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T. Ogino and T. Aoyama Solar-Terrestrial Environment Laboratory

Response of the Earth’s Magnetosphere to the Strength and Rotation of IMF. T. Ogino and T. Aoyama Solar-Terrestrial Environment Laboratory Nagoya University. Background. Plasma energy is transported from the solar wind to the earth’s magnetosphere .

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T. Ogino and T. Aoyama Solar-Terrestrial Environment Laboratory

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  1. Response of the Earth’s Magnetosphere to the Strength and Rotation of IMF T. Ogino and T. Aoyama Solar-Terrestrial Environment Laboratory Nagoya University

  2. Background • Plasma energy is transported from the solar wind to the earth’s magnetosphere. • Magnetic reconnection usually plays most dominant role in the transport processes. • Electric field is usually adopted as measure to estimate reconnection rate. • The quantitative analysis is not enough in the realistic earth’s magnetosphere.

  3. Magnetic Reconnection Magnetic reconnection occurs at the dayside magnetopause and in the tail for southward IMF. IMF Reconnection NENL Solar Wind (Near Earth Neutral Line) magnetopause

  4. Objectives Execute 3D global MHD simulation of solar wind-magnetosphere interaction. Study what parameters of the solar wind and IMF have dominant effects on magnetic reconnection. Compare the electric field of magnetic reconnection with that in the upstream solar wind.

  5. Problem and Solution • Two ways of twice magnetic field and twice velocity to enlarge the electric field (E=-V×B) twice. • Magnetopause approaches the earth due to increase of the solar wind dynamic pressure, Dp=ρV2 when the velocity increases. • The solar wind density needs to be adjusted to realize the same dynamic pressure of solar wind for change of magnetic field. Parameters to be controlled velocity Vx , IMF Bz , density ρ

  6. z 30Re -60Re 30Re x 30Re y Solar-Magnetosphere Coordinate System Solar Wind A quarter model of the earth’s magnetosphere under assumption of dawn-dusk and north-south symmetry grid number (nx,ny,nz)=(450,150,150) grid interval is 0.2-0.1 Re (Re: radius of earth=6370 km)

  7. (nT) Estimate electric field 50 Bz 0 bow shock magnetopause -50 (mV/m) 10 NENL Ey dayside reconnection electric field 0 -10 tail reconnection electric field ρ x 30Re 0 -60Re

  8. X X MP MP simulation simulation theory theory Shue model Shue model (Re) (Re) (km/s) Change of Magnetopause 1 Vx=300 (km/s) ρ=20 (/cc) ρ=20 (/cc) Bz=-5 (nT) 1200 IMF Bz (nT) for change of IMF Bz for change of Vx Difference of the positions of magnetopause is larger for change of IMF Bz due to erosion as the result of magnetic reconnection.

  9. Change of Magnetopause Position 2 before of onset of tail reconnection Shue’s model Statistical model from observations includes all time of data. It naturally includes the case that dayside reconnection occurs to bring erosion and tail reconnection does not yet. MHD simulation Long time duration of southward IMF and magnetopause is pushed back by compensation of magnetic flux due to tail reconnection. after of onset of tail reconnection

  10. : electric field in solar wind : reconnection electric field twice large for change of velocity dayside dayside tail tail (mV/m) (mV/m) (mV/m) (mV/m) sw rec rec sw E E E E y y y y Reconnection Electric Field (maximum for 10 min) Ey=VxBz Vx=300 (km/s) ρ=20 (/cc) ρ=20 (/cc) Bz=-5 (nT) Bz (nT) -50 -30 -10 Vx (km/s) 300 600 900 Change of Vx change of IMF Bz

  11. velocity IMF Bz density =3 (mV/m) =4.5 (mV/m) 2 Dp=ρV x (mV/m) (mV/m) sw rec E E y y Reconnection Electric Field for the Same Dynamic Pressure (dayside) Blue and green are the same dynamic pressure of solar wind. Ey=VxBz

  12. velocity IMF Bz density =3 (mV/m) =4.5 (mV/m) 2 Dp=ρV x (mV/m) (mV/m) rec sw E E y y Reconnection Electric Field for the Same Solar Wind Dynamic Pressure (Tail) Blue and green are the same pressure Reconnection electric field is larger when the velocity is larger in the dayside magnetopause and tail even for the same dynamic pressure. Ey=VxBz

  13. Difference of Reconnection Electric Field Same dynamic pressure of solar wind mV/m mV/m 5 5 -5 -5 K K 0 0 X=-10Re X=-10Re For large velocity, through the bow shock due to Rankine-Hugoniot relationship T large ρ magnetosphere T large small decrease of inertia Vx large large Ey

  14. Sec 4 DiscussionDifference of Reconnection Electric Field Electric field in the upstream solar wind and dynamic pressure are same. (a) (b) Bz=-5 nT ρ=20 /cc Vx=900 km/s Bz=-15 nT ρ=180 /cc Vx=300 km/s electric fieldEy temperatureT densityρ

  15. Sec. 4 DiscussionDifference of Reconnection Electric Field For large solar wind velocity • Plasma temperature increases by Rankine-Hugoniot relationship through bow shock. • In reconnection regions of the dayside magnetopause 2 and tail 3, temperature increases and density decreases due to the constant pressure. Ｖｘ 3 1 2 Reconnection electric field increases because the flow velocity increases due to decrease of the plasma inertia.

  16. (mV/m) (mV/m) sw sw φ(+) E E y y φ(－) φ(+) －φ(－) Polar cap potential 1 80° 70° 60° Vx=300 (km/s) ρ=20 (/cc) ρ=20 (/cc) Bz=-5 (nT) polar cap potential (kV) polar cap potential (kV) Bz (nT) -50 -30 -10 Vx (km/s) 300 600 900 For change of Vx For change of IMF Bz Tendency of saturation for increase of IMF Bz

  17. Polar cap potential 2 For increase of southward IMF Reconnected field lines are difficult to remove from the reconnection region toward the tail when the velocity is small like 300 km/s. Saturation happens because the new IMF lines are hard to reconnect. For increase of solar wind velocity Saturation little happens because the reconnected field lines do not stagnate and are quickly propagate tailward.

  18. Oscillations of electric field on sun-earth line Vsw = 300 km/s for t < 120 min Vsw = 600 km/s for t > 120 min

  19. dayside dayside tail tail (mV/m) (mV/m) rec rec y y E E Oscillations of Reconnection Electric Field 1 Vx=900 (km/s) Bz=-5 (nT) Vx=300 (km/s) ρ=20 (/cc) ρ=20 (/cc) Bz=-50 (nT) Time (min) Time (min) Stronger oscillation for large velocity

  20. dayside dayside tail tail Time (min) (mV/m) (mV/m) 18 15 rec rec y y E E 12 9 6 3 0 0 5 10 15 20 25 30 35 40 45 Oscillations of Reconnection Electric Field 2 (dependence due to spatial resolution) Vx=600 (km/s) Bz=-5 (nT) ρ=20 (/cc) Time (min) Δx=0.1 Re Δx=0.2 Re Oscillations rather become stronger for high spatial resolution.

  21. Oscillation of Reconnection Electric Field 3 No balance between dayside and tail reconnection. Tendency of occurrence of patchy and intermittent reconnection.

  22. Sec. 5 Conclusions • We have quantitatively studied which parameters of the solar wind and the IMF have an important effect to magnetic reconnection from 3D MHD simulation. • It becomes clear that reconnection electric field efficiently increases for increase of the solar wind velocity than that of southward IMF. • For large magnetic storms the solar wind velocity as well as southward IMF plays an important role, even though importance of southward IMF only is particularly stressed. • Oscillations of the reconnection electric field increase for larger velocity, and reconnection region becomes narrower in the tail.

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