Relativistic Electron Microbursts: Properties and Possible Plasma Wave Drivers

Abstract

Relativistic (>1 MeV) electron microbursts (<1 s duration) are particle precipitation events, thought to be significant contributors to radiation belt losses. Although these events are known to be intense, the exact properties of this intensity are unknown. There are two different plasma waves (whistler mode chorus and Electromagnetic Ion Cyclotron (EMIC) waves) capable of causing relativistic microbursts. However, the dominant scattering mechanism has yet to be experimentally verified. The O’Brien et al. [2003] and Blum et al. [2015] algorithms are used to identify relativistic microbursts in the Solar Anomalous Magnetospheric Particle Explorer (SAMPEX) satellite >1.05 MeV electron fluxes. A total of 193,694 relativistic microbursts were detected from 23 August 1996 to 11 August 2007, spanning nearly a full solar cycle. Statistically processing this large database of relativistic microbursts confirms that their occurrence is largely confined from L = 3 – 8. Their peak occurrence of 0.012 microbursts/s is located at L = 5, while their peak flux magnitude of 1,554 (MeV cm^2 sr s)^(-1) occurs at L = 4.5. Relativistic microbursts are more frequent in the morning Magnetic Local Time (MLT) region, from 0 – 13 MLT. A secondary MLT peak occurs in the premidnight MLT region, from 20 – 24 MLT, with occurrence frequencies 33% lower than the morning MLT region. There is little variation in the average flux magnitude over MLT. Overall, we observe orders of magnitude variation in the relativistic microburst occurrence frequencies while we observe only factors of 2–3 variation in the relativistic microburst durations and flux magnitudes. A comparison with trapped flux levels indicates microburst flux magnitudes are not limited by the trapped flux population. However, more microbursts occur when the trapped population is enhancing, suggesting relativistic microburst occurrence may be linked to the acceleration processes causing the increasing trapped fluxes. Lower band equatorial chorus waves in the 21 – 15 MLT (morning side) region and EMIC waves in the 08 – 12 and 18 – 24 MLT regions overlap with both enhanced microburst occurrence frequency and flux magnitude. Off equatorial chorus waves have reduced overlap with microburst activity. Case studies between SAMPEX and Halley present the first experimental evidence of the proposed EMIC scattering mechanism, while also providing evidence of chorus wave driven microburst activity. A superposed epoch analysis of the Halley VLF instrument shows an increase in the 2 kHz wave amplitude associated with relativistic microburst clusters, identified as chorus wave activity. A superposed epoch analysis of the Halley magnetometer shows an increase in the 0.1 – 0.8 Hz wave power associated with relativistic microburst clusters, identified as increased broadband noise and not EMIC wave activity. We suggest the dominant relativistic microburst scattering mechanism is individual lower band equatorial chorus wave elements generated by strong geomagnetic substorm activity. Furthermore, we suggest relativistic microburst scattering is occurring alongside acceleration processes resulting in increasing trapped fluxes. The evidence presented in the case studies does not allow us to rule out EMIC waves as a secondary, and possibly rare, driver of relativistic microbursts.