Polar Plasma Wave Instrument (PWI), High Frequency Waveform Receiver, 25kHz bandwidth, 6-channel fields

PWI>Plasma Wave Instrument Reference: Gurnett, D.A. et al, The Polar Plasma Wave Instrument, Space Science Reviews, Vol. 71, pp. 597-622, 1995. [e-mail] An FFT on 1024 values was used in calibrating the data; i.e., perform FFT, calibrate in frequency domain, perform inverse FFT to get calibrated time series. Coordinate system used: local magnetic field-aligned, a spacecraft centered coordinate system where Z is parallel to the local B-field determined from Polar MFE, X points outward and lies in the plane defined by the Z-axis and the radial vector from the earth to the spacecraft, and Y completes a right-handed system and points eastward. The X- and Z-axes are contained in the north-south plane. Effective bandwidth is 1.5*delta_f, where delta_f depends on the size of the FFT used to convert to the frequency domain, and delta_t. These data come in snapshots of 31816 points per channel, every 9.2 seconds, where the duration of each snapshot is 0.045 seconds. The time for individual samples is the epoch time of the snapshot incremented by delta_t in milliseconds for each successive sample. That is: sample_time = epoch_time + sample_offset * delta_t The data in this file will be in sets of 31744 (31*1024) points per channel because the FFT size does not come out even within the number of points per snapshot. To obtain the time for each point in the snapshot, increment each Epoch time after the first with Delta_T (in ms). The unattenuated frequency range for this file type is 20Hz to 25000Hz. The cadence value given here is computed from the inverse of the sampling rate (71.43 kHz). For a description of the PWI receivers and the different operational modes see: http://www-pw.physics.uiowa.edu/plasma-wave/istp/polar/modes.html

等离子波仪器（Plasma Wave Instrument, PWI）参考文献：Gurnett, D.A. 等人，《极轨等离子体波仪器》，《空间科学评论》，第71卷，第597-622页，1995年。联系邮箱：[e-mail]
数据校准采用1024点快速傅里叶变换（Fast Fourier Transform, FFT）流程：先对数据执行FFT变换，在频域完成校准，再通过逆FFT变换得到校准后的时间序列。
采用的坐标系为局地磁场对齐的航天器中心坐标系：Z轴与由Polar极轨卫星磁强计（MFE）测得的局地磁场B平行；X轴向外，位于Z轴与地球到航天器的径向矢量构成的平面内；Y轴构成右手坐标系并指向东向。X轴与Z轴位于南北平面内。
有效带宽为1.5×Δf，其中Δf取决于转换至频域所用的FFT尺寸，Δt为采样间隔。该数据以快照形式存储，每个通道含31816个数据点，每9.2秒生成一组快照，单条快照的持续时长为0.045秒。
单个采样点的时刻为快照的历元时刻，按每后续一个采样点递增Δt（单位：毫秒）的规则计算，即：采样时刻 = 历元时刻 + 采样偏移量 × Δt
由于单条快照的数据点数无法与FFT尺寸整除，本文件中每个通道的数据为31744（31×1024）个点。若要获取快照中每个数据点的时刻，需从首个历元时刻开始，依次以ΔT（单位：毫秒）递增得到后续每个点的时刻。
该类型文件的无衰减频率范围为20Hz至25000Hz。此处给出的采样间隔由采样率（71.43 kHz）的倒数计算得到。
关于PWI接收机及其不同工作模式的详细说明，请参阅：http://www-pw.physics.uiowa.edu/plasma-wave/istp/polar/modes.html