Does a Local B‐Minimum Appear in the Tail Current Sheet During a Substorm Growth Phase?

• Published in: Geophysical research letters 2018-03, Vol.45 (6), p.2566-2573
• Main Authors: Sergeev, V. A., Gordeev, E. I., Merkin, V. G., Sitnov, M. I.
• Format: Article
• Language: English
• Subjects: Auroral oval; Auroral zones; Condensation; Configurations; Electron precipitation; Electrons; energetic particle precipitation; Flight; Fluid dynamics; Fluid flow; Growth; Ionosphere; Lines; Low altitude; Magnetic field; Magnetic fields; Magnetic properties; Magnetism; Magnetohydrodynamic simulation; Magnetospheres; Magnetotails; Maxima; Outer radiation belt; Precipitation rate; Radiation; Regions; Remote sensing; Satellites; solar electrons; Solar storms; Spaceborne remote sensing; Spacecraft; Stability; substorm growth phase; tail current sheet stability
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1002/2018GL077183

Magnetic configurations with dBz/dr > 0 in the midtail current sheet are potentially unstable to various instabilities associated with the explosive substorm onset. Their existence is hard to confirm with observations of magnetospheric spacecraft. Here we use remote sensing by low‐altitude spacecraft that measured the loss cone filling rate during electron‐rich solar particle event, providing information about magnetic properties of the tail current sheet. We found a latitudinally localized anisotropic 30 keV electron loss cone region embedded inside an extended region of isotropic solar electron precipitation. It was persistently observed for more than 0.5 h during isolated growth phase event by six Polar Operational Environmental Satellites spacecraft, which crossed the premidnight auroral oval. The embedded anisotropic region was observed 1° poleward of the outer radiation belt boundary over 4–5 h wide magnetic local time sector, suggesting a persistent ridge‐type Bz2/j maximum in the equatorial plasma sheet at distances 15–20 RE. We discuss infrequent observations of such events taking into account recent results of global magnetohydrodynamic simulations. Plain Language Summary What causes a sudden onset of substorm in the magnetotail is still a puzzle for both scientists and the public. One interesting possibility is that closed magnetic field lines in the tail current sheet may occasionally form peculiar condensations (configuration with Bz component minima and maxima somewhere in the tail), and these regions may get unstable to initiate substorms. Unfortunately, there are very little chances to identify such unstable regions by satellites flying in the magnetotail just because the tail volume is huge, whereas the spacecraft are very few. Fortunately enough, the precipitation rate of energetic electrons into the ionosphere from the tail current sheet may help us to manifest large/small Bz values in the equatorial parts of corresponding magnetic field lines remotely. We explore this opportunity and demonstrate that a few low‐orbiting spacecraft passing one after another across the nightside auroral region during a substorm growth phase see the same modulation of precipitation which might indicate a presence of magnetic tube condensations at 15–20 RE distance in the tail. Whereas a new method was successfully demonstrated, the magnetic condensation signatures are not always observed. Further investigation is required to understand the role of these phe