D. L. Carpenter’s research while affiliated with Stanford University and other places

Application of whistler mode [Sonwalkar et al., 2004, 2011a, b] and Z mode [Carpenter et al., 2003; Sonwalkar et al., 2004] radio sounding from IMAGE has led to remote sensing along the geomagnetic field line of electron density, ion composition (H+, He+, O+) , plasma density irregularities, ducts, and Z-mode cavities in the inner magnetosphere. An analysis of whistler mode radio sounding data from ˜30 cases led to measurements of electron density and ion compositions over 90 -- 4000 km altitude range for a wide range of latitudes. Our measurements compare well with those from DMSP (Ne, H+, He+, O+) at ˜850 km and CHAMP (Ne) at ˜350 km but deviate significantly from those obtained from IRI and GCPM models. More than half the cases of whistler mode echoes examined indicated presence of ˜10-100 m scale size field aligned irregularities. A case study of the variation of plasma parameters along flux tubes near L˜2 and MLT˜15 during the development of a storm, from quiet conditions and to subsequent recovery, showed that relative to the preceding quiet time, in the first two days of the recovery phase of the storm :1) at F2 peak electron density (Ne) on the first day of the recovery increased (34%) and on the second day decreased (14%); 2) at higher altitudes (>1500 km), on the first day of the recovery Ne increased by 10%, and on the second day it decreased by ˜60%. 3) on the first day of the recovery phase the O+/H+ transition height increased to 1380 km (quiet time 1150 km) and on the second day it further increased to 1590 km; 4) on the first day of recovery phase fractional He+ concentration decreased from >10% at ˜1000 km (quiet time) to <1%, and on the second day it returned close to its quiet time value. Using Z mode echoes the parameters of Z mode ducts and cavities are measured. Analysis of 9 cases of ducted fast Z mode echoes observed in the low to mid latitude region (L=1.3-3.2) show that ducts of half-width Δ L ˜0.005-0.06 (˜30-400 km at equator) and density depletions of ˜5% - 75%, covering the altitude range ˜1000 -- 10000 km are required to trap Z mode waves and to reproduce the observed features of the echoes. Further measurements from whistler and Z mode sounding will lead to new empirical models of magnetospheric plasma density, ion composition, density irregularities, and ducts that are important for developing physics-based models of the magnetosphere and also for testing the performance of such models. The physical principles underlying the generation, propagation, reflection, and scattering of whistler and Z mode echoes provide new insights in understanding of the naturally occurring waves. Overall, we believe that our findings about whistler mode propagation and echoing in an irregular medium have important implications for the connection between whistler mode waves and the Earth's radiation belts. References: Carpenter et al. (2003), Z-mode sounding within propagation ``cavities'' and other inner magnetospheric regions by the RPI instrument on the IMAGE satellite, J. Geophys. Res., 108, 1421. Sonwalkar et al. (2004), Diagnostics of magnetospheric electron density and irregularities at altitude <5000 km using whistler and Z mode echoes from radio sounding on the IMAGE satellite, J. Geophys. Res., 109, A11212. Sonwalkar et al. (2011a), Magnetospherically reflected, specularly reflected, and backscattered whistler mode radio-sounder echoes observed on the IMAGE satellite: 1. Observations and interpretation, J. Geophys. Res., 116, A11210. Sonwalkar et al. (2011b), Magnetospherically reflected, specularly reflected, and backscattered whistler mode radio-sounder echoes observed on the IMAGE satellite: 2. Sounding of electron density, ion effective mass (meff), ion composition (H+, He+, O+), and density irregularities along the geomagnetic field line, J. Geophys. Res., 116, A11211.

Application of whistler mode [Sonwalkar et al., 2004, 2011] and Z mode [Carpenter et al., 2003] radio sounding has led to new measurements, along the geomagnetic field, of electron density, ion effective mass (meff) and composition (H+, He+, O+) , and of plasma density ducts in the inner magnetosphere. Using whistler mode echoes from radio sounding at 6-63 kHz, electron density and ion compositions are measured at altitudes (~90 – 3000 km) for invariant latitude 40 o to 60 o (L=1.7-4.2). Our measurements compare well with those from DMSP and CHAMP. Relative to whistler mode sounding measurements IRI2007 model overestimates Ne by ~20-100% at F2 and underestimates Ne by ~30-70% at 2000 km. A study of the variation of plasma parameters during the development of a storm, from quiet conditions and subsequent recovery, shows that relative to the preceding quiet time heavy ions(O+) increase and the electron density first increases and then decreases during the storm time. Z mode echoes from radio sounding at 50-1000 kHz are used to determine the parameters of Z mode ducts at mid latitudes. Analysis of 9 cases of ducted fast Z mode echoes observed from low to mid latitude region shows that ducts of width ~20-200 km and density depletions of ~ 5% - 75% covering the altitude range ~1000 – 10000 km are required to trap Z mode waves and to reproduce observed time-delays. The significance of our results lies in its focus upon probing an altitude region (<5000 km) that is important for understanding the effects of solar variability on the Earth’s magnetosphere and ionosphere, and which is also elusive in terms of accessibility for measurement by previously applied methods.

A survey of echoes detected in 2004-2005 during pulse transmissions from the Radio Plasma Imager (RPI) instrument on the IMAGE satellite has revealed several new features of sounder generated whistler mode (WM) echoes and has indicated ways in which the echoes may be used for remote sensing of the Earth's plasma structure at altitudes <5000 km. In this paper we describe the frequency versus travel time (f - t) forms of the WM echoes as they appear on RPI plasmagrams and discuss qualitatively their raypaths and diagnostic potentials. Based on their reflection mechanism, the WM echoes can be classified as: magnetospherically reflected (MR), specularly reflected (SR), or backscattered (BS). The MR echoes are reflected at altitudes where the local lower hybrid frequency (flh) is equal to the transmitted pulse frequency f, a phenomenon familiar from both theory and passive recordings of WM wave activity. The SR echoes (previously reported in a higher frequency range) are reflected at the Earth-ionosphere boundary, either with wave vector at normal incidence or, more commonly (and unexpectedly, due to ray bending in the layered ionosphere), at oblique incidence. The BS echoes are the result of scattering from small scale size 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 raypaths. The WM echoes described in this paper have been observed at altitudes less than 5,000 km and at all latitudes and at most MLTs. The diagnostic potential of these phenomena for remotely studying the distribution of plasma density and composition along the geomagnetic field line B0, as well as the presence of FAIs of varying scale sizes, is enhanced by the tendency for SR and MR echoes to be observed simultaneously along with the upward propagating signals from a spatial distribution of communication VLF transmitters. 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 Earth's radiation belts. In a companion paper by Sonwalkar et al. (2011), we employ ray tracing and refractive index diagrams in quantitative support of this paper and also present two diagnostic case studies of plasma density, ion effective mass, and ion composition along B0.

A companion paper by Sonwalkar et al. (2011) provided new details of whistler mode radio sounding of the altitude range below ˜5000 km by the Radio Plasma Imager (RPI) instrument on the IMAGE satellite. That paper presented frequency-vs- group time delay records of echoes whose raypaths either 1) reversed direction through refraction at altitudes above the ionosphere where the wave frequency was approximately equal to the local lower hybrid resonance frequency flh (magnetospherically reflected or MR echoes), or 2) returned to IMAGE from reflection points along the sharp lower boundary of the ionosphere at ˜90 km (obliquely incident (OI) or normally incident (NI) specularly reflected (SR) echoes). The MR and OI echo paths were shown to form narrow loops, while the NI echo followed the same raypath down and back. Furthermore, the echoes were found to be discrete or broadened in time delay either by multipath propagation or by scattering from field aligned irregularities (FAIs). We begin with a direct interpretive approach, employing a combination of refractive index diagrams, ray tracings, and a plasma density model to predict the detailed frequency-vs-time properties of echoes detected when the sounder is operated over a wide range of whistler mode frequencies (typically 6 kHz to 63 kHz) and the satellite is either above or below the altitude of the maximum flh along the geomagnetic field line B0 in the upper ionosphere. 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 and ion effective mass meff, we use the observed frequency-vs- group time delay (tg - f) 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 regime above. Our results on electron density and ion composition measurements are in general agreement with those obtained from in situ measurements on the IMAGE and DMSP-F15 satellites, with bottomside sounding results from nearby Ionosondes, and with values obtained from the IRI-2007 model. We also demonstrate a method of estimating the scale sizes and locations of FAIs located along or near WM echo paths.

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 prev- - ailing 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 ob

The operating frequency of the Radio Plasma Imager (RPI) instrument on the IMAGE satellite extended from 3 kHz to 3 MHz. This wide range made possible free-space O and X mode sounding from altitudes on both sides of the plasmasphere boundary layer (PBL) while also making possible wave injection in the whistler-mode and Z-mode domains at altitudes less than ≈ 10,000 km. We briefly review new findings in four areas: (i) density irregularities in the PBL and within the plasmasphere; (ii) upward Z-mode probing along geomagnetic field lines from within a Z-mode propagation 'cavity' (iii) downward probing from 4500-7000 km altitude using whistler-mode waves that undergo two fundamentally different types of reflection; (iv) strong coupling of RPI pulses to the local proton plasma

RPI on IMAGE designed to sweep from 3 kHz to 3 MHz permitted sounding in all five cold plasma wave modes including whistler- , slow Z-, fast Z-, LO- and RX-mode. Previous studies presented simultaneous observations of two or three echoes on IMAGE each propagating in distinct plasma wave mode (e.g. Reinisch et al. [2001], Carpenter et al. [2003], Sonwalkar et al. [2004]). We define multimode echoes to be simultaneously occurring echoes that have each propagated in distinctive plasma wave mode. We present here first simultaneous observations of multimode echoes containing four or five echoes. These echoes were typically observed below 5000 km in the middle to high geomagnetic latitude region. From the ~10,000 RPI transmissions in 20 kHz to 1000 kHz frequency range, during 2003-05 period, multimode echoes containing up to two echoes were found in ~2500 cases, up to three echoes in ~1000 cases, up to four echoes in ~200 cases, and up to five in ~20 cases. Because different wave modes have unique propagation characteristics (e.g. frequency, wave length, cut offs, refractive index surface), they propagate along different paths, undergo distinctive reflections, and are affected differently by field aligned irregularities of various scale sizes. With the help of ray tracing analysis of one case, 27 July 2003, when whistler-, fast Z- , slow Z- , LO- and RX-mode echoes were observed, we illustrate propagation of echoes in each mode. At this time IMAGE was at 43°S geomagnetic latitude, 2045 km altitude, L=2.5, and 5 MLT. We found that whistler mode waves propagated in a nonducted mode and reflected at the Earth-ionosphere boundary; fast Z-mode waves propagated in a duct, both above and below the satellite, and reflected at an altitude where fZ=f, where f is the transmission frequency and fZ is the fast Z-mode cutoff; the slow Z-mode waves propagated obliquely with respect to magnetic field and were scattered back by field aligned irregularities; LO mode waves propagated in a ducted mode and reflected from the conjugate hemisphere; RX mode waves propagated in a nonducted mode and reflected below the satellite at an altitude where fX=f, where fX is the RX-mode cutoff frequency. This example demonstrates the potential of multimode echoes for radio sounding and the advantages that it presents over single mode radio sounding. Propagation analysis of multimode echoes should provide us with better understanding of the propagation, reflection, and scattering of naturally occurring plasma waves.

This paper presents observations of whistler mode (WM) echoes on the IMAGE satellite at altitudes less than Rflhmax~2,000 km, where Rflhmax is the altitude at which lower hybrid frequency (flh) along the geomagnetic field line (B) passing through the satellite attains a local maximum value. Previous observations of WM echoes were typically above Rflhmax altitude [ Sonwalkar et al., URSI XXIX General Assembly, Chicago, 2008]. Three types of WM echoes are observed: magnetospherically reflected (MR), normally incident specularly reflected (NISR), and obliquely incident specularly reflected (OISR) whistler mode echoes. MR-WM echoes reflect at an altitude where flh~f, where f is the transmitted frequency, and NISR- and OISR-WM reflect near ~90 km. All three types of echoes frequently show spread in time delays indicative of multipath propagation or scattering due to field aligned irregularities. WM echo observations reported here possess the following new features:(1) MR-WM echoes reported here reflect at an altitude above the satellite and below Rflhmax, whereas those reported previously reflect at an altitude below the satellite and above Rflhmax. (2) First simultaneous observation of NISR- and OISR-WM echoes. (3) First observation of an SR echo reflecting from the hemisphere opposite to that of the satellite. Assuming that the magnetospheric plasma can be described by a diffusive equilibrium model, dispersion of MR- and SR-WM echoes allow determination of electron density and ion composition (H+, He+, O+) between Rflhmax and ~90 km along B. WM sounding observations from 21 June 2004 (Altitude =1280 km, lambdam=62°, MLT=6) are used to illustrate how the observed dispersion of MR- and SR-WM echoes combined with ray tracing simulations leads to the determination of electron density and ion composition along B between Rflhmax~2,070 km and ~90 km. The density model determined has an O+/H+ transition height at 1,820 km and F2 peak electron density of 2.6×105el-cm-3 at ~240 km, in general consistent with past observations. Analysis of WM echo observations reported here provides a better understanding of lightning-generated subprotonospheric (SP) whistlers. In general, the significance of WM echo observations on IMAGE lies in its potential for probing the low altitude region that is important for understanding ionosphere-magnetosphere coupling.

Ground-based instruments and a number of space missions have contributed to our knowledge of the plasmasphere since its discovery half a century ago, but it is fair to say that many questions have remained unanswered. Recently, NASA's IMAGE and ESA's CLUSTER probes have introduced new observational concepts, thereby providing a non-local view of the plasmasphere. IMAGE carried an extreme ultraviolet imager producing global pictures of the plasmasphere. Its instrumentation also included a radio sounder for remotely sensing the spacecraft environment. The CLUSTER mission provides observations at four nearby points as the four-spacecraft configuration crosses the outer plasmasphere on every perigee pass, thereby giving an idea of field and plasma gradients and of electric cur-rent density. This paper starts with a historical overview of classical single-spacecraft data interpretation, discusses the non-local nature of the IMAGE and CLUSTER measurements, and emphasizes the importance of the new data interpretation tools that have been developed to extract non-local information from these observations. The paper reviews these innova-tive techniques and highlights some of them to give an idea of the flavor of these methods. 8 J. De Keyser et al. In doing so, it is shown how the non-local perspective opens new avenues for plasmaspheric research.

The electric field and magnetic field are basic quantities in the plasmasphere measured since the 1960s. In this review, we first recall conventional wisdom and remaining problems from ground-based whistler measurements. Then we show scientific results from Cluster and Image, which are specifically made possible by newly introduced features on these spacecraft, as follows. 1.In situ electric field measurements using artificial electron beams are successfully used to identify electric fields originating from various sources. 2.Global electric fields are derived from sequences of plasmaspheric images, revealing how the inner magnetospheric electric field responds to the southward interplanetary magnetic fields and storms/substorms. 3.Understanding of sub-auroral polarization stream (SAPS) or sub-auroral ion drifts (SAID) are advanced through analysis of a combination of magnetospheric and ionospheric measurements from Cluster, Image, and DMSP. 4. Data from multiple spacecraft have been used to estimate magnetic gradients for the first time.