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NONLINEAR ENERGIZATION OF IONOSPHERICIONS BY ELECTROSTATIC FIELDS*

Abhay K. RamPlasma Science and Fusion Center

Massachusetts Institute ofTechnologyCambridge, M4 02139 USA

There are two observations that are particularly ubiquitous inthe auroral ionospheric plasmas: intense electric fields indensity depleted regions, and transverse energization of ionsemanating from such regions.' The observed gain in ionenergies is sufficiently large to allow the ions togravitationally escape the ionosphere and populate themagnetosphere. The gain in energy cannot be explained by alinear model of wave-particle interaction between the ionsand the observed electrostatic fields. We have been studyingthe nonlinear interactions of ionospheric ions withelectrostatic fields to understand the observed energization ofions. The electrostatic fields are assumed to be representedeither by a set of plane waves propagating obliquely to thegeomagnetic field, or localized field structures withtransverse spatial dimensions that are small compared to thethermal Larmor radii of the ions. The dynamics of ions iscompared and contrasted for the two cases. For obliquelypropagating waves, nonlinear, coherent transverseenergization of low energy ions is possible for certainconditions that depend on the frequency spectrum, wavenumbers, and relative obliqueness of the waves.23

The interaction of low energy ions with localized electrostaticfield structures is significantly different from their interactionwith waves. We find that the phase space of the energizedions is chaotic and, for long-time interactions, the ions canundergo large energy gains akin to Levy flights. Detailedanalytical and numerical results comparing the energizationof ions by localized field structures with energization byplane waves will be discussed. From our studies weconclude that nonlinear wave-particle interactions are neededto explain the observed gains in ion energies.

1. K.A. Lynch et al., J Geophys. Res. 104, 28,515 (1999).2. A.K. Ram, A. Bers, and D. Benisti, J Geophys. Res. 103,9431 (1998).3. D.J. Strozzi, A.K. Ram, and A. Bers, Phys. Plasmas 10,2722 (2003).

* Work supported by NSF Grant No. ATM-98-06328.

TEMPORAL EVOLUTION OF ION VELOCITYDISTRIBUTION FUNCTION (IVDF) IN A PULSED,

CURRENT-FREE, HELICON GENERATED,EXPANDING MAGNETOPLASMA*

Costel BiloiuPhysics Department, West Virginia University, Morgantown,

WV26506 USA

Current-free plasma expansion in a divergent magnetic fieldis surprisingly conmnon and is found on a variety of spatialscales and in a variety of applications. Plasma expansion isessentially equivalent to a pressure gradient arising from achange in the plasma density. The density gradient can giverise to a potential gradient that retards motion of the lighterplasma electrons but accelerates the more massive ionsdownstream. The solar wind expansion and correspondingcreation of the interplanetary electric field is a classicexample of this process. Also, there is strong experimentalevidence in support of Alfven's hypothesis that the auroraresults from energetic electrons precipitating onto the upperatmosphere and that the electrons in space could beaccelerated by double layer (DL) electric fields withcomponents parallel to the terrestrial magnetic field. Undercertain exernal conditions, theoretical simulations andexperimental observations showed that in a helicon plasmaexpanding into a weaker magnetic field, a DL with a width ofa few tens of Debye lengths can form at the end of heliconplasma. The DL accelerates ions to Mach numbers of order2.1-2 In this work, we present the temporal evolution of bothparallel and perpendicular to the magnetic field argon ionvelocity distribution functions (ivdf's) in pulsed heliconplasmas obtained by using a time resolved laser inducedfluorescence technique with 1 ms resolution.3 At 600 W of rfpower and 1 mTorr pressure, for pulses of 200 ms at a dutycycle of 0.5, the DL forms in the first 40 ms of the discharge.The parallel argon ion flow speed rises from 400 - 500 m/s atthe beginning of the pulse to 3000 - 3500 m/s at the end ofthe pulse. Perpendicular measurements showed no change inthe perpendicular flow speed, consistent with a DL electricfield parallel to the background magnetic field.

1. C. Charles and R. W. Boswell, "Current-free double-layerformation in a high-density helicon discharge", Appl. Phys.Lett. 82, (2003), pp.1356 - 1359.2. X. Sun, C. Biloiu, R. Hardin, and E. Scime, "Parallelvelocity and temperature of argon ions in an expandinghelicon source driven plasma", Plasma Sources Sci. Technol.13, (2004), pp. 359- 370.3. E. Scime, C. Biloiu, C. Compton, F. Doss, J. Heard, D.Ventura, E. Choueiri, and R. Spector, "Laser inducedfluorescence in a pulsed argon plasma", Rev. Sci. Instrum. 76,(2005), pp. 026107-1- 026107-3.

* Work supported by NSF grant 0315356.

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