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At least 217 records · Page 12

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NASA NTRS1987

Magnetospheric environments of outer planet rings - Influence of Saturn's axially symmetric magnetic field

Saturn's main rings exist within a zone of negligible magnetospheric losses and surface alteration effects, substantially due to the solid-body absorption of inwardly diffusing magnetospheric particles. This process is presently shown to be especially efficient in the inner magnetosphere of Saturn, due to the near-axial symmetry of the planetary magnetic field relative to the equatorial rotation plane; under the assumption of comparable diffusion rates, the inward magnetospheric particle transport is far more inhibited in the inner Saturnian magnetosphere than in the same regions of Jupiter and Uranus, even when only rings of comparable widths and depths are considered. In light of this, ring particle surface exposure to the ion fluxes of the radiation belt remains a prepossessing rationale for low Uranian ring albedos.

Hood, L. L.↗

NASA NTRS2002

Interplanetary Magnetic Field Control of the Entry of Solar Energetic Particles into the Magnetosphere

We have investigated the entry of energetic ions of solar origin into the magnetosphere as a function of the interplanetary magnetic field orientation. We have modeled this entry by following high energy particles (protons and 3 He ions) ranging from 0.1 to 50 MeV in electric and magnetic fields from a global magnetohydrodynamic (MHD) model of the magnetosphere and its interaction with the solar wind. For the most part these particles entered the magnetosphere on or near open field lines except for some above 10 MeV that could enter directly by crossing field lines due to their large gyroradii. The MHD simulation was driven by a series of idealized solar wind and interplanetary magnetic field (IMF) conditions. It was found that the flux of particles in the magnetosphere and transport into the inner magnetosphere varied widely according to the IMF orientation for a constant upstream particle source, with the most efficient entry occurring under southward IMF conditions. The flux inside the magnetosphere could approach that in the solar wind implying that SEPs can contribute significantly to the magnetospheric energetic particle population during typical SEP events depending on the state of the magnetosphere.

Richard, R. L.↗

NASA NTRS1994

Landau damping of magnetospherically reflected whistlers

Unducted VLF signals produced by lightning activity can form a population of magnetospherically reflected (MR) whistlers in the inner magnetosphere. It has been suggested recently that in the absence of significant attenuation such waves could merge into a broadband continuum with sufficient intensity to account for plasmaspheric hiss. To test this conjecture we have evaluated the path-integrated attenuation of MR whistlers along representative ray paths using the HOTRAY code. Using a realistic plasma distribution modeled on in-situ data, we find that the majority of MR waves experience significant damping after a few transits across the equator. This is primarily due to Landau resonance with suprathermal (0.1-1 keV) electrons. The attenuation is most pronounced for waves that propagate through the outer plasmasphere; this can readily account for the infrequent occurrence of multiple-hop MR waves for L greater than or equal to 3.5. Selected waves that originate at intermediate latitudes (15 deg is less than or equal to lambda is less than or equal to 35 deg) and whose ray paths are confined to the inner plasma- sphere may experience up to 10 magnetospheric reflections before substantial attentuation occurs. These waves should form the population of observed MR waves. Wave attenuation becomes more pronounced at higher frequencies; this can account for the absence of multiple-hop waves above 5 kHz. Weakly attenuated MR waves tend to migrate outward to the L shell, where their frequency is comparable to the equatorial lower hybrid frequency. The enhanced concentration of waves due to a merging of ray paths would produce a spectral feature that rises in frequency at lower L. This is quite distinct from the reported properties of plasmaspheric hiss, which maintains a constant frequency band throughout the entire plasmasphere. Furthermore, in the absence of mode conversion, waves below 500 Hz, which often form an important if not dominant part of the spectral properties of hiss, are unable to escape from the topside ionosphere in the whistler mode. Consequently, we conclude that unducted lightning signals cannot account for the origin of plasmaspheric hiss.

Thorne, Richard M.↗

NASA NTRS1993

Origin and maintenance of the oxygen torus in Saturn's magnetosphere

Observations of thermal ions in Saturn's inner magnetosphere suggest distributed local sources rather than diffusive mass loading from a source located further out. We suggest that the plasma is produced and maintained mainly by 'self-sputtering' of E ring dust. Sputtered particles are 'picked up' by the planetary magnetospheric field and accelerated to corotation energies (of the order of 8 eV/amu). The sputter yield for oxygen on ice at, for example, 120 eV is about 5, which implies that an avalanche of self-sputtering occurs. The plasma density is built up until it is balanced by local losses, presumably pitch angle scattering into the loss cone and absorption in the planet's ionosphere. The plasma density determines the distribution of dust in the E ring through plasma drag. Thus a feedback mechanism between the plasma and the E ring dust is established. The model accounts for the principal plasma observations and simultaneously the radial optical depth profile of the E ring.

Morfill, G. E.↗

NASA NTRS1983

Modeling planetary magnetospheres

Recent advances in the development of models of the macroscopic properties of the terrestrial and planetary magnetospheres are reviewed. Particular attention is given to work on semiempirical models of magnetic and electric fields in the earth's magnetosphere, the modeling of magnetospheric storms and substorms in the inner magnetosphere, and the self-consistent modeling of processes in the magnetotail, including reconnection. Magnetohydrodynamic models of the dayside magnetosphere and the magnetotail which are based on calculations of the interaction of the solar wind with the magnetosphere are also considered. Finally, work on the modeling of the magnetospheres of Mercury, Venus, Jupiter, Saturn and Uranus is presented.

Walker, R. J.↗

NASA NTRS1976

The Jovian magnetosphere and magnetopause

The characteristics of the planet Jupiter's inner magnetosphere are examined, taking into account the Pioneer 10 and 11 magnetometer data. Data on the reliability of spherical harmonic expansions are presented in a table. The properties of the Jovian magnetosheath and magnetopause are described. Bow shock and magnetopause crossings were securely identified in plasma data and were usually identified in plasma data and were usually identifiable in the magnetometer data. Explanations for the large number of observed crossings are discussed. It is pointed out that in the case of the outer magnetosphere the observed field strength is nearly an order of magnitude larger than would be expected from Jupiter's dipole moment. The distinguishing characteristics of the magnetic field in the middle magnetosphere are also considered.

Davis, L., Jr.↗

NASA NTRS1994

Energetics of Superstorms

For the most intense storms of the space era, for which there is good solar wind data coverage, we discuss the total energy and power degradation during the storm development, beginning at the solar wind, then at the solar wind-magnetosphere interface (magnetopause) and at the magnetospheric tail, ending at the two basic inner magnetospheric dissipation regions: the ring current and the auroral ionosphere.

solar wind magnetosphere↗

NASA NTRS2021

Why Atmospheric Backscatter Is Important in the Formation of Electron Precipitation in the Diffuse Aurora

In addition to wave particle scattering in the magnetosphere, atmospheric backscatter of magnetospheric electrons is an important process that contributes to the formation of the precipitated electrons in the region of diffuse aurora. Two magnetically conjugate regions are involved in a complex magnetosphere-ionosphere (MI) particle and energy interplay. Based on synthesizing previous theoretical/modeling studies and experimental evidence, we demonstrate the need for improving the quantification of magnetospheric electrons backscatter processes that can affect inner magnetospheric electrodynamics, transport and loss in a way that is not easily predicted. We discuss how these complex and energy-dependent MI coupled processes can be treated in magnetospheric modeling.

diffuse aurora↗

NASA NTRS2022

Explicit IMF By-Dependence of Energetic Protons and the Ring Current

The most important parameter driving the solar wind-magnetosphere interaction is the southward (Bz) component of the interplanetary magnetic field (IMF). While the dawn-dusk (By) component of the IMF is also known to play an important role, its effects are usually assumed to be independent of its sign. Here we demonstrate for the first time a seasonally varying, explicit IMF By-dependence of the ring current and Dst index. Using satellite observations and a global magnetohydrodynamic model coupled with a ring current model, we show that for a fixed level of solar wind driving the flux of energetic magnetospheric protons and the growth-rate of the ring current are greater for By < 0 (By > 0) than for By > 0 (By < 0) in Northern Hemisphere summer (winter). While the physical mechanism of this explicit By-effect is not yet fully understood, our results suggest that IMF By modulates magnetospheric convection and plasma transport in the inner magnetosphere.

L Holappa↗

NASA NTRS2016

Global Images of Trapped Ring Current Ions During Main Phase of 17 March 2015 Geomagnetic Storm as Observed by TWINS

A unique view of the trapped particles in the inner magnetosphere provided by energetic neutral atom (ENA) imaging is used to observe the dynamics of the spatial structure and the pitch angle anisotropy on a global scale during the last 6 h of the main phase of a large geomagnetic storm (minimum SYM-H 230 nT) that began on 17 March 2015. Ion flux and pressure anisotropy obtained from Two Wide-angle Imaging Neutral-atom Spectrometers (TWINS) ENA images are shown. The ion flux shows two peaks, an inner one at approximately radii 34 RE in the dusk-to-midnight sector and an outer peak at radii 89 RE prior to midnight. The inner peak is relatively stationary during the entire period with some intensification during the final steep decline in SYM-H to its minimum. The outer peak shows the significant temporal variation brightening and dimming and finally disappearing at the end of the main phase. The pressure anisotropy shows the expected perpendicular pitch angles inside of L 6 but shows parallel pitch angles at greater L values. This is interpreted as consistent with pitch angle-dependent drift as modeled in the Tsy05 magnetic field and Comprehensive Inner Magnetosphere-Ionosphere simulations. The TWINS results are compared directly with Radiation Belt Storm Probes Ion Composition Experiment (RBSPICE)-A measurements. Using 15 min snapshots of flux and pressure anisotropy from TWINS along the path of RBSPICE-A during the 6 h focused upon in this study, the essential features displayed in the TWINS global images are supported.

Perez, J. D.↗

NASA NTRSDate not supplied

The Circulation of the Plasmasphere Fluid during the Erosion Event on September 8, 2017

A strong solar wind pressure pulse triggered the magnetic storm on September 7-8, 2017. Near the end of September 7, the z-component of the IMF Bz dropped from 9 to -10 nT in 30 min and then had another rapid drop to -31 nT in 30 min. The sudden plunge of IMF Bz and the associated strong convection electric field stirred up the storm main phase with Dst falling from 0 to -122 nT from 2200 UT on September 7 to 0200 UT on the 8th. Severe plasmasphere erosion was observed on September 8 by multiple spacecraft, such as the Van Allen Probes and the Arase satellite. In this study, we examine the fate of the eroded plasmasphere particles during this event by model simulation as well as satellite data analysis. The simulation tool we use is the SWMF/BATS-R-US model coupled with the CIMI model. The cold plasmas in the plasmasphere is treated as a separate fluid in the MHD equations. As a result, the transport and circulation of the plasmasphere fluid in the global magnetosphere can be traced and the impacts of this cold fluid on the global magnetosphere can be evaluated. In this paper, we will show how the drainage plume is formed during the storm and how the plasmasphere fluid is transported to the flank and lobe regions and eventually to the plasma sheet and reenters into the inner magnetosphere. We will validate our simulation by plasmasphere signatures observed in both the inner and outer magnetosphere.

Mei-Ching Fok↗

NASA NTRS1979

The dynamics of the Jovian magnetosphere

The current status of the understanding of the dynamics of Jupiter's magnetosphere is reviewed. A brief summary is presented of the concepts and processes which were identified as being of probable importance by pre-Pioneer 10 and 11 work (both theoretical and observational). The insights provided by the in situ Pioneer flights are then discussed. The Jovian magnetosphere consists of several relatively distinct regions: the inner magnetosphere, the intermediate magnetosphere, the outer magnetosphere, a transition region just inside the magnetopause, and the magnetosheath. The basic particle and magnetic field characteristics of these regions are summarized, and the dynamical processes which are currently thought to be significant in each of them are reviewed. Finally, some outstanding questions and problems are identified for future treatment based on Pioneer data or on data from the upcoming Voyager and Galileo missions.

Goertz, C. K.↗

NASA NTRS1999

What is the Relationship between the Solar Wind and Storms/Substorms?

The interplanetary magnetic field (IMF) carried past the Earth by the solar wind has long been known to be the principal quantity that controls geomagnetic storms and substorms. Intervals of strong southward IMF with durations of at least a significant fraction of a day produce storms, while more typical, shorter intervals of less-intense southward fields produce substorms. The strong, long-duration southward fields are generally associated with coronal mass ejections and magnetic clouds or else they are produced by interplanetary dynamics initiated by fast solar wind flows that compress preexisting southward fields. Smaller, short-duration southward fields that occur on most days are related to long period waves, turbulence, or random variations in the IMF. Southward IMF enhances dayside reconnection between the IMF and the Earth's dipole with the reconnected field lines supplementing open field lines of the geomagnetic tail and producing an expanded polar cap and increased tail energy. Although the frequent storage of solar wind energy and its release during substorms is the most common mode of solar wind/magnetosphere interaction, under certain circumstances, steady southward IMF seems to produce intervals of relatively steady magnetosphere convection without substorms. During these latter times, the inner magnetosphere remains in a stressed tail-like state while the more distant magnetotail has larger northward field and more dipolar-like field lines. Recent evidence suggests that enhanced magnetosphere particle densities associated with enhanced solar wind densities allow more particles to be accelerated for the ring current, thus creating larger storms.

Fairfield, D. H.↗

NASA NTRS2008

Plasma Sheet Circulation Pathways

Global simulations of Earth's magnetosphere in the solar wind compute the pathways of plasma circulation through the plasma sheet. We address the pathways that supply and drain the plasma sheet, by coupling single fluid simulations with Global Ion Kinetic simulations of the outer magnetosphere and the Comprehensive Ring Current Model of the inner magnetosphere, including plasmaspheric plasmas. We find that the plasma sheet is supplied with solar wind plasmas via the magnetospheric flanks, and that this supply is most effective for northward IMF. For southward IMF, the innermost plasma sheet and ring current region are directly supplied from the flanks, with an asymmetry of single particle entry favoring the dawn flank. The central plasma sheet (near midnight) is supplied, as expected, from the lobes and polar cusps, but the near-Earth supply consists mainly of slowly moving ionospheric outflows for typical conditions. Work with the recently developed multi-fluid LFM simulation shows transport via plasma "fingers" extending Earthward from the flanks, suggestive of an interchange instability. We investigate this with solar wind ion trajectories, seeking to understand the fingering mechanisms and effects on transport rates.

Moore, Thomas E.↗

NASA NTRS1982

Some consequences of corotating magnetospheric convection

A comparison is conducted of the expected signature of the inflow region of the proposed corotating convection pattern with relevant magnetic field and plasma flow observations made by Pioneers 10 and 11 and Voyagers 1 and 2 in the Jovian magnetosphere. A region of net plasma inflow would be characterized by superrotation and a negative radial current in the equatorial plane within a characteristic distance L. It is found that no value of L exists that is consistent with both plasma and magnetic field observations. Hence it is concluded that at the time of these flybys, such a simple large-scale convection pattern did not dominate the plasma transport in the outer magnetosphere, although the existence of such a pattern in the inner magnetosphere is not ruled out. A more detailed test is proposed to determine whether or not a superposition of corotating convection and radial diffusion is consistent with the observations.

Hill, T. W.↗

NASA NTRS1983

Jupiter's magnetic field and magnetosphere

Among the planets of the solar system, Jupiter is unique in connection with its size and its large magnetic moment, second only to the sun's. The Jovian magnetic field was first detected indirectly by radio astronomers who postulated its existence to explain observations of nonthermal radio emissions from Jupiter at decimetric and decametric wavelengths. Since the early radio astronomical studies of the Jovian magnetosphere, four spacecraft have flown by the planet at close distances and have provided in situ information about the geometry of the magnetic field and its strength. The Jovian magnetosphere is described in terms of three principal regions. The inner magnetosphere is the region where the magnetic field created by sources internal to the planet dominates. The region in which the equatorial currents flow is denoted as the middle magnetosphere. In the outer magnetosphere, the field has a large southward component and exhibits large temporal and/or spatial variations in magnitude and direction in response to changes in solar wind pressure.

Acuna, M. H.↗

NASA NTRS1992

Energetic particle signatures of satellites and rings in Neptune's magnetosphere

The cosmic ray system on Voyager 2 found a trapped radiation environment in Neptune's inner magnetosphere which is controlled primarily by absorption at the rings and satellite surfaces. The intensity of electrons with kinetic energies approximately greater than 1 MeV shows particularly strong and narrow signatures associated with absorption by the satellite 1989N1 at an orbital radius of 4.75 Neptune radii. Closer to the planet are several signatures of the inner satellites and rings. Absorption limits the intensity of the inner radiation belt sufficiently for the maximum intensity to occur outside the orbit of 1989N1 at a magnetic L shell of about 7. Radial profiles of the electron phase space density show that electrons diffuse inward from a source in the outer magnetosphere. Many of the inward-diffusing electrons are absorbed upon reaching a satellite orbital radius, but the finite absorption efficiency allows some of the electrons to pass by unaffected. The locations of the satellite and ring signatures also provide constraints on the nondipolar components of the planetary magnetic field.

Selesnick, R. S.↗

NASA NTRS2001

Transport of Photoelectrons in the Nightside Magnetosphere

Kinetic modeling results are analyzed to examine the transport of photoelectrons through the nightside inner magnetosphere. A natural filter exists on the nightside for these particles, with Coulomb collisions eroding the distribution at low energies and low L shells, and magnetospheric convection compressing the flux tubes as they corotate towards dawn. This leads to an electron population with very distinctive features. It is shown that, for low activity levels, a band of photoelectrons forms between L=4 and 6 that extends throughout the nightside local times and into the morning sector. Modeled energy and pitch angle distributions are presented to show the development and features of this electron band, as well as their influences on the thermal plasma. Such a population should be observable in satellite spectrometer measurements.

Khazanov, G. V.↗