Sawtooth events in the Earth?s magnetosphere are global, large-amplitude oscillations of energetic plasma particle fluxes at geosynchronous orbit. Sawtooth events are one of the primary magnetospheric modes during extreme solar wind driving periods cause large disturbances in the magnetosphere and ionosphere. It is thought that sawtooth events are periodic intense magnetospheric substorms. There are still a number of questions concerning sawtooth events that remain to be answered: (1) Under what magnetospheric condition can a solar wind change trigger substorm onset? (2) How does the magnetospheric convection near sawtooth onset vary with the solar wind? (3) What are the loading and unloading rates of the magnetotail magnetic flux during sawtooth oscillations? (4)? Can the effective scale length of the dayside magnetopause reconnection line be derived? (5) What determines the period of sawtooth oscillations? (6) Can the evolution of the magnetotail magnetic flux be calculated simply from the solar wind input? This project will use statistical analyses, as well as case studies, to examine these questions.
The project will use a new method to determine the effective scale length of the dayside magnetopause reconnection line during sawtooth events and to determine how this scale length varies with the solar wind. The solar wind parameters will be used as input to calculate the evolution of the magnetotail/polar cap magnetic flux during sawtooth events.
The new methods for determining the scale length of the reconnection line, and the evolution of magnetic flux in the magnetotail will provide guidance for similar studies of isolated substorms. The empirical formulas of the magnetospheric parameters have the potential to be useful in forecasting and nowcasting of space weather events.
Sawtooth events (STEs) are nearly periodic down-and-up structures that appear in energetic proton fluxes-versus-time plots. They were discovered in the mid-1990s by Los Alamos National Laboratory (LANL) satellites flying in geosynchronous orbits. On compressed time scales they seem to occur simultaneously at all local times. On more expanded times scale it is apparent that they reach local noon about ten minutes after injections near midnight. Prior studies show that they occur in some, but not all magnetic storm. The objectives of the present study were to understand the physical mechanism responsible for creating STEs during some magnetic storms and inhibiting their development in others. Our first study concentrated on a magnetic storm that produced a 4-tooth sequence. We noticed that at the beginning of each event the Sym-H index measured by ground magnetometers behaved in a strange way. Sym-H was designed to measure effects of the the westward ring current that is generated during storms in the inner magnetosphere by energetic protons circling the Earth. During STE injections of ring-current protons we expected Sym-H to become more negative. Following each injection just the opposite behavior was seen. We realized that while the inner magnetosphere was growing the ring current, another current across the magnetotail that contributes to Sym-H was being lost. We found two independent ways that successfully tested this conjecture. The answer was the same: large amounts of open magnetic flux builds up to a critical level over 2 to 3 hours. On reaching the critical level a near-Earth neutral line develops in the magnetotail. Over a several minute period large quantities of open flux convert into closed magnetic field lines that carry ring current ions into the inner magnetosphere and interplanetary flux that returns cross tail current particle into the solar wind. The second major study focused on magnetic superstorms in which no STE signatures are found. After verifying that this was indeed the case we turned to previously published studies of the magnetosphere’s state during very large disturbances. They showed that under these circumstances the ring current inflates the inner magnetosphere to nearly twice its normal size near local midnight. The high particle pressure acts to stabilize the magnetotail against the formation of the near-Earth neutral lines and thus the rapid unloading cycles that mark storms with STEs. We believe that the impact of the reported research will positively affect space weather modeling capabilities. The behavior of the Sym-H index provides a readily available remote sensing tool for determining when and if large quantities of open flux is being removed from the magnetotail as happens during STEs. While magnetospheric inflation by the ring current has long been known, it affects for stabilizing the magnetotail against was not appreciated.
