ИСТИНА

Direct measurements of the interplanetary magnetic field on spacecraft have shown that Solar wind magnetic clouds are quite often the observed type of coronal plasma flows [Kilpua et al., 2013]. Interest in the study of magnetic clouds is associated with their high geoeffectiveness. The structure of magnetic cloud can be divided into a shock wave at the front of the cloud, a turbulent sheath between the shock wave and the leading edge of the magnetic cloud, the leading field and the axial field contained in the cloud body, and the tail field. Geomagnetic disturbance is usually associated with the cloud body parameters and the conditions for the Earth's magnetosphere passage through it. In the works [Farrugia et al., 1998; Barkhatov et al., 2011] noted however that the presence of a sheath in magnetic clouds makes noticeable adjustments to the predicted level of geomagnetic disturbance. In this regard, the analysis of parameters evolution for cloud shells is important to determining their geoeffectiveness. In the present work the ULF spectral composition for sheathes and bodies front edge of magnetic clouds of varying length for 33 events was investigated. An analysis of the dynamics of the interplanetary magnetic field modulus, Solar wind velocity and concentration made it possible to establish the cloud sheathes boundaries and mark the transition areas to the cloud body. For all cases when going from the sheath to the cloud body, a sharp weakening of disturbance intensity for all analyzed parameters is noted. The characteristic range of recorded ULF disturbances periods in the event sheathes is from 15 to 25 minutes in the low-frequency spectrum band and about 5 minutes for the high-frequency band. In the bodies of the events studied, both high-frequency disturbances with characteristic periods of 2.5–10 minutes and low-frequency ones with periods 15–25 minutes are also recorded. A detailed analysis of study results Fourier and wavelet spectra for all events sheathes demonstrates significant difference between the two selected groups. As it turned out, the first group included clouds with small length sheathes, less than 2 hours (2 events) and the second group - clouds with long sheathes for more than 2 hours (31 events). The frequency-spatial distribution of disturbance intensity in the cloud sheathes shows that intense disturbances are recorded for the first group events throughout all sheathes with a spectral maxima cascade to the high-frequency region with gradual damping of turbulent processes on a small scale. This confirms the presence of a turbulent regime in the entire sheath for such cloud. Analysis of the wavelet spectra over the sheath demonstrates an increase in the disturbances frequency to their back end. It should be noted that the most intense oscillations of magnetic field occur at the sheath leading edge forfirst group events, while the ULF disturbances ofSolar wind concentration and velocity have the most intense spectral maxima in the sheath middle and back parts. This result is consistent with [Kilpua et al., 2013], where it is argued that ULF oscillations for interplanetary magnetic field and pressure have different profiles inside sheath: for the ULF magnetic field they are strongest in the sheath leading part, while pressure is increasing to their back end. More extended sheathes (second group events) are structures with strong fluctuations of magnetic field, Solar wind velocity and concentration, exceeding their values in the cloud body. Such sheathes are characterized by disturbances in the range 5–25 minutes, and there is a spatial distribution of spectral maxima over the sheath — magnetic field and Solar wind parameters disturbances follow a series. Within each series, cascades of spectral maxima to the high-frequency region are recorded. It indicates that the turbulent regime is not in the whole sheath, but only in these areas. This may mean that long sheathes are not fully turbulent areas. The studies were carried out with the financial support of the Russian Foundation for Basic Research, Project No. 18-35-00430 (Barkhatova O.M., Dolgova D.S.), and within the framework of the State Mission of the Ministry of Education and Science of the Russian Federation No. 5.5898.2017 / 8.9 (Barkhatov N.A., Revunov S.Е.). References FarrugiaC.J. ,et . al., Geoeftectiveness of three Wind magnetic clouds: a comparative study // J. Geophys. Res., V.103, N А8, Р.17261-17278, 1998. Kilpua E. K. J., Isavnin A., Vourlidas A., Koskinen H. E. J., and Rodriguez L. On the relationship between interplanetary coronal mass ejections and magnetic clouds // Ann. Geophys., 31, 1251–1265, 2013. doi:10.5194/angeo-31-1251-2013. Barkhatov N. A., Levitin A. E., Revunova E. A. Geomagnetic Storm Intensity Forecast Caused by Magnetic Clouds of Solar Wind // Geomagnetism and Aeronomy. 2014. V. 54. N. 6. P. 718–726. © Pleiades Publishing, Ltd., 2014. DOI: 10.1134/S001679321406005X