Cosmic ray modulation in the 3-D heliosphere

The basic physical processes that lead to the long-term modulation of cosmic rays by the solar wind have been known for many years. However our knowledge of the structure of the heliosphere, which determines which processes are most important for the modulation, and of the variation of this structure with time and solar activity level is still incomplete. Study of the modulation provides a tool for probing the scale and structure of the heliosphere. While the Pioneer and Voyager spacecraft are surveying the radial structure and extent of the heliosphere at modest heliographic latitudes, theUlysses mission is the first to undertake a nearly complete scan of the latitudinal structure of the modulated cosmic ray intensity in the inner heliosphere (R<5.4 AU).Ulysses will reach latitudes of 80°S in September 1994 and 80°N in July 1995 during the approach to minimum activity in the 11 year solar cycle. We present a first report of measurements extending to latitudes of 52°S, which show surprisingly little latitudinal effect in the modulated intensities and suggest that at this time modulation in the inner heliosphere may be much more spherically symmetric than had generally been believed based upon models and previous observations.     

Modulation of Galactic Cosmic Rays at Solar Minimum

Cosmic ray particles respond to the heliospheric magnetic field in the expanding solar wind and its turbulence and therefore provide a unique probe for conditions in the changing heliosphere. During the last four years, concentrated around the solar minimum period of solar cycle 22, the exploration of the solar polar regions by the joint ESA/NASA mission Ulysses revealed the three-dimensional behavior of cosmic rays in the inner and middle heliosphere. Also during the last decades, the Pioneer and Voyager missions have greatly expanded our understanding of the structure and extent of the outer heliosphere. Simultaneously, numerical models describing the propagation of galactic cosmic rays are becoming sophisticated tools for interpreting and understanding these observations. We give an introduction to the subject of the modulation of galactic cosmic rays in the heliosphere during solar minimum. The modulation effects on cosmic rays of corotating interaction regions and their successors in the outer heliosphere are discussed in more detail by Gazis, McDonald et al. (1999) and McKibben, Jokipii et al. (1999) in this volume. Cosmic-ray observations from the Ulysses spacecraft at high heliographic latitudes are also described extensively in this volume by Kunow, Lee et al. (1999). This revised version was published online in August 2006 with corrections to the Cover Date.     

Simultaneous Observations of Solar Energetic Particle Events by imp 8 and the Ulysses Cospin High Energy Telescope at High Solar Latitudes

With Ulysses approaching the south solar polar latitudes during a period of high solar activity, it is for the first time possible to study the distribution of solar energetic particles (SEPs) in solar latitude as well as in radius and longitude. From July 1997 to August 2000, Ulysses moved from near the solar equator at ∼5 AU to ∼67° S latitude at ∼3 AU. Using observations of >∼30 MeV protons from Ulysses and IMP-8 at Earth we find good correlation between large SEP increases observed at IMP and Ulysses, almost regardless of the relative locations of the spacecraft. The observations show that within a few days after injection of SEPs, the flux in the inner heliosphere is often almost uniform, depending only weakly on the position of the observer. No clear effect of the increasing solar latitude of Ulysses is evident. Since the typical latitudinal extent of CMEs, which most likely accelerate the SEPs, is only ∼30°, this suggests that the enhanced cross-field propagation for cosmic rays and CIR-accelerated particles deduced from Ulysses’ high latitude studies near solar minimum is also true for SEPs near solar maximum. This revised version was published online in August 2006 with corrections to the Cover Date.     

The Onset of Long Term Modulation in the Heliosphere in Cycle 23

The combination of Voyager 1 (77.9 AU, 34.4° N) and Voyager 2 (61.2 AU, 24.5° S) at moderate heliolatitudes in the distant heliosphere and Ulysses with its unique latitudinal surveys in the inner heliosphere along with IMP 8 and other satellites at 1 AU constitutes a network of observatories that are ideally suited to study cosmic rays over the solar minimum of cycle 22 and the onset of solar activity and the long term cosmic ray modulation of cycle 23. Through 2000.7 there have been three well-defined step decreases in the cosmic ray intensity at 1 AU with the cumulative effect being in good agreement with the net decrease in cycle 21 at a comparable time in the solar cycle. Over this period the intensity changes at Ulysses are similar to those at 1 AU. In the distant heliosphere the initial decreases appear to be smaller than those at 1 AU. However the full effects of the interplanetary disturbances producing the most recent and largest step decrease in the inner heliosphere have not yet reached V-2. This revised version was published online in August 2006 with corrections to the Cover Date.     

Modulation Near Solar Maximum at High Solar Latitudes Observations From the Ulysses Cospin High Energy Telescope

The three-dimensional structure of the solar maximum modulation of cosmic rays in the heliosphere can be studied for the first time by comparing observations from Ulysses at high solar latitudes to those from in-ecliptic spacecraft, such as IMP-8. Observations through mid-2000 show that changes in modulation remain well correlated at Earth and Ulysses up to latitudes of ∼60° south. The observed changes seem to be best correlated with changes in the inclination of the heliospheric current sheet. The spectral index of the proton spectra at energies <100 MeV in the ecliptic and at high latitudes remain roughly consistent with the T ⁺¹ spectrum expected from modulation models, while the spectral index of the helium spectrum at both locations has changed smoothly from the flat or even negative index spectra characteristic of anomalous component fluxes toward the T ⁺¹ galactic spectrum with increasing modulation. Intensities near the equator and at high latitude remain nearly equal, and latitudinal gradients for nucleonic cosmic rays thus remain small (<1% deg⁻¹) at solar maximum. In the most recent data fluxes of protons and helium with energies less than ∼100 MeV nucl⁻¹ measured by Ulysses are smaller than those measured at IMP-8, suggesting that the gradients may have switched to become negative toward the poles even before a clear reversal of polarity for the solar magnetic dipole has been completed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Modulation of Cosmic Rays in the Heliosphere From Solar Minimum to Maximum: a Theoretical Perspective

The modulation of galactic cosmic rays in the heliosphere seems to be dominated by four major mechanisms: convection, diffusion, drifts (gradient, curvature and current sheet), and adiabatic energy losses. In this regard the global structure of the solar wind, the heliospheric magnetic field (HMF), the current sheet (HCS), and that of the heliosphere itself play major roles. Individually, the four mechanisms are well understood, but in combination, the complexity increases significantly especially their evolvement with time - as a function of solar activity. The Ulysses observations contributed significantly during the past solar minimum modulation period to establish the relative importance of these major mechanisms, leading to renewed interest in developing more sophisticated numerical models, and in the underlying physics, e.g., what determines the diffusion tensor. With increased solar activity, the relative contributions of the mentioned mechanisms change, but how they change and what causes these changes over an 11-year solar cycle is not well understood. It can therefore be expected that present and forthcoming observations during solar maximum activity will again produce very important insights into the causes of long-term modulation. In this paper the basic theory of solar modulation is reviewed for galactic cosmic rays. The influence of the Ulysses observations on the development of the basic theory and numerical models are discussed, especially those that have challenged the theory and models. Model-based predictions are shown for what might be encountered during the next solar minimum. Lastly, modulation theory and modelling are discussed for periods of maximum solar activity when a global reorganization of the HMF, and the HCS, occurs. This revised version was published online in August 2006 with corrections to the Cover Date.     

CME-Driven Solar Wind Disturbances at High Heliographic Latitudes

We have identified 20 coronal mass ejections, or CMEs, in the solar wind in the Ulysses data obtained between S30° and S75° during the second polar orbit. Unlike CME-driven disturbances observed at high latitudes during Ulysses’ first polar orbit, these disturbances had plasma and magnetic field characteristics similar to those observed in the ecliptic plane near 1 AU when one allows for evolution with heliocentric distance. Here we provide a brief overview of CME observations at high latitudes both close to and far from the Sun, with emphasis on the recent Ulysses measurements on the rising portion of solar cycle 23. This revised version was published online in August 2006 with corrections to the Cover Date.     

Ulysses out-of-ecliptic observations of “27-day” variations in high energy cosmic ray intensity

We report the discovery that for latitudes above 40°S, the observed recurring modulation of cosmic rays and anomalous nuclei occurs without the detection byUlysses of the solar wind velocity and magnetic field recurring enhancements that have, heretofore at lower latitudes, defined corotating interaction regions—i.e., the mechanism producing the recurring intensity variations >40°S appears to be located beyond the radial range ofUlysses.     

The modulation of cosmic rays in the high latitude heliosphere: A computer simulation

Modulation models based on the numerical solution of Parker's transport equation for galactic cosmic rays in the heliosphere make clear predictions about modulation in the high latitude heliosphere. However, for these predictions certain assumptions have to be made, for example, what the heliospheric magnetic field (HMF) looks like above the solar poles and what the spatial dependence of the diffusion coefficients are. For this presentation the general predictions of a standard drift model for the modulation of cosmic rays in the high latitude heliosphere, in particular predictions for the Ulysses trajectory, are discussed and critically reviewed. Preliminary results from Ulysses show a significant increase in the solar wind speed towards higher latitudes. The effects of this strong latitudinal dependence together with different modifications of the HMF at these high latitudes on the apparently too large diffusion and drifts predicted by current models are also shown.     

The Evolution of the Anomalous Cosmic Ray Oxygen Spectra From 1995 to 1998: Ulysses Observations

Moraal and Steenberg (1999), showed that the peak energy in the anomalous cosmic ray spectra is independent of the radial distance up to a few AU away from the termination shock but dependent on the solar wind speed, the radius of the termination shock and the scattering strength. In this paper we will discuss the variation of the cosmic ray oxygen energy spectrum as measured by the Ulysses EPAC and the COSPIN/LET on board Ulysses. We found that the peak energy decreased from ∼5 MeV nucl⁻¹, when Ulysses was at high northern heliographic latitudes embedded in the fast solar wind to ∼3.5 MeV n⁻¹, in the streamer belt. The shift towards lower energy might also be caused by changing modulation although Voyager measurements indicate no variation of the ACR Oxygen spectrum at ∼60 AU. This revised version was published online in August 2006 with corrections to the Cover Date.     

The Inner Heliosphere - Outer Heliosphere Comparison for Cosmic Ray Modulation

This paper summarizes cosmic ray data on both galactic and anomalous particles in the inner and outer heliosphere near the sunspot minimum in 1995 and 1996 at the end of solar cycle 22. These data come from the IMP spacecraft in the inner heliosphere and the Voyager and Pioneer spacecraft in the outer heliosphere. In the inner heliosphere, the cosmic ray intensities at all energies in 1996 have recovered to almost the same maximum values they had at the last sunspot minimum in 1987 and the intensities are an even closer match to those observed two 11-year cycles earlier in 1976. In the outer heliosphere beyond 40 AU the intensity recovery is very slow and the intensities at all energies and for all species are almost constant in 1995-96 indicating that little further recovery can be expected in this cycle. The intensity of galactic cosmic rays in 1996 is only 0.3-0.5 of that observed at the same radius of 42 AU in 1987 and for anomalous cosmic rays this ratio is only 0.1-0.2. This suggests a dramatically different entry of particles into the heliosphere in the two cycles for both types of particles as well as significantly different particle flow characteristics in the outer heliosphere. The net result of these different characteristics is that near the Earth only a relatively small intensity difference is observed between successive 11-year solar cycles whereas in the outer heliosphere the differences between cycles become very large and may even dominate the overall modulation.     

Cosmic Rays Through the Solar Hale Cycle

The Ulysses spacecraft had been the first to orbit the Sun over its poles and to explore the heliosphere at these high heliolatitudes. It has now completed three fast latitude scans, two at solar minimum and one at solar maximum. Since its launch in October 1990, this mission has led to several surprising discoveries concerning energetic particles, cosmic rays, Jovian electrons, the solar wind, the heliospheric magnetic field and the global features of the heliosphere. This review addresses the propagation and modulation of cosmic rays and other charged particles from an observational point of view with emphasis on what has been learned from exploring the inner heliosphere to high heliolatitudes.     

Cosmic Rays at High Heliolatitudes

The Ulysses spacecraft has been the first to orbit the Sun over its poles and to explore the heliosphere at these high heliolatitudes. It has now completed two fast latitude scans, one at solar minimum and one at solar maximum. Since its launch in October 1990, this mission has led to several surprising discoveries concerning energetic particles, cosmic rays, Jovian electrons, the solar wind, the heliospheric magnetic field and the global features of the heliosphere. This review addresses mainly the propagation and modulation of cosmic rays and other charged particles, from both an observational and theoretical point of view, with emphasis on what has been learned from exploring the inner heliosphere to high heliolatitudes. This is done for solar minimum and maximum conditions. The review is concluded with a summary of the main scientific discoveries and insights gained so far from the Ulysses mission.     

26-day Analysis of Energetic Ion Observations at High and Low Heliolatitudes: Ulysses and ACE

We present observations of energetic (0.34–8 MeV) ions from the Ulysses spacecraft during its second ascent to southern high latitude regions of the heliosphere. We cover the period from January 1999 until mid-2000 as Ulysses moved from 5.2 AU and 18° S to 3.5 AU and 55° S. In contrast to the long-lived and well-defined ∼26-day recurrences that were observed throughout Ulysses‘ first southern pass, energetic ion fluxes during the first portion of the Ulysses’ second polar orbit are highly irregular. Although corotating interaction regions (CIRs) are clearly present in solar wind and magnetic field data throughout the first half of 1999, their effects on energetic ion intensities are quite different from what they were in 1992–1993. No dominant strictly recurrent ion flux increases are observed in association with the arrival of these CIRs. Correspondingly, there is no stable structure of large polar coronal holes during the same period. Isolated transient solar energetic particle (SEP) events are observed at low and high latitudes. We compare energetic ion observations from the ACE and Ulysses spacecraft during the first half of 1999 to determine the influence of these SEP events in the observed recurrent CIR structure. Such SEP events occurred only occasionally during 1992–1993, but when they occurred, they obscured the recurrences in a manner similar to that observed in 1999–2000. We therefore conclude that the basic differences in the behavior of energetic ion events between the first and second southern passes are due to the short life of the corotating structure and the higher frequency of SEP events occurring in 1999–2000. This revised version was published online in August 2006 with corrections to the Cover Date.     

ICMEs at High Latitudes and in the Outer Heliosphere

Interplanetary coronal mass ejections (ICMEs) propagate into the outer heliosphere, where they can have a significant effect on the structure, evolution, and morphology of the solar wind, particularly during times of high solar activity. They are known to play an important role in cosmic ray modulation and the acceleration of energetic particles. ICMEs are also believed to be associated with the large global transient events that swept through the heliosphere during the declining phases of solar cycles 21 and 22. But until recently, little was known about the actual behavior of ICMEs at large heliographic latitudes and large distances from the Sun. Over the past decade, the Ulysses spacecraft has provided in situ observations of ICMEs at moderate heliographic distances over a broad range of heliographic latitudes. More recently, observations of alpha particle enhancements, proton temperature depressions, and magnetic clouds at the Voyager and Pioneer spacecraft have begun to provide comparable information regarding the behavior of ICMEs at extremely large heliocentric distances. At the same time, advances in modeling have provided new insights into the dynamics and evolution of ICMEs and their effects on cosmic rays and energetic particles.     

Cosmic Rays in the Inner Heliosphere: Insights from Observations, Theory and Models

The global modulation of galactic cosmic rays in the inner heliosphere is determined by four major mechanisms: convection, diffusion, particle drifts (gradient, curvature and current sheet drifts), and adiabatic energy losses. When these processes combine to produce modulation, the complexity increases significantly especially when one wants to describe how they evolve spatially in all three dimensions throughout the heliosphere, and with time, as a function of solar activity over at least 22 years. In this context also the global structure and features of the solar wind, the heliospheric magnetic field, the wavy current sheet, and of the heliosphere and its interface with the interstellar medium, play important roles. Space missions have contributed significantly to our knowledge during the past decade. In the inner heliosphere, Ulysses and several other missions have contributed to establish the relative importance of these major mechanisms, leading to renewed interest in developing more sophisticated theories and numerical models to explain these observations, and to understand the underlying physics that determines galactic cosmic ray modulation at Earth. An overview is given of some of the observational and modeling highlights over the past decade.     

Associating the Solar Wind Measured by Ulysses with its Source at the sun

Radio occultation, ultraviolet, and white-light measurements have expanded our knowledge of the morphology of density and velocity in polar coronal holes, and made it possible to carry out the first systematic comparisons between the Ulysses solar wind measurements and quantitative white-light observations of the solar corona. This paper summarizes the rationale and salient features of this new approach which has been used to relate the solar wind observed by Ulysses in 1993–1995 to the inner corona. The statistical characteristics (average, standard deviation, and autocorrelation function) of the Ulysses density measurements of the fast wind are found to be mirrored in those of polarized brightness measurements of path-integrated density made by the High Altitude Observatory (HAO) Mauna Loa K-coronagraph at 1.15 R _⊙. These results reinforce the conclusions from comparisons between measurements of the outer and inner corona. They show that the polar coronal hole extends radially into the solar wind, and that sources of the fast wind are not limited to coronal holes. This revised version was published online in August 2006 with corrections to the Cover Date.     

Heliospheric and Interstellar Phenomena Deduced From Pickup ion Observations

Pickup ions, created by ionization of slow moving atoms and molecules well inside the heliosphere, provide us with a new tool to probe remote regions in and beyond the heliosphere and to study injection and acceleration processes in the solar wind. Comprehensive and continuous measurements of H, He, C, N, O, Ne and other pickup ions, especially with the Solar Wind Ion Composition Spectrometer (SWICS) on both Ulysses and ACE, have given us a wealth of data that have been used to infer chemical and physical properties of the local interstellar cloud. With SWICS on Ulysses we discovered a new population of pickup ions, produced from atomic and molecular sources deep inside the heliosphere. The velocity distributions and composition of these “inner source” pickup ions are distinctly different from those of interstellar pickup ions, showing effects of strong adiabatic cooling, and a composition resembling that of the solar wind. Strong cooling indicates that the source of these pickup ions lies close to the Sun. The similarity of composition of inner source heavy ions to that of the solar wind implies that the dominant production mechanism for these pickup ions involves the absorption and re-emission of solar wind from interplanetary dust grains. While interstellar pickup ions are the seed population of the main Anomalous Cosmic Rays (ACRs), inner source pickup ions may be an important source of the rarer ACRs such as C, Mg, Si, S, and Fe. We present new results and review previous work with an emphasis on characteristics of the local interstellar cloud and properties of the inner source. This revised version was published online in August 2006 with corrections to the Cover Date.     

Langmuir Wave Activity: Comparing the Ulysses Solar Minimum and Solar Maximum Orbits

We examine the occurrence and intensity of Langmuir wave activity (electrostatic waves at the electron plasma frequency) during the solar minimum and solar maximum orbits of Ulysses. At high latitudes during the solar minimum orbit, occurrences of Langmuir waves in magnetic holes were frequent; in the second orbit, they were less common. This difference, in comparison with observations from the first Ulysses fast heliolatitude scan, suggests that Langmuir wave activity in magnetic holes is enhanced in solar wind from polar coronal holes. This revised version was published online in August 2006 with corrections to the Cover Date.     

Three-dimensional Solar Modulation of Cosmic Ray and Anomalous Components in the Inner Heliosphere

Our picture of modulation in the inner heliosphere has been greatly affected by observations from the Ulysses mission, which since 1992 has provided the first comprehensive exploration of modulation as a function of latitude from 80° S to 80° N heliographic latitude. Among the principal findings for the inner heliosphere are: a) the cosmic ray intensity depends only weakly on heliographic latitude; b) for the nuclear components, and especially for the anomalous components, the intensity increases towards the poles, qualitatively consistent with predictions of drift models for the current sign of the solar magnetic dipole; c) no change in the level of modulation was observed across the shear layer separating fast polar from slow equatorial solar wind near 1 AU; d) 26-day recurrent variations in the intensity persist to the highest latitudes, even in the absence of clearly correlated signatures in the solar wind and magnetic field; e) the surface of symmetry of the modulation in 1994-95 was offset about 10° south of the heliographic equator; f) the intensity of electrons and of low energy (< 100 MeV) protons showed essentially no dependence on heliographic latitude.