Solar Wind Ion Composition Spectrometer Research Articles

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.

Neural Networks for Operational SYM‐H Forecasting Using Attention and SWICS Plasma Features

AbstractIn this work, we present an Artificial Neural Network for operational forecasting of the SYM‐H geomagnetic index up to 2 hr ahead using the Interplanetary Magnetic Field, the solar wind plasma features and previous SYM‐H values. Former works that forecast the SYM‐H index use data measured by ACE, in particular from the MAG and SWEPAM instruments. However, the plasma data present a high amount of missing samples. This issue has been addressed in the literature, often using linear interpolation, which leads to a non‐accurate data reconstruction, specially when the features are missing during the most intense periods of a geomagnetic storm. To overcome that issue, we use ACE's Solar Wind Ion Composition Spectrometer (SWICS) data to fill the missing plasma features. To validate this technique, we compare the results of our forecasting model trained using plasma features in two ways: only using SWEPAM and performing linear interpolation and using SWICS to fill the missing values in SWEPAM. Then, both models are evaluated in an operational scenario, when only SWEPAM data are available and interpolation can only be performed if the missing values are surrounded by valid measurements. In both cases, our model outperforms the current literature forecasting the SYM‐H one and 2 hr ahead, yielding the best results when the training has been done using the data completed using SWICS measurements.

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.

Bulk Properties of Pickup Ions Derived from the Ulysses Solar Wind Ion Composition Spectrometer Data

Nonthermal, pickup ions (PUIs) represent an energetic component of the solar wind (SW). While a number of theoretical models have been proposed to describe the PUI flow, of major importance are in situ measurements providing us with the vital source of model validation. The Solar Wind Ion Composition Spectrometer (SWICS) instrument on board the Ulysses spacecraft was specifically designed for this purpose. Zhang et al. proposed a new, accurate method for the derivation of ion velocity distribution function in the SW frame on the basis of count rates collected by SWICS. We calculate the moments of these distribution functions for protons (H+) and He+ ions along the Ulysses trajectory for a period of 2 months including the Halloween 2003 solar storm. This gives us the time distributions of PUI density and temperature. We compare these with the results obtained earlier for the same interval of time, in which the ion spectra are converted to the SW frame using the narrow-beam approximation. Substantial differences are identified, which are of importance for the interpretation of PUI distributions in the 3D, time-dependent heliosphere. We also choose one of the shocks crossed by Ulysses during this time interval and analyze the distribution functions and PUI bulk properties in front of and behind it. The results are compared with the test-particle calculations and diffusive shock acceleration theory.

Periodic Solar Wind Structures Observed in Measurements of Elemental and Ionic Composition in situ at L1

Mesoscale periodic structures observed in solar wind plasma serve as an important diagnostic tool for constraining the processes that govern the formation of the solar wind. These structures have been observed in situ and in remote data as fluctuations in proton and electron density. However, only two events of this type have been reported regarding the elemental and ionic composition. Composition measurements are especially important in gaining an understanding of the origin of the solar wind as the composition is frozen into the plasma at the Sun and does not evolve as it advects through the heliosphere. Here, we present the analysis of four events containing mesoscale periodic solar wind structure during which the Iron and Magnesium number density data, measured by the Solar Wind Ion Composition Spectrometer (SWICS) on board the Advanced Composition Explorer spacecraft, are validated at statistically significant count levels. We use a spectral analysis method specifically designed to extract periodic signals from astrophysical time series and apply it to the SWICS 12 minute native resolution data set. We find variations in the relative abundance of elements with low first ionization potential, mass dependencies, and charge state during time intervals in which mesoscale periodic structures are observed. These variations are linked to temporal or spatial variations in solar source regions and put constraints on the solar wind formation mechanisms that produce them. Techniques presented here are relevant for future, higher-resolution studies of data from new instruments such as Solar Orbiter’s Heavy Ion Sensor.

Suprathermal ions from coronal holes at 1 au in solar cycles 23 and 24: dependence of ion abundances on solar wind speed

ABSTRACT The relative abundances of thermal and suprathermal C, O, and Fe ions are analysed and compared in solar wind streams from near-equatorial coronal holes (CHs) during quiet periods of nearly two solar cycle minima. Ion fluxes with energies of ∼0.04–2 MeV per nucleon are studied using data from the Ultra Low Energy Isotope Spectrometer (ULEIS) instrument aboard the Advanced Composition Explorer (ACE) spacecraft together with thermal ions in the fast and slow (Maxwellian) solar wind using data from the Solar Wind Ion Composition Spectrometer (SWICS) instrument aboard ACE. The analysis was carried out for quiescent periods in 2006–2012 and 2015–2017 when solar wind flows from near-equatorial CHs were detected at 1 au. Near the minimum of SC23, although they displayed large variability, the C/O and Fe/O ratios of suprathermal ions were, on average, near the corresponding relative abundances of the solar wind. During the decreasing solar activity phase of SC24, suprathermal Fe/O ratios matched those of solar wind ions from CHs. In both cycles, the thermal and suprathermal Fe/O ratios exhibited a similar character of dependence on maximum solar wind speed. Our results suggest that the sources of suprathermal ions from CHs in low-solar activity periods are accelerated solar wind thermal ions. The thermal and suprathermal Fe/O ratios were found to be higher in 2015–2017 in comparison to the ratios measured in 2006–2010. This difference can be attributed to that the second quiet period was selected at times when solar activity has not reached its minimum of SC24 yet.

Determination of Plasma, Pickup Ion, and Suprathermal Particle Spectrum in the Solar Wind Frame of Reference

Abstract Particle spectra directly measured by a plasma or low-energy particle experiment on spacecraft often contain instrumental effects due to a limited field of view or angular resolution. It is because the particle distribution function at low energies is highly anisotropic in the spacecraft frame of reference, in which the measurements are made. In this paper, we present a new mathematical method of transforming the particle spectrum to the solar wind frame of reference, where the particle distribution can be assumed to be nearly isotropic. The transformed particle spectrum allows us to investigate the properties of the solar wind, pickup ions, and suprathermal particles without concern for instrumental effects. We apply the method to the measurements made by the Solar Wind Ion Composition Spectrometer on Ulysses. Our results demonstrate that the transformed spectrum can improve the determination of the solar wind density and temperature from the previously published methods. A brief survey of Ulysses data shows that suprathermal ions in the slow solar wind frequently display a velocity distribution very close to the v −5 power law.

Evolution of an equatorial coronal hole structure and the released coronal hole wind stream: Carrington rotations 2039 to 2050

The Sun is a highly dynamic environment that exhibits dynamic behavior on many different timescales. In particular, coronal holes exhibit temporal and spatial variability. Signatures of these coronal dynamics are inherited by the coronal hole wind streams that originate in these regions and can effect the Earth's magnetosphere. Both the cause of the observed variabilities and how these translate to fluctuations in the in situ observed solar wind is not yet fully understood. During solar activity minimum the structure of the magnetic field typically remains stable over several Carrington rotations (CRs). But how stable is the solar magnetic field? Here, we address this question by analyzing the evolution of a coronal hole structure and the corresponding coronal hole wind stream emitted from this source region over 12 consecutive CRs in 2006. To this end, we link in situ observations of Solar Wind Ion Composition Spectrometer (SWICS) onboard the Advanced Composition Explorer (ACE) with synoptic maps of Michelson Doppler imager (MDI) on the Solar and Heliospheric Observatory (SOHO) at the photospheric level through a combination of ballistic back-mapping and a potential field source surface (PFSS) approach. Together, these track the evolution of the open field line region that is identified as the source region of a recurring coronal hole wind stream. We find that the shape of the open field line region and to some extent also the solar wind properties are influenced by surrounding more dynamic closed loop regions. We show that the freeze-in order can change within a coronal hole wind stream on small timescales and illustrate a mechanism that can cause changes in the freeze-in order. The inferred minimal temperature profile is variable even within coronal hole wind and is in particular most variable in the outer corona.

Suprathermal helium in corotating interaction regions: combined observations from SOHO/CELIAS/STOF and ACE/SWICS

Context. Energetic particle enhancements that are associated with corotating interaction regions (CIRs) are typically believed to arise from the sunward propagation of particles that are accelerated by CIR-driven shocks beyond 1 AU. It is expected that these sunward-travelling particles will lose energy and scatter, resulting in a turnover of the energy spectra below ~0.5 MeV/nuc. However, the turnover has not been observed so far, suggesting that the CIR-associated low-energy suprathermal ions are accelerated locally close to the observer. Aims. We investigate the variability of suprathermal particle spectra from CIR to CIR as well as their evolution and variation as the observer moves away from the rear shock or wave. Methods. Helium data in the suprathermal energy range from the Solar and Heliospheric Observatory/Charge, Element, and Isotope Analysis System/Suprathermal Time-of-Flight (SOHO/CELIAS/STOF) were used for the spectral analysis and were combined with data from the Advanced Composition Explorer/ Solar Wind Ion Composition Spectrometer (ACE/SWICS) in the solar wind energies. Results. We investigated sixteen events: nine clean CIR events, three CIR events with possible contamination from upstream ion events or solar energetic particles (SEPs), and four events that occurred during CIR periods that were dominated by SEPs. Six of the nine clean CIR events showed possible signs of a turnover between ~10−40 keV/nuc in the fast solar wind that trails the compression regions. Three of them even showed this behaviour inside the compressed fast wind. The turnover part of the spectra became flatter and shifted from lower to higher energies with increasing connection distance to the reverse shock. The remaining three clean events showed continuous power-law spectra in both the compressed fast wind and fast wind regions, that is, the same behaviour as reported from previous observations. The spectra of the seven remaining events are more variable, that is, they show power law, turnover, and a superposition of these two shapes.

Observations of high and low Fe charge states in individual solar wind streams with coronal-hole origin

The solar wind originating from coronal holes is comparatively well-understood and is characterized by lower densities and average charge states compared to the so-called slow solar wind. Except for wave perturbations, the average properties of the coronal-hole solar wind are passably constant. In this case study, we {ocus on observations of the Solar Wind Ion Composition Spectrometer (SWICS) on the Advanced Composition Explorer (ACE) of individual streams of coronal-hole solar wind that illustrate that although the O and C charge states are low in coronal-hole wind, the Fe charge distribution is more variable. In particular, \markmeold{we illustrate that the Fe charge states in coronal-hole solar wind are frequently as high as in slow solar wind. We selected individual coronal-hole solar wind} streams based on their collisional age as well as their respective O and C charge states and analyzed their Fe charge-state distributions. Additionally, with a combination of simple ballistic back-mapping and the potential field source surface model, transitions between streams with high and low Fe charge states were mapped back to the photosphere. The relative frequency of high and low Fe charge-state streams is compared for the years 2004 and 2006. We found several otherwise typical coronal-hole streams that include Fe charge states either as high as or lower than in slow solar wind. Eight such transitions in 2006 were mapped back to equatorial coronal holes that were either isolated or connected to the northern coronal-hole. Attempts to identify coronal structures associated with the transitions were so far inconclusive.

Comparison of algorithms for determination of solar wind regimes

AbstractThis 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.

Composition of inner-source heavy pickup ions at 1 AU: SOHO/CELIAS/CTOF observations

Context. Pickup ions in the inner heliosphere mainly originate in two sources, one interstellar and one in the inner solar system. In contrast to the interstellar source that is comparatively well understood, the nature of the inner source has not been clearly identified. Former results obtained with the Solar Wind Ion Composition Spectrometer on-board the Ulysses spacecraft revealed that the composition of inner-source pickup ions is similar, but not equal, to the elemental solar-wind composition. These observations su ered from very low counting statistics of roughly one C + count per day. Aims. Because the composition of inner-source pickup ions could lead to identifying their origin, we used data from the Charge-TimeOf-Flight sensor on-board the Solar and Heliospheric Observatory. It o ers a large geometry factor that results in about 100 C + counts per day combined with an excellent mass-per-charge resolution. These features enable a precise determination of the inner-source heavy pickup ion composition at 1 AU. To address the production mechanisms of inner-source pickup ions, we set up a toy model based on the production scenario involving the passage of solar-wind ions through thin dust grains to explain the observed deviations of the inner-source PUI and the elemental solar-wind composition. Methods. An in-flight calibration of the sensor allows identification of heavy pickup ions from pulse height analysis data by their mass-per-charge. A statistical analysis was performed to derive the inner-source heavy pickup ion relative abundances of N + , O + , Ne + , Mg + , Mg 2+ , and Si + compared to C + . Results. Our results for the inner-source pickup ion composition are in good agreement with previous studies and confirm the deviations from the solar-wind composition. The large geometry factor of the Charge-Time-of-Flight sensor even allowed the abundance ratios of the two most prominent pickup ions, C + and O + , to be investigated at varying solar-wind speeds. We found that the O + /C + ratio increases systematically with higher solar-wind speeds. This observation is an unprecedented feature characterising the production of inner-source pickup ions. Comparing our observations to the toy model results, we find that both the deviation from the solar-wind composition and the solar-wind-speed dependent O + /C + ratio can be explained.

EVIDENCE FOR LOCAL ACCELERATION OF SUPRATHERMAL HEAVY ION OBSERVATIONS DURING INTERPLANETARY CORONAL MASS EJECTIONS

Suprathermal particles are an important seed population for a variety of energetic particles found throughout the heliosphere, but their origin is in debate. We present, for the first time, high-cadence observations of suprathermal heavy ions during interplanetary coronal mass ejections (ICMEs), from the Suprathermal Ion Composition Spectrometer on board the Wind spacecraft, and investigate their ionic composition and compare it to the bulk solar wind plasma composition, observed from the Solar Wind Ion Composition Spectrometer on board the Advanced Composition Explorer. We find that the composition of the suprathermal plasma is related to the local bulk solar wind plasma and not to the plasma upstream of the ICME. This implies that the suprathermal plasma is accelerated from the local bulk solar wind plasma and not the upstream solar wind plasma.

THE EVOLUTION OF 1 AU EQUATORIAL SOLAR WIND AND ITS ASSOCIATION WITH THE MORPHOLOGY OF THE HELIOSPHERIC CURRENT SHEET FROM SOLAR CYCLES 23 TO 24

The solar wind can be categorized into three types based on its 'freeze-in' temperature (T {sub freeze-in}) in the coronal source: low T {sub freeze-in} wind mostly from coronal holes, high T {sub freeze-in} wind mostly from regions outside of coronal holes, including streamers (helmet streamer and pseudostreamer), active regions, etc., and transient interplanetary coronal mass ejections (ICMEs) usually possessing the hottest T {sub freeze-in}. The global distribution of these three types of wind has been investigated by examining the most effective T {sub freeze-in} indicator, the O{sup 7+}/O{sup 6+} ratio, as measured by the Solar Wind Ion Composition Spectrometer on board the Advanced Composition Explorer (ACE) during 1998-2008 by Zhao et al. In this study, we extend the previous investigation to 2011 June, covering the unusual solar minimum between solar cycles 23 and 24 (2007-2010) and the beginning of solar cycle 24. We find that during the entire solar cycle, from the ascending phase of cycle 23 in 1998 to the ascending phase of cycle 24 in 2011, the average fractions of the low O{sup 7+}/O{sup 6+} ratio (LOR) wind, the high O{sup 7+}/O{sup 6+} ratio (HOR) wind, and ICMEs at 1 AU are 50.3%, 39.4%, and 10.3%, respectively;more » the contributions of the three types of wind evolve with time in very different ways. In addition, we compare the evolution of the HOR wind with two heliospheric current sheet (HCS) parameters, which indicate the latitudinal standard deviation (SD) and the slope (SL) of the HCS on the synoptic Carrington maps at 2.5 solar radii surface. We find that the fraction of HOR wind correlates with SD and SL very well (slightly better with SL than with SD), especially after 2005. This result verifies the link between the production of HOR wind and the morphology of the HCS, implying that at least one of the major sources of the HOR wind must be associated with the HCS.« less

Possible modification of the cooling index of interstellar helium pickup ions by electron impact ionization in the inner heliosphere

AbstractInterstellar neutrals penetrating into the inner heliosphere are ionized by photoionization, charge exchange with solar wind ions, and electron impact ionization. These processes comprise the first step in the evolution of interstellar pickup ion (PUI) distributions. Typically, PUI distributions have been described in terms of velocity distribution functions that cool adiabatically under solar wind expansion, with a cooling index of 3/2. Recently, the cooling index has been determined experimentally in observations of He PUI distributions with Advanced Composition Explorer (ACE)/Solar Wind Ion Composition Spectrometer and found to vary substantially over the solar cycle. The experimental determination of the cooling index depends on the knowledge of the ionization rates and their spatial variation. Usually, ionization rates increase with 1/r2 as neutral particles approach the Sun, which is not exactly true for electron impact ionization, because the electron temperature increases with decreasing distance from the Sun due to the complexity of its distributions and different radial gradients in temperature. This different dependence on distance may become important in the study of the evolution of PUI distributions and is suspected as one of the potential reasons for the observed variation of the cooling index. Therefore, we investigate in this paper the impact of electron ionization on the variability of the cooling index. We find that the deviation of the electron ionization rate from the canonical 1/r2 behavior of other ionization processes plays only a minor role.

The in-situ manifestation of solar prominence material

AbstractCoronal mass ejections observed in the corona exhibit a three-part structure, with a leading bright front indicating dense plasma, a low density cavity thought to be a signature of the embedded magnetic flux rope, and the high density core likely containing cold, prominence material. When observed in-situ, as Interplanetary CMEs (or ICMEs), the presence of all three of these signatures remains elusive, with the prominence material rarely observed. We report on a comprehensive and long-term search for prominence material inside ICMEs as observed by the Solar Wind Ion Composition Spectrometer on the Advanced Composition Explorer. Using a novel data analysis process, we are able to identify traces of low charge state plasma created during prominence eruptions associated with ICMEs. We find that the likelihood of occurrence of cold material in the heliosphere is vastly lower than that observed in the corona but that conditions during the eruption do allow low charge ions to make it into the solar wind, preserving their expansion history. We discuss the implications of these findings.

Observations of interstellar helium pickup ions in the inner heliosphere

During the MESSENGER spacecraft's interplanetary trajectory to Mercury, the Fast Imaging Plasma Spectrometer (FIPS) measured the first helium pickup ion distributions at a heliocentric distance (R) ranging between 0.3 and 0.7 AU. From several transits of MESSENGER through the interstellar helium gravitational focusing cone, we map the cone structure in the ecliptic at R=0.3 AU and compare it with observations of He+ at R=1 AU made with the Solar Wind Ion Composition Spectrometer (SWICS) on the Advanced Composition Explorer (ACE). Average downwind enhancements of ≈8 and average cone widths of ≈30° measured by each sensor match known models of neutral helium during this most recent, unusually quiet, solar minimum. The average cone center direction is calculated to be at J2000 ecliptic longitude λ∞=76.0°(±6.0°) and 77.0°(±1.5°) from FIPS and SWICS observations, respectively. These parameters are also in agreement with previous determinations of the downwind direction of interstellar flow and demonstrate the effectiveness of using pickup ion observations inside 1 AU to map the structure of the neutral helium distribution in the heliosphere, as well as to diagnose the conditions of the interstellar medium.

Solar Wind Conditions and Composition During the Genesis Mission as Measured by in situ Spacecraft

We describe the Genesis mission solar-wind sample collection period and the solar wind conditions at the L1 point during this 2.3-year period. In order to relate the solar wind samples to solar composition, the conditions under which the samples were collected must be understood in the context of the long-term solar wind. We find that the state of the solar wind was typical of conditions over the past four solar cycles. However, Genesis spent a relatively large fraction of the time in coronal-hole flow as compared to what might have been expected for the declining phase of the solar cycle. Data from the Solar Wind Ion Composition Spectrometer (SWICS) on the Advanced Composition Explorer (ACE) are used to determine the effectiveness of the Genesis solar-wind regime selection algorithm. The data collected by SWICS confirm that the Genesis algorithm successfully separated and collected solar wind regimes having distinct solar origins, particularly in the case of the coronal hole sample. The SWICS data also demonstrate that the different regimes are elementally fractionated. When compared with Ulysses composition data from the previous solar cycle, we find a similar degree of fractionation between regimes as well as fractionation relative to the average photospheric composition.

LOW IONIZATION STATE PLASMA IN CORONAL MASS EJECTIONS

The Ultraviolet Coronagraph Spectrometer on board the Solar and Heliospheric Observatory (SOHO) often observes low ionization state coronal mass ejection (CME) plasma at ultraviolet wavelengths. The CME plasmas are often detected in O VI (3x10^5K), C III (8x10^4K), LyA, and LyB, with the low ionization plasma confined to bright filaments or blobs that appear in small segments of the UVCS slit. On the other hand, in situ observations by the Solar Wind Ion Composition Spectrometer (SWICS) on board Advanced Composition Explorer (ACE) have shown mostly high ionization state plasmas in the magnetic clouds in interplanetary coronal mass ejections (ICME) events, while low ionization states are rarely seen. In this analysis, we investigate whether the low ionization state CME plasmas observed by UVCS occupy small enough fractions of the CME to be consistent with the small fraction of ACE ICMEs that show low ionization plasma, or whether the CME plasma must be further ionized after passing the UVCS slit. To do this, we determine the covering factors of low ionization state plasma for 10 CME events. We find that the low ionization state plasmas in CMEs observed by UVCS show average covering factors below 10%. This indicates that the lack of low ionization state ICME plasmas observed by the ACE results from a small probability that the spacecraft passes through a region of low ionization plasma. We also find that the low ionization state plasma covering factors in faster CMEs are smaller than in slower CMEs.

Fifteen years of science and space weather studies

Fifteen years ago, on 25 August 1997, NASA launched the Advanced Composition Explorer (ACE) spacecraft. In operation for more than a solar cycle, ACE has provided numerous important scientific results while at the same time becoming a key component of the space weather–monitoring system and a cornerstone of the fleet of distributed spacecraft that make up the Heliophysics Great Observatory. ACE is located on the sunward side of Earth about 1.5 million kilometers away, or 4 times as far from Earth as is the Moon. This puts ACE well outside the Earth's magnetosphere and in an ideal position to monitor the solar wind environment “upwind” of Earth. ACE has six high‐resolution spectrometers that measure the elemental, isotopic, and ionic charge‐state composition of ions from hydrogen to iron; it also includes three particle and field monitors. One of the nine instruments has now failed (Solar Energetic Particle Ionic Charge Analyzer (SEPICA)), and two others are partially degraded (Solar Wind Ion Composition Spectrometer (SWICS) and Solar Wind Electron, Proton, and Alpha Monitor (SWEPAM)); however, the other six are working well. Instrument descriptions are available on the ACE Web site at http://www.srl.caltech.edu/ACE/.