Coronal and heliospheric observations of solar particle events

Observations of accelerated particle beams are used to probe the coronal and interplanetary magnetic field structures over large distances from the Sun on the order of a few AU and for various heliolatitudes. It is shown that the propagation of low energy particles is very much controled by discrete interplanetary magnetic field structures. These discrete magnetic structures are sometimes embedded within interplanetary solar wind plasma disturbances, commonly called CMEs. The connection between the corona and the interplanetary medium is discussed. These observations lead to new insights on the origin of accelerated particles detected in association with CMEs.     

Mean free paths of energetic charged particles parallel and antiparallel to the interplanetary magnetic field

A new method to calculate the mean free paths of energetic particles propagating parallel and anti-parallel to the interplanetary magnetic field, based on quasi-linear theory and the complex spectral polarization analysis of the field, is developed and presented. Applications of the method using HEOS 2 (1 AU), Pioneer 10 (5 AU), Pioneer 11 (20 AU), ICE (Giacobini-Zinner's comet) data have been made, showing that: (a) The mean free paths parallel and anti-parallel to the field can be completely different in various regions of the interplanetary medium and different time periods. (b) Particles are preferentially scattered in one direction. (c) The parallel and anti-parallel mean free paths become equal at certain energy. Comparisons with the results from another computational method are made.     

Spectral polarization analysis of the interplanetary magnetic field fluctuations

A new computational method and algorithm, based on complex Fourier analysis, is used to derive the spectral density of plane and circularly polarized fluctuation components of the interplanetary magnetic field. Applications of the method have been made using HEOS 2 (1 AU), Pioneer 10 (5 AU), Pioneer 11 (20 AU), and ICE (Giocabini-Zinner's comet) data sets. The results show the existence of circularly polarized MHD waves in all cases.     

An indication for an intrinsically different coronal temperature

As is well known, the interplanetary magnetic field is the extension of the solar magnetic field, and the solar wind is the extension of the solar corona (Biermann, 1951; Parker, 1958). Consequently, knowledge of these two interplanetary structures reflects a knowledge of the solar corona itself.Calculations of radial proton temperature gradients within magnetic sector boundaries, observed by Helios-1 and Helios-2 heliocentric satellites between 0.3 and 1.0 AU, revealed lower temperatures in the solar corona in the region where these boundaries merge.     

On experimental evidence of chaotic dynamics over short time scales in solar wind and cometary data using nonlinear prediction techniques

Strong indications of chaotic dynamics underlying in the interplanetary and cometary magnetic field fluctuations over short time scales are identified using HEOS-2 spacecraft (at 1 AU), Pioneer 10 (at 4.8 AU), Pioneer 11 (at 20 AU) and ICE (Giacobini-Zinner cometary environment) high-resolution measurements. Other non-chaotic candidate processes, such as linear deterministic models, fractal Brownian motion, and linear gaussian stochastic models are rejected at a high confidence level using nonlinear prediction methods. Experimental proofs of phase correlations are obtained. Assuming chaotic dynamics, estimations of the Kolmogorov-Sinai entropy are provided.     

Field-Line Draping Around ICMES

Magnetic field orientations in the sheaths of ten fast interplanetary coronal mass ejections (ICMEs) that cover the solar longitude range roughly from 20° East to 33° West (as determined from the associated flare or filament disruption) are overlain on the MHD-computed magnetic field pattern showing draping in Earth’s magnetosheath. The general draping pattern is evident in the ICME sheath orientations including, most importantly, the east flank where draping causes the greatest distortion of the magnetic field away from the general Parker spiral. Deviations from the general draping pattern are also evident which, we suggest, result from the history of accretion of the inhomogeneous interplanetary magnetic field (IMF) into the ICME sheath over a long stretch of solar wind before arriving at one AU. The profiles of magnetic field intensity between the ICME shock and the nose of the ICME deviate significantly from the corresponding profile in Earth’s magnetosheath. The ICME samples are much more irregular and show no general tendency to increase toward the stagnation point. We suggest that again this difference reflects the history of IMF accretion by the ICME sheath. The long stretch of accreted inhomogeneous field (a significant fraction of one AU) can account for the irregularity, and the weakness of the field close to the body possibly reflects a weaker ICME shock closer to the Sun.     

Comparative Analyses of the CSSS Calculation in the UCSD Tomographic Solar Observations

We describe a new method to derive the interplanetary magnetic field (IMF) out to 1 AU from photospheric magnetic field measurements. The method uses photospheric magnetograms to calculate a source surface magnetic field at 15R_⊙. Specifically, we use Wilcox Solar Observatory (WSO) magnetograms as input for the Stanford Current-Sheet Source-Surface (CSSS) model. Beyond the source surface the magnetic field is convected along velocity flow lines derived by a tomographic technique developed at UCSD and applied to interplanetary scintillation (IPS) observations. We compare the results with in situ data smoothed by an 18-h running mean. Radial and tangential magnetic field amplitudes fit well for the 20 Carrington rotations studied, which are largely from the active phase of the solar cycle. We show exemplary results for Carrington rotation 1965, which includes the Bastille Day event.     

Implications of Proton Anisotropy Development Observed by the Erne Instrument During the 9 July 1996 Solar Particle Event

We analysed the solar particle event following the 9 July 1996 solar flare. High-energy protons were detected by the ERNE instrument on board SOHO. Anisotropy of arriving protons revealed very peculiar non-monotonic development. A short period of almost isotropic distribution was imbedded into the prolonged period of beam-like distribution of 14–17 MeV protons. This implies the existence of a narrow magnetic channel with a much smaller mean free path than in the surrounding quiet solar wind plasma. We used Monte Carlo simulations of interplanetary transport to fit the observed anisotropies and intensity–time profiles. Proton injection and transport parameters are estimated. The injection scenario is found to be very close to the scenario of the 24 May 1990 event, but the intensity and the interplanetary transport parameters are different. The extreme anisotropy observed implies prolonged injection of high-energy protons at the Sun and at the interplanetary shock front, and either a very large mean free path (≥ 5 AU) outside the slow transport channel, or alternatively, a somewhat smaller mean free path (≈2 AU) and enhanced focusing between the Sun and the Earth.     

Observation of shock-accelerated protons by GIOTTO and IMP-8 under solar minimum conditions in February 1986

Solar and shock-accelerated protons were observed by the interplanetary S/C GIOTTO and aboard the Earth orbiting satellite IMP-8 during solar minimum conditions in February 1986. Forward and reverse shock configurations developed on 9–11 February, 1986 and could be recognized by sunward and anti-sunward proton propagation. The results are consistent with a general east-west asymmetry in particle fluxes observed within a distance of 1 AU from the Sun and caused by quasi-perpendicular and quasi-parallel shock configurations in the interplanetary magnetic field. The high-energy protons (>1 GeV) measured simultaneously by ground-based neutron monitors showed a complementary enhanced amplitude in the diurnal variation on 9 February, 1986.     

MHD study of temporal and spatial evolution of simulated interplanetary shocks in the ecliptic plane within 1 AU

We utilize a 21/2-D MHD time-dependent model to perform a parametric study of interplanetary shock propagation to 1 AU. The input conditions are represented by the following variables:(1) initial shock velocity, (2) duration of the driving pulse, and (3) width of the pulse at the near-Sun position (18 solar radii). The total net energy added to the solar wind was calculated for each pulse. The forward shock's travel time to, and the peak dynamic pressure at, 1 AU as a function of location along the shock front have been studied over a range of total input pulse energies from 10²⁹ to 10³² ergs. For input pulses with modest angular width and temporal duration, we find that the propagation of the resulting interplanetary fast forward shock waves depends primarily upon the net input energy. The dependence of the transit time upon energy is a power law with a -1/3 index which corresponds to the classical, piston driven case. Reverse shocks are also formed behind all but the lowest energy shocks. Their properties, although also a function of input energy, depend upon the specific values of the input pulse shock velocity, width and duration. We also briefly discuss the propagation of the shocks out to 1 AU, and the conditions for which the interplanetary shocks depart from being symmetric about the input pulse central meridian due to magnetic and dynamic effects.     

Directional discontinuities in the interplanetary magnetic field

It is shown that the interplanetary magnetic field has different characteristics on different scales, and it is noted that a given physical theory may not be applicable or relevant on all scales. Four scales are defined in terms of time intervals on which the data may be viewed. Many discontinuities in the magnetic-field direction are seen on the mesoscale ( 4 days, 1 AU). The characteristics of such directional discontinuities which were observed by Pioneer 6 during the period December 16, 1965-January 4, 1966 are presented, with special emphasis on their distribution in time. Previously, it was suggested that such discontinuities are simply boundaries of spaghetti-like filaments extending from the sun to the earth. Here it is shown that on the mesoscale unique filaments with sharp boundaries containing well-ordered magnetic fields are not always seen although discontinuities are always present at 1 AU. Thus, the interplanetary medium appears to be discontinuous rather than filamentary. The filamentary model implies that discontinuities originate at the sun and are convected with the solar wind. The discontinuous model allows the additional possibility that the discontinuities form in the interplanetary medium far from the sun.     

Study on the onsets of solar energetic electron events

Fine time resolution observations of the angular distributions of the intensities of energetic electrons (220 ≤ E _e ≤ 500 keV) by the IMP-7 and 8 spacecraft during the onsets of solar electron events and the technique of mapping the solar wind to the solar corona have been incorporated in this work in order to obtain the large-angle scattering distance of these particles under different configurations of the large scale structure of the interplanetary medium. It is found that in the presence of stream-stream interaction regions with compressed magnetic fields beyong 1 AU, the large-angle scattering is determined by the distance along the streamlines from the spacecraft to their intersection by a faster solar wind stream. In cases of diverging magnetic fields the estimated large-angle scattering distance exceeds 1 AU.     

Effects of the Weak Polar Fields of Solar Cycle 23: Investigation Using OMNI for the STEREO Mission Period

The current solar cycle minimum seems to have unusual properties that appear to be related to weak solar polar magnetic fields. We investigate signatures of this unusual polar field in the ecliptic near-Earth interplanetary magnetic field (IMF) for the STEREO period of observations. Using 1 AU OMNI data, we find that for the current solar cycle declining phase to minimum period the peak of the distribution for the values of the ecliptic IMF magnitude is lower compared to a similar phase of the previous solar cycle. We investigate the sources of these weak fields. Our results suggest that they are related to the solar wind stream structure, which is enhanced by the weak polar fields. The direct role of the solar field is therefore complicated by this effect, which redistributes the solar magnetic flux at 1 AU nonuniformly at low to mid heliolatitudes.     

Sector-structured interplanetary magnetic field associated with the fast plasma streams in 1985–1996

An analysis of 373 well-defined high-speed solar-wind streams observed at 1 AU during the years 1985–1996 is outlined. The distribution of the occurrence of these streams as a function of Bartels rotation days using the dominant polarity of the interplanetary magnetic field (IMF) associated with the referred fast streams shows that a four-sector pattern for the positive IMF polarity and a two-sector pattern for the negative IMF polarity are the dominant features in the investigated period. The high-speed streams seem to occur at preferred Bartels days: positive polarity streams are most frequent near Bartels days 5 and 18, while negative polarity streams are most frequent in days 14 and 23. Moreover, the corotating streams with positive IMF polarity prefer to occur in days 5 and 18 of the Bartels rotation period, whereas flare-generated streams with negative IMF polarity occur in days 14 and 23. The observed distribution of Bartels days is probably related to the distribution of the solar sources of high-speed solar wind streams as the solar wind carries with it the photospheric magnetic polarity of the solar source region. In addition, the distribution of the streams reveals a similar behaviour during the ascending and the declining phase of the last solar cycle (22nd) in contrast to the previous one where it has an opposite appearance. Determined differences in the characteristics of the sector structured IMF associated with the fast streams of the last cycle with the previous one (21st) and some similarities with the alternate solar cycle (20th) seem to be attributed to the 22-year magnetic cycle and to the polarity reversals of the polar magnetic field of the Sun. As the magnetic sectors are due to multiple crossings of the solar equatorial plane by a large-scale, warped heliospheric current sheet, it is suggested that the two-sector pattern arises from a tilted solar magnetic dipole component and the more commonly observed four-sector pattern from a quadrupole component of the solar interplanetary magnetic field.     

Numerical simulations of high-speed solar wind streams within 1 AU and their signatures at 1 AU

A parametric study of the evolution within, and signatures at, 1 AU of high-speed streams is performed with the use of a MHD, 21/2-D, time-dependent model. This study is an extension of an earlier one by Smith and Dryer (1990) who examined the ecliptic plane consequences of relatively short-duration, energetic solar disturbances. The present study examines both the erupting and corotating parts of long-duration, high-speed streams characteristic of coronal hole flows. By examining the variation of the simulated plasma velocity, density, temperature, and magnetic field at 1 AU, as well as the location of the solar coronal hole sources relative to the observer at 1 AU, we are able to provide some insight into the identification of the solar sources of interplanetary disturbances. We present and discuss two definitions for angle locating the solar source of interplanetary disturbances at 1 AU.We apply our results to the suggestion by Hewish (1988) that low-latitude coronal holes are suitably positioned to be the sources of major geomagnetic storms when the holes are in the eastern half of the solar hemisphere at the time of the commencement of the storm. Our results indicate that, for these cases, the streams emanating from within the hole must be very fast, greater than 1000 km s^–1, or very wide, greater than 60°, at the inner boundary of 18 solar radii in our simulation.     

Multispacecraft Observations of Magnetic Clouds and Their Solar Origins between 19 and 23 May 2007

We analyze a series of complex interplanetary events and their solar origins that occurred between 19 and 23 May 2007 using observations by the STEREO and Wind satellites. The analyses demonstrate the new opportunities offered by the STEREO multispacecraft configuration for diagnosing the structure of in situ events and relating them to their solar sources. The investigated period was characterized by two high-speed solar wind streams and magnetic clouds observed in the vicinity of the sector boundary. The observing satellites were separated by a longitudinal distance comparable to the typical radial extent of magnetic clouds at 1 AU (fraction of an AU), and, indeed, clear differences were evident in the records from these spacecraft. Two partial-halo coronal mass ejections (CMEs) were launched from the same active region less than a day apart, the first on 19 May and the second on 20 May 2007. The clear signatures of the magnetic cloud associated with the first CME were observed by STEREO B and Wind while only STEREO A recorded clear signatures of the magnetic cloud associated with the latter CME. Both magnetic clouds appeared to have interacted strongly with the ambient solar wind and the data showed evidence that they were a part of the coronal streamer belt. Wind and STEREO B also recorded a shocklike disturbance propagating inside a magnetic cloud that compressed the field and plasma at the cloud’s trailing portion. The results illustrate how distant multisatellite observations can reveal the complex structure of the extension of the coronal streamer into interplanetary space even during the solar activity minimum. Electronic Supplementary Material  The online version of this article () contains supplementary material, which is available to authorized users.     

Interplanetary sector structure, 1962–1966

Some properties of the interplanetary magnetic field observed by IMP-3 in the latter half of 1965 are discussed with relation to previous satellite observations of the interplanetary field. The sector property remains a prominent feature of the observations, with the average field direction at the Archimedes spiral angle. The sector pattern has a 27-day recurrence period from 1962 to 1964, and during the year 1964 the pattern appears to be quasi-stable. In 1965 the recurrence period is about 28 days, and the evolution of the pattern is more rapid, with new sectors appearing and expanding.     

Hydromagnetic shocks in the solar wind

The Rankine-Hugoniot relations are applied to shock-like discontinuities measured by both magnetic field and plasma instruments on the satellite Explorer 34 between May 30, 1967 and Jan. 11, 1968.Shock normals were either determined from the magnetic field observations, or from the times of occurrence of the discontinuity at Explorers 33, 34 and 35. The Rankine-Hugoniot relations are obeyed to the accuracy of the observations, and the values of shock velocities, density ratios, and Mach numbers indicate that at 1 AU the typical interplanetary shock is not strong, although all the events studied caused geomagnetic impulses.     

On Fields and Mass Constraints for the Uniform Propagation of Magnetic-Flux Ropes Undergoing Isotropic Expansion

An analytical 3-D magnetohydrodynamic (MHD) solution of a magnetic-flux rope (FR) is presented. This FR solution may explain the uniform propagation, beyond ～?0.05 AU, of coronal mass ejections (CMEs) commonly observed by today’s missions like The Solar Mass Ejection Imager (SMEI), Solar and Heliospheric Observatory (SOHO) and Solar Terrestrial Relations Observatory (STEREO), tracked to tens of times the radius of the Sun, and in some cases up to 1 AU, and/or beyond. Once a CME occurs, we present arguments regarding its evolution based on its mass and linear momentum conservation. Here, we require that the gravitational and magnetic forces balance each other in the framework of the MHD theory for a simple model of the evolution of a CME, assuming it interacts weakly with the steady solar wind. When satisfying these ansätze we identify a relation between the transported mechanical mass of the interplanetary CME with its geometrical parameters and the intensity of the magnetic field carried by the structure. In this way we are able to estimate the mass of the interplanetary CME (ICME) for a list of cases, from the Wind mission records of ICME encountered near Earth, at 1 AU. We obtain a range for masses of ～?10⁹ to 10¹³ kg, or assuming a uniform distribution, of ～?0.5 to 500 cm^?3 for the hadron density of these structures, a result that appears to be consistent with observations.     

Energetic Particle Fluxes during the Bastille Day Solar Eruption

We report on our observations of solar energetic particle fluxes of p, He, C, O, Ne, Mg, Si, and Fe ions measured by the Energetic and Relativistic Nucleon and Electron (ERNE) experiment associated with the Bastille Day solar flare and coronal mass ejection (CME) on 14 July 2000. We observed two clear maxima of the Fe/O ratio at the energies 8.5–15 MeV nucl⁻¹. The first Fe/O maximum occurred ∼ 3 hours after the beginning of the particle event, and the second maximum ∼ 22 hours after the first one at the arrival of the shock associated with the Bastille Day eruption. We also observed a change in the energy spectrum of oxygen concurrent with a change in the direction of the interplanetary magnetic field at the start of the second enhancement of the Fe/O ratio. We propose an interpretation of the particle event where observed interplanetary particle fluxes are associated with two different particle sources near the Sun and in interplanetary space. We suggest that heavy ions observed during the first period of the Fe/O enhancement were released when a coronal shock reached a magnetic foot point connected to 1 AU. The second maximum of Fe/O occurred when spacecraft encountered Fe-rich material stored in magnetic field flux tubes early in the event and was possibly reaccelerated by the interplanetary shock.