Yu. S. Shugay

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.

Comparative analysis of recurrent high-speed solar wind streams influence on the radiation environment of near-earth space in April–July 20101

This paper analyzes variations of flux of relativistic and subrelativistic electrons in the outer radiation belt of the Earth caused by the arrival of recurrent high-speed streams of solar wind during three consecutive solar rotations. The period from April to July 2010 is covered. During this time, an increase in fluxes of relativistic electrons was observed after they had reached a minimum in November 2009–January 2010. Two coronal holes of different polarity, geometry, and location relative to the solar equator were the source of high-speed solar wind streams. The relationship between the efficiency of acceleration of electrons of subrelativistic energies and the amplitude, duration of high-speed streams of solar wind and geomagnetic disturbances, as well as the wave activity in the range of 2–7 mHz, characterized by the ULF index, is confirmed. Significant increases of the flux of relativistic electrons in the outer radiation belt of the Earth were observed during the considered period with an hourly average speed of solar wind streams above 550 km/s and a duration of more than seven days. It is found that the spectrum of electrons in the Earth’s outer radiation belt over the considered period of time was softer during the observation of solar wind streams from the positive polarity coronal hole, even given the amplitude of the solar wind velocity higher than 550 km/s.

Possible causes of the discrepancy between the predicted and observed parameters of high-speed solar wind streams

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.

The effect of solar activity on the evolution of solar wind parameters during the rise of the 24th cycle

The dynamics of parameters of the near-Earth solar wind (SW) and the effect of solar activity on the parameters of three SW components (fast SW from large-scale coronal holes (CHs); slow SW from active regions, streamers, and other sources; and transient flows related to sporadic solar activity) at the beginning of the 24th solar cycle (2009–2011) are analyzed. It is demonstrated that temperaturedependent parameters of ionic composition (C+6/C+5 and O+7/O+6) of the transient SW component in the profound minimum of solar activity in 2009 were correlated with the variation of the rate of weak (type C and weaker) flares. This verifies the presence of a hot component associated with these flares in the SW. The variations in the velocity and the kinetic temperature of fast SW from CHs with an increase in activity are more pronounced in the bulk of the high-speed stream, and the variations of O+7/O+6 and Fe/O ratios and the magnitude of the interplanetary magnetic field are the most prominent in the region of interaction between fast and slow SW streams. The analysis reveals that a value of O+7/O+6 = 0.1 serves as the criterion to distinguish between fast SW streams and interplanetary coronal mass ejections in the 2009 activity minimum. This value is lower than the one (0.145) determined earlier based on the data on the 23rd cycle (Zhao et al., 2009). Therefore, the distinguishing criterion is not an absolute one and depends on the solar activity level.

High-speed streams of the solar wind near the earths’s orbit and their sources on the sun according to stereoscopic observations in the minimum of the 23rd cycle

Based on the results of plasma and magnetic measurements at three different points of the heliosphere and telescopic observations of the Sun from these points we study simultaneously high-speed streams (HSS) of the solar wind (SW) near the Earths’s orbit and coronal holes (CH) that have generated them. The data from spacecraft STEREO-A, STEREO-B, ACE, and SOHO are used together with ground-based observations from March 2007 to May 2008. In this period there existed HSS whose sources represented CH of various polarity, geometry, and location relative to the heliographic and heliomagnetic equators. Dependence of SW parameters on mutual positions of spacecraft with respect to CH and heliospheric current sheet, and also on heliolatitude and geometry of the CH is revealed. A difference of more than 5° in locations of spacecraft with respect to the heliospheric current sheet in November 2007 allowed us to discover a heliolatitude velocity gradient of the SW streams between the STEREO-A and STEREO-B spacecraft. On the average this gradient at that time was equal to 20 km/s per degree. Substantial variations in SW streams associated with variations of the HSS SW sources during a few hours or days were also observed. This variability makes it difficult to use the data of spacecraft STEREO-B for sufficiently accurate prediction of SW properties in the near-Earth space by the method of simple advanced ti me shift due to heliolongitude difference between a spacecraft and the Earth even in solar activity minimum.

Features of solar wind streams on June 21–28, 2015 as a result of interactions between coronal mass ejections and recurrent streams from coronal holes

Coronal sources and parameters of solar wind streams during a strong and prolonged geomagnetic disturbance in June 2015 have been considered. Сorrespondence between coronal sources and solar wind streams at 1 AU has been determined using an analysis of solar images, catalogs of flares and coronal mass ejections, solar wind parameters including the ionic composition. The sources of disturbances in the considered period were a sequence of five coronal mass ejections that propagated along the recurrent solar wind streams from coronal holes. The observed differences from typical in magnetic and kinetic parameters of solar wind streams have been associated with the interactions of different types of solar wind. The ionic composition has proved to be a good additional marker for highlighting components in a mixture of solar wind streams, which can be associated with different coronal sources.

Prediction of geoeffective solar wind streams near earth according to solar observational data

A semi-empirical model for predicting solar wind (SW) streams is developed based on satellite images of the Sun and corona using a hierarchical approach. The model allows simultaneous calculation of the parameters of all three SW components, i.e., slow SW, high-speed streams from coronal holes, and interplanetary coronal mass ejections, as well as the detection of their possible interaction in the heliosphere.

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.

Influence of Heliospheric and Geomagnetic Activity on the Dynamics of the Relativistic Electron Fluxes in the Earth's Outer Radiation Belt around the Minimum of the Solar Activity in 2008-2010

The relativistic electron flux in the Earth’s outer radiation belt generally decreased by almost three orders of magnitude during the minimum of cycle 23 in 2009. Such a behavior was possibly caused by very low geomagnetic activity during an extremely weak interplanetary field in that period. This decrease was replaced by an increase in the relativistic electron flux by two orders of magnitude during several months after the sunspot minimum at the beginning of 2010.