Solar Wind Stream Interaction Regions Research Articles

CME propagation through the heliosphere: Status and future of observations and model development

The ISWAT (International Space Weather Action Teams) heliosphere clusters H1 and H2 have a focus on interplanetary space and its characteristics, especially on the large-scale co-rotating and transient structures impacting Earth. Solar wind stream interaction regions, generated by the interaction between high-speed solar wind originating in large-scale open coronal magnetic fields and slower solar wind from closed magnetic fields, are regions of compressed plasma and magnetic field followed by high-speed streams that recur at the ∼27 day solar rotation period. Short-term reconfigurations of the lower coronal magnetic field generate flare emissions and provide the energy to accelerate enormous amounts of magnetised plasma and particles in the form of coronal mass ejections into interplanetary space. The dynamic interplay between these phenomena changes the configuration of interplanetary space on various temporal and spatial scales which in turn influences the propagation of individual structures. While considerable efforts have been made to model the solar wind, we outline the limitations arising from the rather large uncertainties in parameters inferred from observations that make reliable predictions of the structures impacting Earth difficult. Moreover, the increased complexity of interplanetary space as solar activity rises in cycle 25 is likely to pose a challenge to these models. Combining observational and modeling expertise will extend our knowledge of the relationship between these different phenomena and the underlying physical processes, leading to improved models and scientific understanding and more-reliable space-weather forecasting. The current paper summarizes the efforts and progress achieved in recent years, identifies open questions, and gives an outlook for the next 5–10 years. It acts as basis for updating the existing COSPAR roadmap by Schrijver et al. (2015), as well as providing a useful and practical guide for peer-users and the next generation of space weather scientists.

Upstream proton cyclotron waves at Mars during the passage of solar wind stream interaction regions

Proton cyclotron waves (PCWs) upstream from Mars are generated by ion–ion instabilities due to the interaction between the solar wind and the pickup protons that originate from the extended hydrogen (H) exosphere of Mars, indicating a loss of H to interplanetary space. Their occurrences and properties associated with nominal solar wind conditions have been widely studied, but relatively little is known about PCW activity under extreme solar wind conditions. In this work we characterize the upstream PCWs during the passage of solar wind stream interaction regions (SIRs) utilizing in situ observations by the Mars Atmosphere and Volatile EvolutioN spacecraft from October 2014 to August 2021. The 46 analyzed SIR events are a subset of the events that occurred in the perihelion season of Martian years 32–35. We find that the PCW occurrence rate is increased by a factor of about 1.8 during the SIR phase relative to the pre-SIR and post-SIR phases. Furthermore, the PCW activity tends to occur more frequently in the leading portion of the SIR. The PCWs detected during the SIR phase have more pronounced wave characteristics, namely larger wave amplitudes, smaller propagation angles with respect to the background magnetic field, and closer-to-circular polarization.

Properties of Stream Interactions and Their Associated Shocks near 1.52 au: MAVEN Observations

Abstract The measurements from the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft are analyzed for the basic properties of solar wind stream interaction regions (SIRs) and their associated shocks at 1.52 au, as well as the evolution of SIRs from 1 to 1.52 au. A total of 149 SIRs are identified during the period from 2014 October to 2018 November, and 126 SIRs with high-quality data are selected for this study. The average occurrence rate of SIRs at 1.52 au is 36.3 yr−1, which is comparable to but slightly higher than that (32.4 yr−1) at 1 au, meaning that most SIRs are well formed at 1 au. The average duration of SIRs at 1.52 au is about 37.0 hr, comparable to that at 1 au (36.73 hr), indicating that SIRs have not yet expanded more rapidly as they are convected to 1.52 au. The maximum magnetic field strength and pressure of SIRs decrease significantly from 1 to 1.52 au. The shock association rates of SIRs increase from 20.3% to 33.3% or higher as SIRs evolve from 1 to 1.52 au. The forward shocks tend to occur twice more frequently than the reverse shocks. About 75% of shocks at 1.52 au are quasi-perpendicular shocks. The strength of the shocks becomes weaker and the average shock speed remains almost unchanged from 1 to 1.52 au. These results will help us understand the solar wind conditions at Mars and their potential impact on the Martian space environment.

Solar wind stream interaction regions throughout the heliosphere

This paper focuses on the interactions between the fast solar wind from coronal holes and the intervening slower solar wind, leading to the creation of stream interaction regions that corotate with the Sun and may persist for many solar rotations. Stream interaction regions have been observed near 1 AU, in the inner heliosphere (at sim 0.3–1 AU) by the Helios spacecraft, in the outer and distant heliosphere by the Pioneer 10 and 11 and Voyager 1 and 2 spacecraft, and out of the ecliptic by Ulysses, and these observations are reviewed. Stream interaction regions accelerate energetic particles, modulate the intensity of Galactic cosmic rays and generate enhanced geomagnetic activity. The remote detection of interaction regions using interplanetary scintillation and white-light imaging, and MHD modeling of interaction regions will also be discussed.

Metrics for solar wind prediction models: Comparison of empirical, hybrid, and physics‐based schemes with 8 years of L1 observations

Space weather effects on technological systems originate with energy carried from the Sun to the terrestrial environment by the solar wind. In this study, we present results of modeling of solar corona‐heliosphere processes to predict solar wind conditions at the L1 Lagrangian point upstream of Earth. In particular we calculate performance metrics for (1) empirical, (2) hybrid empirical/physics‐based, and (3) full physics‐based coupled corona‐heliosphere models over an 8‐year period (1995–2002). L1 measurements of the radial solar wind speed are the primary basis for validation of the coronal and heliosphere models studied, though other solar wind parameters are also considered. The models are from the Center for Integrated Space‐Weather Modeling (CISM) which has developed a coupled model of the whole Sun‐to‐Earth system, from the solar photosphere to the terrestrial thermosphere. Simple point‐by‐point analysis techniques, such as mean‐square‐error and correlation coefficients, indicate that the empirical coronal‐heliosphere model currently gives the best forecast of solar wind speed at 1 AU. A more detailed analysis shows that errors in the physics‐based models are predominately the result of small timing offsets to solar wind structures and that the large‐scale features of the solar wind are actually well modeled. We suggest that additional “tuning” of the coupling between the coronal and heliosphere models could lead to a significant improvement of their accuracy. Furthermore, we note that the physics‐based models accurately capture dynamic effects at solar wind stream interaction regions, such as magnetic field compression, flow deflection, and density buildup, which the empirical scheme cannot.

Energetic particle measurements from the <I>Ulysses</I>/COSPIN/LET instrument obtained during the August/September 2005 events

Abstract. We report recent observations of energetic particles at energies 1–40 MeV/n made by the COSPIN/LET instrument onboard the Ulysses spacecraft during the period of intense solar activity in August/September 2005 during the declining phase of solar cycle 23. Ulysses, having started its climb to high southern latitudes for the third time, was located at ~5 AU, at a helio-latitude of ~30 degrees south. It detected the arrival of a solar wind compound stream resulting from the merging of a series of fast halo CMEs ejected from the Sun in late August and early September 2005 and their interaction with the pre-existing pattern of solar wind Stream Interaction Regions (SIRs) in the ambient medium through which they propagated. The heavy ion intensities are observed by COSPIN/LET to remain elevated for at least 20 days following the very intense X17.0/3B solar flare on 7 September and its associated very fast CME (plane of sky projected CME speed ~2400 km s−1). We carry out an analysis of the composition of the particle increases observed at the location of the spacecraft. Although the composition signatures were predominantly Solar Energetic Particle (SEP)-like, after the passage of the compound stream over Ulysses, in association with a characteristic forward and reverse shock pair, the observations showed evidence of an enhanced He content.

Sources and acceleration efficiencies for energetic particles in the heliosphere

Shocks at solar wind stream interaction regions, coronal mass ejections and magnetospheric obstacles have long been known for their intimate link with particle acceleration. Much enhanced capabilities to determine mass and charge composition at interplanetary shocks with ACE and SOHO have enabled us to identify sources and acceleration processes for the energetic particles. Both solar wind and interstellar pickup ions are substantial sources for particle acceleration in corotating interaction regions and at coronal mass ejections driven shocks and that flare particles are re-accelerated. Suprathermal distributions, such as pickup ions and pre-existing flare populations are accelerated much more efficiently than particles out of the solar wind. Recent results of the termination shock crossing by Voyager I and the scientific goals of the upcoming IBEX mission will be discussed.

Solar wind stream interaction regions without sector boundaries

During periods of high solar activity when there are many sources of solar wind on the solar disk, a spacecraft occasionally encounters consecutive solar wind streams with the same magnetic polarity. The low‐speed wind in the region of interaction between the two streams exhibits many of the same features as, but has some differences from, the low‐speed wind that includes crossings of the heliospheric current sheet (HCS) where the direction of the heliospheric magnetic field reverses. The non‐HCS slow wind exhibits many of the same small‐scale structures usually associated with the slow wind around the HCS; these include discontinuous stream interfaces and other discontinuities, magnetic holes, and low‐entropy structures. These entropy holes do not appear to have the same origin as the plasma sheets observed near the HCS, however. The helium abundances and heavy ion charge states in the non‐HCS regions are not significantly different from those in HCS‐associated regions. Some of the dynamical properties of the non‐HCS regions differ from those found near the HCS; the regions between leading and trailing stream interfaces have a shorter duration or scale size, greater minimum speed, and lower peak and average densities. No correlation could be found between the non‐HCS slow wind and visible coronal streamers.

Solar maximum low-energy particle observations at heliographic latitudes above 75 degrees

We present a preliminary analysis of low-energy particle observations by the Ulysses and ACE spacecraft during the time intervals that Ulysses spent at heliographic latitudes above 75 degrees during its solar maximum orbit. The most southerly and northerly latitudes (80.2°) were reached by Ulysses in November 2000 and October 2001, respectively. These two periods were characterized by high levels of solar activity. During its passage through the highest southern latitudes, Ulysses encountered complex magnetic field and solar wind configurations, with numerous energetic particle events of either solar origin or associated with solar wind stream interaction regions. By contrast, during its high northern latitude passage, Ulysses remained immersed in the high-speed (∼700 km s −1) solar wind coronal hole flow bearing magnetic field of negative polarity and detected three intense solar energetic particle (SEP) events. We correlate observations from Ulysses at high heliographic latitudes with 1 AU measurements in the ecliptic from the ACE spacecraft. Large SEP events were observed simultaneously by both spacecraft. Periods with comparable particle intensities at ACE and Ulysses occurred during the decay phase of large SEP events. We discuss the formation of these periods of nearly identical particle intensities (i.e., “reservoirs”) in terms of both fresh injections of solar energetic particles and the formation of compressed magnetic field regions in the heliosphere beyond the Earth orbit (∼2–5 AU). We discuss the implications of these observations for models of solar energetic particle propagation to both high and low heliographic latitudes.

Energetic [formula omitted] intensity ratio under solar maximum and solar minimum conditions: Ulysses observations

We study the solar cycle variability of the heliospheric energetic proton-to-helium abundance ratios. We use 0.5–1.0 MeV nucleon −1 proton and helium intensities observed by the Ulysses spacecraft at both high and low heliographic latitudes. Ulysses observations show that during solar maximum the 0.5–1.0 MeV nucleon −1 H He intensity ratios are, on average, higher than during solar minimum. Under solar minimum conditions the interaction between slow and fast solar wind streams is strong, producing long-lasting and stable corotating interaction regions (CIRs) which are efficient accelerators of pickup He +. During solar maximum, transient events of solar origin (characterized by high H He ratios) are able to globally fill the heliosphere. In addition, the absence of large and stable coronal holes results in a lack of recurrent strong corotating solar wind interactions, and consequently a less efficient acceleration of pickup He +. Even when solar wind stream interaction regions (SIRS) are observed, the H He intensity ratio during solar maximum rarely decreases to the low (∼6) values typically observed during solar minimum CIR events. The latest data collected by Ulysses during its solar maximum descent from northern polar latitudes (mid-2002) show nearly-recurrent CIR events, which occasionally decrease the H/He ratios to these low (∼6) values. The still frequent occurrence of SEP events, however, produces increases of the H/He intensity ratios to high (∼30) values. Although SEP events still dominate the particle population in the inner heliosphere, the low H He ratios observed in these specific CIR events suggest an efficient acceleration of pickup He + rather than the acceleration of remnant SEP material.

Microscale instabilities in stream interaction regions

A theoretical investigation of the microstructure of solar wind stream interaction regions is presented. We discuss the role of several electrostatic kinetic instabilities which may be important within the stream interface and the compression region. Inside of 1 AU the interface is likely to be stable against the electrostatic streaming instabilities considered. Between 1 and 2 AU we argue that the interface will excite the magnetized ion‐ion instability. The compression region is also found to be unstable beyond 1 AU, where the modified two‐stream instability, beam‐cyclotron instability, and ion‐acoustic instability will be important in determining the structure of the compressive pulses as they evolve into forward and reverse shocks. We conclude that the modified two‐stream instability and beam‐cyclotron instability predominately play a role in heating the electrons to the threshold for the ion‐acoustic instability. Various electrostatic plasma waves, ranging in frequency from the lower‐hybrid to harmonics of the electron cyclotron frequency, would be produced by these instabilities. Their signature should also be seen by high time resolution measurements of the temperature of the various plasma species.

Solar wind stream interactions and interplanetary heat conduction

The compression of plasma in solar wind stream interaction regions (where a fast-moving stream overtakes slower-moving material) implies the existence of a heat source for solar wind electrons. The resulting electron thermal structure is largely determined by a balance between the rate of compressive heating in this ‘interaction region’ source and the rate of thermal conduction away from the region. Two different quantitative models of the thermal structure are derived under the common simplifying assumptions of a steady state, a negligible spatial extent for the heat source, and a radial magnetic field line geometry. The models employ contrasting assumptions about the heat conduction process; one is based upon a ‘classical’ thermal conductivity, and the other uses a ‘saturated’ heat conduction flux density. The predicted dependence of electron temperature increase on the characteristics of the stream interaction region is significantly different for the two cases. It is thus suggested that observations of electron temperatures in such regions could be used to examine the nature of the interplanetary heat conduction process.