Remote sensing of the heliospheric boundary Original Research Article

The distant regions of the heliosphere, including the termination shock and the heliopause, are not yet accessible to direct observation in situ. Our ideas about the structure of this region come mostly from theoretical models. Nevertheless, there are some important observations available and more should be forthcoming in the near future. This contribution concentrates on two means of observation of the distant heliosphere: low frequency radio waves and energetic neutral atoms. The experimental data and the underlying theoretical models are reviewed and discussed. Included are: the discovery by Voyager spacecraft of low frequency heliospheric radio emissions and the observations by SOHO (and, more recently, other spacecraft) of energetic neutral hydrogen of possibly heliospheric origin.