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Register a new userMagnetic field-line lengths inside interplanetary magnetic flux ropes
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We report on the detailed and systematic study of field-line twist and length distributions within magnetic flux ropes embedded in Interplanetary Coronal Mass Ejections (ICMEs). The Grad-Shafranov reconstruction method is utilized together with a constant-twist nonlinear force-free (Gold-Hoyle) flux rope model to reveal the close relation between the field-line twist and length in cylindrical flux ropes, based on in-situ Wind spacecraft measurements. We show that the field-line twist distributions within interplanetary flux ropes are inconsistent with the Lundquist model. In particular we utilize the unique measurements of magnetic field-line lengths within selected ICME events as provided by Kahler et al. (2011) based on energetic electron burst observations at 1 AU and the associated type III radio emissions detected by the Wind spacecraft. These direct measurements are compared with our model calculations to help assess the flux-rope interpretation of the embedded magnetic structures. By using the different flux-rope models, we show that the in-situ direct measurements of field-line lengths are consistent with a flux-rope structure with spiral field lines of constant and low twist, largely different from that of the Lundquist model, especially for relatively large-scale flux ropes.
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Magnetic flux rope (MFR) is the core structure of the greatest eruptions, i.e., the coronal mass ejections (CMEs), on the Sun, and magnetic clouds are post-eruption 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 post-eruption 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.
Magnetic flux ropes (MFRs) are one kind of fundamental structures in the solar physics, and involved in various eruption phenomena. Twist, characterizing how the magnetic field lines wind around a main axis, is an intrinsic property of MFRs, closely related to the magnetic free energy and stableness. So far it is unclear how much amount of twist is carried by MFRs in the solar atmosphere and in heliosphere and what role the twist played in the eruptions of MFRs. Contrasting to the solar MFRs, there are lots of in-situ measurements of magnetic clouds (MCs), the large-scale MFRs in interplanetary space, providing some important information of the twist of MFRs. Thus, starting from MCs, we investigate the twist of interplanetary MFRs with the aid of a velocity-modified uniform-twist force-free flux rope model. It is found that most of MCs can be roughly fitted by the model and nearly half of them can be fitted fairly well though the derived twist is probably over-estimated by a factor of 2.5. By applying the model to 115 MCs observed at 1 AU, we find that (1) the twist angles of interplanetary MFRs generally follow a trend of about $0.6frac{l}{R}$ radians, where $frac{l}{R}$ is the aspect ratio of a MFR, with a cutoff at about $12pi$ radians AU$^{-1}$, (2) most of them are significantly larger than $2.5pi$ radians but well bounded by $2frac{l}{R}$ radians, (3) strongly twisted magnetic field lines probably limit the expansion and size of MFRs, and (4) the magnetic field lines in the legs wind more tightly than those in the leading part of MFRs. These results not only advance our understanding of the properties and behavior of interplanetary MFRs, but also shed light on the formation and eruption of MFRs in the solar atmosphere. A discussion about the twist and stableness of solar MFRs are therefore given.
Previous studies indicate that interplanetary small magnetic flux ropes (SMFRs) are manifestations of microflare-associated small coronal mass ejections (CMEs), and the hot material with high charge states heated by related microflares are found in SMFRs. Ordinary CMEs are frequently associated with prominence eruptions,and cool prominencematerialsare found within some magnetic clouds (MCs). Therefore, the predicted small CMEs may also be frequently associated with small prominence eruptions. In this work, we aim to search for cool prominence materials within SMFRs.We examined all the O5+ and Fe6+ fraction data obtained by the Advanced Composition Explorer spacecraft during 1998 to 2008 and found that 13 SMFRs might exhibit low-charge-state signatures of unusual O5+and/or Fe6+abundances.One of the 13 SMFRs also exhibited signatures of high ionic charge states. We also reported a SMFR with highFe6+ fraction, but the values of Fe6+is a little lower than the threshold defining unusualFe6+.However, the SDO/AIA observations confirmed that the progenitor CME of this SMFR is associated with a small eruptive prominence, and the observations also supported the prominence materials were embedded in the CME.These observations are at the edge of the capabilities of ACE/SWICS and it cannot be ruled out that they are solely caused by instrumental effects. If these observations are real, they provide new evidence for the conjecture that SMFRs are small-scale MCs but also imply that the connected small CMEs could be associated with flares and prominence eruptions.
Magnetic flux ropes (MFRs) are thought to be the central structure of solar eruptions, and their ideal MHD instabilities can trigger the eruption. Here we performed a study of all the MFR configurations that lead to major solar flares, either eruptive or confined, from 2011 to 2017 near the solar disk center. The coronal magnetic field is reconstructed from observed magnetograms, and based on magnetic twist distribution, we identified the MFR, which is defined as a coherent group of magnetic field lines winding an axis with more than one turn. It is found that 90% of the events possess pre-flare MFRs, and their three-dimensional structures are much more complex in details than theoretical MFR models. We further constructed a diagram based on two parameters, the magnetic twist number which controls the kink instability (KI), and the decay index which controls the torus instability (TI). It clearly shows lower limits for TI and KI thresholds, which are $n_{rm crit} = 1.3$ and $|T_w|_{rm crit} = 2$, respectively, as all the events above $n_{rm crit}$ and nearly 90% of the events above $|T_w|_{rm crit}$ erupted. Furthermore, by such criterion, over 70% of the events can be discriminated between eruptive and confined flares, and KI seems to play a nearly equally important role as TI in discriminating between the two types of flare. There are more than half of events with both parameters below the lower limits, and 29% are eruptive. These events might be triggered by magnetic reconnection rather than MHD instabilities.
It remains unclear how solar flares are triggered and in what conditions they can be eruptive with coronal mass ejections. Magnetic flux ropes (MFRs) has been suggested as the central magnetic structure of solar eruptions, and their ideal instabilities including mainly the kink instability (KI) and torus instability (TI) provide important candidates for triggering mechanisms. Here using magnetic field extrapolations from observed photospheric magnetograms, we systematically studied the variation of coronal magnetic fields, focusing on MFRs, through major flares including 29 eruptive and 16 confined events. We found that nearly 90% events possess MFR before flare and 70% have MFR even after flare. We calculated the controlling parameters of KI and TI, including the MFRs maximum twist number and the decay index of its strapping field. Using the KI and TI thresholds empirically derived from solely the pre-flare MFRs, two distinct different regimes are shown in the variation of the MFR controlling parameters through flares. For the events with both parameters below their thresholds before flare, we found no systematic change of the parameters after the flares, in either the eruptive or confined events. In contrast, for the events with any of the two parameters exceeding their threshold before flare (most of them are eruptive), there is systematic decrease in the parameters to below their thresholds after flares. These results provide a strong constraint for the values of the instability thresholds and also stress the necessity of exploring other eruption mechanisms in addition to the ideal instabilities.
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