Rigidity Dependence and Time Response of Cosmic Rays to the Modulation Steps in the Rising Part of Solar Cycle 23. Cospin/ket Results

Previous work on the latitudinal gradient and on the amplitude of the recurrent cosmic ray decreases, has shown that their magnitude does not decrease monotonically with the particle rigidity, but it presents a broad maximun around 1–2 GV. We have extended this analysis to study the behaviour of cosmic-ray particles during the modulation steps in the rising part of the solar activity of the present Solar cycle. We found that the ‘depth’ of the modulation step decreases monotonically with increasing rigidity and that the least energetic particles are the last to reach their minimum intensity value. We also considered in this analysis electrons of equal rigidity to study the influence of the charge sign on the particle behaviour during the modulation steps. This revised version was published online in August 2006 with corrections to the Cover Date.     

Variations of the high energy electron flux along the Ulysses trajectory

We describe for the first time the analysis of high energy electrons (above 240 MeV) from the COSPIN/KET experiment onboard Ulysses. The electron time profiles in four energy windows are presented from Oct. 90 to the end of March 94, up to a maximum heliographic latitude of 57 °S. The recovery rates we derived for the electrons are compared to the recovery rates of positively charged particles with the same rigidity.     

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.     

Corotating Particle Events

The corotating particle events give us a unique opportunity to probe the three-dimensional structures of the heliosphere. This is especially true if we have observations over a period of extreme stability of the CIRs, such as existed over the recent solar minimum. We discuss how the observations fit into the context of current heliospheric magnetic field models. The energetic particle signatures of CIRs throughout the regions of the heliosphere covered by the deep-space missions are reviewed. The CIRs accelerate these particles and at the same time modulate both the high energy galactic cosmic rays and the anomalous cosmic rays.     

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.     

Energetic Particle Observations

The characteristics of solar energetic particles (SEP) as observed in interplanetary space provide fundamental information about the origin of these particles, and the acceleration and propagation processes at the Sun and in interplanetary space. Furthermore, energetic particles provide information on the development and structure of coronal mass ejections as they propagate from the solar corona into the interplanetary medium. In this paper we review the measurements of energetic particles in interplanetary space and discuss their implication for our understanding of the sources, and of acceleration and propagation processes.     

High energy cosmic ray nuclei results on Ulysses: 1. Mission overview

The cosmic ray flux observed with the Kiel Electron Telescope on board the ULYSSES spaceprobe varies with solar activity as well as with heliospheric position. Determination of the latitudinal gradients requires a careful analysis of the influences of the current sheet tilt angle, the number of major solar flares, interplanetary shocks and interaction regions evolving in the expanding solar wind. In this paper we concentrate on nuclei with rigidity above 1 GV. We discuss the effects of the variable solar activity in the declining phase of the present solar cycle and the variation with radial distance as a basis for separating latitudinal effects. We show that during this phase of the solar cycle modulation of GV nuclei is ordered by temporal evolution, radial distance and negligible latitudinal effects even at latitudes between 30° and 50° South.     

Corotating Interaction Regions at High Latitudes

Ulysses observed a stable strong CIR from early 1992 through 1994 during its first journey into the southern hemisphere. After the rapid latitude scan in early 1995, Ulysses observed a weaker CIR from early 1996 to mid-1997 in the northern hemisphere as it traveled back to the ecliptic at the orbit of Jupiter. These two CIRs are the observational basis of the investigation into the latitudinal structure of CIRs. The first CIR was caused by an extension of the northern coronal hole into the southern hemisphere during declining solar activity, whereas the second CIR near solar minimum activity was caused by small warps in the streamer belt. The latitudinal structure is described through the presentation of three 26-day periods during the southern CIR. The first at ∼24°S shows the full plasma interaction region including fast and slow wind streams, the compressed shocked flows with embedded stream interface and heliospheric current sheet (HCS), and the forward and reverse shocks with associated accelerated ions and electrons. The second at 40°S exhibits only the reverse shock, accelerated particles, and the 26-day modulation of cosmic rays. The third at 60°S shows only the accelerated particles and modulated cosmic rays. The possible mechanisms for the access of the accelerated particles and the CIR-modulated cosmic rays to high latitudes above the plasma interaction region are presented. They include direct magnetic field connection across latitude due to stochastic field line weaving or to systematic weaving caused by solar differential rotation combined with non-radial expansion of the fast wind. Another possible mechanism is particle diffusion across the average magnetic field, which includes stochastic field line weaving. A constraint on connection to a distant portion of the CIR is energy loss in the solar wind, which is substantial for the relatively slow-moving accelerated ions. Finally, the weaker northern CIR is compared with the southern CIR. It is weak because the inclination of the streamer belt and HCS decreased as Ulysses traveled to lower latitudes so that the spacecraft remained at about the maximum latitudinal extent of the HCS. This revised version was published online in August 2006 with corrections to the Cover Date.     

Origin, Injection, and Acceleration of CIR Particles: Theory Report of Working Group 7

On the basis of the observational picture established in the report of Mason, von Steiger et al. (1999) the status of theoretical models on origin, injection, and acceleration of particles associated with Corotating Interaction Regions (CIRs) is reviewed. This includes diffusive or first-order Fermi acceleration at oblique shocks, adiabatic deceleration in the solar wind, stochastic acceleration in Alfvén waves and oblique propagating magnetosonic waves, and shock surfing as possible injection mechanism to discriminate pickup ions from solar wind ions. This revised version was published online in August 2006 with corrections to the Cover Date.