The plasma of the solar wind incident upon the Earth’s magnetosphere can produce several types of geoeffective events. Among them, an important phenomenon consists of the interrelation of the magnetospheric–ionospheric current systems and the charged-particle population of the Earth’s Van Allen radiation belts. Ultra-low-frequency (ULF) waves resonantly interacting with such particles have been claimed to play a major role in the energetic particle flux changes, particularly at the outer radiation belt, which is mainly composed of electrons at relativistic energies. In this article, we use global magnetohydrodynamic simulations along with in situ and ground-based observations to evaluate the ability of two different solar wind transient (SWT) events to generate ULF (few to tens of mHz) waves in the equatorial region of the inner magnetosphere. Magnetic field and plasma data from the Advanced Composition Explorer (ACE) satellite were used to characterize these two SWT events as being a sector boundary crossing (SBC) on 24 September 2013, and an interplanetary coronal mass ejection (ICME) in conjunction with a shock on 2 October 2013. Associated with these events, the twin Van Allen Probes measured a depletion of the outer belt relativistic electron flux concurrent with magnetic and electric field power spectra consistent with ULF waves. Two ground-based observatories apart in 90∘ longitude also showed evidence of ULF-wave activity for the two SWT events. Magnetohydrodynamic (MHD) simulation results show that the ULF-like oscillations in the modeled electric and magnetic fields observed during both events are a result from the SWT coupling to the magnetosphere. The analysis of the MHD simulation results together with the observations leads to the conclusion that the two SWT structures analyzed in this article can be geoeffective on different levels, with each one leading to distinct ring current intensities, but both SWTs are related to the same disturbance in the outer radiation belt, i.e. a dropout in the relativistic electron fluxes. Therefore, minor disturbances in the solar wind parameters, such as those related to an SBC, may initiate physical processes that are able to be geoeffective for the outer radiation belt.
Bibliographical noteFunding Information:
We would like to thank the Center for Space Environment Modeling (CSEM) at the University of Michigan for providing the numerical SWMF/BATS-R-US Code available at: http://csem.engin.umich.edu/tools/swmf/downloads.php. We acknowledge the Kakioka Magnetic Observatory of the Japan Meteorological Agency and Bureau Central de Magnetisme Terrestre (France) for providing the geomagnetic field 1-second digitized data. We are also in debt to the Van Allen Probes mission teams, NASA?s CDAWeb, OMNI, and NGDC for online data access and data analysis tools. Solar wind parameters presented in Section 2 measured by ACE are available at http://www.srl.caltech.edu/ACE/ASC/DATA/browse-data/browse-curr/. The Van Allen Probes mission datasets presented in Sections 2 and 4 and supporting information are available at CDAWeb http://cdaweb.gsfc.nasa.gov/istp_public/. C.R. Braga and V.M. Souza thank the S?o Paulo research foundation (FAPESP) for grant 2014/24711-6 and 2014/21229-9. A. Dal Lago, M. Rockenbach, M.V. Alves, R.R.S. de Mendon?a and D. Koga thank CNPq for research grant 304209/2014-7, 301495/2015-7, 305373/2010-2, 152050/2016-7, and 112886/2015-9. P.R. Jauer and L.A. da Silva thank the Research Foundation CNPq/PCI for financial support process number 313281/2015-7 and 312743/2015-7. We would also like to thank CT-INFRA-FINEP/INPE 11 number 01.12.0527.00 and EMBRACE/INPE: http://www2.inpe.br/climaespacial/portal/en/. Earth-Affecting Solar Transients Guest Editors: Jie Zhang, Xochitl Blanco-Cano, Nariaki Nitta, and Nandita Srivastava
- Earth’s outer radiation belts
- MHD simulation
- Solar wind geoeffectiveness
- Solar wind transients
- ULF waves