High-Altitude CUSP: The Extremely Dynamic Region in Geospace

Abstract

The high-altitude dayside cusps (both northern and southern) are extremely dynamic regions in geospace. Large diamagnetic cavities with significant fluctuations of the local magnetic field strength have been observed there. These cusp diamagnetic cavities are always there day after day and are as large as 6 R E . Associated with these cavities are charged particles with energies from 20 keV up to 10 MeV. The intensities of the cusp energetic ions have been observed to increase by as much as four orders of the magnitude when compared with regions adjacent to the cusp which includes the magnetosheath. Their seed populations are a mixture of ionospheric and solar wind particles. The measured energetic ion fluxes in the high-altitude cusp are higher than that in both the regions upstream and downstream from the bow shock. Turbulent electric fields with an amplitude of about 10 mV/m are also present in the cusp, and a cusp resonant acceleration mechanism is suggested. The observations indicate that the dayside high-altitude cusp is a key region for transferring the solar wind mass, momentum, and energy into the Earth's magnetosphere.

First terrestrial soft X-ray auroral observation by the Chandra X-ray Observatory

“…The polar cusps are extremely dynamic regions that play a key role in the transfer of mass, momentum, and energy from the solar wind into the magnetosphere and upper atmosphere systems. It has been argued that cusp is an important source region of energetic particles (Fritz, 2001;Chen and Fritz, 2005).…”

Association of cusp energetic ions with geomagnetic storms and substorms

“…Energetic particles within the cusp were observed by the CAMMICE MICS instrument . CEPs were identified from the definition of Chen and Fritz (2005): a MICS > 40 keV ion count rate exceeding the instrument background by 3σ . Applying these criteria to the six years when MICS was in operation (April 1996 through March 2002), we found 1126 passes through the cusp, 915 of which had energetic particle signatures.…”

“…At geosynchronous orbit (6.6R E ), a 40 keV proton (the minimum energy for the CEP definition of Chen and Fritz, 2005) has a drift period of approximately 2.8 h; a 200 keV ion (near the upper end of the MICS range), 0.56 h. Reeves et al (1991) presented measurements of drift periods for electrons at geosynchronous orbit based on drift echos from substorm injections. These periods ranged from nearly 4 h at 37 keV to 0.6 h at 245 keV.…”

Transport of transient solar wind particles in Earth’s cusps

“…We need to evaluate how these particles are related to the energetic particles observed in high-altitude cusps. 9,25 Plans are underway to combine data analysis and modeling to enhance our understanding of the physics of solar wind interaction with the magnetosphere.…”

“…7 Cluster also has found energetic particles up to several hundred keV in the cusps, corroborating earlier observations. 8,9 During disturbed solar wind, even energetic Ͼ20 keV O + ions are detected. 10 Moreover, intense ion fluxes detected at the expected regions of mid-altitude cusps have strengthened the idea that magnetosheath particles are entering the high-altitude cusps.…”