Solar wind variation with the cycle

The cyclic evolution of the heliospheric plasma parameters is related to the time-dependent boundary conditions in the solar corona. “Minimal” coronal configurations correspond to the regular appearance of the tenuous, but hot and fast plasma streams from the large polar coronal holes. The denser, but cooler and slower solar wind is adjacent to coronal streamers. Irregular dynamic manifestations are present in the corona and the solar wind everywhere and always. They follow the solar activity cycle rather well. Because of this, the direct and indirect solar wind measurements demonstrate clear variations in space and time according to the minimal, intermediate and maximal conditions of the cycles. The average solar wind density, velocity and temperature measured at the Earth’s orbit show specific decadal variations and trends, which are of the order of the first tens per cent during the last three solar cycles. Statistical, spectral and correlation characteristics of the solar wind are reviewed with the emphasis on the cycles.     

Global Variations and Asymmetry of the Sun During Extremely High Activity in October–November 2003

The data of the Coronas F satellite and other spacecraft were used to show that, in October–November 2003, global variations in brightness occurred in all spectral regions of the solar electromagnetic radiation. The variations were asymmetric in heliolongitude. This phenomenon was accompanied by an extremely strong energy release in the form of coronal-mass ejections and solar flares. The most powerful of them took place on the solar side that was characterized by an enhanced brightness even before these events. As a result, superimposed corotating and sporadic disturbances, which are partly correlated, can be traced in parameters of the solar atmospheric radiation, flows of the solar-wind plasma, and heliospheric magnetic field.__________Translated from Astronomicheskii Vestnik, Vol. 39, No. 3, 2005, pp. 195–201.Original Russian Text Copyright © 2005 by Veselovsky, Dmitriev, Zhitnik, Zhukov, Zel’dovich, Kuzin, Naumkin, Persiantsev, Ryazanov, Shugai, Yakovchuk, Bogachev, Shestov.     

Forecast of the solar wind velocity and the interplanetary magnetic field radial component polarity at the phase of decay of solar cycle 23

The solar wind velocity and polarity of the B _x-component of the interplanetary magnetic field have been analyzed for the first eight months of 2005. The interplanetary magnetic field had a four-sector structure, which persisted during nine Carrington rotations. Three stable clusters of a high-speed solar wind stream and one cluster of a low-speed stream were observed during one solar rotation. These clusters were associated with the interplanetary magnetic field sectors. The predicted solar wind velocity was calculated since July 2005 one month ahead as an average over several preceding Carrington rotations. The polarity of the B _x-component of the interplanetary magnetic field was predicted in a similar way based on the concept of the sector structure of the magnetic field and its relation to maxima of the solar wind velocity. The results indicate a satisfactory agreement of the forecast for two rotations ahead in July–August 2005 and pronounced violation of agreement for the next rotation due to a sudden reconfiguration of the solar corona and strong sporadic processes in September 2005.     

On the lack of any statistically significant effect of Mercury on the solar wind velocity near the orbit of the Earth

The notion that Mercury modulates considerably the solar wind velocity at the orbit of the Earth (Nikulin, 2014) is erroneous. It is not grounded in experimental data. Quantitative estimates also suggest that this effect should be negligible at such large distances from a planet that small. The assertion that this effect may be used in practice to improve the accuracy of prediction of the solar wind velocity (Nikulin, 2014) is unfounded as well: no credible observational and theoretical evidence in favor of it has been offered.     

Fast damping of acoustic waves in solar coronal loops as a result of their radiation

The problem of the observed fast damping of longitudinal waves in solar coronal magnetic loops is studied. According to some data, the radiation effect, which plays a key role in the cooling of coronal loops that are observed in the extreme UV wavelength range, is used to explain this phenomenon. The dispersion relation for slow magnetosonic modes of a cylindrical magnetic tube has been obtained with regard to the radiation effect. It has been indicated that the radiation actually results in the fast damping of the slow magnetosonic modes of coronal magnetic loops.     

Hierarchical approach to forecasting recurrent solar wind streams

The hierarchical approach to predicting quasi-stationary, high-speed solar wind (SW) streams is described. This approach integrates various types of data into a single forecasting system by means of an ensemble of experts. The input data included the daily values of the coronal hole areas, which were calculated from the ultraviolet images of the Sun, and the speed of the SW streams during the previous solar rotations. The coronal hole areas were calculated from the images taken by the SWAP instrument aboard the PROBA2 satellite in the spectral interval centered at a wavelength of 17.4 nm and by the AIA instrument aboard the SDO spacecraft in the interval of wavelengths centered at 19.3 and 17.1 nm. The forecast was based on the data for 2010, corresponding to the rising phase of the 24th solar cycle. On the first hierarchical level, a few simple model estimates were obtained for the speed of the SW streams from the input data of each type. On the second level of hierarchy, the final 3 day ahead forecast of the SW velocity was formulated on the basis of the obtained estimates. The proposed hierarchical approach improves the accuracy of forecasting the SW velocity. In addition, in such a method of prediction, the data gaps in the records of one instrument do not crucially affect the final result of forecasting of the system as a whole.     

Model of the magnetic field in the inner heliosphere with regard to radial field strength leveling in the solar corona

On the basis of experimental results from the Ulysses spacecraft, a model is proposed for calculating the magnetic field in the corona and the heliosphere in the potential approximation, which is a modification of the potential-field source-surface model. In addition to the photospheric surface and the source surface, a new demarcated spherical surface (leveling surface) is introduced in this model. The magnitude of the radial component of the magnetic field on this surface is assumed to be constant, and its sign abruptly changes from one hemisphere to another. General analytical formulas are given to calculate the potential and field for this model. Calculations are described in detail using the dipole and quadrupole harmonics as examples. Expressions are obtained for the surface currents. The results of visualization of the magnetic field for an axial dipole are discussed.     

Origin of the solar wind: Astrophysical and plasma-physical aspects of the problem

At what evolutionary stage of the Sun as a star the accumulation of its matter from the interstellar medium ended and the dominant outflow of solar wind (SW) streams started is one of the main outstanding questions in the astrophysical aspect of the problem of SW origin. It is unknown when and how this happened in detail, although the onset of thermonuclear reactions in the solar interior undoubtedly played a crucial role in the energetics and dynamics of the star, which could lead to such a change of the regimes. Therefore, it is hypothesized that the accretion or plasma outflow from the star was determined by the preceding evolution, i.e., by the “memory“ and not just by the distribution of instantaneous density, temperature, magnetic field, and other macroscopic parameters of the system that consists of this star, its nearest stellar environment, and interstellar gas. Depending on this, neighboring stars can serve as donors or acceptors of interstellar gas. Some of them can simultaneously or alternately play both roles. The polytropic solution for centrally symmetric flows obtained by Bondi is degenerate in the sign of the radial velocity. It is suitable for describing the quasi-stationary regimes of both types. However, the theory of transient processes has not been developed. Hence, the question of whether there exist stars similar in their internal structure and parameters to the present-day Sun but without stellar wind emission or even with interstellar gas accretion can be answered only observationally. The possibility that such stars exist is not ruled out; it does not contradict any laws of nature. The plasma-physical aspect of the problem of SW origin concerns much shorter time intervals than the main evolutionary time scale measured in billions of years. Hence, this aspect of the problem has been studied much better, although it is also fairly complex and has not yet been solved in much detail due to the multi-scale character of the flow formation processes. The SW as a permanent supermagnetosonic plasma outflow in the radial direction appears against the background of much more powerful nonequilibrium and unsteady motions, which are ordered only partially in the upper solar atmosphere and corona (turbosphere). The instantaneous SW state is controlled by the fluxes of free energy, matter, and momentum that enter the corona from the convection zone and underlying solar atmospheric layers. Although the main physical mechanisms of the transport of free energy of the electromagnetic field and plasma are generally well known, they need a quantitative analysis as applied to specific realizations in frequent and rare particle collisions in the solar corona to ascertain the nonlocal processes of the formation of fields and plasma flows, including the SW.