Time‐dependent magnetohydrodynamic simulations of the inner heliosphere

Abstract

This 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.

A Computationally Efficient, Time-Dependent Model of the Solar Wind for Use as a Surrogate to Three-Dimensional Numerical Magnetohydrodynamic Simulations

“…Whereas an MHD solution to a (27-day) Carrington rotation requires between 30-60 minutes computing time on an NVIDIA TitanXP GPU, HUXt requires approximately 0.05 seconds on a modest desktop CPU. We stress that HUXt is not intended to replace the more complete physics-based approaches of three-dimensional magnetohydrodynamic solar-wind models such as ENLIL, SWMF, HelioMAS, HelioLFM, and EUHFORIA (Odstrcil, 2003;Toth et al, 2005;Riley et al, 2012;Merkin et al, 2016;Pomoell and Poedts, 2018). Instead, we suggest HUXt can act as a surrogate for these models when very large ensembles are required, which would otherwise be computationally prohibitive.…”

“…Solar-wind conditions are then propagated from 30 R to Earth using a heliospheric model (e.g. Riley, Linker, and Mikic, 2001;Odstrcil, 2003;Toth et al, 2005;Merkin et al, 2016;Pomoell and Poedts, 2018). Because the solar wind is both supersonic and super-Alfvénic, information only flows away from the Sun and no outer boundary conditions are required to generate any one model simulation run.…”

CESE‐HLL Magnetic Field‐Driven Modeling of the Background Solar Wind During Year 2008

“…The PNC method was also employed by Hayashi () in his data‐driven coronal simulation. Other heliospheric solar wind simulations are driven above the Alfvénic surface by either empirical coronal models (Linker et al, ; Merkin, Lyon, et al, ) or interplanetary scintillation data (IPS) (Hayashi, ; Jackson et al, ; Kim et al, ; Odstrcil et al, ; Yu et al, ). For these models, the boundary treatment is more straightforward because the solar wind at their inner boundaries is already super‐Alfvénic and all variables may be prescribed directly.…”

Models and data analysis tools for the Solar Orbiter mission

“…For decades diachronic synoptic maps were typically used as input to coronal models, but now the heliospheric community is moving towards using synchronic maps that attempt to estimate the global magnetic field distribution at any moment in time (see, e.g., Ulrich & Boyden 2006;Riley et al 2014 uncertainty with flux transport modeling (e.g., such as the uncertainties in the meridional drift or different rotation rates or lack of observations on the solar far-side), the Air Force Data Assimilative Photospheric flux Transport (ADAPT) model utilizes an ensemble (with typically 12 realizations) of synchronic synoptic maps (based on Worden & Harvey 2000) and state-of-the-art data assimilative techniques (see, Hickmann et al 2015) to represent as realistically as possible the spread in the uncertainty of the state of the global photospheric magnetic field (Arge et al 2010). The ADAPT global magnetic maps, using the available Earth/L1 perspective magnetograms, are publicly available 1 and used within the heliospheric modeling community, e.g., for timedependent MHD simulations of the inner heliosphere (Merkin et al 2016) and ensemble modeling of the large CME during July 2012 (Cash et al 2015). In addition, ADAPT forecast maps are utilized to predict the observed F10.7 values (i.e., the solar radio flux at 10.7 cm) and bands within the VUV (vacuum ultraviolet, between 0.1 and 175 nm) solar irradiance .…”