Search

This paper analyses magnetosphere-ionosphere (MI) coupling from a perspective that is independent of inertial reference frame, explicitly acknowledging the role of special relativity in MI coupling...

We extend our earlier Particle Acceleration and Transport in the Heliosphere (PATH) model to study particle acceleration and transport at a coronal mass ejection (CME)-driven shock. We model the propagation of a CME-driven shock in the ecliptic plane using the ZEUS-3D code from 20 solar radii to 2 AU. As in the previous PATH model, the initiation of the CME-driven shock is simplified and modeled as a disturbance at the inner boundary. Different from the earlier PATH model, the disturbance is now longitudinally dependent. Particles are accelerated at the 2-D shock via the diffusive shock acceleration mechanism. The acceleration depends on both the parallel and perpendicular diffusion coefficients κ|| and κ⊥ and is therefore shock-obliquity dependent. Following the procedure used in Li, Shalchi, et al. (2012), we obtain the particle injection energy, the maximum energy, and the accelerated particle spectra at the shock front. Once accelerated, particles diffuse and convect in the shock complex. The diffusion and convection of these particles are treated using a refined 2-D shell model in an approach similar to Zank et al. (2000). When particles escape from the shock, they propagate along and across the interplanetary magnetic field. The propagation is modeled using a focused transport equation with the addition of perpendicular diffusion. We solve the transport equation using a backward stochastic differential equation method where adiabatic cooling, focusing, pitch angle scattering, and cross-field diffusion effects are all included. Time intensity profiles and instantaneous particle spectra as well as particle pitch angle distributions are shown for two example CME shocks.

A Polar magnetosonic wave (MSW) study was conducted using 1 year of 1996–1997 data (during solar minimum). Waves at and inside the plasmasphere were detected at all local times with a slight preference for occurrence in the midnight-postmidnight sector. Wave occurrence (and intensities) peaked within~±5° of the magnetic equator, with half maxima at ~±10°. However, MSWs were also detected as far from the equator as +20° and 60° MLAT but with lower intensities. An extreme MSW intensity event of amplitude Bw = ~± 1 nT and Ew = ~± 25 mV/m was detected. This event occurred near local midnight, at the plasmapause, at the magnetic equator, during an intense substorm event, e.g., a perfect occurrence. These results support the idea of generation by protons injected from the plasma sheet into the midnight sector magnetosphere by substorm electric fields. MSWs were also detected near noon (1259 MLT) during relative geomagnetic quiet (low AE). A possible generation mechanism is a recovering/expanding plasmasphere engulfing preexisting energetic ions, in turn leading to ion instability. The wave magnetic field components are aligned along the ambient magnetic field direction, with the wave electric components orthogonal, indicating linear wave polarization. The MSW amplitudes decreased at locations further from the magnetic equator, while transverse whistler mode wave amplitudes (hiss) increased. We argue that intense MSWs are always present somewhere in the magnetosphere during strong substorm/convection events. We thus suggest that modelers use dynamic particle tracing codes and the maximum (rather than average) wave amplitudes to simulate wave-particle interactions.

In gradual solar energetic particle (SEP) events, protons and heavy ions are often accelerated to >100 MeV/nucleon at a CME-driven shock. In this work, we study particle acceleration at an oblique shock by extending our earlier particle acceleration and transport in heliosphere (PATH) code to include shocks with arbitrary θ BN , where θ BN is the angle between the upstream magnetic field and the shock normal. Instantaneous particle spectra at the shock front are obtained by solving the transport equation using the total diffusion coefficient κ , which is a function of the parallel diffusion coefficient κ ∥ and the perpendicular diffusion coefficient κ ⊥ . In computing κ ∥ and κ ⊥ , we use analytic expressions derived previously. The particle maximum energy at the shock front as a function of time, the time intensity profiles and particle spectra at 1 AU for five θ BN ’s are calculated for an example shock.

Cosmic rays of solar and galactic origin at energies >100 MeV/n charge and induce spurious forces on free-floating test masses on board interferometers devoted to gravitational wave detection in space. LISA Pathfinder (LISA-PF), the technology testing mission for eLISA/NGO, will carry radiation monitors for on board test-mass charging monitoring. We present here the results of a simulation of radiation monitor performance during the evolution of solar energetic particle (SEP) events of different intensity. This simulation was carried out with the Fluka Monte Carlo package by taking into account for the first time both energy and spatial distributions of solar protons for the SEP events of 23 February 1956, 15 November 1960 and 7 May 1978. Input data for the Monte Carlo simulations was inferred from neutron monitor measurements. Conversely, for the SEP event of 13 December 2006 observed by the PAMELA experiment in space, we used the proton pitch angle distribution (PAD) computed from the Particle Acceleration and Transport in the Heliosphere (PATH) code. We plan to adopt this approach at the time of LISA-PF data analysis in order to optimize the correlation between radiation monitor observations and test-mass charging. The results of this work can be extended to the future space interferometers and other space missions carrying instruments for SEP detection.

The solar and interplanetary causes of major middle latitude ionospheric disturbances are reviewed. Solar flare photons can cause abrupt (within ~5 min), 30% increases in ionospheric total electron content, a feature that can last for tens of minutes to hours, depending on the altitude of concern. Fast interplanetary coronal mass ejection sheath fields and magnetic clouds can cause intense magnetic storms if the field in either region is intensely southward for several hours or more. If the field conditions in both regions are southward, "double storms" will occur. Multiple interplanetary fast forward shocks "pump up" the sheath magnetic field, leading to conditions that can lead to superstorms. Magnetic storm auroral precipitation and Joule heating cause pressure waves that propagate from subauroral latitudes to middle and equatorial latitudes. Shocks can create middle latitude dayside auroras as well as trigger nightside subauroral supersubstorms. Solar wind ram pressure increases after fast shocks can lead to the formation of new radiation belts under proper conditions. Prompt penetration electric fields can cause a dayside ionospheric superfountain, leading to plasma transport from the equatorial region to middle latitudes. The large amplitude Alfven waves present in solar wind highspeed streams cause sporadic magnetic reconnection, plasma injections, and electromagnetic chorus wave generation. Energetic electrons interacting with chorus (and PC5) waves are accelerated to hundreds of keV up to MeV energies.

We compare the efficiency of proton acceleration at oblique strong CME shocks at radial heliocentric distances from 0.13 to 2.5 AU. We use the PATH code to obtain the diffusion coefficients and maximum achievable particle energies at shocks with 15°, 45° and 75° shock angles. In computing the maximum energy, we use the total diffusion coefficient which is a function of the parallel diffusion coefficient and the perpendicular diffusion coefficient. The parallel diffusion coefficient is calculated from the wave intensity of Alfven waves generated by streaming protons in the shock vicinity [1, 2]. The perpendicular diffusion is described in the NLGC approach and depends on the parallel diffusion coefficient and pre-existing turbulence in the solar wind [3, 4, 6]. It is shown that efficiency of particle acceleration at a fast CME-driven shock depends on the shock angle, injection energy and radial heliocentric distance. Quasi-perpendicular strong shocks are more efficient in accelerating higher-energy seed particles, especially at larger heliocentric distances and close to the Sun. By contrast, quasi-parallel strong shocks can accelerate particles more efficiently at distances about 1 AU.

The Particle Acceleration and Transport in the Heliosphere (PATH) numerical code was developed to understand solar energetic particle (SEP) events in the near-Earth environment. We discuss simulation results for the 13 December 2006 SEP event. The PATH code includes modeling a background solar wind through which a CME-driven oblique shock propagates. The code incorporates a mixed population of both flare and shock-accelerated solar wind suprathermal particles. The shock parameters derived from ACE measurements at 1 AU and observational flare characteristics are used as input into the numerical model. We assume that the diffusive shock acceleration mechanism is responsible for particle energization. We model the subsequent transport of particles originated at the flare site and particles escaping from the shock and propagating in the equatorial plane through the interplanetary medium. We derive spectra for protons, oxygen, and iron ions, together with their time-intensity profiles at 1 AU. Our modeling results show reasonable agreement with in situ measurements by ACE, STEREO, GOES, and SAMPEX for this event. We numerically estimate the Fe/O abundance ratio and discuss the physics underlying a mixed SEP event. We point out that the flare population is as important as shock geometry changes during shock propagation for modeling time-intensity profiles and spectra at 1 AU. The combined effects of seed population and shock geometry will be examined in the framework of an extended PATH code in future modeling efforts.

Solar energetic particle (SEP) events are traditionally classified as "impulsive" or "gradual." It is now widely accepted that in gradual SEP events, particles are accelerated at coronal mass ejection-driven (CME-driven) shocks. In many of these large SEP events, particle spectra exhibit double power law or exponential rollover features, with the break energy or rollover energy ordered as (Q/A)^α, with Q being the ion charge in e and A the ion mass in units of proton mass m_p . This Q/A dependence of the spectral breaks provides an opportunity to study the underlying acceleration mechanism. In this paper, we examine how the Q/A dependence may depend on shock geometry. Using the nonlinear guiding center theory, we show that α ~ 1/5 for a quasi-perpendicular shock. Such a weak Q/A dependence is in contrast to the quasi-parallel shock case where α can reach 2. This difference in α reflects the difference of the underlying parallel and perpendicular diffusion coefficients κ_(||) and κ ⊥. We also examine the Q/A dependence of the break energy for the most general oblique shock case. Our analysis offers a possible way to remotely examine the geometry of a CME-driven shock when it is close to the Sun, where the acceleration of particle to high energies occurs.

(Abridged abstract) A theory of incompressible MHD turbulence recently developed by Boldyrev predicts the existence of a scale dependent angle of alignment between velocity and magnetic field fluctuations that is proportional to the lengthscale of the fluctuations to the power 1/4. In this study, plasma and magnetic field data from the Wind spacecraft are used to investigate the angle between velocity and magnetic field fluctuations in the solar wind as a function of the timescale of the fluctuations and to look for the power law scaling predicted by Boldyrev., Particle Acceleration and Transport in the Heliosphere and Beyond, 7th Annual International Astrophysics Conference, Kauai, Hawaii, G. Li, Q. Hu, O. Verkhoglyadova, G. P. Zank, R. P. Lin, J. Luhmann (eds), AIP Conference Proceedings 1039, 81-86

The largest solar energetic particle (SEP) events are thought to be due to particle acceleration at a shock driven by a fast coronal mass ejection (CME). We investigate the efficiency of this process by comparing the total energy content of energetic particles with the kinetic energy of the associated CMEs. The energy content of 23 large SEP events from 1998 through 2003 is estimated based on data from ACE, GOES, and SAMPEX, and interpreted using the results of particle transport simulations and inferred longitude distributions. CME data for these events are obtained from SOHO. When compared to the estimated kinetic energy of the associated coronal mass ejections (CMEs), it is found that large SEP events can extract ~10% or more of the CME kinetic energy. The largest SEP events appear to require massive, very energetic CMEs.

Using data from the Solar Isotope Spectrometer (SIS) on the Advanced Composition Explorer (ACE) mission, heavy ion composition measurements have been made in 26^3He-rich solar energetic particle (SEP) events that occurred between 1998 and 2004. Relative abundances of 13 elements from C through Ni have been investigated, as have the isotopic compositions of the elements Ne and Mg. We find a general tendency for the abundances to follow trends similar to those found in gradual SEP events, in which fractionation can be represented in the form of a power-law in Q/M. However several deviations from this pattern are noted that may provide useful diagnostics of the acceleration process occurring in solar flares.

We have surveyed He-Fe spectra for 41 Corotating Interaction Regions (CIRs) from 1998–2007 observed on ACE. The spectra are similar for all species, and have the form of broken power laws with the spectral break occurring at a few MeV/nucleon. Except for overabundances of He and Ne, the abundances are close to those of the solar wind. We find the rare isotope ^3He is enhanced in ~40% of the events. In individual CIRs the Fe/O ratio correlates strongly with the solar wind Fe/O ratio measured 2–4 days prior to the CIR passage. Taken together with previously reported observations of pick-up He^+ in CIRs, these observations provide evidence that CIRs are accelerated out of a suprathermal ion pool of heated solar wind ions, pick-up ions, and remnant suprathermal ions from impulsive solar energetic particle (SEP) events.

The acceleration of interstellar pick-up ions as well as solar wind species has been observed at a multitude of interplanetary (IP) shocks by different spacecraft. This paper expands upon previous work modeling the phase space distributions of accelerated ions associated with the shock event encountered on day 292 of 1991 by the Ulysses mission at 4.5 AU. A kinetic Monte Carlo simulation is employed here to model the diffusive acceleration process. This exposition presents recent developments pertaining to the incorporation into the simulation of the diffusive characteristics incurred by field line wandering (FLW), according to the work of Giacalone and Jokipii. For a pure field-line wandering construct, it is determined that the upstream spatial ramp scales are too short to accommodate the HI-SCALE flux increases for 200 keV protons, and that the distribution function for H+ somewhat underpopulates the combined SWICS/HI-SCALE spectra at the shock. This contrasts our earlier theory/data comparison where it was demonstrated that diffusive transport in highly turbulent fields according to kinetic theory can successfully account for both the proton distributions and upstream ramp scales, using a single turbulence parameter. The principal conclusion here is that, in a FLW scenario, the transport of ions across the mean magnetic field is slightly less efficient than is required to effectively trap energetic ions within a few Larmor radii of the shock layer and thereby precipitate efficient acceleration. This highlights the contrast between ion transport in highly turbulent shock environs and remote, less-disturbed interplanetary regions., 6 pages, 1 embedded figure, to appear in Proc. of the 7th IGPP International Astrophysics Conference "Particle Acceleration and Transport in the Heliosphere and Beyond" (2008), eds. G. Li, et al. (AIP Conf. Proc., New York)

We present numerical simulations modeling acceleration of ACRs at a blunt termination shock with the spiral field lines intersecting the shock multiple times. The 2‐D model includes parallel and perpendicular diffusion. Our results confirm the qualitative expectations: the nose segment of the blunt shock is inefficient to accelerate particles to ACR energies, which results in a continuing increase of the ACR flux and a gradual unfolding of the ACR spectrum behind the shock, as seen by Voyager‐1. We also present a simple analytically tractable test‐case to illustrate physical effects at a 2‐D shock with a structure along the shock‐front.

Using high-resolution mass spectrometers on board the Advanced Composition Explorer (ACE), we surveyed the event-averaged ~0.1-60 MeV/nuc heavy ion elemental composition in 64 large solar energetic particle (LSEP) events of cycle 23. Our results show the following: (1) The rare isotope ^3He is greatly enhanced over the corona or the solar wind values in 46% of the events. (2) The Fe/O ratio decreases with increasing energy up to ~10 MeV/nuc in ~92% of the events and up to ~60 MeV/nuc in ~64% of the events. (3) Heavy ion abundances from C-Fe exhibit systematic M/g-dependent enhancements that are remarkably similar to those seen in ^3He-rich SEP events and CME-driven interplanetary (IP) shock events. Taken together, these results confirm the role of shocks in energizing particles up to ~60 MeV/nuc in the majority of large SEP events of cycle 23, but also show that the seed population is not dominated by ions originating from the ambient corona or the thermal solar wind, as previously believed. Rather, it appears that the source material for CME-associated large SEP events originates predominantly from a suprathermal population with a heavy ion enrichment pattern that is organized according to the ion's mass-per-charge ratio. These new results indicate that current LSEP models must include the routine production of this dynamic suprathermal seed population as a critical pre-cursor to the CME shock acceleration process.

A brief narrative description of the phenomenon of dynamical complexity is presented. Intermittency characteristics of such phenomena are considered based on the rank—ordered multifractal spectrum, a method recently introduced by Chang and Wu. Analyses of data from MHD simulations and in‐situ solar wind observations are used to illustrate the basic ideas.

We discuss the influence of magnetic fields on the distribution of parameters in the outer heliosphere. The physical model acknowledges the crucial importance of charge exchange and collisions between neutral and charged particles in the partially ionized plasma and takes into account both the interstellar and interplanetary magnetic fields (ISMF and IMF). We summarize the results obtained for different ISMF orientations and discuss possible consequences of increasing the ISMF magnitude for both the 2–3 kHz radio emission and the deflection of the flow of neutral hydrogen in the inner heliosheath from its original direction in the unperturbed interstellar medium.

A brief review of particle acceleration at inner heliospheric interplanetary and coronal mass ejection (CME)‐driven shocks is presented. The discussion is presented in terms of a specific event that was modeled using the Particle Acceleration and Transport in the Heliosphere model (hereafter PATH) code.

In the present solar minimum, corotating interaction regions (CIRs) produce frequent particle enhancements at 1 AU as observed at STEREO and ACE. As the two STEREO spacecraft move apart, differences in CIR time profiles observed at each spacecraft are becoming large. The timing differences are often roughly similar to the corotation time lag between the two spacecraft, however many of the features seen at Ahead and Behind require more than just a time shift. Perhaps transient disturbances in the solar wind affect connection to or transport from the shock, or temporal changes occur in the CIR shock itself. Additional timing differences of >1 day result from the different heliographic latitudes of the two STEREO spacecraft

The RHESSI spacecraft images hard X‐ray emission from solar flares with an angular resolution down to ∼2″ and an energy resolution of 1 keV. For such a Rotating Modulation Collimator (RMC) instrument, imaging information is gathered not as a set of spatial images, but rather as a set of (energy‐dependent) spatial Fourier components (termed visibilities). We report here on a novel technique which uses these spatial Fourier components in count space to derive, via a regularized spectral inversion process, the corresponding spatial Fourier components for the electron distribution, in such a way that the resulting electron visibilities, and so the images that are constructed from them, vary smoothly with electron energy E. “Stacking” such images then results in smooth, physically plausible, electron spectra for prominent features in the flare.Application of visibility‐based analysis techniques has also permitted an assessment of the density and volume of the electron acceleration region, and so the number of ...

The two basic types of particle acceleration at the Sun—a flare‐resident process (or processes) and acceleration at coronal shock waves—were first identified in solar metric radio emissions through their associated type III bursts and type II bursts, respectively. A key question for solar energetic particle (SEP) physics today concerns the relative contributions of flares and shocks to large gradual SEP events, particularly at >30 MeV energies. We address this question by: (1) comparing low‐frequency (∼1 MHz) radio emissions for samples of the largest gradual and impulsive SEP events from cycle 23; and (2) determining SEP associations for a sample of large favorably‐located low‐frequency type III radio bursts. Our results indicate that a strong shock, as commonly manifested by a low‐frequency ( 30 MeV proton event. We propose a revision to the standard two‐class paradigm for SEP events in which we subdivide the current gradual ...

Particle acceleration in Supernova Remnant shocks has been known since the discovery of synchrotron emission from young SNRs. However, it has been difficult to pin down the acceleration efficiency even for the electrons observed. This paper summarizes the observational constraints on electron acceleration, then discusses constraints on the ratio of cosmic ray to gas pressure based on compression and electron temperature, and finally discusses evidence for shock wave precursors and constraints on the cosmic ray diffusion coefficient.

In mid December 2006 several flares on the Sun occurred in rapid succession, spawning several CMEs and bathing the Earth in multiple solar energetic particle (SEP) events. One such SEP event occurring on December 14 was observed at the Earth just as an interplanetary CME (ICME) from a previous flare on December 13 was transiting the Earth. Although solar wind observations during this time show typical energetic proton fluxes from the prior SEP event and IP shock driven ahead of the ICME, as the ICME passes the Earth unusual energetic particle signatures are observed. Measurements from ACE, Wind, and STEREO show proton flux variations at energies ranging from ∼3 MeV up to greater than 70 MeV. Energetic electron signatures from ACE show similar variations. Within the Earth’s magnetosphere Polar HIST also sees these proton flux variations at energies greater than 10 MeV while crossing open field lines in the southern polar cap. Although no such variation in the energetic proton flux is observed at the GOES 11 spacecraft in geosynchronous orbit near the subsolar region, differential fluxes observed at GOES 11 and GOES 12 in the 15–40 MeV energy range do show some variability, indicating the signature is observable near dawn and dusk.

A time‐dependent focused transport model is used to model how solar wind protons are injected from the suprathermal tail of the solar wind distribution into diffusive shock acceleration at both a strong and a weak coronal mass ejection‐driven shock. It is discussed how the temporal evolution of the injection and acceleration process can result in solar energetic particle spectra that differ substantially from the prediction of standard steady state diffusive shock acceleration models.

The specific microphysical mechanism which generates the fast solar wind in collisionless coronal holes is still not known. Observations show that the wind results from strong ion heating perpendicular to the magnetic field which starts at the very bottom of the corona. A major clue toward identifying the responsible mechanism is that it preferentially heats the heavy ions relative to the protons. Since this preferential heating occurs at heights where ion collisions are no longer effective, a kinetic treatment is necessary to understand and test any candidate mechanisms. We describe the current state of a kinetic model we are constructing which will test the preferential heating due to the resonant cyclotron interaction with parallel‐propagating ion cyclotron waves.

Electron and proton acceleration by a super‐Driecer electric field is investigated in a reconnecting current sheet (RCS) with guiding field. The particle trajectories and energy spectra are calculated numerically for different magnetic field topologies by solving motion equation in the test particle and PIC approaches. Three mechanisms of particle acceleration inside an RCS are considered: a drifted electric field caused by the plasma inflows formed during a magnetic reconnection process, polarization electric field induced by the accelerated protons and electrons extracted from PIC simulations and turbulent electric field induced by accelerated particles. Electron and proton densities, energy spectra inside an RCS and at ejection are found to be strongly affected by a magnetic field topology and induced electric fields leading to a strong increases of the energy of accelerated particles in addition to the energy gained by the drifted electric field.

In this short review we report about recent findings related to two fundamental points in the study of solar wind turbulence: a) the verification of the equivalent of the −4/5 law in the solar wind and b) the estimate of the energy cascade along the spectrum and its comparison with the heating rate necessary to heat the solar wind during its expansion as deduced from in‐situ measurements. As a matter of fact, a Yaglom‐like scaling relation has recently been found in both high‐latitude and in‐ecliptic data samples. However, analogous scaling law, suitably modified to take into account compressible fluctuations, has been observed in a much more extended fraction of the same data set recorded at high latitude. Thus, it seems that large scale density fluctuations, despite their low amplitude, play a major role in the basic scaling properties of turbulence. The compressive turbulent cascade, moreover, seems to be able to supply the energy needed to account for the local heating of the non‐adiabatic solar wind.

Observations of hard X‐ray/γ‐ray continuum and γ‐ray line emission show that electrons are accelerated to >∼100 s of MeV and ions up to GeV energies, respectively, in large solar flares. The flare‐accelerated electrons above ∼20 keV and ions above a few MeV often contain >∼10–50% or more of the total energy released, indicating that the particle acceleration is intimately related to the energy release mechanism. RHESSI observations show strong evidence that both the ion and electron acceleration are associated with the process of magnetic reconnection. Direct in situ observations of solar energetic particles (SEPs) near 1 AU indicate that shock waves driven by fast (>∼1000 km/s) coronal mass ejections (CMEs) accelerate ions and electrons to similarly high energies, at altitudes of ∼2–40 solar radii. Both CMEs and large flares involve the transient release of up to ∼1032–1033 ergs. Frequent acceleration of electrons to ∼10 keV is observed in smaller flares and even in microflares. Radio type III bursts ind...

Electric fields provide the largest part of wave‐particle interactions in the important ion cyclotron frequency range at 1 AU. Hence their study is essential for understanding the turbulent solar wind. In particular, this study would be an important objective for spacecraft closer to the sun.At present, the best measurements of solar wind electric fields have been provided by the EFW experiment on Cluster [1,2]. The Cluster spacecraft are spinning. It would seem that a 3‐axis oriented satellite would provide better measurements, since photoelectric effects on spinning satellites are often in the essential frequency range, and often much larger. That is why electric fields have rarely been measured. On the other hand, the 3‐axis stabilized STEREO spacecraft are apparently unable to measure electric fields in this frequency range. Reasons why this is so will be discussed, as will the implications for further measurements.

Radio synchrotron observations have long shown that electrons are accelerated to mildly relativistic energies in supernova remnants, but only in the last few years has it been conclusively demonstrated that particle acceleration in supernova remnants can reach energies of at least 1014 eV. The key observation here has been the detection of a number of young supernova remnants at TeV energies by the HESS (High Energy Stereoscopic System) collaboration. There is however still debate as to whether the dominant signal comes from electrons or protons. The current observational and theoretical situation will be discussed. Some surprising recent observations by the MILAGRO consortium are also discussed.

It is discussed how a time‐dependent focused transport model, using a time series of shock obliquities at the termination shock based on Voyager 1 observations to model magnetic field‐line random walk, can reproduce observational features of energetic ions at the termination shock and in the heliosheath which is beyond the scope of standard cosmic‐ray transport models.

We review recent progress in the large‐scale modeling of the topology and dynamics of magnetic reconnection at Earth’s dayside magnetopause. We use three‐dimensional global magnetohydrodynamic (MHD) and large‐scale kinetic (LSK) simulations to analyze reconnection events observed by multiple spacecraft. In particular, we examine the occurrence and consequences of magnetic merging at high‐latitudes and in the subsolar region of the dayside magnetopause. We then use these results to discuss large‐scale merging patterns and how high‐energy ions could be used as tracers of the global topology of magnetic reconnection at Earth’s dayside magnetopause.

Global MHD models of Coronal Mass Ejections (CMEs) can provide important insights into the physical processes associated with the eruption and evolution of CMEs and the acceleration of SEPs, and are a valuable tool for interpreting both remote solar and interplanetary in situ observations. Moreover, they represent a virtual laboratory for exploring conditions and regions of space that are not conveniently or currently accessible by spacecraft. The most energetic events typically originate from active regions on the Sun. To accurately model such regions, whilst also capturing the global corona, requires an MHD model that includes energy transport (radiative losses, anisotropic thermal conduction, and coronal heating) in the transition region and corona. Equally importantly, the model must reproduce an accurate ambient solar wind through which the CME propagates. In this report, we describe the current status of modeling efforts, and present three applications that we believe are relevant in studies of ener...

Continuous input of solar data to time‐dependent 3D models is necessary for the study of shock propagation in the solar wind. We have performed time‐dependent 3D simulations using two different models, the full MHD based HHMS model and the kinematic HAF model, to study turbulence, particle acceleration and transport, cosmic ray modulation, and other physically significant phenomena. The continuous solar inputs to these models include solar data from source surface maps, Wang‐Sheeley‐Arge parameters, and information on solar events such as coronal mass ejections, flares, etc. Model output options include the time series at any location of specific solar wind and magnetic field parameters, entropy, momentum flux, shock propagation, longitude and latitude distributions of parameters, meridian slices at any orientation for any parameter throughout the 3D heliosphere, flux ropes, interplanetary coronal mass ejections, corotating interaction regions, merged interaction regions, etc. Through comparisons with in‐...

Type II radio bursts, solar energetic particle (SEP) events, and interplanetary (IP) shocks all have a common cause, viz., fast and wide (speed ⩾900 km/s and width ⩾60°)) coronal mass ejections (CMEs). Deviations from this general picture are observed as (i) lack of type II bursts during many fast and wide CMEs and IP shocks, (ii) slow CMEs associated with type II bursts and SEP events, and (iii) lack of SEP events during many type II bursts. I examine the reasons for these deviations. I also show that ground level enhancement (GLE) events are consistent with shock acceleration because a type II burst is present in every event well before the release of GLE particles and SEPs at the Sun.

We report on Voyager 1 and 2 observations of anomalous cosmic rays in the outer heliosphere. The energy spectrum of anomalous cosmic ray helium as each spacecraft crossed the solar wind termination shock into the heliosheath remained modulated. Assuming the intensity gradient between the two spacecraft is purely radial, we find that radial gradients in the heliosheath of He with 11.6–22.3 MeV/nuc and with ∼61–73MeV/nuc∼61–73 MeV/nuc are 4.9±1.2%/AU4.9±1.2%/AU and 0.0±0.5%/AU,0.0±0.5%/AU, respectively. Strong temporal variations of the 11.6–22.3 MeV/nuc He intensity at both spacecraft were observed in 2005 just after Voyager 1 crossed the termination shock and while Voyager 2 was upstream. After 2006.0, the intensity variations are more moderate and likely due to a combination of spatial and temporal variations. As of early 2008, the anomalous cosmic ray He energy spectrum has unfolded to what may be a source spectrum. The spectrum at Voyager 2 remains modulated. We examine three recent models of the origin of anomalous cosmic rays in light of these observations.

Spacecraft observations (ACE, Wind, Ulysses, Voyager1/2) of energetic particles during the just‐completed Solar Cycle 23 (1996–2007) have provided multiple case studies in different regions throughout the heliosphere where weak scattering seems to have dominated the local transport process for energetic ions and electrons over a wide range of energies. This paper analyzes in detail two diverse phenomena observed near Earth: (1) approximately “scatter‐free” (r 1 AU) propagation of near‐relativistic electrons (rigidities R∼0.5 MV) in impulsive solar energetic particle (SEP) events along magnetic field lines connecting the coronal injection region to spacecraft at 1 AU; and (2) the anisotropy of galactic cosmic rays (rigidities R∼1–10 GV) as measured by the diurnal variation detected by ground‐based neutron monitors. In the scatter‐free limit, weak scattering can be mathematically described by a functional equation, rather than by a linear partial differential equation (as in foc...

Recent studies suggest that there may be flux‐tube‐like structures exist in the solar wind, where upon crossing the boundaries of such structures, the magnetic field directions change significantly. In such a scenario, the turbulence in the solar wind is confined in individual flux tubes and is described locally by the underlying solar wind MHD turbulence. The drastic changes of magnetic field direction introduce another source of MHD turbulence intermittency and can affect the transport of energetic particles. However, to theoretically study transport of energetic particles in solar wind, magnetic field is usually assumed to be composed by a uniform large scale background magnetic field with a superposed turbulent field. In this paper we construct a model turbulence of the solar wind by including the flux‐tube‐like structures. We start by cutting the solar wind randomly into smaller cells. In each of these cells, the magnetic field consists of a uniform mean magnetic field B0 with random direction and a ...

Release of stored magnetic energy via particle acceleration is a characteristic feature of astrophysical plasmas. Magnetic reconnection is one of the primary candidate mechanisms for releasing non‐potential energy from magnetized plasmas. A collisionless magnetic reconnection scenario could provide both the energy release mechanism and the particle accelerator in flares. We studied particle acceleration consequences from fluctuating electric fields superposed on an X‐type magnetic field in collisionless hot solar plasma. This system is chosen to mimic generic features of dynamic reconnection, or the reconnective dissipation of a linear disturbance. Time evolution of thermal particle distributions are obtained by numerically integrating particle orbits. A range of frequencies of the electric field is used, representing a turbulent range of waves. Depending on the frequency and amplitude of the electric field, electrons and ions are accelerated to different degrees and often have energy distributions of different forms. Protons are accelerated to gamma‐ray producing energies and electrons to and above hard X‐ray producing energies in timescales of 1 second. The acceleration mechanism could be applicable to all collisionless plasmas.

A magnificent phenomenon in major solar eruptions is the creation of a long current sheet connecting solar flares to coronal mass ejections (CMEs). The sheet is important as the site where the magnetic energy is converted efficiently into heat and bulk kinetic energy and where particles are accelerated to high energy. With the development made recently in studying the CME/flare current sheet itself and particle accelerations in both theories and observations, our knowledge about the current sheet, its interior structures, and various processes occurring inside the sheet has been improved significantly. We are weaving various manifestations of solar eruptive processes related to the reconnecting current sheets, and looking into physical mechanisms that connect them to one another in the present work. Implications of these manifestations and the corresponding mechanisms to particle accelerations in the sheet are discussed, and some results we obtained recently for the particle acceleration are also displayed.

Using ACE data, we selected a number of characteristic “energetic storm particle” (ESP) events, i.e., solar energetic particles events in which the CME‐driven interplanetary shock passes the spacecraft, to compare observed local proton flux profiles with those obtained from hybrid simulations (kinetic ions, electron fluid). The events were selected for undisturbed solar wind and profiles indicating local shock acceleration. Interestingly, in the sub‐MeV range, we find very little variation of peak proton fluxes with shock normal angle. This is in stark contrast to the Earth’s bow shock, which in both observations and simulations only shows weak reflected ion beams, but no significant turbulence or secondary ion acceleration for shock‐normal angles of ∼45° or larger. We find that in sufficiently large simulations of oblique, planar shocks, even the dilute upstream ion beams suffice to generate upstream compressional waves, which change the local shock‐normal angle upon impact on the shock. In a feed‐back m...

This paper will discuss recent magnetic field observations of the termination shock (TS), heliosheath (HSH), and supersonic solar wind (SW) made by Voyager 1 (V1) and Voyager 2 (V2). Voyager 2 crossed the TS at least 5 times at 83.7 AU and 27.5deg South heliographic latitude during 2007/242.14 to 244.80. The structure of the shock, observed on three of the crossings, was highly variable, ranging from a simple quasi-perpendicular supercritical shock to a complex profile suggesting reformation. Two merged interaction regions were observed in the SW upstream of the TS, and the magnetic field increased to relatively high values during the month prior to crossing the TS. Voyager 2 observations in the HSH will also be discussed. Voyager 1 observed a small radial gradient of the magnetic field strength B in the HSH. It is expected that B should increase from the TS to heliopause, but the available predictions of the profile are not adequate to compare with the observations. Voyager 1 has entered a unipolar region, as the heliospheric current sheet moved below the latitude of V1.

Upstream electron plasma oscillations and broadband bursts of electrostatic waves have been observed in the vicinity of the solar wind termination shock by the Voyager 2 plasma wave instrument. The upstream electron plasma oscillations were first detected on August 1, 2007, at a radial distance of 83.4 astronomical units (AU). These oscillations continued sporadically for about a month until, starting on August 31, three well‐defined bursts of broadband electrostatic waves, similar to those observed at planetary bow shocks, were observed at a heliocentric radial distance of about 83.7 AU. Two of these broadband bursts corresponded to shock crossings identified in the magnetometer and plasma data, and one did not. During the crossings labeled TS‐3 and TS‐4 by the magnetometer and plasma teams, the broadband electrostatic bursts corresponded almost exactly with steep ramps in the magnetic field strength. By scaling the frequencies by the upstream electron plasma frequency and the spectral densities by the u...

We consider the acceleration of charged particles and anomalous cosmic rays (ACR) at propagating shocks and especially at the heliospheric termination shock, when upstream variations cause spatial variations along the shock face, or, alternatively, if the global spatial geometry causes the shock to vary. Here we present the results a new analytic solution for the case of a shock in which the injection low‐energy particles varies with position along the shock face. We find that, downstream of the shock, positive spatial gradients together with modulation‐like and multiple‐power‐law spectral features are naturally produced by plausible spatial variations at the shock. Similar effects also result from large‐scale or global spatial structure. These various effects show promising qualitative agreement with observations at and near the heliospheric termination shock, suggesting that they may play an important role.

There are often two particle components which form a major SEP event, one originating from a solar flare and the other from solar wind particles accelerated at a traveling CME‐driven shock [1]. If a CME and a flare are part of the same process, then the interplay between corresponding energetic particle components may yield temporal, spectral, and compositional differences in observations. Depending on spacecraft location and magnetic connection to either a flare site or a CME‐driven shock (or both), we expect to observe distinct signatures in the time intensity profiles. Following an approach by Li and Zank [2], we apply the Particle Acceleration and Transport in the Heliosphere (PATH) one‐dimensional numerical code developed at University of California in Riverside to model the mixed SEP event of December 13, 2006. We initiate the code by modeling a quiet‐time solar wind. Observed shock parameters at 1 AU and flare characteristics then are used as input into the code. We model energetic particle acceleration at a traveling quasi‐parallel CME‐driven shock and subsequent transport throughout the interplanetary medium to 1 AU. Time‐intensity profiles and spectra of proton and heavy ions are presented and compared with in situ measurements by ACE. Contributions from the solar wind suprathermal and flare particles to the resultant SEP event are discussed.

A suite of 64 global heliospheric models, for which the interstellar densities and temperatures are varied within reasonable bounds, is analyzed with respect to the location of the termination shock on and off the stagnation axis, its temperature, and its compression ratio. The empirical relations regarding the termination shock, the heliopause and the interstellar bow shock, are discussed, as are the physical reasons behind these relations.