modelling mercury’s magnetosphere

Modelling Mercury’s MagnetosphereModelling 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 INAF-IFSI Interplanetary Space Physics Institute, Roma - ItalyInterplanetary Space Physics Institute, Roma - Italy

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/NSW, V/VSW and T/TSW), as a function of the SW Mach number (1), with the TSW calculated as a function of both VSW 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 dataThen 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

BC

D

3

A B C D

4

SERENA meeting 2008 - Santa Fe


Ions injection at the dayside magnetopause

1) VSH’ = - VA-SH cos

2) VHT = VSH + VA-SH cos

3) Vmin = VHT cos

4) Vp = VHT cos + VA-SP

5) Vmax = VHT cos + VA-SP + Vth

• 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 VHT.

• 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.


(from Massetti et al., 2007)

A

B

C

D

Vmin=VHTcos

VA_SP Vth

Vpeak=Vmin+VA-SP

Vmax=Vpeak+Vth

A B C D

(from Lockwood, 1997)

Monte Carlo Simulation

• Simulation box (150 x 150 x 150)

-4 RM < X < 2 RM -3 RM < Y < 3 RM -3 RM < Z < 3 RM

by steps of 0.04 RM (~100km)

• Surface impact data stored into a 180 x 360 lat/long grid (1°x1°)

• Magnetosheath (N/NSW , V/VSW , T/TSW) and kinetic (VMIN , VPEAK) 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 105) 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 Vmin , Vpeak (dHT) speeds || B. Particle tracking stops when they hit the planet or exit from the simulation box.

Y

X

Z


H+ energy (keV

)


open m.field on the dayside

open m.field on the nightside

closed m.field lines

H+ energy (keV

)

H+ energy (keV

)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)


LLBL/OPBLCUSP

H+ total flux (cm

-2 s-1)


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)

TSH / 4


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 VSW, DSW, |IMF B| (Spreiter et al., 1966), TSW and distance form the Sun (Lopez & Freeman, 1986) - Right panels

distance from mp nose

T SH /

T SW

T SH (k

m/s

) (T S

W =

2x1

05 K) Perihelion (VSW=350, DSW=60, |IMF|=40)

Aphelion (VSW=430, DSW=32, |IMF|=20)

VSW \ AU 0.29 0.36 0.44

350 1.4 1.1 0.9

400 2.1 1.7 1.4

TSW (x105) according to Lopez & Freeman (1986)

from: Sarantos et al. (2007)

derived from Spreiter et al. (1966)


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(?)


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


H+ log

10 total flux (cm-2 s

-1)H

+ log10 total flux (cm

-2 s-1)

H+ log


-1)H

+ log10 total flux (cm

-2 s-1)



North North

South South




H+ energy (keV

)

nightside

H+ energy (keV

)

nightside

H+ log


-1)

nightside

H+ log


-1)

nightside




H+ energy (keV

)H

+ energy (keV)

H+ energy (keV

)H

+ energy (keV)


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)


H+ log


-1)

North, tilt = +10°

H+ log


-1)

Dayside, tilt = -10°

H+ log


-1)

North, tilt = -10°

H+ log


-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°


H+ log


-1)

North, tilt = +10°

H+ log


-1)

North, tilt = -10°

TILT EFFECTS - Aphelion (0.44 AU)SW (32 cm-3, 400 km/s) IMF (-16,+05,-05) nT

H+ log


-1)

Dayside, tilt = +10°

~ 40°

H+ log


-1)

Dayside, tilt = -10°

~ 55°


TILT EFFECTS – NIGHTIME regionPerihelion (0.29AU) Aphelion (0.44 AU)

H+ log


-1)Night, tilt = +10°

H+ log


-1)

Night, tilt = -10°

H+ log


-1)Night, tilt = +10°

H+ log


-1)

Night, tilt = -10°


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 BX (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 (log10 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

modelling mercury’s magnetosphere

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