[1] The occurrence of triggered and nontriggered substorm are examined in light of current interest in such issues as substorm identification, IMF B y variations, and potentially undetected small-scale solar wind perturbation. Global substorms are identified using a sudden, persistent decrease in the AL index. The onset of this global expansion is taken to be the time of the Pi 2 burst nearest in time to the beginning of the AL decrease. IMF triggers were identified both subjectively through visual scanning of the data and automatically with a computer algorithm. Both northward turnings of the IMF Bz and decreases in the amplitude of the By component were considered as possible triggers. Two different solar wind monitors were used in the investigation: IMP-8 in a circular orbit with a distance 12$35 Re to the Earth-Sun line and ISEE-2 in an elliptical orbit with a distance only 5$10 Re to the Earth-Sun line. The IMP-8 results show that the triggering probability does not depend on the distance of the monitor from the Earth-Sun line in the range 12-35 Re. The ISEE dataset shows that closer than 12 Re the triggering probability is the same as it is in the IMP-8 data set. Thus there appears to be no dependence of triggering on the location of the monitor provided it is within 35 Re of the Earth. We also demonstrate that including the By component does not significantly increase the probability of substorm triggering. Approximately 60% of all substorms appear to be triggered. Of the 40% for which we could not identify a trigger, 10% occurred while the IMF was northward. The data suggest that substorm onset is a consequence of an internal magnetospheric instability that is highly sensitive to changes in magnetospheric convection induced by a sudden change in the IMF, but that these changes are not always necessary.

[1] All known types of auroral zone magnetic activity are associated with closure of open magnetic flux in the magnetotail. As closure is caused by magnetic reconnection we expect to observe fast flows during geomagnetic activity. We have scanned the ion flow data during the first pass of the THEMIS D spacecraft through the tail (December 2007 to May 2008, identifying all flows with |V ?x | > 150 km/s. These flows generally occur in a sequence of several short bursts (bursty bulk flows). Earthward flows are much more common than tailward flows and are faster than tailward flows. Earthward flows have a longer duration; tailward flows are seen alone or after an earthward flow. Both directions of flow are associated with an increase in tail B z (dipolarization). Fast flows in either direction are rarely seen inside of 9 R E . Earthward flows are strongly localized in the local time sector 2100-0100 and have a probability distribution identical to that seen in auroral substorm expansions by the IMAGE spacecraft. Tailward flows are also localized but with a peak shifted to 2330 LT. Very close to midnight the flows are slowed and reflected. At other local times they appear to be deflected around the Earth. Fast flows often follow a reduction in E s (GSM VB s ) and occur close to the time of a sudden decrease in the AL index. Generally, the first flow burst in a sequence is most closely associated with the AL onset, and its peak follows the AL onset by about 2 min. The probability of observing a fast flow at THEMIS D during steady magnetospheric convection (SMC) events is quite low compared with the probability during an interval before the SMC. Since most of the fast flows carry magnetic flux earthward and are associated with substorm onset seen in the aurora by IMAGE and in the AL index, we interpret them as evidence that magnetic reconnection has occurred in the tail. Near 30 R E in the tail plasmoid ejection has also been associated with substorm onset, so we conclude that the fast flows are created by a new X line formed outside the 11.9 apogee of THEMIS D some time earlier than they are seen at THEMIS D.During SMC it appears that fast flows due to reconnection are deflected around the Earth outside the apogee of the satellite.

Steady magnetospheric convection (SMC) events in the Earth's magnetosphere are thought to result from balancing the rate of opening flux through solar wind‐magnetosphere reconnection at the dayside magnetopause to the rate of closing flux through reconnection in the magnetotail. For this to occur, reconnected flux in the tail must return to the dayside to balance the dayside reconnection rate. Using Geotail and THEMIS data over a span of 14 years, we examine the average plasma conditions and fast Earthward flows during SMC intervals and compare them to other types of geomagnetic activity, such as quiet intervals, isolated substorm phases, and the two hours before an SMC (Pre‐SMC intervals). We show that the average total pressure in the inner magnetosphere is higher during SMC events than for other types of activity. This higher pressure region extends to larger radial distances, and causes fast Earthward flows to divert toward the dawn or dusk flanks and continue to the dayside. This pattern is contrasted to substorms, during which flows are directed toward the inner magnetosphere and flux remains there in the “pile‐up region.” We suggest that the SMC pattern of flow deflection carries enough flux from the tail to the dayside to allow for balanced reconnection. Finally, the Pre‐SMC intervals have plasma conditions that are similar to, but slightly weaker than, SMC events. Since most SMCs begin with a substorm, this indicates that preconditioning of the magnetosphere by prior geomagnetic activity is important in setting up the magnetotail for an SMC state.