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Understanding the evolution of radial sizes and instantaneous expansion speeds of coronal mass ejections (CMEs) is crucial for assessing their impact duration on Earth's environment. We introduce a non-conventional approach to derive the CME's radial sizes and expansion speeds at different instances during its passage over a single-point in situ spacecraft. We also estimate the CME's radial sizes and expansion speeds during its journey from the Sun to 1 au using the 3D kinematics of different CME features, including the leading edge, centre, and trailing edge. The continuous 3D kinematics of the CME is estimated by employing the graduated cylindrical shell and stereoscopic self-similar expansion reconstruction methods on multipoint observations from coronagraphs and heliospheric imagers combined with the drag-based model. We choose the 2010 April 3 CME as a suitable case for our study, promising a more accurate comparison of its remote and in situ observations. We show that the introduced non-conventional approach can provide better accuracy in estimating radial sizes and instantaneous expansion speeds of CMEs at different instances. We examine the aspect ratio of the CME, which influences its expansion behaviour and shows the discrepancy between its value in the corona and interplanetary medium. Our study highlights significant inconsistencies in the arrival time, radial size, and expansion speed estimates obtained from remote and in situ observations. We advocate for future studies leveraging multispacecraft in situ observations and our non-conventional approach to analyse them to improve the comprehension of CME dynamics in the solar wind. [ABSTRACT FROM AUTHOR]

In this study, we map the origin, acceleration, and propagation of the high-speed solar wind streams (HSS) and observe their impact on the low-latitude Earth's ionosphere. Data from radio-sounding experiments conducted by the Indian Mars Orbiter Mission (MOM) from 2015 May 9–19 is analysed to understand the solar wind speed's evolution at various helio-centric distances. The slope of the turbulence spectrum from 25 to 35 Rs was in the range of 0.2–0.4, indicative of the underdeveloped turbulence corresponding to the high-flow streams. It coincided with the appearance of the earth-facing coronal holes as observed in the coronal EUV images. The particle bulk velocity at L1 showed that the speeds began to rise from 400 km s−1 on May 11th–12th, reaching a peak of around 800 km s−1 on May13th–14th, followed by a gradual decrease to the average slow speeds. Geomagnetic disturbances during the same period manifested as a dip in the DST index values. The GNSS (Global Navigation Satellite System) data from the InSWIM (Indian network for Space Weather Impact Monitoring) network show an appreciable increase in the VTEC (vertical total electron content) of the ionosphere on disturbed days in entire low-latitude ionospheric region in the Indian sector. All these observed parameters correlate well with the HSS arrival. This is a unique study that connects the propagation of the HSS and its impact on near-Earth's environment from the different vantage points in interplanetary space and proposes the application of Radio beacons to improve space weather forecasting. [ABSTRACT FROM AUTHOR]

Our knowledge of the centennial solar variability is largely based on the time series of international sunspot number (⁠|$S_{N}$|⁠), a composite index based on multiple visual sunspot observers' records from the 18th century onward and maintained by the World Data Center (WDC) 'Sunspot Index and Long-term Solar Observations' (SILSO). However, over the period 1919–1944, our capacity to diagnose the homogeneity of this time series is currently limited, because most of the archived source data of the Zürich Observatory were never published over that interval and are presently still missing. Therefore, it is essential to recover any long-duration series from individual sunspot observers active during this period to bridge this Zürich data gap. In this context, Katsue Misawa has conducted regular sunspot observations from 1921 to 1934 (mean coverage of 25.4 days per month), which were not accessible for the Zürich Observatory and thus form a valuable addition to the data base maintained by the WDC–SILSO. In this study, we digitized his observational records, documented his observing technique, and reconstructed his total and hemispheric SNs. We compared his data with the international SN (current version V2) and evaluated their stability. Misawa's data series generally agrees well with SN V2. However, Misawa's data show a significant transitory drift in 1925–1928 against the SN V2, when the Zürich pilot observer changed from Alfred Wolfer to William Otto Brunner. [ABSTRACT FROM AUTHOR]

Context. The solar energetic particle analysis platform for the inner heliosphere (SERPENTINE) project, funded through the H2020-SPACE-2020 call of the European Union's Horizon 2020 framework program, employs measurements of the new inner heliospheric spacecraft fleet to address several outstanding questions on the origin of solar energetic particle (SEP) events. The data products of SERPENTINE include event catalogs, which are provided to the scientific community. Aims. In this paper, we present SERPENTINE's new multi-spacecraft SEP event catalog for events observed in solar cycle 25. Observations from five different viewpoints are utilized, provided by Solar Orbiter, Parker Solar Probe, STEREO A, BepiColombo, and the near-Earth spacecraft Wind and SOHO. The catalog contains key SEP parameters for 25–40 MeV protons, ~1 MeV electrons, and ~100 keV electrons. Furthermore, basic parameters of associated flares and type II radio bursts are listed, as are the coordinates of the observer and solar source locations. Methods. An event is included in the catalog if at least two spacecraft detect a significant proton event with energies of 25–40 MeV. The SEP onset times were determined using the Poisson-CUSUM method. The SEP peak times and intensities refer to the global intensity maximum. If different viewing directions are available, we used the one with the earliest onset for the onset determination and the one with the highest peak intensity for the peak identification. We furthermore aimed to use a high time resolution to provide the most accurate event times. Therefore, we opted to use a 1-min time resolution, and more time averaging of the SEP intensity data was only applied if necessary to determine clean event onsets and peaks. Associated flares were identified using observations from near Earth and Solar Orbiter. Associated type II radio bursts were determined from ground-based observations in the metric frequency range and from spacecraft observations in the decametric range. Results. The current version of the catalog contains 45 multi-spacecraft events observed in the period from November 2020 until May 2023, of which 13 events were found to be widespread (observed at longitudes separated by at least 80° from the associated flare location) and four could be classified as narrow-spread events (not observed at longitudes separated by at least 80° from the associated flare location). Using X-ray observations by GOES/XRS and Solar Orbiter/STIX, we were able to identify the associated flare in all but four events. Using ground-based and space-borne radio observations, we found an associated type II radio burst for 40 events. In total, the catalog contains 142 single event observations, of which 20 (45) have been observed at radial distances below 0.6 AU (0.8 AU). It is anticipated that the catalog will be extended in the future. [ABSTRACT FROM AUTHOR]

Solar wind is known to have different properties depending on its origin at the Sun. In addition to the differences in plasma and magnetic field parameters, these streams differ due to the properties of turbulent fluctuations involved in the flow. The present study addresses the changes in the turbulence properties in the magnetosheath—the transition region in front of the magnetosphere. This study is based on statistics from the simultaneous measurements of magnetic field fluctuations in the solar wind and in the magnetosheath. Both the dayside and flank magnetosheath regions are focused on to detect the evolution of the turbulent fluctuations during their flow around the magnetosphere. Turbulent cascade is shown to save its properties for fast solar wind streams. Conditions favorable for the preservation of the turbulence properties at the bow shock may correspond to the increased geoefficiency of large-scale solar wind structures. [ABSTRACT FROM AUTHOR]

Within four centuries of sunspot observations, the Maunder Minimum (MM) in 1645–1715 has been considered a unique grand minimum with weak solar cycles in group numbers of sunspots and hemispheric asymmetry in sunspot positions. However, the early part of the MM (1645–1659) is poorly understood in terms of its source records and has accommodated diverse reconstructions of the contemporaneous group number. This study identified their source records, classidied them in three different categories (datable observations, general descriptions, and misinterpreted records), and revised their data. On this basis, we estimated the yearly mean group number using the brightest star method, derived the active day fraction (ADF), reconstructed the sunspot number based on ADF, and compared them with proxy reconstructions from the tree-ring data sets. Our results revised the solar activity in the early MM downward in yearly mean group numbers using the brightest star method and upward in the active day fraction and sunspot number estimates. Our results are consistent with the proxy reconstruction for 1645–1654 and show more realistic values for 1657–1659 (against the unphysical negative sunspot number). These records have paid little attention to sunspot positions, except for Hevelius' report on a sunspot group in the northern solar hemisphere in 1652 April. Therefore, slight caveats are required to discuss if the sunspot positions are located purely in the southern solar hemisphere throughout the MM. [ABSTRACT FROM AUTHOR]

Aims. An energetic particle event related to a corotating interaction region (CIR) structure was observed by the Solar-Terrestrial Relations Observatory-A (STEREO-A) from 21 to 24 August 2016. Based on an analysis of measurement data, we suggest that instead of being accelerated by distant shocks, a local mechanism similar to diffusive shock acceleration (DSA) acting in the compression region could explain the flux enhancements of 1.8–10.0 MeV nucleon−1 protons. We created simulations to verify our hypothesis. Methods. We developed a coupled model composed of a data-driven analytical background model providing solar wind configuration and a particle transport model represented by the focused transport equation (FTE). We simulated particle transport in the CIR region of interest in order to obtain the evolution of proton fluxes and derive the spectra. Results. We find that the simulation is well correlated with the observation. The mechanism of particle scattering back and forth between the trap-like structure of interplanetary magnetic field (IMF) in the compression region is the major factor responsible for the flux enhancements in this energetic particle event, and perpendicular diffusion identified by a ratio of κ⊥/κ|| ∼ 10−2 plays an important role in the temporal evolution of proton fluxes. [ABSTRACT FROM AUTHOR]

Using the SDO /AIA, SOHO /LASCO, STEREO / SECCHI , and ground-based H α, radio observations, we studied a prominence eruption (PE) from the western limb that occurred on 2013 December 4. PE is associated with a moderate coronal mass ejection (CME) and GOES class C4.7 flare. Before a couple of days, the prominence pre-existed as an inverse-S shaped filament lying above fragmented opposite polarities between two active regions. Initially, the prominence appears as kinked or writhed as observed from different vantage points. From a careful study of magnetic field observations, we infer that the flux emergence at one leg of the prominence causes the loss of equilibrium which then initiates the slow upward motion of the prominence followed by onset of the eruption at a projected height of 35 Mm. The fast rise motion is also in synchronization with the flare impulsive phase but the average acceleration is quite small (150 ms−2) compared to strong flare cases. In the LASCO field of view (FOV), the CME continues to accelerate at 3 ms−2 attaining a speed of 450 km s−1 at 16 R⊙. In the extended STEREO-A FOV upto 38 R⊙, the CME decelerates 0.82 m s−2. The PE launched type III bursts delayed by 14 min with respect to the flare peak time (04:58 UT). Since the prominence is lying in the fragmented polarities, it is likely that the sheared arcade has little contribution to the poloidal flux of the rising magnetic flux rope and subsequently weak flare is recorded. This study of PE emphasizes the influence of the magnetic reconnection on the CME speed, launch of type II, III burst, and the CME propagation distance farther away from the Sun. [ABSTRACT FROM AUTHOR]

Historical auroral reports extend our knowledge of solar eruptions and long-term solar variability in the millennial time scale beyond the chronological coverage of instrumental observations, in the decadal to centennial time scales. Such chronological extensions benefit the scientific community, increasing the number of case studies on extreme space weather events with lower frequency but higher potential impact on modern technological infrastructure. So far, the earliest known datable reports of candidate aurorae have reached back to the 7th century BCE. Beyond this time series, we have analysed a celestial report in the Chinese Bamboo Annals that has attracted little scientific interest, probably owing to the controversial interpretations for the physical identity and the chronology of the event. Our philological analysis establishes its probable auroral nature. The textual description can be compared with early modern accounts of visual auroral observations for the multi-colour aurorae. We have located the observational site around Hàojīng (N34°14′, E108°46′) and dated the event to 977 ± 1 or 957 ± 1 BCE. On this basis, we have computed the equatorward extension of the auroral visibility as ≤39.0° in magnetic latitude and reconstructed the equatorward boundary of the auroral oval as ≤45.5° in invariant latitude. Our investigations empirically associate this candidate aurora with an extreme geomagnetic storm. We have compared this space weather event with proxy-based reconstructions of long-term solar variability and characterised it as a unique space-weather reference before the Neo-Assyrian Grand Minimum (alternatively Homeric Grand Minimum) in ca. 810 – 720 BCE. [ABSTRACT FROM AUTHOR]

Abstract Here we review present knowledge of the long-term behaviour of solar activity on a multi-millennial timescale, as reconstructed using the indirect proxy method. The concept of solar activity is discussed along with an overview of the dedicated indices used to quantify different aspects of variable solar activity, with special emphasis on sunspot numbers. Over long timescales, quantitative information about past solar activity is historically obtained using a method based on indirect proxies, such as cosmogenic isotopes $$^{14}$$ 14 C and $$^{10}$$ 10 Be in natural stratified archives (e.g., tree rings or ice cores). We give a historical overview of the development of the proxy-based method for past solar-activity reconstruction over millennia, as well as a description of the modern state of the art. Special attention is paid to the verification and cross-calibration of reconstructions. It is argued that the method of cosmogenic isotopes makes a solid basis for studies of solar variability in the past on a long timescale (centuries to millennia) during the Holocene (the past $$\sim $$ ∼ 12 millennia). A separate section is devoted to reconstructions of extremely rare solar eruptive events in the past, based on both cosmogenic-proxy data in terrestrial and lunar natural archives, as well as statistics of sun-like stars. Finally, the main features of the long-term evolution of solar magnetic activity, including the statistics of grand minima and maxima occurrence, are summarized and their possible implications, especially for solar/stellar dynamo theory, are discussed.

Solar wind is known to have different properties depending on its origin at the Sun. In addition to the differences in plasma and magnetic field parameters, these streams differ due to the properties of turbulent fluctuations involved in the flow. The present study addresses the changes in the turbulence properties in the magnetosheath—the transition region in front of the magnetosphere. This study is based on statistics from the simultaneous measurements of magnetic field fluctuations in the solar wind and in the magnetosheath. Both the dayside and flank magnetosheath regions are focused on to detect the evolution of the turbulent fluctuations during their flow around the magnetosphere. Turbulent cascade is shown to save its properties for fast solar wind streams. Conditions favorable for the preservation of the turbulence properties at the bow shock may correspond to the increased geoefficiency of large-scale solar wind structures.

Context. Galactic cosmic rays (GCRs) and solar particles with energies greater than tens of MeV penetrate spacecraft and instruments hosted aboard space missions. The Solar Orbiter Metis coronagraph is aimed at observing the solar corona in both visible (VL) and ultraviolet (UV) light. Particle tracks are observed in the Metis images of the corona. An algorithm has been implemented in the Metis processing electronics to detect the VL image pixels crossed by cosmic rays. This algorithm was initially enabled for the VL instrument only, since the process of separating the particle tracks in the UV images has proven to be very challenging. Aims. We study the impact of the overall bulk of particles of galactic and solar origin on the Metis coronagraph images. We discuss the effects of the increasing solar activity after the Solar Orbiter mission launch on the secondary particle production in the spacecraft. Methods. We compared Monte Carlo simulations of GCRs crossing or interacting in the Metis VL CMOS sensor to observations gathered in 2020 and 2022. We also evaluated the impact of solar energetic particle events of different intensities on the Metis images. Results. The study of the role of abundant and rare cosmic rays in firing pixels in the Metis VL images of the corona allows us to estimate the efficiency of the algorithm applied for cosmic-ray track removal from the images and to demonstrate that the instrument performance had remained unchanged during the first two years of the Solar Orbiter operations. The outcome of this work can be used to estimate the Solar Orbiter instrument's deep charging and the order of magnitude for energetic particles crossing the images of Metis and other instruments such as STIX and EUI. [ABSTRACT FROM AUTHOR]

This study presents new prediction models of the 11-yr solar activity cycles (SC) 25 and 26 based on multiple activity indicator parameters. The developed models are based on the use of non-linear autoregressive exogenous (NARX) neural network approach. The training period of the NARX model is from July 1749 to December 2019. The considered activity indicator parameters are the monthly sunspot number time series (SSN), the flare occurence frequency, the 10.7-cm solar radio flux, and the total solar irradiance (TSI). The neural network models are fed by these parameters independently and the prediction results are compared and verified. The obtained training, validation, and prediction results show that our models are accurate with an accuracy of about 90 per cent in the prediction of peak activity values. The current models produce the dual-peak maximum (Gnevyshev gap) very well. Based on the obtained results, the expected solar peaks in terms of SSN (monthly averaged smoothed) of the solar cycles 25 and 26 are R SSN = 116.6 (February 2025) and R SSN = 113.25 (October 2036), respectively. The expected time durations of SC 25 and SC 26 cycles are 9.2 and 11 yr, respectively. The activity levels of SC 25 and 26 are expected to be very close and similar to or weaker than SC 24. This suggests that these two cycles are at the minimum level of the Gleissberg cycle. A comparison with other reported studies shows that our results based on the NARX model are in good agreement. [ABSTRACT FROM AUTHOR]

Here we review present knowledge of the long-term behaviour of solar activity on a multi-millennial timescale, as reconstructed using the indirect proxy method. The concept of solar activity is discussed along with an overview of the dedicated indices used to quantify different aspects of variable solar activity, with special emphasis on sunspot numbers. Over long timescales, quantitative information about past solar activity is historically obtained using a method based on indirect proxies, such as cosmogenic isotopes 14 C and 10 Be in natural stratified archives (e.g., tree rings or ice cores). We give a historical overview of the development of the proxy-based method for past solar-activity reconstruction over millennia, as well as a description of the modern state of the art. Special attention is paid to the verification and cross-calibration of reconstructions. It is argued that the method of cosmogenic isotopes makes a solid basis for studies of solar variability in the past on a long timescale (centuries to millennia) during the Holocene (the past ∼ 12 millennia). A separate section is devoted to reconstructions of extremely rare solar eruptive events in the past, based on both cosmogenic-proxy data in terrestrial and lunar natural archives, as well as statistics of sun-like stars. Finally, the main features of the long-term evolution of solar magnetic activity, including the statistics of grand minima and maxima occurrence, are summarized and their possible implications, especially for solar/stellar dynamo theory, are discussed. [ABSTRACT FROM AUTHOR]

An extreme geomagnetic storm has the potential to affect various technologies and activities in space and on the ground, e.g., power grids, oil and gas industries, communications, ground transportation, satellite infrastructure, global navigation satellite systems, aviation, etc. Therefore, it is considered a major source of risk by various governmental agencies and corporations at the international level. All notable space weather events (superstorms) are caused by interplanetary coronal mass ejections (ICMEs). But not every ICME leads to an extreme storm. Moreover, how does an extreme storm form? Or which explicit characteristic of ICME actually is responsible for inducing a superstorm? Here, we re-investigate the ICME characteristics that contribute to the most intense storm of the current century that occurred on 20 November 2003. Interestingly, the studied ICME magnetic cloud shows characteristics of extremely flattened (pancaked) structure i.e. quasi-planar magnetic structure (PMS). The pancaked ICME shows less adiabatic expansion than usual in the compressed direction, which leads to strong magnetic field strength, high plasma density, high solar wind speed, high dynamic pressure, and a high eastward interplanetary electric field. Here, we propose that the ICME that transformed into a quasi-PMS has the aforementioned enhanced features with strong southward magnetic field component that contributes to efficiently transferring plasma and energy into the Earth's magnetosphere to cause the observed superstorm. [ABSTRACT FROM AUTHOR]

Aims. The possible influence of solar superflares on the near-Earth space radiation environment are assessed through the investigation of scaling laws between the peak proton flux and fluence of solar energetic particle (SEP) events with the solar flare soft X-ray peak photon flux. Methods. We compiled a catalog of 65 well-connected (W20-90) SEP events during the last three solar cycles covering a period of ∼34 yr (1984–2020) that were associated with flares of class ≥C6.0, and investigated the statistical relations between the recorded peak proton fluxes (IP) and the fluences (FP) at a set of integral energies from E > 10, > 30, and > 60 to > 100 MeV versus the associated solar flare peak soft X-ray flux in the 1–8 Å band (FSXR). Based on the inferred relations, we calculated the integrated energy dependence of the peak proton flux (IP) and fluence (FP) of the SEP events, assuming that they follow an inverse power law with respect to energy. Finally, we made use of simple physical assumptions, combining our derived scaling laws, and estimated the upper limits for IP and FP focusing on the flare associated with the strongest ground level enhancement (GLE) directly observed to date (GLE 05 on 23 February 1956), and that inferred for the cosmogenic radionuclide-based SEP event of AD774/775. Results. A scaling law relating IP and FP to the solar soft X-ray peak intensity (FSXR) as ∝ F SXR 5 / 6 $ {F}_{\mathrm{SXR}}^{5/6} $ for a flare with a FSXR = X600 (in the revised scale) is consistent with values of FP inferred for the cosmogenic nuclide event of AD774/775. [ABSTRACT FROM AUTHOR]

The Earth's magnetosphere extension is controlled by the solar activity level via solar wind properties. Understanding such a relation in the Solar system is important for predicting the condition of exoplanetary magnetospheres near Sun-like stars. We use measurements of a chromospheric proxy, the Ca  ii K index, and solar wind OMNI parameters to connect the solar activity variations, on decennial time-scales, to the solar wind properties. The data span the period 1965–2021, which almost entirely covers the last five solar cycles. Using both cross-correlation and mutual information analysis, we find a 3.2-yr lag of the solar wind speed with respect to the Ca  ii K index. Analogously, a 3.6-yr lag is found once we consider the dynamic pressure. A correlation between the solar wind dynamic pressure and the solar ultraviolet emission is found and used to derive the Earth's magnetopause standoff distance. Moreover, the advantage of using a chromospheric proxy, such as the Ca  ii K index, creates the possibility to extend the relation found for the Sun to Sun-like stars, by linking stellar variability to stellar wind properties. The model is applied to a sample of Sun-like stars as a case study, where we assume the presence of an Earth-like exoplanet at 1 au. Finally, we compare our results with previous estimates of the magnetosphere extension for the same set of Sun-like stars. [ABSTRACT FROM AUTHOR]

Observations of energetic particles at interplanetary shocks are important to study acceleration mechanisms and their connection with magnetohydrodynamic turbulence. Energetic storm particle (ESP) events are increases in proton fluxes that occur locally at the passage time of interplanetary shocks. These events are more dangerous when they are superimposed on the solar energetic particles (SEPs) produced by the eruption of flares and/or CME-driven shocks propagating from the corona to the interplanetary space. We considered ESP events occurring in association with SEPs on 3 November 2021. We used proton fluxes provided by Solar Orbiter (located at 0.85 AU) in the energy range of 30 keV–82 MeV, by Wind at energies from 70 keV to 72 MeV, and ACE in the range from 40 keV to 5 MeV (both located at the Lagrangian point L1, close to 1 AU along the Sun-Earth direction). In order to broaden the range of analyzed energies (40 keV - 72 MeV), we combine these data with the proton fluxes from the SOHO spacecraft, also located at L1. We analyzed the ESP event and fitted the proton energy spectra at both locations with several distributions to shed light on the mechanisms leading to the acceleration of energetic particles. We also investigated the turbulent magnetic field fluctuations around the shock. The obtained ESP spectra, best reproduced by the so-called double power law function, the spectral differences at the two locations, and the shock features (quasi-parallel geometry, enhanced downstream turbulence) suggest that diffusive shock acceleration is responsible for acceleration of low energy particles, whereas stochastic acceleration contributes to the (re) acceleration of high energies ones.

The most energetic particles accelerated in solar eruptive events are protons and nuclei with energies that may reach a few tens of GeV. They can be detected on the Earth through the secondaries they produce when interacting with the atmosphere. Solar energetic particle events where this happens are called Ground-Level Enhancements (GLEs). Their study is relevant on the one hand because the high particle energies pose particularly strong challenges to the understanding of the acceleration processes. On the other hand, the secondary particles constitute a source of radiation in the atmosphere that may temporarily exceed the permanent dose rate from galactic cosmic rays. This makes the monitoring of radiation doses received by aircrew from GLEs one issue of space weather services for civil aviation. This study addresses the time profiles of GLEs, in the search for commonalities that can be used to constrain models of acceleration and propagation and to forecast the evolution of an ongoing event. We investigate historical GLEs (1971–2012) with the worldwide network of neutron monitors, comparing the rise and the decay as observed by the neutron monitor with the strongest response. The sample comprises 23 events. We evaluate statistical correlations between rise time and decay time inferred from fits to the time profiles and compute a normalised median GLE time profile. An empirical correlation reported in earlier work between the observed rise times and decay times of the neutron monitor count rate profiles is confirmed. We find indications of a statistical relationship between the rise times and the parent eruptive activity. We discuss ideas on the mechanisms behind the correlation of rise and decay times and on its usefulness for space weather services. [ABSTRACT FROM AUTHOR]

The Earth's magnetosphere is permanently influenced by the solar wind. When supersonic and superalfvenic plasma flow interacts with the magnetosphere, the magnetosheath region is formed, which is filled with shocked turbulent plasma. Varying SW parameters influence the mechanisms of formation of this boundary layer, including the dynamics of turbulence behind the bow shock. The effect of the solar wind on the development of turbulence in the magnetosheath was demonstrated recently based on broad statistics of spacecraft measurements. The present study considers the multipoint observations of turbulent fluctuations in the solar wind, in the dayside magnetosheath and at the flanks, to analyze the evolution of the turbulent cascade while the solar wind plasma enters the magnetosheath. Observations of the magnetosheath behind the quasi-perpendicular bow shock are analyzed to exclude the influence of the bow shock topology from consideration. Three basic types of solar wind flows are considered: slow undisturbed solar wind, compressed regions, and interplanetary manifestations of coronal mass ejections. The results show surviving Kolmogorov scaling behind the bow shock for steady solar wind flow and amplification of the compressive fluctuations at the kinetic scales at the magnetosheath flanks for the solar wind associated with compressed plasma streams. During interplanetary manifestations of the coronal mass ejection, the spectra in the dayside magnetosheath substantially deviate from those observed in the solar wind (including the absence of Kolmogorov scaling and steepening at the kinetic scales) and restore at the flanks. [ABSTRACT FROM AUTHOR]

In late 1940 March, at least five significant solar flares were reported. They likely launched interplanetary coronal mass ejections (ICMEs), and were associated with one of the largest storm sudden commencements (SSCs) since 1868, resulting in space weather hazards that would have significant societal impacts should it occur today. The initial solar activity is associated with a solar proton event. Afterwards, another flare was reported in the eastern solar quadrant (N12 E37-38) at 11:30–12:30 ut on March 23, with significant magnetic crochets (up to ≈ |80| nT at Eskdalemuir) during 11:07–11:40 ut. On their basis, we conservatively estimate the required energy flux of the source solar flare as X35 ± 1 in soft X-ray class. The resultant ICMEs caused enormous SSCs (up to >425 nT recorded at Tucson) and allowed us to estimate an extremely inward magnetopause position (estimated magnetopause stand-off position ≈3.4 RE). The time series of the resultant geomagnetic storm is reconstructed using a Dst estimate, which peaked at 20 ut on March 24 at ≈ −389 nT. Around the storm main phase, the equatorward boundary of the auroral oval extended ≤46.3° in invariant latitudes. This sequence also caused a solar proton event and Forbush decrease (≈3 per cent). These sequences indicate pile-up of multiple ICMEs, which even achieved a record value of inward magnetopause position. Our analyses of this historical pioneer event bring more insights into possible serious space weather hazards and provide a quantitative basis for future analyses and predictions. [ABSTRACT FROM AUTHOR]

Based on the data of 35-yr (1987–2021) measurements of magnetospheric electron fluxes with energy >2 MeV in geostationary orbits, solar wind speed, and geomagnetic activity, a catalogue of electron flux enhancements was compiled in which the electron fluence exceeds 108 cm–2 sr–1 d–1. The epoch superposition method performed using the GOES-13 spacecraft data shows that large electron flux enhancements are preceded by a significant increase in the solar wind velocity and the Ap index of geomagnetic activity, and immediately before the increase the relativistic electron flux decreases. For the events of the catalogue, the average characteristics of electron flux enhancements and parameters of the interplanetary and near-Earth medium were calculated: the mean values of the diurnal and total fluences during an event and the average duration of electron enhancements. The average duration of the electron flux enhancement is 5 d, and the maximum duration is 24 d. Based on the calculated mean values of the electron fluences, solar wind velocity, and Ap-index of geomagnetic activity on the day of electron enhancement and on previous days, a typical behaviour of these parameters during and before an electron flux enhancement was obtained. The average characteristics of electron flux enhancements and the parameters of interplanetary and near-Earth medium are calculated before large electron flux enhancements, when the fluence exceeds 3 × 108, 5 × 108, and 109 particles cm–2 sr–1 d–1, respectively. It is shown that the greater the increase in solar wind velocity and geomagnetic activity the larger the subsequent electron flux enhancement. [ABSTRACT FROM AUTHOR]

In this paper, Forbush decreases (FDs) from 1997 to 2020 associated with coronal mass ejections from active and non-active regions are compared between themselves and to FDs caused by high-speed streams from coronal holes. The two types of sporadic FDs are also compared when corresponding solar wind (SW) disturbances contain, or do not contain, magnetic clouds (MCs) near Earth. Cosmic ray density and anisotropy variations, SW speed, interplanetary magnetic field (IMF) strength, and geomagnetic indices have been examined using statistical methods. The results reveal that these parameters are larger for FDs associated with active region (AR) ejections and have highly skewed distributions for both types of sporadic events. In the same ranges of SW parameters, FD magnitude is larger for flare-associated events; more efficient modulation occurs in FDs associated with AR ejections. Differences between FDs associated with AR and non-AR ejections are more pronounced when an MC is registered. For IMF strength and geomagnetic indices, differences between the distributions depend more upon MC presence or absence than on the type of solar source. Correlation of IMF strength and SW speed differs slightly between FDs caused by AR and non-AR ejections regardless of the presence or absence of an MC, akin to the partial correlation between FD magnitude and IMF strength. Difference between the speeds of disturbed and background SW is larger for FDs associated with AR ejections especially when an MC is registered; the interaction region of different-speed SW streams occurs more frequently in interplanetary disturbances induced by AR ejections. [ABSTRACT FROM AUTHOR]

Coronal mass ejections (CMEs) are large-scale eruptions observed close to the Sun. They travel through the heliosphere and possibly interact with the Earth environment, creating interruptions or even damaging new-technology instruments. Most of the time their physical conditions (velocity, density and pressure) are measured in situ at only one point in space, with no possibility of having information on the variation of these parameters during their journey from the Sun to Earth. Our aim is to understand the evolution of the internal physical parameters of a set of three particular fast halo CMEs. These CMEs were launched between 15 and 18 July 2002. Surprisingly, the related interplanetary CMEs (ICMEs), observed near Earth, have a low, and in one case a very low, plasma density. We use the EUropean Heliospheric FORecasting Information Asset (EUHFORIA) model to simulate the propagation of the CMEs in the background solar wind by placing virtual spacecraft along the Sun–Earth line. We set up the initial conditions at 0.1 au, first with a cone model and then with a linear force-free spheromak model. Relatively good agreement between the simulation results and observations concerning the speed, density and arrival times of the ICMEs is obtained by adjustment of the initial CME parameters. In particular, this is achieved by increasing the initial magnetic pressure so that a fast expansion is induced in the inner heliosphere. This resulted in the development of fast expansion for two of the three ICMEs. In contrast, the intermediate ICME is strongly overtaken by the last ICME, so its expansion is strongly limited. First, we show that a magnetic configuration with an out-of-force balance close to the Sun mitigates the EUHFORIA assumptions related to an initial uniform velocity. Second, the overexpansion of the ejected magnetic configuration in the inner heliosphere is one plausible origin for the low density observed in some ICMEs at 1 au. Furthermore, we conclude for one ICME, surrounded by two other ICMEs, that the in situ observed very low density has a possible coronal origin. [ABSTRACT FROM AUTHOR]

Some important data on both lunar tidal and solar effects on mesopause region characteristics, which were obtained by scientists of the Obukhov Institute of Atmospheric Physics in cooperation with colleagues from other organizations, are discussed. The present-day evidence that supports the hypothesis proposed by A.I. Semenov and N.N. Shefov about the existence of oscillations (with periods of the lunar synodic month and its half) in the mesopause region characteristics is considered. The oscillation amplitudes are estimated based on a statistical analysis of measurement data, possible mechanisms for generating these oscillations are indicated. Statistical data on the effect of solar activity on the mesopause region are discussed. It is shown that the effects of solar activity on some atmospheric characteristics on interannual and intraseasonal time scales have opposite signs, which suggests that there are different physical mechanisms of solar-terrestrial relations within these frequency ranges. [ABSTRACT FROM AUTHOR]

Space Weather is receiving more and more attention from the heliophysical scientific community, as it is now well established that an adequate capability of monitoring any Earth-directed heliospheric event and forecasting the most severe perturbations produced by solar activity and their impact on the geo-spatial environment is crucial, given the human increasing reliance on space-related technologies and infrastructures. Predicting how the Sun affects life on Earth and human activities in the short term relies on establishing empirical laws to forecast not only the arrival time on Earth of potentially geo-effective solar drivers, but also, and more importantly, the intensity of induced geomagnetic disturbance (if any). Scientific studies performed on a statistical basis are the key to providing such empirical laws and analytically relating solar-wind properties to geomagnetic indices. This paper summarizes the results achieved by the author in the last few years in the context of Space Weather science, and based on statistical analyses of interplanetary and geomagnetic data.

Solar wind forecasting is a core component of Space Weather, a field that has been the target of many novel machine-learning approaches. The continuous monitoring of the Sun has provided an ever-growing ensemble of observations, facilitating the development of forecasting models that predict solar wind properties on Earth and other celestial objects within the solar system. This enables us to prepare for and mitigate the effects of solar wind-related events on Earth and space. The performance of some simulation-based solar wind models depends heavily on the quality of the initial guesses used as initial conditions. This work focuses on improving the accuracy of these initial conditions by employing a Recurrent Neural Network model. The study's findings confirmed that Recurrent Neural Networks can generate better initial guesses for the simulations, resulting in faster and more stable simulations. In our experiments, when we used predicted initial conditions, simulations ran an average of 1.08 times faster, with a statistically significant improvement and reduced amplitude transients. These results suggest that the improved initial conditions enhance the numerical robustness of the model and enable a more moderate integration time step. Despite the modest improvement in simulation convergence time, the Recurrent Neural Networks model's reusability without retraining remains valuable. With simulations lasting up to 12 h, an 8% gain equals one hour saved per simulation. Moreover, the generated profiles closely match the simulator's, making them suitable for applications with less demanding physical accuracy. [ABSTRACT FROM AUTHOR]

The paper discusses changes in various characteristics of the solar wind, interplanetary magnetic field, geomagnetic activity, and cosmic rays during the registration of paired interacting solar wind disturbances on the Earth using the data base of Forbush effects and interplanetary disturbances – FEID. The cases of pair interaction are considered for 1995–2020: (i) successive coronal mass ejections; (ii) coronal mass ejections and high-speed streams from coronal holes; (iii) successive high-speed streams from coronal holes. It is shown that for the first events from a pair, the times for reaching the maximum values of the interplanetary magnetic field and solar wind velocity are significantly reduced, and the amplitudes of the recorded Forbush decreases decline. It is also found that the presence of interaction enriches the second event at the expense of the resources of the first, increasing geomagnetic efficiency and the degree of cosmic ray modulation for the second event in comparison with isolated events, which is especially pronounced for a pair of interacting coronal mass ejections. The existence of the described effects can be explained by the observed increase in the interplanetary magnetic field magnitude in the second events due to the presence of interaction. [ABSTRACT FROM AUTHOR]

We investigate the rectified interplanetary magnetic field (IMF) components in two coordinate systems, GSEQ and GSM, focusing on the southward pointing component ( B s ) in GSM within the period 1999 – 2017. The analysis is performed for different solar activity levels. The obtained results are valuable for the theoretical interpretation of IMF components variations and variations seen in geomagnetic indices. B s ordered according to the polarity exhibits a "pair of spectacles" pattern. This reveals that B s can also exist for toward/away field in fall/spring. The field is reduced, but it is not zero. Thus, in "unfavorable" seasons, geomagnetic activity can be due to reduced B s and not because the field is northward pointing. We show that patterns of the experimental B s fields are not in agreement with the Russell-McPherron model of B s as widely assumed. The present study contributes in understanding the nature and behavior of B s , which is the important element that controls the reconnection process and has a great influence on terrestrial space weather over the solar cycle. [ABSTRACT FROM AUTHOR]

The paper discusses the importance of the problem of adverse effects of cosmic processes and phenomena (space weather factors) on the ground engineering and technical infrastructure, space-born radio-electronic facilities, etc. It examines the state of the experimental base for research in solar, atmosphere, and near-Earth space physics in Russia and abroad. The necessity of creating new-generation instruments, developed using advanced engineering solutions and technologies, is justified. The complex of the main facilities and instruments of the National Heliogeophysical Complex of the Russian Academy of Sciences is analyzed. The main scientific directions of basic research and applied problems are formulated.

For the experimental study of the relationship between earthquakes and solar activity, we introduce the idea of the statistical sum Z of earthquake ensemble. A number of numerical parameters of seismicity (average planetary magnitude, entropy, fluctuations of magnitude) are expressed through Z. A severe restriction is, however, imposed on the magnitude during the formation of the ensemble. We propose an alternative method. It does not have this specific restriction, although it allows us to calculate only one numerical parameter, namely the global daily magnitude Mg. Over the 20-year period from 1980 to 1999, 7300 values of Mg have been calculated. The comparison between the Mg values and Wolf numbers W made it possible to determine the effect of the Sun on earthquakes at a statistically significant level.

We discuss the relationship of solar activity with the seismicity of Earth and reasons for the differences in the results of studies of various authors. Using the epoch superposition method, we analyze the differences in seismic activity distribution over phases of the 11-year solar cycle for the whole world, hemispheres, sectors, latitudinal belts, and individual regions. The northeastern sector of Earth has been shown to make the main contribution to the planetary distribution of seismic activity over phases of the 11-year solar cycle. We have revealed a pattern in the distribution of seismic activity over latitudinal belts: the solar cycle phases, at which the main maximum of seismic activity occurs, increase with increasing latitude in both hemispheres. For some regions, the results may differ from the generalized results for Earth due to the influence of local geodynamic conditions during the destruction of the earth's crust. In middle latitudes, the maximum number of earthquakes is shifted to the later phases of the solar cycle from west to east, which was not found for the northern regions. We discuss possible reasons for various manifestations of solar-terrestrial relationships for different regions, taking into account their different structure and geodynamic development modes. The presence of pronounced maxima of the seismic activity distribution over the 11-year solar cycle phases allows us to use them for refining the “time” parameter in the medium-term prediction of dangerous earthquakes.

Observations at (sub-)millimeter wavelengths offer a complementary perspective on our Sun and other stars, offering significant insights into both the thermal and magnetic composition of their chromospheres. Despite the fundamental progress in (sub-)millimeter observations of the Sun, some important aspects require diagnostic capabilities that are not offered by existing observatories. In particular, simultaneously observations of the radiation continuum across an extended frequency range would facilitate the mapping of different layers and thus ultimately the 3D structure of the solar atmosphere. Mapping large regions on the Sun or even the whole solar disk at a very high temporal cadence would be crucial for systematically detecting and following the temporal evolution of flares, while synoptic observations, i.e., daily maps, over periods of years would provide an unprecedented view of the solar activity cycle in this wavelength regime. As our Sun is a fundamental reference for studying the atmospheres of active main sequence stars, observing the Sun and other stars with the same instrument would unlock the enormous diagnostic potential for understanding stellar activity and its impact on exoplanets. The Atacama Large Aperture Submillimeter Telescope (AtLAST), a single-dish telescope with 50m aperture proposed to be built in the Atacama desert in Chile, would be able to provide these observational capabilities. Equipped with a large number of detector elements for probing the radiation continuum across a wide frequency range, AtLAST would address a wide range of scientific topics including the thermal structure and heating of the solar chromosphere, flares and prominences, and the solar activity cycle. In this white paper, the key science cases and their technical requirements for AtLAST are discussed., Competing Interests: No competing interests were disclosed., (Copyright: © 2024 Wedemeyer S et al.)

By now several methods have been proposed enabling to determine the kinematic characteristics of coronal mass ejections (CMEs) in 3D space. Many of these methods are based on using the triangulation technique and stereoscopic observations of the CMEs with two and more spacecraft. As a rule, these methods involve relatively complicated procedures. Nevertheless, there is a need for a simple technique to find 3D characteristics of a CME motion fairly quickly. Such a technique, in particular, will enable to estimate on which side of the Sun (front side or back side for an observer on the Earth) the CME emerged, as well as to efficiently solve a problem relevant for solar-terrestrial physics: to determine the time of the CME arrival into Earth's orbit. Such a simple technique is proposed in this article. The technique comprises two stages. First, one identifies the CME motion direction in the solar equatorial plane by using the data from any pair of the COR2 and LASCO (Large Angle and Spectrometric Coronagraph) C3 coronagraphs on board the Solar Terrestrial Relations Observatory (STEREO) A, B and the Solar and Heliospheric Observatory (SOHO), respectively. Next, one measures the angle between the CME motion direction in 3D space and the equatorial plane. We illustrate the technique for five CMEs that emerged either on the front side or back side of the Sun. An important advantage of the proposed technique is the possibility to quickly filter the events, that emerged on the front or back side of the Sun, by using only the information of the CME central position angle from a CME catalog. For the investigated CMEs, we obtained the time dependencies of the motion direction in the equatorial plane (angle φ ), the angle between the CME motion direction in 3D space and the equatorial plane (λ ), the distance from Sun's center to the CME leading edge in 3D space ( R 3 D ). For some investigated events, we compared our values for angle φ and distance R 3 D with those obtained in other articles. [ABSTRACT FROM AUTHOR]

Predicting solar activity is one of the most challenging topics among the various Space Weather and Space Climate issues. In the last decades, the constant enhancement of Space Climate data improved the comprehension of the related physical phenomena and the statistical bases for prediction algorithms. For this purpose, we used geomagnetic indices to provide a powerful algorithm (see Diego et al. [2010. J Geophys Res115: A06103]) for the solar activity prediction, based on evaluating the recurrence rate in the geomagnetic activity. This paper aims to present the validation of our algorithm over solar cycle n. 24, for which a successful prediction was made, and upgrade it to forecast the shape and time as well as the amplitude of the upcoming cycle n. 25. Contrary to the consensus, we predict it to be quite high, with a maximum sunspot number of 205 ± 29, which should be reached in the first half of 2023. This prediction is consistent with the scenario in which the long-term Gleissberg cycle has reached its minimum in cycle n. 24, and the rising phase is beginning. [ABSTRACT FROM AUTHOR]

We present multipoint remote and in situ observations of interplanetary coronal mass ejection (ICME) structures during the year 2011. The selected ICMEs arrived at Earth on 2011 March 11 and 2011 August 6, and led to geomagnetic storms. Around the launch of these CMEs from the Sun, the coronagraphs onboard STEREO-A and -B and SOHO enabled the CMEs to be imaged from three longitudinally separated viewpoints. We attempt to identify the in situ plasma and magnetic parameters of the ICME structures at multiple locations, for example at both STEREO spacecraft and also at the ACE/Wind spacecraft near the first Sun–Earth Lagrangian point (L1), to investigate the global configuration, interplanetary propagation, arrival times and geomagnetic response of the ICMEs. The near-Earth identified ICMEs of March 11 and August 6 formed as a result of the interaction of two successive CMEs observed in the inner corona on March 7 (for the March 11 ICME) and on August 3–4 (for the August 6 ICME). Our study suggests that the structures associated with interacting CMEs, possibly as a result of deflection or large sizes, may reach to even larger longitudinally separated locations in the heliosphere. Our multipoint in situ analysis shows that the characteristics of the same shock, propagating in a pre-conditioned medium, may be different at different longitudinal locations in the heliosphere. Similarly, multiple cuts through the same ejecta/complex ejecta, formed as a result of CME–CME interaction, are found to have inhomogeneous properties. The study highlights the difficulties in connecting the local observations of an ICME from a single in situ spacecraft to its global structures. [ABSTRACT FROM AUTHOR]

The Earth’s magnetosphere is permanently influenced by the solar wind. When supersonic and superalfvenic plasma flow interacts with the magnetosphere, the magnetosheath region is formed, which is filled with shocked turbulent plasma. Varying SW parameters influence the mechanisms of formation of this boundary layer, including the dynamics of turbulence behind the bow shock. The effect of the solar wind on the development of turbulence in the magnetosheath was demonstrated recently based on broad statistics of spacecraft measurements. The present study considers the multipoint observations of turbulent fluctuations in the solar wind, in the dayside magnetosheath and at the flanks, to analyze the evolution of the turbulent cascade while the solar wind plasma enters the magnetosheath. Observations of the magnetosheath behind the quasi-perpendicular bow shock are analyzed to exclude the influence of the bow shock topology from consideration. Three basic types of solar wind flows are considered: slow undisturbed solar wind, compressed regions, and interplanetary manifestations of coronal mass ejections. The results show surviving Kolmogorov scaling behind the bow shock for steady solar wind flow and amplification of the compressive fluctuations at the kinetic scales at the magnetosheath flanks for the solar wind associated with compressed plasma streams. During interplanetary manifestations of the coronal mass ejection, the spectra in the dayside magnetosheath substantially deviate from those observed in the solar wind (including the absence of Kolmogorov scaling and steepening at the kinetic scales) and restore at the flanks.

There were 51 superactive regions (SARs) during solar cycles (SCs) 21–24. We divided the SARs into SARs1, which produced extreme space weather events including ≥X5.0 flares, ground level events (GLEs), and super geomagnetic storms (SGSs, Dst < −250 nT), and SARs2, which did not produce extreme space weather events. The total number of SARs1 and SARs2 are 31 and 20, respectively. The statistical results showed that 35.5%, 64.5%, and 77.4% of the SARs1 appeared in the ascending phase, descending phase, and in the period from two years before to the three years after the solar maximum, respectively, whereas 50%, 50%, and 100% of the SARs2 appeared in the ascending phase, descending phase, and in the period from two years before to the three years after the solar maximum, respectively. The total number of SARs during an SC has a good association with the SC amplitude, implying that an SC with a higher amplitude will have more SARs than that with a lower amplitude. However, the largest flare index of a SAR within an SC has a poor association with the SC amplitude, suggesting that a weak cycle may have a SAR that may produce a series of very strong solar flares. The analysis of the north–south asymmetry of the SARs showed that SARs1 dominated in the southern hemisphere of the sun during SCs 21–24. The SAR2 dominated in the different hemispheres by turns for different SCs. The solar flare activities caused by the SARs with source locations in the southern hemisphere of the sun were much stronger than those caused by the SARs with source locations in the northern hemisphere of the sun during SCs 21–24.

A record of 44Ti activity measured in 20 stony meteorites fallen during CE 1766–2001 was compared to six series of 44Ti activity, calculated from the data on modulation parameter Φ based on cosmogenic isotopes 14C and 10Be. It was shown that both experimental and computed records have appreciable divergences. The secular trends of many model curves are flatter than that of the measured record and the difference in amplitude of the century-scale cycle of Gleissberg is highly significant (p < 5.5 × 10−4). Possible causes of this discrepancy are discussed. The obtained results show that at present the cosmogenic isotope data likely contain mostly qualitative information about the behavior of the cosmic ray flux in the past and not always are suited for quantitative estimations. [ABSTRACT FROM AUTHOR]

We studied the source locations and solar cycle distribution of strong solar proton events (SPEs, ≥1000 pfu) measured at the Earth from 1976 to 2018. There were 43 strong SPEs during this period. 27.9 per cent of the strong SPEs were ground level enhancement (GLE) events. We detect more strong SPEs coming from the Western hemisphere. The strong SPEs were distributed in the region of [E90–W90], extreme SPEs (≥10 000 pfu) appeared within the longitudinal area from E30 to W75, while the SPEs with peak fluxes ≥20 000 pfu concentrated in the range from E30 to W30 and were always accompanied by super geomagnetic storms (Dst ≤−250 nT). The Northern and Southern hemispheres of the Sun have 23 and 20 strong SPEs, respectively. The ranges S0–S19 and N0–N19 have 13 and 11 strong SPEs, respectively. S20–S45 and N20–N45 have 7 and 12 strong SPEs, respectively, indicating that the N-S asymmetry of strong SPEs mainly occurred in the areas with a latitude greater than 20° of the two hemispheres of the Sun. The statistical results showed that 48.8 per cent, 51.2 per cent, and 76.7 per cent of the strong SPEs appeared during the rising phase, declining phase, and in the period from two years before to the three years after the solar maximum, respectively. The number of strong SPEs during a solar cycle has a poor correlation with the solar cycle size. [ABSTRACT FROM AUTHOR]

The relationship between coronal mass ejections (CMEs) and Forbush decreases (FDs) has been investigated in the past. But the selection of both solar events are difficult. Researchers have developed manual and automated methods in efforts to identify CMEs as well as FDs. While scientists investigating CMEs have made significant advancement, leading to several CME catalogues, including manual and automated events catalogues, those analyzing FDs have recorded relatively less progress. Till date, there are no comprehensive manual FD catalogues, for example. There are also paucity of automated FD lists. Many investigators, therefore, attempt to manually select FDs which are subsequently used in the analysis of the impact of CMEs on galactic cosmic-ray (GCR) flux depressions. However, some of the CME versus FD correlation results might be biased since manual event identification is usually subjective, unable to account for the presence of solar-diurnal anisotropy which characterizes GCR flux variations. The current paper investigates the relation between CMEs and FDs with emphasis on accurate and careful Forbush event selection. [ABSTRACT FROM AUTHOR]

Coronal mass ejections (CMEs) largely influence the space weather and cause geomagnetic perturbations. Hence, the statistical studies pertaining to the occurrence of CMEs over the solar cycles and their consequence at the near-Earth region are extremely important. For an in-depth understanding, such studies need to be carried out considering various observational aspects of CMEs. With this motivation, here we present a statistical study of radio-loud (RL) and radio-quiet (RQ) CMEs during solar cycles 23 and 24. We also assess their geo-effectiveness and analyze their influence on cosmic ray intensity (CRI). The RL and RQ CMEs constitute 40% and 60% cases, respectively, of the total population of CMEs that arrive the near-Earth region at 1 AU. The mean speed of RL CMEs (≈1170 km s−1) was found to be significantly higher (almost twice) than the mean speed of RQ CMEs (≈519 km s−1) in the low corona while their speed became comparable (≈536 km s−1 for RL and ≈452 km s−1 for RQ CMEs) at near-Earth region. The yearly-averaged speeds of Earth-reaching CMEs follow solar cycle variations. The CRI and geomagnetic Dst index are found to have good negative correlation with speed of Earth-reaching CMEs. RL CMEs were found to be more effective in producing CRI depressions and geomagnetic storms (GSs) in comparison to RQ CMEs; in about 70% cases RL CMEs produced CRI depression and GSs earlier than the RQ CMEs. Superposed epoch analysis suggests strongest depression in CRI occurs 2-5 days and 4-9 days after the onset of RL and RQ CMEs, respectively. Further, GS events show a time-lag of 1-5 days and 3-8 days, respectively, with respect to RL and RQ CMEs. [ABSTRACT FROM AUTHOR]

The behaviour of the solar wind (SW) proton temperature and velocity and their relationship during Forbush decreases (FDs) associated with various types of solar source – coronal mass ejections (CMEs) and coronal holes (CHs) – have been studied. Analysis of cosmic ray variations, SW temperature, velocity, density, plasma beta, and magnetic field (from 1965–2019) is carried out using three databases: the OMNI database, Variations of Cosmic Rays database (IZMIRAN) and Forbush Effects & Interplanetary Disturbances database (IZMIRAN). Comparison of the observed SW temperature (T) and velocity (V) for the undisturbed SW allows us to derive a formula for the expected SW temperature (T exp, the temperature given by a T – V formula, if V is the observed SW speed). The results reveal a power-law T–V dependence with a steeper slope for low speeds (V < 425 km s−1, exponent = 3.29 ± 0.02) and flatter slope for high speeds (V > 425 km s−1, exponent = 2.25 ± 0.02). A study of changes in the T–V dependence over the last five solar cycles finds that this dependence varies with solar activity. The calculated temperature index KT = T / T exp can be used as an indicator of interplanetary and solar sources of FDs. It usually has abnormally large values in interaction regions of different-speed SW streams and abnormally low values inside magnetic clouds (MCs). The results obtained help us to identify the different kinds of interplanetary disturbance: interplanetary CMEs, sheaths, MCs, corotating interaction regions, high-speed streams from CHs, and mixed events. [ABSTRACT FROM AUTHOR]

Observation of auroras at low latitudes is an extremely rare event typically associated with major magnetic storms due to intense Earth-directed Coronal Mass Ejections. Since these energetic events represent one of the most important components of space weather, their study is of paramount importance to understand the Sun-Earth connection. Due to the rarity of these events, being able to access all available information for the few cases studied is equally important. Especially if we refer to historical periods in which current accurate observations from ground-based instruments or space were not available. Certainly, among these events, we must include the great aurora of February 4, 1872. An event whose effects have been observed in different regions of the Earth. What we could consider today a global event, especially for its effects on the communication systems of the time, such as the transatlantic cable that allowed a connection between the United States and Europe since 1866. In this paper, we describe the main results of the observations and studies carried out by Angelo Secchi at the Observatory of the Roman College and described in his Memoria sull’Aurora Elettrica del 4 Febbraio 1872 for the Notes of the Pontifical Academy of new Lincei. This note is extremely modern both in its multi-instrumental approach to the study of these phenomena and in its association between solar-terrestrial connection and technological infrastructures on the Earth. The Secchi’s note definitely represents the first example of analysis and study of an event on a global scale, such as the Atlantic cable, affecting the Earth. What we nowadays call an extreme space weather event.

Context. Active regions in close proximity to coronal holes, also known as anemone regions, are the best candidates for studying the interaction between closed and open magnetic field topologies at the Sun. Statistical investigation of their source-region characteristics can provide vital clues regarding their possible association with energetic events, relevant from space weather perspectives. Aims. The main goal of our study is to understand the distinct properties of flaring and non-flaring anemone active regions and their host coronal holes, by examining spatial and magnetic field distributions during the rise phase of the solar cycle, in the years 2011–2014. Methods. Anemone regions were identified from the minimum-distance threshold, estimated using the data available in the online catalogs for on-disk active regions and coronal holes. Along with the source-region area and magnetic field characteristics, associated filament and flare cases were also located. Regions with and without flare events were further selected for a detailed statistical examination to understand the major properties of the energetic events, both eruptive and confined, at the anemone-type active regions. Results. Identified anemone regions showed weak asymmetry in their spatial distribution over the solar disk, with yearly average independent from mean sunspot number trend, during the rise phase of solar cycle 24. With the progression in solar cycle, the area and minimum-distance parameters indicated a decreasing trend in their magnitudes, while the magnetic field characteristics indicated an increase in their estimated magnitudes. More than half of the regions in our database had an association with a filament structure, and nearly a third were linked with a magnetic reconnection (flare) event. Anemone regions with and without flares had clear distinctions in their source-region characteristics evident from the distribution of their properties and density analysis. The key differences included larger area and magnetic field magnitudes for flaring anemone regions, along with smaller distances between the centers of the active region and its host coronal hole. [ABSTRACT FROM AUTHOR]

Context. The launch of Parker Solar Probe (PSP) in 2018, followed by Solar Orbiter (SO) in February 2020, has opened a new window in the exploration of solar magnetic activity and the origin of the heliosphere. These missions, together with other space observatories dedicated to solar observations, such as the Solar Dynamics Observatory, Hinode, IRIS, STEREO, and SOHO, with complementary in situ observations from WIND and ACE, and ground based multi-wavelength observations including the DKIST observatory that has just seen first light, promise to revolutionize our understanding of the solar atmosphere and of solar activity, from the generation and emergence of the Sun's magnetic field to the creation of the solar wind and the acceleration of solar energetic particles. Aims. Here we describe the scientific objectives of the PSP and SO missions, and highlight the potential for discovery arising from synergistic observations. Here we put particular emphasis on how the combined remote sensing and in situ observations of SO, that bracket the outer coronal and inner heliospheric observations by PSP, may provide a reconstruction of the solar wind and magnetic field expansion from the Sun out to beyond the orbit of Mercury in the first phases of the mission. In the later, out-of-ecliptic portions of the SO mission, the solar surface magnetic field measurements from SO and the multi-point white-light observations from both PSP and SO will shed light on the dynamic, intermittent solar wind escaping from helmet streamers, pseudo-streamers, and the confined coronal plasma, and on solar energetic particle transport. Methods. Joint measurements during PSP–SO alignments, and magnetic connections along the same flux tube complemented by alignments with Earth, dual PSP–Earth, and SO-Earth, as well as with STEREO-A, SOHO, and BepiColumbo will allow a better understanding of the in situ evolution of solar-wind plasma flows and the full three-dimensional distribution of the solar wind from a purely observational point of view. Spectroscopic observations of the corona, and optical and radio observations, combined with direct in situ observations of the accelerating solar wind will provide a new foundation for understanding the fundamental physical processes leading to the energy transformations from solar photospheric flows and magnetic fields into the hot coronal plasma and magnetic fields and finally into the bulk kinetic energy of the solar wind and solar energetic particles. Results. We discuss the initial PSP observations, which already provide a compelling rationale for new measurement campaigns by SO, along with ground- and space-based assets within the synergistic context described above. [ABSTRACT FROM AUTHOR]

Magnetic flux ropes observed as magnetic clouds near 1 au have been extensively studied in the literature and their distinct features are derived using numerous models. These studies summarize the general characteristics of flux ropes at 1 au without providing an understanding of the continuous evolution of the flux ropes from near the Sun to 1 au. In the present study, we investigate 26 flux ropes observed by the Helios 1 and 2 spacecraft (from 0.3 to 1 au) using the velocity-modified Gold–Hoyle model. The correlation and regression analyses suggest that the expansion speed, poloidal speed, total magnetic helicity and twist per au of the flux rope are independent of heliospheric distance. The study implies that the aforementioned features are more strongly influenced by their internal properties compared with external conditions in the ambient medium. Moreover, the poloidal magnetic flux and magnetic energy of the studied flux ropes exhibit power-law dependence on heliospheric distance. A better understanding of the underlying physics and corroboration of these results is expected from the Parker Solar Probe measurements in the near future. [ABSTRACT FROM AUTHOR]

For over 60 years, neutron monitors have been the main standard and high precision detectors for measuring cosmic rays with energy from 400 MeV to hundreds GeV. In order to obtain sufficiently complete information about the distribution of cosmic rays outside the magnetosphere, it is necessary to have a network of detectors spaced evenly enough around the globe. The ring of stations method is one of the most useful methods for studying the properties of the angular distribution of cosmic rays without expressing the cosmic ray intensity in terms of spherical harmonics. The method allows one to get the hourly longitude distribution of the cosmic ray intensity without modeling. The main objective of this work is to expand the use of the ring of stations method, as it is a convenient and useful method of studying cosmic ray variation. Using the ring of stations method, it is possible to study specific angular distributions of cosmic ray variation that are described poorly by the sum of the first spherical harmonics. The ring of stations method is primarily used to study Forbush decreases. Detailed descriptions of Forbush decrease investigation by the ring of stations method are presented in this study. The application of the method to the study of the precursors of Forbush decreases and cosmic rays behavior inside the solar wind disturbances is shown. [ABSTRACT FROM AUTHOR]

We emphasize the need for a careful and rigorous timing of Forbush decreases (FDs) as well as a correct calculation of FD magnitudes in studies related to cosmic ray (CR) modulation. We have employed Fourier and R -based algorithms for FD event selection, timing and magnitude estimation. The large number of Forbush events that have been identified were employed in correlation and regression analyses to investigate the rigidity and sensitivity dependence of neutron monitors (NMs). It was found that there is a significant difference between the number of FDs identified manually and those selected by the automated method. While the minimum number (238) of FDs occurred at Irkutsk NM, the Novosibirsk CR station observed the largest number (386) of Forbush events. However, within the north high-latitude band (39°N ≤ latitude ≤ 90°N), only 29 FDs have been simultaneously identified using the data from some NMs in the region, including Irkutsk and Novosibirsk. The result obtained using a large number of FDs differs significantly from those employing manual identification of Forbush events. We conclude, among other things, that the automation of FD event selection is essential for understanding the dependence of CR modulation on NM rigidity and altitude, as well as on the contribution from terrestrial modulation agents. [ABSTRACT FROM AUTHOR]