Vol 58, No 8 (2018)

The results of the digitalization of the centennial series of solar activity are considered. The data contain information on sunspots since 1918, sunspot umbra since 1917, plages since 1907, and spectral corona since 1939. In particular the digitalization of prominences from the observations in the Ca II K line in 1910–1954 and solar filaments from the observations in the H-alpha line in 1912–2002 acquired at the Kodaikanal Observatory are described. The methods of constructing composite maps of solar activity and presenting the resulting data series on the Internet are described. The long-term variations in solar activity are analyzed.

The shift of the heliospheric current sheet (HCS) can be judged by the correlation between polarities of the interplanetary magnetic field (IMF) observed near the Earth. Here the IMF polarities which were reconstructed from the geomagnetic data of the mid-latitude stations beginning from 1844 are studied. The correlation of positive and negative IMF sectors is the evidence of a shift of the heliospheric current sheet towards the Northern Hemisphere in cycles 15–19 of solar activity, and towards the south, in cycles 10–14. We suggest that there is a regular secular cycle of the south–north asymmetry of the current sheet. Correspondingly, a similar cycle is also characteristic of the solar magnetic field.

This paper analyzes hard X-ray radiation of solar flares detected by Konus-Wind and RHESSI spacecrafts. X-ray quanta are measured using high energy, time (Konus-Wind), and spatial (RHESSI) resolution. Spectra of accelerated electrons are calculated from the spectra of hard X-rays both via the forward-fitting method and the Tikhonov reconstruction method. The X-ray data for the solar flare of July 8, 2013, 0120:24 UT are analyzed. In Konus-Wind data the irregularities and inversions are detected in the energy spectrum. According to RHESSI data, the hard X-ray (HXR) energy spectrum is smoother, although it contains breaks. The X-ray energy spectrum determined by the forward-fitting method is hard at the peak of the flux, and the spectrum index is γ ≈ 3.5. Over time, the HXR spectrum is significantly softened, and the spectrum index increases to γ ≈ 7.5. X-ray time delays are determined by a correlation analysis of the time series of different energies. The time delay spectrum is U-shaped with a 47-keV break in the energy for the Konus-Wind data and 57 keV for the RHESSI data.

Ionospheric anomaly crest regions are most challenging for scientific community to understand its mechanism and investigation, for this purpose we are investigating some inospheric result for this region. The study is based on the ionogram data recorded by IPS-71 Digital Ionosonde installed over anomaly crust region Bhopal (Geo. Lat. 23.2° N, Geo. Long 77.4° E, Dip latitude 18.4°) over a one year period from January 2010 to December 2010. The studies were examined in four different variations such as: Hourly variation, diurnal variation, seasonal variation and hourly-seasonal variation. The occurrence probability of this layer is highest in summer solstice, moderate during equinox and low during winter solstice. Remarkable occurrence peaks appear from June to July in summer and from December to January in winter. The layer occurrence showed a double peak variation with distinct layer groups, in the morning between 0000–0400 UT and the other during evening between 1100–1600 UT. The morning layer descent was associated with layer density increase indicating the strengthening of the layer while it decreased during the evening layer descent. The result indicates the presence of semi-diurnal tide over the location while the higher descent velocities could be due to the modulation of the ionization by gravity waves along with the tides. The irregularities associated with the gradient-drift instability disappear during the counter electrojet and the current flow is reversed in westward.

A brief comparative analysis determining the magnetic field in optical and radio range prominences is presented; a tendency to increase averaged magnetic fields two to four times in quiescent and active prominences, compared to previously accepted values, is noted. The possibility of indirect data on electric currents in the Hα-prominence from measurements of linear polarization during the total solar eclipse of 29 March 2006 was shown. The 2D sign distributions of a linear polarization angle, interpreted within the Thomson scattering by the free moving electrons of prominences, indirectly indicate the presence of oppositely directed electric currents in the prominence and surrounding corona.

The paper presents a study of the evolution of the photospheric magnetic field of active regions (ARs) of the Sun in which flares (larger than the M9 class in the Geostationary Operational Environmental Satellite (GOES) X-ray classification) occurred in 2010–2017. The purpose of this paper is to detect the precursors of flares. Thirteen ARs at a distance of not more than 45 degrees from the central meridian are selected for analysis out of 31 ARs in which flares larger than M9.0 were detected in the specified period. The magnetographic characteristics of the selected ARs are studied based on the Solar Dynamics Observatory (SDO) data. A flare index is proposed. It is calculated from the data of the Helioseismic and Magnetic Imager of SDO (SDO/HMI) and reflects the distance between regions of opposite magnetic polarity computed between the field boundaries by the threshold value. The analysis showed that a sharp increase in the flare index is detected in all the studied ARs 2–3 days before large flares. Flares larger than the M9.0 class occurred 5–20 h after the global or local maximum of the flare index. It is shown using the example of individual events that regions of the opposite polarity first converged and then relatively quickly separated from each other before large flares. The revealed features of evolution of ARs before large flares can be used to develop methods for predicting them.

This work continues the study of the two sunspot group populations' properties discovered earlier (Nagovitsyn et al., 2009; Nagovitsyn et al., 2012; Nagovitsyn et al., 2017; Nagovitsyn and Pevtsov, 2016; Nagovitsyn et al., 2018; Osipova and Nagovitsyn, 2017). The concepts of “static” indices (when each sunspot group included in statistics once, characterizing the performance of the dynamo process) and “dynamic” indices (regular, when all the days of the group’s existence are present in statistics, describing, in particular, the influence of solar activity on terrestrial processes and heliosphere) are introduced. For static indices, the well-known Gnevyshev-Ohl rule in various formulations is tested—MGO, AGO, and DGO (see Introduction). The main conclusion is that for the number of small short-living groups (SSG) the Gnevyshev-Ohl rule is accurate to the contrary, and such groups form a pair in a 22-year cycle—a single whole in a combination of an odd and subsequent even cycle, with the latter having a smaller value.

Millimeter (93 and 140 GHz) emission of the М6.4 solar flare detected on April 2, 2017 in the NOAA 12644 active region by the RT-7.5 telescope of the Bauman Moscow State Technical University is analyzed using the observational data provided by the Radio Solar Telescope Network (4.9, 8.8, and 15.4 GHz); the SDO/AIA satellites (Etreme Ultraviolet); and GOES, RHESSI, and Konus-Wind (X-rays). It is found that the spectral flux density of millimeter emission increases with frequency throughout the entire burst. The similarity between the profiles of millimeter and soft-X-ray radiation suggests that the burst is of a thermal nature. It follows from the results of calculations of the differential emission measure of coronal plasma based on the SDO/AIA data that its contribution to millimeter emission of the flare is negligible. The simulation of thermal emission of chromospheric flare plasma in the model of Machado (Machado et al., 1980) yields millimeter fluxes that are by several times lower than the observed ones. The physical implications of these results are discussed.

In the scope of the performed work, two problems are solved. The first is the analysis of two existing methods for calculating vertical electric currents, differential and integral. The integral method has some advantages compared to the differential method. Among them, one can mention there is no need to differentiate the magnetic field and a good smoothing of noises without losing significant information about the current structures. The integral method of calculating vertical currents has been tested for the NOAA AR 11158 using the data of two instruments, the Helioseismic and Magnetic Imager of the Solar Dynamics Observatory (HMI/SDO) spacecraft and the Solar Optical Telescope of the Hinode satellite (SOT/Hinode). There was a good correlation between the calculated data (the coefficient of correlation k = 0.77). This result indicates a sufficient reliability of data on currents calculated by the integral method, regardless of the observation instrument. In the second part of the work, we analyze the data on the relation between the flare activity of groups of spots and the magnitude of the vertical current in them. A direct dependence between these quantities has been established: the greater the mean squared vertical current density, the higher the flare activity of the region (the coefficient of correlation k₁ = 0.91).

It is shown that a source generating a slowly varying or quiet component of radio emission from ultracool stars such as the brown dwarf TVLM 513-46546 can be the gyrosynchroton radiation from the magnetic loops system distributed quasi-uniformly over the star’s surface. Such a model explains the low modulation of the flux and makes it possible to understand the pumping mechanism of magnetic loops by high energy electrons. The main parameters of magnetic loops and which part of the star’s surface is the emission source are determined using the information on microwave radiation from the brown dwarf TVLM 513-46546.

Data set acquired by the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO) during 2010–2017 allowed us to classify active regions (ARs) into three categories: A-type— regular bipolar ARs; U-type—unipolar spots; B-type—irregular ARs, violating either Hale polarity law or Joy’s law or having the leading spot less than the main following spot. A separate subset of anti-Hale ARs was formed. We selected 1494 ARs in total and found the following: (i) Pearson correlation coefficient r between the total unsigned flux for a given category and the International Sunspot Number smoothly decreases with transition from A-type (r = 0.57) to B-type (r = 0.53) to anti-Hale ARs (r = 0.31) to U-type (r = 0.18); (ii) yearly contributions into the total flux from categories also gradually decreases: from 50–70% from A-type ARs to 20–40% from B‑type ARs to 10–20% from U-type ARs to 5–11% from anti-Hale ARs. (iii) At the beginning of the solar minimum, the fraction of flux from anti-Hale groups increased from 5 to 9% and amount of flux per magnetogram was constant at about 10²¹ Mx level. The data are compatible with a concept that generation of the magnetic field on the Sun occurs as a united process in a non-linear dynamical dissipative system, i.e., global and local (fluctuation) dynamos are inseparable and operate together. The observed enhancement of the anti-Hale flux during the solar maximum can be due to the combined mechanisms of global mean-field and local fluctuation dynamos.

The Total Solar Irradiance (TSI) index of solar activity attracts the attention of a wide scientific audience due to its direct influence on the Earth’s climate. A number of reconstructions of inter-decadal TSI variability have been extended back to the early Holocene but these estimates still remain uncertain and depend on the model and proxy data used for the particular reconstruction. Here we compare the accuracy of nonlinear forecasts for different reconstructions, confirmed by the global Hurst exponent and pointwise Hölder regularity estimates with the only persistent, regular and predictable reconstruction. The deterministic reconstruction identified predicts a further slow decrease in average TSI level in Cycle 25. We applied the empirical mode decomposition to determine major TSI modes which mainly describe long-term changes in the Sun’s radiative output. We found a crucial role of ~100- and ~200-yr cycles in the occurrence of long-term TSI depressions related to grand minima in the Sun’s magnetic activity. A necessary condition for grand minima occurrence is established in terms of the major TSI modes. Based on the relationship, we conclude that a moderate TSI depression is possible in future decades without a grand minimum.