On the possibility of quasi-static convection in the quiet magnetotail

Abstract The magnetotail is known to serve as a reservoir of energy transferred into the terrestrial magnetosphere from the solar wind. In principle, the stored energy can be dissipated impulsively, as in a substorm, or steadily through the process of steady adiabatic plasma convection. However, some theoretical arguments have suggested that quasi-static adiabatic convection cannot occur throughout the magnetotail because of the structure of the magnetic field. Here we reexamine the question. We show that in a magnetotail of finite width, downtail pressure gradients depend strongly on the ratio of the potential across half the tail to the ion temperature in the far tail (60 RE). For pertinent quiet time ratios (∼3), a Tsyganenko quiet-time magnetic field model is consistent with steady convection

Static magnetic field models consistent with nearly isotropic plasma pressure

Using the empirical magnetospheric magnetic field models of Tsyganenko and Usmanov (TU), we have determined the self-consistent plasma pressure gradients and anisotropies along the midnight meridian in the near-Earth magnetosphere. By “inverting” the magnetic field, we determine what distributions of an anisotropic plasma, confined within the specified magnetic field configuration, are consistent with the magnetohydrostatic equilibrium condition, J × B = ∇ · P. The TU model, parameterized for different levels of geomagnetic activity by the Kp index, provided the magnetic field values from which J × B was numerically evaluated. A best fit solution was found that minimized the average difference between J × B and ∇ · P along an entire flux tube. Unlike previous semi-empirical models, the TU models contain magnetic stresses that can be balanced by a nearly isotropic plasma pressure with a reasonable radial gradient at the equator

Dawn‐dusk asymmetries in rotating magnetospheres: Lessons from modeling Saturn

Spacecraft measurements reveal perplexing dawn‐dusk asymmetries of field and plasma properties in the magnetospheres of Saturn and Jupiter. Here we describe a previously unrecognized source of dawn‐dusk asymmetry in a rapidly rotating magnetosphere. We analyze two magnetohydrodynamic simulations, focusing on how flows along and across the field vary with local time in Saturn’s dayside magnetosphere. As plasma rotates from dawn to noon on a dipolarizing flux tube, it flows away from the equator along the flux tube at roughly half of the sound speed (Cs), the maximum speed at which a bulk plasma can flow along a flux tube into a lower pressure region. As plasma rotates from noon to dusk on a stretching flux tube, the field‐aligned component of its centripetal acceleration decreases and it flows back toward the equator at speeds typically smaller than 12Cs. Correspondingly, the plasma sheet remains far thicker and the field less stretched in the afternoon than in the morning. Different radial force balance in the morning and afternoon sectors produce asymmetry in the plasma sheet thickness and a net dusk‐to‐dawn flow inside of L = 15 or equivalently, a large‐scale electric field (E) oriented from postnoon to premidnight, as reported from observations. Morning‐afternoon asymmetry analogous to that found at Saturn has been observed at Jupiter, and a noon‐midnight component of E cannot be ruled out.Key PointsDescribes a previously unrecognized source of dawn‐dusk asymmetry in rapidly rotating magnetospheresAsymmetry arises from the different rates of plasma expansion and contraction along flux tubes prenoon and postnoonThe mechanism explains Saturn’s noon‐midnight E field and may produce analogous effects at JupiterPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/135239/1/jgra52440.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/135239/2/jgra52440_am.pd

Coupled SKR Emissions in Saturn’s Northern and Southern Ionospheres

Kilometric radiation (SKR) emitted above Saturn’s auroral ionosphere is modulated in intensity at periods close to the planetary rotation period; SKR periods differ slightly for sources in the north and in the south. Although there is good evidence that the signals are generated independently in the two hemispheres, it is also well established that during southern summer power emitted from the northern hemisphere is modulated in intensity not only at the northern period but also at the southern period, an observation that requires an explanation. We examine the idea that the signal in the north at the southern period is a secondary effect of the strong field‐aligned current system centered at 270° that drives the southern signal. Basing our analysis on studies of field‐aligned current systems in the terrestrial and Jovian magnetospheres, we argue that the parallel electric fields that drive electrons into the southern auroral ionosphere and generate SKR are, at least in part, bidirectional and thus capable of accelerating electrons toward the opposite hemisphere where the secondary signal is detected with intensity lower than that of the locally generated signal. This interpretation implies that the atmospheric process that modulates the periodic responses can operate independently in each hemisphere.Plain Language SummaryRadio frequency signals with wavelengths of order 1 km emitted from high latitudes at Saturn vary in intensity at close to the planetary rotation period. Signals emitted from the southern and northern hemispheres are modulated at slightly different periods. It has been shown that these signals are generated in regions above the atmosphere where electrons accelerated to high velocities move toward the planet along the planetary magnetic field, generating intense electric current. Refined analysis has shown that sometimes the emissions are modulated not only at the dominant period for that hemisphere but also at the period of the opposite hemisphere. The mechanism for generating, for example, southern period emissions in the northern hemisphere has not been established. We propose that where electrons are accelerated in the southern hemisphere, they are accelerated both downward and upward along the planetary magnetic field. The upward moving electrons from the south move downward as they approach the northern hemisphere end of the magnetic field line, generating emissions with an intensity modulated at the southern period. This model implies that the peak emission at the southern period should occur at the same time north and south, a feature that has not yet been tested.Key PointsWe describe the generation of coupled north‐south SKR emissions observed at SaturnThe SKR emission in the north at the southern period is interpreted as a secondary effect of the strong field‐aligned current system that drives the southern signalThe signals at the southern period should appear at 270° southern PPO phase in both hemispheresPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/143770/1/grl57125.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/143770/2/grl57125_am.pd

Driving Saturn's magnetospheric periodicities from the upper atmosphere/ionosphere: Magnetotail response to dual sources

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95157/1/jgra22221.pd

Magnetospheric plasma pressures in the midnight meridian: Observations from 2.5 to 35 RE

Plasma pressure data from the ISEE 2 fast plasma experiment (FPE) were statistically analyzed to determine the plasma sheet pressure versus distance in the midnight local time sector of the near-earth (12–35 RE) magnetotail plasma sheet. The observed plasma pressure, assumed isotropic, was mapped along model magnetic field flux tubes (obtained from the Tsyganenko and Usmanov [1982] model) to the magnetic equator, sorted according to magnetic activity, and binned according to the mapped equatorial location. In regions (L ≳ 12 RE) where the bulk of the plasma pressure was contributed by particles in the energy range of the FPE (70 eV to 40 keV for ions), the statistically determined peak plasma pressures vary with distance similarly to previously determined lobe magnetic pressures (i.e., in a time-averaged sense, pressure balance normal to the magnetotail magnetic equator in the midnight meridian is maintained between lobe magnetic and plasma sheet plasma pressures). Additional plasma pressure data obtained in the inner magnetosphere (2.5 \u3c L \u3c 7) by the Explorer 45, ATS 5, and AMPTE CCE spacecraft supplement the ISEE 2 data. Estimates of plasma pressures in the “transition” region (7–12 RE), where the magnetic field topology changes rapidly from a dipolar to a tail-like configuration, are compared with the observed pressure profiles. The quiet time “transition” region pressure estimates, obtained previously from inversions of empirical magnetic field models, bridge observations both interior to and exterior to the “transition” region in a reasonable manner. Quiet time observations and estimates are combined to provide profiles of the equatorial plasma pressure along the midnight meridian between 2.5 and 35 RE

Driving Saturn's magnetospheric periodicities from the upper atmosphere/ionosphere

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95345/1/jgra21800.pd

Anomalous aspects of magnetosheath flow and of the shape and oscillations of the magnetopause during an interval of strongly northward interplanetary magnetic field

On 15 Feb. 1978, the orientation of the interplanetary magnetic field (IMF) remained steadily northward for more than 12 hours. The ISEE 1 and 2 spacecraft were located near apogee on the dawn side flank of the magnetotail. IMP 8 was almost symmetrically located in the magnetosheath on the dusk flank and IMP 7 was upstream in the solar wind. Using plasma and magnetic field data, we show the following: (1) the magnetosheath flow speed on the flanks of the magnetotail steadily exceeded the solar wind speed by 20 percent; (2) surface waves with approximately a 5-min period and very non-sinusoidal waveform were persistently present on the dawn magnetopause and waves of similar period were present in the dusk magnetosheath; and (3) the magnetotail ceased to flare at an antisunward distance of 15 R(sub E). We propose that the acceleration of the magnetosheath flow is achieved by magnetic tension in the draped field configuration for northward IMF and that the reduction of tail flaring is consistent with a decreased amount of open magnetic flux and a larger standoff distance of the subsolar magnetopause. Results of a three-dimensional magnetohydrodynamic simulation support this phenomenological model

Vortex motion in the ionosphere and nonlinear transport

The relation between vorticity and ionospheric flow patterns is investigated by using a fluid mechanics approach in place of the more customary electromagnetic approach. The focus on the fluid features is justified by the observation that in the incompressible limit appropriate to the ionosphere, vorticity can be regarded as the source of the flow field. We show how vorticity can be introduced into the flow by local ionospheric conditions. However, in the cases of greatest interest, the vorticity is imposed by external sources, which can be in the magnetosphere or in the solar wind. As an important application, we consider traveling ionospheric vortices propagating around the polar cap boundary. We show that such traveling disturbances transport both momentum and magnetic flux in the direction of their phase velocity, typically antisunward. Like other intermittent disturbances of small scale, such as flux transfer events, individual traveling ionospheric vortices transport relatively little flux, but multiple disturbances could conceivably transport an important fraction of the polar cap magnetic flux from the dayside to the tail

Ionospheric flow shear associated with the preexisting auroral arc: A statistical study from the FAST spacecraft data

An auroral substorm is a disturbance in the magnetosphere that releases energy stored in the magnetotail into the high‐latitude ionosphere. By definition, an auroral substorm commences when a discrete auroral arc brightens and subsequently expands poleward and azimuthally. The arc that brightens is usually the most equatorward of several auroral arcs that remain quiescent for ~5 to ~60 min before the breakup commences. This arc is often referred to as the “preexisting auroral arc (PAA)” or the “growth‐phase arc.” In this study, we use FAST measurements to establish the statistics of flow patterns near PAAs in the ionosphere. We find that flow shear is present in the vicinity of a preexisting arc. When a PAA appears in the evening sector, enhanced westward flow develops equatorward of the arc, whereas when a PAA appears in the morning sector, enhanced eastward flow develops poleward of the arc. We benchmark locations of the PAAs relative to large‐scale field‐aligned currents (FACs) and convective flows in the ionosphere, finding that the arc forms in the upward current region within ~1° of the Region 1/Region 2 boundary in all local time sectors from 20 MLT to 03 MLT. We also find that near midnight in the Harang region, most of the PAAs lie within 0.5° poleward of the low‐latitude Region 1/Region 2 currents boundary and sit between the westward and eastward flow peak but equatorward of the flow reversal point. Finally, we examine arc‐associated electrodynamics and find that the FAC of the PAA is mainly closed by the north‐south Pedersen current in the ionosphere.Key PointsAn ionospheric flow shear is associated with the preexisting auroral arcThe FAC of the PAA is primarily closed by N‐S Pedersen current in the ionosphereThe PAA is located very close to the R1/R2 boundaryPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/112278/1/jgra51768.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/112278/2/jgra51768-sup-0001-supinfo.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/112278/3/jgra51768-sup-0002-supinfo.pd