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A large number of magnetic clouds (MCs) were observed during the operation period of the Helios mission. Some of the MCs were observed by multi-spacecraft, enabling a detailed study of their extension in the inner heliosphere when they pass by more than one observation location. In the present work, we analyzed 62 MCs, including 16 which were observed by multiple spacecraft and 46 observed by a single one. For those MCs observed by a single spacecraft, the cloud's axis, obtained using minimum variance analysis (MVA) on magnetic field data, could be an explanation for the fact that there were no observations at the other spacecraft. Highly inclined MCs, defined as MCs whose axes are inclined more than 45 ∘ from the ecliptic plane, are less likely to be observed by two spacecraft that are close to the ecliptic plane.

For over an entire solar cycle, from the end of 1974 until the beginning of 1986, the twin Helios spacecraft explored the inner heliosphere. These in situ, high-resolution plasma and magnetic field measurements covered heliocentric distances between 0.3 and 1 AU from the Sun and are of particular interest to studies of space weather phenomena. During this period the two spacecraft detected 395 ICME-driven shocks and these waves were found to be driven by interplanetary coronal mass ejections (ICMEs). Based on these multi-spacecraft measurements, which include a third vantage point with the observations from ISEE-3/IMP-8, the longitudinal extent of the shock waves were measured in the inner heliosphere. It was found that shock waves have about a 50% chance to be observed by two different locations separated by 90 ° . In practice, one can expect with about a 50% chance that the shock driven by a limb coronal mass ejections (CMEs) will hit the Earth, considering the expansion in longitude of shock waves driven by their associated ICMEs. For a larger separation the uncertainty increases, as only a few cases could be observed. With the absence of simultaneous solar disk observations one can then no longer unequivocally identify the shock waves observed at each spacecraft.

The KuaFu mission is designed to explore the physical processes that are responsible for space weather, complementing planned in situ and ground-based programs, and also to make an essential contribution to the space weather application. KuaFu encompasses three spacecraft. KuaFu-A will be located at the L1 libration point and have instruments to observe solar extreme ultraviolet (EUV) and far ultraviolet (FUV) emissions and white-light coronal mass ejections (CMEs), and to measure radio waves, the local plasma and magnetic field, and high-energy particles. KuaFu-B1 and KuaFu-B2 will be in elliptical polar orbits chosen to facilitate continuous (24 h per day 7 days per week) observation of the northern polar aurora oval and the inner magnetosphere. The KuaFu mission is designed to observe globally the complete chain of disturbances from the solar atmosphere to geospace, including solar flares, CMEs, interplanetary clouds, shock waves, and their geo-effects, with a particular focus on dramatic space weather events such as magnetospheric substorms and magnetic storms. The mission start is targeted for the next solar maximum with launch hoped for in 2012. The initial mission lifetime will be 3 years. The overall mission design, instrument complement, and incorporation of recent technologies will advance our understanding of the physical processes underlying space weather, solve several key outstanding questions including solar CME initiation, Earth magnetic storm and substorm mechanisms, and advance our understanding of multi-scale interactions in and system-level behavior of our Sun–Earth space plasma system.

In this paper I will briefly summarize the present status of our knowledge on the four different sorts of solar wind, their sources and their short- and long-term variations. First: the fast solar wind in high-speed streams that emerges from coronal hole regions. Second: the slow solar wind emerging from the non-active Sun near the global heliospheric current sheet above helmet streamers and underlying active regions. Third: the slow solar wind filling most of the heliosphere during high solar activity, emerging above active regions in a highly turbulent state, and fourth: the plasma expelled from the Sun during coronal mass ejections. The coronal sources of these different flows vary dramatically with the solar activity cycle.

The paper presents the dynamics of the solar corona at the minimum phase of the solar cycle (period August–October 1996), as inferred from LASCO-C1 spectral data. LASCO-C1 is an internally occulted coronagraph aboard the SOHO spacecraft. It has a tunable Fabry–Perot interferometer which allows taking spectral scans of selected coronal emission lines. From measured line profiles we deduced physical quantities such as temperature and flow velocities along the line of sight. This way, we obtained information on the flow pattern in the low corona (1.1–1.6 solar radii).

CME disturbances at Earth arise from the sheath that arrives in front of the ICME and from the ICME itself. The geoeffective environment is qualitatively different in the sheath than within the ICME. Consequently two types of forecast procedures using solar observations of phenomena associated with the release of the CME as input parameters have been developed to treat the two types of environment. This chapter surveys efforts that have resulted in implementable (at least in principle) forecast algorithms for sheath and ICME disturbances and discusses uncertainties associated with both.

International audience; CMEs have been observed for over 30 years with a wide variety of instruments. It is now possible to derive detailed and quantitative information on CME morphology, velocity, acceleration and mass. Flares associated with CMEs are observed in X-rays, and several different radio signatures are also seen. Optical and UV spectra of CMEs both on the disk and at the limb provide velocities along the line of sight and diagnostics for temperature, density and composition. From the vast quantity of data we attempt to synthesize the current state of knowledge of the properties of CMEs, along with some specific observed characteristics that illuminate the physical processes occurring during CME eruption. These include the common three-part structures of CMEs, which is generally attributed to compressed material at the leading edge, a low-density magnetic bubble and dense prominence gas. Signatures of shock waves are seen, but the location of these shocks relative to the other structures and the occurrence rate at the heights where Solar Energetic Particles are produced remains controversial. The relationships among CMEs, Moreton waves, EIT waves, and EUV dimming are also cloudy. The close connection between CMEs and flares suggests that magnetic reconnection plays an important role in CME eruption and evolution. We discuss the evidence for reconnection in current sheets from white-light, X-ray, radio and UV observations. Finally, we summarize the requirements for future instrumentation that might answer the outstanding questions and the opportunities that new space-based and ground-based observatories will provide in the future.

The magnetic structure and geomagnetic response of 73 magnetic clouds (MC) observed by the WIND and ACE satellites in solar cycle 23 are examined. The results have been compared with the surveys from the previous solar cycles. The preselected candidate MC events were investigated using the minimum variance analysis to determine if they have a flux-rope structure and to obtain the estimation for the axial orientation (θC, φC). Depending on the calculated inclination relative to the ecliptic we divided MCs into "bipolar" (θCC>45°). The number of observed MCs was largest in the early rising phase, although the halo CME rate was still low. It is likely that near solar maximum we did not identify all MCs at 1AU, as they were crossed far from the axis or they had interacted strongly with the ambient solar wind or with other CMEs. The occurrence rate of MCs at 1AU is also modified by the migration of the filament sites on the Sun towards the poles near solar maximum and by the deflection of CMEs towards the equator due to the fast solar wind flow from large polar coronal holes near solar minimum. In the rising phase nearly all bipolar MCs were associated with the rotation of the magnetic field from the south at the leading edge to the north at the trailing edge. The results for solar cycles 21-22 showed that the direction of the magnetic field in the leading portion of the MC starts to reverse at solar maximum. At solar maximum and in the declining phase (2000-2003) we observed several MCs with the rotation from the north to the south. We observed unipolar (i.e. highly inclined) MCs frequently during the whole investigated period. For solar cycles 21-22 the majority of MCs identified in the rising phase were bipolar while in the declining phase most MCs were unipolar. The geomagnetic response of a given MC depends greatly on its magnetic structure and the orientation of the sheath fields. For each event we distinguished the effect of the sheath fields and the MC fields. All unipolar MCs with magnetic field southward at the axis were geoeffective (DstDstDst≥-100nT).

The LASCO-C1 telescope was designed to perform spectral analysis of coronal structures by means of a tunable Fabry–Perot interferometer acquiring images at different wavelengths. Results from spectral scans of the Fe xiv 5303 A green coronal emission line are presented. Physical quantities like the ion temperature (line widths), and the flow velocity along the line of sight (Doppler shifts) are obtained over the entire corona.

We have analyzed brightness and polarization data of comet 96P/Machholz, obtained with the SOHO-LASCO C3 coronagraph at phase angles up to 167° and 157°, respectively. The polarization data are characteristic of a typical dusty comet. Within error limits the corresponding trigonometric fit describes the new data measured at larger phase angles as well as those of the previously known range. In the phase angle range from 110° to 167° the brightness increases almost linearly by about two orders of magnitude. The gradient is independent of wavelength. From the absence of a diffraction spike we conclude that the grains contributing significantly to the scattered light must have a size parameter $x = 2\pi r/\lambda \ge 20$, i.e. have a radius larger than 1 μ m. Fits of the data with Mie calculations of particles having a power law distribution of power index ≈ 2.5 provide a best fit refractive index $\emph{m}$ = 1.2 + $\emph{i}$0.004. In the framework of effective medium theory and on the assumption of a particle porosity $P= 0.5$ this leads to a complex refractive index of the porous medium $\emph{m}$ = 1.43 + $\emph{i}$0.009. A higher refractive index is possible for more porous grains with very low absorption. The large particle sizes are in qualitative agreement with findings derived from the analysis of the motion of cometary dust under solar radiation pressure (Fulle and coworkers, see [CITE]; [CITE] 1997) and with the in-situ measurements of the dust of Halley's comet.

We have compared characteristics of 38 halo coronal mass ejections observed on the Sun by the Large Angle and Spectrometric Coronagraph onboard SOHO with their corresponding counterparts observed near Earth by the magnetic field and plasma instruments onboard the ACE, WIND and SOHO satellites, in the period from January 1997 to April 2001. We only have selected events that have some associated interplanetary ejecta structure at 1 AU and we have compared the lateral expansion speeds of these halo CMEs and the corresponding ejecta speeds near Earth. We found that there is a high correlation between these two speeds. The results are very similar to the study done by Lindsay et al. (1999) using observations made by Solwind and SMM coronagraphs, and Helios-1 and PVO plasma and interplanetary field data from the period of 1979 to 1988. Also, we reviewed the relation between the CME-related shock transit speed to Earth and the ejecta speeds near Earth. This kind of relation is very important to estimate ejecta speeds of events for which no interplanetary observations are available.

En el presente trabajo presentamos diagramas altura-tiempo de la eyección de masa coronal tipo halo observada en julio 25, 1999 con el Coronógrafo Espectroscópico y de Gran Ángulo (LASCO), a bordo del satélite Observatorio Heliosférico y Solar (SOHO), que puede observar la corona solar de 2 a 32 radios solares. Para obtener estos diagramas se usó una técnica en la cual se dividen las imágenes de LASCO en rebanadas angulares y se coloca lado a lado la misma imagen en diferentes tiempos de observación. Con este método pudimos identificar una pequeña desaceleración de la eyección cerca del polo sur solar.

In this work we have tracked coronal mass ejections observed with the ground based Mirror Coronagraph for Argentina (MICA) and the Large Angle and Spectroscopic Coronagraph (LASCO) C2 and C3 on board of the Solar and Heliospheric Observatory (SOHO). The MICA telescope is located at El Leoncito (31.8 S, 69.3 W), San Juan (Argentina), since 1997 as part of a bilateral scientific project between Germany and Argentina. SOHO is a project of international cooperation between ESA and NASA. Together these instruments are able to observe the solar corona ranging from 1.05 to 32 solar radii. MICA images the Fe XIV emission line corona and LASCO coronagraphs observe the Thomson scattered white light corona. We have selected events for which there are observations from the three coronagraphs. Using the composite data we were able to obtain height-time diagrams for the corresponding dynamical coronal features traveling outwards in order to determine some of their kinematical properties, i.e., plane of sky velocity and acceleration.

In this work we have tracked coronal mass ejections observed with the ground based Mirror Coronagraph for Argentina (MICA) and the Large Angle and Spectroscopic Coronagraph (LASCO) C2 and C3 on board of the Solar and Heliospheric Observatory (SOHO). The MICA telescope is located at El Leoncito (31.8 S, 69.3 W), San Juan (Argentina), since 1997 as part of a bilateral scientific project between Germany and Argentina. SOHO is a project of international cooperation between ESA and NASA. Together these instruments are able to observe the solar corona ranging from 1.05 to 32 solar radii. MICA images the Fe XIV emission line corona and LASCO coronagraphs observe the Thomson scattered white light corona. We have selected events for which there are observations from the three coronagraphs. Using the composite data we were able to obtain height-time diagrams for the corresponding dynamical coronal features traveling outwards in order to determine some of their kinematical properties, i.e., plane of sky velocity and acceleration.

We have selected 57 limb coronal mass ejections observed by LASCO during the period of January1997 to April 2001. We used the related EIT activity close to the limb to define these CMEs as “limbs”. We measured the radial speed of the leading edge close to the center of these CMEs and the lateral expansion speed of the structures. Comparison of both speeds revealed a high correlation between them, the radial speed being around 88% of the expansion speed of the CME. The expansion speed can also be measured for halo CMEs so that it can be used to infer their radial speed toward earth, which is otherwise inaccessible.

The scientific rationale of the Solar Orbiter is to provide, at high spatial (35 km pixel size) and temporal resolution, observations of the solar atmosphere and unexplored inner heliosphere. Novel observations will be made in the almost heliosynchronous segments of the orbits at heliocentric distances near 45 R⊙ and out of the ecliptic plane at the highest heliographic latitudes of 30° – 38°. The Solar Orbiter will achieve its wide-ranging aims with a suite of sophisticated instruments through an innovative design of the orbit. The first near-Sun interplanetary measurements together with concurrent remote observations of the Sun will permit us to determine and understand, through correlative studies, the characteristics of the solar wind and energetic particles in close linkage with the plasma and radiation conditions in their source regions on the Sun. Over extended periods the Solar Orbiter will deliver the first images of the polar regions and the side of the Sun invisible from the Earth.

Coronal mass ejections (CMES) are among the most dramatic forms of transient activity occurring in the solar atmosphere. Despite over twenty years of research, many basic questions related to the physics of CMEs have remained unanswered. Observations with the LASCO and EIT experiments on SOHO, combined with recent theoretical modeling, have provided new insights on some of these outstanding questions and have also raised many new ones that need to be addressed in the future. In this paper, we present some of the new results from SOHO pertaining to the source regions and onsets of CMEs, and their evolution in the corona. We emphasize the important role that studies of CMEs will play in the International Solar Cycle Studies program.

The dominant interplanetary phenomena that are frequently associated with intense magnetic storms are the interplanetary manifestations of fast coronal mass ejections (CMEs). Two such interplanetary structures, involving an intense and long duration Bs component of the IMF are: the sheath region behind a fast forward interplanetary shock, and the CME ejecta itself. Frequently, these structures lead to the development of intense storms with two-step growth in their main phases. These structures, when combined, lead sometimes to the development of very intense storms, especially when an additional interplanetary shock is found in the sheath plasma of the primary structure accompanying another stream. The second stream can also compress the primary cloud, intensifying the Bs field, and bringing with it an additional Bs structure. Thus, at times very intense storms are associated with three or more Bs structures. Another aspect that can contribute to the development of very intense storms refers to the recent finding that magnetic clouds with very intense core magnetic fields tend to have large velocities, thus implying large amplitude interplanetary electric fields that can drive very intense magnetospheric energization.

Charge state distributions of heavy solar wind ions measured in interplanetary space can be used to probe the physical conditions in the solar corona. This paper presents a study of the charge state distributions and the magnetic topology of 56 coronal mass ejections (CMEs) observed in interplanetary space by the Ulysses spacecraft. The analysis of the data from the Solar Wind Ion Composition Spectrometer (SWICS) instrument and the Vector Helium Magnetometer (VHM) experiment onboard Ulysses shows a clear correlation between the charge state distributions and the magnetic topology of CMEs. Almost all CMEs whose charge state distributions are shifted to higher charge states with respect to the ambient solar wind have the structure of magnetic clouds, whereas CMEs with the same charge state distributions as the surrounding solar wind do not show magnetic cloud structure. This correlation is found for CMEs observed at low, mid, and high solar latitudes. On the basis of the numerical solution of the ionization/recombination equations for oxygen and silicon, it is investigated which changes of the electron temperature, electron density, and the speed of the ions in the source region of the CMEs can reproduce the observations. It is shown that the main reason for the observed enhancement of higher charge states in the cloud CMEs is an increased electron temperature. However, the evolution of the density and velocity of the CMEs before the charge states freeze in cannot be neglected.

We report observations, taken with the Solar Ultraviolet Measurements of Emitted Radiation spectrometer, of spatially resolved high red and blue Doppler shifts (up to 650 km s-1) from X-ray-emitting plasma in the corona above a flare. The high Doppler shifts are seen minutes after a fast, faint optical front is seen racing through the same part of the corona in images taken with the Mirror Coronagraph for Argentina. The association of the large-scale fast optical emission front with soft X-ray emission and high Doppler shifts suggests plasma heating and acceleration in the wake of a shock.

The flow directions of solar wind protons were measured in situ by the Helios spacecraft. A long-term average of the velocity shows a systematic drift in the latitudinal flow angle of about +1 north observed with Helios 1 and 1 south observed onboard of Helios 2. The longitudinal flow angle migrates about +1 west over a period of almost 10 years for Helios 1 and 6 years for Helios 2. This systematic change with time of the plasma flow direction may be caused by solar-cycle variations of the orientation of the Sun's magnetic eld which partially corotates with the Sun inside the Alfv en surface (varying in distance between 10 R over the poles and 30 R near the equator). These variations must have been imprinted on the solar wind flow when it detached from corotation with the Sun near the Alfv en point. The angular momentum of the wind is intimately connected with the flow and eld directions. The gain of total angular momentum of the wind equals the loss of angular momentum of the Sun, which is caused by the torque exerted on the rotating Sun through the magnetic eld of the expanding corona. Implications of the Helios observations for models of the magnetic elds of the Sun as well as the solar wind are discussed. We show evidence, that changes of the solar magnetic eld inside the Alfv en surface are responsible for systematic drifts in the solar wind flow direction.

We present observations of a coronal mass ejection (CME) associated with an eruptive prominence during 1998 June 21-22 by LASCO (Large Angle Spectroscopic Coronagraph) aboard SOHO (Solar and Heliospheric Observatory). Various features in the three-part structured, white-light CME as observed by LASCO-C2 and C3 coronagraphs were compared with features in the other wavelengths, for example, in Fe XIV and Fe X emission lines obtained from LASCO C1, in Hα from Helio-Research and at 17 GHz obtained from Nobeyama Radioheliograph. We have investigated conditions in several data sets to understand the eruptive and the pre-eruptive scenario of the CME. The CME and the eruptive prominence accelerate up to ~20 R☉ and then decelerate to the velocity of the ambient slow solar wind. The analysis clearly shows that this particular CME is a typical case of a very slow or gradual CME for which it is difficult to define an exact onset time. The CME could be tracked for about 30 hours until it crossed a distance of 30 R☉ and disappeared from the field of view of the LASCO-C3 coronagraph. The height-time profiles of various features of this CME suggest that the leading edge of the CME and the top of the prominence or the core follow similar pattern, implying a common driver for both the CME and the eruptive prominence. The observations provide strong evidence that the CME and the prominence eruption resulted from a common cause which is the global restructuring of the magnetic field in the corona in an extensive volume of space near and including the CME. The restructuring in turn was a consequence of newly emerging flux regions near and within the neighboring active regions close to the base of the CME.

A comparative study of two events accompanied by both a flare and a CME has been performed. The data analysis has been made by comparing the observations of the LASCO/SOHO coronagraphs with those of the Nancay radioheliograph. The observations show a clear connection between coronal green and red line transient activity, burst radio emission and the CME development which is due to successive loop interactions. Signatures of these interactions are given by the radio emission. One can identify successive sequences in the evolution of the coronal restructuring leading to the full development of the CME. Identification and timing of these sequences result from the radio emission analysis. For flare-CME events, the evolution takes place in the low corona and is extremely fast of the order, on a few minutes.

From Skylab, Helios and Ulysses observations we had learnt already how closely the 3D heliosphere is related to the underlying coronal structure. Around solar activity minimum, large polar coronal holes dominate the major part of the heliosphere, through the high-speed solar wind streams emanating from them. A different type of solar wind is restricted to a narrow near-equatorial belt (about 30 degrees in latitude). The magnetic topology is dominated by strong multipole components and multiple current sheets in the upper corona, and by a large-scale dipole field further outside, respectively. New observations by LASCO on SOHO cover both regions and may reveal clues to the processes responsible for the generation of the different types of solar wind.

Observations from Skylab, Helios, Ulysses, and SOHO have demonstrated how closely the 3D heliosphere is related to the underlying coronal structure. Around solar activity minimum, the large polar coronal holes dominate the major part of the heliosphere, through the high-speed solar wind streams emanating from them. A distinctively different type of solar wind is restricted to a narrow near-equatorial belt (about 30 0 in latitude). Its magnetic topology is dominated by strong multipole components and multiple current sheets. Owing to the new telescopes on SOHO, various effects of CME disturbances propagating through the heliosphere can now be observed in much greater detail: optically from the photosphere out to 32 R s , and later on by in situ spacecraft. It appears that the prediction reliability of space weather at Earth's orbit can be raised substantially in the near future.

The Oporto radiospectrograph and the Nancay radioheliograph recorded a radio event on November 6, 1997, closely related in time with a flare on National Oceanic and Atmospheric Administration (NOAA) active region 8100. At the beginning of the event the radio sources are located on a rather small volume in the vicinity of the flare site. In a timescale of only a few minutes the radio emission sites spread over a large volume in the corona, covering a range of 100° in heliolatitude. During the period of the radio event the Large Angle and Spectrometric Coronagraph (LASCO) on board the Solar and Heliospheric Observatory (SOHO) observed an extremely fast coronal mass ejection (CME), with a velocity around 2000 km s -1 . This CME presents the particularity of having a fast lateral expansion, giving it a shape reminiscent of a coat hanger. There is a very good association between the latitudinal extent and time development of the CME seen by LASCO and the radio sources recorded by the radio instruments.

From October 1996 to January 1997, Ulysses was situated roughly above the west limb of the Sun as observed from Earth at a heliocentric distance of about 4.6 AU and a latitude of about 25 deg. This presents the first opportunity to compare Solar and Heliospheric Observatory (SOHO) limb observations of coronal mass ejections (CMEs) directly with their solar wind counterparts far from the Sun using the Ulysses data. During this interval, large eruptive events were observed above the west limb of the Sun by the Large Angle Spectrometric Coronagraph (LASCO) on SOHO on October 5, November 28, and December 21-25, 1996. Using the combined plasma and magnetic field data from Ulysses, the October 5 event was clearly identified by several distinguishing signatures as a CME. The November 28 event was also identified as a CME that trailed fast ambient solar wind, although it was identified only by an extended interval of counterstreaming suprathermal electrons. The December 21 event was apparently characterized by a six-day interval of nearly radial field and a plasma rarefaction. For the numerous eruptive events observed by the LASCO coronagraph during December 23-25, Ulysses showed no distinct, CMEs, perhaps because of intermingling of two or more of the eruptive events. By mapping the Ulysses observations back in time to the Sun assuming a constant flow speed, we have identified intervals of plasma that were accelerated or decelerated between the LASCO and Ulysses observations.

The near-rigid rotation of the corona above the differential rotation of the photosphere has important implications for the form of the global coronal magnetic field. The magnetic reconfiguring associated with the shear region where the rigidly-rotating coronal field lines interface with the differentially-rotating photospheric field lines could provide an important energy source for coronal heating. We present data on coronal rotation as a function of altitude provided by the Large Angle Spectrometric Coronagraph (LASCO) instrument aboard the Solar and Heliospheric Observatory (SOHO) spacecraft. LASCO comprises of three coronagraphs (C1, C2, and C3) with nested fields-of-view spanning 1.1 R⊙ to 30 R⊙. An asymmetry in brightness, both of the Fe xiv emission line corona and of the broad-band electron scattered corona, has been observed to be stable over at least a one-year period spanning May 1996 to May 1997. This feature has presented a tracer for the coronal rotation and allowed period estimates to be made to beyond 15 R⊙, up to 5 times further than previously recorded for the white-light corona. The difficulty in determining the extent of differential motion in the outer corona is demonstrated and latitudinally averaged rates formed and determined as a function of distance from the Sun. The altitude extent of the low latitude closed coronal field region is inferred from the determined rotation periods which is important to the ability of the solar atmosphere to retain energetic particles. For the inner green line corona ( 2.5 R⊙) we determine a rotation period of (27.7±0.1) days.

The January 6, 1997 halo CME is the subject of continuing research due to the large geomagnetic effects of this event. This paper presents a very simplified model which explains the key observational features. The CME is modelled as a hemispherical shell with a cone shaped extension toward the sun. The observed circular arc is the projection of the “shoulders” of the CME onto the image plane. The low surface brightness and the fuzzy appearance of the halo CME are naturally explained.

(1) All but two geomagnetic storms with Kp ≥ 6 during the operating period (March 1996 through June 1997) of the Large Angle Spectroscopic Coronagraph (LASCO) experiment on the Solar and Heliospheric Observatory (SOHO) spacecraft can be traced to Coronal Mass Ejections (CMEs). (2) These geomagnetic storms are not related to high speed solar wind streams. (3) The CMEs which cause geomagnetic effects, can be classified into two categories: Halo events and toroidal CMEs. (4) The CMEs are accompanied by Coronal Shock Waves as seen in the Extreme Ultraviolet Imaging Telescope (EIT) Fe XII images. (5) Some CMEs are related to flares, others are not. (6) In many cases, the travel time between the explosion on the Sun and the maximum geomagnetic activity is about 80 hours.

Coronal mass ejections (CMEs) that occur near the center of the solar disk are most likely to impact Earth. Detection of such events as ‘halos’ in white-light coronagraphs has been somewhat controversial in recent years. We present observations from the LASCO coronagraphs on SOHO that provide convincing evidence of the detection of an Earth-directed CME on May 12, 1997. The event began at about 04:35 UT and propagated outwards from the Sun with a projected speed of around 250 km s−1. Using some reasonable assumptions about the geometry of the CME, we estimate the true speed to be around 600 km s−1. The onset of the event in LASCO is coincident (to within measurement uncertainties) with an eruptive event detected in extreme ultraviolet observations of the solar disk by the SOHO EIT. This is the first reported observation of a halo CME at projected distances greater than 10 R⊙, with a clearly identifiable solar origin. We discuss the possibility that at least some of the enhanced brightness observed by LASCO may be due to a compressional wave propagating in the corona.

We investigate the nature and origin of the outward-moving density inhomogeneities ("blobs") detected previously with the Large Angle and Spectrometric Coronagraph on the Solar and Heliospheric Observatory. The blobs are concentrated around the thin plasma layer that surrounds the heliospheric current sheet and that constitutes the outer streamer belt; they represent only a small, fluctuating component of the total density within the plasma sheet. As noted before in Sheeley et al., blobs are characterized by low speeds and are continually emitted from the elongated tips of helmet streamers at 3-4 R from Sun center. We suggest that both the blobs and the plasma sheet itself represent closed-field material injected into the solar wind as a result of footpoint exchanges between the stretched helmet-streamer loops and neighboring open field lines. The plasma sheet is thus threaded by newly reconnected, open magnetic field lines, which lend the white-light streamer belt its filamentary appearance. Since in situ observations at 1 AU show that the slow wind (with speeds below 500 km s−1) spreads over an angular extent much greater than the 3° width of the plasma sheet, we deduce that a major component of this wind must originate outside the helmet streamers (i.e., from just inside coronal holes).

The first X-class flare in four years occurred on 9 July 1996. This X2.6/1B flare reached its maximum at 09:11 UT and was located in active region 7978 (S10° W30°) which was an old-cycle sunspot polarity group. We report the SOHO LASCO/EIT/MDI and SOONSPOT observations before and after this event together with Yohkoh SXT images of the flare, radio observations of the type II shock, and GOES disk-integrated soft X-ray flux during an extended period that included energy build-up in this active region. The LASCO coronagraphs measured a significant coronal mass ejection (CME) on the solar west limb beginning on 8 July at about 09:53 UT. The GOES 8 soft X-ray flux (0.1–0.8 nm) had started to increase on the previous day from below the A-level background (10-8 W m-2). At the start time of the CME, it was at the mid-B level and continued to climb. This CME is similar to many events which have been seen by LASCO and which are being interpreted as disruption of existing streamers by emerging flux ropes. LASCO and EIT were not collecting data at the time of the X-flare due to a temporary software outage. A larger CME was in progress when the first LASCO images were taken after the flare. Since the first image of the 'big' CME was obtained after the flare's start time, we cannot clearly demonstrate the physical connection of the CME to the flare. However, the LASCO CME data are consistent with an association of the flare and the CME. No eruptive filaments were observed during this event. We used the flare evidence noted above to employ in real time a simplified Shock-Time-of-Arrival (STOA) algorithm to estimate the arrival of a weak shock at the WIND spacecraft. We compare this prediction with the plasma and IMF data from WIND and plasma data from the SOHO/CELIAS instrument and suggest that the flare - and possibly the interplanetary consequences of the 'big' CME - was the progenitor of the mild, high-latitude, geomagnetic storm (daily sum of Kp=16+, Ap=8) on 12 July 1996. We speculate that the shock was attenuated enroute to Earth as a result of interaction with the heliospheric current/plasma sheet.

The development of a coronal mass ejection on 9 July 1996 has been analyzed by comparing the observations of the LASCO/SOHO coronagraphs with those of the Nancay radioheliograph. The spatial and temporal evolution of the associated radioburst is complex and involves a long-duration continuum. The analysis of the time sequence of the radio continuum reveals the existence of distinct phases associated with distinct reconnection processes and magnetic restructuring of the corona. Electrons are accelerated in association with these reconnection processes. An excellent spatial association is found between the position and extension of the radio source and the CME seen by LASCO. Furthermore, it is shown that the topology and evolution of the source of the radio continuum involve successive interactions between two systems of loops. These successive interactions lead to magnetic reconnection, then to a large scale coronal restructuring. Thus electrons of coronal origin may have access to the interplanetary medium in a large range of heliographic latitudes as revealed by the Ulysses observations.

We employ coronal extrapolations of solar magnetograph data to interpret observations of the white-light streamer structure made with the LASCO coronagraph in 1996. The topological appearance of the streamer belt during the present minimum activity phase is well described by a model in which the Thomson-scattering electrons are concentrated around a single, warped current sheet encircling the Sun. Projection effects give rise to bright, jet-like structures or spikes whenever the current sheet is viewed edge-on; multiple spikes are seen if the current sheet is sufficiently wavy. The extreme narrowness of these features in polarized images indicates that the scattering layer is at most a few degrees wide. We model the evolution of the streamer belt from 1996 April to 1996 September and show that the effect of photospheric activity on the streamer belt topology depends not just on the strength of the erupted magnetic flux, but also on its longitudinal phase relative to the background field. Using flux transport simulations, we also demonstrate how the streamer belt would evolve during a prolonged absence of activity.

Images of the green line corona made with the LASCO C1 coronagraph on SOHO are analyzed by applying current-free extrapolations to the observed photospheric field. The Fe XIV λ5303 emission is shown to be closely related to the underlying photospheric field strength. By modeling the observed intensity patterns as a function of latitude and height above the solar limb, we derive an approximate scaling law of the form nfoot ∝ Bfoot0.9, where nfoot is the density of the green line-emitting plasma and Bfoot is the average field strength at the footprints of the coronal loop. The observed high-latitude enhancements in the green line corona are attributed to the poleward concentration of the large-scale photospheric field. The strongest such enhancements occur where the high-latitude unipolar fields become reconnected to active region flux at lower latitudes; the global emission pattern rotates quasi-rigidly at the rate of the dominant active region complex. The validity of the current-free approximation is assessed by comparing the topology of the observed and simulated green line structures.

Time-lapse sequences of white-light images, obtained during sunspot minimum conditions in 1996 by the Large Angle Spectrometric Coronagraph on the Solar and Heliospheric Observatory, give the impression of a continuous outflow of material in the streamer belt, as if we were observing Thomson scattering from inhomogeneities in the solar wind. Pursuing this idea, we have tracked the birth and outflow of 50-100 of the most prominent moving coronal features and find that: 1. They originate about 3-4 R☉ from Sun center as radially elongated structures above the cusps of helmet streamers. Their initial sizes are about 1 R☉ in the radial direction and 0.1 R☉ in the transverse direction. 2. They move radially outward, maintaining constant angular spans and increasing their lengths in rough accord with their speeds, which typically double from 150 km s-1 near 5 R☉ to 300 km s-1 near 25 R☉. 3. Their individual speed profiles v(r) cluster around a nearly parabolic path characterized by a constant acceleration of about 4 m s-2 through most of the 30 R☉ field of view. This profile is consistent with an isothermal solar wind expansion at a temperature of about 1.1 MK and a sonic point near 5 R☉. Based on their relatively small initial sizes, low intensities, radial motions, slow but increasing speeds, and location in the streamer belt, we conclude that these moving features are passively tracing the outflow of the slow solar wind.

We present the first observations of the initiation of a coronal mass ejection (CME) seen on the disk of the Sun. Observations with the EIT experiment on SOHO show that the CME began in a small volume and was initially associated with slow motions of prominence material and a small brightening at one end of the prominence. Shortly afterward, the prominence was accelerated to about 100 km s-1 and was preceded by a bright loop-like structure, which surrounded an emission void, that traveled out into the corona at a velocity of 200–400 km s-1. These three components, the prominence, the dark void, and the bright loops are typical of CMEs when seen at distance in the corona and here are shown to be present at the earliest stages of the CME. The event was later observed to traverse the LASCO coronagraphs fields of view from 1.1 to 30 R. Of particular interest is the fact that this large-scale event, spanning as much as 70 deg in latitude, originated in a volume with dimensions of roughly 35″ (2.5 × 104 km). Further, a disturbance that propagated across the disk and a chain of activity near the limb may also be associated with this event as well as a considerable degree of activity near the west limb.

The newly developed C1 coronagraph as part of the Large-Angle Spectroscopic Coronagraph (LASCO) on board the SOHO spacecraft has been operating since January 29, 1996. We present observations obtained in the first three months of operation. The green-line emission corona can be made visible throughout the instrument's full field of view, i.e., from 1.1 R⊙ out to 3.2 R⊙ (measured from Sun center). Quantitative evaluations based on calibrations cannot yet be performed, but some basic signatures show up even now: (1) There are often bright and apparently closed loop systems centered at latitudes of 30° to 45° in both hemispheres. Their helmet-like extensions are bent towards the equatorial plane. Farther out, they merge into one large equatorial ‘streamer sheet’ clearly discernible out to 32 R⊙. (2) At mid latitudes a more diffuse pattern is usually visible, well separated from the high-latitude loops and with very pronounced variability. (3) All high-latitude structures remain stable on time scales of several days, and no signature of transient disruption of high-latitude streamers was observed in these early data. (4) Within the first 4 months of observation, only one single ‘fast’ feature was observed moving outward at a speed of 70 km s-1 close to the equator. Faster events may have escaped attention because of data gaps. (5) The centers of high-latitude loops are usually found at the positions of magnetic neutral lines in photospheric magnetograms. The large-scale streamer structure follows the magnetic pattern fairly precisely. Based on our observations we conclude that the shape and stability of the heliospheric current sheet at solar activity minimum are probably due to high-latitude streamers rather than to the near-equatorial activity belt.

The LASCO experiment on board the SOHO satellite has been making observations of the solar corona out to 30 Ro since first light on 29 December 1995, and routinely since 15 May 1996. Over 120 coronal mass ejections have been observed to date. LASCO represents a significant advance over previous coronagraphs in many ways, but principally in an expanded field of view, increased sensitivity and increased dynamic range. The satellite, orbiting about the Lagrangian point, L1, is in continual sunlight, providing the opportunity to view the corona continuously, uninterrupted by orbit night as is common with near-Earth orbits. While the CMEs observed by LASCO are similar to those observed with previous coronagraphs, there are several new aspects: (1) Many are accompanied by a global response of the solar corona, (2) Many show acceleration to the edge of the field, (3) Disconnection is a frequent occurrence, (4) CMEs are occurring more frequently than had been expected at this minimum phase of the solar activity cycle, and (5) CMEs undergo extensive internal evolution as they move outward.

The Large Angle Spectrometric Coronagraph (LASCO) C1 coronagraph on board the Solar and Heliospheric Observatory (SOHO) is designed to image the corona from 1.1 to 3.0 R☉. The resolution of C1 is defined by the size of its CCD pixels, which correspond to 56, and not by the diffraction limit of the optical system, which may be as small as 3''. The resolution of C1 can be improved using the technique of "dynamic imaging"—the process of acquiring successive images of the same scene using subpixel displacements of the steerable primary mirror. We developed a technique we call the fractional pixel restoration (FPR) algorithm that utilizes these observations to construct an image with improved resolution. Simulations were used to test this algorithm and to explore its limitations. We also applied the direct co-addition and FPR algorithms to laboratory preflight images of a wire mesh grid. These results show that the resolution of the C1 coronagraph can be significantly enhanced, even in the presence of noise and modest differences between successive images. In some cases, the results can even reach the diffraction limit of the telescope.

While fast coronal mass ejections (CMEs) have been uniquely identified as sources of transient shock-disturbances in interplanetary space, it is less understood why different kinds of plasma and magnetic field signatures are found in the individual post-shock flows. It is an open question if CMEs can produce various types of solar wind ejecta, e.g. magnetic flux ropes or flows without highly ordered internal fields. To help further clarify the consequences of fast CMEs in interplanetary space we have examined shock events measured by the Helios 1 spacecraft during the years 1979–1981 for which the associated CME has been directly observed with the coronagraph onboard the P78/1 satellite. Seventeen (68%) out of 25 shock-disturbances were followed by clearly discernible driver gas signatures, but only 7 (41%) of the driver gases were magnetic clouds.

In the field of solar-terrestrial relations a clear and unique terminology is needed in order to abolish and avoid unnecessary confusion between the scientists from several involved disciplines. For example, the widely used abbreviationCME (for coronal mass ejection) has turned out to be somewhat misleading. Early on it had been known that other than coronal material is often involved in such events. The discoverers observed transient events of mass ejections from the sun, which could be observed in the corona owing to the newly available coronagraphs. This article is meant to clarify the terminology, with emphasis on giving credit to the original discoverers and the terms they introduced. With this aim in mind I suggest some minor modifications of the terminology.

Energetic particles (EH2O ≥ 60 keV) as well as plasma ions of a few KeV in energy have been measured in the magnetic pileup region and in the cavity of comet Halley (13/14 March 1986) by the instruments EPA/EPONA, RPA and IMS. The Pileup region of comet Grigg-Skjellerup (G-S) was traversed on 10 July 1992. Similarities and diversities in the encounter conditions, particle fluxes and acceleration processes at both comets are discussed.

Coronal Mass Ejections (CMEs) are large, energetic expulsions of mass and magnetic fields from the Sun; they can significantly affect large volumes of the heliosphere and appear to be a key cause of geomagnetic storms. We have compiled a list of all significant CMEs detected by the HELIOS white light photometers from 1975–1982. We are studying the characteristics of these CMEs, and present preliminary results of their associations with in-situ features, especially magnetic “clouds” and periods of bidirectionally streaming ions, two classes of structures considered indicative of interplanetary loops. Advantages of this data set include reliable association in the interplanetary medium of the white light CME plasma with the in-situ features, and observations of a large number of events over a long time base.