Characteristics of Solitary Waves and Weak Double Layers in the Magnetospheric Plasma

Abstract

The wave experiment of the Viking satellite frequently detected dynamic small-scale (=100 m), large-amplitude, rarefactive (~An/n ( &50%) solitary waves of negative potential (~|S~& 2 V) moving upwards along the magnetic field lines (v =5 to & 50 km/s). The structures, which resemble ion holes, often have an upward-directed net potential drop (& l V) and are then interpreted as weak double layers. PACS numbers: 52.35.Mw, 52.35.gz, 52.35.Sb, 52.70.Ds There is currently an interest in the appearance in space plasmas of small (Debye) scale solitary structures, such as ion acoustic solitions, ion holes, phase-space vortices, and weak double layers. While several laboratory studies, ' computer simulations, and theoretical investigations 5 7 have been performed, partly with space applications in mind, in situ space experiments revealing such structures are rare. The low-frequency wave experiment of the Swedish Viking satellite (launched 22 February 1986, apogee 13527 km, perigee 817 km, inclination 98. 8') has verified the occurrence in the magnetospheric plasma of structures described as solitary waves (SW, with no net potential drop) and weak double layers (WDL, with a small net potential drop pi &kT,/e) first reported from the S3-3 satellite.Our experiment has provided new information on their characteristics, i.e. , spatial and temporal scales, direction of motion, typical velocities, density depletions, and the relation between density and potential variations.The structures are of interest per se, but also for particle energizing processes which may be of universal occurrence. Specifically, it has been proposed that the simultaneous presence of a large number of WDL's along the geomagnetic field lines might, through their cumulative effect, contribute to the auroral particle acceleration, and stochastic variations might provide a mechanism to explain observed energy spectra.The new, two-point measurements of relative plasma density variations (hn/n) provided by two Langmuir probes on Viking proved particularly useful. Two spherical probes of diameter 10 cm were located 80 m apart at the end of wire booms in the spin plane (spin rate 3 rpm) of the spacecraft (inset in Fig. 1). The probes were biased positively ( & 16 V), so that photoelectron currents were negligible, in order to collect electron currents proportional to the plasma density (n). The influence of electron temperature variations is discussed below, and other possible interference sources, such as

Ion and electron characteristics in auroral density cavities associated with ion beams: No evidence for cold ionospheric plasma

“…This result was based upon Langmuir probe sweeps near the ion beams which inferred a 1-10 eV temperature for the electrons [Bostrom et at., 1988]. However, Hilgers et al [1992] show that the Viking Langmuir probe current is dominated by photoelectrons from the wire boom within ion beam regions.…”

“…No mass separation was performed and the cold ions were assumed to be hydrogen. A cold electron (few eV) component with roughly the same density, as required for charge neutrality, was inferred from Langmuir probe measurements [Bostrom et at., 1988; Koskinen et at., 1989]. These Viking observations were made at 8000-11000 km altitudes, in the afternoon-evening sector and in a region whose total accelerating potential was about 1 kV.…”

Acceleration from Field-Aligned Potential Drops

“…Satellite measurements of the high-frequency electric field have revealed the existence of electrostatic ion-cyclotron waves (EICW) and of solitary waves, also named double layers (DL), in which the potential can jump parallel to B by a good fraction of kT e over scales of the order of 10 Debye lengths, X D (Temerin et al 1982;Bostrom et al 1988). The postulated anomalous resistivity may thus be distributed (EICW) or concentrated in a multitude of small ion-acoustic double layers (DL).…”

Electron acceleration by an Alfvénic pulse propagating in an auroral plasma cavity

“…Other evidences of accelerated electrons, with broader distributions in energy and direction, have shown the importance of time-varying electric fields [Hultqvist et al, 1988], possibly carried by Alfvén waves [Kletzing, 1994]. Deep plasma cavities above the Earth auroral zone are a privileged place for electron acceleration [Hilgers et al, 1992] and the subsequent turbulence, characterized by electrostatic coherent structures such as double layers and solitary waves [Bostrom et al, 1988;Eriksson et al, 1997]. With the Freja spacecraft, is was shown that these regions are pervaded by Alfvén wave packets (termed as Solitary Kinetic Alfvén Waves, SKAW).…”