Geomagnetism and Aeronomy

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On the Dependence of the Solar Wind Velocity on the Fractional Area of Coronal Holes in Longitude

Sat, 12/01/2018 - 00:00
Abstract

The relationship between the fractional area of coronal holes and the maximum velocity of the fast solar wind at 1 AU based on AIA/SDO and ACE/SWEPAM observations is considered for the period from June 2015 to March 2017. Assuming the ballistic model of the solar wind propagation it has been shown that the coronal holes within the meridional slice ±10° make a basic contribution to the ecliptic solar wind. The maximum correlation coefficient between the area of coronal holes and the peak solar wind velocity at latitudes within ±40° was found to be equal to 0.762 ± 0.145. The probable causes of the discrepancy between the predicted and observed values of the solar wind velocity are discussed.

Some Features of the Two Sunspot Group Populations’ Properties

Sat, 12/01/2018 - 00:00
Abstract

This paper describes some features of the two sunspot group populations’ properties: Large Long-living Groups, LLG, and Small Short-living, SSG (each distributed lognormally), divided by the lifetime of the group (less and strictly more than 5 days). It is shown that the relative fraction of LLG varies with the latitude from 0.45 at mid-latitudes to 0.25–0.30 at low and high latitudes. The SSG has a wider spread in latitude than the LLG. It is shown that the N-S asymmetry is an overall global process to which both the LLG and the SSG are exposed. At the same time, the asymmetry of LLG shows a slightly higher dispersion of oscillations than the asymmetry of SSG. Other properties of sunspot groups in the context of the two separate populations existence are considered.

The Gnevyshev-Ohl Rule and Two Sunspot Group Populations

Sat, 12/01/2018 - 00:00
Abstract

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.

Secular Variations of a Shift of the Heliospheric Current Sheet

Sat, 12/01/2018 - 00:00
Abstract

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.

New Data about Long-Term Changes of Solar Activity

Sat, 12/01/2018 - 00:00
Abstract

The double Hale cycle (about 44 years) was found in changes of power-law index for integral distributions of equivalent diameters of large groups of sunspots having sizes of 50–90 Mm. According to the Greenwich catalog data, this cycle is traced in the last 13 cycles of solar activity, nos. 12–24. In addition to this periodic change, a unique sporadic phenomenon was discovered, namely, a sharp jump in the maximum diameters of sunspot groups in the 18th cycle (more precisely, in 1946–1947), which has no analogues in other cycles.

Energy Spectra and Time Delays of Hard X-Rays of Solar Flares in Konus-Wind and RHESSI Experiments

Sat, 12/01/2018 - 00:00
Abstract

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.

Undamped Oscillations of Electric Current in Coronal Magnetic Loops and in Magnetic Loop Arcades

Sat, 12/01/2018 - 00:00
Abstract

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.

Features of the Solar Wind Plasma Flow around the Earth’s Magnetosphere

Sat, 12/01/2018 - 00:00
Abstract

The change in the properties of the solar wind flow as it crosses the bow shock wave front and moves in the Earth’s magnetosheath are discussed. Solar wind data are used to study the refraction of magnetohydrodynamic (MHD) shock waves and the stationary tangential discontinuity of the solar wind into the magnetosheath. It is shown that the refraction of the solar-wind rotational discontinuity into the magnetosheath is accompanied by the emergence of a plateau-type plasma inhomogeneity with respect to the density of charged particles, with a simultaneous decrease in the magnetic field intensity. Moreover, the breaking of the secondary MHD contraction wave, reflected from the magnetopause, may be accompanied by the emergence of a fast reverse shock wave.

Compton Scattering of the Hard X-Ray Flux of Solar Flares with Various Angular Anisotropies of Hard X-Ray Sources

Sat, 12/01/2018 - 00:00
Abstract

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.

Protons from the Decay of Solar Neutrons and the Properties of the Interplanetary Medium

Sat, 12/01/2018 - 00:00
Abstract

One of the results of the interaction between flare protons and the solar atmosphere is the generation of neutrons, some of which escape to the interplanetary medium and decay there when propagating. We simulate time profiles of the intensity and anisotropy of “secondary” protons produced in the decay of neutrons. These decay protons are characterized by a distributed (in time and space) source of particles rather than an impulse source, which is typical of primary protons from flares. We use the kinetic transport equation, which includes magnetic focusing, adiabatic losses of energy, and pitch-angle diffusion. We pay special attention to the properties of decay protons from flares on the eastern limb and behind it. Measurements of the time profiles of the intensity and anisotropy of protons produced in such flares may yield estimates of the local pitch-angle scattering coefficient (the mean free path) for protons of a certain energy.

Latitude-Longitude Characteristics of the North–South Asymmetry of Solar Activity

Sat, 12/01/2018 - 00:00
Abstract

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.

Time Evolution of the Energy Spectra of Accelerated Electrons and Hard X-Rays from Local Sources of Solar Flares

Sat, 12/01/2018 - 00:00
Abstract

Modern observations of solar flares in hard X-rays are carried out with a high spatial resolution of ~2–4 arcsec with the RHESSI satellite. In this way one can identify, at least for powerful events, the local spatial structure of hard X-ray sources. The structure of flare X2.8 SOL 2013-5-13T15:50, as detected from RHESSI data and from observations made using Nobeyama and SDO, includes three bright local hard X-ray sources: one at the loop top and two at the footpoints of the flaring loop. The goal of this work is to obtain the energy spectra of hard X-rays, determine the spectrum of delays in hard X-ray radiation, reconstruct the spectra of accelerated electrons for each local source, and determine the parameters of the electron beam and flare plasma. The electron spectra are reconstructed by the forward-fitting method and the Tikhonov regularized inversion. The results of the analysis suggest that explaining the bright hard X-ray source high in the corona requires, simultaneously, a high magnetic field gradient in the footpoints with Bmax/B0 > 7 (in a symmetrical configuration), a background plasma density of n0 > 1010 cm–3, and the presence of turbulence.

Analysis of Oscillatory Modes of the Magnetic Field of Solar Facular Formations

Sat, 12/01/2018 - 00:00
Abstract

Quasi-periodical variations in the magnetic fields of long-living, small-scale magnetic structures, solar facular formations (FFs), are analyzed. We used the method of Empirical Mode Decomposition (EMD) to reveal the empirical modes of time series of magnetic field variations in four FFs and tested them for the existence of coloured noise. We proposed a qualitative interpretation of statistically reliable oscillatory modes.

Reconstruction of the Production Rate of Cosmogenic 14 C in the Earth’s Atmosphere for 17 000–5000 BC

Sat, 12/01/2018 - 00:00
Abstract

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.

Influence of Supergranulation on the Properties of Sunspot Bipoles

Sat, 12/01/2018 - 00:00
Abstract

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.

Traces of Periodicity in the Observational Data on Magnetic Fields of Celestial Bodies and the Dynamo Models

Sat, 12/01/2018 - 00:00
Abstract

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.

Study of Variations of Some Characteristics of the Giant Coronal Hole of 2015–2017

Sat, 12/01/2018 - 00:00
Abstract

Analysis of images of the full solar disk obtained by ground-based (BST-2, CrAO RAS) and space-based (SDO/AIA, SDO/HMI) instruments and the potential approximation method revealed some features of the evolution of a long-lived coronal hole (CH) of 2015–2017. Variations in such CH characteristics as the area, intensity, magnetic field (MF) strength, and magnetic flux are studied. The study of the change in the area and average intensity of the CH contour at different heights has shown that changes in its area throughout the entire period occur almost synchronously within the boundaries determined by model calculations at the photospheric level and by observations of the Sun in the chromosphere and corona. The average CH intensity depends on the moment of its evolution: it varies from the intensity of the undisturbed region to the CH maximum intensity. This process lasts about the same time as the increase in the CH area. The intensity of the He I 10830 Å (He I) line occurs in antiphase with the decrease in the intensity in the Fe XII 193Å (Fe XII) line. This paper shows that this CH has significantly reduced the magnetic flux in a large part of the Sun in a time interval of almost a year and a half. Even when the CH began to collapse noticeably, the magnetic field strength and the magnetic flux continued to decrease.

Large-Scale Magnetostatic Structures in the Solar Corona and a Model of the Polar Coronal Hole

Sat, 12/01/2018 - 00:00
Abstract

A method for the theoretical calculation of large-scale magnetoplasma solar coronal structures with a rotational symmetry in the spherical coordinate system is presented. The method makes it possible to obtain analytical solutions for equilibrium spatial distributions of pressures, densities, and temperatures in any preset axisymmetrical magnetic configuration. The obtained solution is used to plot model polar coronal holes via the introduction of a small power correction in the force free distribution of the magnetic field for subpolar region. The thermodynamic parameters of the coronal hole represented in this model are close to the observed values.

Regularities in the Formation of Coronal Mass Ejections Associated and not Associated with Type II Radio Bursts

Sat, 12/01/2018 - 00:00
Abstract

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.

Manifestation of the Solar Cycle in the Circulation Characteristics of the Lower Atmosphere in the Northern Hemisphere

Sat, 12/01/2018 - 00:00
Abstract

The relationships between circulation characteristics in the lower atmosphere of the Northern Hemisphere and the sunspot cycle are analyzed with the use of the superposed epoch method and elements of correlation analysis on the basis of data on the length of elementary circulation processes of different types (according to Dzerdzeevsky classification) and the time series of Wolf numbers for 1899–2016. It is ascertained that a general intensification of solar activity promotes an increase in the length of meridional forms of circulation, but its effect on different subtypes of elementary circulation mechanisms is different. Seasonal differences in the solar effect on the lower atmosphere are also revealed.

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