Cusp energetic particle events: Implications for a major acceleration region of the magnetosphere

Abstract

Abstract. The Charge and Mass Magnetospheric Ion Composition Experiment(CAMMICE) on board the Polar spacecraft observed 75 energetic particle events in 1996 while the satellite was at apogee. All of these events were associated with a decrease in the magnitude of the local magnetic field measured by the Magnetic Field Experiment (MFE) on Polar. These new events showed several unusual features: (1) They were detected in the dayside polar cusp near the apogee of Polar with about 79% of the total events in the afternoonside and 21% in the morningside; (2) an individual event could last for hours; (3) the measured helium ion had energies up to and many times in excess of 2.4 MeV; (4) the intensity of 1-200 KeV/e helium was anticorrelated with the magnitude of the local geomagnetic field but correlated with the turbulent magnetic energy density; (5) the events were associated with an enhancement of the low-frequency magnetic noise, the spectrum of which typically extends from a few hertz to a few hundreds of hertz as measured by the Plasma Wave Instrument (PWI) on Polar; and (6) a seasonal variation was found for the occurrence rate of the events with a maximum in September. These characterized a new phenomenon which we are calling cusp energetic particle (CEP) events. The observed high charge state of helium and oxygen ions in the CEP events indicates a solar source for these particles. Furthermore, the measured 0.52-1.15 MeV helium flux was proportional to the difference between the maximum and the minimum magnetic field in the event. A possible explanation is that the energetic helium ions are energized from lower energy helium by a local acceleration mechanism associated with the high-altitude dayside cusp. These observations represent a potential discovery of a major acceleration region of the magnetosphere.

Energetic particles observed by ISEE-1 and ISEE-2 in a cusp diamagnetic cavity on 29 September 1978

“…Cusp Energetic Particle (CEP) events are characterized by a decreased and turbulent local magnetic field, as well as increased energetic particle flux (Chen et al, 1997(Chen et al, , 1998Fritz et al, 1999). During these events, electrons show large flux enhancements for 100 eV particles and more subtle flux enhancements up to 200 keV.…”

Dawn-dusk asymmetry in particles of solar wind origin within the magnetosphere

“…These are known as extended cusp-like regions (ECRs) and are sometimes observed during persistent, strong northward IMF B Z . A study by Chen et al (1998) discussed observations of He 2+ ions lasting for a few hours with energies from 1-200 keV/e in the dayside cusp around local noon, with a 79% bias to the afternoon side. On the 29 May 1996 an ECR event, characterised by high fluxes of solar wind ions over a large range of invariant latitudes and magnetic local times, coincided with a persistent, strong northward IMF.…”

Space Weather: Physics, Effects and Predictability

“…The cusps are believed to be another place of transport of plasma into the magnetosphere. Analyzing the POLAR spacecraft data, Chen et al (1998) presented a Cusp Energetic Particle (CEP) event having (a) a decrease in magnetic field magnitude in the dayside cusp, (b) a more than one order of magnitude increase in intensity of 1-10 keV ions, and (c) a substantial (more than 3r, r being the standard deviation of the measured data) increase in intensity above background for [40 keV ions (dominated by protons). These CEP events have shed light on the origin of the energetic particles in the magnetosphere (Fritz et al 2003 a, b) and in upstream ion events (Chen and Fritz 2002a, b).…”

“…The origin of energetic particles in the CEP events was debated to be (i) locally energized in the high-altitude cusp Chen et al 1998), (ii) energized particles from the ring current source in the geomagnetic tail associated with substorms and drifting into the cusp Antonova et al 2000;Chen and Fritz 2001), or (iii) particles energized at the quasi-parallel bow shock and subsequently transported into the cusp (Chang et al 2000). The solar wind plasma in the high altitude dayside cusp regions causes extremely large diamagnetic cavities, which are identified by a combination of low magnetic field strength and high solar wind plasma intensity .…”