Polar wind

Polar wind

Polar wind is "the permanent outflow of ionization from the polar regions of the magnetosphere." [http://amsglossary.allenpress.com/glossary/browse?s=p&p=43 AMS Glossary] ]

Ionospheric Plasma Source

The ‘ground state’ of the ionosphere consists of the baseline conditions that prevail in this plasma domain. This discussion is limited to topics thought to influence the characteristics of the ionosphere as a source of plasma outflows into the magnetosphere, notably the Polar Wind.

Thermospheric Composition

The composition of the ionospheric plasma source is fundamentally constrained by the composition and structure of the thermosphere (the neutral atmosphere above 80 km altitude). An excellent treatment of the thermosphere and ionosphere as a chemically coupled system is given by Banks and Kockarts (1973), while an empirical description of the thermosphere itself is given by Hedin (1983). The composition of the thermosphere is strongly dependent upon its temperature, as illustrated in Figure 1. Atmospheric hydrogen is only marginally bound by gravity at thermospheric temperatures, so a temperature increase produces significant escape of hydrogen atoms into space, increasing the H density in the geocorona above 2000 km altitude and decreasing the H density at lower altitudes through-out the lower thermosphere. The supply of H, available for production of H+ in the ionosphere, is thus reduced when thermospheric temperatures rise. To a lesser degree, the same is true for helium atoms, though they are more strongly bound than hydrogen. Thermospheric heating has completely the opposite effect on the oxygen atom density in the ionospheric altitude range. Oxygen atoms are strongly confined by gravity, so that increasing temperature produces an increase in the scale height of O, leading to orders of magnitude higher O densities at ionospheric heights.

Effect on Plasma Outflows

The resulting O density falls from much larger to much smaller than the H density with altitude, the ‘crossover’ occurring in the topside ionosphere. Ion-neutral chemistry in this region is dominated by the reaction H + O+ <=> H+ + O. This ‘accidentally resonant’ charge exchange tends to maintain the ions in approximately the same mixing ratio as the neutral atoms of these species. Thus, H+ is favoured at altitudes where H is dominant, and upward flows of the dominantly O+ ionosphere through the crossover level produce H+ at the expense of O+. Moore (1980) pointed out this effect and suggested that rapid outflow through this region would be required to increase the O+ content of the outflow. Barakat et al. (1987) and Cannata and Gombosi (1989) studied this effect quantitatively and showed that it does exert a strong influence on the outflow composition, as shown in Figure 2. Because the velocity of ionospheric outflows generally increases with altitude, the elevated crossover level associated with high thermospheric temperatures leads directly to reduced production of H+ and increased content of O+ in the outflow.

Diurnal and Seasonal Effects

The daily rotation of the ionosphere under the Sun, and the seasonal variations in the inclination of Earth’s spin axis, lead to diurnal and seasonal variations in ionospheric density and composition with time scales and amplitudes commensurate with the response time of the ionosphere. The main diurnal variation of the ionospheric density is due to the presence in the day and the absence in the night of the ionization of the neutral gas by the solar UV and EUV radiation. This diurnal effect leads to a large local time variation in the density and mean altitude of the ionosphere. At latitudes below 50 , where the flux tubes are near co-rotation with Earth, this leads to draining of plasma from the high-altitude parts of the flux tubes on the night side, with compensating refilling flows on the day side. The time scale for this latter process is a few days in this region, so the amplitude of the diurnal variation at higher altitudes is small, but perceptible.

A seasonal version of the same effect is observed to produce hemispherically asymmetric conditions in the high altitude, closed field line extension of the ionosphere, the plasmasphere, except near the equinoxes. This affects the composition of the plasmasphere, producing He+ enrichments in the winter hemisphere, hemispherical asymmetries in the plasmasphere (Decreau et al., 1986), and He+ enrichment of plasma outflows (Raitt et al., 1978; Chandler et al., 1991).

Geomagnetic Activity and Solar Cycle Effects

Geomagnetic activity and the solar cycle variation of UV flux both influence the thermospheric temperature and consequently produce changes in the composition of the plasma available for outflow. The variation of thermospheric temperature over the course of a solar cycle ranges from 500 to 2000 K, changing the O scale height by a factor of four, leading to many orders of magnitude variation of O density at F-region peak heights. Combined with the H density drop in this range, the ionosphere is a significantly better source of O+ whenever or wherever the thermospheric temperature is raised. Geomagnetic activity produces thermospheric warming with a duration of up to days, while the solar cycle produces a long term increase in the capability of the ionosphere to provide O+ to the magnetosphere. This change appears to be reflected in the increasing mass flux with solar and/or geomagnetic activity at auroral latitudes. At constant ionospheric electron temperature of 7500 K, the increase of O+ outflow from solar minimum to solar maximum was found to be over an order of magnitude in a modeling study of Cannata and Gombosi (1989).


Plasma Circulation

At higher latitudes, beyond a critical boundary that varies in location with the strength of solar wind interaction, the topology of horizontal circulation streamlines changes abruptly from near co-rotation to a double-celled convection pattern for a southward z-component (Bz) of the interplanetary magnetic field (IMF) and a more complex convection pattern for northward IMF Bz. The typical features of this circulation have been well summarized by Heelis et al. (1982) and Heppner and Maynard (1987). This circulation pattern reflects the mapping of the magnetospheric boundary layer and internal circulation flows through the non-axisymmetric magnetic field to the ionosphere, and usually comprises an anti-sunward flowing region poleward of the auroral ovals and a sunward flowing region equatorward of the auroral ovals. As IMF varies from negative to positive, the distribution of the anti-sunward flow within the polar cap varies from peaked near the noon-midnight meridian for negative Bz, to minimizing and reversing in that region for positive Bz. Under the latter conditions, when there are strong non-uniformities in the polar cap, strong shears are often created in the flow, leading in turn to transpolar auroral features.

Auroral Zone Features

Independent of IMF conditions, the auroral zone is generally the region of flow reversal from anti-sunward to sunward flow. As such, it is the site of strong flow shear regions that are collocated with sheets of field-aligned electric currents flowing to or from the magnetospheric low latitude boundary layers. Auroral observations have often been interpreted as showing that auroral arcs are shear regions so strong that they break up into streets of vortices (Wescott et al., 1993). Space observations have directly confirmed this interpretation, showing evidence that auroral arcs sometimes consist of real plasma vortex streets (Lui et al., 1987a; Moore et al., 1996). Such features of the ionospheric flow field may be significant as sources of free energy in the system, the consequences of which for plasma escape will be discussed below.

Vertical Flows

The ionospheric ‘ground state’ exhibits, in addition to the horizontal or convective flows, the vertical flows which are the main topic of this article.

Low and Mid-Latitude Flows

The typical diurnal cycle in the mid-latitude (i.e. well below auroral latitudes) ionosphere and plasmasphere includes a significant transient triggered by sunrise on the ionosphere, as well as a diurnal ‘breathing’ associated with the varying sun-light and length of the flux tube. For O+ ions, the breathing velocities, especially during drainage from the plasmasphere, are predicted to be transonic, and standing acoustic deceleration shocks can be set up at mid-altitudes in the topside of each hemisphere, though this has not to date been clearly observed. A recent theoretical treatment of this region is given by Guiter et al. (1995).

Near the mean plasmapause position, the plasmasphere undergoes cycles of density erosion by enhanced magnetospheric circulation, followed by refilling to equilibrium densities, over time scales of a few days. The erosive part of the cycle transports ionospheric plasma out of the inner magnetosphere toward its boundary layers. The refilling part of the cycle is a case of transient polar wind outflow, mentioned here only to set the context for higher latitude plasma outflows.

High Latitude Outflows–Polar Wind

Within the high latitude (i.e. at auroral and polar cap latitudes) magnetospheric flow, plasma flux tubes undergo a circulation cycle whose time scale ranges from diurnal to much shorter than diurnal, depending on the strength of the solar wind interaction. During the course of this circulation cycle, a particular plasma flux tube is first stretched from 10 RE to 100 RE in length as it is convected anti-sunward in the low latitude boundary layer, or it may be disconnected from the conjugate hemisphere and connected into the solar wind during part of the cycle, permitting direct plasma exchange between the two media. Later, after reconnection in the tail if disconnection has occurred, the flux tube relaxes back to a length consistent with passage through the inner magnetosphere and back to its starting point. During each circulation cycle, the flux tube volume changes by a factor of order 10^4 .

During the stretch part of the cycle, or when the flux tube is connected to downstream solar wind regions (which pull a vacuum rather than filling the flux tube with solar plasma), ionospheric plasma expands freely into the flux tube as if there were a zero pressure upper boundary condition, introducing H+ /He+ plasma to the flux tube at a rate limited by the available source of plasma. The net result is a very low density, supersonic flux of cold light ions through the polar caps and into the magnetospheric lobes. This outflow is generally called the polar wind and has been observed at ionospheric topside heights by Brinton et al. (1971), Hoffman et al. (1974, 1980), and Blelly et al. (1992), at altitudes of about 1 R_E by Abe et al. (1993a, 1993b) and Yau et al. (1993), around 3 – 4 RE by Nagai et al. (1984) and Olsen et al. (1986) and most recently at altitudes up to 9 R_E by Moore et al. (1997). However, it has been noted by many of these authors that the cold supersonic light ion outflows are often accompanied by comparable fluxes of O+, even under conditions of low solar activity. Understanding this profound difference between predicted and observed outflow composition is the most important goal of research on ionospheric plasma source processes at high latitudes. It is generally agreed that special energizing processes must be involved to produce sizeable O+ ion outflow fluxes.

Photoelectron Effects

Ambipolar coupling of electron energy to the ion plasma is an intrinsic aspect of all polar wind theories and the proper handling of photoelectrons has been argued extensively over the history of polar wind research. An ambipolar potential drop will develop which is sufficient to limit the electron escape flux to the signed sum of the flux of ions escaping and electrons entering the system. Based on contemporary estimates of photoelectron content, Tam et al. (1995) concluded that a large flux of O+ is required to escape unless a supply of high altitude electrons is present. They computed an enhanced ambipolar potential distribution able to extract the required O+, using a one-dimensional flux tube model. However, Khazanov et al. (1997) assume that sufficient numbers of electrons will be available at high altitude to limit this effect, obtaining results more similar to observations (including enhanced O+ outflow) with the same photoelectron population as used by Tam et al. (1995). At present, the role of the high-altitude electrons seems unresolved and there is a need for a more realistic (at least two-dimensional) model of convecting flux tubes with a (magnetosheath) supply of electrons at the upper boundary. Three-dimensional models have been developed (Schunk and Sojka, 1989), but they do not presently account for photoelectrons or structure in the upper boundary condition.

Centrifugal Effects

Cladis (1986) first proposed the centrifugal force in the reference frame of the plasma convecting across the polar cap as an important contributor to the acceleration of O+ . Horwitz et al. (1994a) suggested that this acceleration process is sufficient to significantly enhance the O+ escape flows. Demare and Shunk (1996) confirmed the strong effect of the centrifugal acceleration on the O+ , but argued that it was only effective above a few RE altitude. Thus, some other energization process was required at lower altitude. They argued therefore that the centrifugal acceleration would add little O+ escape. Demars and Schunk cite observations of heating at lower altitudes as evidence against a significant role for centrifugal acceleration in the escape process. Thus it is likely that low altitude heating effects are dominant in enhancing the number flux of O+ escape. The centrifugal acceleration increases the parallel velocity of the O+ flow relative to the transverse or convective flow and enhances O+ penetration down the lobes into the tail of the magnetosphere (Moore and Delcourt, 1995).

Cool/Hot Plasma Contact Surfaces

The ‘ground state’ ionosphere is also influenced by the presence of hot magnetospheric or magnetosheath plasmas through many processes that exceed the scope of this article. Here we consider only the possible formation of contact surfaces or double layers between the cool ionospheric and hot magnetospheric plasmas. The interaction of these two plasmas with wide temperature differences can produce an electric potential difference as was first proposed by Hultqvist (1971). Lemaire and Scherer (1978) suggested that this effect could lead to a double layer above the ionosphere. The formation of a double layer was also reported in simulations of Barakat and Schunk (1984). They have shown that where the hot electron density is substantial and the energy is of the order of several keV, there exists a discontinuity in the kinetic solution. This discontinuity corresponds to a contact surface between the hot and cold electrons, at which a double-layer potential barrier reflects the cold ionospheric electrons and prevents their escape. Barakat and Schunk (1984) have also shown that if the hot/cold electron temperature ratio is small, the polar wind solutions are similar to those obtained previously without hot electrons. When the temperature ratio is high, the supersonic H+ and O+ ion outflow velocities are increased on passage through the surface.

Recent observations have shown that the polar wind is often variable in velocity, at times with amplitudes of a factor of three on time scales of a few minutes or less (Su et al., 1998; Moore et al., 1999), and that the variability is in some way related to the polar rain electron environment. It is likely that enhanced ambipolar potential drops and their spatio-temporal variability are responsible for these variations in polar wind outflow velocity.

Ionospheric Three-Dimensional Circulation

From the preceding, and since light ion polar wind outflows are a pervasive feature of the high latitude ionosphere, we might expect that corresponding slow O+ upflows should be a similarly pervasive feature of the high latitude ionosphere at F-region and topside heights (up to the H/O crossover altitude, just below which O+ is converted to H+). Observations show that, while there is ample evidence of generally upward O+ plasma flows at high latitudes, there is also a superposed enhancement of O+ upflow on the dayside with a downward flow in the nightside polar cap (Heelis et al., 1992). The dayside upward flows are well in excess of what is required to support light ion outflows, both in velocity and flux, even after reducing them by the amount of plasma contained in the nightside downward flows. The difference is thought to represent the amount of heavy ion plasma provided to the magnetosphere, that cannot be accounted for in terms of thermal polar wind processes.

At higher altitudes, only small amounts of O+-escape are anticipated from a ‘ground state’ ionosphere. Nevertheless, relatively cold O+ outflows have been observed to occur in the polar cap at a few RE altitude (Lockwood et al., 1985a; Horwitz and Lockwood, 1985; Waite et al., 1986; Abe et al., 1993a, 1993b), and in the tail lobes (Frank et al., 1977; Sharp et al., 1981; Candidi et al., 1988; Hirahara et al., 1996a). Based upon modeling of trajectories, these outflows were attributed to an origin in the dayside cleft region, with velocity dispersion throughout the polar cap for anti-sunward convection conditions there, and indeed a transition to downward flow was observed in many cases as the spacecraft moved nightward away from the cleft source region. It was clear that stronger convection (inferred from K) spread the O+ outflows tailward across the polarcap, and the IMF component was found to influence the dawn-dusk distribution of O+ outflows within the polar cap (Waite et al., 1986).

In contrast, observations by means of the Akebono spacecraft have prompted consideration of steady O+ outflow enhancements originating throughout the polar cap region, or at least the sunlit parts of it. This work has motivated renewed investigation of the possible role of photoelectrons in liberating the O+ to participate in the polar wind (Tam et al., 1995; Khazanov et al., 1997). It appears that some enhancement of O+ outflow is to be expected when photoelectrons are realistically treated. On the other hand, the plasmasphere contains only a minor component of O+ even though it is connected to ionospheric source regions that are fully illuminated by sunlight. Moreover, Chandler (1995) reported that the DE-1 measurements below 4000 km altitude show behaviour similar to that seen in the topside F-region, i.e. O+ downflow in the polarcap, at least for IMF Bz < 0. The same behaviour is seen in the observations in the 1RE altitude range by means of the Polar satellite. For IMF Bz > 0, the polar cap becomes very small and the remaining polar cap tends to be threaded by transpolar auroral arcs. Thus it may be possible that the O+ outflows, seen to originate from the nominal polar cap region during IMF Bz > 0 periods, are in fact consistent with a non-thermal auroral source. In any case, it is clear that the ionospheric flow field is fully three dimensional and that ion outflow features that originate from auroral processes are not confined to the auroral zone when the convection of outflowing ionospheric plasmas is considered.


(details forthcoming)

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