Inductive-Dynamic Magnetosphere-Ionosphere Coupling via MHD Waves Jiannan Tu Center for Atmospheric Research University of Massachusetts Collaborators: Paul Song, Vytenis Vasyliunas Boston College Seminar 11/5/2013

2 Outline Background Governing Equations Numerical Method Simulation Results Conclusion

3 Background Coupling must be through waves in addition to flow Transient responses of ionosphere/thermosphere to changes in solar wind/IMF variations Conventional approach: electrostatic, quasi-stead state Because (In fact ) in plasma momentum equations and Implication: time averaging or quasi-steady state. Missing wave or oscillation information (Detailed discussion in Vasyliunas, Angeo., 30, 357, 2012)

Governing Equations Plasma and neutral continuity, momentum equations (1) (2) (3) (4) 4

Plasma and neutral dissipation equations [Vasyliunas and Song, JGR, 110, A02301, 2005] (5) (6) (7) Generalized Ohm’s law Faraday’s law and Ampere’s law (8) 5

Eliminate J with Ampere’s law and E with generalized Ohm’s law, and write eq. (1)-(6) and Faraday’s law in a compact form where (9) 6

Numerical Method One-dimensional geometry for the central polar cap (northern hemisphere) with a uniform background magnetic field Eq. (9) is a system of stiff partial differential equations because of very large (up to 10 6 s -1 ) ion-neutral collision frequency v in at low altitudes y z - vertical x – antisunward z = 80 km z = 1000 km 7 A scaled M-IT system

Altitude distribution of ion-neutral, electron-neutral, and electron-ion collision frequencies 8

Fully implicit difference scheme (10) Implicit difference overcomes the stiffness so that 5-6 order of magnitude larger time step (up to tens of sec), compared to explicit difference scheme, can be used while still obtaining stable solutions Eq. (10) is a set of nonlinear algebraic equations, which is solved by an iterative Newton-Krylov subspace method An implicit framework has been developed based on a portable, extensible toolkit for scientific computation (PETSc) package ( ), which includes various kinds of Newton-Krylov solvershttp:// 9

Initialization: background & runtime param Photoionization Chemistry Collision- frequencies Main Routine PETSc Nonlinear Solver (SNES) PETSc Krylov Linear Solver Functions Evaluation Jacobian Matrix Postprocessing Flexible: 1-D, 2-D or 3-D, easy to add variables Time Stepping Optional Large time step (up to tens of seconds) can be used with implicit solver – quite efficient Components of implicit framework for the inductive-dynamic ionosphere/thermosphere model 10

At the top boundary (1000 km), the density, velocity, pressure are linearly extrapolated, the free boundary condition (zero first order derivative) is applied to the perturbation magnetic field. At the bottom boundary (80 km) the plasma and neutral velocities, and perturbation magnetic field is set to zero. The plasma and neutral mass densities are determined by photo-chemical equilibrium. The plasma and neutral pressures are assumed fixed in time. The initial ionosphere/thermosphere is specified by IRI 2011 ionospheric model and NRLMSISE00 thermospheric model for the solar minimum night time polar cap. 11

Spatial cell size Δz = 5 km and time step Δt = 0.01s. Choose Δt = 0.01 s so that Δt is less than the transition time (0.1 s) of the convection velocity imposed at the top boundary, and also less than the time for the Alfven wave to propagate from the top to bottom boundary. If the simulation domain is extended to the magnetopause the Alfven travel time will be about 1 min and we may use a larger time step, say 10 sec. The system is driven by an antisunward convection velocity (V x ) at the top boundary, changing from 0 to 600 m/s in 0.1 s, keeping at 600 m/s for 2 min, and then to 2000 m/s in 0.1 s, keeping at 2000 m/s for 1 min. Simulation results for the last 1 min are shown. 12

13 Time variation of an antisunward convection (V x ) imposed at the top boundary Transient time 0.1 s

14 Simulation Results Oscillations & overshoots. Also V y & B y

The transient to quais-steady state takes ~18 sec (~40 Alfven travel times) for scaled M-IT system but will be proportionally longer with higher altitude of the top boundary so that the Alfven travel time t A is used to scale the time 15 Bristow et al., km 1000 km

Alfven travel time t A ~ 0.46 s. t = 0 equilibrium state. Topside V x increases from 600 m/s to 2000 m/s in 0.1 s, velocity perturbation propagates downward along the field line. Localized enhancement and all-altitude overshoot. 16 Superposition of incident and reflected waves causes oscillations and overshoots

17 High Alfven speed (~10,000 km/s) at high altitude, decreases to ~3000 km/s in the F region. Topside V x increases from 600 m/s to 2000 m/s in 0.1 s. Prompt perturbation in V x at 600 km, about one t A to reach 120 km. Perturbations in V y begins at 120 km when V x perturbation arrives and propagates upward. Dynamic Hall effect not present in electrostatic models

18 Ionosphere behaves as a damped resonator in response to magnetospheric perturbations The oscillation period depends on the ionospheric mass or inertia.

19 Compressional waves similar to acoustic-gravity waves but with much faster propagation speed because of the different parameter range. Pressure oscillations with progressive phase delay with altitudes. The phase velocity is ~ 200 km/s at lower altitudes and ~400 km/s at higher altitudes, and correlate with the plasma temperature variations

20 Variation of total heating rate q with time and height. Strong heating below about 250 km. Heating rate included both frictional and true Joule heating rates, with dominant contribution from the frictional heating rate.

The M-I coupling is through MHD waves and produces strong variation of the ionospheric state variables during the transient stage, which persists for Alfven travel times (20-30 minutes). Inductive-dynamic (non-electrostatic and retaining time derivative terms in the momentum equations) approach is necessary to correctly to describe transient variations The overshoot and oscillation is caused by the superposition of the incident and reflected Alfven waves with the period dependent on IT inertial. The dynamical Hall effect is an inherent aspect of the M-I coupling. The ionosphere-thermosphere responds to magnetospheric driving forces as a damped oscillator. 21 Conclusions