Energetic neutral atom response to solar wind dynamic pressure enhancements

Abstract

[1] We have investigated the response of the ring current to solar wind dynamic pressure (P dyn ) enhancement impacts on the magnetosphere by using energetic neutral atom (ENA) images obtained by the High-Energy Neutral Atom (HENA) imager on board the IMAGE spacecraft. In this work we present several events by distinguishing between pressure events where only the pure compression effects exist (without substorm effects) and those where the substorm is triggered by the P dyn enhancement, and we find notable differences between these two types of events. First, for the pure compression events, we present four events where the P dyn increases by $100% to $450% under northward interplanetary magnetic field (IMF) conditions. The P dyn enhancement results in weak-to-modest ENA emission increases in both the hydrogen and oxygen channels: The total ENA emission rate increases by $25% to $40% relative to averages over the 1-hour period prior to the P dyn enhancement impact. This ENA enhancement is due to ions adiabatically energized by the compression. The increased ENA emission rate drops as P dyn decreases, implying that the ENA responses are directly caused by the adiabatic compression and decompression process. Also, the pure compression events lead to overall global, quasi-simultaneous increases of ENA in contrast to pure substorm-induced ENA enhancements that are initially localized in the nightside region followed by the drift effect. Next we present two events where the P dyn enhancement was $100% and >300%, respectively, under strongly southward IMF conditions, IMF B z À10 nT, and a substorm was triggered. For both events the hydrogen and oxygen ENA emission increase is far more significant than that for the pure compression events, more than 100% even when the low-altitude contribution was excluded. This is primarily owing to energetic ions generated by the triggered substorm, although the compression effect itself occurs as well. Also, the ENA enhancement appears to be not as global in magnetic local time (MLT) as for the pure compression events, because of the predominant substorm effect that starts near midnight and spreads in MLT.Citation: Lee, D.-Y., S. Ohtani, P. C. Brandt, and L. R. Lyons (2007), Energetic neutral atom response to solar wind dynamic pressure enhancements,

Global‐scale observations of ionospheric convection variation in response to sudden increases in the solar wind dynamic pressure

“…This is analogous to an incompressible flow that would increase when passing through a smaller cross‐sectional area. Enhancements in energetic neutral atom (ENA) emissions after pressure pulses have been explained through this compression effect [ Lee et al , 2007] and were particularly clear for SI events. Advanced numerical models of the system also now exist to document more precisely how the currents react to the sudden pressure changes and, later on in SSC cases, to a southward IMF orientation [e.g., Sitnov et al , 2010].…”

Persistent global proton aurora caused by high solar wind dynamic pressure

“…They claim that this behavior is due to adiabatic energization, with a reinforcing effect from the inward motion of the charged particles to areas where the density of neutral atoms is higher. One of the events studied by Lee et al [2007] was the 18 August 2001 event. The flux of <10 keV energetic neutral hydrogen in this event showed a very similar development as the global average proton aurora intensity (Figure 1c).…”

“…So far we have looked only at events when the IMF was mostly northward. Many previous studies show that changes in solar wind dynamic pressure have much more dramatic consequences when it is combined with southward IMF [e.g., Meurant et al , 2004; Lee and Lyons , 2004; Boudouridis et al , 2004; Lee et al , 2007], and we want to see if this holds also for the proton aurora intensity.…”

Recurrent energization of plasma in the midnight-to-dawn quadrant of Saturn's magnetosphere, and its relationship to auroral UV and radio emissions

“…The profile of the intensity of ENA emission is similar, again consistent with a compression of the magnetosphere-as the lobe pressure rises by a factor of 2, the particles in the ring current are compressed and adiabatically accelerated, increasing their intensity at a given energy by a similar factor of about 2 such that the ring current particle pressure balances the increased lobe pressure. Such an effect is commonly seen at Earth as well (e.g., Lee et al, 2007). Up to this point, there is little evidence for more than a passive response by the magnetosphere to the increase in solar wind pressure.…”