Upward flowing ions (UFI)

spaceweb@oulu.fi - last update: 23 December 1998, 1430 UT (RR)


An important part in ionosphere- magnetosphere coupling is the formation of upward ion flows from the ionosphere, as it provides a significant source of magnetospheric plasma. First direct observations of outflowing H+ and O+ beams were by Shelley et al. (1976). The primary region of the outflow is not know, but polar cap (see polar wind), dayside cusp/cleft region (e.g., Lockwood et al., 1985a,b; Thelin et al., 1990; Pollock et al., 1990), auroral oval, and even mid-latitudes (Yeh and Foster, 1990) contribute. In addition to the ions mentioned above, also He+ and O++ have been observed. See also the review by Hultqvist (1991).

Several mechanisms can lead to upward flow of ions. Below altitudes of few hundred kilometers, frictional heating caused by strong perpendicular electric fields form isotropically heated ion distributions. This leads to an increased parallel pressure gradient, and ions flow upward to attain a new equilibrium scale height. Perpendicular ion heating (or transverse ion acceleration, TIA) leads, with the help of magnetic mirror force, to the so-called ion conics, where the ion pitch angle distribution is peaked at oblique pitch angles. When field-aligned acceleration is added, elevated conics or field-aligned ion beams can be produced. Conical distributions occur throughout the auroral zone, while the field-aligned beams are more confined to upward current regions (Cattell et al., 1978).

Mechanism Notes
Frictional (isotropic) heating Thermal plasma outflow; at low altitudes (requires collisions) and low energies
Lower hybrid waves (LH) Chang and Coppi, 1981
Electrostatic ion cyclotron waves (EIC) Kindel and Kennel, 1971; Lysak et al., 1980
Electromagnetic ion cyclotron waves (EMIC) Chang et al., 1986; Temerin and Roth, 1986)
Non-resonant electric field fluctuations Lundin and Hultqvist, 1989; Lundin et al., 1990; Ball et al., 1991
Velocity-shear effects Ganguli et al., 1994

The average ion temperature in the lower ionosphere is about 0.1 eV. Isotropically heated ions are only few times hotter, while perpendicular ion heating can produce energies in the hundreds of eV range.

Isotropic heating

Norqvist et al. (1998) showed that at altitudes between 1000 and 1600 km the isotropic O+ energization dominates at low (< 0.4 eV) energies. Ground-based radar measurements by Wahlund et al. (1992) showed outflow events relating to strong perpendicular electric fields, frictional ion heating, lifted F-region and low electron densities below 300 km, indicating a small amount of auroral precipitation. This could be explained by strong pressure gradients produced by increased ion temperature, and consequent pushing of ions upward (thermal plasma outflow). It is possible that these outflow events are bulk plasma outflows with both electrons and ions moving upward.

Resonant transverse ion heating

Conics were first observed by Sharp et al. (1977). They relate to transverse ion heating events, typically attributed to variety of different resonant waves, like electrostatic ion cyclotron (EIC) or lower hybrid (LH) waves.

It has been show theoretically that EIC waves become unstable to field-aligned currents strengths which are observed in the auroral zone especially relating to auroral arcs (Kindel and Kennel, 1971). Accordingly, they are often observed as low-altitude satellites are crossing electrostatic shock regions, i.e., field lines with parallel electric fields (Kintner et al., 1978). When such instabilities are triggered the ions will be heated to high transverse velocities (e.g., Lysak et al., 1980; Brown et al., 1991). This may result to field-aligned ion outflows via parallel velocity conversion by magnetic mirror force. Typical altitude for these phenomena is about 1 Re, which is also the most unstable region for current-driven instabilities (Lysak and Hudson, 1979).

Lower hybrid waves are found throughout the auroral zone (Mozer et al., 1979). The source of these waves can be, e.g., a linear mode coupling mechanism as electromagnetic (VLF range) auroral hiss scatters from magnetic-field-aligned irregularities in the background mean plasma density (Bell et. al., 1991; similarly, strong VLF/ELF transmitters can be used to heat the magnetospheric ions).

Andre et al. (1998) have related most of the ion heating events observed by Freja satellite to broadband low-frequency electric wave fields covering the important oxygen gyrofrequency.

It seems quite likely that field aligned potential drops play some role in creating elevated conics. However, it has also been shown that elevated conics can be formed without such help by the velocity filter effect (Horwitz, 1986) and the effect of large heating region (Temerin, 1986).

Non-resonant electric field fluctuations

Also stochastic, slow electric field fluctuations of large amplitudes can produce conics (e.g., Lundin and Hultqvist, 1989; Lundin et al., 1990; Ball et al., 1991). The fluctuating field may also have a component along the magnetic field.

Radar observations

Radar observations can be difficult to correlate with satellite measurements, and here we will mention separately the work by Wahlund et al. (1992), who reported on auroral arc related events that showed enhanced electron temperature and field-aligned currents with the bulk ion population moving upward and the bulk electron population moving downward. Authors argued that they may be due to enhanced field-aligned electric fields caused by anomalous resistivity due to low-frequency plasma turbulence (e.g., ion acoustic turbulence). It is to be noted that all arcs are not accompanied by ion outflows, and this may be related to the strength of the filed-aligned current.


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