ENA-H and ENA-O separation and their distribution features during the main phase of a great magnetic storm —TWINS satellite observation
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摘要:
本文采用基于ENA（Energetic Neutral Atoms）次生电子起始脉冲高度分布，统计拟合分离中能段ENA两种主要成分氢和氧的方法，研发了实现ENA氢与氧分离的TWINS卫星原始数据处理软件；其中所需要的脉高分布模型，参照已有理论公式，利用TWINS（Two Wide-angle Imaging Neutral-atom Spectrometers）卫星标定数据进行拟合确定未知参数，再加以计算得到.将上述方法用于TWINS卫星实测数据，分离得到一次大磁暴主相期间ENA-H和ENA-O微分通量随观测视线的分布及其随主相增长的变化.分析发现：（1）ENA-H与ENA-O微分通量的强度和随观测视线的分布特征都有明显差别，从某种角度反映出ENA之源的O+与H+离子强度和分布之间的差异；（2）接近主相极大时，ENA-H有很强的低高度发射（LAE，Low Altitude Emission），出现在磁地方时午夜前极光和亚极光纬度区，意味着该区域较强的等离子片和环电流质子沉降，进入到外层基底以下较低高度大气层；而ENA-O则未有明显LAE产生；ENA-O强通量观测视线主要穿过广大环电流区，磁地方时主要在午夜之后以及黄昏前和黎明前后；（3）在磁暴主相快速增长期，ENA-O平均总通量持续增大，而ENA-H同步减小，ENA-O与ENA-H平均总通量的比率随环电流指数Dst绝对值的增大而大致成正比增长.
Abstract:Applying the method of statistical fitting based on the probability distribution features of start pulse height (STPH) excited by ENA's secondary electron, a set of programs has been developed to process the TWINS (Two Wide-angle Imaging Neutral-atom Spectrometers) satellite Level-0 data aiming to separate compositions of oxygen and hydrogen in ENA. Therein a model of STPH distribution needed for making the separation is set up by fitting the satellite calibration data to the theoretic formula in being. Implementing the separation method to the TWINS satellite measured data during a great magnetic storm, the respective ENA-H and ENA-O differential flux distributions with line-of-sight (LOS) have been obtained, as well as their evolution with the storm main phase growth. It is found that: (1) There are obvious differences in both magnitudes and distributions of differential fluxes between ENA-H and ENA-O, implying from a certain angle the differences between fluxes of energetic O+ and H+ ions as ENAs' sources. (2) During the period approaching the storm main phase, ENA-H displays intense Low Altitude Emissions (LAEs) which appear in the aurora and sub-aurora region before midnight, suggesting strong proton precipitation coming from plasma-sheet and RC (ring current) region into the lower atmosphere below exobase; while the oxygen has no obvious LAEs, but showing intensive fluxes along LOSs that traverse extensive RC region located mainly in the magnetic local time sectors of post-midnight, pre-dusk and around dawn. (3) During the rapid growth of the storm main phase, the averaged total flux (ATF) of ENA-O continues to increase, while the ENA-H to decrease synchronously. Correspondingly, the ATF ratio of ENA-O to ENA-H grows in a way of being roughly proportional to the increase of the absolute value of the ring current index Dst.
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图 1
起始脉冲高度分布标定曲线与理论模型的拟合结果示例
Figure 1.
Examples of fitting the calibration data to the theoretic model of STPH
图 2
模型给出的随能质比变化的脉冲高度分布
Figure 2.
Modeled start pulse height distributions varying with ratio of energy to mass
图 3
模型计算的起始脉冲高度分布与标定实测分布的比较示例
Figure 3.
Comparison of modeled and calibrated STPH distribution
图 4
2012年7月15日磁暴事件Dst指数随时间的变化
Figure 4.
Dst index variation with UT during the magnetic storm of July 15, 2012
图 5
对于给定能质比分离ENA-H与ENA-O的流程
Figure 5.
Process diagram to separate ENA-H and ENA-O for given ratio of energy to mass
图 6
TWINS卫星观测得到的1~32 keV/amu能质比ENA-H所占比例A随观测视线方向的分布
Figure 6.
Distributions of A versus line-of-sights for ENAs measured by TWINS at E/M between 1 and 32 keV/amu
图 7
最小二乘拟合参数A示例
Figure 7.
Examples of determining the parameter of A by the least square method
图 8
拟合相对标准差随观测视线方向的分布.能质比分别为1，2，4，16 keV/amu
Figure 8.
The relative standard deviation distribution with line of sight for E/M ratio of 1, 2, 4, 16 keV/amu, respectively
图 9
多个能段上氢和氧ENA的微分通量分布
Figure 9.
Differential flux distributions of hydrogen and oxygen ENA at various energies
图 10
TWINS两颗卫星S1和S2观测到的16 keV和32 keV ENA-H (a)和ENA-O (b)微分通量随观测视线的分布
Figure 10.
Differential flux distributions versus LOSs of ENA-H (a) and ENA-O (b) at 16 keV and 32 keV energies observed by TWINS satellites of S1 and S2
图 11
磁暴主相快速增长期间5个时段16 keV ENA-H(上)和ENA-O(下)微分通量随卫星观测视线的分布
Figure 11.
16 keV ENA-H (top) and ENA-O (bottom) differential flux distribution with satellite LOS for 5 time periods during the rapid growth of the storm main phase
图 12
(a) ENA-H(蓝色)和ENA-O(红色)平均总通量随时间的变化；(b) ENA-O与ENA-H平均总通量之比率(红色)以及Dst指数(蓝色)随时间的演化
Figure 12.
(a) Variation of averaged total fluxes with time for ENA-H (blue) and ENA-O (red); (b) Time evolution of the averaged total flux ratio (red) of ENA-O to ENA-H and Dst index (blue)
图 13
ENA强通量视线穿过离子源区域的MLT分布
Figure 13.
MLT distribution of ion source areas that are transmitted through by ENAs′ strong flux LOSs
表 1
次生电子有效生成量随能质比变化的模型系数
Table 1.
Model coefficients of secondary electron′s effective yields varying with energy to mass ratio
探头 H O 系数k 系数b 系数k 系数b S1-A 0.2891 -0.0696 0.3663 -0.0935 S1-T 0.3033 0.4348 0.3098 0.4630 S2-A 0.3294 0.0891 0.3576 -0.1576 S2-T 0.3315 0.5489 0.3728 0.4728 
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表 2
平均拟合相对标准差
Table 2.
Mean Relative Standard Error of fitting
E/M(keV/amu) 1 2 3 4 16 32 S1 0.4497 0.4508 0.4447 0.4402 0.4210 0.4534 S2 0.3905 0.4017 0.4091 0.4057 0.4278 0.5513
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表 3
16 keV的ENA-H强通量视线穿过环电流离子源区网格的特征
Table 3.
Features of RC ion source region grids that are passed through by LOSs with intensive fluxes for 16 keV ENA-H
TWINS116 keVENA-H16:45—17:002012-07-15 通量排序 射线方向 网格数 穿过的网格 仰角(°) 方位角(°) 地心高度/L值 磁纬度(°) 磁地方时(h) 1 64 148 7 (1.44~2.12)RE 65 19-23 2 64 144 6 (1.44~2.12)RE 65 18-22 3 68 148 5 (1.44~2.12)RE 65 18-21 4 68 152 8 (1.44~2.12)RE/2.5~3.5 48~65 18-22 5 64 152 10 (1.44~2.12)RE/3.5 48~65 18-23-1 6 68 144 5 (1.44~2.12)RE 65 18-21
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表 4
16 keV的ENA-O强通量视线穿过环电流离子源区网格的特征
Table 4.
Features of RC ion source region grids that are passed through by LOSs with intensive fluxes for 16 keV ENA-O
TWINS1 16 keVENA-O 16:45—17:002012-07-15 通量排序 射线方向 网格数 穿过的网格 仰角(°) 方位角(°) 地心高度/L值 磁纬度(°) 地方时(h) 1 60 148 25 (1.44~2.12)RE/2.25~7.25 -42.5~65 18-5 2 60 144 5 (1.44~2.12)RE 65 17、18、3、4 3 56 144 18 (1.44~2.12)RE/2.25~7.25 -22.5~65 5、6、17 4 56 140 21 (1.44~2.12)RE/2.25~7.25 -22.5~65 5、6、7、8、15、16 5 56 148 21 (1.44~2.12)RE/2.25~7.25 -22.5~65 4、5、17、18 6 60 140 2 (1.44~2.12)RE 65 17
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