LPL Colloquium: Dr. Walter Harris

Tuesday, Feb 11, 2014
3:45 pm
Location: Kuiper Space Sciences
Room: 312

Dr. Walter Harris
Associate Professor
University of Arizona - Lunar and Planetary Laboratory

The Big Shock That Wasn't: Or Was It?

The heliopause is the broad region marking the transition from space defined by the outflowing solar wind and rarefied local interstellar medium (LISM). This region is far too remote for easy in situ study, but fairly simple to conceptualize theoretically. In the classic model the heliopause itself is a standoff between counter-flowing regions with different plasma pressures and densities. Inside the heliopause, the solar wind decelerates across a termination shock and as it encounters the barrier itself. Outside the heliopause, the LISM encounters a similar process, starting with an upstream bowshock and ending with a pile up of plasma at the interface.

The past two decades have marked a sea change in our understanding of this region. Remote observations with the SOHO and Voyager spacecraft have mapped the spatial distribution and velocity of the incoming neutral LISM flow. At the same time, direct, in situ measurements of penetrating LISM from Ulysses and of energetic neutrals produced at the heliopause by IBEX have combined with termination shock and heliospause crossings by the Voyager spacecraft to produce a detailed map of the interface region shape. Combined these have greatly increased our understanding of the magnetic and plasma properties of the region immediately beyond the solar system.

In 2012 a new controversy emerged from an IBEX measurement of heliospheric hydrogen and helium that suggested the relative velocity between the LISM and the solar system is lower than previously thought. The lower velocity, when combined with models of the density, temperature, and magnetic field of the LISM suggested that the plasma Mach number was <1, in which case the long assumed upstream bow shock did not exist.

In this presentation, I will discuss the origins and basis for both the original assumption of a bow shock and the results that suggest it is not present. I will also discuss more recent observations and modeling that indicate flaws in both the original IBEX measurement of heliospheric neutrals and the modeling that led to the no-shock condition, and I will discuss new measurements that could be made to resolve the controversy.