Heliophysics Science Division

Kinetic and magnetic energy spectra in the ecliptic plane near 1 AU are found to exhibit different power-law behaviors in the inertial range, with the magnetic spectrum often having a power-law exponent near 5/3 and the kinetic energy spectrum often having a power-law exponent near 3/2 (the inertial range extends from approximately 5 × 10⁻⁴ to 10⁻¹ Hz). The total energy, kinetic plus magnetic, has a power-law exponent that lies between 3/2 and 5/3, with a value near 1.6. The Alfvén ratio, the ratio of kinetic to magnetic energy, is found to be a slowly increasing function of frequency in the inertial range, increasing from roughly 0.5 to 0.9 in the frequency range from 10⁻³ to 10⁻¹ Hz. These conclusions are based on the analysis of four distinct time intervals of solar wind magnetic field and plasma data obtained by the Wind spacecraft near the end of solar cycle 22 and at different times throughout solar cycle 23. Three 54 day intervals and one 81 day interval are used to compute power spectra in the range from 10⁻⁵ to 1.7 × 10⁻¹ Hz. Power-law exponents are estimated from linear least-squares fits to the logarithm of the power spectral density versus the logarithm of the frequency over the frequency interval from 10⁻³ to 10⁻² Hz. To prevent errors due to spectral aliasing, the last decade of the spectrum is omitted from the calculation of the power-law exponents. The results show that a measurable difference exists between the power-law exponents of velocity and magnetic field fluctuations and that this difference persists throughout the solar cycle.

This chapter focuses on the range of low frequency electromagnetic modes observed at and upstream of collisionless shocks in the heliosphere. It discusses a specific class of whistler mode wave observed immediately upstream of collisionless shock ramps, called a whistler precursor. Though these modes have been (and are often) observed upstream of quasi-parallel shocks, the authors limit their discussion to those observed upstream of quasi-perpendicular shocks. The chapter discusses the various ion velocity distributions observed at and upstream of collisionless shocks. It also introduces some terminology and relevant instabilities for ion foreshock waves. The chapter discusses the most common ultra-low frequency (ULF) wave types, their properties, and their free energy sources. It discusses modes that are mostly Alfvénic (i.e., mostly transverse but can be compressive) in nature.

Abstract The Wind spacecraft, launched on November 1, 1994, is a critical element in NASA’s Heliophysics System Observatory (HSO)—a fleet of spacecraft created to understand the dynamics of the Sun‐Earth system. The combination of its longevity (>25 years in service), its diverse complement of instrumentation, and high resolution and accurate measurements has led to it becoming the “standard candle” of solar wind measurements. Wind has over 55 selectable public data products with over ∼1,100 total data variables (including OMNI data products) on SPDF/CDAWeb alone. These data have led to paradigm shifting results in studies of statistical solar wind trends, magnetic reconnection, large‐scale solar wind structures, kinetic physics, electromagnetic turbulence, the Van Allen radiation belts, coronal mass ejection topology, interplanetary and interstellar dust, the lunar wake, solar radio bursts, solar energetic particles, and extreme astrophysical phenomena such as gamma‐ray bursts. This review introduces the mission and instrument suites then discusses examples of the contributions by Wind to these scientific topics that emphasize its importance to both the fields of heliophysics and astrophysics.

We present an advance toward accurately predicting the arrivals of coronal mass ejections (CMEs) at the terrestrial planets, including Earth. For the first time, we are able to assess a CME prediction model using data over two thirds of a solar cycle of observations with the Heliophysics System Observatory. We validate modeling results of 1337 CMEs observed with the Solar Terrestrial Relations Observatory (STEREO) heliospheric imagers (HI) (science data) from 8 years of observations by five in situ observing spacecraft. We use the self-similar expansion model for CME fronts assuming 60° longitudinal width, constant speed, and constant propagation direction. With these assumptions we find that 23%-35% of all CMEs that were predicted to hit a certain spacecraft lead to clear in situ signatures, so that for one correct prediction, two to three false alarms would have been issued. In addition, we find that the prediction accuracy does not degrade with the HI longitudinal separation from Earth. Predicted arrival times are on average within 2.6 ± 16.6 h difference of the in situ arrival time, similar to analytical and numerical modeling, and a true skill statistic of 0.21. We also discuss various factors that may improve the accuracy of space weather forecasting using wide-angle heliospheric imager observations. These results form a first-order approximated baseline of the prediction accuracy that is possible with HI and other methods used for data by an operational space weather mission at the Sun-Earth L5 point.

We present spatially resolved EUV spectroscopic measurements of pervasive, faint Fe xix 592.2 Å line emission in an active region observed during the 2013 April 23 flight of the Extreme Ultraviolet Normal Incidence Spectrograph (EUNIS-13) sounding rocket instrument. With cooled detectors, high sensitivity, and high spectral resolution, EUNIS-13 resolves the lines of Fe xix at 592.2 Å (formed at temperature T ≈ 8.9 MK) and Fe xii at 592.6 Å (T ≈ 1.6 MK). The Fe xix line emission, observed over an area in excess of 4920 arcsec2 (2.58 × 109 km2, more than 60% of the active region), provides strong evidence for the nanoflare heating model of the solar corona. No GOES events occurred in the region less than 2 hr before the rocket flight, but a microflare was observed north and east of the region with RHESSI and EUNIS during the flight. The absence of significant upward velocities anywhere in the region, particularly the microflare, indicates that the pervasive Fe xix emission is not propelled outward from the microflare site, but is most likely attributed to localized heating (not necessarily due to reconnection) consistent with the nanoflare heating model of the solar corona. Assuming ionization equilibrium we estimate Fe xix/Fe xii emission measure ratios of ∼0.076 just outside the AR core and ∼0.59 in the core.

Abstract Analyses of 15,314 electron velocity distribution functions (VDFs) within ±2 hr of 52 interplanetary (IP) shocks observed by the Wind spacecraft near 1 au are introduced. The electron VDFs are fit to the sum of three model functions for the cold dense core, hot tenuous halo, and field-aligned beam/strahl component. The best results were found by modeling the core as either a bi-kappa or a symmetric (or asymmetric) bi-self-similar VDF, while both the halo and beam/strahl components were best fit to bi-kappa VDF. This is the first statistical study to show that the core electron distribution is better fit to a self-similar VDF than a bi-Maxwellian under all conditions. The self-similar distribution deviation from a Maxwellian is a measure of inelasticity in particle scattering from waves and/or turbulence. The ranges of values defined by the lower and upper quartiles for the kappa exponents are κ ec ∼ 5.40–10.2 for the core, κ eh ∼ 3.58–5.34 for the halo, and κ eb ∼ 3.40–5.16 for the beam/strahl. The lower-to-upper quartile range of symmetric bi-self-similar core exponents is s ec ∼ 2.00–2.04, and those of asymmetric bi-self-similar core exponents are p ec ∼ 2.20–4.00 for the parallel exponent and q ec ∼ 2.00–2.46 for the perpendicular exponent. The nuanced details of the fit procedure and description of resulting data product are also presented. The statistics and detailed analysis of the results are presented in Paper II and Paper III of this three-part study.

Context. On 2020 November 29, the first widespread solar energetic particle (SEP) event of solar cycle 25 was observed at four widely separated locations in the inner (≲1 AU) heliosphere. Relativistic electrons as well as protons with energies > 50 MeV were observed by Solar Orbiter (SolO), Parker Solar Probe, the Solar Terrestrial Relations Observatory (STEREO)-A and multiple near-Earth spacecraft. The SEP event was associated with an M4.4 class X-ray flare and accompanied by a coronal mass ejection and an extreme ultraviolet (EUV) wave as well as a type II radio burst and multiple type III radio bursts. Aims. We present multi-spacecraft particle observations and place them in context with source observations from remote sensing instruments and discuss how such observations may further our understanding of particle acceleration and transport in this widespread event. Methods. Velocity dispersion analysis (VDA) and time shift analysis (TSA) were used to infer the particle release times at the Sun. Solar wind plasma and magnetic field measurements were examined to identify structures that influence the properties of the energetic particles such as their intensity. Pitch angle distributions and first-order anisotropies were analyzed in order to characterize the particle propagation in the interplanetary medium. Results. We find that during the 2020 November 29 SEP event, particles spread over more than 230° in longitude close to 1 AU. The particle onset delays observed at the different spacecraft are larger as the flare–footpoint angle increases and are consistent with those from previous STEREO observations. Comparing the timing when the EUV wave intersects the estimated magnetic footpoints of each spacecraft with particle release times from TSA and VDA, we conclude that a simple scenario where the particle release is only determined by the EUV wave propagation is unlikely for this event. Observations of anisotropic particle distributions at SolO, Wind, and STEREO-A do not rule out that particles are injected over a wide longitudinal range close to the Sun. However, the low values of the first-order anisotropy observed by near-Earth spacecraft suggest that diffusive propagation processes are likely involved.

A new “Paraboloidal” model of Mercury's magnetospheric magnetic field based upon the earlier terrestrial model and using similar techniques is developed. The model describes the field of Mercury's dipole, which is considered to be offset from the planet's center; the magnetopause currents driven by the solar wind; and the tail current system including the cross‐tail currents and their closure currents at the magnetopause. The effect of the interplanetary magnetic field (IMF) is modeled as a partial penetration of the IMF into the magnetosphere. The goals of the present work are (1) to develop an easily usable, yet robust model of Mercury's magnetospheric magnetic field and (2) to produce an improved “unified” determination of Mercury's magnetic dipole moment which fits the measurements taken during both Mariner 10's first and third flybys. This new model of Mercury's magnetosphere is described and used to determine a best Mercury magnetic dipole moment of 192 nT R M 3 , from the two Mariner 10 flybys, a value which is intermediate between the various estimates produced by previous models. The best fit to the Mariner 10 measurements gives the dipole offset 0.18 R M above the equatorial plane. The new Paraboloidal model is used to predict the configuration of this miniature magnetosphere under average and extreme solar wind conditions.

Abstract The dual spacecraft mission BepiColombo is the first joint mission between the European Space Agency (ESA) and the Japanese Aerospace Exploration Agency (JAXA) to explore the planet Mercury. BepiColombo was launched from Kourou (French Guiana) on October 20th, 2018, in its packed configuration including two spacecraft, a transfer module, and a sunshield. BepiColombo cruise trajectory is a long journey into the inner heliosphere, and it includes one flyby of the Earth (in April 2020), two of Venus (in October 2020 and August 2021), and six of Mercury (starting from 2021), before orbit insertion in December 2025. A big part of the mission instruments will be fully operational during the mission cruise phase, allowing unprecedented investigation of the different environments that will encounter during the 7-years long cruise. The present paper reviews all the planetary flybys and some interesting cruise configurations. Additional scientific research that will emerge in the coming years is also discussed, including the instruments that can contribute.

We present a new empirical model of Saturn's bow shock that utilizes observations from the Cassini spacecraft. Shock crossings are identified in magnetic field and plasma observations made by Cassini between June 2004 and August 2005. The Cassini crossings are then combined with the crossings made during the Saturn flybys of Pioneer 11, Voyager 1, and Voyager 2. Solar wind dynamic pressures for the Cassini crossings are estimated using upstream electron densities determined from Langmuir wave observations made by the Radio and Plasma Wave System. The crossing positions are rotated into aberrated coordinates to correct for the effect of the planet's orbital motion. In the case of Saturn this rotation is by ∼1°. To correct for solar wind dynamic pressure variations, the crossing positions are normalized to the average pressure 〈P SW 〉 = 0.048 nPa. The model is then obtained by fitting a conic section to the crossings using a nonlinear least squares technique. To validate the assumptions made in constructing the model, we treat the parameters previously assumed to be constants as variables and fit their values using an optimization routine; this leads to a conic section that is within the positional uncertainty of the model. The spacecraft trajectories are considered, and we conclude that they do not significantly bias the model. The new model is compared to the existing models, and the similarities and differences are discussed. We suggest that the new model gives the most accurate empirical representation of the shape and location of Saturn's bow shock.

The Space Technology 5 (ST‐5) mission successfully placed three micro‐satellites in a 300 × 4500 km dawn‐dusk orbit on 22 March 2006. Each spacecraft carried a boom‐mounted vector fluxgate magnetometer that returned highly sensitive and accurate measurements of the geomagnetic field. These data allow, for the first time, the separation of temporal and spatial variations in field‐aligned current (FAC) perturbations measured in low‐Earth orbit on time scales of ∼10 sec to 10 min. The constellation measurements are used to directly determine field‐aligned current sheet motion, thickness, and current density. In doing so, we demonstrate two multi‐point methods for the inference of FAC current density that have not previously been possible in low‐Earth orbit: 1) the “standard method,” based upon s/c velocity, but corrected for FAC current sheet motion, and 2) the “gradiometer method” which uses simultaneous magnetic field measurements at two points with known separation. Future studies will apply these methods to the entire ST‐5 data set and expand to include geomagnetic field gradient analyses as well as field‐aligned and ionospheric currents.

The total electron flux in the lunar exosphere, measured by the Electron Reflectometer onboard the Lunar Prospector during its passage through the Earth's magnetosphere (June 7–16, 1998), is strongly correlated with variations in the column abundance of equatorial sodium observed coincidentally with the McMath‐Pierce solar telescope at Kitt Peak. At the same time, the exospheric temperature increased from a minimum of 1200 K to a maximum of 3000 K at minimum phase angle. A comparison of different source mechanisms, using the measured electron flux as the proxy for the ion flux, provides strong evidence that ion impact enhances the photon‐stimulated desorption yield. At the Moon photon‐stimulated desorption is shown to be the dominant source of exospheric sodium outside the magnetosphere. Inside the magnetosphere, where the ion flux and the associated photon‐stimulated desorption yield decrease, the contributions from impact vaporization to the observed column abundance and temperature are found to be substantial.

Abstract We present the results from the first ensemble prediction model for major solar flares (M and X classes). The primary aim of this investigation is to explore the construction of an ensemble for an initial prototyping of this new concept. Using the probabilistic forecasts from three models hosted at the Community Coordinated Modeling Center (NASA‐GSFC) and the NOAA forecasts, we developed an ensemble forecast by linearly combining the flaring probabilities from all four methods. Performance‐based combination weights were calculated using a Monte Carlo‐type algorithm that applies a decision threshold P t h to the combined probabilities and maximizing the Heidke Skill Score (HSS). Using the data for 13 recent solar active regions between years 2012 and 2014, we found that linear combination methods can improve the overall probabilistic prediction and improve the categorical prediction for certain values of decision thresholds. Combination weights vary with the applied threshold and none of the tested individual forecasting models seem to provide more accurate predictions than the others for all values of P t h . According to the maximum values of HSS, a performance‐based weights calculated by averaging over the sample, performed similarly to a equally weighted model. The values P t h for which the ensemble forecast performs the best are 25% for M‐class flares and 15% for X‐class flares. When the human‐adjusted probabilities from NOAA are excluded from the ensemble, the ensemble performance in terms of the Heidke score is reduced.

Abstract A statistical analysis of 15,210 electron velocity distribution function (VDF) fits, observed within ±2 hr of 52 interplanetary (IP) shocks by the Wind spacecraft near 1 au, is presented. This is the second in a three-part series on electron VDFs near IP shocks. The electron velocity moment statistics for the dense, low-energy core, tenuous, hot halo, and field-aligned beam/strahl are a statistically significant list of values illustrated with both histograms and tabular lists for reference and baselines in future work. Given the large statistics in this investigation, the beam/strahl fit results in the upstream are now the most comprehensive attempt to parameterize the beam/strahl electron velocity moments in the ambient solar wind. The median density, temperature, beta, and temperature anisotropy values for the core(halo)[beam/strahl] components, with subscripts ec(eh)[eb], of all fit results, respectively, are , , , and . This work will also serve as a 1 au baseline and reference for missions like Parker Solar Probe and Solar Orbiter .

Abstract The intense magnetic storm on 17–18 March 2015 caused large disturbances of the ionosphere. Based on the plasma density ( Ni ) observations performed by the Swarm fleet of satellites, the Gravity Recovery and Climate Experiment mission, and the Communications/Navigation Outage Forecasting System satellite, we characterize the storm‐related perturbations at low latitudes. All these satellites sampled the ionosphere in morning and evening time sectors where large modifications occurred. Modifications of plasma density are closely related to changes of the solar wind merging electric field ( E m ). We consider two mechanisms, prompt penetration electric field (PPEF) and disturbance dynamo electric field (DDEF), as the main cause for the Ni redistribution, but effects of meridional wind are also taken into account. At the start of the storm main phase, the PPEF is enhancing plasma density on the dayside and reducing it on the nightside. Later, DDEF takes over and causes the opposite reaction. Unexpectedly, there appears during the recovery phase a strong density enhancement in the morning/prenoon sector and a severe Ni reduction in the afternoon/evening sector, and we suggest a combined effect of vertical plasma drift, and meridional wind is responsible for these ionospheric storm effects. Different from earlier studies about this storm, we also investigate the influence of storm dynamics on the initiation of equatorial plasma irregularities (EPIs). Shortly after the start of the storm main phase, EPIs appear in the postsunset sector. As a response to a short‐lived decline of E m , EPI activity appears in the early morning sector. Following the second start of the main phase, EPIs are generated for a few hours in the late evening sector. However, for the rest of the storm main phase, no more EPIs are initiated for more than 12 h. Only after the onset of recovery phase does EPI activity start again in the postmidnight sector, lasting more than 7 h. This comprehensive view of ionospheric storm effects and plasma irregularities adds to our understanding of conditions that lead to ionospheric instabilities.

Abstract We investigate the evolution of the suprathermal (ST) proton population as interplanetary shocks cross 1 au. The variability of the ST proton intensities and energy spectra upstream of the shocks is analyzed in terms of the shock parameters, upstream magnetic field configurations, and preexisting upstream populations. Propitious conditions for the observation of ST particles at distances far upstream from the shock occur in parallel shock configurations when particles can easily escape from the shock vicinity. In this situation, ST intensity enhancements show onsets characterized by velocity dispersion effects and energy spectra that develop into a “hump” profile peaking around ∼10 keV just before the arrival of the shock. The observation of field-aligned proton beams at low energies (5–10 keV) is possible under conditions that facilitate the scatter-free propagation of the particles streaming out of the shock. Upstream of perpendicular shocks, ST intensity enhancements are only observed in close proximity to the shock. Power-law proton spectra develop downstream of the shocks. The functional form for the downstream phase-space density proportional to v −5 is observed only over a limited range of ST energies. The absence of ST populations observed far upstream of interplanetary shocks raises questions about whether ST protons contribute as a seed particle population in the processes of particle acceleration at shocks.

We report our findings comparing the geometric factor (GF) as determined from simulations and laboratory measurements of the new Dual Electron Spectrometer (DES) being developed at NASA Goddard Space Flight Center as part of the Fast Plasma Investigation on NASA's Magnetospheric Multiscale mission. Particle simulations are increasingly playing an essential role in the design and calibration of electrostatic analyzers, facilitating the identification and mitigation of the many sources of systematic error present in laboratory calibration. While equations for laboratory measurement of the GF have been described in the literature, these are not directly applicable to simulation since the two are carried out under substantially different assumptions and conditions, making direct comparison very challenging. Starting from first principles, we derive generalized expressions for the determination of the GF in simulation and laboratory, and discuss how we have estimated errors in both cases. Finally, we apply these equations to the new DES instrument and show that the results agree within errors. Thus we show that the techniques presented here will produce consistent results between laboratory and simulation, and present the first description of the performance of the new DES instrument in the literature.

We present an analytical approach to the global magnetic field topology of magnetic clouds (MCs) that considers them like close magnetic structures with torus geometry and with a non-uniform (variable maximum radius) cross section along them. Following our previous approach to the problem of MCs (Hidalgo 2003, 2011), we establish an intrinsic coordinate system for that topology, and then we analytically solve the Maxwell equations in terms of it. The purpose of the present work is to present this model, which will lead us to understand in a more realistic way the physical mechanisms inside MCs. The model has a non-force-free character and also takes into account the time evolution of the cross sections of the MCs in their movement through the interplanetary medium. In this first paper, we obtain the expressions for the components of the magnetic field and the plasma current density imposing a large mean radius of the torus, and imposing a circular cross section with a variable maximum radius. Eventually, we fit the model to data related to four well-known MCs measurements at 1 AU, (three of them with circular cross sections and without expansion, as it is deduced from the experimental data). We compare the results of this toroidal model with those obtained with our previous cylindrical circular cross section model, also with a non-force-free character.

Americanae nace como un proyecto conjunto que surge dentro de la Red Europea de Información y Documentación sobre América Latina (REDIAL), y que ha afrontado la Biblioteca de la Agencia Española de Cooperación Internacional para el Desarrollo (AECID). Esta nueva biblioteca virtual hace más accesibles los libros digitales de tema americanista a los investigadores y usuarios interesados de cualquier parte del mundo.

Aims. We model the energetic storm particle (ESP) event of 14 July 2012 using the energetic particle acceleration and transport model named ‘PArticle Radiation Asset Directed at Interplanetary Space Exploration’ (PARADISE), together with the solar wind and coronal mass ejection (CME) model named ‘EUropean Heliospheric FORcasting Information Asset’ (EUHFORIA). The simulation results illustrate both the capabilities and limitations of the utilised models. We show that the models capture some essential structural features of the ESP event; however, for some aspects the simulations and observations diverge. We describe and, to some extent, assess the sources of errors in the modelling chain of EUHFORIA and PARADISE and discuss how they may be mitigated in the future. Methods. The PARADISE model computes energetic particle distributions in the heliosphere by solving the focused transport equation in a stochastic manner. This is done using a background solar wind configuration generated by the ideal magnetohydrodynamic module of EUHFORIA. The CME generating the ESP event is simulated by using the spheromak model of EUHFORIA, which approximates the CME’s flux rope as a linear force-free spheroidal magnetic field. In addition, a tool was developed to trace CME-driven shock waves in the EUHFORIA simulation domain. This tool is used in PARADISE to (i) inject 50 keV protons continuously at the CME-driven shock and (ii) include a foreshock and a sheath region, in which the energetic particle parallel mean free path, λ ∥ , decreases towards the shock wave. The value of λ ∥ at the shock wave is estimated from in situ observations of the ESP event. Results. For energies below ∼1 MeV, the simulation results agree well with both the upstream and downstream components of the ESP event observed by the Advanced Composition Explorer. This suggests that these low-energy protons are mainly the result of interplanetary particle acceleration. In the downstream region, the sharp drop in the energetic particle intensities is reproduced at the entry into the following magnetic cloud, illustrating the importance of a magnetised CME model.