Multiple magnetic dipolarizations observed by THEMIS during a substorm

Abstract

Abstract. The magnetic field dipolarization in the vicinity of substorm onset and during substorm expansion phase during the period of 06:00-06:40 UT on 15 February 2008 is investigated with observations from multiple probes of THEMIS. It is found that the magnetic dipolarization at the substorm onset (the onset time was about 06:14 UT) was not accompanied by obvious magnetic disturbance and ion bulk speed variation. The magnetic dipolarizations taking place during the substorm expansion phase observed by P4∼(−10.97, 2.04, −3.03) R E and P3∼(−11.32, 1.15, −3.10) R E were mostly accompanied by high speed earthward ion bulk flow, but the magnetic dipolarizations occurring during the substorm expansion phase observed by P5∼(−9.45, 1.07, −2.85) R E were not accompanied by high speed earthward ion bulk flow. Before substorm onset THEMIS P3, P4, P5 all observed the B x component fluctuation with a period of about 300 s. After substorm onset earthward high speed ion bulk flow and significant magnetic disturbances both occurred at P3 and P4 locations. These results indicate that there is no one-to-one relationship between the near-Earth magnetic dipolarization and the earthward ion bulk flow. In particular, the magnetic dipolarization occurring on the earthward side of the inner near-Earth plasma sheet is not accompanied by high speed earthward ion bulk flow. The dipolarization at substorm onset is a local and small scale phenomenon. There are multiple magnetic dipolarizations occurring during the substorm expansion phase. The dipolarization process is very complex and is not simply an MHD process. It is accompanied by some kinds of plasma instabilities, the plasma sheet azimuthal expansion not only by earthward ion bulk flow during substorm. A sharp increase of the AE index does not always give an accurate substorm onset time for substorm analysis.

Tailward leap of multiple expansions of the plasma sheet during a moderately intense substorm: THEMIS observations

“…Some have suggested that it is correlated with earthward flux transport which is piled up at the near tail [ Shiokawa et al , 1997; Birn et al , 1999]. Others, however, claim that dipolarization does not have a one‐to‐one relationship with braking of earthward bulk flows [ Lui et al , 2008; Duan et al , 2011] and that near‐Earth dipolarization is a non‐MHD process [ Lui et al , 1999].…”

Contributions of substorm injections to SYM‐H depressions in the main phase of storms

“…We identify dispersionless ion injections at 12:40 UT from two LANL satellites (LANL91 and LANL94), which are located at nightside at magnetic local time (MLT) = 1.33 and MLT = 22.02. The B z -GSE (Hp component) sharply increases at GOES, indicating the dipolarization process associated with injection [Nagai, 1982;Lui et al, 1999;Duan et al, 2011]. A clear "growth phase dropout" is observed at LANL94.…”

“…(2) The injections occurred during the storm main phases. The B z -GSE (Hp component) sharply increases at GOES, indicating the dipolarization process associated with injection [Nagai, 1982;Lui et al, 1999;Duan et al, 2011]. (b) The dispersionless injection onset time can be well identified.…”

An explanation of auroral intensification during the substorm expansion phase

“…It has been shown that a substorm may have multiple onsets [ Murphy et al , ]. This has been established from the ground geomagnetic field perturbations observed from the ground [ Pytte et al , ], auroral intensifications [ Keiling et al , ; Pu et al , ], global magnetic field dipolarizations in the near‐Earth magnetotail [ Duan et al , ; Palin et al , ], and substorm injections into geosynchronous orbit [ Nagai et al , ]. Previous studies also suggest that multiple auroral intensifications are related to multiple bursty bulk flows [ Nishimura et al , ].…”