[1] Using measurements of the electron density n e found from passive radio wave observations by the IMAGE spacecraft RPI instrument on consecutive passes through the magnetosphere, we calculate the long-term (>1 day) refilling rate of equatorial electron density dn e,eq /dt from L = 2 to 9. Our events did not exhibit saturation, probably because our data set did not include a deep solar minimum and because saturation is an unusual occurrence, especially outside of solar minimum. The median rate in cm À3 /day can be modeled with log 10 (dn e,eq /dt) = 2.22 À 0.006L À 0.0347L 2 , while the third quartile rate can be modeled with log 10 (dn e,eq /dt) = 3.39 À 0.353L, and the mean rate can be modeled as log 10 (dn e,eq /dt) = 2.74 À 0.269L. These statistical values are found from the ensemble of all observed rates at each L value, including negative rates (decreases in density due to azimuthal structure or radial motion or for other reasons), in order to characterize the typical behavior. The first quartile rates are usually negative for L < 4.7 and close to zero for larger L values. Our rates are roughly consistent with previous observations of ion refilling at geostationary orbit. Most previous studies of refilling found larger refilling rates, but many of these examined a single event which may have exhibited unusually rapid refilling. Comparing refilling rates at solar maximum to those at solar minimum, we found that the refilling rate is larger at solar maximum for small L < 4, about the same at solar maximum and solar minimum for L = 4.2 to 5.8, and is larger at solar minimum for large L > 5.8 such as at geostationary orbit (L $ 6.8) (at least to L of about 8). These results agree with previous results for ion refilling at geostationary orbit, may agree with previous results at lower L, and are consistent with some trends for ionospheric density.Citation: Denton, R. E., Y. Wang, P. A. Webb, P. M. Tengdin, J. Goldstein, J. A. Redfern, and B. W. Reinisch (2012), Magnetospheric electron density long-term (>1 day) refilling rates inferred from passive radio emissions measured by IMAGE RPI during geomagnetically quiet times,

Inducing T cell responses requires at least two distinct signals: 1) TCR engagement of MHC-peptide and 2) binding of CD28 to B7.1/2. However, the recent avalanche of newly described costimulatory molecules may represent additional signals which can modify events after the initial two-signal activation. Inducible costimulatory molecule (ICOS) is a CD28 family member expressed on T cells rapidly following activation that augments both Th1 and Th2 T cell responses and has been implicated in sustaining rather than initiating T cell responses. Although it is known that blockade of ICOS-B7-related protein 1 (B7RP-1) in vivo dramatically reduces germinal center formation and Ab production, the mechanism(s) remains unclear. An optimal T cell-dependent Ab response requires T and B cell activation, expansion, differentiation, survival, and migration, and the ICOS-B7RP-1 interaction could be involved in any or all of these processes. Understanding this will have important implications for targeting ICOS-B7RP-1 therapeutically. We have therefore used a double-adoptive transfer system, in which all of the above events can be analyzed, to assess the role of ICOS-B7RP-1 in T cell help for B cell responses. We have shown that ICOS signaling is involved in the initial clonal expansion of primary and primed Th1 and Th2 cells in response to immunization. Furthermore, while ICOS-B7RP-1 interactions have no effect on the migration of T cells into B cell follicles, it is essential for their ability to support B cell responses.

The magnetospheric electron density Ne can often be obtained to within a few percent from passive radio‐wave dynamic spectra when the electron plasma frequency fpe (∝ Ne1/2) is greater than the electron cyclotron frequency fce. This conclusion is based on interleaved active and passive observations from the Radio Plasma Imager (RPI) on the IMAGE satellite in the vicinity of the plasmapause. The Ne determinations are based on the frequency limits of an intense narrowband emission identified as the upper‐hybrid band. The lower limit is identified with fpe and the upper limit with the upper‐hybrid frequency fuh = (fpe2 + fce2)1/2. These frequency limits and the large amplitude of the emission, typically 20 dB or more above background, suggest strong Z‐mode waves, rather than quasi‐thermal fluctuations, as the emission source.

Abstract. The term Z mode is space physics notation for the low-frequency branch of the extraordinary (X) mode. It is an internal, or trapped, mode of the plasma confined in frequency between the cutoff frequency fz and the upper-hybrid frequency f uh which is related to the electron plasma frequency fpe and the electron cyclotron frequency fce by the expression f; fz is a function of fpe and fce. These characteristic frequencies are directly related to the electron number density Ne and the magnetic field strength |B|, i.e., fpe (kHz) 2 ≈ 80.6Ne(cm −3 ) and fce (kHz) 2 ≈ 0.028|B|(nT). The Z mode is further classified as slow or fast depending on whether the phase velocity is lower or higher than the speed of light in vacuum. The Z mode provides a link between the short wavelength λ (large wave number k = 2π/λ ) electrostatic (es) domain and the long λ (small k) electromagnetic (em) domain. An understanding of the generation, propagation and reception of Z-mode waves in space plasma leads to fundamental information on wave/particle interactions, Ne, and field-aligned Ne irregularities (FAI) in both active and passive wave experiments. Here we review Z-mode observations and their interpretations from both radio sounders on rockets and satellites and from plasma-wave receivers on satellites. The emphasis will be on the scattering and ducting of sounder-generated Z-mode waves by FAI and on the passive reception of Z-mode waves generated by natural processes such as Cherenkov and cyclotron emission. The diagnostic applications of the observations to understanding ionospheric and magnetospheric plasma processes and structures benefit from the complementary nature of passive and active plasma-wave experiments.

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