Pinzhong Ye

Multiple magnetic clouds: Several examples during March–April 2001

[1] Multiple magnetic cloud (Multi-MC), which is formed by the overtaking of successive coronal mass ejections (CMEs), is a kind of complex structure in interplanetary space. Multi-MC is worthy of notice due to its special properties and potential geoeffectiveness. Using the data from the ACE spacecraft, we identify the three cases of Multi-MC in the period from March to April 2001. Some observational signatures of Multi-MC are concluded: (1) Multi-MC only consists of several magnetic clouds and interacting regions between them; (2) each subcloud in Multi-MC is primarily satisfied with the criteria of isolated magnetic cloud, except that the proton temperature is not as low as that in typical magnetic cloud due to the compression between the subclouds; (3) the speed of solar wind at the rear part of the front subcloud does not continuously decrease, rather increases because of the overtaking of the following subcloud; (4) inside the interacting region between the subclouds, the magnetic field becomes less regular and its strength decreases obviously, and (5) b value increases to a high level in the interacting region. We find out that two of three Multi-MCs are associated with the great geomagnetic storms (Dst �� 200 nT), which indicate a close relationship between the Multi-MCs and some intense geomagnetic storms. The observational results imply that theMulti-MC is possibly another type of the interplanetary origin of the large geomagnetic storm, though not all of them have geoeffectiveness. Based on the observations from Solar and Heliospheric Observatory (SOHO) and GOES, the solar sources (CMEs) of these Multi-MCs are identified. We suggest that such successive halo CMEs are not required to be originated from a single solar region. Furthermore, the relationship between Multi-MC and complex ejecta is analyzed, and some similarities and differences between them are discussed. INDEX TERMS: 2111 Interplanetary Physics: Ejecta, driver gases, and magnetic clouds; 1739 History of Geophysics: Solar/planetary relationships; 7513 Solar Physics, Astrophysics, and Astronomy: Coronal mass ejections; 2788 Magnetospheric Physics: Storms and substorms; KEYWORDS: multiple magnetic clouds, coronal mass ejections, geomagnetic storms, interaction, interplanetary space

Multiple magnetic clouds in interplanetary space

An interplanetary magnetic cloud (MC) is usually considered the byproduct of a coronal mass ejection (CME). Due to the frequent occurrence of CMEs, multiple magnetic clouds (multi-MCs), in which one MC catches up with another, should be a relatively common phenomenon. A simple flux rope model is used to get the primary magnetic field features of multi-MCs. Results indicate that the magnetic field configuration of multi-MCs mainly depends on the magnetic field characteristics of each member of multi-MCs. It may be entirely different in another situation. Moreover, we fit the data from the Wind spacecraft by using this model. Comparing the model with the observations, we verify the existence of multi-MCs, and propose some suggestions for further work.

Impact of Major Coronal Mass Ejections on Geospace during 2005 September 7-13

We have analyzed five major CMEs originating from NOAA active region (AR) 808 during the period of 2005 September 7–13, when the AR 808 rotated from the east limb to near solar meridian. Several factors that affect the probability of the CMEs’ encounter with the Earth are demonstrated. The solar and interplanetary observations suggest that the second and third CMEs, originating from E67 � and E47 � , respectively, encountered the Earth, while the first CME originating from E77 � missed the Earth, and the last two CMEs, although originating from E39 � and E10 � , respectively, probably only grazed the Earth. On the basis of our ice cream cone mode and CME deflection model, we find that the CME span angle and deflection are important for the probability of encountering Earth. The large span angles allowed the middle two CMEs to hit the Earth, even though their source locations were not close to thesolar centralmeridian.ThesignificantdeflectionmadethefirstCMEtotallymisstheEartheventhoughitalsohad wide span angle. The deflection may also have made the last CME nearly miss the Earth even though it originated close to the disk center. We suggest that, in order to effectively predict whether a CME will encounter the Earth, the factors of the CME source location, the span angle, and the interplanetary deflection should all be taken into account.

MHD simulation of the formation and propagation of multiple magnetic clouds in the heliosphere

A multiple-magnetic-cloud (Multi-MC) structure formed by the overtaking of two successive coronal mass ejections (CMEs) in the heliosphere is studied by using a 2.5-D MHD simulation. This simulation illustrates the process of the formation and propagation of two identical CMEs, which are ejected with speeds of 400 km s −1 and 600 km s −1 respectively and initially separated by 12 h. The results show that it takes ∼18 h for the fast cloud to catch up with the preceding slow one, then the two clouds form a Multi-MC structure that arrives at 1 AU three days later. The fast cloud is slowed down significantly because of the blocking by the preceding slow one. This implies that the travel time of a Multi-MC structure is dominated by the preceding slow cloud. Moreover, most primary observational characteristics of Multi-MC at 1 AU are well represented by the simulation. In addition, by combining observations, theoretical model and the simulation results, differences between Multi-MC and other types of in-situ observed double-flux-rope structure are addressed. A comparison of Multi-MC to coronal- mass-ejection cannibalization near Sun is also given.

A STUDY OF THE ORIENTATION OF INTERPLANETARY MAGNETIC CLOUDS AND SOLAR FILAMENTS

As a kind of eruptive phenomenon associated with coronal mass ejections (CMEs), solar eruptive filaments are thought to be parallel to the axis of surrounding arcade coronal magnetic fields that erupt and develop into interplanetary magnetic clouds (MCs). By investigating three events from 2000 August, 2003 October, and 2003 November, we estimate the axial orientations of the MCs and make a quantitative comparison with the filament orientations. By defining ‘‘tilt angle’’ as the angle between projected orientation on the plane of the sky and the ecliptic, wefind that the

Full-halo coronal mass ejections: Arrival at the Earth

A geomagnetic storm is mainly caused by a frontside coronal mass ejection (CME) hitting the Earth and then interacting with the magnetosphere. However, not all frontside CMEs can hit the Earth. Thus, which CMEs hit the Earth and when they do so are important issues in the study and forecasting of space weather. In our previous work, the deprojected parameters of the full-halo coronal mass ejections (FHCMEs) that occurred from 1 March 2007 to 31 May 2012 were estimated, and there are 39 frontside events that could be fitted by the Graduated Cylindrical Shell model. In this work, we continue to study whether and when these frontside FHCMEs (FFHCMEs) hit the Earth. It is found that 59% of these FFHCMEs hit the Earth, and for central events, whose deviation angles �� , which are the angles between the propagation direction and the Sun-Earth line, are smaller than 45 ◦ , the fraction increases to 75%. After checking the deprojected angular widths of the CMEs, we found that all of the Earth-encountered CMEs satisfy a simple criterion that the angular width (�� ) is larger than twice the deviation angle (�� ). This result suggests that some simple criteria can be used to forecast whether a CME could hit the Earth. Furthermore, for Earth-encountered CMEs, the transit time is found to be roughly anticorrelated with the deprojected velocity, but some events significantly deviate from the linearity. For CMEs with similar velocities, the differences of their transit times can be up to several days. Such deviation is further demonstrated to be mainly caused by the CME geometry and propagation direction, which are essential in the forecasting of CME arrival.

Is there any evident effect of coronal holes on gradual solar energetic particle events

Gradual solar energetic particle (SEP) events are thought to be produced by shocks, which are usually driven by fast coronal mass ejections (CMEs). The strength and magnetic field configuration of the shock are considered the two most important factors for shock acceleration. Theoretically, both of these factors should be unfavorable for producing SEPs in or near coronal holes (CHs). Meanwhile, CMEs and CHs could impact each other. Thus, to answer the question whether CHs have real effects on theintensities of SEP events produced by CMEs, a statistical study is performed. First, a brightness gradient method is developed to determine CH boundaries. Using this method, CHs can be well identified, eliminating any personal bias. Then 56 front-side fast halo CMEs originating from the western hemisphere during 1997‐2003 are investigated as well as their associated large CHs. It is found that neither CH proximity nor CH relative location manifests any evident effect on the proton peak fluxes of SEP events. The analysis reveals that almost all of the statistical results are significant at no more than one standard deviation, � . Our results are consistent with the previous conclusion suggested by Kahler that SEP events can be produced in fast solar wind regions and there is no requirementfor those associated CMEs to be significantly faster.

STRENGTH OF CORONAL MASS EJECTION-DRIVEN SHOCKS NEAR THE SUN AND THEIR IMPORTANCE IN PREDICTING SOLAR ENERGETIC PARTICLE EVENTS

Coronal shocks are important structures, but there are no direct observations of them in solar and space physics. The strength of shocks plays a key role in shock-related phenomena, such as radio bursts and solar energetic particle (SEP) generation. This paper presents an improved method of calculating Alfven speed and shock strength near the Sun.This method is based on using as many observations as possible, rather than one-dimensional global models. Two events, a relatively slow CME on 2001 September 15 and a very fast CME on 2000 June 15, are selected to illustrate the cal- culation process. The calculation results suggest that the slow CME drove a strong shock, with Mach number of 3.43- 4.18, while the fast CME drove a relatively weak shock, with Mach number of 1.90-3.21. This is consistent with the radio observations, which find a stronger and longer decameter-hectometric (DH) type II radio burst during the first event, and a short DH type II radio burst during the second event. In particular, the calculation results explain the observational fact that the slow CME produced a major solar energetic particle (SEP) event, while the fast CME did not. Through a comparison of the two events, the importance of shock strength in predicting SEP events is addressed. Subject headingg acceleration of particles — shock waves — Sun: coronal mass ejections (CMEs)

Why Is a Flare-Rich Active Region Cme-Poor?

Solar active regions (ARs) are the major sources of two kinds of the most violent solar eruptions, namely flares and coronal mass ejections (CMEs). The largest AR in the past 24 years, NOAA AR 12192, crossed the visible disk from 2014 October 17 to 30, unusually produced more than one hundred flares, including 32 M-class and 6 X-class ones, but only one small CME. Flares and CMEs are believed to be two phenomena in the same eruptive process. Why is such a flare-rich AR so CME-poor? We compared this AR with other four ARs; two were productive in both and two were inert. The investigation of the photospheric parameters based on the SDO/HMI vector magnetogram reveals that the flare-rich AR 12192, as the other two productive ARs, has larger magnetic flux, current and free magnetic energy than the two inert ARs, but contrast to the two productive ARs, it has no strong, concentrated current helicity along both sides of the flaring neutral line, indicating the absence of a mature magnetic structure consisting of highly sheared or twisted field lines. Furthermore, the decay index above the AR 12192 is relatively low, showing strong constraint. These results suggest that productive ARs are always large and have enough current and free energy to power flares, but whether or not a flare is accompanied by a CME is seemingly related to (1) if there is mature sheared or twisted core field serving as the seed of the CME, (2) if the constraint of the overlying arcades is weak enough.

Source Region of the Decameter–Hectometric Type II Radio Burst: Shock–Streamer Interaction Region

D–H type II radio bursts are widely thought to be caused by coronal mass ejections (CMEs). However, it is still unclear where the exact source of the type IIs on the shock surface is. We identify the source regions of the decameter–hectometric (D–H) type IIs based on imaging observations from SOHO/LASCO and the radio dynamic spectrum from Wind/Waves. The analysis of two well-observed events suggests that the sources of these two events are located in the interaction regions between shocks and streamers, and that the shocks are enhanced significantly in these regions.