MMS 2 Electron Drift Instrument (EDI) Ambient Electron Flux, Projection Method 1 (PM2), Level 2 (L2), Survey Mode, 31.25 ms Data

Electron Drift Instrument (EDI) Ambient Survey, Level 2, 0.03125 s Data (32 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. Starting January 4, 2016, the anodes were chosen such that the projection of the magnetic field vector was best aligned with the center of the first (that is, outer) of the four anodes. This provides coverage of a larger range of pitch angles in general. Data taken in this configuration are identified by the term "amb-pm2" in the data product names. In the burst data where four channels (corresponding to the four adjacent sensor anode pads) are sampled per GDU, channel 1 represents best the pitch angle of 0 degrees (or 180 degrees). 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/.

电子漂移仪器（Electron Drift Instrument, EDI）环境探测二级数据，采样间隔0.03125秒（32样本/秒）。EDI具备两种科学数据采集模式，即电场模式与环境模式。在电场模式下，仪器发射两束编码电子束，它们在环境磁场与电场中回旋一次或多次后返回探测器。通过发射方向与飞行时间，可推导得出漂移速度与电场信息。环境模式下不使用电子束。探测器具有大几何因子及快速调整视场的能力，可在选定的pitch角（pitch angle）与固定且可选的能量下，对环境电子进行连续采样。为确定能击中探测器的波束方向，EDI以0.22°/ms的固定角速率在垂直于磁场B的平面内扫描各波束，直至探测器接收到信号。信号捕获后，波束会来回扫描以保持对准目标。波束检测并非直接通过计数率变化判定，而是基于波束计数平方与环境电子背景计数的比值——即瞬时信噪比（signal-to-noise ratio, SNR）的平方。该数值由枪-探测器电子学系统中的相关器计算得出，该相关器同时生成施加于出射波束的编码模式。若SNR平方超过阈值，则判定波束返回探测器。SNR阈值根据背景通量选定，需在避免虚假击中（由背景电子通量剧烈变化引发）与减少真实波束击中遗漏之间取得平衡。控制EDI运行的基础软件循环每2毫秒执行一次。由于波束击中探测器的时间既不与遥测同步，也不等距，因此EDI数据无固定时间分辨率，数据通过遥测时隙上报。在探测模式下，采用标准打包模式0时，每个陀螺探测器单元（Gyn Detector Unit, GDU）每秒有8个遥测时隙。前一时隙检测到的最后一束波束将在当前时隙上报；若未检测到波束，则数据质量设为零。突发遥测模式下，每个GDU每秒有128个时隙。每个时隙的数据包含波束发射方向信息（以解析枪偏转电压形式存储）、飞行时间（若测量成功）、质量指标、波束击中时间戳及若干辅助相关器信息。当EDI未处于电子漂移模式时，将启用环境电子模式。该模式可在90度pitch角、0/180度（场向）采样，或在90度与场向之间交替采样（驻留时间可选）。尽管所有选项均在调试阶段得到验证，但常规运行阶段仅使用场向模式。能量选项包括250电子伏特（eV）、500 eV及1千电子伏特（keV）。两个探测器分别朝向相反半球，视向严格相反：一个沿磁场B方向观测，另一个沿反平行于B的方向观测（分别对应180度与0度pitch角）。随着磁场矢量尖端旋转出一个探测器的视场并进入另一个探测器的视场，两个探测器每经过航天器半圈自旋便切换一次角色。自2016年1月4日起，阳极选择方式调整为：磁场矢量投影与四个阳极中的第一个（即最外侧）阳极中心最佳对齐。此举通常可覆盖更大范围的pitch角。该配置下采集的数据在产品名称中以‘amb-pm2’标识。在突发数据中，每个GDU采样四个通道（对应四个相邻传感器阳极垫），其中通道1最能代表0度（或180度）pitch角。EDI仪器相关论文参见：http://link.springer.com/article/10.1007%2Fs11214-015-0182-7；EDI仪器数据产品指南参见：https://lasp.colorado.edu/mms/sdc/public/datasets/fields/。