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Structures in the solar corona are the main drivers of space weather processes that might directly or indirectly affect the Earth. Thanks to the most recent space-based solar observatories, with capabilities to acquire high-resolution images continuously, the structures in the solar corona can be monitored over the years with a time resolution of minutes. For this purpose, we have developed a method for automatic segmentation of solar corona structures observed in EUV spectrum that is based on a deep learning approach utilizing Convolutional Neural Networks. The available input datasets have been examined together with our own dataset based on the manual annotation of the target structures. Indeed, the input dataset is the main limitation of the developed model's performance. Our \textit{SCSS-Net} model provides results for coronal holes and active regions that could be compared with other generally used methods for automatic segmentation. Even more, it provides a universal procedure to identify structures in the solar corona with the help of the transfer learning technique. The outputs of the model can be then used for further statistical studies of connections between solar activity and the influence of space weather on Earth., Comment: accepted for publication in Monthly Notices of the Royal Astronomical Society; for associated code, see https://github.com/space-lab-sk/scss-net

Heavy rainfall events causing floods and flash floods are examined in the context of solar wind coupling to the magnetosphere–ionosphere–atmosphere system. The superposed epoch (SPE) analyses of solar wind variables have shown the tendency of severe weather to follow arrivals of high-speed streams from solar coronal holes. Precipitation data sets based on rain gauge and satellite sensor measurements are used to examine the relationship between the solar wind high-speed streams and daily precipitation rates over several midlatitude regions. The SPE analysis results show an increase in the occurrence of high precipitation rates following arrivals of high-speed streams, including recurrence with a solar rotation period of 27 d. The cross-correlation analysis applied to the SPE averages of the green (Fe XIV; 530.3 nm) corona intensity observed by ground-based coronagraphs, solar wind parameters, and daily precipitation rates show correlation peaks at lags spaced by solar rotation period. When the SPE analysis is limited to years around the solar minimum (2008–2009), which was dominated by recurrent coronal holes separated by ∼ 120∘ in heliographic longitude, significant cross-correlation peaks are found at lags spaced by 9 d. These results are further demonstrated by cases of heavy rainfall, floods and flash floods in Europe, Japan, and the USA, highlighting the role of solar wind coupling to the magnetosphere–ionosphere–atmosphere system in severe weather, mediated by aurorally excited atmospheric gravity waves.

The active regions (ARs) on the Sun, which are distinguished by high values of the flare index and simultaneously developed outside activity complexes (ACs), have been analyzed. Thirteen such objects were identified in cycle 24. We showed that these ARs belong to the category of relatively long-lived large groups of sunspots, characterized by an increased rate of area growth. These ARs are part of the AR complexes, connect with other AR systems of high coronal systems, including trans-equatorial ones. We associated some of these ARs with short-lived (less than 3 Carrington rotations) ACs and some ARs are components of complexes of ARs. All 13 considered ARs were located near coronal holes.

An overview is presented of the main results of the study of the polar coronal hole over the North pole of the Sun based on observations of the solar eclipse on March 29, 2006, using the RATAN-600 radio telescope in a wide cm-range of wavelengths (1.03, 1.38, 2.7, 6.2, 13.0, 30.7 cm) along with observational and theoretical data about the properties of Solar coronal holes published by various authors. The results obtained are discussed: the distribution of brightness temperatures of polar coronal holes over the North pole of the Sun at distances of 1.005–2.0 in units of the Sun optical disk radius from the center of the solar disk; the increase in microwave radiation of polar coronal holes recorded at short wavelengths; the identity of the temperature properties of the polar coronal holes and low-latitude coronal holes on the Sun during the period of minimum solar activity. Comparison of the obtained brightness temperatures of the polar coronal hole with the brightness temperatures of large low-latitude coronal holes at similar wavelengths observed earlier (1973–1976, 1984–1987 yr) were done. This comparison has indicated an identity of the temperature properties of coronal holes regardless of their location on the Sun and the organization of coronal holes during the period of minimum solar activity.

A real-time model for predicting the quasistationary solar wind speed at the near-Earth orbit is presented. The forecast of the high-speed solar wind stream velocity is obtained with an empirical model linking the areas of coronal holes to the solar wind speed. The forecast of the slow solar wind is based on data on the observed solar wind speed from the previous solar rotation. Over the whole analyzed period from May 2010 to December 2019, the coefficient of correlation between the observed and predicted solar wind speed values is 0.45, and the standard deviation is 88 km/s. The accuracy of forecasting the speed of quasistationary solar wind streams is comparable to the results of foreign models.

The results of a statistical study of the polarization characteristics of irregular geomagnetic Pi3 pulsations observed in polar regions of the magnetosphere and the effect of interplanetary conditions on them are presented. One-minute digital observations of the magnetic field of the Heiss Island (HIS, Φ' = 74.80°, Λ' = 144.46°) and Mirny (MIR, Φ' = –76.93°, Λ' = 122.92°) observatories are used. Pi3 pulsations are characterized by right-handed, left-handed, and mixed polarizations (R, L, and R&L types, respectively). Regardless of the polarization type, Pi3 pulsations have been observed mainly in the night sector of the magnetosphere at the final stage of the explosive phase of the substorm. The duration of the time delay between the substorm onset and the occurrence of Pi3 pulsations depends significantly on their polarization type. HIS and MIR show no dependence of the main characteristics (amplitude, frequency, ellipticity, and tilt of the main axis of the polarization ellipse) of Pi3 pulsations of R, L, and R&L types on local time. In HIS, R- and R&L-type Pi3 pulsations are observed mainly during slow solar-wind streams, and L-type pulsations are observed during high-speed streams from coronal holes. In MIR, Pi3 pulsations of the three polarization types are observed mainly during high-speed solar-wind streams. Small-scale closed structures of the magnetic field in the flow of slow and fast solar-wind currents determine the polarization type of Pi3 pulsations. It is revealed that the excitation of Pi3 pulsations of the R, L, and R&L types occurs at different levels of plasma turbulence in the magnetotail.

Based on the data obtained with the CHIMERA algorithm, we consider the evolution of a long-lived low-latitude coronal hole during its central meridian passage over the period from February 15, 2012, to October 14, 2012. The correlation coefficient between the photospheric magnetic field strength of the coronal hole and its area in nine Carrington rotations is $$R={-}0.55$$ . It differs noticeably from $$R={-}0.82$$ given in Heinemann et al. The results suggest a significant dependence of the area of coronal holes on the method of determining their boundaries. This can have a noticeable effect both on the prediction of geomagnetic activity and on the understanding of the nature of solar phenomena related to these structures.

The relative abundances of thermal and suprathermal C, O, and Fe ions are analysed and compared in solar wind streams from near-equatorial coronal holes (CHs) during quiet periods of nearly two solar cycle minima. Ion fluxes with energies of ∼0.04–2 MeV per nucleon are studied using data from the Ultra Low Energy Isotope Spectrometer (ULEIS) instrument aboard the Advanced Composition Explorer (ACE) spacecraft together with thermal ions in the fast and slow (Maxwellian) solar wind using data from the Solar Wind Ion Composition Spectrometer (SWICS) instrument aboard ACE. The analysis was carried out for quiescent periods in 2006–2012 and 2015–2017 when solar wind flows from near-equatorial CHs were detected at 1 au. Near the minimum of SC23, although they displayed large variability, the C/O and Fe/O ratios of suprathermal ions were, on average, near the corresponding relative abundances of the solar wind. During the decreasing solar activity phase of SC24, suprathermal Fe/O ratios matched those of solar wind ions from CHs. In both cycles, the thermal and suprathermal Fe/O ratios exhibited a similar character of dependence on maximum solar wind speed. Our results suggest that the sources of suprathermal ions from CHs in low-solar activity periods are accelerated solar wind thermal ions. The thermal and suprathermal Fe/O ratios were found to be higher in 2015–2017 in comparison to the ratios measured in 2006–2010. This difference can be attributed to that the second quiet period was selected at times when solar activity has not reached its minimum of SC24 yet.

The dynamics of coronal holes (CHs) during the declining phase of solar cycle 24 is studied based on observational data from the Atmospheric Imaging Assembly (AIA), the 19.3 nm channel, of the Solar Dynamic Observatory (SDO). Some features of changes in the areas of polar and mid- and low-latitude CHs in the period April 1, 2014–December 31, 2019, are revealed by a comparison of the variations in the daily total CH area with the index of the sunspot number. The division of all CHs observed in the considered period into polar and nonpolar shows that polar-CH areas dominate and increase while nonpolar-CH areas constantly decrease toward the cycle minimum. Polar-CH areas account for ~80% of the total area of all CHs considered over the studied period. The total polar-CH area increases toward cycle minimum, which is consistent with the general concept of polar CHs as the main source of the Sun’s dipole magnetic field. However, the second subset, which contains selected isolated CHs, shows an interesting behavior in the declining phase of solar activity that was not previously observed. Their areas slowly decrease from the maximum to minimum of the solar cycle. This trend is qualitatively consistent with a decrease in sunspot activity toward the minimum, which suggests a physical relationship between the two phenomena.

The rotational effect observed in variations of areas of polar coronal holes with a period of 14 days is analyzed with SDO data by using Coronal Hole Identification with the Multithermal Emission Recognition Algorithm (CHIMERA) algorithm and the algorithm for the identification of solar structures from neural networks. A physical interpretation of the observed rotational effect associated with the asymmetry of coronal holes is proposed.

This work is devoted to the study of properties of spectra of turbulent solar wind fluctuations in plasma compression regions, such as CIR (the plasma compression regions in front of high-speed streams from coronal holes) and SHEATH (the compression regions in front of interplanetary manifestations of coronal mass ejections EJECTA and magnetic clouds MC). The spectra of ion flux fluctuations, on both magnetohydrodynamic and ion-kinetic scales, are considered on the basis of data from the BMSW spectrometer on the SPEKTR-R spacecraft with a high (up to 31 ms) time resolution. The comparison of turbulent characteristics in the plasma compression regions and in the undisturbed solar wind is carried out both on a separate example and using extensive statistical material. It is shown in the paper that the turbulent cascade characteristics on the kinetic interval can significantly change in the plasma compression regions, and the signatures of a change in the main processes that determine the energy dissipation are revealed in them, which may cause increased heating in the regions under consideration.

The SDO/AIA 193 A and ACE/SWEPAM data acquired in 2019 are used to compare the effects of polar and equatorial coronal holes (CHs) on solar wind (SW) characteristics under conditions of low solar activity. As expected, most geomagnetic storms in this period were caused by high-speed SW streams ( $${>}500{-}600$$ km s $${}^{-1}$$ ) originating from equatorial CHs. At the same time, it has been shown that at a deep solar activity minimum polar CHs can exert a noticeable influence on SW characteristics at the Earth’s orbit. A correlation with a correlation coefficient of 0.8 has been found for the integrated polar CH area and the SW speed in the investigated period. The southern polar CH, for which a correlation with the SW speed with a correlation coefficient of 0.82 was found in the spring of 2019 (in the period when the south solar pole was maximally tilted to the Earth), exerted a particularly significant influence on the SW speed. The northern polar CH had virtually no effect on the SW speed. An anticorrelation of the polar CH area with the SW speed at the Earth’s orbit was found in the fall of 2019, in the period when the north solar pole was tilted to the Earth. We discuss a possible mechanism for the influence of polar CHs on SW characteristics and propose an interpretation of the results obtained.

The physical conditions in the chromosphere of polar and equatorial coronal holes in the decay phase of the 24-th solar activity cycle are studied. Models of the chromosphere are constructed based on the intensities of the HeI 10830 A and Hα lines and using a non-LTE program. The calculations showed that the temperature in equatorial coronal holes is similar to that of polar coronal holes. The temperature in coronal holes is lower than that of the unperturbed chromosphere and increases depending on height in the chromosphere. The difference between the atmospheres of coronal holes and the quiet Sun begins at the level of the upper photosphere- lower chromosphere.

The relationship between the magnetic fluxes and areas of coronal holes (CH) at mid and low latitudes |φ |< 60° and the mean magnetic field (MMF) of the sun over the period from January 19 through December 12, 2012, is studied using ultraviolet images and magnetograms obtained with the Solar Dynamics Observatory satellite. It is shown that the boundaries of the CH determined with the algorithms CHIMERA ( https://solarmonitor.org ) and SPoCA ( https://helioviewer.org ) differ significantly. The correlation coefficient between the weighted total area of CHs taking into account the magnetic field polarity and MMF are in the range from 0.82 to 0.88. The CH can make a substantial contribution to the MMF since their fields have an open configuration and are not balanced on the solar disk. In light of the solar dynamo theory and the results obtained here, the annual time delays between the maxima of the Wolf numbers and the values of the MMF are discussed.

Strong flare events with an extended phase of gamma ray emission from the catalog by Share et al. have been analyzed in order to explain the characteristics of the impulsive and extended phases of such flares and reveal their connection with activity complexes and coronal holes. It is shown that 74% of such events are closely related to activity complexes. It has been qualitatively demonstrated that the particle acceleration processes during the flare development are associated with the changes in the magnetic topology of the flare region and the evolution of the coronal mass ejection. The possible relation of coronal holes to activity complexes and the role of “exchange” reconnection in these processes are discussed.

The regularities in the formation of coronal holes in the growth phase of cycle 23 are studied. The period from January 1, 1997, to March 1, 2000 (Carrington rotations 1918–2059), is considered in detail. The evolution of the global magnetic field of the Sun from a zonal to a sectorial structure of the field distribution is analyzed. The zonal structure is shown to be quasi-stable. The sum of zonal harmonics dominates until Carrington rotation 1941, although a stable four-sector structure of the global magnetic field is formed in rotation 1932. In Carrington rotations 1941–1950, the contribution of the zonal and sectorial components becomes approximately equal, and, starting with Carrington rotation 1950, the sectorial structure of the global magnetic field is dominant. The sectorial structure undergoes sharp changes, from four-sector at the beginning of the growth of sector harmonics (Carrington rotation 1926) to two-sector, then back to four-sector, and then again to the two-sector structure. Coronal holes unambiguously trace all evolutionary changes in the global magnetic field. The structure of the polarity of the global magnetic field uniquely determines the zones of photospheric magnetic fields in which coronal holes form.

The presence of coronal holes in solar disk plays an important role in influencing the space weather and generation of the solar wind. As such there lies a requirement in proper study and prediction of coronal holes occurs in the solar disk. This, in turn, arises the necessity of detection of coronal holes present in the solar disk. In this work, a Hough transformed inspired fuzzy-energy simulated dual contours-based segmentation technique has been proposed for the detection and extraction of holes in solar disk. In the proposed method Hough transform has been induced to initialize the contour for the contour-based method of segmentation. In the algorithm, two contours (active and static) have been initiated and made to evolve based on the energy function by incorporating the gray-scale intensity. Here in the work one contour is made to deform its shape while the other contour is kept static for the coronal holes detection purpose. The experiment has been carried out on few benchmark datasets and the corresponding outcomes have been compared with the results of other existing algorithms. The comparison results highlight the performance of the proposed technique in detection of coronal holes in solar disk.

The paper proposes a model for predicting the integral daily fluxes (fluences) of relativistic electrons (RE) (E > 2 MeV) of the Earth's outer radiation belt in a geostationary orbit using images of the Sun in the ultraviolet range. The results show that the accuracy of the forecast of the RE fluxes three or four days ahead increases significantly when adding the predicted values of solar wind speed at the Earth’s orbit, obtained by processing images of the Sun in the UV range from AIA instrument, SDO Observatory, to the input parameters of the forecasting model.

Using the magnetic field data obtained with the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO), an investigation of magnetic power spectra in the undisturbed solar photosphere was performed. The results are as follows. 1) To get a reliable estimate of a magnetic power spectrum from the uniformly distributed quiet-sun magnetic flux, a sample pattern of no less than 300 pixels length should be adopted. With smaller patterns, energy on all observable scales might be overestimated. 2) For patterns of different magnetic intensity (e.g., a coronal hole, a quiet-sun area, an area of supergranulation), the magnetic power spectra in a range of (2.5-10) Mm exhibit very close spectral indices of about -1. The observed spectrum is more shallow than the Kolmogorov-type spectrum (with a slope of -5/3) and it differs from steep spectra of active regions. Such a shallow spectrum cannot be explained by the only direct Kolmogorov’s cascade, but it can imply a small-scale turbulent dynamo action in a wide range of scales: from tens of megameters down to at least 2.5 Mm. On smaller scales, the HMI/SDO data do not allow us to reliably derive the shape of the spectrum. 3) Data make it possible to conclude that a uniform mechanism of the small-scale turbulent dynamo is at work all over the solar surface outside active regions.

The behavior of high-energy magnetospheric electrons with energies of >2 MeV in geostationary orbit during solar cycles 22–24 is considered. It is shown that the most increases in electrons occurs during the declining phase of solar activity, when a greater number of geoeffective coronal holes are observed. The 2003–2015 data show there is generally a correspondence between the behavior of the number of increases in electrons and the area of geoeffective coronal holes.

Research on the detection of solar events has been conducted over many years. Recently, deep learning and data-driven approaches have been applied to solar event recognition. In this study, we present solar event detection using deep-learning-based object detection methods for real-time space weather monitoring. First, we construct a new object detection dataset using imaging data obtained by the Solar Dynamics Observatory with bounding boxes as labels for three representative features: coronal holes, sunspots, and prominences. Second, we train two representative object detection models: the Single Shot MultiBox Detector (SSD) and the Faster Region-based Convolutional Neural Network (R-CNN) using the new dataset. The results show that both models perform similarly well for coronal hole and sunspot detection. For prominence detection, the SSD and Faster R-CNN exhibited relatively low performance. This study demonstrates that deep-learning-based object detection can successfully detect multiple types of solar events, and it may be extended to detect other solar events. In addition, we provide the dataset for further achievements of object detection studies in solar physics.

Long-term periodicities in the solar irradiance are often observed with periods proportional to the solar rotational period of 27 days. These periods are linked either to some internal mechanism in the Sun or said to be higher harmonics of the rotation without further discussion of their origin. In this article, the origin of the peaks in periodicities seen in the solar extreme ultraviolet (EUV) and ultraviolet (UV) irradiance around the 7, 9, and 14 days periods is discussed. Maps of the active regions and coronal holes are produced from six images per day using the Spatial Possibilistic Clustering Algorithm (SPoCA), a segmentation algorithm. Spectral irradiance at coronal, transition-region/chromospheric, and photospheric levels are extracted for each feature as well as for the full disk by applying the maps to full-disk images (at 19.3, 30.4, and 170 nm sampling in the corona/hot flare plasma, the chromosphere/transition region, and the photosphere, respectively) from the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO) from January 2011 to December 2018. The peaks in periodicities at 7, 9, and 14 days as well as the solar rotation around 27 days can be seen in almost all of the solar irradiance time series. The segmentation also provided time series of the active regions and coronal holes visible area (i.e. in the area observed in the AIA images, not corrected for the line-of-sight effect with respect to the solar surface), which also show similar peaks in periodicities, indicating that the periodicities are due to the change in area of the features on the solar disk rather than to their absolute irradiance. A simple model was created to reproduce the power spectral density of the area covered by active regions also showing the same peaks in periodicities. Segmentation of solar images allows us to determine that the peaks in periodicities seen in solar EUV/UV irradiance from a few days to a month are due to the change in area of the solar features, in particular, active regions, as they are the main contributors to the total full-disk irradiance variability. The higher harmonics of the solar rotation are caused by the clipping of the area signal as the regions rotate behind the solar limb.

Understanding the deflection of coronal mass ejections (CMEs) is of great interest to the space weather community because of their implications for improving the prediction of CME. This paper aims to shed light into the effects of the coronal magnetic field environment on CME trajectories. We analyze the influence of the magnetic environment on the early development of a particular CME event. On 2011 January 24 an eruptive filament was ejected in association with a CME that suffered a large deflection from its source region and expected trajectory. We characterize the 3D evolution of the prominence material using the tie-pointing/triangulation reconstruction technique on EUV and white-light images. To estimate the coordinates in 3D space of the apex of the CME we use a forward-modeling technique that reproduces the large-scale structure of the flux rope-like CME, the Graduated Cylindrical Shell model. We found that the deflection amounts to 42 ° in latitude and 20 ° in longitude and that most of it occurs at altitudes below 4 R ⊙ . Moreover, we found a non-negligible deflection at higher altitudes. Combining images of different wavelengths with the extrapolated magnetic field obtained from a potential field source surface model we confirm the presence of two magnetic structures near the erupting event. The magnetic field environment suggests that field lines from the southern coronal hole act as a magnetic wall that produces the large latitudinal deflection; while a nearby pseudostreamer and a northward extension of the southern coronal hole may be responsible for the eastward deflection of the CME.

The geoeffectiveness of single and complex transient structures of the solar wind (SW), formed by coronal mass ejections (CMEs) and associated Forbush decreases (FDs) in the cosmic ray flux is considered. By the example of the period of 2010–2011, it is shown that complex SW structures resulting from the interaction of CMEs with high-speed streams from coronal holes or with other CMEs in the heliosphere more often lead to moderate and strong geomagnetic storms (Dst < –50 nT) in comparison with single CMEs. It is shown that the FD increases with the growth of the SW transient structure duration and the average magnitude of the interplanetary magnetic field of the transient and takes the highest values when the complex structures arrive at the Earth.

The current 24th solar activity cycle differs substantially from previous cycles in a number of parameters, especially in terms of a large asymmetry in the number of sunspots (Sp) during the second peak of its maximum. From March 2013 through December 2015. a significant predominance was observed in the number of sunspots in the southern hemisphere. The main purpose of this paper is to clarify the behavior of coronal holes (CH) during this period. This study is based on an analysis of data recorded by the 19.3 nm channel of the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamic Observatory (SDO). Two methods of detecting CH (simplified visual and the Spatial Possibilistic Clustering Algorithm (SPoCA)) are used to obtain time series of daily total CH areas for the Sun’s northern and southern hemispheres. The two methods agree on the estimated areas of the CH. A comparison of the observed variations in the areas of the CH with the numbers and areas of sunspots showed that in this period, activity predominates in the S-hemisphere both in terms of sunspots and in terms of the total CH area. Here a rise in the area of the CH follows a rise in sunspot activity by roughly half a year. It is suggested that the CH and spots are associated elements of the overall magnetic activity of the Sun, in qualitative agreement with studies of activity complexes. A dipole poloidal field in the form of open CH fields and a toroidal field in the form of active regions are related on time scales substantially shorter than the solar cycle.

The Van Allen radiation belts consist of megaelectron volt particles trapped in the Earth’s magnetic field mainly with protons at low altitudes and electrons at high altitudes. These concentric, donut-shaped radiation belts are constantly changing due to a variety of physical processes caused by specific types of heliospheric structure. The radiation poses risks for astronauts and spacecraft systems. The understanding of these processes that operate across the universe can help with designs of future space missions as well as the understanding of the universe. The data gathered from the last three months of the Radiation Belt Storm Probes mission (RBSP mission) is examined for patterns and trends in the dynamics of the radiation belts and their relationships with solar activities. From the data, it is shown that the patterns of the high-energy particle dynamics in the belts follow the changes in solar activity. When the Sun’s magnetic field weakens, cosmic rays from deep space are able to carry an increased flux of energetic particles into the solar system. The outer belt exhibits a pattern of electron energization during cycles lasting an average of 27 days, corresponding to the time taken for one solar rotation. The sudden outward shift of the belts on July 19th, 2019 was most likely caused by the shutdown of one of the two Van Allen probes, as the relative position of the probe to the belts changed. Two solar wind streams that arrived from August 27th to the 28th caused the “dropout” event on August 28th, as the contact between the solar wind streams and magnetosphere pushed the magnetosphere’s boundary and enabled the particles outside the shifted boundary to escape into space. A positive polarity coronal hole high speed stream (CH HSS) contributed to the intense September electron re-energization process in the radiation belts starting on September 1st.

Проведено исследование спектров мощности магнитного поля в невозмущенной фотосфере Солнца по данным инструмента Helioseismic and Magnetic Imager (HMI) на борту станции Solar Dynamics Observatory (SDO). Результаты можно сформулировать следующим образом. 1) Для достоверной оценки спектра мощности в обширной зоне квазиравномерного распределения поля следует выбирать участок длиной не менее 300 пикселей. По оценкам по меньшему участку, мощность на всех доступных частотах оказывается завышенной. 2) Для магнитных зон разной интенсивности, а именно для корональной дыры, спокойного Солнца и супергрануляционной сетки, спектр мощности в диапазоне (2.5-10) Мм проявляет одинаковый спектральный индекс, близкий к -1. Наблюдаемый спектр более пологий, чем колмогоровский (с наклоном -5/3), и отличается от более крутых спектров активных областей. Такой пологий спектр нельзя объяснить только прямым колмогоровским каскадом. Его можно объяснить дополнительной накачкой магнитной энергии за счет мелкомасштабного турбулентного динамо, работающего в широком диапазоне масштабов: от десятков мегаметров до, по крайней мере, 2.5 Мм. На масштабах менее 2.5 Мм SDO/HMI-данные не позволяют адекватно оценить форму спектра. 3) Данные позволяют заключить, что однообразный механизм мелкомасштабного турбулентного динамо работает по всей поверхности Солнца вне активных областей.

During the solar minimum, when the Sun is at its least active, the solar wind1,2 is observed at high latitudes as a predominantly fast (more than 500 kilometres per second), highly Alfvenic rarefied stream of plasma originating from deep within coronal holes. Closer to the ecliptic plane, the solar wind is interspersed with a more variable slow wind3 of less than 500 kilometres per second. The precise origins of the slow wind streams are less certain4; theories and observations suggest that they may originate at the tips of helmet streamers5,6, from interchange reconnection near coronal hole boundaries7,8, or within coronal holes with highly diverging magnetic fields9,10. The heating mechanism required to drive the solar wind is also unresolved, although candidate mechanisms include Alfven-wave turbulence11,12, heating by reconnection in nanoflares13, ion cyclotron wave heating14 and acceleration by thermal gradients1. At a distance of one astronomical unit, the wind is mixed and evolved, and therefore much of the diagnostic structure of these sources and processes has been lost. Here we present observations from the Parker Solar Probe15 at 36 to 54 solar radii that show evidence of slow Alfvenic solar wind emerging from a small equatorial coronal hole. The measured magnetic field exhibits patches of large, intermittent reversals that are associated with jets of plasma and enhanced Poynting flux and that are interspersed in a smoother and less turbulent flow with a near-radial magnetic field. Furthermore, plasma-wave measurements suggest the existence of electron and ion velocity-space micro-instabilities10,16 that are associated with plasma heating and thermalization processes. Our measurements suggest that there is an impulsive mechanism associated with solar-wind energization and that micro-instabilities play a part in heating, and we provide evidence that low-latitude coronal holes are a key source of the slow solar wind. Measurements from the Parker Solar Probe show that slow solar wind near the Sun’s equator originates in coronal holes.

Specific features of the behavior of streams of relativistic magnetospheric electrons at energies higher than 2 MeV on geostationary orbits during different types of interplanetary disturbances are studied. More than 30 years (1986–2019) of observations of these particles onboard the GOES satellites is analyzed. It is shown that plasma coronal ejections and high-speed streams from coronal holes differently influence the behavior of high-energy electrons. High-speed streams from coronal holes are more effective In formation of electron flux rises than solar plasma ejections.

Variations of the muon flux of cosmic rays (using data from the URAGAN muon hodoscope) and perturbations of the Earth’s magnetic field (OMNI database) were analyzed for August 2018, when there were almost no active regions on the solar disk and the number of sunspots was close to zero. The Kp and Dst geoefficiency coefficients were found to be elevated when the Earth entered the region of fast solar wind from coronal holes. Analysis of the non-stationarity factor C(t) for the muon count time series Nµ(t) shows that there are always elevated C(t) values that are ahead of the fast solar wind from coronal holes by (1–3) days. A predictor was also found in the sequence of values C(t) which is 1-day ahead of the geo-effective storm (SSC-power G3) of the solar CME eruption on August 20.

The features of daytime high latitude geomagnetic disturbances and geomagnetic pulsations during the recent strong magnetic storm on August 25–26, 2018, which occurred at the end of the decline phase of the 24th solar activity cycle with a very low level of solar flare activity, are considered. As a rule, during this phase of the solar activity cycle, magnetic storms are caused by high-speed solar wind flows from coronal holes; however, the magnetic storms during the decay of the 24th cycle were caused by coronal mass ejections (CMEs). It was shown that, despite very weak disturbances on the Sun and a low solar wind speed, a rather strong magnetic storm occurred in the Earth’s magnetosphere in August 2018 (Dst = –171 nT). The storm SC with a slight (~25 nT) jump in the Dst index caused quite intense daytime geomagnetic pulsations ipcl at the latitudes of the possible position of the daytime polar cusp. A feature of the recovery phase of this storm was the development of a magnetosphere substorm on a global scale, i.e., the appearance of a negative magnetic bay recorded simultaneously in the auroral night sector and in the polar latitudes of the day sector. A possible interpretation is considered.

We discuss elemental abundances of noble gases in targets exposed to the solar wind (SW) onboard the “Genesis” mission during the three different SW “regimes”: “Slow” (interstream, IS) wind, “Fast” (coronal hole, CH) wind and solar wind related to coronal mass ejections (CME). To this end we first present new Ar, Kr, and Xe elemental abundance data in Si targets sampling the different regimes. We also discuss He, Ne, and Ar elemental and isotopic abundances obtained on Genesis regime targets partly published previously. Average Kr/Ar ratios for all three regimes are identical to each other within their uncertainties of about 1% with one exception: the Fast SW has a 12% lower Xe/Ar ratio than do the other two regimes. In contrast, the He/Ar and Ne/Ar ratios in the CME targets are higher by more than 20% and 10%, respectively, than the corresponding Fast and Slow SW values, which among themselves vary by no more than 2–4%. Earlier observations on lunar samples and Genesis targets sampling bulk SW wind had shown that Xe, with a first ionisation potential (FIP) of ∼12 eV, is enriched by about a factor of two in the bulk solar wind over Ar and Kr compared to photospheric abundances, similar to many “low FIP” elements with a FIP less than ∼10 eV. This behaviour of the “high FIP” element Xe was not easily explained, also because it has a Coulomb drag factor suggesting a relatively inefficient feeding into the SW acceleration region and hence a depletion relative to other high FIP elements such as Kr and Ar. The about 12% lower enrichment of Xe in Genesis’ Fast SW regime observed here is, however, in line with the hypothesis that the depletion of Xe in the SW due to the Coulomb drag effect is overcompensated as a result of the relatively short ionisation time of Xe in the ion-neutral separation region in the solar chromosphere. We will also discuss the rather surprising fact that He and Ne in CME targets are quite substantially enriched (by 20% and 10%, respectively) relative to the other solar wind regimes, but that this enrichment is not accompanied by an isotopic fractionation. The Ne isotopic data in CMEs are consistent with a previous hypothesis that isotopic fractionation in the solar wind is mass-dependent.

We studied the limb brightening of SOHO EIT daily images of the solar corona taken in 195 A wavelength line from 1996 to 2018. Using special software, we studied the distribution of the background intensity of the entire disk in the direction of rotation (horizontally) and perpendicular to the direction of rotation (vertical). The study shows that the intensity distribution from the center to the limb of the solar disk is not polar-symmetric. A data-derived study showed that the coronal limb brightening has rather elliptical than circular isophotes. The pattern of the limb brightening correlates with the cycle of solar activity. The moving average patterns with the window of the year give good results for removing the limb brightening when preparing coronal images for further study.

Coronal holes can be identified as the darkest regions in EUV or soft X-ray images with predominantly unipolar magnetic fields (LIRs) or as the regions with open magnetic fields (OMF). Our study reveals that only 12% of OMF regions are coincident with LIRs. The aim of this study is to investigate the conditions that affect the EUV intensity of OMF regions. Our results indicate that the EUV intensity and the magnetic field expansion factor of the OMF regions are weakly positively correlated when plotted in logarithmic scale, and that the bright OMF regions are likely to locate inside or next to the regions with closed field lines. We empirically determined a linear relationship between the expansion factor and the EUV intensity. The relationship is demonstrated to improve the consistency from 12% to 23%. The results have been published in Astrophysical Journal (Huang et al. 2019).

Though generated deep inside the convection zone, the solar magnetic field has a direct impact on the Earth space environment via the Parker spiral. It strongly modulates the solar wind in the whole heliosphere, especially its latitudinal and longitudinal speed distribution over the years. However the wind also influences the topology of the coronal magnetic field by opening the magnetic field lines in the coronal holes, which can affect the inner magnetic field of the star by altering the dynamo boundary conditions. This coupling is especially difficult to model because it covers a large variety of spatio-temporal scales. Quasi-static studies have begun to help us unveil how the dynamo-generated magnetic field shapes the wind, but the full interplay between the solar dynamo and the solar wind still eludes our understanding. We use the compressible magnetohydrodynamical (MHD) code PLUTO to compute simultaneously in 2.5D the generation and evolution of magnetic field inside the star via an alpha-Omega dynamo process and the corresponding evolution of a polytropic coronal wind over several activity cycles for a young Sun. A multi-layered boundary condition at the surface of the star connects the inner and outer stellar layers, allowing both to adapt dynamically. Our continuously coupled dynamo-wind model allows us to characterize how the solar wind conditions change as a function of the cycle phase, and also to quantify the evolution of integrated quantities such as the Alfv\'en radius. We further assess the impact of the solar wind on the dynamo itself by comparing our results with and without wind feedback., Comment: 9 pages, 5 figures, IAUS 354

Observations in the HeI 10830 A and Hα lines are used to study the physical conditions in a polar coronal hole. The coronal hole was observed in the northern hemisphere from June 2015 through March 2017. The range of intensity variations in the coronal hole in the HeI 10830 A line was 1.0046-1.0355 relative to the intensity of the unperturbed Sun and in the Hα line, 0.9676-0.9753. The intensities of the coronal hole were calculated for different models of the chromosphere in order to explain the observed features. The temperature and density in the coronal hole were determined as functions of the column density in the chromosphere. The calculations were based on calculations of the HeI 10830 A and Hα line profiles using a non-LTE program. At the level of the chromosphere the temperature in the coronal hole is lower by 500-1500 K than the unperturbed chromosphere and the density is lower by a factor of 2-3.

The study of solar irradiance variability is of great importance in heliophysics, Earth’s climate, and space weather applications. These studies require careful identifying, tracking and monitoring of features in the solar photosphere, chromosphere, and corona. Do coronal bright points contribute to the solar irradiance or its variability as input to the Earth atmosphere? We studied the variability of solar irradiance for a period of 10 years (May 2010 – June 2020) using the Large Yield Radiometer (LYRA), the Sun Watcher using APS and image Processing (SWAP) on board PROBA2, and the Atmospheric Imaging Assembly (AIA), and applied a linear model between the segmented features identified in the EUV images and the solar irradiance measured by LYRA. Based on EUV images from AIA, a spatial possibilistic clustering algorithm (SPoCA) is applied to identify coronal holes (CHs), and a morphological feature detection algorithm is applied to identify active regions (ARs), coronal bright points (BPs), and the quiet Sun (QS). The resulting segmentation maps were then applied on SWAP images, images of all AIA wavelengths, and parameters such as the intensity, fractional area, and contribution of ARs/CHs/BPs/QS features were computed and compared with LYRA irradiance measurements as a proxy for ultraviolet irradiation incident to the Earth atmosphere. We modeled the relation between the solar disk features (ARs, CHs, BPs, and QS) applied to EUV images against the solar irradiance as measured by LYRA and the F10.7 radio flux. A straightforward linear model was used and corresponding coefficients computed using a Bayesian method, indicating a strong influence of active regions to the EUV irradiance as measured at Earth’s atmosphere. It is concluded that the long- and short-term fluctuations of the active regions drive the EUV signal as measured at Earth’s atmosphere. A significant contribution from the bright points to the LYRA irradiance could not be found.

This article implements a Convolutional Neural Network (CNN)-based deep-learning model for solar-wind prediction. Images from the Atmospheric Imaging Assembly (AIA) at 193 A wavelength are used for training. Solar-wind speed is taken from the Advanced Composition Explorer (ACE) located at the Lagrangian L1 point. The proposed CNN architecture is designed from scratch for training with four years’ data. The solar-wind has been ballistically traced back to the Sun assuming a constant speed during propagation, to obtain the corresponding coronal-intensity data from AIA images. This forecasting scheme can predict the solar-wind speed well with a RMSE of 76.3 ± 1.87 km s−1 and an overall correlation coefficient of 0.57 ± 0.02 for the year 2018, while significantly outperforming benchmark models. The threat score for the model is around 0.46 in identifying the HSEs with zero false alarms.

This study investigated the geomagnetic disturbances and high-speed solar-wind streams (HSSs) during the period of solar storms emitted from coronal holes. The contemplated long-term HSS observations were taken from 1963 to 2016. On the basis of the intensity of geomagnetic activity, solar storms were divided into four groups: weak (−50 nT

We undertake a five solar-cycle (SC 19 - 23) ≈55-year (December 1954 to August 2009) study of the high latitude polarity inversion lines (PILs) using the recently digitized McIntosh Archive (McA) of solar synoptic (Carrington) maps. We looked at the evolution of the median solar latitudes of primary and secondary PILs, and of the polar coronal hole (CH) boundary for all 732 Carrington Rotations (CRs). We found hemispheric differences in the “Rush to the Poles” (RttP) where the polar CH gaps are often longer in the southern hemisphere (SH), and the secondary PIL reaches its polemost latitude at the end of its RttP later and more poleward than in the northern hemisphere (NH). The latitude oscillations found after this poleward peak are also stronger and often longer in the SH than in the NH, and exhibit a 22-year variation. The location variations in the CH boundaries and PILs appear to be at least partly associated with similar variations in the magnetic field. We also found equatorward expansions of the polar CHs by ≈50% and equatorward shifts in the PILs that were part of a disturbance that propagated ≈15°/CR from the SH to the NH in the descending phase of SC 23.

Both coronal holes and active regions are source regions of the solar wind. The distribution of these coronal structures across both space and time is well known, but it is unclear how much each source contributes to the solar wind. In this study we use photospheric magnetic field maps observed over the past four solar cycles to estimate what fraction of magnetic open solar flux is rooted in active regions, a proxy for the fraction of all solar wind originating in active regions. We find that the fractional contribution of active regions to the solar wind varies between 30% to 80% at any one time during solar maximum and is negligible at solar minimum, showing a strong correlation with sunspot number. While active regions are typically confined to latitudes $\pm$30$^{\circ}$ in the corona, the solar wind they produce can reach latitudes up to $\pm$60$^{\circ}$. Their fractional contribution to the solar wind also correlates with coronal mass ejection rate, and is highly variable, changing by $\pm$20% on monthly timescales within individual solar maxima. We speculate that these variations could be driven by coronal mass ejections causing reconfigurations of the coronal magnetic field on sub-monthly timescales., Comment: 18 pages, 7 figures. Published in Solar Physics

Streamers and pseudostreamers structure the corona at the largest scales, as seen in both eclipse and coronagraph white-light images. Their inverted-goblet appearance encloses broad coronal loops at the Sun and tapers to a narrow radial stalk away from the star. The streamer associated with the global solar dipole magnetic field is long-lived, predominantly contains a single arcade of nested loops within it, and separates opposite-polarity interplanetary magnetic fields with the heliospheric current sheet (HCS) anchored at its apex. Pseudostreamers, on the other hand, are transient, enclose double arcades of nested loops, and separate like-polarity fields with a dense plasma sheet. We use numerical magnetohydrodynamic simulations to calculate, for the first time, the formation of pseudostreamers in response to photospheric magnetic-field evolution. Convective transport of a minority-polarity flux concentration, initially positioned under one side of a streamer, through the streamer boundary into the adjacent preexisting coronal hole forms the pseudostreamer. Interchange magnetic reconnection at the overlying coronal null point(s) governs the development of the pseudostreamer above—and of a new satellite coronal hole behind—the moving minority polarity. The reconnection dynamics liberate coronal-loop plasma that can escape into the heliosphere along so-called separatrix-web (“S-Web”) arcs, which reach far from the HCS and the solar equatorial plane, and can explain the origin of high-latitude slow solar wind. We describe the implications of our results for in situ and remote-sensing observations of the corona and heliosphere as obtained, most recently, by Parker Solar Probe and Solar Orbiter.

Heavy rainfall events causing floods and flash floods are examined in the context of solar wind coupling to the magnetosphere-ionosphere-atmosphere system. The superposed epoch (SPE) analyses of solar wind variables have shown a tendency of severe weather to follow arrivals of high-speed streams from solar coronal holes (Prikryl et al., 2018). Precipitation datasets based on rain-gauge and satellite sensor measurements are used to examine the relationship between the solar wind high-speed streams and daily precipitation rates over several mid-latitude regions. The SPE analysis results show an increase in occurrence of high precipitation rates following arrivals of high-speed streams, including recurrence with a periodicity of 27 days. The cross-correlation analysis applied to the SPE averages of the green (Fe XIV, 530.3 nm) corona intensity observed by ground-based coronagraphs, solar wind parameters and daily precipitation rates show correlation peaks at lags spaced by solar rotation period. When the SPE analysis is limited to years around the solar minimum (2008–2009), which was dominated by recurrent coronal holes separated by ~120˚ in heliographic longitude, significant cross-correlation peaks are found at lags spaced by 9 days. These results are further demonstrated by cases of heavy rainfall, floods and flash floods in Europe, Japan, and the U.S., highlighting the role of solar wind coupling to the magnetosphere-ionosphere-atmosphere system in severe weather, mediated by aurorally excited atmospheric gravity waves.

The slow solar wind is thought to consist of a component originating close to the Heliospheric Current Sheet (HCS) in the streamer belt and a component from over-expanded coronal hole boundaries. In order to understand the roles of these contributions with different origin, it is important to separate and characterise them. By exploiting the fact that Parker Solar Probe’s fourth and fifth orbits were the same and the solar conditions were similar, we identify intervals of slow polar coronal hole wind sampled at approximately the same heliocentric distance and latitude. Here, solar wind properties are compared, highlighting typical conditions of the slow coronal hole wind closer to the Sun than ever before. We explore different properties of the plasma, including composition, spectra and microphysics, and discuss possible origins for the features that are observed.

Spectroscopic observations at extreme- and far-ultraviolet wavelengths have revealed systematic upflows in the solar transition region and corona. These upflows are best seen in the network structures of the quiet Sun and coronal holes, boundaries of active regions, and dimming regions associated with coronal mass ejections. They have been intensively studied in the past two decades because they are likely to be closely related to the formation of the solar wind and heating of the upper solar atmosphere. We present an overview of the characteristics of these upflows, introduce their possible formation mechanisms, and discuss their potential roles in the mass and energy transport in the solar atmosphere. Although past investigations have greatly improved our understanding of these upflows, they have left us with several outstanding questions and unresolved issues that should be addressed in the future. New observations from the Solar Orbiter mission, the Daniel K. Inouye Solar Telescope, and the Parker Solar Probe will likely provide critical information to advance our understanding of the generation, propagation, and energization of these upflows.

Although the solar wind flows primarily outward from the Sun to interplanetary space, there are times when small-scale plasma inflows are observed. Inward-propagating density fluctuations in polar coronal holes were detected by the COR2 coronagraph on board the STEREO-A spacecraft at heliocentric distances of 7 to 12 solar radii, and these fluctuations appear to undergo substantial deceleration as they move closer to the Sun. Models of linear magnetohydrodynamic waves have not been able to explain these deceleration patterns, so they have been interpreted more recently as jets from coronal sites of magnetic reconnection. In this paper, we develop a range of dynamical models of discrete plasma parcels with the goal of better understanding the observed deceleration trend. We found that parcels with a constant mass do not behave like the observed flows, and neither do parcels undergoing ablative mass loss. However, parcels that accrete mass in a snowplow-like fashion can become decelerated as observed. We also extrapolated OMNI in situ data down to the so-called Alfven surface and found that the initial launch-point for the observed parcels may often be above this critical radius. In other words, in order for the parcels to flow back down to the Sun, their initial speeds are probably somewhat nonlinear (i.e., supra-Alfvenic) and thus the parcels may be associated with structures such as shocks, jets, or shear instabilities., Comment: Accepted for publication in the Astrophysical Journal. 16 pages, 8 figures

We present the sympathetic eruption of a standard and a blowout coronal jets originating from two adjacent coronal bright points (CBP1 and CBP2) in a polar coronal hole, using soft X-ray and extreme ultraviolet observations respectively taken by the Hinode and the Solar Dynamic Observatory. In the event, a collimated jet with obvious westward lateral motion firstly launched from CBP1, during which a small bright point appeared around CBP1's east end, and magnetic flux cancellation was observed within the eruption source region. Based on these characteristics, we interpret the observed jet as a standard jet associated with photosperic magnetic flux cancellation. About 15 minutes later, the westward moving jet spire interacted with CBP2 and resulted in magnetic reconnection between them, which caused the formation of the second jet above CBP2 and the appearance of a bright loop system in-between the two CBPs. In addition, we observed the writhing, kinking, and violent eruption of a small kink structure close to CBP2's west end but inside the jet-base, which made the second jet brighter and broader than the first one. These features suggest that the second jet should be a blowout jet triggered by the magnetic reconnection between CBP2 and the spire of the first jet. We conclude that the two successive jets were physically connected to each other rather than a temporal coincidence, and this observation also suggests that coronal jets can be triggered by external eruptions or disturbances, besides internal magnetic activities or magnetohydrodynamic instabilities., Comment: 11 pages, 5 figures, accepted for publication in ApJL