A. Reddy

Z mode radio sounding from IMAGE of field aligned electron density and ducts

Radio sounding from IMAGE has led to the observations of a large variety of ducted and nonducted fast Z mode echoes. The ducted echoes were observed when IMAGE was located inside a Z mode “cavity,” whereas the nonducted echoes were observed both inside and outside of the cavity. We present a new ray tracing method to measure field aligned electron density and duct and Z mode cavity parameters from the observed cutoff frequencies and dispersion of the ducted and nonducted fast Z mode echoes. The method is applied to four case studies to obtain field aligned electron densities and duct parameters. Our results demonstrate that Z mode radio sounding observations from IMAGE provide a powerful new way to measure field aligned electron density and duct parameters that play important roles in the physics of whistler and Z mode wave propagation, wave particle interactions, and that of ionosphere magnetosphere coupling.

Variation of plasmaspheric field-aligned electron and ion densities as a function of geomagnetic storm activity

This paper presents whistler mode (WM) radio sounding measurements of field-aligned electron and ion density in the 90–4000 km altitude as a function of 30 August major storm activity. Electron and ion density measurements were obtained at altitudes below 4000 km including F2 peak, L∼2 and MLT∼15. The plasmapause was located at L>2.5. WM sounding measurements were augmented by in situ measurements from the CHAMP (350 km) and DMSP (850 km) satellites. Even at such low L-shells significant variations in electron and ion densities in both the ionosphere (< O⁺/H⁺ transition height) and plasmasphere (> O⁺/H⁺ transition height) were observed as a function of storm activity. The variation of electron density below and above O⁺/H⁺ transition height (∼1000 km) is different. Our results show that: (1) O⁺/H⁺ transition height increased during the main phase and it remained high during the recovery phase; (2) below transition height, Ne and O⁺ significantly increased during storms main phase and decreased close to their average quiet time values during the recovery phase; (3) Above transition height, Ne oscillated about their quiet time values as a function of storm activity. it decreased during the onset, increased during the main phase and progressively decreased during first two days of recovery phase; The same is true for H⁺ at all altitudes (4) He⁺ (at all altitudes) increased during storm onset, decreased during main phase and increased during the recovery phase. Our results on the variations of electron density below 1000 km, O⁺ and He⁺ densities are consistent with the past results. Our results also showed significant variations in Ne above 1000 km and H⁺ density at low L-shells which were not observed in the past. Our results obtained here combined with physics based model simulations such as SAMI3 provides a unique method to study the relative importance of neutral winds and dynamo and prompt penetrations electric fields in determining the storm time variations in electron and ion densities.

Magnetospheric (90 – 4000 km) field aligned electron and ion (H+, He+, O+) densities as a function of geomagnetic storm activity

Field aligned electron (Ne) and ion densities and their variations as a function of geomagnetic storm activity are important parameters in the physics of the thermosphere, ionosphere, and magnetosphere. Whistler mode (WM) radio sounding [Sonwalkar et al., JGR, 2011] from the IMAGE satellite during 24 Aug – 18 Sep 2005, covering 3 major, 3 moderate, and one minor storm, has permitted measurements of electron and ion densities (90–4000 km) as a function of geomagnetic storm activity. During this period, WM echoes were observed on dayside (MLT∼14, 1.7<L<3.5) and nightside (MLT∼3, 1.8<L<4.5). Here we present results from the analysis of WM echoes observed on dayside between 1.8<L<2.2. The analysis of echoes at other L-shell and nighttime is ongoing. From the analysis of WM echoes observed during quiet times nominal density models were built for Ne and relative ion concentrations (αH+, αHe+, αO+).

Whistler mode radio sounding from IMAGE of field aligned electron and ion densities in the plasmasphere below 4000 km

Whistler mode sounding from IMAGE is a unique and novel method that permits remote sensing of field aligned electron density and ion composition in the altitude range 90-4000 km. These measurements are not possible with topside radio sounders utilizing other plasma wave modes. We present results from the radio sounding of the plasmasphere. Preliminary analysis of more than 250 cases of whistler mode echoes observed on IMAGE was performed to obtain: (1) dayside and nightside distribution of electron and H+, O+, and He+ ion densities in the L-shell range 1.6 to 4 during geomagnetically quiet to moderate conditions, and (2) variations in electron and ion densities as a function of geomagnetic storm activity. The results from whistler mode sounding will lead to new empirical models of field aligned electron and ion densities within the plasmasphere as a function L-shell, MLT, and geomagnetic activity. These models can be used to build as well as to test physics based models of the ionosphere and plasmasphere and of the coupling between the two and the underlying thermosphere.

Whistler mode radio sounding from the RPI instrument on the IMAGE satellite

This paper presents the results obtained to date on the whistler mode (WM) sounding from the RPI instrument on the IMAGE satellite. Based on their reflection mechanism, the WM echoes are classified as magnetospherically reflected (MR), specularly reflected (SR), or back scattered (BS) echoes. The MR echoes are reflected at altitudes where the local lower hybrid frequency (fih) is equal to the transmitted pulse frequency f The SR echoes are reflected at the Earth-ionosphere boundary near ∼90 km, either with wave vector at normal incidence (NI echoes) or at oblique incidence (OI echoes). The MR and OI echo paths form narrow loops, while the NI echo follows the same ray path down and back. The BS echoes are the result of scattering from small scale plasma density irregularities close to IMAGE. The echoes are described as discrete, multipath, and diffuse, depending upon the amount of travel time spreading caused by the presence of field aligned density irregularities (FAIs) along echo ray paths. A large number (>2000) WM echoes have been observed at all latitudes below ∼7000 km during both low and high geomagnetic activity. The upper altitude limit (7000 km) is the result of experimental limitation. The WM sounding provides a new remote sensing method to measure the plasma density and ion composition along the geomagnetic field line B0 passing through the satellite and to determine the locations of FAIs of varying scale sizes present along the echo ray paths. In a direct interpretive approach, we employ a combination of refractive index diagrams, ray tracings, and a plasma density model to predict the detailed frequency versus group time delay (f-tg) properties of echoes detected when the sounder is either above or below the altitude of the maximum flh along B0 and when the sounding frequency is varied over the range of possible whistler mode frequencies. We then consider the inverse problem, estimation of the parameters of the prevailing plasma density model from the observed echo properties. Thanks to variations in the sensitivity of the various echo forms to the altitude profiles of electron density (Ne) and effective ion mass meff, we use the observed f-tg details of simultaneously received MR and SR echoes to infer the properties of a diffusive equilibrium model of the plasma, including estimates of the ion composition in the important transition region from the O+ dominated ionosphere to the light ion (H+ and He+) regime above. We also demonstrate a method of estimating the scale sizes and locations of FAIs (10 m–100 km) located along or near WM echo paths. We demonstrate/illustrate the power/potential of the WM radio sounding method by applying it to two cases when MR and OI echoes were received by RPI: 22 Oct 2005 (Alt=3403, λm= 31.9° N, MLT= 11.2) and 26 Oct 2005 (Alt=2574, λm=38.8° N, MLT= 12.1). The inversion of radio sounding data has yielded the following new results: (1) electron density and ion effective mass between the satellite altitude and 90 km along the field line passing through IMAGE. (2) Measurement of O+-H+ transition heights of ∼ 1100 km (22 October) and ∼ 1150 km (26 October). (3) Measurement of H+, O+, and He+ density, assuming diffusive equilibrium density model in which the scale heights of individual ion species are inversely proportional to their atomic weight. (4) Measurement of scale sizes (∼ 10–100 m) and locations (∼ 1400–2500 km) of field aligned irregularities on 26 October 2005. (5) Our results in both cases agree well with Ne and H+, O+, and He+ density measurements on DMSP-15 satellite at 850 km along the same L-shell and nearby MLT and F2 peak electron density and height measured by ionosondes on nearby L-shells, and electron and ion densities obtained from IRI model up to 2000 km. These results establish WM sounding as new magnetospheric tool for measuring electron and ion densities along the geomagnetic field in the important O+-H+ transition region, not accessible by other radio sounding methods. We believe that our findings about WM propagation and echoing in an irregular medium have important implications for the connection between WM waves and the Earths radiation belts.