Vol 58, No 7 (2018)

A mechanism for acceleration of electrons driven by the oscillations of the electric current in solar magnetic loops is considered. The magnetic loop is presented as an equivalent RLC-circuit with the electric current generated by convective motions in the photosphere. Eigen oscillations of the current in a loop induce the electric field directed along to the loop axis. It is shown that pulsating type III bursts and the sudden reductions that occur in the course of type IV continuum in solar flares provide evidence for acceleration and storage of energetic electrons in the coronal magnetic loops. Energization rate of electrons accelerated by sub-Dreicer electric field are determined. The energy spectra of fast electrons under both intermediate and strong regimes of pitch-angle diffusion are discussed. Two examples of the electron acceleration in the pulsating type III solar bursts are considered. We also discuss the efficiency of suggested mechanism as compared with the electron acceleration during 5-min photospheric oscillations and with the acceleration driven by the magnetic Rayleigh-Taylor instability.

This study investigates the impacts of geomagnetic storms on the performance of the International Reference Ionosphere 2016 (IRI-2016) foF2 predictions at different latitudes during geomagnetically disturbed periods in different seasons of the year 1989 which is around maximum solar activity. For this purpose, hourly foF2 data measured from the ionosonde stations Manila (14.7° N, 121.1° E), Rome (41.8° N, 12.5° E), Argentia NF (47.3° N, 54.0° W) and Uppsala (59.8° N, 17.6° E) that are located at low, middle and high latitudes, and hourly foF2 data calculated from the IRI-2016 for the same locations are used. In our study, a comparison between observational results and that of the IRI-2016 foF2 predictions is made. Planetary geomagnetic activity “3h-K_p” and “3h-a_p” indices are used as geomagnetic activity indicators. In order to test the performance of the IRI-2016 for disturbed geomagnetic conditions, the percentile deviations and root mean square errors (RMSE) are calculated using the foF2 data from the IRI-2016 and the ionosonde stations. These analyzes are done for four seasons based on geomagnetic stormy days around equinoxes (March 21, September 23) and solstices (June 21, December 21). Therefore, it is found that during geomagnetically disturbed periods, the IRI-2016 foF2 predictions deviate considerably from the foF2 data taken from the ionosonde stations and this deviation differs according to the seasons and latitudes. The results show that the last version of the IRI model that includes the foF2 storm model is not sufficient to properly represent the real conditions of a disturbed ionosphere and there is still space for improving the IRI model for geomagnetically disturbed conditions.

The possibility of the existence of undamped oscillations of the electric current in coronal magnetic loops and loop arcades due to the presence of a photospheric electromotive force arising in the base of the coronal magnetic loop as a result of the engagement of photospheric convection and the magnetic field of the loop is demonstrated. This photospheric EMF plays the part of negative resistance in the equation of the equivalent electric circuit and leads to a “soft” excitation of electric current self-oscillations. The frequency, amplitude, and phase of the oscillations have been found as functions of the electric current and supercriticality of the photospheric EMF. The possibility of the existence of self-oscillation regimes in arcades of coronal magnetic loops has been considered.

From the viewpoint of dynamo theory, periodic behavior of the magnetic field is interpreted as a manifestation of the eigenmode in magnetohydrodynamic equations describing the dynamo. The alternative explanation is that we are dealing with a demonstration of different fluctuations of the dynamo control parameters rather than with a periodic process. A simplified geodynamo model is considered; it is able to reproduce a scale of inversions that is similar to the real scale and a priori does not describe excitations of a periodically varying magnetic field. The analysis shows that, according to the currently available data on the scale of geomagnetic field polarities, the presence of a eigen frequency corresponding to a period of about 50 Ma is not fixed in the geodynamo problem.

We analyze physical characteristics of late-type stars wherein the Kepler mission registered superflares. We use the revised stellar fundamental parameters, i.e. effective temperatures T_eff and surface gravity accelerations log g, from the Kepler archive published in 2017, as compared to previous studies by Balona (2015) based on the release of 2011. Among superflare stars there are both single objects and members of eclipsing binaries. We select the late-type stars (with T_eff < 6500 K) wherein occured the most powerful flares with the total flare energy >10³⁵ erg and consider their locations in the T_eff – log g diagram. Both components of binaries and single stars appear to reside mostly in between the main sequence and the subgiant branches and therefore have larger radii compared to that of the Sun. Besides, as a rule these single stars are fast rotators and can be considered as young objects that it is difficult to attribute to “solar-type stars”. Extremely high flare energy of these stars requires quite strong magnetic fields that cannot be generated even due to scaling of the solar dynamo. Apparently, for explanation of the strongest non-stationary phenomena on stars considered, it would be worthwhile to attract another regime of the dynamo mechanism that can be realized in these objects.

Manifestations of the activity of the YZ CMi (M4.5Ve) flare star with a rotation period of 2.77 days, which is included in the group of activity-saturated stars, are considered based on long-term photometric data, including photographic measurements over the interval from 1926 to 2009. Long-term changes in the yearly mean brightness of the star found on the time scale of 27.5 years with an amplitude of 0.2–0.3^m, that indicate on variations in the development of surface inhomogeneities and a large degree of spottedness in the maximum activity. The spots are distributed unevenly over the surface with an increased concentration at certain longitudes spaced by intervals equal to 0.6 phases of the rotation period. The recovery of the active longitude (a possible “fliр-flop” effect) occurs over a period of about 6 years. We note differences in activity of this star from typical for the Sun and solar-like stars, which are associated with its dynamic characteristics P_cyc and P_rot and its inner structure.

Carbon isotope ¹⁴С is produced in the Earth’s atmosphere by energetic cosmic-ray (CR) particles. The data on its atmospheric abundance are used to reconstruct the rate of its production and to analyze past levels of CR intensity and solar activity. These data are obtained via measurement of the ¹⁴С abundance in tree rings, with their real age determined dendrochronologically. The ¹⁴С abundance depends on the carbon dioxide concentration in the atmosphere and on climatic changes that affect the processes of carbon exchange between natural reservoirs. The present study reports the results of reconstruction of the production rate for cosmogenic ¹⁴С in the Earth’s atmosphere in the interval from the onset of deglaciation (~17 000 BC) to the mid-Holocene. The climatic factors mentioned above were taken into account in this reconstruction.

The characteristics of the latitude-longitude distribution of the north–south (NS) asymmetry of the number of sunspots for the period of 1874–2013 are studied. It is shown that there is an NS asymmetry of the sunspot number in all latitude-longitude ranges in which solar activity is manifested. Longitude ranges that exist for quite a long time in which the activity of the northern or southern hemisphere predominates are selected. It is found that the NS asymmetry of the sunspot number for the Sun as a whole is determined to a greater extent by the asynchronous development of activity in the northern and southern hemispheres.

We consider fast sausage solitons and super nonlinearity in straight homogeneous magnetic tubes having coronal parameters. The solitonic behavior is described by the Nonlinear Schrödinger Equation-(NLSE) obtained from the ideal magneto hydrodynamic equations with suitable coronal conditions. For the first time we demonstrated that fast sausage waves are subjected to super nonlinearity and likewise introduced the super nonlinear function that defines this phenomenon. We have obtained classical localized sausage soliton and Peregrine sausage breather soliton solutions for coronal conditions. And finally we have carried out extensive numerical simulations of the evolution of a wave packet governed by the NLS equation with real nonlinear parameters and demonstrated the existence and domain of the above mentioned solitonic modes.

The properties of solar bipoles based on data from magnetic field observations at Mount Wilson Observatory during the period of 1917–2016 are considered. Special attention is given to variations in the size of the bipole magnetic axis during the solar cycle. It is shown that the distribution of the length of the bipole magnetic axis is close to lognormal with the exception of secondary local peaks. Secondary peaks are also present in the distribution of the tilt angles of bipoles at different latitudes. The influence of matter flows in supergranules on the length of the magnetic axis and the tilt angles of sunspot bipoles is discussed.

Possible causes of the ~60-year periodicity observed in climatic characteristics and the evolution of long-term correlation links between the circulation of the lower atmosphere and phenomena caused by solar activity are explored in this paper. It is shown that the character of solar activity and galactic cosmic ray variation effects on the intensity of cyclonic processes at extratropical latitudes depends on epochs of the large-scale atmospheric circulation, which may be associated with the state of the stratospheric polar vortex. Global temperature variations associated with long-term oscillations of total solar irradiance are supposed to be a possible cause of changes in the state of the vortex and corresponding transformations of the large-scale circulation.

Regularities in the formation of coronal mass ejections (CMEs) associated (Radio Loud, RL CME) and not associated (Radio Quiet, RQ CME) with type II radio bursts (RBIIs) in cycles 23 and 24 are reviewed. It is shown that the number of observed RQ and RL CME events and their parameters are quite different in cycles 23 and 24. The influence of the magnetic field and plasma parameters on the number of observed RQ and RL CME events and their parameters have been studied. The magnetic field in the region of RL CME detection was computed based on the observed, large-scale, photospheric magnetic field of the Sun, while we used direct observations of the plasma density in the Earth’s orbit to calculate for the plasma density. This enabled us to obtain the Alfven speed and to examine the influence of cyclical variation of the magnetic field, plasma density, and Alfven speed on the number of events and RQ and RL CME parameters. The results show that magnetic field and plasma values may be among the key factors determining the conditions, which are favorable for the formation of RQ CMEs or RL CMEs.

The contribution of photons reflected from the solar photosphere to the intensity and the change in the slope of the energy spectrum of solar flare hard X-rays were analyzed as a function of the X-ray anisotropy. The angular and energy distributions of primary hard X-rays and the position of the flare loop on the solar disk were the main parameters in calculations. The contribution of the reflected component to the total flux for an anisotropic source may be as high as 70% at an energy range of 30–40 keV if the source is shifted relative to the solar disk center. The maximum change in the X-ray spectral index is 0.5.