Solar Drivers Research Articles

A Comparison of geomagnetic activity during the 24th Solar cycle to geomagnetic activities during the preceding solar cycles 19 to 23

Abstract Geomagnetic storms occurring during the period from 1957 to 2018 encompassing six Solar Cycles viz. cycle 19, 20, 21, 22, 23, and 24 have been identified by observing the hourly Dst indices from the World Data Centre Kyoto. The temporal variation of these storms in three different time scales viz. variation around a solar cycle, seasonal variation, and variation about solar rotation period were examined. Geomagnetic activity during the 24th cycle was found to be a cycle of unusually low geomagnetic activity as compared to other cycles of roughly the same solar activity, which can be attributed to absence of adequate interplanetary conditions in addition to absence of solar drivers. It was found that geomagnetic activity during the 24th solar cycle showed attributes more or less similar to those of cycles 19 to 23, though on a much smaller scale. Distinct equinoxial maxima with absence of asymmetry was found in case of seasonal variation. Storms occurring during solar minima indicated a peak around solar rotation period confirming the recurrent solar drivers.

Strong Relativistic Electron Flux Events in GPS Orbit

AbstractRelativistic electrons cause internal charging on satellites and are a significant space weather hazard. In this study we analyze approximately 20 years of data from the US Global Positioning System (GPS) satellite NS41 to determine the conditions associated with the largest daily averaged fluxes of = 2.0 MeV relativistic electrons. The largest flux events at = 4.5 and = 6.5 were associated with moderate to strong CME‐driven geomagnetic storms. However, the majority of the 50 largest flux events at = 4.5 (30 out of 50) and = 6.5 (37 out of 50) were associated with high speed solar wind streams from coronal holes. Both solar drivers are thus very important for relativistic electron flux enhancements in GPS orbit. The 1 in 3 year flux level was not exceeded following any of the 15 largest geomagnetic storms as monitored by the index, showing that the largest geomagnetic storms, most often associated with extreme space weather, do not result in significantly larger relativistic electron flux events in GPS orbit.

Probabilistic Short‐Term Solar Driver Forecasting With Neural Network Ensembles

AbstractSpace weather indices are used to drive forecasts of thermosphere density, which directly affects objects in low‐Earth orbit (LEO) through atmospheric drag force. A set of proxies and indices (drivers), F10.7, S10.7, M10.7, and Y10.7 are used as inputs by the JB2008, (https://doi.org/10.2514/6.2008‐6438) thermosphere density model. The United States Air Force (USAF) operational High Accuracy Satellite Drag Model (HASDM), relies on JB2008, (https://doi.org/10.2514/6.2008‐6438), and forecasts of solar drivers from a linear algorithm. We introduce methods using long short‐term memory (LSTM) model ensembles to improve over the current prediction method as well as a previous univariate approach. We investigate the usage of principal component analysis (PCA) to enhance multivariate forecasting. A novel method, referred to as striped sampling, is created to produce statistically consistent machine learning data sets. We also investigate forecasting performance and uncertainty estimation by varying the training loss function and by investigating novel weighting methods. Results show that stacked neural network model ensembles make multivariate driver forecasts which outperform the operational linear method. When using MV‐MLE (multivariate multi‐lookback ensemble), we see an improvement of RMSE for F10.7, S10.7, M10.7, and Y10.7 of 17.7%, 12.3%, 13.8%, 13.7% respectively, over the operational method. We provide the first probabilistic forecasting method for S10.7, M10.7, and Y10.7. Ensemble approaches are leveraged to provide a distribution of predicted values, allowing an investigation into robustness and reliability (R&R) of uncertainty estimates. Uncertainty was also investigated through the use of calibration error score (CES), with the MV‐MLE providing an average CES of 5.63%, across all drivers.

Solar influences on the Earth’s atmosphere: solved and unsolved questions

Solar influences on the Earth’s atmosphere: solved and unsolved questions

Prediction Capability of Geomagnetic Events from Solar Wind Data Using Neural Networks

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.

The dependence of radial diffusion coefficients on solar/interplanetary drivers

Radial diffusion driven by Ultra Low Frequency (ULF) waves is very important for magnetospheric dynamics, as it contributes to both acceleration and loss of relativistic electrons in the outer Van Allen radiation belt, as well as to other planetary magnetospheres. The dependence of ULF wave power spectral density and radial diffusion coefficients (DLL) on solar wind parameters has already been investigated but their dependence on the various solar and interplanetary drivers is poorly studied. In this study, we conduct a statistical analysis of radial diffusion coefficients DLL to look into their dependence on different interplanetary drivers, i.e. Interplanetary Coronal Mass Ejections (ICMEs) and Stream Interaction Regions (SIRs), which originate from different solar drivers: CMEs and coronal holes. We use the ”SafeSpace” database (https://synergasia.uoa.gr/modules/document/?course=PHYS120), which includes radial diffusion coefficients DLL and ULF wave power spectral density, created using magnetic and electric field measurements by the THEMIS satellites in the 2011–2019 time period. We study how the properties of these solar wind drivers influence the behavior of radial diffusion coefficients. The results indicate significant differences between drivers of different solar origin at the ratio of the electric over the magnetic DLL component (DLLE over DLLB), especially when the solar wind dynamic pressure is maximized due to shock existence. Furthermore, differences are observed between SIRs with shock and SIRs without shock. This feature introduces a significant energy dependence to the radial diffusion coefficients, which is further depending on the radial distance and the different first adiabatic invariant (μ) values. These results emphasize the importance of the energy dependence of the magnetic component of the DLL, especially during the occurrence of interplanetary shocks, which is neglected by most semi-empirical models used in radiation belt simulations.

Energy Input of EUV, Solar Wind, and SEPs at Mars: MAVEN Observations During Solar Minimum

AbstractSolar extreme ultraviolet (EUV) radiation, the solar wind, and solar energetic particles (SEPs) are variable sources of ionization and heating to the Martian atmosphere. Mars Atmosphere Volatile Evolution’s (MAVEN's) elliptical orbit provides a means to characterize these solar drivers immediately upstream of Mars. We have calculated the energy fluxes of EUV, solar wind, and SEPs while MAVEN is outside of the Martian induced magnetosphere. By time‐averaging observations over 2–5 month periods, we reduced short‐term variability to resolve seasonal and solar cycle trends. For the duration spanning the declining phase of solar cycle 24 and three Mars years, the calculated energy fluxes in units of 109 eV/cm2/s were 650–1,400 for EUV, 77–180 for solar wind ions, 2.4–7.4 for solar wind electrons, 0.01–2.7 for SEP ions, and 0–0.4 for SEP electrons. We estimated the fraction of these energy fluxes that would reach the atmosphere and determined that precipitating particle fluxes on the dayside would need to exceed 1012 eV/cm2/s to compare to EUV. We also predicted that SEPs may impart as much or more energy flux than solar wind electrons on the nightside during periods of strong and weak coronal mass ejection activity. We then discussed and decoupled seasonal variation from the solar drivers to reveal trends and outliers as a function of solar cycle. Finally, we compared MAVEN observations during a weak solar cycle to prior estimates of EUV, solar wind, and SEPs from the young sun, and identified times when MAVEN‐observed peak energy fluxes were close to the steady‐state energy fluxes of the ancient solar system.

Solar Rotation Effects in Earth's and Mars' Thermospheric Densities as Revealed by Concurrent MAVEN, Swarm‐C, and GOES Observations

AbstractThe responses of Earth's and Mars' thermospheric densities to quasi‐periodic (∼27‐day) solar rotation variations in flux were measured contemporaneously by the Mars Atmosphere and Volatile EvolutioN, Geostationary Operational Environmental Satellites, and Swarm‐C satellites. While large solar rotation variability is found in both planetary thermospheres, correlation analyses performed on over 6 years of observations reveal that, independently of extreme ultraviolet flux level, Earth's daytime density response is about 10%–50% larger than Mars' at a similar density level. Important altitude dependencies in the density sensitivity to the solar rotation in solar flux are found in the Martian thermosphere, while the terrestrial thermosphere is shown to exhibit only small (±5%) day/night and latitude variations in the response. Detailed analyses focused on correlative periods in 2015–2016 and 2020 indicate important solar cycle effects in the sensitivities of both planetary thermospheres, with increased slopes under low solar flux conditions. These results provide important new insights into processes relevant to the interpretation of the sources of short‐term density variability in Mars' and Earth's thermospheres associated with solar drivers and point to the need for targeted modeling efforts along with dedicated data analyses to help resolve current unknowns in thermal balance processes.

Dependence of radiation belt flux depletions at geostationary orbit on different solar drivers during intense geomagnetic storms

The loss of electron flux of the outer radiation belt has been widely studied in terms of the mechanism that brings in these losses. There are a few studies which have attempted to explain the interplanetary conditions that favor the depletions. As the Sun is the prime cause of any change happening in the magnetosphere, it is important to look at the solar drivers that bring in such changes. In this study, we attempt to understand the effect of solar structures and substructures on the loss of radiation belt high-energy electrons during intense geomagnetic storms. The superposed epoch analysis is used to observe any peculiar changes in GOES electron flux data during the storms that are associated with solar structures such as CME and CIR, ICME substructures such as the magnetic cloud, magnetic cloud with sheath, ejecta, ejecta with sheath, and only sheath. The long-term data also give an opportunity to compare the flux decrease during solar cycles 23 and 24. It has been observed that 1) CIR-associated storms cause a comparatively higher flux decrease than CME-associated storms, 2) sheath-related storms bring out a higher flux decrease, and 3) there is no significant change in flux for the storms of both the solar cycles. The flux decrease in intense storms at the geostationary orbit is essentially triggered by the “Dst effect.” Apart from this, the minimum IMF Bz and northward IMF Bz before turning southward add to the flux decrease. These results hold true for the electron depletions occurring only during intense geomagnetic storms and may alter otherwise.

Statistical Methods Applied to Space Weather Science

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.

The first ground-level enhancement of solar cycle 25 on 28 October 2021

Aims. The first relativistic solar proton event of solar cycle 25 was detected on 28 October 2021 by neutron monitors (NMs) on the ground and particle detectors on board spacecraft in near-Earth space. This is the first ground-level enhancement (GLE) of the current cycle. A detailed reconstruction of the NM response together with the identification of the solar eruption that generated these particles is investigated based on in situ and remote-sensing measurements. Methods. In situ proton observations from a few MeV to ∼500 MeV were combined with the detection of a solar flare in soft X-rays, a coronal mass ejection, radio bursts, and extreme ultraviolet (EUV) observations to identify the solar origin of the GLE. Timing analysis was performed, and a relation to the solar sources was outlined. Results. GLE73 reached a maximum particle rigidity of ∼2.4 GV and is associated with type III, type II, and type IV radio bursts and an EUV wave. A diversity of time profiles recorded by NMs was observed. This points to the event having an anisotropic nature. The peak flux at E > 10 MeV was only ∼30 pfu and remained at this level for several days. The release time of ≥1 GV particles was found to be ∼15:40 UT. GLE73 had a moderately hard rigidity spectrum at very high energies (γ ∼ 5.5). Comparison of GLE73 to previous GLEs with similar solar drivers is performed.

A User’s Guide to the Magnetically Connected Space Weather System: A Brief Review

This article provides a concise review of the main physical structures and processes involved in space weather’s interconnected systems, emphasizing the critical roles played by magnetic topology and connectivity. The review covers solar drivers of space weather activity, the heliospheric environment, and the magnetospheric response, and is intended to address a growing cross-disciplinary audience interested in applied aspects of modern space weather research and forecasting. The review paper includes fundamental facts about the structure of space weather subsystems and special attention is paid to extreme space weather events associated with major solar flares, large coronal mass ejections, solar energetic particle events, and intense geomagnetic perturbations and their ionospheric footprints. This paper aims to be a first step towards understanding the magnetically connected space weather system for individuals new to the field of space weather who are interested in the basics of the space weather system and how it affects our daily lives.

Alfvénicity-related Long Recovery Phases of Geomagnetic Storms: A Space Weather Perspective

Abstract This paper reports, for the first time on a statistical basis, on the key role played by the Alfvénic fluctuations in modulating the recovery phase of the geomagnetic storms, slowing down the restoration of the magnetosphere toward its pre-storm equilibrium state. Using interplanetary and geomagnetic measurements collected over more than one solar cycle, a high correlation between the durations of Alfvénic streams and concurrent recovery phases is found, pointing to a clear coupling between Alfvénic turbulence and magnetospheric ring current dynamics. By exploiting current solar wind models, these observations also provide space weather opportunities of predicting the total duration of any geomagnetic storm induced by any solar driver provided that it is followed by an Alfvénic stream, a crucial piece of information for ground technologies and infrastructures that are affected by time-integrated effects throughout the duration of the storm.

Improving the Medium-Term Forecasting of Space Weather: A Big Picture Review From a Solar Observer's Perspective

We have improved considerably our scientific understanding of the key solar drivers of Space Weather, i.e., Coronal Mass Ejections, flares, in the last 20+ years thanks to a plethora of space missions and modeling advances. Yet, a major breakthrough in assessing the geo-effectiveness of a given CME and associated phenomena still escapes us, holding back actionable medium-term (up to 7 days) forecasting of Space Weather. Why is that? I adopt a two-pronged approach to search for answers. First, I assess the last 20+ years of research on solar drivers by identifying lessons-learned and paradigm shifts in our view of solar activity, always in relation to Space Weather concerns. Then, I review the state of key observation-based quantities used in forecasting to isolate the choke points and research gaps that limit medium-term forecasting performance. Finally, I outline a path forward along three vectors—breakthrough capabilities, geo-effective potential, and actionable forecast—with the strongest potential to improve space weather forecasting horizon and robustness.

Ionospheric Sluggishness: A Characteristic Time‐Lag of the Ionospheric Response to Solar Flares

AbstractThe term “sluggishness” was coined by E. V. Appleton in the 1950s to describe the time delay between peak irradiance at solar noon and the resulting peak in ionospheric electron density. Sluggishness can be understood as an inertial property of the ionosphere that manifests as a lag of the ionospheric response to a solar driver. As shown by Appleton, estimates of sluggishness can be used to study the chemistry of the lower ionosphere, of the D‐region in particular. In this study, for the first time, we have examined ionospheric sluggishness in terms of the time delay between the peak irradiance during a solar flare and the resulting peak in ionospheric electron density using HF instruments. Estimates of the delay are obtained using HF observations from riometers and SuperDARN radars that are primarily sensitive to absorption in the D‐region. Two new methods for measuring delay are introduced. Sluggishness is shown to be anti‐correlated with peak solar X‐ray flux and positively correlated with zenith angle and latitude. The choices of instrument, method, and reference solar waveband affect the sluggishness estimation. A simulation study was performed to estimate the effective recombination coefficient in the D‐region. The coefficient was found to vary by orders of magnitude with peak flare intensity. We argue that the variation in effective recombination coefficient with peak flare intensity is highly sensitive to changes in the negative and positive ion chemistry of the D‐region, which is sensitive to the incoming solar X‐ray and EUV radiation.

Seasonal dependence of the Earth's radiation belt – new insights

Abstract. Long-term variations in the relativistic (∼MeV) electrons in the Earth's radiation belt are explored to study seasonal features of the electrons. An L-shell dependence of the seasonal variations in the electrons is reported for the first time. A clear ∼6 month periodicity, representing one/two peaks per year, is identified for 1.5–6.0 MeV electron fluxes in the L shells between ∼3.0 and ∼5.0. The relativistic electron flux variation is strongest during solar cycle descending to minimum phases, with weaker/no variations during solar maximum. If two peaks per year occur, they are largely asymmetric in amplitude. The peaks essentially do not have an equinoctial dependence. Sometimes the peaks are shifted to solstices, and sometimes only one annual peak is observed. No such seasonal features are prominent for L<3.0 and L>5.0. The results imply varying solar/interplanetary drivers of the radiation belt electrons at different L shells. This has a potential impact on the modeling of the space environment. Plausible solar drivers are discussed.

Consequences of a Solar Wind Stream Interaction Region on the Low Latitude Ionosphere: Event of 7 October 2015

In this article, we present a study of the perturbations occurring in the Earth’s environment on 7 October 2015. We use a multi-instrument approach, including space and ground observations. In particular, we study the ionospheric conditions at low latitudes. Two ionospheric storms are observed at the low latitude station of Tucuman ( $26^{\circ}$ $51'$ S, $65^{\circ}$ $12'$ W). We observe a negative ionospheric storm followed by a positive one. These ionospheric perturbations were triggered by two sudden storm commencements (SSCs) of a strong geomagnetic storm. Preliminary results show that the main mechanism involved in both ionospheric storms is the prompt penetration of electric fields (PPEFs) from the magnetosphere. Furthermore, in the positive storm, disturbed dynamo electric fields are observed acting in combination with the PPEFs. The impact of the solar wind on the Earth’s environment is analyzed using geomagnetic data and proxies, combined with data acquired in the Tucuman Low Latitude Observatory for the Upper Atmosphere. We also investigate the solar and interplanetary drivers of this intense perturbation. We find that, although typically interplanetary coronal mass ejections (ICMEs) are the most geoeffective transient interplanetary events, in this case, a corotating interaction region (CIR) is responsible for these strong perturbations to the geospace.

Design and Implementation of Solar OLED Lighting Driver Circuit with Frequency Modulation Control

This paper proposes a single-stage single-ended primary inductor converter (SEPIC) converter circuit, which is applied to the organic light-emitting diodes (OLED) driver circuit. The circuit proposed in this paper replaces the output Schottky diode from the original SEPIC with a power switch. Deadtime is added to prevent the on-state overlapping of two switches with zero voltage switching (ZVS), and the circuit operates in triangular current mode. The digital control methods are maximum power point tracking and frequency modulation using a battery to supply the converter and illuminate the OLED at night. Finally, a prototype is implemented to show the feasibility under the DC input voltage range of 10–40 V. The DC output is 12 V/1 A/12 W, and the conversion efficiency is up to 96.3%.

Effect of Some Solar Energetic Events on Cosmic Ray (CR) Ground Level Enhancement (GLE)

Ground Level Enhancements (GLEs) are sudden and short-lived increases in cosmic rays intensities registered by neutron monitors and usually associated with powerful solar explosive events. This work examines the key features of forty-nine (49) Ground level enhancement (GLE) events with a view to understanding the possible link to their solar sources. These constitute the 22nd -71st GLE as recorded by the Oulu Neutron monitor. We have computed the increase rates of the GLE and have compared them with the associated solar activity parameters. It is observed that GLEs are more associated with Solar flares than CMEs during the minimum and ascending phase of the 11 year solar cycle when compared with GLEs that occurred during the maximum phase of the solar cycle. In general, more GLE events were associated with solar flares originating from the Northern hemisphere of the solar disk as compared with the southern hemisphere flares. 80% of GLE with percentage increase rate (PIR) <10% were linked with strong flares. There was no/weak statistical association between intensity of solar energetic events (solar flares, CMEs, etc) and increase rate of GLE. The harder solar energetic fluxes seem to be responsible for GLEs with high PIR and that the softer fluxes may be responsible for those with low PIR. These results have important implications for our present understanding of potential solar drivers of the GLEs.

Variability of Precipitating Ion Fluxes During the September 2017 Event at Mars

AbstractIn this work, we study the influence of the September 2017 solar event on the precipitating heavy ion fluxes toward Mars' atmosphere as seen by Mars Atmosphere and Volatile EvolutioN/Solar Wind Ion Analyzer, an energy and angular ion spectrometer and by Mars Atmosphere and Volatile EvolutioN/Suprathermal and Thermal Ion Composition instrument, an energy, mass, and angular ion spectrometer. After a careful reconstruction of the background induced by the Solar Energetic Particle event in the Mars Atmosphere and Volatile EvolutioN/Solar Wind Ion Analyzer spectrometer, we investigate the precipitating ion flux responses to the space weather events that took place in September 2017. This period is a unique opportunity to analyze the respective role of various possible drivers of heavy ion precipitation into Mars' atmosphere with a wide range of different space weather events occurring during the same month. This study shows an increase in the precipitation flux by more than 1 order of magnitude during the arrival of the September Interplanetary Coronal Mass Ejection compared to the average flux during quiet solar conditions. We also showed that among the possible solar drivers, the solar wind dynamic pressure is the most significant during September 2017.