Megavolt Parallel Potentials Arising from Double-Layer Streams in the Earth’s Outer Radiation Belt

Huge numbers of double layers carrying electric fields parallel to the local magnetic field line have been observed on the Van Allen probes in connection with in situ relativistic electron acceleration in the Earth’s outer radiation belt. For one case with adequate high time resolution data, 7000 double layers were observed in an interval of 1 min to produce a 230 000 V net parallel potential drop crossing the spacecraft. Lower resolution data show that this event lasted for 6 min and that more than 1 000 000 volts of net parallel potential crossed the spacecraft during this time. A double layer traverses the length of a magnetic field line in about 15 s and the orbital motion of the spacecraft perpendicular to the magnetic field was about 700 km during this 6 min interval. Thus, the instantaneous parallel potential along a single magnetic field line was the order of tens of kilovolts. Electrons on the field line might experience many such potential steps in their lifetimes to accelerate them to energies where they serve as the seed population for relativistic acceleration by coherent, large amplitude whistler mode waves. Because the double-layer speed of $3100\mathrm{km}/\mathrm{s}$ is the order of the electron acoustic speed (and not the ion acoustic speed) of a 25 eV plasma, the double layers may result from a new electron acoustic mode. Acceleration mechanisms involving double layers may also be important in planetary radiation belts such as Jupiter, Saturn, Uranus, and Neptune, in the solar corona during flares, and in astrophysical objects.

Figure 1

(a), (b), and (c) The three components of the electric field in magnetic-field-aligned coordinates in which the electric field component parallel to the magnetic field is in (c). The spiky electric field signatures containing net potentials are the double layers. (d),(e),(f), and (g) The four sphere potentials that exhibit low amplitude whistlers in spheres 1 and 2 and double-layer signatures in spheres 3 and 4.

Figure 2

Sphere potentials for four of the double layers in Figs. 1 are given in (a) and the cross correlations between measurements made on opposite sphere pairs for all of the double layers of Fig. 1 are given in (b) for three different analyses.

Figure 3

(a),(b) 1 s examples of double layers observed during 1 min of high time-resolution data. (c) The integrated electric potential over the 1 min of data. The steps in (c) indicate the potential jumps associated with individual double-layer streams.

Figure 4

The double-layer indicator that shows when double-layer events occurred during the 90 min around the event of interest.

Figure 5

(a) Five days of data during which electrons were accelerated to relativistic energies. (b) The nine times that double layers were observed, in red. (c) Perpendicular component of the magnetic field that is associated with field-aligned currents. (d) Whistler waves observed in the 1% of data collected at high time resolution. (e) Expanded view of one whistler wave packet, showing electric fields as large as $300\mathrm{mV}/\mathrm{m}$.