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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113, A05207, doi:10.1029/2007JA012929, 2008

Ion heat flux and energy transport near the magnetotail neutral sheet

Richard L. Kaufmann

Department of Physics, University of New Hampshire, Durham, New Hampshire, USA


William R. Paterson

Department of Atmospheric and Planetary Sciences, Hampton University, Hampton, Virginia, USA


Abstract

Ten-year averages of energy transport rates near the neutral sheet showed that the enthalpy flux density or thermal energy term Q T = (5/2)P V was the largest, where P is the isotropic pressure and V is the bulk flow velocity. The ion heat flux, q i , was the next largest term. Sorting data using a magnetic flux transport parameter showed that q i could become dominant during periods of slow flow. Both q i and the ion bulk velocity V i were duskward on the dusk side of the neutral sheet. This relationship is characteristic of cross-tail drift and a heat flux that can be attributed to the energy dependent gradient and curvature drifts. The q i and V i vectors often pointed in different directions on the dawn side. The x component of q i on the dawn side pointed tailward, suggesting entry through the magnetopause of a suprathermal ion component. On the dusk side the q ix plots that were sorted using a magnetic flux transport parameter showed evidence of plasma sheet reconnection. The long-term averaged x component of Q T pointed earthward almost everywhere in the neutral sheet, and was attributed to periods of very fast plasma flow. The cross-tail component of Q T was separated into two contributions. One part of Q Ty involved a common drift away from midnight during both earthward and tailward fast flows. This feature suggests that thermal energy and plasma flow from the outer plasma sheet toward the neutral sheet near midnight, and then toward the flanks. The other part of Q Ty involved a differential duskward drift during fast earthward flows and a dawnward drift during fast tailward flows. The incremental E fields that would produce such convection point tailward during the fastest earthward flows and earthward during the fastest tailward flows. The dependencies of V i , q i and Q T on the interplanetary magnetic field (IMF) clock angle also were studied. Both V i and Q T were reduced when the IMF was northward and the neutral sheet plasma became cold and dense. However, no dependence of q i on the IMF direction was seen. This shows that the generation of a cold dense plasma sheet does not substantially change the distribution of the suprathermal ions that are most important in the production of q i .

Received 7 November 2007; accepted 7 February 2008; published 10 May 2008.

Keywords: heat flux; energy transport; neutral sheet.

Index Terms: 2764 Magnetospheric Physics: Plasma sheet; 7859 Space Plasma Physics: Transport processes; 2744 Magnetospheric Physics: Magnetotail; 2760 Magnetospheric Physics: Plasma convection (2463).

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Citation: Kaufmann, R. L., and W. R. Paterson (2008), Ion heat flux and energy transport near the magnetotail neutral sheet, J. Geophys. Res., 113, A05207, doi:10.1029/2007JA012929.