Entity

Time filter

Source Type

Columbia, MD, United States

Miyashita Y.,Nagoya University | MacHida S.,Kitashirakawa Oiwake cho | Ieda A.,Nagoya University | Nagata D.,Air Liquide | And 8 more authors.
Journal of Geophysical Research: Space Physics | Year: 2010

We have studied plasma (ion) pressure changes that occurred in association with the dipolarization in the near-Earth plasma sheet around substorm onsets. Using Geotail data, we have performed a superposed epoch analysis in addition to detailed examinations of two individual cases with special emphasis on the contribution of high-energy particles to the plasma pressure. It is found that, unlike previously reported results, the plasma pressure does increase in association with the initial dipolarization at X > ∼-12 RE and -2 < Y < 6 RE, with the increase largely due to high-energy particles. Outside the initial dipolarization region, particularly tailward and duskward of this region, the plasma pressure begins to decrease owing to the magnetic reconnection before onset or before the dipolarization region reaches there. At later times, the plasma pressure tends to increase there, related to the expanding dipolarization region, but the contribution of high-energy particles is not very large. These observations suggest the following. The rarefaction wave scenario proposed in the current disruption model is questionable. The radial and azimuthal pressure gradients may strengthen between the initial dipolarization and outside regions, possibly resulting in stronger braking of fast earthward flows and changes in field-aligned currents. The characteristics of the dipolarization may differ between the initial dipolarization and tailward regions, which would be possibly reflected in the auroral features. Furthermore, we have examined the specific entropy and the ion β. The specific entropy increases in the plasma sheet in the dipolarization region as well as in the midtail region in conjunction with substorm onsets, suggesting from the ideal MHD point of view that the substorm processes are nonadiabatic. The ion β is found to peak at the magnetic equator in the initial dipolarization region around dipolarization onsets. Copyright 2010 by the American Geophysical Union. Source


Carbary J.F.,Johns Hopkins University | Hamilton D.C.,University of Maryland University College | Christon S.P.,Focused Analysis and Research | Mitchell D.G.,Johns Hopkins University | Krimigis S.M.,Johns Hopkins University
Journal of Geophysical Research: Space Physics | Year: 2010

The charge-energy-mass spectrometer instrument in the Cassini spacecraft measured differential fluxes of protons (2.8-236 keV) and oxygen ions (8.8-236 keV) from July 2004 to August 2007. The fluxes were bin-averaged in Saturn longitude system (SLS) longitude within 5 RS of the equator and between 8 and 12 RS in radial distance (1 RS = 60,238 km) to determine their global morphology. The 3-year time period is the range of validity of the SLS, which is based on a variable period of Saturn kilometric radiation. Fluxes at all energies of H+ and O+ display an essentially sinusoidal variation in longitude, often with peak-to-trough ratios of 2:1. For E < 77 keV, the maxima are consistently at ∼70 longitude (the minima are at ∼230). For E > 77 keV; the maxima shift to ∼250. The ion distributions closely resemble those of energetic neutral hydrogen and neutral oxygen atoms observed by the ion neutral camera onboard Cassini. Copyright 2010 by the American Geophysical Union. Source


Ono Y.,Kyoto University | Christon S.P.,Focused Analysis and Research | Frey H.U.,University of California at Berkeley | Lui A.T.Y.,Johns Hopkins University
Journal of Geophysical Research: Space Physics | Year: 2010

We discuss the effect of O+ ions on substorm onsets by examining the relation between the substorm onset location and the distribution of the O+/H+ number density ratio before the onset in the various regions within the plasma sheet (-8 RE > XGSM > -32 RE). We use 9-212 keV/e ion flux data observed by Geotail/Energetic Particles and Ion Composition (EPIC)/Suprathermal Ion Composition Spectrometer (STICS) instrument and the IMAGE/Far Ultra-Violet (FUV) substorm onset list presented by Frey et al. [Frey, H. U., S. B. Mende, V. Angelopoulos, and E. F. Donovan (2004), Substorm onset observations by IMAGE-FUV, J. Geophys. Res., 109, A10304, doi:10.1029/2004JA010607]. The results are summarized as follows. Substorm onsets, which we identify by auroral initial brightenings, are likely to occur in the more dusk-(dawn-)ward region when the O+/H + number density ratio is high in the dusk (dawn) side. This property is observed only in the near-Earth plasma sheet (at -8 RE > XGSM > -14 RE). The above-mentioned property holds in each of two groups: substorm events due to internal instability of the magnetosphere (i.e., internally triggered substorms) and events due to external changes in the solar wind or the interplanetary magnetic field (i.e., externally triggered substorms). Thus, we conclude that the substorm onset location depends on the density of O+ ions in the near-Earth plasma sheet prior to onset, whether the substorm is triggered internally or externally. Copyright 2010 by the American Geophysical Union. Source


Ohtani S.,Johns Hopkins University | Nose M.,Kyoto University | Christon S.P.,Focused Analysis and Research | Lui A.T.Y.,Johns Hopkins University
Journal of Geophysical Research: Space Physics | Year: 2011

The present study statistically examines the characteristics of energetic ions in the plasma sheet using the Geotail/Energetic Particle and Ion Composition data. An emphasis is placed on the O+ ions, and the characteristics of the H+ ions are used as references. The following is a summary of the results. (1) The average O+ energy is lower during solar maximum and higher during solar minimum. A similar tendency is also found for the average H+ energy, but only for geomagnetically active times; (2) The O+-to-H+ ratios of number and energy densities are several times higher during solar maximum than during solar minimum; (3) The average H+ and O+ energies and the O +-to-H+ ratios of number and energy densities all increase with geomagnetic activity. The differences among different solar phases not only persist but also increase with increasing geomagnetic activity; (4) Whereas the average H+ energy increases toward Earth, the average O + energy decreases toward Earth. The average energy increases toward dusk for both the H+ and O+ ions; (5) The O +-to-H+ ratios of number and energy densities increase toward Earth during all solar phases, but most clearly during solar maximum. These results suggest that the solar illumination enhances the ionospheric outflow more effectively with increasing geomagnetic activity and that a significant portion of the O+ ions is transported directly from the ionosphere to the near-Earth region rather than through the distant tail. Copyright 2011 by the American Geophysical Union. Source


Christon S.P.,Focused Analysis and Research | Hamilton D.C.,University of Maryland University College | Difabio R.D.,University of Maryland University College | Difabio R.D.,University of Louisiana at Lafayette | And 4 more authors.
Journal of Geophysical Research: Space Physics | Year: 2013

Suprathermal singly charged molecular ions, O2 + (at ∼32 Da/e) and the Mass-28 ion group 28M+ (ions at ∼28 Da/e, with possible contributions from C2H5 +, HCNH+, N2 +, and/or CO +), are present throughout Saturn's ∼4-20 Rs (1 Saturn radius, Rs = 60,268 km) near-equatorial magnetosphere from mid-2004 until mid-2012. These ∼83-167 keV/e heavy ions measured by Cassini's CHarge-Energy-Mass Spectrometer have long-term temporal profiles that differ from each other and differ relative to the dominant water group ions, W+ (O+, OH+, H2O+, and H3O+). O2 +/W+, initially ∼0.05, declined steadily until equinox in mid-2009 by a factor of ∼6, and 28M +/W+, initially ∼0.007, declined similarly until early-2007 by a factor of ∼2. The O2 +/W+ decline is consistent with Cassini's in situ ring-ionosphere thermal ion measurements, and with proposed and modeled seasonal photolysis of Saturn's rings for thermal O2 and O2 +. The water ice-dominated main rings and Enceladus plume depositions thereon are the two most likely O2 + sources. Enceladus' dynamic plumes, though, have no known long-term dependence. After declining, O2 +/W+ and 28M+/W+ levels remained low until late-2011 when O2 +/W+ increased, but 28M+/W+ did not. The O 2 +/W+ increase was steady and became statistically significant by mid-2012, indicating a clear increase after a decline, that is, a possibly delayed O2 + "seasonal" recovery. Ring insolation is driven by solar UV flux which itself varies with the sun's 11 year activity cycle. The O2 +/W+ and 28M+/W+ declines are consistent with seasonal ring insolation. No O2 +/W+ response to the late-2008 solar-cycle UV minimum and recovery is evident. However, the O2 +/W+ recovery from the postequinox baseline levels in late-2011 coincided with a strong solar UV enhancement. We suggest a scenario/framework in which the O 2 + observations can be understood. Key Points Energized Saturn ring ionosphere O2+ ions show seasonal and solar variation. The long-term O2+/W+ variation is not as anticipated after equinox. Energized local-origin Mass-28 ions initially show likely seasonal variation. ©2013. American Geophysical Union. All Rights Reserved. Source

Discover hidden collaborations