The Sun and the solar wind close to the Sun

Abstract

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.

Morphology, dynamics and plasma parameters of plumes and inter-plume regions in solar coronal holes

“…Alternatively, the shear could have been reduced below these heights, as implied by the decreasing density contrast illustrated in Fig. 17, due to some other interaction (Suess 2000). Plume signatures in temperature, composition and ionization state may still exist unless there is an actual mixing of plasma and magnetic field lines -something otherwise not expected to occur in the fast SW.…”

“…According to model calculations, most of this expansion occurs at the base below ≈ 35 Mm, where the plumes grow in diameter from ≈ 3 Mm by nearly a factor of ten. In the regime of low β (the ratio of the plasma pressure to the magnetic pressure) up to at least 5 R ⊙ , the geometric spreading factors in plumes and IPRs vary together (Suess et al 1998;Suess 2000).…”

Validating Solar Wind Prediction Using the Current Sheet Source Surface Model

“…The corona is not strictly current-free. Particularly, above 1.5 R the existence of large-scale plasma structures (e.g., looplike condensations) suggest appreciable interactions between the magnetic field and plasma (Zhao & Hoeksema 1993;Suess et al 1998;Suess 2000). Therefore, the potential field approach in the PFSS model is an oversimplification of the real corona.…”