Modeling ground and space based cosmic ray observations

After entering our local astrosphere (called the heliosphere), galactic cosmic rays, as charged particles, are affected by the Sun’s turbulent magnetic field. This causes their intensities to decrease towards the inner heliosphere, a process referred to as modulation. Over the years, cosmic ray modulation has been studied extensively at Earth, utilizing both ground and space based observations. Moreover, modelling cosmic ray modulation and comparing results with observations, insight can be gained into the transport of these particles, as well as offering explanations for observed features. We review some of the most prominent cosmic ray observations made near Earth, how these observations can be modelled and what main insights are gained from this modelling approach. Furthermore, a discussion on drifts, as one of the main modulation processes, are given as well as how drift effects manifest in near Earth observations. We conclude by discussing the contemporary challenges, fuelled by observations, which are presently being investigated. A main challenge is explaining observations made during the past unusual solar minimum.     

The physics of cosmic-ray modulation

The theory of the modulation of galactic cosmic rays by the solar wind is reviewed. The basic transport equation is presented, interpreted and then applied to cosmic-ray transport in a model heliosphere immersed in a constant uniform bath of galactic cosmic rays. The results of numerical modelling are presented and the dominant physical effects analyzed. A variety of observational tests of the model which were reported over the last several years are summarized and shown, generally, to support a model in which particle drifts play an important role. Recent measurements which show that the latitudinal gradient of cosmic rays changed sign in the recent sunspot minimum (relative the last sunspot minimum) are shown to provide additional, strong, support for the model. A new picture of the interplanetary magnetic field is presented, which gives promise of improving considerably the agreement between the theory and observations in the few remaining problem areas.     

The transport of galactic and jovian cosmic ray electrons in the heliosphere

An overview is given on what we know about the cosmic ray diffusion process from the modelling of low-energy (MeV) electron transport in the heliosphere. For energies below ∼300 MeV, these electrons give a direct indication of the average mean free paths because they do not experience large adiabatic energy changes and their modulation is largely unaffected by global gradient and curvature drifts. Apart from galactic cosmic ray electrons, the jovian magnetosphere at ∼5 AU in the ecliptic plane is also a relatively strong source of MeV electrons, with energies up to ∼30 MeV. Therefore, when modelling the transport of these particles in the inner heliosphere, a three-dimensional treatment is essential. By comparing these models to observations from the Ulysses, Pioneer and Voyager missions, important conclusions can be made on e.g., the relative contributions of the galactic and jovian electrons to the total electron intensity, the magnitude of the parallel and perpendicular transport coefficients, and the time dependant treatment thereof.     

Fluctuations of cosmic rays and IMF in the vicinity of interplanetary shocks

Fluctuations of cosmic rays and interplanetary magnetic field upstream of interplanetary shocks are studied using data of ground-based polar neutron monitors as well as measurements of energetic particles and solar wind plasma parameters aboard the ACE spacecraft. It is shown that coherent cosmic ray fluctuations in the energy range from 10 keV to 1 GeV are often observed at the Earth’s orbit before the arrival of interplanetary shocks. This corresponds to an increase of solar wind turbulence level by more than the order of magnitude upstream of the shock. We suggest a scenario where the cosmic ray fluctuation spectrum is modulated by fast magnetosonic waves generated by flux of low-energy cosmic rays which are reflected and/or accelerated by an interplanetary shock.     

Propagation modes of energetic charged particles in the heliosphere

Several years ago, the anisotropic diffusion and convective transport accompanied by adiabatic deceleration were considered as the principal means for cosmic ray propagation. Particles of relatively small energies (～ 1 MeV) can propagate along the force lines of the magnetic field without scattering at distances of several astronomical units in the quiet heliosphere. The theory describing the 11-year variation of galactic cosmic ray intensity and the propagation of solar cosmic rays was founded on this basis. However, the anomalies of the 11-year variation of galactic cosmic ray intensity in 1969–1971 revealed the necessity to take into account the influence of the general electromagnetic field of the heliosphere giving rise to a rapid magnetic drift of particles. The particles drift either from the magnetic axis to the ecliptic plane (in the cycle of 1969–1980) or in the opposite direction depending on the sign of the general magnetic field of the sun. The neutral layers along which the drift velocity is comparable to the particle velocity is of great significance. However, in the presence of sector structure, the time of particle propagation along the neutral layer from the boundary of the modulation region to the earth orbit is substantially increased. Thus a marked adiabatic deceleration is here possible. The time delay observed in the recovery of proton intensities at various energies can be explained in terms of a transient phase of the interplanetary field following the polarity reversal.     

Cosmic-ray modulation and the heliospheric magnetic field

The heliospheric magnetic field plays a key role in any model for the modulation of cosmic rays. It enters into all diffusion coefficients, and its magnitude, spatial gradient and direction determine drifts patterns of cosmic rays in the heliosphere. While the first axisymmetric model of E.N. Parker proved quite successful to explain in situ measurements in the ecliptic plane, new insight into the origin and the nature of the field, especially at high heliographic latitudes, has led to the development of complex fully-three-dimensional, time-dependent models. In this review, we discuss a selection of models for the heliospheric magnetic field, and discuss how some of the more recent Fisk-type models affect the modulation of cosmic rays.     

Is there an instrumental drift in the counting rate of some high latitude neutron monitors?

For the last six decades the neutron monitors have provided a continuous string of very reliable data to the heliophysics community. Although neutron monitors are not the primary source of data for the galactic cosmic rays, these data serve as a baseline reference for the data collected by the detectors on board the satellites and deep space probes, far away from earth orbit. The pressure corrected hourly data are available from the World Data Centers. These data have been used to derive deep insights pertaining to the electromagnetic states of the heliosphere and the modes of transport of energetic charged particles in the tangled interplanetary magnetic fields. We present evidence that some of the high latitude neutron monitors are undergoing long-term drifts in their baselines. In particular, we argue that there is no physical basis to justify the observed long-term downward trend in the baseline of the South Pole neutron monitor. The real reason may have to do with its maintenance at a distant location with challenging logistics and an improper normalization of its data after the 26 months break in the 1970s.     

New insights from cross-correlation studies between solar activity indices and cosmic-ray flux during Forbush decrease events

Observed galactic cosmic ray intensity can be subjected to a transient decrease. These so-called Forbush decreases are driven by coronal mass ejection induced shockwaves in the heliosphere. By combining in situ measurements by space borne instruments with ground-based cosmic ray observations, we investigate the relationship between solar energetic particle flux, various solar activity indices, and intensity measurements of cosmic rays during such an event. We present cross-correlation study done using proton flux data from the SOHO/ERNE instrument, as well as data collected during some of the strongest Forbush decreases over the last two completed solar cycles by the network of neutron monitor detectors and different solar observatories. We have demonstrated connection between the shape of solar energetic particles fluence spectra and selected coronal mass ejection and Forbush decrease parameters, indicating that power exponents used to model these fluence spectra could be valuable new parameters in similar analysis of mentioned phenomena. They appear to be better predictor variables of Forbush decrease magnitude in interplanetary magnetic field than coronal mass ejection velocities.     

Modeling the acceleration and modulation of anomalous cosmic ray oxygen

After the solar wind termination shock crossings of the Voyager spacecraft, the acceleration of anomalous cosmic rays has become a very contentious subject. In this paper we examine several topics pertinent to anomalous cosmic ray oxygen acceleration and transport using a numerical cosmic ray modulation model. These include the effects of drifts on a purely Fermi I accelerated spectra, the effects of introducing higher charge states of oxygen into the modulation model, examining the viability of momentum diffusion as a re-acceleration process in the heliosheath and examining energy spectra, and intensity gradients, in the inner heliosphere during consecutive drift cycles.     

The composition and distribution of anomalous cosmic rays

The composition of anomalous cosmic rays (ACR), is thought to reflect that of the neutral atoms in the very local interstellar medium, such as helium, nitrogen and neon. Recent observations in the outer heliosphere have provided the first unambiguous evidence for ACR argon, carbon and hydrogen, as well, and a method has been developed to relate the ACR abundances to those of the interstellar medium. The observations also indicate persistent negative latitudinal gradients, opposite to that observed by Pioneer 11 during the previous minimum in solar activity. These and other results are consistent with the presence of gradient and curvature drift during solar minimum periods when the tilt of the interplanetary neutral sheet is small.     

Cosmic ray modulation in the outer heliosphere: Predictions for cosmic ray intensities up to the heliopause along Voyager 1 and 2 trajectories

Time dependent cosmic ray modulation in the outer heliosphere is calculated and results are compared to Voyager 1 and 2 observations using a two-dimensional time-dependent cosmic ray transport model. We predict possible future 133–242 MeV proton observations along the Voyager 1 and 2 spacecraft trajectories. Recent theoretical advances in cosmic ray transport parameters are introduced in order to provide a time-dependence for the assumed transport parameters used in the model. This leads to results that are in general compatible with the spacecraft observations in the inner and outer heliosphere over multiple solar cycles. However, for the outer heliosphere, we find that the Voyager 1 and 2 spacecraft observations cannot be fitted with an identical set of parameters along both trajectories. This indicates a possible asymmetric heliosphere or a symmetric heliosphere but with different diffusion parameters in the northern and southern hemispheres, respectively. Furthermore, results indicate that Voyager 2 observations are still under the influence of solar cycle related changes because of the large modulation volume between the heliopause and spacecraft location in contrast to Voyager 1 which shows a steady increase in cosmic ray intensities.     

Radial distribution of galactic cosmic rays at solar maximum

In this paper we analyze the spatial distribution of galactic cosmic rays during periods of maximum solar activity of the cycles 21, 22 and 23. We have used a two dimensional model to solve the cosmic ray transport equation. This model includes all relevant physical processes: diffusion, convection, drift and shock effects on cosmic ray propagation inside the heliosphere. We focused on the study of the radial distribution of galactic cosmic rays, and compare our results with the spacecraft observations for two energies (175 MeV H and 265 MeV/n He). Although the radial intensities of galactic cosmic rays can be explained qualitatively with all three local interstellar spectra (LISs) used in this work, we applied a reduced chi-squared analysis to investigate the best LIS that could fit the data.     

Fast heavy ions in the heliosphere

The study of heavy ions in space can provide a greater understanding of contemporary nucleosynthesis in our galaxy, as well as the acceleration and propagation of energetic particles in the interplanetary medium and the magnetosphere. We describe here a satellite experiment that uses plastic track detectors to study the age of cosmic-ray source material and the distribution of pathlengths over which the heaviest cosmic rays travel to earth. The experiment will also search for the singly charged particles, thought to make up the anomalous component, and for low-energy heavy ions deep in the magnetosphere.     

Interplanetary Physics in Mainland China

During the past two years (2016-2018), great achievements have been made in the Chinese research of interplanetary physics, with nearly 100 papers published in the academic journals. The achievements are including but not limited to the following topics:solar corona; solar wind and turbulence; filament/prominence and jets; solar flare; radio bursts; particle acceleration at coronal shocks; magnetic flux ropes; instability; instrument; Coronal Mass Ejections (CMEs) and their interplanetary counterparts; Magnetohydrodynamic (MHD) numerical modeling; solar energetic particles and cosmic rays. The progress further improves our understanding of the eruptions of solar activities, their evolutions and propagations in the heliosphere, and final geoeffects on our Earth. These results were achieved by the Chinese solar and space scientists independently or via international collaborations. This paper will give a brief review of these achievements.      

Improved environment radiation models

The two components of the space radiation environment, galactic cosmic rays and solar energetic particles, are of special importance for the planning of space missions and designing space vehicles for flights in the inner heliosphere. There is a constant need for developing and updating the models for calculating the fluxes of these particles for purposes of forecasting radiation conditions anticipated for future flights, including missions to the Moon and Mars.     

The Living With a Star (LWS) Sentinels

This paper gives a short account of the development of the living with a star Sentinels element. The Sentinels element is the heliospheric portion of the NASA program focused on improving our understanding of geo-effective events in the Sun–Earth connected system. The primary objectives of Sentinels are the investigation of the initiation and evolution of solar transients in the inner heliosphere, the acceleration and propagation of solar energetic particles, and the long term climatic change and structure of the inner heliosphere into which all of these geo-effective structures erupt. Due to the large volume of space to be covered, Sentinels will have to rely on, besides a dedicated mission, the observations of spacecraft from other programs of NASA and international partners along with a robust theoretical and modeling effort. The upcoming Sentinels Science and Technology Definition Team will develop specific details of this element.     

Modeling the time-intensity profile of solar flare generated particle fluxes in the inner heliosphere.

It is possible to model the time-intensity profile of solar particles expected in space after the occurrence of a significant solar flare on the sun. After the particles are accelerated in the flare process, if conditions are favorable, they may be released into the solar corona and then into space. The heliolongitudinal gradients observed in the inner heliosphere are extremely variable, reflecting the major magnetic structures in the solar corona which extend into space. These magnetic structures control the particle gradients in the inner heliosphere. The most extensive solar particle measurements are those observed by earth-orbiting spacecraft, and forecast and prediction procedures are best for the position of the earth. There is no consensus of how to extend the earth-based models to other locations in space. Local interplanetary conditions and structures exert considerable influence on the time-intensity profiles observed. The interplanetary shock may either reduce or enhance the particle intensity observed at a specific point in space and the observed effects are very dependent on energy.     

Cosmic rays in the dynamic heliosheath

Cosmic ray modulation in the outer heliosphere is discussed from a modeling perspective. Emphasis is on the transport and acceleration of these particles at and beyond the solar wind termination shock in the inner heliosheath region and how this changes over a solar cycle. We will show that by using numerical models, and by comparing results to spacecraft observations, much can be learned about the dependence of cosmic ray modulation on solar cycle changes in the solar wind and heliospheric magnetic field. While the first determines the heliospheric geometry and shock structure, the latter results in a time-dependence of the transport coefficients. Depending on energy, both these effects contribute to cosmic ray intensities in the inner heliosheath changing over a solar cycle.