Examining Local Time Variations in the Gains and Losses of Open Magnetic Flux During Substorms. Issue 4 (16th April 2020)

• Record Type: Journal Article
• Authors: Mooney, M. K.; Forsyth, C.; Rae, I. J.; Chisham, G.; Coxon, J. C.; Marsh, M. S.; Jackson, D. R.; Bingham, S.; Hubert, B.
• Is Part Of: Journal of geophysical research. Volume 125:Issue 4(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-04-16
• Subjects: open‐closed field line boundary -- substorms -- superposed epoch analysis -- total flux content -- auroral oval -- IMAGE FUV; Magnetospheric physics -- Periodicals; Space environment -- Periodicals; Cosmic physics -- Periodicals; Planets -- Atmospheres -- Periodicals; Heliosphere (Astrophysics) -- Periodicals; Geophysics -- Periodicals; 523.01
• DOI: 10.1029/2019JA027369
• Languages: English
• ISSNs: 2169-9380

Abstract

Abstract: The open magnetic flux content of the magnetosphere varies during substorms as a result of dayside and nightside reconnection. The open flux can be calculated from the area of the polar cap, delineated by the open‐closed field line boundary (OCB). This study presents a superposed epoch analysis of the location of the OCB and the change in the magnetic flux content in individual nightside MLT sectors during substorm growth, expansion, and recovery phases. Far ultraviolet (FUV) observations from the IMAGE satellite are used to derive a proxy of the OCB location. In the hour prior to substorm onset, the total nightside flux content increases by up to 0.12 GWb on average, resulting in an equatorward expansion of the OCB. Following substorm onset, the OCB contracts toward the pole as the open magnetic flux content decreases by up to 0.14 GWb on average, but the rate of decrease of the total nightside open flux content differs by 5–66% between the three IMAGE far ultraviolet instruments. The OCB does not contract poleward uniformly in all nightside magnetic local time (MLT) sectors after substorm onset. Close to the substorm onset MLT sector, the OCB contracts immediately following substorm onset; however, the OCB in more dawnward and duskward MLT sectors continues to expand equatorward for up to 120 minutes after substorm onset. Despite the continued increase in flux in these sectors after substorm onset, the total nightside flux content decreases immediately at substorm onset, indicating that the nightside reconnection rate exceeds the dayside rate following substorm onset. Plain Language Summary: Earth's magnetic field shields us from the steady stream of particles originating from the Sun, which carry the Sun's magnetic field. At Earth, the solar magnetic field can break open our magnetic field and allow energy to build up inside Earth's magnetic field. This energy can be explosively released during substorms. The auroral oval is a ring of aurora around the magnetic poles which varies in size, shape, and brightness during substorms. As energy is building in the magnetic field, the auroral oval expands. As the energy is released, it contracts. Using satellite images of the auroral oval, we show that prior to substorm onset, the nightside auroral oval expands toward the equator. At substorm onset, the auroral oval also rapidly moves poleward. The poleward motion initially occurs in a localized region as the rest of the nightside oval continues to expand. The poleward motion then spreads eastward and westward around the entire nightside oval over the following 20–120 minutes. Our results show that although most of the auroral oval continues to expand equatorward after substorm onset, the total area of the auroral oval decreases resulting in an overall net decrease in the energy stored in the Earth's magnetic field. Key Points: We perform a statistical analysis of open flux content in the nightside ionosphere during substorms At substorm onset, the open flux decreases overall and in the substorm onset sector but increases in the dusk and dawn sectors The estimated rate of closure of the total nightside open flux content differs by up to 66% between the three IMAGE FUV instruments