# Stefan E. S. FerreiraNorth-West University | NWU · Centre for Space Research

Stefan E. S. Ferreira

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115

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## Publications

Publications (115)

Luminous blue variable type stars are massive O-type stars which show variable outflow parameters. These stars can also undergo eruptions where a large amount of mass is ejected in a very short duration. The astrospheres of these stars are modelled using a magnetohydrodynamic model including the effect of radiative cooling. The variation in outflow...

Thanks to dedicated long-term missions like Voyager and GOES over the past 50 years, much insight has been gained on the activity of our Sun, the solar wind, its interaction with the interstellar medium, and, thus, about the formation, the evolution, and the structure of the heliosphere. Additionally, with the help of multi-wavelength observations...

The interpretation of recent observations of bow shocks around O-stars and the creation of corresponding models require a detailed understanding of the associated (magneto-)hydrodynamic structures. We base our study on three-dimensional numerical magneto-)hydrodynamical models, which are analyzed using the dynamically relevant parameters, in partic...

The study addresses the question of the origin of low-energy electrons measured by Voyager 1 in the multi-keV range in the inner heliosheath. It intends to demonstrate that the observed keV-fluxes of electrons are consistent with their transmission through the termination shock under the influence of the associated electrostatic field. A power-law...

Simulations of astrospheres around hot and cool stars have recently move into the focus of scientific research. We describe here the differences between the astrospheres around hot and cool stars. While those around the former are huge (on pc scales) because of their high stellar wind momentum flow, those around cool stars are on a AU scale and the...

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...

The time-dependent modulation of galactic cosmic rays in the heliosphere is studied over different polarity cycles by computing 2.5 GV proton intensities using a two-dimensional, time-dependent modulation model. By incorporating recent theoretical advances in the relevant transport parameters in the model, we showed in previous work that this appro...

In the heliosphere, especially in the inner heliosheath, mass-, momentum-,
and energy loading induced by the ionization of neutral interstellar species
plays an important, but for some species, especially Helium, an underestimated
role. We discuss the implementation of charge exchange and electron impact
processes for interstellar neutral Hydrogen...

Composite supernova remnants consist of a pulsar wind nebula located inside a
shell-type remnant. The presence of a shell has implications on the evolution
of the nebula, although the converse is generally not true. The purpose of this
paper is two-fold. The first aim is to determine the effect of the pulsar's
initial luminosity and spin-down rate,...

A spatially independent model that calculates the time evolution of the electron spectrum in a spherically expanding pulsar wind nebula (PWN) is presented, allowing one to make broadband predictions for the PWN's non-thermal radiation. The source spectrum of electrons injected at the termination shock of the PWN is chosen to be a broken power law....

Galactic cosmic rays are charged particles created in our galaxy and beyond. They propagate through interstellar space to eventually reach the heliosphere and Earth. Their transport in the heliosphere is subjected to four modulation processes: diffusion, convection, adiabatic energy changes and particle drifts. Time-dependent changes, caused by sol...

Results from a newly developed hybrid cosmic ray (CR) modulation model are presented. In this approach, the transport of CRs is computed by incorporating the plasma flow from a magnetohydrodynamic model for the heliospheric environment, resulting in representative CR transport. The model is applied to the modulation of CRs beyond the heliopause (HP...

The propagation of Jovian electrons in interplanetary space was modelled by solving the relevant transport equation numerically through the use of stochastic differential equations. This approach allows us to calculate, for the first time, the propagation time of Jovian electrons from the Jovian magnetosphere to Earth. Using observed quiet-time inc...

In a Pulsar Wind Nebula (PWN), the lifetime of inverse Compton (IC) emitting
electrons exceeds the lifetime of its progenitor pulsar (as well as its
shell-type remnant), but it also exceeds the age of those that emit via
synchrotron radiation. Therefore, during its evolution, the PWN can remain
bright in IC so that its GeV-TeV gamma-ray flux remain...

The interaction of the solar wind with the surrounding interstellar medium (ISM) forms a cavity in the ISM. This cavity is called the heliosphere (influence sphere) of the Sun. Numerical models were developed using hydrodynamic (HD) and magneto-hydrodynamic (MHD) equations to simulate the interaction of the different fluids in the heliosphere. Thes...

This work aims to present results from a numerical time dependent cosmic ray mod-ulation model. These calculations are compared to a selected data set of the Ulysses spacecraft at a certain rigidity to show compatibility. Model calculations at other rigidi-ties are also presented to show the 11 year solar and 22 year magnetic cycle present in these...

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, util...

Two of the paradigms in modeling the transport of galactic cosmic rays
are that the modulation boundary is the heliopause and that the local
interstellar spectra are identical to the galactic cosmic ray spectra.
Here we demonstrate, that the proton spectrum is already modulated, due
to an altered interstellar diffusion coefficient in the outer
heli...

The propagation of energetic charged particles in the heliospheric magnetic field is one of the fundamental problems in heliophysics. In particular, the structure of the heliospheric magnetic field remains an unsolved problem and is discussed as a controversial topic. The first successful analytic approach to the structure of the heliospheric magne...

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 ra...

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...

Two of the paradigms in modeling the transport of galactic cosmic rays are that the modulation boundary is the heliopause and that the local interstellar spectra are identical to the galactic cosmic ray spectra. Here we demonstrate that the proton spectrum is already modulated due to an altered interstellar diffusion in the outer heliosheath as a c...

We review recent advances in the field of galactic cosmic ray transport in the distant heliosphere. The advent of global MHD models brought about a better understanding of the three-dimensional structure of the interface between the solar system and the surrounding interstellar space, and of the magnetic field topology in the outer heliosphere. The...

Time-dependent cosmic ray modulation is calculated over multiple solar cycles using our well established two-dimensional time-dependent modulation model. Results are compared to Voyager 1, Ulysses and IMP cosmic ray observations to establish compatibility. A time-dependence in the diffusion and drift coefficients, implicitly contained in recent exp...

In recent years one could witness tremendous progress regarding the
physics of the transport of cosmic rays (CRs) in the heliosphere. This
progress derives from both theoretical advances and new measurements
from the outer boundary region of the heliosphere. At the same time
theory and observations give new constraints on the local interstellar
CR...

A two-dimensional time-dependent modulation model based on the Parker (1965) transport equation is used to compute the cosmic ray modulation in the heliosphere. The model results are compared to Voyager 1, Ulysses and IMP 8 protons observations over various solar cycles. Recent theoretical work on transport parameters were introduced in our model t...

This paper discusses a numerical modulation model to describe anomalous cosmic ray acceleration and transport in the heliosheath, the portion of the heliosphere between the termination shock and the heliopause. The model is based on the well known Parker transport equation and includes, in addition to diffusive shock acceleration at the solar wind...

During the crossings of the Voyager 1 and 2 spacecraft of the solar wind termination shock (TS), the power-law spectra expected from diffusive shock acceleration of anomalous cosmic rays (ACRs) at the TS were not observed. The spectra seem to be modulated, but unfolding to the expected form as the spacecraft move away from the TS into the helioshea...

After the Voyager 1 and 2 crossings of the solar wind termination shock (TS), the focus on anomalous cosmic ray (ACR) modulation has shifted to improving our basic understanding of how and where exactly the ACRs are accelerated in the heliosheath. This study is focused on two prominent additional modulation effects in this re-gion: adiabatic heatin...

The propagation of energetic particles in the heliosphere is described by the Parker transport equation including the physical processes of di usion, drift, convection and adiabatic energy changes. For the inner heliosphere the Jovian magnetosphere appears to be the dominant source of energetic electrons. Therefore, the so-called Jovian electrons a...

Context. Since the crossings of the solar wind termination shock by the
Voyager 1 and 2 spacecraft, much speculation has surrounded the
acceleration mechanism and region where the anomalous cosmic ray
component is accelerated. A peculiar, and mostly overlooked feature of
the observed anomalous oxygen spectrum near the termination shock, is
the powe...

It is well known that the galactic cosmic ray (GCR) flux is modulated by corotating interaction regions (CIR) in the vicinity of Earth. When Ulysses first explored high latitude regions in 1996, it was found that the flux of GCRs was still modulated on the time scale of one solar rotation, although neither the solar wind nor the interplanetary magn...

In this paper we explore the evolution of a PWN while the pulsar is spinning down. An MHD approach is used to simulate the evolution of a composite remnant. Particular attention is given to the adiabatic loss rate and evolution of the nebular field strength with time. By normalising a two component particle injection spectrum (which can reproduce t...

A termination shock numerical model is used to study the effects of higher ionic states on the modulation of anomalous cosmic ray (ACR) Oxygen, especially on the high energy range of the spectrum. In the model, the ionic state is scaled as a function of energy in accordance with SAMPEX observational results. The acceleration and modulation effects...

After the Voyager 1 and 2 crossings of the solar wind termination shock (TS), the focus on anomalous cosmic ray (ACR) modulation has shifted to improving our basic understanding of how and where exactly the ACRs are accelerated in the heliosheath. This study is focused on two prominent additional modulation effects in this region: Adiabatic heating...

In recent years the variability of the cosmic ray flux has become one of the main issues not only for the interpretation of the abundances of cosmogenic isotopes in cosmochronic archives like, e.g., ice cores, but also for its potential impact on the terrestrial climate. It has been re-emphasized that the cosmic ray flux is not only varying due to...

With the Voyager 1 spacecraft approaching the solar wind termination shock, much emphasis is put on numerical models to simulate the physical parameters that can be expected at and beyond the shock. This work emphasizes the modulation of cosmic-ray electrons in a realistic, nonspherical heliosphere; e.g., the effects on electron modulation of a pol...

A time-dependent model based on a numerical solution of Parker's transport equation is used to model the modulation of cosmic-ray protons, electrons, and helium for full 11 year and 22 year modulation cycles using a compound approach. This approach incorporates the concept of propagating diffusion barriers, increases in the heliospheric magnetic fi...

Galactic and anomalous cosmic-ray modulation in a dynamic realistic heliosphere is studied. We present the time evolution of cosmic-ray spectra from solar minima to maxima as computed by a newly developed hybrid model. This model consists of a hydrodynamic part to model the plasma flow of protons in the solar wind and in the LISM, neutral H atoms,...

The Ulysses trajectory provides a unique opportunity to study the propagation of MeV electrons in a wide range of heliographic latitudes and during varying conditions in the inner heliosphere. From the Ulysses launch up to the beginning of 1998, the 3-10 MeV electron count rate of the COSPIN/KET instrument has been consistently described by modulat...

Magnetized flow and cosmic-ray transport in the local astrosphere are studied. A hybrid numerical model is used to calculate the heliospheric interface, the heliospheric magnetic field, and cosmic-ray modulation. Assuming that the transport parameters scale inversely proportional to the magnetic field, the amplification of the field in the inner he...

The H.E.S.S. telescopes detected VHE gamma-rays from G21.5-0.9, which is most likely the result of IC scattered VHE gamma-rays from the central pulsar wind nebula (PWN). The ~1 kyr age constrains the birth period in the range 50 to 55 ms, which also places a tight constraint on the available energetics for IC radiation. The fact that the reverse sh...

Ulysses, launched in 1990 October in the maximum phase of solar cycle 22, completed its third out-of-ecliptic orbit in 2008 February. This provides a unique opportunity to study the propagation of cosmic rays over a wide range of heliographic latitudes during different levels of solar activity and different polarities in the inner heliosphere. Comp...

This work aims to give a brief overview on the topic of cosmic ray modulation in the heliosphere. The heliosphere, heliospheric magnetic field, transport parameters and the transport equation together with modulation models, which solve this equation in various degree of complexity, are briefly discussed. Results from these models are then presente...

A brief discussion on the transport and acceleration of anomalous cosmic rays in the inner heliosheath is given with emphasis on stochastic acceleration and adiabatic heating of particles. Results from a numerical model is shown and compared to observations. We show that these acceleration processes play a major role in explaining Voyager 1 anomalo...

http://dx.doi.org/10.1051/0004-6361:200809561 http://www.aanda.org/articles/aa/pdf/2008/28/aa09561-08.pdf

While the high-energy part of the Galactic cosmic ray spectrum is well observed, its nature at energies below about 1 GeV nucleon-1 is still not known well. Recent in situ measurements made with the Voyager 1 spacecraft in the heliosheath between the solar wind termination shock and the heliopause have added further constraints on the local interst...

Aims.In this work numerical simulations showing the time evolution of supernova remnants (SNRs) in uniform and non-uniform interstellar medium (ISM) are presented. Methods.We use a hydrodynamic model including a kinematic calculation of the interstellar magnetic field. Important parameters influencing SNR evolution include the ejecta mass and energ...

The transport and acceleration of anomalous cosmic rays in the inner heliosheath is studied. A unique two-dimensional hydrodynamic numerical model is used to calculate the interaction of the solar wind and the local interstellar medium, neutral hydrogen, and pickup ions. The divergence of the flow, heliospheric magnetic field, and Alfven speed are...

Since the 1970's interplanetary electrons in the MeV energy range, of Jovian origin, have been extensively studied from close to the Sun to beyond the Kronian orbit, near the ecliptic. The Ulysses trajectory allowed to study the propagation of these particles, in a wide range of heliographic latitudes. The location of Jupiter with respect to the st...

The termination shock position in time is calculated from Voyager 2 solar wind data, using a multi species hydrodynamic model, also including the latitudinal variations in th e solar wind velocity and density due to the 11-year solar cycle. The results are compared to those obtained by Webber (2005) and the possibility of Voyager 2 crossing the TS...

In recent years the variability of the cosmic ray flux has become one of the main issues interpreting cosmogenic elements
and especially their connection with climate. In this review, an interdisciplinary team of scientists brings together our
knowledge of the evolution and modulation of the cosmic ray flux from its origin in the Milky Way, during...

In recent years, a lot of effort was put in modeling of the heliospheric
structures and their effects on cosmic ray transport. The interest has
risen as Voyager 1 recently passed the solar wind termination shock
(Dec. 2004). While a lot of emphasis was put on the global modeling of
the shock structure and plasma conditions, questions regarding the...

A numerical model based on Parker's transport equation, including diffusive shock acceleration, is applied to the region of the heliosphere at and beyond the solar wind termination shock, with an asymmetrical geometry. Several scenarios relevant to the outer heliosphere and supported by hydrodynamic modeling are studied e.g., changing the solar win...

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 spa...

The heliospheric interface is calculated using a three-fluid hydrodynamic model Using this approach the effects of different solar wind profiles as they may occur during different levels of solar activity on the heliospheric structure and the distribution of hydrogen and pickup ions are calculated self-consistently We present a time-dependent param...

Results from a new magnetohydrodynamic approach are presented The Euler equations together with Faraday s law under the assumption of ideal MHD are solved simultaneously to calculate the heliospheric interface as well as the heliospheric magnetic field within We compare this field to previous kinetic approaches to highlight the differences Furtherm...

The discrepancy between cosmic ray model predictions representing solar minimum conditions in the heliosphere and the 3–10 MeV post-1998 electrons observations by the Kiel Electron Telescope (KET) onboard Ulysses suggests the need for consistent changes in model parameters with increasing solar activity. In order to reduce this discrepancy, an effo...

Based on the five species hybrid model by Scherer & Ferreira (2005, ASTRA, 1, 17) we describe the observations of energetic cosmic ray particles by the Voyager 1 spacecraft for solar minimum and maximum conditions during both the A0 heliomagnetic cycle. Without going into a detailed data and model analysis, we will show that the model (BoPo-code) i...

Time dependent cosmic ray modulation in the inner heliosphere is studied by comparing results from a 2-D, time-dependent cosmic ray transport model with Ulysses observations. A compound approach, which combines the effects of the global changes in the heliospheric magnetic field magnitude with drifts to establish a realistic time-dependence, in the...

In this paper we present a new five particle species hybrid model for calculating cosmic ray particle transport and acceleration in a dynamic heliospheric environment. In particular the effects of solar cycle related changes in the solar wind speed on the heliospheric geometry, solar wind flow and cosmic ray distribution are discussed, when a polar...

Ulysses observations have indicated the presence of low-energy electron 'jets' with extraordinary anisotropies off the equatorial plane as far as 2.2 AU from Jupiter. Jovian electron 'jets' were also observed during Ulysses' first encounter with Jupiter in 1992 and still needs a satisfactory explanation. These observations are a very interesting fe...

A new cosmic-ray transport model, which investigates the modulation of cosmic-ray electrons in the outer heliospheric regions, including all major modulation mechanisms and the asymmetry of the heliospheric shock structures, is developed. The transport of Galactic cosmic rays into the heliosphere is directly calculated from the local interstellar m...

Much work has been done on Corotating Interaction Regions (CIRs), see e.g. \citet{balo99}. Apart from the actual impact of CIRs on the solar wind plasma environment, the influence of CIRs on energetic particle populations has been of special interest. In this paper we study the influence of a CIR on the propagation of Jovian electrons to a spacecra...

The propagation of cosmic rays in turbulent magnetic fields can be studied in detail by way of in‐situ measurements of energetic particles in the three‐dimensional heliosphere. Measurements of 3–20 MeV electrons from 1990 to 2003 have been made by the Kiel Electron Telescope (KET) onboard the Ulysses spacecraft during varying solar conditions. In o...

A three-dimensional, steady state, numerical model was used to model the
propagation of 7 MeV jovian and galactic electrons in the heliosphere.
The computations are compared with 3-10 MeV Ulysses/KET electron
observations which are still not well explained after ˜1998. To
explain these observations, coinciding with solar maximum conditions, it
is s...

Using a three-dimensional numerical modulation model, based on the trans-port equation, the transport of 7 MeV electrons within the inner heliosphere is modeled and compared to observations of 3-10 MeV electrons made by the Kiel Electron Telescope (KET) aboard the Ulysses spacecraft. Previous studies have shown this model can successfully predict t...

The Ulysses mission has provided a wealth of data, particularly regarding the transport of low-energy cosmic ray electrons. These data have been used to derive significant constraints for the anisotropic spatial diffusion of these particles. Detailed model simulations allowed, in addition, to determine the relative contributions of galactic and Jov...

One of the fundamental problems in modern astrophysics is the propagation of cosmic rays in turbulent magnetic fields, which can be studied by way of in-situ measurements of energetic particles in the heliosphere. Between 1 and 10 AU Jovian, and galactic particles contribute continuously to the few-MeV electron intensities. During it's recent appro...

The Ulysses orbit provides a unique opportunity to study the propagation of cosmic rays in a wide range of heliographic latitudes and during different solar activity and polarity in the inner heliosphere. Around July 2000 the solar magnetic field reversed its global polarity. In late 2000, the electron to proton ratio at the rigidities of 1.2 GV an...

We investigate the effects on the transport of a few MeV ($7 MeV) Jovian and Galactic electrons for differ-ent solar wind speed scenarios applicable to different heliospheric conditions. A three-dimensional, steady state model is used based on numerically solving Parker's transport equation, including the Jovian electron source, to compare model co...

The heliospheric modulation of galactic and jovian electrons is studied using a fully three-dimensional, steady-state model based on Parker's transport equation including the jovian electron source. The model is used to study the transport of 7 MeV joviair and galactic electrons in the heliosphere by changing the solar wind speed profile from solar...

A fully time-dependent cosmic ray modulation model based on the numerical solution of Parker's transport equation is used to study the amount of particle drifts needed to explain the observed 1.2 GV electron to Helium ratio at Earth and the 2.5 GV electron to proton ratio (e/p) observed by the KET instrument on the Ulysses spacecraft. The model suc...

We employ our recently developed ansatz for the time-dependent 3-D modelling of Jovian and galactic electrons in the inner heliosphere. The main goal of the study is to check whether the form of the diffusion tensofi as determined recently by Ferreira et al. (2001a,b) with an extended parameter study of the steady state is also useful for compariso...

The Ulysses trajectory provides a unique opportunity to study the propagation of MeV electrons in a wide range of heliographic latitudes and during varying conditions in the inner heliosphere. In recent papers we reported Ulysses Kiel Electron Telescope (KET) observations from the Ulysses launch up to the end of the second fast latitude scan in lat...

The ‘tilt angle’ of the wavy current sheet in the hehosphere has been used very successfully as a proxy for solar activity in cosmic ray (CR) modulation models with particle drifts included. The “Hoeksema tilt angles α” are calculated using two potential field methods: (1) The classic ‘line-of-sight’ computation and (2) the more recent ‘radial’ pho...

The heliospheric modulation of galactic and jovian electrons is studied using a three-dimensional, steady-state model based on Parker's transport equation including the jovian source. The model is developed to study both jovian and galactic electrons. It is illustrated how the electron intensity-time profile along the Ulysses trajectory is effected...

A three-dimensional numerical modulation model, based on the transport
equation, is used to model the transport of a few-MeV electrons in the
inner heliosphere. In particular the modulation of jovian and galactic
electrons is studied with emphasis on explaining the 3-10 MeV
Ulysses/KET observations after mid1998. Analysis of SOHO/CST 4 MeV
electron...

A time-dependent model based on a numerical solution of Parker's transport equation is used to model the modulation of cosmic ray protons, electrons and helium for full 11-year and 22-year modulation cycles using a compound approach. This approach incorporates the concept of propagating diffusion barriers based on global increases in the heliospher...