On the origin of high-energy particles in the cusp diamagnetic cavity K. Nykyri a,n , A. Otto b , E. Adamson b , E. Kronberg c , P. Daly c a Embry-Riddle Aeronautical University, Daytona-Beach, FL, USA b Geophysical Institute, University of Alaska Fairbanks, Fairbanks, AK, USA c Max Planck Institute for Solar System Research, Germany article info Article history: Received 14 March 2011 Received in revised form 25 June 2011 Accepted 23 August 2011 Available online 1 October 2011 Keywords: Cusp Particle acceleration MHD and Test Particle Modeling Cluster Spacecraft Observations abstract We have analyzed Cluster magnetic field and plasma data during a high-altitude cusp crossing in 2003. The Cluster separation was  5000 km and provided unique measurements of high energy particle properties both inside the DiaMagnetic Cavity (DMC) and surrounding magnetosheath. Most of the high energy electrons and protons had pitch angles of  901 in the cavity and the high energy particle intensities dropped as a function of distance from the cavity boundary. By assuming conservation of the first adiabatic invariant for the electrons our analysis indicates that most of the high-energy electrons in the diamagnetic cavity cannot directly originate from the magnetosheath or from the magneto- sphere. Our test particle simulations in a local 3-D high-resolution MHD cusp model show that particles can gain up to 40 keV and their pitch angles become nearly 901 in the local cusp geometry due to gradients in reconnection ‘quasi-potential’ agreeing with the Cluster RAPID observations. These results strongly support a local acceleration of particles in the cusp diamagnetic cavities. & 2011 Elsevier Ltd. All rights reserved. 1. Introduction The magnetosheath plasma has most direct access to the iono- sphere through the high-altitude cusps (Frank and Ackerson, 1971; Heikkil¨ a and Winningham, 1971). The cusps are a key structural element in the magnetosphere around which the geomagnetic field rotates by 3601. Therefore, there is always a region in the vicinity of the cusp and dayside magnetosphere where the geomagnetic field lines are anti-parallel to the Interplanetary Magnetic Field (IMF) making magnetic reconnection possible. When reconnection opens geomagnetic field, denser solar wind plasma gets injected on these field lines reducing the magnetic pressure in this region (Lavraud et al., 2004). The large scale cusp structure is determined by the occurrence of magnetic reconnection at the high-latitude magneto- pause for northward IMF (Burch et al., 1980; Lavraud et al., 2005a) and at the low-latitude magnetopause for southward IMF (Lavraud et al., 2005a; Reiff et al., 1977). This region of depressed magnetic field at the high-altitude cusp is called the Cusp Diamagnetic Cavity and it is frequently filled with high-energy particles (Chen and Fritz, 1998, 2001; Fritz et al., 1999; Niehof et al., 2008; Nykyri et al., 2011a; Walsh et al., 2007, 2010; Whitaker et al., 2006, 2007; Zhang et al., 2005). Zhang et al. (2005) showed that ions with energies greater 28 keV are present during 80% of the crossings while electrons above 40 keV are present only during 22.5% of the exterior cusp crossings. The origin of these particles has been a long-standing and controversial topic. Chen and Fritz (1998) and Chen (2008) have suggested that energetic ion populations up to MeV energies are generated by acceleration via ULF wave ‘turbulence’ present in DMC’s. In addition, Chen and Fritz (2001) and Fritz et al. (2003) showed that cusp particles consist of singly ionized ionospheric (Oþ ) and higher charge state solar wind ions ðO 4 þ 3 and He þþ ). Vogiatzis et al. (2008) have shown examples of case studies of DMCs where supra-thermal Oþ ions are observed simultaneously with broadband low frequency waves concluding that Oþ ions are locally accelerated via wave–particle interactions. It also has been proposed that particles are not accelerated in the cusp at all. Chang et al. (1998, 2000), Lin et al. (2007), and Trattner et al. (1999, 2001, 2010) have argued that the quasi-parallel bow shock is the source for energetic ions in the cusp. Trattner et al. (2001) showed that characteristic spectral breaks observed in the DMC are similar to ions accelerated at the quasi-parallel bow-shock. Trattner et al. (2010) argued that there is a strong dependence of energetic ions in the cusp and magnetic connection to the bow-shock. They showed an example of a DMC event that had fewer energetic ions and demonstrated the lack of connection to the quasi-parallel bow-shock, which would prevent significant energetic particle fluxes from reaching the DMC at the northern cusp. Global hybrid simulations (Lin et al., 2007) have further confirmed that Fermi-type acceleration in quasi-parallel shock and foreshock regions are generating energetic ions that can access the cusp region. However, the bow shock source would not explain high-energy electrons and singly ionized Oxygen ions often observed in the DMC. It also has been suggested that energetic cusp particles originate from the magnetosphere (Asikainen and Mursula, 2005, 2006; Blake, 1999; Delcourt and Sauvaud, 1998, 1999; Kremser et al., 1995; Lavraud et al., 2005b). Delcourt and Sauvaud (1999) showed that particles Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/jastp Journal of Atmospheric and Solar-Terrestrial Physics 1364-6826/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jastp.2011.08.012 n Corresponding author. Tel.: þ1 386 226 6714. E-mail address: nykyrik@erau.edu (K. Nykyri). Journal of Atmospheric and Solar-Terrestrial Physics 87–88 (2012) 70–81