Empirical Studies Related to Open Questions Regarding Geomagnetic Storms

Earth’s magnetosphere is subject to disturbances, as evidenced by variations of the geomagnetic field in space and on the ground. It is generally understood that most such disturbances are controlled by variations in the solar wind, with interplanetary magnetic field orientations directed southward opposite to Earth’s dipole magnetic axis being most conducive to energy transfers into the magnetosphere, thus resulting in more disturbed intervals. However, the exact functional form for solar wind driving of the magnetosphere has been widely studied, with proposed functional forms varying from the simple half-wave electric rectifier to expressions with a much more complicated dependence upon solar wind parameters. We provide evidence that past empirical results favoring more complicated expressions can statistically emerge from simpler expressions when solar wind parameters are time averaged and that expressions found in past empirical studies can be at least partly explained by the use of time-averaged solar wind parameters having hourly timescales, leading to the pitfall of assigning profound physical meaning to a statistical accident. Suggestions are offered to avoid this pitfall in future investigations. The strongest and most expansive disturbances in the magnetospheric system are magnetic storms. The signature of a geomagnetic storm is the reduction in the strength of Earth’s magnetic field at low latitudes. The conventional explanation for this storm-time geomagnetic depression is a ring shaped current system in the near Earth magnetosphere. In recent years, this conventional view has been called into question by researchers who argue that much of the depression is caused by currents in the more distant region called the magnetotail. Many researchers in the field continue to accept the conventional view. The relative contributions of the current systems are still debated. We construct impulse response functions (IRFs) for storm-time depression to shed light on this controversy. We show that the reduced driving of the geomagnetic index SYM/H (used to measure storm magnitude) during intervals of low density solar wind is due to energy diversion to the ionosphere via burstier events called substorms. As substorm energy is derived from the magnetotail, this reduced driving of storms when substorms are enhanced implies that tail currents are significant to storm-time indices. We also note that the storm-time magnetic depression IRF has a second development several (2-7) hours after the solar wind transfers energy to the magnetosphere, which is more prominent when energy is diverted from the tail to the ionosphere. The IRF of that part