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In this work, more than a dozen solar energetic particle (SEP) events are identified where the source region is magnetically well-connected to at least one spacecraft at 1~au. The observed intensity-time profiles, for all available proton and electron energy channels, are compared to results computed using a numerical 1D SEP transport model in order to derive the parallel mean free paths (pMFPs) as a function of energy (or rigidity) at 1~au. These inversion results are then compared to theoretical estimates of the pMFP, using observed turbulence quantities with observationally-motivated variations as input. For protons, a very good comparison between inversion and theoretical results is obtained. It is shown that the observed inter-event variations in the inversion pMFP values can be explained by natural variations in the background turbulence values. For electrons, there is relatively good agreement with pMFPs derived assuming the damping model of dynamical turbulence, although the theoretical values are extremely sensitive to the details of the turbulence dissipation range which themselves display a high level of variation., Comment: Accepted to ApJ

In this paper we explore the idea of using multi-spacecraft observations of Jovian electrons to measure the 3D distribution of these particles in the inner heliosphere. We present simulations of Jovian electron intensities along selected spacecraft trajectories for 2021 and compare these, admittedly qualitatively, to these measurements. Using the data-model comparison we emphasize how such a study can be used to constrain the transport parameters in the inner heliosphere, and how this can lead to additional insight into energetic particle transport. Model results are also shown along the expected trajectories of selected spacecraft, including the off-ecliptic phase of the Solar Orbiter mission from 2025 onward. Lastly, we revisit the use of historical data and discuss upcoming missions that may contribute to Jovian electron measurements., Comment: Accepted for publication in ApJ

Solar radio emissions provide several unique diagnostics to estimate different physical parameters of the solar corona, which are otherwise simply inaccessible. However, imaging the highly dynamic solar coronal emissions spanning a large range of angular scales at radio wavelengths is extremely challenging. At GHz frequencies, MeerKAT radio telescope is possibly globally the best-suited instrument at present for providing high-fidelity spectroscopic snapshot solar images. Here, we present the first published spectroscopic images of the Sun made using the observations with MeerKAT in the 880-1670 MHz band. This work demonstrates the high fidelity of spectroscopic snapshot MeerKAT solar images through a comparison with simulated radio images at MeerKAT frequencies. The observed images show extremely good morphological similarities with the simulated images. Our analysis shows that below ~900 MHz MeerKAT images can recover essentially the entire flux density from the large angular scale solar disc. Not surprisingly, at higher frequencies, the missing flux density can be as large as ~50%. However, it can potentially be estimated and corrected for. We believe once solar observation with MeerKAT is commissioned, it will enable a host of novel studies, open the door to a large unexplored phase space with significant discovery potential, and also pave the way for solar science with the upcoming Square Kilometre Array-Mid telescope, for which MeerKAT is a precursor., Comment: Published at the Astrophysical Journal, 15 pages, 9 figures

The processes responsible for the acceleration of solar energetic particles (SEPs) are still not well understood, including whether SEP electrons and protons are accelerated by common or separate processes. Using a numerical particle transport model that includes both pitch-angle and perpendicular spatial diffusion, we simulate, amongst other quantities, the onset delay for MeV electrons and protons and compare the results to observations of SEPs from widely-separated spacecraft. Such observations have previously been interpreted, in a simple scenario assuming no perpendicular diffusion, as evidence for different electron and proton sources. We show that, by assuming a common particle source together with perpendicular diffusion, we are able to simultaneously reproduce the onset delays for both electrons and protons. We argue that this points towards a common accelerator for these particles. Moreover, a relatively broad particle source is required in the model to correctly describe the observations. This is suggestive of diffusive shock acceleration occurring at large shock structures playing a significant role in the acceleration of these SEPs., Comment: Accepted to ApJ

Context. A solar eruption on 17 April 2021 produced a widespread Solar Energetic Particle (SEP) event that was observed by five longitudinally well-separated observers in the inner heliosphere at heliocentric distances of 0.42 to 1 au: BepiColombo, Parker Solar Probe, Solar Orbiter, STEREO A, and near-Earth spacecraft. The event produced relativistic electrons and protons. It was associated with a long-lasting solar hard X-ray flare and a medium fast Coronal Mass Ejection (CME) with a speed of 880 km/s driving a shock, an EUV wave as well as long-lasting radio burst activity showing four distinct type III burst. Methods. A multi-spacecraft analysis of remote-sensing and in-situ observations is applied to attribute the SEP observations at the different locations to the various potential source regions at the Sun. An ENLIL simulation is used to characterize the interplanetary state and its role for the energetic particle transport. The magnetic connection between each spacecraft and the Sun is determined. Based on a reconstruction of the coronal shock front we determine the times when the shock establishes magnetic connections with the different observers. Radio observations are used to characterize the directivity of the four main injection episodes, which are then employed in a 2D SEP transport simulation. Results. Timing analysis of the inferred SEP solar injection suggests different source processes being important for the electron and the proton event. Comparison among the characteristics and timing of the potential particle sources, such as the CME-driven shock or the flare, suggests a stronger shock contribution for the proton event and a more likely flare-related source of the electron event. Conclusions. We find that in this event an important ingredient for the wide SEP spread was the wide longitudinal range of about 110 degrees covered by distinct SEP injections.

The radiation environment over the African continent, at aviation altitudes, remains mostly uncharacterized and unregulated. In this paper we present initial measurements made by a newly developed active dosimeter on-board long-haul flights between South Africa and Germany. Based on these initial tests, we believe that this low-cost and open-source dosimeter is suitable for continued operation over the Africa continent and can provide valuable long-term measurements to test dosimteric models and inform aviation policy, Comment: Accepted to appear in Journal of Space Weather and Space Climate

Both the recent 2009 and 2020 solar minima were classified as unusually quiet and characterized with unusually high galactic cosmic ray (GCR) levels. However, unlike the trends from previous decades in which anomalous cosmic ray (ACR) and GCR levels strongly agreed, the ACR intensities did not reach such high record-setting levels.This discrepancy between the behaviour of GCRs and ACRs is investigated in this work by simulating the acceleration and transport of GCR and ACR oxygen under different transport conditions. After using recent observations to constrain any remaining free parameters present in the model, we show that less turbulent conditions are characterized by higher GCR fluxes and low ACR fluxes due to less efficient ACR acceleration at the solar wind termination shock. We offer this as an explanation for the ACR/GCR discrepancy observed during 2009 and 2020, when compared to previous solar cycles., Comment: Accepted to ApJ

This white paper recognizes gaps in observations that will, when addressed, much improve solar radiation hazard and geomagnetic storm forecasting. Radiation forecasting depends on observations of the entire "Solar Radiation Hemisphere" that we will define. Mars exploration needs strategic placement of radiation-relevant observations. We also suggest an orbital solution that will improve geomagnetic storm forecasting through improved in situ and solar/heliospheric remote sensing., Comment: Heliophysics 2050 White Paper

In this study we examine the radial dependence of the inertial and dissipation range indices, as well as the spectral break separating the inertial and dissipation range in power density spectra of interplanetary magnetic field fluctuations using {\it Parker Solar Probe} data from the fifth solar encounter between $\sim$0.1 and $\sim$0.7 au. The derived break wavenumber compares reasonably well with previous estimates at larger radial distances and is consistent with gyro-resonant damping of Alfv\'enic fluctuations by thermal protons. We find that the inertial scale power law index varies between approximately -1.65 and -1.45. This is consistent with either the Kolmogorov (-5/3) or Iroshnikov-Kraichnan (-3/2) values, has a very weak radial dependence with a possible hint that the spectrum becomes steeper closer to the Sun. The dissipation range power law index, however, has a clear dependence on radial distance (and turbulence age), decreasing from -3 near 0.7 au (4 days) to -4 [$\pm$0.3] at 0.1 au (0.75 days) closer to the Sun., Comment: Accepted for publication in ApJ

We discuss a newly established neutron monitor station installed at the ENTOTO Observatory Research Center outside of Addis Ababa, Ethiopia. This is a version of a mini-neutron monitor, recently upgraded to detect individual neutrons and able to calculate the waiting time distribution between neutron pulses down to $\sim 1 $ $\mu$s. From the waiting time distribution we define and calculate a new quantity, the correlation ratio, as the ratio of correlated to uncorrelated neutrons measured inside the monitor. We propose that this quantity can, in future, be used as a proxy for spectral index of atmospheric particles incident on the monitor and show that this quantity has a weak pressure dependence. We believe that future measurements from the ENTOTO mini-neutron monitor will contribute towards the understanding of cosmic ray acceleration and transport in the heliosphere., Comment: Submitted to Advances in Space Research spacial issue: Space and Geophysical Observations and Recent Results related to the African Continent

The strongly diverging magnetic field lines in the very inner heliosphere, through the associated magnetic focusing/mirroring forces, can, potentially, lead to highly anisotropic galactic cosmic ray distributions close to the Sun. Using a simplified analytical approach, validated by numerical simulations, we study the behaviour of the galactic cosmic ray distribution in this newly explored region of the heliosphere and find that significant anisotropies can be expected inside 0.2 au., Comment: Accepted for publication in ApJ

Particle drifts perpendicular to the background magnetic field have been proposed by some authors as an explanation for the very efficient perpendicular transport of solar energetic particles (SEPs). This process, however, competes with perpendicular diffusion caused by magnetic turbulence, which can also disrupt the drift patterns and reduce the efficiency of drift effects. The latter phenomenon is well known in cosmic ray studies, but not yet considered in SEP models. Additionally, SEP models which do not include drifts, especially for electrons, use turbulent drift reduction as a justification of this omission, without critically evaluating or testing this assumption. This article presents the first theoretical step for a theory of drift suppression in SEP transport. This is done by deriving the turbulence-dependent drift reduction function with a pitch-angle dependence, as applicable for anisotropic particle distributions, and by investigating to what extent drifts will be reduced in the inner heliosphere for realistic turbulence conditions and different pitch-angle dependencies of the perpendicular diffusion coefficient. The influence of the derived turbulent drift reduction factors on the transport of SEPs are tested, using a state-of-the-art SEP transport code, for several expressions of theoretically-derived perpendicular diffusion coefficients. It is found, for realistic turbulence conditions in the inner heliosphere, that cross-field diffusion will have the largest influence on the perpendicular transport of SEPs, as opposed to particle drifts., Comment: This is a pre-print of an article accepted for publication in The Astrophysical Journal

Neutron monitors have been the premier ground-based instruments for monitoring the near-Earth cosmic ray flux for more than 70 years. It is essential to continue with such measurements in order to extend this unique long-term time series. Moreover, with the recent interest of the aviation industry to space weather effects, and especially the radiation risk posed by solar energetic particles and galactic cosmic rays, it is vital to extend the current neutron monitor network in order to provide near-real-time measurements to the space weather community. In this paper we discuss a new electronics system that was retrofitted to the SANAE neutron monitor in Antarctica. We present initial results from this system, featuring very high temporal resolution and discuss the techniques applied to the data analysis. Based on these successful upgrades, we are confident that this system can be used to rejuvenate the aligning neutron monitor network, and even possibly to revive some of the decommissioned instruments., Comment: Accepted to Advances in Space Research

The basics of focused transport as applied to solar energetic particles are reviewed, paying special attention to areas of common misconception. The micro-physics of charged particles interacting with slab turbulence are investigated to illustrate the concept of pitch-angle scattering, where after the distribution function and focused transport equation are introduced as theoretical tools to describe the transport processes and it is discussed how observable quantities can be calculated from the distribution function. In particular, two approximations, the diffusion-advection and the telegraph equation, are compared in simplified situations to the full solution of the focused transport equation describing particle motion along a magnetic field line. It is shown that these approximations are insufficient to capture the complexity of the physical processes involved. To overcome such limitations, a finite-difference model, which is open for use by the public, is introduced to solve the focused transport equation. The use of the model is briefly discussed and it is shown how the model can be applied to reproduce an observed solar energetic electron event, providing insights into the acceleration and transport processes involved. Past work and literature on the application of these concepts are also reviewed, starting with the most basic models and building up to more complex models., Comment: This is a pre-print of an article published in Space Science Reviews. The final authenticated version is available online at: https://doi.org/10.1007/s11214-020-00771-x

Jovian electrons serve as an important test-particle distribution in the inner heliosphere and have been used extensively in the past to study the (diffusive) transport of cosmic rays in the inner heliosphere. With new limits on the Jovian source function (i.e. the particle intensity just outside the Jovian magnetosphere), and a new set of in-situ observations at 1 AU for both cases of good and poor magnetic connection between the source and observer, we revisit some of these earlier simulations. We aim to find the optimal numerical set-up that can be used to simulate the propagation of 6 MeV Jovian electrons in the inner heliosphere. Using such a set-up, we further aim to study the residence (propagation) times of these particles for different levels of magnetic connection between Jupiter and an observer at Earth (1 AU). Using an advanced Jovian electron propagation model based on the stochastic differential equation (SDE) approach, we calculate the Jovian electron intensity for different model parameters. A comparison with observations leads to an optimal numerical set-up, which is then used to calculate the so-called residence (propagation) times of these particles. Comparing to in-situ observations, we are able to derive transport parameters that are appropriate to study the propagation of 6 MeV Jovian electrons in the inner heliosphere. Moreover, using these values, we show that the method of calculating the residence time applied in former literature is not suited to being interpreted as the propagation time of physical particles. This is due to an incorrect weighting of the probability distribution. We propose and apply a new method, where the results from each pseudo-particle are weighted by its resulting phase-space density (i.e. the number of physical particles that it represents). Thereby we obtain more reliable estimates for the propagation time., Comment: accepted for publication in A&A

The acceleration of thermal solar wind protons at spherical interplanetary shocks driven by coronal mass ejections is investigated. The solar wind velocity distribution is represented using $\kappa$-functions, which are transformed in response to simulated shock transitions in the fixed-frame flow speed, plasma number density, and temperature. These heated solar wind distributions are specified as source spectra at the shock from which particles with sufficient energy can be injected into the diffusive shock acceleration process. It is shown that for shock-accelerated spectra to display the classically expected power-law indices associated with the compression ratio, diffusion length scales must exceed the width of the compression region. The maximum attainable energies of shock-accelerated spectra are found to be limited by the transit times of interplanetary shocks, while spectra may be accelerated to higher energies in the presence of higher levels of magnetic turbulence or at faster-moving shocks. Indeed, simulations suggest fast-moving shocks are more likely to produce very high-energy particles, while strong shocks, associated with harder shock-accelerated spectra, are linked to higher intensities of energetic particles. The prior heating of the solar wind distribution is found to complement shock acceleration in reproducing the intensities of typical energetic storm particle events, especially where injection energies are high. Moreover, simulations of $\sim$0.2 to 1 MeV proton intensities are presented that naturally reproduce the observed flat energy spectra prior to shock passages. Energetic particles accelerated from the solar wind, aided by its prior heating, are shown to contribute substantially to intensities during energetic storm particle events., Comment: 26 pages, 21 figures, 5 tables. Accepted for publication in the Astrophysical Journal

A complete theory for the complex interaction between solar energetic particles and the turbulent interplanetary magnetic field remains elusive. In this work we aim to contribute towards such a theory by modelling the propagation of solar energetic particle electrons in plasma wave turbulence. We specify a background turbulence spectrum, as constrained through observations, calculate the transport coefficients from first principles, and simulate the propagation of these electrons in the inner heliosphere. We have also, for the first time, included dynamical effects into the perpendicular diffusion coefficient. We show that such a "physics-first" approach can lead to reasonable results, when compared qualitatively to observations. In addition, we include the effect of wave growth/damping due to streaming electrons and show that these particles can significantly alter the turbulence levels close to the Sun for the largest events., Comment: Submitted to ApJ

The processes responsible for the effective longitudinal transport of solar energetic particles (SEPs) are still not completely understood. We address this issue by simulating SEP electron propagation using a spatially 2D transport model that includes perpendicular diffusion. By implementing, as far as possible, the most reasonable estimates of the transport (diffusion) coefficients, we compare our results, in a qualitative manner, to recent observations {at energies of 55 -- 105 keV}, focusing on the longitudinal distribution of the peak intensity, the maximum anisotropy and the onset time. By using transport coefficients which are derived from first principles, we limit the number of free parameters in the model to: (i) the probability of SEPs following diffusing magnetic field lines, quantified by $a \in [0,1]$, and (ii) the broadness of the Gaussian injection function. It is found that the model solutions are extremely sensitive to the magnitude of the {perpendicular} diffusion coefficient and relatively insensitive to the form of the injection function as long as a reasonable value of $a=0.2$ is used. We illustrate the effects of perpendicular diffusion on the model solutions and discuss the viability of this process as a dominant mechanism by which SEPs are transported in longitude. Lastly, we try to quantity the effectiveness of perpendicular diffusion as an interplay between the magnitude of the relevant diffusion coefficient and the SEP intensity gradient driving the diffusion process. It follows that perpendicular diffusion is extremely effective early in a SEP event when large intensity gradients are present, while the effectiveness quickly decreases with time thereafter.

The multitude of recent multi-point spacecraft observations of solar energetic particle (SEP) events have made it possible to study the longitudinal distribution of SEPs in great detail. SEPs, even those accelerated during impulsive events, show a much wider than expected longitudinal extent, bringing into question the processes responsible for their transport perpendicular to the local magnetic field. In this paper we examine some aspects of perpendicular transport by including perpendicular diffusion into a numerical SEP transport model that simulates the propagation of impulsively accelerated SEP electrons in the ecliptic plane. We find that: (i) The pitch-angle dependence of the perpendicular diffusion coefficient is an important, and currently mainly overlooked, transport parameter. (ii) SEP intensities are generally asymmetric in longitude, being enhanced towards the west of optimal magnetic connection to the acceleration region. (iii) The maximum SEP intensity may also be shifted (parameter dependently) away from the longitude of best magnetic connectivity at 1 AU. We also calculate the maximum intensity, the time of maximum intensity, the onset time and the maximum anisotropy as a function of longitude at Earth's orbit and compare the results, in a qualitative fashion, to recent spacecraft observations.

Diffusive shock acceleration (DSA), as an acceleration process for Galactic electrons at the solar wind termination shock (TS), is investigated with a comprehensive numerical model which incorporates shock-acceleration, particle drifts and other major modulation processes in the heliosphere. It is known from our previous work that the efficiency of DSA depends on the shape of electron spectra incident on the TS, which in turn depends on the spectral shape of the very local interstellar spectrum. Modulation processes also influence the efficiency of DSA. We find that TS accelerated electrons can make contributions throughout the heliosphere to intensity levels, especially at lower energies. An interesting result is that increases caused by DSA at the TS are comparable in magnitude to electron intensity enhancements observed by Voyager 1 ahead of the TS crossing. These intensity increases are large enough to account for observed intensity peaks, and thus supports the view that DSA is involved in their production. Additionally, the energy spectra observed by Voyager 1 throughout the heliosheath are reproduced satisfactorily, as well as the PAMELA spectrum at Earth at higher energies. We also find that an increase in the rigidity dependence of the diffusion coefficients for these low-energy electrons seems required to reproduce the spectral shape of observed modulated spectra in the heliosheath at kinetic energy E < ~ 4 MeV. This is different from intermediate energies where rigidity independent diffusion explains observations satisfactory., Comment: 8 pages, 6 figures

We study the temporal intensity profile, or pulse shape, of cosmic ray ground level enhancements (GLEs) by calculating the rise $(\tau_\mathrm{r})$ and decay $(\tau_\mathrm{d})$ times for a small subset of all available events. Although these quantities show very large inter-event variability, a linear dependence of $\tau_\mathrm{d} \approx 3.5 \tau_\mathrm{r}$ is found. We interpret these observational findings in terms of an interplanetary transport model, thereby including the effects of scattering (in pitch-angle) as these particles propagate from (near) the Sun to Earth. It is shown that such a model can account for the observed trends in the pulse shape, illustrating that interplanetary transport must be taken into account when studying GLE events, especially their temporal profiles. Furthermore, depending on the model parameters, the pulse shape of GLEs may be determined entirely by interplanetary scattering, obscuring all information regarding the initial acceleration process, and hence making a classification between impulsive and gradual events, as is traditionally done, superfluous.

This paper summarizes the results obtained by the team "Heliosheath Processes and the Structure of the Heliopause: Modeling Energetic Particles, Cosmic Rays, and Magnetic Fields" supported by the International Space Science Institute in Bern, Switzerland. We focus on the physical processes occurring in the outer heliosphere, especially at its boundary called the heliopause (HP), and in the LISM. The importance of magnetic field, charge exchange between atoms and ions, and solar cycle on the heliopause topology and observed heliocentric distances to different heliospheric discontinuities are discussed. It is shown that time-dependent boundary conditions are necessary to describe the heliospheric asymmetries detected by the Voyager spacecraft. We also discuss the structure of the HP, especially due to its instability and magnetic reconnection. It is demonstrated that the Rayleigh-Taylor instability of the nose of the HP creates consecutive layers of the interstellar and heliospheric plasma which are magnetically connected to different sources. This may be a possible explanation of abrupt changes in the galactic and anomalous cosmic ray fluxes observed by Voyager 1 when it was crossing the HP structure for a period of about one month in the summer of 2012. This paper also discusses the plausibility of fitting simulation results to a number of observational data sets obtained by in situ and remote measurements. The distribution of magnetic field in the vicinity of the HP is discussed in the context of Voyager measurements. We discuss the transport of energetic particles in the inner and outer heliosheath, concentrating on the anisotropic spatial diffusion diffusion tensor and the pitch-angle dependence of perpendicular diffusion and demonstrate that the latter can explain the observed pitch-angle anisotropies of both the anomalous and galactic cosmic rays in the outer heliosheath., Comment: Publication resulting from an International ISSI Team of the same name. Submitted to Space Science Reviews

Huge astrospheres or stellar wind bubbles influence the propagation of cosmic rays at energies up to the TeV range and can act as small-scale sinks decreasing the cosmic ray flux. We model such a sink (in 2D) by a sphere of radius 10\,pc embedded within a sphere of a radius of 1\,kpc. The cosmic ray flux is calculated by means of backward stochastic differential equations from an observer, which is located at $r_{0}$, to the outer boundary. It turns out that such small-scale sinks can influence the cosmic ray flux at the observer's location by a few permille (i.e\ a few 0.1\%), which is in the range of the observations by IceCube, Milagro and other large area telescopes., Comment: 10 pages, 4 Figures

A numerical model for the solar modulation of cosmic rays, based on the solution of a set of stochastic differential equations, is used to illustrate the effects of modifying the heliospheric magnetic field, particularly in the polar regions of the heliosphere. To this end, the dfferences in the modulation brought about by each of three choices for the heliospheric magnetic field, i.e. the unmodified Parker field, the Smith-Bieber modified field, and the Jokipii-Kota modified field, are studied. It is illustrated that both the Jokipii-Kota and Smith-Bieber modifications are effective in modifying the Parker field in the polar regions. In addition, it is argued that the modification of Smith and Bieber is based on observational evidence and has a firm physical basis, while these motivations are lacking in the case of the Jokipii-Kota modification. From a cosmic ray modulation point of view, we found the Smith-Bieber modification to be the most suitable choice for modifying the heliospheric magnetic field. The features and effects of these three modifications are illustrated both qualitatively and quantitatively. It is also shown how the Smith-Bieber modified field can be applied in cosmic ray modulation models to reproduce observational cosmic ray proton spectra from the PAMELA mission during the solar minimum of 2006 - 2009. These results are compared with those obtained in previous studies of this unusual solar minimum activity period and found to be in good qualitative agreement., Comment: 25 pages, 11 figures

The transport environment for particles in the heliosphere, e.g. galactic cosmic rays (GCRs) and MeV electrons (including those originating from Jupiters magnetosphere), is defined by the solar wind flow and the structure of the embedded heliospheric magnetic field. Solar wind structures, such as co-rotating interaction regions (CIR), can result in periodically modulation of both particles species. A detailed analysis of this recurrent Jovian electron events and galactic cosmic ray decreases measured by SOHO EPHIN is presented here, showing clearly a change of phase between both phenomena during the cause of the years 2007 and 2008. This effect can be explained by the change of difference in heliolongitude between the Earth and Jupiter, which is of central importance for the propagation of Jovian electrons. Furthermore, the data can be ordered such that the 27-day Jovian electron variation vanishes in the sector which does not connect the Earth with Jupiter magnetically using observed solar wind speeds.