Small-scale Transients Research Articles

Inferring the Solar Wind Velocity in the Outer Corona Based on Multiview Observations of Small-scale Transients by STEREO/COR2

Based on the Heliospheric Imager-1 images of the STEREO twin spacecraft, we established the CORrelation-Aided Reconstruction (CORAR) technique to locate and reconstruct the 3D structures of solar wind transients in interplanetary space. Here, we extend the CORAR method to images of COR2 on board STEREO to study the evolution of small-scale transients in the outer corona from 2010 January to May. We confirm that the transients can be located and reconstructed well by comparing the results with those of a self-similar expanding model. The speed distribution of the reconstructed transients generally shows the typical characteristics of the slow solar wind. We further study the sources of the transients on the Sun, and find that most reconstructed transients are located near the top of streamer belts or the heliospheric current sheet and can be tracked back to the boundaries of the closed-field and open-field regions along the field lines extrapolated by corona models. The formation mechanisms of these transients in the slow solar wind are also discussed.

Beyond small-scale transients: A closer look at the diffuse quiet solar corona

Aims. Within the quiet Sun corona imaged at 1 MK, much of the field of view consists of diffuse emission that appears to lack the spatial structuring that is so evident in coronal loops or bright points. Our aim is to determine if these diffuse regions are categorically different in terms of their intensity fluctuations and spatial configuration from the better-studied dynamic coronal features. Methods. We analyzed a time series of observations from Solar Orbiter’s High Resolution Imager in the extreme ultraviolet to quantify the characterization of the diffuse corona at high spatial and temporal resolutions. We then compared this to the dynamic features within the field of view, mainly a coronal bright point. Results. We find that the diffuse corona lacks visible structuring, such as small embedded loops, and that this is persistent over the 25 min duration of the observation. The intensity fluctuations of the diffuse corona, which are within ±5%, are significantly smaller in comparison to the coronal bright point; however, the total intensity observed in the diffuse corona is on the same order as the bright point. Conclusions. It seems inconsistent with our data that the diffuse corona is a composition of small loops or jets or that it is driven by discrete small heating events that follow a power-law-like distribution. We speculate that small-scale processes such as magnetohydrodynamic turbulence might be energizing the diffuse regions, but at this point we cannot offer a conclusive explanation for the nature of this feature.

Reconstructing Synoptic Maps of Solar Wind Radial Velocity between 20 and 60 R ⊙ Based on STEREO/HI1 Images

Previously, we developed a correlation-aided reconstruction method to recognize and locate solar wind transients observed by the Heliospheric Imager-1 (HI1) on board the Solar Terrestrial Relations Observatory and then developed a technique to infer the radial velocity distribution in each solar wind transient. Considering that the common field of view of HI1 may cover the full longitudes through the solar rotation, we further apply these methods to small-scale transients (STs) in a complete Carrington rotation to reconstruct a synoptic map of the solar wind radial velocity. Our test suggests that the reconstructed synoptic map is in agreement with the preset synthetic STs in latitude, longitude, and radial velocity. Then, Carrington rotation 2095 between 2010 March 26 and April 22 is selected to demonstrate the new technique. The derived synoptic map shows that the solar wind radial velocity corresponding to STs is in the range of 250–550 km s−1 between 20 and 60 R ⊙ in the low-to-middle latitudes. We extrapolate the in situ observation near 1 au to 20 R ⊙ and find that it matches the synoptic velocity map well. The magnetohydrodynamic simulations of the solar wind radial velocity are consistent with the synoptic map near the heliospheric current sheet (HCS) but usually overestimate the velocity values for STs far away from the HCS. We expect that this technique will be a powerful tool to learn about and monitor the solar wind in the inner heliosphere, where the number of human probes is limited.

Optimal stereoscopic angle for 3D reconstruction of synthetic small-scale coronal transients using the CORAR technique

Context.In previous studies, we applied the CORrelation-Aided Reconstruction (CORAR) technique to reconstruct three-dimensional (3D) structures of transients in the field of view (FOV) of Heliospheric Imager-1 (HI-1) on board the spacecraft STEREO-A/B. The reconstruction quality depends on the stereoscopic angle (θSun), that is, the angle between the lines connecting the Sun and two spacecraft.Aims.To apply the CORAR technique on images from the coronagraphs COR-2 on board STEREO, the impact ofθSunon the reconstruction of coronal transients should be explored, and the optimalθSunfor reconstruction should be found.Methods.We apply the CORAR method on synthetic COR-2 images containing the small-scale transient, namely the blob, in the case of variousθSun. Based on a comparison of the synthetic blob and the corresponding reconstructed structure in location and 3D shape, we assess its level of reconstruction quality. According to the reconstruction-quality levels of blobs in various positions with various attributes, we evaluate the overall performance of reconstruction in the COR-2 FOV to determine the optimalθSunfor reconstruction.Results.In the case ofθSun > 90°, we find that the range of suitableθSun, in which the small-scale transients in the COR-2 FOV typically have high reconstruction quality, is between 120° and 150°, and the optimalθSunfor reconstruction is close to 135°. In the case ofθSun < 90°, the global reconstruction performance is similar to that of (180° −θSun). We also discuss the spatial factors in determining the range of suitableθSun, and study the influence of blob properties on the reconstruction. Our work can serve as a foundation for the design of future missions containing coronagraphs from multiple perspectives, such as the newly proposed SOlar Ring mission (SOR).

Spectroscopic and imaging observations of transient hot and cool loops by IRIS and SDO

ABSTRACT Coronal loops are the basic building blocks of the solar atmosphere and are observed on various length scales. However, their formation mechanism is still unclear. In this paper, we present spectroscopic and imaging observations of small-scale transients and the subsequent formation of transient loops. For this purpose, we have utilized the multiwavelength observations recorded by the Atmospheric Imaging Assembly (AIA) and the Interface Region Imaging Spectrograph (IRIS) slit-jaw imager (SJI), along with spectroscopic measurements provided by IRIS. For the photospheric magnetic field data, we obtained line-of-sight magnetogram data provided by the Helioseismic and Magnetic Imager (HMI). Small-scale transients are simultaneously observed with several EUV and UV passbands of AIA and IRIS/SJI. The HMI magnetogram provides evidence of negative flux cancellations beneath these transients. Differential emission measure (DEM) analysis shows that one of the transients attains temperatures up to 8 MK whereas another one only reaches 0.4 MK. These transients further lead to the formation of small-scale loops with similar temperature distributions, thus termed hot and cool loops respectively. During the course of events, the IRIS slit was rastering the region and thus provided spectroscopic measurements at both transients and associated loops. This enabled us to perform in-depth investigations of the hot and cool loops. Using a density-sensitive O iv line pair, we obtained average electron densities along the hot and cool loops to be 1011.2 and 1010.8 cm−3 respectively. Energy estimates suggest that flux cancellation could easily power the hot transient, while it is insufficient for the cool transient. Lifetime estimates and magnetic field extrapolation suggest the presence of small-scale and fine structures within these loops. The results provide crucial ingredients for the physics of loop formation and the thermodynamics involved.

Radial velocity map of solar wind transients in the field of view of STEREO/HI1 on 3 and 4 April 2010

Context. The solar wind transients propagating out in the inner heliosphere can be observed in white-light images from Heliospheric Imager-1 (HI1), an instrument of the Sun Earth Connection Coronal and Heliospheric Investigation on board the Solar Terrestrial Relations Observatory (STEREO), from two perspectives. The spatial velocity distribution inside solar wind transients is key to understanding their dynamic evolution processes. Aims. We generated a velocity map of transients in 3D space based on 2D white-light images and used it to estimate the expansion rate as well as some kinematic properties of solar wind transients. Methods. Based on the recently developed correlation-aided reconstruction method in our previous work, which can recognize and locate 3D solar wind transients from STEREO/HI1 image data, we further developped a new technique for deriving the spatial distribution of the radial velocities of the most pronounced features inside solar wind transients. Results. The technique was applied to events including a coronal mass ejection (CME) and three small-scale transients, so-called blobs, observed by HI1 on 3–4 April 2010 to reconstruct their radial velocity maps. The results match the forward-modeling results, simulations, and in situ observations at 1 AU fairly well. According to the obtained spatial distributions of height and radial velocity of the CME, we analyzed the self-similarity of the radial expansion of the CME ejecta. The dimensionless radial expansion rate of the northern and middle parts of the CME ejecta varies in the range of 0.7−1.0 at heliocentric distance between 25 R⊙ and 55 R⊙ and the rate of the southern part in the range of 0.3−0.5, suggesting that the CME structure was distorted and shaped by the ambient solar wind. The technique we developed is expected to be applied to more events.

Second-order discontinuous Galerkin flood model: Comparison with industry-standard finite volume models

Finite volume (FV) numerical solvers of the two-dimensional shallow water equations are core to industry-standard flood models. The second-order Discontinuous Galerkin (DG2) alternative, although a viable way forward to improve current FV-based flood models, is yet under-studied and rarely used to support flood modelling applications. This paper systematically explores and compares the predictive properties of a robust DG2 flood model to those of prominent FV-based industrial flood models. To identify the simplest and most efficient DG2 configuration suitable for flood inundation modelling, two variants – with and without local slope limiting – are considered. The numerical conservation properties of the DG2 variants are compared to those of a first-order FV (FV1) and a second-order FV (FV2) counterparts. The DG2 variants are then tested over five realistic flooding scenarios, recommended by the UK Environment Agency to validate 2D flood model capabilities, while comparing their performance against that of four FV-based commercial models (i.e. TUFLOW-FV1, TUFLOW-FV2, TUFLOW-HPC and Infoworks ICM). Results reveal that the DG2 variant without local limiting (DG2-NL) is capable to simulate shockless flood flows featured in a wide range of flood modelling applications. The DG2-NL shows closer predictions to commercial model outputs at twice-coarser spatial resolution, and can run twice faster to produce more informative hydrograph with small-scale transients over long-range simulations, even when the sampling is far away from the flooding source.

THE EFFECTS OF TRANSIENTS ON PHOTOSPHERIC AND CHROMOSPHERIC POWER DISTRIBUTIONS

ABSTRACT We have observed a quiet-Sun region with the Swedish 1 m Solar Telescope equipped with the CRISP Imaging SpectroPolarimeter. High-resolution, high-cadence, Hα line scanning images were taken to observe different layers of the solar atmosphere from the photosphere to upper chromosphere. We study the distribution of power in different period bands at different heights. Power maps of the upper photosphere and the lower chromosphere show suppressed power surrounding the magnetic-network elements, known as “magnetic shadows.” These also show enhanced power close to the photosphere, traditionally referred to as “power halos.” The interaction between acoustic waves and inclined magnetic fields is generally believed to be responsible for these two effects. In this study we explore whether small-scale transients can influence the distribution of power at different heights. We show that the presence of transients, like mottles, Rapid Blueshifted Excursions (RBEs), and Rapid Redshifted Excursions (RREs), can strongly influence the power maps. The short and finite lifetime of these events strongly affects all power maps, potentially influencing the observed power distribution. We show that Doppler-shifted transients like RBEs and RREs that occur ubiquitously can have a dominant effect on the formation of the power halos in the quiet Sun. For magnetic shadows, transients like mottles do not seem to have a significant effect on the power suppression around 3 minutes, and wave interaction may play a key role here. Our high-cadence observations reveal that flows, waves, and shocks manifest in the presence of magnetic fields to form a nonlinear magnetohydrodynamic system.

DYNAMICS OF ON-DISK PLUMES AS OBSERVED WITH THE INTERFACE REGION IMAGING SPECTROGRAPH, THE ATMOSPHERIC IMAGING ASSEMBLY, AND THE HELIOSEISMIC AND MAGNETIC IMAGER

We examine the role of small-scale transients in the formation and evolution of solar coronal plumes. We study the dynamics of plume footpoints seen in the vicinity of a coronal hole using the Atmospheric Imaging Assembly (AIA) images, the Helioseismic and Magnetic Imager magnetogram on board the Solar Dynamics Observatory and spectroscopic data from the Interface Region Imaging Spectrograph (IRIS). Quasi-periodic brightenings are observed in the base of the plumes and are associated with magnetic flux changes. With the high spectral and spatial resolution of IRIS, we identify the sources of these oscillations and try to understand what role the transients at the footpoints can play in sustaining the coronal plumes. IRIS sit-and-stare observations provide a unique opportunity to study the evolution of footpoints of the plumes. We notice enhanced line width and intensity, and large deviation from the average Doppler shift in the line profiles at specific instances, which indicate the presence of flows at the footpoints of plumes. We propose that outflows (jet-like features) as a result of small-scale reconnections affect the line profiles. These jet-like features may also be responsible for the generation of propagating disturbances (PDs) within the plumes, which are observed to be propagating to larger distances as recorded from multiple AIA channels. These PDs can be explained in terms of slow magnetoacoustic waves.

Coronal hole boundaries at small scales

We study the magnetic properties of small-scale transients in coronal hole. We found all brightening events are associated with bipolar regions and caused by magnetic flux emergence followed by cancellation with the pre-existing and newly emerging magnetic flux. In the coronal hole, 19 of 22 events have a single stable polarity which does not change its position in time. In eleven cases this is the dominant polarity. The dominant flux of the coronal hole form the largest concentration of magnetic flux in terms of size while the opposite polarity is distributed in small concentrations. In the coronal hole the number of magnetic elements of the dominant polarity is four times higher than the non-dominant one. The supergranulation configuration appears to preserve its general shape during approximately nine hours of observations although the large concentrations in the network did evolve and were slightly displaced, and their strength either increased or decreased. The emission fluctuations seen in the X-ray bright points are associated with reoccurring magnetic cancellation in the footpoints. Unique observations of an X-ray jet reveal similar magnetic behaviour in the footpoints, i.e. cancellation of the opposite polarity magnetic flux. We found that the magnetic flux cancellation rate during the jet is much higher than in bright points. Not all magnetic cancellations result in an X-ray enhancement, suggesting that there is a threshold of the amount of magnetic flux involved in a cancellation above which brightening would occur at X-ray temperatures. Our study demonstrates that the magnetic flux in coronal holes is continuously recycled through magnetic reconnection which is responsible for the formation of numerous small-scale transient events. The open magnetic flux forming the coronal-hole phenomenon is largely involved in these transient features.

Experimental Study of Low Temperature Diesel Combustion Sensitivity to Engine Operating Parameters

This work elucidated which engine operating parameters have the greatest influence on Low temperature diesel combustion (LTC) and emissions. Key parameters were selected and evaluated at low and intermediate speed and load conditions using fractional factorial and Taguchi orthogonal experimental designs. The variations investigated were: about ± 5% in EGR rate, fuel injection quantity and engine speed respectively; and ± 10 °C in intake charge temperature. The half-fractional factorial results showed that the interactions among these parameters were negligible for a specific load/speed point. The Taguchi orthogonal method could be used as an efficient DoE tool for studying the multi-parameter ‘small-scale transients’ that a diesel engine would be likely to encounter when operating in LTC modes. LTC showed the most significant sensitivity to EGR rate variations, where an increase from 60% to 63% in EGR rate doubled THC and CO emissions and reduced combustion stability. LTC was also sensitive to the fuel injection quantity with an increase in injected mass lowering the overall oxygen-fuel ratio and thereby increasing THC and CO emissions. These two parameters influenced the oxygen concentration in the intake charge; which was identified to be a decisive parameter for the LTC combustion and emissions. Intake charge temperature affected the total charge quantity trapped in the cylinder and showed noticeable influence on CO emissions for the low speed intermediate load condition. Variations in engine speed showed a negligible influence on the LTC combustion processes and emissions.

Statistics of counter-streaming solar wind suprathermal electrons at solar minimum: STEREO observations

Abstract. Previous work has shown that solar wind suprathermal electrons can display a number of features in terms of their anisotropy. Of importance is the occurrence of counter-streaming electron patterns, i.e., with "beams" both parallel and anti-parallel to the local magnetic field, which is believed to shed light on the heliospheric magnetic field topology. In the present study, we use STEREO data to obtain the statistical properties of counter-streaming suprathermal electrons (CSEs) in the vicinity of corotating interaction regions (CIRs) during the period March–December 2007. Because this period corresponds to a minimum of solar activity, the results are unrelated to the sampling of large-scale coronal mass ejections, which can lead to CSE owing to their closed magnetic field topology. The present study statistically confirms that CSEs are primarily the result of suprathermal electron leakage from the compressed CIR into the upstream regions with the combined occurrence of halo depletion at 90° pitch angle. The occurrence rate of CSE is found to be about 15–20% on average during the period analyzed (depending on the criteria used), but superposed epoch analysis demonstrates that CSEs are preferentially observed both before and after the passage of the stream interface (with peak occurrence rate >35% in the trailing high speed stream), as well as both inside and outside CIRs. The results quantitatively show that CSEs are common in the solar wind during solar minimum, but yet they suggest that such distributions would be much more common if pitch angle scattering were absent. We further argue that (1) the formation of shocks contributes to the occurrence of enhanced counter-streaming sunward-directed fluxes, but does not appear to be a necessary condition, and (2) that the presence of small-scale transients with closed-field topologies likely also contributes to the occurrence of counter-streaming patterns, but only in the slow solar wind prior to CIRs.

JETS IN POLAR CORONAL HOLES

Here, we explore the nature of small-scale jet-like structures and their possible relation to explosive events and other known transient features, like spicules and macrospicules, using high-resolution spectroscopy obtained with the Solar and Heliospheric Observatory/Solar Ultraviolet Measurements of Emitted Radiation instrument. We present a highly resolved spectroscopic analysis and line parameter study of time-series data for jets occurring on-disk and off-limb in both a northern and a southern coronal hole. The analysis reveals many small-scale transients which rapidly propagate between the mid-transition region (N IV 765 A line formation: 140,000 K) and the lower corona (Ne VIII 770 A line formation: 630,000 K). In one example, a strong jet-like event is associated with a cool feature not present in the Ne VIII 770 A line radiance or Doppler velocity maps. Another similar event is observed, but with a hot component, which could be perceived as a blinker. Our data reveal fast, repetitive plasma outflows with blueshift velocities of 145 km s–1 in the lower solar atmosphere. The data suggest a strong role for smaller jets (spicules), as a precursor to macrospicule formation, which may have a common origin with explosive events.

A tractable inhomogeneous closure theory for flow over mean topography

The quasi-diagonal direct interaction approximation (QDIA) is shown to be a computationally tractable closure theory for inhomogeneous two-dimensional turbulent flow over mean (single-realization) topography. In this paper numerical results for the QDIA are compared to direct numerical simulation (DNS) at moderate Reynolds number for two cases with quite different topographic and mean field amplitudes. The QDIA is found to be in excellent agreement with DNS for cases where the small-scale topographic amplitude is significant. For cases where the small-scale topography is weak, the QDIA closely reproduces the evolving mean field and large-scale energy containing transients but under represents the amplitudes of the small-scale transients in a similar way to the homogeneous DIA. We discuss the prospects of ameliorating the small-scale deficiencies using a regularization of the interaction coefficients.