Large-scale Solar Wind Structures Research Articles

Transient upstream mesoscale structures: drivers of solar-quiet space weather

In recent years, it has become increasingly clear that space weather disturbances can be triggered by transient upstream mesoscale structures (TUMS), independently of the occurrence of large-scale solar wind (SW) structures, such as interplanetary coronal mass ejections and stream interaction regions. Different types of magnetospheric pulsations, transient perturbations of the geomagnetic field and auroral structures are often observed during times when SW monitors indicate quiet conditions, and have been found to be associated to TUMS. In this mini-review we describe the space weather phenomena that have been associated with four of the largest-scale and the most energetic TUMS, namely, hot flow anomalies, foreshock bubbles, travelling foreshocks and foreshock compressional boundaries. The space weather phenomena associated with TUMS tend to be more localized and less intense compared to geomagnetic storms. However, the quiet time space weather may occur more often since, especially during solar minima, quiet SW periods prevail over the perturbed times.

Permutation entropy and complexity analysis of large-scale solar wind structures and streams

Abstract. In this work, we perform a statistical study of magnetic field fluctuations in the solar wind at 1 au using permutation entropy and complexity analysis and the investigation of the temporal variations of the Hurst exponents. Slow and fast wind, magnetic clouds, interplanetary coronal mass ejection (ICME)-driven sheath regions, and slow–fast stream interaction regions (SIRs) have been investigated separately. Our key finding is that there are significant differences in permutation entropy and complexity values between the solar wind types at larger timescales and little difference at small timescales. Differences become more distinct with increasing timescales, suggesting that smaller-scale turbulent features are more universal. At larger timescales, the analysis method can be used to identify localised spatial structures. We found that, except in magnetic clouds, fluctuations are largely anti-persistent and that the Hurst exponents, in particular in compressive structures (sheaths and SIRs), exhibit a clear locality. Our results shows that, in all cases apart from magnetic clouds at the largest scales, solar wind fluctuations are stochastic, with the fast wind having the highest entropies and low complexities. Magnetic clouds, in turn, exhibit the lowest entropy and highest complexity, consistent with them being coherent structures in which the magnetic field components vary in an ordered manner. SIRs, slow wind and ICME sheaths are intermediate in relation to magnetic clouds and fast wind, reflecting the increasingly ordered structure. Our results also indicate that permutation entropy–complexity analysis is a useful tool for characterising the solar wind and investigating the nature of its fluctuations.

Changes in and Recovery of the Turbulence Properties in the Magnetosheath for Different Solar Wind Streams

Solar wind is known to have different properties depending on its origin at the Sun. In addition to the differences in plasma and magnetic field parameters, these streams differ due to the properties of turbulent fluctuations involved in the flow. The present study addresses the changes in the turbulence properties in the magnetosheath—the transition region in front of the magnetosphere. This study is based on statistics from the simultaneous measurements of magnetic field fluctuations in the solar wind and in the magnetosheath. Both the dayside and flank magnetosheath regions are focused on to detect the evolution of the turbulent fluctuations during their flow around the magnetosphere. Turbulent cascade is shown to save its properties for fast solar wind streams. Conditions favorable for the preservation of the turbulence properties at the bow shock may correspond to the increased geoefficiency of large-scale solar wind structures.

Ultra-low-frequency waves for below threshold and thousand times threshold flux events

A recent study by Shah et al. [Astrophys. Space Sci. 366(2), 22 (2021)] explored the effects of large-scale solar wind structures on relativistic electron fluxes in the slot region and categorized the flux events into four classes, including those with the events of maximum peak fluxes, TTF (thousand times threshold flux), and the events of smallest fluxes, BTF (below threshold flux). This study compares ultra-low-frequency (ULF) waves inside a magnetosphere and at the magnetopause boundary for the BTF and TTF events. It is found that during the TTF event, the electron radial drift velocity peaked very early at the position of an inner Van Allen radiation belt (L = 2–3). The delay between the radial velocity peak and the peak of the toroidal mode electric field was 12 min. For the BTF event, strong ULF waves were absent in the outer Van Allen radiation belt. In contrast, peak powers of ULF waves in the outer radiation belt approached 80 (mV/m)2/Hz for the TTF event, and this peak exceeded the peak power of the toroidal mode electric field at the magnetopause boundary (L = 11) for the BTF event. Our findings are critical for understanding the transport of ULF wave-driven charged particles from the magnetopause boundary to the inner magnetosphere.

Characteristic Scales of Complexity and Coherence within Interplanetary Coronal Mass Ejections: Insights from Spacecraft Swarms in Global Heliospheric Simulations

Many aspects of the 3D structure and evolution of interplanetary coronal mass ejections (ICMEs) remain unexplained. Here, we investigate two main topics: (1) the coherence scale of magnetic fields inside ICMEs, and (2) the dynamic nature of ICME magnetic complexity. We simulate ICMEs interacting with different solar winds using the linear force-free spheromak model incorporated into the EUHFORIA model. We place a swarm of ∼20,000 spacecraft in the 3D simulation domain and characterize ICME magnetic complexity and coherence at each spacecraft based on the simulated time series. Our simulations suggest that ICMEs retain a lower complexity and higher coherence along their magnetic axis, but that a characterization of their global complexity requires crossings along both the axial and perpendicular directions. For an ICME of initial half angular width of 45° that does not interact with other large-scale solar wind structures, global complexity can be characterized by as little as 7–12 spacecraft separated by 25°, but the minimum number of spacecraft rises to 50–65 (separated by 10°) if interactions occur. Without interactions, ICME coherence extends for 45°, 20°–30°, 15°–30°, and 0°–10° for B, B ϕ , B θ , and B r , respectively. Coherence is also lower in the ICME west flank compared to the east flank due to Parker spiral effects. Moreover, coherence is reduced by a factor of 3–6 by interactions with solar wind structures. Our findings help constrain some of the critical scales that control the evolution of ICMEs and aid in the planning of future dedicated multispacecraft missions.

Interplanetary mesoscale observatory (InterMeso): A mission to untangle dynamic mesoscale structures throughout the heliosphere

Mesoscale dynamics are a fundamental process in space physics, but fall within an observational gap of current and planned missions. Particularly in the solar wind, measurements at the mesoscales (100s RE to a few degrees heliographic longitude at 1 au) are crucial for understanding the connection between the corona and an observer anywhere within the heliosphere. Mesoscale dynamics may also be key to revealing the currently unresolved physics regulating particle acceleration and transport, magnetic field topology, and the causes of variability in the composition and acceleration of solar wind plasma. Studies using single-point observations do not allow for investigations into mesoscale solar wind dynamics and plasma variability, nor do they allow for the exploration of the sub-structuring of large-scale solar wind structures like coronal mass ejections (CMEs), co-rotating/stream interaction regions (CIR/SIRs), and the heliospheric plasma sheet. To address this fundamental gap in our knowledge of the heliosphere at these scales, the Interplanetary Mesoscale Observatory (InterMeso) concept employs a multi-point approach using four identical spacecraft in Earth-trailing orbits near 1 au. Varying drift speeds of the InterMeso spacecraft enable the mission to span a range of mesoscale separations in the solar wind, achieving significant and innovative science return. Simultaneous, longitudinally-separated measurements of structures co-rotating over the spacecraft also allow for disambiguation of spatiotemporal variability, tracking of the evolution of solar wind structures, and determination of how the transport of energetic particles is impacted by these variabilities.

Rate of Change of Large-Scale Solar-Wind Structure

Quantifying the rate at which the large-scale solar-wind structure evolves is important for both understanding the physical processes occurring in the corona and for space-weather forecast improvement. Models of the global corona and heliosphere typically assume that the ambient solar-wind structure is steady and corotates with the Sun, which is generally expected to be more valid at solar minimum than solar maximum, but this has not been well tested. Similarly, assimilation of solar-wind observations into models requires quantitative knowledge of how the reliability of the observations changes with age. In this study we examine 25 years of near-Earth in situ solar-wind observations and 45 years of observation-constrained solar-wind simulations to determine how much the 1-AU solar-wind speed, V, and radial magnetic-field component, B_{R}, vary between consecutive Carrington rotations (CRs). For the in situ spacecraft observations, we find the rate of change of V and B_{R} is similar during solar maximum and minimum, particularly when transient interplanetary coronal mass ejections are removed from the data. This is somewhat counter to expectations. Conversely, the rate of change in V and B_{R} obtained from global heliospheric simulations is strongly correlated with the solar cycle, with the corona and heliosphere being more variable at solar maximum, as expected. Limiting the analysis of the simulations to the solar equatorial region, however, strongly reduces the difference between solar maximum and minimum, bringing the result into close agreement with the in situ observations. This latitudinal sensitivity is explained in terms of the global solar-wind structure over the solar cycle. For the purposes of assimilating in-ecliptic solar-wind observations, we suggest the uncertainty in V should increase by around 3 km s−1 per day since the observation was made and 0.1 nT per day for B_{R}. For observations made at higher latitude, the effect of observation age will be solar-cycle dependent.

Magnetosheath Jet Occurrence Rate in Relation to CMEs and SIRs.

Magnetosheath jets constitute a significant coupling effect between the solar wind (SW) and the magnetosphere of the Earth. In order to investigate the effects and forecasting of these jets, we present the first‐ever statistical study of the jet production during large‐scale SW structures like coronal mass ejections (CMEs), stream interaction regions (SIRs) and high speed streams (HSSs). Magnetosheath data from Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft between January 2008 and December 2020 serve as measurement source for jet detection. Two different jet definitions were used to rule out statistical biases induced by our jet detection method. For the CME and SIR + HSS lists, we used lists provided by literature and expanded on incomplete lists using OMNI data to cover the time range of May 1996 to December 2020. We find that the number and total time of observed jets decrease when CME‐sheaths hit the Earth. The number of jets is lower throughout the passing of the CME‐magnetic ejecta (ME) and recovers quickly afterward. On the other hand, the number of jets increases during SIR and HSS phases. We discuss a few possibilities to explain these statistical results.

Causes and Consequences of Magnetic Complexity Changes within Interplanetary Coronal Mass Ejections: A Statistical Study

Abstract We present the first statistical analysis of complexity changes affecting the magnetic structure of interplanetary coronal mass ejections (ICMEs), with the aim of answering the questions: How frequently do ICMEs undergo magnetic complexity changes during propagation? What are the causes of such changes? Do the in situ properties of ICMEs differ depending on whether they exhibit complexity changes? We consider multispacecraft observations of 31 ICMEs by MESSENGER, Venus Express, ACE, and STEREO between 2008 and 2014 while radially aligned. By analyzing their magnetic properties at the inner and outer spacecraft, we identify complexity changes that manifest as fundamental alterations or significant reorientations of the ICME. Plasma and suprathermal electron data at 1 au, and simulations of the solar wind enable us to reconstruct the propagation scenario for each event, and to identify critical factors controlling their evolution. Results show that ∼65% of ICMEs change their complexity between Mercury and 1 au and that interaction with multiple large-scale solar wind structures is the driver of these changes. Furthermore, 71% of ICMEs observed at large radial (>0.4 au) but small longitudinal (<15°) separations exhibit complexity changes, indicating that propagation over large distances strongly affects ICMEs. Results also suggest that ICMEs may be magnetically coherent over angular scales of at least 15°, supporting earlier theoretical and observational estimates. This work presents statistical evidence that magnetic complexity changes are consequences of ICME interactions with large-scale solar wind structures, rather than intrinsic to ICME evolution, and that such changes are only partly identifiable from in situ measurements at 1 au.

Study of Mars Magnetosheath Fluctuations using the Kurtosis Technique: Mars Express Observations

Planetary magnetosheaths are characterized by high plasma wave and turbulence activity. The Martian magnetosheath is no exception; both upstream and locally generated plasma waves have been observed in the region between its bow shock and magnetic boundary layer, its induced magnetosphere. This statistical study of wave activity in the Martian magnetosheath is based on 12 years (2005–2016) of observations made during Mars Express (MEX) crossings of the planet’s magnetosheath — in particular, data on electron density and temperature data collected by the electron spectrometer (ELS) of the plasma analyzer (ASPERA-3) experiment on board the MEX spacecraft. A kurtosis parameter has been calculated for these plasma parameters. This value indicates intermittent behavior in the data when it is higher than 3 (the value for a normal or Gaussian distribution). The variation of wave activity occurrence has been analyzed in relation to solar cycle, Martian orbit, and distance to the bow shock. Non-Gaussian properties are observed in the magnetosheath of Mars on all analyzed scales, especially in those near the proton gyrofrequency in the upstream region of the Martian magnetosphere. We also report that non-Gaussian behavior is most prominent at the smaller scales (higher frequencies). A significant influence of the solar cycle was also observed; the kurtosis parameter is higher during declining and solar maximum phases, when the presence of disturbed solar wind conditions, caused by large scale solar wind structures, increases. The kurtosis decreases with increasing distance from the bow shock, which indicates that the intermittence level is higher near the bow shock. In the electron temperature data the kurtosis is higher near the perihelion due to the higher incidence of EUV when the planet is closer to the Sun, which causes a more extended exosphere, and consequently increases the wave activity in the magnetosheath and its upstream region. The extended exosphere seems to play a lower effect in the electron density data.

Solar Wind Anomalies at 1 au and Their Associations with Large-scale Structures

Abstract We study solar wind anomalies and their associations with solar wind structures using the STEREO solar wind and suprathermal electron (STE) data from IMPACT and PLASTIC. We define solar wind anomalies as temporary and local excursions from the average solar wind state, regardless of their origins, for six anomalies: sunward strahls, counterstreaming suprathermal electrons, suprathermal electron depletions, nearly radial magnetic field episodes, anomalously low proton temperatures, and anomalously low proton beta. We first establish the solar wind synoptic contour displays, which show the expected variations in solar wind structure during the solar cycle: recurrent corotating heliospheric magnetic field (HMF) and stream structures are dominant during solar quiet times around the solar minimum (2008 December) preceding cycle 24, while complex structures characterize solar active times around the solar maximum (2014 April). During the declining phase of the cycle (2016–2019), the stream structures remain complex, but the HMF sectors show the structures of the solar minimum. We then systematically study the six anomalies by analyzing the STE data using automated procedures. All anomalies present some degree of dependence on the large-scale solar wind structure, especially around the solar minimum, implying that the solar wind structure plays a role in either the generation or transportation of these anomalies. One common feature of all of the anomalies is that the distributions of the durations of the anomalous episodes all peak at the 1 hr data resolution, but monotonically decrease over longer durations, which may arguably imply that solar anomalies occur on a continuum of temporal and spatial scales.

An Entropy-stable Ideal EC-GLM-MHD Model for the Simulation of the Three-dimensional Ambient Solar Wind

Abstract The main aim of the current work is to apply the Roe+Lax–Friedrichs (LF) hybrid entropy-stable scheme to the simulation of the three-dimensional ambient solar wind. The governing equations for the solar wind flow and magnetic field utilize the entropy-consistent nine-wave magnetic field divergence diminishing ideal magnetohydrodynamics (MHD) equations, which are symmetric and Galilean invariant with some nonconservative terms proportional to the divergence of magnetic field or the gradient of the Lagrange multiplier ψ. By using solenoidality-preserving and non-negativity-preserving reconstruction, the divergence error is further constrained, and the densities and pressures are reliably guaranteed. Moreover, the entropy is used as an auxiliary equation to completely avoid the appearance of negative pressure, which is independent of any numerical flux and can be retrofit into any MHD equations straightforwardly. All the properties referred to above make the newly developed scheme more handy and robust to cope with the high Mach number or low plasma β situations. After the experiments of the entropy consistency and the robustness of the proposed entropy-stable scheme through two simple tests, we carry out the simulation of the large-scale solar wind structures for Carrington Rotation 2183 (CR 2183) in a six-component grid system with the initial potential field obtained from the Helioseismic and Magnetic Imager magnetogram by retaining spherical harmonics of degree 50. The comparisons of the numerical results with the remote sensing observations and in situ data show that the new model has the capability to produce structured solar wind.

Evolution of Interplanetary Coronal Mass Ejection Complexity: A Numerical Study through a Swarm of Simulated Spacecraft

Abstract In-situ measurements carried out by spacecraft in radial alignment are critical to advance our knowledge on the evolutionary behavior of coronal mass ejections (CMEs) and their magnetic structures during propagation through interplanetary space. Yet, the scarcity of radially aligned CME crossings restricts investigations on the evolution of CME magnetic structures to a few case studies, preventing a comprehensive understanding of CME complexity changes during propagation. In this Letter, we perform numerical simulations of CMEs interacting with different solar wind streams using the linear force-free spheromak CME model incorporated into the EUropean Heliospheric FORecasting Information Asset model. The novelty of our approach lies in the investigation of the evolution of CME complexity using a swarm of radially aligned, simulated spacecraft. Our scope is to determine under which conditions, and to what extent, CMEs exhibit variations of their magnetic structure and complexity during propagation, as measured by spacecraft that are radially aligned. Results indicate that the interaction with large-scale solar wind structures, and particularly with stream interaction regions, doubles the probability to detect an increase of the CME magnetic complexity between two spacecraft in radial alignment, compared to cases without such interactions. This work represents the first attempt to quantify the probability of detecting complexity changes in CME magnetic structures by spacecraft in radial alignment using numerical simulations, and it provides support to the interpretation of multi-point CME observations involving past, current (such as Parker Solar Probe and Solar Orbiter), and future missions.

Coupling a Global Heliospheric Magnetohydrodynamic Model to a Magnetofrictional Model of the Low Corona

Abstract Recent efforts coupling our Sun-to-Earth magnetohydrodynamics (MHD) model and lower-corona magnetofrictional (MF) model are described. Our Global Heliospheric MHD (GHM) model uses time-dependent three-component magnetic field data from the lower-corona MF model as time-dependent boundary values. The MF model uses data-assimilation techniques to introduce the vector magnetic field data from the Solar Dynamics Observatory/Helioseismic and Magnetic Imager, hence as a whole this simulation coupling structure is driven with actual observations. The GHM model employs a newly developed interface boundary treatment that is based on the concept of characteristics, and it properly treats the interface boundary sphere set at a height of the sub-Alfvénic lower corona (1.15 R ⊙ in this work). The coupled model framework numerically produces twisted nonpotential magnetic features and consequent eruption events in the solar corona in response to the time-dependent boundary values. The combination of our two originally independently developed models presented here is a model framework toward achieving further capabilities of modeling the nonlinear time-dependent nature of magnetic field and plasma, from small-scale solar active regions to large-scale solar wind structures. This work is a part of the Coronal Global Evolutionary Model project for enhancing our understanding of Sun–Earth physics to help improve space weather capabilities.

Catalogue of events with cumulative fluxes below as well as comparable and exceeding those for RBSP discovered third radiation belt

The formation of a temporary third radiation belt in the slot region of Earth’s magnetosphere was reported (Baker et al. in Science 340:186-190, 2013a) from the early data of the radiation belt storm probes (RBSP) mission. The cumulative flux of relativistic electrons in L = 3-3.5 for the third radiation belt of that study is calculated here and defined as threshold flux (TF) (105 cm−2 s−1 sr−1 MeV−1). A catalogue of 133 events during Sep 2012 – Feb 2019 is presented. Events are distributed into, below threshold flux (BTF), of the order of, one hundred and one thousand times the threshold flux, (OTF), (HTF), and (TTF) categories respectively. Finding high 2 MeV flux threshold at L = 3-3.5 is a way to find ideal condition for the possible formation of a third radiation belt, aware that attaining TF does not necessarily mean formation of the third radiation belt. It is found that BTF occurred 29 times and 105 times cumulative flux exceeded TF but TTF occurred very rarely. The effects of large scale solar wind structures (LSSS) and number of main phase Dst dips on BTF-TTF events are also studied. The findings of this study may be helpful for understanding the third radiation belt formation in the slot region of Earth’s magnetosphere.

Supersubstorms and Conditions in the Solar Wind

This article examines the effect of various large-scale solar-wind structures and streams on the occurrence of a special type of substorms—the so-called supersubstorms (SSS), which are very intense substorms defined by the indices SML –50 nT, the events occur mostly right after the sudden onset (SC) of a storm (11%) and, very rarely, happen late in the storm recovery phase (1.2%). The space weather conditions associated with SSS events differ sharply from those associated with other types of high-latitude substorms, such as polar and expanded substorms.

Large-scale structure of solar wind beyond Earth’s orbit reconstructed by using data of two-site interplanetary scintillation observations at decameter radio waves

Large-scale structure of solar wind beyond Earth’s orbit reconstructed by using data of two-site interplanetary scintillation observations at decameter radio waves

Large-Scale Structure of Solar Wind beyond the Earth’s Orbit: Reconstruction Using the Data of Two-Site Measurements of Interplanetary Scintillations in the Decameter Radio Range

Solar wind is a set of flows with different parameters (the speed, the exponent of the spectrum of heterogeneities, the width, etc.). A bimodal character of the speed distribution of the solar wind was determined in spaceborne experiments. The measurements onboard the Ulysses spacecraft confirmed that the bimodal structure of solar wind continues to persist at relatively large distances from the Sun (to several astronomical units). However, there is one more possibility to determine the stream structure of solar wind. This is the method of interplanetary scintillations. The purpose of the paper is to reconstruct the stream structure of solar wind beyond the Earth’s orbit using the data on interplanetary scintillations obtained at two observational sites. The experiments were carried out at decameter wavelengths, since they are rather strongly scattered by the rarefied interplanetary plasma beyond the Earth’s orbit. The experimental data on interplanetary scintillations analyzed in this work were obtained in synchronous observations with the UTR-2 and URAN-2 radio telescopes. The parameters of solar wind and its stream structure were determined by comparison of the characteristics of interplanetary scintillations measured in the experiment (the dependences of the harmonic velocity of the cross-spectrum of scintillations and the power spectra) to those calculated with the models. To separate the interplanetary and ionospheric scintillations, the spectral, spatial, and frequency criteria were used. The results of this analysis show that solar wind beyond the Earth’s orbit consists of several streams that replace each other on the line of sight toward the radio source. These investigations prove the reliability and efficiency of the interplanetary scintillation method for reconstructing the stream structure of solar wind.

Solar Wind Streams of Different Types and High-Latitude Substorms

The effects of different large-scale solar wind structures on magnetospheric substorms at high geomagnetic latitudes are studied. Two types of high-latitude substorm disturbances are considered: substorms observed in quiet conditions, when the auroral oval contracts and shifts towards high latitudes (contracted oval substorms, or polar substorms), and those observed in disturbed conditions, when the auroral oval expands (expanded oval substorms, or expanded substorms). Ground-based magnetic observations from the Scandinavian IMAGE network are compared with the OMNI solar wind database and the catalog of large-scale solar wind phenomena ( ftp://ftp.iki.rssi.ru/omni/ ). The study involves the following types of solar wind streams: (1) high-speed streams from coronal holes (FAST); (2) interplanetary manifestations of coronal mass ejections, i.e., magnetic clouds (MCs) or EJECTA; and (3) plasma compression regions ahead of these streams, i.e., corotating interaction regions (CIRs) and SHEATH. The study includes 186 polar and 202 expanded substorms in 1995, 1996, 1999, and 2000. It is shown that ~75% of expanded substorms are observed in FAST streams or plasma compression regions ahead of these streams (CIRs), and only ~18% of such substorms are observed in interplanetary manifestations of coronal mass ejections EJECTA and in SHEATH compression regions. While ~67% of polar substorms are observed in SLOW streams, ~19% of such substorms have been recorded in SHEATH and EJECTA but only against the background of a slow solar wind.

Large-Scale and Small-Scale Solar Wind Structures Formed during Interaction of Streams in the Heliosphere

The paper considers the classification of solar wind streams in magnetohydrodynamic parameters (MHD-types); combinations of proton speed, density, temperature, and magnetic field strength, in addition to the classical solar wind separation into high-speed streams from coronal holes, transient streams of coronal mass ejections, and the slow solar wind from the streamer belt. Two classifications of solar wind properties were compared for the events in August 2010 and May 2011, when one could observe the interaction of two coronal mass ejections and a coronal mass ejection with a high-speed solar wind stream from the coronal hole, respectively. It is shown that the classical description of a large-scale structure of wind streams (the ion wind composition especially) in scales of hours and days allows one to determine the type and source of streams, whereas the MHD-parameters allow one to more accurately describe the small-scale structure (in minutes), especially in the cases of several streams interaction in the heliosphere. The detailed study of a small-scale structure of stream interaction regions provides the information required for developing MHD-models describing the processes of propagation and interaction of streams in the heliosphere and for predicting their geoefficiency.