分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： We investigate the generation and evolution of switchbacks (SBs), the nature of the sub-Alfv\'enic wind observed by Parker Solar Probe (PSP), and the morphology of the Alfv\'enic transition, all of which are key issues in solar wind research. First we highlight a special structure in the pristine solar wind, termed a low Mach-number boundary layer (LMBL). An increased Alfv\'en radius and suppressed SBs are observed within an LMBL. A probable source on the Sun for an LMBL is the peripheral region inside a coronal hole with rapidly diverging open fields. The sub-Alfv\'enic wind detected by PSP is an LMBL flow by nature. The similar origin and similar properties of the sub-Alfv\'enic intervals favor a wrinkled surface for the morphology of the Alfv\'enic transition. We find that a larger deflection angle tends to be associated with a higher Alfv\'en Mach number. The magnetic deflections have an origin well below the Alfv\'en critical point, and deflection angles larger than $90^{\circ}$ seem to occur only when $M_{\rm A} \gtrsim 2$. The velocity enhancement in units of the local Alfv\'en speed generally increases with the deflection angle, which is explained by a simple model. A nonlinearly evolved, saturated state is revealed for SBs, where the local Alfv\'en speed is roughly an upper bound for the velocity enhancement. In the context of these results, the most promising theory on the origin of SBs is the model of expanding waves and turbulence, and the patchy distribution of SBs is attributed to modulation by reductions in the Alfv\'en Mach number. Finally, a picture on the generation and evolution of SBs is created based on the results.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： On the Sun, Doppler shifts of bidirectional outflows from the magnetic-reconnection site have been found only in confined regions through spectroscopic observations. Without spatially resolved spectroscopic observations across an extended region, the distribution of reconnection and its outflows in the solar atmosphere cannot be made clear. Magnetic reconnection is thought to cause the splitting of filament structures, but unambiguous evidence has been elusive. Here we report spectroscopic and imaging analysis of a magnetic-reconnection event on the Sun, using high-resolution data from the Interface Region Imaging Spectrograph and the Solar Dynamics Observatory. Our findings reveal that the reconnection region extends to an unprecedented length of no less than 14,000 km. The reconnection splits a filament structure into two branches, and the upper branch erupts eventually. Doppler shifts indicate clear bidirectional outflows of ~100 km/s, which decelerate beyond the reconnection site. Differential-emission-measure analysis reveals that in the reconnection region the temperature reaches over 10 MK and the thermal energy is much larger than the kinetic energy. This Letter provides definite spectroscopic evidence for the splitting of a solar filament by magnetic reconnection in an extended region.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： On the Sun, Doppler shifts of bidirectional outflows from the magnetic-reconnection site have been found only in confined regions through spectroscopic observations. Without spatially resolved spectroscopic observations across an extended region, the distribution of reconnection and its outflows in the solar atmosphere cannot be made clear. Magnetic reconnection is thought to cause the splitting of filament structures, but unambiguous evidence has been elusive. Here we report spectroscopic and imaging analysis of a magnetic-reconnection event on the Sun, using high-resolution data from the Interface Region Imaging Spectrograph and the Solar Dynamics Observatory. Our findings reveal that the reconnection region extends to an unprecedented length of no less than 14,000 km. The reconnection splits a filament structure into two branches, and the upper branch erupts eventually. Doppler shifts indicate clear bidirectional outflows of ~100 km/s, which decelerate beyond the reconnection site. Differential-emission-measure analysis reveals that in the reconnection region the temperature reaches over 10 MK and the thermal energy is much larger than the kinetic energy. This Letter provides definite spectroscopic evidence for the splitting of a solar filament by magnetic reconnection in an extended region.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Xinping Zhou
Yuandeng Shen
Ying D. Liu
Huidong Hu
Jiangtao Su
Zehao Tang
Chengrui Zhou
Yadan Duan
Song Tan
摘要： Large-scale Extreme-ultraviolet (EUV) waves are frequently observed as an accompanying phenomenon of flares and coronal mass ejections (CMEs). Previous studies mainly focus on EUV waves with single wavefronts that are generally thought to be driven by the lateral expansion of CMEs. Using high spatio-temporal resolution multi-angle imaging observations taken by the Solar Dynamic Observatory and the Solar Terrestrial Relations Observatory, we present the observation of a broad quasi-periodic fast propagating (QFP) wave train composed of multiple wavefronts along the solar surface during the rising phase of a GOES M3.5 flare on 2011 February 24. The wave train transmitted through a lunate coronal hole (CH) with a speed of 840 +/-67 km/s, and the wavefronts showed an intriguing refraction effect when they passed through the boundaries of the CH. Due to the lunate shape of the CH, the transmitted wavefronts from the north and south arms of the CH started to approach each other and finally collided, leading to the significant intensity enhancement at the collision site. This enhancement might hint the occurrence of interference between the two transmitted wave trains. The estimated magnetosonic Mach number of the wave train is about 1.13, which indicates that the observed wave train was a weak shock. Period analysis reveals that the period of wave train was $\sim$90 seconds, in good agreement with that of the accompanying flare. Based on our analysis results, we conclude that the broad QFP wave train was a large-amplitude fast-mode magnetosonic wave or a weak shock driven by some non-linear energy release processes in the accompanying flare.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Homologous coronal mass ejections (CMEs) are an interesting phenomenon, and it is possible to investigate the formation of CMEs by comparing multi-CMEs under a homologous physical condition. AR 11283 had been present on the solar surface for several days when a bipole emerged on 2011 September 4. Its positive polarity collided with the pre-existing negative polarity belonging to a different bipole, producing recurrent solar activities along the polarity inversion line (PIL) between the colliding polarities, namely the so-called collisional PIL (cPIL). Our results show that a large amount of energy and helicity were built up in the form of magnetic flux ropes (MFRs), with recurrent release and accumulation processes. These MFRs were built up along the cPIL. A flux deficit method is adopted and shows that magnetic cancellation happens along the cPIL due to the collisional shearing scenario proposed by Chintzoglou et al. The total amount of canceled flux was $\sim$0.7$\times$10$^{21}$ Mx with an uncertainty of $\sim$13.2$\%$ within the confidence region of the 30$^\circ$ sun-center distance. The canceled flux amounts to 24$\%$ of the total unsigned flux of the bipolar magnetic region. The results show that the magnetic fields beside the cPIL are very sheared, and the average shear angle is above 70$^\circ$ after the collision. The fast expansion of the twist kernels of the MFRs and the continuous eruptive activities are both driven by the collisional shearing process. These results are important for better understanding the buildup process of the MFRs associated with homologous solar eruptions.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： We investigate the generation and evolution of switchbacks (SBs), the nature of the sub-Alfv\'enic wind observed by Parker Solar Probe (PSP), and the morphology of the Alfv\'enic transition, all of which are key issues in solar wind research. First we highlight a special structure in the pristine solar wind, termed a low Mach-number boundary layer (LMBL). An increased Alfv\'en radius and suppressed SBs are observed within an LMBL. A probable source on the Sun for an LMBL is the peripheral region inside a coronal hole with rapidly diverging open fields. The sub-Alfv\'enic wind detected by PSP is an LMBL flow by nature. The similar origin and similar properties of the sub-Alfv\'enic intervals favor a wrinkled surface for the morphology of the Alfv\'enic transition. We find that a larger deflection angle tends to be associated with a higher Alfv\'en Mach number. The magnetic deflections have an origin well below the Alfv\'en critical point, and deflection angles larger than $90^{\circ}$ seem to occur only when $M_{\rm A} \gtrsim 2$. The velocity enhancement in units of the local Alfv\'en speed generally increases with the deflection angle, which is explained by a simple model. A nonlinearly evolved, saturated state is revealed for SBs, where the local Alfv\'en speed is roughly an upper bound for the velocity enhancement. In the context of these results, the most promising theory on the origin of SBs is the model of expanding waves and turbulence, and the patchy distribution of SBs is attributed to modulation by reductions in the Alfv\'en Mach number. Finally, a picture on the generation and evolution of SBs is created based on the results.
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