A survey of electron cyclotron waves in the magnetosphere and the diffuse auroral electron precipitation

Abstract

Natural electron cyclotron harmonic emissions in the outer magnetosphere are often cited as the electron scattering mechanism which results in the diffuse auroral precipitation. A survey is presented of the characteristics of these waves using data from both the SCATHA and Active Magnetospheric Particle Tracer Explorers (AMPTE) IRM plasma wave instruments. The emissions were observed most often in the 0300–0600 LT sector at L ∼ 4–8 and magnetic latitudes in the range ±10°. In this region, emissions exceeding 35 µV m−1 were detected only 25% of the time, and those exceeding 12 µV m−1 were detected 60% of the time. In agreement with Belmont et al. (1983), these amounts are grossly insufficient to account for the diffuse auroral electron precipitation by quasi‐linear pitch angle diffusion.

Observations of correlated broadband electrostatic noise and electron cyclotron emissions in the plasma sheet

“…If this peak is deleted from the spectrum, the remaining broadband emission has a total integrated amplitude of 0.1 mV/m. The ECH feature itself has an amplitude of 0.15 mV/m, a center frequency of 1.25 fce, and a relative bandwidth of 18%, consistent with previously observed ECH wave emissions [Roeder and Koons, 1989].…”

“…These "n+1/2," or electron cyclotron harmonic (ECH) emissions, have been detected in the near-earth plasma sheet [Roeder and Koons, 1989] and in the more distant tail regions [Gurnett et al, 1976]. Thes emissions have been attributed to instabilities involving a positive perpendicular gradient in the hot electron velocity distribution, such as a loss cone [AshourAbdalla and Kennel, 1978].…”

Transport and loss of the inner plasma sheet electrons: THEMIS observations

“…In the 1970s, the calculations of pitch angle scattering rate [Lyons,1974] based on the initial OGO-5 wave observations [Kennel et al, 1970] indicated that the amplitudes of ECH waves are large enough to scatter the electrons with energies from a few hundred eV to several keV, suggesting that the ECH waves are responsible for the loss of the plasma sheet electrons. However, in the 1980s, statistical surveys of occurrence rate of ECH waves indicated that the amplitude of ECH waves rarely have the sufficient power to scatter the plasma sheet electrons [e.g., Belmont et al, 1983;Roeder and Koons, 1989]. The new calculation of pitch angle diffusion done by Horne and Thorne [2000] suggested that the ECH waves can scatter the plasma sheet electrons when the waves lie the favored frequency ranges.…”

“…Thus, the wave-particle interactions are an important loss mechanism of the plasma sheet electrons. Both electron cyclotron harmonic (ECH) waves and the whistler mode chorus resonate with the plasma sheet electrons [Horne et al, 2003a], and the main contributor of the diffuse aurora (i.e., sweeper of the plasma sheet electrons) has been discussed by many researchers, but it is still a subject of controversy in magnetospheric physics [e.g., Belmont et al, 1983;Roeder and Koons, 1989;Horne and Thorne, 2000;Ni et al, 2008].…”

Statistical studies of fast and slow Z-mode plasma waves in and beyond the equatorial plasmasphere based on long-term Akebono observations

“…3) is unique one compared with other plasma waves observed in the inner magnetosphere, such as the electromagnetic ion cyclotron (EMIC) waves (Anderson et al, 1992), chorus (Tsurutani and Smith, 1977), ESCH (Roeder and Koons, 1989), and kilometric continuum (Hashimoto et al, 1999). It is well known that occurrence regions of these waves tend to have clear local time dependence.…”