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
2Center
of Physics, Weber State University, 2508 University Circle, Ogden, Utah 84408-2508, USA
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, He 2+, 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 He 2+ 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