A 2D column-normalized histogram of the electron temperature anisotropy, A_e, [N/A], vs. the parallel electron plasma beta, β_e,∥ [N/A]. Note that Appendix B provides Figure 6, which shows the distribution of data for each of these parameters so the reader can properly interpret the statistical significance of any given feature. We have overplotted the whistler anisotropy (dashed line) and electron firehose (solid line) instability thresholds for reference (e.g., S. P. Gary & J. Wang 1996; S. P. Gary & K. Nishimura 2003).