Cosmic-ray variations related to solar,geomagnetic and interplanetary disturbances (23 March–7 April, 1976)

During the second interval of the Study of Travelling Interplanetary Phenomena (STIP, 20 March–5 May, 1976) a series of solar, interplanetary, geomagnetic and cosmic-ray events have occurred. These are surprising events, since this period falls into the minimum of the solar activity of the past solar cycle. The present analysis is concentrated on Forbush decreases, cosmic-ray increases, geomagnetic variations and the related solar wind disturbances recorded by the heliocentric satellites Helios-1, 2 and the geocentric IMP-8, in the period 23 March–7 April, 1976. The cosmic-ray enhancements on 26 March and 1 April were of geomagnetic origin and particularly expressed in middle latitude stations during the largeDst magnetic field depressions. The detected multiple Forbush decreases are related with the type IV solar flares, all produced by the same active region (McMath Plage 14143). The relative positions among the satellites Helios-1, 2, the Sun, and the Earth were very favorable in this period for studying these events, since Helios-1 approached the Sun to its perihelion and Helios-2 was lined-up with the Earth. Helios-2 detected two shock fronts on 30 March and 1 April, respectively, and Helios-1 detected a tangential discontinuity on 26 March. An attempt is made to relate these shock fronts with the erupted solar flares and Storm Sudden Commencements (SSC) recorded on the Earth and to estimate a lower limit of the deceleration distance of the involved shock waves.     

Relationship between interplanetary (IP) parameters and geomagnetic indices during IP shock events of 2005

In the present study, we investigate the possible relationship of IP parameters of solar wind and interplanetary magnetic field with ground-based geomagnetic indices. To carry out the study, we take all the IP shock events listed by Proton Monitor onboard Solar and Heliospheric Observatory (SOHO) during 2005, and plot the time variations of all the IP parameters and geomagnetic parameters (±5 days), centered at the shock arrival time. Next, we obtain scatter plots of absolute values of solar wind parameters such as Vsw, Nsw and Interplanetary Magnetic Field (IMF) components Bx, By, Bz and total B with the values of geomagnetic parameters such as Dst, Kp indices, dayside Magnetopause (MP) distance and Cosmic-Ray Neutron Monitor count (CRNM). The scatter plots show that before the IP shock, the pattern is random with no clear relationship. Following the shock, a clear pattern emerges with a type of relationship being seen — clear for SHARP shocks and less clear for DIFFUSE shocks. A total of 10 shock events for 2005 have been studied. Typical examples of this behaviour are the shock events of January 21, 2005 and May 15, 2005. Our study suggests a definite correlation between changes in the solar wind and interplanetary magnetic field parameters and ground-based geomagnetic response. We are trying to obtain quantitative relationships between these for shock events of 2005.     

How Does Large Flaring Activity from the Same Active Region Produce Oppositely Directed Magnetic Clouds?

We describe the interplanetary coronal mass ejections (ICMEs) that occurred as a result of a series of solar flares and eruptions from 4 to 8 November 2004. Two ICMEs/magnetic clouds occurring from these events had opposite magnetic orientations. This was despite the fact that the major flares related to these events occurred within the same active region that maintained the same magnetic configuration. The solar events include a wide array of activities: flares, trans-equatorial coronal loop disappearance and reformation, trans-equatorial filament eruption, and coronal hole interaction. The first major ICME/magnetic cloud was predominantly related to the active region 10696 eruption. The second major ICME/magnetic cloud was found to be consistent with the magnetic orientation of an erupting trans-equatorial filament or else a rotation of 160° of a flux rope in the active region. We discuss these possibilities and emphasize the importance of understanding the magnetic evolution of the solar source region before we can begin to predict geoeffective events with any accuracy.     

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.     

Magnetic field of the solar wind

Relationship between the geoefficiency of the solar flares as well as of the active regions passing the central meridian of the Sun and the configuration of the large scale solar magnetic field is studied.It is shown that if the tangential component of the large scale magnetic field at the active region or at the flare region is directed southwards, that region and that flare produce geomagnetic storm. In case when the tangential magnetic field is directed northward, the active region and the flares occurring at that region do not cause any geomagnetic disturbance.An index of the geoefficiency of the solar flares and of the active regions is proposed.     

On the possibility of radio emission from quasi-parallel and quasi-perpendicular propagation of shocks

A set of 21 solar type II radio bursts observed using Hiraiso radio spectrograph have been analysed to study the direction of propagation of coronal shocks. A simple analysis is carried out to find the approximate angle between the shock normal and magnetic field by solving the Rankine-Hugoniot MHD relation with assumption of Alfven speed and plasma beta. From this analysis, it is suggested that both quasi-parallel shocks (favourable) and quasi-perpendicular shocks can generate type II bursts depending upon the circumstances of the corona.     

The Interplanetary Responses to the Great Solar Activities in Late October 2003

Based on the observations of the Sun and the interplanetary medium, a series of solar activities in late October 2003 and their consequences are studied comprehensively. Thirteen X-ray flares with importance greater than M-class, six frontside halo coronal mass ejections (CMEs) with span angle larger than 100 and three associated eruptions of filament materials are identified by examining lots of solar observations from October 26 to 29. All these flares were associated with type III radio bursts, all the frontside halo CMEs were accompanied by type II or type II-like radio bursts. Particularly, among these activities, two major solar events caused two extraordinary enhancements (exceeding 1000 particles/(cm²s^–1sterMev^–1) of solar energetic particle (SEP) flux intensity near the Earth, two large ejecta with fast shocks preceding, and two great geomagnetic storms with Dst peak value of –363 and –401 nT, respectively. By using a cross correlation technique and a force-free cylindrical flux rope model, the October 29 magnetic cloud associated with the largest CME are analyzed, including its orientation and the sign of its helicity. It is found that the helicity of the cloud is negative, contrary to the regular statistical pattern that negative- and positive-helical interplanetary magnetic clouds would be expected to come from northern and southern solar hemisphere. Moreover, the relationship between the orientation of magnetic cloud and associated filament is discussed. In addition, some discussion concerning multiple-magnetic-cloud structures and SEP events is also given.     

An Investigation of Solar Maximum Metric Type ii Radio Bursts: do two Kinds of Coronal Shock Sources Exist?

A detailed statistical investigation of solar Type II radio bursts during the last solar maximum period 1999–2001 has been made to address the question if there exist two kinds of coronal shock sources. For this, the Type II bursts were classified into two classes: (i) those associated with flares only (Class I); and (ii) those associated with flares and CMEs (Class II) according to their temporal association. While the properties of all the type IIs agree in general with the common range of values, the properties of the shocks of the two classes differ slightly. For example, while the duration and shock speed for Class II are higher than those of Class I, the ending frequency for Class II is significantly lower. We have also examined in detail the physical association with other solar and interplanetary activities (Type IV bursts, Long Duration Events, Wind/WAVES deca-hectometric Type IIs, and interplanetary shocks) using the data in 2000. As a result, we have found noticeable differences between these two classes in terms of the following physical characteristics: First, the associations of these activities for Class II are much higher than those of Class I. Second, the correlation values between the flare parameters and the Type II properties for Class II are significantly smaller. Third, observed double Type IIs exist in only Class II events. The above results suggest that there are two kinds of coronal shocks or, rather, two general classes of coronal shock sources.     

Nonplanar magnetic reconnection associated with conical slow shocks

We study a nonplanar model of magnetic reconnection associated with conical slow shocks, assuming that the shock surfaces are two identical cones with circular cross sections symmetrical about the ±x-axis. In the inflow region upstream of the shocks, two oppositely directed magnetic fields are separated by a current sheet. The model treats the current sheet as a tangential discontinuity and treats shocks and tangential discontinuity as surfaces of zero thickness. The dynamical structure of the global magnetic field in the continuous regions is studied using compressible, non-resistive MHD equations. In the inflow region, nonplanar magnetic field lines first move toward the current sheet. Near the sheet, the middle sections of the field lines become highly flattened, almost parallel to the sheet. Eventually, then oppositely directed field lines merge across the tangential discontinuity between the two shocks, and the magnetic lines are reconnected at the intersection of the shock and the tangential discontinuity. Reconnected magnetic lines are carried away at high speeds by the MHD flow in the outflow region, downstream of the shocks.     

The Role of Magnetic Fields in Transient Seismic Emission Driven by Atmospheric Heating in Flares

Transient seismic emission in flares remains largely mysterious. Its discoverers proposed that seismic transients are driven by impulsive heating of the flaring chromosphere. Simulations of such heating show strong shocks, but these are damped by heavy radiative losses as they proceed downward. Because compression of the gas the shock enters both heats it and increases its density, the radiative losses increase radically with the strength of the shock, leaving doubt that sufficient energy can penetrate into the solar interior to explain helioseismic signatures. We note that simulations to date have no account for strong, inclined magnetic fields characteristic of transient-seismic-source environments. A strong horizontal magnetic field, for example, greatly increases the compressional modulus of the chromospheric medium, greatly reducing compression of the gas, hence radiative losses. Inclined magnetic fields, then, must be fundamental to the role of impulsive heating in transient seismic emission.     

Connection among spacecrafts and ground level observations of small solar transient events

An overview of the results of the search for small solar transient events, in association with muon enhancements (deficits) registered at ground-level by the Tupi muon telescopes, is presented. Among the events, there are three interplanetary shocks and two solar flares of small scale whose X-ray emission flux is much smaller than 10^???5 W m^?2 at 1 AU (GOES-Tupi connection). Two of the interplanetary shocks are cataloged as corotating interaction region and the third shock is due to the passage of a CME (coronal mass ejection) ejecta (ACE and SOHO-Tupi connection) in the Earth’s proximities. In most cases, the particles excess (deficit) coming from these events have only been observed with spacecraft instruments. However, the Tupi telescopes are located at sea level and within the South Atlantic Anomaly (SAA), a region where the shielding effect of the magnetosphere is not perfectly spherical and shows a ‘dip’. This fact enables the muon telescopes to achieve a low rigidity of response to primary and secondary charged particles (≥?0.1 GV). Muon excesses (deficits) with significances above 4σ have been found. These events observed at ground admit a temporal correlation with solar transient events observed by spacecrafts, which suggests strongly a real connection between them. Details of these observations are reported.     

Three solar filament disappearances associated with interplanetary low-energy particle events

Three low-energy particle events (35–1600 keV) associated with interplanetary shocks, detected at 1 AU by ISEE-3, have been identified as originating in solar disappearing filaments instead of large flares. This increases to fourteen the number of events of this kind presently known. The observational characteristics of these non-flare generated events are similar to the ones of the other eleven events already known (i.e., absence of type II or IV bursts, weak X-ray emission, H brightening in the surroundings of the filament disappearance, frequent presence of a double-ribbon event, slow propagation of the generated interplanetary shock, lack of shock deceleration).     

Interplanetary Transients and Cosmic-Ray Anisotropy

Cosmic-ray intensity data recorded with the ground-based neutron monitor at Deep River have been investigated taking into account the associated interplanetary magnetic field and solar-wind plasma data during 1981 – 1994. A large number of days having abnormally high or low amplitudes for five or more successive days as compared to the annual average amplitude of diurnal anisotropy have been taken as high- or low-amplitude anisotropic wave-train events. The amplitude of the diurnal anisotropy of these events is found to increase on days with a magnetic cloud as compared to the days prior to the event, and it is found to decrease during the later period of the event as the cloud passes the Earth. The high-speed solar-wind streams do not play any significant role in causing these types of events. However, corotating solar-wind streams produce significant deviations in cosmic-ray intensity during high- and low-amplitude events. The interplanetary disturbances (magnetic clouds) are also effective in producing cosmic-ray decreases. Hα solar flares have a good positive correlation with both the amplitude and direction of the anisotropy for high-amplitude events, while the principal magnetic storms have a good positive correlation with both amplitude and direction of the anisotropy for low-amplitude events. The source responsible for these unusual anisotropic wave trains in cosmic rays has been proposed.     

The Automatic Predictability of Super Geomagnetic Storms from halo CMEs associated with Large Solar Flares

We investigate the relationship between magnetic structures of coronal mass ejection (CME) source regions and geomagnetic storms, in particular, the super storms when the D _st index decreases below −200 nT. By examining all full halo CMEs that erupted between 1996 and 2004, we selected 73 events associated with M-class and X-class solar flares, which have a clearly identifiable source region. By analyzing daily full-disk MDI magnetograms, we found that the horizontal gradient of the line-of-sight magnetic field is a viable parameter to identify a flaring magnetic neutral line and thus can be used to predict the possible source region of CMEs. The accuracy of this prediction is about 75%, especially for those associated with X-class flares (up to 89%). The mean orientation of the magnetic structures of source regions was derived and characterized by the orientation angle θ, which is defined to be ≤ 90^∘ in the case of the southward orientation and ≥ 90^∘, when the magnetic structure is northwardly oriented. The orientation angle was calculated as the median orientation angle of extrapolated field lines relative to the flaring neutral line. We report that for about 92% of super storms (12 out of 13 events) the orientation angle was found to be southward. In the case of intense and moderate storms (D _st≥ −200 nT), the relationship is less pronounced (70%, 21 out of 30 events). Our findings demonstrate that the approach presented in this paper can be used to perform an automatic prediction of the occurrence of large X-class flares and super geomagnetic storms.     

Generalized analytical models of Syrovatskii’s current sheet

Two-dimensional stationary magnetic reconnection models that include a thin Syrovatskii-type current sheet and four discontinuous magnetohydrodynamic flows of finite length attached to its endpoints are considered. The flow pattern is not specified but is determined from a self-consistent solution of the problem in the approximation of a strong magnetic field. Generalized analytical solutions that take into account the possibility of a current sheet discontinuity in the region of anomalous plasma resistivity have been found. The global structure of the magnetic field in the reconnection region and its local properties near the current sheet and attached discontinuities are studied. In the reconnection regime in which reverse currents are present in the current sheet, the attached discontinuities are trans-Alfvénic shock waves near the current sheet endpoints. Two types of transitions from nonevolutionary shocks to evolutionary ones along discontinuous flows are shown to be possible, depending on the geometrical model parameters. The relationship between the results obtained and numerical magnetic reconnection experiments is discussed.     

On one mechanism for the generation of a reverse solar wind shock in the magnetosheath in front of the Earth’s magnetosphere

A possible mechanism for the generation of a reverse fast shock in the magnetosheath in the solar wind flow around the Earth’s magnetosphere is considered. It is shown that such a shock can emerge through the breaking of a nonlinear fast magnetosonic compression wave reflected from the magnetopause toward the bow shock rear. In this case, the magnetopause is represented as a tangential discontinuity with a zero normal magnetic field component at it and the mechanism under consideration is assumed to be secondary with respect to the sudden disturbance of the bow shock-Earth’s magnetosphere system by a nonstationary solar wind shock. A possible confirmation of the process under study by in-situ SC3 experimental observations of the bow shock front motion on the Cluster spacecraft is pointed out.     

Two-dimensional,time-dependent MHD description of interplanetary disturbances: Simulation of high speed solar wind interactions

A two-dimensional, time-dependent, magnetohydrodynamic, numerical model is used to investigate multiple, transient solar wind flows which start close to the Sun and then extend into interplanetary space. The initial conditions are assumed to be appropriate for steady, homogeneous solar wind conditions with an average, spiral magnetic field configuration. Because both radial and azimuthal dimensions are included, it is possible to place two or more temporally-developing streams side-by-side at the same time. Thus, the evolution of the ensuing stream interaction is simulated by this numerical code. Advantages of the present method are as follows: (1) the development and decay of asymmetric MHD shocks and their interactions are clearly indicated; and (2) the model allows flexibility in the specification of evolutionary initial conditions in the azimuthal direction, thereby making it possible to gain insight concerning the interplanetary consequences of real physical situations more accurately than by use of the one-dimensional approach. Examples of such situations are the occurrence of near-simultaneous solar flares in adjacent active regions and the sudden appearance or enlargement of coronal holes as a result of a transient re-arrangement from a closed to an open magnetic field topology.     

Simulation studies of electron acceleration by ion ring distributions in solar flares

Using a 2 1/2-D fully relativistic electromagnetic particle-in-cell code (PIC) we have investigated a potential electron acceleration mechanism in solar flares. The free energy is provided by ions which have a ring velocity distribution about the magnetic field direction. Ion rings may be produced by perpendicular shocks, which could in turn be generated by the super-Alfvénic motion of magnetic flux tubes emerging from the photosphere or by coronal mass ejections (CMEs). Such ion distributions are known to be unstable to the generation of lower hybrid waves, which have phase velocities in excess of the electron thermal speed parallel to the field and can, therefore, resonantly accelerate electrons in that direction. The simulations show the transfer of perpendicular ion energy to energetic electrons via lower hybrid wave turbulence. With plausible ion ring velocities, the process can account for the observationally inferred fluxes and energies of non-thermal electrons during the impulsive phase of flares. Our results also show electrostatic wave generation close to the plasma frequency: we suggest that this is due to a bump-in-tail instability of the electron distribution.     

Analysis of the Properties of Super Solar Proton Events and Associated Phenomena

The solar flares, the speeds of shocks propagated in the solar-terrestrial space and driven by coronal mass ejections (CMEs), the heliographic longitudes and Carrington longitudes of source regions, and the geomagnetic storms, which are accompanied by the super solar proton events with a peak ?ux equal to or exceeding 10 000 pfu, have been studied by using the data of ground-based and space observations. The results show that the heliographic longitudes of source regions of super solar proton events distributed in the range from E30? to W75°. The Carrington longitudes of source regions of super solar proton events distributed in the two longitudinal belts, 130°∼220° and 260°∼320°, respectively. All super solar proton events were accompanied by major solar flares and fast CMEs. The averaged speeds of shocks propagated from the sun to the Earth were greater than 1 200 km/s. Eight super solar proton events were followed by major geomagnetic storms (Dst≤−100 nT), except that one super solar proton event was followed by a geomagnetic storm with the geomagnetic activity index Dst=−96 nT, a little smaller than that of major geomagnetic storms.     

Predictions of Energy and Helicity in Four Major Eruptive Solar Flares

In order to better understand the solar genesis of interplanetary magnetic clouds (MCs), we model the magnetic and topological properties of four large eruptive solar flares and relate them to observations. We use the three-dimensional Minimum Current Corona model (Longcope, 1996, Solar Phys. 169, 91) and observations of pre-flare photospheric magnetic field and flare ribbons to derive values of reconnected magnetic flux, flare energy, flux rope helicity, and orientation of the flux-rope poloidal field. We compare model predictions of those quantities to flare and MC observations, and within the estimated uncertainties of the methods used find the following: The predicted model reconnection fluxes are equal to or lower than the reconnection fluxes inferred from the observed ribbon motions. Both observed and model reconnection fluxes match the MC poloidal fluxes. The predicted flux-rope helicities match the MC helicities. The predicted free energies lie between the observed energies and the estimated total flare luminosities. The direction of the leading edge of the MC’s poloidal field is aligned with the poloidal field of the flux rope in the AR rather than the global dipole field. These findings compel us to believe that magnetic clouds associated with these four solar flares are formed by low-corona magnetic reconnection during the eruption, rather than eruption of pre-existing structures in the corona or formation in the upper corona with participation of the global magnetic field. We also note that since all four flares occurred in active regions without significant pre-flare flux emergence and cancelation, the energy and helicity that we find are stored by shearing and rotating motions, which are sufficient to account for the observed radiative flare energy and MC helicity.