Convective ionospheric storms: a review
TL;DR: A major goal of the National Space Weather Program, and of C/NOFS, is predicting these storms, analogous to thunderstorms in the lower atmosphere due to their adverse effects on communication and navigation signals.
Abstract: [1] Equatorial spread F (ESF) was discovered almost a century ago using the first radio wave instrument designed to study the upper atmosphere: the ionosonde. The name came from the appearance of reflections from the normally smooth ionosphere, which were spread over the altitude frequency coordinates used by the instrument. Attempts to understand this phenomenon in any depth activated such tools as radars and in situ probes such as rockets and satellites in the 1960s. Over the next 15 years, these tools expanded our experimental understanding enormously, and new nonlinear theoretical methods developed in the late 1970s, which led to proposing a name revision from ESF to convective ionospheric storms. Interest in these phenomena continues, but a new, practical aspect has developed from the associated turbulence effects on communications (transionosphere) and navigation (GPS). The first satellite to specifically investigate this problem and the associated goal of predicting occurrences is under the umbrella of the Communications/Navigation Outage Forecast System (C/NOFS). In contemplating the successful first years of the C/NOFS program, reviewing the state of the art in our knowledge of convective ionospheric storms seems appropriate. We also present some initial results of this satellite program. A major goal of the National Space Weather Program, and of C/NOFS, is predicting these storms, analogous to thunderstorms in the lower atmosphere due to their adverse effects on communication and navigation signals. Although ambitious, predictive capability is a noble and important goal in the current technological age and is potentially within our reach during the coming decade.
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TL;DR: In this article, the authors introduce a simplistic modelling framework that can integrate the various drivers to explain the emergence of bistability for shrub-encroached grassland systems and identify the basic stages in the transition from grassland to shrubland.
Abstract: Many arid grasslands around the world are affected by woody plant encroachment and by the replacement of a relatively continuous grass cover with shrub patches bordered by bare soil. This shift in plant community composition is often abrupt in space and time, suggesting that it is likely sustained by positive feedbacks between vegetation and environmental conditions (e.g. resource availability) or disturbance regime (e.g. fire or freeze). These feedbacks amplify the effects of drivers of shrub encroachment, i.e. of conditions favouring a shift from grass to shrub dominance (e.g. overgrazing, climate change). Here, we review some major drivers and feedbacks and identify the basic stages in the transition from grassland to shrubland. We discuss some possible scenarios of interactions between drivers and feedbacks that could explain the transition from a stage to the next and the potential irreversibility of the shift from grass to shrub dominance. We introduce a simplistic modelling framework that can integrate the various drivers to explain the emergence of bistability for shrub-encroached grassland systems. Published 2011. This article is a U.S. Government work and is in the public domain in the USA.
325 citations
01 Jul 2018
TL;DR: A brief review of the recent developments in the understanding of two major phenomena in low and mid-latitude ionosphere, the equatorial ionization anomaly (EIA) and involved equatorial plasma fountain (EPF) and ionospheric irregularities is presented in this article.
Abstract: Following a brief history and progress of ionospheric research, this paper presents a brief review of the recent developments in the understanding of two major phenomena in low and mid latitude ionosphere—the equatorial ionization anomaly (EIA) and involved equatorial plasma fountain (EPF) and ionospheric irregularities. Unlike the easy-to-understand misinterpretations, the EPF involves field perpendicularE×B plasma drift and field-aligned plasma diffusion acting together and plasma flowing in the direction of the resultant at all points along the field lines at all altitudes. The EIA is formed mainly from the removal of plasma from around the equator by the upward E×B drift creating the trough and consequently the crests with small accumulation of plasma at the crests when the crests are within ~±20° magnetic latitudes and no accumulation when they are beyond ~±25° magnetic latitudes. The strong EIA under magnetically active conditions arises from the simultaneous impulsive action of eastward prompt penetration electric field and equatorward neutral wind. Intense ionospheric irregularities develop in the post-sunset bottom-side equatorial ionosphere when it rises to high altitudes, and evolve nonlinearly into the topside. Pre-reversal enhancement (PRE) of the vertical upward E×B drift and its fluctuations amplified during PRE provide the driving force and seed, with neutral wind and gravity waves being the primary sources. At low solar activity especially in summer when fast varying PRE is absent, the slow varying gravity waves including large scale waves (LSW) seem to act as both driver and seed for weak irregularities. At mid latitudes, the irregularities are weak and associated with medium scale traveling ionospheric disturbances (MSTIDs). A low latitude minimum in the occurrence of the irregularities at March equinox predicted by theoretical models is identified. The minimum occurs on the poleward side of the EIA crest and shifts equatorward from ~25° magnetic latitudes at high solar activity to below 17° at low solar activity.
132 citations
Cites background from "Convective ionospheric storms: a re..."
...The irregularities are also called convective ionospheric storms (CIS) which may be more appropriate for their various forms of appearance and underlying physical processes (Kelley et al., 2011)....
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TL;DR: In this paper, ultraviolet images of emissions from the Earth's nighttime ionosphere were examined to determine the location of the equatorial ionization anomaly, regions of enhanced ionization that result in bands of nighttime airglow emission that typically appear parallel to the magnetic equator near +15° and −15° magnetic latitude.
Abstract: The National Aeronautics and Space Administration Global‐scale Observations of the Limb and Disk ultraviolet spectrograph has been imaging the equatorial ionization anomaly (EIA), regions of the ionosphere with enhanced electron density north and south of the magnetic equator, since October 2018. The initial 3 months of observations was during solar minimum conditions, and they included observations in December solstice of unanticipated variability and depleted regions. Depletions are seen on most nights, in contrast to expectations from previous space‐based observations. The variety of scales and morphologies also pose challenges to understanding of the EIA. Abrupt changes in the EIA location, which could be related to in situ measurements of large‐scale depletion regions, are observed on some nights. Such synoptic‐scale disruptions have not been previously identified. Plain Language Summary In this study, ultraviolet images of emissions from the Earth's nighttime ionosphere were examined to determine the location of the equatorial ionization anomaly, regions of enhanced ionization that result in bands of nighttime airglow emission that typically appear parallel to the magnetic equator near +15° and −15° magnetic latitude. We found that gaps in the anomaly are observed much more frequently in these observations than in previous space‐based observations. These gaps, sometimes referred to as ionospheric bubbles or depletions, are important because they are associated with ionospheric changes that can cause disruptions in communications and satellite navigation that depend on satellites, such as GPS. The location of the anomaly was also observed to vary significantly, by as much as 15°, from the typical latitudes. The observed level of variability seen during the unusually quiet geomagnetic conditions during which the observations occurred suggests that accurate predictions of the location and variability of the equatorial ionization anomaly requires significant advances in understanding the causes of this variability.
86 citations
TL;DR: In this article, an analysis of the occurrence of equatorial plasma bubbles (EPBs) around the world during the 2015 St. Patrick's Day geomagnetic storm is presented.
Abstract: An analysis of the occurrence of equatorial plasma bubbles (EPBs) around the world during the 2015 St. Patrick's Day geomagnetic storm is presented. A network of 12 Global Positioning System receivers spanning from South America to Southeast Asia was used, in addition to colocated VHF receivers at three stations and four nearby ionosondes. The suppression of postsunset EPBs was observed across most longitudes over 2 days. The EPB observations were compared to calculations of the linear Rayleigh-Taylor growth rate using coupled thermosphere-ionosphere modeling, which successfully modeled the transition of favorable EPB growth from postsunset to postmidnight hours during the storm. The mechanisms behind the growth of postmidnight EPBs during this storm were investigated. While the latter stages of postmidnight EPB growth were found to be dominated by disturbance dynamo effects, the initial stages of postmidnight EPB growth close to local midnight were found to be controlled by the higher altitudes of the plasma (i.e., the gravity term). Modeling and observations revealed that during the storm the ionospheric plasma was redistributed to higher altitudes in the low-latitude region, which made the plasma more susceptible to Rayleigh-Taylor growth prior to the dominance of the disturbance dynamo in the eventual generation of postmidnight EPBs.
84 citations
Cites background from "Convective ionospheric storms: a re..."
...Research on equatorial plasma bubbles (EPBs) has been driven in recent years by their negative influence on trans-ionospheric radio signals that are widely used for positioning, navigation, timing, and satellite communications [e.g., Kelley et al., 2011, 2014, and references therein]....
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TL;DR: A review of the current state of understanding regarding two classes of irregularities causing mesoscale structuring (hundreds of kilometers) in the nighttime ionosphere at low and mid-latitudes is presented in this paper.
Abstract: We present a review of the current state of understanding regarding two classes of irregularities causing mesoscale structuring (hundreds of kilometers) in the nighttime ionosphere at low- and mid-latitudes. Additionally, current state of understanding of equatorial plasma bubbles at low latitudes, and medium-scale traveling ionospheric disturbances at mid latitudes and their relationship to possible seeding from lower altitudes are described. In each case, well-developed linear theories exist to explain the general properties of the irregularities. However, these linear theories have growth rates too low to explain the actual observations, giving rise to the need to invoke seeding mechanisms. We describe the observational databases that have been compiled over the decades and discuss possible coupling and seeding mechanisms that would overcome the low growth rate and explain the observed structuring at the mesoscale. Future research directions are also briefly discussed.
70 citations
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TL;DR: The MSIS-86 empirical model of thermospheric temperature, density and composition as discussed by the authors uses new temperature and composition data from the Dynamics Explorer satellite to improve the representation of polar region morphology over that in theMSIS-83 model.
Abstract: The MSIS-86 empirical model of thermospheric temperature, density and composition uses new temperature and composition data from the Dynamics Explorer satellite to improve the representation of polar region morphology over that in the MSIS-83 model. Terms were added or changed to better represent seasonal variations in the polar regions under both quiet and magnetically disturbed conditions. Local time variations in the magnetic activity effect were added. In addition a new species, atomic nitrogen, was added to the previous list of N2, O2, He, O, H, and Ar covered by the model.
1,699 citations
TL;DR: In this paper, the results of backscatter observations of the F region irregularities made with the large 50MHz radar at Jicamarca, Peru, during a few days of observations are presented.
Abstract: The paper presents some results of backscatter observations of the F region irregularities made with the large 50-MHz radar at Jicamarca, Peru, during a few days of observations. The results were obtained by using three observational techniques: the modified range-time-intensity technique, the digital power mapping technique, and the digital raw data recording technique. Backscatter intensity maps as a function of altitude and time are presented, which can be interpreted as radar pictures of F region irregularities. A classification of spread F spectral signatures resulting from approximately 30,000 spectra obtained in sets of 64 simultaneous heights under a variety of conditions is also given.
917 citations
Book•
01 Jan 1989
TL;DR: In this article, the authors describe the buffeting of the ionosphere from above by the sun and from below by the lower atmosphere, and explore the plasma physics and electrodynamics of the system.
Abstract: Although interesting in its own right, due to the ever-increasing use of satellites for communication and navigation, weather in the ionosphere is of great concern. Every such system uses trans-ionospheric propagation of radio waves, waves which must traverse the commonly turbulent ionosphere. Understanding this turbulence and predicting it are one of the major goals of the National Space Weather program. Acquiring such a prediction capability will rest on understanding the very topics of this book, the plasma physics and electrodynamics of the system. *Fully updated to reflect advances in the field in the 20 years since the first edition published *Explores the buffeting of the ionosphere from above by the sun and from below by the lower atmosphere *Unique text appropriate both as a reference and for coursework.
827 citations
TL;DR: Immel et al. as discussed by the authors showed that ionospheric densities vary with the strength of nonmigrating, diurnal atmospheric tides that are, in turn, driven mainly by weather in the tropics.
Abstract: [1] A newly discovered 1000-km scale longitudinal variation in ionospheric densities is an unexpected and heretofore unexplained phenomenon. Here we show that ionospheric densities vary with the strength of nonmigrating, diurnal atmospheric tides that are, in turn, driven mainly by weather in the tropics. A strong connection between tropospheric and ionospheric conditions is unexpected, as these upward propagating tides are damped far below the peak in ionospheric density. The observations can be explained by consideration of the dynamo interaction of the tides with the lower ionosphere (E-layer) in daytime. The influence of persistent tropical rainstorms is therefore an important new consideration for space weather. Citation: Immel, T. J., E. Sagawa, S. L. England, S. B. Henderson, M. E. Hagan, S. B. Mende, H. U. Frey, C. M. Swenson, and L. J. Paxton (2006), Control of equatorial ionospheric morphology by atmospheric tides, Geophys. Res. Lett., 33, L15108, doi:10.1029/2006GL026161. [2] The ionosphere is the region of highest plasma density in Earth’s space environment. It is a dynamic environment supporting a host of plasma instability processes, with important implications for global communications and geo-location applications. Produced by the ionization of the neutral atmosphere by solar x-ray and UV radiation, the uppermost ionospheric layer has the highest plasma density with a peak around 350–400 km altitude and primarily consists of O + ions. This is called the F-layer and it is considered to be a collisionless environment such that the charged particles interact only weakly with the neutral atmosphere, lingering long after sunset. The E-layer is composed of molecular ions and is located between 100–150 km where collisions between ions and neutrals are much more frequent, with the result that the layer recombines and is reduced in density a hundredfold soon after sunset [Rees ,1 989;Heelis, 2004]. The respective altitude regimes of these two layers are commonly called the E- and F-regions. [3] The ionosphere glows as O + ions recombine to an excited state of atomic oxygen (O I) at a rate proportional to
597 citations
"Convective ionospheric storms: a re..." refers background in this paper
...This interesting tidal behavior was first seen in airglow data [Immel et al., 2006]....
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TL;DR: In this article, the authors present a global empirical model for the F region equatorial vertical drifts based on combined incoherent scatter radar observations at Jicamarca and Ion Drift Meter observations on board the Atmospheric Explorer E satellite.
Abstract: We present the first global empirical model for the quiet time F region equatorial vertical drifts based on combined incoherent scatter radar observations at Jicamarca and Ion Drift Meter observations on board the Atmospheric Explorer E satellite. This analytical model, based on products of cubic-B splines and with nearly conservative electric fields, describes the diurnal and seasonal variations of the equatorial vertical drifts for a continuous range of all longitudes and solar flux values. Our results indicate that during solar minimum, the evening prereversal velocity enhancement exhibits only small longitudinal variations during equinox with amplitudes of about 15–20 m/s, is observed only in the American sector during December solstice with amplitudes of about 5–10 m/s, and is absent at all longitudes during June solstice. The solar minimum evening reversal times are fairly independent of longitude except during December solstice. During solar maximum, the evening upward vertical drifts and reversal times exhibit large longitudinal variations, particularly during the solstices. In this case, for a solar flux index of 180, the June solstice evening peak drifts maximize in the Pacific region with drift amplitudes of up to 35 m/s, whereas the December solstice velocities maximize in the American sector with comparable magnitudes. The equinoctial peak velocities vary between about 35 and 45 m/s. The morning reversal times and the daytime drifts exhibit only small variations with the phase of the solar cycle. The daytime drifts have largest amplitudes between about 0900 and 1100 LT with typical values of 25–30 m/s. We also show that our model results are in good agreement with other equatorial ground-based observations over India, Brazil, and Kwajalein.
571 citations