Total Pressure Balance Research Articles

Total Magnetic Field Perturbations at Martian Low Altitudes (≤500 km): MAVEN Observations in 2014–2023

AbstractMagnetic perturbations in the Martian ionosphere are often considered as representing plasma irregularities, but thorough statistical comparisons between the two independent phenomena have been rarely conducted. In this study, we investigate the statistical relationship between total magnetic field perturbations and those of O2+ density as observed by the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft at altitudes below 500 km in 2014–2023. From the 1 Hz time series of magnetic field strength, perturbation intensity is estimated by subtracting a signal smoothed with a Savitzky‐Golay filter (window size ∼36 km). Statistically, the magnetic perturbations are stronger on the dayside than at night, in the local summer hemisphere than in winter, and away from crustal magnetic anomalies than nearby. Also, day‐to‐day variability of the magnetic perturbations exhibits a weak but significant correlation with that of solar wind electron density. The statistical behavior of total magnetic field perturbations is not the same as that of O2+ density perturbations: that is, the former is not a perfect proxy for the latter. The seemingly counter‐intuitive discrepancy can be explained by effects of inhomogeneous background magnetic field strength (participating in total pressure balance across plasma perturbations) and localized ionospheric currents at altitudes unreachable by MAVEN.

Are the Heliosphere, Very Local Interstellar Medium, and Local Cavity in Pressure Balance with Galactic Gravity?* *Based on observations made with the NASA/ESA Hubble Space Telescope obtained from the Data Archive at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS AR-09525.01A. These observations are associated with

The Voyager spacecraft are providing the first in situ measurements of physical properties in the outer heliosphere beyond the heliopause. These data, together with data from the IBEX and Hubble Space Telescope and physical models consistent with these data, now provide critical measurements of pressures in the heliosphere and surrounding interstellar medium. Using these data, we assemble the first comprehensive survey of total pressures inside and outside of the heliopause, in the interstellar gas surrounding the heliosphere, and in the surrounding Local Cavity to determine whether the total pressures in each region are in balance with each other and with the gravitational pressure exerted by the galaxy. We intercompare total pressures in each region that include thermal, nonthermal, plasma, ram, and magnetic pressure components. An important result is the role of dynamic (ram) pressure. Total pressure balance at the heliopause can only be maintained with a substantial contribution of dynamic pressure from the inside. Also, total pressure balance between the outer heliosphere and pristine very local interstellar medium (VLISM) and between the pristine VLISM and the Local Cavity requires large dynamic pressure contributions.

A dynamic pressure regain method for uniform air supply design

The Dynamic Pressure Regain Method, a new design method for uniform air supply, was proposed in this paper, based on the principle of total pressure balance. It can achieve uniform air supply by keeping equal dynamic pressure in adjacent sections of the main duct that opens up a new field for the design of uniform air supply duct system. Research was carried out through numerical simulation and full-scale experiment. First, the design steps of this method were proposed according to the flow characteristics in the duct. Then the recommended range of air velocity ratio was determined by numerical simulation calculation, and the resistance calculation formulas of the duct under this method were fitted. Finally, the full-scale experiment was carried out for the working conditions of inlet air velocity of 4 m/s and 5 m/s to verify the results of numerical simulation, including the total pressure balance, the uniformity of air supply system, the resistance calculation formulas of duct and the verticality of outlet airflow. The results showed that with a constant air velocity of the main duct sections, the variation of the total pressure loss was within 5 %, thus satisfying the total pressure balance; With a 0.6–1.0 air velocity ratio of the branch to main duct, the air volume imbalance rate of the branch duct was lower than 10 %; The error of local resistance calculation formulas of the duct system was no more than 3 %. The airflow deflection angle travelling out along the branch duct walls was within 20°. This complete design method of uniform air supply satisfies the total pressure balance and features simplicity, which decreases the air volume imbalance rate by 84 % compared with the static pressure regain method. Besides, when the size of the inlet duct remains constant, that of the design model will not change with the inlet air velocity. In addition, this study proposed the resistance calculation formulas that conform to the characteristics of its design model.

Mirror-mode Wave Identification Methods and Their Application to Martian Magnetosheath

Mirror-mode waves are structures usually seen in plasma with temperature anisotropy, identifiable through features in magnetic field and particle distribution and fluctuation. Two identification methods are analyzed and compared in this paper. Method A uses magnetic field data only, while Method B combines magnetic field and particle data. Method A is based mainly on features of magnetic field variation such as large amplitude fluctuation along background field direction, using magnitude of magnetic field fluctuation ∆<italic>B</italic>/|<italic>B</italic>| and angles between background field and maximum/minimum variation direction θ_max, θ_min as criteria. Method B is based on features such as wave compression, total pressure balance and zero velocity in plasma frame. Identification using data from MAVEN probe in the Martian magnetosheath shows that Method A can cause misidentification under certain circumstances, <italic>e.g.</italic> magneto-sonic waves. Results of identification using Method A with varying criteria ∆<italic>B</italic>/|<italic>B</italic>| and θ_min/θ_max are studied in the Martian magnetosheath, suggesting threshold values: θ_min> 40º, θ_max< 40º and ∆<italic>B</italic>/|<italic>B</italic>| > 80% can yield satisfying results.

Properties of the circumgalactic medium in cosmic ray-dominated galaxy haloes

ABSTRACT We investigate the impact of cosmic rays (CRs) on the circumgalactic medium (CGM) in FIRE-2 simulations, for ultra-faint dwarf through Milky Way (MW)-mass haloes hosting star-forming (SF) galaxies. Our CR treatment includes injection by supernovae, anisotropic streaming and diffusion along magnetic field lines, and collisional and streaming losses, with constant parallel diffusivity $\kappa \sim 3\times 10^{29}\, \mathrm{cm^2\ s^{-1}}$ chosen to match γ-ray observations. With this, CRs become more important at larger halo masses and lower redshifts, and dominate the pressure in the CGM in MW-mass haloes at z ≲ 1–2. The gas in these ‘CR-dominated’ haloes differs significantly from runs without CRs: the gas is primarily cool (a few ${\sim}10^{4}\,$ K), and the cool phase is volume-filling and has a thermal pressure below that needed for virial or local thermal pressure balance. Ionization of the ‘low’ and ‘mid’ ions in this diffuse cool gas is dominated by photoionization, with O vi columns ${\gtrsim}10^{14.5}\, \mathrm{cm^{-2}}$ at distances ${\gtrsim}150\, \mathrm{kpc}$. CR and thermal gas pressure are locally anticorrelated, maintaining total pressure balance, and the CGM gas density profile is determined by the balance of CR pressure gradients and gravity. Neglecting CRs, the same haloes are primarily warm/hot ($T\gtrsim 10^{5}\,$K) with thermal pressure balancing gravity, collisional ionization dominates, O vi columns are lower and Ne viii higher, and the cool phase is confined to dense filaments in local thermal pressure equilibrium with the hot phase.

Anomalous mix induced by a collisionless shock wave in an inertial confinement fusion hohlraum

The anomalous mix at the high-Z and low-Z plasma interfaces in an inertial confinement fusion hohlraum is a current topic of interest. The mechanism for such an anomalous mix in the interpenetration layer at the high-Z and low-Z plasma interface and its effects on the laser plasma instabilities have been investigated by particle-in-cell simulations. It is found that a diffusion-driven collisionless shock wave can be generated from an initially sharp high-Z and low-Z plasma interface with total pressure balance and constant temperature in the laser propagation channel. This purely electrostatic shock wave propagates into the high-Z plasma and leads to mix of different species of ions which is significantly faster than a classical mix in the presence of the large electric field. The mix layer width, measured as a separation distance affected by the shock, grows as , where . The effect of the anomalous mix on the linear growth rate of laser plasma instabilities is evaluated.

Pressure balance in a lower collisionality, attached tokamak scrape-off layer

Previously the gyrokinetic neoclassical code XGCa found that the pressure balance (or momentum balance) in the diverted scrape-off layer (SOL) does not follow that from the fluid description based on the Chew–Goldberger–Low (CGL) theory. In this paper a gyrocenter parallel momentum moment equation is derived for pressure balance (or momentum balance) in a tokamak SOL, for use in interpreting this difference. The new gyrocenter-fluid pressure balance equation allows identifying from the XGCa code results which physical processes dominate the setting of pressure variation in the scrape-off layer. This pressure balance equation is applied to the simulation of a DIII-D H-mode discharge, with a lower ion collisionality in the SOL, and the Coulomb and atomic collisions are not strong enough to yield a detached divertor plasma. It is found that the total pressure balance is much better matched using the gyrocenter parallel momentum moment equation. Electrons are shown to be dominantly adiabatic, while ions have multiple contributions to pressure balance, including terms originating from particle drifts. These results show that in strong gradient, low collisionality regions of the SOL, the typical fluid reductions miss important effects captured in the gyrokinetic model.

Gyrokinetic theory of slab universal modes and the non-existence of the gradient drift coupling (GDC) instability

A gyrokinetic linear stability analysis of a collisionless slab geometry in the local approximation is presented. We focus on k∥=0 universal (or entropy) modes driven by plasma gradients at small and large plasma β. These are small scale non-MHD instabilities with growth rates that typically peak near k⊥ρi∼1 and vanish in the long wavelength k⊥→0 limit. This work also discusses a mode known as the Gradient Drift Coupling (GDC) instability previously reported in the gyrokinetic literature, which has a finite growth rate γ=β/[2(1+β)]Cs/|Lp| with Cs2=p0/ρ0 for k⊥→0 and is universally unstable for 1/Lp≠0. We show that the GDC instability is a spurious, unphysical artifact that erroneously arises due to the failure to respect the total equilibrium pressure balance p0+B02/(8π)=constant, which renders the assumption B0′=0 inconsistent if p0′≠0.

Total fluid pressure imbalance in the scrape-off layer of tokamak plasmas

Simulations using the fully kinetic neoclassical code XGCa (X-point included guiding- center axisymmetric) were undertaken to explore the impact of kinetic effects on scrape-off layer (SOL) physics in DIII-D H-mode plasmas. XGCa is a total-f, gyrokinetic code which self-consistently calculates the axisymmetric electrostatic potential and plasma dynamics, and includes modules for Monte Carlo neutral transport.Previously presented XGCa results showed several noteworthy features, including large variations of ion density and pressure along field lines in the SOL, experimentally relevant levels of SOL parallel ion flow (Mach number ∼ 0.5), skewed ion distributions near the sheath entrance leading to subsonic flow there, and elevated sheath potentials (Churchill 2016 Nucl. Mater. Energy 1–6).In this paper, we explore in detail the question of pressure balance in the SOL, as it was observed in the simulation that there was a large deviation from a simple total pressure balance (the sum of ion and electron static pressure plus ion inertia). It will be shown that both the contributions from the ion viscosity (driven by ion temperature anisotropy) and neutral source terms can be substantial, and should be retained in the parallel momentum equation in the SOL, but still falls short of accounting for the observed fluid pressure imbalance in the XGCa simulation results.

Magnetic reconnection with a fast perpendicular sheared flow

AbstractMagnetic reconnection at the Earth's low‐latitude magnetopause near the flank region is likely associated with a large sheared flow, being frequently quasi‐perpendicular to the antiparallel magnetic field components. The magnitude of a fast sheared flow can be super‐Alfvénic and even overcome the local fast mode speed. A scaling analysis implies a contradiction between the Walén relation and the balance of the total pressure for magnetic reconnection with a supercritical perpendicular sheared flow. This study uses one‐ and two‐dimensional magnetohydrodynamic (MHD) simulations to demonstrate that the traditional reconnection layer violates the Walén relation but still maintains the total pressure balance in such a configuration. The results show an expanded outflow region, consistent with the presence of divergent normal flow, and a significant decrease of the plasma density as well as the thermal pressure in the outflow region. In contrast, the magnitude of the magnetic field in the outflow region matches the value in the inflow region due to the total pressure balance, which is fundamentally different from the classical reconnection layer under sub‐Alfvénic perpendicular sheared flow conditions. In three‐dimensional geometry, the fast sheared flow without being stabilized by the magnetic field is expected to be Kelvin‐Helmholtz unstable. However, the three‐dimensional MHD simulation suggests that such structure can be KH stable. Although, the presence of surface waves modulates some two‐dimensional features, the major characteristics of the expanded outflow region are likely to be observed by in situ satellites.

In situ properties of small and large flux ropes in the solar wind

AbstractTwo populations of twisted magnetic field tubes, or flux ropes (hereafter, FRs), are detected by in situ measurements in the solar wind. While small FRs are crossed by the observing spacecraft within few hours, with a radius typically less than 0.1 AU, larger FRs, or magnetic clouds (hereafter, MCs), have durations of about half a day. The main aim of this study is to compare the properties of both populations of FRs observed by the Wind spacecraft at 1 AU. To do so, we use standard correlation techniques for the FR parameters, as well as histograms and more refined statistical methods. Although several properties seem at first different for small FRs and MCs, we show that they are actually governed by the same propagation physics. For example, we observe no in situ signatures of expansion for small FRs, contrary to MCs. We demonstrate that this result is in fact expected: small FRs expand similar to MCs, as a consequence of a total pressure balance with the surrounding medium, but the expansion signature is well hidden by velocity fluctuations. Next, we find that the FR radius, velocity, and magnetic field strength are all positively correlated, with correlation factors than can reach a value &gt;0.5. This result indicates a remnant trace of the FR ejection process from the corona. We also find a larger FR radius at the apex than at the legs (up to 3 times larger at the apex), for FR observed at 1 AU. Finally, assuming that the detected FRs have a large‐scale configuration in the heliosphere, we derived the mean axis shape from the probability distribution of the axis orientation. We therefore interpret the small FR and MC properties in a common framework of FRs interacting with the solar wind, and we disentangle the physics present behind their common and different features.

Radial profile of pressure in a storm ring current as a function of D st

Using satellite data obtained near the equatorial plane during 12 magnetic storms with amplitudes from −61 down to −422 nT, the dependences of maximum in L-profile of pressure (Lm) of the ring current (RC) on the current value of Dst are constructed, and their analytical approximations are derived. It is established that function Lm(Dst) is steeper on the phase of recovery than during the storm’s main phase. The form of the outer edge of experimental radial profiles of RC pressure is studied, and it is demonstrated to correspond to exponential growth of the total energy of RC particles on a given L shell with decreasing L. It is shown that during the storms’ main phase the ratio of plasma and magnetic field pressures at the RC maximum does not practically depend on the storm strength and Lm value. This fact reflects resistance of the Earth’s magnetic field to RC expansion, and testifies that during storms the possibilities of injection to small L are limited for RC particles. During the storms’ recovery phase this ratio quickly increases with increasing Lm, which reflects an increased fraction of plasma in the total pressure balance. It is demonstrated that function Lm(Dst) is derived for the main phase of storms from the equations of drift motion of RC ions in electrical and magnetic fields, reflecting the dipole character of magnetic field and scale invariance of the pattern of particle convection near the RC maximum. For the recovery phase it is obtained from the Dessler-Parker-Sckopke relationship. The obtained regularities allow one to judge about the radial profile of RC pressure from ground-based magnetic measurements (data on the Dst variation).

Comparison of the thin flux tube approximation with 3D MHD simulations

The structure and dynamics of small vertical photospheric magnetic flux concentrations has been often treated in the framework of an approximation based upon a low-order truncation of the Taylor expansions of all quantities in the horizontal direction, together with the assumption of instantaneous total pressure balance at the boundary to the non-magnetic external medium. Formally, such an approximation is justified if the diameter of the structure (a flux tube or a flux sheet) is small compared to all other relevant length scales (scale height, radius of curvature, wavelength, etc.). The advent of realistic 3D radiative MHD simulations opens the possibility of checking the consistency of the approximation with the properties of the flux concentrations that form in the course of a simulation. We carry out a comparative analysis between the thin flux tube/sheet models and flux concentrations formed in a 3D radiation-MHD simulation. We compare the distribution of the vertical and horizontal components of the magnetic field in a 3D MHD simulation with the field distribution in the case of the thin flux tube/sheet approximation. We also consider the total (gas plus magnetic) pressure in the MHD simulation box. Flux concentrations with super-equipartition fields are reasonably well reproduced by the second-order thin flux tube/sheet approximation. The differences between approximation and simulation are due to the asymmetry and the dynamics of the simulated structures.

On the Expansion of Quasi‐Static, Twisted Coronal Loops in Uniform Gravity

Coronal loop emission profiles are often of remarkably constant width along their entire lengths, contradicting expectations based on model coronal magnetic field strengths decreasing with height. Meanwhile P. Bellan has produced a theoretical model in which an initially empty, twisted force-free loop, on being filled with plasma via upflow at each footpoint, in the absence of significant gravitational effects, forms a narrow, filamentary loop of constant cross section. In this paper, we focus on equilibrium states that include stratification by uniform gravity while retaining the effects of magnetic field twist. Comparing these with related force-free equilibria, it is found that injection of low-? plasma under coronal conditions is not likely to change the shape of a loop significantly. These linear equilibria apply to the interiors and boundaries of loops only, with external influences modeled by boundary total pressures. The effects of total pressure balance with surroundings and of gravitational stratification are to inhibit the pinching of a loop to a constant cross section. Only if the plasma ? were high enough for the plasma to reconfigure the external field and the hydrostatic scale height much greater than the loop size could the final state have nearly constant cross section. We do not expect this to occur in the corona.

Investigations of MHD wave coupling in a 3-D numerical model: effects of temperature gradients

The inhomogeneity of the plasma pressure becomes important near the boundary between cold and hot plasmas. The Alfven speed undergoes significant variations in such regions in order to satisfy the total pressure balance. We have developed a new three-dimensional (3-D) MHD wave model that allows for finite plasma pressure. Based on this time-dependent model, we study the resonant absorption properties when temperature gradients are significant. We consider two models, one corresponding to the inner edge of the plasma sheet while the second models the plasma sheet boundary layer. We discuss how both transverse and compressional waves are affected by inhomogeneous pressure and temperature by examining the wave spectra. Our results show that localized field-aligned currents are strongly excited at the boundary between the plasma sheet and its surrounding regions.

Behaviour of upstream separatrix density in JET H-mode plasmas

The variation of n Sep e / n e in JET H-modes is described. The upstream separatrix density, n Sep e, was determined using total pressure balance, in conjunction with `onion-skin' modelling and reciprocating Langmuir probe measurements. It was found that the ratio n Sep e / n e was strongly influenced by ELM frequency increasing, proportional to the rate of particle loss to the scrape-off layer (SOL) from ELMs, towards the pedestal density and then saturating. The initial n Sep e / n e ratio and rate of increase were different for horizontal and vertical target configurations with the latter having an initial n Sep e / n e ratio of 0.12. Related information on the behaviour of the density SOL width and the variation of n Sep e / n e with confinement are also presented.

Detection of localized, plasma‐depleted flux tubes or bubbles in the midtail plasma sheet

Recent studies have shown that most Earthward transport hi the midtail, high‐beta plasma sheet takes place in the form of short‐lived, high‐speed plasma flow bursts. Bursty bulk flows are observed both when the plasma sheet is thin, such as during substorm expansion, and when it is thick, such as during substorm recovery. We present multi‐instrument observations from the ISEE1 and ISEE 2 spacecraft to argue that when the plasma sheet becomes thick and close to its equilibrium state, the plasma and magnetic field signatures of high‐speed flow events are consistent with the theoretically predicted signatures of plasma‐depleted flux tubes or “bubbles” [Pontius and Wolf, 1990; Chen and Wolf, 1993]. These signatures consist of a decrease in the plasma pressure and an increase in the Bz‐component of the magnetic field accompanying the high speed flow. We show that the Earthward moving bubbles are separated from the plasma ahead of them by a sharp tangential discontinuity. The layer ahead of the bubbles exhibits flow and magnetic field shear consistent with flow around an Earthward moving obstacle. The bubble is in approximate total pressure balance with the surrounding medium. We show that there is a systematic difference in the orientation of the discontinuity measured at ISEE 1 and 2, implying a small (about 1–3 RE) cross‐tail size of the bubbles.

Magnetopause crossings without a boundary layer

The microstructure of pristine magnetopause crossings has been analyzed by using high‐resolution particle and field data obtained by the Active Magnetospheric Particle Tracer Explorers (AMPTE) Charge Composition Explorer (CCE) and ISEE 2 spacecraft. These crossings are pristine in the sense that they exhibit no adjoining magnetospheric boundary layer, or, at most, a low‐density plateau. The CCE crossings include the low‐latitude near‐noon region not typically sampled by ISEE 2, which covers all other local time sectors in a complementary way. Magnetopause crossings without a boundary layer are found to occur to all local times, and such crossings constitute about 10% of all magnetopause crossings. Total pressure balance across the magnetopause is observed to within experimental errors; however, electron data, full‐energy composition measurements, and occasionally field stress are needed to fully evaluate pressure balance. The microstructure of the magnetopause current layer is also found to depend on local time. Crossings within about 1 hour local time of the noon meridian often exhibit very sharp density gradients on scale lengths down to a few plasma skin depths. These gradients are reduced for crossings farther from local noon such that, for cases near the dawn‐dusk meridian, the scale length for density gradients and the magnetopause current are roughly comparable. Magnetopause crossings without a boundary layer impose severe constraints on various theories of boundary layer formation. Pristine magnetopause crossings may be direct cuts through the diffusion region for reconnection. With this interpretation our results are in qualitative agreement with recent simulations of the diffusion region and associated turbulence by Drake et al. [1994], who propose the current convective instability as the dominant process for current transport at the magnetopause.

Simulation study of the Riemann problem associated with the magnetotail reconnection

The structure of reconnection layer in the distant magnetotail is studied by solving the Riemann problem for the evolution of an initial current sheet using one‐dimensional MHD and hybrid simulations. Initially, the current sheet with total pressure balance separates the plasmas and fields in the two lobes. In the presence of a nonzero normal magnetic field, which is due to the magnetic reconnection, the initial current sheet evolves to form the reconnection layer, which consists of various MHD discontinuities. First, we study the symmetric case with equal magnetic field strengths, equal plasma densities, and exactly antiparallel magnetic fields (By=0) in the two lobes. The Petschek (1964) reconnection layer which contains two switch‐off slow shocks, whose intermediate Mach number MI=1, is obtained. In the hybrid simulation, the switch‐off shock possesses a large‐amplitude, left‐hand‐polarized rotational wave train of magnetic field in the downstream region. Each slow shock propagates to either lobe in the magnetotail. A strong temperature anisotropy with T∥&gt;T⊥ appears in most region of the reconnection layer. This is due to the interpenetrating of ions between the two lobes and the backstreaming of ions from the downstream of each slow shock to the lobe. Second, the presence of a finite guide field in lobes (By≠0) is considered in the simulation. It is found that the slow shocks becomes nonswitch‐off shocks with an intermediate Mach number MI&lt;1, and two rotational discontinuities are also present in the hybrid simulation to bound the reconnection layer from, respectively, the two lobes. In addition, our hybrid simulations show that the presence of the finite By destroys the coherent wave train in the reconnection layer if the lobe By0≥0.08Bx0. Finally, the simulations are extended to the asymmetric cases in which the plasma densities and/or the magnetic fields in the two lobes are unequal. For asymmetric cases with By=0 in the two lobes, it is found that an intermediate shock and a nonswitch‐off slow shock are present in the reconnection layer and that the large‐amplitude rotational wave structure does not exist in the entire reconnection layer if the density ratio between the two lobes N1/N0 ≥ 1.5, where N0 and N1 are the ion number densities in the two lobes, receptively. For general cases with both density asymmetry and By0≠0, two rotational discontinuities and two slow shocks are present in the reconnection layer, and the critical value of the density ratio, above which the coherent wave train does not exist, is reduced. The absence of a downstream wave train for the slow shocks observed in the magnetotail may be associated with the presence of a finite By0.

Evidence for Langmuir oscillations and a low density cavity in the Venus magnetotail

We report the discovery of Langmuir oscillations in a very low plasma density region in the Venus magnetotail. These waves are observed more often at 30 kHz, but also at 5.4 kHz indicating densities as low as 0.3 cm−3 in the central tail lobe. The Langmuir probe on board the Pioneer Venus Orbiter cannot resolve such a low plasma density. We use the magnetic field strength and the assumption of total pressure balance to infer the electron temperature as a test of the Langmuir wave interpretation. By investigating the spatial distribution of this wave activity we find that the plasma cavity is ordered in a coordinate system defined by the interplanetary magnetic field and is found at either side of the central tail current sheet.