[1] Observations of the electron density n e 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 n e = n e0 (R max /R) a , where n e0 is the equatorial electron density, R max % LR E is the maximum geocentric radius R to any point on the field line, and a = a model = 8.0 À 3.0 log 10 n e0 + 0.28(log 10 n e0 ) 2 À 0.43(R max /R E ), for all categories of plasma (plasmasphere and plasmatrough). (In the formula for a model , n e0 is expressed in cm À3 .) 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 R E the electron density tends to level out at large R max to a constant value $100 cm À3 . 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 n e0 versus R max 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 n e0 scales with respect to R max like R max À3.4 , but in the region for which our plasmatrough data is most reliable (L 6), it is well fit by the R max À4.0 scaling of Sheeley et al. or the R max À4.5 scaling of Carpenter and Anderson. We present a simple interpretation for the field line dependence of the density. For large n e0 , such as occurs in the plasmasphere, a is close to zero on average (implying that n e is roughly constant along field lines). When n e0 decreases, so does n e at R = 2 R E , but the value there does not decrease much below 100 cm À3 . (It is unclear if this value is an absolute lower density limit because most often the upper hybrid resonance emission disappears at R $ 2 R E because f p /f ce < 1, where f p / ffiffiffiffi ffi n e p is the plasma frequency and f ce is the electron cyclotron frequency.) Finally, we examined the dependence of a and the density at the equator and at R $ 2 R E on the average hKpi (Kp averaged with a 3-day timescale). There is no clear dependence of the average a À a model on hKpi or on MLT. In the plasmasphere, n e0 decreases with respect to increasing hKpi.

Plasma, magnetic‐field and dc electric‐field observations from Dynamics Explorers 1 and 2 are used to investigate the morphology of solar‐wind ion injection, Birkeland currents, and plasma convection in the morning sector for both positive and negative interplanetary magnetic field (IMF) By components. The results of the study are used to construct a By‐dependent global convection model for southward IMF. A significant element of the model is the coexistence of three types of convection cells (“merging cells,” “viscous cells,” and “lobe cells”). This model can account for observations of a nearly stationary (in local time) convection “throat,” a sunward‐antisunward convection reversal zone at the polar‐cap boundary in both the morning and afternoon quadrants, the morphology of solar‐wind ion injection and transport in the mid‐altitude polar cusp, and the By‐dependent dawn‐dusk asymmetry of polar‐cap electron fluxes.

Plasma wave and plasma measurements from the Dynamics Explorer 1 (DE 1) spacecraft are used to investigate an intense broadband spectrum of low‐frequency, < 100 Hz, electric and magnetic noise observed at low altitudes over the auroral zones. This noise is detected by DE 1 on essentially every low‐altitude pass over the auroral zone and occurs in regions of low‐energy, 100 ev to 10 keV, auroral electron precipitation and field‐aligned currents. The electric field is randomly polarized in a plane perpendicular to the static magnetic field. Correlation measurements between the electric and magnetic fields show that the perpendicular (∼ north‐south) electric field fluctuations are closely correlated with the perpendicular (east‐west) magnetic field fluctuations and that the Poynting flux is directed downward, toward the earth. The total electromagnetic power flow associated with the fluctuations is large, approximately 108 W. Two general interpretations of the low‐frequency noise are considered: first, that the noise is produced by static fields imbedded in the ionosphere and, second, that the noise is due to Alfven waves propagating along the auroral field lines. For the static interpretation the ratio of the magnetic to electric field strengths at the base of the ionosphere is determined by the Pedersen conductivity, B/(µ0E) = Σp, whereas for the Alfven wave interpretation it is determined by the Alfven index of refraction, cB/E = nA. Measurements show that the magnetic to electric field ratio decreases rapidly with increasing height. This height dependence is in strong disagreement with the static model if the magnetic field lines are assumed to be equipotentials (E∥ = 0). At present, no satisfactory model is available for comparison with the data if an electrostatic potential drop is assumed to exist along the magnetic field (E∥ ≠ 0). The Alfven wave model is in good agreement with the general form of the height dependence of the magnetic to electric field ratio but disagrees in certain details. The cB/E ratio tends to decrease with increasing frequency and is usually somewhat larger than the computed value of the Alfven index of refraction. Some of these difficulties could be accounted for by reflections at the base of the ionosphere or propagation at large angles to the magnetic field (kinetic Alfven waves). For both the static model and the Alfven wave model the source must be located at high altitudes, since the average Poynting flux is always directed downward, even at radial distances up to 2 RE.

According to existing theory, electrons are accelerated up to ultra-relativistic energies 1 inside Jupiter's magnetic field by betatron and Fermi processes as a result of radial diffusion towards the planet and conservation of the first two adiabatic invariants 2-4 . Recently, it has been shown that gyro-resonant electron acceleration by whistler-mode waves 5,6 is a major, if not dominant 7 , process for accelerating electrons inside the Earth's outer radiation zone, and has redefined our concept for producing the Van Allen radiation belts 8 . Here, we present a survey of data from the Galileo spacecraft at Jupiter, which shows that intense whistler-mode waves are observed outside the orbit of the moon Io and, using Fokker-Planck simulations, are strong enough to accelerate electrons to relativistic energies on timescales comparable to that for electron transport. Gyroresonant acceleration is most effective between 6 and 12 jovian radii (R j ) and provides the missing step in the production of intense synchrotron radiation from Jupiter 1,9 . At Jupiter, volcanic activity on the moon Io provides a major source of gaseous material 10 which is ionized by electron impact and solar radiation and forms a torus-like structure around the planet near 6R j . Centrifugal force from Jupiter's rapid rotation drives magnetic flux interchange instabilities 11,12 whereby outward transport of cold dense plasma is replaced by inward transport of low-density higher-energy (∼1-100 keV) plasma. As the plasma is transported into increasing magnetic-field strength, a temperature anisotropy develops whereby the temperature perpendicular to the magnetic field increases faster than that parallel to the field. This anisotropy can excite very low-frequency whistler-mode chorus waves which resonate with electrons [13][14][15] . Figure 1 shows data from the Galileo spacecraft where a broad frequency band of whistler-mode chorus waves can be identified below the electron gyrofrequency (f ce = |e|B/(2πm e ), where |e| is the electron charge, m e is the electron mass and B is the ambient magnetic-field strength obtained from the fluxgate magnetometer onboard the spacecraft). The band rises and falls in frequency each time the spacecraft crosses the magnetic equator. The waves were detected between 9 and 12R j in a region where magnetic flux interchange takes place. A more general survey of all Galileo wave data recorded between 27 June 1996 and 5 November 2002 (Fig. 3a) shows that whistler-mode wave intensity (averaged over all longitudes) peaks in a region between 6 and 10R j . Thus magnetic flux interchange beyond the moon Io provides a natural and plentiful source of energy to drive the waves unstable.To determine whether the waves detected by Galileo interact with relativistic electrons, we calculated the electron resonant energy by solving the cold plasma dispersion relation with the Doppler-shifted cyclotron resonance condition given bywhere γ is the relativistic correction factor, n is the harmonic number (n = 0, ±1, ±2, . . .), |Ω e | i...

We present a survey of whistler mode chorus emissions based on high‐telemetry rate data of the Plasma Wave Instrument on board the Polar spacecraft. Using simultaneous measurements of full vectors of the electric field and the magnetic field we calculate the Poynting vectors of chorus, and we parameterize the observations by their L* coordinate. Our new analysis of the angle between the direction of the Poynting vector and the magnetic field line confirms previous results on propagation of the main chorus band away from the equator. Our systematic results also prove the existence of the magnetospherically reflected component of chorus found previously by case studies. We observe weak outer zone waves that propagate toward the equator. These waves can be attributed to a poleward reflected component of equatorial chorus or chorus generated in the dayside pockets of a low magnetic field strength. We analyze the probability distribution of the Poynting flux of chorus, and we find that it shows a “heavy tail” feature that can be modeled by a power law or lognormal model. This result has consequences for theories of chorus generation and for methods of statistical characterization of chorus intensities in the frame of radiation belt modeling. We find that the Poynting flux of dayside chorus increases with L* toward the outer zone at L* > 6. We estimate that approximately 1% of the Poynting flux of chorus in the outer zone could be sufficient to accelerate electrons in the outer Van Allen radiation belt on the timescale of days.

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[1] Observations of the electron density n e 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 n e = n e0 (R max /R) a , where n e0 is the equatorial electron density and R max % LR E is the maximum geocentric radius R to any point on the field line, and model the statistical average of a as a model = 8.0 À 3.0 log 10 n e0 + 0.28 (log 10 n e0 ) 2 À 0.43(R max /R E ) 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 2.5R E R max 8.5R E , 2R E R R max , and 2 n e0 1500 cm À3. There is no remaining dependence of the average a -a model on MLT or Kp.

It has been known for many years that Saturn emits intense radio emissions at kilometer wavelengths and that this radiation is modulated by the rotation of the planet at a rate that varies slowly on time scales of years. Recently it has been shown that the radio emission consists of two components that have different rotational modulation rates, one emitted from the northern auroral region and the other emitted from the southern auroral region. In this paper we show using radio measurements from the Cassini spacecraft that the rotational modulation rates of the northern and southern components reversed near Saturn's recent equinox, which occurred on 11 August 2009. We show that a similar reversal was also observed by the Ulysses spacecraft near the previous equinox, which occurred on 19 November 1995. The solar control implied by these reversals has important implications on how Saturn's rotation is coupled into the magnetosphere.