What role do energetic particles present in the dayside cusp play in magnetospheric processes? THEODORE A. FRITZ Center for Space Physics at Boston University Polar PI Telecon March 7, 2008 Thanks to Jiasheng Chen, Jon Niehof, Mike Klida, Kate Whitaker, Brian Walsh, Charlotte Wong, Zhang Hui, the Polar CAMMICE team and the Cluster RAPID team

The McIlwain E3 model does not look anything like the classical ideas of the origin of the cross- tail electric field Lyons and Williams, 1984 What does this have to do with the cusp?

The conclusion to draw from this: Within the cusp there appear to be embedded regions containing diamagnetic cavities (CDC). Within the CDCs the field and particle pressures are strongly anti- correlated whereas within the cusp outside the CDCs the anti-correlation no longer holds. Within the cusp, ions with energies >20 keV up to many 100s of keV (CEP) are present only in those portions of the cusp associated with diamagnetic cavities (CDC). Polar sees CDC that are very large (>6 R E and lasting for hours) At the Polar team meeting in October I presented strong evidence from a combination of Polar, Cluster, and ISEE that the CEPs are energized locally in the cusp from a source population of shocked solar wind ions and up-flowing ionospheric ions.

These CEP/CDC regions are present all of the time independent of solar wind and IMF conditions—the only criteria from the perspective of the Polar satellite being that a satellite must be in the right position to observe them. These CEP particles have pitch angle distributions in which the anti-field aligned direction (  =180 0 ) is full with the opposite direction basically empty. These particles will paint the magnetopause with a layer of energetic particles. The questions that now remain are what effects, if any, these particles have on magnetospheric processes? Are they the source of the ring current via gradient and curvature drifts to the nightside inner magnetosphere? Do these particles create, via charge separation, the cross polar-cap potential?

Conclusions Since these particles are mostly on “open” field lines they will paint the magnetopause with a layer of energetic particles and will populate the upstream medium when an appropriate IMF orientation is present. In addition many of these energetic ions in the cusp will gradient and curvature drift, --ions to the west and electrons to the east, -- separating and drifting along what we have described as a Shabansky orbit that will carry the particles to the equatorial plane around dusk and dawn. These energetic particles on the magnetopause will reenter the magnetosphere -- ions along the dawn flank and electrons along the dusk flank.

Only slight imbalances of these charges is sufficient to establish large charge-separation electric fields. Such charge-separation electric fields can explain most of the features of the McIlwain E3 model.

The E3 model of Carl E. McIlwain was based on plasma measurements at 6.6 R E Note the strong radial field in the post- midnight/pre-dawn sector and a dawn to dusk field only near the midnight sector. McIlwain, 1976