Storm time dynamics of ring current protons and the implications for the long-term energy budget in the inner magnetosphere: Van Allen Probes observations

The ring current plays a key role in the global electrodynamics of the magnetosphere. Pressure gradients associated with ring current particles can shield the near-Earth region from solar wind induced electric fields. Also, the distortion of the Earth's magnetic field in the inner magnetosphere due to the ring current formation, affects the transport and loss of particles contributing both to ring current and radiation belts. Therefore, understanding the long-term evolution of the ring current energy content is essential. The Van Allen Probes provide continuous monitoring of the region inside geosynchronous orbit, with unprecedented high-energy resolution and wide-energy-range measurements of proton intensities, thus allowing us to revisit the proton dynamics inside geosynchronous orbit in the context of the long-term variability of the energy budget in the inner magnetosphere. We present here a quantitative assessment of the energy budget in the inner magnetosphere, its variations, and the dominant energy contributors during storm time and non-storm time periods. Our investigation of the long-term ring current proton pressure evolution in Earth's inner magnetosphere shows drastically different behavior of the low- and high- energy components of the ring current proton population with respect to the SYM-H index variation, an index that has traditionally been used to define geomagnetic storm intervals. We found that while the low-energy component of the protons (<80 keV) is strongly governed by convective timescales and is very well correlated with the absolute value of SYM-H index, the high-energy component (>100 keV) varies on much longer timescales and shows either no correlation or anti-correlation with the absolute value of SYM-H index. Our study also shows that the contributions of the low- and high- energy protons to the inner magnetosphere energy content are comparable. Thus, our results conclusively demonstrate that proton dynamics, and as a result the energy budget in the inner magnetosphere, do not vary strictly on storm time timescales as those are defined by the SYM-H index.