MMS 3 Electron Drift Instrument (EDI) Electric Field, Level 2 (L2), Burst Mode, 0.9765625 ms Data

Electron Drift Instrument (EDI) Electric Field Burst Survey, Level 2, 0.0009765625 s Data (1024 samples/s). EDI has two scientific data acquisition modes, called electric field mode and ambient mode. In electric field mode, two coded electron beams are emitted such that they return to the detectors after one or more gyrations in the ambient magnetic and electric field. The firing directions and times-of-flight allow the derivation of the drift velocity and electric field. In ambient mode, the electron beams are not used. The detectors with their large geometric factors and their ability to adjust the field of view quickly allow continuous sampling of ambient electrons at a selected pitch angle and fixed but selectable energy. To find the beam directions that will hit the detector, EDI sweeps each beam in the plane perpendicular to B at a fixed angular rate of 0.22 °/ms until a signal has been acquired by the detector. Once signal has been acquired, the beams are swept back and forth to stay on target. Beam detection is not determined from the changes in the count-rates directly, but from the square of the beam counts divided by the background counts from ambient electrons, i.e., from the square of the instantaneous signal-to-noise ratio (SNR). This quantity is computed from data provided by the correlator in the Gun-Detector Electronics that also generates the coding pattern imposed on the outgoing beams. If the squared SNR ratio exceeds a threshold, this is taken as evidence that the beam is returning to the detector. The thresholds for SNR are chosen dependent on background fluxes. They represent a compromise between getting false hits (induced by strong variations in background electron fluxes) and missing true beam hits. The basic software loop that controls EDI operations is executed every 2 ms. As the times when the beams hit their detectors are neither synchronized with the telemetry nor equidistant, EDI data have no fixed time-resolution. Data are reported in telemetry slots. In Survey, using the standard packing mode 0, there are eight telemetry slots per second and Gyn Detector Unit (GDU). The last beam detected during the previous slot will be reported in the current slot. If no beam has been detected, the data quality will be set to zero. In Burst telemetry there are 128 slots per second and GDU. The data in each slot consists of information regarding the beam firing directions (stored in the form of analytic gun deflection voltages), times-of-flight (if successfully measured), quality indicators, time stamps of the beam hits, and some auxiliary correlator-related information. Whenever EDI is not in electron drift mode, it uses its ambient electron mode. The mode has the capability to sample at either 90 degrees pitch angle or at 0/180 degrees (field aligned), or to alternate between 90 degrees and field aligned with selectable dwell times. While all options have been demonstrated during the commissioning phase, only the field aligned mode has been used in the routine operations phase. The choices for energy are 250 eV, 500 eV, and 1 keV. The two detectors, which are facing opposite hemispheres, are looking strictly into opposite directions, so while one detector is looking along B the other is looking antiparallel to B (corresponding to pitch angles of 180 and 0 degrees, respectively). The two detectors switch roles every half spin of the spacecraft as the tip of the magnetic field vector spins outside the field of view of one detector and into the field of view of the other detector. This is the primary data product generated from data collected in electric field mode. The science data generated are drift velocity and electric field data in various coordinate systems. They are derived from triangulation and/or time-of-flight analysis. Where both methods are applicable, their results will be combined using a weighting approach based on their relative errors. The EDI instrument paper can be found at: http://link.springer.com/article/10.1007%2Fs11214-015-0182-7. The EDI instrument data products guide can be found at https://lasp.colorado.edu/mms/sdc/public/datasets/fields/.

电子漂移仪（Electron Drift Instrument, EDI）电场爆发勘测二级数据集，采样间隔为0.0009765625秒（对应1024样本/秒）。EDI具备两种科学数据采集模式，分别为电场模式与环境模式。在电场模式下，仪器会发射两路编码电子束，使其在周围磁场与电场中经历一次或多次回旋运动后返回探测器；通过电子束的发射方向与飞行时间（time-of-flight），可推导得到漂移速度与电场强度。在环境模式下，仪器无需启用电子束，该模式的探测器具备较大的几何因子，且可快速调整视场（Field of View, FOV），能够以选定的投掷角（pitch angle）对背景电子进行连续采样，采样能量固定且可灵活选择。

为找到可击中探测器的电子束方向，EDI会以0.22°/毫秒的固定角速度在垂直于磁场B的平面内扫描每一路电子束，直至探测器捕获到信号；捕获信号后，仪器会来回扫描电子束以保持瞄准目标。电子束的探测并非直接通过计数率的变化判定，而是通过电子束计数的平方除以背景电子的背景计数得到的瞬时信噪比（Signal-to-Noise Ratio, SNR）平方值来确定，即基于瞬时信噪比的平方进行判断。该参数由枪-探测器电子学单元（Gun-Detector Electronics）中的相关器生成的数据计算得到，该单元同时还会对出射电子束施加编码模式。当信噪比平方值超过阈值时，则判定电子束已返回至探测器。信噪比阈值的选取需依据背景电子通量而定，需在误触发（由背景电子通量的剧烈波动引发）与漏检真实电子束信号之间取得平衡。

控制EDI运行的基础软件循环每2毫秒执行一次。由于电子束击中探测器的时刻既未与遥测帧同步，也非等间隔分布，因此EDI数据并无固定的时间分辨率。数据以遥测时隙的形式进行上报。在采用标准打包模式0的勘测模式中，每秒可为每个陀螺探测器单元（Gyn Detector Unit, GDU）提供8个遥测时隙；前一个时隙内探测到的最后一束电子束将在当前时隙中上报，若未探测到电子束，则将数据质量标记为0。在爆发遥测模式下，每秒可为每个GDU提供128个遥测时隙，每个时隙内的数据包含：电子束发射方向信息（以解析形式存储的枪偏转电压）、飞行时间（若成功测得）、质量指标、电子束击中时刻的时间戳，以及部分与相关器相关的辅助信息。

当EDI未处于电子漂移模式时，将自动切换至背景电子采样模式。该模式支持以90°投掷角或0°/180°（磁场对齐（field aligned）方向）进行采样，也可在90°投掷角与磁场对齐方向之间交替采样，且驻留时间（dwell times）可选择。尽管在调试阶段已验证了所有采样选项，但常规运行阶段仅使用了磁场对齐模式。可选的采样能量为250 eV、500 eV与1 keV。两台探测器分别朝向相反的半球，严格沿相反方向观测：一台沿磁场B方向观测，另一台则沿B的反方向观测（分别对应投掷角180°与0°）。随着磁场矢量的尖端在一台探测器的视场外旋转并进入另一台探测器的视场，两台探测器会在航天器每半圈自旋时切换观测角色。

本数据集为电场模式下采集数据生成的核心数据产品，所生成的科学数据为不同坐标系下的漂移速度与电场强度数据，其通过三角测量法和/或飞行时间分析法推导得到。当两种方法均可适用时，将基于二者的相对误差采用加权方法融合其结果。EDI仪器的相关研究论文可参阅：http://link.springer.com/article/10.1007%2Fs11214-015-0182-7。EDI仪器数据产品指南可参阅：https://lasp.colorado.edu/mms/sdc/public/datasets/fields/。