Observed instability constraints on electron heat flux in the solar wind

We present for the first time the joint distribution of electron heat flux and electron plasma beta, and the joint distribution of core electron drift velocity and electron plasma beta using a statistically large dataset of solar wind electrons at 1AU. We calculate the growth rates of linear instabilities and show compelling experimental evidence that the heat-flux-driven Alfv{\'e}n instabilities constrain the electron core drift, and therefore the electron heat flux, in low-beta plasmas. This result is relevant to understanding energy transport in low-beta solar/stellar coronae and winds.