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Modelling Mercury’s Magnetosphere S. Massetti, S. Orsini, A. Milillo, A. Mura, E. De Angelis, V. Mangano S. Massetti, S. Orsini, A. Milillo, A. Mura, E. De Angelis, V. Mangano INAF-IFSI Interplanetary Space Physics Institute, Roma - Italy INAF-IFSI Interplanetary Space Physics Institute, Roma - Italy

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Develop of a magnetospheric model able to reproduce Mercury’s basic features: by means of an ad hoc modification of the Toffoletto & Hill TH93 magnetospheric model (IMF Bx interconnected) by using the Spreiter’s gasdynamic approx to describe the ion magnetosheath key parameters (N/N SW, V/V SW and T/T SW ), as a function of the SW Mach number (1), with the T SW calculated as a function of both V SW and heliocentric distance (Lopez & Freeman, 1986). The model was “checked” for consistency with available Mariner 10 data (fly-by III through the model) (2)... which seem basically confirmed by first Messenger data Then the kinetic properties of the magnetosheath ions crossing the magnetopause and precipitating through the open field areas (3-4) have been derived following the Cowley & Owen approach, by means of the de Hoffman- Teller (dHT) reference frame. 1 2 A B C D 3 ABCD 4 SERENA meeting 2008 - Santa Fe

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Ions injection at the dayside magnetopause 1) V SH ’ = - V A-SH cos 2) V HT = V SH + V A-SH cos 3) V min = V HT cos 4) V p = V HT cos + V A-SP 5) V max = V HT cos + V A-SP + V th the kinetic properties of the m.sheath plasma that crosses the m.pause can be derived by assuming the existence of a de-Hoffman-Teller (dHT) reference frame (e.g.: Cowley and Owen 1989; Lockwood and Smith 1994; Cowley 1995; Lockwood 1995, 1997; Onsager et al., 1995 ). the dHT frame moves with the rotational discontinuity (IMF + inner field) along the m.pause, at speed V HT. in the dHT frame the convection electric field is null, and the plasma just flow across the discontinuity at the local Alfvénic speed. in the planet frame the ions are accelerated/decelerated depending on the theta angle (0˚-90 ˚/ 90 ˚ -180 ˚) and the Alfvénic speeds at both the m.sheath and m.sphere sides.

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SERENA meeting 2008 - Santa Fe (from Massetti et al., 2007) ABCD V min =V HT cos V A_SP V th V peak =V min +V A-SP V max =V peak +V th ABCD (from Lockwood, 1997)

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Monte Carlo Simulation Simulation box (150 x 150 x 150) -4 R M < X < 2 R M -3 R M < Y < 3 R M -3 R M < Z < 3 R M by steps of 0.04 R M (~100km) Surface impact data stored into a 180 x 360 lat/long grid (1°x1°) Magnetosheath (N/N SW, V/V SW, T/T SW ) and kinetic (V MIN, V PEAK ) key parameters are computed over a 2°x 2° m.pause grid, Monte Carlo simulation is achieved by launching a number of test particles (several 10 5 ) that is proportional to the ion density at the magnetopause, with an initial speed randomly chosen within a bi- Maxwellian distribution that take into account V min, V peak (dHT) speeds || B. Particle tracking stops when they hit the planet or exit from the simulation box. Y X Z SERENA meeting 2008 - Santa Fe

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H + energy (keV) SERENA meeting 2008 - Santa Fe open m.field on the dayside open m.field on the nightside closed m.field lines H + energy (keV)

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H + impacts (a.u.) H + total flux (cm -2 s -1 ) Pure southward IMF run sample: SW (60 cm -3, 400 km/s) IMF ( 0, 0, -20) nT the actual value of energy and flux depends upon the Alfvénic speed on both magnetosheath and magnetospheric side of the magnetopause, (i.e. on local B strength and ion density) SERENA meeting 2008 - Santa Fe LLBL/OPBLCUSP

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H + total flux (cm -2 s -1 ) SERENA meeting 2008 - Santa Fe κ adiabatic parameter Non-adiabatic effects on the dayside H + precipitation top-left - K parameter mapped at the magnetoapuse (sq. root of min. field line curvature / max Larmor radius, Büchner and Zelenyi, 1989) bottom-left - H + total flux at planetary surface bottom-right – the same, but cooling the m.sheath ions by a factor 4 H + total flux (cm -2 s -1 ) T SH / 4

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SERENA meeting 2008 - Santa Fe We performed numerical simulations for Mercury at both perihelion and aphelion, by using the most probable values of the Solar Wind and IMF, accordingly to the statistical analysis of Helios I end II data published by Sarantos et al. (2007) – Left panel Magnetosheath H+ temperature has been consistently computed as a function of V SW, D SW, |IMF B| (Spreiter et al., 1966), T SW and distance form the Sun (Lopez & Freeman, 1986) - Right panels distance from mp nose T SH / T SW T SH (km/s) (T SW = 2x10 5 K) Perihelion (V SW =350, D SW =60, |IMF|=40) Aphelion (V SW =430, D SW =32, |IMF|=20) V SW \ AU0.290.360.44 3501.41.10.9 4002.11.71.4 T SW (x10 5 ) according to Lopez & Freeman (1986) from: Sarantos et al. (2007) derived from Spreiter et al. (1966)

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SERENA meeting 2008 - Santa Fe H + log 10 density (cm -3 ) Aphelion (0.44 AU) SW (32 cm -3, 400 km/s) - IMF (-16,+05,-05) nT H + log 10 density (cm -3 ) Perihelion (0.29 AU) SW (60 cm -3, 350 km/s) - IMF (-34,+12,-10) nT diamagnetic effect(?)

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SERENA meeting 2008 - Santa Fe Perihelion (0.29 AU) SW (60 cm -3, 350 km/s) IMF (-34,+12,-10) nT Aphelion (0.44 AU) SW (32 cm -3, 400 km/s) IMF (-16,+05,-05) nT H + energy (keV) North H + energy (keV) North H + energy (keV) South H + energy (keV) South

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SERENA meeting 2008 - Santa Fe H + log 10 total flux (cm -2 s -1 ) Perihelion (0.29 AU) SW (60 cm -3, 350 km/s) IMF (-34,+12,-10) nT Aphelion (0.44 AU) SW (32 cm -3, 400 km/s) IMF (-16,+05,-05) nT North South

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SERENA meeting 2008 - Santa Fe Perihelion (0.29 AU) SW (60 cm -3, 350 km/s) IMF (-34,+12,-10) nT Aphelion (0.44 AU) SW (32 cm -3, 400 km/s) IMF (-16,+05,-05) nT H + energy (keV) nightside H + energy (keV) nightside H + log 10 total flux (cm -2 s -1 ) nightside H + log 10 total flux (cm -2 s -1 ) nightside

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SERENA meeting 2008 - Santa Fe Perihelion (0.29 AU) SW (60 cm -3, 350 km/s) IMF (-34,+12,-10) nT Aphelion (0.44 AU) SW (32 cm -3, 400 km/s) IMF (-16,+05,-05) nT H + energy (keV)

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SERENA meeting 2008 - Santa Fe V|| to B (km/s) NB:sign is wrong Perihelion (0.29 AU) H+ parallel speed to B within Mercury’s magnetosphere (blue/red = toward/away to planet) (NB: colorscale is contrained within -100 / +100 km/s)

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SERENA meeting 2008 - Santa Fe H + log 10 total flux (cm -2 s -1 ) North, tilt = +10° H + log 10 total flux (cm -2 s -1 ) Dayside, tilt = -10° H + log 10 total flux (cm -2 s -1 ) North, tilt = -10° H + log 10 total flux (cm -2 s -1 ) Dayside, tilt = +10° TILT EFFECTS - Perihelion (0.29 AU) SW (60 cm -3, 350 km/s) IMF (-34,+12,-10) nT ~ 30°-35° ~ 50°

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SERENA meeting 2008 - Santa Fe H + log 10 total flux (cm -2 s -1 ) North, tilt = +10° H + log 10 total flux (cm -2 s -1 ) North, tilt = -10° TILT EFFECTS - Aphelion (0.44 AU) SW (32 cm -3, 400 km/s) IMF (-16,+05,-05) nT H + log 10 total flux (cm -2 s -1 ) Dayside, tilt = +10° ~ 40° H + log 10 total flux (cm -2 s -1 ) Dayside, tilt = -10° ~ 55°

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SERENA meeting 2008 - Santa Fe TILT EFFECTS – NIGHTIME region Perihelion (0.29AU) Aphelion (0.44 AU) H + log 10 total flux (cm -2 s -1 ) Night, tilt = +10° H + log 10 total flux (cm -2 s -1 ) Night, tilt = -10° H + log 10 total flux (cm -2 s -1 ) Night, tilt = +10° H + log 10 total flux (cm -2 s -1 ) Night, tilt = -10°

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SERENA meeting 2008 - Santa Fe Summary magnetospheric open regions equivalent to those of the Earth, but extending over broader areas; cusp precipitation could be reduced depending on the local B intensity and H + thermal speed in the magnetosheath (due to non-adiabatic effects); IMF B X (pos./neg.) causes strong hemispheric asymmetries, in both the dayside (cusp areas) and the nightside; Perihelion / Aphelion SW-IMF condition causes different dayside H + precipitation, by an order of magnitude (log 10 flux 9-9.5 / 8.5 cm -2 s -1 ); nighttime H + flux shows to be lower but wider during aphelion; Dipole tilt (?) causes the displacement of the open/cusp areas; nighttime H + flux appears to increase for a negative tilt (Northern hemisphere IMF-reconnected).... to be checked with new data coming from MESSENGER

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