Abstract
Abstract. A physical model for the existence of bipolar structures in the electric field that are parallel to the magnetic field and observed in the auroral ionosphere, is established by deriving the "Sagdeev potential" from the two-fluid equations in a cylindrical coordinate system. The model shows that the bipolar electric field structure can develop not only from an ion acoustic wave, but also from an ion cyclotron wave, when the Mach number and the initial electric field satisfy certain conditions. Moreover, in the auroral region, it shows that the polarity of this electric field structure can be oriented either negative to positive or the reverse polarity, its amplitude can be varied from 35 to 330 mV/m, and its duration can be 7 ms to 23 ms. These results are in agreement with observation. Therefore, our two-fluid model can interpret the bipolar structures observed in the auroral ionosphere.
Highlights
The Cluster satellites have observed Electric Field Solitary (EFS) waves, moving parallel to the magnetic field direction, in many space plasma regions, such as the solar wind, magnetosheath, magnetotail, and in the auroral zone (Pickett et al, 2004; Marklund et al, 2004)
Mozer et al (1997), for example, reported a Polar satellite observation of electrostatic solitary waves (i.e. EFS) with amplitudes larger (∼200 mV/m) than any previously reported in the auroral acceleration region, moving along the magnetic field lines at frequencies associated with the proton cyclotron mode
We obtain, ω2=a Cs2k2 and ω2= 2i, which show that an ion acoustic wave and an ion cyclotron wave, respectively, can be excited in the plasma described by our model and can probably develop into nonlinear waves
Summary
The Cluster satellites have observed Electric Field Solitary (EFS) waves, moving parallel to the magnetic field direction, in many space plasma regions, such as the solar wind, magnetosheath, magnetotail, and in the auroral zone (Pickett et al, 2004; Marklund et al, 2004). Shukla and Yu (1978) and Ray (1979) showed that finite amplitude ion acoustic density solitons propagating obliquely to an external magnetic field can occur. By considering the structure of bipolar electric field solitary waves, we present a two-fluid model for ions in a low-β magnetized plasma with cylindrical symmetry and derive the “Sagdeev potential” from the two-fluid equations. The results from our model show that the bipolar electric field structure propagating along the magnetic field line can develop from ion acoustic waves and from ion cyclotron waves.
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