Harm Moraal

The cosmic radiation in the heliosphere at successive solar minima

Comparative cosmic ray studies at the time of successive solar minima are of special importance in establishing the relative role of large-scale drift effects in the modulation process and in estimating the size of the modulation region and the local interstellar spectra of low- and medium-energy cosmic rays. In this paper the 1 AU cosmic ray observations are compared for the last three solar minimum periods along with the 1977/1978 and 1987 Pioneer 10 and Voyager 1 and 2 data from the outer heliosphere. There is good agreement between the 1965 and 1987 galactic cosmic ray H and He spectra at 1 AU. However, there are significant and complex differences between the 1977/1978 and 1987 measurements of the galactic and anomalous cosmic ray components at 1 and 15 AU. In the outer heliosphere there are large negative latitudinal gradients that reach their maximum magnitude when the inclination of the outer heliosphere current sheet is at a minimum. The radial gradients decrease with heliocentric distance as ∼1/r0.7 and do not differ significantly at the successive solar minima. While these effects, along with the shift in the intensity maximum of anomalous helium (He+), are in the direction expected from large-scale gradient and curvature drifts in the heliosphere, they are about an order of magnitude smaller than those calculated using the standard Parker model of the interplanetary magnetic field. The measured radial and latitudinal gradients are used to estimate the particle transport parameters in the outer heliosphere. The slope of the H spectra measured at Pioneer 10 for both solar minimum periods is given by γH ≈ 0.5 down to energies of 40 MeV. Using the local interstellar He spectrum of Webber et al. (1987), it is estimated that the modulation boundary is of the order of 160 AU.

The cosmic radiation in the heliosphere at successive solar minima: 3. Steady state drift solutions of the transport equation

Cosmic ray intensities and density gradients, as observed by the Pioneer 10 and 11, Voyager 1 and 2, and IMP 8 spacecraft during the 1977 and 1987 solar minimum periods, are interpreted in terms of a two-dimensional version of the cosmic ray transport equation that includes drifts. This paper is a follow-up of an earlier no-drift version to elucidate the necessity and limitations of drift effects as demanded by actual observations.

Measurement and simulation of neutron monitor count rate dependence on surrounding structure

Neutron monitors are the premier instruments for precise measurements of time variations (e.g., of solar origin) in the galactic cosmic ray (GCR) flux in the range of ∼1–100 GeV. However, it has proven challenging to accurately determine the yield function (effective area) versus rigidity in order to relate a neutron monitors count rate with those of other monitors worldwide and the underlying GCR spectrum. Monte Carlo simulations of the yield function have been developed, but there have been few opportunities to validate these observationally, especially regarding the particular environment surrounding each monitor. Here we have precisely measured the count rate of a calibration neutron monitor near the Princess Sirindhorn Neutron Monitor (PSNM) at Doi Inthanon, Thailand (18.59∘N, 98.49∘E, 2560 m altitude), which provides a basis for comparison with count rates of other neutron monitors worldwide that are similarly calibrated. We directly measured the effect of surrounding structure by operating the calibrator outside and inside the building. Using Monte Carlo simulations, we clarify differences in response of the calibrator and PSNM, as well as the calibrator outside and inside the building. The dependence of the calibrator count rate on surrounding structure can be attributed to its sensitivity to neutrons of 0.5–10 MeV and a shift of sensitivity to nucleons of higher energy when placed inside the building. Simulated calibrator to PSNM count rate ratios inside and outside agree with observations within a few percent, providing useful validation and improving confidence in our ability to model the yield function for a neutron monitor station.

THE HIGH-ENERGY IMPULSIVE GROUND-LEVEL ENHANCEMENT

We have studied short-lived (21 minute average duration), highly anisotropic pulses of cosmic rays that constitute the first phase of 10 large ground-level enhancements (GLEs), and which extend to rigidities in the range 5‐20 GV. We provide a set of constraints that must be met by any putative acceleration mechanism for this type of solar-energetic-particle (SEP) event. The pulses usually have very short rise-times (three to five minutes) at all rigidities, and exhibit the remarkable feature that the intensity drops precipitously by 50% to 70% from the maximum within another three to five minutes. Both the rising and falling phases exhibit velocity dispersion, which indicates that there are particles with rigidities in the range 1 90 MeV gamma-ray bursts, indicating that freshly accelerated SEPs had impinged on higher-density matter in the chromosphere prior to the departure of the SEP pulse for Earth. This study was based on an updated archive of the 71 GLEs in the historic record, which is now available for public use.

Form of the anomalous cosmic ray spectrum at the solar wind termination shock

New insights regarding the form of the accelerated anomalous cosmic ray spectrum at the solar wind termination shock, obtained from solutions of the cosmic ray transport equation, are presented. A simple analytical expression for the spectrum on the shock is derived, and its dependence on the acceleration parameters is shown. Particular attention is paid to the high-energy cutoff of this spectrum. This expression and its parameterization should be applicable for the acceleration of charged particles in any spherical shock.

Rapid fluctuations of stratospheric electric field following a solar energetic particle event

[1] During January, 2005, there were several large X-class solar flares and associated solar energetic particle (SEP) events. Coincidentally, the MINIS balloon campaign had multiple payloads aloft in the stratosphere above Antarctica measuring dc electric fields, conductivity and x-ray flux. One-to-one increases in the electrical conductivity and decreases to near zero of both the vertical and horizontal electric field components were observed in conjunction with an increase in particle flux at SEP onset. Combined with an atmospheric electric field mapping model, these data are consistent with a shorting out of the global electric circuit and point toward substantial ionospheric convection modifications. Additionally, two subsequent, rapid changes were detected in the vertical electric field component several hours after SEP onset. These changes result in similar fluctuations in the calculated vertical current density. We will describe how rigidity cut-off dynamics may be crucial in understanding these sudden jumps in the vertical electric field.

GLOBAL PROCESSES THAT DETERMINE COSMIC RAY MODULATION

The global processes that determine cosmic ray modulation are reviewed. The essential elements of the theory which describes cosmic ray behavior in the heliosphere are summarized, and a series of discussions is presented which compare the expectations of this theory with observations of the spatial and temporal behavior of both galactic cosmic rays and the anomalous component; the behavior of cosmic ray electrons and ions; and the 26-day variations in cosmic rays as a function of heliographic latitude. The general conclusion is that the current theory is essentially correct. There is clear evidence, in solar minimum conditions, that the cosmic rays and the anomalous component behave as is expected from theory, with strong effects of gradient and curvature drifts. There is strong evidence of considerable latitude transport of the cosmic rays, at all energies, but the mechanism by which this occurs is unclear. Despite the apparent success of the theory, there is no single choice for the parameters which describe cosmic ray behavior, which can account for all of the observed temporal and spatial variations, spectra, and electron vs. ion behavior.

Cosmic ray energy changes at the termination shock and in the heliosheath

Voyager 1 crossed the termination shock of the solar wind in December 2004 at 94 AU and currently measures the cosmic ray intensity in the heliosheath. To better understand this modulation region beyond the shock, where adiabatic energy changes should be small, we review the net effect of energy changes during the modulation process, including adiabatic deceleration in the solar wind, acceleration at the termination shock, and the possibility that stochastic acceleration in the heliosheath may also make a contribution.

The Effect of Diffusion on the Particle Spectra in Pulsar Wind Nebulae

A possible way to calculate particle spectra as a function of position in pulsar wind nebulae is to solve a Fokker-Planck transport equation. This paper presents numerical solutions to the transport equation with the processes of convection, diffusion, adiabatic losses, and synchrotron radiation included. In the first part of the paper, the steady-state version of the transport equation is solved as a function of position and energy. This is done to distinguish the various effects of the aforementioned processes on the solutions to the transport equation. The second part of the paper deals with a time-dependent solution to the transport equation, specifically taking into account the effect of a moving outer boundary. The paper highlights the fact that diffusion can play a significant role in reducing the amount of synchrotron losses, leading to a modification in the expected particle spectra. These modified spectra can explain the change in the photon index of the synchrotron emission as a function of position. The solutions presented in this paper are not limited to pulsar wind nebulae, but can be applied to any similar central source system, e.g., globular clusters.

The influence of cosmic-ray modulation at high heliospheric latitudes on the solar diurnal variation observed at earth

During the solar minimum period of 1954 the cosmic-ray diurnal variation as observed by neutron monitors and muon telescopes underwent a dramatic swing in its direction of maximum intensity, from the normal value between 16:00 and 18:00 local time to as early as 08:00. It is shown that this swing can be explained as being due to a negative radial density gradient of cosmic rays in the inner heliosphere and that this negative gradient is caused by large radial and latitudinal diffusion mean free paths that bring in particles from high latitudes. In principle, such large diffusion mean free paths should simultaneously cause high intensities, as were observed in 1954.