Data assimilation of low‐altitude magnetic perturbations into a global magnetosphere model

Space Weather RESEARCH ARTICLE 10.1002/2015SW001330 Key Points: • Initial approach to data assimilation in a global magnetosphere model tested • Low-altitude magnetic perturbations assimilated • Approach found promising for assimilation of data sets like AMPERE Correspondence to: V. G. Merkin, Slava.Merkin@jhuapl.edu Citation: Merkin, V. G., D. Kondrashov, M. Ghil, and B. J. Anderson (2016), Data assimilation of low-altitude magnetic perturbations into a global magnetosphere model, Space Weather, 14, 165–184, doi:10.1002/2015SW001330. Received 30 OCT 2015 Accepted 29 JAN 2016 Accepted article online 4 FEB 2016 Published online 29 FEB 2016 Data assimilation of low-altitude magnetic perturbations into a global magnetosphere model V. G. Merkin 1 , D. Kondrashov 2 , M. Ghil 2 , and B. J. Anderson 1 1 The Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA, 2 Department of Atmospheric and Oceanic Sciences and Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California, USA Abstract The ionosphere is the only region of the terrestrial magnetosphere-ionosphere system where in situ observations with high temporal resolution and approaching global spatial scale are possible. Ionospheric measurements of magnetic ﬁelds with such spatiotemporal coverage have become available from the Active Magnetosphere and Planetary Electrodynamics Response Experiment, combining data from the Iridium Ⓡ satellites. Motivated by the emergence of this data set, we report here on the ﬁrst results of assimilation of low-altitude ionospheric magnetic perturbations into the Lyon-Fedder-Mobarry (LFM) global magnetospheric model coupled with the Rice Convection Model (RCM). Our assimilation approach relies on the assumption of a quasi-steady, linear approximate relation between equatorial magnetospheric pressure and ionospheric Region 2 ﬁeld-aligned currents. This approximation is implemented numerically by perturbing large-scale modes from the Fourier decomposition of equatorial magnetospheric pressures and computing responses of the corresponding modes in the ionospheric magnetic ﬁeld. This methodology was validated by using model-based assimilation tests of the so-called “fraternal twins” type. In this approach, the LFM-RCM model with one set of parameters is used to generate synthetic observations, while a model version with diﬀerent parameters is used to assimilate the ionospheric observations and calculate the magnetospheric pressure corrections which are then applied to reproduce the synthetic observations. The model with assimilated synthetic data responded correctly by modifying ionospheric currents and magnetic perturbations in the expected way. We thus found the approach proposed herein to be promising for future assimilation of real data. 1. Introduction and Motivation The terrestrial magnetosphere is a strongly coupled nonlinear physical system populated by fully ionized plas- mas immersed in the Earth’s magnetic ﬁeld. Together with the ionosphere, solar wind, and corona, they form the largest natural plasma laboratory accessible to in situ observations. Studies of this system are therefore of fundamental physical interest. The magnetosphere-ionosphere system is also home to geomagnetic dis- turbances that are primarily caused by this system’s interaction with the solar wind. These disturbances are collectively called space weather whose extreme manifestations can have signiﬁcant socioeconomic impacts. Studying complex physical systems with a large number of degrees of freedom requires building computer models. The most complete description of the magnetosphere can in theory be achieved by solving the full set of Maxwell and Vlasov-Boltzmann equations using particle simulations throughout. Due to the extremely wide range of spatiotemporal scales involved—spanning 10 2 – 10 9 m and microseconds to days—this goal will remain elusive for the foreseeable future, at least for explicit codes [Lapenta, 2012]. The Vlasov description can be reduced to a single-ﬂuid approximation by taking moments of the ion and electron distribution functions. This magnetohydrodynamic (MHD) approximation allows a global description of the magnetosphere, including its most basic structural elements such as the bow shock, the magnetopause, and the magnetotail. MHD-based computer models of the magnetosphere have been developed since the early days of space science [LeBoeuf et al., 1978; Lyon et al., 1981; Ogino and Walker, 1984] and have become the workhorses of magnetospheric and space weather research ever since [e.g., Lyon et al., 2004; Toth et al., 2007; Raeder et al., 2008; Janhunen et al., 2012]. ©2016. American Geophysical Union. All Rights Reserved. MERKIN ET AL. Global MHD magnetospheric models have matured to the point at which they can be used to predict space weather processes [e.g., Pulkkinen and et al., 2011; Rastatter et al., 2011]. These models, though, still face the problem of insuﬃciently or poorly speciﬁed boundary and initial data, as well as of incomplete physics. ASSIMILATION OF MAGNETIC PERTURBATIONS