We investigate quasi-adiabatic dynamics of charged particles in strong current sheets (SCSs) in the solar wind, including the heliospheric current sheet (HCS), both theoretically and observationally. A self-consistent hybrid model of an SCS is developed in which ion dynamics is described at the quasi-adiabatic approximation, while the electrons are assumed to be magnetized, and their motion is described in the guiding center approximation. The model shows that the SCS profile is determined by the relative contribution of two currents: (i) the current supported by demagnetized protons that move along open quasi-adiabatic orbits, and (ii) the electron drift current. The simplest modeled SCS is found to be a multi-layered structure that consists of a thin current sheet embedded into a much thicker analog of a plasma sheet. This result is in good agreement with observations of SCSs at ∼1 au. The analysis of fine structure of different SCSs, including the HCS, shows that an SCS represents a narrow current layer (with a thickness of ∼104 km) embedded into a wider region of about 105 km, independently of the SCS origin. Therefore, multi-scale structuring is very likely an intrinsic feature of SCSs in the solar wind.

In this paper we examine the dynamic response of a magnetoplasma to an external time-dependent current source in the context of electronmagnetohydrodynamics (EMHD). A combined analytic and numerical technique is developed to address this problem. The set of cold electron plasma and Maxwell’s equations are first solved analytically in the (k,ω) space. Inverse Laplace and three-dimensional complex Fast Fourier Transform techniques are used subsequently to numerically transform the radiation fields and plasma currents from the (k,ω) space to the (r,t) space. The results show that the electron plasma responds to a time-varying current source imposed across the magnetic field by exciting whistler/helicon waves and forming an expanding local current loop, driven by field-aligned plasma currents. The current loop consists of two antiparallel field-aligned current channels concentrated at the ends of the imposed current and a cross-field Hall current region connecting these channels. The characteristics of the current closure region are determined by the background plasma density, the magnetic field, and the time scale of the current source. The results are applied to the ionospheric generation of extremely low-frequency (ELF) and very low-frequency (VLF) radiation using amplitude modulated high-frequency heating. It is found that contrary to previous suggestions the dominant radiating moment of the ELF/VLF ionospheric source is an equivalent horizontal magnetic dipole.

The interaction of the low frequency (10−2 Hz) MHD waves, observed upstream of comets, with the structured plasma near the cometary bow wave is examined. It is suggested that the waves undergo resonant absorption due to either ambient density gradients or localized shear in the background magnetic field. The absorption process can give rise to rapid heating of the solar wind protons, in agreement with observations from comet Halley. Since the free energy for the generation of MHD waves came from deceleration (without accompanying heating) of the solar wind protons during the pick up of cometary ions, the subsequent reabsorption of the energy is equivalent to a nonlocal transformation of ordered to random energy and can be described as nonlocal viscosity.

The satellite observations at comet Halley have shown strong heating of solar wind alpha particles over an extended region dominated by high-intensity, low-frequency turbulence. These waves are excited by the water group pickup ions and can energize the solar wind plasma by different heating processes. The alpha particle heating by the Landau damping of kinetic Alfven waves and the transit time damping of low-frequency hydromagnetic waves in this region of high plasma beta are studied in this paper. The Alfven wave heating was shown to be the dominant mechanism for the observed proton heating, but it is found to be insufficient to account for the observed alpha particle heating. The transit time damping due to the interaction of the ions with the electric fields associated with the magnetic field compressions of magnetohydrodynamic waves is found to heat the alpha particles preferentially over the protons. Comparison of the calculated heating times for the transit time damping with the observations from comet Halley shows good agreement. These processes contribute to the thermalization of the solar wind by the conversion of its directed energy into the thermal energy in the transition region at comet-solar wind interaction. IntroductionA dominant feature of the solar wind interaction with comets Giacobini-Zinner and Halley is the presence of highintensity, low-frequency magnetohydrodynamic (MttD) turbulence over an extended region. At comet Giacobini-Zinner the amplitudes of the MttD fluctuations detected by the magnetometer and plasma instruments aboard the ICE spacecraft were as high as 104 nT2/I-Iz and extended over a region of size > 106 km [Tsurutani and Smith, 1986a]. At comet Halley the fluctuations had smaller amplitudes and were detected by the instruments aboard the satellites Giotto [Glassmeier et al., 1987; Johnstone et al., 1987], Sakigake [Yumoto et al., 1986], and VEGA [Galeev et al., 1986]. The high-intensity turbulence in the comet-solar wind interaction region is excited by the cometary ions picked up by the solar wind and is thus an e•ted feature of the cometary magnetosphere [Sagdeev et al., 1986]. The presence of high-intensity turbulence in the transition region separating the solar wind from the cometary plasma makes the comet-solar wind interaction quite different from the case of planets [Galeev, 1987; Hizanidis et al., 1988; Sharma et al. Tsurutani, 1991]. In the familia_r terrestrial case the transition from supermagnetosonic to submagnetosonic flow takes place through the formation of a bow shock across which there is a sharp change in the magnetic field and plasma parameters. The transition in the case of comets is more complex, and while the existence of a shock structure is a topic of debate, it is clear that the region is dominated by intense low-frequency fluctuations. Before the spacecr• encounters with comets Giacobini-Zinner and Halley it was recogni?ed that the shock, when it existed, would be weak and also different from the planetary bow shocks [Wallis, 1972;Omidi et al., 1...

Abstract. Riometers monitor the changes in ionospheric conductivity by measuring the absorption of very high frequency radio noise of galactic origin passing through the ionosphere. In this Letter the absorption of radio signals by a thin layer of ionospheric plasma, produced by ionization due to energetic precipitating electrons, is modeled by taking into account strong turbulent heating caused by instabilities. The precipitating electron population is obtained from a global MHD simulation of the magnetosphere, along with the electric fields which excite the Farley-Buneman instability and lead to turbulent electron heating. A comparison, the first of its kind, of the data from polar and sub-auroral riometers for the magnetic cloud event of January 10, 1997 shows good agreement. The ionospheric conductance modified by turbulent electron heating can be used to improve the magnetosphereionosphere coupling in the current global MHD models.