Spectral characteristics of large-scale radio emission areas in coronal holes

The spectra of the coronal hole radio emission in solar cycles 23 and 24 have been studied based on RATAN-600 data in the 4–16.5 GHz range at frequencies of 5.7 and 17 GHz and 327 MHz. It has been found that bright features of coronal hole microwave emission at 17 GHz and dark features at 5.7 GHz can exist in coronal holes when the spectral index is 1.25–1.5 in the 6.5–16.5 GHz range; the radio spectrum in this range is flat when coronal holes are indiscernible against the background of a quiet Sun. The possible vertical scale of the solar atmosphere over coronal holes is discussed.     

Analysis of the Physical Characteristics of the Polar Coronal Hole on the Sun in the Microwave Wavelength Range

Geomagnetism and Aeronomy - The results of a study of a polar coronal hole based on data from observations of the solar eclipse of March 29, 2006, with the RATAN-600 radio telescope in a wide...     

Physical characteristics of the radio emission of solar coronal holes

This paper presents a brief overview of works dedicated to the studies of solar coronal holes (CH). Special attention is paid to CH studies at millimeter and centimeter wavelengths. The observational data in the millimeter range, combined with satellite observations in the ultraviolet and soft X-ray range, as well as solar eclipse observations of March 29, 2006, at centimeter wavelengths with RATAN-600, provide new results for understanding the physical nature of coronal holes on the Sun.     

Chromospheric and Coronal Radio Sources from Observations of the Partial Solar Eclipse of March 20, 2015, at the Mountain Astronomical Station of the Central Astronomical Observatory

This paper presents the results of processing the data on the partial solar eclipse that occurred on March 20, 2015, and was observed with the RT-3 (λ = 4.9 cm) and RT-2 (λ = 3.2 cm) radio telescopes of the Kislovodsk Mountain Astronomical Station, Central Astronomical Observatory, Russian Academy of Sciences (MAS CAO RAS). They were compared with observations in the optical and X-ray ranges. The local radio sources at the limb and on the disc of the Sun were identified: an eruptive and a quiet prominence; filaments; a coronal hole; facular plages; and a sunspot group. The curves of the center-to-limb variations in the radio brightness of the undisturbed regions of the Sun were plotted for λ = 4.9 and λ = 3.2 cm. The solar radio maps were presented. The altitude of the radiating layer in the chromosphere above the sunspot and the facular sources for λ = 4.9 cm λ = 3.2 cm was compared.     

Studying correlations between the coronal hole area,solar wind velocity,and local magnetic indices in the Canadian region during the decline phase of cycle 23

Geomagnetic disturbances in the Canadian region are compared with their solar and heliospheric sources during the decline phase of solar activity, when recurrent solar wind streams from low-latitude coronal holes were clearly defined. A linear correlation analysis has been performed using the following data: the daily and hourly indices of geomagnetic activity, solar wind velocity, and coronal hole area. The obtained correlation coefficients were rather low between the coronal hole areas and geomagnetic activity (0.17–0.48), intermediate between the coronal hole areas and the solar wind velocity (0.40–0.65), and rather high between the solar wind velocity and geomagnetic activity (0.50–0.70). It has been indicated that the correlation coefficient values can be considerably increased (by tens of percent in the first case and about twice in the second case) if variations in the studied parameters related to changes in the ionosphere (different illumination during a year) and variations in the heliolatitudinal shift of the coordinate system between the Earth, the Sun, and a spacecraft are more accurately taken into account.     

Comparative analysis of the distributions of brightness temperatures in the solar polar region based on observational data obtained with RATAN-600 in the microwave range during the solar eclipse of March 29, 2006

The results of microwave observations of the solar eclipse of March 29, 2006, with the RATAN-600 radiotelescope are presented. The observations were carried out using the northeastern sector of the radiotelescope in a broad wavelength range (1.03, 1.38, 2.70, 6.20, 13.00, and 30.70 cm) in the intensity channel. The aim of the present work is to conduct a comparative analysis of the distributions of brightness temperatures in the solar atmosphere at a distance (ranging from one to two solar radii) from the center of the optical solar disk. The data for the analysis come from the RATAN-600 observations of the solar eclipse of March 29, 2006, earlier observations with the RATAN-600 radiotelescope, and calculations of brightness temperatures that were carried out using the Baumbach-Allen formula which describes the electron density in the solar corona. The differences between the distributions obtained by the above-mentioned methods are discussed.     

Coronal Holes of Cycle 24 in Observations at the Solar Dynamics Observatory

Geomagnetism and Aeronomy - The dynamics of the areas of coronal holes and their localization on the Sun in solar cycle 24 and the minimum of cycles 24–25 were analyzed. The study is based on...     

Finding of low-contrast formations in the solar corona using a low contrast method

Coronal holes, bright coronal points, filaments, and prominences are among the initial factors responsible for variability of the space weather conditions. Radioheliographic data on low-contrast formations contain valuable information necessary for studying conditions of origination, peculiarities of evolution, and prediction of solar-terrestrial relations. It is important to identify these formations on the solar disk when physical properties of coronal holes are revealed. The algorithm based on the Wiener-Tikhonov filter modification with controlled parameters and a high-frequency contrast filter was developed in order to isolate low-contrast formations in the solar corona brightness distributions obtained at a wavelength of 5.2 cm from the Siberian solar radio telescope observations. In this case low-contrast sources are isolated in two main stages: (1) HF noise smoothing based on an evolutionary filter with controlled parameters and (2) contrasting of sources using an HF filter. The evolutionary filter regularization parameters and the dimensions of an HF contrast filter mask are selected depending on the signal-to-noise ratio and dimensions of the studied region based on the results of preliminary data processing. The corresponding software has been developed in order to identify low-contrast objects on the Sun’s radio images using this method. The algorithm is used to isolate filaments and coronal holes and the results of this usage are presented in this work.     

Statistical Studies of Coronal Mass Ejections and Coronal Holes

Statistical studies of coronal mass ejections (CMEs) and coronal holes (CHs) are reviewed. The work summarizes the historical and current results of statistical studies of CMEs and CHs and their parameters that have been obtained by various authors who considered these phenomena as independent manifestations of solar activity, as well as their mutual effect on geomagnetic activity, based on both ground and space observations.     

Studying local sources in the radio range based on the partial solar eclipse of January 4, 2011, at the Mountain Astronomical Station, Central Astronomical Observatory, Russian Academy of Sciences

Data are presented on a partial solar eclipse, which occurred on January 4, 2011, and was observed with RT-3 (?? = 4.9 cm) and RT-2 (?? = 3.2 cm) radio telescopes at the Mountain Astronomical Station, Central Astronomical Observatory, Russian Academy of Sciences (MAS CAO RAS). The radioemission flux in two channels was registered using digital methods with a time resolution of 0.5 s. Comparisons were performed with observations in the optical, UV, and X-ray ranges. The following local sources of increased radioemission on the solar disk have been identified: sunspot groups 1 (NOAA 1142) and 126 (NOAA 1141), unipolar sunspot 127 (NOAA 1140), facula areas, and polar and midlatitude coronal holes. It has been indicated that the brightness of a unipolar sunspot (for ?? = 4.9 cm, I _rel = 29.5; for ?? = 3.2 cm, I _rel = 10.1) and two sunspot groups (for ?? = 4.9 cm, I _rel = 10.1 and 14.2; for ?? = 3.2 cm, I _rel = 5.1 and 6.2) is maximal. The radioemission flux of all found coronal holes is decreased, and the decrease is more contrasting in the 4.9-cm range as compared to such a decrease in the 3.2-cm range. Radio maps of the Sun and changes in the radioemission flux of undisturbed solar regions from the center to the limb for ?? = 4.9 and 3.2 cm have been constructed based on the eclipse data.     

Specific features of radio emissions of coronal holes based on eclipse and noneclipse observations at a solar activity minimum

Solar coronal holes (CHs) at a minimum of the 23rd activity cycle were investigated using Solar and Heliospheric Observatory (SOHO) data, ground-based observational data from the radio telescopes of the Kislovodsk Solar Station, Pulkovo Observatory (KSS PO), Russian Academy of Sciences, and radioheliograph data from the Nobeyama Observatory (Japan). The 2006–2008 period was characterized by a small number of active regions on the solar disk; nevertheless, this period is favorable for studying low-contrast objects in the radio band (CHs). We investigate the evolution of CH areas, the location of CHs on the solar disk, and the features of their radiation in the radio band. We present the results of observations of the total (March 29, 2006) and partial (August 1, 2008) solar eclipses by the RT-3 and RT-2 radio telescopes of the KSS PO. Based on the eclipse observation data, compact sources were identified on the solar disk and the contribution of CHs to the integral radio emission flux was estimated. A rare effect (increased radio emissions of high-latitude CHs at a wavelength of 4.9 cm) was revealed, which may be caused by X-ray bright points in CHs. Here, polar CHs are characterized by low levels of radio emission.     

冕洞与太阳风高速流关系的统计研究:到达时间、持续时间和峰值强度

冕洞是太阳风高速流的源区.当冕洞出现在中低纬区域时,太阳风高速流会扫过地球并引发地球空间环境扰动,如地磁暴和高能电子暴等.在太阳活动周下降年和低年,这种类型的扰动占据主导地位.因此,冕洞高速流的到达时间、峰值时间、峰值强度和持续时间等,是空间天气预报的重要内容.本文基于2010年5月到2016年12月的SDO/AIA太阳极紫外图像以及1AU处ACE和WIND卫星的太阳风观测数据,确定了160个冕洞-太阳风高速流事件,定量计算了他们的特征参数,包括冕洞与太阳风高速流的开始时间、峰值时间、峰值强度和结束时间,分析了各个特征参数的分布规律,对冕洞-高速流之间的关系进行了统计研究,并提出了一种新的预报方法,为基于冕洞成像观测的太阳风高速流的精准预报提供了依据.

     

Features of Microwave Radiation and Magnetographic Characteristics of Solar Active Region NOAA 12242 Before the X1.8 Flare on December 20, 2014

This paper continues the cycle of authors’ works on the detection of precursors of large flares (M5 and higher classes) in active regions (ARs) of the Sun by their microwave radiation and magnetographic characteristics. Generalization of the detected precursors of strong flares can be used to develop methods for their prediction. This paper presents an analysis of the development of NOAA AR 12242, in which an X1.8 flare occurred on December 20, 2014. The analysis is based on regular multiazimuth and multiwavelength observations with the RATAN-600 radio telescope in the range 1.65–10 cm with intensity and circular polarization analysis and data from the Solar Dynamics Observatory (SDO). It was found that a new component appeared in the AR microwave radiation two days before the X-flare. It became dominant in the AR the day before the flare and significantly decreased after the flare. The use of multiazimuth observations from RATAN-600 and observations at 1.76 cm from the Nobeyama Radioheliograph made it possible to identify the radio source that appeared before the X-flare with the site of the closest convergence of opposite polarity fields near the neutral line in the AR. It was established that the X-flare occurred 20 h after the total gradient of the magnetic field of the entire region calculated from SDO/HMI data reached its maximum value. Analysis of the evolution of the microwave source that appeared before the X-flare in AR 12242 and comparison of its parameters with the parameters of other components of the AR microwave radiation showed that the new source can be classified as neutral line associated sources (NLSs), which were repeatedly detected by the RATAN-600 and other radio telescopes 1–3 days before the large flares.     

Jet phenomena above null points of the coronal magnetic field

Short-lived plasma jets of various scales, from giant X-ray jets more than 300 Mm in extent to numerous small jets with sizes typical of macrospicules, are the phenomena observed in the solar corona in extreme ultraviolet and X-ray emission. Small jets are particularly prominent in polar coronal holes. They are close neighbors of tiny bright loops and coincide in time with their sudden brightening and increase in size. The geometric shape of the jets and their location suggest that they arise near singular null points of the coronal magnetic field. These points appear in coronal holes due to the emergence of small bipolar or unipolar magnetic structures within large-scale unipolar cells. Polar jets show a distinct vertical plasma motion in a coronal hole that introduces significant momentum and mass into the solar wind flow. Investigating the dynamics of polar jets can elucidate certain details in the problem of fast solar wind acceleration.     

Sporadic and recurrent geomagnetic disturbances in 1859–1860 according to the archived data from the Russian network of stations

Based on an analysis of the available archived data from the Russian network of geomagnetic stations, it has been indicated that the known event of August–September 1859 was the first and the greatest event in the series of the recurrent geomagnetic storms. Similar series were repeatedly observed in the next years. These series are caused by the processes on the Sun and in the heliosphere related to the superposition of the solar wind flows. The sporadic and regular components in joint activity of the complex, including active regions and coronal holes on the rotating Sun, play the role of the Bartels M regions responsible for initiation and development of geomagnetic storms. Neither coronal holes nor active regions can separately explain observations. During interpretation, active regions and coronal holes should be considered as a unified complex.     

Microwave radiation of solar active regions before X flares according to the RATAN-600 observations in 2011

The evolution of the microwave radiation from four active regions, where strong X-ray flares (X-class, GOES) occurred in 2011, has been studied. Daily multiwavelength RATAN-600 radio observations of the Sun in the 1.6–8.0 cm range have been used. It has been indicated that the radiosource above the photospheric magnetic field neutral line (above the region with the maximal convergence of the fields opposite in sign) becomes predominant in the structure of the active region microwave radiation one to two days before a powerful flare as in the eruptive events previously studied with RATAN-600. The appearance of such a radiosource possibly reflects the current sheet formation in the corona above the active region. The energy necessary for a flare is stored in the magnetic field of active region, which can be considered as a factor for predicting a powerful flare.     

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

Geomagnetism and Aeronomy - 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...     

Local maximum in the microwave spectrum of solar active regions as a factor in predicting powerful flares

We discuss the results of a study of microwave radiation from three flare-active regions??NOAA 10300, 10930, and 11158??with powerful eruptive events (X-class flares and coronal mass ejections) recorded on July 15, 2002; December 13, 2006; and February 15, 2011, when the regions were in the central part of the disk. There exists evidence of a ??-configuration in the structure of the photospheric magnetic field formed one or two days prior to the eruptive process as a result of the emergence of a new magnetic flux and shifting movements of the sunspots and accompanied by changes in the spectral characteristics of the microwave radiation of the active regions (ARs), which suggests the development of a peculiar radio source. The analysis of these regions continues a series of studies of eruptive events carried out at RATAN-600 in the 1980s?C1990s and gives a reason to conclude that early detections of peculiar sources in the microwave radiation of ARs, which are essentially areas of high energy release in the solar atmosphere, can be used as a factor in predicting powerful eruptive (geoeffective) processes on the Sun.     

The spatial relationship between active regions and coronal holes and the occurrence of intense geomagnetic storms throughout the solar activity cycle

We study the annual frequency of occurrence of intense geomagnetic storms (Dst < –100 nT) throughout the solar activity cycle for the last three cycles and find that it shows different structures. In cycles 20 and 22 it peaks during the ascending phase, near sunspot maximum. During cycle 21, however, there is one peak in the ascending phase and a second, higher, peak in the descending phase separated by a minimum of storm occurrence during 1980, the sunspot maximum. We compare the solar cycle distribution of storms with the corresponding evolution of coronal mass ejections and flares. We find that, as the frequency of occurrence of coronal mass ejections seems to follow very closely the evolution of the sunspot number, it does not reproduce the storm profiles. The temporal distribution of flares varies from that of sunspots and is more in agreement with the distribution of intense geomagnetic storms, but flares show a maximum at every sunspot maximum and cannot then explain the small number of intense storms in 1980. In a previous study we demonstrated that, in most cases, the occurrence of intense geomagnetic storms is associated with a flaring event in an active region located near a coronal hole. In this work we study the spatial relationship between active regions and coronal holes for solar cycles 21 and 22 and find that it also shows different temporal evolution in each cycle in accordance with the occurrence of strong geomagnetic storms; although there were many active regions during 1980, most of the time they were far from coronal holes. We analyse in detail the situation for the intense geomagnetic storms in 1980 and show that, in every case, they were associated with a flare in one of the few active regions adjacent to a coronal hole.