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Radial diffusion in planetary radiation belts is a dominant transport mechanism resulting in the energization and losses of charged particles by large-scale electromagnetic fluctuations. In this work we exploit the extensive radial diffusion coefficients (D LL) database created in the framework of the Horizon 2020 SafeSpace project, which spans 9 years of hourly D LL coefficients, to investigate the spatio-temporal distributions of the coefficients. Our results indicate that the radial distribution of the magnetic and electric component of the D LL , as well as their sum, the total D LL , can be well described by a power law function of L* in the [4.3–7.7] range. We show that the L*-dependent spectral index varies significantly and is far from constant as assumed and implemented in many semi-empirical models. We examine the quasi-periodic behavior of the radial profiles of the D LL throughout most of the 24th Solar cycle, which the data cover, and find an approximately 420 days dominant periodicity. This periodic behavior is linked (in terms of cross-wavelet analysis) with solar activity, nevertheless, its origin remains unclear. The uncovered features are important for understanding D LL behavior and drivers as well as for current and future modelling efforts. [ABSTRACT FROM AUTHOR]

Whistler-mode waves have been extensively observed and investigated in terrestrial space. In this study, we present the dynamic response of whistler-mode waves to different solar wind conditions in the dayside terrestrial space based on Magnetospheric Multiscale (MMS) data. Statistical results show that the occurrence rate, amplitudes, and corresponding electron temperature anisotropy of whistler-mode waves increase with P sw in the dayside terrestrial space, which is attributed to the compression of magnetic fields in these magnetosheath and outer magnetosphere. Furthermore, whistler-mode waves under the southward interplanetary magnetic fields (IMFs) show a higher occurrence rate than that under the northward IMFs, mostly corresponding to T e ⊥/ T e ∥ > 1, and have a higher occurrence rate during quasi-radial IMFs. These results present that whistler-mode waves in these magnetosheath and outer magnetosphere are also modulated by the solar wind as clearly as the inner magnetosphere. This work advanced our understanding in the solar–terrestrial interaction. [ABSTRACT FROM AUTHOR]

Modeling of time profiles of solar energetic particle (SEP) observations often considers transport along a large-scale magnetic field with a fixed path length from the source to the observer. Here, we point out that variability in the turbulent field line path length can affect the fits to SEP data and the inferred mean free path and injection profile. To explore such variability, we perform Monte Carlo simulations in representations of homogeneous 2D MHD + slab turbulence adapted to spherical geometry and trace trajectories of field lines and full particle orbits, considering proton injection from a narrow or wide angular region near the Sun, corresponding to an impulsive or gradual solar event, respectively. We analyze our simulation results in terms of field line and particle path length statistics for 1° × 1° pixels in heliolatitude and heliolongitude at 0.35 and 1 au from the Sun, for different values of the turbulence amplitude b / B 0 and turbulence geometry as expressed by the slab fraction f s . Maps of the most probable path lengths of field lines and particles at each pixel exhibit systematic patterns that reflect the fluctuation amplitudes experienced by the field lines, which in turn relate to the local topology of 2D turbulence. We describe the effects of such path length variations on SEP time profiles, both in terms of path length variability at specific locations and the motion of the observer with respect to turbulence topology during the course of the observations. [ABSTRACT FROM AUTHOR]

Terrestrial planets currently in the habitable zone around M dwarfs are estimated to have been in runaway greenhouse conditions for up to ∼1 Gyr due to the long-term pre-main-sequence phase of M dwarfs. These planets likely lose a significant portion of water during the pre-main-sequence phase owing to H2O photolysis followed by hydrogen and oxygen loss to space. However, the effects of H2O reproduction reactions and UV shielding by chemical products that reduce photolysis-induced water loss have yet to be estimated. Here, we apply a 1D photochemical model to a H2O-dominated atmosphere of an Earth-like planet around a pre-main-sequence M dwarf to estimate these effects. We find that water loss is suppressed by efficient H2O reproduction reactions and by UV shielding due to O2. The water loss rate decreases by several to several hundred times compared to that in previous studies, with the assumption that the water loss rate is limited by stellar X-ray and extreme-ultraviolet flux or hydrogen diffusion through the atmosphere. Our results imply that terrestrial planets currently in the habitable zone around M dwarfs are more likely to retain surface water than previously estimated. [ABSTRACT FROM AUTHOR]

The largest geomagnetic storm in two decades occurred in 2024 May with a minimum D _st of −412 nT. We examine its solar and interplanetary origins by combining multipoint imaging and in situ observations. The source active region, NOAA AR 13664, exhibited extraordinary activity and produced successive halo eruptions, which were responsible for two complex ejecta observed at the Earth. In situ measurements from STEREO A, which was 12.°6 apart, allow us to compare the “geo-effectiveness” at the Earth and STEREO A. We obtain key findings concerning the formation of solar superstorms and how mesoscale variations of coronal mass ejections affect geo-effectiveness: (1) the 2024 May storm supports the hypothesis that solar superstorms are “perfect storms” in nature, i.e., a combination of circumstances resulting in an event of an unusual magnitude; (2) the first complex ejecta, which caused the geomagnetic superstorm, shows considerable differences in the magnetic field and associated “geo-effectiveness” between the Earth and STEREO A, despite a mesoscale separation; and (3) two contrasting cases of complex ejecta are found in terms of the geo-effectiveness at the Earth, which is largely due to different magnetic field configurations within the same active region.

Telescopic sunspot observations began in 1610 and captured subsequent solar cycles. In combination with proxy reconstructions on an annual scale, these data sets indicate a gradual transition between regular solar cycles and the Maunder Minimum. The telescopic sunspot observations missed the beginning of the first telescopic solar cycle (Solar Cycle −13), leaving room for considerable uncertainty as to its temporal evolution. Before these early telescopic observations, however, Kepler made solar observations using camerae obscurae and recorded a sunspot group in three solar drawings in 1607. Here, we make use of Kepler’s sunspot drawings and descriptive texts to identify his observational sites and time stamps. We have deprojected his sunspot drawings and compared the reported positions with our calculations of the inclination of the solar equator as seen from these sites at that time. These results locate the reported sunspot group near the solar equator eastward from the central meridian. This contrasts with telescopic sunspot drawings from the 1610s that show sunspot groups in the higher heliographic latitudes. Therefore, what Kepler saw was probably a sunspot group from Solar Cycle −14, rather than one from Solar Cycle −13. These records allow us to place the beginning of Solar Cycle −13 between 1607 and 1610. In comparison with the ^14 C-based solar-cycle reconstructions, our result supports regular solar-cycle durations around the 1610s, rather than any suggested extreme extensions of the solar-cycle duration(s) around the 1610s.

On 2022 March 28 two successive coronal mass ejections (CMEs) were observed by multiple spacecraft and resulted in a magnetic cloud (MC) at 1 au. We investigate the propagation and interaction properties of the two CMEs correlated with the MC using coordinated multipoint remote sensing and in situ observations from Solar Orbiter, STEREO-A, Solar and Heliospheric Observatory, and Wind. The first CME was triggered by a filament eruption with a high inclination angle. Roughly 9 hr later, the second CME originating from the same active region erupted with a smaller tilt angle and faster speed compared to the first one. The second CME overtook the preceding CME and formed a merged front at approximately 75 R _☉ , which developed into a complex ejecta at 1 au. The descending speed and low proton temperature inside the complex ejecta suggest that the two CMEs have fully merged before reaching 1 au, leading them to begin expanding rather than compressing against each other. The complex ejecta appears to have the magnetic field and plasma signatures of an MC, although there is a discontinuity in the magnetic field implying previous interactions. The cross section of the complex ejecta, reconstructed from in situ data using a Grad–Shafranov technique, exhibits a right-handed flux rope structure. These results highlight that an MC-like complex ejecta lacking interaction features could arise from the complete merging of two CMEs.

High-speed solar wind streams (HSSs) interact with the preceding ambient solar wind to form stream interaction regions (SIRs), which are a primary source of recurrent geomagnetic storms. However, HSSs may also encounter and subsequently interact with interplanetary coronal mass ejections (ICMEs). In particular, the impact of the interaction between slower ICMEs and faster HSSs represents an unexplored area that requires further in-depth investigation. This specific interaction can give rise to unexpected geomagnetic storm signatures, diverging from the conventional expectations of individual SIR events sharing similar HSS properties. Our study presents a comprehensive analysis of solar wind data spanning from 1996 to 2020, capturing 23 instances where such encounters led to geomagnetic storms ( SymH < −30 nT). We determined that interaction events between preceding slower ICMEs and faster HSSs possess the potential to induce substantial storm activity, statistically nearly doubling the geoeffective impact in comparison to SIR storm events. The increase in the amplitude of the SymH index appears to result from heightened dynamic pressure, often coupled with the concurrent amplification of the CMEs rearward ∣ B ∣ and/or B _z components.

Clette recently showed that F _10.7 systematically approaches a quiet Sun daily value of 67 solar flux units (sfu) at solar minima as the number of spotless days on the Sun increases. Previously, a floor of ∼2.8 nT had been proposed for the solar wind (SW) magnetic field strength (B). F _10.7 , which closely tracks the Sun's unsigned photospheric magnetic flux, and SW B exhibit different relationships to their floors at 11 yr solar minima during the last ∼50 yr. While F _10.7 approaches 67 sfu at each minimum, the corresponding SW B is offset above ∼2.8 nT by an amount approximately proportional to the solar polar field strength—which varied by a factor of ∼2.5 during this interval. This difference is substantiated by ∼130 yr of reconstructed F _10.7 (via the range of the diurnal variation of the East-component (rY) of the geomagnetic field) and SW B (based on the interdiurnal variability geomagnetic activity index). For the last ∼60 yr, the contribution of the slow SW to SW B has exhibited a floor-like behavior at ∼2 nT, in contrast to the contributions of coronal mass ejections and high-speed streams that vary with the solar cycle. These observations, as well as recent SW studies based on Parker Solar Probe and Solar Dynamics Observatory data, suggest that (1) the Sun has a small-scale turbulent dynamo that is independent of the 11 yr sunspot cycle; and (2) the small-scale magnetic fields generated by this nonvarying turbulent dynamo maintain a constant open flux carried to the heliosphere by the Sun's floor-like slow SW.

Predictions of solar activity in the future are difficult to make due to the chaotic state of solar dynamo and the high nonlinearity of physical processes on the Sun. Therefore, the Climate Intercomparison Project Phase 6 (CMIP6) used a statistical approach and recommended two different solar forcing scenarios for the simulations. The reference scenario was developed as the standard forcing, whereas the alternative forcing has lower solar activity (EXT CMIP6). In this study, we use both forcings in a set of experiments to explore the importance of the alternative CMIP6 solar forcing for future climate and ozone layer variability. In general, the difference in solar forcing scenarios is small, and thus most changes at the surface and at high altitudes are not significant. In addition, only the active phases of the Sun, which have the largest difference in amplitude of the forcing, are investigated. In this case, some statistically significant patterns emerge, mostly in the stratosphere, but still, the magnitude of the changes is not very large and a noticeable surface climate response to these changes is not expected and also not found. Our results indicate that low amplitude solar forcings such as the EXT CMIP6 or similar are not worthwhile considering during the next CMIP type of activities. The proposed solar irradiance decline does not represent any danger to the ozone layer. Plain Language Summary: Predictions of future solar activity are very difficult a make. Therefore, the Climate Model Intercomparison Project Phase 6 (CMIP6) used statistical methods to develop solar forcing scenarios and recommended two different forcings. One is the reference forcing and one is a low solar activity forcing scenario. In general, the difference between the two solar forcing scenarios is small and thus most changes at the surface and at high altitudes are not significant. In addition, only the active phases of the sun are investigated. In this case, some statistically significant patterns emerge, but still, the response signal is weak. Our results suggest that low amplitude solar forcings as proposed for CMIP6 are not worthwhile considering in the next CMIP type of activity. Key Points: The proposed Climate Intercomparison Project Phase 6 (CMIP6) lower solar activity scenario is investigated with an Earth System ModelThe analysis shows no pronounced impact of the CMIP6 lower solar activity on future climate change, as the change in forcing is very smallThe impact on the ozone layer is also very small [ABSTRACT FROM AUTHOR]

We investigate the detailed properties of electron inflow in an electron-only reconnection event observed by the four Magnetospheric Multiscale (MMS) spacecraft in the Earth's turbulent magnetosheath downstream of the quasi-parallel bow shock. The lack of ion coupling was attributed to the small-scale sizes of the current sheets, and the observed bidirectional super-Alfvénic electron jets indicate that the MMS spacecraft crossed the reconnecting current sheet on both sides of an active X-line. Remarkably, the MMS spacecraft observed the presence of large asymmetries in the two electron inflows, with the inflows (normal to the current sheet) on the two sides of the reconnecting current layer differing by as much as a factor of four. Furthermore, even though the four MMS spacecraft were separated by less than seven electron skin depths, the degree of inflow asymmetry was significantly different at the different spacecraft. The asymmetry in the inflow speeds was larger with increasing distances downstream from the reconnection site, and the asymmetry was opposite on the two sides of the X-line. We compare the MMS observations with a 2D kinetic particle-in-cell (PIC) simulation and find that the asymmetry in the inflow speeds stems from in-plane currents generated via the combination of reconnection-mediated inflows and parallel flows along the magnetic separatrices in the presence of a large guide field. [ABSTRACT FROM AUTHOR]

Abstract Predictions of solar activity in the future are difficult to make due to the chaotic state of solar dynamo and the high nonlinearity of physical processes on the Sun. Therefore, the Climate Intercomparison Project Phase 6 (CMIP6) used a statistical approach and recommended two different solar forcing scenarios for the simulations. The reference scenario was developed as the standard forcing, whereas the alternative forcing has lower solar activity (EXT CMIP6). In this study, we use both forcings in a set of experiments to explore the importance of the alternative CMIP6 solar forcing for future climate and ozone layer variability. In general, the difference in solar forcing scenarios is small, and thus most changes at the surface and at high altitudes are not significant. In addition, only the active phases of the Sun, which have the largest difference in amplitude of the forcing, are investigated. In this case, some statistically significant patterns emerge, mostly in the stratosphere, but still, the magnitude of the changes is not very large and a noticeable surface climate response to these changes is not expected and also not found. Our results indicate that low amplitude solar forcings such as the EXT CMIP6 or similar are not worthwhile considering during the next CMIP type of activities. The proposed solar irradiance decline does not represent any danger to the ozone layer.

A technique has recently been developed for tracking short-term spectral variations in Galactic cosmic rays (GCRs) using data from a single neutron monitor (NM), by collecting histograms of the time delay between successive neutron counts and extracting the leader fraction L as a proxy of the spectral index. Here we analyze L from four Antarctic NMs from 2015 March to 2023 September. We have calibrated L from the South Pole NM with respect to a daily spectral index determined from published data of GCR proton fluxes during 2015–2019 from the Alpha Magnetic Spectrometer (AMS-02) on board the International Space Station. Our results demonstrate a robust correlation between the leader fraction and the spectral index fit over the rigidity range 2.97–16.6 GV for AMS-02 data, with uncertainty of 0.018 in the daily spectral index as inferred from L . In addition to the 11 yr solar activity cycle, a wavelet analysis confirms a 27 day periodicity in the GCR flux and spectral index corresponding to solar rotation, especially near sunspot minimum, while the flux occasionally exhibits a strong harmonic at 13.5 days. The magnetic field component along a nominal Parker spiral (i.e., the magnetic sector structure) is a strong determinant of such spectral and flux variations, with the solar wind speed exerting an additional, nearly rigidity-independent influence on flux variations. Our investigation affirms the capability of ground-based NM stations to accurately and continuously monitor cosmic-ray spectral variations over the long-term future.

Variations in X-ray and extreme ultraviolet (EUV) irradiance during solar flares lead to a noticeable increase in the electron concentration in the illuminated part of the Earth’s ionosphere. Due to the large amount of experimental data accumulated by global navigation satellite systems, the total electron content (TEC) response to the impulsive phase of a solar flare has been studied quite well. However, recent studies have shown that a large fraction of X-class flares have a second strong peak of warm coronal emission (which is called “EUV late phase”), whose influence on the ionization of ionospheric layers is not yet clear. A combined analysis of successive solar emissions and the caused TEC changes made it possible to numerically estimate the ionospheric response to the impulsive, gradual, and late phases of the X 2.9 solar flare that occurred on 2011 November 3 and demonstrate the high geoeffectiveness of the rather weak Fe xv 28.4 nm solar emission during the EUV late phase. It was found that the ionospheric response to the relatively weak emissions of the EUV late phase of the X 2.9 solar flare amounted to almost a third of the TEC increase during the impulsive phase.

We develop a preliminary interplanetary disturbance index ( J ${}_{{\rm{s}}}^{{\rm{W}}}$ ) by applying the spectral whitening method to an energy coupling function with solar wind measurements during the years 1998–2014 as the input, which can be used as an indicator of perturbations in the near-Earth solar wind. The correlation and temporal variation between J ${}_{{\rm{s}}}^{{\rm{W}}}$ and the geomagnetic disturbance index ( J ${}_{{\rm{p}}}^{{\rm{G}}}$ ) constructed from the same method have been analyzed in detail for 167 geomagnetic storms with the minimum D _st index less than or equal to −50 nT. The time delay between J ${}_{{\rm{s}}}^{{\rm{W}}}$ and J ${}_{{\rm{p}}}^{{\rm{G}}}$ is clearly observable and varies for different events, according to which J ${}_{{\rm{s}}}^{{\rm{W}}}$ is shifted backward with respect to J ${}_{{\rm{p}}}^{{\rm{G}}}$ . We obtain a fairly good negative correlation between the shifted J ${}_{{\rm{s}}}^{{\rm{W}}}$ and the J ${}_{{\rm{p}}}^{{\rm{G}}}$ indices for the majority of events, and the significance level for 88% of the events (i.e., 147 events) does not exceed 0.05. A statistical analysis of the shifted J ${}_{{\rm{s}}}^{{\rm{W}}}$ and the J ${}_{{\rm{p}}}^{{\rm{G}}}$ indices for 147 selected events reveals that larger values of J ${}_{{\rm{s}}}^{{\rm{W}}}$ and smaller magnitudes of J ${}_{{\rm{p}}}^{{\rm{G}}}$ are commonly accompanied by enhanced southward magnetic fields, which implies that more solar wind energy is entering the magnetosphere and thus causing strong geomagnetic storms. Furthermore, a linear fit of the two indices suggests that the evolution of J ${}_{{\rm{p}}}^{{\rm{G}}}$ can be predicted about 2 hr in advance based on J ${}_{{\rm{s}}}^{{\rm{W}}}$ , indicating that J ${}_{{\rm{s}}}^{{\rm{W}}}$ can provide early warnings of possible disturbances in the geomagnetic fields, which is crucial for space weather monitoring and operational forecasting.

Coronal mass ejections (CMEs) exhibit a wide range of quasiperiodic variations and are crucial for our understanding of the cyclical evolution of large-scale magnetic fields. However, the mid-term periodicities of different types of CMEs associated with different processes at the source location need to be clearly understood. Based on the CDAW catalog released by the Large Angle and Spectroscopic Coronagraph mission on the Solar and Heliospheric Observatory, we investigated the period of CMEs based on the speeds and accelerations using the continuous wavelet transformation method. Our results revealed that the distribution of CMEs over time is quite distinctly different for different speeds, and there are Rieger-type periods and quasi-biennial oscillations of the CMEs. The two types of periodic signals show significant differences in solar cycles 23 and 24. Furthermore, the periodicity patterns for the northern hemisphere differ from those in the southern hemisphere. The potential mechanisms and explanations of the results are also discussed.

Whistler-mode waves have been extensively observed and investigated in terrestrial space. In this study, we present the dynamic response of whistler-mode waves to different solar wind conditions in the dayside terrestrial space based on Magnetospheric Multiscale (MMS) data. Statistical results show that the occurrence rate, amplitudes, and corresponding electron temperature anisotropy of whistler-mode waves increase with P _sw in the dayside terrestrial space, which is attributed to the compression of magnetic fields in these magnetosheath and outer magnetosphere. Furthermore, whistler-mode waves under the southward interplanetary magnetic fields (IMFs) show a higher occurrence rate than that under the northward IMFs, mostly corresponding to T _e _⊥ / T _e _∥ > 1, and have a higher occurrence rate during quasi-radial IMFs. These results present that whistler-mode waves in these magnetosheath and outer magnetosphere are also modulated by the solar wind as clearly as the inner magnetosphere. This work advanced our understanding in the solar–terrestrial interaction.

Modeling of time profiles of solar energetic particle (SEP) observations often considers transport along a large-scale magnetic field with a fixed path length from the source to the observer. Here, we point out that variability in the turbulent field line path length can affect the fits to SEP data and the inferred mean free path and injection profile. To explore such variability, we perform Monte Carlo simulations in representations of homogeneous 2D MHD + slab turbulence adapted to spherical geometry and trace trajectories of field lines and full particle orbits, considering proton injection from a narrow or wide angular region near the Sun, corresponding to an impulsive or gradual solar event, respectively. We analyze our simulation results in terms of field line and particle path length statistics for 1° × 1° pixels in heliolatitude and heliolongitude at 0.35 and 1 au from the Sun, for different values of the turbulence amplitude b / B _0 and turbulence geometry as expressed by the slab fraction f _s . Maps of the most probable path lengths of field lines and particles at each pixel exhibit systematic patterns that reflect the fluctuation amplitudes experienced by the field lines, which in turn relate to the local topology of 2D turbulence. We describe the effects of such path length variations on SEP time profiles, both in terms of path length variability at specific locations and the motion of the observer with respect to turbulence topology during the course of the observations.

The Zeeman effect has been routinely used to image and quantify the solar photospheric magnetic field (B). Such a direct measuring technique is not yet available for the corona (Lin et al. 2004). Since almost all transient nonthermal radio emissions from the corona are either partially or fully circularly polarized, observing their polarization signatures over broad frequency ranges would be of help to estimate B as a function of heliocentric height. This article aims to describe the design and development of a Cross-polarized Log-Periodic Dipole Antenna (CLPDA), an integral part of a radio spectro-polarimeter, which works in the 50–500 MHz frequency-range and to explain the tests that were carried out to characterize it. The above frequency range corresponds to a heliocentric height range ≈1.03 < r < 2.5 R _⊙ ( R _⊙ = photospheric radius ), wherein the numerous coronal nonthermal transients associated with space-weather effects are observed to originate. The CLPDA is used to determine the strength and sense of polarization of the received radio signal. The uncertainty involved in the determination depends on the polarization-isolation (PI) between the two orthogonal components of a CLPDA. Some of the recent advancements made in the antenna design concepts at high frequencies (∼GHz) were adopted to reduce the PI at low frequencies (∼MHz). Throughout the above frequency range, the CLPDA has a gain, return loss, and PI of ≈6.6 dBi, ≲−10 dB, and ≲−27 dB, respectively. The average PI of the CLPDA varies from −30 to −24 dB over an azimuthal angle range 0° to ±45° within which the observations are performed regularly.

Observational studies of the Earth’s radiation belts indicate that Alfvénic fluctuations in the frequency range of 2–25 mHz accelerate electrons to relativistic energies. For decades, statistical models of radiation belts have quantified the impact of Alfvénic waves in terms of quasi-linear diffusion. However, quasi-linear models are inadequate to quantify Alfvénic radial transport occurring on timescales comparable to the azimuthal drift period of 0.1–10 MeV electrons. With recent advances in observational methodologies offering coverage of the Earth’s radiation belts on fast timescales, a theoretical framework that distinguishes between fast and diffusive radial transport can be tested for the first time in situ. In this report, we present a drift-kinetic description of radial transport for planetary radiation belts. We characterize fast linear processes and determine the conditions under which higher-order effects become dynamically significant. In the linear regime, wave–particle interactions are categorized in terms of resonant and nonresonant responses. We demonstrate that the phenomenon of zebra stripes is nonresonant and can originate from injection events in the inner radiation belts. We derive a radial diffusion coefficient for a field model that satisfies Faraday’s law and that contains two terms: one scaling as L ^10 independent of the azimuthal number m , and a second scaling as m ^2 L ^6 . In the higher-order regime, azimuthally symmetric waves with properties consistent with in situ measurements can energize 10–100 keV electrons in less than a drift period. This process provides new evidence that acceleration by Alfvénic waves in radiation belts cannot be fully contained within diffusive models.

The Galactic cosmic-ray spectrum manifests pronounced variations over the 11 yr sunspot cycle and more subtle variations over the 22 yr solar magnetic cycle. An important tool to study these variations is repeated latitude surveys with neutron monitors (NMs) on board icebreakers in conjunction with land-based references. We revisit 13 annual latitude surveys from 1994 to 2007 using reference data from the Mawson NM instead of McMurdo NM (which closed in 2017). We then consider two more latitude surveys (2018 and 2019) with a monitor similar to the 3NM64 in the previous surveys but without lead rings around the central tube, a so-called “semi-leaded neutron monitor.” The new surveys extend the linear relationship among data taken at different cutoff rigidity ranges. They also confirm the “crossover” measured near solar minima during epochs of opposite solar magnetic polarity and the absence of a crossover for epochs having the same solar magnetic polarity.

Coronal mass ejections (CMEs), a kind of violent solar eruptive activity, can exert a significant impact on space weather. When arriving at the Earth, they interact with the geomagnetic field, which can boost the energy supply to the geomagnetic field and may further result in geomagnetic storms, thus having potentially catastrophic effects on human activities. Therefore, accurate forecasting of the transit time of CMEs from the Sun to the Earth is vital for mitigating the relevant losses brought by them. XGBoost, an ensemble model that has better performance in some other fields, is applied to the space weather forecast for the first time. During multiple tests with random data splits, the best mean absolute error (MAE) of ∼5.72 hr was obtained, and in this test, 62% of the test CMEs had absolute arrival time error of less than 5.72 hr. The average MAE over all random tests was ∼10 hr. It indicates that our method has a better predictive potential and baseline. Moreover, we introduce two effective feature importance ranking methods. One is the information gain method, a built-in method of ensemble models. The other is the permutation method. These two methods combine the learning process of the model and its performance to rank the CME features, respectively. Compared with the direct correlation analysis on the sample data set, they can help select the important features that closely match the model. These two methods can assist researchers to process large sample data sets, which often require feature selection in advance.

The flux of energetic particles originating from the Sun fluctuates during the solar cycles. It depends on the number and properties of active regions (ARs) present in a single day and associated solar activities, such as solar flares and coronal mass ejections. Observational records of the Space Weather Prediction Center NOAA enable the creation of time-indexed databases containing information about ARs and particle flux enhancements, most widely known as solar energetic particle (SEP) events. In this work, we utilize the data available for solar cycles 21–24 and the initial phase of cycle 25 to perform a statistical analysis of the correlation between SEPs and properties of ARs inferred from the McIntosh and Hale classifications. We find that the complexity of the magnetic field, longitudinal location, area, and penumbra type of the largest sunspot of ARs are most correlated with the production of SEPs. It is found that most SEPs (≈60%, or 108 out of 181 considered events) were generated from an AR classified with the “k” McIntosh subclass as the second component, and these ARs are more likely to produce SEPs if they fall in a Hale class containing a δ component. The resulting database containing information about SEP events and ARs is publicly available and can be used for the development of machine learning models to predict the occurrence of SEPs.

Multiple neural network architectures, with different structural composition and complexity, are implemented in this study with the aim of providing multi-hour-ahead warnings of severe geomagnetic disturbances, based on in situ measurements of the solar wind plasma and magnetic field acquired at the Lagrangian point L1. First, a statistical analysis of the interplanetary data was performed to point out which are the most relevant parameters to be provided as input to the neural networks, and a preprocessing of the data set was implemented to face its heavy imbalance (the Earth’s magnetosphere is in fact mostly at rest). Then, neural networks were tested to evaluate their performance. It turned out that, in a binary classification problem, recurrent approaches are best at predicting critical events both 1 and 8 hr in advance, achieving a balanced accuracy of 94% and 70%, respectively. Finally, in an attempt at multistep prediction of the criticality of future geomagnetic events from 1–8 hr ahead, more complex neural networks, built by merging the different types of basic convolutional and recurrent architectures, have been shown to outperform single-step and state-of-the-art approaches with a balanced accuracy of at least 70%. Interestingly, the accuracy peaks at 4 hr, corresponding to the waiting time between the detection of solar drivers at L1 and the onset of the geomagnetic storm (as previously obtained by statistical investigations), suggesting that on average this is the time the Earth’s magnetosphere takes to react to the solar event.

This work studies the dynamic response of solar wind charge exchange (SWCX) soft X-ray emission in the Earth’s magnetosphere to the solar wind proton flux. Unlike previous studies that attempted to use complex magnetohydrodynamic models to match the details of observed SWCX of a necessarily limited number of cases, this work focuses on determining the changes over individual observations in a much larger sample. To provide the cleanest test, we selected XMM-Newton observations when the solar wind proton flux changed suddenly by a factor greater than 1.5 and calculated the correlation coefficient between the SWCX emission in the 0.5–0.7 keV band and the proton flux. We find that the dynamical response is weak when the solar wind proton flux is low (

We investigate the detailed properties of electron inflow in an electron-only reconnection event observed by the four Magnetospheric Multiscale (MMS) spacecraft in the Earth's turbulent magnetosheath downstream of the quasi-parallel bow shock. The lack of ion coupling was attributed to the small-scale sizes of the current sheets, and the observed bidirectional super-Alfvénic electron jets indicate that the MMS spacecraft crossed the reconnecting current sheet on both sides of an active X-line. Remarkably, the MMS spacecraft observed the presence of large asymmetries in the two electron inflows, with the inflows (normal to the current sheet) on the two sides of the reconnecting current layer differing by as much as a factor of four. Furthermore, even though the four MMS spacecraft were separated by less than seven electron skin depths, the degree of inflow asymmetry was significantly different at the different spacecraft. The asymmetry in the inflow speeds was larger with increasing distances downstream from the reconnection site, and the asymmetry was opposite on the two sides of the X-line. We compare the MMS observations with a 2D kinetic particle-in-cell (PIC) simulation and find that the asymmetry in the inflow speeds stems from in-plane currents generated via the combination of reconnection-mediated inflows and parallel flows along the magnetic separatrices in the presence of a large guide field.

In particular during the descending phase of the solar cycle, Alfvén waves in the high-speed solar wind streams are a major form of interplanetary disturbances. The fluctuating southward interplanetary magnetic field (IMF) of Alfvén waves has been suggested to induce geomagnetic activities through intermittent magnetic reconnection at the magnetopause. In this study, we provide in situ observational evidence for dayside magnetopause reconnection induced by such interplanetary Alfvén waves. Using multipoint conjunction observations, we show that the IMF B _z from interplanetary Alfvén waves is transmitted through and amplified by the Earth’s bow shock. Associated with the intensified southward B _z to the magnetopause, in situ signatures of magnetic reconnection are detected. Repetitively, interplanetary Alfvén waves transmit the intensified B _z to the magnetosheath, leading to intervals of large magnetic shear angles across the magnetopause and magnetopause reconnection. Such intervals are promptly followed by hundreds of nanoTesla (nT) increases in the auroral electrojet indices (AE and AU) within 10–20 minutes. These observations are confirmed in multiple events in corotating interaction region-driven geomagnetic storms. To put the observations into context, we propose a phenomenological model of a strongly driven substorm. The substorm electrojet is linked to the enhanced magnetopause reconnection in the short timescale of re-establishing the ionosphere electric field and the two-cell convection. These results provide insights on the temporal patterns of solar wind magnetosphere–ionosphere coupling, especially during the descending phase of the solar cycle.

Using multipoint observations over 10 yr near 1 au, we investigate the spectra (5 minutes to 2 hr) of interplanetary Alfvén waves and the responses in the geomagnetic activities. We compute the two-point correlations of the wave magnetic field between the ACE and the THEMIS spacecraft, which are separated by ∼200 Earth radius ( R _E ) in the solar wind. Alfvén waves associated with high two-point correlations exhibit steep spectra (spectra index ∼−1.63). Such Alfvén waves occur mostly in slow-speed streams. By contrast, Alfvén waves with low two-point correlations exhibit flatter spectra (spectra index ∼−1.51) with a relative enhancement of power above 2 × 10 ^−4 Hz. The occurrence of Alfvén waves with low two-point correlations is more equally distributed between high-speed and low-speed streams. In general, interplanetary Alfvén waves show correlations with moderate geomagnetic responses in symmetric ring-current intensity, SuperMAG electrojet (SME), and Kp indices. Statistical analyses indicate that the Alfvén waves with flat spectra correspond to stronger responses in the geomagnetic indices than those with steep spectra, suggesting the importance of the tens of minutes (30–90 minutes) Alfvénic power spectra in the generation of SME/Auroral Electrojets. These observations may shed light on the response of the magnetosphere to fluctuating interplanetary magnetic field B _z .

Coronal mass ejections (CMEs) are energetic expulsions of organized magnetic features from the Sun. The study of CME quasiperiodicity helps establish a possible relationship between CMEs, solar flares, and geomagnetic disturbances. We used the angular width of CMEs as a criterion for classifying the CMEs in the study. Based on 25 yr of observational data, we systematically analyzed the quasiperiodic variations corresponding to the CME occurrence rate of different angular widths in the northern and southern hemispheres, using frequency and time–frequency analysis methods. There are various periods for CMEs of different angular widths: 9 months, 1.7 yr, and 3.3–4.3 yr. Compared with previous studies based on the occurrence rate of CMEs, we obtained the same periods of 1.2 (±0.01), 3.1 (±0.04), and ≈6.1 (±0.4) months, and 1.2 (±0.1) and 2.4 (±0.4) yr. We also found additional periods of all CMEs that appear only in one hemisphere or during a specific solar cycle. For example, 7.1 (±0.2) months and 4.1 (±0.2) yr in the northern hemisphere, 1 (±0.004) and 5.9 (±0.2) months and 1 (±0.1), 1.4 (±0.1), and 2.4 (±0.4) yr in the southern hemisphere, 6.1 (±0.4) months in solar cycle 23, and 6.1 (±0.4) months and 1.2 (±0.1) and 3.7 (±0.2) yr in solar cycle 24. The analysis shows that quasiperiodic variations of the CMEs are a link among oscillations in coronal magnetic activity, solar flare eruptions, and interplanetary space.

The letters published in the ‘Focus issue on high energy particles and atmospheric processes’ serve to broaden the discussion about the influence of high energy particles on the atmosphere beyond their possible effects on clouds and climate. These letters link climate and meteorological processes with atmospheric electricity, atmospheric chemistry, high energy physics and aerosol science from the smallest molecular cluster ions through to liquid droplets. Progress in such a disparate and complex topic is very likely to benefit from continued interdisciplinary interactions between traditionally distinct science areas.

Abstract: Non-typical long lasting quasi-periodic (QP) VLF emissions have been recorded in Northern Finland at L∼5.3 during the recent Finnish VLF campaign held in December 2011. Contrary to the typical daytime QP emissions, the night-time and early morning (00–05UT) event reported here for the first time is a sequence of 1.5–3.5kHz noise bursts lasting for several tens of seconds with an unusually long repetition period which gradually decreases from ∼700s to ∼50s. These QP emissions were observed under conditions of very quiet geomagnetic activity (K p =0). In spite of that, the interplanetary magnetic field generally had a small southward component, and a high-latitude substorm occurred on the night-side. After this substorm, the repetition period of the VLF bursts suddenly dropped from ∼200s to∼60s and the spectral structure of QP wave changed. We attribute these QP emissions to auto-oscillations of the cyclotron instability of the Earth's radiation belts. According to the theory, the repetition period of the QP should be inversely proportional to the flux of the gyroresonant energetic electrons. Thus the increased flux of energetic electrons injected by the substorm probably led to the decreasing QP repetition periods. [Copyright &y& Elsevier]