The Experts below are selected from a list of 318 Experts worldwide ranked by ideXlab platform
S. Arya - One of the best experts on this subject based on the ideXlab platform.
-
Application of the empirically derived Polytropic Index for the solar wind to models of solar wind propagation
Journal of Geophysical Research: Space Physics, 1996Co-Authors: T. L. Totten, J. W. Freeman, S. AryaAbstract:An empirically derived Polytropic Index for the solar wind is applied to two magnetohydrodynamic models of solar wind propagation. The first of these models is time-independent and can be used to define an initial solar wind state for the second, time-dependent model. The time-dependent model simulates the propagation of disturbances, such as shocks that originate at the Sun, through the interplanetary medium to Earth as a space weather forecasting tool. Both models assume adiabatic flow of the solar wind. Changes are made to these models to include the effects of heating of the solar wind by applying the empirically derived Polytropic Index. The adjustments made to the time-independent model produce a moderate solar wind state that compares well with Helios 1 data. Data concerning several shocks observed at the Sun and corresponding storm sudden commencements at Earth are used to investigate the predictions for shock arrival times made by the time-dependent model. Changes in the predictions for shock arrival times resulting from the application of the empirically derived Polytropic Index for the solar wind are discussed.
-
An empirical determination of the Polytropic Index for the free‐streaming solar wind using Helios 1 data
Journal of Geophysical Research, 1995Co-Authors: T. L. Totten, J. W. Freeman, S. AryaAbstract:Observations of solar wind proton temperatures indicate that the solar wind is heated as it moves outward toward the orbit of Earth. This heating, which may be the result of electron heat conduction and perhaps MHD waves, has proven difficult to quantify and hence is often neglected in MHD models of the solar wind. An alternate approach to finding explicit heating terms for the MHD energy equation is to use a Polytropic approximation. This paper discusses the properties of the Polytropic approximation and its application to the solar wind plasma. By using data from the Helios 1 spacecraft, an empirical value for the Polytropic Index of the free-streaming solar wind is determined. Various corrections to the data are made to account for velocity gradients, nonuniformity in radial sampling, and stream interaction regions. The Polytropic Index, as derived from proton data, is found to be independent of speed state, within statistical error, and has an average value of 1.46. If magnetic pressure is included, the Polytropic Index has an average value of 1.58.
-
an empirical determination of the Polytropic Index for the free streaming solar wind using helios 1 data
Journal of Geophysical Research, 1995Co-Authors: T. L. Totten, J. W. Freeman, S. AryaAbstract:Observations of solar wind proton temperatures indicate that the solar wind is heated as it moves outward toward the orbit of Earth. This heating, which may be the result of electron heat conduction and perhaps MHD waves, has proven difficult to quantify and hence is often neglected in MHD models of the solar wind. An alternate approach to finding explicit heating terms for the MHD energy equation is to use a Polytropic approximation. This paper discusses the properties of the Polytropic approximation and its application to the solar wind plasma. By using data from the Helios 1 spacecraft, an empirical value for the Polytropic Index of the free-streaming solar wind is determined. Various corrections to the data are made to account for velocity gradients, nonuniformity in radial sampling, and stream interaction regions. The Polytropic Index, as derived from proton data, is found to be independent of speed state, within statistical error, and has an average value of 1.46. If magnetic pressure is included, the Polytropic Index has an average value of 1.58.
George Livadiotis - One of the best experts on this subject based on the ideXlab platform.
-
Radial profile of the Polytropic Index of solar wind plasma in the heliosphere
arXiv: Space Physics, 2020Co-Authors: George LivadiotisAbstract:We combine different measurements of the Polytropic Index of the proton plasma in the heliosphere: i) near-adiabatic Index in the inner heliosphere ~1AU, ii) subadiabatic indices in the outer heliosphere ~20-40AU, and iii) near-zero indices in the inner heliosheath. These observations are unified by a single theoretical model of the Polytropic Index throughout its radial extent in the heliosphere; the corresponding fitting reveals the decreasing trend of the Polytropic Index with increasing heliocentric distance R. We anticipate that with increasing R, (i) the Debye length and mean-free-path decreases; (ii) the Landau damping is less effective, transferring thus less wave energy to particles; and (iii) the collisionality degree increases, indicating that the proton plasma in the inner heliosheath might be collisionless.
-
Relationship between Polytropic Index and temperature anisotropy in space plasmas
arXiv: Space Physics, 2020Co-Authors: George Livadiotis, George NicolaouAbstract:The paper develops a theoretical relationship between the Polytropic Index and the temperature anisotropy that may characterize space plasmas. The derivation is based on the correlation among the kinetic energies of particles with velocities described by anisotropic kappa distributions. The correlation coefficient depends on the effective dimensionality of the velocity distribution function, having its shape determined by the temperature anisotropies caused by the ambient magnetic field; on the other hand, the effective dimensionality is directly dependent on the Polytropic Index. This analysis leads to the connection between the correlation coefficient, effective dimensionality of velocity space, and the Polytropic Index, with the ratio of temperature anisotropy. Moreover, a data and statistical analysis is performed to test the application of the theoretically developed model, in the solar wind proton plasma near 1 AU. The derived theoretical relationship is in good agreement with observations, showing that the lowest and classical value of the adiabatic Polytropic Index occurs in the isotropic case, while higher indices characterize anisotropic plasmas. Finally, possible extensions of the theory considering (i) non-adiabatic Polytropic behavior, and (ii) more general distributions, are further discussed.
-
Polytropic Behavior of Solar Wind Protons Observed by Parker Solar Probe
arXiv: Space Physics, 2020Co-Authors: Georgios Nicolaou, George Livadiotis, Robert T. Wicks, Daniel Verscharen, Bennett A. MarucaAbstract:A Polytropic process describes the transition of a fluid from one state to another through a specific relationship between the fluid density and temperature. The value of the Polytropic Index that governs this relationship determines the heat transfer and the effective degrees of freedom during the process. In this study, we analyze solar wind proton plasma measurements, obtained by the Faraday cup instrument on-board Parker Solar Probe. We examine the large-scale variations of the proton plasma density and temperature within the inner heliosphere explored by the spacecraft. We also address a Polytropic behavior in the density and temperature fluctuations in short-time intervals, which we analyze in order to derive the effective Polytropic Index of small time-scale processes. The large-scale variations of the solar wind proton density and temperature which are associated with the plasma expansion through the heliosphere, follow a Polytropic model with a Polytropic Index ~5/3. On the other hand, the short time-scale fluctuations which may be associated with turbulence, follow a model with a larger Polytropic Index. We investigate possible correlations between the Polytropic Index of short time-scale fluctuations and the plasma speed, plasma beta, and the magnetic field direction. We discuss the scenario of mechanisms including energy transfer or mechanisms that restrict the particle effective degrees of freedom.
-
Connection of Turbulence with Polytropic Index in the Solar Wind Proton Plasma
Entropy, 2019Co-Authors: George LivadiotisAbstract:This paper improves our understanding of the interplay of the proton plasma turbulent heating sources of the expanding solar wind in the heliosphere. Evidence is shown of the connections between the Polytropic Index, the rate of the heat absorbed by the solar wind, and the rate of change of the turbulent energy, which heats the solar wind in the inner and outer heliosphere. In particular, we: (i) show the theoretical connection of the rate of a heat source, such as the turbulent energy, with the Polytropic Index and the thermodynamic process; (ii) calculate the effect of the pick-up protons in the total proton temperature and the relationship connecting the rate of heating with the Polytropic Index; (iii) derive the radial profiles of the solar wind heating in the outer and inner heliosphere; and (iv) use the radial profile of the turbulent energy in the solar wind proton plasma in the heliosphere, in order to show its connection with the radial profiles of the Polytropic Index and the heating of the solar wind.
-
On the Calculation of the Effective Polytropic Index in Space Plasmas
Entropy, 2019Co-Authors: Georgios Nicolaou, George Livadiotis, Robert T. WicksAbstract:The Polytropic Index of space plasmas is typically determined from the relationship between the measured plasma density and temperature. In this study, we quantify the errors in the determination of the Polytropic Index, due to uncertainty in the analyzed measurements. We model the plasma density and temperature measurements for a certain Polytropic Index, and then, we apply the standard analysis to derive the Polytropic Index. We explore the accuracy of the derived Polytropic Index for a range of uncertainties in the modeled density and temperature and repeat for various Polytropic indices. Our analysis shows that the uncertainties in the plasma density introduce a systematic error in the determination of the Polytropic Index which can lead to artificial isothermal relations, while the uncertainties in the plasma temperature increase the statistical error of the calculated Polytropic Index value. We analyze Wind spacecraft observations of the solar wind protons and we derive the Polytropic Index in selected intervals over 2002. The derived Polytropic Index is affected by the plasma measurement uncertainties, in a similar way as predicted by our model. Finally, we suggest a new data-analysis approach, based on a physical constraint, that reduces the amount of erroneous derivations.
T. L. Totten - One of the best experts on this subject based on the ideXlab platform.
-
Application of the empirically derived Polytropic Index for the solar wind to models of solar wind propagation
Journal of Geophysical Research: Space Physics, 1996Co-Authors: T. L. Totten, J. W. Freeman, S. AryaAbstract:An empirically derived Polytropic Index for the solar wind is applied to two magnetohydrodynamic models of solar wind propagation. The first of these models is time-independent and can be used to define an initial solar wind state for the second, time-dependent model. The time-dependent model simulates the propagation of disturbances, such as shocks that originate at the Sun, through the interplanetary medium to Earth as a space weather forecasting tool. Both models assume adiabatic flow of the solar wind. Changes are made to these models to include the effects of heating of the solar wind by applying the empirically derived Polytropic Index. The adjustments made to the time-independent model produce a moderate solar wind state that compares well with Helios 1 data. Data concerning several shocks observed at the Sun and corresponding storm sudden commencements at Earth are used to investigate the predictions for shock arrival times made by the time-dependent model. Changes in the predictions for shock arrival times resulting from the application of the empirically derived Polytropic Index for the solar wind are discussed.
-
An empirical determination of the Polytropic Index for the free‐streaming solar wind using Helios 1 data
Journal of Geophysical Research, 1995Co-Authors: T. L. Totten, J. W. Freeman, S. AryaAbstract:Observations of solar wind proton temperatures indicate that the solar wind is heated as it moves outward toward the orbit of Earth. This heating, which may be the result of electron heat conduction and perhaps MHD waves, has proven difficult to quantify and hence is often neglected in MHD models of the solar wind. An alternate approach to finding explicit heating terms for the MHD energy equation is to use a Polytropic approximation. This paper discusses the properties of the Polytropic approximation and its application to the solar wind plasma. By using data from the Helios 1 spacecraft, an empirical value for the Polytropic Index of the free-streaming solar wind is determined. Various corrections to the data are made to account for velocity gradients, nonuniformity in radial sampling, and stream interaction regions. The Polytropic Index, as derived from proton data, is found to be independent of speed state, within statistical error, and has an average value of 1.46. If magnetic pressure is included, the Polytropic Index has an average value of 1.58.
-
an empirical determination of the Polytropic Index for the free streaming solar wind using helios 1 data
Journal of Geophysical Research, 1995Co-Authors: T. L. Totten, J. W. Freeman, S. AryaAbstract:Observations of solar wind proton temperatures indicate that the solar wind is heated as it moves outward toward the orbit of Earth. This heating, which may be the result of electron heat conduction and perhaps MHD waves, has proven difficult to quantify and hence is often neglected in MHD models of the solar wind. An alternate approach to finding explicit heating terms for the MHD energy equation is to use a Polytropic approximation. This paper discusses the properties of the Polytropic approximation and its application to the solar wind plasma. By using data from the Helios 1 spacecraft, an empirical value for the Polytropic Index of the free-streaming solar wind is determined. Various corrections to the data are made to account for velocity gradients, nonuniformity in radial sampling, and stream interaction regions. The Polytropic Index, as derived from proton data, is found to be independent of speed state, within statistical error, and has an average value of 1.46. If magnetic pressure is included, the Polytropic Index has an average value of 1.58.
J. W. Freeman - One of the best experts on this subject based on the ideXlab platform.
-
Application of the empirically derived Polytropic Index for the solar wind to models of solar wind propagation
Journal of Geophysical Research: Space Physics, 1996Co-Authors: T. L. Totten, J. W. Freeman, S. AryaAbstract:An empirically derived Polytropic Index for the solar wind is applied to two magnetohydrodynamic models of solar wind propagation. The first of these models is time-independent and can be used to define an initial solar wind state for the second, time-dependent model. The time-dependent model simulates the propagation of disturbances, such as shocks that originate at the Sun, through the interplanetary medium to Earth as a space weather forecasting tool. Both models assume adiabatic flow of the solar wind. Changes are made to these models to include the effects of heating of the solar wind by applying the empirically derived Polytropic Index. The adjustments made to the time-independent model produce a moderate solar wind state that compares well with Helios 1 data. Data concerning several shocks observed at the Sun and corresponding storm sudden commencements at Earth are used to investigate the predictions for shock arrival times made by the time-dependent model. Changes in the predictions for shock arrival times resulting from the application of the empirically derived Polytropic Index for the solar wind are discussed.
-
An empirical determination of the Polytropic Index for the free‐streaming solar wind using Helios 1 data
Journal of Geophysical Research, 1995Co-Authors: T. L. Totten, J. W. Freeman, S. AryaAbstract:Observations of solar wind proton temperatures indicate that the solar wind is heated as it moves outward toward the orbit of Earth. This heating, which may be the result of electron heat conduction and perhaps MHD waves, has proven difficult to quantify and hence is often neglected in MHD models of the solar wind. An alternate approach to finding explicit heating terms for the MHD energy equation is to use a Polytropic approximation. This paper discusses the properties of the Polytropic approximation and its application to the solar wind plasma. By using data from the Helios 1 spacecraft, an empirical value for the Polytropic Index of the free-streaming solar wind is determined. Various corrections to the data are made to account for velocity gradients, nonuniformity in radial sampling, and stream interaction regions. The Polytropic Index, as derived from proton data, is found to be independent of speed state, within statistical error, and has an average value of 1.46. If magnetic pressure is included, the Polytropic Index has an average value of 1.58.
-
an empirical determination of the Polytropic Index for the free streaming solar wind using helios 1 data
Journal of Geophysical Research, 1995Co-Authors: T. L. Totten, J. W. Freeman, S. AryaAbstract:Observations of solar wind proton temperatures indicate that the solar wind is heated as it moves outward toward the orbit of Earth. This heating, which may be the result of electron heat conduction and perhaps MHD waves, has proven difficult to quantify and hence is often neglected in MHD models of the solar wind. An alternate approach to finding explicit heating terms for the MHD energy equation is to use a Polytropic approximation. This paper discusses the properties of the Polytropic approximation and its application to the solar wind plasma. By using data from the Helios 1 spacecraft, an empirical value for the Polytropic Index of the free-streaming solar wind is determined. Various corrections to the data are made to account for velocity gradients, nonuniformity in radial sampling, and stream interaction regions. The Polytropic Index, as derived from proton data, is found to be independent of speed state, within statistical error, and has an average value of 1.46. If magnetic pressure is included, the Polytropic Index has an average value of 1.58.
David B. Jess - One of the best experts on this subject based on the ideXlab platform.
-
The Polytropic Index of solar coronal plasma in sunspot fan loops and its temperature dependence.
The Astrophysical Journal, 2018Co-Authors: S. Krishna Prasad, J. O. Raes, T. Van Doorsselaere, N. Magyar, David B. JessAbstract:Observations of slow magneto-acoustic waves have been demonstrated to possess a number of applications in coronal seismology. Determination of the Polytropic Index ($\gamma$) is one such important application. Analysing the amplitudes of oscillations in temperature and density corresponding to a slow magneto-acoustic wave, the Polytropic Index in the solar corona has been calculated and based on the obtained value it has been inferred that thermal conduction is highly suppressed in a very hot loop in contrast to an earlier report of high thermal conduction in a relatively colder loop. In this study, using SDO/AIA data, we analysed slow magneto-acoustic waves propagating along sunspot fan loops from 30 different active regions and computed Polytropic indices for several loops at multiple spatial positions. The obtained $\gamma$ values vary from 1.04$\pm$0.01 to 1.58$\pm$0.12 and most importantly display a temperature dependence indicating higher $\gamma$ at hotter temperatures. This behaviour brings both the previous studies to agreement and perhaps implies a gradual suppression of thermal conduction with increase in temperature of the loop. The observed phase shifts between temperature and density oscillations, however, are substantially larger than that expected from a classical thermal conduction and appear to be influenced by a line-of-sight integration effect on the emission measure.
