Energetic Particle Intensities Research Articles

Observations of the 2022 September 5 Solar Energetic Particle Event at 15 Solar Radii

On 2022 September 5, Parker Solar Probe (Parker) observed a large solar energetic particle (SEP) event at the unprecedented distance of only 15 R S from the Sun. The observations from the Integrated Science Investigation of the Sun (IS⊙IS) obtained over the course of this event are remarkably rich, and an overview is presented here. IS⊙IS is capable of measuring ions from 20 keV to over 100 MeV nuc−1 and electrons from 30 keV to 6 MeV; here, we primarily focus on the proton and helium measurements above 80 keV. Among the surprising results are evidence of inverse velocity dispersion at energies above 1 MeV during the onset of the event, a sharp decrease in the energetic particle intensities at all energies at the interplanetary shock crossing, and repeated short durations of highly anisotropic sunward flow. Many changes in the SEP intensities, anisotropy, and spectral steepness are coincident with solar wind structure boundaries identified using the Parker solar wind magnetic field and plasma data. However, there are significant changes that are not correlated with any clearly visible solar wind variation. The observations presented here serve as an introduction to a complex event with numerous opportunities for future, more in-depth studies.

Correlation of Coronal Mass Ejection Shock Temperature with Solar Energetic Particle Intensity

Solar energetic particle (SEP) events have been observed by the Parker Solar Probe (PSP) spacecraft since its launch in 2018. These events include sources from solar flares and coronal mass ejections (CMEs). The IS⊙IS instrument suite on board PSP is measuring ions over energies from ∼ 20 keV nucleon−1 to 200 MeV nucleon−1 and electrons from ∼ 20 keV to 6 MeV. Previous studies sought to group CME characteristics based on their plasma conditions and arrived at general descriptions with large statistical errors, leaving open questions on how to properly group CMEs based solely on their plasma conditions. To help resolve these open questions, the plasma properties of CMEs have been examined in relation to SEPs. Here, we reexamine one plasma property, the solar wind proton temperature, and compare it to the proton SEP intensity in a region immediately downstream of a CME-driven shock for seven CMEs observed at radial distances within 1 au. We find a statistically strong correlation between proton SEP intensity and bulk proton temperature, indicating a clear relationship between SEPs and the conditions in the solar wind. Furthermore, we propose that an indirect coupling of SEP intensity to the level of turbulence and the amount of energy dissipation that results is mainly responsible for the observed correlation between SEP intensity and proton temperature. These results are key to understanding the interaction of SEPs with the bulk solar wind in CME-driven shocks and will improve our ability to model the interplay of shock evolution and particle acceleration.

Relationship between solar energetic particle intensity and coronal mass ejections and its associated type II radio bursts

Based on the multiple-vantage observations of STEREO, SOHO, wind and other spacecraft, the fast and wide coronal mass ejections (CME) during the 24th solar cycle from January 2010 to September 2014 are selected in this paper. Using the outputs of Richardson’s (2014) empirical model of solar energetic particle (SEP) intensity under different conditions, the effects of its associations such as CME, pre-CME, and type II radio bursts, on SEP intensity are analyzed, and the relationship between SEP event and these characteristics is also discussed. The main conclusions are as follows. 1) The presence or absence of pre-CME within 13 h before fast CME significantly improves the model prediction effect and has a significant influence on whether fast CME produces SEP event. Compared with the events without pre-CMEs, the events with pre-CMEs have a low proportion of false alarms (FR: 47.7% <i>vs.</i> 70%). However, the number of pre-CMEs does not improve the model output. 2) CMEs with type-II radio bursts have significantly lower FR to generate SEP events than fast CMEs without type-II radio bursts (42% <i>vs.</i> 68%). And selecting type-II radio bursts as a constraint will filter out some small/weak SEP events, the relationship between model predictions and observations especially for large SEP events (e.g. <i>I</i><sub>p</sub> ≥ 0.01 pfu/MeV) will stand out. Moreover, if the type-II radio enhancement is taken into account, FR can be further reduced to 29.4%, and the proportion of hits can be further increased (HR: 48.5%), and the model prediction is significantly improved. 3) The larger the start frequency of type II radio bursts, the smaller the end frequency is, and FR decreases slightly, but at the same time, a large number of SEP events are excluded by this condition, and the results show that the constraints on the start/end frequency of type-II radio bursts do not improve the model predictions distinctly. 4) If the sub-classification of type-II radio bursts is considered as the model constraint, the CMEs associated with multi-band type-II radio bursts have better model predictions than those with single-band events. For example, m-DH-km type-II radio bursts have lower FR (35.4%) and higher HR (48%), and the accuracy of empirical model is higher. In summary, we find that in addition to the velocity and angular width of CME, the associations of pre-CME, type II radio bursts and their enhancement, and multi-band sub-classification are the favorable conditions for CME to generate SEP events. The SEP intensities obtained by the empirical model have better consistency with the observations, and better predictions can be obtained. This investigation indicates that SEP events are more likely generated by fast and wide CMEs accompanied by pre-CMEs, multi-band type II radio bursts and their enhancements, which seem to serve as discriminative signal for SEP-rich and SEP-poor CMEs.

Empirical forecasting models for peak intensities of energetic storm particles at 1 AU

We have investigated the dependence of the peak intensities of energetic storm particles (ESPs) on various parameters characterising the coronal mass ejections (CMEs) and associated phenomena. The aim of this study is to suggest empirical models for forecasting the peak intensities of ESP events at 1 AU based on solar and interplanetary (IP) space observations.For this study we searched for the associations of front-side full and partial halo CMEs with linear speeds >400 km s−1during the years 1996–2015 with IP shocks at 1 AU and ESP events observed near the time when the shock passes the observer. We found 88 CME-driven IP shocks associated with ESP events at proton energy range 5.0–7.2 MeV (nominal energy 6.0 MeV) and 59 shocks at the energy range 15.1–21.9 MeV (nominal energy 18.2 MeV). At these two energies 71% and 68% of the ESP events were associated with solar energetic particle (SEP) events, 85% and 84% were associated with decametric–hectometric (DH) type II radio bursts while 67% and 66% were associated with both.For each CME - shock pair we calculated the predicted shock transit speed (VTR) by using the method of Belov et al. (2022) and used this as the primary parameter in the investigation. We performed correlation analyses between the logarithm of the peak intensities of the ESP events (log10 [IESPpeak]) and the solar parameters related to the CMEs, solar flares, IP shocks, SEP events, and type II radio bursts. When using a single explanatory variable, we found best correlation coefficients for VTR(0.68 ±0.05 and 0.71 ±0.06), the CME space speed (VCMEspace) (0.59 ± 0.05 and 0.68 ± 0.07), and the logarithm of SEP peak intensity (log10 [ISEPpeak]) (0.55 ± 0.08 and 0.70 ±0.08) at 6.0 and 18.2 MeV, respectively. Weak to moderate correlations were found for the logarithm of the soft X-ray flux (log10 [SXRF]) and the logarithm of the duration of DH type II radio burst (log10 [DTII]).Using linear combinations of two or more variables improved the correlations. The best two-variable combination explaining log10 [IESPpeak] was VTRcombined with log10 [ISEPpeak] and the best three- and four-variable combinations also included these two parameters. We found two methods for forecasting ESP peak intensities, one of which can be used for long lead time and the other for medium lead time forecasting. For long lead time forecasting VTR, VCMEspaceand log10 [SXRF] are used. The correlation coefficients between the calculated and observed log10 [IESPpeak] were 0.71 ±0.05 at 6.0 MeV and 0.74 ±0.06 at 18.2 MeV. This method only depends on the coronagraph and X-ray observations at the Sun. For medium lead time forecasting the four parameters used are VTR, log10 [ISEPpeak], VCMEspace(or log10 [SXRF]), and log10 [DTII]. The correlation coefficients were 0.80 ±0.04 at 6.0 MeV and 0.84 ±0.05 at 18.2 MeV. Coronagraph observations at the Sun and solar energetic particle and DH type II burst measurements in IP space are required for this method. The medium lead time forecasting provides an average warning time of 30 ±16 h.

Analyses of ∼0.05–2 MeV Ions Associated with the 2022 February 16 Energetic Storm Particle Event Observed by Parker Solar Probe

We present analyses of 0.05–2 MeV ions from the 2022 February 16 energetic storm particle event observed by Parker Solar Probe's (PSP) IS⊙IS/EPI-Lo instrument at 0.35 au from the Sun. This event was characterized by an enhancement in ion fluxes from a quiet background, increasing gradually with time with a nearly flat spectrum, rising sharply near the arrival of the coronal mass ejection (CME)–driven shock, becoming nearly a power-law spectrum, then decaying exponentially afterward, with a rate that was independent of energy. From the observed fluxes, we determine diffusion coefficients, finding that far upstream of the shock the diffusion coefficients are nearly independent of energy, with a value of 1020 cm2 s−1. Near the shock, the diffusion coefficients are more than 1 order of magnitude smaller and increase nearly linearly with energy. We also determine the source of energetic particles, by comparing ratios of the intensities at the shock to estimates of the quiet-time intensity to predictions from diffusive shock acceleration theory. We conclude that the source of energetic ions is mostly the solar wind for this event. We also present potential interpretations of the near-exponential decay of the intensity behind the shock. One possibility we suggest is that the shock was overexpanding when it crossed PSP and the energetic particle intensity decreased behind the shock to fill the expanding volume. Overexpanding CMEs could well be more common closer to the Sun, and this is an example of such a case.

Unexpected energetic particle observations near the Sun by Parker Solar Probe and Solar Orbiter

Solar energetic particles (SEPs) from suprathermal (few keV) up to relativistic (∼few GeV) energies are accelerated at the Sun in association with solar flares and coronal mass ejection-driven shock waves. Although our knowledge of the origin, acceleration, and transport of these particles from close to the Sun through the interplanetary medium has advanced dramatically in the last 40 years, many puzzles have still remained unsolved due to the scarcity of in situ measurements well inside 1 AU. Furthermore, energetic particle intensity enhancements associated with high-speed streams or stream interaction regions (SIRs) have been routinely observed at interplanetary spacecraft near Earth orbit since the 1960s. Since only a small sample of SIR events were observed by the Helios spacecraft inside 1 AU, additional observations well inside 1 AU were also needed to further investigate the energization and transport effects of SIR-associated ions and to compare with expectations from contemporary SIR-associated particle acceleration and transport models and theories. The Solar Orbiter (SolO) and Parker Solar Probe (PSP) pioneering missions have been providing unprecedented measurements of energetic particles in the near-Sun environment. This review presents the unexpected observations of SEP and SIR-related ion events as measured by the PSP/IS⊙IS and SolO/EPD experiments, which revealed surprises that challenge our understanding.

An Analytical Model of Turbulence in Parker Spiral Geometry and Associated Magnetic Field Line Lengths

Understanding the magnetic connections from the Sun to interplanetary space is crucial for linking in situ particle observations with the solar source regions of the particles. A simple connection along the large-scale Parker spiral magnetic field is made complex by the turbulent random walk of field lines. In this paper, we present the first analytical model of heliospheric magnetic fields where the dominant 2D component of the turbulence is transverse to the Parker spiral. The 2D wave field is supplemented with a minor wave field component that has asymptotic slab geometry at small and large heliocentric distances. We show that turbulence spreads field lines from a small source region at the Sun to a 60° heliolongitudinal and heliolatitudinal range at 1 au, with a standard deviation of the angular spread of the field lines of 14°. Small source regions map to an intermittent range of longitudes and latitudes at 1 au, consistent with dropouts in solar energetic particle intensities. The lengths of the field lines are significantly extended from the nominal Parker spiral length of 1.17 au up to 1.6 au, with field lines from sources at and behind the west limb considerably longer than those closer to the solar disk center. We discuss the implications of our findings for understanding charged particle propagation and the importance of understanding the turbulence properties close to the Sun.

Multiannual Variability of the North Atlantic Pressure and Temperature Imposed by the Lower-Stratospheric Ozone

Abstract Analyses of the Northern Hemisphere’s sea level pressure, air surface temperature, and lower-stratospheric ozone during the period 1900–2019 reveal an existing coherence in their temporal variability. The coherence is heterogeneously distributed over the globe, and the patterns of ozone impact on the pressure and temperature are different. More specifically, the strongest ozone influence on the sea level pressure is found in the main “centers of action”—that is, the Aleutian low and the region of NAO formation. The ozone influence is localized mainly in the latitudinal belt 40°–75°N, where the ozone mixing ratio at 70 hPa is reduced during most of the twentieth century (relative to the first decade of the twenty-first century). This peculiarity of ozone spatial distribution we attribute to the energetic particles trapped in Earth’s radiation belts, activating ion-molecular reactions of ozone production in the region of Regener–Pfotzer ionization maximum. Consequently, the spatial–temporal variations of the lower-atmospheric ionization could be a good explanation for irregularly distributed ozone and its regionally specified impact on the climatic variables. Significance Statement We tried to understand the regional character of the Northern Hemisphere’s winter weather conditions. The latter is usually attributed to the North Atlantic Oscillation (NAO), but we actually do not know the factors impacting the NAO variability itself. We found that, at multiannual time scales, the surface pressure is only weakly related to the temperature variations, whereas its correlation with the ozone at 70 hPa is unexpectedly strong—especially in the active regions of the weather phenomena formation. We attribute the ozone variability itself to the variable intensity of energetic particles precipitating in the lower atmosphere—where they activate ion-molecular reactions producing ozone. This finding opens new horizons for understanding the regionality of atmospheric variation at different time scales.

Dynamical Coupling between Anomalous Cosmic Rays and Solar Wind in Outer Heliosphere

Voyager 2 (V2) observed that pickup ions (PUIs) and anomalous cosmic rays (ACRs) have a significant influence on the solar wind structures in the outer heliosphere. In particular, the maxima in energetic particle intensities often lagged behind the shock front, while the flow velocity in some cases featured a precursor in front of the shock. These two effects are believed to be caused by the cease of ACR injection from PUIs at large heliocentric distances, and the backward diffusion of ACRs, respectively. This paper investigates the dynamical coupling between the ACRs and the solar wind in the outer heliosphere by means of a time-dependent numerical MHD simulation, in which the ACRs are treated as a massless fluid that only contributes its pressure to the system. Two types of inner boundary conditions were used, namely a synthetic short-term shock event and an extended period of data-driven solar wind variability based on the OMNI database. The lag of the ACR pressure maximum relative to the shock front, and the extended shock precursor were reproduced by the numerical results. The increase rate of the lag is related to the corresponding diffusion coefficient and the injection efficiency from PUIs to ACRs. The model was also applied to the termination shock, where simulations likewise showed that the peak in the ACR distribution can be located a short distance downstream of the shock front, indicating that the time-dependent diffusive shock acceleration mechanism is a candidate to interpret the lag between the ACR pressure peak and the shock front observed by V2.

The Response of Saturn's Dawn Field‐Aligned Currents to Magnetospheric and Ring Current Conditions During Cassini's Proximal Orbits: Evidence for a Region 2 Response at Saturn

AbstractCassini's 2017 proximal orbits provided the opportunity to examine the auroral field‐aligned currents in the northern hemisphere dawn sector in relation to wider magnetospheric conditions. We combine three recent studies to examine the response of the dawn region auroral field‐aligned currents and the azimuthal ring currents to compressions and expansions of the Saturnian magnetosphere. For compressions of Saturn's magnetosphere resulting in tail reconnection, the currents within the downward current sheet, located equatorward of the main auroral oval, increases in strength with increasing total ring current and location of the peak downwards current moves inwards toward Saturn. While the inverse relation occurs during intervals of quiet or expanded magnetospheric conditions. During compression events there is an increase in the energetic particle intensities, in particular in the protons (35–506 keV), within the downward current region. This current system is akin to an Earth‐like “region 2” field aligned current within Saturn's magnetosphere, with tail reconnection occurring when the magnetosphere is compressed resulting in a partial nightside ring current closed by a downward current near to dawn. Within the upward current sheet, mapping to Saturn's main auroral oval, both non‐rotating subcorotating current and the rotating Planetary Period Oscillations (PPOs) currents flow. The upward current is strongly modulated by the PPOs but also increases in strength, with enhanced high‐energy protons, during intervals of magnetospheric compressions and tail reconnection. We conclude that the enhanced plasma injected into the midnight‐dawn sector during tail reconnection events results in an enhanced subcorotation current system.

Observation-based modelling of the energetic storm particle event of 14 July 2012

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.

The Effects of Magnetic Boundary on the Uniform Distribution of Energetic Particle Intensities Observed by Multiple Spacecraft

Abstract In the decay phase of solar energetic particle (SEP) events, particle intensities observed by widely separated spacecraft usually present comparable intensities (within a factor of 2–3) that evolve similarly in time. The phenomenon of SEP events is called a reservoir, which could be observed frequently in intensive gradual SEP events. In this work, we find the effects of the magnetic boundary could help to form the reservoir phenomenon in energetic proton and electron events. In the 1978 January 1 and the 2000 November 8 SEP events, we find the effects of the magnetic boundary associated with the reservoir phenomenon were observed simultaneously in the sheath of magnetic cloud/interplanetary coronal mass ejection. Based on the observations, we suggest that the effects of the magnetic boundary could be due to the magnetic mirrors and/or the small diffusion coefficients in the sheath region and they could help to form the reservoir phenomenon in both the energetic proton and electron events in some large SEP events.

Relationship between solar energetic particle intensities and coronal mass ejection kinematics using STEREO/SECCHI field of view

Solar energetic particles (SEPs) accelerated from shocks driven by coronal mass ejections (CMEs) are one of the major causes of geomagnetic storms on Earth. Therefore, it is necessary to predict the occurrence and intensity of such disturbances. For this purpose we analyzed in detail 38 non-interacting halo and partial halo CMEs, as seen by the Solar and Heliospheric Observatory/Large Angle and Spectrometric Coronagraph, generating SEPs (in > 10 MeV, > 50 MeV, and > 100 MeV energy channels) during the quadrature configuration of the Solar TErrestrial RElations Observatory (STEREO) twin spacecrafts with respect to the Earth, which marks the ascending phase of solar cycle 24 (i.e., 2009–2013). The main criteria for this selection period is to obtain height–time measurements of the CMEs without significant projection effects and in a very large field of view. Using the data from STEREO/Sun Earth Connection Coronal and Heliospheric Investigation (STEREO/SECCHI) images we determined several kinematic parameters and instantaneous speeds of the CMEs. First, we compare instantaneous CME speed and Mach number versus SEP fluxes for events originating at the western and eastern limb; we observe high correlation for the western events and anticorrelation for the eastern events. Of the two parameters, the Mach number offers higher correlation. Next we investigated instantaneous CME kinematic parameters such as maximum speed, maximum Mach number, and the CME speed and Mach number at SEP peak flux versus SEP peak fluxes. Highly positive correlation is observed for Mach number at SEP peak flux for all events. The obtained instantaneous Mach number parameters from the emperical models was verified with the start and end time of type II radio bursts, which are signatures of CME-driven shock in the interplanetary medium. Furthermore, we conducted estimates of delay in time and distance between CME, SEP, and shock parameters. We observe an increase in the delay in time and distance when SEPs reach peak flux with respect to CME onset as we move from the western to the eastern limb. Western limb events (longitude 60°) have the best connectivity and this decreases as we move towards the eastern limb. This variation is due to the magnetic connectivity from the Sun to the Earth, called the Parker spiral interplanetary magnetic field. Comparative studies of the considered energy channels of the SEPs also throw light on the reacceleration of suprathermal seed ions by CME-driven shocks that are pre-accelerated in the magnetic reconnection.

Dependence of fishbone cycle on energetic particle intensity in EAST low-magnetic-shear plasmas

The dependence of fishbone cycle on energetic particle intensity has been investigated in EAST low-magnetic-shear plasmas. It is observed that the fishbone mode growth rate, saturation amplitude as well as fishbone cycle frequency clearly increase with increasing neutral beam injection (NBI) power. Moreover, enhanced electron density and temperature perturbations as well as energetic particle loss were observed with greater injected NBI power. Simulation results using M3D-K code show that as the NBI power increases, the resonant frequency and the energy of the resonant particles become higher, and the saturation amplitude of the mode also changes, due to the non-perturbative energetic particle contribution. The relationship between the calculated energetic particle pressure ratio and fishbone cycle frequency is obtained as ${f_{\textrm{FC}}} = 2.2{(1000{\beta _{\textrm{ep,calc}}} - 0.1)^{5.9 \pm 0.5}}$ . Results consistent with the experimental observations have been achieved based on a predator–prey model.

Particle Dynamics in the Earth's Radiation Belts: Review of Current Research and Open Questions

AbstractThe past decade transformed our observational understanding of energetic particle processes in near‐Earth space. An unprecedented suite of observational systems was in operation including the Van Allen Probes, Arase, Magnetospheric Multiscale, Time History of Events and Macroscale Interactions during Substorms, Cluster, GPS, GOES, and Los Alamos National Laboratory‐GEO magnetospheric missions. They were supported by conjugate low‐altitude measurements on spacecraft, balloons, and ground‐based arrays. Together, these significantly improved our ability to determine and quantify the mechanisms that control the buildup and subsequent variability of energetic particle intensities in the inner magnetosphere. The high‐quality data from National Aeronautics and Space Administration's Van Allen Probes are the most comprehensive in situ measurements ever taken in the near‐Earth space radiation environment. These observations, coupled with recent advances in radiation belt theory and modeling, including dramatic increases in computational power, have ushered in a new era, perhaps a “golden era,” in radiation belt research. We have edited a Journal of Geophysical Research: Space Science Special Collection dedicated to Particle Dynamics in the Earth's Radiation Belts in which we gather the most recent scientific findings and understanding of this important region of geospace. This collection includes the results presented at the American Geophysical Union Chapman International Conference in Cascais, Portugal (March 2018) and many other recent and relevant contributions. The present article introduces and review the context, current research, and main questions that motivate modern radiation belt research divided into the following topics: (1) particle acceleration and transport, (2) particle loss, (3) the role of nonlinear processes, (4) new radiation belt modeling capabilities and the quantification of model uncertainties, and (5) laboratory plasma experiments.

Spreading protons in the heliosphere: a note on cross-field diffusion effects

We study how a high-speed solar wind stream embedded in a slow solar wind affects the transport and energy changes of solar energetic protons in interplanetary space, assuming different levels of cross-field diffusion. This is done using a particle transport model that computes directional particle intensities and first order parallel anisotropies in a background solar wind generated by the magnetohydrodynamic model EUHFORIA. In particular, we consider a mono-energetic 4 MeV proton injection over an extended region located at a heliographic radial distance of 0.1 AU. By using different values for the perpendicular proton mean free path, we study how cross-field diffusion may affect the energetic particle spread and intensity profiles near a high-speed solar wind stream and a corotating interaction region (CIR). We find that both a strong cross-field diffusion and a solar wind rarefaction region are capable of dispersing SEPs efficiently, producing overall low particle intensities which can in some cases prevent the SEPs from being detected in-situ, since their intensity may drop below the detected preevent intensity levels. We also discuss how accelerated particle populations form on the reverse and forward shock waves, separated by the stream interface inside the CIR. Under strong levels of cross-field diffusion, particles cross the SI and hence both accelerated particle populations merge together.

The Enhancement of the Energetic Particle Intensities in ICMEs

The Enhancement of the Energetic Particle Intensities in ICMEs

Acceleration of Solar Wind Particles by Traveling Interplanetary Shocks

Abstract The acceleration of thermal solar wind (SW) protons at spherical interplanetary shocks driven by coronal mass ejections is investigated. The SW velocity distribution is represented using κ-functions, which are transformed in response to simulated shock transitions in the fixed-frame flow speed, plasma number density, and temperature. These heated SW distributions are specified as source spectra at the shock from which particles with sufficient energy can be injected into the diffusive shock acceleration process. It is shown that for shock-accelerated spectra to display the classically expected power-law indices associated with the compression ratio, diffusion length scales must exceed the width of the compression region. The maximum attainable energies of shock-accelerated spectra are found to be limited by the transit times of interplanetary shocks, while spectra may be accelerated to higher energies in the presence of higher levels of magnetic turbulence or at faster-moving shocks. Indeed, simulations suggest that fast-moving shocks are more likely to produce very high energy particles, while strong shocks, associated with harder shock-accelerated spectra, are linked to higher intensities of energetic particles. The prior heating of the SW distribution is found to complement shock acceleration in reproducing the intensities of typical energetic storm particle (ESP) events, especially where injection energies are high. Moreover, simulations of ∼0.2–1 MeV proton intensities are presented that naturally reproduce the observed flat energy spectra prior to shock passages. Energetic particles accelerated from the SW, aided by its prior heating, are shown to contribute substantially to intensities during ESP events.

Why the Shock-ICME Complex Structure Is Important: Learning from the Early 2017 September CMEs

Abstract In the early days of 2017 September, an exceptionally energetic solar active region AR 12673 aroused great interest in the solar physics community. It produced four X class flares, more than 20 coronal mass ejections (CMEs), and an intense geomagnetic storm, for which the peak value of the Dst index reached up to −142 nT at 2017 September 8 02:00 UT. In this work, we check the interplanetary and solar source of this intense geomagnetic storm. We find that this geomagnetic storm was mainly caused by a shock-interplanetary CME (ICME) complex structure, which was formed by a shock driven by the 2017 September 6 CME propagating into a previous ICME, which was the interplanetary counterpart of the 2017 September 4 CME. To better understand the role of this structure, we conduct a quantitative analysis on the enhancement of ICME’s geoeffectiveness induced by the shock compression. The analysis shows that the shock compression enhanced the intensity of this geomagnetic storm by a factor of two. Without shock compression, there would have been only a moderate geomagnetic storm with a peak Dst value of ∼−79 nT. In addition, the analysis of the proton flux signature inside the shock-ICME complex structure shows that this structure also enhanced the solar energetic particle intensity by a factor of approximately five. These findings illustrate that the shock-ICME complex structure is a very important factor in solar physics study and space weather forecast.

Large Energetic Particle Pressures in Solar Cycles 23 and 24

We study periods of elevated energetic particle intensities observed at the L1 Sun-Earth Lagrangian point when the partial energy density associated with energetic (≥80 keV) particles (PEP) dominates that of the local magnetic field (PB) and thermal plasma populations (PPLS). These periods are not uncommon and are frequently observed prior to the passage of interplanetary (IP) shocks. Because of the significant decreases in key solar wind parameters observed during solar cycle 24 [e.g., 1], we were motivated to perform a comparative statistical analysis to determine if the occurrence rate of periods when PEP exceeded PB or PPLS, or both, differed between solar cycles 23 and 24. We find that the general decrease of PB and PPLS in solar cycle 24 was also accompanied by a general decrease of periods with elevated PEP. The result is that solar cycle 24 showed a lower number of time intervals dominated by PEP. We analyze whether these differences can be related to the properties of the IP shocks observed at L1. Incomplete datasets of shock parameters do not show significant differences between solar cycles 23 and 24 that would allow us to explain the difference in the number of periods with PEP>PB and PEP>PPLS. We analyze then the averaged plasma parameters measured in the upstream region of the shocks and find significantly lower solar wind proton temperatures and magnetic field magnitude upstream of IP shocks in solar cycle 24 compared with those in solar cycle 23. These factors, together with the lower level of solar activity, may explain the lower particle intensities in solar cycle 24 and hence the fewer events with PEP>PB and PEP>PPLS.