The Auroral Zone: Potential Structures in Field and Density Gradients

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1 The Auroral Zone: Potential Structures in Field and Density Gradients David Schriver May 8, 2007

2 Global Kinetic Modeling: week 10 Foreshock (week 3) Auroral zone (week 7) (week 8) Radiation Belt (week 8) (week 4) Space Physics Simulations: week 2 Memorial Day: week 9

3 Aurora observed in 1681 in Hungary (Paul Urbano, respectable citizen and his whole household )

4 Aurora seen in Alaska

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6 Aurora on April 2, 2001 Glendale, CA photo by Vahe Peroomian

7 VIS Instrument L. Frank (U. Iowa)

8 300 km Space Shuttle 200 km Aurora 100 km Shooting Star Ozone Layer Concorde Mt. Pinatubo

9 Schematic diagram of Earth s internal dipole magnetic field

10 Magnetotail Configuration

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12 Auroral Zone Schematic

13 Aurora Generation Mechanism

14 Scientific Goal Determine the drivers of auroral acceleration from the magnetotail examine sources of free-energy that propagate from the magnetosphere into the ionosphere (satellite data) understand the detailed physics of how these drivers lead to field-aligned acceleration in the auroral zone (plasma simulations)

15 Approach Examine data when FAST and Polar satellites are along conjunctive field lines in the auroral region - use particle data and distribution functions - electric and magnetic field data (waves, currents) Use a specially adapted particle in cell (PIC) simulation to model a portion of the auroral zone - use satellite data to determine boundary conditions - examine formation of quasi-static parallel electric fields and wave-particle interactions self-consistently

16 FAST/Polar Conjunction On June 9, 1997 at 04:32 UT, FAST and Polar were in near-magnetic conjunction in the auroral zone FAST orbit altitude ~ 2500 km - invariant latitude ~ longitude ~ 20.3 MLT Polar orbit altitude ~ 3.8 R E (~ 24,000 km) - invariant latitude ~ longitude ~ 19.8 MLT Note: Event during a magnetic storm (Dst = 84 nt)

17 FAST/Polar Conjunction

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21 Polar Ion (Hydra) and Magnetic Field (MFE) Data FAC FAC+

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23 Takahashi and Hones, 1988

24 Auroral zone (week 8) (week 4)

25 Polar/FAST Observation Summary Latitude Poleward edge Poleward Between Equatorward Driver Field-aligned Acceleration Alfvén waves electrons earthward Fieldaligned current (primary) electrons earthward; ion beam tailward PSBL ion beams electrons earthward; ion beam tailward Fieldaligned current (return) electrons tailward; ions tailward Observed during magnetically active times. During quiet times only field-aligned currents are observed.

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27 Boundary Conditions Determined by the auroral satellite observations (Polar, FAST) Ionospheric plasma at low altitudes cold and dense use cold reflection (i.e., a particle hitting low altitude boundary is replaced by a cold ionospheric particle) Plasma sheet boundary at high altitudes is hot and tenuous with magnetotail free energy Drive system with beam or anisotropic input

28 Need for Variable Grid System Plasma populations with different temperature and/or density in different parts of the system non-uniform Debye length Allows the ability to simulate meso-scale systems in real space

29 Auroral Zone Gradients Density and temperature gradients from low to high altitudes (based on observations): n(z) = n o e (z z o)/h + 17(z 1) 1.5 T(z) = T 1 e z/h + T o Magnetic field (dipolar along the field) B(z) = B o z o3 /(z + z o ) 3

30 Grid Size Consideration Ideally the grid size in a PIC code should be equal to the electron Debye length: λ e = (k B T e /4πn e q 2 ) 1/2 Since the temperature and density vary with altitude, adjust the grid size to match the local electron Debye length: Δ j = λ e (z)

31 Set up grid system according to Debye length at x = 0 and allow grid size to increase according to local n and T. Load particles uniformly with increasing temperature. Determine grid (j) nearest to each particle at t = 0 and store information in array. At each subsequent time step, search only nearest grid stored for each particle (particle should not move more than 1 grid per time step).

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33 Simulation Model One dimensional electrostatic particle in cell code with variable grid spacing Include cold dense ionosphere, hot tenuous magnetosphere and magnetic field gradient Drive system with magnetotail input (ion beam) Push particles using: ma = qe - μ B - mg E = 4πρ Non-periodic, finite difference, leapfrog

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36 Ion Phase Space and Electrostatic Potential

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39 Ionospheric Ions km altitude t = 0

40 Ion Distribution Function - POLAR Satellite

41 Electrons km altitude t = 0

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44 FAST Satellite - Electric and Magnetic Fields

45 Conclusions Specialized simulation with variable grid system is well suited to study large-scale (~ R E ) auroral dynamics Alignment of satellites useful for setting realistic boundary conditions Good qualitative agreement between simulations and data for the event studied - accelerated ion and electron distribution functions - wave spectrum

46 Future Directions Include Alfvén wave input from high altitude boundary (large amplitude, low frequency waves) solve wave equation in time with particles limited feedback of particles on waves Generalize simulation to two dimensions examine inverted V structure transverse wave-particle interactions (e.g. conics) long-thin electromagnetic system to include Alfvén waves self-consistently

47 Lectures: Contact: Course Information (Physics 290) Week Date Topics Monday 2:00 3:30 PM, Room: PAB Office: 3871 Slichter Hall Phone: Office Hours: Wednesdays 2 pm or by appointment 1 April 2 Introduction; magnetospheric physics 2 April 9 Simulation techniques to study space plasma processes 3 April 16 Upstream solar wind: electron beams in density gradients 4 April 23 Magnetotail: ion beam and shell instabilities 5 April 30 no lecture 6 May 7 no lecture 7 May 14 Auroral zone: potential structures in field and density gradients 8 May 21 Large scale kinetic simulations of global ion and electron transport or Radiation belt: whistler wave chorus emissions 9 May 28 Holiday (Memorial Day) 10 June 4 Hybrid simulations; solar wind interaction with Earth s Moon and Mercury

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51 Alfvén Wave Injection

52 Alfvén Wave Injection (electrons)

53 FAST/Polar Conjunction June 9, 1997 FAST Polar Altitude ~ 2500 km ~ km Latitude ~ 71 o ~ 71 o Longitude ~ 20.3 MLT ~ 19.8 MLT

54 Observed Magnetotail Drivers Alfvén wave Poynting flux (high latitude poleward edge) intense earthward streaming electrons (~ 1 kev) Structured field-aligned current primary current region (higher latitude): earthward streaming electrons, tailward streaming oxygen beams (~ 1 kev) return current (lower latitude): upwelling ions/electrons (< 1 kev) High-energy PSBL ion beams (mid-lower latitude) earthward streaming electrons, tailward streaming oxygen beam (~ 1 kev) Observed during magnetically active times. During quiet times only field-aligned currents are observed.

55 Jovian Aurora