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The origin of whistler mode radiation in the plasmasphere is examined from three years of plasma wave observations from the Dynamics Explorer and three years from the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) spacecraft. These data are used to construct plasma wave intensity maps of whistler mode radiation in the plasmasphere. The highest average intensities of the radiation in the wave maps show source locations and/or sites of wave amplification. Each type of emission is classified based on its magnetic latitude and longitude rather than any spectral feature. Equatorial electromagnetic (EM) emissions (approx. 30-330 Hz), plasmaspheric hiss (approx. 330 Hz - 3.3 kHz), chorus (approx. 2 kHz - 6 kHz), and VLF transmitters (approx. 10-50 kHz) are the main types of waves that are clearly delineated in the plasma wave maps. Observations of the equatorial EM emissions show that the most intense region is on or near the magnetic equator in the afternoon sector and that during times of negative B(sub z) (interplanetary magnetic field),the maximum intensity moves from L values of 3 to less than 2. These observations are consistent with the origin of this emission being particle-wave interactions in or near the magnetic equator. Plasmaspheric hiss shows high intensity at high latitudes and low altitudes (L shells from 2 to 4) and in the magnetic equator over L values from 2 to 3 in the early afternoon sector. The longitudinal distribution of the hiss intensity (excluding the enhancement at the equator) is similar to the distribution of lightning: stronger over continents than over the ocean, stronger in the summer than winter, and stronger on the dayside than nightside. These observations strongly support lightning as the dominant source for plasmaspheric hiss, which through particle-wave interactions, maintains the slot region in the radiation belts. The enhancement of hiss at the magnetic equator is consistent with particle-wave interactions. The chorus emissions are most intense on the morning side as previously reported. At frequencies from approx. 10-50 kHz VLF transmitters dominate the spectrum. The maximum intensity of the VLF transmitters is in the late evening or early morning with enhancements all along L shells from 1.8 to 3.

[1] We report an event observed by the Low-Energy Neutral Atom (LENA) imager on 18 April 2001, in which enhanced neutral atom emission was observed coming from the direction of the Sun and from the general direction of the subsolar magnetopause. The enhanced neutral atom emission is shown to be primarily a result of increased solar wind charge exchange with the Earth's hydrogen exosphere, that is, enhanced neutral solar wind formation, occurring in conjunction with a southward turning of the interplanetary magnetic field (IMF) which moves the magnetopause closer to the Earth. It is shown that the neutral atom flux under compressed magnetopause conditions is extremely sensitive to changes in the IMF north-south component.

[1] Several year's worth of observations from the plasma wave instruments on both Magnetopause-to-Aurora Global Exploration (IMAGE) and Polar spacecraft are used to study the seasonal and solar cycle variations in the spectrum of auroral kilometric radiation (AKR). Only AKR observations when the spacecraft were in the Northern Hemisphere emission cones were used. The results from the seasonal analysis show significant changes in the AKR spectrum as a function of dipole tilt. The average AKR spectral peak for positive dipole tilt is ∼150 kHz but is ∼300 kHz during times of negative dipole tilt. In addition, the average emission spectrum for positive tilt is up to two orders of magnitude weaker over the 200–500 kHz frequency range when compared with the average emission spectrum for negative tilt. Assuming the cyclotron maser mechanism for AKR, these results imply that the AKR source region (the auroral density cavity) moves to higher altitudes during the summer and to lower altitudes during the winter. Using data from the DE-1 plasma wave instrument, the magnetic local time of average AKR source region is also investigated with dipole tilt. From these observations it is found that for negative dipole tilt a broad AKR source region exists, ranging from ∼18 to ∼24 MLT, with peak emission coming from ∼20 MLT. In comparison, under positive dipole tilt the source region narrows (∼20 to ∼24 MLT) with peak emission at ∼22 MLT. Taking into account the above seasonal effect, a comparison of the average spectra from IMAGE and Polar plasma wave data also demonstrates a solar cycle effect. The average AKR spectrum at solar maximum has the same structure with dipole tilt as at solar minimum but is typically lower (by as much as 1–2 orders of magnitude). The observations presented support the concept that the expected increases in ionospheric densities (with positive dipole tilt for the Northern Hemisphere and solar EUV flux increases during solar maximum) play a significant role in magnetospheric-ionospheric coupling by: (1) shortening the altitude range of the auroral plasma cavity, (2) confining the cavity to a smaller range of MLT and closer to midnight, and (3) decreasing the overall intensity of AKR by lessening the density depth of the auroral density cavity. The results of this study should be taken into account in future studies of using AKR as a substorm index and other statistical emission cone studies at both low and high frequencies.

[1] A year's worth of observations of kilometric continuum (KC) from the plasma wave instrument (PWI) on GEOTAIL and extreme ultraviolet (EUV) images of the plasmasphere from IMAGE are compared. In the vast majority (∼94%) of the 87 cases when simultaneous data from both spacecraft were available, KC was observed to be associated with density depletions or notch structures in the plasmasphere. From a careful analysis of 1 month of EUV data comprising 13 notch structures, only one notch structure was found in which no accompanying KC was observed when GEOTAIL was in a low-latitude position and therefore should have observed the emission if it were generated. IMAGE observations from the radio plasma imager (RPI) during passage through a plasmaspheric notch structure found that KC was generated in or very near the magnetic equator at steep gradients in density and associated with emissions in the upper hybrid resonance band as previously reported by others at lower frequencies. Statistical analysis of the KC events associated with plasmaspheric notch structures shows that the typical source region is at an equatorial radial distance of ∼2.4 RE (Earth radii) in the magnetic equator and produces an emission cone that is ∼40° in longitude and ∼20° in latitude. These results show that a density depletion or notch structure in the plasmasphere is typically a critical condition for the generation of KC but that the notch structures do not always provide the conditions necessary for the generation of the emission.

Received 22 May 2002; revised 16 April 2003; accepted 2 May 2003; published 15 August 2003. [1] Recent IMAGE Extreme Ultraviolet Imager (EUV) observations showed the first global images of plasmaspheric convection plumes, which have been interpreted as the plasmaspheric tails predicted theoretically 3 decades earlier. Using observations by the IMAGE Radio Plasma Imager (RPI), we show that these convection plumes have large latitudinal extent. These results complement those recently made by others in correlating IMAGE EUV data with measurements of total electron content in the ionosphere. By correlating in situ RPI density measurements with global plasmaspheric EUV images, we have shown that apparently detached plasma structures, as appear in RPI dynamic spectrograms, are in many cases plasmaspheric convection plumes. The temporal separation between the RPI and EUVobservations help constrain the interpretation of one data set in the context of the other, thereby enabling an examination of the threedimensional plasma density structures outside the core plasmasphere. The data sets are mutually reinforcing because the data are collected within a few hours of one another. We used the EUV data to provide unambiguous identification of density enhancements in the region outside the plasmasphere and used the RPI data to obtain accurate number densities and extend information from the EUV data set by measuring densities below the EUV sensitivity threshold. INDEX TERMS: 2768 Magnetospheric Physics: Plasmasphere; 2740 Magnetospheric Physics: Magnetospheric configuration and dynamics; 2730 Magnetospheric Physics: Magnetosphere—inner; 2788 Magnetospheric Physics: Storms and substorms; KEYWORDS: plasmasphere, plasmapause, inner magnetosphere, convection plumes, RPI, EUV

[1] Among the objectives of the Radio Plasma Imager (RPI) on IMAGE is the observation of the Earth's plasmasphere from the satellite's polar orbit, with apogee ≈8 RE geocentric distance and perigee near 1200 km altitude. This objective is here pursued by (1) remote sounding from high-altitude regions outside the main plasmasphere, (2) sounding within the plasmasphere, and (3) in situ passive measurements of natural wave activity. During sounding of the plasmasphere from both outside and inside the plasmapause, RPI echoes that follow non-field-aligned ray paths are usually not the discrete traces on range-versus-frequency records (plasmagrams) that are predicted by ray-tracing simulations in smooth magnetospheric density models. Instead, such RPI echoes exhibit various amounts of spreading, from ≈0.5 RE to ≈2 RE in virtual range (range assuming free-space speed of light propagation). The range spreading is attributed to scattering from, partial reflection from, and propagation along geomagnetic field-aligned electron density irregularities. There exists a substantial body of theoretical work on mechanisms that might explain the appearance of such irregularities both within and beyond the plasmasphere. That the spread-producing irregularities are field-aligned is suggested by the efficiency with which RPI excites discrete echoes that propagate along the geomagnetic field, sometimes into both hemispheres. The spatial distributions and scale sizes of the spread-producing irregularities remain to be investigated. The RPI echo data, however, coupled with earlier evidence from topside sounders and whistler mode instruments, suggest that they can have cross-field scale sizes within a range from ≈200 m to over 10 km and electron densities within ≈10% of background. RPI is found capable of detecting plasmapause locations from distances of ≈3 RE or more. When minimal signal integration is used, the location and range of density values of a steep plasmapause can be determined from distances of order 1 RE, and echoes can at times be returned from points extending inward from the plasmapause to locations where the electron density reaches ≈3000 cm−3, which is usually at L < 3.