Microburst Scale Size Derived From Multiple Bounces of a Microburst Simultaneously Observed With the FIREBIRD‐II CubeSats

Abstract

We present the observation of a spatially large microburst with multiple bounces made simultaneously by the Focused Investigation of Relativistic Electron Bursts: Intensity, Range, and Dynamics II (FIREBIRD‐II) CubeSats on 2 February 2015. This is the first observation of a microburst with a subsequent decay made by two coorbiting but spatially separated spacecraft. From these unique measurements, we place estimates on the lower bounds of the spatial scales as well as quantify the electron bounce periods. The microburst's lower bound latitudinal scale size was 29 ± 1 km and the longitudinal scale size was 51 ± 1 km in low Earth orbit. We mapped these scale sizes to the magnetic equator and found that the radial and azimuthal scale sizes were at least 500 ± 10 km and 530 ± 10 km, respectively. These lower bound equatorial scale sizes are similar to whistler mode chorus wave source scale sizes, which supports the hypothesis that microbursts are a product of electron scattering by chorus waves. Lastly, we estimated the bounce periods for 200‐ to 800‐keV electrons and found good agreement with four common magnetic field models.

Statistical Analysis of Transverse Size of Lower Band Chorus Waves Using Simultaneous Multisatellite Observations

“…Moreover, chorus waves are one of the most important loss mechanisms of plasma sheet electrons via pitch angle scattering (e.g., Horne et al, ). The scattered electrons into the upper atmosphere could then generate microbursts (e.g., Breneman et al, ; Nakamura et al, ; Shumko et al, ), pulsating aurora (e.g., W. Li et al, ; Nishimura et al, ; Nishimura, Bortnik, Li, Thorne, Lyons, et al, ; Nishimura, Bortnik, Li, Thorne, Chen, et al, ; Ozaki et al, , ), and diffuse aurora (e.g., Ni et al, ; Thorne et al, ).…”

“…At a higher L shell, that is, L ~ 11, Agapitov et al () showed a case with a larger scale size of chorus element, around 3,000 km, using Time History of Events and Macroscale Interactions during Substorms (THEMIS) observations. Moreover, the scale size of chorus waves is estimated by mapping the size of microburst and pulsating aurora onto the equatorial plane (e.g., Breneman et al, ; Nishimura, Bortnik, Li, Thorne, Lyons, et al, ; Nishimura, Bortnik, Li, Thorne, Chen, et al, ; Ozaki et al, , ; Shumko et al, ). On the nightside at L ~ 8, the latitudinal (longitudinal) size of pulsating aurora is found to be a few tens (hundreds) kilometers, which is roughly 3,000–7,000 km when mapping onto the equatorial plane (Nishimura, Bortnik, Li, Thorne, Chen, et al, ).…”

“…Using a similar method, Ozaki et al () estimated the scale size of chorus wave to be smaller, ~900 km at L ~ 5. Shumko et al () showed the scale size of microburst to be ~50 and 30 km in the latitudinal and longitudinal directions, respectively (from two points FIREBIRD CubeSat measurements of bouncing microburst at L = 4.7), and the corresponding chorus spatial extent at the geomagnetic equator is estimated to be 500–550 km. More recently, Agapitov et al () statistically analyzed the scale size of chorus waves by utilizing chorus wave amplitudes from the THEMIS filter bank data (FBK) data set.…”

“…Li et al, 2007;Li, Mourenas, et al, 2014;Li, Thorne, et al, 2014;Ma et al, 2018;Meredith et al, 2003;Shen et al, 2017;Thorne et al, 2013;Tu et al, 2014;Turner et al, 2019;Xiao et al, 2014). Moreover, chorus waves are one of the most important loss mechanisms of plasma sheet electrons via pitch angle scattering (e.g., Horne et al, 2003 could then generate microbursts (e.g., Breneman et al, 2017;Nakamura et al, 2000;Shumko et al, 2018), pulsating aurora (e.g., W. Li et al, 2012;Nishimura et al, 2010;Nishimura, Bortnik, Li, Thorne, Chen, et al, 2011;Ozaki et al, 2018Ozaki et al, , 2019, and diffuse aurora (e.g., Ni et al, 2008;Thorne et al, 2010).…”

The FIREBIRD-II CubeSat mission: Focused investigations of relativistic electron burst intensity, range, and dynamics

“…This implies that the spatial size of those microbursts must have been at least 11 km and represents the first direct measurement of a microburst's scale size. Shumko et al 25 identified a bouncing packet of microburst electrons observed by both FIREBIRD units and calculated a minimum latitudinal scale size of 30 km based on satellite position and a longitudinal scale size of 50 km using drift time analysis. FIREBIRD has observed thousands of microbursts at a variety of geomagnetic conditions, allowing an in depth look at the spectral properties of microbursts, which is currently underway.…”