Analysis of the complex solar particle event on April 29–30, 1973

Based on the data of the high-apogee satellite Prognoz-3, the April 29–30, 1973 solar particle event is analysed. The event's complex energetic particle, interplanetary magnetic field and solar wind plasma properties are discussed. The unusual behaviour of solar particles up to energies 100 MeV can well be explained in terms of the interaction with an interplanetary shock wave system passing the Earth. Assuming that the structure of the interplanetary shock wave system is similar to that considered first by Parker (1961) and Gold (1959) and reviewed later by Hundhausen (1972) and Dryer (1974, 1975), the main characteristics of the energetic particle fluxes, solar wind and interplanetary magnetic field can be understood.     

SCR Steady State Distribution Function and Scattering Properties of the Interplanetary Medium

The propagation of energetic particles in the interplanetary space is considered on the basis of kinetic equation describing the scattering of charged particles by magnetic irregularities and the particle focusing by regular magnetic field. Our analysis confirms that angular distribution of solar cosmic rays contains a valuable information about properties of the particle scattering in the interplanetary magnetic field. Steady state solutions of the kinetic equation are applied to the analysis of solar proton events.     

A comment on the detection of closed magnetic structures in the solar wind

It has recently been suggested that the large scale structure of the interplanetary magnetic field can be deduced solely from solar wind speed measurements. Here it is emphasized that, in addition to speed measurements, direct measurements of the interplanetary field and indirect diagnostics such as measurements of the solar wind kinetic temperature and galactic and solar energetic particle modulations and anisotropics are required to distinguish between open and closed magnetic structures in the solar wind.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Interplanetary dust particles: Disintegration and orbital motion

Abrupt or gradual disintegration of the interplanetary dust particle causes increase of its distance from the Sun due to the solar radiation pressure. The problem of the orbital evolution of the interplanetary dust particles under such disintegration processes is discussed. The process of gradual disintegration due to the solar wind particles is calculated in detail. Obtained results represent corrections to the changes of orbital elements for the Poynting-Robertson effect and effect of the solar wind.     

Long term storage of relativistic particles in the solar corona

A model is presented which shows that large numbers of energetic electrons (0.3-> 10 MeV) and protons (1–30 MeV) can be stored in the solar corona at altitudes around 3 × 10⁵ km for periods in excess of 5 days. Specific reference is made to the time period July 6–16 1968 as an excellent example of energetic solar particle storage. Time histories of interplanetary charged particle intensities observed by the IMP-4 and Pioneer 8 satellites are used to substantiate this contention. Detailed reference is also made to solar X-ray, optical and radio data obtained during the period in question, in addition to interplanetary magnetometer data. This model provides a unique solution to many hitherto unexplained solar particle events, and can also account for the lack of prompt particle emission from certain large solar flares recorded in the past.     

Influence of solar wind variability on the recurrence of droughts

The observed effects of solar flares and interplanetary sector crossings seem to indicate that particle precipitation in the Earth's upper atmosphere decreases cyclonic activity in the troposphere. As an extrapolation to longer term effects, it is suggested that the recurrence of prolonged periods of enhanced solar wind particle precipitation in the upper atmosphere during alternate solar minima could cause the recurrence of extreme droughts.     

Interplanetary dust particles and impact erosion

Motion of the interplanetary dust particle under the action of collisions with much smaller interplanetary dust particles is investigated. The equation of motion is derived. Perturbation equations of celestial mechanics are also discussed. The results are compared with the Poynting-Robertson effect and the effect of solar wind on the motion of the interplanetary dust particles, from the point of view of observational data.     

SCR Distribution Function in the Radial IMF Approximation

The transport of cosmic rays in the interplanetary medium is considered in terms of the kinetic equation describing the energetic particle scattering by magnetic irregularities and their focusing by the regular interplanetary magnetic field. The analytical expression for solar cosmic ray distribution function in the approximation of radial regular magnetic field is obtained and the evolution of energetic particle angular distribution is analyzed. The obtained results can be used for the analysis of ground-level enhancements of cosmic ray intensity.     

Solar Energetic Particle Event of 2005 January 20： Release Times and Possible Sources

Based on cosmic ray data obtained by neutron monitors at the Earth's surface, and data on near-relativistic electrons measured by the WIND satellite, as well as on solar X-ray and radio burst data, the solar energetic particle (SEP) event of 2005 January 20 is studied. The results show that this event is a mixed event where the flare is dominant in the acceleration of the SEPs, the interplanetary shock accelerates mainly solar protons with energies below 130 MeV, while the relativistic protons are only accelerated by the solar flare. The interplanetary shock had an obvious acceleration effect on relativistic electrons with energies greater than 2 MeV. It was found that the solar release time for the relativistic protons was about 06:41 UT, while that for the near-relativistic electrons was about 06:39 UT. The latter turned out to be about 2 min later than the onset time of the interplanetary type III burst.     

A Hall MHD model to study energetic charged particle events produced during fluctuations in the interplanetary magnetic field intensity

Energetic charged particles, which are often observed in solar active regions, may be also produced in interplanetary space due to the decoupling of ions and electrons in plasma. The Hall term in general Ohm's law is generally thought to be responsible for the decoupling of electrons and ions in plasma during magnetic reconnection. In this paper, a Hall MHD model is developed to study energetic charged particle events produced during fluctuations in the interplanetary magnetic field intensity. Two energetic charged particle events are used to test this model. It is concluded that the Hall effect does not only play the important role in the process of magnetic reconnection, but also in energetic charged particle events produced during fluctuations in the interplanetary magnetic field intensity.     

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.     

Influence of the magnetosheath on solar proton penetration into the magnetosphere

The observation of solar protons (1–9 MeV) aboard HEOS-2 in the high-latitude magnetotail and magnetosheath on 9 June 1972, and their comparison with simultaneous measurements on Explorers 41 and 43, both in interplanetary space, indicate the existence of a distinct region of the inner magnetosheath (about 3 Earth radii thick) near the high-latitude magnetopause in which the solar particle flow is almost reversed with respect to the flow observed in interplanetary space. The region can also be seen by comparing magnetic field measurements on the three spacecraft. The observations in the outer layer of the magnetotail show solar protons predominantly entering the magnetosphere somewhere near the Earth, perhaps the cusp region.     

Heliospheric propagation of galactic cosmic rays depending on the scattering characteristics of the turbulent interplanetary magnetic field

The effect of the solar wind on the spectrum of cosmic rays accelerated in the Galaxy is studied. The coefficient of cosmic-ray diffusion in the interplanetary turbulent magnetic field is assumed to be independent of the particle energy and a power-law function of the distance from the Sun. The particle spectrum at the heliospheric boundary is specified as a power-law function of the total particle energy.     

Persistent particle anisotropies and magnetospheric models

The influence of an interplanetary particle anisotropy on the asymmetry of solar particle entry into the magnetotail is analysed in the diffusion as well as in the reconnection model. By time dependent diffusion calculations with an asymmetric boundary condition in a cylindrical tail lobe it can be shown that a north-south interplanetary anisotropy leads in the open as well as in the closed magnetosphere to essentially the same polar cap structures when observed with a dawn-dusk polar orbiting satellite. However, depending on the satellite orbit, an east-west interplanetary anisotropy can serve to distinguish between rival magnetospheric models. Comparison of our diffusion calculations with polar cap measurements during an east-west interplanetary anisotropy, as presented in Morfill and Scholer (1972), show a large discrepancy, whereas an open tail model fits these observations best.     

Simulation of solar flare particle interaction with interplanetary shock waves

In order to study the propagation of solar cosmic rays in interplanetary space a computer program has been developed using a Monte-Carlo technique, which traces the histories of particles released impulsively at the Sun. The particle propagation model considers the adiabatic deceleration during the convective and diffusive transport of the particles, and the model of the interplanetary medium incorporates a radially expanding blast wave which exerts a sweeping action on the particles and accelerates them through the first-order Fermi process. It is shown that energetic storm particle events cannot be simulated by assuming a pure sweeping action of the interplanetary blast wave, but that energization of the particles while reflected at the shock can explain many observed features of such events.     

The motion of charged dust particles in interplanetary space—I. The zodiacal dust cloud

The problem of electromagnetic perturbations of charged dust particle orbits in interplanetary space has been re-examined in the light of our better understanding of the large scale spatial and temporal interplanetary plasma and field topology. Using both analytical and numerical solutions for particle propagation it was shown that: (1) stochastic variations induced by electromagnetic forces are unimportant for the zodiacal dust cloud except for the lowest masses, (2) systemetic variations in orbit inclinations are unimportant if orbital radii are larger than 10 a.u. This is due to the solar cycle variation in magnetic polarity which tends to cancel out systematic effects, (3) systematic variations in orbital parameters (inclination, longitude of ascending node, longitude of perihel) induced by electromagnetic forces inside 1 a.u. tend to shift the plane of symmetry of the zodiacal dust cloud somewhat towards the solar magnetic equatorial plane, (4) inside 0.3 a.u. there is a possibility that dust particles may enter a region of “magnetically resonant” orbits for some time. Changes in orbit parameters are then correspondingly enhanced, (5) the observed similarity of the plane of symmetry of zodiacal light with the solar equatorial plane may be the effect of the interaction of charged interplanetary dust particles with the interplanetary magnetic field. Numerical orbit calculation of dust particles show that one of the results of this interaction is the rotation of the orbit plane about the solar rotational axis.     

Solar Energetic Particle Events in the 23rd Solar Cycle: Interplanetary Magnetic Field Configuration and Statistical Relationship with Flares and CMEs

We study the influence of the large-scale interplanetary magnetic field configuration on the solar energetic particles (SEPs) as detected at different satellites near Earth and on the correlation of their peak intensities with the parent solar activity. We selected SEP events associated with X- and M-class flares at western longitudes, in order to ensure good magnetic connection to Earth. These events were classified into two categories according to the global interplanetary magnetic field (IMF) configuration present during the SEP propagation to 1 AU: standard solar wind or interplanetary coronal mass ejections (ICMEs). Our analysis shows that around 20 % of all particle events are detected when the spacecraft is immersed in an ICME. The correlation of the peak particle intensity with the projected speed of the SEP-associated coronal mass ejection is similar in the two IMF categories of proton and electron events, ≈?0.6. The SEP events within ICMEs show stronger correlation between the peak proton intensity and the soft X-ray flux of the associated solar flare, with correlation coefficient r=0.67±0.13, compared to the SEP events propagating in the standard solar wind, r=0.36±0.13. The difference is more pronounced for near-relativistic electrons. The main reason for the different correlation behavior seems to be the larger spread of the flare longitude in the SEP sample detected in the solar wind as compared to SEP events within ICMEs. We discuss to what extent observational bias, different physical processes (particle injection, transport, etc.), and the IMF configuration can influence the relationship between SEPs and coronal activity.     

Access of solar electrons to the polar regions

The interaction between the geomagnetic and interplanetary magnetic fields is studied through its effects upon the intensities of solar electrons reaching the polar caps during times of strongly anisotropic electron fluxes in the magnetosheath. During the particle event of 18 November 1968, electrons of solar origin were observed outside the magnetopause with detectors aboard OGO-5. This is the only case on record for which high resolution directional flux observations are available for determining in detail the electron angular distribution, and thus the electron density in the magnetosheath. Correlative studies of these satellite observations and concurrent measurements by riometers and ionospheric forward scatter systems in both polar regions have revealed that the initial stage of the associated Polar Cap Absorption event is attributable to the prompt arrival of solar electrons. The electron flux precipitating into the south polar region was equal to or larger than the mean directional flux in interplanetary space, whereas over the north pole it was equal to or less than the backscattered flux. This evidence of a north-south asymmetry in the solar electron flux at a time when the interplanetary magnetic field vector was nearly parallel with the ecliptic plane supports an open magnetospheric model. The ratio of particle intensities in the High Polar Latitude and Low Polar Latitude regions in the southern hemisphere is consistent with that determined at times when the interplanetary electron fluxes were isotropic. The analysis indicates that an anisotropic electron flux may be isotropized at the magnetopause before propagating into the polar regions.     

Hybrid Solar Energetic Particle Events Observed on Board Soho

We summarize ERNE/SOHO observations of solar energetic particle events associated with impulsive soft X-ray flares and LASCO coronal mass ejections (CMEs). The new observational data support an idea that the >10 MeV proton acceleration may be initiated at different coronal sources, operating in the flaring active region and on the global coronal scale, in concert with CME development. However, the particle acceleration continues beyond the coronal scales and may culminate at the interplanetary CME well after the flare. We emphasize the importance of CME liftoff/aftermath processes in the solar corona and the possible role of seed particle re-acceleration, which may explain the existence of hybrid solar energetic particle events.     

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.