CME Propagation Characteristics from Radio Observations

We explore the relationship among three coronal mass ejections (CMEs), observed on 28 October 2003, 7 November 2004, and 20 January 2005, the type II burst-associated shock waves in the corona and solar wind, as well as the arrival of their related shock waves and magnetic clouds at 1 AU. Using six different coronal/interplanetary density models, we calculate the speeds of shocks from the frequency drifts observed in metric and decametric radio wave data. We compare these speeds with the velocity of the CMEs as observed in the plane-of-the-sky white-light observations and calculated with a cone model for the 7 November 2004 event. We then follow the propagation of the ejecta using Interplanetary Scintillation measurements, which were available for the 7 November 2004 and 20 January 2005 events. Finally, we calculate the travel time of the interplanetary shocks between the Sun and Earth and discuss the velocities obtained from the different data. This study highlights the difficulties in making velocity estimates that cover the full CME propagation time.     

Predicting the Arrival Time of Shock Passages at Earth

The purpose of this parametric study is to predict the arrival time at Earth of shocks due to disturbances observed on the Sun. A 3D magnetohydrodynamic (MHD) simulation code is used to simulate the evolution of these disturbances as they propagate out to 1 AU. The model in Han, Wu and Dryer (1988) uses solar data for input at 0.08 AU (18 solar radii). The initial shock speed (ISS) is assumed to be constant from the corona to 0.08 AU. We investigate how variations of this ISS affect the arrival times of the shock at Earth. This basic parametric study, however, does not consider inhomogeneous background solar wind structures such as corotating interaction regions and their precursor stream–stream interactions, nor interplanetary manifestations of complex coronal mass ejecta such as magnetic clouds. In the latter case, only their associated shocks are considered. Because the ambient (pre-existing background) solar wind speed is known to affect the shock arrival time at 1 AU, we also simulated events with various background solar wind speeds (BSWS) to investigate this effect. The results show that the shock arrival time at Earth depends on the BSWS, the speed of solar disturbances, their size, and their source location at the Sun. However, it is found that for a sufficiently large momentum input, the shock arrival time at Earth is not significantly affected by the pre-existing solar wind speed.     

The Radial Variation of Interplanetary Shocks in the Inner Heliosphere: Observations by <Emphasis Type="Italic">Helios</Emphasis>, MESSENGER,and STEREO

The two major sources of collisionless shocks in the solar wind are interplanetary coronal mass ejections (ICMEs) and stream interaction regions (SIRs). Previous studies show that some SIR-associated shocks form between Venus and Earth while most form beyond 1 AU. Here we examine the high-resolution magnetometer records from Helios 1 and 2 obtained between 0.28 and 1 AU and from MESSENGER obtained between 0.3 and 0.7 AU to further refine our understanding as to where, and in what context, shocks are formed in the inner solar system. From Helios data (Helios 1 from 1974 to 1981 and Helios 2 from 1976 to 1980), we find there were only a few shocks observed inside the orbit of Venus with the closest shock to the Sun at 0.29 AU. We find that there is a strong correlation between shock occurrence and solar activity as measured by the sunspot number. Most of the shocks inside of the orbit of Venus appear to be associated with ICMEs. Even the ICME-associated shocks are quite weak inside the orbit of Venus. By comparing MESSENGER and STEREO results, from 2007 to 2009, we find that in the deep solar minimum, SIR-driven shocks began to form at about 0.4 AU and increased in number with heliocentric distance.     

Direct observations of a flare related coronal and solar wind disturbance

Numerous mass ejections from the Sun have been detected with orbiting coronagraphs. Here for the first time we document and discuss the direct association of a coronagraph observed mass ejection, which followed a 2B flare, with a large interplanetary shock wave disturbance observed at 1 AU. Estimates of the mass (2.4 × 10¹⁶ g) and energy content (1.1 × 10³² erg) of the coronal disturbance are in reasonably good agreement with estimates of the mass and energy content of the solar wind disturbance at 1 AU. The energy estimates as well as the transit time of the disturbance are also in good agreement with numerical models of shock wave propagation in the solar wind.     

The Bastille day Shocks and Merged Interaction Region

Near 1 AU the solar wind structure associated with the solar flare of 14 July 2000 (Bastille Day) consisted of a large high-speed stream of 15 July and five nearby small streams during a 10-day period. At the leading edge of the large high-speed stream, in less than 6 hours, the flow speed increased from 600 km s⁻¹ to 1100 km s⁻¹, the magnetic field intensity increased from 10 nT to 60 nT, and an interaction region was identified. The interaction region was bounded between the pair of a forward shock F and a reverse shock R. Additional forward shocks were also identified at the leading edge of each of the five smaller streams. This paper presents a magnetohydrodynamics (MHD) simulation using ACE plasma and magnetic field data near 1 AU as input to study the radial evolution of the Bastille Day solar wind event. The two shocks, F and R, propagated in opposite directions away from each other in the solar wind frame and interacted with neighboring shocks and streams; the spatial and temporal extent of the interaction region continued to increase with the heliocentric distance. The solar wind was restructured from a series of streams at 1 AU to a huge merged interaction region (MIR) extending over a period of 12 days at 5.5 AU. Throughout the interior of the MIR bounded by the shock pair F and R the magnetic field intensity was a few times stronger than that outside the MIR. The simulation shows how merging of shocks, collision of shocks, and formation of new shocks contributed to the evolution process.     

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).     

Propagation of Fast Coronal Mass Ejections and Shock Waves Associated with Type II Radio-Burst Emission: An Analytic Study

Coronal mass ejections (CMEs) are large-scale eruptive events in the solar corona. Once they are expelled into the interplanetary (IP) medium, they propagate outwards and “evolve” interacting with the solar wind. Fast CMEs associated with IP shocks are a critical subject for space weather investigations. We present an analytic model to study the heliocentric evolution of fast CME/shock events and their association with type II radio-burst emissions. The propagation model assumes an early stage where the CME acts as a piston driving a shock wave; beyond this point the CME decelerates, tending to match the ambient solar wind speed and its shock decays. We use the shock speed evolution to reproduce type II radio-burst emissions. We analyse four fast CME halo events that were associated with kilometric type II radio bursts, and in-situ measurements of IP shock and CME signatures. The results show good agreement with the dynamic spectra of the type II frequency drifts and the in-situ measurements. This suggests that, in general, IP shocks associated with fast CMEs evolve as blast waves approaching 1 AU, implying that the CMEs do not drive their shocks any further at this heliocentric range.     

A New Prediction Method for the Arrival Time of Interplanetary Shocks

Solar transient activities such as solar flares, disappearing filaments, and coronal mass ejections (CMEs) are solar manifestations of interplanetary (IP) disturbances. Forecasting the arrival time at the near Earth space of the associated interplanetary shocks following these solar disturbances is an important aspect in space weather forecasting because the shock arrival usually marks the geomagnetic storm sudden commencement (SSC) when the IMF Bz component is appropriately southward and/or the solar wind dynamic pressure behind the shock is sufficiently large. Combining the analytical study for the propagation of the blast wave from a point source in a moving, steady-state, medium with variable density (wei, 1982; wei and dryer 1991) with the energy estimation method in the ISPM model (smith and dryer 1990, 1995), we present a new shock propagation model (called SPM below) for predicting the arrival time of interplanetary shocks at Earth. The duration of the X-ray flare, the initial shock speed and the total energy of the transient event are used for predicting the arrival of the associated shocks in our model. Especially, the background speed, i.e., the convection effect of the solar wind is considered in this model. Applying this model to 165 solar events during the periods of January 1979 to October 1989 and February 1997 to August 2002, we found that our model could be practically equivalent to the prevalent models of STOA, ISPM and HAFv.2 in forecasting the shock arrival time. The absolute error in the transit time in our model is not larger than those of the other three models for the same sample events. Also, the prediction test shows that the relative error of our model is ≤10% for 27.88% of all events, ≤30% for 71.52%, and ≤50% for 85.46%, which is comparable to the relative errors of the other models. These results might demonstrate a potential capability of our model in terms of real-time forecasting.     

Geoeffectiveness of interplanetary shocks during solar minimum (1995–1996) and solar maximum (2000)

Plasma and magnetic field parameter variations across fast forward interplanetary shocks are analyzed during the last solar cycle minimum (1995–1996, 15 shocks), and maximum year 2000 (50 shocks). It was observed that the solar wind velocity and magnetic field strength variation across the shocks were the parameters better correlated with Dst. Superposed epoch analysis centered on the shock showed that, during solar minimum, B _z profiles had a southward, long-duration variation superposed with fluctuations, whereas in solar maximum the B _z profile presented 2 peaks. The first peak occurred 4 hr after the shock, and seems to be associated with the magnetic field disturbed by the shock in the sheath region. The second peak occurred 19 hr after the shock, and seems to be associated with the ejecta fields. The difference in shape and peak in solar maximum (Dst peak =−50 nT, moderate activity) and minimum (Dst peak =−30 nT, weak activity) in average Dst profiles after shocks are, probably, a consequence of the energy injection in the magnetosphere being driven by different interplanetary southward magnetic structures. A statistical distribution of geomagnetic activity levels following interplanetary shocks was also obtained. It was observed that during solar maximum, 36% of interplanetary shocks were followed by intense (Dst≤−100 nT) and 28% by moderate (−50≤Dst <−100 nT) geomagnetic activity. During solar minimum, 13% and 33% of the shocks were followed by intense and moderate geomagnetic activity, respectively. Thus, during solar maximum a higher relative number of interplanetary shocks might be followed by intense geomagnetic activity than during solar minimum. One can extrapolate, for forecasting goals, that during a whole solar cycle a shock has a probability of around 50–60% to be followed by intense/moderate geomagnetic activity.     

Stream Interactions and Interplanetary Coronal Mass Ejections at 5.3 AU near the Solar Ecliptic Plane

We have performed a survey of the characteristics of two types of large spatial-scale solar-wind structures, stream interaction regions (SIRs), and interplanetary coronal mass ejections (ICMEs), near 5.3 AU, using solar-wind observations from Ulysses. Our study is confined to the three aphelion passes of Ulysses, and also within ± 10° of the solar ecliptic plane, covering a part of 1992, 1997 – 1998, and 2003 – 2005, representing three slices of different phases of the solar activity cycle. Overall, there are 54 SIRs and 60 ICMEs in the survey. Many are merged in hybrid events, suggesting that they have undergone multiple interactions prior to reaching Jovian orbit. About 91% of SIRs occur with shocks, with 47% of such shocks being forward – reverse shock pairs. The solar-wind velocity sometimes stays constant or even decreases within the interaction region near 5.3 AU, in contrast with the gradual velocity increase during SIRs at 1 AU. Shocks are driven by 58% of ICMEs, with 94% of them being forward shocks. Some ICMEs seem to have multiple small flux ropes with different scales and properties. We quantitatively compare various properties of SIRs and ICMEs at 5.3 AU, and study their statistical distributions and variations with solar activity. The width, maximum dynamic pressure, and peak perpendicular pressure of SIRs all become larger than ICMEs. Dynamic pressure (P _dyn) is expected to be important for Jovian magnetospheric activity. We have examined the distributions of P _dyn of SIRs, ICMEs, and general solar wind, but these cannot explain the observed bimodal distribution of the location of the Jovian magnetopause. By comparing the properties of SIRs and ICMEs at 0.72, 1, and 5.3 AU, we find that the ICME expansion slows down significantly between 1 and 5.3 AU. Some transient and small streams in the inner heliosphere have merged into a single interaction region. Electronic Supplementary Material  The online version of this article () contains supplementary material, which is available to authorized users.     

Low density drivers of strong interplanetary shocks

The theory that most, if not all, interplanetary shocks are caused by coronal mass ejections (CMEs) faces serious problems in accounting for the strongest shocks. The difficulties include (i) a remarkable absence of very strong shocks during solar maximum 1980 when CMEs were prolific, (ii) unrealistic initial speeds near the Sun for impulsive models, (iii) the absence of rarefaction zones behind the shocks and (iv) sustained high speed flows following shocks which are not easily explained as consequences of CME eruptions. Observations of the proton temperature near 1 AU indicate that strong shock drivers have properties similar to high speed streams emitted by coronal holes. Eruptions of fast solar wind from coronal holes influenced by solar activity can explain the occurrence of the strongest interplanetary shocks.     

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.     

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.     

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.     

A piston-driven shock in the solar corona

A solar flare that occurred on the west limb at 1981, March 25, 2038 UT generated a massive, rapidly-expanding optical coronal transient, which moved outward with an approximately constant velocity of 800 km s^–1. An associated magnetohydrodynamic shock travelled out ahead of the transient with a velocity estimated to be approximately 1000 km s^–1. The optical and radio data on the transient and shock fit well with general theories concerning piston-driven shocks and with current MHD models for propagation of such shocks through the solar corona.     

Sixty-five years of solar radioastronomy: flares, coronal mass ejections and Sun–Earth connection

This paper will review the input of 65 years of radio observations to our understanding of solar and solar–terrestrial physics. It is focussed on the radio observations of phenomena linked to solar activity in the period going from the first discovery of the radio emissions to present days. We shall present first an overview of solar radio physics focussed on the active Sun and on the premices of solar–terrestrial relationships from the discovery to the 1980s. We shall then discuss the input of radioastronomy both at metric/decimetric wavelengths and at centimetric/millimetric and submillimetric wavelengths to our understanding of flares. We shall also review some of the radio, X-ray and white-light signatures bringing new evidence for reconnection and current sheets in eruptive events. The input of radio images (obtained with a high temporal cadence) to the understanding of the initiation and fast development in the low corona of coronal mass ejections (CMEs) as well as the radio observations of shocks in the corona and in the interplanetary medium will be reviewed. The input of radio observations to our knowledge of the interplanetary magnetic structures (ICMEs) will be summarized; we shall show how radio observations linked to the propagation of electron beams allow to identify small scale structures in the heliosphere and to trace the connection between the Sun and interplanetary structures as far as 4AU. We shall also describe how the radio observations bring useful information on the relationship and connections between the energetic electrons in the corona and the electrons measured in-situ. The input of radio observations on the forecasting of the arrival time of shocks at the Earth as well as on Space Weather studies will be described. In the last section, we shall summarize the key results that have contributed to transform our knowledge of solar activity and its link with the interplanetary medium. In conclusion, we shall indicate the instrumental radio developments at Earth and in space, which are from our point of view, necessary for the future of solar and interplanetary physics.     

Correlation Analyses Between the Characteristic Times of Gradual Solar Energetic Particle Events and the Properties of Associated Coronal Mass Ejections

It is generally believed that gradual solar energetic particles (SEPs) are accelerated by shocks associated with coronal mass ejections (CMEs). Using an ice-cream cone model, the radial speed and angular width of 95 CMEs associated with SEP events during 1998 – 2002 are calculated from SOHO/LASCO observations. Then, we investigate the relationships between the kinematic properties of these CMEs and the characteristic times of the intensity-time profile of their accompanied SEP events observed at 1 AU. These characteristic times of SEP are i) the onset time from the accompanying CME eruption at the Sun to the SEP arrival at 1 AU, ii) the rise time from the SEP onset to the time when the SEP intensity is one-half of peak intensity, and iii) the duration over which the SEP intensity is within a factor of two of the peak intensity. It is found that the onset time has neither significant correlation with the radial speed nor with the angular width of the accompanying CME. For events that are poorly connected to the Earth, the SEP rise time and duration have no significant correlation with the radial speed and angular width of the associated CMEs. However, for events that are magnetically well connected to the Earth, the SEP rise time and duration have significantly positive correlations with the radial speed and angular width of the associated CMEs. This indicates that a CME event with wider angular width and higher speed may more easily drive a strong and wide shock near to the Earth-connected interplanetary magnetic field lines, may trap and accelerate particles for a longer time, and may lead to longer rise time and duration of the ensuing SEP event.     

A global 3-D simulation of interplanetary dynamics in June 1991

The global dynamics of the solar wind and interplanetary magnetic field in June 1991 is simulated based on a fully three-dimensional, time-dependent numerical MHD model. The numerical simulation includes eight transient disturbances associated with the major solar flares of June 1991. The unique features of the present simulation are: (i) the disturbances are originated at the coronal base (1R _s) and their propagation through inhomogeneous ambient solar wind is simulated out to 1.5 AU; (ii) as a background for the transients, the global steady-state solar wind structure inferred from the 3-D steady-state model (Usmanov, 1993c) is used. The parameters of the initial pulses are prescribed in terms of the near-Sun shock velocities (as inferred from the metric Type II radio burst observations) relative to the preshock steady-state flow parameters at the flare sites. The computed parameters at the Earth's location for the period 1–18 June, 1991 are compared with the available observations of the interplanetary magnetic field, solar wind velocity, density, and with variation of the geomagnetic activityK _pindex.     

Interplanetary propagation of relativistic solar protons

Particle fluxes and pitch angle distributions of relativistic solar protons at Earth's orbit have been determined by Monte Carlo calculations. The analysis covers two hours after the release of the particles from the Sun and total of 8 × 10⁶ particle trajectories were simulated. The pitch angle scattering was assumed to be isotropic and the scattering mean free path was varied from 0.1 to 4 AU.The intensity-time profiles after a delta-like injection from the Sun show that the interplanetary propagation is clearly non-diffusive at scattering mean-free paths above 0.5 AU. All pitch angle distributions have a steady minimum at 90 °, and they become similar about 20 min after the arrival of first particles.As an application, the solar injection profile and the interplanetary scattering mean-free path of particles that gave rise to the GLE on 7 May, 1978 were determined. In contrast to the values of 3–5 AU published by other authors, the average scattering mean-free path was found to be about 1 AU.     

Gamma-ray lines and neutrons from solar flares

An analysis, with a representative (canonical) example of solar-flare-generated equatorial disturbances, is presented for the temporal and spatial changes in the solar wind plasma and magnetic field environment between the Sun and one astronomical unit (AU). Our objective is to search for first order global consequences rather than to make a parametric study. The analysis - an extension of earlier planar studies - considers all three plasma velocity and magnetic field components (V _r, V_φ, V₀, and B _r, B₀, B_φ) in any convenient heliospheric plane of symmetry such as the ecliptic plane, the solar equatorial plane, or the heliospheric equatorial plane chosen for its ability (in a tilted coordinate system) to order northern and southern hemispheric magnetic topology and latitudinal solar wind flows. Latitudinal velocity and magnetic field gradients in and near the plane of symmetry are considered to provide higher-order corrections of a specialized nature and, accordingly, are neglected, as is dissipation, except at shock waves. The representative disturbance is examined for the canonical case in which one describes the temporal and spatial changes in a homogeneous solar wind caused by a solar-flare-generated shock wave. The ‘canonical’ solar flare is assumed to produce a shock wave that has a velocity of 1000 km s^#X2212;1 at 0.08 AU. We have examined all plasma and field parameters at three radial locations: central meridian and 33° W and 90° W of the flare's central meridian. A higher shock velocity (3000 km s^#X2212;1) was also used to demonstrate the model's ability to simulate a strongly-kinked interplanetary field. Among the global (first-order) results are the following: (i) incorporation of a small meridional magnetic field in the ambient magnetic spiral field has negligible effect on the results; (ii) the magnetic field demonstrates strong kinking within the interplanetary shocked flow, even reversed polarity that - coupled with low temperature and low density - suggests a viable explanation for observed ‘magnetic clouds’ with accompanying double-streaming of electrons at directions ～ 90° to the heliocentric radius.