Plasma entry in the Mercury’s Plasma entry in the Mercury’s magnetosphere magnetosphere

  S. MassettiS. Massetti

INAF-IFSI INAF-IFSI Interplanetary Space Physics Institute, Roma - ItalyInterplanetary Space Physics Institute, Roma - Italy

At IFSI we developed an analytical-empirical model mainly focused on the study of the dayside SW plasma entry:

• derived from 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 (2) (fly-by III through the model). A check with new data from Messenger is in progress...

• The kinetic properties of the m.sheath H+ ions crossing the m.pause and entering through the open field areas (3-4) are calculated 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

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SERENA meeting 2009 - Mykonos

(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 Simulations

• 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 simulations are achieved by launching a number of test particles that is

proportional to the H+ density at the magnetopause, with an initial speed randomly chosen within a bi-Maxwellian distribution, which take also into account Vmin , Vpeak (dHT) speeds || B. Particle

tracking stops when H+ ions hit the planet or exit from the simulation box (i.e. in the tail).

Y

X

Z

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SERENA meeting 2009 - Mykonos

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

TS

H /

TS

W

TS

H (

km/s

) (T

SW =

2x1

05K

)

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)

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log1

0 H+ density (cm

-3)

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

log1

0 H+ density (cm

-3)

Perihelion (0.29 AU)SW (60 cm-3, 350 km/s) - IMF (-34,+12,-10) nT As expected, H+ entry is in general

greater at perihelion due to both higher SW density and IMF intensity, which in turn implies wider open field regions

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log1

0 H+ to

tal flux (cm-2 s

-1)log

10 H

+ total flux (cm

-2 s-1)

log1

0 H+ to

tal flux (cm-2 s

-1)log

10 H

+ 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 North

South South

H+ energy (keV

)H

+ imp

acts (a.u.)

H+ to

tal flux (cm-2 s

-1)Dayside magnetospheric regions

(pure southward IMF case)

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)

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LLBL/OPBLCUSP

log1

0 H+ to

tal flux (cm-2 s

-1)

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κ adiabatic param

eter

Non-adiabatic effects on H+ dayside precipitation

top-left - parameter mapped at the magnetoapuse (sq. root of min. field line curvature / max Larmor radius, Büchner and Zelenyi, 1989) , if < 3 particles do not behave adiabatically

bottom-left - H+ total flux at planetary surface

bottom-right – same as bottom-left, but with m.sheath

ions temperature reduced by a factor 4

log 10 H

+ total flux (cm

-2 s-1)

TSH / 4

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PA distribution low-latitudes PA distribution high-latitudes

log1

0 H+ density (cm

-3)

TOWARD

log1

0 H+ density (cm

-3)

AWAY

MMO

MPO

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PA distribution low-latitudes PA distribution high-latitudes

log1

0 H+ density (cm

-3)

TOWARD

log1

0 H+ density (cm

-3)

AWAY

Same SW/IMF conditions but with an higher m.sheath H+ temperature

MMO

MPO

log1

0 H+ flu

x (cm-2 s

-1)

AWAY

log1

0 H+ flu

x (cm-2 s

-1)

TOWARD

log1

0 H+ flu

x (cm-2 s

-1)

TOWARD

log1

0 H+ flu

x (cm-2 s

-1)

AWAY

*Low H+ Temp * *Low H+ Temp *

*High H+ Temp * *High H+ Temp *

MMO

MPO

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H+ Vdown || B H+ Vdown |_ B

H+ Vup || B H+ Vup |_ B

MMO

MPO

* High H+ Temp *

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Summary

• magnetospheric open regions probably equivalent to those of the Earth, but

extending over broader areas

• IMF BX (pos./neg.) causes strong hemispheric asymmetries, in both the dayside

(cusp areas) and the nightside (not shown here)

• SW-IMF condition at perihelion / aphelion causes different dayside H+

precipitation, by about an order of magnitude (log10 flux 9-9.5 / 8.5 cm-2 s-1)

• SW precipitation on the dayside depends on both local B intensity and H+ thermal

speed in the magnetosheath (due to non-adiabatic effects)

• the H+ thermal speed profile along the magnetosheath is a critical parameter in

the simulations (e.g. to estimate ion sputtering and SW @MPO and MMO)