节点文献
内磁层“象鼻状”重离子结构的统计研究
【作者】 张浩；
【导师】 吕建永；
【作者基本信息】 南京信息工程大学 ， 空间天气学， 2022， 硕士
【摘要】 被地球准偶极磁场捕获的不同能量的带电粒子在内磁层形成相对论辐射带、高能环电流和冷等离子体层,这些粒子群与时变背景电磁场之间的相互作用使内磁层成为一个高度动态的空间环境。理论研究发现,等离子体片粒子在磁暴或亚暴期间可以被注入到内磁层。卫星也观测到了等离子体片离子进入内磁层的明显特征,其中一个特征以能谱特征的形式出现,如:鼻状结构、楔状结构、离子间隙、指状结构和象鼻结构等。这些不同的能谱结构归因于沿漂移路径的单一或组合效应。离子能谱结构的研究之所以非常重要,是因为它们是新粒子从磁尾等离子体片注入内磁层的重要标志,对离子结构形成机制的研究有助于理解离子从等离子体片向内磁层的传输、加速和损失。本工作聚焦于目前研究很少的“象鼻”结构。这种结构的主要特征是随着地心距离的减小,峰值通量对应的能量会逐渐减小。本文基于Van Allen Probe A的观测数据,对在内磁层的He~+和O~+中观察到的“象鼻”结构进行了统计,研究了象鼻结构的发生频率、时间演化、空间和能量分布以及对不同地磁指数的依赖性等特征,并分析了象鼻结构的形成机制。本文的主要工作和结果如下:1.典型个例的分析研究。本文研究了2018年2月17日观测到的He~+和O~+象鼻结构,分析了其在能谱图中的特征形状和参数,利用地磁指数考察了该象鼻结构形成前地磁指数的变化。结果表明该象鼻结构发生前24小时内无磁暴发生,但有3个连续的亚暴出现,且象鼻结构出现在亚暴恢复相的末尾阶段。2.象鼻结构数据库的建立。本文利用2012年11月至2019年6月的Van Allen Probe A上HOPE仪器的数据,共选取了862个He~+和202个O~+象鼻结构,获得了每个象鼻根部和尖部的UT、L壳、磁地方时(MLT)以及能量信息。数据筛选时没有观测到H~+象鼻结构,这可能是由于H~+沿漂移路径的电荷交换寿命最短之故。3.象鼻结构的时空分布和能量依赖分析。本文统计分析了筛选出的象鼻结构的时空分布和能量依赖,发现:(1)象鼻结构通常位于L～1.5-4.0范围内,在MLT～18-24扇区具有优先位置。MLT～14-16扇区是一个没有象鼻根部的禁区,在夜侧有一个类似的区域(MLT～3-5),只有几个象鼻根部处于这个扇区。象鼻离子是从夜侧沿开放轨道漂移到日侧,并以非常小的方位漂移速度进入闭合轨道区域。因为漂移速度变小,象鼻离子需要更长的漂移时间才能到达日侧形成象鼻结构的区域,尤其是午后扇区。这意味着离子在到达日侧之前,更可能因为电荷交换和库仑碰撞而被损耗殆尽。这可能就是为什么象鼻结构在夜侧有一个优先位置,而禁区在下午扇区存在的原因。(2)He~+象鼻的能量在黄昏附近最大,且沿逆时针方向逐渐减小,而O~+象鼻的能量在MLT和L中分布相对均匀。被E×B漂移所主导的低能离子向东漂移,由梯度曲率漂移控制的高能离子向西漂移,高能象鼻离子更可能存在于昏侧而非晨侧,低能象鼻离子很难通过晨侧和日侧到达黄昏附近。因此,在黄昏附近形成了许多高能He~+象鼻。由于高能象鼻离子在闭合轨道上漂移非常缓慢,且O~+比He~+更容易损失,导致在黄昏附近不存在高能O~+象鼻聚集。4.象鼻结构的地磁条件依赖研究。本文利用三种地磁指数研究了象鼻结构与地磁活动的依赖性,发现强烈的地磁活动将抑制象鼻结构的形成,较为平静的空间环境更有利于象鼻的出现。随着地磁扰动和亚暴强度的增加,象鼻结构在日侧的发生率逐渐增加,说明主要是夜侧象鼻的形成受到抑制。因为更强的对流电场使闭合轨道区域变得更小,象鼻离子更难通过日侧到达夜侧。夜侧象鼻数量减少,日侧象鼻发生率自然增加。
【Abstract】 Charged particles with different energies trapped by the quasi-dipole magnetic field form the relativistic radiation belt,energetic ring current,and cold plasmasphere in the inner magnetosphere.The interaction between these particles and the time-varying background electromagnetic fields makes the inner magnetosphere a highly dynamic space environment.As the main source of the inner magnetospheric particles,plasma sheet particles are injected into the inner magnetosphere during magnetic storm or substorm.Several satellite missions have observed distinct features of plasma sheet ions entering the inner magnetosphere.One of these features appears in the form of energy spectrum features such as“nose-like”structures,“wedge-like”structures,ion spectral gaps,“finger-like”structures,and“trunk-like”structures.The study of the energy spectrum is very important because they are a significant sign of new particles’injection into the inner magnetosphere from the magnetotail plasma sheet.Different spectrums in the inner magnetosphere are attributed to the single or combined effects along the drift paths.The investigations of the formation mechanism of ion spectral structures are helpful to understand the transport,acceleration,and loss of ions from the plasma sheet.We present a statistical study of“trunk-like”structures observed in He~+and O~+in the inner magnetosphere.The main characteristic of this structure is that the energy of the peak flux decreases Earthward.We investigate the behavior of trunks in terms of occurrence frequency,temporal evolution,spatial and energy distribution,as well as dependence on different geomagnetic indices,and analyze the formation mechanism of the trunk structures.The main work and results of this paper are as follows:1.Analysis of typical case.We study in detail the characteristics of the typical“trunk”structure on February 17,2018.We use the geomagnetic indices to analyze the variation of geomagnetic activity before the formation of this trunk structure.No magnetic storm occurred within 24 hours before the trunk structure,but three consecutive substorms appeared,and the trunk appeared at the end of the substorm recovery phase.2.A database of trunk structures is created.Using observations from the HOPE instrument onboard Van Allen Probe A,we obtain 862 He~+trunk spectral structures and 202 O~+trunk spectral structures observed from November 2012 to June 2019 and determine the UT,L,MLT,and energy of each trunk’s root and tip.The failure of H~+trunk observation is probably due to the short charge exchange lifetime along the drift paths.3.Investigation of the spatial distribution and energy dependence of trunk structures.We find that(1)the trunk structures are always located in the range of L～1.5-4.0 and have a preferential location mainly concentrated in the range of MLT～18-24.The MLT～14-16 sector is a forbidden zone without trunk roots,and there is a similar sector(MLT～3-5)on the nightside with a few roots.Trunk ions drift from the nightside along the open trajectory to the dayside and enter the closed trajectory region with very small azimuthal drift velocities.Because the velocity becomes smaller,trunk ions take a longer drift time to reach the afternoon sector and are more easily to be depleted due to charge exchange and Coulomb collision.This is probably the reason why the trunk structures have a preferential location in the nightside and the forbidden region exists in the afternoon sector.(2)The energy of the He~+trunks is the largest near dusk and gradually decreases in the counterclockwise direction,while the energy of O~+trunks is relatively evenly distributed with MLT and L.Ions with relatively low energies dominated by the E×B drift will drift eastward.Ions with high enough energies dominated by the gradient-curvature drift will drift westward.Therefore,high-energy trunk ions are more likely to exist on the dusk-side than on the dawn-side.It is difficult for low-energy trunk ions to reach dusk through the dawn-side and the dayside.As a result,many high-energy He~+trunks are formed near dusk.Since high-energy trunk ions drift very slowly on the closed trajectory and O~+is easier to be lost than He~+,therefore there is no high-energy O~+trunk gathering near dusk.4.Investigation of the geomagnetic condition dependence of trunk structures.Three geomagnetic indices are used to analyze the geomagnetic activity dependence of trunks.We find that strong geomagnetic activity will inhibit the formation of trunks and a relatively quiet space environment is more conducive to the occurrences of trunks.However,more trunks on the dayside with higher geomagnetic disturbance and substorm intensity indicates that the trunks in the nightside are mainly inhibited.This is reasonable since a stronger convective electric field makes the closed orbit region smaller,and it is more difficult for trunk ions to reach the nightside through the dayside.The number of trunks on the nightside decreases,and the occurrence rate on the dayside increases naturally.