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At least 775 records · Page 43
All sourcesNASA NTRSDOE OSTI
NASA NTRS1999

Constructing the Coronal Magnetic Field by Correlating Parameterized Magnetic Field Lines with Observed Coronal Plasma Structures

A method is presented for constructing the coronal magnetic field from photospheric magnetograms and observed coronal loops. A set of magnetic field lines generated from magnetogram data is parameterized and then deformed by varying the parameterized values. The coronal flux tubes associated with this field are adjusted until the correlation between the field lines and the observed coronal loops is maximized. A mathematical formulation is described which ensures that (1) the normal component of the photospheric field remains unchanged, (2) the field is given in the entire corona over an active region, (3) the field remains divergence-free, and 4electric currents are introduced into the field. It is demonstrated that a parameterization of a potential field, comprising a radial stretching of the field, can provide a match for a simple bipolar active region, AR 7999, which crossed the central meridian on 1996 November 26. The result is a non-force-free magnetic field with the Lorentz force being of the order of 10(exp -5.5) g per s(exp 2) resulting from an electric current density of 0.79 micro A per m(exp 2). Calculations show that the plasma beta becomes larger than unity at a strong non-radial currents requires low height of about 0.25 solar radii supporting the non-force-free conclusion. The presence of such strong non-radial currents requires large transverse pressure gradients fo maintain a magnetostatic atmosphere, required by the relatively persistent nature of the coronal structures observed in AR 7999. This scheme is an important tool in generating a magnetic field solution consistent with the coronal flux tube observations and the observed photospheric magnetic field.

Allen, Gary G.↗
NASA NTRS2013

Low Frequency Plasma Oscillations in a 6-kW Magnetically Shielded Hall Thruster

The oscillations from 0-100 kHz in a 6-kW magnetically shielded thruster are experimen- tally characterized. Changes in plasma parameters that result from the magnetic shielding of Hall thrusters have the potential to significantly alter thruster transients. A detailed investigation of the resulting oscillations is necessary both for the purpose of determin- ing the underlying physical processes governing time-dependent behavior in magnetically shielded thrusters as well as for improving thruster models. In this investigation, a high speed camera and a translating ion saturation probe are employed to examine the spatial extent and nature of oscillations from 0-100 kHz in the H6MS thruster. Two modes are identified at 8 kHz and 75-90 kHz. The low frequency mode is azimuthally uniform across the thruster face while the high frequency oscillation is concentrated close to the thruster centerline with an m = 1 azimuthal dependence. These experimental results are discussed in the context of wave theory as well as published observations from an unshielded variant of the H6MS thruster.

BPT-4000↗
NASA NTRS1997

Towards Inner Magnetosphere Particle and Field Models

We report on the initial stages of an effort to construct comprehensive empirical models of the plasma and fields in the inner magnetosphere. The models are based not only on archival data (e.g., SCATHA) but also on current spacecraft mission data (e.g., POLAR). In this component of the effort, we incorporate the recently archived data provided by the SCATRA (Spacecraft Charging AT High Altitude) satellite. The SCAT14A satellite was in a near-geostationary orbit and was in operation for more than a decade. In this paper, we focus on the SCATRA plasma and magnetic field data from approximately the first two years of operation. The time-series data are binned according to spatial location and geomagnetic activity. Examples of statistical, empirical models from this initial effort are presented; even the simply-constructed preliminary models reveal such subtle features as the previously reported warm (greater than 100 eV) plasma density enhancement near the magnetic equator.

Spence, H. E.↗
NASA NTRS1979

Theory of the large-amplitude plane magnetoacoustic wave propagating transverse to the magnetic field in a hot collisionless plasma

An exact solution of the kinetic and electromagnetic equations for a large-amplitude plane magnetoacoustic wave propagating transverse to the magnetic field in a hot collisionless plasma is presented. The solution gives simple relations among the magnetic-field strength, density, stress tensor, and plasma velocity, all of which are measurable in the interplanetary plasma. These relations are independent of the electron and ion velocity distributions, subject to certain restrictions on 'high-velocity tails.' The magnetic field of the wave is linearly polarized. The wave steepens to form a shock much as the analogous waves of MHD theory do.

Barnes, A.↗
NASA NTRS2012

What Properties of CMEs are Most Important for Space Weather?

Severe space weather is characterized by intense particle radiation from the Sun and major geomagnetic storm caused by magnetized solar plasmas arriving at Earth. Coronal mass ejections (CMEs) are key players in both these aspects. CMEs traveling at super-Alfv nic speeds drive fast-mode MHD shocks that create the high levels of particle radiation. When a CME arrives at Earth, the CME-associated magnetic fields reconnect with Earth s magnetopause fields resulting in solar plasma entry into the magnetosphere and a geomagnetic storm depending on the magnetic structure of the CME. Particle radiation starts affecting geospace as soon as the CMEs leave the Sun and the geospace may be immersed in the radiation for several days. On the other hand, the geomagnetic storm happens only upon CME arrival at Earth. The requirements for the production of particles and magnetic storms by CMEs are different in a number of respects: solar source location, CME magnetic structure, conditions in the ambient solar wind, and shock-driving ability of CMEs. Intense shocks arriving at Earth have additional space weather effects such as sudden impulse that shrinks the magnetosphere often exposing satellites in geosynchronous orbit to the solar wind and energetic storm particle events. This paper highlights these space weather effects using CME observations space and ground based instruments during of solar cycles 23 and 24.

Gopalswamy, Nat↗
NASA NTRS1980

An empirical model of the Venusian outer environment. I - The shape of the dayside solar wind-atmosphere interface. II - The shape and location of the bow shock

Solar wind plasma and magnetic field data and ionospheric data obtained from the Pioneer Venus orbiter are discussed. The variation in the magnetic field pressure within the magnetic barrier is shown to be similar to that expected for the solar wind pressure variations along an obstacle's boundary when a more realistic approximation of the shape of the ionosphere is included. Simultaneous solar wind pressure, ionospheric pressure, and magnetic barrier pressure data reveal that the ionospheric pressure below the ionopause is approximately equal to the solar wind pressure. The magnetic barrier pressure is found to be equal to approximately 2/3 to 3/4 of both the solar wind pressure and the ionospheric pressure. Estimates of the hot plasma pressure contribution to the total pressure in the magnetic barrier vary from 1/4 to 1/3. Just below the ionopause the ionospheric pressure deviations from the mean ionospheric pressure are significant, especially below approximately 400 km; they appear to be indicative of the adjustment of the ionospheric structure to changeing solar wind conditions. A first-order model of the ionopause pressure variations as a function of height and solar-zenith angle is proposed. Venera 9 and Venera 10 bow shock crossings are then analyzed for solar zenith angles in the range from approximately 25 deg to approximately 153 deg. A comparison of Venera 9 and Venera 10 bow shock crossings with those observed by the Pioneer Venus orbiter suggests that both the secular variation and the latitudinal asymmetry may be responsible for the closer shock crossings of the Venera 9 and Venera 10 spacecraft.

Vaisberg, O. L.↗
NASA NTRS1991

A large-amplitude rotational wave in the Venusian ionosheath

Results are reported of a preliminary analysis of a large-amplitude rotational wave in the Venusian ionosheath. The results are based on an analysis of the Pioneer Venus Orbiter plasma and magnetic field observations from six orbits in the first (1979) tail season. This wave appears to be the standing super-Alfvenic wing. It is located within the shocked plasma flow outside the boundary of the tail. The rotation of the magnetic field by about 90 deg across the wave occurs through several successive cycles making the wave similar to the group velocity wing consisting of phase velocity waves. The transition through the wave is accompanied by the vector change of the plasma velocity with the magnitude of the plasma velocity jump comparable to the vector jump of the Alfven velocity. The observed super-Alfvenic wing appears to originate upstream and closer to the planet, possibly near the upper boundary of the magnetic barrier on the dayside.

Fedorov, A. O.↗
NASA NTRS1980

The dynamic behavior of the Venus ionosphere in response to solar wind interactions

The dynamics of the Venus ionosphere relates to the variations in the solar wind and the ionosheath magnetic fields as demonstrated by the electron density and temperature measurements of the Pioneer Venus orbiter electron probe. The mean ionopause height increases from 330 km at the subsolar point to 700 km at the dusk terminator, and to 1000 km at the dawn terminator; the dayside ionopause expands and contracts with solar wind pressure variations. Extreme spatial irregularities in the shape of holes, horizontally stratified layers, and detached plasma clouds are observed in the nightside ionosphere. The ion pickup on the dayside is described in terms of solar wind discontinuities inducing a wavelike pattern at the ionopause which is penetrated by the ionosheath plasma and magnetic fields which remove the plasma in the form of detached plasma clouds.

Brace, L. H.↗
NASA NTRS2022

Modeling the Lunar Wake Response to a CME Using a Hybrid PIC Model

In the solar wind, a low-density wake region forms downstream of the nightside lunar surface. In this study, we use a series of 3D hybrid particle-in-cell simulations to model the response of the lunar wake to a passing coronal mass ejection (CME). Average plasma parameters are derived from the Wind spacecraft located at 1 au during three distinct phases of a passing halo (Earth-directed) CME on 2015 June 22. Each set of plasma parameters, representing the shock/plasma sheath, a magnetic cloud, and plasma conditions we call the mid-CME phase, are used as the time-static upstream boundary conditions for three separate simulations. These simulation results are then compared with results that use nominal solar wind conditions. Results show a shortened plasma void compared to nominal conditions and a distinctive rarefaction cone originating from the terminator during the CME's plasma sheath phase, while a highly elongated plasma void reforms during the magnetic cloud and mid-CME phases. Developments of electric and magnetic field intensification are also observed during the plasma sheath phase along the central wake, while electrostatic turbulence dominates along the plasma void boundaries and 2–3 lunar radii RM downstream in the central wake during the magnetic cloud and mid-CME phases. The simulations demonstrate that the lunar wake responds in a dynamic way with the changes in the upstream solar wind during a CME.

Anthony P. Rasca↗
NASA NTRS2010

Turbulence and Global Properties of the Solar Wind

The solar wind shows striking characteristics that suggest that it is a turbulent magnetofluid, but the picture is not altogether simple. From the earliest observations, a strong correlation between magnetic fluctuations and plasma velocity fluctuations was noted. The high corrections suggest that the fluctuations are Alfven waves. In addition, the power spectrum of the magnetic fluctuation showed evidence of an inertial range that resembled that seen in fully-developed fluid turbulence. Alfven waves, however, are exact solutions of the equations of incompressible magnetohydrodynamics. Thus, there was a puzzle: how can a magnetofluid consisting of Alfven waves be turbulent? The answer lay in the role of velocity shears in the solar wind that could drive turbulent evolution. Puzzles remain: for example, the power spectrum of the velocity fluctuations is less steep than the slope of the magnetic fluctuations. The plasma in the magnetic tail of Earth's magnetosphere also shows aspects of turbulence, as does the plasma in the dayside magnetosphere near the poles the dayside cusps. Recently, new analyses of high time resolution magnetic field data from Cluster have offered a glimpse of how turbulence is dissipated, thus heating the ambient plasma.

Goldstein, Melvyn L.↗
NASA NTRS1979

The prediction of AE, ap, and Dst at time lags between 0 and 30 hours

The magnetic activity indexes AE, ap, and Dst are correlated with approximately 35,000 hours of interplanetary plasma and magnetic field measurements acquired near the Earth (assembled by NASA/NSSDC into a composite data sat). Lag times between the indexes and solar wind parameters ranged from 0 to 30 hours. Correlations at lags less than 6 hours yield results in agreement with previous studies. At greater lags, the correlation coefficients between the solar wind parameters and AE and ap approach these parameters' autocorrelation (persistence) values. For Dst the correlation with solar wind parameters is lower than that with AE and ap in the 3 to 4 hour lag range whereas the autocorrelation of Dst is significantly higher over the entire 0 to 30 hour lag range. The implications of these differences between AE, ap, and Dst are discussed in terms of persistence of solar wind structure.

Smart, D. F.↗
NASA NTRS1994

The magnetosheath of Neptune: Models and observations

This paper gives a complete overview of the plasma and magnetic field in the magnetosheath of Neptune. A more sophisticated method is used to reanalyze the plasma data. This method provides better values of plasma density, velocity, and temperature in the magnetosheath both inbound and outbound from the planet than published previously. The data are compared to results obtained from a gasdynamic model of low around a planetary magnetosphere. Agreement of theory and data is good for some parameters and poor for others. We show that some discrepancies between the model predictions and observations may be due to changes in the solar wind parameters. The model results and data are used to check the hypothesis that Voyager encountered the high-latitude plasma mantle downstream from Neptune. The deviations of the data from the model results in this region are consistent with those expected for a mantle crossing, with the exception of the plasma temperature which decreases in one crossing and increases in the other. This may indicate that our knowledge of the nature of the mantle-magnetosheath boundary is not complete.

Richardson, John D.↗
NASA NTRS1997

Heliospheric Plasma Sheet and Coronal Streamers

In-situ measurements of solar wind plasma and magnetic field between 0.3 and 1 AU are used to investigate the structure of the heliospheric plasma sheet, namely the region of enhanced plasma density that surrounds the helioshperic current sheet.

solar wind↗
NASA NTRS2003

Dynamical Evolution of the Inner Heliosphere Approaching Solar Activity Maximum: Interpreting Ulysses Observations Using a Global MHD Model

In this study we describe a series of MHD simulations covering the time period from 12 January 1999 to 19 September 2001 (Carrington Rotation 1945 to 1980). This interval coincided with: (1) the Sun s approach toward solar maximum; and (2) Ulysses second descent to the southern polar regions, rapid latitude scan, and arrival into the northern polar regions. We focus on the evolution of several key parameters during this time, including the photospheric magnetic field, the computed coronal hole boundaries, the computed velocity profile near the Sun, and the plasma and magnetic field parameters at the location of Ulysses. The model results provide a global context for interpreting the often complex in situ measurements. We also present a heuristic explanation of stream dynamics to describe the morphology of interaction regions at solar maximum and contrast it with the picture that resulted from Ulysses first orbit, which occurred during more quiescent solar conditions. The simulation results described here are available at: http://sun.saic.com.

Riley, Pete↗
NASA NTRS1981

Evidence for magnetic field reconnection at the Earth's magnetopause

Eleven passes of the ISEE satellites through the frontside terrestrial magnetopause were identified, where the plasma velocity in the magnetopause and boundary layer was substantially larger than in the magnetosheath. The nature of the plasma flow, magnetic field, and energetic particle fluxes in these regions were examined, with a view to determining whether the velocity enhancements can be explained by magnetic field reconnection.

Sonnerup, B. U. O.↗
NASA NTRS2001

A Prolonged He(+) Enhancement within a Coronal Mass Ejection in the Solar Wind

A coronal mass ejection and magnetic cloud containing an unusually large enhancement of He+ was observed in the solar wind by the plasma and magnetic field instruments on the Advanced Composition Explorer (ACE) spacecraft on May 2-4, 1998. The He+/He++ ratio during this event exceeded 0.5% for a period of more than 24 hours, and reached values as high as 100%. The high He+/He++ ratio indicates the presence of prominence material, and in fact a disappearing filament and prominence were observed at the Sun in association with this event. The prolonged observation of He+ indicates that prominence material extended through mu ch of this CME, the first such observation in a CME in the solar wind.

Skoug, R. M.↗
NASA NTRS2008

Four Point Measurements of the Foreshock

Hybrid code numerical simulations accurately predict the properties of the Earth's foreshock, a region populated by solar wind particles heated and reflected by their interaction with the bow shock. The thermal pressures associated with the reflected population suffice to substantially modify the oncoming solar wind, substantially reducing densities, velocities, and magnetic field strengths, but enhance temperatures. Enhanced thermal pressures cause the foreshock to expand at the expense of the ambient solar wind, creating a boundary that extends approx.10 RE upstream which is marked by enhanced densities and magnetic field strengths, and flows deflected away from the foreshock. We present a case study of Cluster plasma and magnetic field observations of this boundary.

Sibeck, D. G.↗