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We identify and investigate cosmic ray Forbush decreases (FDs) observed in the E > 50 MeV data from the Helios 1 and 2 spacecraft, spanning from 1974–1985 and covering heliocentric distances in the range 0.28–1 au. A Helios FD catalogue is compiled, including the characteristics of the cosmic ray variations, as well as those of the solar wind (SW) and the interplanetary magnetic field (IMF) of the associated interplanetary disturbances. An extended statistical study considers the radial dependence of the FD magnitude, the SW velocity, and the IMF intensity in these disturbances. It is found that the Helios FD sizes at different distances from the Sun are determined by the parameters of the interplanetary disturbances. In particular, the FD magnitudes observed at Helios, as well as near Earth, correlate well with VB, which is the product of the maximum SW velocity and the IMF intensity when normalized by the average values of these parameters at the radial distance of the observations. However, we found that, on average, the Helios FD sizes are statistically independent of the radial distance in the range of 0.28–1 au.

We analyze two specific features of the intense solar energetic particle (SEP) event observed by Parker Solar Probe (PSP) between 2020 November 29 and 2020 December 2. The interplanetary counterpart of the coronal mass ejection (CME) on 2020 November 29 that generated the SEP event (hereafter ICME-2) arrived at PSP (located at 0.8 au from the Sun) on 2020 December 1. ICME-2 was preceded by the passage of an interplanetary shock at 18:35 UT on 2020 November 30 (hereafter S2), that in turn was preceded by another ICME (i.e., ICME-1) observed in situ on 2020 November 30. The two interesting features of this SEP event at PSP are the following: First, the presence of the intervening ICME-1 affected the evolution of the ≲8 MeV proton intensity-time profiles resulting in the observation of inverted energy spectra throughout the passage of ICME-1. Second, the sheath region preceding ICME-2 was characterized by weak magnetic fields compared to those measured immediately after the passage of the shock S2 and during the passage of ICME-2. Comparison with prior SEP events measured at 1 au but with similar characteristics indicates that (1) low-energy particles accelerated by S2 were excluded from propagating throughout ICME-1, and (2) the low magnetic fields measured in the sheath of ICME-2 resulted from the properties of the upstream solar wind encountered by ICME-2 that was propagated into the sheath, whereas the energy density of the high-energy particles in the sheath did not play a dominant role in the formation of these low magnetic fields.

An interval of exceptional solar activity was registered in early September 2017, late in the decay phase of solar cycle 24, involving the complex Active Region 12673 as it rotated across the western hemisphere with respect to Earth. A large number of eruptions occurred between 4-10 September, including four associated with X-class flares. The X9.3 flare on 6 September and the X8.2 flare on 10 September are currently the two largest during cycle 24. Both were accompanied by fast coronal mass ejections and gave rise to solar energetic particle (SEP) events measured by near-Earth spacecraft. In particular, the partially-occulted solar event on 10 September triggered a ground level enhancement (GLE), the second GLE of cycle 24. A further, much less energetic SEP event was recorded on 4 September. In this work we analyze observations by the Advanced Composition Explorer (ACE) and the Geostationary Operational Environmental Satellites (GOES), estimating the SEP event-integrated spectra above 300 keV and carrying out a detailed study of the spectral shape temporal evolution. Derived spectra are characterized by a low-energy break at few/tens of MeV; the 10 September event spectrum, extending up to ~1 GeV, exhibits an additional rollover at several hundred MeV. We discuss the spectral interpretation in the scenario of shock acceleration and in terms of other important external influences related to interplanetary transport and magnetic connectivity, taking advantage of multi-point observations from the Solar Terrestrial Relations Observatory (STEREO). Spectral results are also compared with those obtained for the 17 May 2012 GLE event., Comment: Accepted for publication in Space Weather; 24 pages, 10 figures, 1 table

Geomagnetic storms are an important aspect of space weather and can result in significant impacts on space- and ground-based assets. The majority of strong storms are associated with the passage of interplanetary coronal mass ejections (ICMEs) in the near-Earth environment. In many cases, these ICMEs can be traced back unambiguously to a specific coronal mass ejection (CME) and solar activity on the frontside of the Sun. Hence, predicting the arrival of ICMEs at Earth from routine observations of CMEs and solar activity currently makes a major contribution to the forecasting of geomagnetic storms. However, it is clear that some ICMEs, which may also cause enhanced geomagnetic activity, cannot be traced back to an observed CME, or, if the CME is identified, its origin may be elusive or ambiguous in coronal images. Such CMEs have been termed "stealth CMEs." In this review, we focus on these "problem" geomagnetic storms in the sense that the solar/CME precursors are enigmatic and stealthy. We start by reviewing evidence for stealth CMEs discussed in past studies. We then identify several moderate to strong geomagnetic storms (minimum Dst < -50 nT) in solar cycle 24 for which the related solar sources and/or CMEs are unclear and apparently stealthy. We discuss the solar and in situ circumstances of these events and identify several scenarios that may account for their elusive solar signatures. These range from observational limitations (e.g., a coronagraph near Earth may not detect an incoming CME if it is diffuse and not wide enough) to the possibility that there is a class of mass ejections from the Sun that have only weak or hard-to-observe coronal signatures. In particular, some of these sources are only clearly revealed by considering the evolution of coronal structures over longer time intervals than is usually considered. We also review a variety of numerical modelling approaches..., 60 pages, 25 figures, 5 tables. Accepted for publication in Space Science Reviews

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 ∼1100 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.

A sequence of discrete solar wind structures within the sheath region of an interplanetary coronal mass ejection on November 6, 2015, caused a series of compressions and releases of the dayside magnetosphere. Each compression resulted in a brief adiabatic enhancement of ions (electrons) driving bursts of electromagnetic ion cyclotron (EMIC; whistler mode chorus) wave growth across the dayside magnetosphere. Fine‐structured rising tones were observed in the EMIC wave bursts, resulting in nonlinear scattering of relativistic electrons in the outer radiation belt. Multipoint observations allow us to study the spatial structure and evolution of these sheath structures as they propagate Earthward from L1 as well as the spatio‐temporal characteristics of the magnetospheric response. This event highlights the importance of fine‐scale solar wind structure, in particular within complex sheath regions, in driving dayside phenomena within the inner magnetosphere., Key Points A sequence of discrete structures within an interplanetary coronal mass ejection sheath are tracked via multipoint measurementsEach structure results in magnetospheric compression, adiabatic enhancement of ions and electrons, and cyclotron wave growthThis event highlights the importance of fine‐scale solar wind structure and sheath regions for driving dayside magnetospheric phenomena

We present a new empirical model to predict solar energetic particle (SEP) event-integrated and peak intensity spectra between 10 and 130 MeV at 1 AU, based on multi-point spacecraft measurements from the Solar TErrestrial RElations Observatory (STEREO), the Geostationary Operational Environmental Satellites (GOES), and the Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics (PAMELA) satellite experiment. The analyzed data sample includes 32 SEP events occurring between 2010 and 2014, with a statistically significant proton signal at energies in excess of a few tens of MeV, unambiguously recorded at three spacecraft locations. The spatial distributions of SEP intensities are reconstructed by assuming an energy-dependent 2D Gaussian functional form, and accounting for the correlation between the intensity and the speed of the parent coronal mass ejection (CME), and the magnetic-field-line connection angle. The CME measurements used are from the Space Weather Database Of Notifications, Knowledge, Information (DONKI). The model performance, including its extrapolations to lower/higher energies, is tested by comparing with the spectra of 20 SEP events not used to derive the model parameters. Despite the simplicity of the model, the observed and predicted event-integrated and peak intensities at Earth and at the STEREO spacecraft for these events show remarkable agreement, both in the spectral shapes and their absolute values.

We assess whether a formula obtained by Richardson et al. (2014,https://doi.org/10.1007/s11207-014-0524-8) relating the peak intensity of 14- to 24-MeV protons in a solar energetic particle (SEP) event at 1 AU to the solar event location and the speed of the associated coronal mass ejection (CME) may be used in a scheme to predict the intensity of an SEP event at any location at this heliocentric distance. Starting with all 334 CMEs in the CCMC/SWRC DONKI database in October 2011 to July 2012, we use the CME speed and direction to predict the proton intensity at Earth and the two Solar Terrestrial Relations Observatory spacecraft using this formula. Since most (85%)of these CMEs were not in fact associated with SEP events, many SEP events are predicted that are not actually observed. Such cases may be reduced by considering whether type II or type III radio emissions accompany the CMEs, or by selecting faster, wider CMEs. This method is also applied to predict the SEP intensities associated with 1,100 CMEs observed by the Solar and Heliospheric Observatory Large Angle and Spectrometric Coronagraph during 1997-2006 in solar cycle 23. Various skill scores are calculated, which assess different aspects of the skill of the SEP predictions. We conclude that the Richardson et al. (2014) formula has potential as a simple empirical SEP intensity prediction tool.

We discuss observations of the journey throughout the Solar System of a large interplanetary coronal mass ejection (ICME) that was ejected at the Sun on 14 October 2014. The ICME hit Mars on 17 October, as observed by the Mars Express, MAVEN, Mars Odyssey and MSL missions, 44 hours before the encounter of the planet with the Siding-Spring comet, for which the space weather context is provided. It reached comet 67P/Churyumov-Gerasimenko, which was perfectly aligned with the Sun and Mars at 3.1 AU, as observed by Rosetta on 22 October. The ICME was also detected by STEREO-A on 16 October at 1 AU, and by Cassini in the solar wind around Saturn on the 12 November at 9.9 AU. Fortuitously, the New Horizons spacecraft was also aligned with the direction of the ICME at 31.6 AU. We investigate whether this ICME has a non-ambiguous signature at New Horizons. A potential detection of this ICME by Voyager-2 at 110-111 AU is also discussed. The multi-spacecraft observations allow the derivation of certain properties of the ICME, such as its large angular extension of at least 116°, its speed as a function of distance, and its magnetic field structure at four locations from 1 to 10 AU. Observations of the speed data allow two different solar wind propagation models to be validated. Finally, we compare the Forbush decreases (transient decreases followed by gradual recoveries in the galactic cosmic ray intensity) due to the passage of this ICME at Mars, comet 67P and Saturn.

We suggest an analogy between energetic particle and magnetic field observations made by the Voyager 1 spacecraft in the distant heliosheath at 122 AU in August 2012, and those made in the distant geomagnetic tail by the ISEE 3 spacecraft in 1982–1983, despite large differences in the time and distance scales. The analogy suggests that in August, 2012, Voyager 1 may not have moved from the anomalous cosmic ray (ACR)-dominated heliosheath into the interstellar medium but into a region equivalent to the “lobes” of the geomagnetic tail, composed of heliospheric field lines which have reconnected with the interstellar medium beyond the spacecraft and so are open to the entry of cosmic rays, while heliospheric particles (e.g., ACRs) are free to escape, and which maintain a ∼Parker spiral configuration. The heliopause, analogous to the magnetopause forming the outer boundary of the lobes, may then lie beyond this so-called “heliocliff”. Even if this analogy is incorrect, the remarkable similarities between the energetic particle and magnetic field observations in these very different regions are worth noting.

This is a companion to Savani et al. (2015) that discussed how a first-order prediction of the internal magnetic field of a coronal mass ejection (CME) may be made from observations of its initial state at the Sun for space weather forecasting purposes (Bothmer-Schwenn scheme (BSS) model). For eight CME events, we investigate how uncertainties in their predicted magnetic structure influence predictions of the geomagnetic activity. We use an empirical relationship between the solar wind plasma drivers and Kp index together with the inferred magnetic vectors, to make a prediction of the time variation of Kp (Kp(BSS)). We find a 2σ uncertainty range on the magnetic field magnitude (|B|) provides a practical and convenient solution for predicting the uncertainty in geomagnetic storm strength. We also find the estimated CME velocity is a major source of error in the predicted maximum Kp. The time variation of Kp(BSS) is important for predicting periods of enhanced and maximum geomagnetic activity, driven by southerly directed magnetic fields, and periods of lower activity driven by northerly directed magnetic field. We compare the skill score of our model to a number of other forecasting models, including the NOAA/Space Weather Prediction Center (SWPC) and Community Coordinated Modeling Center (CCMC)/SWRC estimates. The BSS model was the most unbiased prediction model, while the other models predominately tended to significantly overforecast. The True skill score of the BSS prediction model (TSS = 0.43 ± 0.06) exceeds the results of two baseline models and the NOAA/SWPC forecast. The BSS model prediction performed equally with CCMC/SWRC predictions while demonstrating a lower uncertainty.

This work presents a statistical analysis of near-relativistic solar energetic electron event spectra near 1 au. We use measurements of the Solar Electron and Proton Telescope (SEPT) on board STEREO in the energy range of 45–425 keV and utilize the SEPT electron event list containing all electron events observed by STEREO A and STEREO B from 2007 through 2018. We select 781 events with significant signal-to-noise ratios for our analysis and fit the spectra with single or broken-power-law functions of energy. We find 437 events showing broken power laws and 344 events only showing a single power law in the energy range of SEPT. For those events with broken power laws, we find a mean break energy of about 120 keV. We analyze the dependence of the spectral index on the rise times and peak intensities of the events as well as on the presence of relativistic electrons. The results show a relation between the power law spectral index and the rise times of the events with softer spectra belonging to rather impulsive events. Long rise-time events are associated with hard spectra as well as with the presence of higher-energy (>0.7 MeV) electrons. This group of events cannot be explained by a pure flare scenario but suggests an additional acceleration mechanism, involving a prolonged acceleration and/or injection of the particles. A dependence of the spectral index on the longitudinal separation from the parent solar source region was not found. A statistical analysis of the spectral indices during impulsively rising events where the rise times are below 20 minutes is also shown.

Despite the progress made over the past decades, the physical mechanisms underlying the origin of solar energetic particles (SEPs) are still debated. The largest uncertainties concern the most energetic ($\gtrsim$500 MeV) SEP events, which are difficult to characterize due to the relatively few and indirect observations such as those made by neutron monitors. The Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics (PAMELA) satellite experiment has recently offered a unique opportunity to study SEPs with energies between 80 MeV and few GeV, including their energy spectra, composition and pitch angle distributions. In particular, PAMELA has measured for the first time with good accuracy the spectral features at moderate and high energies for 26 SEP events occurring between 2006 December and 2014 September, providing important constraints for current SEP models. Reported spectral shapes exhibit a high-energy rollover that can be attributed to particles escaping the shock region during acceleration, as a consequence of its limited extension and lifetime. PAMELA observations also allow the relationship between low-energy SEPs detected by in-situ spacecraft and the high-energy SEPs registered by the worldwide network of neutron monitors during the rare ground-level enhancements (GLEs) to be investigated. No qualitative distinction between the spectra of GLE and non-GLE events was observed, suggesting that GLEs are not a separate class, but rather are a subset of a continuous distribution of SEP events that are more intense at high energies. In this work we combine data from PAMELA and other near-Earth spacecraft in order to determine the SEP spectral shapes at 1 AU from tens of keV to a few GeV.

Long Duration Gamma-Ray Flares (LDGRFs) are characterized by delayed and long-duration gamma-ray emission above $\sim$50 MeV. Despite dozens of observations in the last decade with $\it{Fermi}$/LAT, the nature of this emission has been a challenge to explain. The highest energy emission has generally been attributed to the decay of pions produced by the interaction of high-energy protons with ambient solar material. The fact that the $\gamma$-ray emission is delayed from the onset of the initial eruption and that the emission is, in some cases, unusually long in duration suggests that particle acceleration occurs within large volumes extending to high altitudes, either by stochastic acceleration within large coronal loops or by back-precipitation from CME-driven shocks. We have tested these models by a making direct comparisons between the properties of the accelerated ion population at the flare derived from the observations of $\it{Fermi}$/LAT and those of solar energetic particles detected at Earth by PAMELA at comparable high energies. We investigated 27 high-energy gamma ray events (from \cite{ref:SHARE2018}), and for 14 events we compare the two populations (SEPs in space and the interacting population at the Sun) and discuss the implications in terms of potential sources of the LDGRFs.

Little is known about the origin of the high-energy and sustained emission from solar Long-Duration Gamma-Ray Flares (LDGRFs), identified with the Compton Gamma Ray Observatory (CGRO), the Solar Maximum Mission (SMM), and now Fermi. Though Fermi/Large Area Space Telescope (LAT) has identified dozens of flares with LDGRF signature, the nature of this phenomenon has been a challenge to explain both due to the extreme energies and long durations. The highest-energy emission has generally been attributed to pion production from the interaction of >300 MeV protons with the ambient matter. The extended duration suggests that particle acceleration occurs over large volumes extending high in the corona, either from stochastic acceleration within large coronal loops or from back precipitation from coronal mass ejection driven shocks. It is possible to test these models by making direct comparison between the properties of the accelerated ion population producing the gamma-ray emission derived from the Fermi/LAT observations, and the characteristics of solar energetic particles (SEPs) measured by the Payload for Matter-Antimatter Exploration and Light Nuclei Astrophysics (PAMELA) spacecraft in the energy range corresponding to the pion-related emission detected with Fermi. For fourteen of these events we compare the two populations -- SEPs in space and the interacting particles at the Sun -- and discuss the implications in terms of potential sources. Our analysis shows that the two proton numbers are poorly correlated, with their ratio spanning more than five orders of magnitude, suggesting that the back precipitation of shock-acceleration particles is unlikely the source of the LDGRF emission., Accepted for publication in ApJ

The rigidity dependence of all Forbush decreases (FDs) recorded from 1995 to 2015 has been determined using neutron monitor (NM) and Solar and Heliospheric Observatory (SOHO) (EPHIN) spacecraft data, covering the energy (rigidity) range from ~433 MeV (1 GV) to 9.10 GeV (10 GV). We analyzed a total of 421 events and determined the spectrum in rigidity with an inverse power-law fit. As a result, the mean spectral index was identified to be 〈γ F 〉 = 0.46 ± 0.02. The majority (~66%) of the FDs have γ F within the range 0.3–0.7. The remaining one-third of the events (~33%) have either (very) soft or hard FD spectra, with the latter being more common than the former. Significant variations of γ F occur within almost every FD event. During the initial FD decay phase the spectrum becomes gradually harder, in contrast to the recovery phase, when it becomes softer. Additionally, low energies (rigidities) seem to be better suited for studying the fine structure of interplanetary disturbances (primarily interplanetary coronal mass ejections) that lead to FDs. In particular, FDs recorded by the EPHIN instrument on SOHO better capture a two-step structure than FDs observed by NMs. Finally, the ejecta of an ICME, especially when identified as a magnetic cloud, often leads to abrupt changes in the slope of γ F.

This study compares the designation of different solar wind flow regimes (transient, coronal hole, and streamer belt) according to two algorithms derived from observations by the Solar Wind Ion Composition Spectrometer, the Solar Wind Electron Proton Alpha Monitor, and the Magnetometer on the ACE spacecraft, with a similar regime determination performed on board the Genesis spacecraft. The comparison is made for the interval from late 2001 to early 2004 when Genesis was collecting solar wind ions for return to Earth. The agreement between hourly regime assignments from any pair of algorithms was less than two thirds, while the simultaneous agreement between all three algorithms was only 49%. When the results of the algorithms were compared to a catalog of interplanetary coronal mass ejection events, it was found that almost all the events in the catalog were confirmed by the spacecraft algorithms. On the other hand, many short transient events, lasting 1 to 13 h, that were unanimously selected as transient like by the algorithms, were not included in the catalog.

Trapped electrons in Earth's outer Van Allen radiation belt are influenced profoundly by solar phenomena such as high-speed solar wind streams, coronal mass ejections (CME), and interplanetary (IP) shocks. In particular, strong IP shocks compress the magnetosphere suddenly and result in rapid energization of electrons within minutes. It is believed that the electric fields induced by the rapid change in the geomagnetic field are responsible for the energization. During the latter part of March 2015, a CME impact led to the most powerful geomagnetic storm (minimum Dst = −223 nT at 17 March, 23 UT) observed not only during the Van Allen Probe era but also the entire preceding decade. Magnetospheric response in the outer radiation belt eventually resulted in elevated levels of energized electrons. The CME itself was preceded by a strong IP shock whose immediate effects vis-a-vis electron energization were observed by sensors on board the Van Allen Probes. The comprehensive and high-quality data from the Van Allen Probes enable the determination of the location of the electron injection, timescales, and spectral aspects of the energized electrons. The observations clearly show that ultrarelativistic electrons with energies E > 6 MeV were injected deep into the magnetosphere at L ≈ 3 within about 2 min of the shock impact. However, electrons in the energy range of ≈250 keV to ≈900 keV showed no immediate response to the IP shock. Electric and magnetic fields resulting from the shock-driven compression complete the comprehensive set of observations that provide a full description of the near-instantaneous electron energization.

Energetic charged particle precipitation associated with solar wind perturbations causes enhanced high-frequency radiowave absorption in the high-latitude ionosphere. This study models 38.2 MHz cosmic noise absorption (CNA) by utilising measurements from the Imaging Riometer for Ionospheric Studies (IRIS) at Kilpisjarvi, Finland obtained during solar cycle 23 (1996–2009) associated with the passage of interplanetary coronal mass ejections (ICMEs) past Earth; ICMEs are a major driver of enhanced geomagnetic activity. Superposed epoch analysis suggests that the r6absorption vs. time profile depends on whether ICME arrival occurs in the day-time (10–14 MLT) or night-time (22-02 MLT) for IRIS, with peak absorption occurring ~2–3 h ahead of ICME arrival or ~4 h after ICME arrival, respectively. We determine which combinations of solar wind and IMF parameters show the best correlation with the absorption associated with day-time or night-time arriving ICMEs using superposed epoch analysis and the least squares estimation method. Various combinations of solar wind parameters (including bulk velocity v, density n, and the interplanetary magnetic field north and south components Bz and the SYMH geomagnetic index), have been ranked to obtain the best coupling function for the absorption associated with day- and night-time arriving ICMEs. The absorption for day-time events is found to correlate closely with the solar wind dynamic pressure, SYMH, and the northward direction of the Bz while the absorption for night-time events is most closely related to the direction of the Bz and SYMH. The coupling functions are found to model the observed absorption successfully, with correlation coefficients of ~0.7–0.8 between the observed and modelled absorption.

The current understanding of the characteristics of solar and inner heliospheric electron events is inferred almost entirely from observations made by spacecraft located at 1 astronomical unit (au). Previous observations within 1 au of the Sun, by the Helios spacecraft at ∼0.3–1 au, indicate the presence of electron events that are not detected at 1 au or may have merged during transport from the Sun. Parker Solar Probe’s close proximity to the Sun at perihelion provides an opportunity to make the closest measurements yet of energetic electron events. We present an overview of measurements of electrons with energies between ∼17 keV and ∼1 MeV made by the Parker Solar Probe Integrated Science Investigation of the Sun (IS⊙IS) instrument suite during Encounter 2 (2019 March 31–April 10 with perihelion of ∼0.17 au), including several small electron events. We examine these events in the context of the electromagnetic and solar wind environment measured by the FIELDS and SWEAP instruments on Parker Solar Probe. We find most of these electron enhancements to be associated with type III radio emissions that reach the local plasma frequency and one enhancement that appears to be primarily associated with abrupt changes in the local magnetic field. Together, these associations suggest that these are indeed the first measurements of energetic electron events within 0.2 au.

We acknowledge the NASA Parker Solar Probe mission team and the SWEAP team led by J. C. Kasper, the ISeIS team led by D. J. McComas, and the FIELDS team led by S. D. Bale, for the use of PSP data. The data used in this paper can be downloaded from spdf.gsfc.nasa.gov, www.srl.caltech.edu/ACE/ASC/, and stereo.gsfc.nasa.gov. Parker Solar Probe was designed, built, and is now operated by the Johns Hopkins Applied Physics Laboratory as part of NASA’s Living with a Star (LWS) program (contract NNN06AA01C). Support from the LWS management and technical team has played a critical role in the success of the Parker Solar Probe mission. Simulation results have been provided by the Community Coordinated Modeling Center at Goddard Space Flight Center through their public Runs on Request system (ccmc.gsfc.nasa. gov; run david_lario_012820_SH_1). The ENLIL-Cone Model was developed by D. Odstrcil of George Mason University. We thank the STEREO/SECCHI, STEREO/IMPACT, STEREO/ PLASTIC, SOHO/LASCO, and SDO/AIA teams and Predictive Science Inc. for providing the data used in this study. The STEREO SECCHI data are produced by a consortium of RAL (UK), NRL (USA), LMSAL (USA), GSFC (USA), MPS (Germany), CSL (Belgium), IOTA (France), and IAS (France). SOHO is a mission of international cooperation between ESA and NASA. The SDO/AIA data are provided by the Joint Science Operations Center (JSOC) Science Data Processing (SDP). We acknowledge all of the science instrument teams for making their data used in this paper available. This work utilizes data produced collaboratively between the Air Force Research Laboratory (AFRL) and the National Solar Observatory (NSO). The ADAPT model development is supported by AFRL. We appreciate the reviewer’s comprehensive reading of the article. This work was partially supported under NASA contract NNN06AA01C. D.L. was also supported by NASA-HGI grant NNX16AF73G and NASA/LWS grant NNX15AD03G. L.J. acknowledges support from the NASA Program NNH16ZDA001N-HSR and NNH17ZDA001-HSR. D.L., L.B., and I.R. acknowledge the support from the NASA Program NNH17ZDA001N-LWS.

Changes in the Dst index, or the similarly constructed high-resolution SYM-H index, are thought to indicate changes in the total energy content of the ring current. However, this is not always the case. In this paper we examine an intense (SYM-H ∼ −435 nT) magnetic storm that occurred on March 31, 2001. The arrival at Earth of strongly southward IMF produced an immediate negative response in the SYM-H index. While energetic particle and magnetometer data from geosynchronous orbit and inner magnetosphere energetic neutral atom imaging indicate that two substorm injections took place during the main phase, there was about one hour when the SYM-H decreased more than 200 nT with no evidence in the data for ring current enhancement. Instead the near-Earth magnetotail exhibited a growth phase indicative of a strong, growing cross-tail current, with the large substorm expansion phase and the associated injection of energetic particles coming significantly later. Data from the DMSP spacecraft demonstrate that the polar cap flux grew rapidly in response to the strongly southward IMF. We present observations showing that the decrease in SYM-H occurred when polar cap flux was increasing and there was no evidence of injection into the ring current. Our findings strongly support the relationship between Dst and the polar cap flux proposed by theoretical studies that determined that the tail current system could be a significant contributor to Dst.

During early November 2013, the magnetosphere experienced concurrent driving by a coronal mass ejection (CME) during an ongoing high-speed stream (HSS) event. The relativistic electron response to these two kinds of drivers, i.e., HSS and CME, is typically different, with the former often leading to a slower buildup of electrons at larger radial distances, while the latter energizing electrons rapidly with flux enhancements occurring closer to the Earth. We present a detailed analysis of the relativistic electron response including radial profiles of phase space density as observed by both Magnetic Electron and Ion Sensor (MagEIS) and Relativistic Electron Proton Telescope instruments on the Van Allen Probes mission. Data from the MagEIS instrument establish the behavior of lower energy (

The process by which the Sun affects the terrestrial environment on short timescales is predominately driven by the amount of magnetic reconnection between the solar wind and Earth's magnetosphere. Reconnection occurs most efficiently when the solar wind magnetic field has a southward component. The most severe impacts are during the arrival of a coronal mass ejection (CME) when the magnetosphere is both compressed and magnetically connected to the heliospheric environment. Unfortunately, forecasting magnetic vectors within coronal mass ejections remain elusive. Here we report how, by combining a statistically robust helicity rule for a CME's solar origin with a simplified flux rope topology, the magnetic vectors within the Earth-directed segment of a CME can be predicted. In order to test the validity of this proof-of-concept architecture for estimating the magnetic vectors within CMEs, a total of eight CME events (between 2010 and 2014) have been investigated. With a focus on the large false alarm of January 2014, this work highlights the importance of including the early evolutionary effects of a CME for forecasting purposes. The angular rotation in the predicted magnetic field closely follows the broad rotational structure seen within the in situ data. This time-varying field estimate is implemented into a process to quantitatively predict a time-varying Kp index that is described in detail in paper II. Future statistical work, quantifying the uncertainties in this process, may improve the more heuristic approach used by early forecasting systems.

The physical parameters of the solar wind observed in-situ near 1 AU have been studied for several decades, and relationships between them, such as the positive correlation between the solar wind plasma temperature, $T$ , and velocity, $V$ , and the negative correlation between density, $N$ , and velocity, $V$ , are well known. However, the magnetic field intensity, $B$ , does not appear to be well correlated with any individual plasma parameter. In this article, we discuss previously under-reported correlations between $B$ and the combined plasma parameters $\sqrt{N V^{2}} $ as well as between $B$ and $\sqrt{NT}$ . These two correlations are strong during periods of corotating interaction regions and high-speed streams, and moderate during intervals of slow solar wind. The results indicate that the magnetic pressure in the solar wind is well correlated both with the plasma dynamic pressure and the thermal pressure.

New results on the short-term galactic cosmic-ray (GCR) intensity variation (Forbish decrease) in 2006 December measured by the PAMELA instrument are presented. Forbush decreases are sudden suppressions of the GCR intensities, which are associated with the passage of interplanetary transients such as shocks and interplanetary coronal mass ejections (ICMEs). Most of the past measurements of this phenomenon were carried out with groundbased detectors such as neutron monitors or muon telescopes. These techniques allow only the indirect detection of the overall GCR intensity over an integrated energy range. For the first time, thanks to the unique features of the PAMELA magnetic spectrometer, the Forbush decrease, commencing on 2006 December 14 and following a CME at the Sun on 2006 December 13, was studied in a wide rigidity range (0.4-20 GV) and for different species of GCRs detected directly in space. The daily averaged GCR proton intensity was used to investigate the rigidity dependence of the amplitude and the recovery time of the Forbush decrease. Additionally, for the first time, the temporal variations in the helium and electron intensities during a Forbush decrease were studied. Interestingly, the temporal evolutions of the helium and proton intensities during the Forbush decrease were found to be in good agreement, while the low rigidity electrons (

Despite the significant progress achieved in recent years, the physical mechanisms underlying the origin of solar energetic particles (SEPs) are still a matter of debate. The complex nature of both particle acceleration and transport poses challenges to developing a universal picture of SEP events that encompasses both the low-energy (from tens of keV to a few hundreds of MeV) observations made by space-based instruments and the GeV particles detected by the worldwide network of neutron monitors in ground-level enhancements (GLEs). The high-precision data collected by the Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics (PAMELA) satellite experiment offer a unique opportunity to study the SEP fluxes between $\sim$80 MeV and a few GeV, significantly improving the characterization of the most energetic events. In particular, PAMELA can measure for the first time with good accuracy the spectral features at moderate and high energies, providing important constraints for current SEP models. In addition, the PAMELA observations allow the relationship between low and high-energy particles to be investigated, enabling a clearer view of the SEP origin. No qualitative distinction between the spectral shapes of GLE, sub-GLE and non-GLE events is observed, suggesting that GLEs are not a separate class, but are the subset of a continuous distribution of SEP events that are more intense at high energies. While the spectral forms found are to be consistent with diffusive shock acceleration theory, which predicts spectral rollovers at high energies that are attributed to particles escaping the shock region during acceleration, further work is required to explore the relative influences of acceleration and transport processes on SEP spectra., Comment: 26 pages, 13 figures, 2 tables

We study the latitudinal distribution of geomagnetic activity in 1966–2009 with local geomagnetic activity indices at 26 magnetic observatories. Using the principal component analysis method we find that more than 97% of the variance in annually averaged geomagnetic activity can be described by the two first principal components. The first component describes the evolution of the global geomagnetic activity, and has excellent correlation with, e.g., the Kp/Ap index. The second component describes the leading pattern by which the latitudinal distribution of geomagnetic activity deviates from the global average. We show that the second component is highly correlated with the relative (annual) fraction of high-speed streams (HSS) in solar wind. The latitudinal distribution of the second mode has a high maximum at auroral latitudes, a local minimum at subauroral latitudes and a low maximum at midlatitudes. We show that this distribution is related to the difference in the average location and intensity between substorms related to coronal mass ejections (CMEs) and HSSs. This paper demonstrates a new way to extract useful, quantitative information about the solar wind from local indices of geomagnetic activity over a latitudinally extensive network.

Previous studies have discussed the identification of interplanetary coronal mass ejections (ICMEs) near the Earth based on various solar wind signatures. In particular, methods have been developed of identifying regions of anomalously low solar wind proton temperatures (T p) and plasma compositional anomalies relative to the composition of the ambient solar wind that are frequently indicative of ICMEs. In this study, similar methods are applied to observations from the Ulysses spacecraft that was launched in 1990 and placed in a heliocentric orbit over the poles of the Sun. Some 279 probable ICMEs are identified during the spacecraft mission, which ended in 2009. The identifications complement those found independently in other studies of the Ulysses data, but a number of additional events are identified. The properties of the ICMEs detected at Ulysses and those observed near the Earth and in the inner heliosphere are compared.

In this study we synthesize the results of four previous studies on the global energetics of solar flares and associated coronal mass ejections (CMEs), which include magnetic, thermal, nonthermal, and CME energies in 399 solar M and X-class flare events observed during the first 3.5 years of the Solar Dynamics Observatory (SDO) mission. Our findings are: (1) The sum of the mean nonthermal energy of flare-accelerated particles ($E_{\mathrm{nt}}$), the energy of direct heating ($E_{\mathrm{dir}}$), and the energy in coronal mass ejections ($E_{\mathrm{CME}}$), which are the primary energy dissipation processes in a flare, is found to have a ratio of $(E_{\mathrm{nt}}+E_{\mathrm{dir}}+ E_{\mathrm{CME}})/E_{\mathrm{mag}} = 0.87 \pm 0.18$, compared with the dissipated magnetic free energy $E_{\mathrm{mag}}$, which confirms energy closure within the measurement uncertainties and corroborates the magnetic origin of flares and CMEs; (2) The energy partition of the dissipated magnetic free energy is: $0.51\pm0.17$ in nonthermal energy of $\ge 6$ keV electrons, $0.17\pm0.17$ in nonthermal $\ge 1$ MeV ions, $0.07\pm0.14$ in CMEs, and $0.07\pm0.17$ in direct heating; (3) The thermal energy is almost always less than the nonthermal energy, which is consistent with the thick-target model; (4) The bolometric luminosity in white-light flares is comparable with the thermal energy in soft X-rays (SXR); (5) Solar Energetic Particle (SEP) events carry a fraction $\approx 0.03$ of the CME energy, which is consistent with CME-driven shock acceleration; and (6) The warm-target model predicts a lower limit of the low-energy cutoff at $e_c \approx 6$ keV, based on the mean differential emission measure (DEM) peak temperature of $T_e=8.6$ MK during flares. This work represents the first statistical study that establishes energy closure in solar flare/CME events., 35 pages, 10 Figures, accepted for publication in The Astrophysical Journal

We summarize observations of around a thousand solar energetic particle (SEP) events since 1967 that include ∼25 MeV protons, made by various near-Earth spacecraft (IMPs 4, 5, 7, 8, ISEE 3, SOHO), that encompass Solar Cycle 20 to the current cycle (24). We also discuss recent observations of similar SEP events in Cycle 24 made by the STEREO spacecraft. The observations show, for example, that the time distribution of ∼25 MeV proton events varies from cycle to cycle. In particular, the time evolution of the SEP occurrence rate in Cycle 24 is strongly asymmetric between the northern and southern solar hemispheres, and tracks the sunspot number in each hemisphere, whereas Cycle 23 was more symmetric. There was also an absence of 25 MeV proton events during the solar minimum preceding Cycle 24 (other minima show occasional, often reasonably intense events). So far, events comparable to the exceptionally intense events detected in Cycles 22 and 23 have not been observed at Earth in Cycle 24, though Cycle 21 (the largest of the cycles considered here) also apparently lacked such events. We note a correlation between the rates of intense 25 MeV proton events and “ground level enhancements” (GLEs) observed by neutron monitors, since 1967, and conclude that the number of “official” GLEs (1) observed to date in Cycle 24 appears to be significantly lower than expected (5 to 7 ± 1) based on the rate of intense 25 MeV proton events in this cycle.

We present evidence that $>25~\mbox{MeV}$ solar proton events show a clustering in time at intervals of about six months that persisted during the rising and peak phases of Solar Cycle 24. This phenomenon is most clearly demonstrated by considering events originating in the northern or southern solar hemispheres separately. We examine how these variations in the solar energetic particle (SEP) event rate are related to other phenomena, such as hemispheric sunspot numbers and areas, rates of coronal mass ejections, and the mean solar magnetic field. Most obviously, the SEP event rate closely follows the sunspot number and area in the same hemisphere. The variations of about six months are associated with features in many of the other parameters we examine, indicating that they are just one signature of the episodic development of Cycle 24. They may be related to periodicities of about 150 days reported in various solar and interplanetary phenomena during previous solar cycles. The clear presence of periodicities of about six months in Cycle 24 that evolve independently in each hemisphere contradicts a scenario suggested by McIntosh et al. (Nature Com. 6, 6491, 2015) for the variational timescales of solar magnetism.

In-situ measurements of interplanetary coronal mass ejections (ICMEs) display a wide range of properties. A distinct subset, “magnetic clouds” (MCs), are readily identifiable by a smooth rotation in an enhanced magnetic field, together with an unusually low solar wind proton temperature. In this study, we analyze Ulysses spacecraft measurements to systematically investigate five possible explanations for why some ICMEs are observed to be MCs and others are not: i) An observational selection effect; that is, all ICMEs do in fact contain MCs, but the trajectory of the spacecraft through the ICME determines whether the MC is actually encountered; ii) interactions of an erupting flux rope (FR) with itself or between neighboring FRs, which produce complex structures in which the coherent magnetic structure has been destroyed; iii) an evolutionary process, such as relaxation to a low plasma-β state that leads to the formation of an MC; iv) the existence of two (or more) intrinsic initiation mechanisms, some of which produce MCs and some that do not; or v) MCs are just an easily identifiable limit in an otherwise continuous spectrum of structures. We apply quantitative statistical models to assess these ideas. In particular, we use the Akaike information criterion (AIC) to rank the candidate models and a Gaussian mixture model (GMM) to uncover any intrinsic clustering of the data. Using a logistic regression, we find that plasma-β, CME width, and the ratio O7/O6 are the most significant predictor variables for the presence of an MC. Moreover, the propensity for an event to be identified as an MC decreases with heliocentric distance. These results tend to refute ideas ii) and iii). GMM clustering analysis further identifies three distinct groups of ICMEs; two of which match (at the 86 % level) with events independently identified as MCs, and a third that matches with non-MCs (68 % overlap). Thus, idea v) is not supported. Choosing between ideas i) and iv) is more challenging, since they may effectively be indistinguishable from one another by a single in-situ spacecraft. We offer some suggestions on how future studies may address this.