Solar Isotope Spectrometer Research Articles

Chemical composition of solar energetic particle events and their association with ground level enhancement

Solar energetic particle (SEP) events originate from solar flares and coronal mass ejections. Ground level enhancement (GLE) is a sudden increase in the count rate of a ground neutron monitor. When GLE occurs, the associated SEP event shows a large peak intensity and fluence in the proton intensity of the high energy channels in the Geostationary Operational Environmental Satellite (GOES). This study examined the chemical composition ratio at the moment of SEP events to determine what is occurring at the SEP event origin site on the solar surface. This study used the data of the Solar Isotope Spectrometer (SIS) onboard the Advanced Composition Explorer (ACE) satellite for SEP events, which was confirmed by proton intensity data from the GOES satellite. The chemical compositions during the SEP events with and without GLE related to the solar flares in M5 class were compared. In addition, the solar wind composition in a normal quiet time and the solar photosphere abundance observed by spectroscopy were compared with the selected SEP events. As expected, all elements except helium showed less than a hundredfold increase in intensity at the SEP event periods than on quiet-day events. The helium intensity surged more than a thousandfold increase for SEP events with and without GLE. The abundance of the heavy elements at the periods of the SEP event accompanied by GLE was greater than that of the solar photosphere. Although it is generally believed that the erupted emission of electromagnetic radiation, including gamma and x-rays, called a flare, escapes from the deep undersurface core, the relatively higher abundance of helium compared to other elements at the moment of the flare blowing out the SEP events suggests that nuclear fusion by the magnetic confinement is occurring near the solar surface.

Statistical Study and Live Catalog of Multispacecraft 3He-rich Time Periods over Solar Cycles 23, 24, and 25

Using ion measurements from Ultra-Low-Energy Isotope Spectrometer observations on board Advanced Composition Explorer and Solar Isotope Spectrometer observations on board the Solar Terrestrial Observatory (STEREO)-A and STEREO-B spacecraft, we have identified 854 3He-rich time periods between 1997 September and 2021 March. We include all event types with observed 3He enhancements such as corotating interaction regions, gradual solar energetic particle (SEP) events, interplanetary shocks, and impulsive SEP events. We employ two different mass separation techniques to obtain 3He, 4He, Fe, and O fluences for each event, and we determine the 3He/4He and Fe/O abundance ratios between 0.32–0.45 and 0.64–1.28 MeV nucleon−1. We find a clear correlation in the 3He/4He and Fe/O abundance ratios between both energy ranges. We find two distinct trends in the 3He/4He versus Fe/O relation. For low 3He/4He values, there is a positive linear correlation between 3He/4He and Fe/O. However, at 3He/4He ∼ 0.3, Fe/O appears to reach a limit and the correlation weakens significantly. We provide a live catalog of 3He-rich time periods that includes the robust determination of the onset and end times of the 3He enhancements in SEP-associated periods for different types of events observed by multiple spacecraft. This catalog is available for public use. New releases will follow after major additions such as adding new periods from new missions (e.g., Parker Solar Probe and Solar Orbiter), identifying event types (impulsive SEP events, etc.), or adding new parameters such as remote observations detailing characteristics of the events’ active regions.

A Mechanism for the Fractionation of Isotopes in 3He-rich Solar Energetic Particle Events

Abstract By employing our charge-consistent acceleration model we demonstrate a possibility to explain the isotopic ratios observed in a number of 3He-rich events. We investigate the dependence of isotopic ratios of heavy ions on the energy of the particles, the acceleration efficiency, the product of acceleration time, and the number density of ambient electrons, of the plasma temperature, and of the spectral index of the magnetic fluctuations by which the ions are energized in the acceleration region. On the basis of the energy spectra of heavy ions and their isotopic ratios observed in the event of 2002 August 20 by the Solar Isotope Spectrometer on board the Advanced Composition Explorer (SIS/ACE) we put constraints on the plasma parameters in the acceleration region. Our investigation gives evidence that the spectral index of the fluctuations with which the ions interact is greater than 2, and that the plasma temperature is about 1 MK.

Temporal Evolution of Heavy-Ion Spectra in Solar Energetic Particle Events

Solar energetic particles (SEPs) are released into the heliosphere by solar flares and coronal mass ejections (CMEs). They are mostly protons, with smaller amounts of heavy ions from helium to iron, and lesser amounts of species heavier than iron. The spectra of heavy ions have been previously studied mostly by using the fluence of the particles in an event-integrated spectrum in a small number of spectral snapshots. In this article, we analyze the temporal evolution of the heavy-ion spectra using two large SEP events (27 January 2012 and 7 January 2014) from the Solar TErrestrial Relations Observatory (STEREO) era using Advanced Composition Explorer (ACE) Solar Isotope Spectrometer (SIS) and Ultra Low Energy Isotope Spectrometer (ULEIS), Energetic Particles: Acceleration, Composition and Transport (EPACT) onboard Wind, and the STEREO-A (Ahead) and -B (Behind) Low-Energy Telescope (LET) and Suprathermal Ion Telescope (SIT) instruments, taking a large number of snapshots covering the temporal evolution of the event. We find large differences in the spectra of the ions after the main flux enhancement in terms of the grouping of similar species, but also in terms of the location of the instruments. Although it is somewhat less noticeable than in the case of the temporal evolution of protons (Doran and Dalla, Solar Phys. 291, 2071, 2016), we observe a wave-like pattern travelling through the heavy ion spectra from the highest energies to the lowest, creating an “arch” structure that later straightens into a power law after 18 to 24 hours.

Cosmic Ray Enhancements in Lunar Radiation Environment Observed by CRaTER on the LRO

The Cosmic Ray Telescope for the Effects of Radiation (CRaTER) instrument onboard the Lunar Reconnaissance Orbiter (LRO) characterizes the global lunar radiation environment and its biological impacts by measuring cosmic ray (CR) radiation. By using CRaTER data, we identify the lunar CR enhancements, which are similar to the terrestrial ground level enhancements (GLEs). GLE is a sudden and short increase in CR intensity recorded by the Earth’s ground neutron monitors. We examine the origins and the characteristics of CR enhancements in the lunar space environment by using the CR intensity and dose rate according to CRaTER data. In order to determine origins of CR enhancements, we also use solar proton event (SPE) data, CR data of the Advanced Composition Explorer (ACE) equipped with a Solar Isotope Spectrometer (SIS), and proton flux data of Geostationary Operational Environmental Satellite 15 (GOES15). We identified 96 CRaTER enhancements (CREs) as increases in CR events in the lunar space environment during the period June 2009-December 2017. Unlike terrestrial GLEs, CREs are much longer and more frequent. Of the 96 CREs, 43 events are associated with SPE. The values of their physical characteristics are statistically larger than those of the CREs without associated SPEs. However, such non-SPE CREs are considered to originate from solar ejections. All CREs are associated with an increase in He flux by ACE/SIS. Even though CREs are associated with a small increase in He flux, this does indicate that some materials are ejected from the Sun; rather, it indicates that all CREs are associated with solar eruptions. Because of the very weak magnetic field and extremely rare atmosphere, the lunar space environment responds to even weak solar activity. An increase in He flux observed by ACE/SIS can be useful for monitoring the eruption of energetic particles from the Sun regardless of accompanying SPEs.

Identification and replacement of proton‐contaminated real‐time ACE solar wind measurements

AbstractReal‐time solar wind speed measurements derived from measurements by the Solar Wind Electron, Proton, and Alpha Monitor (SWEPAM) aboard the Advanced Composition Explorer (ACE) satellite are critical to specifying and forecasting current space weather and its impacts. When solar energetic protons produce high background levels in SWEPAM, the real‐time solar wind speed measurements can be corrupted due to errors in the onboard algorithm. We analyzed 14 years of ACE real‐time solar wind data for the years 1998 through 2011 to determine how to identify the contaminated measurements and what proxy might be substituted for these corrupt data. We find that good criteria for flagging contaminated data are that (1) the measured solar wind speeds are below 305 km s–1, and (2) the >10 MeV ion fluxes measured by the ACE Solar Isotope Spectrometer (SIS) are above 180 pfu. We also compare several potential proxies for the contaminated solar wind and determine that the best proxy depends on the last valid measured wind speed, vinitial, and the duration of the contamination period. For the first 25 h, the best proxy is simply vinitial. At later times, if vinitial < 400 km s–1, then vinitial continues to be the best proxy, while if vinitial > 400 km s–1, the best proxy is a linear function of the Kp geomagnetic index.

Solar Elemental Composition Based on Studies of Solar Energetic Particles

Solar abundances can be derived from the composition of the solar wind and solar energetic particles (SEPs) as well as obtained through spectroscopic means. Past comparisons have suggested that all three samples agree well, when rigidity-related fractionation effects on the SEPs were accounted for. It has been known that such effects vary from one event to the next and should be addressed on an event-by-event basis. This paper examines event variability more closely, particularly in terms of energy-dependent SEP abundances. This is now possible using detailed SEP measurements spanning several decades in energy from the Ultra Low Energy Isotope Spectrometer (ULEIS) and the Solar Isotope Spectrometer (SIS) on the ACE spacecraft. We present examples of the variability of the elemental composition with energy and suggest they can be understood in terms of diffusion from the acceleration region near the interplanetary shock. By means of a spectral scaling procedure, we obtain energy-independent abundance ratios for 14 large SEP events and compare them to reported solar wind and coronal abundances as well as to previous surveys of SEP events.

Solar Isotopic Composition as Determined Using Solar Energetic Particles

Solar energetic particles (SEPs) provide a sample of the Sun from which solar composition may be determined. Using high-resolution measurements from the Solar Isotope Spectrometer (SIS) onboard NASA’s Advanced Composition Explorer (ACE) spacecraft, we have studied the isotopic composition of SEPs at energies ≥20 MeV/nucleon in large SEP events. We present SEP isotope measurements of C, O, Ne, Mg, Si, S, Ar, Ca, Fe, and Ni made in 49 large events from late 1997 to the present. The isotopic composition is highly variable from one SEP event to another due to variations in seed particle composition or due to mass fractionation that occurs during the acceleration and/or transport of these particles. We show that various isotopic and elemental enhancements are correlated with each other, discuss the empirical corrections used to account for the compositional variability, and obtain estimated solar isotopic abundances. We compare the solar values and their uncertainties inferred from SEPs with solar wind and other solar system abundances and find generally good agreement.

An Update on Ultra-Heavy Elements in Solar Energetic Particles above 10 MeV/Nucleon

Measurements below several MeV/nucleon from Wind/LEMT and ACE/ULEIS show that elements heavier than Zn (Z=30) can be enhanced by factors of ∼100 to 1000, depending on species, in 3He-rich solar energetic particle (SEP) events. Using the Solar Isotope Spectrometer (SIS) on ACE we find that even large SEP (LSEP) shock-accelerated events at energies from ∼10 to >100 MeV/nucleon are often very iron rich and might contain admixtures of flare seed material. Studies of ultra-heavy (UH) SEPs (with Z>30) above 10 MeV/nucleon can be used to test models of acceleration and abundance enhancements in both LSEP and 3He-rich events. We find that the long-term average composition for elements from Z=30 to 40 is similar to standard solar system values, but there is considerable event-to-event variability. Although most of the UH fluence arrives during LSEP events, UH abundances are relatively more enhanced in 3He-rich events, with the (34 10 MeV/nucleon, the most extreme event in terms of UH composition detected so far took place on 23 July 2004 and had a (34<Z<40)/O enhancement of ∼250–300 times the standard solar value.

Heavy ion abundances and spectra from the large solar energetic particle events of October–November 2003

Observations from the Solar Isotope Spectrometer and the Ultra Low Energy Isotope Spectrometer on the ACE spacecraft during the extremely large events of October and November 2003 are combined to create heavy ion spectra over more than 3 decades in energy. The resulting spectra differed substantially in shape from event to event, as well as from element to element within a given event, resulting in energy‐dependent abundance ratios. Although the effects of strong local shock acceleration are apparent in the intensities of the 28 and 29 October events, these do not explain the order of magnitude differences between the event‐integrated abundances obtained at 0.64–0.91 MeV/nucleon and those at 12–60 MeV/nucleon. The higher‐energy abundances relative to the lower‐energy ones show trends with nuclear charge or charge‐to‐mass ratio that are similar for all the events and suggest that heavier ions are less efficiently accelerated to high energies. The position of the breaks in the energy spectra of O, Ne, Mg, Si, S, Ca, and Fe can be understood in terms of leakage from the shock region, if the mean free path is assumed to be a power law in rigidity. The resulting rigidity dependence is consistent with a source of wave turbulence in the vicinity of the shock when the ions are accelerated.

Elemental Fractionation in Small Solar Energetic Particle Events

Using the Solar Isotope Spectrometer on the Advanced Composition Explorer (ACE), we have measured the 11.0-21.8 MeV nucleon^(-1) heavy (Z ≥ 6) element abundances of 39 small solar energetic particle (SEP) events that occurred between 1998 April 3 and 2002 February 26. Using He isotope data from the Ultra-Low-Energy Isotope Spectrometer on ACE, we have classified the events according to their 0.5-2.0 MeV nucleon^(-1) ^3He/^4He ratios. We find that their average heavy-element composition is similar to that of either large gradual events or ^3He-rich events, depending on their ^3He/^4He ratio. As seen in recent studies of small SEP events, we find that the heavy-element intensities relative to C increase with the ^3He/^4He ratios. The dependence of the heavy-element abundances on first ionization potential (FIP) has been derived, using a model consisting of a power law in Q/M (and Z) times a step function in FIP. We report the magnitude of the FIP factor in each event and find that it varies from ~2 to 7 with no clear dependence on the ^3He/^4He ratio.

Variability of spectra in large solar energeticparticle events

Using data from the Solar Isotope Spectrometer on the Advanced Composition Explorer obtained during 36large solar energetic particle events (SEPs) that occurred during 1997–2002 we have examined the spectral characteristics of oxygen and iron. Based on the shape of the oxygen spectrum during the decay phase following the peak in particle intensity, each SEP event was categorized as either exponential (7 events) or power law (29 events). We find that the exponential events were typically the larger events (in terms of peak oxygen intensity) and had Fe/O ratios that strongly decreased with increasing energy. Event-averaged Fe/O ratios (integrated over 12 to 60 MeV/nucleon) were at or below coronal abundances for nearly all these events, while the ratios obtained in the power law events were typically enhanced over coronal values. The majority of the power law events had the same spectral index for both O and Fe resulting in an Fe/O ratio independent of energy. However 6 of the 29 power law events had Fe/O ratios that increased with increasing energy due to an Fe spectral index less negative than that of O. We consider simple diffusion theory in an effort to understand the nature of these events and obtain preliminary but promising results.

Energetic particle composition at 1 au during periods of moderate interplanetary particle fluxes

It has recently been proposed that residual energetic ions from preceding impulsive solar energetic particle (SEP) events may contribute to the source population for the larger gradual events accelerated by CME-driven shocks (Mason et al., 1999). This suggests that there may be impulsive SEP material left in interplanetary space during periods relatively free of energetic particle activity. To test this idea we have studied the heavy ions and 3He present at L1 during periods of low—to—moderate energetic particle fluxes between February 1998 and April 2000. Using the Solar Isotope Spectrometer on the Advanced Composition Explorer, we have measured the energy spectra of heavy elements (Z≥6) and 3He over the ranges ∼8–20 MeV/nucleon and ∼4–15 MeV/nucleon, respectively. In these energy spectra, we have looked for spectral and compositional signatures typically characteristic of impulsive or gradual SEP events. We discuss the implications of these observations for the origin of particles accelerated in gradual SEP events.

Forecasting the arrival of shock‐accelerated solar energetic particles at Earth

Energetic particles accelerated at interplanetary shocks can result in an increased radiation dose for astronauts as well as an increased risk to satellite hardware. These shocks are known to occasionally accelerate protons to energies greater than 100 MeV and can result in very high fluxes. Advanced warning of the arrival of strong interplanetary shocks can enable steps to be taken to minimize the potential risks. In an effort to monitor and assess the radiation risk to astronauts, two real‐time count rate monitors were implemented in the Solar Isotope Spectrometer (SIS) on the ACE spacecraft. These rates measure protons with energies &gt;10 and &gt;30 MeV. Since ACE is located at the L1 Lagrangian point, these rates provide information and warning up to 1 hour before an interplanetary shock reaches Earth. Using ACE and GOES, data we have examined examples of shocks and associated energetic particles that have been detected at ACE and were later observed near Earth in an effort to develop more fully the forecasting capability of ACE. Because of the limited amount of solar activity during 1997–1999, this current work is primarily a proof of concept.

Event‐to‐event variations in the isotopic composition of neon in solar energetic particle events

Using the Solar Isotope Spectrometer on the Advanced Composition Explorer, we have measured the isotopic composition of neon at 24–72 MeV/nucleon in nine solar energetic particle events between 4 November 1997 and 16 November 1998. The 22Ne/20Ne ratio varies from ∼0.7 to ∼2 times the solar wind value from event to event, indicating significant mass fractionation in the acceleration and/or transport of these particles. The mass fractionation is strongly correlated with the Fe/O and Na/Mg element abundance ratios, suggesting that elemental and isotopic fractionation are both governed by the ionic charge to mass ratio.

New observations of heavy‐ion‐rich solar particle events from ACE

Following launch of the Advanced Composition Explorer in August 1997, the Solar Isotope Spectrometer measured the composition of nine solar energetic particle events. We have used isotopic measurements of Ne to determine the degree of charge‐to‐mass‐dependent fractionation and infer the charge states of C‐Ni in the four most heavy‐ion‐rich of the nine events. The results indicate a source temperature of ∼4×106 K; this and the measured abundances suggest that these four events are more characteristic of impulsive events than gradual. Although the ³He/4He ratios are not enhanced to the level commonly ascribed to impulsive events, there are sizable enhancements over typical solar wind values measured in three of the events.

Unusual isotopic composition of solar energetic particles observed in the November 6, 1997 event

Using the Solar Isotope Spectrometer on the Advanced Composition Explorer, we have measured the isotopic composition of solar energetic particles (SEPs) with energies of tens of MeV/nucleon during the November 6, 1997 solar event, including isotopes never before reported in SEPs, such as 34S, 38Ar, 44Ca, 54Fe, and 60Ni, as well as isotopes of C, N, O, Ne, Mg, and Si. Abundances are found to be enhanced by factors of ∼2 relative to the standard solar system values for 22Ne/20Ne and 26Mg/24Mg. Overall, the enrichments of the isotope ratios show a correlation with the ratio of masses, M2/M1, scaling approximately as (M2/M1)7, although significant deviations from this trend are evident.

Inferred charge states of high energy solar particles from the solar isotope spectrometer on ACE

The large solar event on November 6, 1997, was enriched in heavy elements (Fe/O ∼ 1) with very hard energy spectra (power law index of −2). There was also a large enrichment in heavier isotopes, suggesting unusually strong charge to mass (Q/M) fractionation (∼(Q/M)−7). Assuming that the fractionation is dominated by Q/M and first ionization potential (FIP) related processes, we have inferred the charge states of twelve elements with energies of 12–60 MeV/nucleon. These results suggest a source temperature of 3–6 × 106K, significantly higher than deduced at lower energies.

The Solar Isotope Spectrometer for the Advanced Composition Explorer

The Solar Isotope Spectrometer (SIS), one of nine instruments on the Advanced Composition Explorer (ACE), is designed to provide high-resolution measurements of the isotopic composition of energetic nuclei from He to Zn (Z = 2 to 30) over the energy range from ∼ 10 to ∼ 100 MeV nucl-1. During large solar events SIS will measure the isotopic abundances of solar energetic particles to determine directly the composition of the solar corona and to study particle acceleration processes. During solar quiet times SIS will measure the isotopes of low-energy cosmic rays from the Galaxy and isotopes of the anomalous cosmic-ray component, which originates in the nearby interstellar medium. SIS has two telescopes composed of silicon solid-state detectors that provide measurements of the nuclear charge, mass, and kinetic energy of incident nuclei. Within each telescope, particle trajectories are measured with a pair of two-dimensional silicon-strip detectors instrumented with custom, very large-scale integrated (VLSI) electronics to provide both position and energy-loss measurements. SIS was especially designed to achieve excellent mass resolution under the extreme, high flux conditions encountered in large solar particle events. It provides a geometry factor of ∼ 40 cm2 sr, significantly greater than earlier solar particle isotope spectrometers. A microprocessor controls the instrument operation, sorts events into prioritized buffers on the basis of their charge, range, angle of incidence, and quality of trajectory determination, and formats data for readout by the spacecraft. This paper describes the design and operation of SIS and the scientific objectives that the instrument will address.