Multiple spacecraft observations of energetic ions during a high solar wind pressure event

Abstract

[1] On 28 June 1999 the Wind spacecraft (near the forward libration point) observed a large solar wind pressure spike from 0445 UT to 0600 UT. The Polar satellite at 7 hours magnetic local time detected an energetic particle event in the high-altitude region associated with turbulent diamagnetic cavities from 0512 UT to 0627 UT. The particles and cavities are very similar to those that were previously found in the high-altitude dayside cusp region. They are independent of both the solar wind velocity and the interplanetary magnetic field (IMF) vectors. The enhancements of the magnetic field fluctuations in the ultra-low frequency range measured by the Wind were also observed by the Polar. Most of the time, during the event period, the IMF had a duskward component, suggesting that cusp diamagnetic cavities also existed in the postnoon sector in the Northern Hemisphere. Energetic ions of both ionospheric origin and solar wind origin were observed by the Polar spacecraft during this event period. The He ++ /H + ratio in the diamagnetic cavities was a factor of four higher than in the quasi-trapping region before the event onset, while the He + /He ++ ratio in the cavities was more than one order of the magnitude lower. In this event, the measured cusp ions had energies up to 4 MeV. No clear relationship between the cusp energetic ion flux and the IMF cone angle was found. The Interball 1 spacecraft located just upstream of the bow shock in the prenoon sector measured an upstream ion event from about 0516 UT to 0600 UT. The onset of the energetic ions observed by Interball 1 in the upstream event was the same for different energies; the ion energy spectra were independent of the solar wind velocity and their intensities were independent of the bow shock geometry and the solar wind density. The energetic ion event onset was first detected in the cusp by Polar at 0512 UT, then near the bow shock in the prenoon by Interball 1 at 0516 UT, and then in the far upstream by Wind at 0523 UT. The measured energetic ion intensity decreased with increasing distance from the cusp diamagnetic cavities. These observational facts together with the IMF directions suggest that (1) this large solar wind pressure event produced an extremely large diamagnetic cavity (>10 R E ) within the magnetosphere, (2) the bow shock was not the main source of both the cusp and upstream energetic ions, and (3) the upstream energetic ions most likely came from the cusp.

Particle acceleration at coronal mass ejection–driven interplanetary shocks and the Earth's bow shock

“…Such events are characterized by short durations (∼1–2 h), steeply falling spectra ( j ∝ E −4 ), large (>100:1) field‐aligned sunward anisotropies [ Mitchell and Roelof , 1983; Müller‐Mellin et al , 2007], and positive correlations with the solar wind speed and geomagnetic indices [e.g., Desai et al , 2000]. Despite the wealth of information available, however, it is still not clear whether these ions are accelerated at the bow shock [e.g., Lee , 1982; Trattner et al , 2003] or inside the Earth's magnetosphere [ Sheldon et al , 2003; Anagnostopoulos et al , 2005; Chen et al , 2005].…”

The spatial distribution of upstream ion events from the Earth's bow shock measured by ACE, Wind, and STEREO

“…Such upstream ion events are characterized by short durations (∼1–2 hours), steeply falling spectra ( j ∞ E −4 ), large (>100:1) field‐aligned sunward anisotropies [ Mitchell and Roelof , 1983; Müller‐Mellin et al , 2007b], and positive correlations with the solar wind speed and geomagnetic indices [e.g., Desai et al , 2000]. Despite the wealth of information available, however, it is still not clear which fraction of these ions is accelerated at the bow shock [e.g., Lee , 1982; Trattner et al , 2003] or inside the Earth's magnetosphere [ Sheldon et al , 2003; Anagnaostpoulos et al , 2005; Chen et al , 2005] and under which circumstances.…”

Evidence for particle acceleration in the magnetospheric cusp

“…Recent multiple satellite (POLAR and CLUSTER) measurements reveal that the Earth's magnetospheric cusp is a broad and dynamic region. It has a size of several R E (Earth radii) (Chen and Fritz, 2002;Chen et al, 2005a). Associated with this region are charged particles with energies from 20 keV up to 15 MeV that are potentially an additional independent source of the Correspondence to: J.…”