The February 1986 solar activity: A comparison of GIOTTO,VEGA-1, and IMP-8 solar wind measurements with MHD simulations

Large disturbances in the interplanetary medium were observed by several spacecraft during a period of enhanced solar activity in early February 1986. The locations of six solar flares and the spacecraft considered here encompassed more than 100° of heliolongitude. These flares during the minimum of cycle 21 set the stage for an extensive multi-spacecraft comparison performed with a two-dimensional, magnetohydrodynamic (MHD) numerical experiment. The plasma instruments on the European Space Agency (ESA)'s GIOTTO spacecraft, on its way to encounter Comet Halley in March 1986, made measurements of the solar wind for up to 8 hours per day during February. We compare solar wind measurements from the Johnstone Plasma Analyzer (JPA) experiment on GIOTTO with the MHD simulation of the interplanetary medium throughout these events. Using plasma data obtained by the IMP-8 satellite in addition, it appears that an extended period of high solar wind speed is required as well as the simulated flares to represent the interplanetary medium in this case. We also compare the plasma and magnetometer data from VEGA-1 with the MHD simulation. This comparison tends to support an interpretation that the major solar wind changes at both GIOTTO and VEGA-1 on 8 February, 1986 were due to a shock from a W05° solar flare on 6 February, 1986 (06:25 UT). The numerical experiment is considered, qualitatively, to resemble the observations at the former spacecraft, but it has less success at the latter one.     

Bidirectional distribution of energetic protons in the 18 February 1979 interplanetary shock event

Pitch angle distributions of 35–1000 keV protons in the 18 January 1979 interplanetary shock event are presented and discussed in this paper. The observed, apparently unusual distributions can be well explained by assuming a magnetic bottle configuration of the interplanetary magnetic field immediately prior to the shock passage. The implication of such a magnetic configuration on the acceleration of charged particles by shock waves is also discussed.     

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.     

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.     

Observation of solar wind heavy ions

Plasma experiment on board of the ESRO satellite HEOS-1 observed heavy ions with energy per unit charge 2.5 and 4 times higher than protons. Observations refer to the driving piston of the interplanetary shock occurred on 31 March 1970.     

Sources of SEP Acceleration during a Flare – CME Event

A high-speed, halo-type coronal mass ejection (CME), associated with a GOES M4.6 soft X-ray flare in NOAA AR 0180 at S12W29 and an EIT wave and dimming, occurred on 9 November 2002. A complex radio event was observed during the same period. It included narrow-band fluctuations and frequency-drifting features in the metric wavelength range, type III burst groups at metric – hectometric wavelengths, and an interplanetary type II radio burst, which was visible in the dynamic radio spectrum below 14 MHz. To study the association of the recorded solar energetic particle (SEP) populations with the propagating CME and flaring, we perform a multi-wavelength analysis using radio spectral and imaging observations combined with white-light, EUV, hard X-ray, and magnetogram data. Velocity dispersion analysis of the particle distributions (SOHO and Wind in situ observations) provides estimates for the release times of electrons and protons. Our analysis indicates that proton acceleration was delayed compared to the electrons. The dynamics of the interplanetary type II burst identify the burst source as a bow shock created by the fast CME. The type III burst groups, with start times close to the estimated electron-release times, trace electron beams travelling along open field lines into the interplanetary space. The type III bursts seem to encounter a steep density gradient as they overtake the type II shock front, resulting in an abrupt change in the frequency drift rate of the type III burst emission. Our study presents evidence in support of a scenario in which electrons are accelerated low in the corona behind the CME shock front, while protons are accelerated later, possibly at the CME bow shock high in the corona.     

Multi-spacecraft observations of particle events and interplanetary shocks during November/December 1982

We present a sample of solar energetic particle events observed between November 18 and December 31, 1982 by the HELIOS 1, the VENERA 13, and IMP 8 spacecraft. During the entire time period all three spacecraft were magnetically connected to the western hemisphere of the Sun with varying radial and angular distances from the flares. Eleven proton events, all of them associated with interplanetary shocks, were observed by the three spacecraft. These events are visible in the low-energy (about 4 MeV) as well as the high-energy (30 MeV) protons. In the largest events protons were observed up to energies of about 100 MeV. The shocks were rather fast and in some cases extended to more than 90% east of the flare site. Assuming a symmetrical configuration, this would correspond to a total angular extent of some interplanetary shocks of about 180%. In addition, due to the use of three spacecraft at different locations we find some indication for the shape of the shock front: the shocks are fastest close to the flare normal and are slower at the eastern flank. For particle acceleration we find that close to the flare normal the shock is most effective in accelerating energetic particles. This efficiency decreases for observers connected to the eastern flank of the shock. In this case, the efficiency of shock acceleration for high-energy protons decreases faster than for low-energy protons. Observation of the time-intensity profiles combined with variations of the anisotropy and of the steepness of the proton spectrum allows one in general to define two components of an event which we term solar and interplanetary. We attempt to describe the results in terms of a radially variable efficiency of shock acceleration. Under the assumption that the shock is responsible not only for the interplanetary, but also for the solar component, we find evidence for a very efficient particle acceleration while the shock is still close to the Sun, e.g., in the corona. In addition, we discuss this series of strong flares and interplanetary shocks as a possible source for the formation of a superevent.     

Solar protons E > 100 MeV incident over antarctica during January–February 1967

Commencing at 0825 ₊₃ ^–1 UT on January 28, 1967, a large and prolonged increase in the intensity of penetrating charged particles was observed by balloon-borne instruments floating over Byrd Station, Antarctica. (80°S, 120°W). A peak intensity of approximately 60 protons per cm²-secsteradian with E> 100 MeV occurred at about 1230 UT on the 28th. The event was under observation almost continuously over a period of about 100 hours until the intensity decayed below cosmic-ray background on February 1. The initial decay was rapid but, some 40 hours after onset, went over into a slow exponential decay characterized by a 20 hour time-constant. The decay phase of an additional, though considerably less intense, event was observed on February 3 and 4. Presumably both events had their origins in major disturbances on the far side of the sun since neither event has been definitely linked to any feature which existed on the visible disk within an appropriate time interval.Results pertaining to the time-intensity profile and to the energy spectrum for protons E> 100 MeV are presented for the January 28 event. Comparison of the balloon results with neutron-monitor and satellite measurements and with models of interplanetary diffusion has led to some conclusions regarding the role of small-angle scattering by irregularities and by the random walk of magnetic lines of force relative to the mean interplanetary field within the orbit of earth.     

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.     

The Bastille day Shock and Merged Interaction Region at 63 au: Voyager 2 Observations

A transient flow system containing several streams and shocks associated with the Bastille Day 2000 solar event was observed by the WIND and ACE spacecraft at 1 AU. Voyager 2 (V2) at 63 AU observed this flow system after it moved through the interplanetary medium and into the distant heliosphere, where the interstellar pickup protons strongly influence the MHD structures and flow dynamics. We discuss the Voyager 2 magnetic and plasma observations of this event. Increases in the magnetic field strength B, density N, temperature T and speed V were observed at the front of a stream at V2, consistent with presence of a shock related to the Bastille Day shock at 1 AU. However, the jumps occurred in a 16.9-hour data gap, so that the shock was not observed directly, and the properties of the candidate shock cannot be determined precisely. The candidate shock was followed by a merged interaction region (MIR) that moved past V2 for at least 10 days. The first part of this MIR contains a structure that might be a magnetic cloud. Just ahead of the shock there was an abrupt increase in density associated with a decrease in temperature such that the solar wind thermal pressure was constant across it. Just behind the shock there was an abrupt decrease in density associated with a net increase in magnetic field strength. This appears to be a pressure balanced structure in which the interstellar pickup protons make a significant contribution.     

Energetic protons from the solar flare of March 24, 1966

Ten to 100 meV protons from the solar flare of March 24, 1966 were observed on the University of California scintillation counter on OGO-I. The short rise and decay times observed in the count rates of the 32 channels of pulse-height analysis show that scattering of the protons by the interplanetary field was much less important in this event than in previously observed proton flares. A diffusion theory in which D = M _r is found to be inadequate to account for the time behavior of the count rates of this event. Small fluctuations of the otherwise smooth decay phase may be due to flare protons reflected from the back of a shock front, which passed the earth on March 23.     

Origin of Large Solar Proton Events

A prompt increase of solar protons shortly after a flare has been thought to originate in the flare itself. We give new evidence for the shock-wave acceleration of these prompt protons that is called `solar component' in a gradual-type event. In this study, a recompiled data set of gradual particle events is evaluated, in order to test proportionality between the numbers of interplanetary protons and -ray fluencies of the parent flares. The result does show some proportionality only in western-hemisphere events despite a small sample of events. For eastern hemisphere events, on the other hand, no prompt protons are observed in spite of large prompt relativistic-electron fluxes. It is suggested from these observational facts that prompt protons in gradual events originate in shock waves driven by CMEs very close to the Sun and that the flare-origin hypothesis seems implausible.     

Particle acceleration and Alfvén wave generation by an interplanetary shock

Based on the event observed by ISEE 3 near the Earth’s orbit at 01:21 UT on April 5, 1979, we investigate the diffusive shock acceleration of ions and the generation of Alfvén waves by accelerated particles near the quasi-parallel parts of interplanetary shock fronts within a quasi-linear approach. The theory is shown to give an excessively high level of Alfvén wave generation by accelerated particles at significant deflection angles of the interplanetary magnetic field from the normal to the shock front. At the Earth’s orbit, the Alfve´ n waves produced by accelerated ions are confined within the frequency range 5 × 10^?2?0.5 Hz, and their spectral peak with a wave amplitude δB ≤ B corresponds to a frequency ν = (1?2)×10^?2 Hz. The high-frequency part of the wave spectrum (ν ≥ 0.5 Hz) is subjected to damping on thermal protons. The calculated spectra of the accelerated ions and Alfvén waves generated by them reproduce the main features observed in experiments.     

Possible crossing of the termination shock in the solar wind by the Voyager 1 spacecraft

The solution of the classical problem of two-dimensional magnetohydrodynamic (MHD) interaction between two shocks (the angle between the interacting shocks and the slope of the magnetic field are arbitrary) obtained by Pushkar' (1995) is applied to the problem of interaction between interplanetary shocks and the solar wind termination shock (TS). The self-consistent kinetic-gasdynamic model of solar wind interaction with the supersonic flow of a three-component (electrons, protons, and hydrogen atoms) interstellar medium developed for the axisymmetric, steady-state case by Baranov and Malama (1993) is used as the stationary background against which the physical phenomenon under consideration takes place. The main physical process in this model is the resonant charge exchange between protons and hydrogen atoms. This paper is a natural continuation of our previous papers (Baranov et al. 1996a, 1996b). However, whereas attention in these papers was focused on the TS interaction with an interplanetary forward shock moving away from the Sun, here we consider the TS interaction with an interplanetary reverse shock (RS) moving toward the Sun with a velocity lower than the solar-wind velocity. We show that the TS-RS interaction can give rise to a new TS' that moves toward the Sun, i.e., toward Voyager 1 and Voyager 2. This phenomenon may be responsible for the unexpected suggestion made by some of the scientists that Voyager 1 already crossed TS in the past year. This conclusion was drawn from the interpretation of the intensity, energy spectra, and angular distributions of ions in the energy range from 10 keV to 40 MeV measured from this spacecraft. Our results show that Voyager 1 could cross TS' rather than TS.     

Pulsations in the solar wind and on the ground

Measurements of the pulsation activity recorded by the HEOS-1 satellite in the solar wind upstream from the Earth's bow shock are compared with records of Pc 3–4 activity at the Soviet Borok Observatory. Selecting only events in the 0300–1000 U.T. range most suitable for observing at Borok, we obtained eight events with closely similar periods at the satellite and at Borok, while another event showed similar onset times but had rather different dominant periodicities at the two locations.The time delay between the start of the event at HEOS and at Borok depends on the distance between the satellite and the bow shock in a way which suggests that the pulsation activity at the satellite is produced by protons which are counter-streaming along the interplanetary field lines as a result of reflection or energisation at the shock. When the interplanetary field is directed away from the Sun, the period of the disturbance at Borok is most closely similar to the period at HEOS and both are inversely proportional to the magnitude of the interplanetary magnetic field. This coincident dominant periodicity at HEOS and Borok did not seem to exist during periods of Sunward directed interplanetary field.These results are discussed in terms of the possible generating mechanisms for Pc 3–4 activity.     

Solar flare injection and propagation of low-energy protons and electrons in the event of 7–9 July, 1966

Simultaneous observations of the 7–9 July 1966 solar particle event by energetic particle detectors on three satellites, IMP-III, OGO-III and Explorer 33 are utilized to show that large spatial gradients are present in the fluxes of 0.5–20 meV protons and 45 keV electrons. The event is divided into three parts: the ordinary diffusive component, the halo, and the core. The core corotates with the interplanetary field, and therefore it and the surrounding halo are interpreted as spatial features which are connected by the interplanetary magnetic field lines to the vicinity of the flare region. Upper limits to the interplanetary transverse diffusion coefficient for 4–20 meV protons at 1 AU are derived from the width of the halo. These are at least two orders of magnitude less than the parallel diffusion coefficient for the same energy particles.It is argued that the observed flux variations cannot be explained by an impulsive point source injection for any physically reasonable diffusion model. Instead, since the interplanetary transverse-diffusion coefficient is small for these low-energy particles, the observed spatial features are interpreted as the projection to 1 AU by the interplanetary field lines of an extensive injection profile at the sun. The geometry of the injection mechanism is discussed and it is suggested that some temporary storage of the flare particles occurs near the sun.Now at NASA, Goddard Space Flight Center, Greenbelt, Md., U.S.A.     

Post-Impulsive-Phase Acceleration in a Wide Range of Solar Longitudes

We have analyzed five solar energetic particle (SEP) events observed aboard the SOHO spacecraft during 1996–1997. All events were associated with impulsive soft X-ray flares, Type II radio bursts and coronal mass ejections (CMEs). Most attention is concentrated on the SEP acceleration during the first 100 minutes after the flare impulsive phase, post-impulsive-phase acceleration, being observed in eruptions centered at different solar longitudes. As a representative pattern of a (nearly) well-connected event, we consider the west flare and CME of 9 July 1996 (S10 W30). Similarities and dissimilarities of the post-impulsive-phase acceleration at large heliocentric-angle distance from the eruption center are illustrated with the 24 September 1997 event (S31 E19). We conclude that the proton acceleration at intermediate scales, between flare acceleration and interplanetary CME-driven shock acceleration, significantly contributes to the production of ≳10 MeV protons. This post-impulsive-phase acceleration seems to be caused by the CME lift-off.     

Influence of the magnetic field on the density distribution of solar wind protons and cometary ions in the shock layer ahead of cometary ionospheres

The “mass loading” of the solar wind by cometary ions produced by the photoionization of neutral molecules outflowing from the cometary nucleus plays a major role in the interaction of the solar wind with cometary atmospheres. In particular, this process leads to a decrease in the solar wind velocity with a transition from supersonic velocities to subsonic ones through the bow shock. The so-called single-fluid approximation, in which the interacting plasma flows are considered as a single fluid, is commonly used in modeling such an interaction. However, it is occasionally necessary to know the distribution of parameters for the components of the interacting plasma flows. For example, when the flow of the cometary dust component in the interplanetary magnetic field is considered, the dust particle charge, which depends significantly on the composition of the surrounding plasma, needs to be known. In this paper, within the framework of a three-dimensional magnetohydrodynamic model of the solar wind flow around cometary ionospheres, we have managed to separately obtain the density distributions of solar wind protons and cometary ions between the bow shock and the cometary ionopause (in the shock layer). The influence of the interplanetary magnetic field on the position of the point of intersection between the densities with the formation of a region near the ionopause where the proton density is essentially negligible compared to the density of cometary ions is investigated. Such a region was experimentally detected by the Vega-2 spacecraft when investigating Comet Halley in March 1986. The results of the model considered below are compared with some experimental data obtained by the Giotto spacecraft under the conditions of flow around Comets Halley and Grigg–Skjellerup in 1986 and 1992, respectively. Unfortunately, our results of calculations on Comet Churyumov–Gerasimenko are only predictive in character, because the trajectory of the Rosetta spacecraft, which manoeuvred near its surface for several months, is complex.     

Interplanetary proton (0.61 < E p < 3.41 MeV) events observed with Pioneer 11, 1973–1986 and out to 22.4 AU

A survey of interplanetary proton (0.61 < E _p < 3.41 MeV) events is summarized in graphical and tabular form for the period April 1973–December 1986. The observations were obtained by an effectively continuous data stream from the University of Iowa instrument on the Ames Research Center/NASA spacecraft Pioneer 11 as it moved outward in the solar system from 1.0 to 22.4 AU. Two hundred and sixty-five distinct events are identified. The spectra and intensities of the protons, presumed to be originally of solar origin, are influenced dramatically by propagative and accelerative processes in the interplanetary medium.     

Multispacecraft observations of the east-west asymmetry of solar Energetic Storm Particle events

Multispacecraft observations of energetic protons (E _p 500 keV) were obtained by the APL/JHU instruments on board the IMP-7 and 8 spacecraft and the Voyager-1 and 2 deep space probes, in order to study the generation of solar flare Energetic Storm Particle (ESP) events at widely separated locations on the same shock front. These locations are presumably characterized, on the average, by different interplanetary magnetic field-shock front configurations, i.e. quasi-perpendicular (quasi-parallel) shocks for eastern (western) solar flare sites. The multispacecraft energetic proton observations show that substantial differences in the ESP proton intensity enhancements (defined as the ratio of intensity increases near the shock over the ambient solar proton population) are detected at these energies for locations on the shock front with wide heliolongitude separations. In particular, large ESP proton intensity enhancements are detected at locations on the shock front for which the solar flare site generating the shock is to the east of the spacecraft meridian, whereas only weak ESP events are observed at locations on the same shock for which the flare site is to the west of the spacecraft meridian. The results indicate that acceleration of ESP protons to E _p 500 keV takes place exclusively at the quasi-perpendicular shock front domain, consistent with the shock drift acceleration mechanism (Armstrong et al., 1977).