Solar polar coronal hole - A mathematical simulation

Suess, S. T.; Richter, A. K.; Winge, C. R.; Nerney, Steven

doi:10.1086/155579

Cited by 57 publications

(29 citation statements)

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“…It is often assumed (e.g., Hundhausen, 1977;Suess et al , 1977) that coronal holes have unique properties independent of their position on the sun; in particular that the photospheric magnetic field strength is always the same within coronal holes, namely about 1 mT. During the interval of our observations (May 1976 -August 1977 there were only a few and ill-defined equatorial holes (Solar Geophysical Data, 1976.…”

Section: Coronal Holesmentioning

confidence: 77%

The strength of the Sun's polar fields

1978

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IntroductionThe magnetic field strength within the polar caps, of the sun is an important parameter for both the solar activity cycle and for our understanding of the interplanetary magnetic field.Measurements In addition, extensive background fields are apparent including welldefined polar fields. Referring to Figure 1 we shall be particularly interested in the field measured in the polar cap, i.e. the polemost scan line in each hemisphere. The average latitude of the equatoro ward limit of that aperture is 55 . At the present time that is also the equatorward boundary of the polar coronal holes (Solar Geophysical Data, 1976. In accordance with that, the measured polar fields have invariably been unipolar (over the aperture) throughout the interval covered by the daily magnetograms -16 May 1976 through August 1977 -positive or outwards in the north and negative or inwards in the southern polar cap. The position of an aperture along the scan line is measured by the parameter x, being 0 at central meridian, and ±11 respectively at the west -and east equatorial limbs ( Figure 1). We shall be concerned with apertures at x = 0 (black in Figure 1) and at x = ±2 (dotted areas in Figure 1).During the course of a year the solar rotation axis tips toward and later away from the observer by l\ . We thus get a kind of stereoscopic view of the polar fields provided that they do not change appreciably over a timescale of up to a year.In order to interpret measurements of the polar cap fields, it is necessary to investigate if the fact thtt the data is taken near the limb is distorting the data in any way. It has been suggested by Howard and Stenflo (1972) that measurements of the magnetic field in the \525.02 nm line are systematically in error by a factor that 2 depends on the angle between the radius and the line of sight.This error changes by more than a factor of two as one goes from the center of the disk to the limb. By following specific areas as they cross the disk from east limb to west limb in the course of solar rotation we may directly verify the existence of this systematic error under the weak assumption that the intrinsic field does not change considerably during disk passage. In the following section we report the results of an extensive analysis of measured field strengths as a function of the distance from disk center. Our conclusion is that no systematic error of the kind suggested by Howard and Stenflo (1972) is apparent in the Stanford data. Center to limb variation of field strengthWhen an aperture is placed at central meridian an area is defined on the solar surface. The area can be assigned heliographic coordinates and can be identified on magnetograms both before and after central Figure 4 shows the average B« as a function of cosL. The analysis was performed separately for weak fields (|B| < 150 n,T) and for strong fields (lB|i 150 tiT). The magnetic field strength at central meridian passage was used as the selection parameter. Furthermore, the data was analyzed separately for each polarity of ...

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Section: Coronal Holesmentioning

confidence: 77%

The strength of the Sun's polar fields

1978

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“…There are good observational reasons to believe that close to the Sun, the heating of the wind is sufficient to keep it approximately isothermal (Suess et al 1977;Roussev et al 2003). Far from the Sun, in situ measurements by multiple spacecraft have shown that the wind is hotter than would be expected from adiabatic expansion, and therefore must continue to be heated out to several AU (Smith & Wolfe 1979;Totten et al 1995;Ebert et al 2009).…”

Section: Stellar Winds In Theory and Practicementioning

confidence: 99%

“…Determining α in is more difficult because in situ measurements of the solar wind do not extend closer to the Sun than 0.3 AU, but all indications suggest that the solar corona and the inner regions of the wind are almost isothermal. For example, Suess et al (1977) analysed observations of a polar coronal hole between 2 R and 5 R and found a value of α of 1.05 gives a good fit. This value has been used extensively in solar and stellar wind simulations (e.g.…”

Section: Numerical Modelmentioning

confidence: 99%

Stellar winds on the main-sequence

Johnstone

Güdel

Lüftinger

et al. 2015

A&A

100

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Aims. We develop a method for estimating the properties of stellar winds for low-mass main-sequence stars between masses of 0.4 M and 1.1 M at a range of distances from the star. Methods. We use 1D thermal pressure driven hydrodynamic wind models run using the Versatile Advection Code. Using in situ measurements of the solar wind, we produce models for the slow and fast components of the solar wind. We consider two radically different methods for scaling the base temperature of the wind to other stars: in Model A, we assume that wind temperatures are fundamentally linked to coronal temperatures, and in Model B, we assume that the sound speed at the base of the wind is a fixed fraction of the escape velocity. In Paper II of this series, we use observationally constrained rotational evolution models to derive wind mass loss rates. Results. Our model for the solar wind provides an excellent description of the real solar wind far from the solar surface, but is unrealistic within the solar corona. We run a grid of 1200 wind models to derive relations for the wind properties as a function of stellar mass, radius, and wind temperature. Using these results, we explore how wind properties depend on stellar mass and rotation. Conclusions. Based on our two assumptions about the scaling of the wind temperature, we argue that there is still significant uncertainty in how these properties should be determined. Resolution of this uncertainty will probably require both the application of solar wind physics to other stars and detailed observational constraints on the properties of stellar winds. In the final section of this paper, we give step by step instructions for how to apply our results to calculate the stellar wind conditions far from the stellar surface.

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“…The consequence is that by 5Rs the radial magnetic field strength is essentially independent of position across a coronal hole. This simple physical argument is fully supported by numerical (Wang et al 1995) and analytic (Suess et al, 1977) solutions and by physical estimates . Figure 3.…”

Section: The Geometric Spreading Of Coronal Holesmentioning

confidence: 58%

The Sun and the solar wind close to the Sun

Suess

2000

Advances in Space Research

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One of the benefits from the Ulysses, SOHO, and YOHKOH missions has been a strong stimulus to better understand the magnetohydrodynamic processes involved in coronal expansion. Three topics for which this has been especially true are described here. These are: (i) The observed constancy of the radial interplanetary magnetic field strength (as mapped to constant radius).(ii) The geometric spreading of coronal plumes and coronal holes, and the fate of plumes.(iii) The plasma _ in streamers and the physics of streamer confinement. INTRODUCTIONIt is said that the structure of the solar corona is imprinted onto the solar wind. This has been demonstrated in many ways, but SOHO and Ulysses are modifying the concept -for both fast and slow solar wind. More specifically, the results are modifying ideas about the physical role of the magnetic field as interpreted using MHD models of coronal expansion.Consider first fast solar wind, which originates in coronal holes that expand in area by factors of two to tenfold between one and five solar radii. MHD and potential field-based models of the coronal magnetic field make specific, verifiable predictions. Yet, these predictions differ from each other in how geometric expansion varies across a hole and how wind speed depends on the expansion. The differences are still being reconciled, but each prediction must be consistent with the observation that fast wind and the radial magnetic field strength (normalized to constant radius) are surprisingly uniform. What has been added to this is that the solar wind within coronal holes appears to be permeated by filamentary structures in addition to MHD waves. The most well known form of this filamentary structure is the coronal plume. Plumes then apparently becomes thoroughly mixed with the interplume plasma inside I/3 AU because outside that distance all that is observed is an evolving field of MHD turbulence superimposed on the high speed wind.Consider next slow wind, which seems to somehow come from inside or from the flanks of streamers and may never achieve a state of steady flow. This puts to rest the old idea of "quiet solar wind" and raises many new questions.There are divergent ideas for the how streamers might be the source of slow wind, but these are being strongly constrained by new data on composition and flow speed, as well as on the properties of streamer cores. It will be important to develop a broader range of streamer models to clarify the main physical processes.The brief discussion of the above points will concentrate on modeling and the magnetohydrodynamic structure of coronal holes and streamers.

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Solar polar coronal hole - A mathematical simulation

Cited by 57 publications

References 0 publications

The strength of the Sun's polar fields

The strength of the Sun's polar fields

Stellar winds on the main-sequence

The Sun and the solar wind close to the Sun

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