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:
The pinkish region is the outer Van Allen radiation belt. In the context, 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:
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 è 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 -when we convert some of the plasma waves surrounding the Earth into sound (acoustic) waves. It's been found that when such conversion occurs the space around Earth sounds like a "jungle" with different species of particles all emitting distinct "calls". These are illustrated in the graph below, with the 'calls' in the form of plasma waves of different frequency (w) - some left (L) circularly polarized, others right (R) circularly polarized:
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.
The source of the higher density plasmasphere hiss remained largely unknown until work in the 1980s by LC. Lee at the Geophysical Institute at the University of Alaska- Fairbanks. Lee started with the presumption that increased solar wind dynamic pressure can compress the magnetosphere, potentially discarding energetic electrons and leading to a decrease in both the amplitude and occurrence rate of a hiss.
·
Whistler
· Lower-Hybrid
·
Ion-Acoustic
· Non-MHD waves
In the magnetosheath.
Together, high-resolution satellite data has revealed that the plasmaspheric hiss, certainly in the higher density magnetosheath, often contains a complex fine structure of coherent rising and falling tones. These include H or chorus waves that originate in the lower density magnetosphere outside the plasmasphere, then leak into the high-density plasmasphere. There they devolve into incoherence, propagating multiple times across the equator finally developing into the incoherent band of hiss.(Roughly 200 Hz to 2 kHz).
Lee also found the propagation of hiss is guided and sometimes blocked by magnetic field variations. For instance, magnetic dips (localized reductions in the magnetic field) can reflect hiss waves, causing them to suddenly disappear from certain regions. Lee traced the whistlers to a species of wave for which the high speed components of the wave packet travel faster than its low frequency components. Thus the hypothetical observers at some distance from the source (say a lighting strike) would detect a whistle starting at high frequencies and descending to low ones.
The graph below is instructive in separating out the whistlers from other waves, e.g electron cyclotron:
The latter were found to be associated with the electron-cyclotron instability, where unstable distributions of energetic (substorm-injected) electrons transfer energy to the waves. These could account for localized, low frequency hiss (below 100 Hz), generated locally within the plasmasphere.
From the graph we see the electron-cyclotron wave has a portion for which Vg = d w/ dk (group velocity) increases as w decreases. This is the whistler wave. Lee traced the whistlers to a species of wave for which the high speed components of the wave packet travel faster than its low frequency components. Thus the hypothetical observers at some distance from the source (say a lighting strike) would detect a whistle starting at high frequencies and descending to low ones. Note the critical slope in particular, defined by: c = w/ k.
Meanwhile, the R-waves of which the whistler forms the lower edge are called right circularly polarized. they have a higher phase speed than the L-wave. We now know if a plane wave is incident on a plasma along the B-field its two normal components R and L travel at different speeds and the plane of polarization of the wave rotates as it travels. (Circular polarization).
Lastly, I note that Lee found that a largely perpendicular shock leads to a more pronounced pressure anisotropy with larger perpendicular pressure. The pressure anisotropy can be further enhanced as plasma travels from the bow shock toward the magnetopause because field aligned flow cools the parallel pressure component This has particular importance for the presence of mirror waves in the magnetosheath.
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