An empirical determination of the polytropic index for the free‐streaming solar wind using Helios 1 data

Totten, Tracy L.; Freeman, J. W.; Arya, Sharda

doi:10.1029/94ja02420

Cited by 201 publications

(169 citation statements)

References 18 publications

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“…Démoulin & Dasso (2009) have shown theoretically that the main origin of MC expansion is the decrease of the total SW pressure with solar distance D. With a SW pressure decreasing as D −n P , they found that ζ ≈ n P /4 independently of the magnetic structure of the flux rope forming the MC. In the present work, we re-analyzed the total SW pressure variation with D, revising previous studies that also analyzed Helios data (Mariani & Neubauer 1990;Schwenn 2006;Totten et al 1995;Marsch et al 1989). We found n P = 2.91 ± 0.31, which implies n P /4 = 0.73 ± 0.08, in agreement with our estimation of ζ from the measured velocity in MCs.…”

Section: Summary and Discussion Of Main Resultsmentioning

confidence: 99%

Global and local expansion of magnetic clouds in the inner heliosphere

Gulisano

Démoulin²,

Dasso

et al. 2009

A&A

121

219

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Context. Observations of magnetic clouds (MCs) are consistent with the presence of flux ropes detected in the solar wind (SW) a few days after their expulsion from the Sun as coronal mass ejections (CMEs). Aims. Both the in situ observations of plasma velocity profiles and the increase of their size with solar distance show that MCs are typically expanding structures. The aim of this work is to derive the expansion properties of MCs in the inner heliosphere from 0.3 to 1 AU. Methods. We analyze MCs observed by the two Helios spacecraft using in situ magnetic field and velocity measurements. We split the sample in two subsets: those MCs with a velocity profile that is significantly perturbed from the expected linear profile and those that are not. From the slope of the in situ measured bulk velocity along the Sun-Earth direction, we compute an expansion speed with respect to the cloud center for each of the analyzed MCs. Results. We analyze how the expansion speed depends on the MC size, the translation velocity, and the heliocentric distance, finding that all MCs in the subset of non-perturbed MCs expand with almost the same non-dimensional expansion rate (ζ). We find departures from this general rule for ζ only for perturbed MCs, and we interpret the departures as the consequence of a local and strong SW perturbation by SW fast streams, affecting the MC even inside its interior, in addition to the direct interaction region between the SW and the MC. We also compute the dependence of the mean total SW pressure on the solar distance and we confirm that the decrease of the total SW pressure with distance is the main origin of the observed MC expansion rate. We found that ζ was 0.91 ± 0.23 for non-perturbed MCs while ζ was 0.48 ± 0.79 for perturbed MCs, the larger spread in the last ones being due to the influence of the solar wind local environment conditions on the expansion.

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Section: Summary and Discussion Of Main Resultsmentioning

confidence: 99%

Global and local expansion of magnetic clouds in the inner heliosphere

Gulisano

Démoulin²,

Dasso

et al. 2009

A&A

121

219

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“…The small § dependence of with distance was unexpected. The temperature of the background solar wind in the inner heliosphere decreases as § 2 3 £ [Totten et al, 1995]. Since ICMEs expand with distance, the temperature in ICMEs should decrease faster than in the background solar wind due to adiabatic cooling; instead it decreases less quickly.…”

Section: Radial Evolution Of Icmesmentioning

confidence: 99%

“…Ulysses was launched in 1990 and orbits over the solar poles at 1.3 -5.4 AU (although most ICMEs detected are at low latitudes). Since the temperature decreases with distance, we normalized to 1 AU assuming a § 2 3 £ dependence [Totten et al, 1995; see also Steinitz and Eyni, 1981;Lopez and Freeman, 1995] before applying the temperature criteria. The…”

Section: Identification Of Icmesmentioning

confidence: 99%

Propagation and Evolution of ICMES in the Solar Wind

Richardson

Liu

Belcher

2005

NATO Science Series II: Mathematics, Physics and Chemistry

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Interplanetary coronal mass ejections (ICMEs) evolve as they propagate outward from the Sun. They interact with and eventually equilibrate with the ambient solar wind. One difficulty in studying this evolution is that ICMEs have no unique set of identifying characteristics, so boundaries of the ICMEs are difficult to identify. Two characteristics present in some ICMEs but generally not present in the ambient solar wind, high helium/proton density ratios and low temperature/speed ratios, are used to identify ICMEs. We search the Helios 1 and 2, WIND, ACE, and Ulysses data for ICMEs with these characteristics and use them to study the radial evolution of ICMEs. We find that the magnetic field magnitude and density decrease faster in ICMEs than in the ambient solar wind, but the temperature decreases more slowly than in the ambient solar wind. Since we also find that ICMEs expand in radial width with distance, the protons within ICMEs must be heated. Scale sizes for He structures are smaller than for proton structures within ICMEs.

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“…The heliospheric model uses g = 5/3 to describe fully ionized solar wind plasma, which allows for obtaining accurate shock strengths. We have also performed heliospheric computations with an empirically based g = 1.5 which reflects nonideal processes in the solar wind [Totten et al, 1995]. The dynamical differences are small and g = 5/ 3 is used in this paper since the aim is to track the steepening of coronal pressure waves into interplanetary shocks.…”

Section: Numerical Modelsmentioning

confidence: 99%

Merging of coronal and heliospheric numerical two‐dimensional MHD models

et al. 2002

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[1] Space weather research requires investigation of a complex chain of coupled dynamic phenomena occurring simultaneously on various spatial and temporal scales between the Sun and Earth. Specialized physically based numerical models have been developed to address particular aspects of the entire system. However, an integrated modeling approach is necessary to provide a complete picture suitable for interpretation of various remote and in situ observations and for development of forecasting capabilities. In this paper we demonstrate merging of coronal and heliospheric MHD models for a two-dimensional hypothetical case involving a magnetic cloud, shock, streamer belt, and current sheet. The disruption of a sheared helmet streamer launches a coronal mass ejection (CME) (simulated by the coronal model), which evolves during its propagation through interplanetary space (simulated by the heliospheric model). These models employ different physical approximations and numerical grids to simulate physical phenomena over their respective spatial and temporal domains. The merging of the models enables accurate tracking of a CME from its origin in the solar atmosphere to its arrival at Earth.

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An empirical determination of the polytropic index for the free‐streaming solar wind using Helios 1 data

Cited by 201 publications

References 18 publications

Global and local expansion of magnetic clouds in the inner heliosphere

Global and local expansion of magnetic clouds in the inner heliosphere

Propagation and Evolution of ICMES in the Solar Wind

Merging of coronal and heliospheric numerical two‐dimensional MHD models

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