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.     

Statistical Comparison of Magnetic Clouds with Interplanetary Coronal Mass Ejections for Solar Cycle 23

We compare the number and characteristics of interplanetary coronal mass ejections (ICMEs) to those of magnetic clouds (MCs) by using in-situ solar wind plasma and magnetic field observations made at 1 AU during solar cycle 23. We found that ≈ 28% of ICMEs appear to contain MCs, since 103 magnetic clouds (MCs) occurred during 1995  – 2006, and 307 ICMEs occurred during 1996 – 2006. For the period between 1996 and 2006, 85 MCs are identified as part of ICMEs, and six MCs are not associated with ICMEs, which conflicts with the idea that MCs are usually a subset of ICMEs. It was also found that solar wind conditions inside MCs and ICMEs are usually similar, but the linear correlation between geomagnetic storm intensity (Dst _min) and relevant solar wind parameters is better for MCs than for ICMEs. The differences between average event duration (Δt) and average proton plasma β (〈β〉) are two of the major differences between MCs and ICMEs: i) the average duration of ICMEs (29.6 h) is 44% longer than for MCs (20.6 hours), and ii) the average of 〈β〉 is 0.01 for MCs and 0.24 for ICMEs. The difference between the definition of a MC and that for an ICME is one of the major reasons for these average characteristics being different (i.e., listed above as items i) and ii)), and it is the reason for the frequency of their occurrences being different.     

Near-Earth Interplanetary Coronal Mass Ejections During Solar Cycle 23 (1996 – 2009): Catalog and Summary of Properties

In a previous study (Cane and Richardson, J. Geophys. Res. 108(A4), SSH6-1, 2003), we investigated the occurrence of interplanetary coronal mass ejections in the near-Earth solar wind during 1996 – 2002, corresponding to the increasing and maximum phases of solar cycle 23, and provided a “comprehensive” catalog of these events. In this paper, we present a revised and updated catalog of the ≈300 near-Earth ICMEs in 1996 – 2009, encompassing the complete cycle 23, and summarize their basic properties and geomagnetic effects. In particular, solar wind composition and charge state observations are now considered when identifying the ICMEs. In general, these additional data confirm the earlier identifications based predominantly on other solar wind plasma and magnetic field parameters. However, the boundaries of ICME-like plasma based on charge state/composition data may deviate significantly from those based on conventional plasma/magnetic field parameters. Furthermore, the much studied “magnetic clouds”, with flux-rope-like magnetic field configurations, may form just a substructure of the total ICME interval.     

Interplanetary Coronal Mass Ejections,Associated Features,and Transient Modulation of Galactic Cosmic Rays

Interplanetary structures such as shocks, sheaths, interplanetary counterparts of coronal mass ejections (ICMEs), magnetic clouds, and corotating interaction regions (CIRs) are of special interest for the study of the transient modulation of galactic cosmic rays (GCRs). These structures modulate the GCR intensity with varying amplitudes and recovery-time profiles. It is known that ICMEs are mainly responsible for Forbush decreases in the GCR intensity. However, not all of the ICMEs produce such decreases in GCR intensity. We utilize GCR intensity data recorded by neutron monitors and solar-wind plasma/field data during the passage of ICMEs with different features and structures, and we perform a superposed-epoch analysis of the data. We also adopt the best-fit approach with suitable functions to interpret the observed similarities and differences in various parameters. Using the GCR-effectiveness as a measure of the cosmic-ray response to the passage of ICMEs, about half of the ICMEs identified during 1996?–?2009 are found to produce moderate to very large intensity depressions in GCR intensity. The ICMEs associated with halo CMEs, magnetic-cloud (MC) structures, bidirectional superthermal electron (BDE) signatures, and those driving shocks are 1.5 to 4 times more GCR effective than the ICMEs not associated with these structures/features. Further, the characteristic recovery time of GCR intensity due to shock/BDE/MC/halo-CME-associated ICMEs is larger than those due to ICMEs not associated with these structures/features.     

On the Interplanetary Coronal Mass Ejection Shocks in the Vicinity of the Earth

We studied the relation between the near-Earth signatures of the interplanetary coronal mass ejections (ICMEs) shocks such as sudden storms commencement (SSC), and their counterparts of coronal mass ejections (CMEs) observed near-Sun by solar and heliospheric observatory (SOHO)/large angle and spectrometric coronagraph (LASCO) coronagraph during 1996?C2008. Our result showed that there is a good correlation between the travel time of the ICMEs shocks and their associated radial speeds. Also we have separated the ICME shocks into two groups according to their effective acceleration and deceleration. The results showed that the faster ICME shocks (with negative accelerations which decelerated by solar wind plasma) are more correlated to their associated travel time than those with positive accelerations.     

Interplanetary Coronal Mass Ejections Observed by <Emphasis Type="Italic">Ulysses</Emphasis> Through Its Three Solar Orbits

An extended Ulysses interplanetary coronal mass ejections (ICMEs) list is used to statistically study the occurrence rate of ICMEs, the interaction of ICMEs with solar wind, and the magnetic field properties in ICMEs. About 43% of the ICMEs are identified as magnetic clouds (MCs). It is found that the occurrence rate of ICMEs approximately follows the solar activity level, except for the second solar orbit; the rate is higher in the southern heliolatitude than in the northern heliolatitude; and it roughly decreases with the increase of ICME speeds. Our results show that the speed difference between the ICME and the solar wind in front of it shows a slight decrease with increasing heliocentric distance for ICMEs preceded by a shock, whereas no such dependence is found for the ICMEs without shock association. It is also found that approximately 23% of the ICMEs are associated with radial field events, in which the interplanetary magnetic field with near-radial direction lasts for many hours, in the Ulysses detected range, and these associated events are not necessarily confined to fast ICMEs or the trailing portions of ICMEs. Nearly all these associated events occur during the period of declining solar wind speed and most of them occur at low heliolatitudes.     

The “Floor” in the Interplanetary Magnetic Field: Estimation on the Basis of Relative Duration of ICME Observations in Solar Wind During 1976 – 2000

Measurement of the floor in the interplanetary magnetic field and estimation of the time-invariant open magnetic flux of the Sun require knowledge of closed magnetic flux carried away by coronal mass ejections (CMEs). In contrast with previous papers, we do not use global solar parameters to estimate such values: instead we identify different large-scale types of solar wind for the 1976 – 2000 interval to obtain the fraction of interplanetary CMEs (ICMEs). By calculating the magnitude of the interplanetary magnetic field B averaged over two Carrington rotations, the floor of the magnetic field can be estimated from the B value at a solar cycle minimum when the number of ICMEs is minimal. We find a value of 4.65±0.6 nT, in good agreement with previous results.     

Statistical Study of ICMEs and Their Sheaths During Solar Cycle 23 (1996 – 2008)

We have created a new catalog of 325 interplanetary coronal mass ejections (ICMEs) using their in-situ plasma signatures from 1996 to 2008; this time period includes Solar Cycle 23. The data set came from the OMNI near-Earth database. The one-minute resolution data that we used include magnetic-field strength, solar-wind speed, proton density, proton temperature, and plasma β. We compared this new catalog with other published catalogs. For every event, we indicated the presence of an ICME-driven shock. We identified the boundaries of ICMEs and their sheaths, and examined the statistical properties of characteristic parameters. We derived the duration and radial width of ICMEs and sheaths in the region near Earth. The statistical analysis of all events shows that, on average, sheaths travel faster than ICMEs, which indicates the expansion of CMEs in the interplanetary medium. They have higher mean magnetic-field strength values than ICMEs, and they are denser. They have higher mean proton temperature and plasma β than ICMEs, but they are smaller than ICMEs and last for a shorter time. The events were divided into different categories according to whether they included a shock and according to the phase of Solar Cycle 23 in which they are observed, i.e. ascending, maximum, or descending phase. We compared the different categories. We present a catalog of events available to the scientific community that studies ICMEs, and show the distribution and statistical properties of various parameters during these phenomena that govern the solar wind, the interplanetary medium, and space weather.     

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.     

A Comparative Study of Coronal Mass Ejections with and Without Magnetic Cloud Structure near the Earth: Are All Interplanetary CMEs Flux Ropes?

An outstanding question concerning interplanetary coronal mass ejections (ICMEs) is whether all ICMEs have a magnetic flux rope structure. We test this question by studying two different ICMEs, one having a magnetic cloud (MC) showing smooth rotation of magnetic field lines and the other not. The two ICMEs are chosen in such a way that their progenitor CMEs are very similar in remote sensing observations. Both CMEs originated from close to the central meridian directly facing the Earth. Both CMEs were associated with a long-lasting post-eruption loop arcade and appeared as an elliptical halo in coronagraph images, indicating a flux rope origin. We conclude that the difference in the in-situ observation is caused by the geometric selection effect, contributed by the deflection of flux ropes in the inner corona and interplanetary space. The first event had its nose pass through the observing spacecraft; thus, the intrinsic flux rope structure of the CME appeared as a magnetic cloud. On the other hand, the second event had the flank of the flux rope intercept the spacecraft, and it thus did not appear as a magnetic cloud. We further argue that a conspicuous long period of weak magnetic field, low plasma temperature, and density in the second event should correspond to the extended leg portion of the embedded magnetic flux rope, thus validating the scenario of the flank-passing. These observations support the idea that all CMEs arriving at the Earth include flux rope drivers.     

Implications of Non-cylindrical Flux Ropes for Magnetic Cloud Reconstruction Techniques and the Interpretation of Double Flux Rope Events

Magnetic clouds (MCs) are a subset of interplanetary coronal mass ejections (ICMEs) which exhibit signatures consistent with a magnetic flux rope structure. Techniques for reconstructing flux rope orientation from single-point in situ observations typically assume the flux rope is locally cylindrical, e.g., minimum variance analysis (MVA) and force-free flux rope (FFFR) fitting. In this study, we outline a non-cylindrical magnetic flux rope model, in which the flux rope radius and axial curvature can both vary along the length of the axis. This model is not necessarily intended to represent the global structure of MCs, but it can be used to quantify the error in MC reconstruction resulting from the cylindrical approximation. When the local flux rope axis is approximately perpendicular to the heliocentric radial direction, which is also the effective spacecraft trajectory through a magnetic cloud, the error in using cylindrical reconstruction methods is relatively small (≈ 10^∘). However, as the local axis orientation becomes increasingly aligned with the radial direction, the spacecraft trajectory may pass close to the axis at two separate locations. This results in a magnetic field time series which deviates significantly from encounters with a force-free flux rope, and consequently the error in the axis orientation derived from cylindrical reconstructions can be as much as 90^∘. Such two-axis encounters can result in an apparent ‘double flux rope’ signature in the magnetic field time series, sometimes observed in spacecraft data. Analysing each axis encounter independently produces reasonably accurate axis orientations with MVA, but larger errors with FFFR fitting.     

Interplanetary coronal mass ejection influence on high energy pick-up ions at Venus

We have used the ion mass analyzer (IMA) and magnetometer (MAG) on Venus Express (VEX) to study escaping O⁺ during interplanetary coronal mass ejections (ICMEs). Data from 389 VEX orbits during 2006 and 2007 revealed 265 samples of high energy pick-up ion features in 197 separate orbits. Magnetometer data during the same time period showed 17 ICMEs. The interplanetary conditions associated with the ICMEs clearly accelerate the pick-up ions to higher energies at lower altitudes compared to undisturbed solar wind. However, there is no clear dependence of the pick-up ion flux on ICMEs which may be attributed to the fact that this study used data from a period of low solar activity, when ICMEs are slow and weak relative to solar maximum. Alternatively, atmospheric escape rates may not be significantly changed during ICME events.     

Small Solar Wind Transients and Their Connection to the Large-Scale Coronal Structure

It has been realized for some time that the slow solar wind with its embedded heliospheric current sheet often exhibits complex features suggesting at least partially transient origin. In this paper we investigate the structure of the slow solar wind using the observations by the Wind and STEREO spacecraft during two Carrington rotations (2054 and 2055). These occur at the time of minimum solar activity when the interplanetary medium is dominated by recurrent high-speed streams and large-scale interplanetary coronal mass ejections (ICMEs) are rare. However, the signatures of transients with small scale-sizes and/or low magnetic field strength (comparable with the typical solar wind value, ～?5 nT) are frequently found in the slow solar wind at these times. These events do not exhibit significant speed gradients across the structure, but instead appear to move with the surrounding flow. Source mapping using models based on GONG magnetograms suggests that these transients come from the vicinity of coronal source surface sector boundaries. In situ they are correspondingly observed in the vicinity of high density structures where the dominant electron heat flux reverses its flow polarity. These weak transients might be indications of dynamical changes at the coronal hole boundaries or at the edges of the helmet streamer belt previously reported in coronagraph observations. Our analysis supports the idea that even at solar minimum, a considerable fraction of the slow solar wind is transient in nature.     

Deformation of ICMEs/MCs along their path

Interplanetary coronal mass ejections (ICMEs) and their subset, magnetic clouds (MCs), are important manifestations of solar activity which have substantial impact on the geomagnetic field. We re-analyze events already identified in Wind and Voyager 2 data and estimate changes of their geometry along the path from the Sun. The analysis is based on the thickness of the sheath between a shock and a particular ICME or MC which is proportional to the apparent curvature radius of ICMEs/MCs. We have found that this apparent radius of curvature increases with the Mach number and this effect is attributed to the larger deformation of the fast ICME/MC. Further, the relative sheath thickness that is proportional to the flux rope oblateness decreases with the magnetic field intensity inside the ICME/MC and increases with the heliospheric distance.     

Galactic Cosmic Ray Intensity Response to Interplanetary Coronal Mass Ejections/Magnetic Clouds in 1995 – 2009

We summarize the response of the galactic cosmic ray (CGR) intensity to the passage of the more than 300 interplanetary coronal mass ejections (ICMEs) and their associated shocks that passed the Earth during 1995 – 2009, a period that encompasses the whole of Solar Cycle 23. In ∼ 80% of cases, the GCR intensity decreased during the passage of these structures, i.e., a “Forbush decrease” occurred, while in ∼ 10% there was no significant change. In the remaining cases, the GCR intensity increased. Where there was an intensity decrease, minimum intensity was observed inside the ICME in ∼ 90% of these events. The observations confirm the role of both post-shock regions and ICMEs in the generation of these decreases, consistent with many previous studies, but contrary to the conclusion of Reames, Kahler, and Tylka (Astrophys. J. Lett. 700, L199, 2009) who, from examining a subset of ICMEs with flux-rope-like magnetic fields (magnetic clouds) argued that these are “open structures” that allow free access of particles including GCRs to their interior. In fact, we find that magnetic clouds are more likely to participate in the deepest GCR decreases than ICMEs that are not magnetic clouds.     

Study of the Cosmic-Ray Modulation During the Passage of ICMEs and CIRs

We compare the cosmic-ray response to interplanetary coronal mass ejections (ICMEs) and corotating interaction regions (CIRs) during their passage in near-Earth space. We study the relative importance of various structures/features identified during the passage of the ICMEs and CIRs observed during Cycle 23 (1995?–?2009). The identified ICME structures are the shock front, the sheath, and the CME ejecta. We isolate the shock arrival time, the passage of the sheath region, the arrival of ejecta, and the end time of their passage. Similarly, we isolate the CIR arrival, the associated forward shock, the stream interface, and the reverse shock during the passage of a CIR. For the cosmic-ray intensity, we utilize the data from high counting rate neutron monitors. In addition to neutron monitor data, we utilize near-simultaneous and same time-resolution data of interplanetary plasma and field, namely the solar-wind velocity, the interplanetary magnetic field (IMF) vector, and its variance. Further, we also utilize some derived interplanetary parameters. We apply the method of the superposed-epoch analysis. As the plasma and field properties are different during the passage of different structures, both in ICMEs and CIRs, we systematically vary the epoch time in our superposed-epoch analysis one by one. In this way, we study the role and effects of each of the identified individual structures/features during the passage of the ICMEs and CIRs. Relating the properties of various structures and the corresponding variations in plasma and field parameters with changes of the cosmic-ray intensity, we identify the relative importance of the plasma/field parameters in influencing the amplitude and time profiles of the cosmic-ray intensity variations during the passage of the ICMEs and CIRs.     

Geoeffectiveness of the interplanetary manifestations of coronal mass ejections and solar-wind stream–stream interactions

Coronal mass ejections and high-speed streams from the Sun, and related structures formed and evolved in interplanetary space, i.e. interplanetary manifestations of CMEs (ICMEs) and stream interaction regions (SIRs)/corotating interaction regions (CIRs), are mainly responsible for geomagnetic disturbances in the Earth’s magnetic environment. However, the presence or absence of associated/finer structures of ICMEs (e.g., shock/sheath, magnetic cloud) and SIRs/CIRs (forward and reverse shocks, stream interface) might influence their geoeffectiveness as these features within large-scale structures of ICMEs and SIRs display different and varying plasma and field characteristics. In this work, we analyze the solar-wind plasma and field parameters (plasma velocity, density and pressure, magnetic field, its north-south component and electric field) together with geomagnetic activity parameters (kp and Dst), applying the method of superposed epoch analysis. By systematically changing the time of passage of different features as epochs, e.g. discontinuities/shocks, CMEs/magnetic clouds in ICMEs and discontinuities/forward shocks in SIRs/CIRs, we study the relative geoeffectiveness of not only the large-scale structures (ICMEs/SIRs/CIRs), but of their finer features also. We critically analyze the differences in geoeffectiveness due to different structures and features, with distinct plasma/field characteristics, and we utilize these results to understand the mechanism during their interaction with geospace.     

Helium abundance variations in the solar wind

Helium abundance variations in the solar wind have been studied using data obtained with Los Alamos plasma instrumentation on IMP 6, 7, and 8 from 1971 through 1978. For the first time, average flow characteristics have been determined as a function of helium abundance, A(He). Low and average values of A(He) are each preferentially identified with a different characteristic plasma ‘state’ these correspond to what have previously been recognized as the signatures of interplanetary magnetic field polarity reversals and high speed streams, respectively. Helium enhancements at 1 AU also can be identified with a characteristic plasma state, which includes high magnetic field intensity and low proton temperature. This is further evidence that such enhancements are a signal of coronal transient mass ejections. Long-term averages of A(He) at least partially reflect the relative frequency with which coronal streamers, holes, and transients extend their influence into the ecliptic plane at 1 AU. As a result, there is a real and pronounced solar cycle variation of solar wind H(He).     

Simulated (STEREO) Views of the Solar Wind Disturbances Following the Coronal Mass Ejections of 1 August 2010

Images observed by the twin spacecraft Solar TErrestrial RElations Observatory (STEREO) A and B appear as complex structures for two coronal mass ejections (CMEs) on 1 August 2010. Therefore, a series of sky maps of Thomson-scattered white light by interplanetary coronal mass ejections (ICMEs) on 1 August 2010 are simulated using the Hakamada–Akasofu–Fry (HAF) three-dimensional solar-wind model. A comparison between the simulated images and observations of STEREO-A and -B clarifies the structure and evolution of ICMEs (including shocks) in the observed images. The results demonstrate that the simulated images from the HAF model are very useful in the interpretation of the observed images when the ICMEs overlap within the fields of view of the instruments onboard STEREO-A and -B.     

Sharp Changes of Solar Wind Ion Flux and Density Within and Outside Current Sheets

Analysis of the Interball-1 spacecraft data (1995 – 2000) has shown that the solar wind ion flux sometimes increases or decreases abruptly by more than 20% over a time period of several seconds or minutes. Typically, the amplitude of such sharp changes in the solar wind ion flux (SCIFs) is larger than 0.5×10⁸ cm⁻² s⁻¹. These sudden changes of the ion flux were also observed by the Solar Wind Experiment (SWE), on board the Wind spacecraft, as the solar wind density increases and decreases with negligible changes in the solar wind velocity. SCIFs occur irregularly at 1 AU, when plasma flows with specific properties come to the Earth’s orbit. SCIFs are usually observed in slow, turbulent solar wind with increased density and interplanetary magnetic field strength. The number of times SCIFs occur during a day is simulated using the solar wind density, magnetic field, and their standard deviations as input parameters for a period of five years. A correlation coefficient of ∼0.7 is obtained between the modelled and the experimental data. It is found that SCIFs are not associated with coronal mass ejections (CMEs), corotating interaction regions (CIRs), or interplanetary shocks; however, 85% of the sector boundaries are surrounded by SCIFs. The properties of the solar wind plasma for days with five or more SCIF observations are the same as those of the solar wind plasma at the sector boundaries. One possible explanation for the occurrence of SCIFs (near sector boundaries) is magnetic reconnection at the heliospheric current sheet or local current sheets. Other probable causes of SCIFs (inside sectors) are turbulent processes in the slow solar wind and at the crossings of flux tubes.