Sustained heating of lower ionospheric electrons by thundercloud fields, as recently suggested by Inan et al. [1996], may lead to the production of enhanced infrared (IR) emissions, in particular 4.3‐ µ m CO 2 emission. The excitation rate for N 2 (v) via electron collisions is calculated using a new steady‐state two‐dimensional electrostatic‐heating (ESH) model of the upward coupling of the thundercloud (TC) electric fields. The vibrational energy transfer to CO 2 and 4.3‐ µ m radiative transfer are then computed using a line‐by‐line non‐LTE (non‐local thermodynamic equilibrium) radiation model. Limb‐viewing radiance profiles at 4.3‐ µ m and typical radiance spectra are estimated for five different TC charge distributions and ambient ionic conductivities. Broadband 4.3‐ µ m enhancements of greater than a factor of two above ambient nighttime levels are predicted for tangent heights (TH) in the range ∼80 to >130 km for the most perturbed case, with larger enhancements in selected narrower spectral regions. The predicted IR enhancements should be observable to an orbiting IR sensor.
48.5 kHz signals from a transmitter in Silver Creek, Nebraska, propagating to Huntsville (HU), Alabama over a ∼1200 km Great Circle Path (GCP) exhibit characteristic amplitude changes which appear within 20 ms of cloud‐to‐ground (CG) flashes located within 50 km of the path. Data are consistent with the heating of ionospheric electrons by the electromagnetic (EM) pulse from lightning producing ionization changes in the D‐region over the thunderstorm.
Simultaneous observations of early/fast Very Low Frequency (VLF) events at nine closely spaced (∼65 km) sites are used together with a numerical model of the propagation and scattering of VLF signals in the earth‐ionosphere waveguide to directly measure the scattering pattern of associated ionospheric disturbances. In cases when the causative lightning is within 700 km of the north‐south array of observing sites, early/fast VLF events are typically observed at no more than 2 or 3 sites, which indicates a narrow beam of the scattered signal in the forward direction. In the different cases studied, forward scattering patterns exhibit 15 dB beamwidths of less than 30° consistent with horizontal extent of 90±30 km.
Observations on the night of 21 July 2003 of the ionospheric effects of a thunderstorm in central France are reported. From 0200 to 0315 UT, a camera system in the Pyrenees Mountains captured 28 sprites, triggered by +CG lightning as observed by the French METEORAGE lightning detection system. A narrowband VLF receiver located on Crete, at ∼2200 km southeast of the storm, observed subionospheric VLF signals from six ground‐based transmitters. The amplitude of one of the VLF signals, originating at a transmitter located ∼150 km west of the storm and passing through the storm region, exhibited rapid onset perturbations occurring in a nearly one‐to‐one relationship with the optical sprites. These “early” VLF events are consistent with a process of narrow‐angle forward scattering from a volume of enhanced ionization above the storm with lateral sizes larger than the VLF radio wavelength. The many +CG and −CG discharges that did not produce sprites were also found to not be associated with detectable VLF amplitude perturbations, even though some of these discharges reached relatively large peak currents. The rapid onsets of several of the sprite‐related VLF perturbations were followed by relatively long onset durations, ranging from ∼0.5 to 2.5 s, indicating that these events were early but not “fast.” These “early/slow” events may suggest a slow process of ionization build‐up in the lower ionosphere, following intense lightning discharges that also lead to sprites. A limited number of early VLF perturbation events were also associated with whistler‐induced electron precipitation events, or classic Trimpi perturbations, undoubtedly produced by the precipitation of electrons due to whistler‐mode waves injected into the magnetosphere by the same lightning flash that led to the production of the sprite.
Global images of the plasmasphere obtained by the Extreme Ultraviolet (EUV) imager on the IMAGE satellite are used to study the evolving structure of the plasmasphere during two geomagnetic disturbances. By tracking the location of the plasmapause as a function of L shell and magnetic local time, quantitative measurements of radial and azimuthal motions of the boundary are made for intervals ≥7 hours in duration with a time resolution of 10 min. The two cases presented are 26–27 June 2001, a relatively weak but isolated geomagnetic disturbance, and 9–10 June 2001, a moderate event with a multistaged onset and recurring substorm activity after the main disturbance. In both cases the onset of the disturbance, correlated with a southward turning of the IMF, is characterized by inward motion or erosion of the plasmapause and a smoothing of any existing azimuthal variations across the nightside. Over a period of many hours, a plasmaspheric plume forms in the afternoon sector as a result of sunward flows from dusk and corotational flows across the dayside. Azimuthal variations in the plasmapause radius tend to form in the local time sector from dawn to the western edge of the plume, including mesoscale (≤0.5 in L and ≤2 hours in local time) crenulations and larger‐scale shoulder features, while the nightside boundary remains featureless. In the 26–27 June 2001 case, the magnetosphere entered a period of deep quiet after the main disturbance, and the plasmaspheric plume began to corotate with the main plasmasphere from the afternoon sector across the nightside. In contrast, the plume in the 9–10 June 2001 event became wrapped around the main plasmasphere and a second plume formed in the afternoon sector, perhaps due to continued substorm activity. In situ density data for these events show highly irregular density structure within the plumes as measured at geosynchronous orbit, whereas a measurement by IMAGE RPI suggests that there may be less structure near the base of the plume closer to the main plasmasphere.
We present results from numerical studies of whistler mode wave propagation in the Earth's magnetosphere. Numerical simulations, based on the novel algorithm, solving one‐dimensional electron‐MHD equations in the dipole coordinate system, demonstrate that the amplitude (and power) of the whistler mode waves generated by the ground‐based transmitter can be significantly increased in some particular location along the magnetic field line (for example, at the equatorial magnetosphere) by the frequency modulation of the transmitted signal. The location where the amplitude of the signal reaches its maximum is defined by the time delay between different frequency components of the signal. Simulations reveal that a whistler mode wave with a discrete frequency modulation (where the frequency changes by a finite step) in the range from 1 to 3 kHz can be compressed as efficiently as a signal with a continuous frequency modulation when the frequency difference between components of the discrete‐modulated signal is not greater than 100 Hz.
We analyze nightside measurements of the DEMETER spacecraft related to lightning activity. At the 707 km altitude of DEMETER, we observe 3‐D electric and magnetic field waveforms of fractional‐hop whistlers. At the same time, the corresponding atmospherics are recorded by a very low frequency (VLF) ground‐based station located in Nançay (France). The source lightning strokes are identified by the METEORAGE lightning detection network. We perform multidimensional analysis of the DEMETER measurements and obtain detailed information on wave polarization characteristics and propagation directions. This allows us for the first time to combine these measurements with ray‐tracing simulation in order to directly characterize how the radiation penetrates upward through the ionosphere. We find that penetration into the ionosphere occurs at nearly vertical wave vector angles (as was expected from coupling conditions) at distances of 100–900 km from the source lightning. The same distance is traveled by the simultaneously observed atmospherics to the VLF ground station. The measured dispersion of fractional‐hop whistlers, combined with the ionosonde measurements at the Ebro observatory in Spain, allows us to derive the density profile in the topside ionosphere.
[1] Bell et al. (2009) proposed that the source region for banded chorus consists of whistler mode ducts of depleted electron density (Ne) for upper band (UB) chorus for all wave normal angles (θ) and ducts of either enhanced or depleted Ne for lower band (LB) chorus for small θ and θ near or greater than the Gendrin angle, respectively. This paper provides support for this model using new high resolution (17 km) Ne observations from the Cluster WHISPER and EFW instruments. Data is examined from January 20, 2004, when strong banded chorus was observed over 3000 km of the Cluster 2 orbit, ending at the magnetic equator. Previous analysis of LB chorus on this day indicated the wave normal angles were larger than the Gendrin angle. Using the Ne data, we show that the LB chorus is generated within depletion ducts in the source region and that the half-width of these ducts (∼70 km) is comparable to the transverse scale (∼100 km) of the chorus source region. Making use of the fact that the group velocity of the LB chorus waves has a significant cross-L component, we show that the source region extends over 500 km near the magnetic equator.
A test particle approach is used to compare gyroresonant pitch angle scattering of energetic electrons by coherent versus incoherent whistler mode waves, for the case in which the coherent wave amplitude is below the nonlinear phase trapping threshold. Wave packets of 400 ms duration propagating along the magnetic field at L = 4 within the plasmasphere are considered, and the wave‐induced pitch angle scattering along the propagation path from one hemisphere to the other and the resulting precipitation flux are computed. An incoherent wave spectrum is simulated by random modulation of the wave frequency at intervals of 1 ms, thereby generating signals with nearly constant power spectral density over a bandwidth of 2 kHz centered at 5.5 kHz. The associated pitch angle scattering is compared with that of a monochromatic 5.5‐kHz signal of 400 ms duration. Results of the test particle analysis are compared with those expected on the basis of a classical diffusion treatment, and an expression is derived for an effective “diffusion” coefficient for pitch angle scattering by coherent waves. The trajectory followed by a particle when interacting with incoherent waves essentially represents a random walk in velocity space, while for coherent waves the pitch angle of the particle varies in a well‐defined manner. In spite of the fact that individual particle scatterings are typically larger for coherent waves, the peak precipitation fluxes induced by incoherent waves are found to be approximately the same as those for coherent waves having the same total power. This results from the fact that incoherent waves interact with particles over a wider range of energies. As a consequence, the energy spectrum and the temporal extent of transient precipitation pulses due to incoherent wave packets are broader than those for equivalent coherent ones.
