Plasma entry in the Mercury’s magnetosphere S. Massetti S. Massetti INAF-IFSI Interplanetary 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/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 (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 B C D 3 ABCD 4 SERENA meeting Mykonos

(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)

Monte Carlo Simulations 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 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 V min, V peak (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 SERENA meeting 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 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 \ AU T SW (x10 5 ) according to Lopez & Freeman (1986) from: Sarantos et al. (2007) derived from Spreiter et al. (1966)

SERENA meeting Mykonos log 10 H + density (cm -3 ) Aphelion (0.44 AU) SW (32 cm -3, 400 km/s) - IMF (-16,+05,-05) nT log 10 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

SERENA meeting Mykonos 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 South

H + energy (keV) H + impacts (a.u.) H + total 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) SERENA meeting Mykonos LLBL/OPBLCUSP

log 10 H + total flux (cm -2 s -1 ) SERENA meeting Mykonos κ adiabatic parameter 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 ) T SH / 4

SERENA meeting Mykonos PA distribution low-latitudesPA distribution high-latitudes log 10 H + density (cm -3 ) TOWARD log 10 H + density (cm -3 ) AWAY MMO MPO

SERENA meeting Mykonos PA distribution low-latitudesPA distribution high-latitudes log 10 H + density (cm -3 ) TOWARD log 10 H + density (cm -3 ) AWAY Same SW/IMF conditions but with an higher m.sheath H + temperature MMO MPO

log 10 H + flux (cm -2 s -1 ) AWAY log 10 H + flux (cm -2 s -1 ) TOWARD log 10 H + flux (cm -2 s -1 ) TOWARD log 10 H + flux (cm -2 s -1 ) AWAY *Low H+ Temp * *High H+ Temp * MMO MPO

SERENA meeting Mykonos H + V down || B H + V down |_ B H + V up || B H + V up |_ B MMO MPO * High H+ Temp *

SERENA meeting Mykonos Summary magnetospheric open regions probably equivalent to those of the Earth, but extending over broader areas IMF B X (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 (log 10 flux / 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 and MMO)