Sunday, November 8, 2015

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    =   2  (A  +  s 2 ) /  (A  +  2 )


Where 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

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