Generation of short‐burst radiation through Alfvénic acceleration of auroral electrons

Abstract

[1] A comparison of modeled simulation results with in situ observations is presented in an effort to better understand the generation mechanism of short-burst radiation (AKR) observed in the Alfvénic acceleration region. The simulation is performed using a linear gyrofluid Alfvén model to generate propagating inertial Alfvén waves with a test particle scheme to study the behavior of electrons under the influence of waves. The observations are taken from the wave and plasma instruments on board the Fast Auroral Snapshot (FAST) satellite. We find that single-and multiple-electron shell distributions are produced as a result of parallel electric fields generated from inertial Alfvén waves. Electrons outside the loss cone are reflected back up the flux tube because of the mirror force forming electron conics. Distributions obtained from this simulation resemble the observed electron distributions in the Alfvénic auroral acceleration region where short-burst radiations were present. Multiple-shell distributions as well as electron conics are unstable, making them candidates for triggering the electron-cyclotron maser instability. Hence we suggest that these unstable distributions, resulting from the effects of inertial Alfvén wave, could be the source of the observed short-burst radiation in the terrestrial auroral zone magnetosphere-ionosphere coupling region. We also briefly discuss the application of this general mechanism to explain the Io-associated S-bursts observed emanating from the Jovian magnetosphere.

Electromagnetic interaction between Jupiter's ionosphere and the Io plasma torus

“…As stated in the maser theory, unstable electron distributions are the source of remotely observed auroral radiation with a condition that the plasma frequency is much less than the electron cyclotron frequency (ω pe /ω ce << 1), however, in-situ observations are currently unavailable near the ionosphere of the Jupiter-Io flux tube to confirm the Alfvén-driven maser hypothesis. Su et al (2007; supported the Alfvén-driven maser theory as the generation mechanism of S-bursts on the basis of Earth-based satellite measurements.…”

“…The Alfvén wave producing the ionospheric resonator is also responsible for electron acceleration (Su et al 2007). Unstable electron distributions, such as shell, ring, or conics, are generated by parallel electric fields associated with this inertial Alfvén wave (Su et al 2007;Hess et al 2007;.…”

“…Unstable electron distributions, such as shell, ring, or conics, are generated by parallel electric fields associated with this inertial Alfvén wave (Su et al 2007;Hess et al 2007;. As stated in the maser theory, unstable electron distributions are the source of remotely observed auroral radiation with a condition that the plasma frequency is much less than the electron cyclotron frequency (ω pe /ω ce << 1), however, in-situ observations are currently unavailable near the ionosphere of the Jupiter-Io flux tube to confirm the Alfvén-driven maser hypothesis.…”

“…Radiations excited near the local electron cyclotron frequency are amplified through a gyro-resonant interaction from a perpendicular-driven instability (∂f /∂v ⊥ > 0, where f is the electron phase-space distribution function). In the 1990s, an important modification was made to the original maser Su et al 2007) theory that strong auroral radiations are driven by a shell (or horseshoe) distribution in the auroral density cavity due to quasi-static parallel electric fields (Ergun et al 2000b;, because the loss-cone maser does not provide sufficient energy to power AKRs. The electron distributions responsible for the maser instability are illustrated in Figure 5.…”

Short‐burst auroral radiations in Alfvénic acceleration regions: FAST observations

“…The negative‐sloped stripes have a frequency extent of about 4 kHz and a typical time duration of less than 2 s. The upper limit on the frequency bandwidth of an individual stripe is at Δ f / f ∼ 4 × 10 −3 . Recently, Ergun et al [2006] and Su et al [2007] reported an example of possible Earth‐based short bursts detected by the FAST satellite at the polar cap boundary during a substorm expansion. These short‐burst emissions at Earth are analogous to S‐bursts remotely observed from Jupiter.…”