Journal of Geophysical Research: Space Physics
Comparison of Relativistic Microburst Activity Seen
by SAMPEX With Ground-Based Wave
Measurements at Halley, Antarctica
, Craig J. Rodger
, Mark A. Clilverd
, Aaron T. Hendry
Mark J. Engebretson
, and Marc R. Lessard
Department of Physics, University of Otago, Dunedin, New Zealand,
British Antarctic Survey, NERC, Cambridge, UK,
Department of Physics, Augsburg University, Minneapolis, MN, USA,
Department of Physics, University of New Hampshire,
Durham, NH, USA
Relativistic electron microbursts are a known radiation belt particle precipitation phenomenon;
however, experimental evidence of their drivers in space have just begun to be observed. Recent modeling
eﬀorts have shown that two diﬀerent wave modes (whistler mode chorus waves and electromagnetic
ion cyclotron (EMIC) waves) are capable of causing relativistic microbursts. We use the very low
frequency/extremely low frequency Logger Experiment and search coil magnetometer at Halley, Antarctica,
to investigate the ground-based wave activity at the time of the relativistic microbursts observed by the
Solar Anomalous Magnetospheric Particle Explorer. We present three case studies of relativistic microburst
events, which have one or both of the wave modes present in ground-based observations at Halley.
To extend and solidify our case study results, we conduct superposed epoch analyses of the wave activity
present at the time of the relativistic microburst events. Increased very low frequency wave amplitude is
present at the time of the relativistic microburst events, identiﬁed as whistler mode chorus wave activity.
However, there is also an increase in Pc1–Pc2 wave power at the time of the relativistic microburst events,
but it is identiﬁed as broadband noise and not structured EMIC emissions. We conclude that whistler mode
chorus waves are, most likely, the primary drivers of relativistic microbursts. However, case studies conﬁrm
the potential of EMIC waves as an occasional driver of relativistic microbursts.
Relativistic electron microbursts are small-timescale (
1 s) intense precipitation events of
1 MeV electrons
from the outer radiation belt into the atmosphere (Blake et al., 1996), typically observed in morning magnetic
local times (MLT) (Blum et al., 2015; Johnston & Anderson, 2010; Nakamura et al., 2000; O’Brien et al., 2003;
Thorne et al., 2005). It is believed relativistic electron microbursts are signiﬁcant contributors to radiation belt
losses, with the suggestion that a single storm containing relativistic microbursts could empty the entire outer
radiation belt relativistic electron population (Clilverd et al., 2006; Dietrich et al., 2010; Lorentzen, Looper, &
Blake, 2001). The net ﬂux in the radiation belts is delicate balance between loss and energization (Reeves
et al., 2003); therefore, we require better understanding of the conditions under which relativistic microbursts
occur, and moreover, the physical processes in space driving this type of precipitation.
It is well known that lower-energy electron microbursts (energy of tens to hundreds of keV) are a result of
wave particle interactions with whistler mode chorus waves (Fennell et al., 2014; Lorentzen, Blake, et al., 2001).
For some time it has been suggested that relativistic microbursts are also a result of pitch angle scattering of
radiation belt electrons by whistler mode chorus waves. However, there is little direct experimental evidence
in the existing literature to demonstrate this. There are a number of experimental studies published in support
of the chorus wave driver of relativistic microbursts. These are primarily based on the overlap in L and MLT of
large-scale regions of relativistic microburst occurrence and whistler mode chorus wave occurrence or power
(e.g., Anderson et al., 1977; Johnston & Anderson, 2010; Kersten et al., 2011; Kurita et al., 2016; Lorentzen, Blake,
et al., 2001; Nakamura et al., 2000). A recent study by Breneman et al. (2017) shows the ﬁrst direct evidence
of simultaneous observations of relativistic microbursts and whistler mode chorus waves during a single case
study. Modeling eﬀorts show that rising tone elements of whistler mode chorus waves propagating away from
• Case studies of relativistic microbursts
with EMIC and/or chorus waves
occurring are presented
• Statistically, there is an increase in
VLF wave amplitude at the time
of relativistic microbursts, consistent
• Statistically, there is no increase in
EMIC activity at the time of relativistic
Douma, E., Rodger, C. J., Clilverd, M. A.,
Hendry, A. T., Engebretson, M. J.,
& Lessard, M. R. (2018). Comparison
of relativistic microburst activity seen
by SAMPEX with ground-based wave
measurements at Halley, Antarctica.
Journal of Geophysical Research:
Space Physics, 123, 1279–1294.
Received 6 SEP 2017
Accepted 31 JAN 2018
Accepted article online 5 FEB 2018
Published online 22 FEB 2018
©2018. American Geophysical Union.
All Rights Reserved.
DOUMA ET AL.