A Review of Cluster Wideband Data Multi‐Spacecraft Observations of Auroral Kilometric Radiation

Abstract

We review important advances in the understanding of auroral kilometric radiation (AKR) resulting from observations by the Wideband Data instruments on the four Cluster spacecraft. AKR is an intense radio emission originating in the Earth's auroral regions with frequencies typically in the range 50–700 kHz, usually observed from space. It is now widely accepted that AKR is generated by the cyclotron maser instability (CMI) in density cavities in the auroral acceleration region. Multi‐point observations by the Cluster spacecraft with a time delay of arrival technique have allowed the source locations of many individual AKR bursts to be determined. The position uncertainty is around 500 km at the source region or about 200 km when mapped on to the auroral zone. AKR is emitted in a narrow beam close to the tangent to the magnetic field vector in the source region. This has important implications for the possible generation mechanisms, being incompatible with filled or hollow cone beaming models. It also implies that an observer at a given location can only see AKR from a fraction of possibly active source regions. The complex frequency time structure of AKR sometimes shows regular striations or pulsations. Cluster observations of these phenomena have been interpreted as modulation of the CMI by disturbances propagating through the generation region. Exceptionally, AKR can sometimes be observed from low altitude spacecraft or even on the ground. Recent work has involved simultaneous observations of AKR on the Cluster spacecraft and on the ground at the South Pole.

Electron Cyclotron Maser Emission and the Brightest Solar Radio Bursts

“…Then an important question is whether ECM from solar flares is relevant for such systems where the ECM is not necessarily associated with flares. AKR exhibits a lot of fine structure, usually with pronounced frequency drift (e.g., Benson et al 1988;Yearby & Pickett 2022;Taubenschuss et al 2023, and references therein) that is attributed to motion of the source region up or down magnetic field lines with particularly low plasma density (hence f p < Ω B ) in the auroral regions of the Earth's magnetosphere. Electric fields play a major role in both accelerating electrons into maser-favoring "horseshoe" velocity distributions and evacuating the field lines (Ergun et al 2000), and in turn the auroral electric fields are affected by the impact of the solar wind on the magnetosphere.…”

Auroral Kilometric Radiation -- the Electron Cyclotron Maser Paradigm

“…1 some other satellites occasionally reported observations of AKR from remote and closer in (among them were ISIS 1 (Benson et al, 1979), POLAR (Menietti et al, 2000;Mogilevsky et al, 2008)), ISEE 3 (Benson et al, 1991;Gallagher & Gurnett, 1979;Imhof et al, 2000;Hashimoto et al, 2016;Grach et al, 2020;Green et al, 2004).) Galileo (Menietti et al, 1996), ERG (Kolpak et al, 2021), (Gurnett, 1974) Voyager 1& 2 waves/plasma (Kaiser et al, 1978) AMPTE IRM ±(15 − 18) waves/plasma (LaBelle et al, 1989) Geotail −(10 − 30) PWI/LEP (Nishida, 1994;Kokubun et al, 1994) (Mukai et al, 1994;Matsumoto et al, 1994a, b) CLUSTER > | ± 10| PWI/plasma (Mutel et al, 2003;Balogh et al, 1997) (Yearby & Pickett, 2022) STEREO waves (Panchenko et al, 2009) MMS ∼ −20 PWI/plasma (Burch et al, 2016;Torbert et al, 2016) Cassini 1.2 to −6400 PWI (Anderson et al, 2005;Lamy et al, 2010) Wind < | ± 300| waves/plasma (Bougeret et al, 1995;Lepping et al, 1995) (Wilson III et al, 2021) DE 1 < 5 PWI/plasma (Mellott et al, 1986;Hoffman et al, 1981) (Mellott et al, 1984) Hawkeye < 3 plasma (Green et al, 1977;Voots et al, 1977) Fast < 2 PWI/plasma (Carlson et al, 1998, a;Pfaff et al, 1998) Viking < 3 PWI/plasma (Hultqvist, 1990;Ungstrup et al, 1990) (Bahnsen ...…”

ECMI resonance in AKR revisited: Hyperbolic resonance, harmonics, and wave–wave interaction

“…AKR [for a recent review of observations, cf., e.g., [10]] propagates in the X-mode polarization and is radiated from the auroral zone close to Earth where the magnetic field is strong, and the thermal plasma on various scales becomes locally diluted, frequently with a very small plasma-to-cyclotron frequency ratio ω e /ω ce ≪ 1. Under such circumstances, the ECMI sets on if only its necessary and sufficient conditions are both met simultaneously.…”