Time-varying Heliospheric Distance to the Heliopause

Using a three-dimensional MHD simulation, we examine the time-varying outer heliospheric structure and distance to the heliopause. Voyager 2 (V2) solar-wind observations show that a global merged interaction region (GMIR) with a ram-pressure of the order of several nPa normalized at 1 au enters the distant solar wind at an average rate of about one per year. This series of GMIRs adds an additional perturbative increase to the solar-wind ram-pressure in the inner heliosheath, and it also reduces the surrounding interstellar medium pressure acting on the heliopause; consequently, our simulation results in the distance to the heliopause being similar to 14 au larger when compared to the case when a series of GMIRs is not taken into account. In addition, OMNI data show that the solar-wind ram-pressure near the Earth increases from similar to 1.3 nPA in 2010 and before to 1.7-2.4 nPa after that until the present time. These variations in the overall ram-pressure of the solar wind are also included in our simulation. The inclusion of the time variable solar-wind ram-pressure and the series of GMIRs allows us to illustrate how the realistic distance to the heliopause varies in response to both long-and short time variability in solar activity. This simulation study also explains the puzzle of why V2 has not yet crossed the heliopause, although it is now almost 5 years since Voyager 1 crossed the heliopause in 2012.