TY - JOUR

T1 - On the triggering of auroral substorms by northward turnings of the interplanetary magnetic field

AU - Wild, J.A.

AU - Woodfield, E.E.

AU - Morley, S.K.

PY - 2009/9/25

Y1 - 2009/9/25

N2 - Some studies over the last decade have indicated that the instability responsible for substorm expansion phase onset may require an external trigger such as a northward turning of the interplanetary magnetic field (IMF). Statisti- cal investigations have lead to contrasting interpretations re- garding the relationship between proposed solar wind trig- gers and substorm onsets identified from geomagnetic data. We therefore present the results of a study into the possible triggering of 260 substorms between 2001–2005, exploiting data from the Cluster and IMAGE satellite missions. We find that only a small fraction (<25%) of the substorms stud- ied are associated with northward turnings of the IMF. How- ever, the majority of the observed onsets are associated with a growth phase characterised using a subset of the criteria employed to define northward-turning IMF triggers. Based upon a case-by-case investigation and the results of an anal- ysis using the statistics of point processes, we conclude that northward-turning structures in the IMF, while sometimes coinciding with the initial phase of individual substorms, are not required to trigger the magnetospheric instability associ- ated with substorm expansion phase onset.

AB - Some studies over the last decade have indicated that the instability responsible for substorm expansion phase onset may require an external trigger such as a northward turning of the interplanetary magnetic field (IMF). Statisti- cal investigations have lead to contrasting interpretations re- garding the relationship between proposed solar wind trig- gers and substorm onsets identified from geomagnetic data. We therefore present the results of a study into the possible triggering of 260 substorms between 2001–2005, exploiting data from the Cluster and IMAGE satellite missions. We find that only a small fraction (<25%) of the substorms stud- ied are associated with northward turnings of the IMF. How- ever, the majority of the observed onsets are associated with a growth phase characterised using a subset of the criteria employed to define northward-turning IMF triggers. Based upon a case-by-case investigation and the results of an anal- ysis using the statistics of point processes, we conclude that northward-turning structures in the IMF, while sometimes coinciding with the initial phase of individual substorms, are not required to trigger the magnetospheric instability associ- ated with substorm expansion phase onset.

KW - Substorms

KW - Cluster

KW - Triggering DCS-publications-id

KW - art-961

KW - DCS-publications-credits

KW - iono-fa

KW - DCS-publications-personnel-id

KW - 104

KW - 140

U2 - 10.5194/angeo-27-3559-2009

DO - 10.5194/angeo-27-3559-2009

M3 - Journal article

VL - 27

SP - 3559

EP - 3570

JO - Annales Geophysicae

JF - Annales Geophysicae

SN - 0992-7689

IS - 9

ER -