No abstract

Magnetic flux rope (MFR) is the core structure of the greatest eruptions, that is, the coronal mass ejections (CMEs), on the Sun, and magnetic clouds are posteruption MFRs in interplanetary space. There is a strong debate about whether or not a MFR exists prior to a CME and how the MFR forms/grows through magnetic reconnection during the eruption. Here we report a rare event, in which a magnetic cloud was observed sequentially by four spacecraft near Mercury, Venus, Earth, and Mars, respectively. With the aids of a uniform‐twist flux rope model and a newly developed method that can recover a shock‐compressed structure, we find that the axial magnetic flux and helicity of the magnetic cloud decreased when it propagated outward but the twist increased. Our analysis suggests that the “pancaking” effect and “erosion” effect may jointly cause such variations. The significance of the pancaking effect is difficult to be estimated, but the signature of the erosion can be found as the imbalance of the azimuthal flux of the cloud. The latter implies that the magnetic cloud was eroded significantly leaving its inner core exposed to the solar wind at far distance. The increase of the twist together with the presence of the erosion effect suggests that the posteruption MFR may have a high‐twist core enveloped by a less‐twisted outer shell. These results pose a great challenge to the current understanding on the solar eruptions as well as the formation and instability of MFRs.

The geoeffectiveness of interplanetary coronal mass ejections (ICMEs) is an important issue in space weather research and forecasting. Based on the ICME catalog that we recently established and the Dst indices from the World Data Center, we study and compare the geoeffectiveness of ICMEs of different in situ signatures and different solar phases from 1995 to 2014. According to different in situ signatures, all ICMEs are divided into three types: isolated ICMEs (I‐ICMEs), multiple ICMEs (M‐ICMEs), and shock‐embedded ICMEs (S‐ICMEs), resulting in a total of 363 group events. The main findings of this work are as follows: (1) Fifty‐eight percent of ICMEs caused geomagnetic storms with Dstmin≤−30 nT. Further, large fraction (87%) of intense geomagnetic storms are caused by ICME groups and their sheath regions. (2) Numbers of ICME groups and the probabilities of ICME groups in causing geomagnetic storms varied in pace with the solar cycle. Meanwhile, the ICME groups and the probabilities of them in causing geomagnetic storms in Solar Cycle 24 are much lower than those in Solar Cycle 23. (3) The maximum value of the intensity of the magnetic field (B), south component of the magnetic field (Bs), and dawn‐dusk electric field vBs are well correlated with the intensity of the magnetic storms. (4) Shock‐embedded ICMEs have a high probability in causing geomagnetic storms, especially intense geomagnetic storms. (5) The compression of shock on the south component of magnetic field is an important factor to enhance the geoeffectiveness of S‐ICMEs structures.

Stream interaction regions (SIRs) are important sources of geomagnetic storms. In this work, we first extend the end time of the widely used SIR catalog developed by Jian et al. (2006, https://doi.org/10.1007/s11207-006-0132-3), which covered the period from 1995 to 2009, to the end of 2016. Based on this extended SIR catalog, the geoeffectiveness of SIRs is discussed in detail. It was found that 52% of the SIRs caused geomagnetic storms with Dstmin ≤−30 nT, but only 3% of them caused intense geomagnetic storms with Dstmin ≤−100 nT. Furthermore, we found that 10 of the intense geomagnetic storms caused by SIRs were associated with complex structures due to interactions between SIRs and interplanetary coronal mass ejections (ICMEs). In such a structure, an ICME is embedded in the SIR and located between the slow and fast solar wind streams. In addition, we found that the geoeffectiveness of SIRs interacting with ICMEs is enhanced. The possibility of SIR‐ICME interaction structures causing geomagnetic storms is markedly higher than that of isolated SIRs or isolated ICMEs. In particular, the geoeffectiveness of SIR‐ICME interaction structures is similar to that of the Shock‐ICME interaction structures, which have been demonstrated to be the main causes of geomagnetic storms.