1. Introduction

[3]  Spacecraft measurements have shown that the solar corona expansion leads to formation of a supersonic plasma flow (solar wind) and extension of the solar magnetic field lines. The configuration of the interplanetary field near the ecliptic plane is mainly determined by the current in the closed heliospheric current sheet. The current sheet separates the regions with the field lines directed toward the Sun and away from the Sun. The structure of the heliospheric current sheet was studied at crossings of the sheet by spacecrafts. However, it is impossible to reveal a complete picture of the magnetic field distribution inside the sheet from individual crossings [Crooker et al., 2001; Smith, 2001]. The possibility of generation of current sheets in the corona was earlier reported by Pneuman and Kopp [1971], Somov and Syrovatsky [1971], and Podgorny and Podgorny [1995]. The interplanetary magnetic field in the vicinity of the current sheet was first considered in the two-dimensional geometry [Pneuman and Kopp, 1971; Somov and Syrovatsky, 1971]. In these papers, gravitation was neglected, which can be justified by a high thermal velocity of hydrogen ions in the corona, comparable with the cosmic velocity. However, the simplifying assumption of the existence of a neutral line above the arched magnetic lines is not correct. In the work of Pneuman and Kopp [1971], the plasma motion across the magnetic field was forbidden. With this condition, continuous evaporation from the Sun's surface would lead to an unlimited growth in the corona density in the regions with closed field lines if there is no plasma motion across the field lines. The main conclusions of these works on the absence of the normal magnetic field component in the current sheet and existence of two types of magnetic lines (closed and open) are even more dubious. The neutral sheet cannot exist for a long time because of instability [see, e.g., Laval et al., 1966]. The current sheet stability can be provided by the normal component and flow along the sheet [Podgorny and Podgorny, 1995], and therefore the heliospheric current sheet cannot be neutral. Some critical remarks on the formulation of the problem by Pneuman and Kopp [1971] were given by Koutchmy and Livshits [1992].

[4]  The consideration of the steady state flow of magnetized plasma of the corona taking into account the Sun's rotation but assuming that the corona is isothermal was given by Pisanko [1985]. However, like in the works of Pneuman and Kopp [1971] and Somov and Syrovatsky [1971], the normal component of the magnetic field in the current sheet was assumed to be zero.

[5]  The incorrectness of the conclusion on the absence of the normal magnetic field component in the heliospheric current sheet is obvious. Such a current cannot arise in a neutral ring stationary current sheet. To generate current in the sheet, the flow of a conducting material across the magnetic field must exist. In the MHD approximation, the Ohm's law has the form j/s = V B/c. The polarization electric field E = - V B/c arises when a bounded conductor moves across the field, because in this case j = 0 and there are charges of opposite signs at the conductor ends. In the ring current formed at radial expansion of the solar corona, charges move unhindered under the action of the Lorentz force e V B/c, and no polarization arises. The Ohm's law in the case of the ring current acquires the form j/s = V B/c. There are no other reasons for the ring current generation. It can be generated by the flow of a conducting material only in the case of the nonzero magnetic field component normal to the sheet B_n. Actually, in this case we have the MHD generator connected to a load with zero resistance. The current strength in this generator is determined by internal resistance. The presence of B_n means that division of the magnetic field lines into closed and open ones, as done by Pneuman and Kopp [1971], is incorrect. All the field lines emerging from a Sun's hemisphere enter the other hemisphere after crossing the current sheet.

[6]  MHD simulation of the stationary solar wind in the solar magnetic field by Usmanov [1993] has shown that there is no reason to believe that the current sheet is neutral, and the data reported by Usmanov [1999] clearly indicate that the normal magnetic field component exists. However, Usmanov [1993, 1999] adopted several frequently used simplifications. In particular, thermal balance and temperature gradient in the expanding plasma were ignored. Three-dimensional numerical MHD simulation of the solar corona expansion in the dipole magnetic field of the Sun [Minami et al., 2002; Podgorny et al., 4] taking into account dissipative processes and nonisothermality of plasma has revealed that the normal component of the magnetic field is an integral feature of the heliospheric current sheet. Below we present results of three-dimensional MHD simulation that demonstrate the role of gravitation in the corona expansion leading to formation of the current sheet. In the calculations, the rotation of the Sun that gives rise to the spiral structure of the interplanetary field is not taken into consideration. The spiral structure means that a radial component of the Lorentz force B_z wr/c appears, and, in addition to the azimuthal current component j_j, the sheet has a radial component j_r. Here, r is the distance to the Sun, and w is the angular frequency of the Sun's rotation. The idea that the radial current component must exist in the equatorial plane was first advanced by Alfvén [1981], and the generation of this component was demonstrated by Israelevich et al. [2001].

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