A Statistical Study of Pulsating Aurora Using THEMIS Ground Camera Images
Pulsating aurora is a frequently occurring phenomenon that often develops during the recovery phase of a substorm and results in widespread luminosity corresponding to a significant transfer of power from the magnetosphere to the ionosphere. Statistical studies have been performed to evaluate possible relationships between parameters such as patch size and pulsation period but no extensive (on the order of a year) statistics have been collected. The focus of this research is the development of a database of occurrences of pulsating aurora and the statistical analysis of these data to provide important information regarding occurrence rates vs. MLT and invariant latitude as well as the typical size of the pulsating region, which will act as a measure of the relative importance of pulsating aurora in terms of energy transfer to the ionosphere. A preliminary analysis of 8 substorm-related pulsating aurora events with good optical data from September through early December 2007 show the region of pulsating aurora occurring mostly post-midnight within the range of approximately 63- 68 degrees magnetic latitude and spanning 1-2 hours in magnetic local time.
The Relative Timing of Changes in the Inner Magnetospheric Magnetic Field and Plasma Configuration to Substorm Expansion Onset
It is generally accepted that the synchronous magnetic field becomes more tail-like during the growth phase and more dipole-like during the expansion phase of magnetospheric substorms. The cause of this transition at expansion onset is still one of the central issues of substorm research. There are several models that attempt to explain the expansion onset. The near near-earth neutral line (NENL) model suggests that reconnection is the trigger of expansion onset. In this model, the plasma flows, caused by reconnection, brake in the near earth region, and thus cause flux pile-up. However, it has been claimed that the detection of a fast flow in the magnetotail is not always followed by ground substorm activity. Furthermore, a negative Bz spike (explosive growth phase) is frequently observed in the near-earth region just before the onset. It has been suggested that this feature implies that the flux pile up may not be the only factor responsible for the substorm expansion. In this study we examine a number of substorms from the THEMIS tail passages for which we have detailed information about the time history of activity. Detailed timing relationship between plasma flows, geosynchronous local onsets, Pi 2 pulsations at different locations will be examined as well as their occurrence frequency at substorm onsets. The understanding of the occurrence frequency and relative timing can help us understand the evolution of substorms.
Tail and Ionospheric Signatures of Tail Fast Flows Associated with PBIs and with Substorms
Earthward convection of the tail plasma sheet is often organized in bursts of fast ion flows restricted in azimuthally narrow channels. It has been shown that Auroral Poleward Boundary Intensifications (PBIs) are often the ionospheric signature of such fast flow channels in the midtail. Equatorward flow bursts have been observed in the ionosphere, and have been shown to be the ionospheric mapping of the tail fast flow channels in few case studies. We focus on identifying such ionospheric signatures and understanding the physics of this magnetosphere-ionosphere interaction via conjunctions of the THEMIS probes with the Sondrestrom radar. We find fundamental differences between the tail fast flows that are associated with substorm onsets and those associated with PBIs, as well as between their respective ionospheric flow signatures. The tail fast flows that produce PBIs are observed in the midtail. They do not typically penetrate to the inner magnetosphere and they are accompanied by plasma sheet expansion signatures in the mid tail. No dipolarization signatures are observed in the inner magnetosphere. The ionospheric signatures associated with such tail flows are PBI- type aurora and substantially enhanced equatorward flows. Tail fast flows that are associated with substorm onsets are typically observed only by the inner magnetosphere probes, only occasionally being seen also in the midtail. Clear dipolarizations are seen with such flows in the inner magnetosphere but not in the midtail. The ionospheric flow associated with such tail fast flows is far distinct, enhanced westward flows being occasionally seen at the higher latitude part of the Sondrestrom field of view with enhanced eastward flows observed at the lower latitudes. Enhanced equatorward flows are not seen.
What is the difference between a Steady Magnetospheric Convection event and a Substorm?
All types of geomagnetic activity are associated with magnetospheric convection driven by the solar wind. Known types of activity include substorms, steady magnetospheric convection (SMC), poleward boundary intensifications, and sawtooth injection events. In the original model of reconnection the rates of dayside merging, transport over the polar caps, nightside reconnection, and return to the dayside were all balanced. However, typically one of these rates does not balance another. Which type of geomagnetic activity occurs depends on the nature of this imbalance. At the present time it is not known what causes a transition from one mode to another. Is it dependent upon solar wind input or is it controlled by internal magnetospheric processes? Is it possible that both solar wind and internal magnetospheric processes can influence dynamic behavior? Most previous studies utilize one or two satellites to investigate this problem. With the launch of THEMIS mission, we have multi-point measurements simultaneously in the magnetosphere and on the ground providing us with a good opportunity to examine this subject. In this study, we have identified several interesting intervals which included isolated substorms, a substorm to SMC transistion, and a SMC to substorm transition. A detailed comparison of solar wind, magnetotail, and ground observations provides us a good opportunity to examine the important processes (quantities) responsible for different modes of activities. A preliminary result suggests that the tail lobe energy may be a good indicator to determine whether a substorm or SMC event will develop. During a SMC event, the tail lobe energy is elevated but sustained at a high level for about an hour while the substorm shows a typical loading and unloading of tail energy. The auroral electrojets also show interesting behavior. While substorm shows typical substorm current wedge formation, it seems that SMC has a high latitude current that forms near the boundary of the polar cap. The ground Pi 2 also shows interesting results. During a SMC event, the high latitude Pi 2 is unorganized but the low-latitude Pi 2s exhibits periodic pulsation trains.