1. 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

2. 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
Modified from Haerendel et al. JGR 1978

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

4. 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.

6. 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

7. 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

8. 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

9. 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?

10. Brian Walsh has done a survey of cusp crossing of CDC occurrences using Cluster data
Extent: 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)

11. Jonathan Niehof has done a survey of cusp crossing of CDC occurrences using Polar data
Extent: 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 %

12. 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

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

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

15. 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

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

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

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

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

20. March 18, 2006
Polar

21. B field - Cluster, Polar

22. 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 :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

23. 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