Simulations and observations of heating of auroral ion beams (original) (raw)

1989, Journal of Geophysical Research

In the auroral zone, quasi-static parallel electric fields produce beams of ionospheric ions (e.g., H + , He + •md O+), which flow outward into the magnetosphere, providing a significant source of ions for the ring current and plasma sheet. Because the velocities to which these beams axe accelerated is dependent on the mass of the ions, differential flows between the various ion species can develop which are unstable to an ion-ion streaming instability. Particle simulations and observations from DE 1 are used to investigate the heating of the ion beams produced by this instability. It is shown that there is net transfer of energy from the light ions to the heavy ions, with the heavy ions reaching maximum velocities near the beam velocity of the light ions. Bulk heating of the heavy ions occurs when their relative density is low while high-energy tails are produced when their relative density is high. The heating is primarily parallel to the magnetic field if the difference in the heavy and light ion beam velocities is subsonic while both perpendicular and parallel heating can occur if it is supersonic. In the latter case, very strong heating of an intermediate ions species such as He + can also occur. Comparison with observations shows features consistent with heating via the ion-ion instability including perpendicular heating in the supersonic regime and parallel heating in the subsonic regime and a change in the heating between these regimes as the ratio of the H + beam speed to the local sound speed is observed to decrease. This heating is, however, not always observed in association with enhanced wave emissions. This lack of waves is attributed to reabsorption of the waves as the ions become heated. 1. INTRODUCTION On occasions, "bimodal" distributions are observed where Along auroral field lines, ionospheric ions are observed the outflowing ions show evidence of both strong perpento be accelerated outward into the magnetosphere and dicular and parallel acceleration [Kiumpar et al., 1984]. are consequently a significant source of ions for the ring Such distributions can also be produced by extended current and plasma sheet [Ghappeli et ai., 1987]. This transverse heating and velocity-filter effects. ion outflow is in p•rt produced by quasi-static parallel Several mechanisms for this perpendicular acceleration electric fields known to occur along auroral field lines or heating utilizing wave-particle interactions have been [Hoffman and Evans, 1968; Whalen and McDiarmid, proposed. These mechanisms include heating by (1) 1972; Arnoidy et al., 1974; Evans, 1974; Chiu and Shuiz, electrostatic ion cyclotron waves [e.g., Lysak et al., 1980; 1978]. This potential drop accelerates ions along the Dusenbery and Lyons, 1981; Ashour-Abdaila and Okuda, magnetic field to produce field-aligned ion beams with 1984], (2) lower hybrid waves [Chang and Coppi, 1981] energies of several keV [Shelley et ai., 1976]. Such beams and (3) shear Alfv•n waves [Chang et ai., 1986; Winglee are rarely observed below 5000 km and appear to be et al., 1987, 1988b]. While many of these waves have heated appreciably in both parallel and perpendicular been observed on auroral field lines, observations from directions [Coilin et al., 1986; Ghielmetti et al., 1986]. S3-3 [Kintner and Gorney, 1984; Kintner, 1986] and DE 1 [Peterson et ai., 1988] have yet to estabhsh a Ionospheric ions can also experience strong perpendicular acceleration. Evidence for this comes from direct association between conics and these waves. observations of ion conic distributions where the pitch Recently, Winglee et al. [1988a] used particle simulations and observations from DE 1 to show that conics angles of the ions are confined to angles nearly perpendicular to the magnetic field [Sharp et al., 1977; could easily be generated by the quasi-static perpendic-Gorney et al., 1981; Klumpar, 1986]. Conics typically ular electric fields associated with V-shaped potentials have energies less than about 500 eV. Due to the mirror in discrete aurora] arcs. Within these potentials there force associated with gradients in the geomagnetic field, is an enhanced upward current as the magnetospheric this perpendicular energy can be converted to parallel electrons are accelerated downward by the quasi-parallel energy and thereby lead to enhanced ionospheric outflow. electric field. In adjacent regions, ionospheric electrons are accelerated outward, forming the return current. Plasma ions are accelerated across the field hnes to dose these spatial]y separate currents. This acceleration