D-region Altitudes Research Articles

Influence of meteoric smoke particles on the incoherent scatter measured with EISCAT VHF

Abstract. Meteoric ablation in the Earth's atmosphere produces particles of nanometer size and larger. These particles can become charged and influence the charge balance in the D region (60–90 km) and the incoherent scatter observed with radar from there. Radar studies have shown that, if enough dust particles are charged, they can influence the received radar spectrum below 100 km, provided the electron density is sufficiently high (>109 m3). Here, we study an observation made with the EISCAT VHF radar on 9 January 2014 during strong particle precipitation so that incoherent scatter was observed down to almost 60 km altitude. We found that the measured spectra were too narrow in comparison to the calculated spectra. Adjusting the collision frequency provided a better fit in the frequency range of ± 10–30 Hz. However, this did not lead to the best fit in all cases, especially not for the central part of the spectra in the narrow frequency range of ±10 Hz. By including a negatively charged dust component, we obtained a better fit for spectra observed at altitudes of 75–85 km, indicating that dust influences the incoherent-scatter spectrum at D-region altitudes. The observations at lower altitudes were limited by the small number of free electrons, and observations at higher altitudes were limited by the height resolution of the observations. Inferred dust number densities range from a few particles up to 104 cm−3, and average sizes range from approximately 0.6 to 1 nm. We find an acceptable agreement with the dust profiles calculated with the WACCM-CARMA (Whole Atmosphere Community Climate Model-Community Aerosol Radiation Model for Atmospheres) model. However, these do not include charging, which is also based on models.

Solar Flare Effects on the Earth’s Lower Ionosphere

Solar flares significantly impact the conditions of the Earth's ionosphere. In particular, the sudden increase in X-ray flux during a flare penetrates down to the lowest-lying D-region and dominates ionization at these altitudes (60-100 km). Measurements of very low frequency (VLF: 3-30kHz) radio waves that reflect at D-region altitudes provide a unique remote-sensing probe to investigate the D-region response to solar flare emissions. Here, using a combination of VLF amplitude measurements at 24kHz together with X-ray observations from the Geostationary Operational Environment Satellite (GOES) X-ray sensor, we present a large-scale statistical study of 334 solar flare events and their impacts on the D-region over the past solar cycle. Focusing on both GOES broadband X-ray channels, we investigate how the flare peak fluxes and position on the solar disk dictate an ionospheric response and extend this to investigate the characteristic time delay between incident X-ray flux and the D-region response. We show that the VLF amplitude linearly correlates with both the 1-8 A and 0.5-4 A channels, with correlation coefficients of 0.80 and 0.79, respectively. Unlike higher altitude ionospheric regions for which the location of the flare on the solar disk affects the ionospheric response, we find that the D-region response to solar flares does not depend on the flare location. By comparing the time delays between the peak X-ray fluxes in both GOES channels and VLF amplitudes, we find that there is an important difference between the D-region response and the X-ray spectral band. We also demonstrate for several flare events that show a negative time delay, the peak VLF amplitude matches with the impulsive 25-50 keV hard X-ray fluxes measured by the Ramaty High Energy Solar Spectroscopic Imager (RHESSI).

The Influence of Solar X-ray Flares on SAR Meteorology: The Determination of the Wet Component of the Tropospheric Phase Delay and Precipitable Water Vapor

In this work, we study the impact of high-energy radiation induced by solar X-ray flares on the determination of the temporal change in precipitable water vapor (ΔPWV) as estimated using the synthetic aperture radar (SAR) meteorology technique. As recent research shows, this radiation can significantly affect the ionospheric D-region and induces errors in the estimation of the total electron content (TEC) by the applied models. Consequently, these errors are reflected in the determination of the phase delay and in many different types of measurements and models, including calculations of meteorological parameters based on SAR observations. The goal of this study is to quantify the impact of solar X-ray flares on the estimation of ΔPWV and provide an estimate of errors induced if the vertical total electron content (VTEC) is obtained by single layer models (SLM) or multiple layer models (MLM) (these models do not include ionosphere properties below the altitude of 90 km as input parameters and cannot provide information about local disturbances in the D-region). The performed analysis is based on a known procedure for the determination of the D-region electron density (and, consequently, the vertical total electron content in the D-region (VTECD)) using ionospheric observations by very low frequency (VLF) radio waves. The main result indicates that if the D-region, perturbed by medium-sized and intense X-ray flares, is not modeled, errors occur in the determination of ΔPWV. This study emphasizes the need for improved MLMs for the estimation of the TEC, including observational data at D-region altitudes during medium-sized and intense X-ray flare events.

Anomalous variations of VLF sub-ionospheric signal and Mesospheric Ozone prior to 2015 Gorkha Nepal Earthquake

The subject of pre-earthquake ionospheric signatures has always been contentious and debatable. Some of the previous reports have documented unforeseen and unusual variations in some of the atmospheric and ionospheric parameters well before an earthquake. Here, we analyze the ionospheric response from the Indian Subcontinent to Nepal Gorkha Earthquakes occurred between April and May 2015, which were the most powerful and disastrous natural calamities in past ~80 years over the Himalayan region left ~9000 causalities and more than ~20000 people injured with the property damage of the order of several billion dollars. In view of severe earthquakes occurrences, their prior information on the shorter time scales are warranted for mitigation of associated disasters. Here, we report for the first time, a case which shows a strong link in anomalous variations between VLF sub-ionospheric signal and mesospheric ozone prior to both April 25, 2015 (Mw = 7.8) earthquake and its biggest aftershock on May 12, 2015 (Mw = 7.3). Observations show an unusual variation in VLF signals amplitude /shift in terminator time (TT) strongly linked with positive (negative) mesospheric ozone anomaly in D-region altitudes prior to the Gorkha Nepal earthquakes. It is surmised that simultaneous continuous observations of both VLF waves and mesospheric ozone can be considered as an important tool to identify the prior earthquake signatures in the vicinity of the extremely earthquake-prone zone such as Himalayan region. In this context, the current report opens up a new dimension in lithosphere-atmosphere-ionosphere coupling during the earthquake preparation processes itself.

Influence of Atmospheric Solar Radiation Absorption on Photodestruction of Ions at D-Region Altitudes of the Ionosphere

The influence of atmospheric solar radiation absorption on the photodetachment, dissociative photodetachment, and photodissociation rate coefficients (photodestruction rate coefficients) of O−, Cl−, O2 −, O3 −, OH−, NO2 −, NO3 −, O4 −, OH−(H2O), CO3 −, CO4 −, ONOO−, HCO3 −, CO3 −(H2O), NO3 −(H2O), O2 +(H2O), O4 +, N4 +, NO+(H2O), NO+(H2O)2, H+(H2O) n for n = 2–4, NO+(N2), and NO+(CO2) at D-region altitudes of the ionosphere is studied. A numerical one-dimensional time-dependent neutral atmospheric composition model has been developed to estimate this influence. The model simulations are carried out for the geomagnetically quiet time period of 15 October 1998 at moderate solar activity over the Boulder ozonesonde. If the solar zenith angle is not more than 90° then the strongest influence of atmospheric solar radiation absorption on photodestruction of ions is found for photodissociation of CO4 − ions when CO3 − ions are formed. It follows from the calculations that decreases in the photodestruction rate coefficients of ions under consideration caused by this influence are less than 2 % at 70 km altitude and above this altitude if the solar zenith angle does not exceed 90°.

Erratum to: Photochemistry of Ions at D-region Altitudes of the Ionosphere: A Review

The effective photoelectron ionization cross sections of the formation of N2 ? and N ions from N2 found from the ratios of photoelectron impact ionization to direct photoionization for N2 ? and N ions (Solomon and Qian 2005) were considered in the early version of the review by Pavlov (2014). In Table 1 of the review by Pavlov (2014), these effective photoelectron ionization cross sections were presented instead of the average photoionization cross sections of the formation of N2 ? and N ions from N2 whose values taken from Solomon and Qian (2005) are given here in Table 1. The review by Pavlov (2014) contains a typographic error in the total rate coefficient, Ktot, of the ONOO ? O3 ? O3 ? NO ? O2 and ONOO ? O3 ? NO2 ? 2O2 reactions [reactions 100 and 101 of Table 10 given by Pavlov (2014)]. The correct value of Ktot is equal to 7.5 9 10 -10 (300/Tn) 1.16 cm s where Tn is the neutral temperature expressed in Kelvins. This rate coefficient was measured by Viggiano et al. (2006) at 200 and 300 K temperature, and it was assumed by Pavlov (2014) that the temperature dependence of Ktot follows a power law. Over 87 % of the ONOO ? O3 reaction proceeded by producing O3 ? NO ? O2, and the amount of NO2 and 2O2 was less than 13 % of the products (Viggiano et al. 2006).

Photochemistry of Ions at D-region Altitudes of the Ionosphere: A Review

The current state of knowledge of the D-region ion photochemistry is reviewed. Equations determining production rates of electrons and positive ions by photoionization of atmospheric neutral species are presented and briefly discussed. Considerable attention is given to the progress in the chemistry of O+(4S), O+(2D), O+(2P), N+, N2+, O2+, NO+, N4+, O4+, NO+(N2), NO+(CO2), NO+(CO2)2, NO+(H2O)n for n = 1–3, NO+(H2O)(N2), NO+(H2O)2(N2), NO+(H2O)(CO2), NO+(H2O)2(CO2), O2+(H2O), H3O+(OH), H+(H2O)n for n = 1–8, O−, O2−, O3−, O4−, OH−, CO3−, CO4−, NO2−, NO3−, ONOO−, Cl−, Cl−(H2O), Cl−(CO2), HCO3−, CO3−(H2O), CO3−(H2O)2, NO3−(H2O), NO3−(H2O)2, OH−(H2O), and OH−(H2O)2 ions. The analysis of the D-region rocket ion mass spectrometer measurements shows that, among these ions, O2+, NO+, NO+(H2O), and H+(H2O)n for n = 1–7 can make the main contribution to the total positive ion number density, and O−, O2−, Cl−, OH−(H2O), CO3−, HCO3−, NO3−, ONOO−, CO4−, NO3−(H2O), NO3−(H2O)2, and 35Cl−(CO2) ions can be responsible for the main contribution to the total negative ion number density. Photodetachment of electrons from O−, Cl−, O2−, O3−, OH−, NO2−, and NO3−, dissociative electron photodetachment of O4− and OH−(H2O), and photodissociation of O3−, O4−, CO3−, CO4−, ONOO−, HCO3−, CO3−(H2O), NO3−(H2O), O2+(H2O), O4+, N4+, NO+(H2O), NO+(H2O)2, H+(H2O)n for n = 2–4, NO+(N2), and NO+(CO2) are studied, and the photodetachment and photodissociation rate coefficients are calculated using the current state of knowledge on the cross sections of these processes and fluxes of solar radiation.

Statistical signature of active D-region HF heating in IRIS riometer data from 1994–2004

Abstract. In this paper we study the effect of artificial HF heating on cosmic radio noise absorption in the D-region ionosphere. The effect has earlier been studied theoretically in idealised cases and without experimental verification. Here we present a 3-dimensional modelling of the effect, taking into account the directivity patterns of the vertical beam of the EISCAT Heater at Tromsø, Norway, and the intersecting beam of the IRIS imaging riometer at Kilpisjärvi, Finland. The heater-induced enhancement of cosmic radio noise absorption at the IRIS frequency (38.2 MHz) is estimated to be between 0.02 dB and 0.05 dB in the most representative model cases. However, a statistical study of IRIS data from a selected set of heating experiments carried out during the years 1994–2004 shows that the median effect is between 0.002 dB and 0.004 dB, i.e. an order of magnitude less than theoretically predicted. This indicates that the actual HF heating effect at D-region altitudes is substantially overestimated by the present theory.

First E- and D-region incoherent scatter spectra observed over Jicamarca

Abstract. We present here the first Jicamarca observations of incoherent scatter radar (ISR) spectra detected from E- and D-region altitudes. In the past such observations have not been possible at Jicamarca due a combined effect of strong equatorial electrojet (EEJ) clutter and hardware limitations in the receiving system. The observations presented here were made during weak EEJ conditions (i.e., almost zero zonal electric field) using an improved digital receiving system with a wide dynamic range and a high data throughput. The observed ISR spectra from E- and D-region altitudes are, as expected, narrow and get even narrower with decreasing altitude due to increasing ion-neutral collision frequencies. Therefore, it was possible to obtain accurate spectral measurements using a pulse-to-pulse data analysis. At lower altitudes in the D-region where signal correlation times are relatively long we used coherent integration to improve the signal-to-noise ratio of the collected data samples. The spectral estimates were fitted using a standard incoherent scatter (IS) spectral model between 87 and 120 km, and a Lorentzian function below 110 km. Our preliminary estimates of temperature and ion-neutral collisions frequencies above 87 km are in good agreement with the MSISE-90 model. Below 87 km, the measured spectral widths are larger than expected, causing an overestimation of the temperatures, most likely due to spectral distortions caused by atmospheric turbulence.

Ion chemistry of the lower thermosphere

Three questions of ion photochemistry in the lower thermosphere (80–130 km) are discussed. First, a complete explanation of the behaviour of metal ions in the D- and E-regions still presents a problem. Experimental data show that the width of the metal ion layers decreases as altitude increases from around 90 to 130 km. There seems also to be a diurnal variability of this width at altitudes below 100 km. The Fe + Mg + ratio changes from 2.2 in the lower part of the region to 0.7 in the upper part, the latter value being close to the Fe + Mg + ratio in chondrites. Secondly, at D-region altitudes, below 90–95 km, a principal role is played by the photochemistry of the cluster ions, governing the ionization recombination cycle. The ratio of cluster to normal ion concentrations, ƒ +, shows strong variability, depending on local time, season, various disturbances, etc. The main aspects of that variability have been described in terms of a strong ƒ + dependence on atmospheric temperature, humidity and ionization rate. As a next step in the ion composition data systematization the variability of the parameter R (describing the proportion between proton hydrates and other clusters) is considered. It is shown that R is a maximum in summer during NLC events and minimum during WA events. Thirdly, the major ions above 90 km are NO + and O + 2. Their concentration ratio at a fixed height changes depending on local time and on various disturbances. The behaviour of this ratio is in principle described by ion chemistry (dissociative recombination and ion-molecular reactions), and it is possible to estimate neutral nitric oxide concentrations in the E-region from the ion composition data.

Investigation of D-region plasma by modulation techniques

Ionospheric electrons of the D-region were investigated by means of cylindrical Langmuir probes. It was found that the energy distribution and the density of the electrons are affected by the presence of negative ions of ionospheric origin and by the high energetic protons which cause an auroral activity at arctic latitudes. A measurable difference is detected between the electron temperature and the neutral gas temperature at D-region altitudes. The fast protons of the aurora are believed to be the origin of the partial heating.

Some aspects of the metal ion chemistry of the Earth's atmosphere

Measurements of some metal ion reactions with ozone have been made in a flowing afterglow reaction system. The ions Fe+, Mg+, and Ca+ react rapidly to form the metal oxide ion; Na+ and K+ do not. Three-body reactions of the same metals with O2 and a third body argon to form the metal dioxide ions have also been measured. Again Fe+, Mg+, and Ca+ have large rate constants, but Na+ and K+ do not. The rapid Mg+ + O3 → MgO+ + O2 reaction is not effective in deionizing sporadic E layers of the ionosphere because it is followed by a rapid reaction MgO+ + O → Mg+ + O2, which has also been measured and which keeps the Mg+/MgO+ ratio large in the E region. A similar situation will probably occur for the other metal ions. Even efficient three-body association reactions are too slow for effective ion loss at E-region altitudes. It thus appears likely that ion chemistry does not play a significant role in the deionization of sporadic E layers in the ionosphere. The three-body ion association reactions may play an important role in meteor trail deionization at lower D-region altitudes.

The role of nonflare X radiation in theDregion

A compilation of short wavelength nonflare solar X-ray spectrums is used to evaluate the role of X radiation in the formation of the unperturbed D region. lonization contributions by solar hydrogen Lyman α and galactic cosmic radiation are compared with the contribution by X radiation. It is concluded that the pronounced dependence of X-ray flux on solar activity results in a model in which the D region is controlled by X radiation. At solar maximum the D region is found to be produced by X radiation and cosmic radiation. Lyman α becomes important at middle D-region altitudes about halfway between solar maximum and solar minimum. At solar minimum the D region is produced by cosmic, Lyman α, and X radiation in the lower, middle, and upper altitudes, respectively.

Effects of nuclear testing at medium frequencies

Numerous reports have appeared in recent months relating to the geophysical effects of high-altitude nuclear explosions. The observations of electromagnetic phenomena associated with the detonations have been primarily restricted to frequencies in the VLF range or to frequencies in the 10- to 100-Mc/s range. The effects have been observed in regions considerably remote from the blast area and have generally been attributed to enhanced ionization at D-region altitudes. Such enhancements would also be expected to affect the propagation of signals in the medium-frequency band.