Flux Tube Volume Research Articles

To the Interchange Instability Criterion in the Magnetosphere in the Presence of Velocity Shear

AbstractIn this paper, we address the issue of interchange instability excitation criterion in the magnetospheric plasma in the presence of the velocity shear using a magnetohydrodynamic approach. We conducted our analysis for the arbitrary β plasma configuration using a WKB approximation for wavelengths, λ, much smaller than the characteristic plasma inhomogeneity scale, L, and found two branches of waves. These two wave branches can exist due to the plasma velocity shear and one branch can be unstable for arbitrary angles between the plasma entropy parameter and magnetic flux tube volume gradients. The implications of these results for magnetospheric physics as well as comparison of our results with corresponding analysis of other authors are discussed.

Impact of a cryogenic baffle system on the suppression of radon-induced background in the KATRIN Pre-Spectrometer

The KATRIN experiment will determine the effective electron anti-neutrino mass with a sensitivity of 200 meV/c2 at 90% CL. The energy analysis of tritium β-decay electrons will be performed by a tandem setup of electrostatic retarding spectrometers which have to be operated at very low background levels of <10−2 counts per second. This benchmark rate can be exceeded by background processes resulting from the emanation of single 219,220Rn atoms from the inner spectrometer surface and an array of non-evaporable getter strips used as main vacuum pump. Here we report on the impact of a cryogenic technique to reduce this radon-induced background in electrostatic spectrometers. It is based on installing a liquid nitrogen cooled copper baffle in the spectrometer pump port to block the direct line of sight between the getter pump, which is the main source of 219Rn, and the sensitive flux tube volume. This cold surface traps a large fraction of emanated radon atoms in a region outside of the active flux tube, preventing background there. We outline important baffle design criteria to maximize the efficiency for the adsorption of radon atoms, describe the baffle implemented at the KATIRN Pre-Spectrometer test set-up, and report on its initial performance in suppressing radon-induced background.

On application of asymmetric Kan-like exact equilibria to the Earth magnetotail modeling

Abstract. A specific class of solutions of the Vlasov–Maxwell equations, developed by means of generalization of the well-known Harris–Fadeev–Kan–Manankova family of exact two-dimensional equilibria, is studied. The examined model reproduces the current sheet bending and shifting in the vertical plane, arising from the Earth dipole tilting and the solar wind nonradial propagation. The generalized model allows magnetic configurations with equatorial magnetic fields decreasing in a tailward direction as slow as 1∕x, contrary to the original Kan model (1∕x3); magnetic configurations with a single X point are also available. The analytical solution is compared with the empirical T96 model in terms of the magnetic flux tube volume. It is found that parameters of the analytical model may be adjusted to fit a wide range of averaged magnetotail configurations. The best agreement between analytical and empirical models is obtained for the midtail at distances beyond 10–15 RE at high levels of magnetospheric activity. The essential model parameters (current sheet scale, current density) are compared to Cluster data of magnetotail crossings. The best match of parameters is found for single-peaked current sheets with medium values of number density, proton temperature and drift velocity.

Experimental Study of the Plasmasphere Boundary Layer Using MAGION 5 Data

AbstractThe in situ cold plasma measurements onboard MAGION 5 were carried out with very good time resolution, and this permitted to analyze thin plasmasphere boundary layer (PBL) near the plasmapause. In this layer the plasma density N is decreasing exponentially with L: N~exp((LPP − L)/WB), where WB corresponds to the characteristic width of the PBL, the distance in L within which the density varies by a factor of e, and LPP is the position of the plasmapause. The density in the boundary layer is inversely proportional to the volume of the unit magnetic flux tube, whereas its width is proportional to the volume of magnetic flux tube. The characteristic width of the PBL linearly depends on the time elapsed since the most recent maximum value of KP. Empirical relation for the dependence of the PBL width on most recent maximum value of KP and on the lapse time between this maximum and the plasmapause observations is proposed.

Perturbation of the magnetic field in the Earth’s magnetosphere due to plateau creation in the radial distribution of plasma pressure

The nonlinear perturbation of a dipole field by a system of transverse currents, which arises due to the radial pressure distribution when the pressure is almost independent of the radial distance, is analyzed. This distribution of pressure was observed in the experiment. The radial dependences of the magnetic field depression, transverse current density, and volumes of magnetic flux tubes have been obtained at different values of the plasma parameter via nonlinear simulation. It is shown that a dependence of the volume of magnetic flux tube on the radial distance can change and a region of the negative gradient of volume can appear at some plasma-parameter values.

Magnetotail energy dissipation during an auroral substorm.

Violent releases of space plasma energy from the Earth’s magnetotail during substorms produce strong electric currents and bright aurora. But what modulates these currents and aurora and controls dissipation of the energy released in the ionosphere? Using data from the THEMIS fleet of satellites and ground-based imagers and magnetometers, we show that plasma energy dissipation is controlled by field-aligned currents (FACs) produced and modulated during magnetotail topology change and oscillatory braking of fast plasma jets at 10-14 Earth radii in the nightside magnetosphere. FACs appear in regions where plasma sheet pressure and flux tube volume gradients are non-collinear. Faster tailward expansion of magnetotail dipolarization and subsequent slower inner plasma sheet restretching during substorm expansion and recovery phases cause faster poleward then slower equatorward movement of the substorm aurora. Anharmonic radial plasma oscillations build up displaced current filaments and are responsible for discrete longitudinal auroral arcs that move equatorward at a velocity of about 1km/s. This observed auroral activity appears sufficient to dissipate the released energy.

A model of force balance in Jupiter's magnetodisc including hot plasma pressure anisotropy

AbstractWe present an iterative vector potential model of force balance in Jupiter's magnetodisc that includes the effects of hot plasma pressure anisotropy. The fiducial model produces results that are consistent with Galileo magnetic field and plasma data over the whole radial range of the model. The hot plasma pressure gradient and centrifugal forces dominate in the regions inward of ∼20 RJ and outward of ∼50 RJ, respectively, while for realistic values of the pressure anisotropy, the anisotropy current is either the dominant component or at least comparable with the hot plasma pressure gradient current in the region in between. With the inclusion of hot plasma pressure anisotropy, the ∼1.2 and ∼2.7° shifts in the latitudes of the main oval and Ganymede footprint, respectively, associated with variations over the observed range of the hot plasma parameter Kh, which is the product of hot pressure and unit flux tube volume, are comparable to the shifts observed in auroral images. However, the middle magnetosphere is susceptible to the firehose instability, with peak equatorial values of βh∥e−βh⊥e≃1 − 2, for Kh=2.0 − 2.5 × 107 Pa m T−1. For larger values of Kh,βh∥e−βh⊥e exceeds 2 near ∼25 RJ and the model does not converge. This suggests that small‐scale plasmoid release or “drizzle” of iogenic plasma may often occur in the middle magnetosphere, thus forming a significant mode of plasma mass loss, alongside plasmoids, at Jupiter.

Anharmonic oscillatory flow braking in the Earth's magnetotail

AbstractPlasma sheet bursty bulk flows often oscillate around their equilibrium position at about 10 RE downtail. The radial magnetic field, pressure, and flux tube volume profiles usually behave differently earthward and tailward of this position. Using data from five Time History of Events and Macroscale Interactions during Substorms (THEMIS) probes, we reconstruct these profiles with the help of an empirical model and apply thin filament theory to show that the oscillatory flow braking can occur in an asymmetric potential. Thus, the thin filament oscillations appear to be anharmonic, with a power spectrum exhibiting peaks at both the fundamental frequency and the first harmonic. Such anharmonic oscillatory braking can explain the presence of the first harmonic in Pi2 pulsations (frequency doubling), which are simultaneously observed by magnetometers on the ground near the conjugate THEMIS footprints.

Dipole magnetic-field disturbance and generation of current systems by asymmetric plasma pressure

Nonlinear disturbance of the dipole field by nonaxisymmetric plasma pressure distribution was analyzed under the assumption of magnetostatic equilibrium for finite values of the plasma parameter at the pressure maximum area. The distributions of isolines of the constant value of magnetic-field component BZ and the volume of magnetic flux tube in the equatorial plane were obtained. At a finite plasma pressure, local minima and maxima of the magnetic field are formed. The formation of these local maxima and minima leads to the formation of contours (not surrounding the Earth) Bmin = const, where Bmin is the minimum magnetic field on the magnetic field line. This changes the direction of the gradient of the volume of the magnetic flux tube. The configuration of appearing field-aligned currents was determined. The results obtained are discussed in terms of their use in explaining a number of effects observed in the Earth’s magnetosphere.

The quiet evening auroral arc and the structure of the growth phase near‐Earth plasma sheet

AbstractThe plasma pressure and current configuration of the near‐Earth plasma sheet that creates and sustains the quiet evening auroral arc during the growth phase of magnetospheric substorms is investigated. We propose that the quiet evening arc (QEA) connects to the thin near‐Earth current sheet, which forms during the development of the growth phase enhancement of convection. The current sheet's large polarization electric fields are shielded from the ionosphere by an Inverted‐V parallel potential drop, thereby producing the electron precipitation responsible for the arc's luminosity. The QEA is located in the plasma sheet region of maximal radial pressure gradient and, in the east‐west direction, follows the vanishing of the approximately dawn‐dusk‐directed gradient or fold in the plasma pressure. In the evening sector, the boundary between the Region1 and Region 2 current systems occurs where the pressure maximizes (approximately radial gradient of the pressure vanishes) and where the approximately radial gradient of the magnetic flux tube volume also vanishes in an inflection region. The proposed intricate balance of plasma sheet pressure and currents may well be very sensitive to disruption by the arrival of equatorward traveling auroral streamers and their associated earthward traveling dipolarization fronts.

On the role of pressure and flow perturbations around dipolarizing flux bundles

As a dipolarizing flux bundle (DFB) moves earthward, it creates pressure and flow perturbations. These perturbations may play a significant role in controlling DFB motion and generating field‐aligned currents (FACs) which render the DFB a “wedgelet”, a traveling building block of the substorm current wedge. To investigate this hypothesis, we use DFB observations from the Time History of Events and Macroscale Interactions during Substorms mission to reconstruct the spatial profiles of the thermal and total (thermal plus magnetic) pressures and of the plasma flow near the DFB. The total pressure reaches maximum inside the dipolarization front (DF, the leading edge of the DFB). The resultant pressure gradient force pushes ambient plasma in the direction normal to the front and exerts a gradient force density of ~0.15 nPa/RE against the DFB motion. The thermal pressure in the equatorial plane is strongest immediately ahead of the DFB's leading point; it decreases with distance from that peak: toward the ambient plasma, toward the DFB interior, and toward the DFB flanks. Combining our estimate of the flux tube volume distortion with the measured equatorial thermal pressure distribution, we obtain a region‐1‐sense FAC inside the DF layer and region‐2‐sense FAC in the ~1 RE thick region immediately ahead of it. This system of FACs is indeed consistent with a wedgelet.

Empirical reconstruction of storm time steady magnetospheric convection events

We investigate the storm‐scale morphology of the magnetospheric magnetic field as well as underlying distributions of electric currents, equatorial plasma pressure, and entropy for four steady magnetospheric convection (SMC) events that occurred during the May 2000 and October 2011 magnetic storms. The analysis is made using the empirical geomagnetic field model TS07D, in which the structure of equatorial currents is not predefined and it is dictated by data. The model also combines the strengths of statistical and event‐oriented approaches in mining data for the reconstruction of the magnetic field. The formation of a near‐Earth minimum of the equatorial magnetic field in the midnight sector is inferred from data without ad hoc assumptions of a special current system postulated in earlier empirical reconstructions. In addition, a new SMC class is discovered where the minimum equatorial field is substantially larger and located closer to Earth. The magnetic field tailward of the minimum is also much larger, and the corresponding area of accumulated magnetic flux may occupy a very short tail region. The equatorial current and plasma pressure are found to be strongly enhanced far beyond geosynchronous orbit and in a broad local time interval covering the whole nightside region. This picture is consistent with independent recent statistical studies of the SMC pressure distributions, global MHD, and kinetic Rice Convection Model‐Equilibrium (RCM‐E) simulations. Distributions of the flux tube volume and entropy inferred from data reveal different mechanisms of the magnetotail convection crisis resolution for two classes of SMC events.

On near‐tail bubble penetration into geosynchronous altitude

Dipolarization of the magnetic field at the near‐Earth tail is usually associated with the local reduction of pV5/3 compared to that of the background, where p is the plasma pressure and V is the volume of the unit magnetic flux tube. This can be interpreted as a bubble, which can propagate earthward by the interchange process. How deep such a bubble can penetrate earthward, and what is the critical factor are critical questions that need to be answered. In this paper, we examine these issues by comparing near‐tail observations by inner probes of THEMIS with geosynchronous magnetic observations by GOES. We identified a number of bubble events associated with near‐tail dipolarization, which we call “tail bubble,” and checked geosynchronous disturbances. We find a statistical trend that geosynchronous disturbance is more likely to occur when associated with (or when hit by) an earthward moving tail bubble with a more‐depleted pV5/3. We estimated the background pV5/3 profile statistically and used it to determine expected equilibrium (or stop) positions for earthward moving bubbles where the bubble's pV5/3 is equal to that of the background. Statistically, we find that the equilibrium position is more inward for tail bubbles with a lower pV5/3, for which the probability of causing geosynchronous disturbance is higher. For example, the probability of a tail bubble being associated with geosynchronous disturbance is 75% if the bubble's equilibrium position is &lt;8 RE. However, for all the events studied here, the bubble equilibrium positions are still outside the geosynchronous altitude. Although this result may be subject to change due to the uncertainty in estimating pV5/3 and the limited number of the events identified near geosynchronous altitude, we suggest that an overshooting of the penetrating bubbles beyond equilibrium positions is a possible explanation.

Plasmapause and Increase in Whistler Mode Wave’s Growth

Singh et al have studied in detail the Whistler mode VLF waves intensity enhancement, by assuming change in VII , volume of flux tube, EII change due to a change of position of plasmapause. Enhancing this work, we in place of energy increase, consider the wave growth increase of whistler mode waves. The results are in agreement with reported observations and suggest that plasmapause plays an important role in wave propagation. It is shown that this guidance is possible at both the inner and outer edges of the Plasmapause and that more efficient guide occurs as the Plasmapause gradients become stronger. In the case of strong gradients, waves coming from a wide latitudes range (~8 0 ) are focused tightly about the plasmapause field lines, resulting in a wave intensity increase of about 3dB near the magnetic equator plane 12 .Since plasmapause has inner & outer edges, we can say that it has a width pp) which depends upon Kpmax values, MLT as well as L the McIlwain parameters. pp variation is discussed in next section.

3D MHD simulations of electric current development in a rotating sunspot: active region NOAA 8210

Context. Active region (AR) 8210 was the host of many flares and coronal mass ejections (CMEs). Studies of its temporal evolution indicated that the clockwise rotation of a negative magnetic polarity, together with the motion of a positive polarity located close to it are a major source of magnetic energy to the corona above. Aims. Search for the mechanisms of energy storage and release above AR8210. For this sake we locate the current system generated by the photospheric plasma motion suggested as source of coronal energy supply above AR8210 and compare it with the location of identified flaring regions. Methods. We simulated the reaction of the corona using a three-dimensional (3D) MagnetoHydroDynamic (MHD) model. The initial coronal magnetic field is extrapolated out of the observed line-of-sight (LOS) component of the photospheric magnetic field. The corresponding photospheric plasma motion is imposed close to the bottom boundary of the simulation box. Current dependent resistivity and compressibility of the plasma are considered in the model. Results. The horizontal plasma motion causes a current system that spatially coincides with the flaring region associated to AR8210. Particularly, the rotation of the plasma over the main negative polarity gives rise to strong currents localized over the main negative polarity, i.e. close to the position of a flare site. Above this region a strong magnetic field divergence is indicated by large differential flux tube volumes. Intense currents also form along the eastern border of a positive polarity region, over a polarity inversion line (PIL), in a site where a flare appears later. The southward motion of a positive flux concentration generates a current system extending mainly along the eastern border of it, over a polarity inversion line. Magnetic energy is deposited mainly over the main negative polarity, where the major flare activity is observed at later times. Conclusions. The two patterns of photospheric plasma motion suggested as being responsible for the flaring activity of AR8210 generate current systems spatially coincident with the flaring area associated to this active region. These currents generate magnetic fields that contribute to the increase in magnetic energy inside the simulation volume. The transport of magnetic flux by the photospheric plasma motion also contributes to the redistribution of magnetic energy. The increase in magnetic energy occurs mainly over the main negative polarity and close to where a strong current system is formed. We conclude that in the case of AR8210 the rotation of the negative polarity region is the main contributor to its flare activity. The southward motion of a positive magnetic flux concentration plays an important role in the formation of current systems at its eastern border.

Some statistical properties of flow bursts in the magnetotail

We investigated the properties of physical parameters during earthward flow burst (FB) events in the near‐Earth magnetic tail. For the investigation, we used the measurements made by two of the Cluster satellites, C1 and C4, from 2001 to 2003 and selected FB events based on a set of strict criteria. First, while we confirmed the well‐known fact that the FBs are characterized by generally having a lower density than that of their surrounding medium, we newly found that the maximum flow speed of FBs is statistically proportional to the density depletion amount relative to the surrounding medium density. Namely, more density‐depleted FBs relative to the surrounding medium density move faster toward the Earth. We statistically determined the flux tube entropy parameter pV5/3, where p is the ion pressure and V is the flux tube volume of unit magnetic flux, and found that nearly all of the FB events indicate a decrease as compared to the background values. These results clearly support the bubble model of the flow burst events examined here. Also we found that the FBs can be of high or low temperature or ion pressure: while the majority of the FB events show an increased temperature and a decreased ion pressure as compared to those of background, ∼30% of the events indicate an opposite behavior. This does not contradict the bubble idea as long as they are associated with a reduced pV5/3. In addition, we found a few other interesting features which can all be interpreted within the bubble model. First, the temperature is found to be overall nearly isotropic during some FB events and only weakly anisotropic during some other FB events. However, we newly found that when it is anisotropic, T⊥ is higher than T∥ for ∼63% of the FB events and T⊥/T∥ increases during FBs as compared to that of the preceding quiet time interval. One implication of this result is that the Betatron acceleration takes place more effectively over the Fermi acceleration during the course of the earthward transportation of bubble flux tubes. Also, we newly found that for ∼23% of the FB events, even the small amount of the anisotropy exceed the classic mirror instability limit mainly due to a high plasma beta condition while the firehose instability limit is rarely violated. Last, the total pressure (plasma pressure plus magnetic pressure) decreases for ∼74% of the FB events and does not for the others. A more interesting feature is that while the total pressure change is small for most of the FBs, some events reveal a nonnegligible unbalance with the surrounding medium. This implies the existence of the total pressure gradient force acting on the bubble flux tubes of such events, which likely decelerates the earthward motion of the flux tube, in addition to the accelerating magnetic tension force.

A kinetic ballooning/interchange instability in the magnetotail

Three‐dimensional electromagnetic particle‐in‐cell simulations are used to investigate the stability properties of a plasma sheet equilibrium with a minimum in the magnetic normal (Bz) component. Such a configuration is found to be unstable to a ballooning/interchange type mode that is localized tailward of the Bz minimum. The mode has a relatively short dawn‐dusk wavelength of the order of the ion Larmor radius in the Bz field (∼2000 km) and a phase velocity in the direction of the ion diamagnetic drift with a magnitude about one‐fifth of the drift speed. The real frequency is about 60% of the midplane ion cyclotron frequency. The dominant mode polarization is δϕ and δB∥. A linear kinetic analysis including bounce and drift resonance interactions for the electrons and an orbit average over the flux tube volume for the Boltzmann term in the ion density perturbation produces agreement with the simulation mode properties and permits identification of the mode as the low‐frequency extension of the lower hybrid drift instability in straight magnetic geometry. In its nonlinear evolution, the mode develops Rayleigh‐Taylor fingers that extend across the Bz minimum and into the near‐Earth dipole region. These fingers transport magnetic flux earthward, and the flux is redistributed by electron Hall currents that flow around the fingers. This mode is likely to contribute to the nearly continuous presence of turbulence in the center of the plasma sheet.

Northward interplanetary magnetic field plasma sheet entropies

The plasma sheet pressure, temperature, and density profiles inferred from DMSP observations are used to investigate northward interplanetary magnetic field plasma sheet specific entropy s = p/ργ or p/nγ (for a single ion species). Profiles of s for hot and cold populations are considered. The hot ion population s profile suggests a duskward heat flux that is consistent with the curvature and gradient drift. In contrast, s of the cold population is approximately conserved in the X direction but has a strong gradient toward the midnight meridian. The total entropy (S = ∫p1/γ dl/B) and s under various entry mechanisms are estimated for comparisons with observed values. The cold population s is higher than that of the magnetosheath, suggesting the entry process may heat the ions. Cusp reconnection by itself may not increase the magnetosheath ion s, but cusp reconnection with kinetic Alfvén waves (KAW) may provide the necessary heating. Localized reconnection in Kelvin‐Helmholtz vortices may increase s if the plasma expands nonadiabatically into the large magnetospheric flux tube volume. KAWs may lead to diffusion and heating consistent with the observations. The cold population s increases by a factor of 5 from the flanks to the midnight meridian, which provides a constraint for transport mechanism(s) within the plasma sheet. A simple calculation is performed to demonstrate that the spatial gradients of s in the plasma sheet could result from the temporal dependence of transport processes in the magnetopause boundary layer.

Ion and electron temperature evolution during equatorial spread F

The first simulation study of ion (O+ and H+) and electron temperature evolution during equatorial spread F (ESF) is presented. The simulation results are based on the Naval Research Laboratory (NRL) SAMI3/ESF three‐dimensional code. It is found that the ions and electrons undergo both cooling and heating during bubble evolution. The main cause of cooling is adiabatic, associated with the increase of the flux tube volume as the plasma bubble rises. Ion heating is primarily caused by the compression of ions as they stream down the converging magnetic field. The electrons are heated by collisional coupling with the ions. Additionally, it is found that the electrons are heated at high altitudes (≳1200 km) because of thermal conduction, and that hydrogen ions can be heated at relatively low altitudes (≃300 km) because of ion‐neutral frictional heating. We compare the simulation results with observations from the ROCSAT and Hinotori satellites.

Plasma sheet P5/3 and n and associated plasma and energy transport for different convection strengths and AE levels

To understand the large‐scale plasma sheet thermodynamics, we have used Geotail data and a formula for estimating flux tube volume to investigate statistically the equatorial distributions of P5/3 and n for slow flowing plasma in the nightside plasma sheet and compare them with the physical bases of ideal MHD and the Rice Convection Model (RCM). We have examined the distributions under three conditions: (1) weak convection and low AE, (2) enhanced convection and low AE, and (3) enhanced convection and high AE. The overall n decreases significantly with increasing convection or AE, while the overall P5/3 remains similar. We found that P5/3 drops significantly earthward along the estimated electric drift paths near midnight, inconsistent with ideal MHD. Examination of Pk5/3 and the electric and magnetic drift paths of different energy invariants, where Pk is the partial pressure of a specific energy invariant, shows that the strong duskward drift of the above thermal‐energy particles due to magnetic drift, together with there being significantly fewer higher‐energy particles from the dawn flank than from the tail, results in the strong earthward decrease of P5/3. We also found that d(Pk5/3)/dt = 0 and d(nk)/dt = 0 along the electric and magnetic drift paths used in the RCM are good approximations for slow flowing pressure‐bearing plasma sheet ions. The distributions of n indicate that low‐energy plasma becomes dominant when convection and AE are low, and their transport from the flanks may be affected by nonadiabatic processes not included in ideal MHD or the RCM.