日冕物質(zhì)拋射及其驅(qū)動激波的多波段和多視角研究

應(yīng)蓓麗

(1 中國科學(xué)院紫金山天文臺 南京 210023)

(2 中國科學(xué)技術(shù)大學(xué) 合肥 230026)

日冕物質(zhì)拋射(Coronal Mass Ejection, CME)是太陽大氣中劇烈的爆發(fā)現(xiàn)象之一. 其爆發(fā)通常能釋放大量的能量并拋射大量磁化等離子體. CME所驅(qū)動的激波能進一步導(dǎo)致太陽高能粒子事件(Solar Energetic Particle,SEP)的發(fā)生, 并可能影響航天器和宇航員的安全. 因此,研究CME及其驅(qū)動激波的形成機制和性質(zhì)有利于我們更加清晰地了解及監(jiān)測它們的運動過程, 降低它們帶來的災(zāi)害性空間天氣的影響. 本文主要以分析觀測數(shù)據(jù)為主對不同CME事件及其驅(qū)動激波進行了多方面研究. 我們利用多個儀器的觀測分析了4個不同尺度的CME事件.觀測數(shù)據(jù)主要來自SOHO (Solar and Heliospheric Observatory)、SDO(Solar Dynamics Observatory)和STEREO(Solar Terrestrial Relations Observatory) 3個衛(wèi)星.

首先, 我們分析了一個發(fā)生于2015年11月4日的小尺度短時標的太陽爆發(fā)事件, 與其相關(guān)的M1.9級耀斑脈沖相持續(xù)時間很短(＜4 min). 與大尺度的CME爆發(fā)事件相比, 這個CME熱通道主加速相持續(xù)時間短(＜2 min)、最大加速度大(～50 km·s-2)以及峰值速度高(～1800 km·s-1)的特性十分突出. CME的快速脈沖式運動驅(qū)動了一個活塞型的快模激波. 與該激波相關(guān)的II型射電暴的起始基頻高達～320 MHz, 且形成時間不晚于CME熱通道主加速相的2 min以內(nèi). 通過分析II型射電暴的頻帶分裂, 我們估算了激波的壓縮比以及激波上游的磁場強度等參量. 通過對CME(～4×1030erg)與耀斑消耗的磁能(～1.6×1030erg)估算, 我們發(fā)現(xiàn)大、小尺度爆發(fā)事件的CME和耀斑可能具有相同的耗能機制. 依據(jù)爆發(fā)磁通量繩模型的預(yù)測, 該CME的運動學(xué)特征可能與相關(guān)磁通量繩的足點間距小有關(guān).

其次, 我們分析了一個發(fā)生在2010年8月31日的與噴流相關(guān)的CME事件, 該CME鼻端驅(qū)動了一個激波. 我們通過對CME及其驅(qū)動激波進行三維重構(gòu)來研究其真實的運動學(xué)性質(zhì). 考慮到激波頂點的運動速度與CME頂點的速度基本一致, 以及激波鼻端具有弓激波形狀, 我們推測該激波鼻端遵循弓激波的形成機制. 通過“區(qū)域擬合(mask-fitting)”方法, 我們可獲得非對稱CME頂點的最大、最小主曲率半徑及其曲率半徑的演化. 我們發(fā)現(xiàn)CME的最大主曲率半徑是最小主曲率半徑的2–4倍, 這表明僅假設(shè)CME具有一個曲率半徑將會導(dǎo)致日冕參量的估算產(chǎn)生較大誤差. 依據(jù)阿爾芬馬赫數(shù)與比值δ的關(guān)系, 我們還估算了日冕的阿爾芬速度及磁場強度等參量.

然后, 利用磁流體動力學(xué)(magnetohydrodynamics,MHD)數(shù)值模擬的結(jié)果, 我們合成得到白光圖像, 并首次使用互相關(guān)方法分析了日冕白光圖像序列, 進而獲得了CME內(nèi)部瞬時等離子體的二維速度分布圖. 該方法首先利用MHD數(shù)值模擬結(jié)果合成的白光圖像進行測試, 然后再應(yīng)用于2010年10月28日真實CME事件的速度測量中.我們還研究了CME內(nèi)部的動能演化和分布, 以及機械能在CME核心和前沿不同部分的分配情況. 將來, 新一代的日冕儀將對CME提供白光和紫外(H I Lyα)波段的同時觀測, 比如搭載在Solar Obiter衛(wèi)星上的Metis日冕儀和搭載在中國先進天基太陽天文臺(the Chinese Advanced Spacebased Solar Observatory)上的Lyα太陽望遠鏡(LyαSolar Telescope,LST).互相關(guān)方法可用于將來CME的速度測量,限制Lyα多普勒暗化效應(yīng), 以便我們進一步分析CME相關(guān)物理參數(shù).

最后, 我們通過結(jié)合SOHO/LASCO (Large Angle Spectroscopic Coronagraph)的白光觀測和SOHO/UVCS(UV Coronagraph Spectrometer)在2.45R⊙的紫外(O VI 103.2 nm和H I Lyα121.6 nm)和白光的觀測分析了一個快速CME. 首次基于UVCS的白光數(shù)據(jù), 我們利用偏振度方法得到了CME的傳播位置角. 結(jié)合紫外和白光數(shù)據(jù), 我們得到了UVCS視場中CME核心及暗腔處等離子體的電子溫度和有效運動溫度. CME核心的通過導(dǎo)致電子溫度下降至105K. CME前沿在Lyα強度圖上出現(xiàn)明顯的暗化現(xiàn)象. 我們利用LASCO白光圖像推導(dǎo)的CME二維徑向速度分布來限制Lyα譜線多普勒暗化效應(yīng), 以此重構(gòu)將來的Metis和LST紫外觀測圖像.

總的來說, 我們利用不同的地面和空間觀測儀器,對CME及其驅(qū)動激波進行了多角度多波段的觀測分析.結(jié)合已有的白光和Lyα波段觀測, 我們利用相對應(yīng)的研究方法推導(dǎo)CME的速度、密度和溫度等性質(zhì), 為將來新的觀測儀器(Metis和LST)提供必要的科學(xué)工具和準備.

Coronal Mass Ejections (CMEs) are one of the most fierce explosion phenomena in the solar atmosphere. CMEs usually release a large amount of energy and eject massive magnetized plasma. CMEdriven shocks can further lead to solar energetic particle (SEP) events and affect the safety of spacecraft and astronauts. Therefore,researches on CME initiation,shock formation and the evolution and propagation of CMEs in interplanetary space are essential aspects of space weather. Combining observational data from different instruments, we have analyzed four CMEs and their driven shocks at different scales and with other behaviors. The observational data mainly come from three satellites, including SOHO (Solar and Heliospheric Observatory), SDO (Solar Dynamics Observatory)and STEREO(Solar Terrestrial Relations Observatory).

First, we analyze a small-scale, short-duration solar eruption occurred on November 4, 2015. The impulsive phase of the associated M1.9-class flare is very short (＜4 min). The kinematic evolution of the CME hot channel reveals some exceptional characteristics, including a very short duration of the main acceleration phase (＜2 min), a rather high maximal acceleration rate (～50 km·s-2), and peak velocity (～1800 km·s-1). The fast and impulsive motion subsequently results in a piston-driven shock.The starting fundamental frequency of the type II radio burst reaches up to～320 MHz. The type II source formed less than～2 min after the onset of the main acceleration phase. Through the band-splitting of the type II burst, we estimate the shock strength and the magnetic field strength of the shock upstream and so on. Besides, the CME (～4×1030erg) and flare (～1.6×1030erg) consume similar amounts of magnetic energy,implying that small-and large-scale events possibly share a similar relationship between CMEs and flares. The kinematic characteristics of this event are perhaps related to the small footpointseparation distance of the associated magnetic flux rope,as predicted by the Erupting Flux Rope model.

Then,we analyze a CME associated with jets on August 31, 2010. The CME nose drives a shock. For this CME and its driven shock, we perform threedimensional (3D) reconstructions of these structures to study their kinematic features. Given the almost equal speed between the shock and CME, and the bow-shock shape of the shock nose, we infer that the nose part of the shock might follow the formation mechanism of a bow shock. With the aid of the mask fitting method, we obtain two principal radii of curvature of the asymmetrical CME and their evolution with time. We find that the maximal radius of curvature (ROC) is 2 to 4 times the minimal ROC of the CME, inferring that the assumption of one radius of curvature of a CME will result in the high uncertainty in estimations of coronal parameters. Based on the relationship between the ratioδand the Alfv′en Mach number, coronal plasma parameters have been investigated, including the Alfv′enic speed and the coronal magnetic strength.

Using the data obtained from the magnetohydrodynamics (MHD) numerical simulation, we synthesize white-light (WL) images and develop the crosscorrelation method to calculate the two-dimensional(2D) velocity distribution of the CME firstly. The technique is first tested by analyzing synthetic WL images through the MHD numerical simulation and then applied to measure the speed distribution of a real CME occurred on October 28, 2010. The results of this work allow us to characterize the distribution and time evolution of kinetic energy inside the CME,as well as the mechanical energy (combined with the kinetic and potential energy) partition between the core and front of the CME. In the future, new generations of coronagraphs, such as Metis onboard the Solar Orbiter mission and the LyαSolar Telescope(LST) onboard the Chinese Advanced Space-based Solar Observatory mission, will observe CMEs simultaneously in WL and ultraviolet(UV,H I Lyα)bandpasses. The cross-correlation method can be used to measure the speed of CME and constrain the effect of LyαDoppler dimming,so that we can further analyze the relevant physical parameters of CMEs in the future.

We study a fast CME with the combination of WL images acquired by SOHO/LASCO (the Large Angle Spectroscopic Coronagraph), and intensities measured by SOHO/UVCS (The UV Coronagraph Spectrometer)at 2.45R⊙both in UV(H I Lyα121.6 nm lines and O VI 103.2 nm lines)and WL channels.This CME generates a shock. Data from the UVCS WL channel have been employed to measure the CME position angle with a polarization-ratio technique for the first time. Electron temperature and effective kinetic temperatures of the plasma at the CME core and cavity have been estimated combining the UV and WL data. The transit of the CME core results in a decrease of the electron temperature down to 105K. The CME front is observed as a significant dimming in the Lyαintensity. The 2D distribution of plasma speed within the CME body has been reconstructed from LASCO images and employed to constrain the Doppler dimming of the Lyαline and simulate future observations by Metis and LST.

In this dissertation, we have analyzed the CMEs and their driven shocks with multi-perspective and multi-wavelength observations obtained from different space and ground instruments. Combining WL and UV Lyαline observations, we have derived the velocity,density and temperature properties of CMEs based on the corresponding methods, and try to provide data analysis tools for the new instruments(such as Metis and LST) in the future.