14 May JIM M. RAINES University of Michigan DANIEL J. GERSHMAN, THOMAS H. ZURBUCHEN, JAMES A. SLAVIN, HAJE KORTH, and BRIAN J. ANDERSON Magnetospheric Cusp Structure and Dynamics: MESSENGER FIPS Measurements at Mercury

Previous results 14 May FIPS measurements show highest concentration of planetary ions in cusp [Zurbuchen et al. 2011] Size of Mercury’s cusp [Winslow et al. 2012] more like Earth’s outer cusp Na + -group ions are ~2 times more abundant than solar wind He 2+ in the cusp area on the average [Raines et al., 2012] Jim M. Raines, Univ. of Michigan Updated version from Zurbuchen et al., 2011

Produced in the cusp by solar wind sputtering from the surface Solar wind Na + Produced upstream (magnetosheath and beyond) and driven into the cusp Na Na + hν Driving Question 14 May 20133Jim M. Raines, Univ. of Michigan Where and how are the Na + -group ions observed in and around the cusp produced?

REPRESENTATIVE CUSPS 14 May 20134Jim M. Raines, Univ. of Michigan

Typical case 1: Expected cusp 14 May 20135Jim M. Raines, Univ. of Michigan Small spatial extent Na + -group ions grouped with protons Na + -group ions >1 keV (energies vary) cusp QUIET cusp

Typical case 3: Active 6 Hot Na + -group ions, mostly > 2keV Low energy bins below detection threshold at these low densities Dayside Na + -group ions FTEs ACTIVE cusp 14 May 2013Jim M. Raines, Univ. of Michigan

Active with lots of Na+, low E dayside 14 May 20137Jim M. Raines, Univ. of Michigan Colder ions at lower LAT than cusp Hot ions in cusp FTEs Trapped in dayside closed-field region? ~55 km gyroradius at equator is small enough to fit inside dayside magnetosphere Atypical: Low energy Na +

COLLECTIVE PROPERTIES 14 May 20138Jim M. Raines, Univ. of Michigan

Cusp Na + -group Properties Surveyed 518 cusp crossings, Sept 2011 – May Significant in ~30% of cusps –Active (~20%), has significant dayside component –Quiet (~10%), NO significant dayside component Peak ~900 km altitude ~ 2.7 keV Low energy, eV, <1% of observed particles (in <5% of cusp crossings) Most active and highest energies coincident with FTE activity 14 May 20139Jim M. Raines, Univ. of Michigan LAT only

Na + -group Pitch Angle Distributions 14 May Jim M. Raines, Univ. of Michigan Not consistent with beaming distributions QuietActiveActive (dayside portion)

Seasonal Variability 14 May Jim M. Raines, Univ. of Michigan Cusp only: LAT LT 9-15

Seasonal Variability (2) 14 May Jim M. Raines, Univ. of Michigan Cusp only: LAT LT 9-15 TAA ~135 TAA ~240 Z MSO (R M ) Y MSO (R M ) X MSO (R M ) Na + -group ion variability shows patterns with TAA TAA and LT of periapsis are phased locked Variability could be seasonal (TAA) or spatial (LT)

DISCUSSION 14 May Jim M. Raines, Univ. of Michigan

Acceleration Mechanisms Initial ion energy <10 eV from direct surface processes or photoionization Local to cusp –Centrifugal from E x B drift »Cleft ion fountain observed at Earth »Predicted at Mercury »Can accelerate to 10x initial energy but only a small fraction within cusp –Wave-particle interactions »Can accelerate up to 10-20x initial energy –Not enough for 10 eV  2.7 keV Away from cusp –Solar wind pickup »160 km/s magnetosheath flow gives 3-12 keV Na + –Dayside reconnection »3 keV in range for Alfven speed expected from outflow 14 May Jim M. Raines, Univ. of Michigan Delcourt et al., 2012 Na + trajectories in the X-Z plane. The test ions are launched from the planet surface at 65 latitude in the dayside sector, with 0.01 eV or 0.1 eV energy, 170 pitch angle and arbitrary gyrophase. Blue, green and orange-red colors correspond to cross-polar cap potential drops of 2 kV, 5 kV, and 10 kV, respectively. Adapted from Delcourt et al., 2012.

Circulation? Could < 10 eV Na + ions be accelerated into the tail then circulate back to the cusp with keV energies? 14 May Jim M. Raines, Univ. of Michigan Small clearance within dayside magnetosphere makes circulation by >2 keV ions unlikely. Ions in the 200 eV range do fit – energy filter?

Source of Na + -group Ions Na is ionized in the magnetosheath (and beyond) and the resulting Na + -group ions are swept into the cusp Majority of Na + -group ions observed >2 keV Observed energies consistent with ion pickup or reconnection heating Dayside component and association with magnetic activity may mean FTEs are carrying ions into cusp Na + -group ions are sputtered off of the surface at low energy, < 10 eV, by solar wind ions funneling down the cusp. Known local heating/acceleration mechanisms cannot account for high energy (> 2 keV) of observed ions Observed density maximum is in upper half of cusp altitudes Pitch angles do not show beaming up from surface Low energy ( eV) Na + - group ions observed only in minority of cusps (< 5%) 16 Strongly indicated 14 May 2013Jim M. Raines, Univ. of Michigan

Summary and Conclusions Na + -group ions are routinely observed in the cusp region with energies of keV and observed densities of cm -3. Na + -group ions in the cusp show substantial variability. The highest densities and energies of Na + -group ions correlate with magnitude of diamagnetic decrease and level of FTE activity. The high energies (2-4 keV) of Na + group ions regularly observed in the cusp strongly favor ionization in the magnetosheath (and beyond) then sweeping into the cusp, possibly by reconnection. Low energy ( eV) Na + ions are occasionally present at lower dayside latitudes than the cusp. These ions may be accelerated locally, recirculated from the magnetotail and/or trapped in the closed-field dayside magnetosphere. 14 May Jim M. Raines, Univ. of Michigan

Jupiter and Mercury, 13 Mar Image: Pete Lawrence via NASA. 14 May 2013Jim M. Raines, Univ. of Michigan18