Showing papers on "Heliosphere published in 2017"
TL;DR: This review focuses on the current understanding of observational signatures and properties of ICMEs and the associated sheath regions based on five decades of studies, and specially emphasize the different origin, properties and consequences of the sheaths andICMEs.
Abstract: Interplanetary coronal mass ejections (ICMEs) are large-scale heliospheric transients that originate from the Sun. When an ICME is sufficiently faster than the preceding solar wind, a shock wave develops ahead of the ICME. The turbulent region between the shock and the ICME is called the sheath region. ICMEs and their sheaths and shocks are all interesting structures from the fundamental plasma physics viewpoint. They are also key drivers of space weather disturbances in the heliosphere and planetary environments. ICME-driven shock waves can accelerate charged particles to high energies. Sheaths and ICMEs drive practically all intense geospace storms at the Earth, and they can also affect dramatically the planetary radiation environments and atmospheres. This review focuses on the current understanding of observational signatures and properties of ICMEs and the associated sheath regions based on five decades of studies. In addition, we discuss modelling of ICMEs and many fundamental outstanding questions on their origin, evolution and effects, largely due to the limitations of single spacecraft observations of these macro-scale structures. We also present current understanding of space weather consequences of these large-scale solar wind structures, including effects at the other Solar System planets and exoplanets. We specially emphasize the different origin, properties and consequences of the sheaths and ICMEs.
292 citations
TL;DR: In this article, the authors provide a fresh perspective on the post-eruption and interplanetary evolution of CMEs, focusing on the physical processes that define the many complex interactions of the ejected plasma with its surroundings as it departs the corona and propagates through the heliosphere.
Abstract: As observed in Thomson-scattered white light, coronal mass ejections (CMEs) are manifest as large-scale expulsions of plasma magnetically driven from the corona in the most energetic eruptions from the Sun. It remains a tantalizing mystery as to how these erupting magnetic fields evolve to form the complex structures we observe in the solar wind at Earth. Here, we strive to provide a fresh perspective on the post-eruption and interplanetary evolution of CMEs, focusing on the physical processes that define the many complex interactions of the ejected plasma with its surroundings as it departs the corona and propagates through the heliosphere. We summarize the ways CMEs and their interplanetary CMEs (ICMEs) are rotated, reconfigured, deformed, deflected, decelerated and disguised during their journey through the solar wind. This study then leads to consideration of how structures originating in coronal eruptions can be connected to their far removed interplanetary counterparts. Given that ICMEs are the drivers of most geomagnetic storms (and the sole driver of extreme storms), this work provides a guide to the processes that must be considered in making space weather forecasts from remote observations of the corona.
221 citations
TL;DR: In this article, two propagation packages, GALPROP and HelMod, are combined to provide a single framework that is run to reproduce direct measurements of cosmic-ray (CR) species at different modulation levels and at both polarities of the solar magnetic field.
Abstract: Local interstellar spectra (LIS) for protons, helium, and antiprotons are built using the most recent experimental results combined with state-of-the-art models for propagation in the Galaxy and heliosphere. Two propagation packages, GALPROP and HelMod, are combined to provide a single framework that is run to reproduce direct measurements of cosmic-ray (CR) species at different modulation levels and at both polarities of the solar magnetic field. To do so in a self-consistent way, an iterative procedure was developed, where the GALPROP LIS output is fed into HelMod, providing modulated spectra for specific time periods of selected experiments to compare with the data; the HelMod parameter optimization is performed at this stage and looped back to adjust the LIS using the new GALPROP run. The parameters were tuned with the maximum likelihood procedure using an extensive data set of proton spectra from 1997 to 2015. The proposed LIS accommodate both the low-energy interstellar CR spectra measured by Voyager 1 and the high-energy observations by BESS, Pamela, AMS-01, and AMS-02 made from the balloons and near-Earth payloads; it also accounts for Ulysses counting rate features measured out of the ecliptic plane. The found solution is in a good agreement with proton, helium, and antiproton data by AMS-02, BESS, and PAMELA in the whole energy range.
132 citations
TL;DR: A review of the different aspects associated with the interaction of successive coronal mass ejections (CMEs) in the corona and inner heliosphere, focusing on the initiation of series of CMEs, their interaction in the helIosphere, the particle acceleration associated with successive CME, and the effect of compound events on Earth's magnetosphere is presented in this article.
Abstract: We present a review of the different aspects associated with the interaction of successive coronal mass ejections (CMEs) in the corona and inner heliosphere, focusing on the initiation of series of CMEs, their interaction in the heliosphere, the particle acceleration associated with successive CMEs, and the effect of compound events on Earth’s magnetosphere. The two main mechanisms resulting in the eruption of series of CMEs are sympathetic eruptions, when one eruption triggers another, and homologous eruptions, when a series of similar eruptions originates from one active region. CME – CME interaction may also be associated with two unrelated eruptions. The interaction of successive CMEs has been observed remotely in coronagraphs (with the Large Angle and Spectrometric Coronagraph Experiment – LASCO – since the early 2000s) and heliospheric imagers (since the late 2000s), and inferred from in situ measurements, starting with early measurements in the 1970s. The interaction of two or more CMEs is associated with complex phenomena, including magnetic reconnection, momentum exchange, the propagation of a fast magnetosonic shock through a magnetic ejecta, and changes in the CME expansion. The presence of a preceding CME a few hours before a fast eruption has been found to be connected with higher fluxes of solar energetic particles (SEPs), while CME – CME interaction occurring in the corona is often associated with unusual radio bursts, indicating electron acceleration. Higher suprathermal population, enhanced turbulence and wave activity, stronger shocks, and shock – shock or shock – CME interaction have been proposed as potential physical mechanisms to explain the observed associated SEP events. When measured in situ, CME – CME interaction may be associated with relatively well organized multiple-magnetic cloud events, instances of shocks propagating through a previous magnetic ejecta or more complex ejecta, when the characteristics of the individual eruptions cannot be easily distinguished. CME – CME interaction is associated with some of the most intense recorded geomagnetic storms. The compression of a CME by another and the propagation of a shock inside a magnetic ejecta can lead to extreme values of the southward magnetic field component, sometimes associated with high values of the dynamic pressure. This can result in intense geomagnetic storms, but can also trigger substorms and large earthward motions of the magnetopause, potentially associated with changes in the outer radiation belts. Future in situ measurements in the inner heliosphere by Solar Probe+ and Solar Orbiter may shed light on the evolution of CMEs as they interact, by providing opportunities for conjunction and evolutionary studies.
116 citations
TL;DR: In this paper, the origin and early structures of coronal mass ejections (CMEs) and solar flares are discussed from a multi-wavelength observational perspective, with the aim of improving the prediction of the occurrence of CMEs/flares and their effects on geospace and the heliosphere.
Abstract: Coronal mass ejections (CMEs) and solar flares are the large-scale and most energetic eruptive phenomena in our solar system and able to release a large quantity of plasma and magnetic flux from the solar atmosphere into the solar wind. When these high-speed magnetized plasmas along with the energetic particles arrive at the Earth, they may interact with the magnetosphere and ionosphere, and seriously affect the safety of human high-tech activities in outer space. The travel time of a CME to 1 AU is about 1–3 days, while energetic particles from the eruptions arrive even earlier. An efficient forecast of these phenomena therefore requires a clear detection of CMEs/flares at the stage as early as possible. To estimate the possibility of an eruption leading to a CME/flare, we need to elucidate some fundamental but elusive processes including in particular the origin and structures of CMEs/flares. Understanding these processes can not only improve the prediction of the occurrence of CMEs/flares and their effects on geospace and the heliosphere but also help understand the mass ejections and flares on other solar-type stars. The main purpose of this review is to address the origin and early structures of CMEs/flares, from multi-wavelength observational perspective. First of all, we start with the ongoing debate of whether the pre-eruptive configuration, i.e., a helical magnetic flux rope (MFR), of CMEs/flares exists before the eruption and then emphatically introduce observational manifestations of the MFR. Secondly, we elaborate on the possible formation mechanisms of the MFR through distinct ways. Thirdly, we discuss the initiation of the MFR and associated dynamics during its evolution toward the CME/flare. Finally, we come to some conclusions and put forward some prospects in the future.
107 citations
TL;DR: A review of the current understanding of the interrelated problems of coronal heating and the acceleration of the solar wind can be found in this paper, where the authors also discuss where the community stands in its ability to forecast how variations in solar wind (i.e., fast and slow wind streams) impact the Earth.
Abstract: The Sun’s outer atmosphere is heated to temperatures of millions of degrees, and solar plasma flows out into interplanetary space at supersonic speeds. This paper reviews our current understanding of these interrelated problems: coronal heating and the acceleration of the ambient solar wind. We also discuss where the community stands in its ability to forecast how variations in the solar wind (i.e., fast and slow wind streams) impact the Earth. Although the last few decades have seen significant progress in observations and modeling, we still do not have a complete understanding of the relevant physical processes, nor do we have a quantitatively precise census of which coronal structures contribute to specific types of solar wind. Fast streams are known to be connected to the central regions of large coronal holes. Slow streams, however, appear to come from a wide range of sources, including streamers, pseudostreamers, coronal loops, active regions, and coronal hole boundaries. Complicating our understanding even more is the fact that processes such as turbulence, stream-stream interactions, and Coulomb collisions can make it difficult to unambiguously map a parcel measured at 1 AU back down to its coronal source. We also review recent progress—in theoretical modeling, observational data analysis, and forecasting techniques that sit at the interface between data and theory—that gives us hope that the above problems are indeed solvable.
105 citations
TL;DR: In this article, a comprehensive set of propagation models, with or without reacceleration or convection, has been discussed and compared, and it was found that only the models with re-celeration can selfconsistently fit both the proton and boron-to-carbon ratio (B/C) data.
Abstract: In this work we use the newly reported boron-to-carbon ratio (B/C) from AMS-02 and the time-dependent proton fluxes from PAMELA and AMS-02 to constrain the source and propagation parameters of cosmic rays in the Milky Way. A linear correlation of the solar modulation parameter with solar activities is assumed to account for the time-varying cosmic ray fluxes. A comprehensive set of propagation models, with or without reacceleration or convection, has been discussed and compared. We find that only the models with reacceleration can self-consistently fit both the proton and B/C data. The rigidity dependence slope of the diffusion coefficient, $\ensuremath{\delta}$, is found to be about 0.38--0.50 for the diffusion-reacceleration models. The plain diffusion and diffusion-convection models fit the data poorly. We compare different model predictions of the positron and antiproton fluxes with the data. We find that the diffusion-reacceleration models overproduce low energy positrons, while nonreacceleration models give better fit to the data. As for antiprotons, reacceleration models tend to underpredict low energy antiproton fluxes, unless a phenomenological modification of the velocity dependence of the diffusion coefficient is applied. Our results suggest that there could be important differences of the propagation for nuclei and leptons, in either the Milky Way or the solar heliosphere.
99 citations
TL;DR: In this paper, the authors show that the heliosphere is bubble-shaped and not tail-shaped, as usually schematized, and the heliosheath ions are the source of ENA.
Abstract: For more than five decades, the shape and interactions of the heliosphere with the local interstellar medium have been discussed in the context of two competing models, posited in 19611: a magnetosphere-like heliotail and a more symmetric bubble shape. Although past models broadly assumed the magnetosphere-like concept, the accurate heliospheric configuration remained largely undetermined due to lack of measurements. In recent years, however, Voyagers 1 and 2 (V1 and V2) crossed the termination shock — the boundary where the solar wind drops — north and south of the ecliptic plane at 94 au2,3 and 84 au4 in 2004 and 2007, respectively, and discovered the reservoir of ions and electrons that constitute the heliosheath, while Cassini remotely imaged the heliosphere5 for the first time in 2003. Here we report 5.2–55 keV energetic neutral atom (ENA) global images of the heliosphere obtained with the Cassini/Ion and Neutral Camera (INCA). We compare them with 28–53 keV ions measured within the heliosheath by the low-energy charged particle (LECP) experiment onboard V1 and V2 over an 11-year period (2003–2014). We show that the heliosheath ions are the source of ENA. These observations also demonstrate that the heliosphere responds promptly, within ~2–3 years, to outward propagating solar wind changes in both the nose and tail directions. These results, together with the V1 measurement of a ~0.5 nT interstellar magnetic field6 and the enhanced ratio between particle pressure and magnetic pressure in the heliosheath7, strongly suggest a diamagnetic bubble-like heliosphere with few substantial tail-like features. Our results are consistent with recent modelling8–11. The authors put together measurements of ions and neutral atoms from Cassini and the two Voyagers and find that the heliosphere responds quickly (with a lag of 2–3 years) to the solar cycle and that it is bubble-shaped and not tail-shaped, as usually schematized.
89 citations
TL;DR: In this paper, a global magnetohydrodyanmic (MHD) simulation of the fast coronal mass ejection (CME) that occurred on 2011 March 7 was performed using the newly developed Alfv\'en Wave Solar Model (AWSoM), which describes the background solar wind starting from the upper chromosphere and extends to 24 R$(n)
Abstract: We perform and analyze results of a global magnetohydrodyanmic (MHD) simulation of the fast coronal mass ejection (CME) that occurred on 2011 March 7. The simulation is made using the newly developed Alfv\'en Wave Solar Model (AWSoM), which describes the background solar wind starting from the upper chromosphere and extends to 24 R$_{\odot}$. Coupling AWSoM to an inner heliosphere (IH) model with the Space Weather Modeling Framework (SWMF) extends the total domain beyond the orbit of Earth. Physical processes included in the model are multi-species thermodynamics, electron heat conduction (both collisional and collisionless formulations), optically thin radiative cooling, and Alfv\'en-wave turbulence that accelerates and heats the solar wind. The Alfv\'en-wave description is physically self-consistent, including non-Wentzel-Kramers-Brillouin (WKB) reflection and physics-based apportioning of turbulent dissipative heating to both electrons and protons. Within this model, we initiate the CME by using the Gibson-Low (GL) analytical flux rope model and follow its evolution for days, in which time it propagates beyond STEREO A. A detailed comparison study is performed using remote as well as \textit{in situ} observations. Although the flux rope structure is not compared directly due to lack of relevant ejecta observation at 1 AU in this event, our results show that the new model can reproduce many of the observed features near the Sun (e.g., CME-driven extreme ultraviolet (EUV) waves, deflection of the flux rope from the coronal hole, "double-front" in the white light images) and in the heliosphere (e.g., shock propagation direction, shock properties at STEREO A).
78 citations
TL;DR: Novel limits covering a very extended DM particle mass range, from MeV to TeV are obtained, and these bounds have a strength similar to those obtained with the cosmic microwave background-they are even more stringent for p-wave annihilation.
Abstract: MeV dark matter (DM) particles annihilating or decaying to electron-positron pairs cannot, in
principle, be observed via local cosmic-ray (CR) measurements because of the shielding solar
magnetic field. In this Letter, we take advantage of the fact that the Voyager I spacecraft, which
does measure CRs, has crossed the heliopause in 2012 and can actually, since then, detect
interstellar CRs. This opens up a new avenue to probe DM in the sub-GeV
energy/mass range that we first exploit here. From a complete description of the transport of
electrons and positrons at low energy, we derive predictions for both the secondary astrophysical
background and the pair production mechanisms relevant to DM annihilation or decay down
to the MeV mass range. Interestingly, we show that reacceleration may push positrons up to
energies larger than the DM particle mass. We combine the constraints from the Voyager and
AMS-02 data to get novel limits covering a very extended DM particle mass range, from MeV to TeV.
In the MeV mass range, our limits reach annihilation cross sections of order
$\langle \sigma v\rangle \sim 10^{-28}{\rm cm^3/s}$. Though extracted from a completely
different and new probe, these bounds have a strength similar to those obtained with the
cosmic microwave background (CMB) --- they are even more stringent for p-wave annihilation.
77 citations
TL;DR: In this article, the authors present a model for bow-shaking pulsar wind nebulae with bow shocks (BSPWNe) and estimate their contribution to the positron flux observed at the Earth.
Abstract: Pulsars with high spin-down power produce relativistic winds radiating a non-negligible fraction of this power over the whole electromagnetic range from radio to gamma-rays in the pulsar wind nebulae (PWNe). The rest of the power is dissipated in the interactions of the PWNe with the ambient interstellar medium (ISM). Some of the PWNe are moving relative to the ambient ISM with supersonic speeds producing bow shocks. In this case, the ultrarelativistic particles accelerated at the termination surface of the pulsar wind may undergo reacceleration in the converging flow system formed by the plasma outflowing from the wind termination shock and the plasma inflowing from the bow shock. The presence of magnetic perturbations in the flow, produced by instabilities induced by the accelerated particles themselves, is essential for the process to work. A generic outcome of this type of reacceleration is the creation of particle distributions with very hard spectra, such as are indeed required to explain the observed spectra of synchrotron radiation with photon indices $\varGamma\lesssim 1.5$
. The presence of this hard spectral component is specific to PWNe with bow shocks (BSPWNe). The accelerated particles, mainly electrons and positrons, may end up containing a substantial fraction of the shock ram pressure. In addition, for typical ISM and pulsar parameters, the $e^{+}$
released by these systems in the Galaxy are numerous enough to contribute a substantial fraction of the positrons detected as cosmic ray (CR) particles above few tens of GeV and up to several hundred GeV. The escape of ultrarelativistic particles from a BSPWN—and hence, its appearance in the far-UV and X-ray bands—is determined by the relative directions of the interstellar magnetic field, the velocity of the astrosphere and the pulsar rotation axis. In this respect we review the observed appearance and multiwavelength spectra of three different types of BSPWNe: PSR J0437-4715, the Guitar and Lighthouse nebulae, and Vela-like objects. We argue that high resolution imaging of such objects provides unique information both on pulsar winds and on the ISM. We discuss the interpretation of imaging observations in the context of the model outlined above and estimate the BSPWN contribution to the positron flux observed at the Earth.
TL;DR: In this paper, the authors show that charge exchange modifies the LISM plasma to such extent that the contribution of a shock transition to the total variation of plasma parameters becomes small even if the LIS velocity exceeds the fast magnetosonic speed in the unperturbed medium.
Abstract: The heliosphere is formed due to interaction between the solar wind (SW) and local interstellar medium (LISM). The shape and position of the heliospheric boundary, the heliopause, in space depend on the parameters of interacting plasma flows. The interplay between the asymmetrizing effect of the interstellar magnetic field and charge exchange between ions and neutral atoms plays an important role in the SW-LISM interaction. By performing three-dimensional, MHD plasma / kinetic neutral atom simulations, we determine the width of the outer heliosheath - the LISM plasma region affected by the presence of the heliosphere - and analyze quantitatively the distributions in front of the heliopause. It is shown that charge exchange modifies the LISM plasma to such extent that the contribution of a shock transition to the total variation of plasma parameters becomes small even if the LISM velocity exceeds the fast magnetosonic speed in the unperturbed medium. By performing adaptive mesh refinement simulations, we show that a distinct boundary layer of decreased plasma density and enhanced magnetic field should be observed on the interstellar side of the heliopause. We show that this behavior is in agreement with the plasma oscillations of increasing frequency observed by the plasma wave instrument onboard Voyager 1. We also demonstrate that Voyager observations in the inner heliosheath between the heliospheric termination shock and the heliopause are consistent with dissipation of the heliospheric magnetic field. The choice of LISM parameters in this analysis is based on the simulations that fit observations of energetic neutral atoms performed by IBEX.
TL;DR: The Interstellar Boundary Explorer (IBEX) has now operated in space for 7 years and returned nearly continuous observations that have led to scientific discoveries and reshaped our entire understanding of the outer heliosphere and its interaction with the local interstellar medium.
Abstract: The Interstellar Boundary Explorer (IBEX) has now operated in space for 7 years and returned nearly continuous observations that have led to scientific discoveries and reshaped our entire understanding of the outer heliosphere and its interaction with the local interstellar medium. Here we extend prior work, adding the 2014-2015 data for the first time, and examine, validate, initially analyze, and provide a complete 7-year set of Energetic Neutral Atom (ENA) observations from ~0.1 to 6 keV. The data, maps, and documentation provided here represent the 10th major release of IBEX data and include improvements to various prior corrections to provide the citable reference for the current version of IBEX data. We are now able to study time variations in the outer heliosphere and interstellar interaction over more than half a solar cycle. We find that the Ribbon has evolved differently than the globally distributed flux (GDF), with a leveling off and partial recovery of ENAs from the GDF, owing to solar wind output flattening and recovery. The Ribbon has now also lost its latitudinal ordering, which reflects the breakdown of solar minimum solar wind conditions and exhibits a greater time delay than for the surrounding GDF. Together, the IBEX observations strongly support a secondary ENA source for the Ribbon, and we suggest that this be adopted as the nominal explanation of the Ribbon going forward.
TL;DR: In this paper, a study comparing coronal flux ropes inferred from eruption data with their interplanetary counterparts constructed from in situ data was conducted, and it was found that the CME magnetic field in the corona is significantly higher than the ambient magnetic field at a given heliocentric distance.
TL;DR: In this article, the authors performed large-scale particle-in-cell simulations of the oblique rotator to capture the multi-scale evolution of the wind and found that the current sheet breaks up into a dynamical chain of magnetic islands separated by secondary thin current sheets.
Abstract: Context. Rapidly rotating neutron stars blow a relativistic, magnetized wind mainly composed of electron-positron pairs. The free expansion of the wind terminates far from the neutron star where a weakly magnetized pulsar wind nebula forms, implying efficient magnetic dissipation somewhere upstream.Aims. The wind current sheet that separates the two magnetic polarities is usually considered as the most natural place for magnetic dissipation via relativistic reconnection, but its efficiency remains an open question. Here, the goal of this work is to revisit this issue in light of the most recent progress in the understanding of reconnection and pulsar electrodynamics.Methods. We perform large two-dimensional particle-in-cell simulations of the oblique rotator to capture the multi-scale evolution of the wind. Our simulations are limited to the equatorial plane.Results. We find that the current sheet breaks up into a dynamical chain of magnetic islands separated by secondary thin current sheets. The sheet thickness increases linearly with radius while the Poynting flux decreases monotonically as reconnection proceeds. The radius of complete annihilation of the stripes is given by the plasma multiplicity parameter at the light cylinder. Current starvation within the sheets does not occur before complete dissipation as long as there is enough charges where the sheets form. Particles are efficiently heated up to a characteristic energy set by the magnetization parameter at the light cylinder. Energetic pulsed synchrotron emission peaks close to the light cylinder, and presents sub-pulse variability associated with the formation of plasmoids in the sheet.Conclusions. This study suggests that the striped component of the wind dissipates far before reaching the termination shock in isolated pulsars, even in very-high-multiplicity systems such as the Crab pulsar. Pulsars in binary systems may provide the best environments to study magnetic dissipation in the wind.
TL;DR: In this paper, the authors present a new model, Multi-Ivy-VP, that computes the three-dimensional structure of the solar wind which includes the chromosphere, the transition region, and the corona and low heliosphere.
Abstract: We present a new model, MULTI-VP, that computes the three-dimensional structure of the solar wind which includes the chromosphere, the transition region, and the corona and low heliosphere. MULTI-VP calculates a large ensemble of wind profiles flowing along open magnetic field-lines which sample the whole three-dimensional atmosphere or, alternatively, on a given region of interest. The radial domain starts from the photosphere and extends, typically, to about 30 $R_{sun}$ . The elementary uni-dimensional wind solutions are based on a mature numerical scheme which was adapted in order to accept any flux-tube geometry. We discuss here the first results obtained with this model. We use Potential Field Source-Surface (PFSS) extrapolations of magnetograms from the Wilcox Solar Observatory (WSO) to determine the structure of the background magnetic field. Our results support the hypothesis that the geometry of the magnetic flux-tubes in the lower corona controls the distribution of slow and fast wind flows. The inverse correlation between density and speed faraway from the Sun is a global effect resulting from small readjustments of the flux-tube cross-sections in the high corona (necessary to achieve global pressure balance and a uniform open flux distribution). In comparison to current global MHD models, MULTI-VP performs much faster and does not suffer from spurious cross-field diffusion effects. We show that MULTI-VP has the capability to predict correctly the dynamical and thermal properties of the background solar wind (wind speed, density, temperature, magnetic field amplitude and other derived quantities) and to approach real-time operation requirements.
TL;DR: In this article, the authors examined the evolution of three-dimensional magnetic flux ropes formed at the top of the helmet streamer belt by reconnection in the heliospheric current sheet (HCS), and simulated remote and in situ signatures of the large-scale torsional Alfven wave (TAW) which propagates along an S-Web arc to high latitudes.
Abstract: The slow solar wind exhibits strong variability on timescales from minutes to days, likely related to magnetic reconnection processes in the extended solar corona. Higginson2017b presented a numerical magnetohydrodynamic simulation which showed interchange magnetic reconnection is ubiquitous and most likely responsible for releasing much of the slow solar wind, in particular along topological features known as the Separatrix-Web (S-Web). Here, we continue our analysis, focusing on two specific aspects of structured slow solar wind variability. The first type is present in the slow solar wind found near the heliospheric current sheet, and the second we predict should be present everywhere S-Web slow solar wind is observed. For the first type, we examine the evolution of three-dimensional magnetic flux ropes formed at the top of the helmet streamer belt by reconnection in the heliospheric current sheet (HCS). For the second, we examine the simulated remote and in situ signatures of the large-scale torsional Alfven wave (TAW) which propagates along an S-Web arc to high latitudes. We describe the similarities and differences between the reconnection-generated flux ropes in the HCS, which resemble the well-known "streamer blob'' observations, and the similarly structured TAW. We discuss the implications of our results for the complexity of the HCS and surrounding plasma sheet, and the potential for particle acceleration, as well as the interchange reconnection scenarios which may generate TAWs in the solar corona. We discuss predictions from our simulation results for the dynamic slow solar wind in the extended corona and inner heliosphere.
TL;DR: In this article, the authors used local stellar wind characteristics observationally derived in the studies of atmospheric evaporation of the warm-neptune GJ436 b to derive the global characteristics of the wind of its M-dwarf host.
Abstract: Winds of cool dwarfs are difficult to observe, with only a few M dwarfs presenting observationally-derived mass-loss rates (Mdot), which span several orders of magnitude. Close-in exoplanets are conveniently positioned in the inner regions of stellar winds and can, thus, be used to probe the otherwise-unobservable local properties of their host-stars' winds. Here, we use local stellar wind characteristics observationally-derived in the studies of atmospheric evaporation of the warm-neptune GJ436 b to derive the global characteristics of the wind of its M-dwarf host. Using an isothermal wind model, we constrain the stellar wind temperature to be in the range [0.36,0.43] MK, with Mdot=[0.5,2.5] x 10^{-15} Msyn/yr. By computing the pressure balance between the stellar wind and the interstellar medium, we derive the size of the astrophere of GJ436 to be around 25 au, significantly more compact than the heliosphere. We demonstrate in this paper that transmission spectroscopy, coupled to planetary atmospheric evaporation and stellar wind models, can be a useful tool for constraining the large-scale wind structure of planet-hosting stars. Extending our approach to future planetary systems discoveries will open new perspectives for the combined characterisation of planetary exospheres and winds of cool dwarf stars.
TL;DR: In this paper, a perspective is given of the solar modulation of galactic cosmic rays in the heliosphere during the extraordinary quiet solar minimum period from 2006 to 2009, and the results from comprehensive modeling are discussed.
TL;DR: In this article, the effect of photospheric flows on the open and closed magnetic flux of a model corona with a dipole magnetic field and an isothermal solar wind was investigated.
Abstract: Remote and in situ observations strongly imply that the slow solar wind consists of plasma from the hot, closed-field corona that is released onto open magnetic field lines. The Separatrix Web theory for the slow wind proposes that photospheric motions at the scale of supergranules are responsible for generating dynamics at coronal-hole boundaries, which result in the closed plasma release. We use three-dimensional magnetohydrodynamic simulations to determine the effect of photospheric flows on the open and closed magnetic flux of a model corona with a dipole magnetic field and an isothermal solar wind. A rotational surface motion is used to approximate photospheric supergranular driving and is applied at the boundary between the coronal hole and helmet streamer. The resulting dynamics consist primarily of prolific and efficient interchange reconnection between open and closed flux. The magnetic flux near the coronal-hole boundary experiences multiple interchange events, with some flux interchanging over 50 times in one day. Additionally, we find that the interchange reconnection occurs all along the coronal-hole boundary and even produces a lasting change in magnetic-field connectivity in regions that were not driven by the applied motions. Our results show that these dynamics should be ubiquitous in the Sun and heliosphere. We discuss the implications of our simulations for understanding the observed properties of the slow solar wind, with particular focus on the global-scale consequences of interchange reconnection.
TL;DR: In this article, the authors used the Vasyliunas and Siscoe model functional form to quantify the pickup ion distributions, and while the fit parameters generally lie outside their physically expected ranges, this form allows fits that quantify variations in the pickup H+ properties with distance.
Abstract: We provide the first direct observations of interstellar H+ and He+ pickup ions in the solar wind from 22 AU to 38 AU. We use the Vasyliunas and Siscoe model functional form to quantify the pickup ion distributions, and while the fit parameters generally lie outside their physically expected ranges, this form allows fits that quantify variations in the pickup H+ properties with distance. By ~20 AU, the pickup ions already provide the dominant internal pressure in the solar wind. We determine the radial trends and extrapolate them to the termination shock at ~90 AU, where the pickup H+ to core solar wind density reaches ~0.14. The pickup H+ temperature and thermal pressure increase from 22-38 AU, indicating additional heating of the pickup ions. This produces very large extrapolated ratios of pickup H+ to solar wind temperature and pressure and an extrapolated ratio of the pickup ion pressure to the solar wind dynamic pressure at the termination shock of ~0.16. Such a large ratio has profound implications for moderating the termination shock and the overall outer heliospheric interaction. We also identify suprathermal tails in the H+ spectra and complex features in the He+ spectra, likely indicating variations in the pickup ion history and processing. Finally, we discover enhancements in both H+ and He+ populations just below their cutoff energies, which may be associated with enhanced local pickup. This study serves to document the release and as the citable reference of these pickup ion data for broad community use and analysis.
TL;DR: In this article, the authors show that high-resolution imaging of pulsar wind nebulae with bow shocks provides unique information on pulsar winds and on the interstellar medium (ISM), and estimate the BSPWN contribution to the positron flux observed at the Earth.
Abstract: Pulsars with high spin-down power produce relativistic winds radiating a fraction of the power in the range from radio to gamma-rays in the pulsar wind nebulae (PWNe). The rest of the power is dissipated in the interactions of the PWNe with the interstellar medium (ISM). Some of the PWNe are moving relative to the ISM with supersonic speeds producing bow shocks. In this case, the ultrarelativistic particles accelerated at the termination surface of the pulsar wind may be reaccelerated in the converging flow system formed by the outflow from the wind termination shock and the inflow from the bow shock. An outcome of this reacceleration is the creation of particle distributions with hard spectra, such as required to explain the observed synchrotron spectra with photon indices Gamma <~ 1.5. The presence of this hard component is specific to PWNe with bow shocks (BSPWNe). The accelerated particles may end up containing a substantial fraction of the shock ram pressure. For typical ISM and pulsar parameters, the positrons released by these systems are numerous enough to contribute a substantial fraction of the positrons detected as galactic cosmic ray particles above few tens and up to several hundred GeV. The escape of ultrarelativistic particles from a BSPWN and its appearance in the far-UV and X-ray bands is determined by the directions of the interstellar magnetic field, the velocity of the astrosphere and the pulsar rotation axis. In this respect we review the observed appearance and multiwavelength spectra of three different types of BSPWNe: PSR J0437-4715, the Guitar and Lighthouse nebulae, and Vela-like objects. We argue that high resolution imaging of such objects provides unique information on pulsar winds and on the ISM. We discuss the interpretation of imaging observations and estimate the BSPWN contribution to the positron flux observed at the Earth. (ABRIDGED)
TL;DR: In this paper, the authors used the force field approach to reconstruct the solar modulation potential ϕ from ground-based and spacecraft measurements, showing that ϕ depends not only on the Local Interstellar Spectrum (LIS) but also on the energy range of interest.
Abstract: On their way through the heliosphere, Galactic Cosmic Rays (GCRs) are modulated by various effects before they can be detected at Earth. This process can be described by the Parker equation, which calculates the phase space distribution of GCRs depending on the main modulation processes: convection, drifts, diffusion and adiabatic energy changes. A first order approximation of this equation is the force field approach, reducing it to a one-parameter dependency, the solar modulation potential ϕ. Utilizing this approach, it is possible to reconstruct ϕ from ground based and spacecraft measurements. However, it has been shown previously that ϕ depends not only on the Local Interstellar Spectrum (LIS) but also on the energy range of interest. We have investigated this energy dependence further, using published proton intensity spectra obtained by PAMELA as well as heavier nuclei measurements from IMP-8 and ACE/CRIS. Our results show severe limitations at lower energies including a strong dependence on the solar magnetic epoch. Based on these findings, we will outline a new tool to describe GCR proton spectra in the energy range from a few hundred MeV to tens of GeV over the last solar cycles. In order to show the importance of our modification, we calculate the global production rates of the cosmogenic radionuclide 10Be which is a proxy for the solar activity ranging back thousands of years.
TL;DR: In this paper, a comprehensive three-dimensional drift model is compared to PAMELA spectra to authenticate the modelling approach and then to make predictions of how electrons and protons are differently modulated down to 1 MeV, based on very local interstellar spectra.
Abstract: Context. The PAMELA experiment observed galactic proton and electron spectra down to 70 MV and 400 MV, respectively, from mid-2006 to the end of 2009 during what is called an A Drift theory for the solar modulation of cosmic rays predicts that the intensity of protons at the Earth is expected to show a different rate of recovery towards solar minimum when compared to electrons during A The solutions of a comprehensive three-dimensional drift model are compared to PAMELA spectra to authenticate the modelling approach and then to make predictions of how electrons and protons are differently modulated down to 1 MeV, based on new very local interstellar spectra.Results. The comparison of observations and modelling provides insight into how the rigidity dependence of the three major diffusion coefficients changes during such quiet modulation conditions. How drift effects dissipate above several GeV and below 100 MeV is illustrated for both protons and electrons. The modulation that occurred at the Earth during this quiet period is shown as a function of rigidity and time. The e− /p ratio is computed from 10 MV to 50 GV for this period and a prediction is made for what may be observed in terms of spectra during the next A > 0 solar minimum.Conclusions. The presence of drifts during this quiet period is established based on the presented modelling and PAMELA measurements. Drift effects for protons and electrons are quantified in terms of their rigidity and temporal development from 2006 to 2009.
TL;DR: In this paper, the authors used the Vasyliunas and Siscoe model functional form to quantify the pickup ion distributions, and while the fit parameters generally lie outside their physically expected ranges, this form allows fits that quantify variations in the pickup H+ properties with distance.
Abstract: We provide the first direct observations of interstellar H+ and He+ pickup ions in the solar wind from 22 AU to 38 AU. We use the Vasyliunas and Siscoe model functional form to quantify the pickup ion distributions, and while the fit parameters generally lie outside their physically expected ranges, this form allows fits that quantify variations in the pickup H+ properties with distance. By ~20 AU, the pickup ions already provide the dominant internal pressure in the solar wind. We determine the radial trends and extrapolate them to the termination shock at ~90 AU, where the pickup H+ to core solar wind density reaches ~0.14. The pickup H+ temperature and thermal pressure increase from 22-38 AU, indicating additional heating of the pickup ions. This produces very large extrapolated ratios of pickup H+ to solar wind temperature and pressure and an extrapolated ratio of the pickup ion pressure to the solar wind dynamic pressure at the termination shock of ~0.16. Such a large ratio has profound implications for moderating the termination shock and the overall outer heliospheric interaction. We also identify suprathermal tails in the H+ spectra and complex features in the He+ spectra, likely indicating variations in the pickup ion history and processing. Finally, we discover enhancements in both H+ and He+ populations just below their cutoff energies, which may be associated with enhanced local pickup. This study serves to document the release and as the citable reference of these pickup ion data for broad community use and analysis.
TL;DR: In this article, the authors simulated the signal due to neutral He atoms, observed by Interstellar Boundary Explorer (IBEX), assuming that charge exchange collisions between neutral he atoms and He+ ions operate everywhere between the heliopause and a distant source region in the local interstellar cloud, where the neutral and charged components are in thermal equilibrium.
Abstract: We simulated the signal due to neutral He atoms, observed by Interstellar Boundary Explorer (IBEX), assuming that charge exchange collisions between neutral He atoms and He+ ions operate everywhere between the heliopause and a distant source region in the local interstellar cloud, where the neutral and charged components are in thermal equilibrium. We simulated several test cases of the plasma flow within the outer heliosheath and investigated the signal generation for plasma flows both in the absence and in the presence of the interstellar magnetic field. We found that a signal in the portion of IBEX data identified as due to the Warm Breeze does not arise when a homogeneous plasma flow in front of the heliopause is assumed, but it appears immediately when any reasonable disturbance in its flow due to the presence of the heliosphere is assumed. We obtained a good qualitative agreement between the data selected for comparison and the simulations for a model flow with the velocity vector of the unperturbed gas and the direction and intensity of magnetic field adopted from recent determinations. We conclude that direct-sampling observations of neutral He atoms at 1~AU from the Sun are a sensitive tool for investigating the flow of interstellar matter in the outer heliosheath, that the Warm Breeze is indeed the secondary population of interstellar helium, as it was hypothesized earlier, and that the WB signal is consistent with the heliosphere distorted from axial symmetry by the interstellar magnetic field.
TL;DR: In this paper, the radial and rigidity dependence of the cosmic ray (CR) diffusion tensor is investigated on the basis of a recently developed 2D and slab turbulence transport model using nearly incompressible theory.
Abstract: The radial and rigidity dependence of the cosmic ray (CR) diffusion tensor is investigated on the basis of a recently developed 2D and slab turbulence transport model using nearly incompressible theory. We study CR diffusion coefficients in two regions: 0.29 to 1 au, and 1 to 75 au. In the former case, we use 2D and radial slab turbulence transport models, and in the latter case, 2D and perpendicular slab turbulence transport models. We employ quasilinear theory and nonlinear guiding center theory, respectively, to determine the parallel and perpendicular elements of the CR diffusion tensor. We also present the effect of both weak and moderately strong turbulence on the drift element of the CR diffusion tensor. We find that in the solar wind ecliptic plane (1) the radial mean free path (mfp) is dominated by diffusion parallel to the mean magnetic field and is nearly constant from 0.29 to 1 au; (2) from 1 to 75 au, the role of the perpendicular mfp becomes increasingly important despite the parallel mfp being about three orders of magnitude larger than the perpendicular mfp. The radial mfp initially decays slowly and then faster after ∼6 au; (3) beyond ∼10 au, pickup-ion-driven turbulence is the most important factor in determining the various mean free paths since stream interactions weaken with increasing heliocentric distance; (4) the rigidity (P) dependence of the parallel mfp is proportional to P0.33 from 10 to 10 MV, but in the distant heliosphere, its dependence increases for higher rigidities and is proportional to P2.0. In contrast, the perpendicular mfp is weakly influenced by CR rigidity; (5) the drift length scale is comparable to the perpendicular mfp beyond ∼10 au; and (6) strong turbulence may introduce a new “drift” component, the direction of which is normal to the mean magnetic field.
TL;DR: In this article, the authors analyzed one of the first solar energetic particle (SEP) events of solar cycle 24 observed at widely separated spacecraft in order to assess the reliability of models currently used to determine the connectivity between the sources of SEPs at the Sun and spacecraft in the inner heliosphere.
Abstract: We analyze one of the first solar energetic particle (SEP) events of solar cycle 24 observed at widely separated spacecraft in order to assess the reliability of models currently used to determine the connectivity between the sources of SEPs at the Sun and spacecraft in the inner heliosphere. This SEP event was observed on 2010 August 14 by near-Earth spacecraft, STEREO-A (∼80° west of Earth) and STEREO-B (∼72° east of Earth). In contrast to near-Earth spacecraft, the footpoints of the nominal magnetic field lines connecting STEREO-A and STEREO-B with the Sun were separated from the region where the parent fast halo coronal mass ejection (CME) originated by ∼88° and ∼47° in longitude, respectively. We discuss the properties of the phenomena associated with this solar eruption. Extreme ultraviolet and white-light images are used to specify the extent of the associated CME-driven coronal shock. We then assess whether the SEPs observed at the three heliospheric locations were accelerated by this shock or whether transport mechanisms in the corona and/or interplanetary space provide an alternative explanation for the arrival of particles at the poorly connected spacecraft. A possible scenario consistent with the observations indicates that the observation of SEPs at STEREO-B and near Earth resulted from particle injection by the CME shock onto the field lines connecting to these spacecraft, whereas SEPs reached STEREO-A mostly via cross-field diffusive transport processes. The successes, limitations, and uncertainties of the methods used to resolve the connection between the acceleration sites of SEPs and the spacecraft are evaluated.
TL;DR: In this paper, the authors presented 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 (ISSI).
Abstract: 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 (ISSI) in Bern, Switzerland. We focus on the physical processes occurring in the outer heliosphere, especially at its boundary called the heliopause, and in the local interstellar medium. The importance of magnetic field, charge exchange between neutral 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, data-driven boundary conditions are necessary to describe the heliospheric asymmetries detected by the Voyager spacecraft. We also discuss the structure of the heliopause, especially due to its instability and magnetic reconnection. It is demonstrated that the Rayleigh–Taylor instability of the nose of the heliopause 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 heliopause 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 heliopause is discussed in the context of Voyager measurements. It is argued that a classical heliospheric current sheet formed due to the Sun’s rotation is not observed by in situ measurements and should not be expected to exist in numerical simulations extending to the boundary of the heliosphere. Furthermore, 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.