Period Of Minimum Solar Activity Research Articles

Long-Term Microwave Observations of Middle Atmospheric Ozone in Apatity during Three Winters

The results of ground-based microwave observations of ozone in the middle atmosphere in Apatity(67° N, 33° E) during three winters (2017–2018, 2018–2019 and 2019–2020) are presented. Long-term ozoneobservations were carried out during the period of minimum solar activity for cycles 24 and 25. A mobilemicrowave spectrometer with an operating frequency of 110.8 GHz was used in the measurements, whichallows tracking the behavior of ozone in the middle atmosphere with a 15-minute time resolution. The microwaveozone data from ground-based measurements are compared with the MLS/Aura onboard data. Groundand airborne data are compared with the data of contact measurements with ozonesondes at Sodankyla st.(67° N, 27° E). In addition, MLS/Aura data from mid-atmospheric temperature soundings are used to interpretperturbations in the ozone layer associated with sudden stratospheric warmings. A significant influencesudden stratospheric warming on the ozone vertical distribution at altitudes of 22–60 km was found. At thesame time, the scale of mesospheric ozone variability (60 km) over Apatity is comparable or exceeds theknown model calculations for assessing the impact of solar proton events and auroral electron precipitationon the ozone of the polar regions.

Relation of Microstreams in the Polar Solar Wind to Switchbacks and Coronal X-Ray Jets

Abstract Ulysses data obtained at high solar latitudes during periods of minimum solar activity in 1994 and 2007 are examined to determine the relation between velocity structures called microstreams and folds in the magnetic field called switchbacks. A high correlation is found. The possibility of velocity peaks in microstreams originating from coronal X-ray jets is reexamined; we now suggest that microstreams are the consequence of the alternation of patches of switchbacks and quiet periods, where the switchbacks could be generated by minifilament/flux rope eruptions that cause coronal jets.

Analysis of Study Results of the Polar Coronal Hole on the Sun According to Observations in the Microwave Wavelength Range

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.

Quiet-Sun hydrogen Lyman-α line profile derived from SOHO/SUMER solar-disk observations

Context. The solar radiation in the Lyman-α spectral line of hydrogen plays a significant role in the illumination of chromospheric and coronal structures, such as prominences, spicules, chromospheric fibrils, cores of coronal mass ejections, and solar wind. Moreover, it is important for the investigation of the heliosphere, Earth’s ionosphere, and the atmospheres of planets, moons, and comets. Aims. We derive a reference quiet-Sun Lyman-α spectral profile that is representative of the Lyman-α radiation from the solar disk during a minimum of solar activity. This profile can serve as an incident radiation boundary condition for the radiative transfer modelling of chromospheric and coronal structures. Because the solar radiation in the Lyman lines is not constant over time but varies significantly with the solar cycle, we provide a method for the adaptation of the incident radiation Lyman line profiles (Lyman-α and higher lines) to a specific date. Moreover, we analyse how the change in the incident radiation influences the synthetic spectra produced by the radiative transfer modelling. Methods. We used SOHO/SUMER Lyman-α raster scans obtained without the use of the attenuator in various quiet-Sun regions on the solar disk. The observations were performed on three consecutive days (June 24, 25, and 26, 2008) during a period of minimum solar activity. The reference Lyman-α profile was obtained as a spatial average over eight available raster scans. To take into account the Lyman-α variation with the solar cycle, we used the LISIRD composite Lyman-α index. To estimate the influence of the change in the incident radiation in the Lyman lines on the results of radiative transfer models, we used a 2D prominence fine structure model. Results. We present the reference quiet-Sun Lyman-α profile and a table of coefficients describing the variation of the Lyman lines with the solar cycle throughout the lifetime of SOHO. The analysis of the influence of the change in the incident radiation shows that the synthetic spectra are strongly affected by the modification of the incident radiation boundary condition. The most pronounced impact is on the central and integrated intensities of the Lyman lines. There, the change in the synthetic spectra can often have the same amplitude as the change in the incident radiation itself. The impact on the specific intensities in the peaks of reversed Lyman-line profiles is smaller but still significant. The hydrogen Hα line can also be considerably affected, despite the fact that the Hα radiation from the solar disk does not vary with the solar cycle.

Evolutionary Changes in the Configuration of the Magnetic Field of the Solar Corona in the Epoch of the Minimum

The evolution of the magnetic configuration in the solar corona is studied. The curvature of the K-corona helmets obtained from an analytical approximation of the geometry of the helmet axis was used as an indicator of the configuration change. The period of minimum solar activity was analyzed with solar eclipse materials. As a result of the analysis, the dependence of the curvature value on the latitude of the position of the helmet base and the phase of the solar cycle has been obtained. It has been established that this dependence is different for the period of solar-activity growth (the beginning of the cycle) and its decline. It was found that there is a sharp decrease in the curvature to almost zero at the beginning of the solar cycle. During the decline of the 23rd cycle and at the beginning of the 24th cycle, the change of the curvature of the helmets differs from the trend of the previous solar cycles. The apparent reason for this is a change in the conditions of the generation of the solar magnetic field. The found latitudinal–phase dependences of the curvature of the coronal helmets indicate a difference in the coronal magnetic configuration during periods of decline and increase in solar activity, as well as a sharp reorganization of the configuration at the beginning of the new solar cycle.

Cosmogenic Radionuclides In Meteorites and Solar Modulation of Galactic Cosmic Rays In the Internal Heliosphere

Cosmogenic radionuclides with half-life periods T1/2 ranging from several days to a million years, produced in the nuclear reactions of galactic cosmic rays (GCR) with meteoritic matter, provide valuable information on the GCR intensity variations on a long time scale (∼1 million years) within meteorite orbits 2–4 AU from the Sun. Information on the variations of GCR gradients in the inner heliosphere was obtained by comparing the measured contents of 54Mn and 22Na in stone meteorites (chondrites) with known orbits at the time of their fall to Earth with the calculated production rates of these radionuclides in them using balloon intensity measurements of GCR (E > 100 MeV) in the stratosphere at appropriate times. Although individual gradient values show significant uncertainties, the important information is that cosmogenic radionuclides in chondrites predict low gradients (0–10% per 1 AU) for all periods of minimum solar activity in 1957–2013, according to direct measurements in interplanetary space. High gradients (50–100% per 1 AU) are predicted for periods of maximum solar activity, especially in 1992 and 2012 (up to ∼200% per 1 AU). Average values of gradients are (20 ± 10)% per 1 AU for modern solar cycles (according to the production rate of 22Na), similar to the average values for the last ∼1 million years (according to the production rate of 26Al), which indicates the constancy of the solar modulation of the GCR, at least for the last million years.

Manifestation of the Identity of Brightness Temperatures of the Polar Coronal Hole and Low-Latitude Coronal Holes in the Centimeter Wave Range

This paper is devoted to an analysis of brightness temperatures of the polar and low-latitude coronal holes on the Sun in the cm-wave range during periods of minimum solar activity. Data from observations of the polar coronal hole received by the RATAN-600 radio telescope during the solar eclipse of March 29, 2006, and low-latitude observations of coronal holes and quiet Sun received earlier with the RATAN-600 and BPR radio telescopes in the period of minimum solar activity have been used in the paper. The obtained good agreement between the brightness temperatures of cm-wave emission of the polar coronal hole above the North Pole of the Sun and of the low-latitude coronal holes against the background of the quiet Sun reveals the identity of the temperature properties of large coronal holes, irrespective of the mode of their arrangement and location on the Sun during the periods of minimum solar activity.

NEW EVIDENCE FOR CHARGE-SIGN-DEPENDENT MODULATION DURING THE SOLAR MINIMUM OF 2006 TO 2009

ABSTRACT The PAMELA space experiment, in orbit since 2006, has measured cosmic rays (CRs) through the most recent period of minimum solar activity with the magnetic field polarity as A < 0. During this entire time, galactic electrons and protons have been detected down to 70 MV and 400 MV, respectively, and their differential variation in intensity with time has been monitored with unprecedented accuracy. These observations are used to show how differently electrons and protons responded to the quiet modulation conditions that prevailed from 2006 to 2009. It is well known that particle drifts, as one of four major mechanisms for the solar modulation of CRs, cause charge-sign-dependent solar modulation. Periods of minimum solar activity provide optimal conditions in which to study these drift effects. The observed behavior is compared to the solutions of a three-dimensional model for CRs in the heliosphere, including drifts. The numerical results confirm that the difference in the evolution of electron and proton spectra during the last prolonged solar minimum is attributed to a large extent to particle drifts. We therefore present new evidence of charge-sign-dependent solar modulation, with a perspective on its peculiarities for the observed period from 2006 to 2009.

Suprathermal ions in solar-wind outflows from coronal holes at 1 AU

The energy spectra and relative abundances of 3He, 4He, C, O, and Fe ions with energies of ~0.04–2 MeV/nucleon are studied using data from the ULEIS instrument on board the ACE spacecraft obtained during quiescent periods in 2006–2012. During the unique, prolonged minimum between cycles 23 and 24, 35 quiescent periods were distnguished, during which solar-wind flows from near-equatorial coronal holes (CHs) were detected. It is shown that the C/O and Fe/O ratios for suprathermal ions correspond to the relative abundances of the corresponding thermal ions in the fast and slow (Maxwellian) solar wind (SWICS/ACE), while the 4He/O ratio exceeds the corresponding ratio in the solar wind by a factor of two. The intensities of the 3He, 4He, C, O, and Fe suprathermal ions in outflows from CHs grow with the speed of the solar wind. This indicates that, in periods ofminimumsolar activity, suprathermal ions from CHs represent a high-temperature “tail” of the solar wind. An additional flux of suprathermal helium ions may also be contributed by other external sources.

Factors controlling typhoons and storm rain on the Korean Peninsula during the Little Ice Age

Documenting multi-decadal typhoon and storm-rain variability is useful to prevent future typhoon and flood disasters. We present the history of typhoon and storm-rain activity in East Asia inferred from multi-proxy analyses of Lagoon Hwajin-po sediments along the eastern coast of Korea. Anthropogenic effects were enhanced in Lagoon Hwajin-po since ca. AD 1900, by increasing farming in the catchment. To avoid these human-induced effects, we reconstructed the history of typhoon and storm-rain activity only for the interval AD 1400–1900. The record indicates that typhoon frequency throughout the Korean Peninsula varied in response to the state of the El Nino/Southern Oscillation. Typhoon variability was likely modulated further by the state of the East Asia summer monsoon (EASM) pattern, associated with variation in the magnitude of solar irradiance. During periods of minimum solar activity, such as the early Maunder Minimum (AD 1650–1675), typhoons struck the east China coast and Korean Peninsula more frequently because of a strengthened EASM.

The occurrence of equatorial spread-F at conjugate stations in Southeast Asia

In this study, the probability of equatorial spread-F (ESF) occurrence at the conjugate stations in Southeast Asia: Chiangmai station (CMU), Thailand, and Kototabang station (KTB), Indonesia, and near the magnetic equator, Chumphon station (CPN), Thailand, is presented. We analyze the ionogram data recorded by the Frequency Modulated Continuous Wave (FM/CW) ionosondes for the periods of minimum solar activity from September 2008 to April 2009 and in the equinoctial months (March and April) from 2006 to 2013. The spread-F signatures are manually categorized into three types: the frequency spread-F (FSF), the range spread-F (RSF) and the mixed spread-F (MSF) and the monthly average percentage of the occurrence of each ESF type is presented. The results show that the percentage of RSF occurrence at CPN, which is located around the magnetic equator, is higher than at other stations and the RSF mostly occurs during the equinoctial months. On the other hand, the FSF occurrence at CMU and KTB, that are located in the northern and southern hemispheres, respectively, are higher than at CPN. The RSF occurrence typically has the peaks before midnight, while the maximum occurrence rate of FSF is after midnight. Furthermore, the RSF onsets normally precede the FSF onsets by about 1–2h. As the solar activity levels go up, the percentages of RSF occurrence increase, but the percentages of FSF tends to decrease. In addition, we compare the statistics of observed RSF occurrence with the prediction of the IRI-2012 model. The results show that the IRI model overestimates the observed RSF occurrence at all stations during most of the solar activity levels and seasons, except in June 2008 (21:00–03:00LT), March 2011 (23:30–01:00LT) and March 2013 (01:00–02:30LT) when the IRI model underestimates our observations. However, the IRI model gives closer probability of RSF occurrence to our observed values at CPN, especially in equinoctial months and during the periods of medium solar activity. This work is important for an improvement of the IRI model in the prediction of the spread F occurrence probability in the low-latitude region.

Cosmic-Ray Modulation: an Ab Initio Approach

A better understanding of cosmic-ray modulation in the heliosphere can only be gained through a proper understanding of the effects of turbulence on the diffusion and drift of cosmic rays. We present an ab initio model for cosmic-ray modulation, incorporating for the first time the results yielded by a two-component turbulence transport model. This model is solved for periods of minimum solar activity, utilizing boundary values chosen so that model results are in fair to good agreement with spacecraft observations of turbulence quantities, not only in the solar ecliptic plane but also along the out-of-ecliptic trajectory of the Ulysses spacecraft. These results are employed as inputs for modelled slab and 2D turbulence energy spectra. The latter spectrum is chosen based on physical considerations, with a drop-off at the very lowest wavenumbers commencing at the 2D outerscale. There currently exist no models or observations for this quantity, and it is the only free parameter in this study. The modelled turbulence spectra are used as inputs for parallel mean free path expressions based on those derived from quasi-linear theory and perpendicular mean free paths from extended nonlinear guiding center theory. Furthermore, the effects of turbulence on cosmic-ray drifts are modelled in a self-consistent way, employing a recently developed model for drift along the wavy current sheet. The resulting diffusion coefficients and drift expressions are applied to the study of galactic cosmic-ray protons and antiprotons using a three-dimensional, steady-state cosmic-ray modulation code, and sample solutions in fair agreement with multiple spacecraft observations are presented.

The effects of the solar magnetic polarity and the solar wind velocity on Bz-component of the interplanetary magnetic field

We have studied the effect of both solar magnetic polarity and the solar wind velocity on the Bz-component of the interplanetary magnetic field, IMFBz, for the minimum activity of the solar cycles 21, 22, 23 and 24. We made a statistical study of IMFBz in the first section which is considered as an extension of Lyatsky et al. (2003). They made a statistical study of IMFBz for two periods of minimum solar activity 22 and 23 related to 1985–1987 and 1995–1997 when the solar magnetic field had opposite polarity. Our results seem to be consistent with the results obtained by Lyatsky et al. (2003). We found that there is a dependence of IMFBz on the IMFBx and the solar magnetic polarity for the minimum periods of the selected four solar cycles. In addition, we found that there is a dependence of IMFBz on the solar wind velocity.

The Influence of Solar Activity on the Investors' Behaviour in the Stock Markets (Влияние Солнечной Активности На Поведение Инвесторов На Фондовых Рынках - In Russian)

The main aim of the article is to find a statistically significant correlation between solar activity and some indicators of the stock market. Some experts suppose that the financial crisis of 2008-2009 was partly caused by a period of minimum solar activity. Nevertheless, in this study, based on the analysis of extensive historical data, connection between solar activity and the stock market was not found. Author assume that natural factors, which include the influence of the Sun, create only a background for the decisions made by investors in the markets, and the adequate assessment of their significance will be possible only within the multifactor model, which also includes macroeconomic and other important indicators.

The centenary variations in the solar corona shape in accordance with the observations during the minimal activity epoch

Aims. We investigate the long-term change in coronal large-scale structure at periods of minimum solar activity from 1878 to 2008. Methods. A parameter γ that characterizes the angle between high-latitude boundaries of the large coronal streamers at a distance of 2 Ris used to quantify the large-scale coronal structure at minimum solar activity. The comparative analysis of the solar corona during the minimum epochs in activity cycles 12 to 24 shows that the index has been slow and steadily changing during the past 130 years. Results. The maximum value of the index occurred during activity cycles 17 to 19, i.e. around 1950, which was the period when the global magnetic field of the Sun was closest to a dipole configuration. During activity minima close to the years 1900 and 2000, the large-scale coronal structure corresponded to a quadrupole configuration. The reasons for the variations in the solar coronal structure and its relation with long-term variations in the geomagnetic indices and Gleissberg cycle are discussed.

Regularities of solar energetic particle events and fluxes

Some regularities related to solar energetic proton (SEP) events and fluxes have been established. It is shown that the rate of SEP events is proportional to the Wolf numbers and that the distribution functions of SEP events related to both different levels and cycle phases, divided by the sums of Wolf numbers, are identical. It is concluded that extremely large SEP events may also occur in periods of minimum solar activity. This conclusion is confirmed by the experimental data obtained at the end of 2004 and in 2005.

Absorbed dose from galactic cosmic rays and solar protons as derived from stratospheric measurements

Regular measurements of charged-particle fluxes in the Earth’s atmosphere, performed at the Lebedev Institute of Physics, Russian Academy of Sciences since 1957 up to date, are used to determine the absorbed dose rate in the atmosphere from galactic cosmic rays in the period of minimum solar activity and from solar protons during solar proton events. The values of the absorbed dose rate estimated from the measurement data for some intense solar proton events are reported.

Use of proxy-documentary and instrumental data to assess the risk factors leading to sea flooding in Venice

Due to its geographical position, and because it was built at sea level, Venice is extremely sensitive to climate changes and, in the future, risks being submerged as a consequence of the expansion of oceanic water in response to global warming. A key problem is the increasing frequency of storm surges with the sea flooding the city (locally named acqua alta) that has reached an unsustainable level. After careful examination of both proxy documentary and instrumental data, it has been possible to reconstruct the extreme weather conditions and natural hazards, which have taken place in the last two millennia, with particular reference to surges. A key problem with proxy data from written sources is that periods with missing data may be misinterpreted as ‘normal’, or that inaccurate reporting may lead to duplication or even multiplication of events. For this reason, the accuracy of the proxy data has been checked by comparing it with other known events. An accurate series of the flood surges has been obtained combining instrumental observations (1872–2000) with documentary data for the previous period. The period of minimum solar activity of Spörer (1416–1534) was anomalous for surges. Not only have meteorologically perturbed periods been identified, but also the effects of the accelerated subsidence of the city and of certain works undertaken in the lagoon that varied the exchanges between the sea and the lagoon. The impact of subsidence and of the modified dynamical exchanges of the lagoon on the change in flooding frequency has been established. During the instrumental period, the tide gauge measured a 31-cm rise. The frequency of surges has dramatically increased since the 1960s, reaching about two flooding events per year, the greatest value since 792 AD.

The outer scale of solar-wind turbulence from GALILEO coronal-sounding data

Radio sounding experiments on of the solar plasma were carried out by the GALILEO spacecraft using S-band (2295 MHz) signals in 1995–1996 a period of minimum solar activity. Equatorial regions at heliocentric distances of 7–80 solar radii were studied. The frequency of the received signal was detected by three ground stations. By carrying out continuous observations of unprecedented duration and processing the data using spectral and correlation methods, we have obtained reliable information on large-scale inhomogeneities of the solar-wind density for the first time. The outer turbulence scale increases with heliocentric distance, the dependence being close to linear. We estimate the outer turbulence scale and analyze its dependence on distance from the Sun and local plasma parameters for a model in which the outer scale is formed due to competition between the linear amplification of Alfven waves in the irregular, moving solar-wind plasma and the nonlinear transfer of turbulent energy to higher frequencies. A comparison of predictions for various specific cases of this model with the observational data suggests that the main nonlinear processes responsible for the formation of the inertial range of the spectrum on the investigated scales are three-wave decay processes involving Alfven and magnetoacoustic waves.

Simple analytical solutions for propagating diffusive barriers and application to the 1974 minicycle

The cosmic ray minicycle of 1974 is a medium‐term modulation event where a clear anticorrelation is observed between the cosmic ray intensity and the heliospheric magnetic field magnitude B. It occurs during a period of minimum solar activity in an A > 0 magnetic polarity epoch. Cosmic ray modulation during the event is studied with an analytical, spherically symmetric approximation for a propagating diffusive barrier. Under the assumption that the cosmic ray radial diffusion coefficient K scales inversely with B, the temporal development of the disturbance can be obtained from the observed variation of B. A recovery process is described by a simple exponential, with a constant recovery time throughout the event. The profile of the intensity decrease as predicted by the model varies with the temporal development of the disturbance and with the duration of the disturbance in relation to the recovery timescale. The model predicts a maximum depression which is about a factor of 2.5 too small compared with neutron monitor observations. A replacement of the inverse relation K ∝ 1/B by the more general case K ∝ B−n and a value of n ∼ 2.5 leads to a good agreement between model and observations. It is remarkable how closely the high‐energy cosmic ray intensity follows the temporal structure of the magnetic field magnitude, suggesting that the magnetic field enhancement plays an important role for modulation during the minicycle. For the period under study, radial diffusion is essentially determined by diffusion parallel to the average field. Based on quasilinear theory a variation K‖∝B−2 is suggested, consistent with the value of n = 2.5 ± 1.0 derived in the paper.