Solar physicists have captured the first direct observational signatures of resonant absorption, thought to play an important role in solving the “coronal heating problem” which has defied explanation for over 70 years.
由日本，美国和欧洲的国际研究团队由博士领导。Joten Okamoto和Patrick Antolin结合了Jaxa的Hinode Mission和NASA的IRIS（接口区域成像谱仪）任务的高分辨率观测，以及最先进的数值模拟以及Naoj的Aterui Aterui SuperCupter的建模。在组合数据中，他们能够检测和识别共振吸收的观察性特征。
Resonant absorption is a process where two different types of magnetically driven waves resonate, strengthening one of them. In particular this research looked at a type of magnetic waves known as Alfvénic waves which can propagate through a prominence (a filamentary structure of cool, dense gas floating in the corona). Here, for the first time, researchers were able to directly observe resonant absorption between transverse waves and torsional waves, leading to a turbulent flow which heats the prominence. Hinode observed the transverse motion and IRIS observed the torsional motion; these results would not have been possible without both satellites.
This new information can help explain how the solar corona reaches temperatures of 1,000,000 degrees Celsius; the so called “coronal heating problem.”
The solar corona, the outer layer of the Sun’s atmosphere, is composed of extreme high temperature gas, known as plasma, with temperatures reaching millions of degrees Celsius. As the outer layer of the Sun, the part farthest from the core where the nuclear reactions powering the Sun occur, it would logically be expected to be the coolest part of the Sun. But in fact, it is 200 times hotter than the photosphere, the layer beneath it. This contradiction, dubbed “the coronal heating problem” has puzzled astrophysicists ever since the temperature of the corona was first measured over 70 years ago.
太空传播的任务观察太阳和其他技术进步表明，太阳的磁场在这个谜语中起着至关重要的作用。但是解决“冠状加热问题”的关键是了解如何有效地转化为电晕中的热量。有两种相互竞争的理论。
The first theory involves solar flares. Although each flare converts large amounts of magnetic energy into thermal energy, the overall frequency of solar flares is not high enough to account for all of the energy needed to heat and maintain the solar corona. To solve this discrepancy, the idea of “nanoflares” was introduced. It is thought that nanoflares, miniature solar flares, occur continuously throughout the corona and that the sum of their actions convert enough magnetic energy into heat to make up the difference. Unfortunately, such nanoflares have yet to be detected.
第二个假设基于磁性波。得益于日本“ Hinode”任务等太空任务（2006年推出），我们现在知道太阳氛围已被“Alfvénic”波渗透。这些磁性驱动的波可以沿着磁场线携带大量能量，实际上足够的能量可以加热和维护电晕。但是，要使该理论起作用，需要有一种机制，可以将这种能量转化为热量。
寻找这种转换机制、研究h team combined data from two state-of-the-art missions: Hinode and the IRIS imaging and spectroscopic satellite (the newest NASA solar mission, launched in 2013).
两种仪器都针对相同的太阳突出性（见图1）。突出的是一块丝状丝堆，散布在电晕上。在这里，“酷”是一个相对术语。突出通常约为10,000度。尽管比其他电晕的其他部分都浓密，但由于磁场线的行为像是将其固定在高空的情况下，但它并没有下沉。组成突出的单个细丝，称为螺纹，遵循磁场线。
Hinode’s very high spatial and temporal resolution allowed researchers to detect small motions in the 2-dimensional plane of the image (up/down and left/right). To understand the complete 3-dimensional phenomenon, researchers used IRIS to measure the Doppler velocity (i.e. velocity along the line of sight, in-to/out-of the picture). The IRIS spectral data also provided vital information about the temperature of the prominence.
这些不同的仪器允许卫星检测到不同的Alfvénic波：Hinode可以检测横向波，而Iris可以检测到扭转波。比较这两个数据集表明，这两种波浪确实已经同步，同时，从10,000度到超过100,000度的突出性也会升高。这是第一次在Alfvénic波和突出供暖之间建立这种密切的关系。
But the waves are not synchronized in the way scientists expected. Think of moving a spoon back-and-forth in a cup of coffee: the half-circular torsional flows around the edges of the spoon appear instantly as the spoon moves. But in the case of the prominence threads, the torsional motion is half-a-beat out of sync with the transverse motion driving it: there is a delay between the maximum speed of the transverse motions and the maximum speed of the torsional motion (see Figure 2), like the delay between the motion of the hips of a dancer in a long skirt and the motions of the skirt hem.
To understand this unexpected pattern the team used NAOJ’s ATERUI supercomputer to conduct 3D numerical simulations of an oscillating prominence thread. Of the theoretical models they tested, one involving resonant absorption provides the best match to the observed data. In this model, transverse waves resonate with torsional waves, strengthening the torsional waves; similar to how a child on a swing can add energy to the swing, causing it to swing higher and faster, by moving his body in time with the motion. The simulations show that this resonance occurs within a specific layer of the prominence thread close to its surface (see Figure 3). When this happens, a half-circular torsional flow around the boundary is generated and amplified. This is known as the resonant flow. Because of its location close to the boundary, the maximum speed of this flow is delayed by half-a-beat from the maximum speed of the transverse motion, just like the pattern actually observed (see Figure 2).
模拟进一步表明，沿螺纹表面的这种共振流可以变得湍流。湍流的出现非常重要，因为它有效地将波能转化为热能。这种湍流的另一个重要效果是将预测模型中预测的谐振流扩大到实际观察到的大小。
该模型可以将观测值的主要特征解释为两步过程的结果。首先谐振吸收会将能量转移到扭转运动，从而沿突出线的表面产生共振流。然后，这种增强的谐振流中的湍流将能量转化为热量（见图2）。
This work shows how the power of multiple satellites, such as Hinode and IRIS, can be combined to investigate long-standing astrophysical problems and will serve as an example for other research looking for similar heating in other solar observations.
这些结果发表在The Astrophysical Journal, Vol 809 in August, 2015.
Source: http://www.eurekalert.org/pub_releases/2015-08/nion-hia082015.php
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