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The results from a statistical analysis of solar wind data obtained near the Earth’s orbit by the DSCOVR spacecraft for the period of one year starting August 2016 are discussed. The distribution of the solar wind temperature T for this period was bimodal and exhibited two maxima at 15 × 103 and 190 × 103 K (with a standard deviation of the same order of magnitude, respectively). This distribution can be represented as the sum of two lognormal distributions. The first peak of the T distribution is caused by the slow-cold wind component, the distributions of which over speed V and density n (varying in wide ranges) are also close to lognormal. This component covers slightly more than a quarter of the entire time period. A fast hot wind (the distribution maximum is at 310 × 103 K) can be distinguished from the remainder; this component covers slightly less than a quarter of the period. Its T, V, and n distributions are also close to lognormal. The rest the wind, nearly half, exhibits a complex “jagged” distribution over speed and a double-peak distribution over density. The conclusion is that data averaging is of key importance in the classification of solar wind at the quantitative level. This is shown by the results of data averaging over a minute, hour, day, and week.

The paper considers the classification of solar wind streams in magnetohydrodynamic parameters (MHD-types); combinations of proton speed, density, temperature, and magnetic field strength, in addition to the classical solar wind separation into high-speed streams from coronal holes, transient streams of coronal mass ejections, and the slow solar wind from the streamer belt. Two classifications of solar wind properties were compared for the events in August 2010 and May 2011, when one could observe the interaction of two coronal mass ejections and a coronal mass ejection with a high-speed solar wind stream from the coronal hole, respectively. It is shown that the classical description of a large-scale structure of wind streams (the ion wind composition especially) in scales of hours and days allows one to determine the type and source of streams, whereas the MHD-parameters allow one to more accurately describe the small-scale structure (in minutes), especially in the cases of several streams interaction in the heliosphere. The detailed study of a small-scale structure of stream interaction regions provides the information required for developing MHD-models describing the processes of propagation and interaction of streams in the heliosphere and for predicting their geoefficiency.

At the present time, there is no generally accepted classification of the solar wind flows. There are various approaches to this problem depending on the goal of the study. In our paper, we propose the binary classification of the solar wind types by the main hydrodynamic parameters (velocity, temperature, and density) based on the statistical analysis of the solar wind. The analysis of the OMNIWeb one-minute data is performed for the period from 1996 to 2017, which encompasses solar cycle 23 and current solar cycle 24. Eight types of the solar wind are distinguished: fast-hot-dense, fast-hot-rarefied, fast-cold-dense, fast-cold-rarefied, slow-hot-dense, slow-hot-rarefied, slow-cold-dense, slow-cold-rarefied. These types occur with different frequency and are the consequences of different manifestations of solar activity. Of particular interest are the solar wind flows, the parameters of which deviate from the averages most significantly.

A binary classification of solar wind types is given according to three main hydrodynamic parameters—the proton speed, the proton temperature, and the proton density. Boundaries between the fast–hot–dense, fast–hot–rarefied, fast–cold–dense, fast–cold–rarefied, slow–hot–dense, slow–hot–rarefied, slow–cold–dense, and slow–cold–rarefied solar wind types are specified in the space of these variables. These types arise at different frequencies and originate from different sorts of solar activity. An analysis of 1-minute data from the OMNIWeb database is performed over the period from January 1, 2016, to September 30, 2017. In some cases, this classification scheme, which is quite simple and clear, allows one to identify the sources of the solar wind (coronal holes and coronal mass ejections), as well as to reveal long-term trends. The proposed scheme is compared with the methods used earlier in the literature to classify solar wind types.

This paper discussed whether 17th Century observers left historical records of the plasma tails of comets that would be adequate to enable us to extract the physical parameters of the solar wind. The size of the aberration angle between a comet’s tail and its radius-vector defines the type of the tail: plasma or dust. We considered Bredikhin’s calculations of the parameters for 10 comet tails observed during the Maunder minimum (1645 – 1715). For those comets the angle between the tail’s axis and the radius-vector on average exceeded the value of 10° that is typical for dust tails. It was noted that visual observations of the ion tails of comets are very difficult to make owing to the spectral composition of their radiation, confirming the conclusion that observations of comet tails made in the 17th Century are not suitable for deriving past values of the physical parameters of the solar wind.

We have considered the possible causes of discrepancies between the predicted and observed at 1 AU parameters of the recurrent solar wind (SW) streams in the maximum of the 24th solar cycle. These discrepancies have been observed in both the SW velocity profile and the SW stream arrival time, as well as in the absence of the expected high-speed SW stream. The degree of discrepancy depends on the model used for the SW prediction; however, in some cases, different prediction methods provide a similar discrepancy with the observed SW parameters at 1 AU. For several cases, we show that the probable cause of the discrepancies can be a deflection of the high-speed SW stream from the radial direction due to the interaction with the transient SW streams at certain configuration of the magnetic fields of high-speed and transient SW sources in the solar corona.

In the context of studying the problem of simulation of magnetic fields on the Sun, the structure of the field in the vicinity of two circular current loops with different mutual arrangement in space is considered. When the symmetry in the arrangement is sufficient, a system of magnetic surfaces created by the closed field lines arises. With a reduction in symmetry, isolated closed lines may exist. For the case of two identical current loops coupled perpendicularly, it is shown that the subsystems of these lines may be ordered in space in a complex manner. At large distances, a system of loops is equivalent to a dipole with a high degree of accuracy, while an approximate winding of the lines on the deformed toroids, encircling each of the loops, occurs at small distances. At intermediate distances, there are regions of both ordered and chaotic behavior of field lines. Results were obtained with the use of the numerical simulation method.

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.

This paper addresses the problem of the solar wind behaviour during the Maunder minimum. Records on plasma tails of comets can shed light on the physical parameters of the solar wind in the past. We analyse descriptions and drawings of comets between the eleventh and eighteenth century. To distinguish between dust and plasma tails, we address their colour, shape, and orientation. Based on the calculations made by F.A. Bredikhin, we found that cometary tails deviate from the antisolar direction on average by more than $10^{\circ }$, which is typical for dust tails. We also examined the catalogues of Hevelius and Lubieniecki. The first indication of a plasma tail was revealed only for the great comet C/1769 P1.

We analyzed statistics, solar sources and properties of interplanetary coronal mass ejections (ICMEs) in the solar wind. In comparison with the first eight years of Cycle 23, during the same period of Cycle 24 the yearly numbers of ICMEs were less correlated with the flare numbers (0.68 vs 0.78) and sunspot numbers (0.66 vs 0.81), whereas the ICME correlation with coronal mass ejections (CMEs) was higher (0.77 vs 0.70). For the period January 2010 -- August 2011, we identified solar sources of the ICMEs included in the Richardson and Cane list. The solar sources of ICME were determined from coronagraph observations of the Earth-directed CMEs supplemented by modeling of their propagation in the heliosphere using the kinematic models and the WSA Enlil Cone MHD-based model. A detailed analysis of the ICME solar sources in the period under study showed that in 11 cases out of 23 the observed ICME might be associated with two or more sources. In cases of multiple-source events, the resulting solar wind disturbances may be described as complex (merged) structures occurred due to the stream interactions with properties depending on the type of participating streams. As a reliable marker for identification of interacting streams and their sources, we used the plasma ion composition, as it becomes frozen in the low corona and remains unchanged in the heliosphere. According to the ion composition signatures, we classified these cases into three types: complex ejecta originating from weak and strong CME-CME interactions, as well as merged interaction regions originating from the CME-high-speed stream interactions. We described temporal profiles of the ion composition for the single-source and multi-source solar wind structures and compared them with the ICME signatures determined from the kinematic and magnetic field parameters of the solar wind., 27 pages, 6 figures

General principles of describing secondand higher-order null points of a potential magnetic field are formulated. The potential near a second-order null of the general form can be specified by a linear combination of four basic functions, the list of which is presented. Near secondand higher-order null points, field line equations often cannot be integrated analytically; however, in some cases, it is possible to present a qualitative description of the geometry of null vicinities with consideration of the behavior of field lines near rays outgoing from null, at which the field is radial or equals zero.

The most powerful coronal mass ejection of the 24th solar cycle took place on the opposite side of the Sun on July 23, 2012 and had no geomagnetic consequences. Nevertheless, as a result of passing of the ejection through the heliosphere, variations of galactic cosmic rays flux were observed on the Earth. These variations were registered by the muon hodoscope URAGAN (MEPhI, Moscow). Muon flux angular distributions on the Earth’s surface are reported and analyzed.

This work is a continuation of our previous articles (Yermolaev et al. in J. Geophys. Res. 120, 7094, 2015; Yermolaev et al. in Solar Phys. 292, 193, 2017; Yermolaev et al. in Solar Phys. 293, 91, 2018), which describe the average temporal profiles of interplanetary plasma and field parameters in large-scale solar-wind (SW) streams: corotating interaction regions (CIRs), interplanetary coronal mass ejections (ICMEs including both magnetic clouds (MCs) and ejecta), and sheaths as well as interplanetary shocks (ISs). In this work we analyze the average profile of helium abundance N{\alpha}/Np for interval of 1976-2016. Our results confirm main early obtained results: N{\alpha}/Np is higher in quasi-stationary fast streams than in slow ones; it slowly changes in compression regions CIRs and Sheaths from values in undisturbed solar wind to values in corresponding type of piston (HSS or ICME); in Ejecta it is closed to abundance in undisturbed streams and it is maximal in MCs. We obtained for first time that N{\alpha}/Np correlates with plasma {\beta}-parameter in compression regions CIRs and Sheaths and anti-correlates in ICMEs. The N{\alpha}/Np vs. {\beta} dependence is stronger in MCs than in Ejecta and may be used as an indicator of conditions in CME formation place on the Sun., Comment: 12 pages, 6 figures, calculations for new OMNI2 dataset version (without ACE helium measurements), results unchanged

We investigate the case of disagreement between predicted and observed in-situ parameters of the recurrent high-speed solar wind streams (HSSs) existing for Carrington rotation (CR) 2118 (December 2011) in comparison with CRs 2117 and 2119. The HSSs originated at the Sun from a recurrent polar coronal hole (CH) expanding to mid-latitudes, and its area in the central part of the solar disk increased with the rotation number. This part of the CH was responsible for the equatorial flank of the HSS directed to the Earth. The time and speed of arrival for this part of the HSS to the Earth were predicted by the hierarchical empirical model based on EUV-imaging and the Wang-Sheeley-Arge ENLIL semi-empirical replace model and compared with the parameters measured in-situ by model. The predicted parameters were compared with those measured in-situ. It was found, that for CR 2117 and CR 2119, the predicted HSS speed values agreed with the measured ones within the typical accuracy of ±100 km s−1. During CR 2118, the measured speed was on 217 km s−1 less than the value predicted in accordance with the increased area of the CH. We suppose that at CR 2118, the HSS overtook and interacted with complex ejecta formed from three merged coronal mass ejections (CMEs) with a mean speed about 400 km s−1. According to simulations of the Drag-based model, this complex ejecta might be created by several CMEs starting from the Sun in the period between 25 and 27 December 2011 and arriving to the Earth simultaneously with the HSS. Due to its higher density and magnetic field strength, the complex ejecta became an obstacle for the equatorial flank of the HSS and slowed it down. During CR 2117 and CR 2119, the CMEs appeared before the arrival of the HSSs, so the CMEs did not influence on the HSSs kinematics.

We present study of the complex event consisting of several solar wind transients detected by Advanced Composition Explorer (ACE) on 4 -- 7 August 2011, that caused a geomagnetic storm with Dst$=-$110 nT. The supposed coronal sources -- three flares and coronal mass ejections (CMEs) occurred on 2 -- 4 August 2011 in the active region (AR) 11261. To investigate the solar origin and formation of these transients we studied kinematic and thermodynamic properties of the expanding coronal structures using the Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA) EUV images and the differential emission measure (DEM) diagnostics. The Helioseismic and Magnetic Imager (HMI) magnetic field maps were used as the input data for the 3D magnetohydrodynamic (MHD) model to describe the flux rope ejection. We characterize the early phase of the flux rope ejection in the corona, where the usual three-component CME structure formed. The flux rope ejected with a speed of about 200~km~s$^{-1}$ to the height of 0.25~R$_{\odot}$. The kinematics of the modeled CME front agrees well with the Solar Terrestrial Relations Observatory (STEREO) EUV measurements. Using the results of the plasma diagnostics and MHD modeling, we calculated the ion charge ratios of carbon and oxygen as well as the mean charge state of iron ions of the 2 August 2011 CME, taking into account the processes of heating, cooling, expansion, ionization, and recombination of the moving plasma in the corona up to the frozen-in region. We estimated a probable heating rate of the CME plasma in the low corona by matching the calculated ion composition parameters of the CME with those measured in situ for the solar wind transients. We also consider the similarities and discrepancies between the results of the MHD simulation and the observations., Comment: Title and abstract changed, figure 4c modified, figures' and tables' captions changed, Equations 5-7 modified, proofread was made, text revised (p.2,3,4,5,7,8,24,26). The final publication is available at Springer

The self-consistent 1D kinetic Harris-like model of a collisionless current sheet is developed for the case of the current sheet experiencing the impact of an external uniform gravity field. The ambipolar Pannekoek-Rosseland electric field appears in the system as a result of the additional drift motion of ions and electrons. This produces separation of charges, which is responsible for corresponding changes of the current sheet form. The presence of gravitation leads to formation of asymmetric distributions of the magnetic field as well as the plasma and the current density changes. Our estimations show that gravity-forced disruptions of the current sheet profile may occur in the Mercurial magnetosphere and, most probable, in the Io plasma torus near the Jupiter. Also, the model can be applied to magnetospheres of exoplanets.

The coronal mass ejection (CME) that occurred on April 28, 2015 is analyzed. The passage of the ejection did not cause geoeffective disturbances in the near-Earth space. At the same time, the CME had a significant impact on the flux of cosmic rays registered on the Earth's surface by the muon hodoscope URAGAN.