Advanced Composition Explorer Research Articles

Anisotropy of Density Fluctuations in the Solar Wind at 1 au

A well-known property of solar wind plasma turbulence is the observed anisotropy of the autocorrelations, or equivalently the spectra, of velocity and magnetic field fluctuations. Here we explore the related but apparently not well-studied issue of the anisotropy of plasma density fluctuations in the energy-containing and inertial ranges of solar wind turbulence. Using 10 yr (1998–2008) of in situ data from the Advanced Composition Explorer mission, we find that for all but the fastest wind category, the density correlation scale is slightly larger in directions quasi-parallel to the large-scale mean magnetic field as compared to quasi-perpendicular directions. The correlation scale in fast wind is consistent with isotropic. The anisotropy as a function of the level of correlation is also explored. We find at small correlation levels, i.e., at energy-containing scales and larger, the density fluctuations are close to isotropy for fast wind, and slightly favor more rapid decorrelation in perpendicular directions for slow and medium winds. At relatively smaller (inertial range) scales where the correlation values are larger, the sense of anisotropy is reversed in all speed ranges, implying a more “slablike” structure, especially prominent in the fast wind samples. We contrast this finding with published results on velocity and magnetic field correlations.

Comparison of Ion–Proton Differential Speed between Interplanetary Coronal Mass Ejections and Solar Wind near 1 au

The elemental abundance of interplanetary coronal mass ejections (ICMEs) and solar wind near 1 au is often adopted to represent the abundance in the corresponding coronal sources. However, the absolute abundance of heavy ions (relative to hydrogen) near 1 au might be different from the coronal abundance due to the ion–proton differential speed (V ip ). To illustrate the V ip characteristics and explore whether it influences the absolute abundance analysis for ICMEs and solar wind, we perform a statistical study on the V ip for He2+, C5+, O6+, and Fe10+ in both ICMEs and solar wind based on measurements of Advanced Composition Explorer. The results show that the V ip is negligible within ICMEs and slow solar wind (< 400 km s−1), while obvious in the intermediate (400–600 km s−1) and fast wind (> 600 km s−1). Previous studies showed that the V ip in ICMEs keeps negligible during propagation from 0.3 to 5 au, but in solar wind it increases with the decreasing heliocentric distance. Therefore, it might be questionable to infer the absolute abundance of coronal sources through in situ abundance near 1 au for solar wind. Fortunately, the ion–oxygen (O6+) differential speed (V io ) is negligible for He2+, C5+, and Fe10+ within both ICMEs and solar wind, and previous studies suggested that the V io does not vary significantly with the heliocentric distance. This indicates that various heavy ions always flow at the same bulk speed and their relative abundance (relative to oxygen) near 1 au can represent the coronal abundance for both ICMEs and solar wind.

Orientation of IMF Discontinuity Normals Across the Solar Cycles

AbstractTransient events like hot flow anomalies and foreshock bubbles are common in the Earth's foreshock. These foreshock transients may play an important role in the solar wind‐magnetosphere interaction. They typically occur when backstreaming ions accumulate at the intersection of interplanetary magnetic field (IMF) discontinuities with the bow shock. Discontinuity orientations play a key role in determining when and where transients form in the foreshock and strike the magnetopause. They also play a role in determining the amplitude, and significance, of individual transients. We investigate properties of the IMF discontinuity normals across two solar cycles. We compare Advanced Composition Explorer (ACE) IMF discontinuity observations for 2001, 2002, 2012, and 2014 (solar maximum) and 2008, 2009, 2019, and 2020 (solar minimum). We employ both the Minimum variance analysis (MVA) and Cross‐product methods to determine phase front normals. Most discontinuity normals point earthward and dawnward at longitudes ranging from 180° to 250°. A greater range of normal can be seen during solar minimum than during solar maximum. Normals lie further from the Sun‐Earth line when solar wind densities are low and velocities are high. This provides a natural explanation for the observed tendency of transients to occur for low solar wind densities and high velocities in terms of longer discontinuity interaction times with the bow shock.

Identifying Coronal Sources of L1 Solar Wind Disturbances Using the Fisk Heliospheric Magnetic Field and Potential Field Extrapolations during Three Solar Minima

The solar minima between solar cycles 22–23, 23–24, and 24–25 are the best observed minima on record. In situ solar wind and interplanetary magnetic field measurements by the Wind and ACE spacecraft at L1 with 1 hr cadence are explored using wavelet analyses for the most quiescent year during each minimum. Times of local peaks in periodicities are identified in the solar wind velocity, magnetic field components, and proton number densities. The measured radial velocities at these times are used to trace magnetic field lines to the photosphere using two models. The first is the Fisk heliospheric magnetic field that traces field lines from L1 to the photosphere. They connect exclusively to solar poles and in 88% of instances to locations of polar coronal holes (PCHs). The second model uses the Parker spiral to trace from L1 to the solar source surface and potential-field extrapolations from the source surface to the photosphere. These field lines terminate at equatorial and midlatitude coordinates, of which some are located close to coronal holes (CHs). This study connects for the first time CH signatures in the ecliptic plane at L1 with PCHs using the Fisk field. It shows how sources from both the solar equator and poles influence the solar wind at L1 and how the two models complement each other to identify these sources.

Variations of Heavy Ions in Interplanetary Shock Driven by Interplanetary Coronal Mass Ejections and Stream Interaction Regions

In the solar wind, the fluctuation of heavy ion abundance serves as a crucial physical metric. This not only mirrors the attributes of the solar wind’s originating solar region but also signifies its influence on Earth’s magnetosphere. Utilizing data from the Advanced Composition Explorer satellite, this investigation scrutinizes heavy ion variations in stream interaction region (SIR)- and interplanetary coronal mass ejection (ICME)-driven shocks. We further delineate the disparities in heavy ion fluctuations between these two types of interplanetary shocks across diverse solar activity cycles. Our findings reveal that ICME-driven shocks typically manifest elevated shock velocities and magnetic field strengths relative to their SIR-driven counterparts. Additionally, heavy ion abundance ratios, such as C6+/O4+, O7+/O6+, He/O, Si/O, and Fe/O, are consistently higher in ICME-driven shocks than in SIR-driven shocks. During varying solar activity cycles, these ratios surge postarrival of ICME-driven shocks. At solar maximum, these elevated ratios persist, whereas they revert to baseline levels swiftly during solar minimum. For SIR-driven shocks, the alteration in heavy ion abundance ratios is comparatively subdued, yet a noteworthy correlation with the solar activity cycle is evident.

Forecasting solar wind disturbances and geomagnetic storms from ground-based cosmic ray measurements

This work reports on the methods developed by our group at ShICRA SB RAS to forecast negative manifestations of space weather from ground-based measurements of cosmic rays. Such manifestations are registered on the Earth in the form of decreases in galactic cosmic rays known as the Forbush effects, various types of geomagnetic and ionospheric disturbances, as well as auroras. Only the latters are directly visible to the naked eye, while the rest can only be detected with the help of various instruments. These occurrences are all caused by passage through the Earth’s orbit, such as the substantial influxes of charged particles from both solar and interplanetary sources, interplanetary shock waves, ejections of solar material and high-speed solar wind streams, all of which are attributed to the level of solar flares and coronal activity. The state of near-Earth space weather is determined by their presence. To monitor this state, we use data collected from ground-based cosmic ray stations and employ multiple methods to analyse these measurements. Additionally, we incorporate data from the Dst-index of the geomagnetic field and measurements of the interplanetary environmental parameters gathered by ACE, WIND, SOHO and DSCOVR spacecraft at the L1 libration point to confirm events. This has led to the development of techniques for predicting terrestrial effects of space weather in the short-term (1-3 days). Our findings suggest that it is feasible to predict the occurrence of severe geomagnetic storms in real-time through the utilisation of solely ground-based measurements from cosmic ray stations.

Zone of Preferential Heating for Minor Ions in the Solar Wind

The properties of minor ions in the solar wind are important indicators of the state of the solar corona and the heating processes it undergoes as it expands. Evidence of preferential heating of minor ions has been observed in the solar wind, and is believed to be active up to a certain radial boundary, beyond which thermalization due to Coulomb collisions predominates. Building on previous works that calculated the location of this boundary for alpha particles, this work calculates the outer radial boundary of the zone of preferential heating for selected heavy ions in the solar wind. This analysis uses ion data from the Solar Wind Ion Composition Spectrometer and proton data from the Solar Wind Electron, Proton, and Alpha Monitor aboard the Advance Composition Explorer spanning the years from 1998 through to 2011. Observations of proton and ion temperatures, velocities, and densities, and fixed parameters derived from temperature, density, and velocity scaling laws are used in a model function to predict the radial boundary and excess temperature at the boundary via reduction of the χ 2/degrees of freedom statistic. In this study, the values of the radial boundary of the preferential heating zone were quite high when compared to what was previously found for alpha particles, but a clear scaling relationship between excess temperature and ion properties was observed.

A Comprehensive Comparison of Various Galactic Cosmic-Ray Models to the State-of-the-art Particle and Radiation Measurements

Galactic cosmic rays (GCRs) are the slowly varying background energetic particles that originate outside the solar system, are modulated by the heliospheric magnetic field, and pose ongoing radiation hazards to deep space exploration missions. To assess the potential radiation risk, various models have been developed to predict the GCR flux near Earth based on propagation theories and/or empirical functions. It is essential to benchmark these models by validating against the state-of-the-art measurements. In this work, a comprehensive model–observation comparison of the energy-dependent particle flux has been performed, by combining five typical GCR models and observational data from the Cosmic Ray Isotope Spectrometer on board the Advanced Composition Explorer spacecraft at relatively lower energies and data from the Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics and Alpha Magnetic Spectrometer at higher energies. The analysis shows that, out of the five models investigated in this study, the optimal model, characterized by minimal relative difference or reduced chi-square divergence from measurements, depends on the particle type, energy range, and epoch of interest. Furthermore, a silicon slab is applied to compute the absorbed dose rate using conversion factors applied to GCR model outputs, and the results are compared to measurements from the Cosmic Ray Telescope for the Effects of Radiation. The comparisons in this paper have implications for the strengths and limitations of individual GCR models, advance our comprehension of the underlying GCR transport mechanisms, and also have strong application aspects for mitigating space radiation risks.

Multispacecraft Energetic Particle Enhancements Associated with a Single Corotating Interaction Region

The radial evolution of particles accelerated at corotating interaction regions (CIRs) is not fully understood, particularly the distance range over which this particle acceleration occurs and how the energy spectra are modulated by transport through the inner heliosphere. Here, we present observations of energetic proton enhancements associated with a CIR observed by Parker Solar Probe on 2021 April 25 during the inbound leg of its orbit near ∼46 R s (∼0.21 au). The CIR is identified at additional spacecraft (Solar Terrestrial Relations Observatory, STEREO-A; Solar Orbiter, SolO; and Advanced Composition Explorer, ACE) using a corotation time delay estimation, and energetic proton spectra from each spacecraft are compared. We find that (1) energetic protons are observed near 46 R s streaming sunward ahead of the CIR; (2) the CIR persists for at least one solar rotation and the corresponding energetic proton enhancements are observed at STEREO-A, SolO, and ACE; and (3) the proton energy spectrum is steeper near the Sun and hardens near 1 au. This observation presents the closest in situ energetic particle observation of a CIR to the Sun ever recorded. Results presented here suggest that particles can be accelerated by CIR structures within 1 au and these particles can penetrate very deep into the inner heliosphere.

3D pressure-corrected ballistic extrapolation of solar wind speed in the inner heliosphere

Solar wind parameters at different locations in the inner heliosphere can be estimated using various solar wind extrapolation methods. The simple ballistic method extrapolates solar wind parameters from the point of measurement to a chosen heliospheric position by assuming that major solar wind structures are persistent and arrive relatively unaltered to the target position. The method considers the rotation period of the Sun while assuming a constant solar wind speed during radial propagation. We improve the simple ballistic model by considering the interaction between the slow and the fast solar wind with a pressure correction during the propagation. Instead of extrapolating from the position of a single spacecraft, we apply this pressure-corrected ballistic method to 2D speed maps of the solar source surface available from solar coronal models to determine the solar wind speed in the inner heliosphere in 3D, between latitudes of ±50°. We also take into account the effects of the solar differential rotation in our model. Our method is simple and fast, and it can be applied to different source surface datasets. The results of our model are validated with in situ data from the ACE spacecraft. We find that the pressure-corrected ballistic method can give accurate predictions of the solar wind in 3D.

On the Role of Alfvénic Fluctuations as Mediators of Coherence within Interplanetary Coronal Mass Ejections: Investigation of Multi-spacecraft Measurements at 1 au

Interplanetary coronal mass ejections (ICMEs) are defined as “coherent” if they are capable of responding to external perturbations in a collective manner. This implies that information must be able to propagate across ICME structures, and if this is not the case, single-point in situ measurements cannot be considered as indicative of global ICME properties. Here, we investigate the role of Alfvénic fluctuations (AFs) as mediators of ICME coherence. We consider multipoint magnetic field and plasma measurements of 10 ICMEs observed by the ACE and Wind spacecraft at 1 au at longitudinal separations of 0.5°–0.7°. For each event, we analyze the Alfvénicity in terms of the residual energy and cross helicity of fluctuations, and the coherence in terms of the magnetic correlation between Wind and ACE. We find that ∼65% and 90% of ICME sheaths and magnetic ejecta (MEs), respectively, present extended AFs covering at least 20% of the structure. Cross helicity suggests AFs of solar and interplanetary origin may coexist in the ICME population at 1 au. AFs are mainly concentrated downstream of shocks and in the back of MEs. The magnetic field is poorly correlated within sheaths, while the correlation decreases from the front to the back of the MEs for most magnetic field components. AFs are also associated with lower magnetic field correlations. This suggests either that ICME coherence is not mediated by Alfvén waves, implying that the coherence scale may be smaller than previously predicted, or that the magnetic field correlation is not a measure of coherence.

Analysis of the Motion of Solar Wind (SW) and Other Solar Activities Impacts on the Earth

Solar wind (SW), the powerful outflow of charged particles from the Sun, has a significant impact on Earth's magnetosphere, ionosphere, and atmospheric processes. This paper examines the motion of SW and its effects on Earth, focusing on changes such as slow and fast SWs and their impacts on geomagnetic storms, ionospheric disturbances, and precipitation patterns. Observations from spacecraft, including the Solar and Heliospheric Observatory (SOHO), the Advanced Composition Explorer (ACE), and the Wind spacecraft, have provided valuable data on solar wind properties. However, there are limitations in spatial coverage and the measurement of internal SW properties. Accurately predicting SW behavior remains a challenge. Future research should focus on developing advanced observational techniques, utilizing multiple spacecraft, and improving modeling and data analysis approaches. The forthcoming expeditions, specifically the Solar Orbiter and Parker Solar Probe, are predicted to provide novel understandings into solar activity. Enhancing our understanding of solar activity and improving space weather predictions are crucial for mitigating the potential adverse effects of SW on electricity, communication, and other human activities. This research contributes to advancements in space weather forecasting and the reliable and safe operation of our technological infrastructure.

Solar Energetic Particle Track-production Rates at 1 au: Comparing In Situ Particle Fluxes with Lunar Sample-derived Track Densities

Heavy (Z > 26) solar energetic particles (SEPs) with energies ∼1 MeV nucleon−1 are known to leave visible damage tracks in meteoritic materials. The density of such “solar flare tracks” in lunar and asteroidal samples has been used as a measure of a sample’s exposure time to space, yielding critical information on planetary space weathering rates, the dynamics and lifetimes of interplanetary dust grains, and the long-term history of solar particle fluxes. Knowledge of the SEP track accumulation rate in planetary materials at 1 au is critical for properly interpreting observed track densities. Here, we use in situ particle observations of the 0.50−3.0 MeV nuc−1 Fe-group SEP flux taken by NASA’s Advanced Composition Explorer (ACE) to calculate a flux of track-inducing particles at 1 au of 6.0 × 105 cm−2 yr−1 str−1. Using the observed energy spectrum of Fe-group SEPs, we find that the depth distribution of SEP-induced damage tracks inferred from ACE measurements matches closely to that recently measured in lunar sample 64455; however, the magnitude of the ACE-inferred rate is approximately 25× higher than that observed in the lunar sample. We discuss several hypotheses for the nature of this discrepancy, including inefficiencies in track formation, thermal annealing of lunar samples, erosion via space weathering processing, and variations in the SEP flux at the Moon, yet find no satisfactory explanation. We encourage further research on both the nature of SEP track formation in meteoritic materials and the flux of Fe-group SEPs at the lunar surface in recent and geologic times to resolve this discrepancy.

Scope and limitations of ad hoc neural network reconstructions of solar wind parameters

Context.Solar wind properties are determined by the conditions of their solar source region and transport history. Solar wind parameters, such as proton speed, proton density, proton temperature, magnetic field strength, and the charge state composition of oxygen, are used as proxies to investigate the solar source region of the solar wind. The solar source region of the solar wind is relevant to both the interaction of this latter with the Earth’s magnetosphere and to our understanding of the underlying plasma processes, but the effect of the transport history of the wind is also important. The transport and conditions in the solar source region affect several solar wind parameters simultaneously. Therefore, the typically considered solar wind properties (e.g., proton density and oxygen charge-state composition) carry redundant information. Here, we are interested in exploring this redundancy.Aims.The observed redundancy could be caused by a set of hidden variables that determine the solar wind properties. We test this assumption by determining how well a (arbitrary, non-linear) function of four of the selected solar wind parameters can model the fifth solar wind parameter. If such a function provided a perfect model, then this solar wind parameter would be uniquely determined from hidden variables of the other four parameters and would therefore be redundant. If no reconstruction were possible, this parameter would be likely to contain information unique to the parameters evaluated here. In addition, isolating redundant or unique information contained in these properties guides requirements for in situ measurements and development of computer models. Sufficiently accurate measurements are necessary to understand the solar wind and its origin, to meaningfully classify solar wind types, and to predict space weather effects.Methods.We employed a neural network as a function approximator to model unknown, arbitrary, non-linear relations between the considered solar wind parameters. This approach is not designed to reconstruct the temporal structure of the observations. Instead a time-stable model is assumed and each point of measurement is treated separately. This approach is applied to solar wind data from the Advanced Composition Explorer (ACE). The neural network reconstructions are evaluated in comparison to observations, and the resulting reconstruction accuracies for each reconstructed solar wind parameter are compared while differentiating between different solar wind conditions (i.e., different solar wind types) and between different phases in the solar activity cycle. Therein, solar wind types are identified according to two solar-wind classification schemes based on proton plasma properties.Results.Within the limits defined by the measurement uncertainties, the proton density and proton temperature can be reconstructed well. Each parameter was evaluated with multiple criteria. Overall proton speed was the parameter with the most accurate reconstruction, while the oxygen charge-state ratio and magnetic field strength were most difficult to recover. We also analysed the results for different solar wind types separately and found that the reconstruction is most difficult for solar wind streams preceding and following stream interfaces.Conclusions.For all considered solar wind parameters, but in particular the proton density, proton temperature, and the oxygen charge-state ratio, parameter reconstruction is hindered by measurement uncertainties. The proton speed, while being one of the easiest to measure, also seems to carry the highest degree of redundancy with the combination of the four other solar wind parameters. Nevertheless, the reconstruction accuracy for the proton speed is limited by the large measurement uncertainties on the respective input parameters. The reconstruction accuracy of sector reversal plasma is noticeably lower than that of streamer belt or coronal hole plasma. We suspect that this is a result of the effect of stream interaction regions, which strongly influence the proton plasma properties and are typically assigned to sector reversal plasma. The fact that the oxygen charge-state ratio –a non-transport-affected property– is difficult to reconstruct may imply that recovering source-specific information from the transport-affected proton plasma properties is challenging. This underlines the importance of measuring the heavy ion charge-state composition.

Variability of Interplanetary Shock and Associated Energetic Particle Properties as a Function of the Time Window Around the Shock

We study the effect of sampling windows on derived shock and associated energetic storm particle (ESP) properties in 296 fast-forward interplanetary shocks using Advanced Composition Explorer measurements at 1 au between 1998 February and 2013 August. We vary the time windows from 2 minutes to 20 minutes for the shock properties and from 2 minutes to 540 minutes for ESP properties. Variability is quantified by the median absolute deviation statistic. We find that the magnetic, density, and temperature compression ratios vary from their median values by 17.03%, 20.05%, and 25.91%, respectively; shock speed by 16.26%; speed jump by 45.46%; Alfvénic Mach number by 31.53%; and shock obliquity by 24.25%. Spectral indices in the 2 minute–540 minute windows downstream of the shock vary from the median value of 1.79 by 26.05% and by 30.53% from the 1.70 median value upstream of the shock. Similarity of ESP spectral indices upstream and downstream of the shock suggest that these ESP populations are likely locally accelerated at the shock. Furthermore, we find that for a moving sampling window around the shock, values for the density ratio hold for ∼10 minutes; the magnetic ratio and shock speed jump hold for ∼30 minutes and ∼60 minutes, respectively. Fixing the upstream window to 2 minutes and moving only in the downstream direction, the density ratio holds for ∼60 minutes downstream, magnetic ratio holds for ∼30 minutes, and the shock speed jump holds for ∼110 minutes. Beyond these time windows, derived shock properties are no longer representative of shock properties. These results provide constraints for modeling and forecasting efforts of shock and ESP-associated properties.

Behaviour of 27-Day and 13.5-Day Periodicities in Galactic Cosmic Particles as Observed by Spacecraft and Neutron Monitors During Different Solar Polarity Cycles

An analysis has been made of the behaviour of the 27-day and 13.5-day periodicities in proton, C, and O galactic-cosmic-ray (GCR) particles at different energies as observed by the Advanced Composition Explorer (ACE) and SOHO (Solar and Heliospheric Observatory) spacecraft during both Solar Cycles 23 and 24. In addition, the behaviour of the 27-day and 13.5-day periods in the solar-wind-modulation parameter zeta = BIMF × VSW has been investigated during the same time interval to determine the existence of a possible solar-polarity dependence. Ground-based neutron-monitor (NM) observations, corresponding to different rigidity cutoff [RC] parameters, were also studied to determine the temporal behaviour of both the 27-day and 13.5-day periods during Cycles 23 and 24, revealing a statistically significant solar-polarity correlation. The Lomb–Scargle periodogram technique has been employed to extract spectral information from the above-mentioned observations for each individual year from 2001 – 2009 (Cycle 23) and 2010 – 2019 (Cycle 24). Daily mean energetic ACE and SOHO particle measurements are used to identify how both the 27-day and 13.5-day periodicities vary during each individual year during these cycles as a function of particle mean energy. This spectral analysis of proton, C, and O galactic-cosmic-particle data at different energies revealed that both the 27-day and 13.5-day periods are stronger during the minimum of Solar Cycle 24/25 when A > 0 (solar dipole pointing North) in comparison to the minimum of Cycle 23/24 when A < 0 (solar dipole pointing South) at certain energy levels. This showed a particularly strong energy-dependent behaviour for both periodicities. This article reports for the first time an annual time- and energy-dependent behaviour of both the 27-day and 13.5-day periodicities in daily-mean galactic cosmic particles observed by spacecraft and ground-based neutron monitors during consecutive Solar Cycles 23 and 24, corresponding to opposite solar-magnetic-field orientations. Periodicity behaviour in heliospheric solar-wind data corroborate these results in general.

Annual Variations in the Near-Earth Solar Wind

Earth’s orbit and rotation produces systematic variations in geomagnetic activity, most notably via the changing orientation of the dayside magnetospheric magnetic field with respect to the heliospheric magnetic field (HMF). Aside from these geometric effects, it is generally assumed that the solar wind in near-Earth is uniformly sampled. But systematic changes in the intrinsic solar wind conditions in near-Earth space could arise due to the annual variations in Earth heliocentric distance and heliographic latitude. In this study, we use 24 years of Advanced Composition Explorer data to investigate the annual variations in the scalar properties of the solar wind, namely the solar wind proton density, the radial solar wind speed and the HMF intensity. All parameters do show some degree of systematic annual variation, with amplitudes of around 10 to 20%. For HMF intensity, the variation is in phase with the Earth’s heliocentric distance variation, and scaling observations for distance largely explains the observed variation. For proton density and solar wind speed, however, the phase of the annual variation is inconsistent with Earth’s heliocentric distance. Instead, we attribute the variations in speed and density to Earth’s heliographic latitude variation and systematic sampling of higher speed solar wind at higher latitudes. Indeed, these annual variations are most strongly ordered at solar minimum. Conversely, combining scalar solar wind parameters to produce estimates of dynamic pressure and potential power input to the magnetosphere results in solar maximum exhibiting a greater annual variation, with an amplitude of around 40%. This suggests Earth’s position in the heliosphere makes a significant contribution to annual variations in space weather, in addition to the already well-studied geometric effects.

Comparing Energetic Storm Particle Heavy-ion Properties in Solar Cycles 23 and 24

We examine variations in energetic storm particle (ESP) heavy-ion average intensities and energy spectra between ∼0.1 and 75 MeV nucleon−1 at coronal mass ejection (CME)–driven interplanetary shocks for events observed at the ACE spacecraft. We compare ESP events observed during the weaker solar cycle (SC) 24 and the relatively stronger SC 23 to investigate any effects on the strength of an SC, including the associated transient events, on ESP properties at 1 au. We find that the number of clearly defined heavy-ion ESP events at ACE during SC 23 is about twice that observed during SC 24 (76 versus 41). The average transit speed of the driving interplanetary CMEs (ICMEs) at 1 au is 20% higher during SC 23 than during SC 24 (859.4 km s−1 versus 729.1 km s−1). The correlation of ESP average intensities with ICME speeds shows that lower-speed ICMEs in SC 24 can be as efficient as the higher-speed events in SC 23 at producing ESPs below 2 MeV nucleon−1. The distribution and magnitude of the average intensities for energies below ∼1 MeV nucleon−1 are consistent between both SCs. However, events with intensity enhancements at higher energies (>∼10 MeV nucleon−1) are more frequent and their intensity distributions are harder for SC 23, resulting in an increase in the rollover energy (E 0) for their spectra profiles. This suggests more efficient ESP acceleration at >10 MeV nucleon−1 during SC 23.

Solar Cycle Variation of 0.3–1.29 MeV Nucleon−1 Heavy Ion Composition During Quiet Times Near 1 au in Solar Cycles 23 and 24

We report on the annual variation of quiet-time suprathermal ion composition for C through Fe using Advanced Composition Explorer (ACE)/Ultra-Low Energy Isotope Spectrometer data over the energy range 0.3–1.28 MeV nuc−1 from 1998 through 2019, covering solar cycle 23's rising phase through Solar Cycle 24's declining phase. Our findings are: (1) quiet-time suprathermal abundances resemble CIR-associated particles during solar minima; (2) quiet-time suprathermals are M/Q fractionated in a manner that is consistent with M/Q fractionation in large gradual solar energetic particle events (GSEP) during solar maxima; and (3) variability within the quiet-time suprathermal pool increases as a function of M/Q and is consistent with the analogous variability in GSEP events. From these observations, we infer that quiet-time suprathermal ions are remnants of CIRs in solar minima and GSEP events in solar maxima. Coincident with these results, we also unexpectedly show that S behaves like a low FIP ion in the suprathermal regime, and is therefore drawn from low FIP solar sources.

Distributions of Mesoscale Periodic Structures in the Elemental and Ionic Composition of the Solar Wind

Multiple statistical and event studies based on in situ observations have shown that the solar wind contains mesoscale (∼ 100 – 10,000 Mm) periodic structures in the proton number density. Remote observations of such structures and event studies of concurrent variations in composition have demonstrated that they can form in the solar atmosphere and be preserved while advecting outwards through the heliosphere. Viall, Kepko, and Spence (J. Geophys. Res. (Space Phys.)113, A07101, 2008; J. Geophys. Res. (Space Phys.)114, A01201, 2009) and Kepko, Viall, and Wolfinger (J. Geophys. Res. (Space Phys.)125(8), e28037, 2020) have reported that periodic proton density structures preferentially occur at specific radial length scales and have published their distributions from Wind measurements near L1. Here, we conduct a statistical study of 14 years (1998 – 2011) of 12-minute composition data measured by the Solar Wind Ion Composition Spectrometer instrument aboard the Advanced Composition Explorer spacecraft. We found that the elemental and ionic composition also contain statistically significant mesoscale periodic structures and, for the first time, present occurrence distributions for elemental abundance ratios with low, intermediate, and high first ionization potentials as well as for key solar wind charge states. These distributions set important constraints on solar wind formation in general and the formation of periodic mesoscale solar wind structures specifically, as the elemental and ionic composition are known to be determined at the Sun and to not evolve during advection.