Radial Speed Evolution of Interplanetary Coronal Mass Ejections During Solar Cycle 23

We report radial-speed evolution of interplanetary coronal mass ejections (ICMEs) detected by the Large Angle and Spectrometric Coronagraph onboard the Solar and Heliospheric Observatory (SOHO/LASCO), interplanetary scintillation (IPS) at 327 MHz, and in-situ observations. We analyze solar-wind disturbance factor (g-value) data derived from IPS observations during 1997?–?2009 covering nearly the whole period of Solar Cycle 23. By comparing observations from SOHO/LASCO, IPS, and in situ, we identify 39 ICMEs that could be analyzed carefully. Here, we define two speeds [V _SOHO and V _bg], which are the initial speed of the ICME and the speed of the background solar wind, respectively. Examinations of these speeds yield the following results: i) Fast ICMEs (with V _SOHO?V _bg>500 km?s^?1) rapidly decelerate, moderate ICMEs (with 0 km?s^?1≤V _SOHO?V _bg≤500 km?s^?1) show either gradually decelerating or uniform motion, and slow ICMEs (with V _SOHO?V _bg<0 km?s^?1) accelerate. The radial speeds converge on the speed of the background solar wind during their outward propagation. We subsequently find; ii) both the acceleration and the deceleration are nearly complete by 0.79±0.04 AU, and those are ended when the ICMEs reach a 480±21 km?s^?1. iii) For ICMEs with (V _SOHO?V _bg)≥0 km?s^?1, i.e. fast and moderate ICMEs, a linear equation a=?γ ₁(V?V _bg) with γ ₁=6.58±0.23×10^?6 s^?1 is more appropriate than a quadratic equation a=?γ ₂(V?V _bg)|V?V _bg| to describe their kinematics, where γ ₁ and γ ₂ are coefficients, and a and V are the acceleration and speed of ICMEs, respectively, because the χ ² for the linear equation satisfies the statistical significance level of 0.05, while the quadratic one does not. These results support the assumption that the radial motion of ICMEs is governed by a drag force due to interaction with the background solar wind. These findings also suggest that ICMEs propagating faster than the background solar wind are controlled mainly by the hydrodynamic Stokes drag.     

Minimum Variance Analysis of Interplanetary Coronal Mass Ejections Around Solar Cycle 23 Maximum (1998–2002)

The minimum variance analysis of interplanetary coronal mass ejections (ICMEs) observed close to the Earth's orbit around solar cycle 23 maximum (1998–2002) was performed. The ICMEs were classified in three categories: magnetic clouds (MC), undefined ejecta (UE), and complex ejecta (CE). An analysis of the full ICMEs set shows that the average of minimum variance direction inclination angle is 1.6°± 24.8° in relation to the ecliptic plane, with more than 33% of the events presenting inclination angles lower than 10°. The average of minimum variance direction azimuthal angle (in relation to the Sun–Earth line) was 56°. However, around 60% of the ICMEs presented an azimuthal angle lower than 30°, close to the radial direction. It was also observed that the MC set had lower axial (intermediate variance) inclinations relative to the ecliptic plane than the UE and CE events. The intermediate variance axis is close to 90° to the Sun–Earth line. The results obtained in the present analysis were also compared with previous works, permitting a comparison of the ICMEs orientations in solar cycle 23 with previous sor cycles.     

Interplanetary Coronal Mass Ejections Resulting from Earth-Directed CMEs Using SOHO and ACE Combined Data During Solar Cycle 23

In this work a total of 266 interplanetary coronal mass ejections observed by the Solar and Heliospheric Observatory/Large Angle and Spectrometric Coronagraph (SOHO/LASCO) and then studied by in situ observations from Advanced Composition Explorer (ACE) spacecraft, are presented in a new catalog for the time interval 1996?–?2009 covering Solar Cycle 23. Specifically, we determine the characteristics of the CME which is responsible for the upcoming ICME and the associated solar flare, the initial/background solar wind plasma and magnetic field conditions before the arrival of the CME, the conditions in the sheath of the ICME, the main part of the ICME, the geomagnetic conditions of the ICME’s impact at Earth and finally we remark on the visual examination for each event. Interesting results revealed from this study include the high correlation coefficient values of the magnetic field \(B_{z}\) component against the Ap index (\(r = 0.84\)), as well as against the Dst index (\(r = 0.80\)) and of the effective acceleration against the CME linear speed (\(r = 0.98\)). We also identify a north–south asymmetry for X-class solar flares and an east–west asymmetry for CMEs associated with strong solar flares (magnitude ≥ M1.0) which finally triggered intense geomagnetic storms (with \(\mathrm{Ap} \geq179\)). The majority of the geomagnetic storms are determined to be due to the ICME main part and not to the extreme conditions which dominate inside the sheath. For the intense geomagnetic storms the maximum value of the Ap index is observed almost 4 hours before the minimum Dst index. The amount of information makes this new catalog the most comprehensive ICME catalog for Solar Cycle 23.     

Propagation of Coronal Mass Ejections Observed During the Rising Phase of Solar Cycle 24

In this study, we investigate the interplanetary consequences and travel time details of 58 coronal mass ejections (CMEs) in the Sun–Earth distance. The CMEs considered are halo and partial halo events of width \({>}\,120\)°. These CMEs occurred during 2009?–?2013, in the ascending phase of the Solar Cycle 24. Moreover, they are Earth-directed events that originated close to the centre of the solar disk (within about \(\pm30\)° from the Sun’s centre) and propagated approximately along the Sun–Earth line. For each CME, the onset time and the initial speed have been estimated from the white-light images observed by the LASCO coronagraphs onboard the SOHO space mission. These CMEs cover an initial speed range of \({\sim}\,260\,\mbox{--}\,2700~\mbox{km}\,\mbox{s}^{-1}\). For these CMEs, the associated interplanetary shocks (IP shocks) and interplanetary CMEs (ICMEs) at the near-Earth environment have been identified from in-situ solar wind measurements available at the OMNI data base. Most of these events have been associated with moderate to intense IP shocks. However, these events have caused only weak to moderate geomagnetic storms in the Earth’s magnetosphere. The relationship of the travel time with the initial speed of the CME has been compared with the observations made in the previous Cycle 23, during 1996?–?2004. In the present study, for a given initial speed of the CME, the travel time and the speed at 1 AU suggest that the CME was most likely not much affected by the drag caused by the slow-speed dominated heliosphere. Additionally, the weak geomagnetic storms and moderate IP shocks associated with the current set of Earth-directed CMEs indicate magnetically weak CME events of Cycle 24. The magnetic energy that is available to propagate CME and cause geomagnetic storm could be significantly low.     

The Association of Solar Flares with Coronal Mass Ejections During the Extended Solar Minimum

We study the association of solar flares with coronal mass ejections (CMEs) during the deep, extended solar minimum of 2007?–?2009, using extreme-ultraviolet (EUV) and white-light (coronagraph) images from the Solar Terrestrial Relations Observatory (STEREO). Although all of the fast (v>900 km?s^?1), wide (θ>100^°) CMEs are associated with a flare that is at least identified in GOES soft X-ray light curves, a majority of flares with relatively high X-ray intensity for the deep solar minimum (e.g. ?1×10^?6 W?m^?2 or C1) are not associated with CMEs. Intense flares tend to occur in active regions with a strong and complex photospheric magnetic field, but the active regions that produce CME-associated flares tend to be small, including those that have no sunspots and therefore no NOAA active-region numbers. Other factors on scales similar to and larger than active regions seem to exist that contribute to the association of flares with CMEs. We find the possible low coronal signatures of CMEs, namely eruptions, dimmings, EUV waves, and Type III bursts, in 91 %, 74 %, 57 %, and 74 %, respectively, of the 35 flares that we associate with CMEs. None of these observables can fully replace direct observations of CMEs by coronagraphs.     

Solar Wind and Heavy Ion Properties of Interplanetary Coronal Mass Ejections

Magnetic field and plasma properties of the solar wind measured in near-Earth space are a convolution of coronal source conditions and in-transit processes which take place between the corona and near-Earth space. Elemental composition and heavy ion charge states, however, are not significantly altered during transit to Earth and thus such properties can be used to diagnose the coronal source conditions of the solar wind observed in situ. We use data from the Advanced Composition Explorer (ACE) spacecraft to statistically quantify differences in the coronal source properties of interplanetary coronal mass ejections (ICMEs). Magnetic clouds, ICMEs which contain a magnetic flux-rope signature, display heavy ion properties consistent with significantly hotter coronal source regions than non-cloud ICMEs. Specifically, magnetic clouds display significantly elevated ion charge states, suggesting they receive greater heating in the low corona. Further dividing ICMEs by speed, however, shows this effect is primarily limited to fast magnetic clouds and that in terms of heavy ion properties, slow magnetic clouds are far more similar to non-cloud ICMEs. As such, fast magnetic clouds appear distinct from other ICME types in terms of both ion charge states and elemental composition. ICME speed, rather ICME type, correlates with helium abundance and iron charge state, consistent with fast ICMEs being heated through the more extended corona. Fast ICMEs also tend to be embedded within faster ambient solar wind than slow ICMEs, though this could be partly the result of in-transit drag effects. These signatures are discussed in terms of spatial sampling of ICMEs and from fundamentally different coronal formation and release processes.     

Study of Multiple Coronal Mass Ejections at Solar Minimum Conditions

Solar activity alternates between active and quiet phases with an average period of 11?years, and this is known as the Schwabe cycle. Additionally, solar activity occasionally falls into a prolonged quiet phase (grand solar minimum), as represented by the Maunder Minimum in the 17th century, when sunspots were almost absent for 70?years and the length of the Schwabe cycle increased to 14?years. To examine the consistency of the cycle length characteristics during the grand solar minima, the carbon-14 contents in single-year tree rings were measured using an accelerator mass spectrometer as an index of the solar variability during the grand solar minimum of the 4th century BC. The signal of the Schwabe cycle was detected with a statistical confidence level of higher than 95?% by wavelet analysis. This is the oldest evidence for the Schwabe cycle at the present time, and the cycle length is considered to have increased to approximately 16?years during the grand solar minimum of the 4th century BC. This result confirms the association between the increase of the Schwabe cycle length and the weakening of solar activity, and indicates the possible prolonged absence of sunspots in the 4th century BC as during the Maunder Minimum. Theoretical implications from solar dynamo theory are discussed in order to identify the trigger of prolonged sunspot absence. A possible association between the long-term solar variation around the 4th century BC and terrestrial cooling in this period is also discussed.     

Post-Eruption Arcades and Interplanetary Coronal Mass Ejections

We compare the temporal and spatial properties of posteruption arcades (PEAs) associated with coronal mass ejections (CMEs) at the Sun that end up as magnetic cloud (MC) and non-MC events in the solar wind. We investigate the length, width, area, tilt angle, and formation time of the PEAs associated with 22 MC and 29 non-MC events and we find no difference between the two populations. According to current ideas on the relation between flares and CMEs, the PEA is formed together with the CME flux-rope structure by magnetic reconnection. Our results indicate that at the Sun flux ropes form during CMEs in association with both MC and non-MC events; however, for non-MC events the flux-rope structure is not observed in the interplanetary space because of the geometry of the observation, i.e. the location of the spacecraft when the structure passes through it.     

Identification of Interplanetary Coronal Mass Ejections at Ulysses Using Multiple Solar Wind Signatures

Previous studies have discussed the identification of interplanetary coronal mass ejections (ICMEs) near the Earth based on various solar wind signatures. In particular, methods have been developed of identifying regions of anomalously low solar wind proton temperatures (T _p) and plasma compositional anomalies relative to the composition of the ambient solar wind that are frequently indicative of ICMEs. In this study, similar methods are applied to observations from the Ulysses spacecraft that was launched in 1990 and placed in a heliocentric orbit over the poles of the Sun. Some 279 probable ICMEs are identified during the spacecraft mission, which ended in 2009. The identifications complement those found independently in other studies of the Ulysses data, but a number of additional events are identified. The properties of the ICMEs detected at Ulysses and those observed near the Earth and in the inner heliosphere are compared.     

Properties of Interplanetary Coronal Mass Ejections at One AU During 1995 – 2004

We present a comprehensive survey of 230 interplanetary CMEs (ICMEs) during 1995 – 2004 using Wind and ACE in situ observations near one AU, and examine the solar-cycle variation of the occurrence rate, shock association rate, scale size, velocity change, and other properties of ICMEs. The ICME occurrence rate increases (from 5 in 1996 to 40 in 2001) with solar activity; and 66% of all ICMEs occurred with shock(s). A compound parameter, the total pressure perpendicular to the magnetic field (Pt), i.e., the sum of magnetic and perpendicular plasma thermal pressures, assists us in effectively distinguishing ICMEs from other solar-wind structures such as stream interactions, and in quantifying the interaction strength. We interpret the characteristic signatures of the Pt temporal variation in terms of the inferred distance perpendicular to the flow to the center of the obstacle. Group 1 includes events that appear to be traversed near the ICME center, showing an apparent enhanced central Pt; Group 3 represents ICMEs passed far away from the center, displaying a rapid rise and then gradual decay in Pt; and Group 2 includes events with intermediate signatures. About 36% of 198 classifiable ICMEs are Group 1 events, consistent with the conventional wisdom that at one AU a magnetic cloud is found during crossings of only ~1/3 of ICMEs. Our set of Group 1 ICMEs and the set of magnetic clouds from other researchers have significant overlap and a similar solar-cycle dependence. The rough decline of the Group 1 fraction as solar activity increases, is consistent with rough increases of scale size, shock percentage, and peak Pt. These results call into question the need to have different mechanisms to create differently appearing ICMEs. Rather it is possible that all ICMEs have a central flux rope that is traversed about 33% of the time, but in the majority of cases is missed by the spacecraft. Electronic Supplementary Material Supplementary material is available for this article at     

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.     

Propagation of Interplanetary Coronal Mass Ejections: The Drag-Based Model

We present the “Drag-Based Model” (DBM) of heliospheric propagation of interplanetary coronal mass ejections (ICMEs). The DBM is based on the hypothesis that the driving Lorentz force, which launches a CME, ceases in the upper corona and that beyond a certain distance the dynamics becomes governed solely by the interaction of the ICME and the ambient solar wind. In particular, we consider the option where the drag acceleration has a quadratic dependence on the ICME relative speed, which is expected in a collisionless environment, where the drag is caused primarily by emission of magnetohydrodynamic (MHD) waves. In this paper we present the simplest version of DBM, where the equation of motion can be solved analytically, providing explicit solutions for the Sun–Earth ICME transit time and impact speed. This offers easy handling and straightforward application to real-time space-weather forecasting. Beside presenting the model itself, we perform an analysis of DBM performances, applying a statistical and case-study approach, which provides insight into the advantages and drawbacks of DBM. Finally, we present a public, DBM-based, online forecast tool.     

On the Aerodynamic Drag Force Acting on Interplanetary Coronal Mass Ejections

It is well known that the interaction of an interplanetary coronal mass ejection (ICME) with the solar wind leads to an equalisation of the ICME and solar wind velocities at 1 AU. This can be understood in terms of an aerodynamic drag force per unit mass of the form F _D/M=−(ρ_e AC _D/M)(V _i−V _e)∣V _i−V _e∣, where A and M are the ICME cross-section and sum of the mass and virtual mass, V _i and V _e the speed of the ICME and solar wind, ρ_e the solar wind density, C _D a dimensionless drag coefficient, and the inverse deceleration length γ=ρ_e A/M. The optimal radial parameterisation of γ and C _D beyond approximately 15 solar radii is calculated. Magnetohydrodynamic simulations show that for dense ICMEs, C _D varies slowly between the Sun and 1 AU, and is of order unity. When the ICME and solar wind densities are similar, C _D is larger (between 3 and 10), but remains approximately constant with radial distance. For tenuous ICMEs, the ICME and solar wind velocities equalise rapidly due to the very effective drag force. For ICMEs denser that the ambient solar wind, both approaches show that γ is approximately independent of radius, while for tenuous ICMEs, γ falls off linearly with distance. When the ICME density is similar to or less than that in the solar wind, inclusion of virtual mass effects is essential.     

Coronal Mass Ejections,Magnetic Fields,and the Green Corona in Cycle 23

We study a time – latitudinal distribution of CMEs observed by the SOHO spacecraft, their projected speeds and associated magnetic fields, as well as the north – south (N – S) asymmetry of solar surface magnetic fields, and the coronal green line intensities. We have found that (a) there exists an intricate relation between the average projected velocity of CMEs and the mean value of large-scale magnetic fields; (b) there exists a pronounced N – S asymmetry in both the distribution and the number of CMEs; (c) this asymmetry is in favor of the northern hemisphere at the beginning of the cycle, and of the southern hemisphere from 2001 onward, being, in fact, (d) closely related with the N – S asymmetry in the distribution of large-scale magnetic fields and the coronal green line intensities.     

太阳射电米波爆发与日冕物质喷射

利用云南天文台声光频谱仪观测到的一次特殊的太阳射电米波爆发 ,与对应的光学活动及相关事件 ,我们探讨了 1 991年 6月 7日的日冕物质喷射过程