ARTEMIS: Revolutionary Insights into Lunar Wake Physics

1. ARTEMISFundamental Physics of the Lunar Wake Halekas, Schriver, Travnicek, Farrell 12/15/2007

2. Previous Views of the Wake: Explorer 35 and Apollo • Early observations primarily focused on the magnetic field, finding an enhanced field in the wake, reduced fields in the surrounding expansion region – due to diamagnetic currents • Wake assumed to be an MHD structure, predicted to fill in 5-6 RL downstream • Very limited particle observations Spreiter et al., 1970

3. Previous Views of the Wake: Wind, Nozomi, Geotail • Wake seen at >20 RL • Found anisotropic ion beams accelerated into the central wake, enhanced ion acoustic activity, and whistler turbulence upstream from the wake boundary • Consistent with a kinetic view of the wake as ion sonic disturbance, with non-neutral plasma and ambipolar E-fields at the wake boundary • Still limited to only a few flybys Ogilvie et al., 1996

4. Previous Views of the Wake: Lunar Prospector • Thousands of orbits at <110 km altitude • Showed density dropout of >2 orders of magnitude • Demonstrated the importance of the non-Maxwellian nature of SW electrons • Handicapped by incomplete data (no ions, E-fields, waves) Halekas et al., 2005

5. Current View of the Wake

6. Major Science Issues • Structure: What is the three-dimensional structure of the wake, including asymmetries? • Energetics: How and where are particles accelerated/decelerated and how are waves generated in and around the wake? • Dynamics: How does the wake respond to changing solar wind and magnetospheric conditions?

7. Wake Structure and Asymmetries • Questions: • What is the wake’s 3D structure and extent? • What are the asymmetries in the wake structure? • Why not answered before: • Not enough orbits far enough downstream from the Moon • Insufficient coverage • How ARTEMIS answers: • The orbital coverage and complete plasma instrumentation of ARTEMIS will allow a conclusive settlement to the question of the wake’s extent and structure. The two-point measurements provided by ARTEMIS will allow unambiguous determination of the asymmetries in the wake due to the perturbing influences of solar wind and crustal magnetic fields or other effects.

8. Wake Energetics • Question: • How are ions and electrons accelerated in the lunar wake? • Where and how are waves generated in and around the lunar wake and what role do the waves play in plasma acceleration/deceleration? • Why not answered before: • Insufficient plasma and wave measurements at the wake boundaries near the Moon and in the deep central wake • How ARTEMIS answers: • ARTEMIS’s full suite of plasma instruments including three dimensional ion and electron particle detectors, along with magnetic and electric field analyzers, will allow a detailed study of the plasma physics occurring within the lunar wake that leads to acceleration and energization.

9. Wake Dynamics • Question: • How does the wake respond to changing solar wind conditions? • What is the structure of the wake when it is embedded in the Earth’s magnetosphere? • Why not answered before: • Not enough orbits and not enough coverage • Lack of simultaneous upstream/downstream observations • No lunar wake studies within magnetosphere done at > 100 km altitude • How ARTEMIS answers: • By utilizing dual-probe measurements and extending the superposed epoch analysis used successfully to analyze LP data [Halekas et al., 2005], ARTEMIS will determine the dynamics of the wake and its response to solar wind drivers. ARTEMIS will provide the first detailed measurement of the lunar wake plasma environment within the terrestrial magnetosphere.

10. Support for Science Closure Travnicek et al. • Hybrid and PIC simulations (Travnicek et al., Farrell et al.) • Laboratory experiments (Gekelman et al.) Gekelman et al. Farrell et al.