Comet Research Articles

Oscillations in a Josephson junction lattice stimulated by a common load

We present the analysis of soliton dynamics, current–voltage characteristics and AC power for a two-dimensional lattice of Josephson junctions using an effective alternating direction implicit (ADI) numerical scheme. The lattice is supplemented by a resistive-capacitive load (RC-load) and the bias feed is supplied from boundaries of the structure only and is redistributed between junctions via internal dynamics. At fixed length of such a lattice, its critical current have a maximum as a function of a lattice width, since not all junctions inside the lattice can be biased equally. The use of RC-load allows transferring generated AC power to a load device out of the array, but it changes the dynamics significantly. We show that regimes of solitons with plasma tails, generated due to array discreteness and leading to strong superradiant power, become unstable with matched RC-load and the corresponding zero field steps at the current–voltage characteristics disappear completely. Instead, at matched RC-load, standing wave regimes demonstrate high generation efficiency, reaching 21% of supplied dc power.

The Wagging Plasma Tail of Comet C/2020 S3 (Erasmus)

Long-period comet C/2020 S3 (Erasmus) reached perihelion at 0.398 au on UT 2020 December 12.67, making it a bright, near-Sun object. Images taken between 2020 mid-November and December using the HI-1 camera and COR2 coronagraph on board STEREO-A, as well as the LASCO/C3 coronagraph on board SoHO, show significant variations in the plasma tail position angles. To analyze these variations, a simple technique was developed to calculate the aberration angles. These angles are defined as the angle between the Sun–comet line and the tail axis, measured in the orbital plane. The aberration angles were found to range from 1.°2 to 46.°8, with an average (median) value of approximately 20.°3 (16.°3). By considering the aberration angles, the solar wind radial velocities during the observations were inferred to range from 73.9 to 573.5 km s−1, with a mean (median) value of approximately 205.5 km s−1 (182.3 km s−1). Throughout the observations, two periods were identified where the tails showed forward tilting, which cannot be explained by aberration alone. In one case, this anomalous position angle was sustained for at least 11 days and is possibly due to corotating interaction regions. In the other case, the tail exhibited dramatic excursions from 180° to 150° back to 210° over a limited period of around 34 hr. This behavior is tentatively explained as a consequence of the interaction with a halo coronal mass ejection that was launched from NOAA Active Region 12786 and arrived at comet C/2020 S3 during the time when the tail displayed its wagging behavior.

Interface instability of the thermal plasma jet

This work is first focused to experimentally study the interface instability and expansion mechanism of thermal plasma jet and provide a better understanding of the complex fluid-dynamic interactions occurring on the surface of the plasma bubble due to the Kelvin–Helmholtz effect. The experimental techniques used include a plasma generator, a pulse-forming network based on the capacitive energy storage, pressure measurement system along the capillary tube, and high-speed camera system to trace the development processes of the plasma interface. Results indicate that the plasmas jet has a better advantage of radial expansion with a high light at the beginning. However, the axial expansion velocity is larger than the radial one with time going on; thus, a torch-shaped jet body occurs under the Rayleigh–Taylor effect and can be divided into two parts including a plasma head and tail. With a dissipation of the initial energy and turbulent mixing between the plasmas and the gas, the jet boundary is broken and even the local rupture phenomena occur on the plasma jet surface. The turbulent dissipation is also very violent when the discharge voltage increases to 3000 V; thus, the turbulent mixing layer between the plasma jet and the gas is quite thicker and the plasma jet boundary is also fuzzy resulting in that the fold surfaces with much larger degree exist earlier. These experimental phenomena are also explained further from the mechanism by deriving the momentum equations of the interface of the plasma jet into the gas. Finally, a fitting formula of the surface area as an important factor in the expansion process of the plasma is obtained to analyze the interface characteristic of the plasma jet.

Modeling the energetic tail of a dusty plasma's electron energy distribution and its effect on dust grain charge and behavior

A model for a weakly ionized dusty plasma is proposed in which UV or x-ray radiation continuously creates free electrons at high energy, which then cool through collisions with a cold neutral gas before recombining. The transition of a free electron from high energy at birth to low energy at demise implies that the electron energy distribution is not the simple Maxwellian of an isolated system in thermal equilibrium, but instead has a high-energy tail that depends on the recombination time. This tail can have a major effect on dust grain charging because the flux of tail electrons can be substantial even if the density of tail electrons is small. Detailed analytic and numerical calculations of dust grain charging show that situations exist in which a small high-energy tail dominates charge behavior. This implies that dust grain charge in terrestrial and space dusty plasmas may be significantly underestimated if a Maxwellian distribution is assumed and the non-thermal dynamics are neglected.

A low power microwave room-temperature air plasma jet at atmospheric pressure

An atmospheric pressure low power microwave-induced air plasma source at 2.45 GHz is presented in this Letter. The device was capable of self-igniting air and generating plasma with only 50 W microwave input power and without any extra trigger. Its sustaining power was as low as 10 W, and the gas temperature of plasma tail flame measured by a fiber optic thermometer was about 32 °C. Spectroscopic measurements showed strong OH(A2Σ+→X2Π) bands at 306–310 nm and oxygen atomic lines OI (3p5P → 3s5S) at 777.1 nm and OI (3p3P → 3s3S) at 844.6 nm. NO−γ(A2Σ+→X2Πr) from 200 to 300 nm was also detected in this air plasma jet. This portable plasma apparatus offers potential for air plasma applications in the biomedical fields.

Current distribution and plasma velocity characteristics of parallel-plate accelerator under static pressure

Electromagnetic plasma accelerators which can generate high-density and hypervelocity plasma jets have been widely used in plasma physics research and application fields. An experimental platform of parallel-plate accelerator electromagnetically driven plasma is established in this paper, mainly including a parallel-plate accelerator, a power supply, magnetic probes, photodiodes, a current probe, and an oscilloscope. The current distribution and plasma velocity characteristics of a parallel-plate accelerator under static pressure are studied by using magnetic probe array and photodiode array. The working gas is synthetic air. A mechanical pump is used to pump the vacuum chamber to about 1 Pa, and then synthetic air is injected into the vacuum chamber to a target pressure. The power supply of the parallel-plate accelerator has a sinusoidal oscillation attenuation waveform with a total capacitance of 120 μF and a total inductance of about 400 nH. When the charging voltage is 13 kV, the discharge current is 170 kA and the pulse width is 23.5 μs. The discharge currents are 38, 100, 135 kA, and 170 kA when the pressures are 100, 200, 400 and 1000 Pa, respectively. The current distribution of the parallel-plate accelerator is concentrated, and the discharge mode is consistent with the snowplow mode, when the discharge current is small and the working pressure is high. As the discharge current increases or the working pressure decreases, a diffuse current distribution gradually appears in the parallel-plate accelerator. Two regions are formed, i.e. the plasma front region and the plasma tail region. The diffuse current distribution phenomenon is more remarkable when the discharge current is higher or the working pressure is lower. The plasma front current distribution proportion decreases and the plasma front velocity increases with the increase of discharge current and the decrease of working pressure. However, the plasma velocity proportion increased is much lower than the discharge current proportion increased or working pressure proportion decreased. When the discharge current increases from 38–170 kA, the plasma velocity increases from 25.0 km/s to 33.6 km/s, with the velocity increment being only 34.4%. The plasma front region is subjected to both the Lorentz force and the thermal pressure of the plasma tail region.

Probing the Plasma Tail of Interstellar Comet 2I/Borisov

We present an occultation study of compact radio sources by the plasma tail of interstellar comet 2I/Borisov (C/2019 Q4) both pre- and near-perihelion using the Arecibo and Green Bank radio telescopes. The interplanetary scintillation technique was used to probe the plasma tail at the P band (302–352 MHz), 820 MHz, and the L band (1120–1730 MHz). The presence and absence of scintillation at different perpendicular distances from the central axis of the plasma tail suggests a narrow tail of less than 6′ at a distance of ∼10′ (∼106 km) from the comet nucleus. Data recorded during the occultation of B1019+083 on 2019 October 31 with the Arecibo Telescope covered the width of the plasma tail from its outer region to the central axis. The systematic increase in scintillation during the occultation provides the plasma properties associated with the tail when the comet was at its pre-perihelion phase. The excess level of L-band scintillation indicates a plasma density enhancement of ∼15–20 times that of the background solar wind. The evolving shape of the observed scintillation power spectra across the tail from its edge to the central axis suggests a density spectrum flatter than Kolmogorov and that the plasma density irregularity scales present in the tail range between 10 and 700 km. The discovery of a high-frequency spectral excess corresponding to irregularity scales much smaller than the Fresnel scale suggests the presence of small-scale density structures in the plasma tail, likely caused by interaction between the solar wind and the plasma environment formed by the comet.

The scintillating tail of comet C/2020 F3 (Neowise)

Context. The occultation of a radio source by the plasma tail of a comet can be used to probe structure and dynamics in the tail. Such occultations are rare, and the occurrence of scintillation, due to small-scale density variations in the tail, remains somewhat controversial. Aims. A detailed observation taken with the Low-Frequency Array (LOFAR) of a serendipitous occultation of the compact radio source 3C196 by the plasma tail of comet C/2020 F3 (Neowise) is presented. 3C196 tracked almost perpendicularly behind the tail, providing a unique profile cut only a short distance downstream from the cometary nucleus itself. Methods. Interplanetary scintillation (IPS) is observed as the rapid variation of the intensity received of a compact radio source due to density variations in the solar wind. IPS in the signal received from 3C196 was observed for five hours, covering the full transit behind the plasma tail of comet C/2020 F3 (Neowise) on 16 July 2020, and allowing an assessment of the solar wind in which the comet and its tail are embedded. Results. The results reveal a sudden and strong enhancement in scintillation which is unequivocally attributable to the plasma tail. The strongest scintillation is associated with the tail boundaries, weaker scintillation is seen within the tail, and previously-unreported periodic variations in scintillation are noted, possibly associated with individual filaments of plasma. Furthermore, contributions from the solar wind and comet tail are separated to measure a sharp decrease in the velocity of material within the tail, suggesting a steep velocity shear resulting in strong turbulence along the tail boundary.

RhoA improves cryopreservation of rooster sperm through the Rho/RhoA-associated kinase/cofilin pathway

Cryopreservation of rooster sperm leads to relatively low semen quality due to cytoskeletal damage during the freeze-thawing process. This study aimed to explore how the addition of RhoA recombinant protein affected the viability and subcellular structure of rooster sperm after freeze-thawing and elucidated the molecular mechanisms of sperm cryopreservation. Semen quality and acrosome integrity testing revealed that the addition of 0.5 μg/mL RhoA recombinant protein to the cryoprotectant fluid significantly increased sperm motility, survival rate, linearity, straight-line velocity, and acrosome integrity after freeze-thawing (P < 0.05). Ultrastructure analysis of cryopreserved sperm showed structural damage to the sperm plasma membrane, nuclear membrane, and tail. However, compared to the control, these structural changes were reduced upon the addition of RhoA recombinant protein to the cryoprotective fluid (P < 0.05). Western blotting revealed that the expression of Rho/RhoA-associated kinase and p-cofilin was increased, and cofilin expression was decreased after sperm cryopreservation with recombinant RhoA protein. Treatment with Y-27632, a ROCK antagonist, suppressed ROCK and p-cofilin expression and decreased semen quality, acrosome integrity, and ultrastructure integrity. In summary, we have demonstrated a cryoprotective effect in spermatozoa involving the Rho/ROCK pathway during freeze-thawing. Furthermore, the addition of 0.5 μg/mL RhoA recombinant protein to the cryoprotective fluid improved rooster semen quality and subcellular structural homeostasis after freeze-thawing via the Rho/ROCK pathway. This pathway may regulate the dynamic reorganization of the actin cytoskeleton by regulating the cofilin phosphorylation.

The Role of O+ and He+ in the Propagation of Kinetic Alfvén Waves in the Earth’s Inner Magnetosphere

Interactions between plasma particles and electromagnetic waves play a crucial role in the dynamics and regulation of the state of space environments. From plasma physics theory, the characteristics of the waves and their interactions with the plasma strongly depend on the composition of the plasma, among other factors. In the case of the Earth’s magnetosphere, the plasma is usually composed of electrons, protons, O+ ions, and He+ ions, all with their particular properties and characteristics. Here, using plasma parameters relevant for the inner magnetosphere, we study the dispersion properties of kinetic Alfvén waves (KAWs) in a plasma composed of electrons, protons, He+ ions, and O+ ions. We show that heavy ions induce significant changes to the dispersion properties of KAWs, such as polarization, compressibility, and the electric-to-magnetic amplitude ratio, and therefore the propagation of KAWs is highly determined by the relative abundance of He+ and O+ in the plasma. These results, when discussed in the context of observations in the Earth’s magnetosphere, suggest that for many types of studies based on theory and numerical simulations, the inclusion of heavy ions should be customary for the realistic modeling of plasma phenomena in the inner magnetosphere or other space environments in which heavy ions can contribute a substantial portion of the plasma, such as planetary magnetospheres and comet plasma tails.

Using Single-view Observations of Cometary Plasma Tails to Infer Solar Wind Speed

Abstract A comet plasma tail is a product of the interaction between the solar wind and the comet’s coma, and has long been studied as a natural probe of the solar wind condition. We previously developed a method to derive the solar wind speed from dual-view observations of comet plasma tails. Here we improve the method to use single-view observations by assuming a radially propagating solar wind and apply it to two comets, C/2011 W3 (Lovejoy) and C/2012 S1 (ISON) observed by coronagraphs on board the Solar and Heliospheric Observatory and STEREO. We compare the results to the solar wind simulations and tomography and to the results from our previous dual-view method, and find they are generally consistent, especially when the comets were far away from the Sun or far away from the ecliptic plane and when the observer is high above the comet’s orbital plane. Meanwhile, we notice that this method may suffer from a large error for comets near the ecliptic plane and close to the Sun, where a nonradial component of the solar wind is significant. Using the observations from the first seven orbits of the Parker Solar Probe, we show that the solar wind deviates from a radial direction significantly within around 35 R s. We also notice that, when the nonradial solar wind component is presented, the error may be even larger if the observer is closer to the comet’s orbital plane. This method provides a potentially useful tool to estimate the solar wind speed around comets from only single-view imaging observations.

A Statistical Study on Wide‐Amplitude Kinetic Alfvénic Pulse at 8–12RE in the Magnetotail by THEMIS Spacecraft From 2008 to 2010

AbstractUsing THEMIS spacecraft observations from 2008 to 2010, we perform statistical study on the wide‐amplitude kinetic Hall‐like Alfvénic wave (KAW) pulse in the near‐Earth tail region of 8–12 RE. The KAW pulse is identified by Ez‐By pulse with parallel‐predominantly Poynting flux (|P///PT| &gt; 0.7) and large Ez/By ratio (|∆Ez/∆By | &gt; VA, VA is Alfvénic velocity). Three typical events are presented. Therein, the KAW pulse on 24 March 2009 at the magnetic dipolarization is accompanied by the ion/electron dispersed structure (IDS/EDS; time of flight effect from reconnection source). Statistical study shows that the near‐Earth KAW pulse has a typical temporal scale of ∼15–25 s. The KAW pulses are mainly distributed in the plasma sheet boundary layer (PSBL) and tail lobe regions of β &lt; 0.1 (β is the ratio of thermal pressure to magnetic pressure). The KAW pule at the PSBL/lobe has a wide distribution in the dawn‐dusk direction (−10 RE &lt; |Y| &lt; 10 RE), with the maximum occurrence at the midnight (|Y| &lt; 4 RE). The KAW pulses at the PSBL/lobe are predominantly earthward‐propagating. Only a small number of the KAW pulses are tailward‐propagating. The earthward‐propagating pulse has a higher Poynting flux (PT) than tailward‐propagating pulse. The KAW pulse occurs during quiet time (AE &lt; 200 nT) as well as substorm interval (AE &gt; 200 nT). The substorm‐interval pulse has a higher Poynting flux than the quiet‐time pulse. Finally, tail reconnection is likely the significant source of the near‐Earth KAW pulses at 8–12 RE.

Computational Study of Quenching Effects on Growth Processes and Size Distributions of Silicon Nanoparticles at a Thermal Plasma Tail.

In this paper, quenching effects on silicon nanoparticle growth processes and size distributions at a typical range of cooling rates in a thermal plasma tail are investigated computationally. We used a nodal-type model that expresses a size distribution evolving temporally with simultaneous homogeneous nucleation, heterogeneous condensation, interparticle coagulation, and melting point depression. The numerically obtained size distributions exhibit similar size ranges and tendencies to those of experiment results obtained with and without quenching. In a highly supersaturated state, 40–50% of the vapor atoms are converted rapidly to nanoparticles. After most vapor atoms are consumed, the nanoparticles grow by coagulation, which occurs much more slowly than condensation. At higher cooling rates, one obtains greater total number density, smaller size, and smaller standard deviation. Quenching in thermal plasma fabrication is effectual, but it presents limitations for controlling nanoparticle characteristics.

Author Correction: Far-ultraviolet aurora identified at comet 67P/Churyumov-Gerasimenko

Author Correction: Far-ultraviolet aurora identified at comet 67P/Churyumov-Gerasimenko

Bursty Ion Escape Fluxes at Mars

AbstractBased on the Mars Atmosphere and Volatile Evolution measurements we have observed cases when the fluxes of oxygen ions escaping the Martian ionosphere exceed their median values by more than a factor of 100. In the Martian tail very high fluxes of the more energetic (E &gt; 30 eV) oxygen ions fill the plasma sheet which then becomes much broader than under conditions with median values of ion fluxes. We have analyzed the occurrence of such events in the upper ionosphere near the terminator plane, which is the main source region of ions in the plasma tail. The maximum values of fluxes of oxygen ions with E &gt; 30 eV were observed mostly in the hemisphere where the motional electric field imposed by the solar wind is directed outward from the planet. Although high values of the solar wind dynamic pressure and (or) the motional electric field are favorable for the observation of the extreme values of ion fluxes with E &gt; 30 eV, there must also be other factors which initiate these events. In particular, we found a close relation of the maximum ion fluxes with the values of the simultaneously measured fluxes of solar wind penetrating into the upper ionosphere. Direct interaction of both plasmas might be a critical factor for the strong growth of the oxygen ion escape. Very high fluxes of the low‐energy oxygen ions (E &lt; 30 eV) are often related with ion “clouds” with anomalously large number density observed in the upper ionosphere.

Anti-thrombotic effects mediated by dihydromyricetin involve both platelet inhibition and endothelial protection

Classical antithrombotics and antiplatelets are associated with high frequencies of bleeding complications or treatment failure when used as single agents. The platelet-independent fibrin generation by activated endothelium highlights the importance of vascular protection in addition to platelet inhibition in thrombosis prevention. Dihydromyricetin (DHM), the most abundant flavonoid in Ampelopsis grossedentata, has unique vasoprotective effects. This study aims to characterize the antithrombotic potential of DHM. The effects of DHM on the activation of platelets and endothelial cells were evaluated in vitro. Calcium mobilization and activation of mitogen-activated protein kinases (MAPKs) were examined as the potential targets of DHM based on molecular docking analysis. The in vivo effects of DHM were determined in FeCl3-injured carotid arteries and laser-injured cremasteric arterioles. The results showed that DHM suppressed a range of platelet responses including aggregation, secretion, adhesion, spreading and integrin activation, and inhibited exocytosis, phosphatidylserine exposure and tissue factor expression in activated endothelial cells. Mechanistically, DHM attenuated thrombin-induced calcium mobilization and phosphorylation of ERK1/2 and p38 both in platelets and endothelial cells. Intravenous treatment with DHM delayed FeCl3-induced carotid arterial thrombosis. Furthermore, DHM treatment inhibited both platelet accumulation and fibrin generation in the presence or absence of eptifibatide in the laser injury-induced thrombosis model, without prolonging ex vivo plasma coagulation or tail bleeding time. DHM represents a novel antithrombotic agent whose effects involve both inhibition of platelet activation and reduction of fibrin generation as a result of endothelial protection.

Three case reports on the cometary plasma tail in the historical documents

Cometary tails visually manifest the solar wind and became an initial hint for its discovery. While the solar wind is being directly monitored with satellites, its time series before the space age has been controversially reconstructed with multiple proxies. Recently, archival reports of cometary plasma tails have been subjected to consideration to indirectly measure the solar wind but brought conclusion that no plasma tails had been reported prior to 1769 probably due to their brightness. However, historical records have occasionally reported comets with two tails even before 1769. These cases have been tentatively associated with visual reports of cometary plasma and dust tails. Therefore, we examined three such cases (C/1577 V1, 1P/837, and 1P/760), and compared the descriptions in historical records with calculated direction of their plasma tails. Our comparisons show that the records and calculations agree in these cases and plasma tails were visually recorded corresponding to these three great comets. These cases certify the capability of plasma tail observations with the unaided eye even before 1769, qualitatively imply their extreme brightness, proximities with the Sun and the Earth, relative enhancements of UV radiations, and interaction of cometary neutral atmosphere with solar wind plasma and magnetic field, while the lack of their detailed length or kink hinders us from their quantitative measuring. Further investigations will likely lead to the re-discovery of even more visual evidence of cometary plasma tails and, hence, improve our understanding on past space climate.

In vitro genotoxicity study of silver nanoparticles and titanium dioxide nanoparticles.

Nanoparticles are widely used in cosmetic, pharmaceutical, and food industries, but their safety and genetic toxicity are still unclear. In this study, the genotoxicity of silver nanoparticles (AgNPs) and titanium dioxide nanoparticles (titanium dioxide nanoparticles) were evaluated by in vitro comet assay and PIG-A assay in TK6 cells. We exposed TK6 cells to two types of nanoparticles at the highest concentration of 200 μmol/L for 4 h and conducted the in vitro comet assay. We examined the mutation results of PIG-A gene in vitro after 4 h, 24 ho and 10 days of exposure, respectively. We also examined the endocytosis of nanoparticles in TK6 cells exposed to nanoparticles for 24 h. In the endocytosis assay, with the increase of nano-material concentration, the side scatter (SSC) of TK6 cells in flow cytometry showed a concentration-dependent and time-dependent increase, indicating that TK6 cells could uptake both types of nanoparticles. In the comet assay, AgNPs could induce a concentration-dependent increase in DNA tail intensity. However, titanium dioxide NPs could not induce the concentration-dependent increase of DNA fluorescence intensity of comet tail. In the PIG-A assay, both AgNPs and TiO2NPs did not induce PIG-A gene mutation frequency in TK6 cells. The results showed that AgNPs could induce DNA damage in TK6 cells, but could not induce increase of PIG-A gene mutation frequency. TiO2NPs neither induce DNA damage in TK6 cells nor increase PIG-A mutation frequency. Further tests are needed to determine whether TiO2NPs are genotoxic.

Efficient electron injection by hybrid parametric instability and forward direct laser acceleration in subcritical plasma

The efficient injection of electrons into a propagating relativistic laser pulse with normalized vector potential a 0 ∼ 2 is demonstrated numerically and experimentally in a thin plasma layer with density 0.15–0.3 of the critical value. The injection is due to the wavebreaking of parametric plasma waves. The trapped particles gain multi-MeV (up to 20 MeV) energies by the direct laser acceleration in the plasma channel formed by the laser pulse in the lower density plasma tail. Numerical calculations were supported by experiments with micron-scale films pre-evaporated by an additional nanosecond laser pulse and a TW femtosecond laser facility. The experimentally observed bunch of electrons with energy above 1.6 MeV had a divergence of ∼0.05 rad and charge of ∼50 pC measured with photoneutron Be(g,n) reaction.

Comet 21P/Giacobini-Zinner: Water production activity over 20 years with SOHO/SWAN

In 1985 Comet 21P/Giacobini-Zinner was the first comet visited by a spacecraft, the International Cometary Explorer (ICE) satellite, several months before the armada of Halley spacecraft had their encounters in 1986. ICE was originally the ISEE-3 satellite, designed for magnetospheric measurements near the Earth, and was diverted via a lunar gravity assist to pass through the plasma tail of the comet. The comet has been observed by the all-sky hydrogen Lyman-alpha Solar Wind Anisotropies (SWAN) camera on the SOlar and Heliospheric Observatory satellite during its last four apparitions in 1998, 2005, 2012 and 2018. This paper compares water production rates calculated from the hydrogen images from the 1998 and 2005 results, published by Combi et al. (2011), with new observations from 2012 and 2018. Unlike some comets that have faded over time, except for 2 outbursts seen in the 2012 results, the activity levels for Comet 21P have not changed consistently over time. A power-law fit to the pre-perihelion water production rate vs. heliocentric distance for all four apparitions gives a result of 3.75 × 1028 x r-0.28±0.4, and that for the post-perihelion production rate, discounting 2012 data dominated by outbursts, gives 4.15 × 1028 x r-10.2±0.9. The production rates are in s−1 and heliocentric distances are in AU.