Doppler spectral properties of 50 MHz auroral echoes observed at several magnetic aspect angles are presented. The measurements were made with high resolution CW Doppler systems at 10 different aspect angles ranging between 1 and 14 degrees, during a series of experiments carried out the summers of 1981, 1982 and 1983 in western Canada. The extensive data set was representative of several backscatter periods and different geomagnetic conditions. We found that three basic spectral types associated with different irregularity groups and classified as type 1, 2 and 3, existed throughout the aspect angle range. Type 1 and 2 spectra are reminiscent of those observed in both the equatorial and auroral backscatter at small aspect angles, whereas type 3 is a newly established spectral signature possibly related to electrostatic ion cyclotron waves. The evidence suggests that 3 m primary and secondary plasma waves can exist in aurora with k||/k⊥ wavenumber ratios perhaps larger than 0.25. Statistical analysis of the data showed the magnetic aspect angle effect upon the Doppler spectrum properties to be weak for all types of irregularities at 50 MHz. It is now clear that equatorial E region instability theories, which had been applied directly to aurora, need important modifications, and possibly the inclusion of field aligned current effects, to account for the magnetic aspect angle observations.
Abstract The Upper Mesosphere–Lower Thermosphere (UMLT) region of the atmosphere is known to vary on many temporal and spatial scales. However, this region of the atmosphere is very difficult to measure and monitor continuously. In this paper, we demonstrate an intriguing connection between mesopause temperatures and the intensity of very low frequencies (VLF) narrowband (NB) signals reflected off the lower ionosphere. The temperature data used are from the SABER instrument onboard the TIMED satellite, while the VLF data are obtained from various ground‐based receiving systems. The results of the analysis show a high anticorrelation between temperature and VLF amplitude. It is shown that the variability of the UMLT temperatures and VLF amplitudes can be explained by global seasonal solar irradiance changes (~72% of the variability), while the remaining variability has its origins from other sources (~28%). High‐resolution mesopause temperature estimates might be achieved in the future by combining VLF NB observations and calculated solar irradiance variability (as a function of hour, day, and location, i.e., latitude).
The Valensole high frequency (HF) radar in the south of France is an ionospheric Doppler sounder which can perform E region coherent backscatter measurements over an azimuthal sector of 86°, from 26° E to 58° W, with ∼2° angular resolution. This large azimuthal coverage is taken advantage of in order to study quasiperiodic (QP) echoes in the zonal direction using azimuth‐time‐intensity (ATI) analysis. ATI plots show sequential sloping striations of scatter reminiscent of those detected routinely in the range‐time‐intensity (RTI) plots of midlatitude radars which view the medium at a fixed azimuth about the meridian. It was found that ATI striation periods range from a few minutes to less than 30 min, whereas the striation slopes are systematically negative (motions westward) prior to local midnight, and turn positive (motions eastward) in the post‐midnight hours. The zonal rates, dx/dt , computed from the striation slopes take values between ∼30 and 160 m/s. These are due to real motions of unstable plasma structures, most likely sporadic E patches that drift along with the neutral wind, that have zonal scale lengths of several tens of kilometers. The present observations imply that the mechanism responsible for QP echoes is independent of azimuth and can basically operate effectively in any direction in the horizontal plane.
Dataset and FORTRAN program used to produce Figure 4, of the paper (MS# 2023JA031336) entitled "Understanding the diurnal variation of midlatitude sporadic E. The role of metal atoms- ", which was submitted for publication in the Journal of Geophysical Research - Space Physics. The data file was provided by Qihou Zhou of Miami University in Ohio and the program by co-author Chris Meek, University of Saskatchewan.
The paper reviews recent advances in studies of electric discharges in the stratosphere and mesosphere above thunderstorms, and their effects on the atmosphere. The primary focus is on the sprite discharge occurring in the mesosphere, which is the most commonly observed high altitude discharge by imaging cameras from the ground, but effects on the upper atmosphere by electromagnetic radiation from lightning are also considered. During the past few years, co-ordinated observations over Southern Europe have been made of a wide range of parameters related to sprites and their causative thunderstorms. Observations have been complemented by the modelling of processes ranging from the electric discharge to perturbations of trace gas concentrations in the upper atmosphere. Observations point to significant energy deposition by sprites in the neutral atmosphere as observed by infrasound waves detected at up to 1000 km distance, whereas elves and lightning have been shown significantly to affect ionization and heating of the lower ionosphere/mesosphere. Studies of the thunderstorm systems powering high altitude discharges show the important role of intracloud (IC) lightning in sprite generation as seen by the first simultaneous observations of IC activity, sprite activity and broadband, electromagnetic radiation in the VLF range. Simulations of sprite ignition suggest that, under certain conditions, energetic electrons in the runaway regime are generated in streamer discharges. Such electrons may be the source of X- and Gamma-rays observed in lightning, thunderstorms and the so-called Terrestrial Gamma-ray Flashes (TGFs) observed from space over thunderstorm regions. Model estimates of sprite perturbations to the global atmospheric electric circuit, trace gas concentrations and atmospheric dynamics suggest significant local perturbations, and possibly significant meso-scale effects, but negligible global effects.
Crossed‐beam spectral measurements, made with two bistatic CW Doppler systems operating at 50 MHz, often reveal a type of radio auroral spectrum characterized by narrow peaks near 30 and 55 Hz. The experiment was designed so that spectra could be measured simultaneously from a common scattering region along two directions which differed in azimuth by 65° and which were at magnetic aspect angles of ∼5° and ∼12° from perpendicularity with the earth's field at 110 km altitude. During two magnetically active periods of continuous backscatter, this spectral type was found to occur regularly, especially in the few hours prior to magnetic midnight. Usually, the echoes were strong and lasted several minutes. The resonant peak in the spectrum occurs at either positive or negative shifts, or sometimes both. Mostly, the power lies in a narrow Doppler frequency band centered at either 55 or 30 Hz, but sometimes the power is in both bands, particularly for the path of larger magnetic aspect angle. The spectra can be almost identical in both observing directions but also can show systematic differences. From the relatively fixed locations of the spectral peaks, and from the occurrence of the echoes at angles well away from perpendicularity with the magnetic field, one infers that the echoes are due to primary 3‐m plasma waves having a finite wave vector component k ∥ along the magnetic field lines and with ratio k ∥ /k ⊥ possibly as large as 1/3.5. The spectral evidence suggests the existence of electrostatic ion cyclotron waves near the gyrofrequencies of the main E region ions (i.e., NO + , O 2 + , O + ). The existing kinetic and fluid theories of the ion cyclotron instability explain some, but not all, of the observed properties. Kinetic theory cannot explain the direct excitation of short (3 m) waves in the E region at such large magnetic aspect angles. Yet the frequent observation of these waves shows that the theories must be modified so as to relax the conditions under which the waves occur and to allow for Doppler shifts at almost exactly the gyrofrequencies of the main ions.
There is growing experimental evidence to suggest that mesoscale spread F is linked to the occurrence of midlatitude coherent backscatter from patchy sporadic‐ E layers, which are unstable to the gradient‐drift and Farley‐Buneman plasma instabilities. To validate this suggestion, we have compared E ‐region backscatter and spread‐ F ionosonde recordings from about 100 days of joint operation during summer and found a one‐to‐one relation in the occurrence of both phenomena. Also, midlatitude backscatter studies over the last few years have shown the existence of enhanced electric fields inside patchy sporadic E . These are believed to be polarization fields set up locally by neutral winds that transport the plasma patches horizontally, and by the relatively large Hall‐to‐Pedersen conductivity ratios at E ‐region altitudes. Moreover, midlatitude echoes were found to be associated with mostly westward drifting sporadic‐ E patches with typical scale lengths from 10 to more than 100 km and perturbed eastward electric fields from a few to maybe more than 10 to 15 mV/m. We propose that the enhanced polarization fields set up inside unstable sporadic‐ E patches can easily map up the magnetic field lines to the F region and thus contribute to the formation of midlatitude spread F . This new mechanism for spread‐ F generation is basically an image process that can account for key observational properties of the phenomenon. These include the rapid plasma upwelling and the abrupt changes in height (uplifts) of the F layer, as well as the scale sizes involved and morphological characteristics.
Abstract. Based on recent developments in the formalism governing the spectral resonance structures (SRS) of the Ionospheric Alfvén resonator (IAR) as observed on the Earth's surface, a numerical code was developed to investigate properties of SRS which can in particular be contributed to magnetic inclination effects. Among the theoretical findings are: 1) SRS is discernible in both orthogonal components, 2) the harmonic structure of SRS is not anymore over frequency equidistantly distributed, 3) the frequency scales of SRS differ in the two normal modes. The theoretically predicted properties could be found in the observations of a low latitude and some of them even in the data of a mid latitude station. The verification, however, is not as straight forward because the predicted effects do not only depend on magnetic inclination but also on the wave angle of the lightning induced electromagnetic wave k-vector with the normal to the magnetic meridian passing through the observation site. So far the formalism is simplified as it deals with a single source situation alone whereas the actual observation is a composite of excitations caused by an average of about 60 flashes of lightning operating all the time, world-wide.
'%3e%3cpath fill='%23012169' d='M0 0v30h60V0z'/%3e%3cpath stroke='%23fff' stroke-width='6' d='m0 0 60 30m0-30L0 30'/%3e%3cpath stroke='%23C8102E' stroke-width='4' d='m0 0 60 30m0-30L0 30' clip-path='url(%23b)'/%3e%3cpath stroke='%23fff' stroke-width='10' d='M30 0v30M0 15h60'/%3e%3cpath stroke='%23C8102E' stroke-width='6' d='M30 0v30M0 15h60'/%3e%3c/g%3e%3c/svg%3e)