Stream Interaction Research Articles

Unusual Multiple Excitation of Large‐Scale Gravity Waves by Successive Stream Interactions: The Role of Alfvénic Fluctuations

AbstractInstances of multiple excitation of large‐scale gravity waves in the auroral region by solar wind Alfvénic fluctuations are rare. In this paper we report an unusual instance, in which sequential large‐scale gravity waves launched from northern and southern auroral regions are clearly linked to intermittent southward magnetic fields of Alfvénic fluctuations embedded in three successive stream interaction regions. The continuous gravity wave activity persists for ∼7 days, the longest duration currently tracked. The Alfvénic fluctuations are mainly dominated by Alfvén and slow waves, which might play a role in the solar wind‐magnetosphere interaction, leading to impulsive auroral electrojet activity and in turn exciting the sequential gravity waves.

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.

Properties and Geoeffectiveness of Solar Wind High‐Speed Streams and Stream Interaction Regions During Solar Cycles 23 and 24

AbstractWe study the properties and geoeffectiveness of solar wind high‐speed streams (HSSs) emanating from coronal holes and associated with stream interaction regions (SIRs). This paper presents a statistical study of 588 SIR/HSS events with solar wind speed at 1 AU exceeding 500 km/s during 1995–2017, encompassing the decline of solar cycle 22 to the decline of cycle 24. Events are detected using measurements of the solar wind speed and the interplanetary magnetic field. Events misidentified as or interacting with interplanetary coronal mass ejections are removed by comparison with an existing interplanetary coronal mass ejection list. Using this SIR/HSS event catalog (list given in the supporting information), a superposed epoch analysis of key solar wind parameters is carried out. It is found that the number of SIR/HSSs peaks during the late declining phase of solar cycle (SC) 23, as does their velocity, but that their geoeffectiveness in terms of the AE and SYM‐H indices is low. This can be explained by the anomalously low values of magnetic field during the extended solar minimum. Within SC23 and SC24, the highest geoeffectiveness of SIR/HSSs takes place during the early declining phases. Geoeffectiveness of SIR/HSSs continues to be up to 40% lower during SC24 than SC23, which can be explained by the solar wind properties.

From solar sneezing to killer electrons: outer radiation belt response to solar eruptions.

Electrons in the outer Van Allen (radiation) belt occasionally reach relativistic energies, turning them into a potential hazard for spacecraft operating in geospace. Such electrons have secured the reputation of satellite killers and play a prominent role in space weather. The flux of these electrons can vary over time scales of years (related to the solar cycle) to minutes (related to sudden storm commencements). Electric fields and plasma waves are the main factors regulating the electron transport, acceleration and loss. Both the fields and the plasma waves are driven directly or indirectly by disturbances originating in the Sun, propagating through interplanetary space and impacting the Earth. This paper reviews our current understanding of the response of outer Van Allen belt electrons to solar eruptions and their interplanetary extensions, i.e. interplanetary coronal mass ejections and high-speed solar wind streams and the associated stream interaction regions. This article is part of the theme issue 'Solar eruptions and their space weather impact'.

Spectral Properties and Abundances of Suprathermal Heavy Ions in Compression Regions near 1 au

Abstract We have analyzed spectral properties and abundances of ∼0.02–3.0 MeV nucleon−1 suprathermal (ST) H–Fe ions in 41 stream interaction regions (SIRs) near 1 au observed by Wind and ACE spacecraft from 1995 January through 2008 December. We find that, (i) the event-averaged spectral index is γ ∼ 2.44, with a standard deviation (σ) of 0.67, (ii) γ’s are poorly correlated with the magnetic compression ratios, and 17% of the events group around γ ∼ 1.5, (iii) γ’s for both O and Fe at ∼0.02–0.09 MeV nucleon−1 and 0.09–0.3 MeV nucleon−1 are correlated, but do not exhibit any systematic steepening or flattening as a function of energy, (iv) the ST heavy ion abundance ratios remain constant with increasing energy, implying that the spectral rollovers, defined by the e-folding energy E 0, are independent of the ion’s mass per charge (M/Q), and (v) SIR ST abundances are similar to the corresponding solar wind values, and do not exhibit any systematic behavior when plotted versus the ion’s M/Q or first ionization potential. The above results pose challenges for (1) particle acceleration models that invoke either a corotating interaction region or SIR shocks between ∼3 and 5 au, (2) particle transport models that predict M/Q-dependent spectral rollovers due to interplanetary turbulence effects, and (3) the notion that SIR ST ions originate directly from the bulk solar wind. Instead, we suggest that the SIR ST ions are accelerated out of a pool of material that includes particles accelerated in solar energetic particle events and processed or heated solar wind ions.

Apparent Friction Coefficient Used for Flow Calculation in Straight Compound Channels

Water flow in channels with a compound cross-section involves an exchange of water mass and momentum between the slower flowing water in the floodplains and the faster water in the main channel. This process is called the streams interaction. As a result, the water velocity in the main channel decreases, and at the same time the velocity and depth of flow increase in the part of the floodplains adjacent to the main channel. Diversification of the surface roughness of the main channel and floodplains significantly affects the form of interactions. The results of laboratory experiments were used to characterize the influence of interactions on the discharge capacity of the channel with diversified roughness. The reduction in velocity of the main channel caused by the stream interactions is described with the apparent friction coefficients introduced at the boundary between the main channel and the floodplain. The obtained values of resistance coefficients, supplemented with the values from experiments reported in the literature, were used to establish a relationship useful in assessing the discharge capacity of such channels.

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.

Influence of Stream Interactions on the Carrying Capacity of Two-Stage Channels

AbstractStreams of water in channels that have a compound cross section interact with each other. The most characteristic feature of such channels is the transfer of momentum and mass from the main...

A Study of the Cross-Scale Causation and Information Flow in a Stormy Model Mid-Latitude Atmosphere.

A fundamental problem regarding the storm–jet stream interaction in the extratropical atmosphere is how energy and information are exchanged between scales. While energy transfer has been extensively investigated, the latter has been mostly overlooked, mainly due to a lack of appropriate theory and methodology. Using a recently established rigorous formalism of information flow, this study attempts to examine the problem in the setting of a three-dimensional quasi-geostrophic zonal jet, with storms excited by a set of optimal perturbation modes. We choose for this study a period when the self-sustained oscillation is in quasi-equilibrium, and when the energetics mimick the mid-latitude atmospheric circulation where available potential energy is cascaded downward to smaller scales, and kinetic energy is inversely transferred upward toward larger scales. By inverting a three-dimensional elliptic differential operator, the model is first converted into a low-dimensional dynamical system, where the components correspond to different time scales. The information exchange between the scales is then computed through ensemble prediction. For this particular problem, the resulting cross-scale information flow is mostly from smaller scales to larger scales. That is to say, during this period, this model extratropical atmosphere is dominated by a bottom-up causation, as collective patterns emerge out of independent entities and macroscopic thermodynamic properties evolve from random molecular motions. This study makes a first step toward an important field in understanding the eddy–mean flow interaction in weather and climate phenomena such as atmospheric blocking, storm track, North Atlantic Oscillation, to name a few.

The Response of Earth's Electron Radiation Belts to Geomagnetic Storms: Statistics From the Van Allen Probes Era Including Effects From Different Storm Drivers

AbstractA statistical study was conducted of Earth's radiation belt electron response to geomagnetic storms using NASA's Van Allen Probes mission. Data for electrons with energies ranging from 30 keV to 6.3 MeV were included and examined as a function of L‐shell, energy, and epoch time during 110 storms with SYM‐H ≤−50 nT during September 2012 to September 2017 (inclusive). The radiation belt response revealed clear energy and L‐shell dependencies, with tens of keV electrons enhanced at all L‐shells (2.5 ≤ L ≤ 6) in all storms during the storm commencement and main phase and then quickly decaying away during the early recovery phase, low hundreds of keV electrons enhanced at lower L‐shells (~3 ≤ L ≤ ~4) in upward of 90% of all storms and then decaying gradually during the recovery phase, and relativistic electrons throughout the outer belt showing main phase dropouts with subsequent and generally unpredictable levels of replenishment during the recovery phase. Compared to prestorm levels, electrons with energies >1 MeV also revealed a marked increase in likelihood of a depletion at all L‐shells through the outer belt (3.5 ≤ L ≤ 6). Additional statistics were compiled revealing the storm time morphology of the radiation belts, confirming the aforementioned qualitative behavior. Considering storm drivers in the solar wind: storms driven by coronal mass ejection (CME) shocks/sheaths and CME ejecta only are most likely to result in a depletion of >1‐MeV electrons throughout the outer belt, while storms driven by full CMEs and stream interaction regions are most likely to produce an enhancement of MeV electrons at lower (L < ~5) and higher (L > ~4.5) L‐shells, respectively. CME sheaths intriguingly result in a distinct enhancement of ~1‐MeV electrons around L~5.5, and on average, CME sheaths and stream interaction regions result in double outer belt structures.

Cortical and thalamic connectivity of temporal visual cortical areas 20a and 20b of the domestic ferret (Mustela putorius furo).

The present study describes the ipsilateral and contralateral corticocortical and corticothalamic connectivity of the temporal visual areas 20a and 20b in the ferret using standard anatomical tract-tracing methods. The two temporal visual areas are strongly interconnected, but area 20a is primarily connected to the occipital visual areas, whereas area 20b maintains more widespread connections with the occipital, parietal and suprasylvian visual areas and the secondary auditory cortex. The callosal connectivity, although homotopic, consists mainly of very weak anterograde labeling which was more widespread in area 20a than area 20b. Although areas 20a and 20b are well connected with the visual dorsal thalamus, the injection into area 20a resulted in more anterograde label, whereas more retrograde label was observed in the visual thalamus following the injection into area 20b. Most interestingly, comparisons to previous connectional studies of cat areas 20a and 20b reveal a common pattern of connectivity of the temporal visual cortex in carnivores, where the posterior parietal cortex and the central temporal region (PMLS) provide network points required for dorsal and ventral stream interaction enroute to integration in the prefrontal cortex. This pattern of network connectivity is not dissimilar to that observed in primates, which highlights the ferret as a useful animal model to understand visual sensory integration between the dorsal and ventral streams. The data generated will also contribute to a connectomics database, to facilitate cross species analysis of brain connectomes and wiring principles of the brain.

L1 Stream Deflection and Ballistic Launching at the Disk Bow Shock: An Absorption-line Velocity Analysis in Semi-detached Binaries

Abstract Observations of semi-detached interacting binaries reveal orbital modulation in the optical, UV, and X-ray bands, indicating the presence of absorbing material obscuring the disk and accreting primary star at specific orbital phases consistent with L1 stream material overflowing the disk edge. We simulate the L1 stream interaction with the disk using tests particles within the context of the Roche model in the restricted three-body problem. At the disk bow shock the L1 stream particles are deflected and launched onto ballistic trajectories above the disk (as would normally occurs at the front of a detached shock in the hypersonic flow past a blunt body). At a given scale height, the material is assumed to continue without being affected by the disk, while at lower altitude it is being launched at an increasing elevation, as well as gradually being dragged by the Keplerian flow. Near the disk midplane (z ≪ H) the material is assumed to become part of the disk. We follow the stream material ballistic trajectories over the disk surface, where they reach a maximum height z/r at a binary phase Φ ∼ 0.75 and land on the disk at a smaller radius around phase Φ ∼ 0.5. The phase of the maximum height, the phase of the landing site, and the phase of the hot spot itself, all decrease significantly with decreasing radius. The radial velocity for each L1 stream ballistic trajectory along the line of sight (of the observer) to the hot inner parts of the disk is computed as a function of the orbital phase for a binary configuration matching the dwarf nova U Geminorum. The computed velocity amplitudes, phases, and pattern match the observed velocity offsets of the metal lines in the FUSE spectrum of U Gem during outburst. As ballistic trajectories are much easier to compute than realistic 3D hydrodynamical simulations, we propose the use of the L1 stream deflection and ballistic launching as a way to analyze the absorption-line orbital variability in semi-detached binaries and to assess or confirm, with some limitations, system parameters such as the mass ratio, inclination, and disk outer radius.

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.

Numerical modeling of the pressure distribution on the axisymmetric body surface at the supersonic freestream interaction with a transversely blown gas jet

The results of numerical investigation of the interaction of a supersonic free stream with the Mach number of 3.85 with a transverse gas jet blown from the surface of an axisymmetric model located at zero angle of attack with respect to the flow are presented. The numerical modeling was conducted using the averaged Navier-Stokes equations employing the k-ω SST turbulence model. The investigation of the influence of the gas jet blowing on pressure redistribution over the axisymmetric model surface and on the formation of an additional transverse force and moment has also been conducted. The shift of the blowing orifice along the model length leads to a significant variation of arising forces and moments. A fundamental difference in the distribution of pressures over the surface at the jet blowing from the axisymmetric body and from a flat plate is shown.

Analysis of the Optical Cyclotron Emission of Polar CRTS CSS081231 J071126+440405

We have modeled the cyclotron spectra and photopolarimetric data of polar CRTS CSS081231 J071126+440405. A series of spectra of the polar were obtained in the intermediate brightness state spanning an orbital period. Modeling spectra allowed us to determine the magnetic field strength in the vicinity of both magnetic poles of the white dwarf: B1 = 38 MG and B2 = 51 MG. We obtained a temperature estimate for the accretion spots near both magnetic poles of the white dwarf. The estimate for the first spot is kT ~ 20–30 keV, whereas the second spot turned out to be cooler by about 10 keV. An analysis of the light curves and polarization of the polar obtained in the low state shows that the interaction of the accretion stream with the star’s atmosphere occurs close (within Δθ = 10◦) to the magnetic pole.

LES–RANS of Installed Ultra-High-Bypass-Ratio Coaxial Jet Aeroacoustics with Flight Stream

Using large-eddy simulation–Reynolds-averaged Navier–Stokes (LES–RANS), this paper studies a round coaxial nozzle with an ultra-high bypass ratio of 15, with and without a wing-flap geometry. Depending on engine placement, the nozzle can become extremely close to the wing-flap geometry, introducing strong installation effects. Two different flap deflections of 8 and 14 deg are contrasted with an isolated round nozzle. A flight stream is applied and an Ffowcs Williams-Hawkings (FWH) surface placement procedure for installed jets is proposed. Impressive agreement with available experimental data is economically achieved for far-field overall sound pressure level and spectra as well as near-field spectra. The LES provides a wider range of polar and azimuthal angles at 1 deg increments, providing one of the most detailed acoustics data sets to date. The installed cases generate up to 20 dB more noise at mid-low frequencies due to large flap trailing edge sources. Additional sources are introduced by the interaction of the flight stream with the lifting surfaces. Second-order space-time correlations reveal length and time scales in the flow. Fourth-order space-time correlations indicate increasing magnitudes of the dominant noise source components with increasing flap angle. The three-dimensional unsteady data set produced should lead to improved acoustics models.

Experimental Studies on a Mesoscale Cycloidal Rotor in Hover

This paper details the performance and flowfield measurements on a mesoscale cycloidal rotor (rotor radius of 1 in.), which operates at ultralow Reynolds numbers () and uses a design different from previous cycloidal rotor studies. The mesoscale rotor uses a cantilevered blade with a flat-plate airfoil and low-aspect-ratio elliptical blade planform. A three-component balance was developed to measure the vertical and sideward thrust, torque, and rotation frequency as the number of blades, pitch amplitude, and blade aspect ratio were varied. The studies showed that the highest efficiency was achieved with higher number of blades (4–6), high pitch amplitude (40–45 deg), and moderate aspect ratios. Phase-locked flowfield measurements were conducted using a particle image velocimetry (PIV) technique in two different orientations: chordwise and spanwise. PIV measurements show that the flowfield on the present mesoscale cycloidal rotor is highly three-dimensional and unsteady, characterized by the growth and shedding of leading-edge vortices and a trailing-edge vortex sheet similar to that seen on flapping wings; strong root and tip vortices that interact with each other to create an undulating wake; skewed inflow; and the strong interaction of the slip stream from the upper blade on the lower blade.

The Merging of Two Stream Interaction Regions within 1 au: The Possible Role of Magnetic Reconnection

Abstract As the Sun rotates, a fast stream can overtake a preceding slow stream, leading to the formation of a stream interaction region (SIR). Two neighboring SIRs may eventually coalesce to produce a merged interaction region (MIR) en route to the outer heliosphere. However, instances of significant interaction and merging of two neighboring SIRs within 1 au are thought to be extremely rare. In this Letter, we present a case report of two interacting and merging SIRs observed near 1 au, which was associated with two adjacent low-latitude coronal holes. The two SIRs were filled with outward propagating Alfvénic fluctuations associated with magnetohydrodynamic turbulence. A reconnection exhaust associated with a current sheet was identified. We suggest that magnetic reconnection represented a potentially important mechanism for the merging of two neighboring SIRs. This observation may shed light on the understanding of the structure and formation of a MIR within 1 au.

Geoeffectiveness of Stream Interaction Regions From 1995 to 2016

AbstractStream interaction regions (SIRs) are important sources of geomagnetic storms. In this work, we first extend the end time of the widely used SIR catalog developed by Jian et al. (2006, https://doi.org/10.1007/s11207-006-0132-3), which covered the period from 1995 to 2009, to the end of 2016. Based on this extended SIR catalog, the geoeffectiveness of SIRs is discussed in detail. It was found that 52% of the SIRs caused geomagnetic storms with Dstmin ≤−30 nT, but only 3% of them caused intense geomagnetic storms with Dstmin ≤−100 nT. Furthermore, we found that 10 of the intense geomagnetic storms caused by SIRs were associated with complex structures due to interactions between SIRs and interplanetary coronal mass ejections (ICMEs). In such a structure, an ICME is embedded in the SIR and located between the slow and fast solar wind streams. In addition, we found that the geoeffectiveness of SIRs interacting with ICMEs is enhanced. The possibility of SIR‐ICME interaction structures causing geomagnetic storms is markedly higher than that of isolated SIRs or isolated ICMEs. In particular, the geoeffectiveness of SIR‐ICME interaction structures is similar to that of the Shock‐ICME interaction structures, which have been demonstrated to be the main causes of geomagnetic storms.

Simulation of the Internal Structure of a Tandem-Type Plasmatron

Numerical simulation of a tandem-type plasmatron intended for supersonic plasma spraying of metallic and ceramic materials at atmospheric pressure has been carried out. The calculation results are compared with experimental data. The internal gas dynamics of the plasmatron and the interaction of a plasma stream with spray particles are presented in the form of distributions of the main thermal and kinetic characteristics of stream.