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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 145 records · Page 8
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NASA NTRS1985

Differential measurement of cosmic-ray gradient with respect to interplanetary current sheet

Simultaneous magnetic field and charged particle measurements from the Voyager pacecraft at heliographic latitude separations from 10 deg to 21 deg are used to determine the latitude gradient of the galactic cosmic ray flux with respect to the interplanetary current sheet. By comparing the ratio of cosmic ray flux at Voyager 1 to that at Voyager 2 during periods when both spacecraft are first north and then south of the interplanetary current sheet, we find an estimate of the latitudinal gradient with respect to the current sheet of approximately -0.15 + or- 0.05%/deg under restricted interplanetary conditions.

Christon, S. P.↗
NASA NTRS1985

Differential measurement of cosmic-ray gradient with respect to interplanetary current sheet

Simultaneous magnetic field and charged particle measurements from the Voyager spacecraft at heliographic latitude separations from 10 deg. to 21 deg. are used to determine the latitude gradient of the galactic cosmic ray flux with respect to the interplanetary current sheet. By comparing the ratio of cosmic ray flux at Voyager 1 to that a Voyager 2 during periods when both spacecraft are first nort and then south of the interplanetary current sheet, we find an estimate of the latitudinal gradient with respect to the current sheet of approximately -0.15 + or 0.05% deg under restricted interplanetary conditions.

Christon, S. P.↗
NASA NTRS1978

Structure of current sheets in magnetic holes at 1 AU

Current density profiles in several types of interplanetary magnetic holes were calculated assuming that the currents flow in planar sheets and that the magnetic field varies only in the direction normal to the sheet. The planarity was verified in four holes which were observed by two suitably spaced spacecraft. The structure of the current sheets ranges from very simple in some holes to very complex in others. The observed structures are found to be qualitatively consistent with models based on self-consistent solutions of Vlasov's equation and Maxwell's equations. Examples of complex, irregular magnetic holes are also presented, and they are shown to contain multiple, current sheets in which currents flow parallel to one another.

Fitzenreiter, R. J.↗
NASA NTRS1990

Linear tearing modes of a forced current-sheet equilibrium

Linear tearing modes are studied in a nearly singular forced current-sheet equilibrium, such as could result from global magnetic forces in the solar corona. Growth rates for the tearing modes, determined by solving the linearized reduced MHD (Strauss) equations numerically, were found to scale as (gamma)tau(d) = about S exp 4/5 k(y) exp 4/5, where S is the Lundquist number, k(y) is the wavenumber, and tau(d) is the classical resistive diffusion time. This scaling is in agreement with predictions from analytical theory. Because of the faster S scaling of these modes compared to the tearing modes of a diffuse current-sheet equilibrium, the modes have much higher growth rates (by a factor of about 10,000) for coronal values of S (about 10 to the 12th). For coronal parameters, the growth times of these new modes are estimated to be on the order of several hours to days, as compared to growth times of months to years for the tearing modes in a diffuse current-shear equilibrium. The growth times are comparable to reconnection times scales required in models of coronal heating by magnetic field dissipation.

Liewer, Paulett C.↗
NASA NTRS1990

Quasilinear saturation of forced current sheet tearing modes

Numerical studies of tearing modes in a nearly singular forced current sheet equilibrium (Liewer and Payne, 1990) show that the modes saturate quasilinearly when the width of the magnetic island formed by the reconnection is on the order of several times the linear mode width which scales as approximately (kS) exp -2/5, where S is the Lundquist number and k is the wavenumber. The modes saturate quasilinearly by flattening the current profile, converting magnetic energy into plasma energy. The longer wavelength modes, which saturate at higher levels, release the most energy. These modes may, nonlinearly, play a role in coronal heating when sharp current sheets form as a result of global magnetic stresses.

Liewer, Paulett C.↗
NASA NTRS1981

The orientation and motion of the predawn current sheet and Jupiter's magnetotail

From the observed times when Pioneer 10 and Voyager 1 observe particle flux maxima and minima and when the magnetic field strength is at minimum a geometric description of the current sheet in Jupiter's magnetosphere is derived. It lags the planetary rotation by an angle delta which varies as a function of radial distance as delta equals 22 deg + 0.80 (r - 16.3). The maximum latitude of the current sheet is roughly 10 deg inside of 60 Jovian radii and decreases beyond. Thus there is a hinge point near 60 Jovian radii. The position of the hinge point moves in response to the external solar wind pressure variation. The variation of delta(r) is discussed theoretically and how it is related to the magnetic field topology and plasma flow is shown. It is concluded that the distant field lines do not corotate with Jupiter and that the Alfven speed at 50 Jovian radii is of the order of 1500 km/s. The topology of the current sheet is affected by centrifugal forces, outward mass transport and solar wind-magnetosphere interaction. It is shown how these effects can be related to different current systems.

Goertz, C. K.↗
NASA NTRS2006

Electrodynamics in a Very Thin Current Sheet Leading to Magnetic Reconnection

We study the formation of a very thin current sheet (CS) and associated plasma electrodynamics using three-dimensional (3-D) particle-in-cell simulations with ion to electron mass ratio M/m=1836. The CS is driven by imposed anti-parallel magnetic fields. The noteworthy features of the temporal evolution of the CS are the following: (i) Steepening of the magnetic field profile B,(z) in the central part of the CS, (ii) Generation of three-peak current distribution with the largest peak in the CS center as B,(z) steepens, (iii) Generation of converging electric fields forming a potential well in the CS center in which ions are accelerated. (iv) Electron and ion heating in the central part of the CS by current-driven instabilities (CDI). (v) Re-broadening of the CS due to increased kinetic plasma pressure in the CS center. (vi) Generation of electron temperature anisotropy with temperature perpendicular to the magnetic field being larger than the parallel one. (vii) Current disruption by electron trapping in an explosively growing electrostatic instability (EGEI) and electron tearing instability (ETI). (viii)The onset of EGEI coincides with an increase in the electron temperature above the temperature of the initially hot ions as well as the appearance of new shear in the electron drift velocity. (ix) Bifurcation of the central CS by the current disruption. (x) Magnetic reconnection (MR) beginning near the null in B, and spreading outward. (xi) Generation of highly energized electrons reaching relativistic speeds and having isotropic pitch-angle distribution in the region of reconnected magnetic fields. We compare some of these features of the current sheet with results from laboratory and space experiments.

Singh, Nagendra↗
NASA NTRS1988

A case study of magnetotail current sheet disruption and diversion

On June 1, 1985 the AMPTE/CCE spacecraft (at a geocentric distance of about 8.8 earth radii at the midnight neutral sheet region) observed a dispersionless energetic particle injection and an increase in magnetic field magnitude, which are features commonly attributed to disruption of the near-earth cross-tail current sheet during substorm expansion onsets. An analysis based on high time-resolution measurements from the magnetometer and the energetic particle detector indicates that the current sheet disruption region exhibited localized (less than 1 earth radius) and transient (less than 1 min) particle intensity enhancements, accompanied by complex magnetic field changes with occasional development of a southward magnetic field component. Similar features are seen in other current disruption/diversion events observed by the CCE. The present analysis suggests that the current disruption region is quite turbulent, similar to laboratory experiments on current sheet disruption, with signatures unlike those expected from an X-type neutral line configuration. No clear indication of periodicity in any magnetic field parameter is discernible for this current disruption event.

Lui, A. T. Y.↗
NASA NTRS1988

Current sheets in the solar corona

Coronal magnetic fields are twisted up by motion of their footpoints in the photosphere. When the twist exceeds a critical amount, kink-ballooning instabilities occur. These instabilities are studied numerically, in long, thin, axially bounded magnetic fields. Nonlinearly, the three-dimensional kinking motion compresses magnetic flux, forming a current sheet. Magnetic energy can be dissipated at a rate orders of magnitude greater than without the current sheets. The energy of footpoint motion can then go into coronal heating, via Ohmic dissipation in the current sheets.

Strauss, H. R.↗
NASA NTRS1993

Self-consistent current sheet structures in the quiet-time magnetotail

The structure of the quiet-time magnetotail is studied using a test particle simulation. Vlasov equilibria are obtained in the regime where v(D) = E(y) c/B(z) is much less than the ion thermal velocity and are self-consistent in that the current and magnetic field satisfy Ampere's law. Force balance between the plasma and magnetic field is satisfied everywhere. The global structure of the current sheet is found to be critically dependent on the source distribution function. The pressure tensor is nondiagonal in the current sheet with anisotropic temperature. A kinetic mechanism is proposed whereby changes in the source distribution results in a thinning of the current sheet.

Holland, Daniel L.↗
NASA NTRS1982

Structure of the heliospheric current sheet in the early portion of sunspot cycle 21

The structure of the heliospheric current sheet on a spherical source surface of radius 2.35 solar radii has been computed via the use of a potential field model during the first year and a half after the last sunspot minimum. The solar polar magnetic field that is not fully observed in conventional magnetograph scans was included in the computation. The computed heliospheric current sheet had a quasi-stationary structure consisting of two northward and two southward maxima in latitude per solar rotation. The extent in latitude slowly increased from about 15 deg near the start of the interval to about 45 deg near the end. The magnetic field polarity (away from the sun or toward the sun) at the subterrestrial latitude on the source surface agreed with the interplanetary magnetic field polarity observed or inferred at the earth on 82 percent of the days. The interplanetary field structure observed at the earth at this time is finely tuned to the structure of low-latitude fields on the source surface.

Hoeksema, J. T.↗
NASA NTRS1972

Equatorial current sheet in the magnetosphere.

The delta B distribution deduced from the Ogo 3 and 5 satellites shows large field depressions in the equatorial region inside the plasmasphere. On the basis of the delta B distribution, it is shown that what is usually considered the quiet-time ring current is an equatorial sheet current that is an extension of the neutral sheet current in the magnetospheric tail. The primary source of the large field reductions inside the plasmasphere is a population of protons with energies of 0.1-1 Mev initially detected by Davis and Williamson (1963) and Davis (1965) on Explorer 12, 14, and 15. Low-energy protons extensively measured on Ogo 3 by Frank (1967, 1971) are primarily responsible for the current near and outside the plasmapause.

Sugiura, M.↗
NASA NTRS2020

The Heliospheric Current Sheet in the Inner Heliosphere Observed by the Parker Solar Probe

The Parker Solar Probe (PSP) completed its first solar encounter in 2018 November, bringing it closer to the Sun than any previous mission. This allowed in situ investigation of the heliospheric current sheet (HCS) inside the orbit of Venus. The Parker observations reveal a well defined magnetic sector structure placing the spacecraft in a negative polarity region for most of the encounter. The observed current sheet crossings are compared to the predictions of both potential field source surface and magnetohydrodynamic models. All the model predictions are in good qualitative agreement with the observed crossings of the HCS. The models also generally agree that the HCS was nearly parallel with the solar equator during the inbound leg of the encounter and more significantly inclined during the outbound portion. The current sheet crossings at PSP are also compared to similar measurements made by the Wind spacecraft near Earth at 1 au. After allowing for orbital geometry and propagation effects, a remarkable agreement has been found between the observations of these two spacecraft underlying the large-scale stability of the HCS. Finally, the detailed magnetic field and plasma structure of each crossing is analyzed. Marked differences were observed between PSP and Wind measurements in the type of structures found near the HCS. This suggests that significant evolution of these small solar wind structures takes place before they reach 1 au.

Szabo, Adam↗
NASA NTRS1985

Variation of cosmic rays and solar wind properties with respect to the heliospheric current sheet. I - Five-GeV protons and solar wind speed

One of the principal uncertainties in understanding the propagation of cosmic rays in the heliosphere is related to the role of latitudinal transport by gradient and curvature drifts. The steady state distribution of cosmic rays with respect to the heliospheric current sheet or in latitude may possibly provide some information regarding the existing problems. Newkirk and Lockwood (1981) have considered the current sheet with respect to its role as a natural surface of symmetry for the propagation of cosmic rays in the heliosphere. The present investigation represents an extension of this study. Central objectives are related to a determination of the gradient of 5-GeV cosmic rays with respect to the heliospheric current sheet at 1 AU, the possible effect of the solar cycle upon such gradients, and an evaluation of the ability of several theoretical models to explain the gradients.

Newkirk, G., Jr.↗
NASA NTRS1986

The heliospheric current sheet - 3-dimensional structure and solar cycle changes

The reversal in polarity of the interplanetary current sheet/sector structure is investigated during the recent solar maximum. Multipoint observations by ISEE-3 and Pioneer 11 show that a simple two sector or occasional four sector structure persisted throughout the maximum and out to distances of 10 AU. The polarity reversal occurred between March 1979 and October 1980, without any indication of an abrupt transition that might permit a more precise timing. The reversal coincided approximately with the reversal in the sun's polar cap fields. The current sheet appeared to be highly inclined during the ascending and descending phases of the solar cycle but was apparently too complex to describe as a simple inclined current sheet during solar maximum.

Smith, E. J.↗
NASA NTRS1994

The effect of coronal mass ejections on the structure of the heliospheric current sheet

The existence of a stable heliospheric current sheet (HCS) structure near solar cycle maximum was questioned since the recognition that coronal mass ejections (CME's) occur in coronal helmet streamers. Evidence is presented suggesting that pre-existing helmet streamers disrupted or blown out by CME's reform in a time interval much shorter than the life time of the HCS, and that the concept of the HCS has a meaning at any time of thesolar cycle. It appears that the HCS, the current layer that separates adjacent interplanetary magnetic field regions with opposite magnetic polarity, exists throughout the solar cycle, though not always as a thin disk-like sheet. The sheet may be thickened by embedded magnetic ropes formed by CME's, especially near sunspot maximum. The HCS may be used as timing mark in identifying or predicting CME's in the interplanetary medium.

Zhao, Xuepu↗
NASA NTRS1986

The effect of the heliospheric current sheet on cosmic ray intensities at solar maximum - Two alternative hypotheses

The capabilities of two alternative models for accounting for the reversal of the solar magnetic field polarity at the point of solar maximum and for the associated modulation of cosmic ray intensities at 1 AU are assessed. One model posits a continuous increase in the inclination of the heliospheric current sheet to a point of verticality at maximum, when it overturns and the reversal occurs. The alternative view is that the sun sheds the magnetic field of the previous cycle and generates a new field of opposite polarity. Some data do exist for a tilted current sheet which increases its tilt with proximity to the solar maximum. However, coronal data also support the presence of isolated regions of anomalous polarity which spread over the surface of the sun as maximum approaches, a condition commensurate with flux shedding. Both models predict heliospheric current sheet configurations which would produce some cosmic ray modulations observed over the course of the solar cycle.

Thomas, B. T.↗