TY - JOUR

T1 - Simultaneous THEMIS observations in the near-tail portion of the inner and outer plasma sheet flux tubes at substorm onset

AU - Sergeev, V. A.

AU - Apatenkov, S. V.

AU - Angelopoulos, V.

AU - McFadden, J. P.

AU - Larson, D.

AU - Bonnell, J. W.

AU - Kuznetsova, M.

AU - Partamies, N.

AU - Honary, Farideh

N1 - Copyright (2008) American Geophysical Union. Further reproduction or electronic distribution is not permitted

PY - 2008/11/19

Y1 - 2008/11/19

N2 - We analyzed the measurements made by two Time History of Events and Macroscale Interactions during Substorms (THEMIS) probes in ideal observational conditions (quiet background, near midnight, inside the substorm current wedge) during two distinct isolated substorm onsets, with probe P2 measuring the inner plasma sheet at ∼8 Re and P1 near the plasma sheet–lobe interface at 11–12 Re. The earliest onset-related strong perturbations were observed by P1; they include the increase of both B z (dipolarization) and E y (a few mV/m) as well as the simultaneous drop in total pressure, indicating the unloading process. This was also accompanied by fast inward plasma motion (up to 100 km/s, toward the neutral sheet) and fast plasma sheet thinning while the poleward auroral expansion was in progress in the conjugate ionosphere. These perturbations were followed after 6–8 min by the rapid expansion of the already heated plasma sheet. While in the adjacent lobe during this thinning phase, probe P1 continued to observe intense flux transfer toward the sheet center plane. The inner probe observed intense dipolarization and inward plasma injection but with a smaller flux transfer and starting 1–2 min after the perturbations at P1, supporting the conclusion that onset instability took place tailward of 12 Re. We also demonstrate the global MHD simulations to show that a nontrivial combination of dipolarization and the plasma sheet thinning may be observed simultaneously in the outmost part of the dipolelike region during a sudden increase of the reconnection rate at the nearby active X line, staying a few Re from the observation point. These observations provide constraints for the choice of substorm onset mechanism and indicate near-Earth magnetic reconnection as the most probable source process.

AB - We analyzed the measurements made by two Time History of Events and Macroscale Interactions during Substorms (THEMIS) probes in ideal observational conditions (quiet background, near midnight, inside the substorm current wedge) during two distinct isolated substorm onsets, with probe P2 measuring the inner plasma sheet at ∼8 Re and P1 near the plasma sheet–lobe interface at 11–12 Re. The earliest onset-related strong perturbations were observed by P1; they include the increase of both B z (dipolarization) and E y (a few mV/m) as well as the simultaneous drop in total pressure, indicating the unloading process. This was also accompanied by fast inward plasma motion (up to 100 km/s, toward the neutral sheet) and fast plasma sheet thinning while the poleward auroral expansion was in progress in the conjugate ionosphere. These perturbations were followed after 6–8 min by the rapid expansion of the already heated plasma sheet. While in the adjacent lobe during this thinning phase, probe P1 continued to observe intense flux transfer toward the sheet center plane. The inner probe observed intense dipolarization and inward plasma injection but with a smaller flux transfer and starting 1–2 min after the perturbations at P1, supporting the conclusion that onset instability took place tailward of 12 Re. We also demonstrate the global MHD simulations to show that a nontrivial combination of dipolarization and the plasma sheet thinning may be observed simultaneously in the outmost part of the dipolelike region during a sudden increase of the reconnection rate at the nearby active X line, staying a few Re from the observation point. These observations provide constraints for the choice of substorm onset mechanism and indicate near-Earth magnetic reconnection as the most probable source process.

KW - magnetometer DCS-publications-id

KW - art-943

KW - DCS-publications-credits

KW - iono

KW - samnet

KW - DCS-publications-personnel-id

KW - 5

U2 - 10.1029/2008JA013527

DO - 10.1029/2008JA013527

M3 - Journal article

VL - 113

SP - A00C02

JO - Journal of Geophysical Research

JF - Journal of Geophysical Research

SN - 0148-0227

ER -