Data from the DE 1 satellite show that lightning‐generated whistlers often trigger hiss emissions that endure for up to 10‐ to 20‐s periods. The data consist of the measured electric and magnetic fields in the frequency range of 1.5 kHz to 6.0 kHz, during 22 DE 1 passes during the period December 28, 1986, to January 18, 1987. The 22 passes were nearly identical in terms of their projections on a magnetic meridional plane, and they covered L shells of 3.4 to 5.1 and geomagnetic latitude of 20°N to 40°S in the afternoon (∼1400–1500 MLT) sector. The geographic longitudes of the 22 passes were within ±50° of 80° W, and the geomagnetic activity during the period covered was relatively quiet (ΣKp<20 for most of the days). The whistler‐triggered hiss emissions were observed on 16 of the passes, and they generally exhibited the following characteristics: (1) emission spectra were wide band (1–2 kHz) and rather structureless, (2) well‐defined and sustained fading patterns were observed at twice the spin frequency over 10‐ to 20‐s periods, (3) the spin fading characteristics of the triggered hiss bursts were similar to those reported for background plasmaspheric hiss (Sonwalkar and Inan, 1988), indicating a large wave normal angle with respect to the ambient magnetic field. Our results indicate that lightning‐generated whistlers may be an important embryonic source for magnetospheric hiss and that whistlers and emissions triggered by them often constitute the dominant wave activity in the ∼1.5‐ to 6‐kHz range on L shells of 3.5 to 5 in the afternoon sector during geomagnetically quiet periods. Through cyclotron and Landau resonant scattering, it is likely that these lightning‐generated waves play a dominant role in the loss of ∼0.5‐ to 50‐keV electrons trapped on these field lines in the afternoon sector. Through anisotropic proton instability, these waves can also interact with ring current protons in the range of several tens of keV leading to a loss mechanism for ring current protons.

Ray‐tracing simulations and estimates of whistler wave damping show that magnetospherically reflected whistlers can persist for ∼102 s in a low frequency band (ƒ ∼1 kHz). The combined contribution from whistler rays produced by a single lightning flash but entering the magnetosphere at different points form a continuous hiss‐like signal, as observed at a fixed point. Estimates indicate that the total whistler wave energy input into the magnetosphere from lightning discharges may maintain experimentally observed levels of magnetospheric hiss.

Magnetospherically reflected, specularly reflected, and backscattered whistler mode radio-sounder echoes observed on the IMAGE satellite: 2. Sounding of electron density, ion effective mass (m eff ), ion composition (H + , He + , O + ), and density irregularities along the geomagnetic field line [1] 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 f lh (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 f lh along the geomagnetic field line B 0 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 m eff , we use the observed frequency-vs-group time delay (t g − 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.Citation: Sonwalkar, V. S., A. Reddy, and D. L. Carpenter (2011), Magnetospherically reflected, specularly reflected, and backscattered whistler mode radio-sounder echoes observed on the IMAGE satellite: 2. Sounding of electron density, ion effective mass (m eff ), ion composition (H + , He + , O + ), and den...

Abstract. Auroral hiss is one of the most intense whistler mode plasma wave phenomena observed both on the ground at high latitudes and on spacecraft in the auroral zone. Propagation of auroral hiss from its source region to the ground is poorly understood. The standard whistler mode propagation in a smooth magnetosphere predicts that auroral hiss generated at large wave-normal angles along the auroral field lines by Cerenkov resonance cannot penetrate to the ground. We show that the presence of density depletions along the field lines in the auroral zone and meter-scale density irregularities at altitudes < 5000 km at high latitude permits the auroral hiss propagation to the ground. In our mechanism the auroral hiss generated at high altitudes (> 5000-20,000 km) propagates to lower altitudes (< 3000-5000 km) in two modes: (1) a ducted mode guided by field-aligned density depletions and (2) a nonducted mode. The hiss with large wave-normal angle arriving at < 5000 km altitude is scattered by meter-scale irregularities, and about 0.1% to 10% of the scattered hiss has small wave-normal angles which can penetrate to the ground. Our mechanism explains the following features of auroral hiss observed on the ground: (1) the characteristic spectra of continuous and impulsive auroral hiss, (2) the upper and lower frequency cutoffs, (3) the dispersion of impulsive auroral hiss, (4) the location of ionospheric exit points of auroral hiss with respect to visible aurora, and (5) the 2-5 order of magnitude intensity decrease of auroral hiss observed on the ground relative to that observed on spacecraft. Based on the model presented here, we provide methods to infer parameters of density depletions and intensity of lower hybrid waves stimulated by auroral hiss from the ground measurements of auroral hiss together with optical and radar measurements.

[1] When the Radio Plasma Imager (RPI) on the IMAGE satellite operates in the inner plasmasphere and at moderate to low altitudes over the polar regions, pulses emitted at the low end of its 3-kHz to 3-MHz sounding frequency range can propagate in the whistler mode and/or in the Z mode. During soundings with both 25.6-ms pulses and 3.2-ms pulses, whistler mode echoes have been observed in (1) ''discrete,'' lightning whistlerlike forms and (2) diffuse, widely time spread forms suggestive of mode coupling at the boundaries of density irregularities. Discrete echoes have been observed at altitudes less than %5000 km both inside the plasmasphere and over the auroral and polar regions, being most common inside the plasmasphere. Diffuse echoes have also been observed at altitudes less than 5000 km, being most common poleward of the plasmasphere. Either discrete or diffuse echoes or both have been detected during one or more soundings on at least half of all IMAGE orbits. In regions poleward of the plasmasphere, diffuse Z mode echoes of a kind reported by Carpenter et al. (2003) were found to accompany both discrete and diffuse whistler mode echoes 90% of the time and were also present during 90% of the soundings when no whistler mode echoes were detected. It is proposed that the observed discrete whistler mode echoes are a consequence of RPI signal reflections at the bottom side of the ionosphere and that diffuse whistler mode echoes are a result of scattering of RPI signals by geomagnetic field-aligned electron density irregularities located within 2000 km earthward of the satellite and in directions close to that of the field line passing through IMAGE. Diffuse Z mode echoes are believed to be due to scattering of RPI signals from electron density irregularities within 3000 km of the satellite, particularly those in the generally cross-B direction. Consistent with previous works, our results indicate that the magnetosphere at high latitudes is highly structured, with electron density irregularities that exist over cross-B scales ranging from 10 m to 100 km and that profoundly affect whistler mode propagation. It is demonstrated that both kinds of whistler mode echoes as well as diffuse Z mode echoes have potential for local and remote diagnostics of electron density distributions and structures. , et al. (2004), Diagnostics of magnetospheric electron density and irregularities at altitudes <5000 km using whistler and Z mode echoes from radio sounding on the IMAGE satellite,