The new heliospheric magnetic field: Observational implications

A summary of the new model of the heliospheric magnetic field and its observational implications is presented. We first introduce a global model for the steady-state configuration in the low corona and discuss solar and heliospheric implications of the resulting field configuration. Finally, we compare the effects of this model with random transport of field-lines due to reconnection on the solar surface and to the dynamic turbulent transport of magnetic field-lines. © 1999 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87723/2/87_1.pd

Ubiquitous statistical acceleration in the solar wind

One of the more interesting observations by ACE is the ubiquitous presence of higher energy tails on the distribution functions of solar wind and pickup ions. The tails occur continuously in the slow solar wind, but less so in fast wind. Their presence is not correlated with the passage of shock waves. It is pointed out that statistical acceleration by transit-time damping of propagating magnitude fluctuations in the magnetic field of the solar wind is a likely mechanism to yield the observed tails. © 2000 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87342/2/229_1.pd

Sources, injection and acceleration of heliospheric ion populations

A variety of heliospheric ion populations—from Anomalous Cosmic Rays (ACRs) to particles accelerated in Corotating Interaction Regions (CIRs)—have been observed and studied for several decades. It had been commonly assumed that the solar wind was the source for all of these populations, except for the ACRs, and that shock acceleration produced the energetic particles observed, including the ACRs. For the ACRs the source that had been proposed a long time ago was the interstellar gas that penetrates deep into the heliosphere. Recent measurements of the composition and spectra of suprathermal ions, primarily from Ulysses, ACE and Wind, indicate that pickup ions are likely to be an important source not only of the ACRs but for other heliospheric ion populations as well. In particular, the newly discovered “Inner Source” pickup ions may be a significant source for particles accelerated in the inner heliosphere and may also be the seed material for ACR C, Mg, Si and Fe. Furthermore, the omnipresent suprathermal tails seem to tell us that shock acceleration may not be the primary mechanism energizing particles to ∼0.1 MeV in the heliosphere. Explaining the origin of these persistent high velocity tails remains one of our challenges. © 2000 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87341/2/221_1.pd

Observations of non-thermal properties of heavy ions in the solar wind

Heavy ions in the solar wind are ideal for studying injection processes in the solar wind. We use composition data from Ulysses, ACE, and Wind to examine the properties of heavy ions from thermal energies to several 100 keVs. We show that these particles are observed to gain energy without any association with shocks. This paper provides a survey of recent observations of non-thermal properties of solar wind heavy ions which are consistent with the following picture: At thermal energies coherent wave-particle interactions preferentially heat and accelerate heavy ions with collisional processes limiting subsequent non-thermal properties. At higher energies heavy ion distribution functions are characterized by ubiquitous suprathermal tails. We argue that solar wind heavy ions are a good tracer for acceleration processes which are not directly associated with shocks. These stochastic processes are observed to be relevant for predisposing ions for shock acceleration. © 2000 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87338/2/215_1.pd

Origin of the Solar Wind: Theory

A theory is presented for the origin of the solar wind, which is based on the behavior of the magnetic field of the Sun. The magnetic field of the Sun can be considered as having two distinct components: Open magnetic flux in which the field lines remain attached to the Sun and are dragged outward into the heliosphere with the solar wind. Closed magnetic flux in which the field remains entirely attached to the Sun, and forms loops and active regions in the solar corona. It is argued that the total open flux should tend to be constant in time, since it can be destroyed only if open flux of opposite polarity reconnect, a process that may be unlikely since the open flux is ordered into large-scale regions of uniform polarity. The behavior of open flux is thus governed by its motion on the solar surface. The motion may be due primarily to a diffusive process that results from open field lines reconnecting with randomly oriented closed loops, and also due to the usual convective motions on the solar surface such as differential rotation. The diffusion process needs to be described by a diffusion equation appropriate for transport by an external medium, which is different from the usual diffusion coefficient used in energetic particle transport. The loops required for the diffusion have been identified in recent observations of the Sun, and have properties, both in size and composition, consistent with their use in the model. The diffusive process, in which reconnection occurs between open field lines and loops, is responsible for the input of mass and energy into the solar wind.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43804/1/11214_2004_Article_338798.pd

The inner source for pickup ions

Pickup ions are observed by the Solar Wind Ion Composition Spectrometer on Ulysses which appear to have been picked up close to the Sun. A transport theory for the propagation of these ions is used to constrain the spatial profiles of the ion sources. The composition is like that of the solar wind which suggests that the inner source pickup ions result from solar wind particles that are embedded in dust grains and then released. Through comparison between modeled and observed distributions, it is possible to constrain the radial and latitudinal profiles of the inner source. Inner source protons are also observed and may constitute an energetically important population in the solar wind. © 1999 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87724/2/487_1.pd

Composition variations in fast solar wind streams

The Ulysses spacecraft has now completed its first revolution around the Sun on its nearly-polar orbit. Thereby it has traversed the extended high speed streams from the polar coronal holes (south in 1993/94, north in 1995/96) which were well-developed during that time of close to minimal solar activity. It is evident that the fluctuations of both the kinetic and the compositional parameters are much weaker in the high-speed streams than they are in the slow solar wind, leading Bame to use the term “structure-free” for describing it. It was only the extended time periods Ulysses spent in the polar streams that led to the detection of some structure, the microstreams. From remote observations of the Sun it is clear that the high latitude corona is quite unstructured. The most remarkable features are the polar plumes, which are well detectable because of their higher density and brightness. Also, they are characterized by a difference in composition relative to the coronal hole plasma. These features should in principle be observable in interplanetary space, e.g. by the SWICS mass spectrometer, in the form of abundance variations of heavy ions as well as variations in their charge state composition, which serves as a proxy for the coronal temperature at the site where the stream originated. Using the unique data set of SWICS we examine to what extent polar plumes contribute to fast, coronal hole associated wind. We also study the possible connection between microstreams and polar plumes. © 1999 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87727/2/143_1.pd

On the Slow Solar Wind

A theory for the origin of the slow solar wind is described. Recent papers have demonstrated that magnetic flux moves across coronal holes as a result of the interplay between the differential rotation of the photosphere and the non-radial expansion of the solar wind in more rigidly rotating coronal holes. This flux will be deposited at low latitudes and should reconnect with closed magnetic loops, thereby releasing material from the loops to form the slow solar wind. It is pointed out that this mechanism provides a natural explanation for the charge states of elements observed in the slow solar wind, and for the presence of the First-Ionization Potential, or FIP, effect in the slow wind and its absence in fast wind. Comments are also provided on the role that the ACE mission should have in understanding the slow solar wind.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43787/1/11214_2004_Article_186257.pd

Coronal Hole Boundaries and their Interactions with Adjacent Regions

Coronal hole boundaries are the interfaces between regions where the coronal magnetic field contains a significant component which is open into the heliosphere and regions where the field is primarily closed. It is pointed out that there are constraints on the magnetic field which opens into the heliosphere that must be satisfied in the corona: it must come into pressure equilibrium in the high corona, and the component of the field which connects to the polar regions of the Sun must differentially rotate. A model is presented in which satisfying these constraints determines which field lines are open and which are closed, and thus where the polar coronal hole boundaries occur. Some of the consequences of this model are discussed.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43789/1/11214_2004_Article_233976.pd

The 3-D Heliosphere from the Ulysses and ACE Solar Wind Ion Composition Experiments

The source region of solar wind plasma is observed to be directly reflected in the compositional pattern of both elemental and charge state compositions. Slow solar wind associated with streamers shows higher freeze-in temperatures and larger FIP enhancements than coronal hole associated wind. Also, the variability of virtually all compositional parameters is much higher for slow solar wind compared to coronal hole associated wind. We show that these compositional patterns persist even though stream-stream interactions complicate the identification based on in situ plasma parameters.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43805/1/11214_2004_Article_338816.pd