THE OUTER HELIOSPHERE: SOLAR WIND, COSMIC RAY AND VLF RADIO EMISSION VARIATIONS

45 astronomical units (AU) from Earth. Both spacecraft continue to monitor the outer heliosphere fields-and-particles environment on a daily basis during their journey to the termination shock of the solar wind. Strong transient shocks continue to be detected in the solar wind plasma. The largest of these are associated with Global Merged Interaction Regions (GMIRs) which, in turn, block cosmic ray entry into the inner heliosphere and are apparently responsible for triggering the two major episodes of VLF radio emissions now thought to come from the heliopause. Distance estimates to the termination shock are consistent with those determined from observations of anomalous cosmic rays. Current observations and implications for heliospheric structure are discussed. Knowledge of the Outer Heliosphere The heliosphere is the plasma cavity blown out by the solar wind in the Very Local Interstellar Medium (VLISM)1,2). The outer heliosphere can be defined roughly as that part of the cavity extending from the orbit of Neptune (roughly 30 AU from the Sun) through the interaction region with the VLISM. This region has been directly accessible to solar-system-escaping spacecraft- Pioneer 10 and 11 and Voyager 1 and 2- for less than the last decade. During this time direct measurements of the local medium have been possible. Typical values and variabilities of the solar wind plasma, interplanetary magnetic field and relevant derived quantities are shown in Table 1