Wind SWE/Strahl Detector---2D Angular Distributions

The Wind satellite's strahl detector is a toroidal electrostatic analyzer (Ogilvie et al. 1995), which directly sampled the solar wind electron distribution function (eVDF) at a heliocentric distance r=1 AU. The instrument's 12 anodes are set in a vertical pattern in a plane that contains the spacecraft spin axis, spanning a field of view +/- 28 degrees centered around the ecliptic. Wind's spin axis is set at a right angle with the ecliptic plane, allowing different azimuthal angles to be sampled as the spacecraft spins (3 sec spin period). These azimuthal bins have a fixed separation of 3.53 degrees. Each strahl distribution measured by the spacecraft consists of a 14x12 angular grid of electron counts, that was measured at a fixed energy during a single spacecraft spin. Accompanying each strahl measurement is an analogous 14x12 measurement of the "antistrahl", made at a clock angle 180 degrees with respect to the strahl measurement. The detector voltage was set to a different value each spin, so that 32 energies from 19.34 to 1238 eV would be sampled in as many rotations. 

In the original mode of operation, each measurement grid was centered on the nominal average Parker spiral (in the ecliptic plane, 45 degrees offset from the radial direction). In practice, however, the local magnetic field only fell within the field of view of the detector about half the time. This prompted a revision of the instrument software in February 1999 (Ogilvie et al. 2000), which matched the clock angle of the strahl measurement with the instantaneous measurement of the magnetic field provided by Wind's Magnetic Field Investigation (MFI).

The data set reported here ranges from January 1, 1995 to May 30, 2001, which nearly covers the operational lifetime of the strahl instrument. The strahl detector was reconfigured shortly after this period to serve as a replacement for SWE's Vector Electron/Ion Spectrometer (VEIS), whose power supply had recently failed.

In the CDF files provided here, the raw count data sampled by the strahl detector are contained in the variables F_STRAHL_COUNTS, and F_ANTISTRAHL_COUNTS. Note that there are only 256 discrete count values reported by the detector, which actually represent a broader range of counts than 0-255, because the data were log-compressed before being transmitted by the satellite. Counts are converted into physical units of f(v) (e.g., cm^6 s^-3 ) in the standard fashion by accounting for the detector efficiency and geometric factor, i.e., each bin of the counts distribution is multiplied by a constant value to obtain the physical distribution. The physical distributions are reported in the variables F_STRAHL and F_ANTISTRAHL. The corresponding set of 14 azimuthal bins (phi GSE, in degrees) for the respective distributions are saved in PHI_STRAHL and PHI_ANTISTRAHL. The 12 fixed altitude bins of the distribution are saved in the variable THETA. Note that the 12 anodes are not spaced symmetrically around the ecliptic, and there is a significant gap around theta=0. The time (msec AD) and energy (eV) of each strahl/antistrahl measurement are saved in the variables EPOCH and ENERGY, respectively.

Note that the counts (F_STRAHL_COUNTS, F_ANTISTRAHL_COUNTS) data can be used to estimate error bars of the distribution (F_STRAHL, F_ANTISTRAHL). That is, if assuming Poisson statistics, the error bars sigma(f) of the distribution f for a given bin can be estimated: sigma(f) / f ~ 1/sqrt(N), where N is the raw # of counts in a bin. See e.g., Horaites et al. (2017).

The strahl spectra are saved here with very little alteration or cleaning. Two basic cleaning procedures, however, are applied as a courtesy to the user:

1) sun mask: when the detector was in view of the sun, photoelectrons emitted from the spacecraft surface were registered as spurious counts. This is only relevant for data post-February 1999, because after this time the software revision allowed the strahl detector's sampling domain to sometimes include the sun's position. A simple correction is applied: any time a bin of PHI_STRAHL or PHI_ANTISTRAHL falls within 10 degrees of 180 (the sun's position), the corresponding data in the distribution functions are set to a dummy value (-1e30).
2) saturation: Other, unidentified effects sometimes led broad regions of the distribution function to register anomalous (maximum possible) counts. These events, though quite rare, are corrected for by setting saturated bins of the distribution function to a dummy value (-1e30)

Other cautionary notes:
1) The 12 anodes do not have an even angular spacing. In particular, there is a large data gap arount theta=0. 
2) The strahl tends to become narrower at higher energies. Although the SWE strahl detector had very fine angular resolution, the strahl frequently appears unresolved at high energies.

The data provided here was processed from binary ("*.strahl") files that are stored at NASA's Goddard Space Flight Center. The code used to read the files was adapted in 2016-2017 by Konstantinos Horaites, from code and documentation developed by Richard Fitzenreiter and Matt Holland in the late 1990s and early 2000s. The process of restoring the data was guided through consultation with members of NASA's Wind team (acknowledgments are listed in Horaites et al., 2017). In order to verify the accuracy of the reading procedure, the data were compared with published works that displayed strahl spectra (namely, Fitzenreiter et al., 1998, and Ogilvie et al., 2000). Comments and further validation of the data are welcome.

USEFUL REFERENCES
Ogilvie K. W., et al., 1995, Space Sci. Rev., 71, 55
Fitzenreiter R. J., Ogilvie K. W., Chornay D. J., Keller J., 1998, Geophys. Res. Lett., 25, 249
Ogilvie K. W., Fitzenreiter R., Desch M., 2000, J. Geophys. Res., 105, 27277
K. Horaites, S. Boldyrev, L. B. Wilson, III, A. F. Vinas, and J. Merka. "Kinetic Theory and Fast Wind Observations of the Electron Strahl." ArXiv e-prints, June 2017.

Contact:
Konstantinos Horaites (email: kosta.horaites@gmail.com)