Abstract
Abstract. This study describes a systematic statistical comparison of isolated non-storm substorms, steady magnetospheric convection (SMC) intervals and sawtooth events. The number of events is approximately the same in each group and the data are taken from about the same years to avoid biasing by different solar cycle phase. The very same superposed epoch analysis is performed for each event group to show the characteristics of ground-based indices (AL, PCN, PC potential), particle injection at the geostationary orbit and the solar wind and IMF parameters. We show that the monthly occurrence of sawtooth events and isolated non-stormtime substorms closely follows maxima of the geomagnetic activity at (or close to) the equinoxes. The most strongly solar wind driven event type, sawtooth events, is the least efficient in coupling the solar wind energy to the auroral ionosphere, while SMC periods are associated with the highest coupling ratio (AL/EY). Furthermore, solar wind speed seems to play a key role in determining the type of activity in the magnetosphere. Slow solar wind is capable of maintaining steady convection. During fast solar wind streams the magnetosphere responds with loading–unloading cycles, represented by substorms during moderately active conditions and sawtooth events (or other storm-time activations) during geomagnetically active conditions.
Highlights
Substorms are probably the most common type of magnetic activity
The level of geomagnetic activity as measured by Kp index is rather similar for isolated substorms and steady magnetospheric convection (SMC) events (1– 3), but clearly higher for sawtooth events (4–6)
This reflects the fact that the sawtooth events take place during the magnetic storms, and during stronger magnetospheric convection, while most of the substorms and SMCs are observed during more quiet periods
Summary
Substorms are probably the most common type of magnetic activity. They are an important part of energy circulation through the magnetosphere including reconnection at the dayside magnetopause, storage of energy in the magnetotail and release of the tail energy while reconfiguring the stretched magnetotail into a more dipolar shape. The solar wind speed is typically rather low, and the magnitude of the interplanetary magnetic field (IMF) is moderate and stable To distinguish these events from the other low-activity conditions, the minimum duration of an SMC event is usually required to be 3–4 h (Sergeev et al, 1996), which is longer than the typical time (2–3 h) between recurring substorms (Borovsky et al, 1993), but comparable to the duration of an average substorm. A recent study by Pulkkinen et al (2007) presented a statistical comparison of the typical solar wind driver conditions and ionospheric activity for sawtooth events and substormlike auroral electrojet activations during geomagnetic storms They concluded that sawtooth events are not a specific type of magnetic activity, and that the 2–3 h periodicity, strong stretching of the dusk sector field, and strongly asymmetric ring current are found in association with other types of storm-time activations. We use the sawtooth events as representatives of storm-time activations, because they are generally easier to list, identify and agree on than any other type of storm-time events
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
