Jupiter's magnetotail: Plasma composition, dusk boundary structure and tail motion

Date
2010
Authors
Ebert, Robert Wilkes
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Abstract

This dissertation examines plasma observations from the Solar Wind Around Pluto (SWAP) instrument on New Horizons (NH) to identify the composition of ∼0.035 to 7.5 keV/Q ions, study the dusk boundary structure and investigated the role of the solar wind in driving boundary motion in the ∼150 to 2565 RJ region of Jupiter's magnetotail, a region that was unexplored prior to NH. Plasma and magnetic field observations from all three Ulysses polar orbits of the Sun were also examined to study spatial and solar cycle variations in the properties of the fast and slow solar wind and interplanetary coronal mass ejections (ICMEs) from 1.3 AU out to Jupiter's orbit at ∼ 5 AU. Key results from the solar wind study include the slow wind proton temperature falling less rapidly with distance than the fast wind, only minor latitudinal gradients (< 1% deg-1) being identified in the solar wind properties, and significant decreases in the normalized to 1 AU values for the proton temperature and density, dynamic pressure, mass flux and magnetic field strength along with slight reductions in the proton and alpha particle speeds from the fast polar coronal hole flows for the most recent solar minimum (cycle 23) compared to solar cycle 22.

A plasma composition analysis technique developed for SWAP is described in Chapter 3. Though SWAP was not designed to measure ion composition a technique was developed to distinguish between light (mass ≤ He) and heavy (mass > He) ion counts detected by the instrument. SWAP's mass-dependent response was characterized by analyzing the count ratios from its two channel electron multipliers (CEMs) using calibration data from the flight instrument and another flight-like sensor dubbed “SWAP-II”. Significant differences between the response for light and heavy ions were observed, especially below ∼4 keV, that we attribute to the mass-dependent electron emission yield from SWAP's ultra-thin (∼ 1 µg/cm2) carbon foil.

In Chapter 4 we examine SWAP plasma observations in the vicinity of sixteen magnetopause (MP) crossings from 1654 to 2429 RJ that were identified by transitions between magnetotail/boundary layer and magnetosheath plasmas. These transitions were either sharp, with the MP clearly separating two distinct plasma regimes, or comparatively gradual, where it was difficult to distinguish between different plasma populations. The sheath distributions had high counts, were relatively broad in energy/charge (E/Q) and steadily decreased in speed. The boundary layer plasma was composed of light ions and the counts and mean E/Q of these distributions were generally lower than magnetosheath values indicating a lower density and speed. Estimates of angular displacement for the tail boundary compared favorably with a statistical study of near Jupiter solar wind flow cone angle distributions. We propose that the multiple MP crossings resulted from the deflection and contraction/expansion of the tail cross-section in response to forward and reverse shocks and compression/rarefaction regions in the near Jupiter solar wind.

The SWAP composition analysis technique was applied to the entire set of magnetotail observations in the study presented in Chapter 5. Our analysis shows that the < 1 keV/Q plasma is composed primarily of light ions including cool populations at ∼0.1 keV/Q, hotter high count intervals from ∼0.3 to 1 keV/Q and more tenuous plasmas down to < 0.05 keV/Q. Heavy ions were identified at higher energies, energies where this technique is less effective. We compared our results on ion composition with energetic particle measurements from the PEPSSI instrument on NH and found many of the SWAP heavy ion intervals to correlate with sulfur-rich, velocity dispersive particle bursts identified in the PEPSSI observations.

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Keywords
Jupiter's Magnetotail, Magnetosheath, Magnetospheric Boundary Layer, Plasma Composition, Solar Wind-Magnetosphere Interactions, SWAP plasma instrument
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Department
Physics and Astronomy