Electromagnetic ion cyclotron waves stimulated by modest magnetospheric compressions

Abstract

The relationship between sudden impulses and enhancements or onset of Pc 1 at high-latitude ground stations is well established. It has been proposed that increased hot proton temperature anisotropy resulting from convection during magnetospheric compression is responsible for the enhancement in Pc 1 emissions via generation of electromagnetic ion cyclotron (EMIC) waves in the dayside outer equatorial magnetosphere (e.g., Olson and Lee, 1983). We use AMPTE/CCE magnetic field and particle data to test this suggestion. Compression-induced Pc 1 events are very common in the AMPTE/CCE data set: among isolated sudden (<5 min) magnetic field increases of 10 nT or greater in the 8-16 MLT/L > 6 region, 47% are associated with EMIC wave onset at the satellite. Compressions energize particles via earthward convection, so the association of local Pc 1 onsets with compressions may be either a temporal effect or merely the result of inward transport to the spacecraft of plasma previously unstable to EMIC waves. EMIC waves occur 10-20% of the time in the outer magnetosphere, which is considerably less than the occurrence rate of Pc 1 with compressions, suggesting that convection alone does not account for compression Pc 1 onset. To test whether the plasma is more unstable to EMIC waves after compressions than before, the proton distribution (1-300 keV) measured at the peak of the compression is used together with an image dipole model to calculate the distribution that would result from adiabatic expansion back to the precompression state. Comparison of the EMIC wave growth rates from the measured compression and extrapolated expansion distributions provide an indication of the compressioninduced enhancement of EMIC wave activity. Differences in wave growth of several orders of magnitude are found, indicating significant increases in EMIC wave activity. The results suggest that enhancements in dynamic pressure pump the energetic proton distributions in the outer magnetosphere, driving EMIC waves. Waves are expected to be generated most readily close to the magnetopause, and transient pressure pulses may be associated with bursts of EMIC waves, which would be observed on the ground in association with ionospheric transient signatures.

Location of Pc 1–2 waves relative to the magnetopause

“…When the L-value is normalized to the magnetopause position L mp , the highest probabilities of Pc 1 wave occurrence are close to the magnetopause, with P wav ∼ 0.25 for L norm ≡ L/L mp = 0.8-1.0. These results are consistent with increased convective growth rate at large L (Anderson et al, 1992a), and with the greater effect of magnetosphere compression close to the magnetopause (Anderson and Hamilton, 1993). On the other hand, we only directly observe magnetic field compression for at most about 25% of the wave events.…”

“…At most, about 25% of the observed Pc1 events were associated with enhancements (of at least ∼ 15%) in the magnetic field, which may indicate compressions. Anderson and Hamilton (1993) observed that when the AMPTE/CCE data are screened to include only times of magnetospheric compressions, the Pc 1 occurrence rate was enriched from the normal value ∼ 20-25% to 63%. Our data would be consistent with those of Anderson and Hamilton if such compressions occur about 10% of the time.…”

“…Erlandson et al (1994) present an example of a compression-induced Pc 1 event associated with a magnetospheric substorm in which the Pc 1 waves turn on and off with the precise timing of the compression. Anderson and Hamilton (1993) show that the probability of observing Pc 1 waves in space is significantly enhanced during magnetospheric compressions. In their view, the magnetosphere is usually near marginal stability to EMIC waves, and magnetospheric compressions lead to increased anisotropy A and readily switch the magnetosphere to the unstable state.…”

Resonant scattering of outer zone relativistic electrons by multiband EMIC waves and resultant electron loss time scales

“…The compression of the magnetopause is suggested as another possible source of EMIC waves [e.g., Anderson and Hamilton, 1993;McCollough et al, 2012;Usanova et al, 2008]. While it is frequently thought that EMIC emissions are generated along the field line at the equatorial source region particularly in the inner magnetosphere, information about the wave normal angle of EMIC waves is very limited.…”

An increase in Pc1 wave activity prior to magnetic sudden impulses

“…A short time later, electromagnetic waves in the Pcl (0.2-0.5 Hz) frequency range are often observed in the magnetosphere [Anderson and Hamilton, 1993] Kangas et al, 1986Kangas et al, , 1998]. The excitation of Pcl emissions in the wake of Sis is explained by compressions of the magnetosphere under the action of interplanetary shocks [Olson and Lee, 1983].…”