Localized Microwave and EUV Bright Structures in an Eruptive Prominence
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|1st Author:||Jing HUANG|
|Published:||23 June 2019|
|Previous Nugget:||Broken-up hard X-ray spectra found for a loop-top source during a solar limb flare|
A solar filament (also called a prominence) is an intrusion of "cool" chromospheric material in the "hot" corona, maintaining itself somehow due to its magnetic-field structure. Here "cool" and "hot" mean "hot, about 10,000 K" and "hotter, about 1 MK". This kind of structure is prone to eruption, although this action is not easily predictable; in extreme cases a filament eruption can lead to a powerful solar flare and a coronal mass ejection.
We have studied a solar eruptive prominence with both flare and CME via microwave and EUV observations. Its evolution can be divided into three phases: slow rise, fast expansion, and ejection. In the slow-rise phase, the prominence continuously twists for more than one hour with a patch of bright emission appearing around the top. When the north leg interacts with local smaller magnetic loops, a fast expansion begins and the flare takes place at that location. The prominence grows rapidly, and a series of localized brightenings appear in the whole prominence structure. Then the ejection occurs, followed by a CME. The pattern of brightenings as viewed at these wavelengths allows us to learn a bit more about the mechanisms involved.
The localized microwave and EUV bright structures
Our microwave images (at 17 and 34 GHz, mainly showing bremsstrahlung from ionized hydrogen), can be divided into three components. In the microwave range we measure brightness as "brightness temperature" Tb, which is closely related to the physical temperature and to the geometry of the source. Here the strongest emission Tb 25,000-300,000 K, is related to the bright flare region near the north foot. The medium values of Tb (10,000-20,000 K) outline a series of small-scale bright enhancements scattered along the prominence, and superposed on a weak background with Tb = 5000-10,000 K (Figure 1). These localized bright structures, first appearing at the top and then scattering in the entire prominence structure, are cospatial with EUV bright threads, fibers, or spots in both high- and low-temperature passbands (Figure 2).
By studying the evolution of two bright regions in the prominence, we find significant time variations on the scale of 3-5 s in the microwave observations (Figure 3). Thus the plasma inside the prominence must have complex spatial structure that changes with time in both density and temperature. These small-scale bright structures indicate intermittent changes in these component plasma structures within the prominence. The amplitude of the enhancements is comparable to the smooth background, so they are significant elements of the overall prominence.
The small-scale enhancements in both EUV and microwave emission in the prominence body could be interpreted in the frame of the small-scale and short-term process of energy releases in the twisted magnetic structure. Particle acceleration or heating at small scales would take place in the twisted magnetic structure, which may generate a localized change in the plasma density and temperature and consequently the localized and short-period enhanced emission at both the microwave and EUV bands. The small-scale energy releases can take place during the growth of the twisted magnetic structure, which may be weakened by its expansion or relaxation. Therefore, this study provides observational evidence that small-size energy releases within the twisted magnetic structure may contribute to its instability, manifested in a catastrophic disruption into a flare and CME.
Jing HUANG, Baolin TAN, Satoshi MASUDA, Xin CHENG, Susanta Bisoi KUMAR, and Victor MELNIKOV.