This paper presents a comprehensive review of dispersive Alfven waves in space and laboratory plasmas. We start with linear properties of Alfven waves and show how the inclusion of ion gyroradius, parallel electron inertia, and finite frequency effects modify the Alfven wave properties. Detailed discussions of inertial and kinetic Alfven waves and their polarizations as well as their relations to drift Alfven waves are presented. Up to date observations of waves and field parameters deduced from the measurements by Freja, Fast, and other spacecraft are summarized. We also present laboratory measurements of dispersive Alfven waves, that are of most interest to auroral physics. Electron acceleration by Alfven waves and possible connections of dispersive Alfven waves with ionospheric-magnetospheric resonator and global field-line resonances are also reviewed. Theoretical efforts are directed on studies of Alfven resonance cones, generation of dispersive Alfven waves, as well their nonlinear interactions with the background plasma and self-interaction. Such topics as the dispersive Alfven wave ponderomotive force, density cavitation, wave modulation/filamentation, and Alfven wave self-focusing are reviewed. The nonlinear dispersive Alfven wave studies also include the formation of vortices and their dynamics as well as chaos in Alfven wave turbulence. Finally, we present a rigorous evaluation of theoretical and experimental investigations and point out applications and future perspectives of auroral Alfven wave physics.

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Small scale alfvénic structure in the aurora

A model is developed that provides a theoretical basis for previous numerical results showing a feedback instability with frequencies characteristic of Alfven travel times within the region of the large increase of Alfven speed above the ionosphere. These results have been extended to arbitrary ionospheric conductivity by developing a numerical solution of the cavity dispersion relation that involves Bessel functions of complex order and argument. It is concluded that the large contrast between the magnetospheric and ionospheric Alfven speed leads to the formation of resonant cavity modes with frequencies ranging from 0.1 to 1 Hz. The presence of the cavity leads to a modification of the reflection characteristics of Alfven waves with frequencies that compare to the cavity's normal modes.

Feedback instability of the ionospheric resonant cavity

The electron–cyclotron maser is a process that generates coherent radiation from plasma. In the last two decades, it has gained increasing attention as a dominant mechanism of producing high-power radiation in natural high-temperature magnetized plasmas. Originally proposed as a somewhat exotic idea and subsequently applied to include non-relativistic plasmas, the electron–cyclotron maser was considered as an alternative to turbulent though coherent wave–wave interaction which results in radio emission. However, when it was recognized that weak relativistic corrections had to be taken into account in the radiation process, the importance of the electron–cyclotron maser rose to the recognition it deserves. Here we review the theory and application of the electron–cyclotron maser to the directly accessible plasmas in our immediate terrestrial and planetary environments. In situ access to the radiating plasmas has turned out to be crucial in identifying the conditions under which the electron–cyclotron maser mechanism is working. Under extreme astrophysical conditions, radiation from plasmas may provide a major energy loss; however, for generating the powerful radiation in which the electron–cyclotron maser mechanism is capable, the plasma must be in a state where release of susceptible amounts of energy in the form of radiation is favorable. Such conditions are realized when the plasma is unable to digest the available free energy that is imposed from outside and stored in its particle distribution. The lack of dissipative processes is a common property of collisionless plasmas. When, in addition, the plasma density becomes so low that the amount of free energy per particle is large, direct emission becomes favorable. This can be expressed as negative absorption of the plasma which, like in conventional masers, leads to coherent emission even though no quantum correlations are involved. The physical basis of this formal analogy between a quantum maser and the electron–cyclotron maser is that in the electron–cyclotron maser the free-space radiation modes can be amplified directly. Several models have been proposed for such a process. The most famous one is the so-called loss-cone maser. However, as argued in this review, the loss-cone maser is rather inefficient. Available in situ measurements indicate that the loss-cone maser plays only a minor role. Instead, the main source for any strong electron–cyclotron maser is found in the presence of a magnetic-field-aligned electric potential drop which has several effects: (1) it dilutes the local plasma to such an extent that the plasma enters the regime in which the electron–cyclotron maser becomes effective; (2) it generates energetic relativistic electron beams and field-aligned currents; (3) it deforms, together with the magnetic mirror force, the electron distribution function, thereby mimicking a high energy level sufficiently far above the Maxwellian ground state of an equilibrium plasma; (4) it favors emission in the free-space RX mode in a direction roughly perpendicular to the ambient magnetic field; (5) this emission is the most intense, since it implies the coherent resonant contribution of a maximum number of electrons in the distribution function to the radiation (i.e., to the generation of negative absorption); (6) it generates a large number of electron holes via the two-stream instability, and ion holes via the current-driven ion-acoustic instability which manifest themselves as subtle fine structures moving across the radiation spectrum and being typical for the electron–cyclotron maser emission process. These fine structures can thus be taken as the ultimate identifier of the electron–cyclotron maser. The auroral kilometric radiation of Earth is taken here as the paradigm for other manifestations of intense radio emissions such as the radiation from other planets in the solar system, from exoplanets, the Sun and other astrophysical objects.

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The electron–cyclotron maser for astrophysical application

[1] Whistler mode chorus waves are receiving increased scientific attention due to their important roles in both acceleration and loss processes of radiation belt electrons. A new global survey of whistler-mode chorus waves is performed using magnetic field filter bank data from the THEMIS spacecraft with 5 probes in near-equatorial orbits. Our results confirm earlier analyses of the strong dependence of wave amplitudes on geomagnetic activity, confinement of nightside emissions to low magnetic latitudes, and extension of dayside emissions to high latitudes. An important new finding is the strong occurrence rate of chorus on the dayside at L > 7, where moderate dayside chorus is present >10% of the time and can persist even during periods of low geomagnetic activity. Citation: Li, W., R. M. Thorne, V. Angelopoulos, J. Bortnik, C. M. Cully, B. Ni, O. LeContel, A. Roux, U. Auster, and W. Magnes (2009), Global distribution of whistler-mode chorus waves observed on the THEMIS spacecraft, Geophys. Res. Lett., 36, L09104, doi:10.1029/2009GL037595.

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Global distribution of whistler‐mode chorus waves observed on the THEMIS spacecraft

Plasma interactions of exoplanets with their parent star and associated radio emissions

Dynamics Explorer 1 measurements of intense low-frequency electric and magnetic noise observed at low altitudes over the auroral zone are described. The intensity of both the electric and magnetic fields decreases rapidly with increasing frequency. Most of the energy is at frequencies below the O(+) cyclotron frequency, and some evidence is found for a cutoff or change in spectral slope near that frequency. The magnetic to electric field ratio decreases rapidly with increasing radial distance and also decreases with increasing frequency. The polarization of the electric field in a plane perpendicular to the earth's magnetic field is essentially random. The transverse electric and magnetic fields are closely correlated, with the average Poynting flux directed toward the earth. The total electromagnetic power flow associated with the noise is substantial. Two general models are discussed to interpret these observations, one based on static electric and magnetic fields imbedded in the ionosphere and the other based on Alfven waves propagating along the auroral field lines.

Correlated low‐frequency electric and magnetic noise along the auroral field lines

[1] This paper examines the field line distribution of magnetospheric electron density and mass density. The electron density distributions from IMAGE RPI active sounding are generally monotonic. The density increases with increasing MLAT slightly faster than the dependence found from the field line dependence model of Denton et al. (2002b); in general, a power law dependence ne = ne0 (LRE/R)α with α ∼ 1 appears to be appropriate within the plasmasphere, at least for geocentric radius R > 2 RE. Our comparison to RPI data included also one field line distribution at LT = 7.4, which we fit with α = 2.5, a value typical of the plasmatrough based on previous studies. We calculated the average electron density field line distribution at low MLAT using the CRRES plasma wave data and found that the density was relatively flat near the magnetic equator with no convincing evidence for an equatorial peak. Using the average values of toroidal Afven frequencies, we calculated the mass density field line distributions and found that they were roughly monotonic for LT < 6, with α = 2 appropriate for LT = 4–5 and α = 1 appropriate for LT = 5–6. At LT = 6–8, the distribution was nonmonotonic, with a local peak in mass density at the magnetic equator. Dividing the frequency data into different groups based on activity, we found that the inferred average mass density field line dependence was insensitive to geomagnetic activity at LT = 4–6 but that at LT = 6–8, the tendency for the mass density to be peaked at the magnetic equator increased with respect to larger Alfven wave amplitude and more negative Dst. The average frequency ratios at LT = 6–8 did not change if we limited the data to cases with MLT = 8–16, for which the assumed perfect conductor boundary condition was better justified. Taken together, these results imply that heavy ions are preferentially peaked at the magnetic equator for LT = 6–8, at least during more geomagnetically active periods.

Distribution of density along magnetospheric field lines

[1] Observations of the electron density ne based on measurement of the upper hybrid resonance frequency by the Polar spacecraft Plasma Wave Instrument (PWI) are available for March 1996 to September 1997, during which time the Polar orbit sampled all MLT values three times. In a previous study, we modeled the electron density dependence along field lines as ne = ne0(Rmax/R)α, where ne0 is the equatorial electron density, Rmax ≈ LRE is the maximum geocentric radius R to any point on the field line, and α = αmodel = 8.0 − 3.0 log10ne0 + 0.28(log10ne0)2 − 0.43(Rmax/RE), for all categories of plasma (plasmasphere and plasmatrough). (In the formula for αmodel, ne0 is expressed in cm−3.) Here, we illustrate the field line dependence using several example events. We show that the plasmapause is much more evident on the large radius portion of the orbit and that at R ∼ 2 RE the electron density tends to level out at large Rmax to a constant value ∼100 cm−3. We also present an example of plasmaspheric plasma extending out to at least L ∼ 9 on the dawnside during particularly calm geomagnetic conditions (as indicated by low Kp). Then we present the average equatorial profiles of ne0 versus Rmax for plasmasphere and plasmatrough. Our average plasmasphere profile is found to have values intermediate between those based on the models of Carpenter and Anderson and Sheeley et al. The plasmatrough equatorial density ne0 scales with respect to Rmax like Rmax−3.4, but in the region for which our plasmatrough data is most reliable (L ≤ 6), it is well fit by the Rmax−4.0 scaling of Sheeley et al. or the Rmax−4.5 scaling of Carpenter and Anderson. We present a simple interpretation for the field line dependence of the density. For large ne0, such as occurs in the plasmasphere, α is close to zero on average (implying that ne is roughly constant along field lines). When ne0 decreases, so does ne at R = 2 RE, but the value there does not decrease much below 100 cm−3. (It is unclear if this value is an absolute lower density limit because most often the upper hybrid resonance emission disappears at R ∼ 2 RE because fp/fce < 1, where fp is the plasma frequency and fce is the electron cyclotron frequency.) Finally, we examined the dependence of α and the density at the equator and at R ∼ 2 RE on the average 〈Kp〉 (Kp averaged with a 3-day timescale). There is no clear dependence of the average α − αmodel on 〈Kp〉 or on MLT. In the plasmasphere, ne0 decreases with respect to increasing 〈Kp〉.

/pdf/electron-density-in-the-magnetosphere-3u80k22yag.pdf

Electron density in the magnetosphere

[1] Observations of the electron density ne based on measurement of the upper hybrid resonance frequency by the Polar spacecraft Plasma Wave Instrument (PWI) are available for March, 1996 to September, 1997, during which time the Polar orbit sampled all MLT values three times. Using this data set, we assume a power law form for the electron density dependence along field lines ne = ne0 (Rmax/R)α, where ne0 is the equatorial electron density and Rmax ≈ LRE is the maximum geocentric radius R to any point on the field line, and model the statistical average of α as αmodel = 8.0 − 3.0 log10ne0 + 0.28 (log10ne0)2 − 0.43(Rmax/RE) for all categories of plasma (plasmasphere and plasmatrough), with an average error of 0.65. The data set on which this result is based is limited to 2.5RE ≤ Rmax ≤ 8.5RE, 2RE ≤ R ≤ Rmax, and 2 ≤ ne0 ≤ 1500 cm−3. There is no remaining dependence of the average α – αmodel on MLT or Kp.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2002GL015963

Field line dependence of magnetospheric electron density

Using observations of the electron density, n(sub e), based on measurement of the upper hybrid resonance frequency by the Polar spacecraft Plasma Wave Instrument, we have examined the radial density dependence along field lines in the outer plasmasphere and the near plasmatrough. Sampled L values range from 2.5 to 6.6. Our technique depends on the fact that Polar crosses particular L values at two different points with different radial distance R. In our plasmaspheric data set (n(sub e) > 100/cm3), we find that on average n(sub e) is flat along field lines from the equator up to the latitudes sampled by Polar (R approximately equal to or > 2.0). In the plasmatrough data set (n(sub e) < 100/cm-3), there is on average a mild radial dependence n(sub e) varies as R(exp -1.7).

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2000JA000068

John Menietti

Papers

Correlated low‐frequency electric and magnetic noise along the auroral field lines

Distribution of density along magnetospheric field lines

Electron density in the magnetosphere

Field line dependence of magnetospheric electron density

Latitudinal Density Dependence of Magnetic Field Lines Inferred from Polar Plasma Wave Data