Interplanetary Magnetic Field Research Articles

Machine learning based storm time modeling of ionospheric vertical total electron content over Ethiopia

Geomagnetic storms can cause variations in the ionization levels of the ionosphere, which is commonly studied using the total electron content (TEC). TEC is a crucial parameter to identify the possible effects of ionospheric variations on satellite communication and navigation. This paper assesses the performance of light gradient boosting machine (LGB) and deep neural network (DNN) machine learning algorithms in modeling ionospheric vertical TEC (VTEC) during geomagnetic disturbances. GPS VTEC data for years 2011–2016 from 13 dual-frequency receiver stations over Ethiopia was utilized. Input parameters for the models were derived from the factors that influence VTEC, such as time, location, geomagnetic activity, solar activity, solar wind, and the interplanetary magnetic field. The LGB model improved the predictions of the DNN model from root mean squared error (RMSE), mean absolute percentage error (MAPE), and R2 values of 5.45 TECU, 21%, and 0.93 to 4.98 TECU, 18%, and 0.94 on the testing data, respectively. The two machine learning models significantly outperformed the International Reference Ionosphere (IRI 2020) model during the selected geomagnetic storm periods. This study could provide insight into the impacts of ionosphere variations on satellite communication and navigation systems in the low-latitude ionospheric region.

Investigation of low-latitude nighttime geomagnetic pulsation events occurred under variable IMF and solar conditions

Disturbances in the Interplanetary Magnetic Field (IMF) and solar conditions can affect the geomagnetic field measurements. In the current study, the influence of IMF and solar fluctuations on the nighttime Pc3-5 and Pi2 pulsations at low latitudes is investigated. Geomagnetic data recorded by the Egyptian geomagnetic observatories are used to examine the occurrence of nighttime pulsation events in Egypt. The corresponding daytime data from the Honolulu (HON) INTERMAGNET observatory were used for comparison between nighttime and daytime pulsation activity aiming for a better understanding of their sources. Two pulsation activities occurred on 7 and 27 February 2014 under different IMF and solar conditions were examined. Results of data analysis indicate that geomagnetic field fluctuations (Pc3-5 and Pi2 pulsations) are affected by changes in the IMF and solar conditions. The occurrence of space weather events such as geomagnetic storms and Coronal Mass Ejections (CMEs) can significantly affect the coherence and correlations between the pulsation wave-packets observed at nighttime and daytime suggesting different generation mechanisms for each pulsation event.

Machine learning models for predicting geomagnetic storms across five solar cycles using Dst index and heliospheric variables

This study aims to improve the understanding of geomagnetic storms by utilizing machine learning models and analyzing several heliophysical variables, such as the interplanetary magnetic field, proton density, solar wind speed, and proton temperature. Rather than relying on traditional correlation-based methods, we employ advanced machine learning techniques to examine the complex relationships between these factors and geomagnetic storms. Our analysis covers a large dataset spanning six solar cycles, including the current 25th cycle, to provide comprehensive insights into the dynamics of these storms.Our study highlights the significance of the interplanetary magnetic field as a key predictor of geomagnetic storms, challenging previous beliefs that primarily focused on sunspot activity. By using high-resolution data, we uncover new patterns and provide a more detailed analysis of the factors influencing geomagnetic storms. We emphasize the importance of considering a range of heliophysical variables, such as proton temperature and flow pressure, which offer new insights into the complex dynamics driving these storm events.The application of machine learning models, particularly Random Forest and Gradient Boosting, demonstrated superior predictive accuracy compared to traditional methods. Our results reveal that the Dst-index MIN, scalar B, and alpha/proton ratio are among the most influential factors, accounting for a significant portion of the prediction model’s accuracy. These findings underscore the utility of machine learning in identifying critical drivers of geomagnetic activity and enhancing forecast precision.Additionally, our research underscores the need for comprehensive models that can accurately predict geomagnetic storms by integrating various data sources. This machine learning approach not only improves predictive accuracy but also enhances our understanding of the underlying mechanisms of space weather. The insights gained from this study have important implications for both scientific research and practical applications, such as improving early warning systems for geomagnetic storms and mitigating their potential impacts on Earth.

Statistical Analysis of the Correlation between Geomagnetic Storm Intensity and Solar Wind Parameters from 1996 to 2023

The occurrence of space weather events, notably geomagnetic storms driven by various solar wind structures, can significantly alter Earth’s electromagnetic environment. In this study, we examined the interplanetary origins and statistical distribution of 384 geomagnetic storms (Dstmin ≤ −50 nT) that occurred from September 1996 to December 2023. We statistically analyzed the correlations between storm intensity and solar wind parameters (SWPs) across different subsets. The results indicate that (1) the solar activity level, indicated by the sunspot number (SSN), and the number of geomagnetic storms during the first four years of the 25th solar cycle were intermediate, compared to the first four years of the 23rd and 24th solar cycles. Specifically, ICME-related structures caused 80% of the strong storms (Dstmin ≤ −100 nT) and 34% of the moderate storms (−100 nT < Dstmin ≤ −50 nT) from 2020 to 2023. (2) The storm intensity correlated with the peak and/or time-integral values of the southward interplanetary magnetic field (IMF Bs), the dawn–dusk electric field (Ey), the Akasofu’s function (ε), and dynamic pressure (Psw) to varying extents. Strong storms exhibited higher correlation levels than moderate ones and ICME-related storms showed larger correlation levels compared to those driven by other sources. (3) Compared with the storms from 1996-09 to 2000-08, the storms that occurred from 2020 to 2023 had lower correlations with the peak values of the IMF Bs and Ey but higher correlations with the peak value of ε and the time-integral values of the IMF Bs, Ey, Psw, and ε. (4) Among the 174 events that featured continuous southward IMF during the storm’s main phase, the duration of southward IMF during about 66.7% of moderate storms and 51.5% of strong storms were less than 13 h. Continuous southward IMF resulted in more direct and efficient energy coupling, enhancing the correlation between the peak values of SWPs and storm intensity but weakening the relationships with the time-integral values of SWPs. Notably, when the southward IMF persisted for a longer duration (e.g., ∆t > 13 h), the continuous energy input further enhanced correlations with both peak and integral values of SWPs, leading to stronger overall correlations with storm intensity. This analysis sheds light on the intricate relationships between geomagnetic storms and their solar wind drivers, emphasizing the significant influence of ICME-related structures and the duration of southward IMF on storm intensity.

Spatial distribution of plasma density and magnetic field amplitude in the dayside magnetosheath as a function of the IMF orientation

The properties of the magnetosheath are of pivotal importance in determining the coupling between the magnetosphere and interplanetary medium. In particular, the magnetic flux pileup and plasma depletion layer (PDL) modify the boundary conditions of magnetopause reconnection. However, the spatial distribution of the magnetic field strength and plasma density in the magnetosheath and their functional dependence on the interplanetary magnetic field (IMF) orientation remain poorly understood. This study characterizes these aspects in detail through the statistical processing of decades of data from Cluster, Double Star, THEMIS, and Magnetospheric Multiscale (MMS) missions. The first part of this study focuses on the poorly known variations across the magnetosheath, from the shock to the magnetopause. The magnetic pileup and PDL are significantly correlated, with a strong dependence on the IMF cone angle. Their dependence on the IMF clock angle is found only near the magnetopause, consistent with the expected effect of magnetic reconnection. The second part of this study examines the asymmetry in the magnetic field amplitude and density between the quasi-parallel and quasi-perpendicular sides of the equatorial magnetosheath. These asymmetries are characterized for different relative distances to the magnetopause and bow shock boundaries and for different IMF orientation. The magnetic field amplitude, observed to be higher on the quasi-perpendicular side of the magnetosheath, becomes more symmetric as it approaches the magnetopause. The quasi-parallel magnetosheath exhibits a higher plasma density near the magnetopause. However, this asymmetry reverses at approximately the mid-magnetosheath with a decreasing IMF cone angle.

Global Environmental Constraints on Magnetic Reconnection at the Magnetopause From In Situ Measurements

AbstractProgress in locating the X‐line on the magnetopause beyond the atypical due south interplanetary magnetic field (IMF) condition is hampered by the fact that the global plasma and field spatial distributions constraining where reconnection could develop on the magnetopause are poorly known. This work presents global maps of the magnetic shear, current density and reconnection rate, on the global dayside magnetopause, reconstructed from two decades of measurements from Cluster, Double Star, THEMIS and MMS missions. These maps, generated for various IMF and dipole tilt angles, offer a unique comparison point for models and observations. The magnetic shear obtained from vacuum magnetostatic draping is shown to be inconsistent with observed shear maps for IMF cone angles in 12.5° ± 2.5° ≤ |θco| ≤ 45° ± 5°. Modeled maximum magnetic shear lines fail to incline toward the equator as the IMF clock angle increases, in contrast to those from observations and MHD models. Reconnection rate and current density maps are closer together than they are from the shear maps, but this similarity vanishes for increasingly radial IMF orientations. The X‐lines maximizing the magnetic shear are the only ones to sharply turns toward and follow the anti‐parallel ridge at high latitude. We show the behavior of X‐lines with varying IMF clock and dipole tilt angles to be different as the IMF cone angle varies. Finally, we discuss a fundamental disagreement between X‐lines maximizing a given quantity on the magnetopause and predictions of local X‐line orientations.

How Do Substorms Influence Hemispheric Asymmetries in Equivalent Currents?

AbstractIonospheric dynamics exhibits a distinct hemispheric asymmetry, influenced primarily by the Interplanetary Magnetic Field (IMF) By component, dipole tilt, or a combination of both. Previous studies have indicated a reduction in these asymmetries during substorms. In this study, we conduct a superposed epoch analysis using ground magnetometer data from the northern hemisphere to examine the impact of substorms on ionospheric current asymmetry. This analysis uses the assumption of mirror symmetry between the northern and southern hemispheres when IMF By and dipole tilt are reversed. We observe a significant reduction in nightside equivalent current asymmetry indicating the IMF By and dipole tilt have minimal influence on the substorm current. On the other hand, we find that substorms exert minimal or negligible effects on dayside currents. This difference in response between nightside and dayside currents emphasizes the need to incorporate nightside dynamics into existing climatological models, which presently rely mainly on upstream parameters due to a lack of robust parameters effectively representing them. Our findings provide important insights for future modeling efforts, highlighting the distinct interactions between substorms and ionospheric currents across different hemispheric regions.

Seasonal and Interhemispheric Variations of the Afternoon Auroral Responses to the Interplanetary Magnetic Field By Polarity

AbstractThis work investigates seasonal and interhemispheric variations of the afternoon auroral responses to the interplanetary magnetic field (IMF) By effects. The auroral observations are adopted from the global ultraviolet imager instrument on board the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite during 2002–2007. The results show that in both summer and winter solstices, the stronger afternoon auroral intensity is associated with negative IMF By (By < 0) in the northern hemisphere, and with By > 0 in the southern hemisphere. This suggests stronger contributions from the upward field‐aligned currents (FACs), which can be induced by the By‐associated north‐south oriented electric field and the By‐associated flow shear in the ionosphere. In addition, the strongest afternoon aurora occurs in summer in each hemisphere. In summer, the absolute difference between the auroral peak intensity under the two By polarities is greater and occurs earlier than in winter, which may be related to changes in FACs and conductivity from winter to summer. Differently, in equinoxes stronger auroral intensity favors By conditions associated with more frequent occurrence of southward IMF Bz, such as By < 0 and By > 0 conditions in March and in September, respectively. Therefore, in equinoxes the effects of the favorable By, which were seen in solstices, are masked. We suggest that these are caused by the Russell‐McPherron effect, which leads to more southward Bz conditions, resulting in more energy deposited and subsequent stronger aurora in polar ionosphere. These results contribute to our deeper understanding of the asymmetrical phenomena in the Earth's magnetosphere‐ionosphere induced by IMF By.

Hypotheses Concerning Global Magnetospheric Convection, Magnetosphere‐Ionosphere Coupling, and Auroral Activity at Uranus

AbstractWe investigate the unique magnetosphere of Uranus and its interaction with the solar wind. Following the work of Masters (2014), https://doi.org/10.1002/2014ja020077 and others, we developed and validated a simple yet valuable and illustrative model of Uranus' offset, tilted, and rapidly‐spinning magnetic field and magnetopause (nominal and fit to the Voyager‐2 inbound crossing point) in three‐dimensional space. With this model, we investigated details of the seasonal and interplanetary magnetic field (IMF) orientation dependencies of dayside and flank reconnection along the Uranian magnetopause. We found that anti‐parallel (magnetic field shear angle greater than 170°) reconnection occurs nearly continuously along the Uranian dayside and/or flank magnetopause under all seasons of the 84 (Earth) year Uranian orbit and the most likely IMF orientations. Such active and continuous driving of the Uranian magnetosphere should result in constant loading and unloading of the Uranian magnetotail, which may be further complicated and destabilized by sudden changes in the IMF orientation and solar wind conditions plus the reconfigurations from the rotation of Uranus itself. We demonstrate that unlike the other magnetospheric systems that are Dungey‐cycle driven (i.e., Mercury and Earth) or rotationally driven (Jupiter and Saturn), global magnetospheric convection of plasma, magnetic flux, and energy flow may occur via three distinct cycles, two of which are unique to Uranus (and possibly also Neptune). Our simple model is also used to map signatures of dayside and flank reconnection down to the Uranian ionosphere, as a function of planetary latitude and longitude. Such mapping demonstrates that “spot‐like” auroral features should be very common on the Uranian dayside, consistent with observations from Hubble Space Telescope. We further detail how the combination of Uranus' rapid rotation and unique and very active global magnetospheric convection should be consistent with fueling of the surprisingly intense trapped radiation environment observed by Voyager‐2 during its single flyby. Summarizing, Uranus is a very interesting magnetosphere that offers new insights on the nature, complexity, and diversity of planetary magnetospheric systems and the acceleration of particles in space plasmas, which might have important analogs to exoplanetary magnetospheric systems. Our hypotheses can be tested with further work involving more advanced models, new auroral observations, and unprecedented missions to explore the in situ environment from orbit around Uranus, which should include a complement of magnetospheric instruments in the payload.

Plasma Mixing During Active Kelvin‐Helmholtz Instability Under Different IMF Orientations

AbstractWhen the velocity shear between the two plasmas separated by Earth's magnetopause is locally super‐Alfvénic, the Kelvin‐Helmholtz (KH) instability can develop. A crucial role is played by the interplanetary magnetic field (IMF) orientation, which can stabilize the velocity shear. Although, in a linear regime, the instability threshold is equally satisfied during both northward and southward IMF orientations, in situ measurements show that KH instability is preferentially excited during the northward IMF orientation. We investigate this different behavior by means of a mixing parameter which we apply to two KH events to identify both boundaries and the center of waves/vortices. During the northward orientation, the waves/vortex boundaries have stronger electrons than ions mixing, while the opposite is observed at their center. During the southward orientation, instead, particle mixing is observed predominantly at the boundaries. In addition, stronger local ion and electron non‐thermal features are observed during the northward than the southward IMF orientation. Specifically, ion distribution functions are more distorted, due to field‐aligned beams, and electrons have a larger temperature anisotropy during the northward than the southward IMF orientation. The observed kinetic features provide an insight into both local and remote processes that affect the evolution of KH structures.

Characterizing the Solar Wind‐Magnetosphere Viscous Interaction at Uranus and Neptune

AbstractThe solar wind interaction with planetary magnetospheres dictates the mechanism through which energy is transported across planetary systems. The magnetohydrodynamic plasma description suggests that solar wind conditions in the outer solar system encourage the magnetopause boundaries at Uranus and Neptune to be more Kelvin‐Helmholtz unstable, however, no quantitative assessment has been performed. To characterize the viscous solar wind interaction at Uranus and Neptune, we create an analytical model to determine where Kelvin‐Helmholtz Instabilities (KHIs) may form along their magnetopauses by searching for regions where the minimum condition for KHI formation is satisfied. We run the model at solstice and equinox for a range of Interplanetary Magnetic Field (IMF) strengths, and rotation phases. We find minimal seasonal variation for low IMF strengths (B = 0.01 nT), with ∼70% of the magnetopause surface at Uranus and ∼80% at Neptune, enabling KHI formation. For periods of stronger IMF strength (B > 0.3 nT), KHIs were significantly suppressed. While KHIs depend on both the conditions inside the magnetopause boundary and the shocked solar wind IMF strength, we find that the IMF strength is the most significant criterion in determining whether or not KHIs are allowed to form at the magnetopause boundaries.

The Effect of the Interplanetary Magnetic Field Clock Angle and the Latitude Location of the Intense Crustal Magnetic Field on the Ion Escape at Mars: An MHD Simulation Study

AbstractIn this paper, using a three‐dimensional multifluid MHD model, we studied the effects of the interplanetary magnetic field (IMF) clock angle and the latitude position of the intense crustal magnetic field (ICMF) on the escape of ions O+, , and at Mars. The main results are as follows: (a) The IMF clock angle affects the ion escape at Mars. When the ICMF is on the dayside, the ion escape rate reaches a maximum at the IMF clock angles close to 60°–90° and a minimum at the IMF clock angles close to 120°–150°, because the ICMF can change the topology of the magnetic field and affect the interaction between the solar wind and Mars. The difference between the maximum and minimum ion escape rates due to the IMF clock angle can reach over 50%. (b) Compared with the −ESW hemisphere, the escape flux of and in the +ESW hemisphere is more significant. However, O+ generally has a larger escape flux in the −ESW hemisphere. The different results in the ±ESW hemispheres might be due to the larger distribution of the hot oxygen corona, which changes the flow pattern of O+. (c) The latitude location of the ICMF can also affect the ion escape. When the ICMF is on the dayside, as the subsolar point varies from 25°S to 25°N, that is, the intense crustal magnetic field position keeps shifting southward, the ion escape rate shows a gradual increase.

Statistical Study of Hot Flow Anomaly Induced Ground Magnetic Ultra‐Low Frequency Oscillations

AbstractPc5 ULF waves play an important role in transporting energy and particles in the coupled magnetospheric and ionospheric system. They are known to be initiated by dynamic pressure fluctuations upstream of the magnetopause, including those induced by hot flow anomalies (HFAs). However, the role of HFAs in generating magnetospheric and ground magnetic Pc5 ULF oscillations has not been investigated statistically yet. Thus, in this paper, we investigate the contribution of HFAs to ground magnetic Pc5 ULF oscillations and analyze how the characteristics of HFAs influence these oscillations, based on the coordinated observations between the THEMIS probes and the ground magnetometers at high latitudes during the years 2008, 2009 and 2019. We find that HFAs can serve as a notable source of ground magnetic Pc5 ULF oscillations, with about 18.9% of Interplanetary Magnetic Field (IMF) discontinuity‐induced HFAs associated with discernible enhancements in Pc5 ULF wave power, whereas spontaneous HFAs play a comparatively minor role in generating these oscillations. Furthermore, we observe that the cores of HFAs are likely to contribute more significantly to modulating the induced ground magnetic Pc5 ULF oscillations than their compressed boundaries. More dynamic pressure reductions within HFA cores correspond to stronger ground magnetic Pc5 ULF oscillations. Additionally, HFAs can propagate with the IMF discontinuity along the bow shock, continuously generating ground magnetic Pc5 ULF oscillations during their propagation. This research sheds light on the mechanisms underlying Pc5 ULF wave generation and underscores the role of HFAs in driving magnetospheric‐ionospheric interactions.

Solar Wind‐Magnetosphere Coupling Efficiency and Its Dependence on Solar Activity During Geomagnetic Storms of 23–24 Solar Cycles

AbstractSpace weather forecasts are of utmost importance in safeguarding navigation, communication, and electric power system operations, satellites from orbital drag, and the astronauts in the International Space Station from hazardous space radiation during extreme space weather conditions. The finest space weather prediction requires a clear understanding of solar wind‐magnetosphere coupling. The in‐situ measurements of the solar wind properties give unique information about the Sun and its activity on smaller to longer timescales. The present work investigates the influence of solar activity on the coupling of solar wind and Earth's magnetosphere during 23–24 solar cycles. The geomagnetic storms with Symmetric H‐component (SYMH) ≤ −85 nT during the 23–24 solar cycles are considered. We present the results of statistical analysis and relationships between the various solar wind parameters such as the total strength of interplanetary magnetic field (B) and its three‐axis components (Bx, By, and Bz), solar wind proton density (Nsw), solar wind speed (Vsw), SYMH indices, the amplitude, duration, and profile of the geomagnetic storms. The integrated electric field and integrated SYMH index during storms show the highest correlation of 0.92, implying that integrated SYMH is a better proxy of the injected solar wind energy in the magnetosphere in the form of the ring current. Moreover, we do see the difference in the solar wind‐magnetosphere coupling efficiency during the phases of 23–24 solar cycles which is intriguing.

MeV Electron Precipitation During Radiation Belt Dropouts

AbstractTo gain deeper insights into radiation belt loss into the atmosphere, a statistical study of MeV electron precipitation during radiation belt dropout events is undertaken. During these events, electron intensities often drop by an order of magnitude or more within just a few hours. For this study, dropouts are defined as a decrease by at least a factor of five in less than 8 hours. Van Allen probe measurements are employed to identify dropouts across various parameters, complemented by precipitation data from the CALorimetric Electron Telescope instrument on the International Space Station. A temporal analysis unveils a notable increase in precipitation occurrence and intensity during dropout onset, correlating with the decline of SYM‐H, the north‐south component of the interplanetary magnetic field, and the peak of the solar wind dynamic pressure. Moreover, dropout occurrences show correlations with the solar cycle, exhibiting maxima at the spring and autumn equinoxes. This increase during equinoxes reflects the correlation between equinoxes and the SYM‐H index, which itself exhibits a correlation with precipitation during dropouts. Spatial analysis reveals that dropouts with precipitation penetrate into lower L‐star regions, mostly reaching L‐star <4, while most dropouts without precipitation don't penetrate deeper than L‐star 5. This is consistent with the larger average dimensions of dropouts associated with precipitation. During dropouts, precipitation is predominantly observed in the dusk‐midnight sector, coinciding with the most intense precipitation events. The results of this study provide insight into the contribution of precipitation to radiation belt dropouts by deciphering when and where precipitation occurred.

A statistical study of the impact of the stream interaction regions on the heliospheric current sheet

The heliospheric current sheet (HCS) serves as the boundary that separates two sectors in which the interplanetary magnetic field lines diverge in opposite directions. In this study, we conducted a statistical analysis of 216 HCS events, utilizing data from the Wind spacecraft to examine the impact of stream interaction regions (SIRs) on the HCSs. Our findings revealed that out of all the HCS events, 153 (70.8%) were succeeded by a SIR within 36 h. Conversely, 63 HCS events (29.2%) were not followed by any SIR. The occurrence of HCSs accompanied by SIRs displayed a rough anti-correlation with solar sunspots. The statistical results further indicated that the presence of a SIR exerts some influence on the preceding HCS. Specifically, the thickness of the HCS is significantly reduced by the trailing SIR, and the subsequent SIR can cause the leading HCS to stand more vertically to the ecliptic plane and more aligned with the Sun–Earth line (a smaller angle between the normal of the HCS and the Earth's dawn–dusk line in the ecliptic plane).

Multiple Satellite Observations of the High‐Latitude Cusp Aurora During Northward IMF Conditions

AbstractCusp auroras poleward of the typical auroral oval are ascribed to high‐latitude lobe reconnection when the Interplanetary Magnetic Field (IMF) Bz is predominantly northward. In this study, we further investigate the ionospheric characteristics of a unique high‐latitude cusp region employing multiple satellite observations. A cusp aurora event with wide spatial spread was observed in the postnoon polar cap region. It was found to be associated with northward IMF Bz and positive By components. The cusp aurora was located from 68° to 86° in magnetic latitude and within 15–17 hr in magnetic local time. This broad coverage in the polar cap indicates direct precipitating particles from the magnetosheath. Particle energy is different between the equatorward and poleward edges of the cusp aurora. The precipitating ions at the equatorward side maintain magnetosheath particle characteristics as expected, while ions with higher energies occurred in the poleward side. Further, the poleward edge of the cusp aurora was nearly situated in the center of a convection shear and was associated with an upward field‐aligned current. These observations suggest a lobe cell circulation, hence we attribute the formation of the cusp aurora to the high‐latitude lobe reconnection. Simultaneous observations in the southern hemisphere indicate the absence of cusp aurora. The auroral presence only in the northern hemisphere is probably due to the combination of large dipole tilt angle and positive IMF Bz, which facilitates the lobe reconnection.

Nightside Detached Auroras Associated With Expanding Auroral Oval During the Main and Recovery Phases of a Magnetic Storm

AbstractDetached subauroral proton arcs are commonly observed during the recovery phase of geomagnetic storms, and have been extensively investigated. However, there is limited study on their occurrence during the main phase of storms. This study investigated nightside detached auroras (NDAs) observed by the far‐ultraviolet imager onboard the Defense Meteorological Satellite Program spacecraft. The NDAs occurred in the nightside sector, separated from the equatorward boundary of the auroral oval, and were observed during the main and recovery phases of the geomagnetic storm on 02 October 2013. The occurrence of the NDAs appears to correlate with the expanding auroral oval toward lower latitudes, and is independent of the polarity change in the interplanetary magnetic field Bz component. Particle measurements indicate that the NDAs were generated by energetic protons, primarily above 10 keV, originating from the ring current. These precipitating proton fluxes, predominantly anisotropic, were observed to be detached from the isotropic boundary within the auroral oval. Analysis of Pc1 data obtained by ground stations suggests that electromagnetic ion cyclotron waves account for the generation of the NDAs. The limited latitudinal distribution of the NDAs indicates the wave activity in the magnetospheric source region within a narrow L‐shell region. The observations presented in this study would contribute to our understanding of the coupling processes between the magnetosphere and ionosphere within the subauroral region.

Spectral Indices and Evidence of Wave–Wave Modulation in Observations of the Interplanetary Magnetic Field

We present wave and turbulence observations from the DSCOVR spacecraft during the 2017 September solar flare and coronal mass ejection (CME) events. On September 4–12, the spectral index within the magnetic field power spectral density inertial range was consistent with Kolmogorov −5/3. This is despite the 9 days being composed of a complex mix of different features, including solar flares, solar energetic particle events, and CMEs. When analyzing shorter time periods, the spectral index varies. For two days where there were consecutive CMEs, the index follows Kraichnan–Iroshinikov −3/2, while on two quiet days, it was a mixture of −1, −3/2, and −2. The inertial range spectral index taken over the entire 9 days hides or averages out spectral features that occur over shorter time periods. We use a more realistic estimate of the amount of Doppler shifting into the spacecraft frame to show that the break frequencies on most days were located close to the H+ cyclotron frequency. We present evidence of wave–wave modulation and suggest that lower-frequency waves in the solar wind can modulate the growth rates/propagation of ion cyclotron waves, providing a method to transfer energy in the solar wind to smaller scales. Furthermore, we suggest that the indices in the inertial range can be explained by combining containment due to wave generation/propagation and stochastic Brownian motion in the solar wind. When these two phenomena are equal, they combine to create a −3/2 index.

Refinement of Global Coronal and Interplanetary Magnetic Field Extrapolations Constrained by Remote-sensing and In Situ Observations at the Solar Minimum

Solar magnetic fields are closely related to various physical phenomena on the Sun, which can be extrapolated with different models from photospheric magnetograms. However, the open flux problem (OFP), the underestimation of the magnetic field derived from the extrapolated model, is still unsolved. To minimize the impact of the OFP, we propose three evaluation parameters to quantitatively evaluate magnetic field models and determine the optimal free parameters in the models by constraining the coronal magnetic fields and the interplanetary magnetic fields (IMFs) with real observations. Although the OFP still exists, we find that magnetic field lines traced from the coronal models effectively capture the intricate topological configurations observed in the corona, including streamers and plumes. The OFP is lessened by using the Helioseismic and Magnetic Imager synoptic map instead of the Global Oscillation Network Group daily synoptic maps, and the potential field source surface + potential field current sheet (PFSS+PFCS) model instead of the current sheet source surface (CSSS) model. For Carrington Rotation 2231 at the solar minimum, we suggest that the optimal parameters for the PFSS+PFCS model are R ss = 2.2–2.5 R ☉ and R scs = 10.5–14.0 R ☉, as well as for the CSSS model, are R cs = 2.0–2.4 R ☉, R ss = 11.0–14.7 R ☉, and a = 1.0 R ☉. Despite the IMFs at 1 au being consistent with the measurements by artificially increasing the polar magnetic fields, the IMFs near the Sun are still underestimated. The OFP might be advanced by improving the accuracy of both the weak magnetic fields and polar magnetic fields, especially considering magnetic activities arising from interplanetary physical processes.