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Global-scale coherence modulation of radiation-belt electron loss from plasmaspheric hiss


Over 40 years ago it was suggested that electron loss in the region of the radiation belts that overlaps with the region of high plasma density called the plasmasphere, within four to five Earth radii1,2, arises largely from interaction with an electromagnetic plasma wave called plasmaspheric hiss3,4,5. This interaction strongly influences the evolution of the radiation belts during a geomagnetic storm, and over the course of many hours to days helps to return the radiation-belt structure to its ‘quiet’ pre-storm configuration. Observations have shown that the long-term electron-loss rate is consistent with this theory but the temporal and spatial dynamics of the loss process remain to be directly verified. Here we report simultaneous measurements of structured radiation-belt electron losses and the hiss phenomenon that causes the losses. Losses were observed in the form of bremsstrahlung X-rays generated by hiss-scattered electrons colliding with the Earth's atmosphere after removal from the radiation belts. Our results show that changes of up to an order of magnitude in the dynamics of electron loss arising from hiss occur on timescales as short as one to twenty minutes, in association with modulations in plasma density and magnetic field. Furthermore, these loss dynamics are coherent with hiss dynamics on spatial scales comparable to the size of the plasmasphere. This nearly global-scale coherence was not predicted and may affect the short-term evolution of the radiation belts during active times.

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Figure 1: Comparison of satellite and balloon data showing large-scale coherence.
Figure 2: The extent over which the hiss source region is modulated.
Figure 3: Comparison of plasmaspheric hiss and electron precipitation.


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Special thanks to the BARREL operations team B. Anderson, N. Lavers, K. Yando, D. Milling, D. Smith, A. Liang, G. Bowers and R. Friedel, and the CARISMA ground magnetometer team K. Murphy, I. R. Mann, D. K. Milling and others. Additional thanks to W. S. Kurth, A. Y. Ukhorskiy, K. Kersten and J. W. Bonnell.

Author information




This Letter was written by A.W.B., who performed most of the data processing and analysis. A.H. calculated diffusion coefficients and interpreted BARREL data. Additional assistance with analysis and interpretation was provided by R.M., J.S., M.McC. and L.W. (for BARREL work), J.F. (for MagEIS work), G.H. and C.A.K. (for EMFISIS work), J.R.W., C.A.C. and D.M. (for the Electric Fields and Waves instrument work) and J.G. (who performed plasmapause simulations).

Corresponding author

Correspondence to A. W. Breneman.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 The detection of loss cone electrons by a balloon.

The Van Allen probes (labelled here A and B) pass through the hiss source region at the magnetic equator (3RE–6RE, 40,000 km altitude) on field lines that can connect to the BARREL balloons. The red hatched line shows values of 2RE, 4RE and 6RE. The shaded green volume shows the balloon field of view and the white lines represent electrons propagating along and gyrating about magnetic field lines. At an altitude of 70 km, where bremsstrahlung X-rays are typically created, the cross-section of this field of view is a circle about 100 km in radius. Mapped along magnetic field lines to the magnetic equator, this becomes a circle of radius 0.5RE (about 3,200 km).

Extended Data Figure 2 Comparison of predicted resonance energies to observed energies.

a, Calculation of first-order cyclotron resonance energies (6 January 2014, 20:00–22:00 ut) from in situ data on PA versus L. The three lines are resonant energies determined from the minimum (red), peak (black) and maximum hiss (orange) frequencies. b, MagEIS electron flux levels on PA versus L. The horizontal extent of each shaded box shows the L crossed by the balloons. The vertical extent is a mapping of the range of resonant energies from a across the observed energies from MagEIS.

Extended Data Figure 3 Analysis suggesting that hiss is directly responsible for observed electron loss

Coherence and power spectra from PA and BI of fluctuating quantity pairs /X-rays, hiss amplitude/X-rays, magnetic field/X-rays, density/X-rays, and 54-keV electron flux/X-rays. The left plot in each row shows detrended quantity pairs while the right plot is the respective power spectral comparison for fluctuation periods from 1–20 min. To provide comparisons between spectra with different units, values are presented in decibels relative to the power of each curve at the 16.7-min period.

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Breneman, A., Halford, A., Millan, R. et al. Global-scale coherence modulation of radiation-belt electron loss from plasmaspheric hiss. Nature 523, 193–195 (2015).

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