Small-scale Plasma Research Articles

Electromagnetic emission from plasma with counter-streaming electron beams in the regime of oblique instability dominance

In this study, we investigate the generation of electromagnetic emission near the second harmonic of the plasma frequency induced by pairs of counter-propagating electron beams. Such systems can naturally occur in cosmic plasmas when particle acceleration regions are closely spaced, and they can also be implemented in a laboratory device. We specifically focus on the regime where the oblique beam–plasma instability dominates. The emission mechanism relies on the coalescence of counter-propagating plasma waves with different transverse structures. It has been demonstrated that the parameters of the system necessary for efficient radiation generation can be determined using the exact linear theory of beam–plasma instability. Through particle-in-cell numerical simulations, we show that a high beam-to-radiation conversion efficiency can be achieved when the beams excite small-scale oblique plasma oscillations. Importantly, we find that the efficiency and spectral characteristics of the radiation are not dependent on the thickness of the beams. We explore two scenarios involving pairs of symmetric beams: one with relativistic beams having a directed velocity of vb=0.9c and another with sub-relativistic beams at vb=0.7c. Additionally, we consider the injection of two beams with different velocities. In all cases considered, the beam-to-radiation power conversion efficiency reaches a level of a few percent, a sufficiently high value for beam–plasma systems.

Evolution of a long-duration transpolar arc during very quiet period

In this study, we explore the evolution of long-duration transpolar arcs observed during an exceptionally quiet period by the Fengyun-3D and Defense Meteorological Satellite Program satellites on 17 January 2019. Leveraging the extensive field of view of WAI, we captured the complete structure and evolution of the TPA over a very quiet period exceeding 7 h, with AE<30 nT. The TPA initially manifested at the poleward boundary of the auroral oval in the dawn sector, gradually migrating towards the dusk as the y-component of the interplanetary magnetic field (IMF BY) transitioned from negative to positive. Throughout this phase, the TPA underwent a distortion, transforming from a straight line to an “l-shape”. Following a reversal in interplanetary magnetic field BY to negative, the TPA retraced its path towards dawn and ultimately dissipated when the IMF turned southward. Of particular interest is the observation that when the TPA approached the noon-midnight line, small-scale spiral structures became apparent within the TPA. This suggests the presence of intricate small-scale magnetic field topologies or plasma instabilities/shears in the source region. These observations highlight the likelihood of complex field and particle dynamics in the high-latitude reconnection region and the twisted tail plasma sheet.

Peculiarities of leakage of the electromagnetic waves from striations

It is well-known that an ordinary (O-mode) high frequency (HF) electromagnetic (EM) wave radiated into the ionosphere generates in the F-region upper-hybrid resonance (UHR) oscillations trapped in small-scale elongated plasma depletions (striations). Striations are not ideal resonators for UHR oscillations. There is some leakage of the HF energy from striations in the form of Z-mode waves. This result obtained in the quasi-electrostatic approximation is partly reconsidered in the present paper. It is shown in the 2D case that two EM waves (instead of one Z-mode) propagate from each side of a striation. The lowest eigenmodes of the electrostatic oscillations trapped in a striation excite at particular heights Z-mode waves that propagate significantly higher than the reflection height of the initial O-mode wave. This result exists for different angles of the O-mode radiation with respect to the vertical (both more and less than the critical angle).

Self-similar Outflows at the Source of the Fast Solar Wind: A Smoking Gun of Multiscale Impulsive Reconnection?

We present results of a quantitative analysis of structured plasma outflows above a polar coronal hole observed by the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO) spacecraft. In a 6 hr interval of continuous high-cadence SDO/AIA images, we identified more than 2300 episodes of small-scale plasma flows in the polar corona. The mean upward flow speed measured by the surfing transform technique is estimated to be 122 ± 34 km s−1, which is comparable to the local sound speed. The typical recurrence period of the flow episodes is 10–30 minutes, and the mean duration and transverse size of each episode are about 3–5 minutes and 3–4 Mm, respectively. The largest identifiable episodes last for tens of minutes and reach widths up to 40 Mm. For the first time, we demonstrate that the polar coronal-hole outflows obey a family of power-law probability distributions characteristic of impulsive interchange magnetic reconnection. Turbulent photospheric driving may play a crucial role in releasing magnetically confined plasma onto open field. The estimated occurrence rate of the detected self-similar coronal outflows is sufficient for them to make a dominant contribution to the fast-wind mass and energy fluxes and to account for the wind’s small-scale structure.

Measuring small-scale plasma irregularities in the high-latitude E- and F-regions simultaneously

The ionosphere, Earth’s space environment, exhibits widespread turbulent structuring, or plasma irregularities, visualized by the auroral displays seen in Earth’s polar regions. Such plasma irregularities have been studied for decades, but plasma turbulence remains an elusive phenomenon. We combine scale-dependent measurements from a ground-based radar with satellite observations to characterize small-scale irregularities simultaneously in the bottomside and topside ionosphere and perform a statistical analysis on an aggregate from both instruments over time. We demonstrate the clear mapping of information vertically along the ionospheric altitude column, for field-perpendicular wavelengths down to 1.5 km. Our results paint a picture of the northern hemisphere high-latitude ionosphere as a turbulent system that is in a constant state of growth and decay; energy is being constantly injected and dissipated as the system is continuously attempting an accelerated return to equilibrium. We connect the widespread irregularity dissipation to Pedersen conductance in the E-region, and discuss the similarities between irregularities found in the polar cap and in the auroral region in that context. We find that the effects of a conducting E-region on certain turbulent properties (small-scale spectral index) is near ubiquitous in the dataset, and so we suggest that the electrodynamics of a conducting E-region must be considered when discussing plasma turbulence at high latitudes. This intimate relationship opens up the possibility that E-region conductivity is associated with the generation of F-region irregularities, though further studies are needed to assess that possibility.

Recurrent solar density transients in the slow wind observed with the Metis coronagraph

Aims We aim to investigate and characterize the morphology and dynamics of small-scale coronal plasma density inhomogeneities detected as brighter, denser features propagating outward through the solar corona in the visible-light images of the Metis coronagraph on board Solar Orbiter on February 22, 2021. Our main focus is on investigating their possible origin and contribution to the slow wind variability and dynamics and their dependence on coronal magnetic field configurations and structure. Methods. The method adopted is based on the computations of autocorrelation and cross-correlation functions applied to temporal and spatial series of total brightness as a function of the heliocentric distance and solar latitudes. Results. We find that the plasma density inhomogeneities studied here are small-scale structures with typical radial and transverse sizes, as projected on the plane of sky, on the order of 500 Mm and 40 Mm, respectively, and that they are up to 24 times brighter than the ambient solar wind. The brighter density structures exhibit longer lifetime and more stable shape and dimensions as they travel toward the outer edge of the field of view. The enhanced density structures are ejected with a most probable cadence of about 80 min at or below the inner edge of the Metis field of view (within 3.1 R⊙–5.7 R⊙ at the time of observations) in a wide latitudinal region corresponding to the site of a complex web of separatrix and quasi-separatrix layers, as resulting from the simulated magnetohydrodynamic configuration of the west limb of the solar corona. Some of the moving density enhancements clearly show morphological characteristics compatible with the switchback phenomenon, supporting the results indicating that the switchbacks occur at the coronal level. The enhanced density structures were ejected into the ambient slow wind with a mean velocity of about 240 ± 40 km s−1, which is significantly higher than that deduced for the ambient solar wind on the basis of previous Metis observations during the solar minimum of cycle 24. The absence of acceleration observed across the coronagraph field of view suggests that the ejected plasmoids are progressively reaching the expansion rate of the ambient wind. Conclusions. The results suggest that the quasi-periodic enhanced-density plasmoids might be the consequence of reconnection phenomena occurring in the complex web of the separatrix and quasi-separatrix layers present in the solar corona. Moreover, the structural characteristics of some of the detected plasmoids are in favor of the presence of switchbacks that originate during interchange reconnection processes occurring at or below 3 R⊙ in the S-web. The speed of the plasma ejected in the reconnection process is higher than that of the ambient slow solar wind and is likely to be related to the energy involved in the process generating the propagating structures.

Spectroscopic investigation of atmospheric pressure cold plasma jet produced in dielectric barrier discharge

Non-thermal cold atmospheric pressure plasma jet (APPJ) produced in multi-pulse dielectric barrier discharge (DBD) preferentially heats the electrons that subsequently transfer their energy to the other plasma species converting them into reactive species. Therefore the characterization of different energy groups of electrons is essential for efficient production and control of reactive plasma species contributing to surface modifications. Here we present the spectroscopic investigations of the APPJ produced in multi-pulse DBD using pin electrode configuration. Boltzmann plot relates the emission intensities of the several Ar–I spectral lines to their corresponding threshold excitation energies to give the electron temperature more precisely. The Stark broadening of the Ar–I (696.54 nm) line profile gives the electron number density after de-convoluting the contributions of the Doppler and instrumental broadenings. Plasma measurements correlate the gas flow rate, applied jet power across the electrodes, and a group of electrons contributing to optical emission. Sequential imaging correlates the size of the plasma bullets momentarily with the discharge pulse for different argon flow rates and applied jet powers. With an increasing flow rate of up to 1.4 L/min, the plasma plume of the jet becomes more elongated, intense, and spatially uniform. However, the glow intensity and size of the plume start reducing with a further increase in gas flow rate. Experimental findings propose plasma processing for small-scale localized plasma surface treatments.

Role of the variable solar wind in the dynamics of small-scale magnetosheath structures

Varying solar wind plays a crucial role in the processes inside the magnetosphere. Statistical studies generally reveal the geoeffectiveness of solar wind streams of different origins and types, characterized by various parameters such as dynamic pressure and magnetic field orientation. However, the predictions of the space weather are still not completely reliable. Small-scale structures (observed as high-amplitude variations with frequencies above 0.01 Hz) involved in the turbulent flow of the solar wind and in the magnetosheath may contribute to the models’ inaccuracies. Spacecraft measurements obtained during last 2 decades provide a great amount of new information about small-scale plasma processes in near-Earth space. However, the influence of solar wind on the dynamics of the small-scale structures in the magnetosheath has been rarely addressed. The present review summarizes experimental studies on this influence including features of turbulence around ion scales. The study aims to give a general picture of the problem and underline the gaps in current understanding of the role of the dynamics of the small-scale structures and turbulence in the solar–terrestrial relations.

Thomson microwave scattering for diagnostics of small plasma objects enclosed within glass tubes.

In this work, coherent microwave scattering in the Thomson regime was demonstrated for small-scale plasmas enclosed within a glass tube and validated using a well-known hairpin resonator probe technique. The experiments were conducted in a DC discharge tube with a diameter of 1.5cm and a length of 7cm. Thomson microwave scattering (TMS) diagnostics yielded electron number densities of about 5.9 × 1010 cm-3, 2.8 × 1010 cm-3, and 1.8 × 1010 cm-3 for air pressures in the discharge tube of 0.2, 0.5, and 2.5 Torr, respectively. Measurements using the TMS technique were consistent across the tested microwave frequencies of 3-3.9GHz within the margin of error associated with non-idealities of the IQ mixer utilized in the circuit. The corresponding densities measured with the hairpin resonator probe were 4.8 × 1010, 3.8 × 1010, and 2.6 × 1010 cm-3. Discrepancies between the two techniques were within 30% and can be attributed to inaccuracies in the sheath thickness estimation required for correct interpretation of the hairpin resonator probe results.

On the Mechanisms of Formation of Density Cavities under Instability of Intense Langmuir Oscillations in a Plasma

The paper considers the instability of intense Langmuir oscillations in nonisothermal (Zakharov's model) and cold (Silin's model) 1D plasma. The main attention is paid to the formation of plasma density caverns in the hydrodynamic and hybrid (electrons are described hydrodynamically, ions are described by model particles) representations. In the hydrodynamic representation, with a small number of spectrum modes, large-scale plasma density caverns are observed, which rapidly deepen. This process is supported by the appearance of small-scale perturbations, and phase synchronization of the Langmuir waves of the instability spectrum is observed. This phase synchronization of the spectrum modes is quite capable of fulfilling the role that was previously proposed to be given exclusively to the effect of extrusion of particles from the cavity by the field. In hybrid models, in the region of consideration, ions are described by model particles, the number of which in the one-dimensional case 104-5*105 (which in the three-dimensional case corresponds to the number of particles 1012-1014). The initial spectrum of perturbations is very wide and rather intense, which leads to an explosive growth of perturbations in the Zakharov model and a rapid development of instability in the Silin model. In this case, in the developed instability regime, the formation of many small-scale plasma density caverns is observed. It is the presence of this small-scale modulation due to the Fermi effect that quickly forms the normal distribution of ions over velocities. In this case, the effect of particle heating due to Landau damping loses its primacy. It is shown that the caverns practically do not change their position; phase changes for the spectral components of the plasma density were not observed. Only individual small-scale caverns demonstrate dynamics similar to the development of caverns in the hydrodynamic representation.

Air plasma ablation/erosion test for 4D C/C composites used in the throat of solid rocket motor

A small-scale plasma ablation facility was employed to test the four-directional carbon/carbon (4D C/C) composites under high enthalpy multi-phase jet condition for ablation/erosion performance evaluation with the ablation flow field studied via computational fluid dynamics (CFD). The various ablation rates, ablation behavior and microstructure of the C/C specimens were investigated. The overall mass ablation rate was 0.05715 g·s−1 with 0.05550 mm·s−1and 0.01794 mm·s−1 axial linear ablation rate of inner and outer walls. In the radial direction, the linear ablation rates were 0.07794 and 0.00517 mm·s−1 for the inner and external diameter respectively. Ablation changes the inner wall of the annular specimen into an inverted horn shape, while the edge near the jet becomes rounded corners after the plasma procedure. The interface between the axial carbon rod and the matrix has visible ring fractures. Alumina particles are lodged in cracks and holes as a result of multiphase flow scouring. The regression rate of the composites can be considerably accelerated by particle erosion and the existence of inherent defects. The specimen ablation of top surface and inner wall is attributed to the coupling of the heterogeneous reaction and multiphase erosion. On the back wall of specimen, the regression is mainly caused by thermochemical ablation.

Design and setup of a jet-based technology for localized small scale Plasma electrolytic Polishing

Plasma electrolytic Polishing (PeP) represents an innovative technique for finish-machining of complex parts and micro features. The applicability is usually restricted by limitations of the electric power provided by the process energy source (PES). Besides, addressing locally confined areas or single micro features is not possible, since the whole workpiece or a part of it is immersed into an electrolyte bath and thus machined. To overcome this, the development and verification of an alternative process arrangement for localized PeP is presented in this study.The immersion-based PeP was transferred to a small-scale jet-based process (Jet-PeP). It is shown that Jet-PeP allows for local and adapted treatment of AISI 316 L. For the realization of the Jet-PeP technology a specific PES, which provides the required electric energy with process-specific characteristics, was developed and implemented into a novel prototype rig. The shape of the electrolyte jet resulting from the chosen flow rate and nozzle diameter was simulated and the maximum expected current for process initiation was determined. For the functional verification, a planar workpiece surface was polished with various voltages. Surface roughness was measured before and after Jet-PeP. The results indicate localized polishing effects and significant reductions in roughness.

Observations and modeling of scintillation in the vicinity of a polar cap patch

Small-scale ionospheric plasma structures can cause scintillation in radio signals passing through the ionosphere. The relationship between the scintillated signal and how plasma structuring develops is complex. We model the development of small-scale plasma structuring in and around an idealized polar cap patch observed by the Resolute Bay Incoherent Scatter Radars (RISR) with the Geospace Environment Model for Ion-Neutral Interactions (GEMINI). Then, we simulate a signal passing through the resulting small-scale structuring with the Satellite-beacon Ionospheric-scintillation Global Model of the upper Atmosphere (SIGMA) to predict the scintillation characteristics that will be observed by a ground receiver at different stages of instability development. Finally, we compare the predicted signal characteristics with actual observations of scintillation from ground receivers in the vicinity of Resolute Bay. We interpret the results in terms of the nature of the small-scale plasma structuring in the ionosphere and how it impacts signals of different frequencies and attempt to infer information about the ionospheric plasma irregularity spectrum.

The Relationship Between Solar Wind Dynamic Pressure Pulses and Solar Wind Turbulence

Solar wind dynamic pressure pulses (DPPs) are small-scale plasma structures with abrupt and large-amplitude plasma dynamic pressure changes on timescales of seconds to several minutes. Overwhelming majority of DPP events (around 79.13%) reside in large-scale solar wind transients, i.e., coronal mass ejections, stream interaction regions, and complex ejecta. In this study, the intermittency, which is a typical feature of solar wind turbulence, is determined and compared during the time intervals in the undisturbed solar wind and in large-scale solar wind transients with clustered DPP events, respectively, as well as in the undisturbed solar wind without DPPs. The probability distribution functions (PDFs) of the fluctuations of proton density increments normalized to the standard deviation at different time lags in the three types of distinct regions are calculated. The PDFs in the undisturbed solar wind without DPPs are near-Gaussian distributions. However, the PDFs in the solar wind with clustered DPPs are obviously non-Gaussian distributions, and the intermittency is much stronger in the large-scale solar wind transients than that in the undisturbed solar wind. The major components of the DPPs are tangential discontinuities (TDs) and rotational discontinuities (RDs), which are suggested to be formed by compressive magnetohydrodynamic (MHD) turbulence. There are far more TD-type DPPs than RD-type DPPs both in the undisturbed solar wind and large-scale solar wind transients. The results imply that the formation of solar wind DPPs could be associated with solar wind turbulence, and much stronger intermittency may be responsible for the high occurrence rate of DPPs in the large-scale solar wind transients.

The nighttime ionospheric response and occurrence of equatorial plasma irregularities during geomagnetic storms: a case study

Recent studies revealed that the long-lasting daytime ionospheric enhancements of Total Electron Content (TEC) were sometimes observed in the Asian sector during the recovery phase of geomagnetic storms (e.g., Lei (J Geophys Res Space Phys 123: 3217–3232, 2018), Li (J Geophys Res Space Phys 125: e2020JA028238, 2020). However, they focused only on the dayside ionosphere, and no dedicated studies have been performed to investigate the nighttime ionospheric behavior during such kinds of storm recovery phases. In this study, we focused on two geomagnetic storms that happened on 7–8 September 2017 and 25–26 August 2018, which showed the prominent daytime TEC enhancements in the Asian sector during their recovery phases, to explore the nighttime large-scale ionospheric responses as well as the small-scale Equatorial Plasma Irregularities (EPIs). It is found that during the September 2017 storm recovery phase, the nighttime ionosphere in the American sector is largely depressed, which is similar to the daytime ionospheric response in the same longitude sector; while in the Asian sector, only a small TEC increase is observed at nighttime, which is much weaker than the prominent daytime TEC enhancement in this longitude sector. During the recovery phase of the August 2018 storm, a slight TEC increase is observed on the night side at all longitudes, which is also weaker than the prominent daytime TEC enhancement. For the small-scale EPIs, they are enhanced and extended to higher latitudes during the main phase of both storms. However, during the recovery phases of the first storm, the EPIs are largely enhanced and suppressed in the Asian and American sectors, respectively, while no prominent nighttime EPIs are observed during the second storm recovery phase. The clear north–south asymmetry of equatorial ionization anomaly crests during the second storm should be responsible for the suppression of EPIs during this storm. In addition, our results also suggest that the dusk side ionospheric response could be affected by the daytime ionospheric plasma density/TEC variations during the recovery phase of geomagnetic storms, which further modulates the vertical plasma drift and plasma gradient. As a result, the growth rate of post-sunset EPIs will be enhanced or inhibited.

The INSPIRESat-1: Mission, science, and engineering

INSPIRESat-1 (IS-1) is a small scientific satellite being developed jointly by the Laboratory for Atmospheric and Space Physics (LASP) at the University of Colorado at Boulder in the United States, the Indian Institute of Space Science and Technology (IIST) in India, the National Central University (NCU) in Taiwan and the Nanyang Technological University (NTU) in Singapore. This paper describes the science objectives of the IS-1 mission, the engineering and current development status. The IS-1 has completed all functional and performance tests as well as all environmental tests. The spacecraft is manifested on the Indian Space Research Organization’s (ISRO) Polar Satellite Launch Vehicle (PSLV) C-52 mission in 2021. The IS-1 carries two scientific instruments: the Compact Ionospheric Probe (CIP) developed at NCU for studying Earth’s dynamic ionosphere and the Dual-zone Aperture X-ray Solar Spectrometer (DAXSS) developed at LASP for studying the highly-variable solar X-ray radiation. It also carries a Command and Data Handling board developed by IIST. The primary science objectives of the INSPIRESat-1 are twofold. First, using CIP to characterize the temporal and spatial distributions of small-scale plasma irregularities like plasma bubbles in the ionosphere. Secondly, IS-1 aims to provide a greater understanding of why the Sun’s corona is orders of magnitude hotter than the photosphere, why there is an abundance of elements change during different solar events, and how these events (observed with greater soft x-ray fidelity) affect the earth’s ionosphere. The International Satellite Program in Research and Education (INSPIRE) is a consortium of universities with active space programs, formed to advance space science and engineering.

Study on the anticancer effects of a 7 μm sized helium plasma jet on micro-tumors

As a promising technology for cancer therapy, cold atmospheric plasma has received extensive attention in recent years. However, previous studies pay more attention on how to achieve and amplify the anticancer range of plasma, while neglecting the clinical needs of precision medicine. Hence there is a lack of small-scale and fixed-point plasma technology for micro-tumor therapy. Here, a 7 μm sized micro-plasma jet device was proposed, and its anticancer effects were evaluated based on three-dimensional (3D) bladder micro-tumor spheroids (T24 cell line) constructed in vitro. The experimental results showed that this micro-plasma jet device can effectively inactivate micro-tumors with diameters ranging from 50 to 300 μm in a short time, and the anticancer effects on 3D T24 cells can still be maintained after irradiation. This work makes the highly precise and efficient inactivation of target micro-tumors possible through plasma technology in clinical therapy.

Small-Scale Plasma Membrane Preparation for the Analysis of <em>Candida albicans</em> Cdr1-mGFPHis

This article presents a small-scale plasma membrane isolation protocol for the characterization of Candida albicans ABC (ATP-binding cassette) protein Cdr1, overexpressed in Saccharomyces cerevisiae. A protease-cleavable C-terminal mGFPHis double tag with a 16-residue linker between Cdr1 and the tag was designed to facilitate the purification and detergent-screening of Cdr1.

Small-Scale Plasma Membrane Preparation for the Analysis of Candida albicans Cdr1-mGFPHis.

The successful biochemical and biophysical characterization of ABC transporters depends heavily on the choice of the heterologous expression system. Over the past two decades, we have developed a yeast membrane protein expression platform that has been used to study many important fungal membrane proteins. The expression host Saccharomyces cerevisiae ADΔΔ is deleted in seven major endogenous ABC transporters and it contains the transcription factor Pdr1-3 with a gain-of-function mutation that enables the constitutive overexpression of heterologous membrane protein genes stably integrated as single copies at the genomic PDR5 locus. The creation of versatile plasmid vectors and the optimization of one-step cloning strategies enables the rapid and accurate cloning, mutagenesis, and expression of heterologous ABC transporters. Here, we describe the development and use of a novel protease-cleavable mGFPHis double tag (i.e., the monomeric yeast enhanced green fluorescent protein yEGFP3 fused to a six-histidine affinity purification tag) that was designed to avoid possible interference of the tag with the protein of interest and to increase the binding efficiency of the His tag to nickel-affinity resins. The fusion of mGFPHis to the membrane protein ORF (open reading frame) enables easy quantification of the protein by inspection of polyacrylamide gels and detection of degradation products retaining the mGFPHis tag. We demonstrate how this feature facilitates detergent screening for membrane protein solubilization. A protocol for the efficient, fast, and reliable isolation of the small-scale plasma membrane preparations of the C-terminally tagged Candida albicans multidrug efflux transporter Cdr1 overexpressed in S. cerevisiae ADΔΔ, is presented. This small-scale plasma membrane isolation protocol generates high-quality plasma membranes within a single working day. The plasma membrane preparations can be used to determine the enzyme activities of Cdr1 and Cdr1 mutant variants.

Plasma Dynamics in Low-Electron-Beta Environments

Recentin situmeasurements by the MMS and Parker Solar Probe missions bring interest to small-scale plasma dynamics (waves, turbulence, magnetic reconnection) in regions where the electron thermal energy is smaller than the magnetic one. Examples of such regions are the Earth’s magnetosheath and the vicinity of the solar corona, and they are also encountered in other astrophysical systems. In this brief review, we consider simple physical models describing plasma dynamics in such low-electron-beta regimes, discuss their conservation laws and their limits of applicability.