李瑛

(南京大學(xué)天文與空間科學(xué)學(xué)院南京210093)

太陽耀斑大氣動(dòng)力學(xué)的觀測和模擬

李瑛?

(南京大學(xué)天文與空間科學(xué)學(xué)院南京210093)

博士學(xué)位論文摘要選登

太陽耀斑是發(fā)生在太陽大氣中的一種劇烈的活動(dòng)現(xiàn)象,發(fā)生的時(shí)標(biāo)約為幾分鐘到幾十分鐘.耀斑過程涉及能量釋放、等離子體加熱、粒子加速、物質(zhì)運(yùn)動(dòng)、波動(dòng)等現(xiàn)象.耀斑爆發(fā)能夠釋放出大量的能量,所發(fā)出的輻射基本覆蓋了電磁波的所有波段.耀斑發(fā)生通常還會(huì)伴隨日冕物質(zhì)拋射(CME),從而對空間和地球環(huán)境造成影響.

目前我們對耀斑過程的理解還很不足(定量方面),其中的一些關(guān)鍵問題仍待解決,包括:耀斑能量是在何時(shí)、何地被釋放?釋放過程持續(xù)了多長時(shí)間?能量釋放/傳輸?shù)闹饕问绞鞘裁?耀斑大氣對能量釋放又是如何響應(yīng)的?為此,我們針對這些問題,對耀斑大氣等離子體的加熱和動(dòng)力學(xué)演化進(jìn)行了詳細(xì)的研究.我們首先對耀斑的研究歷史作了概述(第1章),并介紹了相關(guān)的觀測儀器(第2章)和計(jì)算模型(第3章),然后用光譜數(shù)據(jù)分析了耀斑中的色球蒸發(fā)過程(第4章).基于色球蒸發(fā)的研究結(jié)果,我們進(jìn)一步探究耀斑的加熱過程.我們采用由觀測限制的加熱函數(shù)對兩個(gè)耀斑環(huán)進(jìn)行了模擬(第5章),得到的模擬結(jié)果與觀測結(jié)果基本符合.在此基礎(chǔ)上,我們又模擬了兩套不同的耀斑環(huán)系統(tǒng)(第6章),其中的加熱時(shí)標(biāo)有很大的不同,由此產(chǎn)生了不同的動(dòng)力學(xué)效應(yīng).具體內(nèi)容如下:

我們用Hinode/EIS的光譜數(shù)據(jù)研究了2007年1月16日耀斑的色球蒸發(fā)過程.仔細(xì)分析了耀斑帶上的3個(gè)點(diǎn),其中第1個(gè)點(diǎn)位于正的磁極區(qū),第2、3個(gè)點(diǎn)位于負(fù)的磁極區(qū).我們發(fā)現(xiàn)在耀斑脈沖相,這3個(gè)點(diǎn)表現(xiàn)出不同的物質(zhì)運(yùn)動(dòng):在第1個(gè)點(diǎn)處,大多數(shù)譜線都呈現(xiàn)藍(lán)移,其中高溫譜線的藍(lán)移分量相對其靜止分量占主導(dǎo);在第2個(gè)點(diǎn)處,只能探測到較弱的向上運(yùn)動(dòng)(upflow),相反,高溫譜線(形成溫度為2.5～5.0 MK)都表現(xiàn)出顯著的向下運(yùn)動(dòng)(downflow);第3個(gè)點(diǎn)和第2個(gè)點(diǎn)的情況類似,只是物質(zhì)向下運(yùn)動(dòng)時(shí)出現(xiàn)多個(gè)速度分量.第2、3個(gè)點(diǎn)處的向下運(yùn)動(dòng)可解釋為色球壓縮的證據(jù).這3個(gè)點(diǎn)表現(xiàn)出了不同的色球蒸發(fā)類型:溫和式色球蒸發(fā)和爆發(fā)式色球蒸發(fā),表明此耀斑區(qū)域可能存在著不同的加熱機(jī)制.

我們隨后用零維的EBTEL(基于焓的環(huán)的熱演化)模型對耀斑加熱的動(dòng)力學(xué)過程進(jìn)行了研究.我們分析了2011年2月16日的一個(gè)M1.0級耀斑.從EUV圖像可以看出此耀斑由兩個(gè)環(huán)系統(tǒng)組成,在模擬中我們將其當(dāng)作兩個(gè)橫截面積為5′′×5′′的粗環(huán).從光變曲線來看,先是UV 1600波段(輻射主要來自環(huán)足點(diǎn))出現(xiàn)快速增亮,隨后幾個(gè)EUV波段的輻射也順次增強(qiáng).這表明有能量快速沉積,耀斑環(huán)對此產(chǎn)生響應(yīng).我們運(yùn)用最近提出的一個(gè)新方法,即從快速上升的UV光變曲線得到耀斑環(huán)的加熱函數(shù),再結(jié)合EBTEL模型來計(jì)算這兩個(gè)粗環(huán)中等離子體的演化.通過模型計(jì)算,我們得到了各個(gè)EUV波段的輻射,并與SDO/AIA和EIS觀測的流量作了對比.結(jié)果顯示,雖然EBTEL模型具有局限性,但是從模型得到的光變曲線與觀測的光變曲線符合得比較好:它們表現(xiàn)出相同的走勢,絕對數(shù)值也在兩倍范圍之內(nèi).此外,我們還將模型計(jì)算的焓流速度與EIS測量的多普勒速度作了對比,結(jié)果符合得也比較好.這些事實(shí)表明,這兩個(gè)不同的耀斑環(huán),從足點(diǎn)的UV輻射顯示了不同的加熱函數(shù),再結(jié)合不同的環(huán)的長度,最后表現(xiàn)出了不同的演化類型;而這不同的演化類型在模擬和觀測方面都得到了證實(shí).

我們用同樣的方法分析和模擬了另一個(gè)C4.7級耀斑.從AIA的圖像我們辨認(rèn)出了兩套耀斑環(huán)系統(tǒng): EIS的光譜觀測顯示,這兩套耀斑環(huán)的足點(diǎn)在脈沖相時(shí)表現(xiàn)出藍(lán)移.這兩套環(huán)的演化和動(dòng)力學(xué)過程非常不同:第1套環(huán)的足點(diǎn)先是出現(xiàn)藍(lán)移(～10 km/s),持續(xù)約25 min后轉(zhuǎn)變?yōu)榧t移;第2套環(huán)的足點(diǎn)出現(xiàn)較強(qiáng)的藍(lán)移(～20 km/s),且持續(xù)了約1小時(shí),基本伴隨耀斑的整個(gè)過程.長時(shí)間的藍(lán)移說明有持續(xù)的加熱.同時(shí),AIA的UV 1600觀測顯示,第2套環(huán)的足點(diǎn)存在相隔15 min的兩次增亮,而第1套環(huán)的足點(diǎn)只有1次增亮.我們用這兩套環(huán)足點(diǎn)處的UV光變曲線構(gòu)建加熱函數(shù),結(jié)合EBTEL模型計(jì)算了環(huán)中等離子體的演化.結(jié)果顯示,對于第1套環(huán),模型預(yù)測的EUV光變曲線與AIA的6個(gè)波段以及EIS的8條譜線的觀測都比較符合,模型計(jì)算的平均焓流速度與EIS測量的多普勒速度也比較一致;但對于持續(xù)加熱的第2套環(huán)來說,模型預(yù)測的低溫輻射與觀測不甚相符,另外,模型沒有完全重現(xiàn)出持續(xù)的藍(lán)移.模型與觀測的差異,一方面可能源于加熱主要集中在耀斑環(huán)足點(diǎn)附近,而這不能被EBTEL模型所模擬;另一方面可能源于耀斑區(qū)存在未被分辨的、而加熱率非常不同的耀斑環(huán).

Solar flares are one of the most energetic events in solar atmosphere,which last minutes to tens of minutes.The eruption of a solar flare involves energy release,plasma heating,particle acceleration,mass flows,waves,etc.A solar flare releases a large amount of energy,and its emission spans a wide wavelength range.Solar flares are usually accompanied by coronal mass ejections(CMEs);therefore they could significantly a ff ect the space environments between the Earth and the Sun.

At present,we do not fully understand the whole flare process.There are still many important questions to be resolved,such as when and where is the energy released?How long does the energy release last?What are the main ways of energy release?And how does the solar atmosphere respond to the energy release?To address these questions,we study in detail the flare heating and dynamic evolution.We first give a brief review of previous flare studies(Chapter 1),and introduce the observing instruments(Chapter 2)and the modeling method(Chapter 3)related to this thesis work.Then we use spectral data to investigate the chromospheric evaporation(Chapter 4).Based on the results,we further explore the flare heating problem.With observationally inferred heating functions,we model two flare loops, and compare the results with observations(Chapter 5).A consistency is achieved between modeling and observations.In addition,we model two di ff erent sets of flare loop systems with quite di ff erent heating profiles and dynamic evolutions(Chapter 6).The details are described as below.

Firstly,we investigate the chromospheric evaporation in the flare on 2007 January 16 using line profiles observed by the Extreme-ultraviolet(EUV)Imaging Spectrometer (EIS)on board Hinode.Three points with di ff erent magnetic polarities at flare ribbons are analyzed in detail.We find that the three points show di ff erent patterns of upflows and downflows in the impulsive phase of the flare.The spectral lines at the first point are mostly blueshifted,with the hotter lines showing a dominant blueshifted component over the stationary one.At the second point,however,only weak upflows are detected;instead,notable downflows appear at high temperatures(up to 2.5–5.0 MK).The third point is similar to the second one except that it shows evidence of multi-component downflows. While the evaporated plasma falling back down as warm rain is a possible cause of the redshifts at the second and third points,the di ff erent patterns of chromospheric evaporation at the three points imply the existence of di ff erent heating mechanisms in the flaring region.

Then,we study the flare heating and dynamics using the“enthalpy-based thermal evolution of loops”(EBTEL)model.We analyze an M1.0 flare on 2011 February 16.Thisflare is composed of two distinctive loop systems observed in EUV images.The UV 1600emission at the feet of these loops exhibits a rapid rise,followed by enhanced emission in di ff erent EUV channels observed by the Atmospheric Imaging Assembly(AIA)on board the Solar Dynamics Observatory(SDO).Such a behavior is indicative of impulsive energy deposit,and the subsequent response of overlying coronal loops.Using the method recently developed,we infer empirical heating functions from the rapid rise of the UV light curves for the two loop systems,respectively,treated as two big loops with cross-sectional area of 5′′by 5′′,and compute the plasma evolution in the loops using the EBTEL model.We further compute the synthetic EUV light curves,which,with the limitation of the model, agree reasonably with the observed light curves obtained in multiple AIA channels and EIS lines:they show the same evolution trend,and their magnitudes are comparable within a factor of two.We also compare the computed mean enthalpy flow velocity with the Doppler shifts of EIS lines during the decay phase of the two loops.Our results suggest that the two di ff erent loops with di ff erent heating functions as inferred from their footpoint UV emission, combined with their di ff erent lengths as measured from imaging observations,give rise to di ff erent coronal plasma evolution patterns as revealed in both models and observations.

With the same method,we further analyze another C4.7 flare.From AIA imaging observations,we can identify two sets of loops in this event.EIS spectroscopic observations reveal blueshifts at the feet of both sets of loops during the impulsive phase.However,the dynamic evolutions of the two sets of loops are quite di ff erent.The first set of loops exhibits blueshifts(～10 km/s)for about 25 minutes followed by redshifts,while the second set shows stronger blueshifts(～20 km/s)which are maintained for about an hour.The long-lasting blueshifts in the second set of loops are indicative of continuous heating.The UV 1600observation by AIA also shows that the feet of the loops brighten twice with 15 minutes apart.The first set of loops,on the other hand,brighten only once in the UV band.We construct heating functions of the two sets of loops using spatially resolved UV light curves at their footpoints,and model plasma evolution in these loops with the EBTEL model. The results show that,for the first set of loops,the synthetic EUV light curves from the model compare favorably with the observed light curves in six AIA channels and eight EIS spectral lines,and the computed mean enthalpy flow velocities also agree with the Doppler shifts measured in EIS lines.For the second set of loops modeled with twice-heating,there are some discrepancies between modeled and observed EUV light curves at low-temperature lines,and the model does not fully reproduce the prolonged blueshift signatures as observed. The prominent discrepancies between model and observations for the second set of loops may be caused by non-uniform heating localized especially at the loop footpoints which cannot be modeled by the 0D EBTEL model,or by unresolved fine flaring strands in the loops with quite di ff erent heating rates and profiles.

Observations and Modeling of Solar Flare Atmospheric Dynamics

LI Ying
(School of Astronomy and Space Science,Nanjing University,Nanjing 210093)

10.15940/j.cnki.0001-5245.2015.05.013

?2013-06-13獲得博士學(xué)位,導(dǎo)師:南京大學(xué)天文與空間科學(xué)學(xué)院丁明德教授、蒙大拿州立大學(xué)邱炯副教授;yingli@nju.edu.cn