The threshold current for the ion-acoustic branch is shown to be significantly lower than the ioncyclotron branch and insensitive to the ion͞electron temperature ratio if there is a transverse gradient in the relative magnetic field aligned drift, V d , and j͑k y ͞k z ͒ ͑1͞V i ͒ ͑dV d ͞dx͒j is sufficiently large. The effect persists even when jdV d ͞dxj ! 0 provided ͑k z ͞k y ͒ ! 0, where k z and k y are wave vectors along and across the magnetic field and V i is the ion gyrofrequency. Therefore, the ionacoustic branch is more central to the plasma processes in the ionosphere than is currently believed.[S0031-9007 (97)05043-6] PACS numbers: 94.20. -y, 52.35.FpA fundamental reality throughout the space plasmas is the existence of magnetic field-aligned flows and currents. It is well known that a field-aligned current can support a host of plasma fluctuations and that these fluctuations can, in turn, affect the plasma steady state. The work of Kindel and Kennel [1], which considered the effects of a field-aligned current on ionospheric plasmas, has influenced and guided the interpretation and analysis of in situ observations for over two and a half decades. Kindel and Kennel show that, in an infinite homogeneous plasma, the threshold current necessary for the currentdriven electrostatic ion-cyclotron (CDEIC) instability [2] is the lowest for ionospheric conditions and, therefore, it is the most likely source for the observed plasma waves which correlate with a field-aligned current.Although there are some in situ ionospheric observations that support the classical CDEIC instability [3,4], a large number of them are at odds with it [5,6]. In particular, the observed signatures are often in the subcyclotron frequency range and resemble more closely the ion-acoustic branch [7][8][9]. A problem of identifying these as the ion-acoustic mode is that they occur for ion/electron temperature ratios of order unity or larger where the classical ion-acoustic modes are severely ion Landau damped [1,10], and, in addition, they are frequently observed for subthreshold currents. We have earlier shown that the inclusion of a localized transverse dc electric field can introduce substantial modifications to the ion-cyclotron wave properties and these modifications can better account for the observed signatures provided that the spatial gradient in the dc electric field is sufficiently strong [11]. In this Letter, we report that, even in the absence of a transverse dc electric field, an infinitesimal transverse gradient in the field-aligned flow can alter the plasma dispersion characteristics sufficiently and make the ion-acoustic branch dominant even when the ion temperature is greater than the electron temperature. This is in sharp contradiction to the behavior in a homogeneous plasma [1].The effect of a shear in the parallel ion drift was first addressed by D'Angelo [12], who showed the existence of a nonresonant instability whose real frequency is zero in the ion frame but whose growth rate depends on the spatial gradient in the p...

The polar wind is an ambipolar outflow of thermal plasma from the terrestrial ionosphere at high latitudes to the magnetosphere along geomagnetic field lines. The polar wind plasma consists mainly of H+, He+, and O+ ions and electrons. Although it was initially believed that O+ ions play a major role only at low altitudes, it is now clear from observations that relatively large amounts of suprathermal and energetic O+ ions are present in the polar magnetosphere. Recently, thermal O+ outflow has been observed at altitudes of 5000–10,000 km together with H+ and He+ ions. The polar wind undergoes four major transitions as it flows from the ionosphere to the magnetosphere: (1) from chemical to diffusion dominance, (2) from subsonic to supersonic flow, (3) from collision‐dominated to collisionless regimes, and (4) from heavy to light ion composition. The collisions are important up to about 2500 km, after which the ions and electrons exhibit temperature anisotropies. The direction of the anisotropy varies with geophysical conditions. The polar wind outflow varies with season, solar cycle, and geomagnetic activity. The O+ flux exhibits a summer maximum, while the H+ flux reaches a maximum in the spring. The He+ flux increases by a factor of 10 from summer to winter. At both magnetically quiet and active times the integrated H+ ion flux is largest in the noon sector and smallest in the midnight sector. The integrated upward H+ ion flux exhibits a positive correlation with the interplanetary magnetic field. In the sunlit polar cap the photoelectrons can increase the ambipolar electric field, which in turn increases the polar wind ion outflow velocities. The outflowing polar wind plasma flux tubes also convect across the polar cap. When the flux tubes cross the cusp and nocturnal auroral regions, the plasma can be heated and become unstable. Similar mixing of hot magnetospheric plasma with cold polar wind may result in instabilities. A number of free energy sources in the polar wind, including temperature anisotropy, relative drift between species, and spatial inhomogeneities, feed various fluid and kinetic instabilites. The instabilities can produce plasma energization and cross‐field transport, which modify the large‐scale polar wind outflow.

Anomalous electron heat fluxes and recent observations of day‐night asymmetries in polar wind features indicate that photoelectrons may affect polar wind dynamics. These anomalous fluxes require a global kinetic description (i.e., mesoscale particle phase space evolution involving microscale interactions); their impact on the polar wind itself requires a self‐consistent description. In this Letter, we discuss results of a self‐consistent hybrid model that explains the dayside observations. This model represents the first global kinetic collisional description for photoelectrons in a self‐consistent classical polar wind picture. In this model, photoelectrons are treated as test particles, ion properties are based on global kinetic collisional calculations, thermal electron features and the ambipolar field are determined by fluid calculations. The model provides the first global steady‐state polar wind solution that is continuous from the subsonic collisional regime at low altitude to the supersonic collisionless regime at high altitude. Also, the results are consistent with experiments in several aspects, such as order of magnitude of the ambipolar electric potential, qualitative features of the ion outflow characteristics, electron anisotropy and upwardly directed electron heat flux on the dayside.

Abstract. Recent space missions such as FAST and Freja report highly structured plasma flows along the magnetic field. Electrostatic fluctuations that can be supported by such inhomogeneous parallel flows are investigated. It is found that even a small transverse gradient in parallel flow can significantly reduce the critical value of the relative ion-electron field-aligned drift for the current-driven electrostatic ion acoustic modes. It is also shown that the shear-modified ion acoustic mode can be excited without any relative field-aligned drift provided that the flow gradient is sufficiently strong. The instability mechanism can be described

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