Abstract Accurately modeling the solar magnetic field is important for understanding long-term solar activity and space weather, but it is challenging due to limited observations, especially near the Sun’s poles. The surface flux transport (SFT) model simulates how magnetic flux moves across the solar surface and contributes to the polar field, but it parameterizes emerged sunspots as simple symmetric bipolar regions and needs improvement by including more realistic sunspot features. In this study, we reconstruct the century-long evolution of the Sun’s magnetic field, including the polar regions, using an improved SFT model. We incorporate cycle-dependent morphological asymmetry between leading and following sunspots, along with observationally derived tilt angles and sunspot area data for a century (1913–2016), to better represent magnetic flux transport and investigate the impact of asymmetry on polar field development. To study morphological asymmetry, we consider two cases: first, a long-term asymmetry factor calculated from the ratio of leading and following sunspot areas spanning a century; second, the temporal asymmetry factor observed during solar cycle 23 applied to every solar cycle. Our simulated magnetic flux transport with inclusion of morphological asymmetry for both cases gets improved compared to the no-asymmetry case in terms of enhanced low- and midlatitude magnetic flux and matches closely with observations. The simulated polar fields with asymmetry also show a better agreement with polar field observations for most cycles, particularly in capturing the timing of the polar field reversals and the peak amplitude during solar minima, which has severe consequences in solar cycle prediction.

Context. Solar activity, dominated by 11-year cyclic evolution, has been observed directly since 1610 CE. Indirect cosmogenic proxy data have been used to reconstruct solar activity for millennia prior to 1610. The precision of radiocarbon Δ 14 C measurements has recently improved enough to allow cyclic solar activity to be reconstructed over millennia. Aims. We present the first detailed reconstruction of solar activity, represented here by annual sunspot numbers, during the first millennium, 1–969 CE. Methods. The reconstruction of sunspot numbers from Δ 14 C was performed using a physics-based method that involves several steps: First, using the modern carbon-cycle box model, the 14 C production rate, corrected for the contemporary geomagnetic shielding, was computed from the measured concentrations. The open solar magnetic flux was then computed using a model of the heliospheric cosmic-ray modulation. Sunspot numbers were calculated by inverting a model of the evolution of the Sun’s magnetic field. Lastly, the Markov chain Monte Carlo approach was used to directly account for different sources of uncertainty. Results. Annual sunspot numbers were reconstructed for the first millennium CE. This period includes one extreme solar event that occurred in 774 CE and one grand solar minimum, from 650 to 730 CE. We were able to identify 91 solar cycles, of which 26 were well defined, 24 were reasonably well defined, and 41 were poorly defined. The mean cycle length was 10.6 years, but the lengths of individual cycles vary between 8 and 15 years. The existence of empirical Waldmeier relations remains inconclusive with this dataset. No significant periodicities were found beyond the 11-year cycle. Conclusions. This work fills the gap in the solar cycle statistics between the previously reconstructed first millennium BCE and the second millennium CE, providing vital constraints for solar dynamo and irradiance models. A consistent 3-millennium-long reconstruction of sunspot numbers, based on a composite multi-proxy cosmogenic record, is pending.

Abstract Accurate forecasting of future solar cycle amplitudes is crucial because solar activity modulates the near-Earth space environment, yet such predictions remain a long-standing challenge in solar physics. Observations indicate a correlation between the polar field strength at a solar cycle minimum and the subsequent sunspot cycle amplitude, which has been widely relied upon for solar activity predictions. However, it remains an open question which polar precursor—polar field strength or axial dipole moment—serves as the more robust predictor of future solar activity. In this study, we employ an observationally constrained, data-driven surface flux transport model to reconstruct century-scale variations in both the polar flux and the axial dipole moment evolution and perform a comprehensive assessment of their predictive capabilities. Our time-lag and correlation analyses show that the axial dipole moment correlates more strongly with the amplitude of the subsequent sunspot cycle than the polar field. Using the simulation results, we also predict the ongoing solar cycle 25. We find that its peak amplitude is expected to lie between about 123 and 157, indicating a stronger cycle than solar cycle 24, with the cycle maximum occurring on average between 2024.5 and 2025. This work provides stronger evidence supporting the axial dipole moment as the primary magnetic precursor for solar cycle prediction within the framework of the Babcock–Leighton dynamo mechanism.

Axions and axionlike particles have gained immense attention in searches for beyond Standard Model physics. Experiments searching for axions leverage their predicted couplings to Standard Model particles to look for observable signals. Though weak, these couplings allow axions to be produced abundantly in the interiors of stars such as the Sun. Once created, axions can escape the Sun and, while passing through the solar atmosphere, oscillate into photons in the magnetic field, producing x-rays. For the first time, to the best of our knowledge, we use data from the observation of soft x-rays from the quiet Sun during the 2019–2020 solar minimum by the Solar X-ray Monitor, on board India’s Chandrayaan-2 lunar exploration mission, to constrain the coupling of axions to photons ( g a γ γ ). Using the latest models of the solar atmosphere to calculate the magnetic field and plasma frequency, we constrain g a γ γ ≲ ( 0.50 – 2.26 ) × 10 − 10 GeV − 1 at 95% confidence level for axion masses m a ≲ 5 × 10 − 4 eV .

Abstract The interplanetary magnetic field (IMF) measured near Earth can be up to 2 times greater than that derived from models using remote solar observations. We investigate this discrepancy by modeling the IMF using a potential field source surface (PFSS) model using synoptic maps of the photospheric magnetic field from 2010 May to 2024 April. Five types of radial field synoptic maps are used in this work: the B r synoptic maps from vector magnetic field data, the M r synoptic maps from the line-of-sight field data, the rescaled M r synoptic maps rescaled from the M r maps by a center-to-limb distance dependent rescaling factor of B r / M r , and composite and rescaled composite synoptic maps comprised of a combination of strong-field pixels from the B r maps and the rest from either the original M r or rescaled M r maps. The modeled IMFs from all five types of synoptic maps agree with each other well in the solar maximum phase, when they are about 2 times smaller than in situ measurements. The IMF calculated from the B r and both composite and rescaled composite synoptic maps match well with in situ observations during solar minimum from 2017 to 2022. The IMF values modeled from both the M r and rescaled M r synoptic maps are still significantly smaller in this time interval. This suggests that (1) the B r maps represent the radial field better than the M r ; and (2) the PFSS model is appropriate to model the heliospheric magnetic field in solar minimum, but has limitations when used near solar maximum.

Abstract This study examines the statistical properties of coronal mass ejections (CMEs) over two and a half solar cycles (1996–2025) using coronal observations from the SOHO LASCO CDAW and CACTus catalogs, supplemented by sunspot number records and in situ measurements of interplanetary CMEs (ICMEs) from the Ulysses spacecraft. Focusing on differences between CMEs originating at different apparent latitudes (AL), we analyzed CME annual rates, spatial distributions, and kinematic properties. The total yearly number of CMEs follows the solar cycle, with high-AL CMEs showing a stronger link than low-AL ones. A pronounced solar-cycle-dependent variation is observed between the number of CMEs detected in the corona and the ICMEs in the solar wind. During solar maximum, ICME counts are substantially lower than coronal ejection rates, likely due to enhanced deflection and interaction in the complex interplanetary medium, whereas during solar minimum, Ulysses captured several times more high-latitude ICMEs. ICME occurrence at Ulysses follows the solar cycle with a systematic two-year phase lag. Spatially, high-AL CMEs display a south–north asymmetry, being preferentially directed northward across all phases of the solar cycle except the solar maximum. Low-AL CMEs show a slight southward bias at solar maximum. During the rising phase, they are northward on the western side and southward on the eastern side, with the opposite pattern in the declining phase. The linear speed and angular width of both low- and high-AL CMEs track the solar cycle, with the CME angular width variations preceding sunspot activity, which suggests potential prognostic applications.

Abstract Solar magnetic tornadoes are dynamic, spiral-shaped plasma structures characterized by helical magnetic fields and rotating plasma flows in the solar atmosphere. They play a significant role in the transport of energy and mass within the solar environment. Identifying and analyzing solar magnetic tornadoes is challenging due to their transient nature and complex morphology and the large volume of associated observational data. We propose two automated methods for detecting these magnetoplasma structures using modern deep learning techniques. Our models search for twisted prominences in the solar corona visible at the solar limb. Our approach involves analyzing the Solar Dynamics Observatory Atmospheric Imaging Assembly 171 Å images using convolutional and recurrent neural networks. By applying established techniques, the methods proposed can detect previously unknown magnetic tornadoes alongside those documented in the literature. The models are trained on 10,294 instances, which corresponds to detection of ∼100 tornadoes with high precision and recall. Identification of 1,476,885 new instances is performed. The resulting database allows for the first comparative analysis of magnetic tornadoes’ spatial distributions across solar cycle phases. We find that tornadoes can serve as tracers of environments prone to reconnection. During solar minimum, these structures occur at the boundaries of coronal holes with strong current sheets and at the edges of polar conic current sheets. During solar maximum, they appear at the footpoints of magnetic loops and are associated with polarity inversion lines.

Abstract Magnetospheric forcing during geomagnetically “quiet” periods is generally assumed to have little impact on the low‐ and mid—latitude ionospheres, especially during solar minimums. However, recent observational evidence suggests that geomagnetic forcing can exert a measurable impact on the global ionosphere even during geomagnetically quiet times. In this study, the Whole Atmosphere Community Climate Model eXtended is employed to investigate how magnetospheric forcing affects the 30‐day average of ionospheric F2‐region peak electron density (NmF2) and height (HmF2) during a geomagnetically quiet period (average Kp 1.67). Four 5‐member ensemble simulations were conducted with different geomagnetic forcings: (a) no geomagnetic disturbances, (b) real Kp, (c) a constant low Kp of 1.67, and (d) a constant moderate Kp of 3.33, all under the same solar minimum condition with a constant F10.7 of 70 sfu. These ensemble runs were generated by introducing small neutral temperature perturbations. The results reveal that quiet‐time magnetospheric forcing alters the 30‐day mean NmF2 by up to 55% at low and mid latitudes. Daytime HmF2 rises by 20–50 km near the equatorial ionization anomaly crests but decreases by 20–30 km near the dip equator around local sunset. These variations are primarily driven by changes in neutral composition, winds, and plasma vertical E × B drifts. These findings demonstrate that magnetospheric forcing during quiet periods can substantially influence ionospheric NmF2. Our results highlight the need to account for the effects of quiet‐time magnetospheric forcing in space weather forecasts and scientific research.

Abstract Crewed missions to Mars will be a milestone of future space exploration programs. However, the absence of Earth's magnetic field leaves astronauts directly exposed to unattenuated energetic particles in deep space, primarily galactic cosmic rays (GCRs), resulting in significantly higher radiation levels and enhanced health risks. Understanding and quantifying these radiation hazards is thus essential for evaluating the feasibility and safety of long‐duration Mars missions. Based on the dose data from the Trace Gas Orbiter mission and the Cosmic Ray Telescope for the Effects of Radiation (CRaTER), we perform correlation analyses between the measured dose rate and solar modulation conditions, parameterized as solar modulation potential, and develop empirical models that can be extrapolated to a broader range of solar activities. Using these models, the GCR‐cumulative dose for mission scenarios following three different transfer trajectories under varying solar modulation conditions during the past ∼60 years are calculated. Our results indicate that missions operated during solar maximum accumulate 30%–55% less GCR effective dose than those during solar minimum, with the specific percentage depending on their execution period and trajectory. Under similar shielding conditions of measurements used here, missions following the minimum energy trajectory and conducted during relatively active solar cycles can generally maintain the cumulative radiation effective dose below 1,000 mSv, but keeping it below the NASA's new limit of 600 mSv requires restricting the mission duration to the solar maximum. Nevertheless, faster transfer orbits can help satisfy this limit during solar minimum years.

Abstract In this paper, we propose an energy decomposition method combined with a Harten–Lax–van Leer Riemann solver that includes an additional dissipation term in the energy equation to improve the numerical stability of the fully implicit, time-evolving coronal model COolfluid COroNal UnsTructured (COCONUT) and extend its applicability to solar-maximum phases. In MHD simulations that evolve conservative variables in time, the thermal pressure is typically computed by subtracting the magnetic and kinetic energies from the total energy. In low- β (the ratio of thermal to magnetic pressure; <10 −3 ) regions, discretization errors of magnetic energy can be comparable to the thermal pressure, potentially leading to negative thermal pressure and causing the simulation to crash. Therefore, we update the decomposed energy, excluding the magnetic energy, at each time step. It avoids subtracting a large magnetic energy from the total energy to obtain a very small thermal pressure in low- β regions, thereby improving the numerical stability of MHD models. We validate the algorithm using a time-evolving solar-maximum Carrington rotation simulation in 2025, which the previous code failed to run to completion. We also perform quasi-steady-state coronal simulations and 2D benchmark tests to further assess the algorithm’s performance. The simulation results show that the algorithm produces results nearly identical to those obtained using the traditional full energy equation during solar minimum, while significantly improving COCONUT’s ability to simulate coronal evolution under strong magnetic fields, even including fields exceeding 100 G with β < 10 −3 . This method provides a promising approach for performing quasi-realistic coronal simulations during solar maxima.

Abstract Unraveling the origins of the quiet Sun’s magnetic fields remains a key challenge. Using observations from Heliospheric Magnetic Imager on board Solar Dynamics Observatory, we identified over 86 million quiet region (QR) structures in daily full-disk magnetograms spanning 2010–2025, a period covering nearly the entire Solar Cycle 24 and half of Cycle 25. By analyzing the cyclic variations, our key findings are as follows. (1) The area coverage and total magnetic flux of QRs closely follow sunspot cycles, indicating that Cycle 25 is stronger than Cycle 24. Throughout Cycle 24, the QR flux surpasses active region (AR) flux for almost the entire cycle. In the first 5.5 yr of Cycle 25, the QR flux remains greater than AR flux except for nine isolated months. (2) From the detection limit to the large-scale end, both the flux and number of QR structures exhibit a sequential pattern of anticorrelation and positive correlation with sunspots. Notably, anticorrelated structures constitute more than 95% of QR structures at any given time, and dominate the QR flux during solar minimum. (3) We develop an empirical model for QR magnetic sources based on long-term flux variations, revealing that the flux contribution of ARs remnants to QRs varies in the solar cycle. During solar maximum, most of the QR flux and positive-correlated QR flux originate from ARs, while roughly 30% of the anticorrelated flux is suppressed by ARs. These findings suggest that the solar cycle should be characterized by the magnetic fields in both ARs and their associated QRs.

Abstract We present new Hubble Space Telescope (HST) UV spectra of the K2 V star HD 166620, the first star clearly recognized to be in a “magnetic grand minimum” state analogous to the Sun’s “Maunder minimum” in the late 1600s. The stellar H I Ly α surface fluxes are extremely low, about a factor of 2 below fluxes observed during solar minimum, and also significantly lower than those of τ Ceti (G8 V) and HD 191408 (K2.5 V), two stars more similar to HD 166620 in spectral type and age (∼10 Gyr) than the Sun. The τ Ceti data that are compared with HD 166620 include both old archival data and a new HST observation as well. The Ly α data are used to confirm a nondetection of astrospheric Ly α absorption for this star, suggesting a very weak wind with M ̇ < 0.1 M ̇ ⊙ . The very compact astrosphere inferred for τ Ceti indicates that the star’s debris disk is at least partly exposed to the interstellar medium (ISM), and we discuss possible consequences.

Abstract Geomagnetic indices can be used to quantify variations in geomagnetic activity caused by Sun‐Earth interactions across the magnetosphere and ionosphere. The global Kp index is widely used as a global geomagnetic indicator, but it is based mostly on the Northern Hemisphere with no contributions from South American observatories. As a result, regional features such as the South American Magnetic Anomaly (SAMA) are not represented. To address this limitation, the regional Ksa index was developed using magnetometer data collected from the Embrace Magnetometer Network (Embrace MagNet), which consists exclusively of South American stations. This study analyzes data from Ksa and Kp and introduces two additional hybrid indices, Kp * and Ksa *. The Kp * index uses global data from the International Real‐time Magnetic Observatory Network processed with the Embrace MagNet algorithm, while the Ksa * index applies the Finnish Meteorological Institute algorithm to South American data. The analysis was conducted over the year 2020, a period characterized by low solar activity, to investigate the behavior of the indices under quiet conditions during three seasonal periods: December solstice, Equinoxes, and June solstice. Pearson correlation coefficients were computed to evaluate the relationships between the indices. The results revealed significant discrepancies between indices derived from different data sets, even when processed with the same algorithm. These differences emphasize the impact of regional geomagnetic phenomena such as the SAMA and Sq current variability.

Abstract Close to Earth, the solar wind is usually super-Alfvénic, i.e., the speed of the solar wind is much larger than the Alfvén speed. However, in the lower coronal regions, the solar wind is mostly sub-Alfvénic. With the Parker Solar Probe (PSP) crossing the boundary between the sub- and super-Alfvénic flow, R. Bandyopadhyay et al. performed a turbulence characterization of the sub-Alfvénic solar wind with initial data from encounters 8 and 9. In this study, we reexamine the turbulence properties such as turbulence amplitude, anisotropy of the magnetic field variance, intermittency, and switchback strength using PSP data from encounters 8–19. The later orbits probe lower altitudes and experience sub-Alfvénic conditions more frequently, providing a greater statistical coverage to contrast sub- and super-Alfvénic solar wind. These later orbits also extend the observations from near solar minimum at launch to near solar maximum conditions. Also, by isolating the intervals where the solar wind speed is approximately equal to the Alfvén speed, we explore the transition in more detail. We show that the amplitude of the normalized magnetic field fluctuation is smaller for the sub-Alfvénic samples. While solar wind turbulence in general is shown to be anisotropic, the sub-Alfvénic samples are more anisotropic than the super-Alfvénic samples, in general. Further, we show that the sub- and super-Alfvénic samples do not show much distinction in terms of intermittency strength. Finally, consistent with prior results, we find no evidence for polarity reversing >90° switchbacks in the sub-Alfvénic solar wind.

Abstract The investigation of small-scale energy release in the Sun’s atmosphere is important in understanding how the corona is heated. Previous work has been able to study small EUV and SXR brightenings outside of active regions (i.e. the quiet Sun), but with HXRs this has mostly focused on active region transients/microflares due to the sensitivity of available telescopes. In this paper we present observations of the quiet Sun with the Nuclear Spectroscopic Telescope Array (NuSTAR), an X-ray imaging spectrometer with much greater sensitivity than previous instruments, allowing the observation of faint events. During the recent solar minimum, NuSTAR captured seven quiet Sun flares/impulsive brightenings, three on 21 February 2020, and four on 12–13 September 2020. From fitting their NuSTAR HXR spectra we find temperatures of 3.1–4.0 MK and emission measures between (0.75–17.0) × 1043 cm−3, which gives thermal energies between (2.5–8.9) × 1026 erg. Only one event, a mini-filament eruption, showed evidence of slightly higher temperatures emission, confirmed through Differential Emission Measure analysis. None of the events showed evidence of non-thermal emission in their NuSTAR spectra, and we placed upper limits to the accelerated electron population. The thermal parameters for these quiet Sun events seem to scale differently to previously studied active region flares, suggesting a different energy release process might be dominating. However, this conclusion is affected by the different sensitivity and biases introduced by the various instruments and analysis approaches used.

Abstract Ionospheric plasma interaction with charged satellite surfaces can lead to a substantial increase in orbital drag. In this context, natural events related to solar activity can have a detrimental effect on a satellite's lifespan and introduce uncertainty in orbital trajectory predictions. This work investigates the variations in charge drag coefficient during solar minimum and solar maximum conditions for Low Earth Orbits with altitudes in the range 300–1,000 [km]. Two months, August 2008 and January 2014, near the solar minimum and solar maximum activities are selected to represent variations of ion and electron densities and temperatures during the solar cycle. The simulations are performed using pdFOAM, an electrostatic particle‐in‐cell code. The results show that the charge drag coefficient increases with altitude. Variations are more pronounced during solar minimum than maximum. The variation of the charge drag coefficient depends significantly on the total ion density as well as the percent fraction of lighter ions . In the lower part of the ionosphere (300 [km]), dominated by , the charge drag coefficient , varies in the range . At higher altitudes 1,000 [km], dominated by , especially during the night, it varies in the range , with the extreme values being related to the solar minimum. For both solar minimum and solar maximum conditions, the charge drag force can become significant compared to the neutral drag force and it is 2 times higher at altitude 1,000 [km].

With the advent of Ulysses measurements during the solar minima of Solar Cycles 23 and 24, the heliospheric magnetic field and the solar wind were observed to behave much differently than expected. In particular, previous studies showed that the magnetic open flux of the Sun, calculated as the product of the radial component of the Sun’s magnetic field and the squared radial distance from the Sun, was observed to have decreased along with observed changes in solar-wind streams, such as solar wind proton density. This was in contrast to the standing theory, prior to the measurements made by Ulysses, that the “baseline” of the magnetic open flux should remain constant across the minima. Studies conducted after these measurements accounted for the discrepancies by showing that the Total Open Magnetic Flux (TOMF), the total amount of open flux outside the streamer belt, is a conserved “baseline” quantity from solar minimum to minimum. In this work, we examine a range of solar-wind parameters across five solar minima in Cycles 21, 22, 23, 24, and 25. Using measurements of the heliospheric magnetic field, dynamic solar wind, and heavy ion composition from multiple spacecraft, we investigate the width of the Heliospheric Current Sheet (HCS) streamer belt in the recent Solar Cycle 25 solar minimum using Advanced Composition Explorer (ACE) Solar Wind Ion Composition Spectrometer (SWICS) (1.1 and 2.0) and Solar Wind Electron, Proton and Alpha Monitor (SWEPAM) data. We also investigate the width of the HCS-streamer belt for the Solar Cycle 22 minimum using a newly proposed entropy methodology. With this analysis, we show continued validation of the conservation theory for the TOMF at solar minimum for Solar Cycles 22 and 25.

Sunspot numbers and spotless days around solar minimum

The use of Static Global Navigation Satellite System (GNSS) has become widespread in geodetic applications due to its ability to provide highly accurate positioning. One of the main sources of errors in GNSS positioning is the ionospheric delay, which has a significant impact especially during periods of maximum solar activity. Previous solar cycle 24 indicated that solar maximum and solar minimum occurred in 2014 and 2019, respectively. This highlights the need for extensive collection of GNSS data to minimize the effects of the ionosphere. This study aimed to develop a simulator for mission planning to determine the optimum duration for GNSS static observation. The Malaysia Real-Time Kinematic GNSS network (MyRTKnet) was used, focusing on three baseline categories: short (<50km), medium (50km to 100km), and long (>100km). The static GNSS data were processed using Trimble Business Centre (TBC), and statistical adjustment was performed based on the standard deviation of delta X, delta Y, delta Z, horizontal, and vertical components. The results of the study indicated that to achieve higher precision in GPS baseline solutions during high ionosphere activity, an observation duration of at least two hours is necessary, particularly to improve vertical precision. Furthermore, the precision of the baseline solution was higher during solar maximum compared to solar minimum. A comparison between the simulation and field data acquisition demonstrated that the simulator successfully estimated the horizontal and vertical precision through statistical analysis. Ultimately, it is expected that this simulator will assist surveyors in determining the optimum duration for GNSS static observation.

Abstract During solar cycle minimum, polar coronal holes show a prominent radio brightening cap. The analysis of polar microwave enhanced radiation in polar coronal holes is helpful for understanding the magnetic field characteristics and the origin of the solar wind. Using the Koshix synthesis method on Nobeyama Radioheliograph data, we identified microwave bright points (BPs) superposed on the microwave brightening cap of the northern polar coronal hole. These microwave BPs manifested intermittently at fixed locations, with their radiation intensities displaying multiperiodic oscillations (20 s, 50 s, and 2.5 minutes). Multiwavelength analysis revealed that nearly all BPs were adjacent to or coincide with 171 Å open structures, indicating a strong correlation between microwave BPs and solar wind, propagating along the open field lines. At lower layers, BPs were associated with bright ribbons in the chromosphere and local enhanced magnetic structures in photosphere. The 20 s lifetime of microwave BP enhancement corresponds to their shortest oscillation period, which, in the temporal characteristics of Alfvén waves, implies a localized heating or a small-scale magnetic reconnection process modulated by Alfvén waves at the root of open magnetic field lines. The microwave 2.5 minute and 171 Å 5 minute oscillations likely stem from chromospheric and photospheric oscillation leakage/propagation, respectively. And longer-period (12 minute) oscillations in 171 Å open structures may link to large-scale coronal hole evolution. The multiperiod oscillatory processes in the polar coronal hole imply complex plasma dynamics inside, which contributes to our understanding of the origin and propagation of solar wind along the open structures in the coronal hole.