Evolution of Plasma Composition in an Eruptive Flux Rope

Magnetic flux ropes are bundles of twisted magnetic field enveloping a central axis. They harbor free magnetic energy and can be progenitors of coronal mass ejections (CMEs). However, identifying flux ropes on the Sun can be challenging. One of the key coronal observables that has been shown to indicate the presence of a flux rope is a peculiar bright coronal structure called a sigmoid. In this work, we show Hinode EUV Imaging Spectrometer observations of sigmoidal active region (AR) 10977. We analyze the coronal plasma composition in the AR and its evolution as a sigmoid (flux rope) forms and erupts as a CME. Plasma with photospheric composition was observed in coronal loops close to the main polarity inversion line during episodes of significant flux cancellation, suggestive of the injection of photospheric plasma into these loops driven by photospheric flux cancellation. Concurrently, the increasingly sheared core field contained plasma with coronal composition. As flux cancellation decreased and a sigmoid/flux rope formed, the plasma evolved to an intermediate composition in between photospheric and typical AR coronal compositions. Finally, the flux rope contained predominantly photospheric plasma during and after a failed eruption preceding the CME. Hence, plasma composition observations of AR 10977 strongly support models of flux rope formation by photospheric flux cancellation forcing magnetic reconnection first at the photospheric level then at the coronal level.

Small-scale Magnetic Flux Ropes and Their Properties Based on In Situ Measurements from the Parker Solar Probe

We report small-scale magnetic flux ropes via the in situ measurements from the Parker Solar Probe during the first six encounters, and present additional analyses to supplement our prior work in Chen et al. These flux ropes are detected by the Grad–Shafranov-based algorithm, with their durations and scale sizes ranging from 10 s to ≲1 hr and from a few hundred kilometers to 10−3 au, respectively. They include both static structures and those with significant field-aligned plasma flows. Most structures tend to possess large cross helicity, while the residual energy is distributed over wide ranges. We find that these dynamic flux ropes mostly propagate in the antisunward direction relative to the background solar wind, with no preferential signs of magnetic helicity. The magnetic flux function follows a power law and is proportional to scale size. We also present case studies showing reconstructed two-dimensional (2D) configurations, which confirm that both the static and dynamic flux ropes have a common configuration of spiral magnetic field lines (also streamlines). Moreover, the existence of such events hints at interchange reconnection as a possible mechanism for generating flux rope-like structures near the Sun. Lastly, we summarize the major findings, and discuss the possible correlation between these flux rope-like structures and turbulence due to the process of local Alfvénic alignment.

Multiwavelength and Dual-perspective Observations of Eruption and Untwisting of Two Homologous Magnetic Flux Ropes

In this paper, we present a detailed morphological, kinematic, and thermal analysis of two homologous magnetic flux ropes (MFRs) from NOAA 11515 on 2012 July 8–9. The study is based on multiwavelength and dual-perspective imaging observations from the Solar Dynamics Observatory and the Solar Terrestrial Relations Observatory Ahead spacecraft, which can reveal the structure and evolution of the two MFRs. We find that both of the MFRs show up in multiple passbands and their emissions mainly consist of a cold component peaking at a temperature of ∼0.4–0.6 MK and a hot component peaking at ∼7–8 MK. The two MFRs exhibit erupting, expanding, and untwisting motions that manifest distinctive features from two different viewpoints. Their evolution can be divided into two stages—a fast-eruption stage with speeds of about 105–125 km s−1 for MFR-1 and 50–65 km s−1 for MFR-2—and a slow-expansion (or untwisting) stage with speeds of about 10–35 km s−1 for MFR-1 and 10–30 km s−1 for MFR-2 in the plane of the sky. We also find that during the two-stage evolution, the high-temperature features mainly appear in the interface region between MFRs and ambient magnetic structures and also in the center of MFRs, which suggests that some heating processes take place in such places as magnetic reconnection and plasma compression. These observational results indicate that the eruption and untwisting processes of MFRs are coupled with the heating process, among which an energy conversion exists.

Coronal Magnetic Field Measurements along a Partially Erupting Filament in a Solar Flare

Magnetic flux ropes are the centerpiece of solar eruptions. Direct measurements for the magnetic field of flux ropes are crucial for understanding the triggering and energy release processes, yet they remain heretofore elusive. Here we report microwave imaging spectroscopy observations of an M1.4-class solar flare that occurred on 2017 September 6, using data obtained by the Expanded Owens Valley Solar Array. This flare event is associated with a partial eruption of a twisted filament observed in Hα by the Goode Solar Telescope at the Big Bear Solar Observatory. The extreme ultraviolet (EUV) and X-ray signatures of the event are generally consistent with the standard scenario of eruptive flares, with the presence of double flare ribbons connected by a bright flare arcade. Intriguingly, this partial eruption event features a microwave counterpart, whose spatial and temporal evolution closely follow the filament seen in Hα and EUV. The spectral properties of the microwave source are consistent with nonthermal gyrosynchrotron radiation. Using spatially resolved microwave spectral analysis, we derive the magnetic field strength along the filament spine, which ranges from 600 to 1400 Gauss from its apex to the legs. The results agree well with the nonlinear force-free magnetic model extrapolated from the preflare photospheric magnetogram. We conclude that the microwave counterpart of the erupting filament is likely due to flare-accelerated electrons injected into the filament-hosting magnetic flux rope cavity following the newly reconnected magnetic field lines.

Statistical Study of Small-Scale Interplanetary Magnetic Flux Ropes in the Vicinity of the Heliospheric Current Sheet

The small-scale interplanetary magnetic flux ropes (SIMFRs) are common magnetic structures in the interplanetary space, yet their origination is still an open question. In this article, we surveyed 63 SIMFRs found within 6-day window around the heliospheric current sheet (HCS) and investigated their axial direction, as well as the local normal direction of the HCS. Results showed that the majority (48/63) of the SIMFRs were quasi-parallel to the associated HCS (i.e., the axial direction of SIMFRs was quasi-perpendicular to the normal direction of the associated HCS). They also showed that the SIMFRs quasi-parallel to the associated HCS statistically had shorter duration than the cases quasi-perpendicular. The results indicate that most of these SIMFRs may be generated in the nearby HCSs.

Confined and Eruptive Catastrophes of Solar Magnetic Flux Ropes Caused by Mass Loading and Unloading

It is widely accepted that coronal magnetic flux ropes are the core structures of large-scale solar eruptive activities, which have a dramatic impact on the solar-terrestrial system. Previous studies have demonstrated that varying magnetic properties of a coronal flux rope system could result in a catastrophe of the rope, which may trigger solar eruptive activities. Since the total mass of a flux rope also plays an important role in stabilizing the rope, we use 2.5 dimensional magnetohydrodynamic numerical simulations in this article to investigate how a flux rope evolves as its total mass varies. It is found that an unloading process that decreases the total mass of the rope could result in an upward (eruptive) catastrophe in the flux rope system, during which the rope jumps upward and the magnetic energy is released. This indicates that mass unloading processes could initiate the eruption of the flux rope. Moreover, when the system is not too diffusive, there is also a downward (confined) catastrophe that could be caused by mass loading processes via which the total mass accumulates. The magnetic energy, however, is increased during the downward catastrophe, indicating that mass loading processes could cause confined activities that may contribute to the storage of energy before the onset of coronal eruptions.