Neutral Velocity Research Articles

Observation of Quiet‐Time Mid‐Latitude Joule Heating and Comparisons With the TIEGCM Simulation

AbstractJoule heating is a major energy sink in the solar wind‐magnetosphere‐ionosphere system and modeling it is key to understanding the impact of space weather on the neutral atmosphere. Ion drifts and neutral wind velocities are key parameters when modeling Joule heating, however there is limited validation of the modeled ion and neutral velocities at mid‐latitudes. We use the Blackstone Super Dual Auroral Radar Network radar and the Michigan North American Thermosphere Ionosphere Observing Network Fabry‐Perot interferometer to obtain the local nightside ion and neutral velocities at ∼40° geographic latitude during the nighttime of 16 July 2014. Despite being a geomagnetically quiet period, we observe significant sub‐auroral ion flows in excess of 200 ms−1. We calculate an enhancement to the local Joule heating rate due to these ion flows and find that the neutrals impart a significant increase or decrease to the total Joule heating rate of >75% depending on their direction. We compare our observations to outputs from the Thermosphere Ionosphere Electrodynamic General Circulation Model (TIEGCM). At such a low geomagnetic activity however, TIEGCM was not able to model significant sub‐auroral ion flows and any resulting Joule heating enhancements equivalent to our observations. We found that the neutral winds were the primary contributor to the Joule heating rates modeled by TIEGCM rather than the ions as suggested by our observations.

ThunderBoltz: an open-source direct simulation Monte Carlo Boltzmann solver for plasma transport, chemical kinetics, and 0D modeling

Abstract Plasma-neutral interactions, including reactive kinetics, are often either studied in 0D using ODE-based descriptions, or in multi-dimensional fluid or particle-based plasma codes. The latter case involves a complex assembly of procedures that are not always necessary to test effects of underlying physical models and mechanisms for particle-based descriptions. Here we present ThunderBoltz, a lightweight, publicly available 0D direct simulation Monte Carlo code designed to accommodate a generalized combination of species and arbitrary cross sections without the overhead of expensive field solves. It can produce electron, ion, and neutral velocity distributions in applied AC/DC E-field and/or static B-field scenarios. The code is built in the C++ standard library and includes a convenient Python interface that allows for input file generation (including compatibility with cross section data from the LXCat database), electron transport and reaction rate constant post-processing, input parameter constraint satisfaction, calculation scheduling, and diagnostic plotting. These codes can be accessed at the repository: https://github.com/lanl/ThunderBoltz. In this work we compare ThunderBoltz transport calculations against Bolsig+ calculations, benchmark test problems, and swarm experiment data, finding good agreement with all three in the appropriate field regimes. In addition, we present example use cases where the electron, ion, and background neutral particle species are self-consistently evolved to model the background kinetics, a feature that is absent in fixed-background Monte Carlo and n-term Boltzmann solvers. The latter functionality allows for the possibility of particle-based chemical kinetics simulations of the plasma and neutral species as a new alternative to ODE-based approaches.

Laser induced fluorescence using frequency modulated light.

The small signal-to-noise ratio (SNR) of conventional laser induced fluorescence (LIF) measurements using a continuous wave laser, either diode or dye, is typically overcome by amplitude modulating the laser at a specific frequency and then using lock-in amplification to extract the signal from measurement noise. Here, we present LIF measurements of the neutral helium velocity distribution function in an rf plasma using frequency modulated (FM) laser injection. A pulse train of 100% amplitude modulation is generated synthetically with a random sequence of pulse lengths. The FM signal then drives an acoustic optic modulator placed in the path of the injection beam in an LIF measurement. The signal from a fast photomultiplier tube is digitized and cross-correlated with the known modulation signal. The resultant FM-based LIF signal outperforms a conventional lock-in-based LIF measurement on the same plasma in terms of SNR and precision.

A New Lever on Exoplanetary B Fields: Measuring Heavy Ion Velocities

Magnetic fields connect an array of planetary processes, from atmospheric escape to interior convection. Despite their importance, exoplanet magnetic fields are largely unconstrained by both theory and observation. In this Letter, we propose a novel method for constraining the B field strength of hot gas giants: comparing the velocities of heavy ions and neutral gas with high-resolution spectroscopy. The core concept of this method is that ions are directly deflected by magnetic fields. While neutrals are also affected by B fields via friction with field-accelerated ions, ionic gas should be more strongly coupled to the underlying magnetic field than bulk neutral flow. Hence, measuring the difference between the two velocities yields rough constraints on the B field, provided an estimate of the stellar UV flux is known. We demonstrate that heavy ions are particularly well suited for this technique because they are less likely to be entrained in complex hydrodynamic outflows than their lighter counterparts. We perform a proof-of-concept calculation with Ba ii, an ion whose velocity has been repeatedly measured at high confidence with high-resolution spectroscopy. Our work shows that a 10 G magnetic field would produce ∼1 km s−1 ion–neutral velocity differences at a microbar, whereas a 50 G magnetic field would produce ∼20 km s−1 velocity difference. With new leverage on magnetic fields, we will be able to investigate magnetic field generation in the extreme edge cases of hot gas giants, with wide-ranging consequences for planetary interior structure, dynamo theory, and habitability.

Mechanics of the foot and ankle joints during running using a multi-segment foot model compared with a single-segment model.

The primary purpose of this study was to compare the ankle joint mechanics, during the stance phase of running, computed with a multi-segment foot model (MULTI; three segments) with a traditional single segment foot model (SINGLE). Traditional ankle joint models define all bones between the ankle and metatarsophalangeal joints as a single rigid segment (SINGLE). However, this contrasts with the more complex structure and mobility of the human foot, recent studies of walking using more multiple-segment models of the human foot have highlighted the errors arising in ankle kinematics and kinetics by using an oversimplified model of the foot. This study sought to compare whether ankle joint kinematics and kinetics during running are similar when using a single segment foot model (SINGLE) and a multi-segment foot model (MULTI). Seven participants ran at 3.1 m/s while the positions of markers on the shank and foot were tracked and ground reaction forces were measured. Ankle joint kinematics, resultant joint moments, joint work, and instantaneous joint power were determined using both the SINGLE and MULTI models. Differences between the two models across the entire stance phase were tested using statistical parametric mapping. During the stance phase, MULTI produced ankle joint angles that were typically closer to neutral and angular velocities that were reduced compared with SINGLE. Instantaneous joint power (p<0.001) and joint work (p<0.001) during late stance were also reduced in MULTI compared with SINGLE demonstrating the importance of foot model topology in analyses of the ankle joint during running. This study has highlighted that considering the foot as a rigid segment from ankle to MTP joint produces poor estimates of the ankle joint kinematics and kinetics, which has important implications for understanding the role of the ankle joint in running.

Impulsively generated waves in two-fluid plasma in the solar chromosphere: Heating and generation of plasma outflows

Context. We present new insights into impulsively generated Alfvén and magneto-acoustic waves in the partially ionized two-fluid plasma of the solar atmosphere and their contribution to chromospheric heating and plasma outflows. Aims. Our study attempts to elucidate the mechanisms responsible for chromospheric heating and excitation of plasma outflows that may contribute to the generation of the solar wind in the upper atmospheric layers. The main aim of this work is to investigate the impulsively generated waves by taking into account two-fluid effects. These effects may alter the wave propagation leading to attenuation and collisional plasma heating. Methods. The two-fluid equations were solved by the JOint ANalytical Numerical Approach (JOANNA) code in a 2.5-dimensional (2.5D) framework to simulate the dynamics of the solar atmosphere. Here, electrons + ions (protons) and neutrals (hydrogen atoms) are treated as separate fluids, which are coupled via ion-neutral collisions. The latter acts as a dissipation mechanism for the energy carried by the waves in two-fluid plasma and may ultimately lead to the frictional heating of the partially ionized plasma. The waves in two-fluid plasma, which are launched from the top of the photosphere, are excited by perturbations induced by localized Gaussian pulses in the horizontal components of the ion and neutral velocities. Results. In the middle and upper chromosphere, a substantial fraction of the energy carried by large amplitude waves in the two-fluid plasma is dissipated in ion-neutral collisions, resulting in the thermalization of wave energy and generation of plasma outflows. We find that coupled Alfvén and magneto-acoustic waves are more effective in heating the chromosphere than magneto-acoustic waves. Conclusions. Large-amplitude waves in the two-fluid plasma may be responsible for heating the chromosphere. The net flow of ions is directed outward, leading to plasma outflows in the lower solar corona, which may contribute to the solar wind at higher altitudes The primary source of wave energy dissipation in the current paradigm comes from collisions between ions and neutrals.

Atmospheric waves disturbances from the solar terminator according to the VLF radio stations data

The perturbations from the solar terminator in the range of acoustic-gravity waves (AGWs) periods from 5 min to 1 h were analysed with the use of measurements of VLF radio signals amplitudes on the European radio path GQD–A118 (Great Britain–France). These observations provide information on the propagation of waves at altitudes near the mesopause (∼90 km), where VLF radio waves are reflected. On the considered radio path a systematic increase in fluctuations in the amplitudes of radio waves was observed within a few hours after the passage of the evening terminator. For April, June, October 2020 and February 2021 events, the distribution of the number of wave perturbations with large amplitudes over AGWs time periods has been studied. Our results show that the evening terminator for different seasons is dominated by waves in the range of periods of 15–20 min. The amplitudes of the AGWs from the terminator at the heights of the mesosphere (fluctuations in the concentration of neutral particles, velocity components and vertical displacement of the volume element) are approximately determined by the fluctuations of the amplitudes of the VLF radio signals. The amplitudes of the AGWs on the terminator are 12–14% in relative concentration fluctuations, which correspond to the vertical displacement of the atmospheric gas volume of 1.1–1.3 km. Based on the analysis of the AGW energy balance equation, it was concluded that the waves predominantly propagate in a quasi-horizontal direction at the terminator. The possibility of studying the long-term changes in the mesosphere parameters using fluctuations in the amplitudes of VLF radio waves at the terminator is shown.

Hilbert expansion based fluid models for kinetic equations describing neutral particles in the plasma edge of a fusion device

Neutral particles in the plasma edge of fusion devices based on magnetic confinement are described by a transient kinetic equation incorporating ionization, recombination, and charge-exchange collisions. In charge-exchange dominated regimes, the neutral particle velocity distribution approaches the drifting Maxwellian defined by the mean velocity and temperature of the plasma. This enables model order reduction from the kinetic equation to approximate fluid models. We derive transient fluid models consistent with the kinetic equation by exploring a splitting based approach. We split the kinetic equation in sources and sinks on the one hand, and transport combined with charge-exchange on the other hand. Combining transport with charge-exchange collisions allows for deriving Hilbert expansion based fluid models. The retrieved fluid models depend on the assumed importance (scaling) of the different terms in the split equation describing transport and charge-exchange. We explore two scalings: the hydrodynamic scaling and the diffusive scaling. The diffusive scaling fluid model closely resembles phenomenological fluid models for describing neutral particles in the plasma edge that have been derived in the past. Therefore, the Hilbert expansion based fluid models can serve as a theoretical basis for such phenomenological fluid models and elucidate some of their properties. The performance of the fluid models with respect to a discrete velocity model and a Monte Carlo reference solver is assessed in numerical experiments. The code used to perform the numerical experiments is openly available.

Monochromatic Two-Fluid Alfvén Waves in the Partially Ionised Solar Chromosphere

We present new results towards the explanation of the chromospheric-heating problem and the solar-wind origin, using a two-fluid model that takes into account the collisional interaction between ions (protons) and neutrals (hydrogen atoms). Our aim is to further reveal the mechanism behind chromospheric heating and plasma outflows. We simulate and analyse the propagation and evolution of Alfvén waves in the partially ionised solar chromosphere, consisting of ions + electrons and neutral fluids. The simplified model chromosphere is permeated by a vertical, uniform magnetic field. We perform numerical simulations in the framework of a quasi-1.5-dimensional (1.5D), two-fluid model in which Alfvén waves are excited by a harmonic driver in the transverse component of the ion and neutral velocities, operating in the chromosphere. In the case of a small-amplitude driver, Alfvén waves are weakly damped, and for the chosen wave periods of a few seconds, Alfvén waves manage to propagate through the chromosphere and enter the solar corona. Non-linear Alfvén waves excited by a large-amplitude driver cause significant chromospheric heating and plasma outflows. We thus conclude that two-fluid Alfvén waves with larger amplitudes can contribute to chromospheric heating and plasma outflows, which may result higher up in the solar-wind origin.

Numerical simulations of two-fluid magnetoacoustic waves in the solar atmosphere

ABSTRACT We study vertical variations of wave periods of magnetoacoustic two-fluid waves in the partially ionized lower solar atmosphere, consisting of ion (proton) + electron and neutral (atomic hydrogen) fluids, which are coupled by ion–neutral collisions. The study allows finding the wave period cut-offs and their variations in the solar atmosphere, as well as establishing the role of these cut-offs in determining the wave propagation conditions. The atmosphere is permitted by a uniform vertical magnetic field. We perform numerical simulations in the framework of a one-dimensional (1D), two-fluid model in which plane waves are exited by a harmonic driver in the vertical ion and neutral velocities, operating at the bottom of the solar photosphere. We observe excitation of waves with cut-off wave periods in addition to waves set directly by the driver. We also see that some waves exited by that driver can reach the solar corona. Despite of its limitations such as the lack of non-adiabatic and non-ideal terms and a simple 1D structure, the developed two-fluid model of the solar atmosphere sheds a new light on the role of cut-offs in setting up the wave propagation conditions in the solar atmosphere and finding periods of waves that may carry their energy from the solar surface to the corona.

Cooling effect of elevated ambient air velocity on thermal comfort when sitting after walking

Sitting after walking is a common combination of activities in temporally occupied spaces (TOSs). The heat accumulated during walking can deteriorate the thermal comfort of occupants who sit for ≤15 min in the TOSs. Compared to reducing the ambient temperature, increasing the ambient air velocity may ensure thermal comfort and energy saving in TOSs. However, previous studies and current standards failed to consider the cooling effect of air movement for dwell times ≤15 min after walking. In this study, 20 male college students walked at 4 km/h for 15 min at 26 °C and 0.05 m/s. They then sat for 15 min in two operative temperatures (26 °C and 29 °C) and five levels of ambient air velocity (0.05 m/s, 0.42 m/s, 0.67 m/s, 0.92 m/s, and 1.08 m/s), for a total of 10 environmental combinations. The relative humidity was controlled in the range of 35–50%. The results showed that ambient air velocities of 0.42–0.67 m/s and 0.92–1.08 m/s could compensate for operative temperature drops of 3 °C and 6 °C, respectively, after sitting for ≤15 min during summer. The neutral, preferred, and acceptable ambient air velocities were compared. The predicted mean vote considering the cooling effect of the elevated airflow in ASHRAE Std. 55 overestimated the thermal sensitivity of the occupants. A summer comfort zone for the condition of sitting for ≤15 min after walking was obtained to guide the environmental design of TOSs.

Mixing Interstellar Clouds Surrounding the Sun

On its journey through the Galaxy, the Sun passes through diverse regions of the interstellar medium. High-resolution spectroscopic measurements of interstellar absorption lines in spectra of nearby stars show absorption components from more than a dozen warm partially ionized clouds within 15 pc of the Sun. The two nearest clouds—the Local Interstellar Cloud (LIC) and Galactic (G) cloud—move toward each other. Their bulk heliocentric velocities can be compared with the interstellar neutral helium flow velocity obtained from space-based experiments. We combine recent results from Ulysses, IBEX, and STEREO observations to find a more accurate estimate of the velocity and temperature of the very local interstellar medium. We find that, contrary to the widespread viewpoint that the Sun resides inside the LIC, the locally observed velocity of the interstellar neutral helium is consistent with a linear combination of the velocities of the LIC and G cloud, but not with either of these two velocities. This finding shows that the Sun travels through a mixed-cloud interstellar medium composed of material from both these clouds. Interactions between these clouds explain the substantially higher density of the interstellar hydrogen near the Sun and toward stars located within the interaction region of these two clouds. The observed asymmetry of the interstellar helium distribution function also supports this interaction. The structure and equilibrium in this region require further studies using in situ and telescopic observations.

Doppler-velocity Drifts Detected in a Solar Prominence

We analyzed multiline observations of a quiescent prominence from the slit spectrograph located at the Ondřejov Observatory. Dopplergrams and integrated intensity maps of the whole prominence were obtained from observations in six spectral lines: Ca ii H, Hϵ, Hβ, He i D3, Hα, and Ca ii IR. By combining integrated intensity maps with non-LTE radiative-transfer modeling, we carefully identified areas in an optically thin regime. The comparison of the Doppler-velocity maps and scatterplots from different lines shows the existence of differences in the velocity of ions and neutrals called velocity drift. The drift is of a local nature, present mostly at prominence edges in the area with a large velocity gradient, as can be tentatively expected based on multifluid MHD models. We could not explore the time evolution of the drift, since our data set consists of a single scan only. Our paper brings another contribution to a rather controversial problem of the detection of multifluid effects in solar prominences.

Depth profile of frictional properties in the inner Nankai accretionary prism using cuttings from IODP Site C0002

We conduct frictional experiments using cuttings collected at Nankai Trough IODP Site C0002 over 980.5–3262.5 mbsf (meters below seafloor) depth interval to better understand the frictional properties through the accretionary prism. The experiments are conducted at the in situ effective normal stresses (9–37 MPa) under brine-saturated conditions, and the slip velocity is abruptly changed in a stepwise manner to either of 0.3, 3, or 33 µm/s after the steady-state friction is reached. The friction coefficient (μ) of the cuttings samples ranges from 0.45 to 0.60, with a slight increase in μ with increasing depth, related to decreasing smectite content. The velocity dependence of friction (a − b) is positive at all depths and ranges from 0.001 to 0.006, which indicates a velocity-strengthening behavior; these values are consistent with relatively homogeneous deformation microstructures. The critical slip distance (Dc) ranges from 0.5 to 123 μm, with relatively large values obtained for the smectite-rich samples. The changes in both the friction coefficient and rate- and state-friction parameters are likely associated with mineralogical change and consolidation with increasing depth. Although all of the cuttings samples collected from Site C0002 exhibit a velocity-strengthening behavior, a slight decreasing trend in a − b with increasing depth indicates either a nearly neutral velocity dependence or a possible transition to velocity-weakening behavior at greater depths, which may be attributed to the occurrence of slow earthquakes in the Nankai accretionary prism.

Atmospheric Lunar Tide in the Low Latitude Thermosphere-Ionosphere.

We present simultaneous, independent measurements of the atmospheric semidiurnal lunar tide in neutral winds and plasma velocities from NASA's Ionospheric Connection Explorer, and in atomic oxygen 135.6 nm airglow measured by the Global‐scale Observations of the Limb and Disk. Westward tidal winds near 115 km at the magnetic equator occur during part of the upward phase of the in‐situ plasma drift. Vertical motions associated with the field‐aligned plasma velocity occur away from the magnetic equator. The morphology of the lunar tide, and the phasing between the airglow and plasma velocities are consistent with E × B drift as a mechanism for linking neutral wind and plasma perturbations. This work provides the first observational quantification of global‐scale E‐ and F‐region coupling through E × B and field‐aligned vertical drifts.

Measurement of Mach probe on plasma flow velocity in highly collisional plasma jet

A normalized plasma flow velocity in highly collisional plasma formed by a microwave plasma jet, which is dimensionless unit for plasma flow velocity/ion acoustic velocity, was measured by the parallel Mach probe. To deduce the normalized plasma flow velocity under highly collisional plasma conditions, the collisional model of a Mach probe was proposed. In addition, neutral gas flow velocity which assumed to be plasma flow velocity was calculated by the turbulent model. The results for the two different models were compared with those for the collsionless models of the Mach probe. The turbulent model produced 2–4 times reduced values than by measurements with collsionless models. The measured results with the collisional model were shown as approximately 100–250% lower than those for collsionless models. They were obtained to be in good agreement with difference rate of 10–30% when compared to those for the turbulent model.

The dark matter haloes of HI selected galaxies

ABSTRACT We present the neutral hydrogen mass (MHI) function (HIMF) and velocity width (w50) function (HIWF) based on a sample of 7857 galaxies from the 40 per cent data release of the ALFALFA survey (α.40). The low mass (velocity width) end of the HIMF (HIWF) is dominated by the blue population of galaxies whereas the red population dominates the HIMF (HIWF) at the high mass (velocity width) end. We use a deconvolution method to estimate the HI rotational velocity (Vrot) functions (HIVF) from the HIWF for the total, red, and blue samples. The HIWF and HIVF for the red and blue samples are well separated at the knee of the function compared to their HIMFs. We then use recent stacking results from the ALFALFA survey to constrain the halo mass (Mh) function of HI-selected galaxies. This allows us to obtain various scaling relations between MHI−w50−Vrot−Mh, which we present. The MHI−Mh relation has a steep slope ∼2.10 at small masses and flattens to ∼0.34 at masses larger than a transition halo mass, $\log _{10}(M_{\rm{ht}}h_{70}^2/M_{\odot })=10.62$. Our scaling relation is robust and consistent with a volume-limited sample of α.40. The MHI−Mh relation is qualitatively similar to the Mstar−Mh relation but the transition halo mass is smaller by ∼1.4 dex compared to that of the Mstar−Mh relation. Our results suggest that baryonic processes like heating and feedback in larger mass haloes suppress HI gas on a shorter time-scale compared to star formation.

Two-photon absorption laser induced fluorescence measurements of absolute neutral deuterium density, temperature, and bulk flow in Proto-MPEX

Neutral particle control is critical for fusion fueling and confinement. Neutral diagnostics for fusion-relevant plasmas are commonly restricted to line-integrated or ex situ methods. A non-perturbative, two-photon absorption laser induced fluorescence (TALIF) diagnostic is implemented on the Prototype Material Plasma Exposure eXperiment (Proto-MPEX) to probe neutral atomic deuterium in a fusion-relevant plasma at 1 cm intervals along the radius of the vacuum vessel. The diagnostic is situated ∼20 m from the vacuum vessel, and a signal is collected along the laser injection axis, requiring only one line-of-sight. TALIF measurements are absolutely calibrated using xenon and krypton. Absolute atomic densities derived from xenon calibration are compared to absolute atomic densities derived from krypton calibration. Here, preliminary measurements of absolute atomic deuterium density, temperature, and local bulk flow dependence on radial location and input power in Proto-MPEX are presented. Neutral atomic deuterium velocity distribution functions are measured throughout a one-second plasma pulse with a time resolution of 250 ms.

Effect of neutral winds on the creation of non-specular meteor trail echoes

Abstract. Non-specular meteor trail echoes are radar reflections from plasma instabilities that are caused by field-aligned irregularities. Meteor simulations are examined to show that these plasma instabilities, and thus the associated meteor trail echo, strongly depend on the meteoroid properties and the characteristics of the atmosphere in which the meteoroid is embedded. The effects of neutral winds, as a function of altitude, are analyzed to understand how their amplitude variability impacts the temporal–space signatures of non-specular meteor trail echoes present in very high-frequency (VHF) radar observations. It is found that amplitudes of the total horizontal neutral wind smaller than 0.6 m s−1 do not provide the right physical conditions to enable the genesis of non-specular meteor echoes. It is also found that a 0.0316 µg meteoroid traveling at 35 km s−1 can be seen as a meteor trail echo if the amplitudes of horizontal neutral winds are stronger than 15 m s−1. In contrast, a 0.316 µg meteoroid, traveling at the same speed, requires horizontal winds stronger than 1 m s−1 to be visible as a meteor trail echo. The neutral velocity threshold illustrates how simulations show that no trail echo is created below a critical wind value. This critical wind value is not mapped directly to radar observations, but it is used to shed light on the physics of meteor trails and improve their modeling. The meteor simulations also indicate that time delays on the order of hundreds of milliseconds or longer, between head echoes and non-specular echoes, which are present in VHF backscatter radar maps, can be a consequence of very dense plasma trails being affected by weak horizontal neutral winds that are smaller than 1 m s−1.

Magnetic field imaging in a laboratory plasma

In laboratory plasmas, arrays of probes have typically been used to measure the evolution of the magnetic field topology. Here, we present initial image-based measurements of the magnetic topology in a low-temperature plasma using a purely optical diagnostic. Laser induced fluorescence measurements of neutral velocity distribution functions are made using a fast camera, imaging the Zeeman splitting of σ-peaks in neutral argon. The separation of σ-peaks provides spatially resolved magnetic field magnitude measurements with a detection threshold on the order of 10 G.