book review

Nonlinear wave and plasma structures in the auroral and subauroral geospace
by Evgeny V. Mishin and Anatoly V. Streltsov
Elsevier, 2022, 621 pages

Reviewed by T. Nelson

This is the only book on aurora physics that not only describes auroras, but is also filled with color images of auroras and subauroral phenomena like subauroral red arcs and Strong Thermal Emission Velocity Enhance­ment or STEVE, which is an aurora-like phenom­enon that caused quite a stir in the space weather community a few years ago.

It's not for the casual reader: the authors' English is good but they're oriented more toward giving formulas than to explaining things. I'd recommend reading Schunk and Nagy (Ionospheres: Physics, plasma physics, and chemistry) for background on the plasma physics and M.H. Rees (Physics and Chemistry of the Upper Atmosphere) for the chemistry before starting. These books may be older, but they're more clearly written and the science hasn't changed much since 1990.

There's no getting around it: you're either interested in plasma physics, in which case you'll read all three; or you're not. Luckily, the sections on plasma physics are nicely separated from the sections on auroras, almost as if they were written by different authors, so you could skip over the math-heavy sections and read them later if you really wanted to.

There are three types of particles in auroras: electrons, neutral atoms, and ions. Even though they may be in direct thermal contact with each other, they can still have different temperatures. The electrons are the hottest due to their higher mobility, and they are the source of energy for most auroras.

In Chapter 2, you get a full course in space plasma physics that you'll need to under­stand why. It will teach you about Alfvén waves, which are waves of plasma propagating along a magnetic field; magnetosonic waves, which is plasma propagating perpendicular to a magnetic field; and Langmuir waves, which are oscillations in plasma density caused by the motion of electrons. You'll also get formulas on Landau resonance, which is nonlinear trapping of electrons that causes damping due to gain or loss of energy if they're in resonance with the wave.

Then you get physics of plasma instabil­it­ies. There are many kinds of instabilities, the most interesting type being Rayleigh-Taylor instability (RTI), which is when a heavy fluid is placed on top of a light fluid. This is familiar to most people. Although surface tension can keep the liquids stable for a while, ripples at the interface always make the heavy liquid penetrate down and the light liquid penetrate up. In space RTI happens when a nonuniform plasma is supported against gravity by a magnetic field. If the field is curved, the gradient-curvature drift plays the role of gravity.

Chapter 3 covers the magnetosphere, mesoscale plasma flows, Alfvén layers, auroral streamers, plasma bubbles, auroral substorms, different types of aurora (diffuse aurora, pulsating diffuse aurora, discrete auroral arcs, Alfvénic aurora), and black aurora. There are also artificial auroras produced by HAARP, a high-power HF transmitter in Alaska which is the subject of many weird conspiracy theories.

Since the other books cover ordinary auroras with greater clarity (though in less detail), I'll focus on what's unique in this book.

Black auroras

Black auroras are the most interesting, but because they're black and invisible no one cares about them. They're probably one of the least photographed objects in the night sky.

Black auroral arcs are narrow dark strips in an aurora during the late recovery phase of a storm. They're different from ordinary dark areas in an aurora in that their boundary is very sharp. The authors say the black part is caused by suppression of high energy electrons >2 keV, which means the electrons are precip­itat­ing from between the plasmasheet boundary layer (PSBL, the source of discrete aurora) and the central plasmasheet (CPS, the source of diffuse aurora). Some black auroras are next to an anti-black aurora, which is a region of brighter-than-normal aurora.

Subauroral arcs and red arcs

The subauroral ionosphere (discovered in 1974) is the region just equatorward of the auroral boundary. It maps to magnetic field lines in the magnetosphere, including the ring current, the outer radiation belt, and the plasmapause.

It contains stable red arcs or SAR arcs, which Schunk and Nagy call SARARCS. These are mid-latitude horizon-to-horizon red struc­tures that form equatorward during an aurora. They're produced by heating (about 4000 K) instead of excitation from electrons.

The subauroral ionosphere also contains a newly discovered phenomenon called STEVE. STEVE is a narrow, east-west purple/white ribbon that's sometimes accompanied by a bunch of small green auroral arcs lined up like planks on a picket fence, called Picket Fence arcs. STEVE is produced by a strong sub-auroral ion drift or SAID that creates a deep density trough and high temperature (6000–10,000 K). According to the authors, the bright narrow arc is caused by electrons at the inner edge of the electron plasmasheet precipitating toward the Earth. Energetic protons can also precipitate from the turbulent boundary layer and from the subauroral plasmasphere to create a proton aurora.

Although STEVE and SAR arcs have similar shapes and both depend on high temperatures, they differ markedly in their spectra. That means the physics of light emission is different. SAR arcs emit in the red at 630.0 nanometers (nm) from O(¹D), while a STEVE emits in a continuum from 400 to 800 nm (blue to deep red). The green “Picket Fence” arcs that accompany a STEVE emit mostly at 557.7 nm (green) from atomic oxygen. The Picket Fence differs from a regular aurora in that there's no 427.8 nm blue emission from N+2.

The authors say the reason STEVE is so much hotter than SAR is that inelastic collisions shut down thermal excitation, so the remaining source of excitation is the suprathermal population. They say the S in STEVE should stand for ‘suprathermal’, not ‘strong’. One of the advantages of being a big cheese like Mishin is that you can enforce name changes like this.

The chemistry is also different. Airglow and SAR arcs are produced by chemi­lumines­cence and radiative attachment (which is similar to radiative recombination): NO + O →NO*2 → NO₂ + hν (where hν is a photon and ‘*’ means excited state) and O + e⁻ → O⁻ + hν. By contrast, a STEVE is produced by dinitrogen in a vibra­tion­ally highly excited state reacting with neutral oxygen atoms. That's the theory. There are other theories, but this one best explains how a STEVE can emit in such a broad spectrum all the way down to 400 nm. The spectrum isn't Planck-shaped, which seems to rule out blackbody radiation.

VLF Whistlers

Auroras don't just make beautiful visual displays. Geomagnetic storms also interfere with radio communi­ca­tion and even make acoustic noise that humans can hear. They can also make whistlers.

Whistlers are radio waves in the magneto­sphere. They're called whistlers because, just like whistlers caused by lightning, as they travel long distances their frequency decreases due to dispersion. In the magnetosphere they're trapped in field density inhomogeneities called ducts and they interact with energetic electrons via cyclotron resonance.

What is a duct? The authors don't say, but other sources say a duct is a plasma channel that can be a high-density region or a low-density region of plasma. The authors say ducts don't act as waveguides as you might think, but act by refracting the EM wave if the duct is comparable to the wavelength. Whistlers are an important cause of scattering of hazardous energetic particles in radiation belts, and they're potentially valuable because if we could generate them, we could use them in spacecraft and satellites to reduce the number of energetic particles in their path, making space travel safer, like a deflector dish in Star Trek.

One challenge is that these VLF radio waves have long wavelengths. A 10 kHz EM wave has a wavelength of 29 km, so ducts are quite big and a transmitter on a satellite would have to be very powerful to create one.

One of the goals of HAARP is to understand whistlers by creating field-aligned irregularities (i.e. ducts) in the ionosphere by heating it using powerful HF ground transmitters and by transmitting whistler-mode VLF signals. So far, success seems to be limited. The government wanted to shut HAARP down but eventu­ally transferred it to the University of Alaska.

The other two books don't mention whistlers, HAARP, or STEVE, so this book stands alone. If it weren't so unfriendly to read, it might become the standard text in the field.

aug 01, 2025