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S.M.P. McKenna-Lawlor 1 , E. Kallio 2 , R. Jarvinen 2 , M. Alho 2 , and S. Dyadechkin 2

Success of the hybrid modelling technique in simulating the response of the Martian magnetosphere to solar energetic particle irradiation and first steps in applying this technique to study the response at Mercury to disturbed solar circumstances.

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S.M.P. McKenna-Lawlor 1 , E. Kallio 2 , R. Jarvinen 2 , M. Alho 2 , and S. Dyadechkin 2

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  1. Success of the hybrid modelling technique in simulating the response of the Martian magnetosphere to solar energetic particle irradiation and first steps in applying this technique to study the response at Mercury to disturbed solar circumstances • S.M.P. McKenna-Lawlor1, E. Kallio2, R. Jarvinen2, M. Alho2, and S. Dyadechkin2 • Space Technology Ireland, Maynooth, Co. Kildare • Finnish Meteorological Institute, Helsinki, Finland

  2. Motivation The initial motivation of this study was to model data composed of energetic protons with energies in the range 30 keV to a few tens of MeV recorded at Mars in March 1989 by the SLED particle detector aboard the Phobos-2 spacecraft. Later the possibility to apply HYB modelling to extreme conditions at Mercury was recognized and first steps to achieve this were initiated.

  3. Encounter of Phobos-2 with Mars Schematic of the encounter of Phobos-2 with Mars showing part of an elliptical orbit (E) and a circular orbit (C). Also represented are the Bow Shock (BS), the Magnetosheath, the Magnetic Pileup Boundary (MPB), the Magnetotail and the nominal interplanetary magnetic field (IMF). The two conical shapes illustrate the field of view of the SLED instrument when in an elliptical and circular orbit near the terminator plane.

  4. The SLED instrument SLED employed semiconductor detectors in two different telescopes directed at 55o westward of the Sun-Earth line (that is approximately in the nominal direction of the interplanetary magnetic field). The front detector of Te1 was covered with a 15µg/cm2 Al foil. Te2 was fitted in addition with a 500 µg/cm2 Al foil which absorbed protons with energies < 350 keV while allowing the passage of electrons. Subtraction of the counts of Te1 and Te2 allowed ionsand electrons to be separated from each other.

  5. The SLED instrument - Energy channels Te 1 (without foil) Ch. 1 30-50 keV electrons + ions Ch. 2 50-200 keV electrons + ions Ch. 3 200-600 keV electrons + ions Ch. 4 0.6-3.2 MeV ions Ch. 5 3.2-4.5 MeV ions Ch. 6 > 30 MeV background rate Te 2 (with foil) Ch. 1 30-50 keV electrons 350-400 keV ions Ch. 2 50-200 keV electrons 400-500 keV ions Ch. 3 200-600 keV electrons 0.5-1 MeV ions Ch. 4 0.6-3.2 MeV ions Ch. 5 3.2-4.5 MeV ions Ch. 6 > 30 MeV background rate

  6. The SLED instrument The SLED data modelled in the present study were measured while Phobos-2 was executing circular orbits about Mars. In these data it was found that, under extreme solar conditions, the proton count rates measured by SLED were significantly reduced behind the planet whenever the pitch angle distribution of the particles remained relatively stable during the integration time of the instrument (230s).

  7. SLED Data The plot shows particle data recorded by SLED in its Channel 3 (200-600 keV) and Channel 6 (> 30 MeV) of Te 1 from 1-26 March, 1989. Dashed vertical lines indicate recurrent records of depressions in flux (interpreted by McKenna-Lawlor et al. (1992) to be due to magnetic shadowing) as the spacecraft executed consecutive circular orbits around Mars. Particle enhancements due to recurrent bow shock crossings can also be discerned.

  8. SLED data Examples of particle data recorded by SLED in Te1, Channel 4 (0.6-3.2 MeV) in an MSO coordinate system; look direction of SLED top right. Mars is represented at the center of each plot. The relative number of counts recorded during particular orbits is indicated by the magnitude of vertical lines that represent individual readings made at successive locations during an orbit. Gaps correspond to operational switch offs due to telemetry constraints. The depressions are due to magnetic shadowing.

  9. Hybrid Modelling To model these observations, a 3-D, self-consistent, hybrid model (HYB) supplemented by test particle simulations was developed. This model allows the macroscopic properties of, solar related, high energy ion populations near Mars to be investigated and supports study of the motions of individual high energy ions at the planet. It is recalled that Mars neither possesses a significant, global, intrinsic magnetic field nor a dense atmosphere.

  10. Mars Hybrid Model (HYB) General features • Quasineutral: Snion = nelectron • Hybrid: H+, O+, and O2+ ions are particles, • Electrons form a massless fluid • Self-consistent model • Dynamics: • - Electrons “carry” the magnetic field (E = - Ue x B) • - Ions (H+, O+, O2+) are accelerated by the Lorentz force • Hierarchically refined cubic grid • Ion splitting and joining: • splitting: A→ A1 + A2 • joining: B1 + B2 + B3→ B1 + B2 • (note: conserves E and p) • Note: Martial crustal magnetic fields are NOT included • See Kallio et al., 2010, for details of the HYB-Mars model ne ,Ue O2+ O+ H+

  11. The Mars-centered Solar Orbital (MSO) coordinate system • The coordinate system used in the HYB Mars model is MSO where: • The x- axis points from the centre of Mars to the Sun. • The z -axis is perpendicular to the orbital plane of Mars pointing towards the north ecliptic pole. • The y-axis completes the right handed co-ordinate system.

  12. HYB-Mars Model runs In the present study, four different upstream parameters were utilized in order to analyze the response of the Martian plasma environment to different conditions: 1. NOMINAL RUN: Usw=485 km/s, nsw= 2.7 cm-3, IMF = [-1.634, 2.516, 0] nT 2. HIGH VELOCITY RUN: U=970 km/s, nsw= 2.7 cm-3, IMF = [-1.634, 2.516, 0] nT 3. HIGH DENSITY RUN: Usw=485 km/s, nsw= 10.8 cm-3, IMF = [-1.634, 2.516, 0] nT 4. HIGH DENSITY AND IMF RUN: Usw=485 km/s, n= 10.8 cm-3, IMF = [-6.536, 10.064, 0] nT

  13. Procedure The nominal run was first compared with runs made when the dynamic pressure of the solar wind was increased by a factor of four (either by doubling the speed of the solar wind or by increasing the solar wind density by a factor of four). The role of the magnitude of the interplanetary magnetic field (IMF) was then investigated in both the high density and high IMF runs by increasing the magnetic field used in the high density run by a factor of four.

  14. Procedure contd. Next the motion of high energy proton populations was studied with respect to each of the four upstream cases selected. The energies of four high energy solar wind H+ beams were individually set at 50 keV, 200 keV, 600 keV and 3.2 MeV, (corresponding to the highest particle energies recorded in Te1 Channels 1-4 of SLED). In the simulation process the high energy H+ ions were injected into the simulation box using densities of a sufficiently low value that the self consistent solution was not affected.

  15. Mars The figures show the flow lines of the four high energy H+ ions near Mars in a nominal run. The 50 and 200 keV particles are disturbed by the magnetosphere. The colour in the z = 0 plane and on the r ~ RM sphere indicates the normalized density of high energy ions, ñ. Note that some of the stream lines extend below the colour plane to the z < 0 hemisphere. An enhanced density (red colour), is created in Fig a because the v×B force impels ions toward the z < 0 hemisphere

  16. Results of HYB modelling • Overall, the model indicated that: • During extreme SW conditions, the plasma environment at Mars affects the motion of high energy protons. • Ion scattering depends on (a) the energy of the incoming ions and (b) on upstream parameters (especially the SW speed and the IMF direction). • A magnetic shadow is formed which decreases in size as the particles increase in energy (keV to MeV range).

  17. Comparison of simulations with in situ measurements The properties of the high energy ions in the model runs were qualitatively similar to the in situ measurements. The quantitative results varied, however, from one run to another and it therefore transpired to be necessary, in order to more fully understand the in situ observations, to know the actual upstream parameters that were present during the taking of the energetic particle measurements.

  18. Upgrading the HYB model An additional study was then mounted in which plasma and magnetic field data measured contemporaneously with the particle measurements aboard Phobos-2 were input to the HYB model. Further, the model was itself upgraded through incorporating an High Energy Particle Tracing Mode in the global simulation such that energetic ions could be manually injected into the simulation using three different velocity distribution functions.

  19. Upgrading the HYB Model 1.“Beam” distributions composed of six, tight, discrete energy beams were launched along the IMF with energies corresponding to those of the SLED channels. 2.An energy scattered distribution based on SLED data was directed along the IMF in velocity space with a continuous energy distribution. 3. A fully scattered distribution around the IMF was utilized which included both angular and energy scattering. a)

  20. Diagnostics Several diagnostic tools were in addition developed and incorporated into the hybrid model platform in order to support one-to-one comparisons between the simulated and observed particle fluxes. In particular, a virtual SLED instrument was included in the simulation to mimic how the instrument collected particles.

  21. View of the simulated magnetic field around Mars and several magnetic field lines which are connected to the orbit of Phobos2. The “cloud” in the figure is a volumetric plot of the magnitude of the magnetic field. The magnetic field lines are coloured in order to help the eye distinguish different lines from each other. The field lines are sourced from a hybrid model run with upstream parameters nSW = 3m-3, USW = [–600, 0, 0] km s-1 and Bsw = [–4, 4, 0] nT. The view point is on the +z axis. Note that the draped magnetic field lines are three dimensional in the simulation and the figure only shows their projection on the xy plane.

  22. Mars Illustration of the motion of energetic protons near Mars. The “cloud” shows the strength of the magnetic field in 3-D (see the colour bar, top left). The circular white sphere represents the surface of Mars. The lines show trajectories of several energetic protons. The colour of a line represents the initial energy of an ion: The lowest energies (50 keV and 200 keV) are shown in green; the yellow trajectories represent 600 keV protons and the red trajectories 3.2MeV protons (see the colour bar, bottom left). Note also that the colour of the initially green trajectories becomes grey or white when the trajectory is within or behind the strong magnetic field region.

  23. Comparison of the simulated energetic proton fluxes and the particle fluxes measured by the SLED/Phobos-2 instrument along the circular orbit of Phobos-2 on 13 March 1989. The SLED ion data come from Telescope 1, from which the electrons were removed. The five panels display the normalized particle fluxes in five SLED energy channels compared with derived fluxes based on three different velocity distribution models: the beam, the energy scattered, and the fully scattered models Two “fully scattered” fluxes are shown in cases where the particle fluxes were collected from 2 πspace (green solid line) and when the fluxes were collected from the field-of-view (FoV) of the SLED telescopes, looking toward the nominal spiral angle of 55 at Mars. The fully scattered model resulted in the best agreement with the data.

  24. Results of the upgraded study • Comparisons with the SLED Phobos-2 observations showed that the upgraded model successfully reproduced the key features of the observations. The best performance was obtained when using the fully scattered velocity distribution function. • Features successfully simulated included. • A particle flux enhancement near the bow shock recorded in the low energy SLED channels. • A particle flux decrease near the bow shock recorded in the high energy SLED channels. • Formation of a magnetic shadow and indication of how its size decreased with particle energy.

  25. Application of the methodology to Mercury The success obtained through matching the Mars in situ energetic particle measurements with simulated data motivated an effort to apply hybrid modeling to simulate, for a BepiColombo application, the particle environment present at Mercury under extreme solar conditions. In this case, instead of a draped magnetic field, Mercury has a dipole field which was shown by Mercury Messenger measurements to be offset from the geographic poles (Anderson et al., 2011).

  26. Application of the methodology to Mercury FOUR RUNS were made using: H+ from the SW ; nsw (H+) = 72 cm-3 NOMINAL RUN (Usw = 430 km/s) • “North IMF run”: IMF = [0, 0, 10] nT • “Parker IMF run”: IMF = [32,10,0] nT HIGH SPEED RUN (Usw = 1000 km/s) • “High speed north IMF run”: IMF = [0, 0, 10] nT • “High speed Parker IMF run”: IMF = [32,10,0] nT

  27. Magnetosphere and the solar wind density North IMF run (430 km/s) High speed Parker IMF run (1000 km/s) n(H+) [m-3] n(H+) [m-3] Note: The increase in Usw results in a more “compressed” magnetosphere

  28. HIGH ENERGY (~0.5 MeV) PROTONS An ~ 0.5MeV H+ population was injected into the solar wind in order to study how such high energy solar wind H+ ions are “shadowed” by Mercury’s magnetosphere. The population was assumed to move in the solar wind along –BIMF. Two cases were studied: “Nominal Parker IMF run”: IMF = [32,10,0] nT, Usw = 430 km/s “High speed Parker IMF run”: IMF = [32,10,0] nT, Usw = 1000 km/s

  29. Test particle simulations • “Parker IMF run”: IMF = [32,10,0] nT, Usw = 430 km/s • High energy (> 50 keV) SW H+ ions launched along the IMF 50 keV H+ population 200 keV H+ population

  30. Test particle simulations • “Parker IMF run”: IMF = [32,10,0] nT, Usw = 430 km/s • High energy (> 50 keV) SW H+ ions launched along the IMF 600 keV H+ population 3.2 MeV H+ population

  31. High energy SW H+ ions: Y = 0 plane • “Parker IMF run”: IMF = [32,10,0] nT, Usw = 430 km/s High energy SW H+ population (~ 0.5 MeV) Main SW H+ populations (~400 km/s) Log[n(H+)] [m-3] Density of ~ 0.5 MeV H+ ions indicating shielding out of the H+ ions by the magnetic field. Low density of SW population but not so low as on the left hand side

  32. High energy SW H+ ions: Z = 0 plane • “Parker IMF run”: IMF = [32,10,0] nT, Usw = 430 km/s High energy SW H+ population (~ 0.5 MeV) Main SW H+ populations (~400 km/s) X X Y Y Log[n(H+)] [m-3] Log[n(H+)] Normalized so that the undisturbed value is ~ 1e-4

  33. High energy SW H+ ions: X = 0 plane • “Parker IMF run”: IMF = [32,10,0] nT, Usw = 430 km/s High energy SW H+ population (~ 0.5 MeV) Main SW H+ populations (~400 km/s) Z Z Y Y Log[n(H+)] [m-3] Log[n(H+)] Normalized so that the undisturbed value is ~ 1e-4

  34. High energy SW H+ ions: Y = 0 plane • “High speed Parker IMF run”: IMF = [32,10,0] nT, Usw = 1000 km/s High energy SW H+ population (~ 0.5 MeV) Main SW H+ populations (~400 km/s) Log[n(H+)] [m-3] Log[n(H+)] Normalized so that the undisturbed value is ~ 1e-4

  35. High energy SW H+ ions: Z = 0 plane • “High speed Parker IMF run”: IMF = [32,10,0] nT, Usw = 1000 km/s High energy SW H+ population (~ 0.5 MeV) Main SW H+ populations (~400 km/s) X X Y Y Log[n(H+)] [m-3] Log[n(H+)] Normalized so that the undisturbed value is ~ 1e-4

  36. High energy SW H+ ions: X = 0 plane • “High speed Parker IMF run”: IMF = [32,10,0] nT, Usw = 1000 km/s High energy SW H+ population (~ 0.5 MeV) Main SW H+ populations (~400 km/s) Log[n(H+)] [m-3] Log[n(H+)] Normalized so that the undisturbed value is ~ 1e-4

  37. Results of applying HBY Modelling to Mercury • A high solar wind (1000 km/s) results in a “compressed” Hermean magnetosphere. • This magnetosphere provides effective “shielding” against high energy (~0.5 MeV) H+ ions. • Upstream velocity changes affected all the parameters analyzed , namely, the: • size of the magnetosphere • shielding of high energy SW H+ ions • As at Mars, knowledge of upstream conditions is essential in order to interpret the observations.

  38. Future Work • The 0.5 MeV case is for H+ ions launched in a beam along the IMF and is therefore similar to the beam case (a) used in the Mars SEP simulations In the future more realistic velocity distributions will be analyzed as in case b) [energy scattering] and case (c) [full scattering] at Mars. • The number of 0.5 MeV ions used in the simulation was low and we plan to include more ions in future runs and also a bigger simulation box. In this case the “cavity” might be at least partially filled with 0.5 MeV H+ ions. • Mercury’s dipole field was assumed to be 300 nT on the surface of Mercury at the magnetic equator. This is being upgraded, using Mercury Messenger data to ~190 nT and appropriately shifted as observed. • Also, energetic particle data recorded aboard Mercury Messenger can be input to the model with corresponding upstream parameters to derive the properties of an SEP along the orbit of Messenger.

  39. Data ultimately available from BepiColombo for interpretation using HYB • MPO/SIXS: (Solar intensity X-ray and particle spectrometer) • (Huovelin et al., 2010). • X-ray (spectral range: 1–20 keV; including X-ray flare parameters), • Protons (spectral range: 1–30 MeV) and • Electrons (spectral range: 100 keV–3MeV) • MMO/MPPE (Mercury plasma particle experiment) (Saito et al., 2008) • will observe: • electrons (3 eV–700 keV), • ions (5 eV–1.5 MeV), and • energetic neutral atoms (25 eV–3.3 keV) (Milillo et al., 2010)

  40. Conclusions Hybrid modeling has been successfully used to simulate the response of the Martian magnetosphere to solar high energy particle irradiation Initial steps have been performed to develop a hybrid model to study the response of Mercury to extreme solar events. This model can later be used to interpret data recorded aboard both Mercury Messenger and BepiColombo.

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