FG6. Cusp Physics

The Cusp Session (Dayside Research Area)

Chairs: K. Trattner, N. Omidi. and D. Sibeck

The last breakout session of the Dayside Research Area held on Monday afternoon addressed the cusp region and its processes. In addition to a poster session later on Monday evening, 6 speakers had been scheduled who covered the following subjects:

• Sounding of the cusp ion fountain. (Rocket observations)
• Pc 3-4 pulsations at cusp latitudes.
• Simulations of the cusp diamagnetic cavities and particle motions.
• Cluster observations in the cusp and magnetosheath (Fields and RAPID).
• Cluster observations in the cusp and magnetosheath (CIS).
• CEP ions in the cusp (ISEE observations).

M. Lessard presented first results from a study on the cusp ion fountain using SCIFER rocket observations (apogee 1500 km) on Jan 18, 2008 following launch at 07:30:08 UT. The launch occurred from Norway over the EISCAT radar on Svalbard during the occurrence of a Poleward Moving Auroral Form (PMAF). Investigated were the relative significance of Joule heating, soft electron precipitation and waves in ion outflow processes including the altitude dependency of these processes. The observations showed ion outflow with the EISCAT radars during the event, in conjunction with soft electron precipitation. The Japanese spacecraft REIMEI took images during the launch. Additional results were presented during the poster session.

F. Lu studied multi-instrument observations of Pc 3-4 pulsations at cusp latitudes in Svalbard on September 18, 2006. The study of combined magnetometer, radar and satellite data shows that the strongest Pc 3-4 signal on the ground occurs 4-5 degrees equatorward of the cusp, whose location can be accurately determined from the radar backscatter. The study contradicts the direct cusp entry theory, which predicts strongest ground signal right under the cusp, but supports the ionospheric transistor theory by Engebretson et al. [1991].

A. Otto presented results from an MHD simulation of a cusp diamagnetic cavity where they used test particles to investigate ion acceleration in the funnel shaped, low magnetic field region. With time, his test particles reached about 50 keV, consistent with the cusp reconnection ‘potential’ (~50 keV). The particles remained trapped in the cavity while a combination of gradient/curvature motion in the reconnection potential was reported as the acceleration mechanism. The resulting energetic population is highly anisotropic with pitch angles peaking at 90 +/- 45 degrees. Predicted spectra match those observed.

K. Nykyri investigated the Cluster cusp crossing on Feb. 14, 2003 using data from the RAPID and FGM instruments. The cusp crossing exhibits two diamagnetic cavities filled with high energy electrons, protons and helium. By using the four Cluster satellites Katariina reported for the first time an actual spatial size of a diamagnetic cavity (about 1 RE in the direction normal to the magnetopause). The turbulence in the cavity, thought to be one of the methods for accelerating ions, was identified as partly the back and forth motion of the cavity boundary over the satellite while most of it exhibits an FTE-like structure.

K. Trattner investigated the same Cluster cusp crossing as K. Nykyri, using data from the CIS instrument. Applying analyzing tools to pinpoint the location of the reconnection site and IMF field line draping around the magnetopause revealed a reconnection site located poleward of the cusp and a quasi-parallel bow shock region in the southern hemisphere which is magnetically connected to the northern hemisphere cusp region. The 3D capability of the CIS instrument provided observations in the cusp cavity and the magnetopause boundary layer and showed an energetic particle distribution streaming into and towards the cusp, respectively, consistent with a bow shock source for cusp energetic particles.

T. Fritz presented an ISEE-1/2 cusp crossing with orders of magnitude flux increase within the depressed and very turbulent diamagnetic cavity. The energetic particles seem to originate from below the observing spacecraft streaming upward/outward. The electrons demonstrated a distribution peak at 90 degree pitch angle, indicative of being confined within a cusp minimum field trap. The observations were interpreted as being consistent with a local acceleration source.