Transcript Document

Magnetospheric plasma density profiles: Co-ordinated multiinstrument ground-based and IMAGE RPI satellite observations
of the plasmasphere.
Zoë C. Dent1, I. R. Mann1, F. W. Menk2, J. Goldstein3, C. R. Wilford4, M. A. Clilverd5,
L. G. Ozeke1, B. W. Reinisch6
1Department
of Physics, University of York, Heslington, York, YO10 5DD, UK
2Department
of Physics, University of Newcastle, Newcastle, NSW, Australia
3Department
of Physics and Astronomy, Rice University, Texas, USA
4Department
of Applied Mathematics, University of Sheffield, Sheffield, UK
5British
Antarctic Survey, Cambridge, UK
6Environmental,
Earth, and Atmospheric Sciences Department, Center for
Atmospheric Research, University of Massachusetts Lowell, USA
Contact Zoë Dent: [email protected]
Abstract
In-situ electron densities as a function of L-shell, measured using the RPI instrument aboard the IMAGE satellite, are
compared to mass density profiles derived using the cross-phase technique applied to data from SAMNET (UK SubAuroral Magnetometer NETwork), BGS (British Geological Survey) and IMAGE (International Monitor for Auroral
Geomagnetic Effects) ground-based magnetometer networks; to electron densities inferred from VLF whistler
measurements; and to mass density profiles from the Sheffield University Plasmasphere-Ionosphere Model (SUPIM).
Data are presented from the 19th August 2000, a geomagnetically quiet day (Kp ~ 1), during which the
magnetosphere was in a state of recovery following a geomagnetic storm seven days previously. In-situ RPI data
from two IMAGE orbits are compared to the ground-based measurements of density profiles. The plasmasphere is
well defined in the ground-based measurements from L = 2.5 – 5.85. Excellent agreement is obtained between the
ground-based and RPI inferred densities in the plasmasphere, providing important validation of both the ULF
magnetometer and VLF whistler techniques. The SUPIM model derived plasmaspheric densities also agree well with
both the in-situ and ground-based measurements. The density profiles from the first IMAGE orbit at ~ 0300MLT
(0615 – 0812UT) suggest a gradual plasmapause, however the second orbit profile at ~ 0300MLT (1928 – 2214UT)
records a sharper and more well defined plasmapause at L ~ 6.3. The difference in the plasmapause structure as
observed by IMAGE RPI on consecutive orbits may be related to an azimuthal asymmetry in plasmapause
morphology. IMAGE EUV imaging of the He+ plasmasphere may provide a framework within which these features
can be understood.
Introduction
The cold plasma populations of the plasmasphere and plasmatrough
regions can be monitored by a number of techniques, both remotely
and in-situ. Since the discovery of the plasmasphere by Carpenter
(1963) using whistler techniques, and by Gringauz (1963) using
satellite data, the position of the plasmapause as a function of time
and geomagnetic activity has become better understood.
et al., 2000), and exactly how the plasmasphere
recovers after a geomagnetic storm. Such studies
require greater temporal and spatial coverage than
single instrument studies provide.
Most of this work has been carried out using observations of electron
number density. The heavy ion populations are less well understood.
Instruments studying different local times
simultaneously may be used to monitor spatial
density variations such as plumes and bite-outs of the
plasmasphere.
Research is now focusing on what happens to plasma when it has
been stripped away from the plasmasphere (Su et al., 2001, Ganguli
Pictorial images from the IMAGE satellite also allow
the monitoring of plasma dynamics for the first time.
From comparisons of electron density and plasma
density observations, the presence of heavy ions can
be inferred.
Using co-ordinated multi-instrument ground-based
and IMAGE satellite observations we can build a
better picture of the structure and dynamics of the
magnetospheric plasma population.
Figure 1: Map showing the relative positions of the
ground-based magnetometers (green squares); VLF
receivers (Halley and Dunedin), VLF transmitter
(NLK), typical lightning source region (NY) and typical
paths of propagation (blue lines); and the northern
geomagnetic field trace of the IMAGE orbit on 19th
August 2000 (first orbit – black, second orbit – red).
Table 1: Temporal and spatial coverage of each of the data sets.
Instrument
IMAGE
RPI
Radio Plasma Imager.
L-shell
MLT range
UT range
Orbit 1
2.45 – 8.10
~ 0300
0615 – 0812
Orbit 2
3.20 – 7.97
~ 0300
1928 – 2214
2.5
0000
1000
Dunedin
Electron number
density
4.15 – 5.54
0100 – 0300 0400 – 0600
4.04 – 5.06
0330 - 0430
0630 - 0730
Ground-based
magnetometers
2.66 – 5.85
0745 – 0940
0625 - 0750
SUPIM
2.57 – 6.02
VLF
whistler
receivers
Halley
Measurement
Ion mass density
N/A
0700
Passive measurements of the
ambient electric field as a function
of frequency in the 3kHz – 1.1MHz
range allow the local electron
plasma frequency, and thus the
local electron density to be
determined.
In-situ measurements of electron
density were mapped along
geomagnetic field lines to the
equatorial plane by assuming a r -3
radial density distribution.
Case study
Data from the 19th August 2000 are presented. This was a
geomagnetically quiet day (Kp ~ 1) during which the
magnetosphere was in a state of recovery following a
geomagnetic storm 7 days previously.
Figure 3: ~0300MLT meridional projection of the IMAGE
satellite orbits on 19th August 2000. First orbit – black,
second orbit – red.
Figure 2: Map showing the geographic location of the
ground-based magnetometer stations whose data were used
for this study. These stations are operated by the BGS,
SAMNET and IMAGE magnetometer networks. The 79o and
107o geomagnetic meridians are separated in MLT by 2.5
hours.
Figure 1 shows the relative positions of the ground-based
instruments and the IMAGE satellite northern geomagnetic
field trace.
Table 1 shows the temporal and spatial coverage of the
four data sets.
Figure 2 shows the positions of the ground-based
magnetometers in the northern European sector.
Figure 3 shows a meridional projection of the two IMAGE
satellite orbits on 19th August 2000.
SUPIM
The Sheffield University Plasmasphere-Ionosphere Model
is a first principles based model which solves time
dependent equations of continuity, momentum and energy
balance to produce estimates of O+, H+, He+, and e- density
along dipolar plasmaspheric field lines.
VLF
Both naturally generated and artificially transmitted
whistler-mode signals were received in order to obtain
equatorial plane electron number densities.
Ground-based magnetometers.
Alfvén waves supported on geomagnetic field lines can be
used to provide magnetospheric plasma mass density
estimates in the equatorial plane. Field-line resonances are
supported at frequencies which depend upon the ambient
geomagnetic field strength, the field-line length and the
plasma mass density along that field line.
The cross-phase technique (Waters et al., 1991) compares
H-component amplitude and phase spectra of data from two
latitudinally separated ground-based magnetometers to
provide an estimate of the resonant frequency of the field-line
whose footprint lies at the mid-point of the two
magnetometers.
Equatorial plane plasma mass densities were calculated
from field-line resonant frequency observations by assuming
a dipole geomagnetic field and a r –3 radial density
distribution.
Results
IMAGE RPI dynamic spectrograms for 19th August 2000 are
shown in figures 4 and 5. Electron plasma frequency is
determined from these.
Electron number density as a function of L-shell, as
observed by the IMAGE RPI instrument is shown in figure 6.
Both in-situ data and values mapped to the equatorial plane
are shown. The first orbit observes a much more gradual
plasmapause than the second orbit. This feature is present in
both the in-situ and mapped profiles.
Figures 4 and 5: Dynamic spectrograms from the IMAGE RPI instrument.
Density profiles from all four techniques are shown in figure
7. To convert electron number densities (measured using the
VLF technique and IMAGE RPI instrument) to mass densities,
a hydrogen plasma has been assumed.
 At low L-shells below L ~ 4 all profiles show good
agreement, inferring the presence of few heavy ions.
Beyond L ~ 4, all profiles except that from the first IMAGE
orbit show good agreement to L ~ 5.5. This anomaly could be
due to azimuthal structure such as a ‘bite-out’ from the
plasmapause in this LT sector.
Conclusions
A suite of techniques have been used to observe electron
and plasma density profiles for 19th August 2000, a
geomagnetically quiet interval.
Figure 6: IMAGE RPI electron density profiles for both
orbits on 19th August 2000. In-situ measurements are
compared to those mapped to the equatorial plane by
assuming a r -3 radial density distribution.
Excellent agreement between the techniques is shown in
general, which validates the ULF magnetometer and VLF
whistler techniques for remote sensing magnetospheric plasma
structures.
Where the IMAGE RPI derived electron density profile does
not agree with the others, it may well be observing a localised
density bite-out from the plasmapause. IMAGE EUV images
may be able to confirm this hypothesis.
Suites of different techniques could be used for varying
purposes: to observe several different MLT sectors at the same
UT in order to observe smaller scale features, or to observe
the dynamics of ion populations in one meridian.
Such studies should help to improve our understanding of
storm-time plasma dynamics.
References
Figure 7: Plasma mass density profiles for 19th August
2000.
Bailey, G. J. and Sellek, R. (1990), A mathematical model of the Earth’s plasmasphere and its application in a study of He+ at L=3, Annales
Geophysicae, 8(3), 171-190.
Carpenter, D. L. (1963), Whistler evidence of a ‘knee’ in the magnetospheric ionization density profile. Journal of Geophysical Research, 68,
1675.
Ganguli, G., Reynolds, M. A., and Liemohn, M. W. (2000), The plasmasphere and advances in plasmaspheric research. Journal of Atmospheric
and Solar-Terrestrial Physics, 62, 1647-1657, 2000.
Gringauz, K. I. (1963), The structure of the ionized gas envelope of earth from direct measurements in the USSR of local charged particle
concentrations. Planetary and Space Science, 11, 281.
Su, Y.-J., Thomsen, M., Borovsky, J., Elphic, R., Lawrence, D., and McComas, D. (2001), Plasmaspheric observations at geosynchronous orbit.
Journal of Atmospheric and Solar-Terrestrial Physics, 63, 1185-1197.
Waters, C. L. et al., The resonance structure of low latitude Pc3 geomagnetic pulsations, Geophysical Research Letters, 18, 2293, 1991.
Acknowledgements
Thanks to all the data providers, including Neil Thomson, Physics Dept., University of Otago, Dunedin, New Zealand. ZCD wishes to thank
PPARC and the IMAGE workshop organisers for providing funding to attend.