Deep Minimum Of Solar Activity Research Articles

Observation of the Solar Eclipse on June 10, 2021 on the Water Vapor Radiometer of the Institute of Applied Astronomy, Russian Academy of Sciences

The results of observations of the solar eclipse on June 10, 2021, which were carried out at the Svetloeobservatory (60.53° N, 29.78° E, Leningrad Province) for the first time using the water vapor radiometerand radio telescopes RT-13 and RT-32 together are presented. The eclipse curves obtained at the radiometeroperating frequencies of 20.7 and 31.4 GHz were compared with the change in the area of the open part of theSun and show good agreement (match to an accuracy of 2% during the cloudless period). Assuming a uniformdistribution of radio brightness across the solar disk, the brightness temperature was estimated to be 9060 ± 380and 8050 ± 300 K at these frequencies, respectively. It was concluded that even in the epoch of a deep minimumof solar activity, the brightness of the solar disk in the microwave range remains unchanged (independentof the cycle number)

Midlatitude Pc1 Geomagnetic Pulsations: Results of Observations and Statistical Estimates

An algorithm is developed for automated detection of the short-period Pc1 geomagnetic pulsations (frequency band f = 0.2–3 Hz) from the continuous time series of digital recording during 1998–2014 at the midlatitude Borok station. A digital catalog with the indication of time intervals of the presence and main morphological characteristics of Pc1 pulsations is created. Based on this catalog, the annual, seasonal, and diurnal dynamics of the midlatitude Pc1 pulsation activity is studied for 1998–2014. It is shown that the annual variation of the Pc1 occurrence has a maximum in 2005, i.e., at the end of the solar cycle decay phase, just as in the previous cycles. It is found that the minimum of the cases of Pc1 occurrence is observed in 2009, i.e., not at the maximum, just was the case in the previous cycles, but during the deep minimum of solar activity, which testifies to the untypical conditions in the magnetosphere during the unusually long minimum of the 23rd cycle. The seasonal variation of the Pc1 occurrence has a summer minimum when the series of Pc1 pulsations occur almost thrice as rarely as in winter. Besides, there are relatively small maxima at equinox. The diurnal behavior of Pc1 pulsations has the maxima in the morning and midnight sectors of the magnetosphere. By the superposed epoch analysis technique it is established that the maximal number of the cases of occurrence of Pc1 pulsations at the Borok observatory is observed on the fourth day after the global geomagnetic disturbances. The statistical distributions of pulsations amplitude and duration are obtained.

Galactic Cosmic Ray Intensity in the Upcoming Minimum of the Solar Activity Cycle

During the prolonged and deep minimum of solar activity between cycles 23 and 24, an unusual behavior of the heliospheric characteristics and increased intensity of galactic cosmic rays (GCRs) near the Earth’s orbit were observed. The maximum of the current solar cycle 24 is lower than the previous one, and the decline in solar and, therefore, heliospheric activity is expected to continue in the next cycle. In these conditions, it is important for an understanding of the process of GCR modulation in the heliosphere, as well as for applied purposes (evaluation of the radiation safety of planned space flights, etc.), to estimate quantitatively the possible GCR characteristics near the Earth in the upcoming solar minimum (~2019–2020). Our estimation is based on the prediction of the heliospheric characteristics that are important for cosmic ray modulation, as well as on numeric calculations of GCR intensity. Additionally, we consider the distribution of the intensity and other GCR characteristics in the heliosphere and discuss the intercycle variations in the GCR characteristics that are integral for the whole heliosphere (total energy, mean energy, and charge).

New results in studying foF2 trends

It is shown that there are three causes of the difference in values of trends in the F2-layer parameters obtained by different groups of authors: using initial data series of different length, the absence of the allowance for seasonal changes in trends and the absence of the allowance for diurnal changes in trends. The problem how the deep minimum of solar activity of 2008—2009 influenced the trends in the critical frequency of the F2 layer and whether are the negative trends in foF2 conserved in the following five years is considered. It is obtained that in the vast majority of cases, the monotonous change in the curves used to derive the foF2 trend was distorted in the years close to the anomalously low minimum of solar activity in 2008—2009. That reduced the magnitude of the obtained negative trend in the critical frequency as compared with the results based on the data to 2009. The change in the dependence of the F2-layer critical frequency on its height hmF2 is considered based on two sources of initial data. It is obtained that the slope k of the foF2 dependence on hmF2 systematically decreases from the earlier (“etalon”) period 1958–1980 to later periods 1988–2010, 1998–2010, and 1998–2014. As far as the foF2 value depends on the atomic oxygen concentration at heights of the F region much stronger than hmF2, the found decrease in k confirms the concept of a decrease in the atomic oxygen concentration in the thermosphere with time formulated by the authors earlier.

The vorticity of Solar photospheric flows on the scale of granulation

We employ time sequences of images observed with a G-band filter (4305{\AA}) by the Solar Optical Telescope (SOT) on board of Hinode spacecraft at different latitude along solar central me-ridian to study vorticity of granular flows in quiet Sun areas during deep minimum of solar activity. Using a feature correlation tracking (FCT) technique, we calculate the vorticity of granular-scale flows. Assuming the known pattern of vertical flows (upward in granules and downward in inter-granular lanes), we infer the sign of kinetic helicity of these flows. We show that the kinetic helicity of granular flows and intergranular vortices exhibits a weak hemispheric preference, which is in agreement with the action of the Coriolis force. This slight hemispheric sign asymmetry, however, is not statistically significant given large scatter in the average vorticity. The sign of the current he-licity density of network magnetic fields computed using full disk vector magnetograms from the Synoptic Optical Long-term Investigations of the Sun (SOLIS) does not show any hemispheric preference. The combination of these two findings suggests that the photospheric dynamo operating on the scale of granular flows is non-helical in nature.

Trends in the critical frequency foF2 after 2009

The problem of how the deep minimum of solar activity of 2008–2009 influenced trends in the critical frequency of the F2 layer and whether the negative trends in foF2 are conserved in the following five years is considered in the paper. The initial data series used in previous publications by the authors are completed by data to 2014 (2015). Methods of searching for trends that were described in the past several times by the authors were applied. It was found that, in the vast majority of cases, the monotonous change in the curves used to derive the foF2 trend was distorted in years close to the anomalously low minimum of solar activity in 2008–2009. That decreased the obtained trend in the critical frequency as compared with the results based on the data to 2009. However, subsequent years again lead to a tendency toward monotonous decrease in foF2. The causes of the anomalies indicated above lie apparently in the inadequacy of the solar activity index F10.7 for the description of changes in the aeronomical parameters determining the foF2 behavior in the period of a very deep minimum of activity that is widely described in some publications.

Diurnal and longitudinal variations in the earth’s ionosphere in the period of solstice in conditions of a deep minimum of solar activity

The results of studies of longitudinal and LT variations in parameters of the ionosphere–plasmasphere system, obtained using the Global Self-Consistent Model of the Thermosphere, Ionosphere and Protonosphere (GSM TIP), assimilation ionospheric model IRI Real-Time Assimilation Mapping (IRTAM), and satellite and ground-based observational data are presented in the paper. The study of the main morphological features of longitudinal and LT variations in the critical frequency of the ionospheric F2 layer (foF2) and total electron content (TEC) depending on latitude in the winter solstice during a solar-activity minimum (December 22, 2009) is carried out. It is shown that the variations in foF2 and TEC, on the whole, are identical, and so mutually substitutable, while creating empirical models of these parameters in quiet geomagnetic conditions. The longitudinal and LT variations in both foF2 and TEC are within an order of magnitude everywhere except for the equator anomaly region, where LT variation is larger by an order of magnitude than longitudinal variation. According to the results of the study, in the American longitudinal sector at all latitudes of the Southern (summer) Hemisphere, maxima of foF2 and TEC are formed. The near-equatorial and high-latitudinal maxima are separated out from these. The estimate of the contribution into the longitudinal variation in foF2 and TEC for various local time sectors and at various latitudes has been obtained for the first time. In the Southern (summer) Hemisphere, longitudinal variation in foF2 and TEC is formed in the nighttime.

Coronal Radio Occultation Experiments with the Helios Solar Probes: Correlation/Spectral Analysis of Faraday Rotation Fluctuations

The coronal Faraday rotation (FR) experiments using the linearly polarized signals of the Helios-1 and Helios-2 interplanetary probes remain a unique investigation of the magnetic field of the solar corona and its aperiodic and quasi-periodic variations. The unexpectedly long lifetime of these spacecraft (1974 – 1986) enabled studies from very deep solar-activity minimum (1975 – 1976) into the strong activity maximum (1979). Important experimental data were also obtained for the rising (1977 – 1978) and declining (1980 – 1984) branches of the solar-activity cycle. Previous publications have presented results of the initial experimental data only for coronal-sounding experiments performed during individual solar-conjunction opportunities. This report is a more detailed analysis of the Helios FR measurements for the entire period 1975 – 1984. Radial profiles of the FR fluctuation (FRF) intensity recorded during the deepest solar-activity minimum in 1975 – 1976 are shown to differ distinctly from those during the strong solar-activity maximum in 1979. In particular, the decrease of the FRF intensity with solar-offset distance is substantially steeper in 1979 than in 1975/1976. In all cases, however, the FR data reveal quasi-periodic wave-like fluctuations in addition to the random background with a power-law spectrum. The dominant period of these fluctuations, recorded during 35 % of the total measurement time, is found to be close to five minutes. Large-scale FR variations at considerably longer periods (1.1 – 2.7 hours) were observed during 20 % of the measurement time. Knowing the intrinsic motion of the radio ray path from spacecraft to Earth and making a reasonable assumption about the solar-wind velocity, FRF observations at widely spaced ground stations have been used to estimate the velocity of coronal Alfven waves. The velocity values range between 290 and 550 km s−1 at heliocentric distances between 3.5 and 4.5 R⊙ and are marginally lower (150 – 450 km s−1) at distances between 5.5 and 6.5 R⊙. Occasional FR variations with a period near 160 minutes and harmonics with periods 60, 30, and 20 minutes were also observed.

On the influence of climatic factors on the ratio between the cosmogenic isotope 14C and total carbon in the atmosphere in the past

Abstract Radiocarbon 14C is a cosmogenic isotope, which is most extensively used by scientists from a wide variety of fields. Its rate of generation in the atmosphere depends on solar modulation and thus, studying 14C concentration in natural archives, one can reconstruct solar activity level in the past. The paper shows results of box-model calculations of generation of the 14C isotope in the atmosphere and its relative abundance during the time interval 1389–1800 AD, taking into account influence of changing climate. This interval includes the deep minimum of solar activity and period of significant change in atmospheric concentration of CO2 and global temperature. The performed analysis showed that concentration of 14C in the atmosphere reflects not only variations of the galactic cosmic rays intensity but as well changes of temperature and atmospheric CO2 concentration. It is shown that the decrease in CO2 concentration in the atmosphere during 1550–1600 can be connected with absorption of CO2 by the ocean surface layer. Thus, taking into account the climatic changes is an important condition for the reconstruction of solar activity in the past using data based on cosmogenic isotopes.

3D Coronal Density Reconstruction and Retrieving the Magnetic Field Structure during Solar Minimum

Measurement of the coronal magnetic field is a crucial ingredient in understanding the nature of solar coronal phenomena at all scales. We employed STEREO/COR1 data obtained during a deep minimum of solar activity in February 2008 (Carrington Rotation CR 2066) to retrieve and analyze the three-dimensional (3D) coronal electron density in the range of heights from 1.5 to 4 R⊙ using a tomography method. With this, we qualitatively deduced structures of the coronal magnetic field. The 3D electron-density analysis is complemented by the 3D STEREO/EUVI emissivity in the 195 Å band obtained by tomography for the same CR. A global 3D MHD model of the solar corona was used to relate the reconstructed 3D density and emissivity to open/closed magnetic-field structures. We show that the density-maximum locations can serve as an indicator of current-sheet position, while the locations of the density-gradient maximum can be a reliable indicator of coronal-hole boundaries. We find that the magnetic-field configuration during CR 2066 has a tendency to become radially open at heliocentric distances greater than 2.5 R⊙. We also find that the potential-field model with a fixed source surface is inconsistent with the boundaries between the regions with open and closed magnetic-field structures. This indicates that the assumption of the potential nature of the coronal global magnetic field is not satisfied even during the deep solar minimum. Results of our 3D density reconstruction will help to constrain solar coronal-field models and test the accuracy of the magnetic-field approximations for coronal modeling.

The sunspot cycle length – modulated by planets?

The Schwabe frequency band of the sunspot record since 1700 has an average period of 11.06 yr and contains four major cycles, with periods of 9.97, 10.66, 11.01 and 11.83 yr. Analysis of the O-C residuals of the timing of solar cycle minima reveals that the solar cycle length is modulated by a secular period of about 190 yr and the Gleissberg period of about 86 yr. Based on a simple harmonic model with these periods, we predict that the solar cycle length will in average be longer during the 21st century. Cycle 24 may be about 12 yr long, while cycles 25 and 26 are estimated to be about 9 and 11 yr long. The following cycle is estimated to be 14 yr long. In all periods during the last 1000 yr, when the solar cycle length has increased due to the 190 yr cycle, a deep minimum of solar activity has occurred. This is expected to re-occur in the first part of this century. The coherent modulation of the solar cycle length over a period of 400 yr is a strong argument for an external tidal forcing by the planets Venus, Earth, Jupiter and Saturn, as expressed in a spin-orbit coupling model.

Deep solar activity minima, sharp climate changes, and their impact on ancient civilizations

It is shown that, over the past ∼10000 years (the Holocene), deep Maunder type solar minima have been accompanied by sharp climate changes. These minima occurred every 2300–2400 years. It has been established experimentally that, at ca 4.0 ka BP, there occurred a global change in the structure of atmospheric circulation, which coincided in time with the discharge of glacial masses from Greenland to North Atlantic and a solar activity minimum. The climate changes that took place at ca 4.0 ka BP and the deep solar activity minimum that occurred at ca 2.5 ka BP affected the development of human society, leading to the degradation and destruction of a number of ancient civilizations.

Interplanetary scintillations of an ensemble of radio sources during the solar activity minimum of cycles 23/24

The results of observations of interplanetary scintillations of a statistical ensemble of radio sources in the period of 2007–2011 are presented. Observation were carried out in the monitoring regime with the BSA LPI radio telescope at the frequency 111 MHz. Fluctuations of radio emission flux of all sources (a few hundred in total) were recorded 24 hours a day. Those sources were investigated, which had a scintillating flux greater than 0.2 Jy and fell within the sky band of 8° width in declination, corresponding to radio telescope’s 16-beam system. The statistical ensemble of radio sources is characterized by the mean variance of a scintillating radiation flux, which is proportional to the squared scintillation index. It follows from the obtained data that the radial dependence of a mean scintillation index during a deep solar activity minimum of 2008–2009 occurs to be weaker than one could expect in the case of spherically symmetric geometry of the solar wind. Suppression of a radial dependence of the mean scintillation index is explained by the effect of the heliospheric current sheet, which reveals itself in a high density of solar wind’s turbulent plasma in the helioequator plane. It is shown that the level of scintillations, averaged over monthly series of observations, was changing synchronously with the solar activity level.

Measurements of the total electron content of the ionosphere by the radioastronomy Faraday method

In 2008 and 2009, when a deep solar-activity minimum was observed, the total electron content of the ionosphere was measured by the radioastronomy method using the ionospheric Faraday rotation of the polarization plane of a linear-polarized component of the diffuse radio-frequency (RF) radiation of the galaxy. In the measurements performed at a frequency of 290 MHz at the Radioastronomy observatory “Staraya Pustyn” of the Radiophysical Research Institute of Nizhny Novgorod, the previously studied region of the Celestial North Pole was used. The brightness temperature was calibrated by the Cassiopeia A radio source. The time dependence of the Cassiopeia The RF radiation stream density, which was obtained in the period of 1978–2011 from the measurement results at a frequency of 290 MHz, is shown. A new method of accounting for a spurious polarized signal allowing for its time dependence was developed and used, which makes it possible to improve the measurement frequency of the time variations of total electron content of the ionosphere. This method allowed us to more accurately measure the brightness temperature of the linearly polarized component of diffuse galaxy RF radiation in the direction of the Celestial North Pole, which amounted to 0.83 ± 0.07 K at a frequency of 290 MHz. For this region of the horizon, we demonstrate the spectra of the brightness temperature and the position angle of the polarization plane of the linearly polarized component of the galaxy diffuse RF radiation ranging from meter to centimeter wavelengths, which allows us to choose the required frequency for the measurements by the radioastronomy Faraday method in the future. The values of the total electron content of the ionosphere are compared with the corresponding values from the global ionospheric maps obtained from observations of the GPS satellite signals. The difference between the average values of total electron content of the ionosphere is 1.9∙1012 cm-2 according to all radioastronomy measurements and the corresponding means from the global ionospheric maps.

Auroral kilometric radio emission in the period of quiet sun in 1996

The auroral kilometric radio emission (AKR) is the most powerful sporadic radio emission of the terrestrial magnetosphere. It was discovered in 1965 by Soviet scientists in the experiment onboard the Electron-2 satellite [1]. The AKR still continues to stay an object of a large interest and detailed study (see, for example, a review by Gurnett [2]). The mechanism of cyclotron maser instability proposed by Wu and Lee [3] is a commonly accepted mechanism of AKR generation. We have demonstrated the presence of powerful AKR simultaneously in both hemispheres of the Earth in the period from August 1995 to August 1997, including summer-winter periods, on particular examples of registration of this emission in [4] where the directivity and mechanism of the emission were studied. Since in that period AKR was observed in the vicinity of perigees of the satellite orbit in both hemispheres almost at every orbit (3.8 days), we have a possibility to trace in more detail the changes in the emission power from one orbit to another in 1996 during a deep minimum of solar activity.

Experimental evidence for direct penetration of ULF waves from the solar wind and their possible effect on acceleration of radiation belt electrons

The event of March 12–19, 2009, when a moderately high-speed solar wind stream flew around the Earth’s magnetosphere and carried millihertz ultralow-frequency (ULF) waves, has been analyzed. The stream caused a weak magnetic storm (D st min = −28 nT). Since March 13, fluxes of energetic (up to relativistic) electrons started increasing in the magnetosphere. Comparison of the spectra of ULF oscillations observed in the solar wind and magnetosphere and on the Earth’s surface indicated that a stable common spectral peak was present at frequencies of 3–4 mHz. This fact is interpreted as evidence that waves penetrated directly from the solar wind into the magnetosphere. Possible scenarios describing the participation of oscillations in the acceleration of medium-energy (E > 0.6 MeV) and high-energy (E > 2.0 MeV) electrons in the radiation belt are discussed. Based on comparing the event with the moderate magnetic storm of January 21–22, 2005, we concluded that favorable conditions for analyzing the interaction between the solar wind and the magnetosphere are formed during a deep minimum of solar activity.

Interplanetary and Ionosphere Scintillation Monitoring of Radio Sources Ensemble at the Solar Activity Minimum

Results are presented of twenty-four-hour interplanetary and ionosphere scintillation monitoring obtained in Pushchino Radioastronomy Observatory during three observation series: between 18 November and 30 December 2006; between 11 June and 18 June 2007; between 20 October and 4 November 2008. Observations were carried out simultaneously in 16 beams of the radio telescope BSA LPI (Big Scanning Array of Lebedev Physical Institute) at the frequency 111 MHz. All the sources with scintillating flux greater than 0.2 Jy were recorded in the range of declination from +3° to +12°40 by observations in the years 2006, 2008 and from 21° to 28° in the year 2007. The values of radio source flux fluctuation temporal structure functions were measured at the temporal lags of 0.1 s, 1 s and 10 s, which characterize noise, interplanetary scintillations and ionosphere scintillations, respectively. About 1000 scintillating radio sources were observed every day. The number of sources N (σ IPS≥σ IPS,0) with scintillating flux σ IPS greater than the given value σ IPS,0 in the sky area with sizes 8° in declination and 0.5 hours in right ascension was used as the parameter describing turbulent interplanetary plasma. This parameter is shown to be proportional to the mean scintillation index of the radio source statistical ensemble in the given sky area. Similarly, the parameter N (σ IONS≥σ IONS,0) was introduced for a description of the turbulent ionosphere. The mean twenty-four-hour temporal dependence of the scintillation index is found. Weak day-to-day variations of the scintillation index were observed. In general, interplanetary plasma, as well as the ionosphere, was in a quiet state during the observation periods. Unusually a weak dependence of scintillation index on the position of the sources relative to the Sun was observed in the year 2008 at very deep solar activity minimum. Such a weak dependence is explained by a strong elongation of the turbulence level spatial distribution along the solar equatorial plane during a very deep minimum of solar activity.

Beginning of the new solar cycle (cycle 24) in the large-scale open solar magnetic field

It is proposed to determined minimums of the 11-year solar cycles based on a minimal flux of the large-scale open solar magnetic field. The minimal fluxes before the finished cycle 23 (Carrington rotation CR 1904) and the started cycle 24 (CR 2054, April 2007) were equal to 1.8 × 1022 and 1.2 × 1022 μs, respectively. The long-term tendency toward an approach to a deep minimum of solar activity is confirmed. On the assumption that magnetic flux variations from minimums to maximums are proportional to each other, the anticipated value of the maximal Wolf number during cycle 24 is estimated as Wmax = 80.