地球物理学报  2020, Vol. 63 Issue (6): 2131-2140   PDF    
波粒相互作用导致环电流质子沉降的卫星共轭观测
王杰, 袁志刚, 余雄东, 薛祖祥     
武汉大学电子信息学院, 武汉 430072
摘要:波粒相互作用是环电流损失的重要机制之一,但波粒相互作用导致的环电流离子沉降而损失迄今为止缺乏直接的观测证据.基于磁层及电离层卫星的协同观测,本文报道了发生在2015年9月7日,由电磁离子回旋波(EMIC波)导致环电流质子沉降的共轭观测事件.在等离子体层的内边界,Van Allen Probe B卫星观测到,存在EMIC波的区域和不存在EMIC波的区域相比,离子通量的投掷角分布的各向异性变弱.我们将Van Allen Probe B卫星沿着磁力线投影到电离层高度,同时在该投影区域内DMSP 16卫星在亚极光区域观测到环电流质子沉降.而且,通过从理论上计算质子弹跳平均扩散系数,我们进一步证实观测的EMIC波确实能将环电流质子散射到损失锥中.本文的研究工作为EMIC波导致环电流质子沉降提供了直接的观测证据,揭示了环电流衰减的重要物理机制:EMIC波将环电流质子散射到损失锥中,从而沉降到低高度大气层中而损失.
关键词: 电磁离子回旋波      环电流质子      能量质子沉降      波粒相互作用     
Precipitation of ring current protons caused by wave-particle interactions with satellite conjugated observation
WANG Jie, YUAN ZhiGang, YU XiongDong, XUE ZuXiang     
School of Electronic Information, Wuhan University, Wuhan 430072, China
Abstract: Wave-particle interaction is considered as an important mechanism for the loss of ring currents. However, the direct observational proof of the loss of ring currents caused by wave-particle interaction has not been reported so far. Based on the combination of observations from magnetosphere to ionosphere, this paper presents conjugated observations of ring current (RC) proton precipitation caused by electromagnetic ion cyclotron (EMIC) waves on September 7, 2015. In the inner boundary of the plasmasphere, the Van Allen Probe B satellite observed that the ion flux anisotropy of pitch angle distributions was weaker in the region where the EMIC wave existed than the region where the EMIC wave did not appear. While at ionospheric altitudes, the DMSP 16 satellite at the footprint of the Van Allen probe B simultaneously observed the RC proton precipitation. In addition, we demonstrate that observed EMIC waves can truly scatter RC ions into the loss cone through theoretically calculating the pitch angle diffusion coefficients for protons. Our result provides a direct proof of precipitation of ring current protons caused by EMIC waves so as to reveal the important mechanism for the loss of ring currents, i.e., EMIC waves can scatter ring current ions into the loss cone and lead to the precipitation and loss of ring current ions in the low-altitude atmosphere.
Keywords: EMIC waves    Ring current protons    Energetic proton precipitation    Wave-particle interaction    
0 引言

电磁离子回旋波(EMIC波)是一种具有离子特征尺度的等离子体波,其频率小于当地离子回旋频率,极化方式主要是左旋极化并且具有小的传播角,基本上沿着磁力线传播(Rauch and Roux, 1982; Kasahara et al., 1992; Liu et al., 2012; Yu et al., 2015).通常认为EMIC波由离子的温度各向异性(T>T)所激发(Cornwall, 1965; Horne and Thorne, 1993; Fraser and Nguyen, 2001; Lu and Wang, 2006; Lu et al., 2006; Xiao et al., 2011),其激发区域在磁赤道附近,磁纬±11°的区域里面(Loto′aniu et al., 2005),在离子温度各向异性不稳定性的阈值附近,EMIC波在磁赤道附近的发生率最高,具有小传播角和左旋极化的特性,表明这些波是由当地激发;而在离磁赤道较远的区域,EMIC波是左旋极化并具有相对较大的传播角,表明波动是由磁赤道源区传播到更高纬度(Yue et al., 2019a). EMIC波作为地球内磁层中常见的波动之一,在地球内磁层动力学研究过程中起着重要作用.EMIC波通过朗道阻尼共振能够加热等离子体层中的冷电子,通过回旋共振能够加热冷离子(Lu and Li, 2007; Zhou et al., 2013; Yuan et al., 2014a; Yuan et al., 2016).并且EMIC波能通过波粒相互作用沉降环电流离子和辐射带电子,也能导致环电流和辐射带快速损失(Miyoshi et al., 2008; Yuan et al., 2010, 2012b, 2013; Su et al., 2011Gao et al., 2015).

内磁层中被地球磁场捕获的高能带电粒子在由地磁场的梯度和曲率产生的漂移运动下形成自东向西的环形电流称之为环电流.环电流粒子主要包括能量为1~200 keV的氢离子(H+)、氦离子(He+)和氧离子(O+),这些粒子主要是通过磁层对流从磁尾等离子体片注入到内磁层(Daglis et al., 1999; Fok et al., 2001; Fu et al., 2001).磁尾等离子体片能量粒子的注入对于EMIC波的发生及其波动特性具有重要影响(Jun et al., 2019a, 2019b).在强地磁活动期间,包括氧离子在内的大量等离子体片粒子被注入内磁层,形成更强的环电流.同时离子的温度各向异性得到增强,从而激发出EMIC波来释放自身的自由能(Yue et al., 2019b);而激发出的EMIC波可通过波粒相互作用,将环电流粒子散射进入损失锥使其沉降到低高度的电离层而损失(Yuan et al., 2013, 2014b).

导致磁层能量离子沉降到低高度电离层存在着两种主要机制(Sergeev et al., 1983; Liang et al., 2013, 2014; Xiong et al., 2016),一种是在中心等离子体片区域,由于磁力线弯曲导致粒子散射到损失锥中(Sergeev et al., 1983; Liang et al., 2013, 2014),这部分粒子将沉降到大气中,通过与大气中的中性原子、分子相互作用,使其处于激发态,产生光子,从而形成极光区的主质子极光(Frey, 2007);另一种是在环电流区域,EMIC波通过波粒相互作用,将环电流离子散射进损失锥,被散射的这部分离子将沿着磁力线沉降到低层大气中,形成亚极光区分离质子极光弧(Burch et al., 2002; Hubert et al., 2001; Immel et al., 2002; Jordanova et al., 2007; Yahnin and Yahnina, 2007; Yahnin et al., 2009; Spasojević and Fuselier, 2009; Yuan et al., 2010, 2012a, 2012b, 2013Xiong et al., 2016).

Yuan等(2012b)通过卫星共轭观测研究了等离子体层羽状结构里面的波粒相互作用,证实了EMIC波能将环电流离子散射到损失锥中,从而沉降到大气中形成亚极光区分离质子极光.但是由于当时IMAGE卫星轨迹数据的缺失,导致无法直接获取NOAA卫星在IMAGE卫星的质子极光成像图中的投影,缺乏从内磁层(波粒相互作用)到电离层(波粒相互作用导致沉降的环电流离子)沿同一条磁力线的共轭观测证据.为了能够获得波粒相互作用的直接观测证据,我们寻找DMSP 16卫星和Van Allen Probe B卫星的联合观测事件.由于极光区域的主质子极光椭圆是由来自于等离子体片的质子沉降引起的,而亚极光区域的分离质子极光是由来自于环电流的质子沉降引起的(Xiong et al., 2016),并且中心等离子体片的质子能量范围是100 eV~ 10 keV,平均能量是5 keV(Hargreaves and Dessler, 1992),环电流粒子的能量范围是1~200 keV,而DMSP卫星的SSJ仪器的能量测量范围是30 eV~30 keV.因此DMSP卫星能够同时观测到极光区域和亚极光区域的沉降质子,并将观测到的分离质子沉降与分离质子极光联系起来,有效地解决了上述投影数据缺失问题.我们从理论上计算得到的质子弹跳平均扩散系数也表明:EMIC波能将keV的能量环电流质子散射到损失锥中.

1 观测数据来源介绍

本文所利用的数据主要来源于DMSP 16卫星和Van Allen Probe B卫星.从1965年开始,美国国防部启动了国防气象卫星计划(Defense Meteorological Satellite Program, DMSP),发射了F1—F18一系列的低高度地球轨道卫星, 作为极轨卫星,DMSP卫星运行在约840 km的太阳同步轨道,运行周期为101 min,轨道倾角为98.9°,24 h内绕地球14.3次,每天观测地球全貌2次.DMSP卫星上搭载了多个仪器,我们主要用到的是由DMSP 16卫星搭载的沉降电子/质子质谱仪(Special Sensor Pecipitation Electron/Proton Spectrometer, SSJ5),拥有20个能通道, 测量沉降极光粒子的能量范围是30 eV~30 keV(Redmon et al., 2017).

Van Allen Probe卫星由美国宇航局于2012年8月30日发射上天,该卫星计划由两颗卫星组成,旨在探索地球辐射带演化的物理机制.这两颗卫星运行在相同的轨道上,并且搭载相同的仪器设备,运行轨道的近地点在500~675 km之间,远地点在30410~30540 km之间,轨道倾角为10°左右,运行周期大约为9 h(Stratton et al., 2013).卫星上面搭载着5种探测仪器,我们主要用的是电场和磁场科学积分分析仪(EMFISIS)和辐射带风暴离子成分探测仪(RBSPICE).EMFISIS仪器用来测量背景磁场,采样频率为64 Hz, 通过短时傅里叶变化,得到磁场的功率谱密度,利用这一数据进行EMIC波的分析.RBSPICE仪器用来测量环电流离子数据,探测的质子能量范围是0.05~0.6 MeV, 分为14个能级通道,投掷角范围共分为17个区间.我们用该仪器的单向差分质子通量数据来研究环电流质子的分布,选取投掷角为0°~15°和85°~95°,能量为49~60 keV和60~74 keV的环电流质子作为研究对象.

2 事件观测

图 1展示的是DMSP 16卫星和Van Allen Probe B卫星的共轭观测位置,图中的粉色实线表示DMSP 16卫星在2015年9月7日10 : 11—10 : 21 UT时间段的投影轨迹,粗黑色实线表示观测到亚极光区能量质子沉降的时间段;绿色实线表示Van Allen Probe B卫星在09 : 00—13 : 00 UT时间段的投影轨迹,粗红色实线表示观测到EMIC波的区域,对应着图 2中的粗蓝色实线标示的时间段.图 1a—b展示的是在GSE坐标系下,共轭观测的两颗卫星分别在XYXZ平面的位置,图 1c展示的是卫星沿着磁力线投影到电离层120 km高度的投影,其中的小圆圈表示卫星轨迹的起始位置,投影的地磁场模型为TS05(Tsyganenko and Sitnov, 2005),从图中可以看出,Van Allen Probe B卫星和DMSP 16卫星构成了良好的共轭观测.图 2展示的是Van Allen Probe B卫星在2015年9月7日08 : 30—14 : 00 UT观测到的EMIC波事件及相关信息,从上到下分别展示的是背景等离子体密度、磁场功率谱密度、椭圆极化率、传播角、波幅、能量分别为49~60 keV和60~ 74 keV的离子通量.如图 2b所示,在两条品红实线标记的时间范围内,卫星在等离子体层内(5 < L < 6,Ne≈100 cm-3)下午侧(磁地方时MLT=14.6~16.6 h)赤道附近(磁纬MLAT < 3°)观测到了EMIC波,红色小方块表示在观测到EMIC波的同时,DMSP 16卫星在亚极光区域探测到能量质子沉降.图 2b—d中的黑色实线和黑色虚线分别表示当地的氦离子回旋频率和氧离子回旋频率,因此该EMIC波是一个双波段结构,其中一个波段位于氦离子回旋频率之上、质子回旋频率(图中未展示出)之下;另一个波段则位于氧离子回旋频率之上、氦离子回旋频率之下.从图 2c—d可以看出,两个波段的EMIC波都具有小传播角,且极化方式主要为左旋极化,因此这两个波段的EMIC波分别为氢波段EMIC波和氦波段EMIC波(Yu et al., 2015).图 3展示的是在10 : 17—10 : 24 UT时间段内,由DMSP 16卫星搭载的SSJ5仪器测量的沉降粒子的差分能量通量:图 3ab分别表示的是电子差分能量通量和质子差分能量通量,图中的红色垂直实线标示极光区的赤道向边界.粗蓝色实线标示的是在10 : 18—10 : 19 UT时间段内,在亚极光区我们观测到能量为20~30 keV的沉降质子,并且伴随着少量的低能电子沉降(能量小于100 eV);而在极光区,沉降质子的平均能量在10 keV左右.并且在10 : 19 : 00 UT时刻,亚极光区的沉降质子和极光区的沉降质子在分布上存在着纬度分离.根据Burch等(2002)的研究成果,在极光区的赤道侧探测到的沉降的能量质子(能量为3~30 keV)形成亚极光区分离质子极光,而极光区的沉降质子(能量小于10 keV)形成主质子极光椭圆.这一观测结果与Burch等(2002)的结果相一致.因此我们认为,在亚极光区域观测到的能量质子沉降,意味着亚极光区分离质子极光弧的形成.亚极光区的分离沉降发生在MLT≈15.00 h的范围内,而此时,对应的EMIC波集中在MLT≈15.20 h, 它们都是分布在下午侧.并且根据图 1可知,Van Allen Probe B卫星和DMSP 16卫星构成了良好的共轭观测.因此可以认为亚极光区域观测到的能量质子沉降是由于EMIC波将环电流质子散射到损失锥中所导致的.为了揭示EMIC波将环电流离子散射到损失锥的物理机制,磁层卫星的实地观测显得尤为重要.在我们的研究工作中,同时从实地观测和理论分析来证实EMIC波对环电流质子的散射作用.图 2e展示的是EMIC波的波幅大小.由于在EMIC波发生的频率范围内,存在着固定频率的背景噪声,因此在计算波幅的时候,我们滤除了该频率背景噪声,同时将滤除的噪声用平均背景功率谱密度代替.图 2fg分别展示的是能量为49~60 keV和60~74 keV的离子通量大小,该通量是由搭载在Van Allen Probe B卫星上的RBSPICE仪器测量所得,其中红色实线代表的是投掷角为0°~15°的离子通量,而黑色实线表示投掷角为85°~95°的离子通量.为了计算Van Allen Probe B卫星当地的损失锥角,我们假定第一绝热不变量守恒,利用下面的公式来计算Van Allen Probe B卫星当地的损失锥角α.

(1)

图 1 DMSP 16卫星和Van Allen Probe B卫星共轭观测位置图 (a)—(b)在GSE坐标系下,DMSP 16卫星和Van Allen Probe B卫星分别在XY、XZ平面的位置; (c) DMSP 16卫星和Van Allen Probe B卫星投影到电离层120 km高度.粉色实线和绿色实线分别表示DMSP 16卫星和Van Allen Probe B卫星的投影轨迹. Fig. 1 Conjugated observation map of DMSP 16 and Van Allen Probe B satellites (a)—(b) Locations of DMSP 16 and Van Allen Probe B satellites on the XY and XZ planes in the GSE coordinate system, respectively; (c) Locations of the footprint of the DMSP 16 and Van Allen Probe B satellites projected to the ionosphere at 120 km altitude. The pink and green solid lines represent the projection trajectories of the DMSP 16 satellite and the Van Allen Probe B satellite, respectively.
图 2 Van Allen Probe B卫星观测的波动分析 (a)背景电子密度; (b)磁场功率谱密度; (c)椭圆极化率; (d)传播角; (e)波幅; (f)—(g)由RBSPICE仪器测得的能量为49~60 keV和60~74 keV的离子通量,其中红色和黑色实线分别代表的是投掷角为0°~15°和85°~95°的离子通量. Fig. 2 Wave analysis observed by Van Allen Probe B (a) Background electron density; (b) Power spectral density (PSD); (c) Ellipticity; (d) Wave normal angle (WNA); (e) Wave amplitude; (f) — (g) Ion flux with energies of 49~60 keV and 60~74 keV measured by RBSPICE, respectively, and red and black lines denote the ions flux with local pitch angles of 0°~15° and 85°~95°, respectively.
图 3 DMSP 16卫星观测的粒子通量 (a)电子差分能量通量; (b)离子差分能量通量.其中红色垂直实线标示极光区的赤道向边界. Fig. 3 Particle data from DMSP 16 SSJ5 (a) Electron differential energy flux; (b) Ion differential energy flux. Red vertical solid line indicates the equator boundary of the auroral zone.

其中Bsat表示卫星当地的磁场强度,该数据由搭载在卫星上的EMFISIS仪器测量所得;B120表示卫星所在位置沿着磁力线投影到120 km高度的磁场强度,由IGRF2000模型计算所得(Tsyganenko and Sitnov, 2005).通过计算,卫星当地的损失锥角α≈3.2°.因此卫星观测到的投掷角为85°~95°的离子都是损失锥之外的离子,它们都是束缚离子;而投掷角为0°~15°的离子既包括束缚离子,也包括沉降离子.如图 2中左边的品红实线所示,在09 : 30 UT开始出现EMIC波,这个时候投掷角为85°~95°的离子通量有所增加,意味着环电流密度的增强,而投掷角为0°~15°的离子通量急剧增加,相比波动出现之前,离子分布更加趋向各向同性,而在12 : 35 UT波动消失之后,离子的各向同性分布逐渐变弱,更加趋向各向异性,对于能量为60~74 keV的离子,这一现象更加明显,而对于能量为49~60 keV的离子而言,在波动消失之后,离子分布的各向同性并没有迅速减弱,这可能与环电流离子的分布有关.但是整体而言,我们仍可以认为在EMIC波出现的时间段09 : 30—12 : 35 UT内,能量为49~60 keV和60~74 keV的环电流离子分布变得各向同性,这是由EMIC波将环电流离子散射进入损失锥所致.下面我们重点研究图中蓝色虚线所标注的时刻,如图 2ef所示,10 : 02、10 : 15、10 : 30、10 : 55 UT时刻分别对应EMIC波波幅的谷值,而这些时刻对应的能量为49~60 keV、投掷角为0°~15°的离子通量也是谷值,可以认为这部分离子的通量随着波幅大小的变化而变化,这进一步说明了EMIC波对环电流离子的散射作用;而能量为60~74 keV的离子通量似乎没有这一特性,这是由于EMIC波对于不同能量环电流质子的散射强度不一样.另外,在10 : 02—10 : 30 UT这个时间段内,我们发现能量为49~60 keV、投掷角为85°~95°的离子通量分布与EMIC波波幅变化具有较强的相关性,它们几乎同时出现峰值和谷值,并且变化趋势相一致,这说明激发EMIC波的自由能主要是由这部分离子提供的.

3 分析

通过对Van Allen Probe B卫星观测的环电流质子进行实地分析,表明环电流质子更加各向同性分布是由EMIC波通过波粒相互作用将其散射进损失锥所致,这部分质子将沉降到亚极光区电离层,从而被DMSP 16卫星探测到.但是由于DMSP 16 SSJ5仪器测量范围的限制,我们探测到的沉降离子能量限制在30 keV以内,然而在对Van Allen Probe B卫星探测的高能粒子进行局部分析时,分布趋向各向同性的离子能量为49~60 keV和60~74 keV.尽管从能量上看,它们都属于环电流离子,并且我们的观测结果已经表明EMIC波能够将环电流离子散射进损失锥, 但是为了进一步验证DMSP 16卫星和Van Allen Probe B卫星的联合观测结果,我们利用准线性理论(Summers et al., 2007a),从理论上计算观测到的EMIC波对环电流质子的弹跳平均扩散系数〈Dαα〉.为了方便计算,我们对观测到的EMIC波的主要波段进行高斯拟合,由于氦波段EMIC波的强度远大于氢波段,因此拟合的主要波段是氦波段.在我们的研究工作中,利用短时傅里叶变换得到了扰动磁场的功率谱密度,选取的滑动窗的长度为6400个点,而EMFISIS仪器的采样频率为64 Hz, 因此滑动窗的时间长度为100 s.选取10 : 18 : 20—10 : 20 : 00 UT时间段内的EMIC波作为研究对象,如图 4a所示,图中的黑色虚线表示在该时间段内,实际测量的氦波段EMIC波功率谱密度,蓝色实线表示高斯拟合曲线.拟合的结果表明,该波动的中心频率为0.159fcH+(fcH+为氢离子回旋频率),上截止频率为0.249fcH+,下截止频率为0.069fcH+,波幅大小为0.25 nT.利用IGRF2000模型,计算得到Van Allen Probe B卫星所在位置对应的磁赤道面的磁场大小为160 nT,因此磁赤道面损失锥大小为3.2°.由Van Allen Probe B卫星测量的背景等离子体密度为117.38 cm-3,我们将冷离子成分的比例设为[H+] : [He+] : [O+]=85 : 10 : 5(Summer et al., 2007b; Yuan et al., 2018),并且假定EMIC沿着背景磁场平行或反平行传播,其波谱强度和背景电子密度大小沿着磁场线是恒定的(Summer et al., 2007a, 2007b).为了进一步评估观测的EMIC波对能量为1~1000 keV质子的投掷角散射能力, 我们参照Xiong等(2016)通过比较〈Dαα〉与强投掷角散射系数DSD(Kennel and Petschek, 1996)的大小的方法(Xiong et al., 2016).当〈Dαα〉>>DSD时,我们认为波对粒子强扩散,而〈Dαα〉 < < DSD时,此时波对粒子为弱扩散.由于本文计算得到的磁赤道面的损失锥角α≈3.2°,属于小损失锥角,因此DSD可表达为(Summer and Thorne, 2003文中方程(27)):

(2)

图 4 (a) EMIC波的高斯拟合曲线(蓝色实线); (b)质子对应于赤道损失锥角(3.2°)的弹跳平均扩散系数〈Dαα〉(黑线),其中红线表示强投掷角扩散系数DSD; (c)投掷角为0°~90°的质子弹跳平均扩散系数〈Dαα〉,其中黄色虚线对应子图(b)中的〈Dαα Fig. 4 (a) Gaussian fitting (blue solid line); (b) Bounce-averaged pitch angle diffusion rates for protons 〈Dαα〉 (black line) near the equatorial loss cone (3.2°), and the red line represents the rate of strong pitch diffusion (DSD); (c) Bounce-averaged pitch angle diffusion rates for protons 〈Dαα〉 with pitch angles of 0°~90° and the yellow dashed line corresponds to 〈Dαα〉 in subgraph (b)

式中,LE分别表示L值和相对于静止能量的归一化的质子动能.图 4b中的黑色曲线展示的是损失锥角α≈3.2°、能量为1~1000 keV质子对应的弹跳平均系数〈Dαα〉,红色曲线表示L=5.7处的强投掷角扩散系数DSD.从图中可知,对于能量为6~30 keV的质子而言,满足〈Dαα〉>DSD,这说明EMIC波能通过回旋共振将能量为6~30 keV的质子强烈地散射到损失锥,这与DMSP 16卫星在亚极光区域观测到很强的能量质子(10~30 keV)沉降是一致的;而对于能量为30~100 keV的质子,〈Dαα〉略小于DSD,所以EMIC波对这些能量的环电流质子仍有较强的散射作用,这与Van Allen Probe B卫星在波动期间观测到环电流质子更加各向同性分布的结果也是一致的.在图 4c中,我们计算了EMIC波对于投掷角为0°~90°、能量为1~1000 keV质子的弹跳平均扩散系数.结果表明,在较大的投掷角范围和能量范围内,EMIC波能够与环电流质子发生共振作用.随着质子能量的增加,相应的共振区域发生在更大的投掷角范围内.图 4c中的黄色虚线对应于图 4b中的弹跳平均扩散系数〈Dαα〉.对于投掷角为3.2°~15°的质子,整体对应的弹跳平均扩散系数更大,这些大角度的质子将被散射成小角度质子.因此,EMIC波通过共振能够将能量为5~100 keV的环电流质子散射进入损失锥,尤其是对于能量为10~30 keV的质子,弹跳平均扩散系数较大,对质子的散射能力更强.

4 结论

在我们的共轭观测事件中,DMSP 16卫星在亚极光区域观测到了能量质子沉降,同时Van Allen Probe B在磁层高度观测到了EMIC波,并通过对当地环电流质子的各向同性分析以及从理论上计算质子的弹跳平均扩散系数,揭示了EMIC波能够将环电流质子散射进入损失锥,进而沉降到亚极光区形成分离质子极光弧.本研究工作为EMIC波导致环电流质子沉降提供了直接的观测证据,揭示了环电流衰减的重要物理机制.

致谢  本文所用的Van Allen Probe B卫星数据和DMSP 16 SSJ5数据分别来源于https://cdaweb.sci.gsfc.nasa.gov/cgi-bin/eval1.cgi网站和https://satdat.ngdc.noaa.gov/dmsp/data/网站.
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