Energetic Ion Observations of the Earth's Magnetic Cusps During an Extended Geomagnetically

Journal of Advances in Space Research, in press, 2005

ENERGETIC ION OBSERVATIONS OF THE EARTH'S MAGNETIC CUSPS

DURING AN EXTENDED GEOMAGNETICALLY QUIESCENT PERIOD

IN APRIL 2001 USING DETECTORS ON S/C ISTP/POLAR

W. N. Spjeldvik1, T. A. Fritz2, J. Chen2, and Q.-G. Zong2

1Department of Physics, Weber State University, 2508 University Circle, Ogden, Utah 84408-2508, USA

2Center for Space Physics, Boston University, 725 Commonwealth Ave., Boston, Massachusetts 02215, USA

ABSTRACT

We present a method of polar cusp identification using high charge state heavy ions. There is reason to think that these ions would constitute shocked solar wind ions such as doubly charged helium ions, He2+, and multiply charged oxygen ions, O(3-8)+, that appear to have relatively easy access to one or both of the polar cusps essentially at all times. The observations made with the ISTP/POLAR spacecraft were mostly obtained during geomagnetically non-storm times in April 2001 when the spacecraft orbit allowed northern cusp encounters with low trajectory angle between the spacecraft velocity vector and the overall cusp magnetic field orientation. We also show one extended cusp crossing during the modest magnetic storm of April 22, 2001 (with Dst ~ -103 nT). It is found that during the study period, 1-10 KeV high charge state ions are present in the cusp region at virtually all times, during storm or quiet. We also found that what appears to be the interior of the northern polar magnetic cusp region can be rather turbulent both in the local magnetic field direction and in the energetic charged particle kinetic flow characteristics. There is some evidence for temporally varying turbulent acceleration processes, either within the cusp domain itself or in the surrounding magnetosphere. In terms of the present ion observations, there is little to distinguish the dayside low latitude boundary layer (LB) ion signatures from those of the interior of the polar cusp region (high charge state ions, abundant 1-10 KeV ion fluxes).

INTRODUCTION

The thermalized flow of plasma within the Earth's bow shock form the Magnetosheath, and the shocked and thermalized particles flow around the magnetosphere is also the source populations of the dayside Low Latitude Boundary Layer (LLBL), the High Latitude Boundary Layer (HLBL), and the Polar Cusp Plasma Particle Population (e.g., Reiff et al., 1977). Traditionally, the high altitude spatial locations of the Earth's magnetic cusps are recognized by the magnetic signature that a spacecraft observes upon traversing these openings in the geomagnetic field shield against the solar wind flow. Observationally, the polar cusp regions are also associated with broadband plasma waves (e.g., Fung et al., 1997; Chen and Fritz, 1998). Lower energy plasma particles (typically below a few keV/ion) are often used to indicate the apparent cusp region.

A complementary view to that of magnetometer observations and low to medium energy plasma ions detection of the polar cusp region (e.g., Newell and Meng, 1988; Farrell and Van Allen, 1990; Kremser and Lundin, 1990; Aparicio et al., 1991; Potemra et al., 1992; Zhou and Russell, 1997; Zhou et al., 2000; Merka et al., 2002) is now afforded by the direct recordings of the cusp plasma particle populations using mass, energy and charge state sensitive ion detectors on modern spacecraft. Energetic ions are found to exhibit a broad B-field organized flow in the Magnetosheath region (Eccles and Fritz, 2002) while the cusp itself can have variable (temporarily confined or flowing) angular ion and electron distribution characteristics. Thus, our empirical toolkit includes observed charge states, energy spectra and angular characteristics, whereby the flux intensities of the hot and energetic plasma particles within the cusp regions can be directly observed, characterized and related to magnetospheric plasma particles in other topological regions.

In this report we describe the locations where the S/C POLAR observed 1-10 KeV high charge state heavy ions (He2+ and O(3-8)+) which we consider to be a shocked and thermalized component of the solar wind plasma entering into the cusp regions after charge-state preserving passage through the Earth's bow shock and subsolar Magnetosheath region. With this new instrumentation capability we also frequently observe extension of the ion spectrum beyond 10 KeV to 50 or 100 KeV and occasionally higher energies in the cusp region. It is possible that the observed energetic high charge state ions are at least intermittently further energized within the cusp-region itself. Furthermore, we do not find a distinct delineation between the characteristics of the LLBL and the interior of the north polar cusp region.

1-10 KeV HIGH CHARGE STATE ION OBSERVATIONS

Amongst the extensive scientific instrumentation on the ISTP/POLAR spacecraft was the CAMMICE/MICS instrument which through a combination of electrostatic deflection, time-of-flight timing and deposited energy measurements using solid state detectors is capable of distinguishing between low charge state ions, He1+ and O(1-2)+ and the higher charge states of those species, He2+ and O(3-8)+. The distinction is an important one, for low charge state ions could conceivably have their direct source in the Earth's ionosphere and be accelerated to supra-thermal energies by ionosphere-magnetosphere acceleration coupling processes, or they could even stem from within the geomagnetically confined ring current and radiation belt region (and so come to the geomagnetic cusp regions through confinement leakage processes) where collisionally induced ion charge state transformations preferentially favor the lowest attainable charge states below several ten of KeV per ion (e.g., Spjeldvik and Fritz, 1978a,b; Spjeldvik, 1979). In contrast, ions of high charge states would have their origin in the multi-million Kelvin solar corona where helium ions exist preferentially as He2+ and Oxygen ions exist preferentially as O(3-8)+ (e.g., Jordan, 1969), or in the radiation belts of the Earth and the planets (i.e., in the hundreds of KeV and multi-MeV energy ranges).

Even though untrapping of radiation belt particles can and does occur in the outer magnetosphere, it is considered unlikely that the two particle source is the main supplier of substantial fluxes of 1-10 KeV thermalized high charge state ions in high charge states to the Earth's polar cusp regions since the radiation belt charge exchange mechanism favors the lower charge states at these energies. At least high charge state ions from the stable trapping region are not expected to appear in amounts and by persistence required to maintain essentially continual presence of the combination of high charge state and low energy ion species in this weak-field outer magnetosphere region. Indeed, Fe-ions have been used as tracers in the outer magnetosphere (i.e., Perry et al., 2000).

Fig. 1. SM Z versus X location of many ISTP/POLAR polar cusp/HLBL/LLBL boundary encounters as seen in 1-10 KeV He2+ and O(3-8)+ ion fluxes during April 2001. With many crossings, these map the observed locations of the borders of this region.

In order to map the spatial locations where we find the characteristic high charge state 1-10 KeV ions we present the locations of entry into and exit from the characteristic shocked solar wind ion locations. Naturally, the geomagnetic LLBL and cusp regions are subject to localization variations due to the natural variations in the solar wind ram-pressure, and to the direction of the solar magnetic field (which may control the ease of entry of solar particles into either of the cusp regions). We included all of the recognizable cusp and/or LLBL crossings during mostly geomagnetically quiescent conditions in April, 2001. This was the time period after the ISTP/POLAR MICS-sensor was returned to normal operational status at UT=22:00 on April 2, 2001 following an extended test to determine the instruments background response to the Earth's radiation environment.

We de-selected the geomagnetically disturbed period of April 11-12 where apparent LLBL and/or cusp crossings were of too short duration for clear signature recognition. We did, however, retain the April 21-22 modest magnetic storm period (|Dst| < 103 nT) because of the then fortuitous orbit for an eight hour long duration LLBL/HLBL/polar cusp observation, and we recognize time variability during this observation.

We have examined the data record, and we have identified a large number of cusp-crossings, including several that were of multi-hour duration. The list of times and orbital entry and exit locations when POLAR observed appreciable fluxes of He2+ and O(3-8)+ ions at 1-10 KeV for a sufficient length of time to obtain statistically significant data in the polar cusp region is given in Table 1. Remarkably, cusp energetic particles appear to have similar characteristics to energetic ions in the low latitude boundary layer (LLBL), and there appears to be essentially a continuity of energetic particle characteristics between these regions. Thus these regions may well be one topological region.. As indicated in Table 1, most of the crossings of the polar cusp energetic particle flux region, seen in high charge state 1-10 KeV ions, are of limited duration, and that is naturally the case when the spacecraft trajectory velocity vector has a significant angle to the cusp magnetic field direction. At other times, the orbit of POLAR either is at a smaller angle to the (cusp and/or LLBL) magnetic field for an extended segment of the spacecraft orbit or the characteristic region has a significant width, and the POLAR spacecraft finds itself immersed in the cusp-like plasma particle population

Figure 1 shows the SM XZ-projection of the spatial extent of the cusp/LLBL region as seen in high charge state 1-10 KeV ions. This figure in some measure traces out the polar cusp and it smooth transition to the LLBL, at least to the extent of the orbital coverage of the ISTP/POLAR spacecraft. The appearance is that of the lower altitude polar cusp being a bit wider than the apparent thickness of the LLBL, although the former has limited sampling..

SUMMARY AND CONCLUSIONS

If the Earth's magnetosphere were to be topologically perfectly static in time, then such a display would have statistically yielded the spatial structure of the polar cusp region as seen in 1-10 KeV shocked solar wind ions. The magnetosphere is, however, constantly buffered by the rather gusty solar wind even during apparently relatively quiescent conditions with only small Dst variations, so that the statistical distribution of cusp encounters is not an instantaneous snapshot, but rather the record of the spatial-temporal extent of the region in which the polar cusp and the LLBL is found as seen in the shocked solar wind particle population well within the Earth's magnetopause location during April 2001. Using data on shocked solar wind high charge state ions, we have presented a particle perspective of the location of the northern polar cusp region. For the cases of extended immersion of the POLAR spacecraft within the cusp structure, we have seen significant turbulence both in magnetic field structure (as evidenced by the varying recordings of the particle pitch angle distributions, not explicitly shown here) and in preferential particle flow direction.

We have seen that the fluxes of 1-10 KeV high charge state ions are persistently present in the cusp region both during extended geomagnetically calm periods and during one modest geomagnetic storm event (extreme Dst ~ -103 nT). We have also seen evidence of sudden extensions of the high charge state particle spectra from a persistent range of 1-10 KeV to inclusion of high charge state ions at energies of 100 KeV or more with appreciable fluxes. The picture that emerges from these charge state sensitive particle observations is that the cusp region appears preferentially filled with shocked and thermalized hot plasma particles from the sun, and that turbulent processes within or near this region provide for significant structural variability and probable energization within the polar cusp cavities. A larger study using complementary and near simultaneous data sets from POLAR, CLUSTER and other modern well instrumented spacecraft is warranted to more precisely evaluate the polar cusp topology, to study the details of interim particle confinement ans/or accelerationthere, and to delineate between extra-magnetospheric energetic plasma particle supplied to the Earth's polar cusps versus the leakage of pre-existing magnetospheric particles into these low magnetic field topological structures in the polar magnetic cusps.

ACKNOWLEDGMENTS

We acknowledge the large efforts made by the ISTP/POLAR spacecraft team. The research herein was supported by NASA Grants NAG5-7677, NAG5-7841, and NAG5-9562 to Boston University. We gratefully thank Prof. James Sullivan of Boston University for many helpful discussions and insights.

REFERENCES

Aparicio, B, B. Thelin, and R. Lundin: The Polar Cusp from a Particle Point of View: A Statistical Study Based on Viking Data, J. Geophys. Res., 96, 14023-14031, 1991.

Chen, J. and T. A. Fritz: "Correlation of Cusp MeV Helium With Turbulent ULF Power Spectra and its Implications", Geophys. Res. Lett., 25, 4113-4116, 1998.

Eccles, A. A. and T. A. Fritz: "Energetic Particle Observations at the Subsolar Magnetopause, Ann. Geophys., 20, 445-460, 2002.

Farrell, W. M., and J. A. Van Allen: Observations in the Earth's Polar Cleft at Large Radial Distances with the Hawkeye-1 Magnetometer, J. Geophys. Res., 95, 20945-20958, 1990.

Fung, S. F, T. E. Eastman, S. A. Boardsen, and S. -H. Chen: High-Altitude Cusp Positions sampled by the Hawkeye Satellite, Phys. Chem. Earth, 22, No. 7-8, 653-662, 1997.

Jordan, Carole: "The Ionization Equilibrium of Elements Between Carbon and Nickel", Mon. Not. R. Astr. Soc., 142, 501-521, 1969.

Kremser, G and R. Lundin: Average Spatial Distributions of Energetic Particles in the Mid-Altitude Cusp/Cleft Region Observed by Viking, J. Geophys. Res., 95, 5753-5766, 1990.

Merka, J, J. Safrankova and Z Nemecek: Cusp-like Plasma in high altitudes: A Statistical Study of the Width and Location of the Cusp from Magion-4, Annales Geophysicae, 20, 311-320, 2002.

Newell, P. T. and C.-I. Meng: The Cusp and the Cleft Boundary Layer: Low-Altitude Identification and Statistical Local Time Variations, J. Geophys. Res., 93, 14549-14556, 1988.

Perry, C. H., M. Grande, T. H. Zurbuchen, S. Hefti, G. Gloeckler, J. F. Fennell, B. Wilken and T. A. Fritz: "Use of Fe Charge State Changes as a Tracer for Solar Wind Entry to the Magnetosphere", Geophys. Res. Lett., 27, 2441-2444, 2000.

Potemra, T. A., R. E. Erlandsson, R. E. Zanetti, L. J. Arnoldy, J. Woch, and E. Friis-Christensen: The Dynamic Cusp, J. Geophys. Res., 97, 2835-2844, 1992.

Reiff, P. H., T. W. Hill, and J. L. Burch: Solar Wind Plasma Injection at the Dayside Magnetospheric Cusp, J. Geophys. Res., 82, 479-491, 1977.

Spjeldvik, W. N.: "Expected Charge States of Energetic Ions in the Magnetosphere", Space Science Reviews, 23, 449-538, 1979.

Spjeldvik, W. N. and T. A. Fritz: "Energetic Ionized Helium in the Quiet Time Radiation Belts: Theory and Comparison with Observation", J. Geophys. Res., 83, 654-662, 1978a.

Spjeldvik, W. N. and T. A. Fritz: "Theory for Charge States of Energetic Oxygen Ions in the Earth's Radiation Belts", J. Geophys. Res., 83, 1583-1594, 1978b.

Zhou, X. W. and C. T. Russell: The Location of the High-Latitude polar Cusp and the Shape of the Surrounding Magnetopause, J. Geophys. Res., 102, 105-110, 1997.

Zhou, X. W., C. T. Russell and G. Lee: Solar Wind Control of the Polar Cusp at High Altitude, J. Geophys. Res., 105, 245-251, 2000.

Key Words:

Magnetosphere, Energetic Particles, Oxygen Ions, Polar Cusps, Magnetosphere