The CRaTER Special Issue of Space Weather: Building the observational foundation to deduce biological effects of space radiation

[1] The United States is preparing for exploration beyond low-Earth Orbit (LEO). However, the space radiation environment poses significant risks. The radiation hazard is potentially severe but not sufficiently well characterized to determine if long missions outside LEO can be accomplished with acceptable risk [Cucinotta et al., 2001; Schwadron et al., 2010; Cucinotta et al., 2010]. Radiation hazards may be over- or under-stated through incomplete characterization in terms of net quantities such as accumulated dose. Time-dependent characterization often changes acute risk estimates [NCRP, 1989; Cucinotta, 1999; Cucinotta et al., 2000; George et al., 2002]. For example, events with high accumulated doses but sufficiently low dose rates (/h) pose significantly reduced risks. Protons, heavy ions, and neutrons all contribute significantly to the radiation hazard. However, each form of radiation presents different biological effectiveness. As a result, quality factors and radiation-specific weighting factors are needed to assess biological effectiveness of different forms of radiation [e.g., NCRP 116, 1993] (Figure 1). More complete characterization must account for time-dependent radiation effects according to organ type, primary and secondary radiation composition, and acute effects (vomiting, sickness, and, at high exposures, death) versus chronic effects (such as cancer)

Energy dissipation and ion heating at the heliospheric termination shock

The Los Alamos hybrid simulation code is used to examine heating and the partition of dissipation energy at the perpendicular heliospheric termination shock in the presence of pickup ions. The simulations are one-dimensional in space but three-dimensional in field and velocity components, and are carried out for a range of values of pickup ion relative density. Results from the simulations show that because the solar wind ions are relatively cold upstream, the temperature of these ions is raised by a relatively larger factor than the temperature of the pickup ions. An analytic model for energy partition is developed on the basis of the Rankine-Hugoniot relations and a polytropic energy equation. The polytropic index gamma used in the Rankine-Hugoniot relations is varied to improve agreement between the model and the simulations concerning the fraction of downstream heating in the pickup ions as well as the compression ratio at the shock. When the pickup ion density is less than 20%, the polytropic index is about 5/3, whereas for pickup ion densities greater than 20%, the polytropic index tends toward 2.2, suggesting a fundamental change in the character of the shock, as seen in the simulations, when the pickup ion density is large. The model and the simulations both indicate for the upstream parameters chosen for Voyager 2 conditions that the pickup ion density is about 25% and the pickup ions gain the larger share ( approximately 90%) of the downstream thermal pressure, consistent with Voyager 2 observations near the shock

The radial evolution of solar wind speeds

The WSA-ENLIL model predicts significant evolution of the solar wind speed. Along a flux tube the solar wind speed at 1.0 AU and beyond is found to be significantly altered from the solar wind speed in the outer corona at 0.1 AU, with most of the change occurring within a few tenths of an AU from the Sun. The evolution of the solar wind speed is most pronounced during solar minimum for solar wind with observed speeds at 1.0 AU between 400 and 500 km/s, while the fastest and slowest solar wind experiences little acceleration or deceleration. Solar wind ionic charge state observations made near 1.0 AU during solar minimum are found to be consistent with a large fraction of the intermediate-speed solar wind having been accelerated or decelerated from slower or faster speeds. This paper sets the groundwork for understanding the evolution of wind speed with distance, which is critical for interpreting the solar wind composition observations near Earth and throughout the inner heliosphere. We show from composition observations that the intermediate-speed solar wind (400-500 km/s) represents a mix of what was originally fast and slow solar wind, which implies a more bimodal solar wind in the corona than observed at 1.0 AU

Role of coronal mass ejections in the heliospheric Hale cycle

[1] The 11-year solar cycle variation in the heliospheric magnetic field strength can be explained by the temporary buildup of closed flux released by coronal mass ejections (CMEs). If this explanation is correct, and the total open magnetic flux is conserved, then the interplanetary-CME closed flux must eventually open via reconnection with open flux close to the Sun. In this case each CME will move the reconnected open flux by at least the CME footpoint separation distance. Since the polarity of CME footpoints tends to follow a pattern similar to the Hale cycle of sunspot polarity, repeated CME eruption and subsequent reconnection will naturally result in latitudinal transport of open solar flux. We demonstrate how this process can reverse the coronal and heliospheric fields, and we calculate that the amount of flux involved is sufficient to accomplish the reversal within the 11 years of the solar cycle

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

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

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

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