John M. Retterer

Forecasting low‐latitude radio scintillation with 3‐D ionospheric plume models: 1. Plume model

[1] A three‐dimensional model has been developed for the plasma plumes caused by interchange instabilities in the low‐latitude ionosphere to describe the structure and extent of the radio scintillation generated by turbulence in and around the plumes (down to the scale sizes resolvable by the computer model). With the inclusion of the processes that determine the transport of plasma parallel to the geomagnetic field lines as well as transverse to them, the model can predict the extent in latitude of the plumes and their scintillation. To better reflect the day‐to‐day variability of the occurrence of the plumes, the model is closely coupled to a time‐dependent model of the ambient ionosphere to describe the changing conditions under which the plasma instabilities that cause the turbulence must act. Diagnostics presented here will illustrate the density structures found in the models of the plumes, including maps of airglow emissions which show the effect of the density depletions within the plumes. A companion paper presents a phase‐screen calculation of the amplitude scintillation caused by the plumes. Citation: Retterer, J. M. (2010), Forecasting low‐latitude radio scintillation with 3‐D ionospheric plume models: 1. Plume model, J. Geophys. Res., 115, A03306, doi:10.1029/2008JA013839.

Global equatorial plasma bubble occurrence during the 2015 St. Patrick's Day storm

An analysis of the occurrence of equatorial plasma bubbles (EPBs) around the world during the 2015 St. Patricks 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.

Geomagnetic control of equatorial plasma bubble activity modeled by the TIEGCM with Kp

Describing the day-to-day variability of Equatorial Plasma Bubble (EPB) occurrence remains a significant challenge. In this study we use the Thermosphere-Ionosphere Electrodynamics General Circulation Model (TIEGCM), driven by solar (F10.7) and geomagnetic (Kp) activity indices, to study daily variations of the linear Rayleigh-Taylor (R-T) instability growth rate in relation to the measured scintillation strength at five longitudinally distributed stations. For locations characterized by generally favorable conditions for EPB growth (i.e., within the scintillation season for that location), we find that the TIEGCM is capable of identifying days when EPB development, determined from the calculated R-T growth rate, is suppressed as a result of geomagnetic activity. Both observed and modeled upward plasma drifts indicate that the prereversal enhancement scales linearly with Kp from several hours prior, from which it is concluded that even small Kp changes cause significant variations in daily EPB growth.

An analysis of the quiet time day‐to‐day variability in the formation of postsunset equatorial plasma bubbles in the Southeast Asian region

Presented is an analysis of the occurrence of postsunset Equatorial Plasma Bubbles (EPBs) detected using a Global Positioning System (GPS) receiver at Vanimo. The three year data set shows that the EPB occurrence maximizes (minimizes) during the equinoxes (solstices), in good agreement with previous findings. The Vanimo ionosonde station is used with the GPS receiver in an analysis of the day-to-day EPB occurrence variability during the 2000 equinox period. A superposed epoch analysis (SEA) reveals that the altitude, and the change in altitude, of the F layer height is ∼1 standard deviation (1σ) larger on the days for which EPBs were detected, compared to non-EPB days. These results are then compared to results from the Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM), which show strong similarities with the observations. The TIEGCM is used to calculate the flux-tube integrated Rayleigh-Taylor (R-T) instability linear growth rate. A SEA reveals that the modeled R-T growth rate is 1σ higher on average for EPB days compared to non-EPB days, and that the upward plasma drift is the most dominant contributor. It is further demonstrated that the TIEGCMs success in describing the observed daily EPB variability during the scintillation season resides in the variations caused by geomagnetic activity (as parameterized by Kp) rather than solar EUV flux (as parameterized by F10.7). Geomagnetic activity varies the modeled high-latitude plasma convection and the associated Joule heating that affects the low-latitude F region dynamo, and consequently the equatorial upward plasma drift. Key Points Day-to-day EPB occurrence in Southeast Asia investigated using ground-based GPS TIEGCM exhibits similiar daily variability to the EPB observations Small changes in Kp strongly influence daily EPB occurrence variability.

Using solar wind data to predict daily GPS scintillation occurrence in the African and Asian low‐latitude regions

The feasibility of predicting the daily occurrence of Global Positioning System scintillation events using forecasts of common geophysical indices to drive a physics-based model of the system is demonstrated over a 5 month period for the African and Asian longitude sectors. The output from the Wing Kp model, which uses solar wind data to predict the geomagnetic activity level up to 4 h in advance, was used to drive the National Center for Atmospheric Research thermosphere/ionosphere model, from which the strength of the Rayleigh-Taylor instability growth rate was calculated to determine the likelihood of scintillation. It is found that the physics-based model demonstrates superior skill to an empirical scintillation model (Wideband Model (WBMOD)) in forecasting scintillation suppression events during seasons when scintillation is common. However, neither of the models driven in this way possess the ability to forecast isolated scintillation events during transitional and off-peak seasons.

Electromagnetic tornadoes in space: Ion conics along auroral field lines generated by lower hybrid waves and electromagnetic turbulence in the ion cyclotron range of frequencies

Abstract The exotic phenomenon of energetic ion conic formation by plasma waves in the magnetosphere is considered. Two particular transverse heating mechanisms are reviewed in detail: lower hybrid energization of ions in the boundary layer of the plasma sheet and electromagnetic ion cyclotron resonance heating in the central region of the plasma sheet. Mean particle calculations, plasma simulations and analytical treatments of the heating processes are described.

Effects of thruster firings on the shuttle's plasma and electric field environment

Simultaneous plasma and AC/DC electric field measurements taken during the space shuttle mission STS-4 at times of prolonged thruster firings are analyzed and cross correlated. Depending on the orientation of the shuttles velocity vector to the magnetic field, ion densities and electric field wave spectra were enhanced or decreased. The systematic picture of interactions within the shuttles plasma/neutral gas environment of Cairns and Gurnett (1991b) is confirmed and extended. Waves are excited by outgassed and thruster-ejected molecules that ionize in close proximity to the shuttle. On time scales significantly less than an ion gyroperiod, the newly created ions act as beams in the background plasma. These beams are sources of VLF waves that propagate near the shuttle and intensify during thruster firings. Plasma density depletions and/or the shuttles geometry may hinder wave detection in the payload bay. A modified two-stream analysis indicates that beam components propagating at large angles to the magnetic field are unstable to the growth of lower hybrid waves. The beam-excited, lower hybrid waves heat some electrons to sufficient energies to produce impact ionization. Empirical evidence for other wave-growth mechanisms outside the lower-hybrid band is presented.

Observations of a transverse magnetic field perturbation at two altitudes on the equatorward edge of the magnetospheric cusp

On January 28, 1990, the Dynamics Explorer 1 and Akebono satellites crossed a magnetic field structure at the equatorward edge of the polar cusp at altitudes of 22,000 and 5000 km, respectively, within 6 min of each other. Locally measured plasma particles and fields and magnetometer data from a ground station near the foot of the magnetic field line are more consistent with an interpretation of the structure as that of a standing Alfven wave than that of a quasi-steady field-aligned current sheet. We discuss the observations supporting this conclusion and other related observations of field-aligned currents, Alfven waves, and ion energization near the equatorward edge of the cusp. These observations suggest that Alfven waves are commonly present near the equatorward edge of the cusp.

Artificial ionospheric modification: The Metal Oxide Space Cloud experiment

Clouds of vaporized samarium (Sm) were released during sounding rocket flights from the Reagan Test Site, Kwajalein Atoll in May 2013 as part of the Metal Oxide Space Cloud (MOSC) experiment. A network of ground-based sensors observed the resulting clouds from five locations in the Republic of the Marshall Islands. Of primary interest was an examination of the extent to which a tailored radio frequency (RF) propagation environment could be generated through artificial ionospheric modification. The MOSC experiment consisted of launches near dusk on two separate evenings each releasing ~6 kg of Sm vapor at altitudes near 170 km and 180 km. Localized plasma clouds were generated through a combination of photoionization and chemi-ionization (Sm + O → SmO+ + e–) processes producing signatures visible in optical sensors, incoherent scatter radar, and in high-frequency (HF) diagnostics. Here we present an overview of the experiment payloads, document the flight characteristics, and describe the experimental measurements conducted throughout the 2 week launch window. Multi-instrument analysis including incoherent scatter observations, HF soundings, RF beacon measurements, and optical data provided the opportunity for a comprehensive characterization of the physical, spectral, and plasma density composition of the artificial plasma clouds as a function of space and time. A series of companion papers submitted along with this experimental overview provide more detail on the individual elements for interested readers.

Critical points in the 16-moment approximation

The singular points in steady state, field-aligned plasma transport models based on velocity moment theory are examined. In particular, two separate singular points in the equations obtained from the 16-moment approximation are identified. These equations are presented in a form that makes the singularities apparent, and they are solved in a simple illustrative case. The singular points, one occurring at the sonic point and the other at a critical value of the parallel heat flux, give rise to different outflow regimes, characterized generically by different asymptotic behavior. The existence of the different outflow regimes separated by the heat flux critical point has been only hinted at in previous discussions of numerical simulation of the polar wind.