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Smith, Charles W

Publications and source records attributed to Smith, Charles W.

8 records

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DOE OSTI2012

THE TURBULENT CASCADE AND PROTON HEATING IN THE SOLAR WIND DURING SOLAR MINIMUM

The recently protracted solar minimum provided years of interplanetary data that were largely absent in any association with observed large-scale transient behavior on the Sun. With large-scale shear at 1 AU generally isolated to corotating interaction regions, it is reasonable to ask whether the solar wind is significantly turbulent at this time. We perform a series of third-moment analyses using data from the Advanced Composition Explorer. We show that the solar wind at 1 AU is just as turbulent as at any other time in the solar cycle. Specifically, the turbulent cascade of energy scales in the same manner proportional to the product of wind speed and temperature. Energy cascade rates during solar minimum average a factor of 2-4 higher than during solar maximum, but we contend that this is likely the result of having a different admixture of high-latitude sources.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2012

OBSERVATION OF BERNSTEIN WAVES EXCITED BY NEWBORN INTERSTELLAR PICKUP IONS IN THE SOLAR WIND

A recent examination of 1.9 s magnetic field data recorded by the Voyager 2 spacecraft in transit to Jupiter revealed several instances of strongly aliased spectra suggestive of unresolved high-frequency magnetic fluctuations at 4.4 AU. A closer examination of these intervals using the highest resolution data available revealed one clear instance of wave activity at spacecraft frame frequencies from 0.2 to 1 Hz. Using various analysis techniques, we have characterized these fluctuations as Bernstein mode waves excited by newborn interstellar pickup ions. We can find no other interpretation or source consistent with the observations, but this interpretation is not without questions. In this paper, we report a detailed analysis of the waves, including their frequency and polarization, that supports our interpretation.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2012

OBSERVATIONAL CONSTRAINTS ON THE ROLE OF CYCLOTRON DAMPING AND KINETIC ALFVEN WAVES IN THE SOLAR WIND

Certain few intervals with high-{beta} plasma (ratio of gas pressure to magnetic pressure) have been interpreted as containing turbulent fluctuations with wave vectors that are confined to very oblique angles with respect to the mean magnetic field. The fluctuations are theorized to be Kinetic Alfven Waves (KAWs) engaged in an energy cascade that dissipates primarily at electron scales. Dissipation by ions, and by cyclotron damping in particular, is argued to be minimal to non-existent. This interpretation is not supported, generally, by the analysis of larger data sets using other data analysis methods. These prior studies, however, were not conducted for specific {beta} ranges. In this study, we reconsider the analysis for a moderately large set of high-{beta} intervals. The analysis includes magnetic variance, the Bieber ratio test, the cross-helicity versus magnetic helicity correlation, and the implied break frequency versus angle relationship. In our analysis, the results do not support the exclusive KAW interpretation as applied generally to solar wind intervals of high-{beta} while the results do support the presence of cyclotron damping at a significant level.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2011

THIRD MOMENTS AND THE ROLE OF ANISOTROPY FROM VELOCITY SHEAR IN THE SOLAR WIND

We have extended the recent analyses of magnetohydrodynamic third moments as they relate to the turbulent energy cascade in the solar wind to consider the effects of large-scale shear flows. Moments from a large set of Advanced Composition Explorer data have been taken, and chosen data intervals are characterized by the rate of change in the solar wind speed. Mean dissipation rates are obtained in accordance with the predictions of homogeneous shear-driven turbulence. Agreement with predictions is best made for rarefaction intervals where the solar wind speed is decreasing with time. For decreasing speed intervals, we find that the dissipation rates increase with increasing shear magnitude and that the shear-induced fluctuation anisotropy is consistent with a relatively small amount.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2011

INTERPLANETARY MAGNETIC FLUX DEPLETION DURING PROTRACTED SOLAR MINIMA

We examine near-Earth solar wind observations as assembled within the Omni data set over the past 15 years that constitute the latest solar cycle. We show that the interplanetary magnetic field continues to be depleted at low latitudes throughout the protracted solar minimum reaching levels below previously predicted minima. We obtain a rate of flux removal resulting in magnetic field reduction by 0.5 nT yr{sup -1} at 1 AU when averaged over the years 2005-2009 that reduces to 0.3 nT yr{sup -1} for 2007-2009. We show that the flux removal operates on field lines that follow the nominal Parker spiral orientation predicted for open field lines and are largely unassociated with recent ejecta. We argue that the field line reduction can only be accomplished by ongoing reconnection of nominally open field lines or very old closed field lines and we contend that these two interpretations are observationally equivalent and indistinguishable.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2010

EXCITATION OF LOW-FREQUENCY WAVES IN THE SOLAR WIND BY NEWBORN INTERSTELLAR PICKUP IONS H{sup +} AND He{sup +} AS SEEN BY VOYAGER AT 4.5 AU

We report the observation of a spectral enhancement in the magnetic field fluctuations measured by the MAG instrument on the Voyager 2 spacecraft during 4.5 hr on DOY 7, 1979 at a heliocentric radial position of 4.5 AU. This time period is contained within a solar wind rarefaction when the large-scale interplanetary magnetic field was nearly radial. The frequency range and polarization of the enhanced fluctuations are consistent with waves generated by newly ionized interstellar H{sup +} and He{sup +}. We show sunward propagation of the waves via a cross-helicity analysis. We compare the observation with a theoretical model and find reasonable agreement given the model assumptions. This event is the first indication of pickup ion-generated waves seen at Voyager. It is also the first identification of pickup He{sup +} waves by any spacecraft.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2010

TURBULENT HEATING OF THE DISTANT SOLAR WIND BY INTERSTELLAR PICKUP PROTONS IN A DECELERATING FLOW

Previous models of solar wind heating by interstellar pickup proton-driven turbulence have assumed that the wind speed is a constant in heliocentric radial position. However, the same pickup process, which is taken to provide the turbulent energy, must also decelerate the wind. In this paper, we extend our phenomenological turbulence model to include variable wind speed, and then incorporate the deceleration due to interstellar pickup protons into the model. We compare the model results with plasma and field data from Voyager 2, taking this opportunity to present an extended and improved data set of proton core temperature, magnetic field fluctuation intensity, and correlation length along the Voyager trajectory. A particular motivation for including the solar wind deceleration in this model is the expectation that a slower wind would reduce the resulting proton core temperature in the region beyond {approx}60 AU, where the previous model predictions were higher than the observed values. However, we find instead that the deceleration of the steady-state wind increases the energy input to the turbulence, causing even higher temperatures in that region. The increased heating is shown to result from the larger values of the ratio of Alfven speed to solar wind speed that develop in the decelerating wind.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2010

Smith et al. Reply:

A Reply to the Comment by J. J. Podesta.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗