Showing papers on "Heliosphere published in 2006"

In-Situ Solar Wind and Magnetic Field Signatures of Interplanetary Coronal Mass Ejections

Abstract: The heliospheric counterparts of coronal mass ejections (CMEs) at the Sun, interplanetary coronal mass ejections (ICMEs), can be identified in situ based on a number of magnetic field, plasma, compositional and energetic particle signatures as well as combinations thereof. We summarize these signatures and their implications for understanding the nature of these structures and the physical properties of coronal mass ejections. We conclude that our understanding of ICMEs is far from complete and formulate several challenges that, if addressed, would substantially improve our knowledge of the relationship between CMEs at the Sun and in the heliosphere.

Large variations in Holocene solar activity: Constraints from 10Be in the Greenland Ice Core Project ice core

Abstract: [1] Cosmogenic radionuclides extracted from ice cores hold a unique potential for reconstructing past solar activity changes beyond the direct instrumental period. Taking the geomagnetic modulation into account, the solar activity in terms of the heliospheric modulation function can quantitatively be reconstructed in high resolution throughout the Holocene. For this period our results reveal changes in heliospheric modulation of galactic cosmic rays significantly larger than the variations reconstructed on the basis of neutron monitor measurements of galactic cosmic rays for the last 50 years. Moreover, the 10Be data from the Greenland Ice Core Project ice core as well as 14C support a high current solar activity. However, although the reconstruction of solar activity on long timescales is difficult, our result suggests that the modern activity state of the Sun is not that exceptional regarding the entire Holocene. This extended solar activity record provides the basis for further detailed investigations on solar and cosmic ray physics, as well as on solar forcing of the Earth's climate whose importance is suggested by increasing paleoclimatic evidences.

Magnetic field inclination and atmospheric oscillations above solar active regions

Abstract: Recent observational evidence for magnetic field direction effects on helioseismic signals in sunspot penumbrae is suggestive of magnetohydrodynamic (MHD) mode conversion occurring at lower levels. This possibility is explored using wave mechanical and ray theory in a model of the Sun's surface layers permeated by uniform inclined magnetic field. It is found that fast-to-slow conversion near the equipartition depth at which the sound and Alfven speeds coincide can indeed greatly enhance the atmospheric acoustic signal at heights observed by Solar and Heliospheric Observatory/Michelson Doppler Imager and other helioseismic instruments, but that this effect depends crucially on the wave attack angle, i.e. the angle between the wavevector and the magnetic field at the conversion/transmission depth. A major consequence of this insight is that the magnetic field acts as a filter, preferentially allowing through acoustic signal from a narrow range of incident directions. This is potentially testable by observation.

Evolution of Coronal Mass Ejections in the Inner Heliosphere: A Study using White-light and Scintillation Images

Abstract: Knowledge of the radial evolution of the coronal mass ejection (CME) is important for the understanding of its arrival at the near-Earth space and of its interaction with the disturbed/ambient solar wind in the course of its travel to 1 AU and further. In this paper, the radial evolution of 30 large CMEs (angular width > 150∘, i.e., halo and partial halo CMEs) has been investigated between the Sun and the Earth using (i) the white-light images of the near-Sun region from the Large Angle Spectroscopic Coronagraph (LASCO) onboard SOHO mission and (ii) the interplanetary scintillation (IPS) images of the inner heliosphere obtained from the Ooty Radio Telescope (ORT). In the LASCO field of view at heliocentric distances R≤30 solar radii (R ⊙), these CMEs cover an order of magnitude range of initial speeds, V CME≈260–2600 km s−1. Following results have been obtained from the speed evolution of these CMEs in the Sun–Earth distance range: (1) the speed profile of the CME shows dependence on its initial speed; (2) the propagation of the CME goes through continuous changes, which depend on the interaction of the CME with the surrounding solar wind encountered on the way; (3) the radial-speed profiles obtained by combining the LASCO and IPS images yield the factual view of the propagation of CMEs in the inner heliosphere and transit times and speeds at 1 AU computed from these profiles are in good agreement with the actual measurements; (4) the mean travel time curve for different initial speeds and the shape of the radial-speed profiles suggest that up to a distance of ∼80 R ⊙, the internal energy of the CME (or the expansion of the CME) dominates and however, at larger distances, the CME's interaction with the solar wind controls the propagation; (5) most of the CMEs tend to attain the speed of the ambient flow at 1 AU or further out of the Earth's orbit. The results of this study are useful to quantify the drag force imposed on a CME by the interaction with the ambient solar wind and it is essential in modeling the CME propagation. This study also has a great importance in understanding the prediction of CME-associated space weather at the near-Earth environment.

Constraints on the global structure of magnetic clouds: Transverse size and curvature

Abstract: We present direct evidence that magnetic clouds (MCs) have highly flattened and curved cross section resulting from their interaction with the ambient solar wind. Lower limits on the transverse size are obtained for three MCs observed by ACE and Ulysses from the latitudinal separation between the two spacecraft, ranging from 40 degrees to 70 degrees. The cross-section aspect ratio of the MCs is estimated to be no smaller than 6:1. We offer a simple model to extract the radius of curvature of the cross section, based on the elevation angle of the MC normal distributed over latitude. Application of the model to Wind observations from 1995 to 1997 (close to solar minimum) shows that the cross section is bent concavely outward by a structured solar wind with a radius of curvature of similar to 0.3 AU. Near solar maximum, MCs tend to be convex outward in the solar wind with a uniform speed; the radius of curvature is proportional to the heliographic distance of MCs, as demonstrated by Ulysses observations between 1999 and 2003. These results improve our knowledge of the global morphology of MCs in the pre-Stereo era, which is crucial for space weather prediction and heliosphere studies.

Turbulent Heating of the Solar Wind by Newborn Interstellar Pickup Protons

Abstract: Spacecraft missions to the outer heliosphere have shown that the thermal protons that make up the solar wind are hotter than simple adiabatic expansion would predict. We examine a theory that describes this heating by using the time-varying 1 AU measurements as input and comparing the predictions with observations in the outer heliosphere. Inside 20 AU wind shear and shocks provide the dominant energy source to drive the turbulence. Outside 20 AU little remains to inject energy into the fluctuations except newborn interstellar pickup protons. The theory is built on a combination of two-dimensional magnetohydrodynamic turbulence concepts and the latest kinetic theory describing the scattering of newborn interstellar pickup protons. We find that application of the theory to the observations produces encouraging agreement at the same time that it illuminates latitudinal effects associated with solar minimum conditions. A remaining challenge is to close the gap of a factor of 2 between observed and predicted proton temperatures beyond 40 AU. For this, we suggest that further structural development of the theory is needed, rather than ad hoc adjustment of the model parameters, which are reasonably well constrained by theory, simulation, and observations.

Coronal mass ejections of solar cycle 23

Abstract: I summarize the statistical, physical, and morphological properties of coronal mass ejections (CMEs) of solar cycle 23, as observed by the Solar and Heliospheric Observatory (SOHO) mission. The SOHO data is by far the most extensive data, which made it possible to fully establish the properties of CMEs as a phenomenon of utmost importance to Sun-Earth connection as well as to the heliosphere. I also discuss various subsets of CMEs that are of primary importance for their impact on Earth.

The Effects of a Local Interstellar Magnetic Field on Voyager 1 and 2 Observations

Abstract: We show that an interstellar magnetic field can produce a north-south asymmetry in the solar wind termination shock Using Voyager 1 and 2 measurements, we suggest that the angle α between the interstellar wind velocity and the magnetic field is 30° < α < 60° The distortion of the shock is such that termination shock particles could have streamed outward along the spiral interplanetary magnetic field connecting Voyager 1 to the shock when the spacecraft was within ~2 AU of the shock The shock distortion is larger in the southern hemisphere, and Voyager 2 could be connected to the shock when it is within ~5 AU of the shock, but with particles from the shock streaming inward along the field Tighter constraints on the interstellar magnetic field should be possible when Voyager 2 crosses the shock in the next several years

Three-dimensional Features of the Outer Heliosphere due to Coupling between the Interstellar and Interplanetary Magnetic Fields. II. The Presence of Neutral Hydrogen Atoms

Abstract: We investigate the effects of coupling the interstellar and interplanetary magnetic fields (ISMF and IMF) at the heliospheric interface while taking into account the momentum and energy transfer between the plasma and neutral components of the interacting solar wind (SW) and local interstellar medium (LISM). Investigation is performed on the basis of the multifluid model that treats different populations of neutral particles as fluids governed by separate sets of the Euler equations. As the properties of the local interstellar cloud (LIC) are rather poorly examined, with the exception of the interstellar plasma velocity and temperature, we assume several possible orientations of the ISMF. In particular, the case is considered where an ISMF vector is perpendicular to the LISM velocity vector and inclined at an angle of 60° toward the ecliptic plane of the Sun, as suggested in recent publications relating LIC properties to the radio emission observed by Voyager 1. Special attention is paid to the distribution of magnetic fields that can affect the cosmic-ray modulation throughout the heliosphere and the possibility of the heliospheric current sheet bending into one of the hemispheres. Differences are discussed between ideal MHD and multifluid calculations. Possibilities are analyzed for the IMF lines to cross the termination shock multiple times. Parameters are determined that affect the divergence between the neutral hydrogen and helium streamlines in the inner heliosphere.

An explanation of the Voyager paradox: Particle acceleration at a blunt termination shock

Abstract: [1] Voyager 1 recently crossed the termination shock at the edge of our heliosphere. In contrast to the expectations of essentially all prior models, however, Voyager 1 did not observe the source of Anomalous Cosmic Rays (ACRs) as had been widely anticipated. We show here that the dearth of higher energy particles near the nose of the heliosphere is a natural consequence of the magnetic geometry in the region ahead of a flattened shock. Particle energization happens primarily back along the flanks of the shock where the injection energy is lower and where the magnetic field has had progressively longer connection times to accelerate particles. In addition to explaining the most baffling aspects of the Voyager 1 observations, this paradigm makes explicit predictions about what should be observed when Voyager 2 reaches the termination shock, significantly further back from its nose.

Modeling of the heliospheric interface: multi-component nature of the heliospheric plasma

Abstract: We present a new model of the heliospheric interface - the region of the solar wind interaction with the local interstellar medium. This new model performs a multi-component treatment of charged particles in the heliosphere. All charged particles are divided into several co-moving types. The coldest type, with parameters typical of original solar wind protons, is considered in the framework of fluid approximation. The hot pickup proton components created from interstellar H atoms and heliospheric ENAs by charge exchange, electron impact ionization and photoionization are treated kinetically. The charged components are considered self-consistently with interstellar H atoms, which are described kinetically as well. To solve the kinetic equation for H atoms we use the Monte Carlo method with splitting of trajectories, which allows us 1) to reduce statistical uncertainties allowing correct interpretation of observational data; 2) to separate all H atoms in the heliosphere into several populations depending on the place of their birth and on the type of parent protons.

Coronal mass ejections and magnetic flux buildup in the heliosphere

Abstract: [1] To test for magnetic flux buildup in the heliosphere from coronal mass ejections (CMEs), we simulate heliospheric flux as a constant background open flux with a time-varying interplanetary CME (ICME) contribution. As flux carried by ejecta can only contribute to the heliospheric flux budget while it remains closed, the ICME flux opening rate is an important factor. Two separate forms for the ICME flux opening rate are considered: (1) constant and (2) exponentially decaying with time. Coronagraph observations are used to determine the CME occurrence rates, while in situ observations are used to estimate the magnetic flux content of a typical ICME. Both static equilibrium and dynamic simulations, using the constant and exponential ICME flux opening models, require flux opening timescales of ∼50 days in order to match the observed doubling in the magnetic field intensity at 1 AU over the solar cycle. Such timescales are equivalent to a change in the ICME closed flux of only ∼7–12% between 1 and 5 AU, consistent with CSE signatures; no flux buildup results. The dynamic simulation yields a solar cycle flux variation with high variability that matches the overall variability of the observed magnetic field intensity remarkably well, including the double peak forming the Gnevyshev gap.

Effects of a Local Interstellar Magnetic Field on Voyager 1 and 2 Observations

Abstract: We show that that an interstellar magnetic field can produce a north/south asymmetry in solar wind termination shock. Using Voyager 1 and 2 measurements, we suggest that the angle $\alpha$ between the interstellar wind velocity and magnetic field is $30^{\circ} < \alpha < 60^{\circ}$. The distortion of the shock is such that termination shock particles could stream outward along the spiral interplanetary magnetic field connecting Voyager 1 to the shock when the spacecraft was within $\sim 2~AU$ of the shock. The shock distortion is larger in the southern hemisphere, and Voyager 2 could be connected to the shock when it is within $\sim 5~AU$ of the shock, but with particles from the shock streaming inward along the field. Tighter constraints on the interstellar magnetic field should be possible when Voyager 2 crosses the shock in the next several years.

Charge-transfer induced EUV and soft X-ray emissions in the heliosphere

Abstract: We study the EUV/soft X-ray emission generated by charge transfer between solar wind heavy ions and interstellar neutral atoms and variations of the X-ray intensities and spectra with the line of sight direction, the observer location, the solar cycle phase and the solar wind anisotropies, and a temporary enhancement of the solar wind similar to the event observed by Snowden et al. (2004) during the XMM-Hubble Deep Field North exposure. Methods.Using recent observations of the neutral atoms combined with updated cross-sections and cascading photon spectra we have computed self-consistent distributions of interstellar hydrogen, helium and highly charged solar wind ions for a stationary solar wind and we have constructed monochromatic emission maps and spectra. We have evaluated separately the contribution of the heliosheath and heliotail, and included X-ray emission of the excited solar wind ions produced in sequential collisions to the signal. Results.In most practicable observations, the low and medium latitude X-ray emission is significantly higher at minimum activity than at maximum, especially around December. This occurs due to a strong depletion of neutrals during the high activity phase, which is not compensated by an increase of the solar wind flux. For high latitudes the emission depends on the ion species in a complex way. Intensity maps are in general significantly different for observations separated by six-month intervals. Secondary ions are found to make a negligible contribution to the X-ray line of sight intensities, because their density becomes significant only at large distances. The contribution of the heliosheath-heliotail is always smaller than 5%. We can reproduce both the intensity range and the temporal variation of the XMM-HDFN emission lines in the 0.52-0.75 keV interval, using a simple enhanced solar wind spiral stream. This suggests a dominant heliospheric origin for these lines, before, during and also after the event.

Interaction between the solar wind and interstellar gas: A comparison between Monte Carlo and fluid approaches

Abstract: [1] The solar wind is a highly supersonic stream of ionized gas which creates a large bubble of plasma around the Sun. This bubble, termed the heliosphere, is stretched like a raindrop by the supersonic flow of partially ionized interstellar gas. In this paper we present a detailed comparison between the kinetic Monte Carlo and multiple neutral fluid approaches to modeling hydrogen atoms of interstellar and heliospheric origins within an axisymmetric heliosphere, coupled through charge exchange to the ionized plasma. We choose a set of parameters and boundary conditions which allow a further comparison with existing independently obtained results. Our results show a similar overall heliospheric structure, with a heliopause in approximately the same place, for both the kinetic and fluid models. The termination shock, on the other hand, is moved about 5 AU inward and the bow shock is weakened and moved about 8 AU farther out in the kinetic model. More critically, the hydrogen wall amplitude is about 50% above the interstellar value in the fluid case but only 25% in the kinetic case, and the level of hydrogen inside the termination shock is about 20% higher in the kinetic model. The velocity distributions at various locations along the symmetry axis show that while the kinetic model predicts a smooth distribution, the total fluid distribution is more disjointly structured. We also find that neutral atoms produced in the inner heliosheath have a “half-Maxwellian” distribution outside the heliopause, something which cannot be realized within the fluid model.

Coronal Mass Ejections

Abstract: A Brief History of CME Science.- Coronal Mass Ejections: Overview of Observations.- In-Situ Solar Wind and Magnetic Field Signatures of Interplanetary Coronal Mass Ejections.- An Introduction to CMEs and Energetic Particles.- An Introduction to Theory and Models of CMEs, Shocks, and Solar Energetic Particles.- An Introduction to the pre-CME Corona.- Solar Imprint on ICMEs, Their Magnetic Connectivity, and Heliospheric Evolution.- ICMEs in the Outer Heliosphere and at High Latitudes: an Introduction.- Coronal Observations of CMEs.- Understanding Interplanetary Coronal Mass Ejection Signatures.- Energetic Particle Observations.- CME Theory and Models.- The Pre-CME Sun.- Multi-Wavelength Observations of CMEs and Associated Phenomena.- ICMEs in the Inner Heliosphere: Origin, Evolution and Propagation Effects.- ICMEs at High Latitudes and in the Outer Heliosphere.- CME Disturbance Forecasting.- Coronal Mass Ejections.

ICMEs in the inner heliosphere: origin, evolution and propagation effects

Abstract: This report assesses the current status of research relating the origin at the Sun, the evolution through the inner heliosphere and the effects on the inner heliosphere of the interplanetary counterparts of coronal mass ejections (ICMEs). The signatures of ICMEs measured by in-situ spacecraft are determined both by the physical processes associated with their origin in the low corona, as observed by space-borne coronagraphs, and by the physical processes occurring as the ICMEs propagate out through the inner heliosphere, interacting with the ambient solar wind. The solar and in-situ observations are discussed as are efforts to model the evolution of ICMEs from the Sun out to 1 AU.

A new look into the Helios dust experiment data: presence of interstellar dust inside the Earth's orbit

Abstract: An analysis of the Helios in situ dust data for interstellar dust (ISD) is presented in this work. Recent in situ dust measurements with impact ionization detectors on-board various spacecraft (Ulysses, Galileo ,a ndCassini) showed the deep penetration of an ISD stream into the Solar System. The Helios dust data provide a unique opportunity to monitor and study the ISD stream alteration at very close heliocentric distances. This work completes therefore the comprehensive picture of the ISD stream properties within the heliosphere. In particular, we show that gravitation focusing facilitates the detection of big ISD grains (micrometer-size), while radiation pressure prevents smaller grains from penetrating into the innermost regions of the Solar System. A flux value of about 2.6±0.3×10 −6 m −2 s −1 is derived for micrometer-size grains. A mean radiation pressure-to-gravitation ratio (so-called β ratio) value of 0.4 is derived for the grains, assuming spheres of astronomical silicates to modelize the grains surface optical properties. From the ISD flux measured on the Helios trajectory, we infer a lower limit of 3 ± 3 × 10 −25 kg m −3 to the spatial mass density of micron-sized grains in the Local Interstellar Cloud (LIC). In addition, compositional clues for ISD grains are obtained from the data provided by the time-of-flight mass spectrometer subsystem of the Helios instrument. No clustering of single minerals is observed but rather a varying mixture of various minerals and carbonaceous compounds.

Multi-Spacecraft Observations of Solar Flare Particles in the Inner Heliosphere

Abstract: For a number of impulsive solar particle events we examine variations of maximum intensities and times to maximum intensity as a function of longitude, using observations from the two Helios spacecraft and near the Earth. We find that electrons in the MeV range can be detected more than 80 deg. from the flare longitude, corresponding to a considerably wider "well connected" region than that (approx. 20 deg. half width) reported for He-3-rich impulsive solar events. This wide range and the decrease of peak intensities with increasing connection angle revive the concept of some propagation process in the low corona that has a diffusive nature. Delays to the intensity maximum are not systematically correlated with connection angles. We argue that interplanetary scattering parallel to the average interplanetary magnetic field, that varies with position in space, plays an important role in flare particle events. In a specific case variations of the time profiles with radial distance and with particle rigidity are used to quantitatively confirm spatial diffusion. For a few cases near the edges of the well connected region the very long times to maximum intensity might result from interplanetary lateral transport.

Preliminary three‐dimensional analysis of the heliospheric response to the 28 October 2003 CME using SMEI white‐light observations

Abstract: [1] The Solar Mass Ejection Imager (SMEI) has recorded the inner heliospheric response in white-light Thomson scattering to the 28 October 2003 coronal mass ejection (CME). This preliminary report shows the evolution of this particular event in SMEI observations, as we track it from a first measurement at approximately 20° elongation (angular distance) from the solar disk until it fades in the antisolar hemisphere in the SMEI 180° field of view. The large angle and spectrometric coronagraph (LASCO) images show a CME and an underlying bright ejection of coronal material that is associated with an erupting prominence. Both of these are seen by SMEI in the interplanetary medium. We employ a three-dimensional (3-D) reconstruction technique that derives its perspective views from outward flowing solar wind to reveal the shape and extent of the CME. This is accomplished by iteratively fitting the parameters of a kinematic solar wind density model to both SMEI white-light observations and Solar-Terrestrial Environment Laboratory (STELab), interplanetary scintillation (IPS) velocity data. This modeling technique separates the true heliospheric signal in SMEI observations from background noise and reconstructs the 3-D heliospheric structure as a function of time. These reconstructions allow separation of the 28 October CME from other nearby heliospheric structure and a determination of its mass. The present results are the first utilizing this type of 3-D reconstruction with the SMEI data. We determine an excess-over-ambient mass for the southward moving ejecta associated with the prominence material of 7.1 × 1016 g and a total mass of 8.9 × 1016 g. Preliminary SMEI white-light calibration indicates that the total mass of this CME including possible associated nearby structures may have been as much as ∼2.0 × 1017 g spread over much of the earthward facing hemisphere.

Heliospheric Response to Different Possible Interstellar Environments

Abstract: At present, the heliosphere is embedded in a warm, low-density interstellar cloud that belongs to a cloud system flowing through the local standard of rest with a velocity near ~18 km s-1 The velocity structure of the nearest interstellar material (ISM), combined with theoretical models of the local interstellar cloud (LIC), suggest that the Sun passes through cloudlets on timescales of ≤103-104 yr, so the heliosphere has been, and will be, exposed to different interstellar environments over time By means of a multifluid model that treats plasma and neutral hydrogen self-consistently, the interaction of the solar wind with a variety of partially ionized ISM is investigated, with the focus on low-density cloudlets such as are currently near the Sun Under the assumption that the basic solar wind parameters remain/were as they are today, a range of ISM parameters (from cold neutral to hot ionized, with various densities and velocities) is considered In response to different interstellar boundary conditions, the heliospheric size and structure change, as does the abundance of interstellar and secondary neutrals in the inner heliosphere, and the cosmic-ray level in the vicinity of Earth Some empirical relations between interstellar parameters and heliospheric boundary locations, as well as neutral densities, are extracted from the models

Forming a constant density medium close to long gamma-ray bursts

Abstract: Aims. The progenitor stars of long Gamma-Ray Bursts (GRBs) are thought to be Wolf-Rayet stars, which generate a massive and energetic wind. Nevertheless, about 25 percent of all GRB afterglows light curves indicate a constant density medium close to the exploding star. We explore various ways to produce this, by creating situations where the wind termination shock arrives very close to the star, as the shocked wind material has a nearly constant density. Methods. Typically, the distance between a Wolf-Rayet star and the wind termination shock is too large to allow afterglow formation in the shocked wind material. Here, we investigate possible causes allowing for a smaller distance: A high density or a high pressure in the surrounding interstellar medium (ISM), a weak Wolf-Rayet star wind, the presence of a binary companion, and fast motion of the Wolf-Rayet star relative to the ISM. Results. We find that all four scenarios are possible in a limited parameter space, but that none of them is by itself likely to explain the large fraction of constant density afterglows. Conclusions. A low GRB progenitor metallicity, and a high GRB energy make the occurrence of a GRB afterglow in a constant density medium more likely. This may be consistent with constant densities being preferentially found for energetic, high redshift GRBs.

A hybrid heliospheric modeling system: Background solar wind

Abstract: [1] We describe a Sun-to-Earth system of coupled models. Our main goal is to create a real-time, three-dimensional (3-D), MHD-based system to aid in the operational forecasting of geomagnetic activity, but we expect the system to have other uses. We give here our initial survey of the system's characteristics. The Hybrid Heliospheric Modeling System (HHMS) is composed of two physics-based models, combined with two simple empirical models. The physics-based models are a source surface (potential field) current sheet model for the corona and a time-dependent 3-D MHD solar wind model. The system is driven by a sequence of photospheric magnetic maps composed from daily magnetograms. An empirical relationship between magnetic flux tube expansion factor and solar wind speed at 0.1 AU is a key element of the system. The solar wind model gives a predicted time series of MHD parameters at the location of Earth in the model grid; this is verified against Omni, Wind, or ACE satellite data, depending on the time period. The predicted solar wind at Earth is used as input to the second, data-based, empirical model to predict the geomagnetic Ap index. We compare test results for simulated 1 day ahead Ap forecasts for the years 1993 through 2002 with forecast skill of the official Ap forecasts that were issued by the NOAA Space Environment Center in that time interval. Results show the HHMS would have been useful to forecasters in some years. Simulations of transient events such as coronal mass ejections and interplanetary shocks with the HHMS will be reported on later.

Solar Mass Ejection Imager (SMEI) observations of coronal mass ejections (CMEs) in the heliosphere

Abstract: [1] The Solar Mass Ejection Imager (SMEI) on the Coriolis spacecraft has been obtaining white light images of nearly the full sky every 102 minutes for three years. We present statistical results of analysis of the SMEI observations of coronal mass ejections (CMEs) traveling through the inner heliosphere; 139 CMEs were observed during the first 1.5 years of operations. At least 30 of these CMEs were observed by SMEI to propagate out to 1 AU and beyond and were associated with major geomagnetic storms at Earth. Most of these were observed as frontside halo events by the SOHO LASCO coronagraphs.

Inverse Compton Scattering on Solar Photons, Heliospheric Modulation, and Neutrino Astrophysics

Abstract: We study the inverse Compton scattering of solar photons by Galactic cosmic-ray electrons We show that the γ-ray emission from this process is substantial, with the maximum flux in the direction of the Sun; the angular distribution of the emission is broad This previously neglected foreground should be taken into account in studies of the diffuse Galactic and extragalactic γ-ray emission Furthermore, observations by GLAST can be used to monitor the heliosphere and determine the electron spectrum as a function of position from distances as large as Saturn's orbit to close proximity of the Sun, thus enabling unique studies of solar modulation This paves the way for the determination of other Galactic cosmic-ray species, primarily protons, near the solar surface, which will lead to accurate predictions of γ-rays from p-p interactions in the solar atmosphere These albedo γ-rays will be observable by GLAST, allowing the study of deep atmospheric layers, magnetic field(s), and cosmic-ray cascade development The latter is necessary to calculate the neutrino flux from p-p interactions at higher energies (>1 TeV) Although this flux is small, it is a "guaranteed flux" in contrast to other astrophysical sources of neutrinos and may be detectable by km3 neutrino telescopes of the near future, such as IceCube Since the solar core is opaque for very high energy neutrinos, directly studying the mass distribution of the solar core may thus be possible

Forming a constant density medium close to long gamma-ray bursts

Abstract: The progenitor stars of long Gamma-Ray Bursts (GRBs) are thought to be Wolf-Rayet stars, which generate a massive and energetic wind. Nevertheless, about 25 percent of all GRB afterglows light curves indicate a constant density medium close to the exploding star. We explore various ways to produce this, by creating situations where the wind termination shock arrives very close to the star, as the shocked wind material has a nearly constant density. Typically, the distance between a Wolf-Rayet star and the wind termination shock is too large to allow afterglow formation in the shocked wind material. Here, we investigate possible causes allowing for a smaller distance: A high density or a high pressure in the surrounding interstellar medium (ISM), a weak Wolf-Rayet star wind, the presence of a binary companion, and fast motion of the Wolf-Rayet star relative to the ISM. We find that all four scenarios are possible in a limited parameter space, but that none of them is by itself likely to explain the large fraction of constant density afterglows. A low GRB progenitor metallicity, and a high GRB energy make the occurrence of a GRB afterglow in a constant density medium more likely. This may be consistent with constant densities beingpreferentially found for energetic, high redshift GRBs.

Heliospheric Response to Different Possible Interstellar Environments

Abstract: At present, the heliosphere is embedded in a warm low density interstellar cloud that belongs to a cloud system flowing through the local standard of rest with a velocity near ~18 km/s. The velocity structure of the nearest interstellar material (ISM), combined with theoretical models of the local interstellar cloud (LIC), suggest that the Sun passes through cloudlets on timescales of < 10^3 - 10^4 yr, so the heliosphere has been, and will be, exposed to different interstellar environments over time. By means of a multi-fluid model that treats plasma and neutral hydrogen self-consistently, the interaction of the solar wind with a variety of partially ionized ISM is investigated, with the focus on low density cloudlets such as are currently near the Sun. Under the assumption that the basic solar wind parameters remain/were as they are today, a range of ISM parameters (from cold neutral to hot ionized, with various densities and velocities) is considered. In response to different interstellar boundary conditions, the heliospheric size and structure change, as does the abundance of interstellar and secondary neutrals in the inner heliosphere, and the cosmic ray level in the vicinity of Earth. Some empirical relations between interstellar parameters and heliospheric boundary locations, as well as neutral densities, are extracted from the models.

Trains of magnetic holes and magnetic humps in the heliosheath

Abstract: This paper discusses the existence of trains (sequences) of magnetic holes and magnetic humps in the heliosheath, based on Voyager 1 observations made in the intervals DOY 312.9707 - 317.0879, 2005 and DOY 185.2762 - 186.7957, 2005. These two trains represent a class of compressive fluctuations in the heliosheath. Varying from one region or time interval to another, this class of fluctuations probably depends on the varying conditions upstream the Termination Shock and its nature. The trains of magnetic holes in the heliosheath resemble certain magnetic field strength fluctuations observed in planetary magnetosheaths.

A three-dimensional MHD solar wind model with pickup protons

Abstract: [1] We have developed a three-dimensional (3-D) steady-state MHD model of the solar corona and solar wind that covers the region from the coronal base to 100 AU and that accounts for the effects of pickup protons in the distant heliosphere. The model expands the two-region model of Usmanov and Goldstein (2003) to include a region III that extends from 1–100 AU and incorporates a population of interstellar neutral hydrogen and its interaction with solar wind protons. Following the approach of Isenberg (1986) and Whang (1998), we consider the solar wind outside 1 AU as a combination of three comoving species, solar wind protons, electrons, and pickup protons, and solve the 3-D steady-state MHD equations with source terms due to photoionization and charge exchange. Separate energy equations are included for solar wind and pickup protons. We show that the pickup protons cause a deceleration of the solar wind and an increase in average plasma temperature with heliocentric distance beyond ∼10 AU. We compute the global structure of the solar wind from the coronal base to 100 AU and compare our results with Voyager 1 and 2 observations.

Heliospheric magnetic field polarity inversions driven by radial velocity field structures

Abstract: Magnetic field polarity inversions embedded in the predominantly unipolar fast solar wind have been observed by the Ulysses spacecraft at high latitudes. Such reversals have the nature of folded back field lines which we suggest are generated by the interaction of standard large amplitude, low frequency, Alfvenic turbulence with velocity shears in the fast solar wind. We present 2D magnetohydrodynamic simulations of a very low frequency and high amplitude Alfven wave propagating away from the sun embedded in a velocity shear structure such as a microstream and show how reversals in the magnetic field lines are generated naturally on a time-scale consistent with their observation at Ulysses. The generated magnetic field and plasma signals are similar to those observed. We discuss the role turbulence-stream shear interactions might play in limiting differential velocities in the asymptotic high speed solar wind. Citation: Landi, S., P. Hellinger, and M. Velli (2006), Heliospheric magnetic field polarity inversions driven by radial velocity field structures, Geophys. Res. Lett., 33, L14101, doi:10.1029/2006GL026308.