CASCADE EXCITATION IN THE GEOCORONAL HYDROGEN BALMER ALPHA-LINE

This paper reports high-accuracy measurements of geocoronal Balmer alp ha line profiles and demonstrates that the profiles are well fit with a model which includes cascade excitation by solar Lyman series radiat ion from n > 3 in addition to the direct excitation of n = 3 by solar Lyman beta. The increase in the signal-to-noise of our data is made po ssible by the use of the Fabry-Perot annular summing technique impleme nted at our Fabry-Perot facility at the University of Wisconsin's Pine Bluff Observatory. The new sensitivity has allowed us to make a detai led examination of line profile asymmetries and to conclude that they are compatible with predictions that of the order of 10% of the geocor onal Balmer alpha emission is caused by the cascade process. Cascade e xcitation alters the observed profile because it produces Balmer alpha emission along fine structure paths yielding slightly shifted wavelen gths not present in direct Lyman beta excitation, which is the predomi nant excitation mechanism for geocoronal Balmer alpha. We discuss how fine structure excitation affects studies of non-Maxwellian exospheric hydrogen velocity distributions and effective temperatures through Ba lmer alpha line profile measurements. In a broader context, we conside r how inclusion of the cascade excited emission in future radiation mo dels can enhance their accuracy and their potential for assisting in t he isolation in the data of shorter-term solar geophysical effects and longer timescale changes in exospheric hydrogen densities.