Abstract

1. Magnetospheric plasma density profiles: Co-ordinated multi-instrument 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: zoe@aurora.york.ac.uk 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 Sub-Auroral 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 ina 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. • Most of this work has been carried out using observations of electron number density. The heavy ion populations are less well understood. • Research is now focusing on what happens to plasma when it has been stripped away from the plasmasphere (Su et al., 2001, Ganguli 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. • Instruments studying different local times simultaneously may be used to monitor spatial density variations such as plumes and bite-outs of the plasmasphere. • 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).

2. Table 1: Temporal and spatial coverage of each of the data sets. Radio Plasma Imager. • 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. • 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. • 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.

3. 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. • 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. 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. Figure 7: Plasma mass density profiles for 19th August 2000. References 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.