Solar Wind Model Research Articles

Assessment of CESE-HLLD ambient solar wind model results using multipoint observation

For a three-dimensional magnetohydrodynamics solar wind model, it is necessary to carry out assessment studies to reveal its ability and limitation. In this paper, the ambient solar wind results of year 2008 generated by the CESE-HLLD 3D MHD model are compared with multipoint in-situ measurements during the late declining phase of solar cycle 23. The near-ecliptic results are assessed both quantitatively and qualitatively by comparing with in-situ data obtained at the L1 point and by the twin STEREO spacecraft. The assessment reveals the model’s ability in reproducing the time series and statistical characteristics of solar wind parameters, and in catching the change of interplanetary magnetic field polarity and the occurrence of the stream interaction regions. We find that the two-stream structure observed near the ecliptic plane is reproduced, but the differences among observations at L1 and the twin STEREO spacecraft are not caught by the model. The latitudinal variation of the results is assessed by comparing with the Ulysses observation. The characters of variation in different latitudinal ranges are duplicated by the model, but biases of the results are seen, and the boundary layers between fast and slow solar wind are sometimes thicker than observation.

EUropean Heliospheric FORecasting Information Asset 2.0

Aims: This paper presents a H2020 project aimed at developing an advanced space weather forecasting tool, combining the MagnetoHydroDynamic (MHD) solar wind and coronal mass ejection (CME) evolution modelling with solar energetic particle (SEP) transport and acceleration model(s). The EUHFORIA 2.0 project will address the geoeffectiveness of impacts and mitigation to avoid (part of the) damage, including that of extreme events, related to solar eruptions, solar wind streams, and SEPs, with particular emphasis on its application to forecast geomagnetically induced currents (GICs) and radiation on geospace.Methods: We will apply innovative methods and state-of-the-art numerical techniques to extend the recent heliospheric solar wind and CME propagation model EUHFORIA with two integrated key facilities that are crucial for improving its predictive power and reliability, namely (1) data-driven flux-rope CME models, and (2) physics-based, self-consistent SEP models for the acceleration and transport of particles along and across the magnetic field lines. This involves the novel coupling of advanced space weather models. In addition, after validating the upgraded EUHFORIA/SEP model, it will be coupled to existing models for GICs and atmospheric radiation transport models. This will result in a reliable prediction tool for radiation hazards from SEP events, affecting astronauts, passengers and crew in high-flying aircraft, and the impact of space weather events on power grid infrastructure, telecommunication, and navigation satellites. Finally, this innovative tool will be integrated into both the Virtual Space Weather Modeling Centre (VSWMC, ESA) and the space weather forecasting procedures at the ESA SSCC in Ukkel (Belgium), so that it will be available to the space weather community and effectively used for improved predictions and forecasts of the evolution of CME magnetic structures and their impact on Earth.Results: The results of the first six months of the EU H2020 project are presented here. These concern alternative coronal models, the application of adaptive mesh refinement techniques in the heliospheric part of EUHFORIA, alternative flux-rope CME models, evaluation of data-assimilation based on Karman filtering for the solar wind modelling, and a feasibility study of the integration of SEP models.

Identifying Critical Input Parameters for Improving Drag‐Based CME Arrival Time Predictions

AbstractCoronal mass ejections (CMEs) typically cause the strongest geomagnetic storms, so a major focus of space weather research has been predicting the arrival time of CMEs. Most arrival time models fall into two categories: (1) drag‐based models that integrate the drag force between a simplified CME structure and the background solar wind and (2) full magnetohydrodynamic models. Drag‐based models typically are much more computationally efficient than magnetohydrodynamic models, allowing for ensemble modeling. While arrival time predictions have improved since the earliest attempts, both types of models currently have difficulty achieving mean absolute errors below 10 hr. Here we use a drag‐based model ANTEATR (Another Type of Ensemble Arrival Time Results) to explore the sensitivity of arrival times to various input parameters. We consider CMEs of different strengths from average to extreme size, speed, and mass (kinetic energies between and erg). For each scale CME, we vary the input parameters to reflect the current observational uncertainty in each and determine how accurately each must be known to achieve predictions that are accurate within 5 hr. We find that different scale CMEs are the most sensitive to different parameters. The transit time of average strength CMEs depends most strongly on the CME speed, whereas an extreme strength CME is the most sensitive to the angular width. A precise CME direction is critical for impacts near the flanks but not near the CME nose. We also show that the Drag‐Based Model has similar sensitivities, suggesting that these results are representative for all drag‐based models.

Quantifying the latitudinal representivity of in situ solar wind observations

Advanced space-weather forecasting relies on the ability to accurately predict near-Earth solar wind conditions. For this purpose, physics-based, global numerical models of the solar wind are initialized with photospheric magnetic field and coronagraph observations, but no further observation constraints are imposed between the upper corona and Earth orbit. Data assimilation (DA) of the available in situ solar wind observations into the models could potentially provide additional constraints, improving solar wind reconstructions, and forecasts. However, in order to effectively combine the model and observations, it is necessary to quantify the error introduced by assuming point measurements are representative of the model state. In particular, the range of heliographic latitudes over which in situ solar wind speed measurements are representative is of primary importance, but particularly difficult to assess from observations alone. In this study we use 40+ years of observation-driven solar wind model results to assess two related properties: the latitudinal representivity error introduced by assuming the solar wind speed measured at a given latitude is the same as that at the heliographic equator, and the range of latitudes over which a solar wind measurement should influence the model state, referred to as the observational localisation. These values are quantified for future use in solar wind DA schemes as a function of solar cycle phase, measurement latitude, and error tolerance. In general, we find that in situ solar wind speed measurements near the ecliptic plane at solar minimum are extremely localised, being similar over only 1° or 2° of latitude. In the uniform polar fast wind above approximately 40° latitude at solar minimum, the latitudinal representivity error drops. At solar maximum, the increased variability of the solar wind speed at high latitudes means that the latitudinal representivity error increases at the poles, though becomes greater in the ecliptic, as long as moderate speed errors can be tolerated. The heliospheric magnetic field and solar wind density and temperature show very similar behaviour.

Effects of Nearly Frontal and Highly Inclined Interplanetary Shocks on High‐Latitude Field‐Aligned Currents (FACs)

AbstractWe present high‐latitude field‐aligned current (FAC) response to nearly frontal shocks (NFSs) and highly inclined shocks (HISs) through a superposed epoch analysis. The FACs are derived from magnetic perturbation data provided by the Active Magnetosphere and Planetary Electrodynamics Response Experiment program. Forty‐nine events for each group are used for the superposed epoch analysis. The 25%, 50%, and 75% quantiles of the FAC and total current distributions are studied. We found that NFSs are statistically stronger shocks in terms of solar wind parameters such as solar wind speed and interplanetary magnetic field.For the 50% quantiles, both groups of shocks produce rapid increases in total currents after shock arrival, but NFSs result in sharper increase in FACs and more intense FACs compared to HISs. At the 50% and 75% quantiles, NFSs trigger stronger auroral‐zone current disturbance for the first hour after shock arrival than do HISs. Spatially, the difference in FAC response is most notable in (1) the dayside noon region, (2) the duskside Region 2 current system, and (3) the dawnside prenoon Region 1 current system. Our results are consistent with previous numerical simulations that showed more symmetric and stronger compression of the magnetosphere for high‐speed and nearly frontal shocks. We observationally confirm the role of shock impact angle in controlling the subsequent shock geoeffectiveness for fast shocks. We assert that determining the shock impact angle via an upstream solar wind model could provide useful insight in forecasting the geoeffectiveness of a shock prior to its arrival at the magnetopause.

Assessing the Performance of EUHFORIA Modeling the Background Solar Wind

In order to address the growing need for more accurate space-weather predictions, a new model named EUHFORIA (EUropean Heliospheric FORecasting Information Asset) was recently developed. We present the first results of the performance assessment for the solar-wind modeling with EUHFORIA and identify possible limitations of its present setup. Using the basic EUHFORIA 1.0.4 model setup with the default input parameters, we modeled background solar wind (no coronal mass ejections) and compared the obtained results with Advanced Composition Explorer (ACE) in-situ measurements. For the purposes of statistical study we developed a technique of combining daily EUHFORIA runs into continuous time series. The combined time series were derived for the years 2008 (low solar activity) and 2012 (high solar activity), from which in-situ speed and density profiles were extracted. We find for the low-activity phase a better match between model results and observations compared to the high-activity time interval considered. The quality of the modeled solar-wind parameters is found to be rather variable. Therefore, to better understand the results obtained we also qualitatively inspected characteristics of coronal holes, i.e. the sources of the studied fast streams. We discuss how different characteristics of the coronal holes and input parameters to EUHFORIA influence the modeled fast solar wind, and suggest possibilities for the improvement of the model.

Comparisons Between the Field Lines Using an Accelerating and a Constant Solar Wind model

Magnetic field line mapping between the Sun and the Earth is important to trace the nonthermal particles. We generalize a recently developed mapping approach (B stepping) where this approach allows us to map the magnetic field lines by stepping along the local magnetic field direction. We employ an advanced solar wind model which includes acceleration, angular momentum conservation, and intrinsic non-radial velocities and magnetic fields at the inner boundary / source surface. We map the field lines using Wind spacecraft data for two solar rotation periods: one near a solar minimum between CR2118 and CR2119 and other CR1992 near a solar maximum. Maps using the accelerating solar wind model are compared with the maps using a constant solar wind model. On a broad scale, maps using two solar wind models for the same solar rotation periods are very similar. However, in a small scale, there are significant differences, e.g. differences are evident in connectivities, paths, and winding angles. In addition, field lines using the accelerating solar wind model are more azimuthally oriented for during the solar maximum. These differences demonstrate the significance of inclusion of the accelerating radial speed profile, intrinsic azimuthal velocity and magnetic field components.

Mapping Magnetic Field Lines for an Accelerating Solar Wind

Mapping of magnetic field lines is important for studies of the solar wind and the sources and propagation of energetic particles between the Sun and observers. A recently developed mapping approach is generalised to use a more advanced solar wind model that includes the effects of solar wind acceleration, non-radial intrinsic magnetic fields and flows at the source surface/inner boundary, and conservation of angular momentum. The field lines are mapped by stepping along local magnetic field $\bm{B}$ and via a Runge-Kutta algorithm, leading to essentially identical maps. The new model's maps for Carrington rotation CR 1895 near solar minimum (19 April to 15 May 1995) and a solar rotation between CR 2145 and CR 2146 near solar maximum (14 January to 9 February 2014) are compared with the published maps for a constant solar wind model. The two maps are very similar on a large scale near both solar minimum and solar maximum, meaning that the field line orientations, winding angles, and connectivity generally agree very well. However, close inspection shows that the field lines have notable small-scale structural differences. An interpretation is that inclusion of the acceleration and intrinsic azimuthal velocity has significant effects on the local structure of the magnetic field lines. Interestingly, the field lines are more azimuthal for the accelerating solar wind model for both intervals. In addition, predictions for the pitch angle distributions (PADs) for suprathermal electrons agree at the $90$ -- $95\%$ level with observations for both solar wind models for both intervals.

Efficacy of Electric Field Models in Reproducing Observed Ring Current Ion Spectra During Two Geomagnetic Storms

AbstractWe use the UNH‐IMEF, Weimer 1996, https://doi.org/10.1029/96GL02255 and Volland‐Stern electric field models along with a dipole magnetic field to calculate drift paths for particles that reach the Van Allen Probes' orbit for two inbound passes during two large geomagnetic storms. We compare the particle access in the models with the observed particle access using both realistic and enhanced solar wind model parameters. To test the accuracy of the drift paths, we estimate the H+ charge exchange loss along these drift paths. While increasing the strength of the model electric field drives particles further inward, improving agreement, energy‐dependent cutoffs in the spectra do not agree, indicating that potential patterns for highly disturbed times are inaccurate. While none of the models were able to reproduce the observed features of the more dawnward pass during the 17 March 2013 storm, the UNH‐IMEF model with enhanced inputs was able to adequately reproduce the access, charge exchange loss, and H+ particle pressure during the 17 March 2015 storm.

Planetary and lunar ephemeris EPM2021 and its significance for Solar system research

AbstractWe present an updated public version of EPM (Ephemerides of Planets and the Moon). Since the last public version, EPM2017, many improvements were made in both the observational database and the mathematical model. Latest lunar laser ranging observations have been added, as well as radio ranges of Juno spacecraft and more recent ranges of Odyssey and Mars Reconnaissance Orbiter. EPM2021 uses a new improved way to calculate radio signal delays in solar plasma and has a major update in the method of determination of asteroid masses. Also, a delay-capable multistep numerical integrator was implemented for EPM in order to properly account for tide delay in the equations of the motion of the Moon. The improved processing accuracy has allowed to refine existing estimates of the mass of the Sun and its change rate, parameters of the Earth–Moon system, masses of the Main asteroid belt and the Kuiper belt; and also to raise important questions about existing numerical models of solar wind.

Nonequilibrium Ionization Effects on Coronal Plasma Diagnostics and Elemental Abundance Measurements

Abstract Plasma diagnostics and elemental abundance measurements are crucial to help us understand the formation and dynamics of the solar wind. Here we use a theoretical solar wind model to study the effect of nonequilibrium ionization (NEI) on plasma diagnostic techniques applied to line intensities emitted by the fast solar wind. We find that NEI almost always changes the spectral line intensities with up to 120% difference for the lighter elements and for higher charge states of Fe even below 1.5 solar radii (R s ). The measured plasma density, temperature, and differential emission measure are only slightly affected by NEI. However, NEI significantly affects the first-ionization potential (FIP) bias and abundance ratio measurements, producing an error of up to a factor 4 at 1.5 R s for the Mg-to-Ne, Fe-to-S, and Ar-to-Fe ratios when EI is assumed. We conclude that it is very important to consider the NEI effect when spectral line intensities are synthesized and the FIP bias and elemental abundance are measured.

The evolution of Earth’s magnetosphere during the solar main sequence

ABSTRACT As a star spins-down during the main sequence, its wind properties are affected. In this work, we investigate how Earth’s magnetosphere has responded to the change in the solar wind. Earth’s magnetosphere is simulated using 3D magnetohydrodynamic models that incorporate the evolving local properties of the solar wind. The solar wind, on the other hand, is modelled in 1.5D for a range of rotation rates Ω from 50 to 0.8 times the present-day solar rotation (Ω⊙). Our solar wind model uses empirical values for magnetic field strengths, base temperature, and density, which are derived from observations of solar-like stars. We find that for rotation rates ≃10 Ω⊙, Earth’s magnetosphere was substantially smaller than it is today, exhibiting a strong bow shock. As the Sun spins-down, the magnetopause standoff distance varies with Ω−0.27 for higher rotation rates (early ages, ≥1.4 Ω⊙) and with Ω−2.04 for lower rotation rates (older ages, <1.4 Ω⊙). This break is a result of the empirical properties adopted for the solar wind evolution. We also see a linear relationship between the magnetopause distance and the thickness of the shock on the subsolar line for the majority of the evolution (≤10 Ω⊙). It is possible that a young fast rotating Sun would have had rotation rates as high as 30–50 Ω⊙. In these speculative scenarios, at 30 Ω⊙, a weak shock would have been formed, but for 50 Ω⊙, we find that no bow shock could be present around Earth’s magnetosphere. This implies that with the Sun continuing to spin-down, a strong shock would have developed around our planet and remained for most of the duration of the solar main sequence.

Incorporation of Heliospheric Imagery Into the CME Analysis Tool for Improvement of CME Forecasting

AbstractCoronal mass ejections (CMEs) cause the largest geomagnetic disturbances at Earth, which impact satellites, wired communication systems, and power grids. The CME Analysis Tool (CAT) is used to determine a CME's initial longitude, latitude, angular width, and radial speed from coronagraph images. These are the initial conditions for the Wang‐Sheeley‐Arge Enlil solar wind model, along with the ambient solar wind properties derived from magnetograms. However, the coronagraph imagery is limited by field of view. We have incorporated heliospheric imagery (HI) from the Solar Terrestrial Relations Observatory into CAT to create the CME Analysis Tool with Heliospheric Imagery (CAT‐HI). These HI images have a larger field of view, allowing tracking of CMEs to greater distances from the Sun. We have compared the performances of CAT and CAT‐HI by examining the expected arrival times of CMEs at the L1 Lagrange point and found them to be consistent. However, CAT‐HI is advantageous because it could be used to prune ensemble forecasts and issue routine updates for CME arrival time forecasts. Finally, we discuss CAT‐HI in the context of an operational mission at the L4 or L5 Lagrange points.

Reflection-driven magnetohydrodynamic turbulence in the solar atmosphere and solar wind

We present three-dimensional direct numerical simulations and an analytic model of reflection-driven magnetohydrodynamic (MHD) turbulence in the solar wind. Our simulations describe transverse, non-compressive MHD fluctuations within a narrow magnetic flux tube that extends from the photosphere, through the chromosphere and corona and out to a heliocentric distance $r$of 21 solar radii $(R_{\odot })$. We launch outward-propagating ‘$\boldsymbol{z}^{+}$fluctuations’ into the simulation domain by imposing a randomly evolving photospheric velocity field. As these fluctuations propagate away from the Sun, they undergo partial reflection, producing inward-propagating ‘$\boldsymbol{z}^{-}$fluctuations’. Counter-propagating fluctuations subsequently interact, causing fluctuation energy to cascade to small scales and dissipate. Our analytic model incorporates dynamic alignment, allows for strongly or weakly turbulent nonlinear interactions and divides the$\boldsymbol{z}^{+}$fluctuations into two populations with different characteristic radial correlation lengths. The inertial-range power spectra of$\boldsymbol{z}^{+}$and$\boldsymbol{z}^{-}$fluctuations in our simulations evolve toward a$k_{\bot }^{-3/2}$scaling at$r>10R_{\odot }$, where$k_{\bot }$is the wave-vector component perpendicular to the background magnetic field. In two of our simulations, the$\boldsymbol{z}^{+}$power spectra are much flatter between the coronal base and$r\simeq 4R_{\odot }$. We argue that these spectral scalings are caused by: (i) high-pass filtering in the upper chromosphere; (ii) the anomalous coherence of inertial-range$\boldsymbol{z}^{-}$fluctuations in a reference frame propagating outwards with the$\boldsymbol{z}^{+}$fluctuations; and (iii) the change in the sign of the radial derivative of the Alfvén speed at$r=r_{\text{m}}\simeq 1.7R_{\odot }$, which disrupts this anomalous coherence between$r=r_{\text{m}}$and$r\simeq 2r_{\text{m}}$. At$r>1.3R_{\odot }$, the turbulent heating rate in our simulations is comparable to the turbulent heating rate in a previously developed solar-wind model that agreed with a number of observational constraints, consistent with the hypothesis that MHD turbulence accounts for much of the heating of the fast solar wind.

Near‐Earth Solar Wind Forecasting Using Corotation From L5: The Error Introduced By Heliographic Latitude Offset

AbstractRoutine in situ solar wind observations from L5, located 60° behind Earth in its orbit, would provide a valuable input to space weather forecasting. One way to utilize such observations is to assume that the solar wind is in perfect steady state over the 4.5 days it takes the Sun to rotate 60°, and thus, near‐Earth solar wind in 4.5 days time would be identical to that at L5 today. This corotation approximation is most valid at solar minimum when the solar wind is slowly evolving. Using STEREO data, it has been possible to test L5‐corotation forecasting for a few months mostly at solar minimum, but the various contributions to forecast error cannot be disentangled. This study uses 40+ years of magnetogram‐constrained solar wind simulations to isolate the effect of latitudinal offset between L5 and Earth due to the inclination of the ecliptic plane to the solar rotational equator. Latitudinal offset error is found to be largest at solar minimum, due to the latitudinal ordering of solar wind structure. It is also a strong function of time of year: maximum at the solstices and very low at equinoxes. At solstice, the latitudinal offset alone means L5‐corotation forecasting is expected to be less accurate than numerical solar wind models, even before accounting for time‐dependent solar wind structures. Thus, a combination of L5‐corotation and numerical solar wind modeling may provide the best forecast. These results also highlight that three‐dimensional solar wind structure must be accounted for when performing solar wind data assimilation.

Towards Construction of a Solar Wind \u201cReanalysis\u201d Dataset: Application to the First Perihelion Pass of Parker Solar Probe

Accurate reconstruction of global solar-wind structure is essential for connecting remote and in situ observations of solar plasma, and hence understanding formation and release of solar wind. Information can routinely be obtained from photospheric magnetograms, via coronal and solar-wind modelling, and directly from in situ observations, typically at large heliocentric distances (most commonly near 1 AU). Magnetogram-constrained modelling has the benefit of reconstructing global solar-wind structure, but with relatively large spatial and/or temporal errors. In situ observations, on the other hand, make accurate temporal measurements of solar-wind structure, but are highly localised. We here use a data assimilative (DA) approach to combine these two sources of information as a first step towards producing a solar-wind “reanalysis” dataset that optimally combines model and observation. The physics of solar wind stream interaction is used to extrapolate in heliocentric distance, while the assumption of steady-state solar-wind structure enables extrapolation in longitude. The major challenge is extrapolating in latitude. Using solar-wind speed during the interval of the first perihelion pass of Parker Solar Probe (PSP) in November 2018 as a test bed, we investigate two approaches. The first is to assume the solar wind is two-dimensional and thus has no latitudinal structure within the {pm}, 7^{circ } bounded by the heliographic equatorial and ecliptic planes. The second assumes in situ solar-wind observations are representative of some (small) latitudinal range. We show how observations of the inner heliosphere, such as will be provided by PSP, can be exploited to constrain the latitudinal representivity of solar-wind observations to improve future solar-wind reconstruction and space-weather forecasting.

Offset Power-law Dependence of the Sun’s Radial Electron Density Profile: Evidence and Implications

Abstract The radial electron density profile n e (r) of the Sun’s corona and solar wind contains information on the sources, heating, and acceleration of the coronal and solar wind plasma. Currently, several empirically derived density models are used to describe the corona, with varying degrees of success and little physical justification or predictive power. The offset power-law (OPL) profile , with radial offset r 0 and power-law index α, models radial outflow from r 0 that conserves total electron number and may be accelerated and heated (affecting α), thus having physical significance and predictive power. We fit the OPL model to multiple sets of published radial density profiles obtained from spectroscopic, white light, and radio data from different regions on the Sun and during different periods of solar activity. The spectroscopic and white light data yield r 0 = (1.02 ± 0.06) R S , where the uncertainties are standard errors of the mean, and , consistent with plasma originating near the chromosphere and acceleration similar to the nominal Parker solar wind model. Comparisons with time-lapse coronagraph and spectroscopic observations are favorable and show evidence for significant variations with position and time. These are expected given the corona’s well-known asymmetries, three-dimensional structures, and time variability. Radio burst data yield flatter profiles α < 2, suggesting that pre-flare activity alters the density profile by increasing the coronal density at large heights. We discuss the possible interpretations and implications for coronal physics and solar radio bursts.

On the usefulness of existing solar wind models for pulsar timing corrections

Dispersive delays due to the Solar wind introduce excess noise in high-precision pulsar timing experiments, and must be removed in order to achieve the accuracy needed to detect, e.g., low-frequency gravitational waves. In current pulsar timing experiments, this delay is usually removed by approximating the electron density distribution in the Solar wind either as spherically symmetric, or with a two-phase model that describes the contributions from both high- and low-speed phases of the Solar wind. However, no dataset has previously been available to test the performance and limitations of these models over extended timescales and with sufficient sensitivity. Here we present the results of such a test with an optimal dataset of observations of pulsar J0034-0534, taken with the German stations of LOFAR. We conclude that the spherical approximation performs systematically better than the two-phase model at almost all angular distances, with a residual root-mean-square (rms) given by the two-phase model being up to 28% larger than the result obtained with the spherical approximation. Nevertheless, the spherical approximation remains insufficiently accurate in modelling the Solar-wind delay (especially within 20 degrees of angular distance from the Sun), as it leaves timing residuals with rms values that reach the equivalent of 0.3 microseconds at 1400 MHz. This is because a spherical model ignores the large daily variations in electron density observed in the Solar wind. In the short term, broadband observations or simultaneous observations at low frequencies are the most promising way forward to correct for Solar-wind induced delay variations.

A Dynamical Model of the Heliosphere with the Adaptive Mesh Refinement

A three-dimensional time-dependent heliospheric model is developed with the adaptive mesh refinement (AMR) technique. The AMR code, SFUMATO, is adopted, which has been originally developed for star-formation problems. The solar wind model is based on the space weather forecast system, SUSANOO, and it is imposed on the inner boundary at r = 25 R⊙. The solar wind model is derived from magnetogram synoptic maps, which are observed daily. Using the potential field source surface models, the WS formula, and a few empirical laws, the MHD parameters at the inner boundary are reproduced. The heliospheric model here extends up to 14 au, and the AMR grid resolves the heliospheric current sheets and co-rotating interaction regions with refined resolutions. The AMR grid brings considerable improvement in the fine features in the outer region of r ≳ 2 au compared to the concentric nested grid, of which resolution has the relationship Δx ∝ r, similar to spherical coordinates. The refinement criteria are based mainly on the distance from the Sun, the anti-parallel toroidal magnetic fields, and the density of the solar wind.

Solar Terrestrial Relations Observatory (STEREO) Observations of Stream Interaction Regions in 2007 - 2016: Relationship with Heliospheric Current Sheets, Solar Cycle Variations, and Dual Observations.

We have conducted a survey of 575 slow-to-fast stream interaction regions (SIRs) using Solar Terrestrial Relations Observatory (STEREO) A and B data, analyzing their properties while extending a Level-3 data product through 2016. Among 518 pristine SIRs, 54% are associated with heliospheric current sheet (HCS) crossings, and 34% are without any HCS crossing. The other 12% of the SIRs often occur in association with magnetic sectors shorter than three days. The SIRs with HCS crossings have slightly slower speeds but higher maximum number densities, magnetic-field strengths, dynamic pressures, and total pressures than the SIRs without an HCS. The iron charge state is higher throughout the SIRs with an HCS than the SIRs without an HCS, by about 1/3 charge unit. In contrast with the comparable phases of Solar Cycle 23, slightly more SIRs and higher recurrence rates are observed in the years 2009 - 2016 of Cycle 24, with a lower HCS association rate, possibly attributed to persistent equatorial coronal holes and more pseudo-streamers in this recent cycle. The solar-wind speed, peak magnetic field, and peak pressures of SIRs are all lower in this cycle, but the weakening is less than for the comparable background solar-wind parameters. Before STEREO-B lost contact in October 2014, 151 SIR pairs were observed by the twin spacecraft. Of the dual observations, the maximum speed is the best correlated of the plasma parameters. We have obtained a sample of plasma-parameter differences analogous to those that would be observed by a mission at Lagrange points 4 or 5. By studying several cases with large discrepancies between the dual observations, we investigate the effects of HCS relative location, tilt of stream interface, and small transients on the SIR properties. To resolve the physical reasons for the variability of SIR structures, mesoscale multi-point observations and time-dependent solar-wind modeling are ultimately required.