Cross‐Scale Quantification of Storm‐Time Dayside Magnetospheric Magnetic Flux Content

M. Akhavan‐tafti , D. Fontaine (1) , J. A Slavin , O. Le Contel (1) , D. Turner

1 LPP - Laboratoire de Physique des Plasmas

A clear understanding of storm-time magnetospheric dynamics is essential for a reliable storm forecasting capability. The dayside magnetospheric response to an interplanetary coronal mass ejection (ICME; dynamic pressure P dyn > 20 nPa and storm-time index SYM-H < −150 nT) is investigated using in situ OMNI, Geotail, Cluster, MMS, GOES, Van Allen Probes, and THEMIS measurements. The dayside magnetic flux content is directly quantified from in situ magnetic field measurements at different radial distances. The arrival of the ICME, consisting of shock and sheath regions preceding a magnetic cloud, initiated a storm sudden commencement (SSC) phase (SYM-H~+50 nT). At SSC, the magnetopause standoff distance was compressed earthward at ICME shock encounter at an average rate~−10.8 Earth radii per hour for~10 min, resulting in a rapid 40% reduction in the magnetospheric volume. The "closed" magnetic flux content remained constant at 170 ± 30 kWb inside the compressed dayside magnetosphere, even in the presence of dayside reconnection, as evident by an outsized flux transfer event containing 160 MWb. During the storm main and recovery phases, the magnetosphere expanded. The dayside magnetic flux did not remain constant within the expanding magnetosphere (110 ± 30 kWb), resulting in a 35% reduction in pre-storm flux content during the magnetic cloud encounter. At that stage, the magnetospheric magnetic flux was eroded resulting in a weakened dayside magnetospheric field strength at radial distances R ≥ 5 R E. It is concluded that the inadequate replenishment of the eroded dayside magnetospheric flux during the magnetosphere expansion phase is due to a time lag in storm-time Dungey cycle. Plain Language Summary A clear understanding of Earth's magnetospheric dynamics is essential for a reliable space weather forecasting capability. To achieve this, we take advantage of the Heliophysics System Observatory's (HSO) multitude of in situ observations in order to, for the first time, quantify the amount of magnetic flux stored in the dayside magnetosphere. The stored magnetic flux shields our ground-based and space-borne assets from adverse space weather events. We examine the dayside magnetic flux content during an encounter with an interplanetary coronal mass ejection (ICME). ICME is a large-scale bundle of magnetic flux and charged particles originating from the Sun. Upon arrival, the ICME which occupied nearly one third of the space between the Sun and Earth forced the dayside magnetosphere to rapidly shrink down to geosynchronous orbit where most communications and weather satellites are located. Though the dayside magnetosphere significantly shrunk, its magnetic flux content remained constant. It was only when the dayside magnetosphere started to expand that the dayside magnetospheric flux content gradually reduced by 35%. It is concluded that, during large ICME encounters, the rate at which dayside magnetic flux is transported to the magnetotail is faster than the rate at which magnetic flux is recycled, via a process known as the Dungey cycle. In addition to the observed loss in magnetic flux, this time lag in Dungey cycle can further cause magnetopause shadowing, wherein significant population of magnetospheric charged particles is lost to solar wind.

Domaines: Planète et Univers [physics]

M. Akhavan‐tafti, D. Fontaine, J. A Slavin, O. Le Contel, D. Turner. Cross‐Scale Quantification of Storm‐Time Dayside Magnetospheric Magnetic Flux Content. Journal of Geophysical Research Space Physics, 2020, 125 (10), ⟨10.1029/2020JA028027⟩. ⟨hal-03000303⟩