MMS 2 Electron Drift Instrument (EDI) Quality 0 Counts, Level 2 (L2), Burst Mode, 7.8125 ms Data

Electron Drift Instrument (EDI) Q0 Burst Survey, Level 2, 0.0078125 s Data (128 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. These data are a by-product generated from data collected in electric field mode. Whenever no return beam is found in a particular time slot by the flight software to be reported will be flagged with the lowest quality level (quality zero). The ground processing generates a separate data product from these counts data. 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）Q0爆发巡天二级数据，采样时长0.0078125秒（即128采样点/秒）。该仪器具备两种科学数据采集模式：电场模式与背景环境模式。在电场模式下，仪器发射两束编码电子束，这些电子束在背景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，可推导出等离子体漂移速度与电场强度。背景环境模式下无需启用电子束，该探测器具备较大几何因子且可快速调整视场，能够以选定的俯仰角（pitch angle）与固定但可配置的能量，对背景电子进行连续采样。为定位能够命中探测器的电子束指向，仪器会在垂直于磁场B的平面内，以0.22°/毫秒的固定角速度扫描每一束电子，直至探测器捕获信号；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变化判断，而是通过电子束计数与背景电子计数的比值的平方，即瞬时信噪比（Signal-to-Noise Ratio, SNR）的平方来确定。该计算量由枪-探测器电子学单元中的相关器提供的数据完成，该相关器同时会生成施加于出射电子束的编码模式。当信噪比平方值超过阈值时，即可判定电子束已返回探测器；信噪比阈值的选取取决于背景电子通量，其设置需在误触发（由背景电子通量剧烈波动引发）与漏检真实电子束信号之间取得平衡。控制仪器运行的基础软件循环每2毫秒执行一次。由于电子束命中探测器的时刻既不与遥测（telemetry）数据同步，亦非等间隔分布，因此该仪器的数据不存在固定时间分辨率，数据以遥测时隙为单位上报。在巡天模式下采用标准打包模式0时，每秒可为每个枪探测器单元（Gun-Detector Unit, GDU）提供8个遥测时隙，上一个时隙中探测到的最后一束电子束将在当前时隙上报；若未探测到任何电子束，则将数据质量标记为0。在突发遥测模式下，每秒可为每个枪探测器单元提供128个遥测时隙；每个遥测时隙内的数据包含以下信息：电子束发射方向（以解析型枪偏转电压形式存储）、飞行时间（若成功测得）、质量指标、电子束命中时刻戳，以及部分与相关器相关的辅助信息。当仪器未处于电子漂移模式时，将自动切换至背景电子采集模式；该模式支持以90°俯仰角、0°/180°俯仰角（即与磁场对齐）进行采样，也可在90°俯仰角与磁场对齐模式之间切换，并可配置停留时长。尽管所有采样模式均在调试阶段完成验证，但在轨常规运行阶段仅使用了磁场对齐模式。可选采样能量为250电子伏特（eV）、500电子伏特（eV）与1千电子伏特（keV）。两台探测器分别朝向相反半球，严格沿相反方向观测：其中一台沿磁场B方向观测，另一台则沿磁场B的反平行方向观测，分别对应180°与0°俯仰角。随着航天器自旋半周，磁场矢量的端点会从一台探测器的视场移出并进入另一台探测器的视场，此时两台探测器的观测角色将互换。此类背景电子模式的数据为电场模式采集过程中产生的副产物。若飞行软件在特定时隙中未发现返回的电子束，则待上报的数据将被标记为最低质量等级（质量值为0）。地面处理系统会基于这些计数数据生成独立的数据产品。电子漂移仪的相关学术论文可通过以下链接获取：http://link.springer.com/article/10.1007%2Fs11214-015-0182-7。电子漂移仪的数据产品指南可通过以下链接获取：https://lasp.colorado.edu/mms/sdc/public/datasets/fields/.