A two-fluid solar wind model with anisotropic proton temperature

A two-fluid model of the solar wind with anisotropic proton temperature and allowing for extended coronal proton-heating is considered for the case of a purely radial and of a spiral magnetic field. Proton-proton Coulomb-collisions together with a spiral interplanetary magnetic field are found to be sufficient to reduce the thermal anisotropy in the proton gas to a value in agreement with observations. Reasonable values are obtained for the flow-velocity, number density and the protontemperature near the orbit of the Earth.This work was supported by the Norwegian Research Council for Science and the Humanities (E. Leer) and the National Aeronautics and Space Administration under Contract No. NGR-05-009-081.     

The solar radio event of January 14, 1971

A study has been made of the behaviour in time of the positions, flux density and polarization for the radio emissions at frequencies of 237 and 408 MHz (from Trieste observations), taking into account also the published results at 169 and 160–200 MHz (from Nançay and Utrecht) connected with the solar flare which occurred on January 14, 1971, at 11^h21^m UT.The flare and the subsequent emissions may have been excited by a fall of coronal matter along the lines of force of the magnetic field in the relevant active region, exciting radio emissions of two different types: a type II-like emission and a type III group apparently located at different positions.This discrepancy can be easily explained if we assume that the type III group was due to second harmonic emission in backward direction, strongly refracted by the coronal plasma.The observed spike emission was due to a source high in the corona, excited by the type II-like perturbation which originated in the flare region, and this fact clearly points out the relationship between all the observed peculiar emissions from type IV sources, as has already been stated by this author.     

The solar longitude dependence of proton event delay time

The relationship between heliographic longitude and the delay between flare occurrence and solar proton observation is studied using results obtained aboard HEOS A1 during 1969. The result obtained differs from previous findings. We ascribe this to the formation of a long-lived magnetic field configuration close to the Sun associated with a particular group of active regions.CNRS.     

Snow carrots after the Chelyabinsk event and model implications for highly porous solar system objects

After the catastrophic disruption of the Chelyabinsk meteoroid, small fragments formed funnels in the snow layer covering the ground. We constrain the pre‐impact characteristics of the fragments by simulating their atmospheric descent with the atmospheric entry model. Fragments resulting from catastrophic breakup may lose about 90% of their initial mass due to ablation and reach the snow vertically with a free‐fall velocity in the range of 30–90 m s^?1. The fall time of the fragments is much longer than their cooling time, and, as a consequence, fragments have the same temperature as the lower atmosphere, i.e., of about ?20 °C. Then, we use the shock physics code iSALE to model the penetration of fragments into fluffy snow, the formation of a funnel and a zone of denser snow lining its walls. We examine the influence of several material parameters of snow and present our best‐fit model by comparing funnel depth and funnel wall characteristics with observations. In addition, we suggest a viscous flow approximation to estimate funnel depth dependence on the meteorite mass. We discuss temperature gradient metamorphism as a possible mechanism which allows to fill the funnels with denser snow and to form the observed “snow carrots.” This natural experiment also helps us to calibrate the iSALE code for simulating impacts into highly porous matter in the solar system including tracks in the aerogel catchers of the Stardust mission and possible impact craters on the 67P/Churyumov‐Gerasimenko comet observed recently by the Rosetta mission.     

A large proton event associated with solar filament activity

We report observations made from several interplanetary spacecraft, of the large low-energy particle event of 23–27 April, 1979 associated with solar filament activity. We discuss the intensity, spectral and directional evolution of the event as observed in the energy range 35–1600 keV on ISEE-3, located ～ 0.99 AU from the Sun upstream of the Earth. We demonstrate that the shock disturbance propagating through the interplanetary medium and observed at ISEE-3 on 24/25 April strongly controls the particle event. From a comparison of the ISEE-3 observations with those on other spacecraft, in particular on Helios-2, located at 0.41 AU heliocentric distance near the Sun-Earth line, we identify the solar filament erupting on late 22 April near central meridian as the trigger for the propagating shock disturbance. This disturbance which comprises a forward shock and a reverse shock at the orbit of ISEE-3 is found to be the main source of the energetic proton population observed.     

The unusual anisotropic solar particle event of November 18, 1968

The ground level event of November 18, 1968, was unusual in that pronounced anisotropy persisted essentially until the cessation of the arrival at the earth of relativistic protons. Real and significant differences were observed in the intensity profiles at different neutron monitor stations. By a method of successive approximations, the geographical coordinates of the axis of symmetry were determined as: latitude = - 25° ± 10° and longitude = -35° ± 10°. The exponent in a power law spectral representation over the narrow range covered by the nucleonic intensity measurements was 5.The onset times of the GLE were related to the angular differences between the mean directions of viewing and the axis of symmetry. The event differed markedly from earlier isotropic events that were well described by diffusion theory, but was compatible with the predictions of a model envisaging diffusion only in the vicinity of the Sun (radius R of solar envelope 34 solar radii). The diffusion coefficient D 3 × 10²¹ cm²/s is consistent with that deduced for earlier isotropic events, but the values of other parameters are smaller than those derived for the only previously observed event to which the model of diffusion in an inhomogeneous medium was applicable (May 4, 1960).It is concluded that the observed one hundred percent anisotropy was a direct consequence of the propagation mechanism rather than other postulated causes. Furthermore, the observed large pitchangle distribution was produced by scattering from small scale field irregularities.This research was sponsored by the National Science Foundation and Air Force Cambridge Research Laboratories, Office of Aerospace Research, under Contract No. F19628-70-C-0190, but the report does not necessarily reflect endorsement by the sponsor.     

A Monte Carlo model for the gardening of the lunar regolith

The processes of movement and turnover of the lunar regolith are described by a Monte Carlo model, which includes the effects of collisions by particles from 10^?7 g to 10¹⁰ g. The movement of material by the direct cratering process is the dominant mode, but slumping is also included for angles exceeding the static angle of repose. Using a group of interrelated computer programs a large number of properties are calculated, including topography, formation of layers, depth of the disturbed layer, nuclear track distributions, cosmogenic nuclides and others. In the most complex program, the history of a 36 point square array is followed for times up to 4 × 10⁸ yr. As expected the histories generated are complex and exhibit great variety. Because a crater covers much less area than its ejecta blanket, there is a tendency for the height change at a test point to exhibit the ‘gambler's ruin’ phenomenon: periods of slow accumulation followed by sudden excavation. In general the agreement with experiment and observation seems good. Two areas of disagreement stand out. First, the calculated surface is rougher than that observed. This problem is understood, and will not occur in a newer version of the model. Second, the observed bombardment ages, of the order of 4 × 10⁸ yr, are shorter than expected (by perhaps a factor of 5). We cannot accept Fireman's (1974) explanation; this remains an important puzzle.     

Monte Carlo study of interaction between solar wind plasma and Venusian upper atmosphere

Isolated events of proton and alpha particle precipitation in the Venusian atmosphere were recorded with the use of the ASPERA-4 analyzer on board the ESA Venus Express spacecraft. Using a Monte Carlo simulation method for calculation of proton and alpha particle precipitations in the Venusian atmosphere, reflected and upward directed particle fluxes have been found. It has been found that only a vanishing percentage of protons and alpha particles are backscattered to the Venusian exosphere when neglecting the induced magnetic field and under conditions of low solar activity. Accounting for the induced field drastically changes the situation: the backscattered by the atmosphere energy fluxes increase up to 44% for the horizontal magnetic field B = 20 nT, measured for Venus, for the case of precipitating protons, and up to 64%, for alpha particles. The reflected energy fluxes increase to about 100% for both protons and alpha particles as the field grows to 40 nT, i.e., the atmosphere is protected against penetration of solar wind particles.     

Change of solar flare proton to alpha ratios during an energetic storm particle event

Observations of the temporal behavior of energetic storm protons and alpha particles are presented for the event associated with the storm sudden commencement observed on Earth on March 8, 1970. The data are obtained on board the low altitude polar orbiting satellite GRS-A/AZUR by means of two particle telescopes. Large changes in the proton to alpha ratios for particles of equal energy and for particles of equal energy per nucleon are observed, whereas no significant change in the equal energy per charge ratio is observed. Electric fields, Fermi acceleration and cyclotron resonance are discussed as possible modulation mechanisms.     

Solar proton events during solar cycles 19, 20, and 21

Solar proton events have been studied for over thirty years and a great deal of lore has grown around them. It is the purpose of this paper to test some of this lore against the actual data. Data on solar proton events now exist for the period from 1956 to 1985 during which time 140 events took place in which the event integrated fluxes for protons of energy > 30 MeV was larger than 10⁵ particles cm^-2. We have studied statistical properties of event integrated fluxes for particles with energy > 10 MeV and for particles with energy > 30 MeV. Earlier studies based on a single solar cycle had resulted in a sharp division of events into ordinary and anomalously large events.Two such entirely separate distributions imply two entirely separate acceleration mechanisms, one common and the other very rare. We find that the sharp division is neither required nor justified by this larger sample. Instead the event intensity forms a smooth distribution for intensities up to the largest observed implying that any second acceleration mechanism cannot be rare. We have also studied the relation of event sizes to the sunspot number and the solar cycle phase. We find a clear bimodal variation of annual integrated flux with solar cycle phase but no statistically significant tendency for the large events to avoid sunspot maximum. We show there is almost no relation between the maximum sunspot number in a solar cycle and the solar cycle integrated flux. We also find that for annual sunspot numbers greater than 35 (i.e., non-minimum solar cycle conditions) there is no relation whatsoever between the annual sunspot numbers and annual integrated flux.     

Flare evolution and polarization changes in fine structures of solar radio emission in the 2013 April 11 event

The measurement of positions and sizes of radio sources in observations is important for understanding of the flare evolution. For the first time, solar radio spectral fine structures in an M6.5 flare that occurred on 2013 April 11 were observed simultaneously by several radio instruments at four different observatories: Chinese Solar Broadband Radio Spectrometer at Huairou(SBRS/Huairou), Ondˇrejov Radio Spectrograph in the Czech Republic(ORSC/Ondˇrejov), Badary Broadband Microwave Spectropolarimeter(BMS/Irkutsk), and spectrograph/IZMIRAN(Moscow, Troitsk). The fine structures included microwave zebra patterns(ZPs), fast pulsations and fiber bursts. They were observed during the flare brightening located at the tops of a loop arcade as shown in images taken by the extreme ultraviolet(EUV) telescope onboard NASA's satellite Solar Dynamics Observatory(SDO). The flare occurred at 06:58–07:26 UT in solar active region NOAA 11719 located close to the solar disk center. ZPs appeared near high frequency boundaries of the pulsations, and their spectra observed in Huairou and Ondˇrejov agreed with each other in terms of details. At the beginning of the flare's impulsive phase, a strong narrowband ZP burst occurred with a moderate left-handed circular polarization. Then a series of pulsations and ZPs were observed in almost unpolarized emission. After 07:00 UT a ZP appeared with a moderate right-handed polarization.In the flare decay phase(at about 07:25 UT), ZPs and fiber bursts become strongly right-hand polarized.BMS/Irkutsk spectral observations indicated that the background emission showed a left-handed circular polarization(similar to SBRS/Huairou spectra around 3 GHz). However, the fine structure appeared in the right-handed polarization. The dynamics of the polarization was associated with the motion of the flare exciter, which was observed in EUV images at 171 ?A and 131 ?A by the SDO Atmospheric Imaging Assembly(AIA). Combining magnetograms observed by the SDO Helioseismic and Magnetic Imager(HMI) with the homologous assumption of EUV flare brightenings and ZP bursts, we deduced that the observed ZPs correspond to the ordinary radio emission mode. However, future analysis needs to verify the assumption that zebra radio sources are really related to a closed magnetic loop, and are located at lower heights in the solar atmosphere than the source of pulsations.     

Modeling the 2017 September 10 solar energetic particle event using the iPATH model

On 2017 September 10, a fast coronal mass ejection(CME) erupted from the active region(AR)12673, leading to a ground level enhancement(GLE) event at Earth. Using the 2D improved Particle Acceleration and Transport in the Heliosphere(iPATH) model, we model the large solar energetic particle(SEP) event of 2017 September 10 observed at Earth, Mars and STEREO-A. Based on observational evidence, we assume that the CME-driven shock experienced a large lateral expansion shortly after the eruption, which is modeled by a double Gaussian velocity profile in this simulation. We apply the in-situ shock arrival times and the observed CME speeds at multiple spacecraft near Earth and Mars as constraints to adjust the input model parameters. The modeled time intensity profiles and fluence for energetic protons are then compared with observations. Reasonable agreements with observations at Mars and STEREO-A are found. The simulated results at Earth differ from observations of GOES-15. However, the simulated results at a heliocentric longitude 20° west to Earth fit reasonably well with the GOES observation. This can be explained if the pre-event solar wind magnetic field at Earth is not described by a nominal Parker field.Our results suggest that a large lateral expansion of the CME-driven shock and a distorted interplanetary magnetic field due to previous events can be important in understanding this GLE event.     

Monte Carlo modelling of cometary dynamics

Monte Carlo simulations can either be viewed as a numerical method for solving evolutionary equations or as a way for statistically modelling the outcome of chaotic dynamical systems. Thus they are well suited for treating many aspects of cometary dynamics. We present a critical review of past applications of Monte Carlo simulations for both long- and short-period comets indicating the strength and weaknesses of the methods with suggestions for future applications.     

The ionospheric effects in D-layer and solar proton precipitation zones during the 16 February 1984 event

The results of the geophysical and VLF (10–16 kHz) radio propagation measurements on the net of observatories during the solar proton event on 16 February 1984 are analysed. It is shown that the abnormal ionization region caused by solar protons consisted of two parts. One of them was the direct access zone in the middle and morning side of the polar cap, the other was the precipitation region of the quasi-trapped particles in the mid-day and evening sectors of the auroral zone. The probable profiles of the lower ionosphere electron density are determined from the VLF and satellite data of the energetic spectra at the maximum penetration. It is shown that the effective electron concentration at the height 45 km was close to 10³ cm⁻³.     

Heavy solar cosmic rays in the January 25, 1971 solar flare

A detailed study of the charge composition of heavy solar cosmic rays measured in the January 25, 1971 solar flare including differential fluxes for the even charged nuclei from carbon through argon is presented. The measurements are obtained for varying energy intervals for each nuclear species in the energy range from 10 to 35 MeV nucleon^?1. In addition, abundances relative to oxygen are computed for all the above nuclei in the single energy interval from 15 to 25 MeV nucleon^?1. This interval contains measurements for all of the species and as a result requires no spectral extrapolations. An upper limit for the abundance of calcium nuclei is also presented. These measurements, when combined with other experimental results, enable the energy dependence of abundance measurements as a function of nuclear charge to be discussed. It is seen that at energies above about 10 MeV nucleon^?1, the variations of abundance ratios are limited to about a factor of 3 from flare to flare, in spite of large variations in other characteristics of these solar events.     

Monte Carlo Simulation of Solar Active-Region Energy

A Monte Carlo approach to solving a stochastic-jump transition model for active-region energy (Wheatland and Glukhov: Astrophys. J. 494, 858, 1998; Wheatland: Astrophys. J. 679, 1621, 2008) is described. The new method numerically solves the stochastic differential equation describing the model, rather than the equivalent master equation. This has the advantages of allowing more efficient numerical solution, the modeling of time-dependent situations, and investigation of details of event statistics. The Monte Carlo approach is illustrated by application to a Gaussian test case and to the class of flare-like models presented in Wheatland (Astrophys. J. 679, 1621, 2008), which are steady-state models with constant rates of energy supply, and power-law distributed jump transition rates. These models have two free parameters: an index (δ), which defines the dependence of the jump transition rates on active-region energy, and a nondimensional ratio ( ) of total flaring rate to rate of energy supply. For the nondimensional mean energy of the active-region satisfies , resulting in a power-law distribution of flare events over many decades of energy. The Monte Carlo method is used to explore the behavior of the waiting-time distributions for the flare-like models. The models with δ≠0 are found to have waiting times that depart significantly from simple Poisson behavior when . The original model from Wheatland and Glukhov (Astrophys. J. 494, 858, 1998), with δ=0 (i.e., no dependence of transition rates on active-region energy), is identified as being most consistent with observed flare statistics.