Cusp Energetic Particle Events Research Articles

Investigating the relationship between cusp energetic particle events and cusp diamagnetic cavities

The magnetospheric cusps in the high latitude magnetosphere are characterized by open field lines and precipitating magnetosheath ions. Contrary to the well-understood precipitating thermal magnetosheath ion population, the origins of energetic ions in the cusp regions are still a matter of controversy. It has been suggested that these cusp energetic particles (CEP) with significant fluxes up to several hundred keV/e are accelerated in the cusp by local magnetic turbulence in strongly depressed magnetic field regions called cusp diamagnetic cavities (CDC). Alternative explanations for these CEP events suggest the magnetosphere and also the quasi-parallel bow shock from where energetic ions are transported downstream and enter the cusp along newly reconnected field lines. Composition and energy spectra of these CEPs resemble those of bow shock energetic diffuse ions and support this model.In this study we investigate the relationship between CEP events and CDCs. The survey contains 822 high altitude cusp crossings observed by the TIMAS instrument on the Polar spacecraft for which we document local magnetic field conditions and the flux of energetic ions at 102keV and 191keV. We find that high fluxes of energetic ions are independent of the local magnetic field conditions. This lack of correlation between CEP and CDCs suggests that the source of these energetic ions is not local acceleration in the cusp.

Hybrid simulation of foreshock waves and ion spectra and their linkage to cusp energetic ions

A three‐dimensional global hybrid simulation is carried out to investigate energetic ions and electromagnetic waves in the quasi‐parallel (Q‐∥) bow shock and cusp for a typical interplanetary magnetic field configuration during the cusp energetic particle events. The bow shock, magnetosheath, and dayside magnetosphere form by interaction between the solar wind and geomagnetic dipole field. Ions of solar wind characteristics injected into the system evolve along with electromagnetic waves in a self‐consistent manner. Several important features are yielded from the simulation. Solar wind ions are accelerated by the waves and turbulence at the bow shock and foreshock as indicated by the ion distribution. The shock‐accelerated ions possess an exponential energy spectrum with their differential flux scaled by the field‐aligned distance to the bow shock, consistent with satellite observations. Second, the compressive and transversal waves in the foreshock are strongly correlated with the diffuse ion dynamics. Third, energetic ions in the magnetosheath and cusp downstream from the Q‐∥ shock also exhibit a spectrum similar to those at the shock. By tracing trajectories of cusp energetic ions in the simulation, the origin of these ions is revealed. Their source is predominantly associated with the Fermi acceleration at the Q‐∥ shock and foreshock. Waves in the magnetosheath and cusp, as well as magnetic reconnection at the high‐latitude magnetopause, are relatively insignificant in accelerating cusp energetic particles.

Features of energetic ions near the compressed magnetosphere

A possible cusp acceleration mechanism has been investigated during a major geomagnetic storm on May 4, 1998 when the magnetosphere was compressed and eroded. At 5:00–12:05 UT, the WIND spacecraft was about 213 R E upstream from the Earth, the GEOTAIL was in the post dusk magnetosheath, and the POLAR traveled in its outbound orbit from the equatorial radiation belt to the high-altitude dayside cusp and crossed the magnetopause into the magnetosheath. Many instances of in situ ion energization (to >MeV) were observed by POLAR during this period. Simultaneous observations indicated that no comparable flux was observed by WIND. Ion fluxes measured by POLAR were higher than that measured by GEOTAIL, indicating ion source regions near or within the magnetosphere. The measured ions show some interesting features: (1) In the radiation belt, in the magnetopause boundary layer, and in the dayside cusp, most 1– 200 keV/ e fluxes were around pitch angles of 90°; while in the magnetosheath, the fluxes came from a broad hemispherical direction between ∼sunward and earthward. (2) At around 6:00 UT, the MeV ion flux was lower in the magnetosheath than in the adjacent magnetosphere. (3) There were two CEP (cusp energetic particle) events with 2–3 orders of magnitudes enhancements of MeV ion fluxes. (4) The MeV ions in one CEP event had a peak flux higher than that of the intense outer radiation belt in the equatorial plane. (5) The 100 keV/ e O <+3 ions which originated from the ionosphere were observed in both the outer cusp and the magnetopause boundary layer. (6) There were no large magnetic field fluctuations upstream before the bow shock. These observations suggest a likely source of substantial particle energization exists in the cusp region.

The discovery of a new acceleration and possible trapping region of the magnetosphere

A new acceleration and possible trapping region of the magnetosphere has been discovered by the POLAR spacecraft through the observations of the cusp energetic particle (CEP) events. The events were detected in the dayside high-altitude cusp and were associated with a dramatic decrease and large fluctuations in the local magnetic field strength. These events could last for hours, in which the measured helium ions had energies up to 8 MeV (Chen, 1997a, 1998). A fundamental question to be addressed is the origins of the cusp energetic ions. Simultaneous observations indicated that the ion fluxes in the CEP events were higher than that in both upstream and downstream from the bow shock. We have determined the power spectra of the local magnetic field turbulence calculated over the September 18, 1996 CEP event periods for fluctuations in the ultra-low frequency (ULF) ranges, corresponding to periods of about 0.33–500 seconds. It is found that the mirror parameter, defined as the ratio of the square root of the integration of the parallel turbulent spectral component over the ULF ranges to the local mean field, is correlated with the intensity of the MeV helium flux. A statistical study of the CEP events indicated that the MeV helium flux was also proportional to the difference between the maximum and the minimum magnetic field in the event. These results represent a discovery that the high-altitude dayside cusp is a new acceleration and possible trapping region of the magnetosphere.

Features of the cusp energetic particle events

In 1996, the POLAR spacecraft discovered a new phenomenon called a cusp energetic particle (CEP) event, in which the He ions had energies up to 8 MeV. A total of 75 CEP events have been reported (Chen et al., 1998). The CEP events showed six unusual features: (1) They were detected in the high altitude dayside cusp; (2) large fluxes of energetic (> 20 keV) ions were observed for periods of hours where such “trapped” ions should not be stably trapped; (3) the associated magnetic field was turbulent and greatly depressed appearing to reach a null value in many cases; (4) the 1–200 keV/e helium ions are He ++; (5) compared to O >+2, the O <+3 is negligible; and (6) at 1–200 keV/e, the He ++ particles are the dominant heavy ions with an intensity of about one order of magnitude larger than the oxygen ions. During periods when the GEOTAIL was around the bow shock and the POLAR was in the cusp, the simultaneous observations indicated that the ion fluxes in the CEP events were higher than that in the bow shock. We have also compared the proton flux in the October 14, 1996 CEP event with that in the equatorial radiation belt ( L = 6.6) at midnight, and found that both fluxes were comparable. These new features and observations imply that the discovery of the CEP events may open a new avenue for understanding the magnetosphere.

Plasma waves observed during cusp energetic particle events and their correlation with Polar and akebono satellite and ground data

We present Polar plasma wave data during cusp energetic particle (CEP) events at 6–9 R E. These data suggest the presence of coherent electrostatic structures that are highly localized and that have typical velocities on the order of hundreds to thousands of km/s along the ambient magnetic field. Some of the wave signatures are solitary waves and some are wave packets. The Polar wave instrument also provides evidence that some of the bursts of electromagnetic waves (with frequencies of a few hundred Hz and just below the electron cyclotron frequency around 800 Hz to 1–2 kHz) that are observed are cohenrent and propagating both up and down the field lines. Electron cyclotron harmonic (ECH) waves are oftern detected but their duration is usually short (< 1 s). Low Frequency (<1 kHz), broadband, bursty electromagnetic waves are also present. The Polar wave data results are used to obtain a better understanding of the macro/microphysics during a CEP event that takes place on September 11, 1996, by correlating various Polar (∼ 7.0 R E) and Akebono (∼ 1.4 R E) data while both spacecraft are in or near the cusp/cleft region and nearly on the same field line, and magnetometer data from the Canadian Intermagnet and Canopus ground stations, which lie near the base of the magnetic pootprint passing through Polar. Solar wind and magnetic field data from the interplanetary medium and magnetosheath are provided by the Geotail and IMP-8 satellites, respectively. Some of the cusp waves may be indicators of the reconnection process taking place through the cusp, the result of mixing of magnetosheath with magnetospheric plasma, and the consequence of an anisotropic electron population in a depressed magnetic field. The low frequency electromagnetic waves are still under study to determine their role, if any, in the heating and acceleration of the MeV He ions during CEP events.

Cusp energetic particle events measured by POLAR spacecraft

The Charge and Mass Magnetospheric Ion Coimposition Experiment (CAMMICE) on the board the POLAR spacecraft observed more than 70 cusp energetic particle (CEP) eventin 1996. 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. This is an important discovery, which reveals six interesting features for the CEP events: (1) They all were detected in the dayside at high latitude near apogee; (2) their energies were in excess of 2.4 MeV; (3) an individual CEP event could last for hours; (4) the event-averaged 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 electromagnetic noise; and (6) a possible seasonal variation was found for the occurence of the events with a maximum in September. The measured high charges state of helium and oxygen ions in the CEP events indicates a solar source for these particles. A possible explanation is that the energetic helium ions are energized from lower energy helium by a local acceleration mechanism associated within the high-altitude dayside cusp. These observations represent a discovery of a major acceleration region of the magnetosphere.

Correlation of cusp MeV helium with turbulent ULF power spectra and its implications

A new magnetospheric phenomenon called a cusp energetic particle (CEP) event was observed by the POLAR spacecraft. CEP events were detected in the dayside polar cusp near the apogee of POLAR and could last for hours, in which the measured helium ions had energies up to 8 MeV [Chen et al., 1997a, 1998]. A fundamental question is where do the cusp MeV ions come from? To answer this question, we have compared the ion flux in the CEP events with that in both the upstream and the downstream from the bow shock and found that bow shock acceleration cannot explain the measured ion flux in the CEP events. We have further determined the power spectra of the local magnetic field turbulence calculated over the September 18, 1996 CEP event periods for fluctuations in the ultra‐low frequency (ULF) ranges, corresponding to periods of about 0.33–500 seconds. It is found that the integrated power of the turbulent spectra over the ULF ranges is correlated with the intensity of the MeV helium flux. These new results provides the first direct observation evidence that the high‐altitude dayside cusp is a new acceleration region of the magnetosphere.

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

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.