Time‐dependent magnetohydrodynamic simulations of the inner heliosphere

Abstract

AbstractThis paper presents results from a simulation study exploring heliospheric consequences of time‐dependent changes at the Sun. We selected a 2 month period in the beginning of year 2008 that was characterized by very low solar activity. The heliosphere in the equatorial region was dominated by two coronal holes whose changing structure created temporal variations distorting the classical steady state picture of the heliosphere. We used the Air Force Data Assimilate Photospheric Flux Transport (ADAPT) model to obtain daily updated photospheric magnetograms and drive the Wang‐Sheeley‐Arge (WSA) model of the corona. This leads to a formulation of a time‐dependent boundary condition for our three‐dimensional (3‐D) magnetohydrodynamic (MHD) model, LFM‐helio, which is the heliospheric adaptation of the Lyon‐Fedder‐Mobarry MHD simulation code. The time‐dependent coronal conditions were propagated throughout the inner heliosphere, and the simulation results were compared with the spacecraft located near 1 astronomical unit (AU) heliocentric distance: Advanced Composition Explorer (ACE), Solar Terrestrial Relations Observatory (STEREO‐A and STEREO‐B), and the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft that was in cruise phase measuring the heliospheric magnetic field between 0.35 and 0.6 AU. In addition, during the selected interval MESSENGER and ACE aligned radially allowing minimization of the effects of temporal variation at the Sun versus radial evolution of structures. Our simulations show that time‐dependent simulationsreproduce the gross‐scale structure of the heliosphere with higher fidelity, while on smaller spatial and faster time scales (e.g., 1 day) they provide important insights for interpretation of the data. The simulations suggest that moving boundaries of slow‐fast wind transitions at 0.1 AU may result in the formation of inverted magnetic fields near pseudostreamers which is an intrinsically time‐dependent process. Finally, we show that heliospheric current sheet corrugation, which may result in multiple sector boundary crossings when observed by spacecraft, is caused by solar wind velocity shears. Overall, our simulations demonstrate that time‐dependent heliosphere modeling is a promising direction of research both for space weather applications and fundamental physics of the heliosphere.