Examining Local Time Variations in the Gains and Losses of Open Magnetic Flux During Substorms

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.

FTA: A Feature Tracking Empirical Model of Auroral Precipitation

“…Recently, Mooney et al. (2020) studied the open‐closed field line boundary (OCB) movement during substorms based on IMAGE FUV images. It was found that within ∼10 min of the beginning of the substorms, in the MLT prior to the onset MLT sectors (MLT difference approximately from −4 to −1 h), the OCB moved equatorward due to the increasing open flux during the growth phase, while in the MLT sectors near and after the onset MLT (MLT differences from 0 to 4 h), the OCB tended to contract poleward or showed no changes in the latitude location.…”

“…However, the previous study indicated that with activity increasing, the EB moved to lower latitudes at a faster rate in the post-noon sector, while the PB moved equatorward in the post-noon sector and poleward on the entire nightside statistically, which is different from the results here. Recently, Mooney et al (2020) studied the open-closed field line boundary (OCB) movement during substorms based on IMAGE FUV images. It was found that within ∼10 min of the beginning of the substorms, in the MLT prior to the onset MLT sectors (MLT difference approximately from −4 to −1 h), the OCB moved equatorward due to the increasing open flux during the growth phase, while in the MLT sectors near and after the onset MLT (MLT differences from 0 to 4 h), the OCB tended to contract poleward or showed no changes in the latitude location.…”

Properties of Storm‐Time Magnetic Flux Transport

“…Additionally, storm‐time reconnection is strong and can extend across a sizeable portion of the nightside oval, leading to a significant deformation of the assumed circular polar cap boundary. This deformation can lead to an overestimate of the open flux of the magnetosphere (Mooney et al., 2020).…”

Evaluating Auroral Forecasts Against Satellite Observations

“…We note that, in this example, there is a lack of observed auroral boundaries in some dayside MLT sectors. While the method of Longden et al (2010) aims to identify the poleward and equatorward auroral luminosity boundaries in each MLT sector, the number of successful boundary identifications in dayside sectors is much lower than on the nightside (Mooney et al, 2020). The dayside aurora tends to be dimmer and thinner (Carbary, 2005;Holzworth & Meng, 1975) and is more contaminated with dayglow making it more difficult to identify the dayside auroral boundaries.…”

“…The solar wind driven OP-2013 model is unable to forecast substorm activity and the 30-min resolution of the operational forecasts cannot capture substorm dynamics, however, the change in width of the auroral oval during substorms occurs within the average predicted auroral oval location. Mooney et al (2020) showed that during substorms, the poleward boundary of the auroral oval moves by up to 3° in the substorm onset MLT sectors. During substorms, the typical width of the auroral oval varies by 10°-17° (Walach et al, 2017).…”