the Hermean exosphere sodium emission. By referring to existing models, recently proposed, we try to interpret our images respect to the model expectations SERENA-HEWG meeting, Visby, Sweden, Aug 24-27, 2010, ORSINI et al.

3. Upper part of the atmosphere where the column density is so low (less than 10 14 cm -2 ) that the collision frequency between particles becomes negligible. Exobase is the boundary between collisional and not collisional regime (Earth case about 500 km altitude). In the case of absence of an atmosphere (Mercury case), we refer to surface-bounded exospheres. It is the result of a complex dynamic equilibrium with the surrounding systems, mainly: surface, magnetosphere and outer space. In this case the exobase is considered the surface itself. Exosphere SERENA-HEWG meeting, Visby, Sweden, Aug 24-27, 2010, ORSINI et al.

4. THEMIS 0.90 m Solar Telescope SERENA-HEWG meeting, Visby, Sweden, Aug 24-27, 2010, ORSINI et al.

5. THEMIS 0.90 m Solar Telescope THEMIS – Observatorio del Teide, Tenerife Latitude: N 28° 18' 12.42" Longitude: W 16° 30’ 32.04" Elevation: 2429 m F/16 Ritchey-Chretien telescope in alt-az mounting Helium filled telescope tube MTR mode for multiline spectropolarimetry Spectral range 400 to 1000 nm at : R ~ 220,000 Slit: 0.5" & 120 " long R ~ 220,000 Slit: 0.5" & 120 " long R ~ 400,000 Slit: 0.25" & 70 " long R ~ 400,000 Slit: 0.25" & 70 " long Four years of campaign (2007-2010)  96 days of observation Spectral resolution 0.027 Å to 0.016 Å Spectral dispersion 10.2 to 6 mÅ Four individual cameras, 2 for low and 2 for high resolution scanning of sodium D lines: D1 Na at 5896 Å D1 Na at 5896 Å D2 Na at 5889 Å D2 Na at 5889 Å SERENA-HEWG meeting, Visby, Sweden, Aug 24-27, 2010, ORSINI et al.

6. Raw data are scans of a slit (0.5 x 120 low resolution or 0.25 x 70 high resolution) normal to the solar equator, in the E-W direction. The scans result in a datacube λ x X x Y giving both 2D spatial dimensions and wavelength. A text header with additional information about the exposure is associated to the scan. Raw data Spatial direction S N WE SSPSEP SUN Wavelenght Disk of Mercury (reflected solar continuum) Exospheric sodium D emission Solar Fraunhofer D line

7. Calibrated 2D maps Sun is at the bottom kR (kRayleigh) is the unit used to measure the emission intensity, and is defined as the isotropic emission of 10 10 photons from a sphere of 1 m 2 radius. Sodium 2D maps, are the result of the analysis (code by Francois Leblanc) of the raw data, that pass through several steps: Wavelenght calibration. Cleaning from cosmic rays and instrumental anysotropies along the CCD. Sky substraction. Na D identification and extraction (pixel by pixel, step by step). The resulting 2D maps are spatially expressed in Rm (both x and y), and show the sodium emission superposed to the disk of Mercury. white cross is the SEP dashed zone is the not-illuminated side of Mercury dashed horizontal curve is the subsolar meridian SERENA-HEWG meeting, Visby, Sweden, Aug 24-27, 2010, ORSINI et al.

8. Analyzed data samples/1 July 13, 2008 Same, interpolated SERENA-HEWG meeting, Visby, Sweden, Aug 24-27, 2010, ORSINI et al.

9. Analyzed data samples/2 April 28, 2008November 7, 2008April 22, 2009 June 30, 2009October 22, 2009 SERENA-HEWG meeting, Visby, Sweden, Aug 24-27, 2010, ORSINI et al.

10. Sodium variability (Leblanc et al., 2008) Na ground based observations reveal high to mid-latitude enhancements. Recently, Leblanc et al. 2008 registered intensity and width variation of the D 1 and D 2 spectral-line in time scale of 1 hour. K ground based observations reveal similar mid-latitude enhancements (Killen et al., 2007). SERENA-HEWG meeting, Visby, Sweden, Aug 24-27, 2010, ORSINI et al.

11. (Schleicher et al., 2004) dawn-dusk and low-high latitudes asymmetries Sodium asymmetries SERENA-HEWG meeting, Visby, Sweden, Aug 24-27, 2010, ORSINI et al.

12. Kameda et al., 2009 The distance from IPD symmetry plane is correlated with Na density. Similar tendency in Feb 2009. SERENA-HEWG meeting, Visby, Sweden, Aug 24-27, 2010, ORSINI et al.

13. 13 ・ Short-term spatial variability? Concentration at high latitudes Concentration at high latitudes and small variability of average density still cannot be explained. SERENA-HEWG meeting, Visby, Sweden, Aug 24-27, 2010, ORSINI et al.

14. 14 Negative Bx component of IMF causes reconnection in the North Emisphere (Sarantos et al., 2003, Kallio et al, 2003, Massetti et al., 2005) Reconnection in the North Emisphere causes higher S/W proton precipitation fluxes North South (Fm Mura et al., 2006) SERENA-HEWG meeting, Visby, Sweden, Aug 24-27, 2010, ORSINI et al.

15. 15 Day Night North-South Asymmetry Simulated precipitation flux using Montecarlo single- particle model, 10 6 test-particles/run (Fm Mura et al., 2008) SERENA-HEWG meeting, Visby, Sweden, Aug 24-27, 2010, ORSINI et al.

16. Sodium Variability Dawn Dusk Rotation Ions PSD Ions TD PSD Ions TD BepiColombo SWT meeting, Blois From Mura et al. 2008 SERENA-HEWG meeting, Visby, Sweden, Aug 24-27, 2010, ORSINI et al.

17. OBSERVATIONS SIMULATION Sodium distribution ParameterDataModel Density (max)2500 cm –3 1000 cm –3 Column dens. (max) 7  10 10 cm –2 5  10 10 cm –2 Total amount 4  10 27 5  10 27 (*) Scale height200... 500 km~1000 km parallel doppler width1.6 km/s1.4 km/s (Mura et al., 2008) Is this complex process able to explain the short term variability? SERENA-HEWG meeting, Visby, Sweden, Aug 24-27, 2010, ORSINI et al.

18. ..by applying the Mura et al., 2008 model Impulsive ion precipitation event See the next simulation movie SERENA-HEWG meeting, Visby, Sweden, Aug 24-27, 2010, ORSINI et al.

20. Selected periods July 2008, 7 days; April 2009, 8 days; June 2009,6 days; October 2009,9 days. Data analysis SERENA-HEWG meeting, Visby, Sweden, Aug 24-27, 2010, ORSINI et al.

21. We rotate the disk of Mercury into a new reference frame, with SubSolar point in the middle of the image, and we consider the following parameters: Mean Intensity = average value (/pixel) of total emission = sum of the emission (kR) coming from all the region upper than 30 degrees latitude North = sum of the emission (kR) coming from all the region upper than 30 degrees latitude South =sum of the emission (kR) coming from all the region lower than -30 degrees latitude Equator = sum of the emission (kR) coming from all the region between +30 and -30 latitude High / Low Lat Asymmetry = (North + South)/Equator Data analysis Data analysis /2 SERENA-HEWG meeting, Visby, Sweden, Aug 24-27, 2010, ORSINI et al.

22. Relevant intensity profile decrease, two evident peaks Data analysis Data analysis /3 SERENA-HEWG meeting, Visby, Sweden, Aug 24-27, 2010, ORSINI et al.

23. Earth view (sun to the left) Subsolar point in the middle of the image in the middle of the image (Density column integral assumed as invariant) Data analysis Data analysis /4 2008, July 13 2008, July 14 2008, July 15 2008, July 16 2008, July 17 2008, July 18 2008, July 19 SERENA-HEWG meeting, Visby, Sweden, Aug 24-27, 2010, ORSINI et al.

24. Data analysis/5 1 2 3 4 5 6 7 The asymmetry and the Mean intensity show good correlation Asimmetry profile verus time SERENA-HEWG meeting, Visby, Sweden, Aug 24-27, 2010, ORSINI et al.

25. Conclusions 1.We have collected several images of the exospheric Na intensity at Mercury, by means of ground observations from the Themis solar telescope. 2.Strong signal variability has been observed 3.By selecting a specific period, when the mean intensity was strongly decreasing versus time, we have seen that this profile could be due to PSD emission, induced by impulsive ion precipitations at high latitudes, followed by a progressive decrease and diffusion towards lower latitudes. 4. Further studies for other similar cases will be performed in the next. SERENA-HEWG meeting, Visby, Sweden, Aug 24-27, 2010, ORSINI et al.