Direct observation of charge state abundances of energetic He,C, O,and Fe emitted in solar flares

The ionic charge states of helium, carbon, oxygen, and iron have been determined for three solar particle enhancements by an electrostatic deflection analyzer, which is combined with a thin window proportional counter dE/dX vs. E system. The observations are obtained during the periods September 23 to 29, 1978, June 6 to 8, 1979, and September 15 to 26, 1979, with an instrument onboard the ISEE-3 spacecraft. The mean ionic charge states for He, C, and oxygen exhibit a high degree of ionization with values of Q = 2, 6, and 7.2, respectively. The charge state of iron is near 13 charge units. Variations from flare to flare and within the September 23, 1978 flare are small. The most surprising feature of the charge state measurement is the observation of a small (～10%) but finite contribution of singly ionized helium.     

Prediction of energetic particle behavior at high solar latitudes

Emphasis is placed on predictions of the galactic cosmic-ray flux at high heliographic latitudes. Recent work on gradient and curvature drifts in the large-scale heliospheric magnetic field have modified the traditional argument that the cosmic rays should be essentially unmodulated over the solar poles. In fact, drift effects during the next solar cycle, when the International Solar Polar Mission is to fly, are predicted to cause considerable modulation in the polar regions. However, it is pointed out that the use of conventional drift formulae in the solar wind, which contains magnetic-field fluctuations with large amplitudes and perhaps systematic properties, is highly suspect. Prediction of cosmic-ray behavior over the solar poles is thus currently uncertain. Also considered is the behavior of the anomalous cosmic-ray component at high heliographic latitudes.     

Exploring the heliosphere in three dimensions a keynote presentation

The 28th ESLAB Symposium marks the beginning of the sprint of the Ulysses mission to the very high heliographic latitudes and the pole-to-pole passage. The more than twenty-year quest to understand the Sun and the heliosphere in three dimensions is about to be realized. It is perhaps worthwhile, as we are poised to begin this journey, to ask how history is likely to judge this mission, or equivalently: what questions need to be answered so that the judgment will be kind?     

Particle Acceleration in the Heliosphere: Implications for Astrophysics

There has been a remarkable discovery concerning particles that are accelerated in the solar wind. At low energies, in the region where the particles are being accelerated, the spectrum of the accelerated particles is always the same: when expressed as a distribution function, the spectrum is a power law in particle speed with a spectral index of ?5, and a rollover at higher particle speeds that can often be described as exponential. This common spectral shape cannot be accounted for by any conventional acceleration mechanism, such as diffusive shock acceleration or traditional stochastic acceleration. It has thus been necessary to invent a new acceleration mechanism to account for these observations, a pump mechanism in which particles are pumped up in energy through a series of adiabatic compressions and expansions. The conditions under which the pump acceleration is the dominant acceleration mechanism are quite general and are likely to occur in other astrophysical plasmas. In this paper, the most compelling observations of the ?5 spectra are reviewed; the governing equation of the pump acceleration mechanism is derived in detail; the pump acceleration mechanism is applied to acceleration at shocks; and, as an illustration of the potential applicability of the pump acceleration mechanism to other astrophysical plasmas, the pump mechanism is applied to the acceleration of galactic cosmic rays in the interstellar medium.     

Element and Isotopic Fractionation in Closed Magnetic Structures

Recent papers have suggested that the slow solar wind is a super-position of material which is released by reconnection from large coronal loops. This reconnection process is driven by large-scale motions of solar magnetic flux driven by the non-radial expansion of the solar wind from the differentially rotating photosphere into more rigidly rotating coronal holes. The elemental composition of the slow solar wind material is observed to be fractionated and more variable than the fast solar wind from coronal holes. Recently, it has also been reported that fractionation also occurs in 3He/4He. This may be interpreted in the frame-work of an existing model for fractionation on large coronal loops in which wave-particle interactions preferentially heat ions thereby modifying their scale-heights. This revised version was published online in June 2006 with corrections to the Cover Date.     

Confidence Limits for the Pointing Error of Gimbaled Sensors

The confidence limits for the pointing error of a gimbaled sensor relative to an off-gimbal reference are derived in this paper. The various misalignment angles and errors that lead to discrepancies between the indicated and the actual pointing direction are discussed in detail. A mathematical model is developed from which the statistics of the pointing error are derived. The confidence limits are obtained for a variety of cases.     

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. This revised version was published online in August 2006 with corrections to the Cover Date.     

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. This revised version was published online in June 2006 with corrections to the Cover Date.     

On compositional variations of heavy ions during solar particle events

Intensity-time profiles of protons, alpha particles, and heavy ions (C, O, Fe) in the MeV/nucleon energy range have been analyzed for one solar particle event following the solar flare on September 23, 1978. The data have been obtained with the wide angle double dE/dx-E sensor of the Max-Planck-Institut/University of Maryland experiment onboard ISEE-3. We found time variations in the iron to helium ratio of up to 2 orders of magnitude and a significant variation of the O/He ratio during this event, whereas the C/O-ratio at the same energy/nucleon appears to be time independent. We investigated the influence of a rigidity dependent mean free path in interplanetary space and of rigidity dependent coronal propagation on heavy ion ratios during solar particle events. We found that both the magnitude and time scale of the ratio changes during the September 23 event cannot be explained by rigidity dependent interplanetary or coronal propagation alone. These ratio changes are probably caused by multiple injection at the sun.     

Implications of a weak termination shock

Recent observations from the Voyager spacecraft have suggested that the spectrum of the anomalous cosmic ray component is relatively steep at the termination shock, which is believed to be responsible for accelerating these particles. This conclusion argues that the termination shock must be weak, which in turn requires that the upstream Mach number in the solar wind must be quite low, 2.4. It is pointed out that such conditions are unlikely to prevail at all locations along the shock front. However, it is possible for such conditions to exist at the interface between high speed streams at high heliographic latitudes and the region at low latitudes where high and low speed streams have interacted and come into equilibrium. This discussion suggests a preferred location for the injection of the anomalous component into the shock acceleration process.