Variation of electron density along magnetospheric field lines as determined from POLAR spacecraft plasma wave observations

Details Variation of electron density along magnetospheric field lines as determined from POLAR spacecraft plasma wave observations Variation of electron density along magnetospheric field lines as determined from POLAR spacecraft plasma wave observations

May 2002

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agusmsm41b..08d&link_type=abstract

American Geophysical Union, Spring Meeting 2002, abstract #SM41B-08

Computer Science

Sound

2730 Magnetosphere: Inner, 2731 Magnetosphere: Outer, 2753 Numerical Modeling, 2768 Plasmasphere

Scientific paper

During the first year and a half of its operation (March of 1996 to September of 1997), the Plasma Wave Instrument onboard the POLAR spacecraft collected high frequency electric field data from which the electron density could be inferred. In its initial orbit, the POLAR spacecraft had its perigee above the South Pole. As it traveled toward or away from perigee, it crossed magnetospheric field lines at two different altitudes. By comparing the density at these two altitudes, we can observe how the electron density varies down to the lowest altitudes sampled by POLAR ( ~ 2 RE). While in some cases there is little correlation between the low and high altitude data (because of variation in MLT and UT), often there is a high correlation indicating that the azimthal and temporal variation is small (at least for ΔMLT ~ ΔUT <= 1.5 with UT measured in hours). Under these not-uncommon conditions, examples of POLAR data show strikingly that the electron density is very flat along field lines within the plasmasphere, but that there is a steeper parallel density dependence in the plasmatrough (ne ~ R-α with α ~ 1--2) at the altitudes sampled by POLAR (R ~ 2--L RE). Note, however, that even α ~ 2 implies a less steep dependence than collisionless theories with α = 3 or 4. Often as POLAR cuts across field lines at R ~ 2 RE, the density levels off at large L to a value near 100 cm-3 despite the fact that during the high altitude portion of the orbit, the density decreases significantly with respect to increasing L. In the plasmasphere, however, the density has a strong L dependence even at R ~ 2 RE. Another way of putting it is that the distribution of density is roughly radial at R ~ 2 RE in the plasmatrough (independent of L), but has a strong L dependence at the equator. This is in agreement with some observations from the IMAGE radio sounder (Dennis Gallagher, private communication). One interesting effect of this is that the plasmapause is not as easilly discernable for L shell crossings at low altitude as it is at high altitude. In at least some cases, the scale length for parallel density variation ~ L / √ α is roughly constant across L shell in the plasmatrough, and this implies that the distribution of density is roughly radial beyond R ~ 2 RE as well. We also present statistical results from the POLAR data set that support the conclusions formed from the individual events, α ~ 0--0.5 in the plasmasphere and α ~ 1--2 in the plasmatrough (for R >= 2 RE).

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