PSP FIELDS Digital Fields Board (DFB) AC-coupled Searchcoil Magnetometer, SCM, Spectra, Low Frequency, High Gain, f-component, Sensor coordinates, Level 2 (L2), 0.873813 s Data

PSP FIELDS Digital Fields Board, DFB, SCMflfhg data:

The DFB is the low frequency, less than 75 kHz, component of the FIELDS experiment on the Parker Solar Probe spacecraft, see reference [1] below. For a full description of the FIELDS experiment, see reference [2]. For a description of the DFB, see reference [3].

DFB AC spectra data consist of power spectral densities as a function of frequency and time. These spectra are averaged over both frequency and time as described in [3]. The spectra have pseudo-logarithmically spaced frequency bins with the bin central frequencies reported in the metadata. The AC spectra are duty-cycled such that spectral averaging takes place over the first 1/8 of any given NYsecond when the data cadence is equal to one NYsecond. Less data are averaged by a factor of 2^N for data cadences that are faster than one NYsecond by 2^N. For cadences slower than one NYsecond, the first 1/8 of each NYsecond of data included are averaged together to construct the reported data.

The Level 2 data products contained in this data file have been calibrated for:

* 1) The Hanning window used in the spectral calculation
* 2) DFB in-band gain
* 3) DFB analog filter gain response
* 4) DFB digital filter gain response
* 5) The search coil preamplifier response, when applicable
* 6) The bandwidth of each spectral bin

Note that compensation for the DFB digital filters will introduce a non-physical positively sloped power trend at high frequencies when the non-corrected signal is dominated by noise. This effect should be examined carefully when determining spectral slopes and features at the highest frequencies. Calibrations for the FIELDS preamplifiers have not been implemented as the preamplifier response is flat and equal to one through the DFB frequency range. Corrections for plasma sheath impedance gain and antenna effective length have not been applied to voltage sensor signals. These corrections will be applied in the Level 3 DFB data products. Therefore, all voltage sensor quantities when present in these Level 2 data products are expressed by using units of Volts squared per Hertz. Likewise, all magnetic field quantities when present in these Level 2 data product are expressed by using units of nanoTesla squared per Hertz.

The Level 2 data products contained in this data file are expressed in sensor coordinates: e.g. dV12, dV34 for voltage measurements. For solar orbits 1 and 2, the search coil magnetometer spectral data are rotated into a non-intuitive coordinate system with components [d,e,f]. For solar orbits 3 and beyond, the magnetic field spectral data are in expressed in search coil magnetometer sensor coordinates with components [u,v,w].

To rotate from [d,e,f] coordinates into [u,v,w] search coil sensor coordinates, use the following matrix, written in IDL notation, and the following equation: spectra_uvw_vector = R ## spectra_def_vector.

R = [[ 0.46834856, -0.81336422 , 0.34509170]
[ -0.66921924, -0.071546954, 0.73961249]
[ -0.57688408, -0.57733845 , -0.57782790]]

The time resolution of the DFB AC spectral data can vary by multiples of 2^N. During encounter when PSP is within 0.25 AU of the Sun, the DFB AC spectra data cadence is typically NYsecond NYsecond [2]. Timestamps correspond to the center time of each window.

References:

* 1) Fox, N.J., Velli, M.C., Bale, S.D. et al., Space Sci Rev (2016) 204:7. https://doi.org/10.1007/s1121401502116
* 2) Bale, S.D., Goetz, K., Harvey, P.R. et al., Space Sci Rev (2016) 204:49. https://doi.org/10.1007/s1121401602445
* 3) Malaspina, D.M., Ergun, R.E., Bolton, M. et al., JGR Space Physics (2016), 121, 5088-5096. https://doi.org/10.1002/2016JA022344

帕克太阳探测器（Parker Solar Probe, PSP）FIELDS实验的数字场板（Digital Fields Board, DFB）、SCMflfhg 数据集：

DFB是帕克太阳探测器FIELDS实验中频率低于75 kHz的低频分量，详见下文参考文献[1]。如需了解FIELDS实验的完整描述，请参见参考文献[2]；关于DFB的详细说明，请参阅参考文献[3]。

DFB交流频谱数据包含随频率和时间变化的功率谱密度。如参考文献[3]所述，此类频谱会在频率和时间维度上进行平均。该频谱采用伪对数间隔的频段（frequency bins），频段中心频率已在元数据（metadata）中给出。DFB交流频谱采用占空循环模式：当数据帧率为1 NYsecond时，频谱平均过程会在每个NYsecond的前1/8时段内完成。若数据帧率快于1 NYsecond达2^N倍，则平均的数据量会减少至原有的1/(2^N)。若数据帧率慢于1 NYsecond，则会将所包含的每个NYsecond时段内的前1/8数据进行平均，以生成最终上报的数据。

本数据文件中包含的二级（Level 2）数据产品已完成以下校准：
* 1) 频谱计算中使用的汉宁窗（Hanning window）校正
* 2) DFB带内增益校正
* 3) DFB模拟滤波器增益响应校正
* 4) DFB数字滤波器增益响应校正
* 5) 搜索线圈磁强计（search coil magnetometer）前置放大器响应校正（如适用）
* 6) 各频谱频段的带宽校正

需要注意的是，当未校正信号以噪声为主时，对DFB数字滤波器的校正会在高频段引入非物理性的正斜率功率趋势。在确定最高频段的频谱斜率与特征时，需仔细核查该效应。由于FIELDS前置放大器的响应在DFB的整个频率范围内均为平坦且等效为1，因此未对其进行校准。电压传感器信号未进行等离子体鞘层阻抗增益与天线有效长度的校正，此类校正将在三级（Level 3）DFB数据产品中完成。因此，本二级数据产品中所有电压传感器相关量的单位均为伏特平方每赫兹（V²/Hz）；同理，所有磁场相关量的单位均为纳特斯拉平方每赫兹（nT²/Hz）。

本数据文件中的二级数据产品采用传感器坐标系表示：例如电压测量量为dV12、dV34。在太阳轨道1和轨道2阶段，搜索线圈磁强计的频谱数据会被转换至一个非直观的坐标系，其分量为[d,e,f]。在太阳轨道3及以后的阶段，磁场频谱数据将采用搜索线圈磁强计的传感器坐标系，分量为[u,v,w]。

若需从[d,e,f]坐标系转换至[u,v,w]搜索线圈磁强计传感器坐标系，可使用以下以交互式数据语言（Interactive Data Language, IDL）表示的矩阵与下述公式：
`spectra_uvw_vector = R ## spectra_def_vector`

转换矩阵R为：
[[ 0.46834856, -0.81336422 , 0.34509170]
[ -0.66921924, -0.071546954, 0.73961249]
[ -0.57688408, -0.57733845 , -0.57782790]]

DFB交流频谱数据的时间分辨率可为2^N的整数倍。当帕克太阳探测器在太阳0.25天文单位（Astronomical Unit, AU）范围内飞掠时，DFB交流频谱数据的帧率通常为1 NYsecond。时间戳对应每个积分窗口的中心时刻。

参考文献：
* 1) Fox, N.J., Velli, M.C., Bale, S.D. 等, Space Sci Rev (2016) 204:7. https://doi.org/10.1007/s1121401502116
* 2) Bale, S.D., Goetz, K., Harvey, P.R. 等, Space Sci Rev (2016) 204:49. https://doi.org/10.1007/s1121401602445
* 3) Malaspina, D.M., Ergun, R.E., Bolton, M. 等, JGR Space Physics (2016), 121, 5088-5096. https://doi.org/10.1002/2016JA022344