Plasmasphere Refilling Rates Inferred from Polar and IMAGE Satellite Spectrogram Data

1. Plasmasphere Refilling Rates Inferred from Polar and IMAGE Satellite Spectrogram Data T. Huegerich(1), J. Goldstein(1), P.H. Reiff(1), B.W. Reinisch(2) (1) Department of Physics and Astronomy, Rice University, Houston, TX (2) Center for Atmospheric Research, University of Massachusetts Lowell, Lowell, MA

2. Abstract. In situ measurements of electron number density, as inferred from dynamic spectrogram data recorded by the Polar Plasma Wave Instrument (PWI) (1996-97) and IMAGE Radio Plasma Imager (RPI) (2000-01), are used to calculate plasmasphere refilling rates for several refilling events. In order to compare subsequent measurements of electron density at the same local longitude of the corotating plasmasphere, the measurement at a given Magnetic Local Time (MLT) must be compared with the corresponding measurement taken either approximately 12n (n = 1, 3, 5,…) hours later at the opposite MLT or 24n (n = 1, 2, 3,…) hours later at the same MLT. The orbit of the Polar satellite provides subsequent measurements of this sort separated by time intervals of 15 and 38 hours, while the IMAGE satellite provides appropriate measurements separated by 12 and 16 hours. Temporal refilling effects are thereby isolated as much as possible from spatial effects due to irregularities in the plasmasphere. This method of observing refilling allows the calculation of refilling rates for a range of L-shells (3 Re < L < 7 Re) and MLT positions. ________ 1 Department of Physics and Astronomy, Rice University, 6100 Main Street, MS 108, Houston, TX 77005, USA 2 Center for Atmospheric Research, University of Massachusetts Lowell, 600 Suffolk Street, Lowell, MA 01854, USA Plasmasphere Refilling Rates Inferred from Polar and IMAGE Satellite Spectrogram Data Timothy Huegerich1, Jerry Goldstein1, Patricia H. Reiff1, and Bodo W. Reinisch2

3. Observing Refilling from Polar Orbit: Basic Idea • Subsequent plasmasphere density profiles, extracted from dynamic spectrograms, exhibit refilling (Jan. 15, 1997) • Saturated values in orange, from (Carpenter and Anderson, 1992). • Low Kp indicates probable refilling.

4. Challenges • Density measurements are from varying latitudes rather than equatorial. • Flux tubes co-rotate as they refill, making it difficult to measure the same flux tube twice in succession with satellites following polar orbits. • Flux tubes may expand and contract in both individual volume and collective L-shell position in different MLT’s, confusing attempts to observe refilling. • Measurement error, as well as error in accepted saturation values.

5. Accounting for Varying Latitudes Using a power law model of density distribution along field lines (Goldstein, et al., 2001), we map the in situ density measurements (orange) to projected equatorial values (black). Polar orbit on Jan. 15, 1997: • Orbital period of Polar: 18 hrs. • IMAGE: 14 hrs. • During each orbit, each satellites has two ~1 hr passes through the plasmasphere (Inbound = X, Outbound = O) separated by ~2 hrs.

6. Observing Co-Rotating Flux Tubes: Polar Subsequent views of the co-rotating (hypothetically shaped) plasmasphere from above (EUV perspective), with satellite passes marked: Composite view of passes through the co-rotating plasmasphere: An inbound pass and the outbound pass of the orbit after next sample nearly the same section of the co-rotating plasmasphere, as do an outbound pass and the inbound pass of the next orbit.

7. Observing Co-Rotating Flux Tubes: IMAGE Subsequent views of the co-rotating (hypothetically shaped) plasmasphere from above (EUV perspective), with satellite passes marked: Composite view of passes through the co-rotating plasmasphere: An inbound pass and the outbound pass of the next orbit sample the same section of the co-rotating plasmasphere, while an outbound pass and the inbound pass of the next orbit sample reasonably close to the same region. (As a consequence, the passes of every other orbit sample nearly the same region, at the same MLT.)

8. The Rasmussen Refilling Model (Rasmussen et al., 1993) plasmasphere refilling rate model: Where t is a constant with the units of time that we wish to calculate for different events, seeking to find its dependence on L, Kp, or other factors. Modeled t values, assuming Carpenter and Anderson saturation values, from (Rasmussen et al., 1993) From two subsequent measures of equatorial electron density in a given flux tube, and a value for the equatorial electron density of a saturated flux tube, we may calculate t as:

9. Refilling: Polar- October 5, 1996

10. Refilling: Polar- October 5, 1996 • t values are the correct order of magnitude but significantly lower than modeled (at near solar minimum) • t varies somewhat with L, as expected. • L-shift inward of about 0.1 L on night side is suggested by profiles. (and may account for lack of consistent L-dependence of t.)

11. Refilling: IMAGE- November 27, 2001

12. Refilling: IMAGE- November 27, 2001 • t values are close to modeled values. • Average refilling rate at 6.6 L is consistent with geosynchronous refilling studies, which report rates of 10-25 electrons/cc per day. (Singh and Horwitz, 1992)

13. A Control Case: IMAGE- June 5, 2001

14. A Control Case: IMAGE- June 5, 2001 • Little to no refilling observed, as expected on night side. • Errant calculated t values may be recognized as non-refilling by noticing the irregularity in the density profile, but they illustrate the difficulty in distinguishing refilling, especially slow refilling, from noise and other effects.

15. Conclusions • This methodology provides useful refilling observations. • More work is needed to support general conclusions about refilling. Future Work • Use more sophisticated techniques for accounting for latitude variation. • Study more cases, and study many cases statistically to find how refilling varies with L, MLT, Kp, time of year, etc.

16. Acknowledgements We would like to thank Doug Menietti and Don Gurnett at the University of Iowa for providing the PWI density data, Ivan Galkin at U. Mass Lowell for developing BinBrowser, which enabled us to extract densities from RPI data, and Don Carpenter at Stanford for providing helpful comments. References Carpenter, D. L. and R. R. Anderson, An ISEE/whistler model of equatorial electron density in the magnetosphere, J. Geophys. Res., 97, 1097, 1992. Goldstein, J., R. E. Denton, M. K. Hudson, E. G. Miftakhova, S. L. Young, J. D. Menietti, and D. L. Gallagher, Latitudinal density dependence of magnetic field lines inferred from Polar plasma wave data, J. Geophys. Res., 106, 6195, 2001. Rasmussen, C. E., S. M. Guiter, and S. G. Thomas, A two-dimensional model of the plasmasphere: refilling time constants, Planet. Space Sci., 41, 35, 1993. Singh, N., and J. L. Horwitz, Plasmasphere Refilling: Recent Observations and Modeling, J. Geophys. Res., 97, 1049, 1992.