17th Cluster workshop Uppsala, Sweden , May 12-15, 2009 The role of the cusp in coupling between the magnetosphere and ionosphere Theodore Fritz, Brian Walsh, Jon Niehof Center for Space Physics at Boston University Thanks to Jiasheng Chen and Chris Havlin at BU as well as the Polar CAMMICE team and the Cluster RAPID team Tuesday, May 12, 2009

The Magnetosphere Earth’s Dipole field providing an obstacle for the solar wind Cusp: Plasma entry into magnetosphere Magnetic field lines threading cusp create magnetopause Magnetic null point http://www.windows.ucar.edu/tour/link=/earth/images/earth_magneto_image.html Modified from Haerendel et al. JGR 1978

The Exterior Cusp Magnetic fields poorly predicted in models Significantly overestimating magnetic field strength Lavraud Ann. Geophys. 2004

At the 15th Cluster SWT held on Tenerife I argued that the size of the cusp observed by Polar was on average much larger than the view of a narrow funnel shaped cusp that has come from the analysis of the Cluster data. Here is an example of a Polar crossing of the cusp.

What is a Cusp Diamagnetic Cavity (CDC)? When enough plasma gains entry to the cusp, pressure balance forces the magnetic field to decrease, forming a diamagnetic cavity. Magnetosheath Tail Lobe CDC Cluster 3 Signatures Shocked solar wind plasma Depressed and variable magnetic field HIA RAPID T96 Data from ESA Cluster Active Archive

Importance of the Diamagnetic Cavity Trapped energetic particle population Strong wave power Polar Spacecraft nT Cm-3 Frequency (Hz) 95 keV - 1MeV Electrons Sheldon et al. GRL 1998 Pickett et al. JGR 2001

Large Acceleration Region? Solar wind and ionospheric plasma up to MeV Pitch angle near 90 degrees Correlation with strong wave activity Resonant acceleration (Chen et al. GRL 1998) Polar Solar Wind Ionospheric T96 Chen and Fritz, GRL 2001

Two of my graduate students have undertaken to study the Polar and Cluster data with the following objectives: 1. Where are cusp diamagnetic cavities located? How large are diamagnetic cavities? Are CDCs and CEPs related to one another and, if so, is the relationship one-to-one?

Brian Walsh has done a survey of cusp crossing of CDC occurrences using Cluster data Extent: 2002 - Spring 2008 One spacecraft: C4 Over 400 events in 956 orbits Variety of IMF and solar wind parameters 1. Location: Very close to the magnetopause 2. Size: Ranges from >0 to 6 RE centered of 1.5 to 2 RE. 3. Not yet answered but the cross correlation is >90 % Centered on 1.5 – 2.0 Re Size = (End coordinates) – (Start coordinates)

Jonathan Niehof has done a survey of cusp crossing of CDC occurrences using Polar data Extent: 1996 - Spring 2002 Over 1380 events in 2500 orbits Variety of IMF and solar wind parameters 1. Location: From magnetopause to deeper into throat and closer to Earth 2. Size: Observed from 8<MLT< to 16 MLT over large latitude range 3. Not yet answered but the cross correlation is >80 %

Case Studies: Simultaneous Cluster and Polar CDC Observations: 10 March 2006 Both Polar and Cluster are in southern hemisphere cusp together 18 March 2006 Polar is in the southern hemisphere cusp but the Cluster s/c are in the northern cusp

Simultaneous Observations Cluster Polar CODIF RAPID Polar TIMAS Diamagnetic cavity 10-11 March 2006 Combined Cluster spacecraft

Extended Spatial Extent Cluster Tetrahedron Boundary normals are identified with Minimum Variance Analysis (Sonnerup & Cahill, JGR 1967) T96 10-11 March 2006

Size Estimate Size: 1.6 x 9 Re in XZ plane MLAT: -81° to -42 ° 10-11 March 2006 T96 1.6 Re 9 Re “Pancake” CDC in the exterior cusp

C1 Combo flux,B (smoothed flux) March 18, 2006 C1 Combo flux,B (smoothed flux)

C2 Combo flux,B (smoothed flux) March 18, 2006 C2 Combo flux,B (smoothed flux)

C3 Combo flux,B (smoothed flux) March 18, 2006

C4 Combo flux,B (smoothed flux) March 18, 2006

March 18, 2006 Polar

B field - Cluster, Polar

DY= 8 RE Cusp: C1 (black),C2 (red),C3 (green) 14:30 to 18:00 UT C4 (blue) 16:00 to 18:00 UT Polar 14:00 to 20:00 UT s/c dx [Re] dy [Re] dz [Re] dr [Re] C1-C3 0.012 0.893 0.3985 0.978 C2-C3 0.074 0.057 1.067 1.071 C4-C3 1.647 0.137 0.209 1.66

Conclusions Cusp diamagnetic cavities are a common feature in a variety of solar wind conditions. Diamagnetic cavities are found in the exterior cusp usually pressed up against the magnetopause near the point of magnetic minimum but also extend well into the throat. A cusp diamagnetic cavity can be extended spatially to at least 1.6 x 9 RE in the XZ plane and possibly 8 RE in Y CDCs and CEPs can exist simultaneously in both the northern and southern cusps