Matteini, L., Landi, S., Hellinger, P., Pantellini, F., Maksimovic, M., Velli, M., et al. (2007). Evolution of the solar wind proton temperature anisotropy from 0.3 to 2.5 AU. Geophysical Research Letters, 34: L20105. doi:10.1029/2007GL030920.