The "Hissing" In The Plasmasphere: What's the Source?

A question that's recently come to the fore is: What is the source of the hiss detected in the plasmasphere?  Recall the plasmasphere is the toroidal region  around the Earth, replete with low energy plasma that we already saw when considering the source of magnetosonic waves, i.e. the green donut-shaped region shown below:

Then the pinkish region is the outer Van Allen radiation belt. In the context, we saw all plasma waves are distinguished by their phase velocities, viz. w / k  where w  is the plasma frequency and k the wave number vector. In the case of the standard magnetosonic wave we have:

w² / k ²    =   c ²  (v _A ²   +  v _s ² ) /  (v _A ²   +  c ² )

Where v _A  is the Alfven velocity, v _s  is the ion sound speed and c the velocity of light. In the limit of low magnetic fields, for which  v _A  -> 0  the wave becomes an ordinary ion acoustic wave.  Basically the waves under review get their energy by interacting with protons trapped in Earth's magnetic field spiraling around magnetic field lines.

Of particular interest in plasma physics research is what occurs - or what do we get - when we convert some of the plasma waves surrounding the Earth into sound waves. Well, it's been found that when these waves are converted to sound - somewhat analogous to a radio playing FM broadcasts- the space around Earth sounds like a "jungle" with different species of particles all emitting distinct "calls".

One of the mysterious sounds isolated resembles what can best be called a plasmaspheric hiss: an ever present sibilance in the inner regions of Earth's magnetic field. To be more specific, it sounds like pure static spanning 100 Hz to several kilohertz. This is a frequency range roughly equivalent to that produced by the middle third of a piano. We already know, for reference, that this hiss plays a crucial role in shaping the structure of the Earth's radiation belts, in particular disrupting them by knocking their energetic particles out into the atmosphere.

Despite knowing what the waves do, the source of the hiss is still unknown. One theory is that it arises from spiraling electrons high over the Earth's equator. Another proposes that it consists of the remnants of distant whistlers or chorus waves that devolve into incoherence. The graph below is useful to separate out the whistlers from other waves, e.g electron cyclotron:

The R-waves of which the whistler forms the lower edge are called right circularly polarized.  The electron cyclotron waves are an example at higher plasma frequencies (given by the ordinate). From the graph we can see that V_g = d w/ dk (group velocity) decreases as the plasma frequency  w increases.

This is called the whistler wave because the high frequency components of the wave packet travel faster than its low frequency components. As a prosaic example, an observer some distance away from a lightning strike will then hear a whistle (for the associated sound wave) starting at high frequencies and descending to lower ones.  Note the critical slope in particular, defined by: c = w/ k.

The most interesting aspect is that plasma physicists previously assumed this hiss was just random white noise with no coherent features. Now, however,  Summers et al (Journal of Geophysical Research: Space Physics, 2014) have analyzed NASA satellite measurements of the hiss from 2013 and fond something quite different.

That is, after breaking the signal down into its spectrum of frequencies they discovered barely detectable rising and falling tones similar to those generated by whistlers, but at frequencies rising to roughly middle C and falling for about two octaves. The authors conjecture this is made possible by the high resolution of the instruments on the satellites: NASA's Van Allen probes and their particularly useful orbit - which keeps them mostly within Earth's radiation belts.

A cautionary note: Though the waves with the plasmaspheric hiss resemble whistler tones (and I lead toward that association) and share similarities in mathematical wave theory, the physical mechanism that generates the hiss is still wide open for debate.

Summers et al hope that their  recent findings will generate renewed interest in the subject of plasma waves and the plasmaspheric hiss and perhaps drive vigorous inquiry to pin down the source.

Rising Tone Magnetosonic Waves Detected By THEMIS Spacecraft

A previously unknown type of magnetosonic wave was detected by the THEMIS spacecraft as it traveled from the Earth's plasmasphere (green central 'blob') to the outer radiation belt (pink).

Let's accept most people would not know a magnetosonic wave from a magnetometer, or a plasma wave from any other. Despite that the use of plasma waves in actually studying near Earth plasmas has increased in importance given major solar events - such as large flares - can adversely impact the ionosphere as well as atmosphere of Earth.  Among the effects we can include the Ottawa power grid going down in 1989, and also recurrent short wave and other band blackouts - including affecting GPS and satellite TV broadcasts. All the underlying plasma phenomena come under the heading of 'space weather'.

Space weather data are assembled from across a wide spectrum as may be expected when we are trying to ascertain the effects of the Sun and solar wind on our Earth.  One of the more important diagnostics are plasma waves in the near Earth space environment.  Now, the THEMIS ('Time History of Events and Macroscale Interactions During Substorms') spacecraft has evidently detected a new kind of magnetosonic plasma wave which may play an important role in space weather forecasts.

For reference, the THEMIS spacecraft orbits in the magnetosphere near Earth's magnetic equator and collects data from magnetic storms especially near the boundary of the magnetosphere on the dayside as well as from Earth's radiation belts.

Now, two events recorded in June and August, 2010, appear to confirm the existence of a special type of magnetosonic (MS) wave, best described as "rising tone".  All plasma waves are distinguished by their phase velocities, viz. w / k  where w  is the plasma frequency and k the wave number vector. In the case of the standard magnetosonic wave we have:

w² / k ²    =   c ²  (v _A ²   +  v _s ² ) /  (v _A ²   +  c ² )

Where v _A  is the Alfven velocity, v _s  is the ion sound speed and c the velocity of light. In the limit of low magnetic fields, for which  v _A  -> 0  the wave becomes an ordinary ion acoustic wave.  Basically the waves under review get their energy by interacting with protons trapped in Earth's magnetic field spiraling around magnetic field lines.

Historically, the frequencies of MS waves were believed to be temporally continuous, or essentially like the smoothly varying notes from a trombone player - going from one note to the next.  This indicated a simple, linear relation between MS waves and protons.

Now, Fu et al, writing in Geophysical Research Letters(2014) have incorporated the 2010 THEMIS detections to interject a possible complication. That is, a sharp rising tone in their spectrogram (see graphic) indicated a more complex, nonlinear series of interactions between MS waves and protons. This would be more analogous to a flute player performing a series of runs and trills.

To be sure, space and plasma physicists have observed rising tone phenomena in other plasma waves including electromagnetic ion cyclotron waves. In these waves it is the web of forces between  the particles which create currents that enhance the wave's frequency. However, this nonlinear behavior bas never before been observed in MS waves.

It remains to be seen just how useful this new form will be in space weather forecasts but for many of us in the space sciences we look forward to productive research and more refined forecasts