Abstract
AbstractWe present observations that provide the strongest evidence yet that discrete whistler mode chorus packets cause relativistic electron microbursts. On 20 January 2016 near 1944 UT the low Earth orbiting CubeSat Focused Investigations of Relativistic Electron Bursts: Intensity, Range, and Dynamics (FIREBIRD II) observed energetic microbursts (near L = 5.6 and MLT = 10.5) from its lower limit of 220 keV, to 1 MeV. In the outer radiation belt and magnetically conjugate, Van Allen Probe A observed rising‐tone, lower band chorus waves with durations and cadences similar to the microbursts. No other waves were observed. This is the first time that chorus and microbursts have been simultaneously observed with a separation smaller than a chorus packet. A majority of the microbursts do not have the energy dispersion expected for trapped electrons bouncing between mirror points. This confirms that the electrons are rapidly (nonlinearly) scattered into the loss cone by a coherent interaction with the large amplitude (up to ∼900 pT) chorus. Comparison of observed time‐averaged microburst flux and estimated total electron drift shell content at L = 5.6 indicate that microbursts may represent a significant source of energetic electron loss in the outer radiation belt.
Highlights
Microbursts (Anderson & Milton, 1964) are localized, subsecond electron precipitation events thought to constitute a major loss mechanism in the radiation belts during storm main and early recovery phases (Lorentzen, Blake, et al, 2001; Lorentzen, Looper, et al, 2001; Millan and Thorne, 2007; O’Brien et al, 2004; Thorne et al, 2005; Tsurutani et al, 2013)
Using Van Allen Probes observations around the time of the conjunction, we find that the MLT extent plausibly ranges from ∼1 h, the size of the local region containing chorus with similar properties (Figure 2), to 5 h, the extent over which chorus is observed throughout the dayside outer belt
Simultaneous Van Allen Probes chorus and FIREBIRD microburst observations on a magnetic flux tube with equatorial diameter less than the size of a chorus wave packet show that microbursts are directly linked to chorus
Summary
Microbursts (Anderson & Milton, 1964) are localized, subsecond electron precipitation events thought to constitute a major loss mechanism in the radiation belts during storm main and early recovery phases (Lorentzen, Blake, et al, 2001; Lorentzen, Looper, et al, 2001; Millan and Thorne, 2007; O’Brien et al, 2004; Thorne et al, 2005; Tsurutani et al, 2013). Their impulsive nature implies that they were abruptly scattered by violation of the first adiabatic invariant. Our observations establish a direct relationship between chorus and electron microbursts from subrelativistic (∼200 keV) to relativistic (∼1 MeV) energies
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