2016 THEMIS SCIENCE NUGGETS
Relativistic electrons produced by foreshock disturbances observed upstream of the Earth's bow shock
by Lynn B. Wilson III
NASA Goddard Space Flight Center
For over 100 years we have observed ultra high energy cosmic rays – charged particles moving near the speed of light – bombarding Earth’s atmosphere. In the late 1940s and early 1950s, Enrico Fermi imagined a mechanism that could explain some of these observations, which involved charged particles accelerating by reflecting off of moving magnetic clouds – called Fermi acceleration. In the 1960s, scientists discovered that shock waves could form in space and by the 1970s it became clear that these magnetized shocks could act as the magnetic clouds to which Fermi had referred decades earlier. However, to this day there has been a critical missing piece to this theory.
|Figure 1. Artist's depiction of electron energization within the ion foreshock region (orange area), upstream of the Earth's bowshock (red) and magnetosphere (blue). Image Credit: E. Masongsong UCLA EPSS.|
To explain the issue, imagine a tennis match between two players running towards the net at a constant speed. Assume they hit a ball back-and-forth, until one of them gets to the net, and each time the ball is hit it gains a little speed. Therefore, to gain a lot of speed the ball needs to be hit as many times as possible. If the speed gain in each hit and the speed at which the tennis players run are fixed, then the total speed gain of the ball is determined by the number of times it is hit. The only way to increase the number of hits is to start with a faster moving ball so it can cross the net more times before either player reaches the net. However, this is akin to not knowing by whom, by what, and why/how the ball is served to start the volley in the first place.
This minimum energy threshold for gaining a lot of energy is known as the "injection problem" – how charged particles are pre-energized to a high enough energy that they can bounce back-and-forth across the shock enough times to get to the ultra high energies observed. We observe evidence of Fermi acceleration directly near the Earth's bow shock and upstream of interplanetary shocks and indirectly through the electromagnetic emissions of astrophysical phenomena like supernova. Thus, we know that Fermi acceleration occurs but we have not been able to determine the "what," "where," and "why/how" particles are pre-energized to high enough energies to undergo Fermi acceleration.
Unlike shock waves in a collisional medium (e.g., Earth's atmosphere), magnetized shock waves exist in a collisionless medium and can reflect and accelerate particles producing a region in communication with the shock itself, called the foreshock. The foreshock is mostly permeated by suprathermal (i.e., particles energized beyond average energies of the incident flow) positively charged ions that can generate foreshock disturbances – transient (~5–10 per day), large-scale (i.e., tens to thousands of thermal ion Larmor radii) electromagnetic structures. We observe low energy electrons being energized up to at least 300 keV (i.e., energy increases by factors of ~1000–10000) within these foreshock disturbances using THEMIS field and particle data. Though energetic electrons have been previously observed in the foreshock, they were due to geomagnetic or solar activity. Examination of STEREO and Wind data showed that these energetic electrons are not associated with any solar activity. In addition, examination of THEMIS ground magnetometers show no geomagnetic activity. Finally, our observations show that the electrons are being energized within the foreshock to energies not even thought possible by the Earth's bow shock, let alone these much weaker foreshock disturbances.
None of the known shock acceleration models that rely on large-scale, quasi-static fields can explain the peak energies, the range of energies being enhanced, or the isotropic, single power-law electron distributions from 10s of eV to 100s of keV. Further, the probability of the electrons being energized outside of the foreshock disturbances and propagating into them roughly one part in three trillion. Thus, the energization must be occurring within the foreshock disturbances. Finally, the observations suggest that small-scale (i.e., much smaller than the foreshock disturbances) electromagnetic waves may be resposible for the energetic electron enhancements.
|Figure 2.Three example foreshock disturbances with energetic electron enhancements. All data were observed by THEMIS-C, with disturbance centers indicated by the vertical magenta lines. Magnetic fields are shown in units of nanotesla (nT) in the geocentric solar ecliptic coordinate basis. Particle data are shown in units of intensity [cm-2 s-1 sr-1 eV-1] (or flux) as omnidirectional averages in the bulk flow rest frame with uniform color schemes (legends at far right) and vertical axis ranges by row. The low and high energy electron and high energy ion data are all shown as stacked line plots of intensity versus time, where each line corresponds to a different energy. The low energy ion data are shown as a dynamic energy spectrogram of energy versus time with a color scale [right-hand side of (l)] for intensity. (a)–(c) |Bo| (nT) at 4 samples per second. (d)–(f) Bo (nT) at 4 samples per second. (g)–(i) low energy electron (i.e., ~50 eV–12 keV) intensity. (j)–(l) Low energy ion (i.e., ~10 eV–25 keV) intensity. (m)–(o) High energy electron (i.e., ~30–300 keV) intensity. (p)–(r) High energy ion (i.e., ~30–430 keV) intensity.|
We present the discovery of an entirely new phenomenon involving electron acceleration within foreshock disturbances located far upstream of the Earth’s bow shock. The resulting energetic electrons can provide a seed population for the higher order Fermi acceleration at the bow shock. Thus, our results present the discovery of the "what" and "where" of electron pre-energization, resolving two parts of the electron injection problem. Contrary to previous work that looked for pre-energization at the main shock ramp, we observe the energization occurring upstream in the foreshock. Given that foreshocks are ubiquitous upstream of collisionless astrophysical shocks, it is likely that foreshock disturbances to be ubiquitous as well. Therefore, the concept of pre-energization upstream of the main shock ramp could fundamentally change our understanding of collisionless shocks.
ReferenceWilson III, L.B., D.G. Sibeck, D.L. Turner, A. Osmane, D. Caprioli, and V. Angelopoulos "Relativistic electrons produced by foreshock disturbances observed upstream of the Earth's bow shock," Phys. Rev. Lett. Vol. 117(21), doi:10.1103/PhysRevLett.117.215101, 2016.
Lynn B. Wilson III is a civil servant plasma physicist at NASA's Goddard Space Flight Center and the Wind spacecraft project scientist. His research primarily focuses on observations of collisionless shock waves and wave-particle interaction and kinetic theory.
Please send comments/suggestions to Emmanuel Masongsong / emasongsong @ igpp.ucla.edu