Suprathermal Electron Pitch Angle Data and Plots from ACE/SWEPAM

Caveats and Warnings

These pitch angle distributions are calculated automatically by a computer code, and have not been validated by a person. It is therefore likely that they include some artifacts and erroneous values.

The SWEPAM electron Channel Electron Multipliers (CEM) detectors have undergone gradual gain degradation over their 15 years of operation, and the instrument has been operating at the highest voltage level since Sept 25, 2005. Currently, 3 of the 7 electron CEMs have quite low gains, precluding measurement of the full suprathermal electron pitch angle distribution under some conditions. Under appropriate combinations of the magnetic field direction and spacecraft pointing, full pitch angle distributions can be and are still measured. Thus, these data are still very useful part of the time and we continue to provide them. However, extra caution is needed at times when unusual distributions are observed and we strongly encourage directly interacting with the SWEPAM team when using them. In addition, we continue to monitor in-flight calibration data and apply calibration factors as required. Please contact Dr. R. Skoug ( ) with any questions regarding the use of these data.

Data file documentation

These data files contain suprathermal electron pitch angle distribution functions (s^3 cm^-6) at 272 eV from the ACE/SWEPAM-E instrument. The energy channel is ~12% wide, centered at 272 eV. Pitch angle bins are 9 degrees wide, and run from 0 to 180 degrees (so 0-9, 9-18, 18-27 degrees, etc.).

The break between the core and suprathermal electrons is generally around 70 eV at 1 AU, but varies around this value, at times exceeding 100 eV. Electrons at 272 eV are generally well within the suprathermal range, and the distribution at this energy typically shows very little contribution from the core population. The measured count rates at 272 eV energy provide excellent statistical significance. In addition, because of the high count rates, very few artifacts (for example due to sunlight contamination) are seen in the data at this energy.

The data are in the solar wind frame, and have been corrected for the spacecraft potential. The spacecraft potential determination requires measurement of the lower energy electrons, and so the potentials calcuated at these times have been interpolated to the times of the suprathermal electron measurements. Transformation to the solar wind frame requires interpolation of the measurements (each at a different energy in the solar wind frame) to the desired energy (272 eV).

Also note that the detector gains on the SWEPAM-E instrument change over time, including both slowly decreasing gains and abrupt increases when the instrument voltages are increased. The use of these data for long-term studies of the distribution function amplitudes is therefore strongly discouraged.

Documentation for the plots

These plots show suprathermal electron pitch angle distribution functions (s^3 cm^-6) at 10 energies from 73 eV (bottom) to 1.37 keV (top) from the ACE/SWEPAM-E instrument. Each energy channel is approximately 12% wide. Pitch angle bins are 9 degrees wide, and run from 0 to 180 degrees (so 0-9, 9-18, 18-27 degrees, etc.).

The color scale runs from blue (low values) to red (high values of the distribution function). Note that the color scale is different for each panel (each energy). The thin bar next to each panel shows the overall range (1.0e-33 to 300.0e-29), while the thick bar and the numerical scale show the portion of that range covered for a given energy; that is, the range of values from blue to red for that energy. So, at 1.37 keV, blue corresponds to 1.0e-33, and red to 10.0e-33. At 987 eV, blue corresponds to 2.0e-33 and red to 50.0e-33.

The break between the core and suprathermal electrons is generally around 70 eV at 1 AU, but varies around this value, at times exceeding 100 eV. Data in the lower energy channels (particularly 73, 100, and 142 eV) can often show contributions from the core electron population. The core electrons are not typically field-aligned. In addition, some artifacts are seen in the higher energy channels. In particular, a signal due to sunlight contamination is frequently present at the highest energies. This signal is aligned with the Sun direction, and so moves through a pitch angle plot as the magnetic field direction changes. An example of this can be seen on Dec 14, 2000, as the non-field aligned signal seen from 0-12 UT in the 712, 987 and 1370 eV panels.

Solar wind suprathermal electrons typically show a unidirectional beam, or strahl, directed outward from the Sun along the interplanetary magnetic field. This strahl can appear at either 0 degree pitch angle or 180 degree pitch angle, depending on whether the IMF is directed inward or outward from the Sun. At times, counterstreaming electrons are present, identified by two oppositely directed strahls (appearing at both 0 and 180 degree pitch angles). Counterstreaming electrons can be an indication of coronal mass ejection associated closed magnetic field lines, but can also result from connection to the Earth's bow shock, connection to an interplanetary shock or wave, or a depletion at 90 degree pitch angle. In addition, particularly at the lower energies, apparent counterstreaming in these plots can result from a thermal anisotropy in the distribution. Care must be taken in distinguishing these various sources.

Last Updated: May 2007

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