Whistler waves, double layers, magnetic reconnection

[StefanR]

Internal structure and spatial dimensions of whistler wave regions in
the magnetopause boundary layer

Abstract.
We use whistler waves observed close to the magnetopause
as an instrument to investigate the internal structure
of themagnetopause-magnetosheath boundary layer. We
find that this region is characterized by tube-like structures
with dimensions less than or comparable with an ion inertial
length in the direction perpendicular to the ambient magnetic
field. The tubes are revealed as they constitute regions
where whistler waves are generated and propagate. We believe
that the region containing tube-like structures extend
several Earth radii along the magnetopause in the boundary
layer. Within the presumed wave generating regions we find
current structures moving at the whistler wave group velocity
in the same direction as the waves.

1 Introduction
The magnetopause constitutes the outer frontier defense of
the magnetosphere. The border is not completely closed but
the entry of the solar wind energy is restricted and regulated
by the so-called reconnection process (Dungey, 1961;
Paschmann, 1979). Ever since the magnetopause first was
suggested to exist (Chapman and Ferraro, 1931) and even
more since the first in situ observations where made (e.g.
Sonett et al., 1959), there has been a huge interest in revealing
its secrets. Numerous studies are devoted to describe, an-
alyze and model the structure and dynamics of this complex
boundary.

Whistler-mode waves have been observed by a number of
spacecraft in the outer parts of of the magnetosphere. Electromagnetic
waves in a broad frequency range are reported
from regions close to the magnetopause, in the outer cusp or
in association with the bowshock by, e.g., Smith et al. (1967)
(OGO-1); Olson et al. (1969) (OGO-3); Rodriguez and Gurnett
(1975) (Imp-6); Tsurutani and Smith (1977) (OGO-5);
Gurnett et al., 1979 (ISEE 1 and 2); LaBelle and Treumann
(1988) (AMPTE/IRM); Zhang et al. (1998) (Geotail); Pickett
et al. (1999) (Polar) and Maksimovic et al. (2001) (Cluster).
In this paper we examine the whistler wave-sheets reported
by Stenberg et al. (2005) in closer detail. In particular, we
determine the extension of these sheets in all three spatial dimensions
and we investigate the internal structure of the presumed
wave generating regions. We reveal that the whistler
wave source regions are characterized by current structures
moving at the whistler wave group velocity in the same direction
as the waves themselves. We estimate the size (perpendicular
to the background magnetic field) of these structures
to be about or less than 20 km, which corresponds to about 6
electron inertial lengths (c/!pe). Simultaneously, our analysis
suggests that the whistler wave emitting regions extend
several Earth radii along the magnetic field lines.

11 Summary of conclusions

The focus of this paper is whistler mode waves observed
close to the magnetopause in the boundary layer on the magnetospheric
side of the boundary. In a time-frequency spectrogram
the waves appear as stripes, narrow in time and
broader in frequency. With the help of the four-spacecraft
mission Cluster we are able to determine both the threedimensional
size of the wave emissions and their internal
structure. The main conclusions are summarized below:
– The whistler mode waves are observed in tube-like
structures in space, extending a long way along the field
lines (see below) but limited to about an ion inertial
length (or less) in the perpendicular direction.
– Some of the whistler wave elements are non-dispersive.
An investigation of the wave vector direction reveals a
sudden and distinct change of the azimuthal component
in the middle of such emission elements. We argue that
these are regions, where waves are generated. As a consequence,
plasma of the boundary layer is organized in
tubes, which favor whistler wave generation.
– Within the presumed locally generated whistler waves
we detect ion-scale current structures, that propagate
with the whistler wave group velocity in the same direction
as the waves. The amplitude of the current structures
is comparable to the whistler wave amplitude.
– Other whistler emission elements are clearly dispersive.
They manifest themselves as risers or as fallers. Using
simple plasma models we conclude that it is possible to
explain both cases as a propagation effect. This allows
us to estimate the extension of the emissions along the
ambient field. We find that the whistler waves propagate
at least 10 RE.
– The large amount of detected generation regions (identified
as non-dispersive emission elements) indicates a
high density of the tube-like structures in the plasma.
Referring to Stenberg et al. (2005), where a whistler
generation region is believed to couple directly to the reconnection
site, we speculate that the observed pattern
of tube-like regions reflects ongoing intermittent and/or
patchy reconnection.

http://www.ann-geophys.net/25/2439/2007 ... 9-2007.pdf

[StefanR]

VORTEX STRUCTURES OF WHISTLER WAVES

Starting with two-dimensional nonlinear Scroedinger equation for a parallel electric
field of spatially localized beam of whistler waves we investigate formation, evolution
and stability of nonlinear whistler waveguides (or ducts) which are frequently
observed during heating active experiments in the ionosphere.

http://www.cosis.net/abstracts/EGS02/06 ... -06468.pdf

[StefanR]

VLF Whistlers seen by Thunderstorm III

Whistlers are a type of electromagnetic wave which result from lighting strokes. When a whistler wave travels through the ionosphere, the low frequencies have a smaller group velocity than the high frequency portions of the signal, resulting in a wave which is dispersed in time.

http://spp.ece.cornell.edu/rockets/T-III/whistler.html

[StefanR]

Polar Sound Descriptions

VLF Plasma Waves

Whistlers Saucers Chorus Emissions
Auroral Hiss Auroral Kilometric Radiation (AKR)

Whistlers were first detected during World War I. They are audio frequency electromagnetic waves produced by lightning. Once produced, these waves travel along closed magnetic field lines from one hemisphere to the other in the right-hand polarized, whistler mode of propagation. The duration of the whistling tone is related to the length of the propagation path. Because of anisotropies in the index of refraction, the wave energy is confined within a cone that makes an angle of 19° 28' with respect to the local magnetic field.

On a high-resolution wideband spectrogram, the whistler's characteristic spectral feature is a clearly defined tone descending rapidly in frequency over several seconds. The name "whistler" refers to this characteristic whistling sound in the audio frequency range.

The first spectrogram is a 48-second wideband spectrogram taken from a nightside plasmaspheric pass on March 26, 1996. Initially the wideband receiver was connected to the electric Eu antenna, but was switched to the Bu magnetic search coil antenna at 07:59:06 UT. A series of brief whistlers is evident throughout this interval below 1.5 kHz.

The second spectrogram is a 48-second wideband spectrogram taken from a dayside plasmaspheric pass on May 10, 1996. The wideband receiver was connected to the magnetic loop antenna throughout this interval. Two clusters of whistlers of varying duration are seen below 8 kHz at 00:16:25 UT and 00:16:44 UT.

The third spectrogram is a 48-second wideband spectrogram taken from a nightside plasmaspheric pass on June 12, 1996. The wideband receiver was again connected to the magnetic loop antenna. Some whistlers can be seen up to 9 kHz (13:58:24 UT and 13:58:29 UT) and several more below 4 kHz (13:58:32 UT and 13:58:44 UT).

http://www-istp.gsfc.nasa.gov/istp/pola ... descs.html

[StefanR]

Magnetospheric whistler waves were first observed by Storey (1953), who interpreted them as evidence of plasma around Earth. The found region was, of course, plasmasphere, and ground-based whistler observations of have subsequently been used to monitore the plasmapause location. Whistlers have also provided an indirect way to estimate plasmaspheric electric fields (Carpenter et al., 1972). This is because the whistler ducts move radially under the influence of the azimuthal electric field.

Whistler mode waves can create auroral region lower hybrid waves (see ion outflow event). They can also produce isolated bursts of precipitating electrons in mid-latitudes (Rosenberg et al., 1971). In addition, it has been shown that they can be generated by electrical transmission lines: these are the so-called power line harmonics.

http://www.oulu.fi/~spaceweb/textbook/whistler.html

[StefanR]

New generation mechanism of the planetary-scale ULF
electromagnetic wave structures in the ionosphere

The results of theoretical investigation of the dynamics of generation and propagation
of planetary (with wavelength 103 km and more) ultra-low frequency (ULF) electromagnetic
wave structures in the dissipative ionosphere are given. The physical mechanism
of generation of the planetary electromagnetic waves is proposed. It is established,
that the global factor, acting permanently in the ionosphere – inhomogeneity
(latitude variation) of the geomagnetic field and angular velocity of the earth’s rotation
– generates the fast and slow planetary ULF electromagnetic waves. The waves
propagate along the parallels to the east as well as to the west.

It is established,
that planetary ULF electromagnetic waves, at their interaction with the local
shear winds, can self-localize in the form of nonlinear solitary vortices, moving along
the latitude circles westward as well as eastward with velocity, different from phase
velocity of corresponding linear waves. The vortices are weakly damped and longlived.
They cause the geomagnetic pulsations stronger than the linear waves by one
order. The vortex structures transfer the trapped particles of medium and also energy
and heat. That is why such nonlinear vortex structures can be the structural elements
of strong macroturbulence of the ionosphere.

http://www.cosis.net/abstracts/IAGA2005 ... -00495.pdf

[StefanR]

Electron Phase-Space Tornados: Observations in Space, Simulations, and Animations


(Phase-space image of electron tornadoes: The x-axis lies parallel to the magnetic field and the y-axis is perpendicular.)

Abstract:
Four Earth orbiting satellites have recently measured streams of isolated electric field pulses traveling along the Earth's magnetic fields in regions as diverse as the auroral ionosphere, the Earth's bow-shock, and the magnetotail. These measurements result from stable electron density depletions and contain some of the highest electric field energy densities found in space plasmas. In 1D, these depletions result from "phase-space electron holes" where rotating trapped electron vortices in (X-Vx) phase space are contained by a strong positive potential. In 2D, electron holes form tornadoes when viewed in (X-Y-Vx) phase space. This poster presents some images of simulations of phase-space electron holes and tornadoes. In addition, one may watch computer animations of electron phase-space tornados.

Conclusions
Electron phase-space tornadoes commonly occur in energetic regions of the ionosphere, magnetosphere, and near-Earth solar wind. Studying this nonlinear plasma phenomenon using massively parallel simulations has taught us that these structures are fundamentally unstable and convert their energy to a particular plasma wave, the electrostatic-whistler wave. By understanding this evolution, we hope to further our understanding of dynamics in the space plasma environment of the Earth.

http://scv.bu.edu/visualization/gallery ... index.html

[StefanR]

UCLA BPPL
Invited talk on whistlers in space and laboratory plasmas, IPELS 97

This Web page presents figures displayed at an invited talk at the 1997 IPELS meeting on Maui, Hawaii, entitled "Whistler Waves in Space and Laboratory Plasmas" given by Professor Reiner L. Stenzel in collaboration with Dr. J. M. Urrutia. The presentation was subsequently converted into a review article [Whistler waves in space and laboratory plasmas, R. L. Stenzel, J. Geophys. Res. 104, 14379-14395 (1999)].

The topic discussed in some slides overlaps material summarized in the research section of Prof. Stenzel and Dr. Urrutia's Web page. References pertinent to each figure are given in each caption.A list of the references is added at the end of the figure section.

Fig. 1. Typical spectrogram of ionospheric whistler waves observed on the ground. The received waveform (top trace) is Fourier analyzed and displayed as an intensity plot vs. frequency and real time (bottom plot). The descending frequency produces falling tones which gave rise to the name "whistlers". [Courtesy of S. P. McGreevy].

Fig. 6. Example of 3D multipoint measurements of whistler wave fields. A wave packet is excited by a current pulse in a loop antenna. In repeated experiments the vector magnetic field is recorded with a movable probe at >10,000 positions vs time. Displayed is a snapshot of the wave magnetic field as isosurface with cut showing contours of constant field strength inside the packet [from Rousculp et al, Pulsed currents carried by whistlers. V. Detailed new results of magnetic antenna excitation (1.9 MB), Phys. Plasmas 2, 4083-4093 (1995). (Link to original publication)].

Fig. 13. Whistler wave ducting and filamentation processes observed in a large laboratory plasma. Schematic sketch at top shows that antenna-launched whistlers in a uniform collisionless plasma exhibit an amplitude decay due to beam diverge. However, a field-aligned density depression guides (ducts) the whistler wave resulting in a constant amplitude with distance. The density depression is produced by the radiation pressure and thermal effects of a large amplitude whistler. Bottom traces show measured interferometer traces of linear and filamented whistlers [from R. L. Stenzel, Filamentation instability of a large amplitude whistler wave (669 kB), Phys. Fluids 19, 865-871 (1976). (Link to original publication).]

Fig. 14. Measured vector magnetic field of a whistler wave packet showing a 3D vortex topology as emphasized by the linked solenoidal and toroidal field lines [from Urrutia et al, Pulsed currents carried by whistlers. III. Magnetic fields and currents excited by an electrode (2.8 MB), Phy. Plasmas 2, 1100-1113 (1995). (Link to original publication).]

Fig. 15. Ideal spherical vortex (e.g., Hill's vortex, spheromak) to which the observed whistler pulse of Fig. 14 resembles. Whistler wave vortices exhibit right-handed linkage (positive helicity) when propagating along the dc magnetic field and left-handed linkage for the opposite direction of propagation.

Fig. 23. Characteristics of EMHD currents to a pulsed electrode in a uniform magnetoplasma. The current density lines form right-handed spirals due to the presence of both field-aligned currents and azimuthal electron Hall currents. The J-lines from the electrode penetrate a finite distance into the plasma before spiraling back to the return electrode in the back of the disk electrode. A current tube has been constructed from the measured data. It's enclosed current is conserved and the J-lines lie on its surface.

Fig. 26. Whistler wings in the perturbed magnetic field of a current loop moving across the magnetic field. The wing is constructed by Huygens principle from a superposition of delayed whistler wavelets from displaced loop positions [Urrutia et al, Magnetic dipole antennas in moving plasmas: A laboratory simulation, in "Solar System Plasmas in Space and Time," J. L. Burch and J. H. Waite, Jr., editors, Geophysical Monograph 84, AGU, Washington, DC, 129-133, 1994]. Similar structures have been observed behind magnetized asteroids in the solar wind [Kivelson et al, Magnetic signatures near Galileo's closest approach to Gaspra, Science 261, 331, 1993].

http://www.physics.ucla.edu/plasma-exp/ ... index.html

[StefanR]

Session bMoaP1 - Poster Session: Waves, Flows, Shocks, and Dusty Plasmas.

http://flux.aps.org/meetings/YR97/BAPSD ... /S300.html

[StefanR]

Cluster Finds Giant Gas Vortices At The Edge Of Earth’s Magnetic Bubble

ESA’s quartet of space-weather watchers, Cluster, has discovered vortices of ejected solar material high above the Earth. The superheated gases trapped in these structures are probably tunnelling their way into the Earth’s magnetic ‘bubble’, the magnetosphere. This discovery possibly solves a 17-year-mystery of how the magnetosphere is constantly topped up with electrified gases when it should be acting as a barrier.

This figure shows a three-dimensional cut-away view of Earth’s magnetosphere. The curly features sketched on the boundary layer are the Kelvin-Helmholtz vortices discovered by Cluster. They originate where two adjacent flows travel with different speed. In this case, one of the flows is the heated gas inside the boundary layer of the magnetosphere, the other the solar wind just outside it. The arrows show the direction of the magnetic field, in red that associated with the solar wind and in green the one inside Earth’s magnetosphere. The white dashed arrow shows the trajectory followed by Cluster. (Image Credit: H. Hasegawa, Dartmouth College)

The Earth’s magnetic field is our planet’s first line of defence against the bombardment of the solar wind. The solar wind itself is launched from the Sun and carries the Sun’s magnetic field throughout the Solar System. Sometimes this magnetic field is aligned with Earth’s and sometimes it points in the opposite direction.

When the two fields point in opposite directions, scientists understand how ‘doors’ in Earth’s field can open. This phenomenon, called ‘magnetic reconnection’, allows the solar wind to flow in and collect in the reservoir known as the boundary layer. On the contrary, when the fields are aligned they should present an impenetrable barrier to the flow. However, spacecraft measurements of the boundary layer, dating back to 1987, present a puzzle because they clearly show that the boundary layer is fuller when the fields are aligned than when they are not. So how is the solar wind getting in? Thanks to the data from the four formation-flying spacecraft of ESA’s Cluster mission, scientists have made a breakthrough. On 20 November 2001, the Cluster flotilla was heading around from behind Earth and had just arrived at the dusk side of the planet, where the solar wind slides past Earth’s magnetosphere. There it began to encounter gigantic vortices of gas at the magnetopause, the outer ‘edge’ of the magnetosphere.

“These vortices were really huge structures, about six Earth radii across,” says Hiroshi Hasegawa, Dartmouth College, New Hampshire who has been analysing the data with help from an international team of colleagues. Their results place the size of the vortices at almost 40 000 kilometres each, and this is the first time such structures have been detected.

These vortices are known as products of Kelvin-Helmholtz instabilities (KHI). They can occur when two adjacent flows are travelling with different speeds, so one is slipping past the other. Good examples of such instabilities are the waves whipped up by the wind slipping across the surface of the ocean. Although KHI-waves had been observed before, this is the first time that vortices are actually detected.

When a KHI-wave rolls up into a vortex, it becomes known as a ‘Kelvin Cat’s eye’. The data collected by Cluster have shown density variations of the electrified gas, right at the magnetopause, precisely like those expected when travelling through a ‘Kelvin Cat’s eye’.

Scientists had postulated that, if these structures were to form at the magnetopause, they might be able to pull large quantities of the solar wind inside the boundary layer as they collapse. Once the solar wind particles are carried into the inner part of the magnetosphere, they can be excited strongly, allowing them to smash into Earth’s atmosphere and give rise to the aurorae.

Cluster’s discovery strengthens this scenario but does not show the precise mechanism by which the gas is transported into Earth’s magnetic bubble. Thus, scientists still do not know whether this is the only process to fill up the boundary layer when the magnetic fields are aligned. For those measurements, Hasegawa says, scientists will have to wait for a future generation of magnetospheric satellites.

Adapted from materials provided by European Space Agency.

http://www.sciencedaily.com/releases/20 ... 081623.htm

[StefanR]

I'll give a link with the New Discovery at Jupiter, as it seems to have some connection

New Discovery at Jupiter

[StefanR]

The curious name ``whistler wave'' for the branch of the dispersion relation lying between the ion and electron cyclotron frequencies is originally derived from ionospheric physics. Whistler waves are a very characteristic type of audio-frequency radio interference, most commonly encountered at high latitudes, which take the form of brief, intermittent pulses, starting at high frequencies, and rapidly descending in pitch. Figure 13 shows the power spectra of some typical whistler waves.

Whistlers were discovered in the early days of radio communication, but were not explained until much later. Whistler waves start off as ``instantaneous'' radio pulses, generated by lightning flashes at high latitudes. The pulses are channeled along the Earth's dipolar magnetic field, and eventually return to ground level in the opposite hemisphere.

http://farside.ph.utexas.edu/teaching/p ... ode50.html

[StefanR]

Here a .ppt (powerpoint):
Collisionless Magnetic Reconnection

Collisionless reconnection is ubiquitous

* Inductive electric fields typically exceed the Dreicer runaway field
o classical collisions and resistivity not important
* Earth’s magnetosphere
o magnetopause
o magnetotail
* Solar corona
o solar flares
* Laboratory plasma
o sawteeth
* astrophysical systems?

Conclusions

* Fast reconnection requires either the coupling to dispersive waves at small scales or a mechanism for anomalous resistivity
* Coupling to dispersive waves
o rate independent of the mechanism which breaks the frozen-in condition
o Can have fast reconnection with a guide field
* Turbulence and anomalous resistivity
o strong electron beams near the x-line drive Buneman instability
o nonlinear evolution into “electron holes” and lower hybrid waves
+ seen in the ionospheric and magnetospheric satellite measurements
* Electron Energization
o Large scale density cavities that develop during reconnection with a guide field become large scale electron accelerators
o Secondary islands facilitate multiple interactions of electrons with this acceleration cavity and the production of very energetic electrons

http://www.newton.cam.ac.uk/programmes/ ... /drake.ppt

[StefanR]

Electromagnetic Radiation from Double Layers

Abstract
The background motivation, and some preliminary results, are reported for a recently begun
investigation of a potentially important mechanism for electromagnetic radiation from space,
Double Layer Radiation (DL-radiation). This type of radiation is proposed to come from DLassociated,
spatially localized high frequency (hf) spikes that are driven by the electron beam
on the high-potential side of the double layer. It is known, but only qualitatively, from
laboratory experiments that double layers radiate in the electromagnetic spectrum. In our
experiment the spectrum has high amplitude close to the plasma frequency, several hundred
MHz. No clear theoretical model exists today. The quantitative evaluation is complicated
because measurements are made in the near field of the radiating structure, and in the vicinity
of conducting laboratory hardware that distorts the field. Because the localized electrostatic
wavelengths (approximately 1 cm) can be relatively small compared to the emitted
electromagnetic wavelengths, the situation is further complicated. We discuss the mutual
influence between the ion density profile and hf-spike formation, and propose that some kind
of self-organization of the density profile through the ponderomotive force of the hf spike
might be operating. First results regarding the electromagnetic radiation are reported: the
frequency, the time variation of the amplitude, and the spatial distribution in the discharge
vessel.

5. Summary.
Three steps are necessary for the electromagnetic double-layer radiation mechanism we want
to study. First, a double layer must be created in a current-carrying plasma. Second, the
electron beam created on the high potential side of the double layer must create a localized hf
spike. Third, the hf spike must couple to electromagnetic waves and act as a sender-antenna.
If all three are fulfilled, there is a possibility of very high efficiency in converting electric
energy to radiation.
The first of these two steps have been studied earlier in experiments and models. It has been
known for a long time that double layers can be created quite easily, in a wide variety of
apparatus configurations and parameter regimes. They basically require that some critical
limit in electric current density be exceeded. Double layers have also recently been observed
with certainty for the first time in space, in the auroral current system. Experiments during the
last decade have furthermore shown that hf spikes generally are formed on the high potential
side of double layers.
This work reports some first results regarding studies of the last step, the production of
electromagnetic radiation from a hf-spike. Preliminary conclusions are that there is indeed
em-radiation most of the time, that it is dominated by frequencies around the local plasma
frequency, and that it is bursty and irregular in time on the ion acoustic time scale. This
electromagnetic radiation however does not penetrate the high density plasma on the high
potential side of the double layer. In that part or the plasma, there is instead a lower frequency
wave excited, which might be a whistler wave. Future work will be directed towards
understanding of the excitation mechanisms, and in quantifying the efficiency in converting
the electric energy deposited in the double layer to radiation.

http://arxiv.org/ftp/physics/papers/0410/0410194.pdf

[StefanR]

The Jovian Magnetopause Boundary Layer

There are plasma boundary layers at the magnetopause of Jupiter and the Earth (Eastman et al.,
1979; Sonnerup et al., 1981; Scudder et al., 1981). It can be assumed that there is a boundary
layer at Saturn’s magnetopause as well. What are these boundary layers and how are they
formed? Under what solar wind conditions? Figure 1 shows an example of the Earth’s low
latitude boundary layer using AMPTEKHEM ion data (Gary and Eastman, 1979). Ions of solar
wind origin (Heff, CNO) are found to have decreasing fluxes from the magnetosheath radially
inward. Energetic singly ionized Oxygen and Helium ions (whose origins must ultimately be the
ionosphere), have decreasing fluxes from the magnetosphere proper radially outward. Clearly,
the magnetopause bounding layer must be a region where cross-field diffusion of plasma is
occurring. Particles are diffusing from this layer in both directions.

The plasma boundary layers at Earth and Jupiter have ELFNLF magnetic and electric waves
present. These waves appear to be broadbanded when the data is viewed in spectrum channel
format or as power spectra (Gurnett et al., 1979; Tsurutani et al., 1981; 1989; 1997; Rezeau et
al., 1989, LaBelle and Treumann, 1988; Belmont et al., 1995). Gurnett et al. (1979)
and Tsurutani et al. (1997) interpreted the magnetic waves as broadband whistler mode waves with a
f2.6 power law spectrum. Because the wave magnetic to electric amplitude ratio (BJE,) did not
fit parallel propagating whistlers well, and there were electric signals at frequencies above f,,, the
above authors speculated that there must be additional electrostatic wave power present as well.
Tsurutani et al. (1997) have determined the electric and magnetic wave spectrum at Jupiter and
found that the magnetic waves similarly have a “broadband” spectral range with a superposition
of broadband electric waves. Because plasma wave magnetic spectral shape at Jupiter
(f2.4) isclose to that at Earth, it is surmised that the generation mechanisms are most likely the same at
the two planets.

Why are these waves apparently “broadbanded”, what are their sources of generation, and are
they responsible for any coupling between the magnetosphere and the ionosphere? First, the
waves are not really “broadbanded”. Using very high time resolution data from the Earth-
orbiting POLAR spacecraft, Tsurutani et al. (1998) have shown that the magnetic waves are
indeed propagating in the electromagnetic whistler mode. However, the waves are
not continuously present, but occur in millisecond packet bursts (Figure
2). There are presumably
bursts at a variety of frequencies, leading to the “broadband magnetic spectra” when averaged
over minutes. The superposed electric spectra is also not a “broadband” signal. This spectra is
caused by electrostatic bipolar pulses (Franz et al., 1998; Tsurutani et al., 1998) similar to those
detected at the ionospheric altitudes by instruments onboard the Freja (Miilkki et al., 1993) and
FAST (Mozer et al., 1997; Ergun et al., 1998) satellites. High-time resolution data are available
from the Galileo plasma wave detector for the Jovian case, but the nature of these plasma waves
have not been reported yet.

The exact mechanisms for wave generation in the
Earth’s magnetopause boundary layer has not
been identified. From resonance condition arguments, it has been speculated that the high
frequency portion (3-5 kHz) of the electromagnetic whistler mode waves are locally generated
by E,, < 1 keV field-aligned beams of electrons. Low-frequency (-300 Hz) whistler mode waves
may be first generated (by ions) as ion cyclotron waves near the ionosphere, with subsequent
mode conversion to the whistler mode. The electric waves may first be generated as lower
hybrid waves with field-aligned currents as a source of free energy (Lakhina and Tsurutani,
1999), with eventual evolution to bipolar pulses. Most workers in the field (Omura et al., 1996;
Goldman et al., 1999), have explained the bipolar pulses as electron holes propagating along the
ambient magnetic field lines.

Other processes such as ionospheric double layers may be the
operative acceleration process. These double layers are speculated to be related to electrostatic
bipolar pulses (electron holes), but the exact nature is unknown.
The Cassini mission to Saturn will address
many of the above issues. Does Saturn’s
magnetopause have a boundary layer and how is it maintained? What is the nature
of ELFNLF plasma waves in this region of space and what roles do they play? Are there electromagnetic ion
cyclotron waves or whistler mode waves responsible for cross-field ion diffusion, leading to the
formation of the boundary layer? Do the electromagnetic whistler mode waves cause significant
electron pitch angle diffusion such that the polar aurora can be explained? Are there bipolar
electric pulses or double-layers that are responsible direct particle acceleration and discrete
auroral arcs? These are some of the pertinent Saturnian boundary layer questions.

Http link of .pdf