Thomas E. Holzer

Dynamics of coronal hole regions

The hydrodynamic properties of a steadily expanding corona are explored for situations in which departures from spherically symmetric outflow are large, in the sense that the geometrical cross section of a given flow tube increases outward from the Sun faster than r2 in some regions. Assuming polytropic flow, it is shown that in certain cases the flow may contain more than one critical point. We derive the criterion for determining which of these critical points is actually crossed by the transonic solution which begins at the Sun and extends continuously outward. Next, we apply the theory to geometries which exhibit rapid spreading of the flow tubes in the inner corona, followed by more-or-less radial divergence at large distances. This is believed to be the type of geometry found in coronal hole regions. The results show that, if this initial divergence is sufficiently large, the outflow becomes supersonic at a critical point encountered low in the corona in the region of high divergence, and it remains supersonic at all greater heights in the corona. This feature strongly suggests that coronal hole regions differ from other open-field regions of the corona in that they are in a ‘fast’, low density expansion state over much of their extent. Such a dynamical configuration makes it possible to reconcile the low values of electron density observed in coronal holes with the large particle fluxes in the associated high speed streams seen in the solar wind.

Active and Eruptive Prominences and Their Relationship to Coronal Mass Ejections

In order to understand better the dynamical processes in the solar atmosphere that are associated with coronal mass ejections (CMEs), we have carried out a study of prominence activity using Hα observations obtained at the Mauna Loa Solar Observatory (MLSO). After developing clear definitions of active prominences (APs) and eruptive prominences (EPs), we examined 54 Hα events to identify distinguishing characteristics of APs and EPs and to study the relationship between prominence activity and CMEs. The principal characteristics we found to distinguish clearly between APs and EPs are maximum projected radial height, projected radial velocity, and projected radial acceleration. We determined CME associations with Hα events by using white-light data from the Mk III K-Coronameter at MLSO and the LASCO C2 Coronagraph on SOHO. We found that EPs are more strongly associated with CMEs than are APs and that the CMEs associated with EPs generally have cores, while those associated with APs do not. A majority of the EPs in the study exhibit separation of escaping material from the bulk of the prominence—the latter initially lifting away from and then returning toward the solar surface. This separation tends to occur in the height range from 1.20 to 1.35 R0, and we infer that it involves the formation of an X-type neutral line in this region, which allows disconnection of part of the prominence material. This disconnection view of prominence eruption seems most consistent with flux rope models of prominence support.

Line broadening of Mg X 609 and 625 A coronal emission lines observed above the solar limb

A University of Colorado sounding rocket experiment on March 17, 1988, provided high-resolution EUV spectra along a solar diameter and out to 1.2 solar radius with spatial resolution of 20 x 60 arcsec. Each spectrum contains transition region and coronal emission lines in the wavelength range 605-635 A and 1210-1270 A, including the emission lines Mg X 609 and 625 A, Fe XII 1242 A, O V 629 A, N V 1238 and 1242 A, corresponding to a wide range of temperatures of formation. Increased line broadening is observed above the limb for all lines, and this effect is illustrated by presenting observed line widths as a function of height above the limb for the higher temperature lines Mg X 609 and 625 A. On the basis of calculations, the most likely cause of the increased broadening above the limb appears to be the presence of hydromagnetic waves in the corona.

Acceleration of the Solar Wind

In this review, we discuss critically recent research on the acceleration of the solar wind, giving emphasis to high-speed solar wind streams emanating from solar coronal holes. We first explain why thermally driven wind models constrained by solar and interplanetary observations encounter substantial difficulties in explaining high speed streams. Then, through a general discussion of energy addition to the solar wind above the coronal base, we indicate a possible resolution of these difficulties. Finally, we consider the question of what role MHD waves might play in transporting energy through the solar atmosphere and depositing it in the solar wind, and we conclude by examining, in a simple way, the specific mechanism of solar wind acceleration by Alfven waves and the related problem of accelerating massive stellar winds with Alfven waves.

The Role of Helium in the Outer Solar Atmosphere

We construct models of the outer solar atmosphere comprising the region from the mid-chromosphere and into the solar wind in order to study the force and energy balance in models with a significant helium abundance. The corona is created by dissipation of an energy flux from the Sun. The energy flux is lost as radiation from the top of the chromosphere and as gravitational and kinetic solar wind energy flux. We find that in models with significant ion heating of the extended corona most of the energy flux is lost in the solar wind. The ion temperatures are higher than the electron temperature in these models, and the α-particle temperature is much higher than the proton temperature, so there is energy transfer from the α-particle fluid to the protons and electrons, but this energy exchange between the different species is relatively small. To a fairly good approximation we can say that the energy flux deposited in the protons and α-particles is lost as kinetic and gravitational energy flux in the proton and α-particle flow. How this energy flux is divided between gravitational and kinetic energy flux (i.e., how large the particle fluxes and flow speeds are) depends upon details of the heating process. We also find that mixing processes in the chromosphere play an important role in determining the coronal helium abundance and the relative solar wind proton and α-particle fluxes. Roughly speaking, we find that the relative α-particle and proton fluxes are set by the degree of chromospheric mixing, while the speeds are set by the details of the coronal heating process.

The Structure and Evolution of a Sigmoidal Active Region

Solar coronal sigmoidal active regions have been shown to be precursors to some coronal mass ejections. Sigmoids, or S-shaped structures, may be indicators of twisted or helical magnetic structures, having an increased likelihood of eruption. We present here an analysis of a sigmoidal regions three-dimensional structure and how it evolves in relation to its eruptive dynamics. We use data taken during a recent study of a sigmoidal active region passing across the solar disk (an element of the third Whole Sun Month campaign). While S-shaped structures are generally observed in soft X-ray (SXR) emission, the observations that we present demonstrate their visibility at a range of wavelengths including those showing an associated sigmoidal filament. We examine the relationship between the S-shaped structures seen in SXR and those seen in cooler lines in order to probe the sigmoidal regions three-dimensional density and temperature structure. We also consider magnetic field observations and extrapolations in relation to these coronal structures. We present an interpretation of the disk passage of the sigmoidal region, in terms of a twisted magnetic flux rope that emerges into and equilibrates with overlying coronal magnetic field structures, which explains many of the key observed aspects of the regions structure and evolution. In particular, the evolving flux rope interpretation provides insight into why and how the region moves between active and quiescent phases, how the regions sigmoidicity is maintained during its evolution, and under what circumstances sigmoidal structures are apparent at a range of wavelengths.

Observational Interpretation of an Active Prominence on 1999 May 1

This paper presents an observational study of an active prominence observed in He I 1083 nm intensity and velocity data obtained at the Mauna Loa Solar Observatory, which provide physical insight into dynamical processes associated with prominences. We compare these observations with existing theoretical prominence models, which fall into two main classes: dip models and flux rope models. Dip models use sagging magnetic arches to explain prominence support, while flux rope models are characterized by helical magnetic field lines that trap prominence material at the bottom of the rope. The prominence on which we focus in the present paper has four interesting components of activity, all of which we attempt to explain using each of three different prominence models: the normal and inverse polarity flux rope models and the dip model. Our objective is to test the viability of each of these models in describing this type of activity. The model that appears consistent with the observed activity in this particular prominence is the inverse polarity flux rope model. We suggest that the process of vertical reconnection between an inverse polarity flux rope and an underlying magnetic arcade may best describe the observed prominence activity.

Ionospheric response to the interplanetary magnetic field southward turning: Fast onset and slow reconfiguration

[1] This paper presents a case study of ionospheric response to an interplanetary magnetic field (IMF) southward turning. It is based on a comprehensive set of observations, including a global network of ground magnetometers, global auroral images, and a SuperDARN HF radar. There is a clear evidence for a two-stage ionospheric response to the IMF southward turning, namely, fast initial onset and slow final reconfiguration. The fast onset is manifested by nearly simultaneous (within 2 min) rise of ground magnetic perturbations at all local times, corroborated by a sudden change in the direction of line-of-sight velocity near local midnight and by the simultaneous equatorward shift of the auroral oval. The slow reconfiguration is characterized by the different rising rate of magnetic perturbations with latitudes: faster at high latitude than at lower latitudes. Furthermore, a cross-correlation analysis of the magnetometer data shows that the maximum magnetic perturbation is reached first near local noon, and then spread toward the nightside, corresponding to a dayside-to-nightside propagation speed of ∼5 km/s along the auroral oval. Global ionospheric convection patterns are derived based on ground magnetometer data along with auroral conductances inferred from the Polar UV images, using the assimilative mapping of ionospheric electrodynamics (AMIE) procedure. The AMIE patterns, especially the residual convection patterns, clearly show a globally coherent development of two-cell convection configuration following the IMF southward turning. While the foci of the convection patterns remain nearly steady, the convection flow does intensify with time and the cross-polar-cap potential drop increases. The overall changes as shown in the AMIE convection patterns therefore are fully consistent with the two-stage ionospheric response to the IMF southward turning.

Neutral Atom Diffusion in a Partially Ionized Prominence Plasma

The support of solar prominences is normally described in terms of a magnetic force on the prominence plasma that balances the solar gravitational force. Because the prominence plasma is only partially ionized, this support needs to be understood in terms of the frictional coupling between the neutral and ionized components of the prominence plasma, the efficacy of which depends directly on the ion density. More specifically, the frictional force is proportional to the relative flow of neutral and ion species, and for a plasma with a sufficiently small vertical ion column density, this flow must be relatively large to produce a frictional force that balances gravity. A large relative flow, of course, implies significant draining of neutral particles from the prominence. We evaluate the importance of this draining effect for a hydrogen-helium plasma and consider the variation of the draining with a variety of prominence parameters. Our calculations show that the loss timescale for hydrogen is much longer than that for helium, which for typical prominence parameters is about one day.

Helmet Streamers Gone Unstable: Two-Fluid Magnetohydrodynamic Models of the Solar Corona

The equations of magnetohydrodynamics (MHD) are used to study heating of electrons and protons in an axially symmetric model of the solar corona, extending from the coronal base to 15 solar radii. To study heating of electrons and protons separately, as well as the collisional coupling between the particle species, we use a two-fluid description of the electron-proton plasma. A steady coronal heat input, uniform base pressure, and dipole field boundary conditions produce a magnetic field configuration similar to that seen with white-light coronagraphs during quiet-Sun conditions: a helmet streamer is formed in the inner corona around the equator, surrounded by coronal holes at higher latitudes. The plasma inside the helmet streamer is in hydrostatic equilibrium, while in the coronal holes a transonic solar wind is accelerated along the field. The collisional coupling between electrons and protons becomes weak close to the coronal base. In the case of proton heating, the thermal structure along open and closed field lines is very different, and there is a large pressure jump across the streamer-coronal hole boundary. When the equations are integrated on a long timescale, the helmet streamer becomes unstable, and massive plasmoids are periodically released into the solar wind. These plasmoids contribute significantly to the total mass and energy flux in the solar wind. The mass of the plasmoids is reduced when electrons are heated.