Pc 1 waves and associated unstable distributions of magnetospheric protons observed during a solar wind pressure pulse

Abstract

[1] We present observations of Pc 1 waves ($0.6 Hz) that occurred shortly after a strong (>20 nPa) compression of Earth's magnetosphere at 1321 UT, 18 March 2002. Intense Pc 1 waves were observed at several high-latitude ground stations in Antarctica and Greenland from 1321 UT to beyond 1445 UT. Two wave bursts were recorded at the Polar satellite at 1338 and 1343-1344 UT as it passed outbound in the Southern Hemisphere at 1154 local time (SM magnetic latitude of À22°and near L = 7.5) in good magnetic conjunction with the Antarctic. The pressure increase created a significant population of protons between a few hundred eV and several keV, whose fluxes were mostly perpendicular to B. These protons seem to have replaced the quiescent stream of protons (presumably convected from the plasma sheet) that existed before this increase. There was also a nearly two-order-of-magnitude increase in the population of thermal/suprathermal (0.32-410 eV) protons. The generation of ion cyclotron waves is expected to limit the proton temperature anisotropy A, defined as T ? /T k À 1. The ion cyclotron instability driven by the observed hot ion temperature anisotropy is studied using two models, with and without the presence of cold background plasma. Peaks in the calculated instability as a function of time show excellent agreement with the times of the Polar wave bursts, which were measured a few tens of seconds after maxima in the instability calculation. The time delay is consistent with the propagation time to the spacecraft from a source nearer to the equatorial plane. The hot proton population at Polar appears to be driven back to stability by a sudden increase in very field-aligned protons having energies less than the hot perpendicular population, suggesting a different source for the two populations. These observations confirm the importance of both the energization and/or increase in population of protons transverse to B in the several keV range (possibly betatron acceleration as a result of the pressure pulse), and the presence of greatly increased fluxes of lower energy protons (100s of eV to a few keV), predominantly aligned along B, in determining whether the particle population is unstable at a given time.Citation: Arnoldy, R. L., et al. (2005), Pc 1 waves and associated unstable distributions of magnetospheric protons observed during a solar wind pressure pulse,

Crucial role of ring current H+ in electromagnetic ion cyclotron wave dispersion relation: Results from global simulations

“…This suggests that the suprathermal plasma plays an important role in destabilization of a more energetic RC and/or plasma sheet H + , and the high energy anisotropic RC and/or plasma sheet proton distributions appear to be a necessary but not sufficient condition for occurrence of EMIC waves. Similarly, studying Pc 1 -2 events in the dayside outer magnetosphere, Engebretson et al [2002] and Arnoldy et al [2005] found that greatly increased fluxes of low energy protons were crucial for destabilization of the higher energy anisotropic RC protons. The analysis by Engebretson et al [2007] suggests that the ion cyclotron wave growth rate could be significantly increased by the addition of cool/ suprathermal H + consistent with their observations.…”

“…(Note that the most intense burst of Pc 1 waves studied by Arnoldy et al [2005] had a local normalized frequency X = 0.2, so the waves were on He + -branch.) For the purpose of comparison with our previous results, in the present study we kept the ion percentage the same as before, i.e., 77% + -mode is well beyond the scope of the current study, and it should be done separately.…”

“…This suggests that a suprathermal plasma plays an important role in the destabilization of the more energetic RC and/or plasma sheet ions, and high energy anisotropic RC and/or plasma sheet protons appear to be a necessary but not sufficient condition for the occurrence of EMIC waves. Similarly, studying Pc 1 -2 events in the dayside outer magnetosphere, Engebretson et al [2002] and Arnoldy et al [2005] found that greatly increased fluxes of low energy protons are crucial for the instability driven by anisotropic RC H + . These observations provide strong evidence that both the cold plasmaspheric plasma and the suprathermal (]1 keV) ions injected from the plasma sheet control EMIC wave excitation.…”

The Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS) on RBSP

“…EMIC waves can cause rapid scattering and loss for ring current ions (Jordanova et al 2001b) and relativistic electrons above 0.5 MeV (Thorne and Kennel 1971;Lyons and Thorne 1973;Albert 2003;Summers and Thorne 2003;Meredith et al 2003b). Favored regions for EMIC excitation include the overlap between the ring current and the plasmasphere (Pickett et al 2010), dayside drainage plumes (Morley et al 2009), and the outer dayside magnetosphere in association with solar wind pressure fluctuations (Arnoldy et al 2005;Usanova et al 2008;McCollough et al 2009). Theoretical global modeling of EMIC wave excitation has confirmed the plasmapause and plume as favored regions of cyclotron resonant instability (Jordanova et al 2007;Chen et al 2010a) and demonstrated that the wave excitation can also be enhanced by density fluctuations within a plume (Chen et al 2009a).…”

Excitation of EMIC waves detected by the Van Allen Probes on 28 April 2013

“…The hot H + distribution, with a temperature anisotropy sufficiently high to satisfy the EMIC instability, is called the wave free-energy provider [e.g., Cornwall, 1965;Rauch and Roux, 1982]. Besides contributing to the increase of the EMIC wave growth rate, energetic H + are able to greatly modify the real part of the wave dispersion relation, dramatically changing the wave generation [e.g., Arnoldy et al, 2005;Engebretson et al, 2007;Gamayunov and Khazanov, 2008].…”