THE EXOSPHERE OF MERCURY:

SODIUM OBSERVATIONS AND MAGNETOSPHERIC MODELLING

Mikonos 2009

Valeria Mangano & Stefano Massetti

Interplanetary Space Physics Institute (IFSI) Roma, Italy

SERENA Meeting

TNG (June 2005)

Telescopio Nazionale Galileo Roque de Los Muchachos, La

Palma

Alt-Az telescope

Primary Mirror 3.58 m

SARG

Echelle spectrograph

Nasmyth telescope focus B

Resolution: 46000-164000 (in our case 115000)

Na filter 60 Ǻ (no order overlapping)

Long slit (26.7” x 0.4”)

CCD dimension 2 K x 4 K pixels

Altitude 2387 m a.s.l.

Lat. 28º 45’ 28” N Long. 17º 53’ 38” W

Observations performed between 29th June and 1st July, at sunset (about 19:00 UT, 1 h)

Sun-Mercury Distance 0.422-0.423 AUIlluminated fraction 57 -54 %Angular Diameter 6.7-7.0 ”Apparent Magnitude 0.05-0.15Elongation (East) 24.2-24.9ºTrue Anomaly Angle 124-130º Sun-Mercury-Earth phase angle 81-85ºExposure time: 120” Scan in the N-S direction: equator ± 6”

Sodium of the SOLAR ATMOSPHERE: ABSORPTION (shifted of the sum of Mercury-Earth BANDS and Sun-Mercury relative velocities)

Sodium in the EXOSPHERE of MERCURY: EMISSION (shifted by the Mercury-Earth rel. velocity) LINE

• Cut + bias and flat-field corrections

• Straightening + CalibrationD2 D1

Pre-Analysis

D2 (D.shifted

from 5895.92 Ǻ)

D1 (D.shifted

from 5889.92 Ǻ)

Analysis/1

1. Correcting spectra for diffuse light

2. Looking for the best profile to fit the Fraunhofer absorptions and further extract the emission lines

Analysis/2

4. Comparison of the Hapke model with the observed continuum to derive the conversion: ADU kiloRayleigh

3. Creation of a surface reflectance model devoted to reproduce the soil emission (following the Hapke theory)

Finally, spatial plots of the D1 and D2 lines are deduced.

Results

July 1st, 2005

Two main features:

• Higher intensities during the second night

• Southern hemisphere peak

June 29th, 2005

June 30th, 2005

Interpretation/1(Possible related solar events)

• Coronal hole on the Sun and an associated high speed stream:

• Three different CMEs from the active region AR0781 occurred during the observations:

coronal hole

SoHO/EIT (Fe XV 284 Å) image

Active Region AR0781 CMEs)

• stream at 650-700 km/s

• arrived at Mercury 2-3 days before

• carried with it an IMF Bx > 0 for several days

SOHO/LASCO CME catalog (http://cdaw.gsfc.nasa.gov/CME_list/)

CME 1: (Halo CME)

28th June halo CME 16:30 UT, observed initial speed V0 = 1750 km/s.

It was estimates that the CME decelerated from 1750 to 400km/s, in about 6h (~0.144 AU), then it reached Mercury at 0.42 AU after about 30h (hence: 30th June 2005, at about 05:00 UT).

CME 2:

28th June 20:06 UT, PA = 97˚, AW = 68˚, V(?) 600 km/s

Estimated arrival time @0.42 AU:

• 30-June, 03:00 UT (assuming the speed of 600 km/s)

• 30-June, 15:00 UT (assuming the speed of 400 km/s)

CME 3:

29th June 07:31 UT , PA = 83˚, AW = 49˚,V(?) 350 km/s

Estimated arrival time @0.42 AU: 01-July, 17:00 UT

29-Jun (180) 30-Jun (181) 01-Jul (182)

20:00 UT 20:00 UT 20:00 UT

Time sequence of the events

Fast Stream(176)

IMF Bx < 0

CME#1

CME#2

CME#3

time

Interpretation/2(Possible relation with the surface)

• Peak coordinates: 250-290º E, 10-40º S

• Radar bright spots: (A. Sprague) A (15° E, 25° S), B(15° E, 55° N),C(120° E,15°N)

(J. Harmon) A (12° E, 34° S), B(17° E, 58° N),C(114° E, 11°N),

(18° E, 27° S), Bartok crater

No corrispondence can be found!

The ion sputtering ion sputtering process (or chemical sputtering + PSD!) caused by plasma precipitation from the magnetosheath through the dayside cusp regions, is highly affected by both the orientation of the Interplanetary Magnetic Field and the solar wind dynamic pressure.In particular, high hemispherical asymmetries are expected because of the dominating radial IMF component at Mercury.

Interpretation/3(Southern Hemisphere peak)

June 30th, 2005

PROBLEM: the knowledge of the IMF orientation at the orbit of Mercury is needed to compute the magnetospheric configuration (true both for ground- and space-based observations)

An indirect estimation of the IMF direction at Mercury can be achieved on the basis of the measured solar magnetic field …

Wilcox Solar Obs. photospheric field CR#2031

(-) → IMF < 0

(+) → IMF > 0

The coronal magnetic field is calculated from photospheric field observations with a potential field model. The field is forced to be radial at the source surface to approximate the effect of the accelerating solar wind on the field configuration

(-) → IMF < 0 → IMF Bx (GSE) > 0

(+) → IMF > 0 → IMF Bx (GSE) < 0

Taking into account the propagation times (3-5 days), we can find the relationship between the coronal magnetic sectors (WSO, ground obs.) and the sign of the IMF radial component at Earth (ACE, L1), and then try to estimate the most likely IMF orientation at Mercury

188 184 180 176 172

?time

IMF Bx (GSE)

A more consistent estimation can be derived by means of 3-D numerical simulations of the solar wind/IMF propagation into the heliosphere:

1) Hakamada-Akasofu-Fry (HAF) kinematic solar wind model (gse.gi.alaska.edu)

2) ENLIL magnetohydrodynamic helispheric model (ccmc.gsfc.nasa.gov)

+

(adapted from original)

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-

Mercury

-

+

-

Simulations diverge progressively as the distance from the source surface (Sun) increases, but are in relatively good agreement at the orbit of Mercury

In the time-period 29th June - 1st July there were 3 or 4 IMF magnetic sectors (depending on the simulation) with Mercury very likely embedded in a negative IMF (i.e. toward the Sun)

In the planet’s MSO reference frame (Mercury-Solar-Orbital) a negative

IMF means a positive IMF Bx component, which imply a direct IMF

reconnection, hence, preferred H+ precipitation in the southern hemisphere …

Mercury

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+

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Tail Analysis

Potter et al., 2002:

• June 5th 2000 TAA= 114º

• May 26th 2001 TAA= 125º

Our data:

• June 29th 2005

• July 1st 2005 TAA= 124-130º

• All dates display similar shape similarTAAs

• Dependance more on TAA values than on ejecting phenomena (SW)

Conclusions• Sodium observations campaign are performed at both TNG

(2002/03/05/06/09) and THEMIS (2007/08/09… full-disk) observatories, and since June 2006 are part of an international observational campaign in preparation of the BepiColombo mission.

• Observations precision need to be improved -> full-disk (THEMIS obs.)

• Coordinated Earth-based observations are needed to cross-check the measurements, in particular longer runs are needed to better study the hermean environment and quantify the contribution to the exosphere of the different processes.

• They evidenced definite features and suggest a relationship with the Solar Events and/or with the Interplanetary Magnetic Field.

• Heliospheric IMF/SW numerical simulations are needed to analyze ground- and space-based data from the magnetospheric point of view.

• Paper just published on ICARUS (Mangano et al. 2009)