Showing papers by "Arthur D. Richmond published in 2013"

How Does the Thermosphere and Ionosphere React to a Geomagnetic Storm

Abstract: Unraveling the ionospheric and thermospheric response to a geomagnetic storm has been a challenge for many decades, due largely to the complex interactions between the plasma and neutral species. Geomagnetic storm sources to the upper atmosphere are caused by an increase in the convective electric field and auroral precipitation, that give rise to Joule heating, the primary driver of global atmospheric change. Driven by the impulsive energy input, wave surges propagate and interact globally, and are dependent on Universal Time (UT) and the time history of the source. There is a strong preference for surges to maximize on the nightside and in the longitude sector adjacent to the magnetic pole. Equatorward wind surges drive the plasma upwards and can initiate a positive ionospheric change. The divergent nature of the wind field causes upwelling and changes to the neutral composition that can be transported by the storm and background wind fields. Negative ionospheric phases result from increased molecular species. Ionosondes have recorded the apparently chaotic ionospheric response for more than 50 years, but it is only recently that local time (LT) and seasonal dependencies have been quantified. Numerical models have shed light on the physical processes; the LT response is caused by the diurnal wind field migration of the composition bulge, and the seasonal dependence is controlled through the transport of the bulge by the summer-to-winter prevailing circulation. Neutral density changes and satellite airglow observations support this basic concept. At low latitudes, electrodynamic changes are initiated by penetration of magnetospheric fields followed by rapid shielding. After shielding, the electrodynamics is forced by dynamo action of the disturbed neutral atmosphere, driving a sequence of equatorial plasma drifts for more than a day. The precise mechanisms responsible for this equatorial response have yet to be defined, but it is tempting to associate the time-scales with those of the global dynamical and composition response of the neutral atmosphere. Despite an increase in our understanding of the causes of the mid-latitude ionospheric response, simulation of a real storm has yet to confirm theory. We are limited by accurate knowledge of the source function, and by the lack of comprehensive data coverage of both the neutral and ionospheric parameters. Both are needed before theory and models can be thoroughly tested.

A theory of ionospheric response to upward‐propagating tides: Electrodynamic effects and tidal mixing effects

Abstract: [1] The atmospheric tide at ionospheric heights is composed of those locally generated and those propagated from below. The role of the latter in producing the variability of the daytime ionosphere is examined using the National Center for Atmospheric Research Thermosphere-Ionosphere-Electrodynamics General Circulation Model. The impact of upward-propagating tides is evaluated by running simulations with and without tidal forcing at the lower boundary (approximately 96 km), which imitates the effect of tides from below. When migrating diurnal and semidiurnal tides at the lower boundary is switched on, the intensity of E region currents and the upward velocity of the equatorial F region vertical plasma drift rapidly increase. The low-latitude ionospheric total electron content (TEC) first increases, then gradually decreases to below the initial level. The initial increase in the low-latitude TEC is caused by an enhanced equatorial plasma fountain while the subsequent decrease is due to changes in the neutral composition, which are characterized by a global-scale reduction in the mass mixing ratio of atomic oxygen O1. The results of further numerical experiments indicate that the mean meridional circulation induced by dissipating tides in the lower thermosphere is mainly responsible for the O1 reduction; it acts like an additional turbulent eddy and produces a “mixing effect” that enhances net downward transport and loss of O1. It is stressed that both electrodynamic effects and mixing effects of upward-propagating tides can be important in producing the variability of ionospheric plasma density. Since the two mechanisms act in different ways on different time scales, the response of the actual ionosphere to highly variable upward-propagating tides is expected to be complex.

Attribution of ionospheric vertical plasma drift perturbations to large-scale waves and the dependence on solar activity

Abstract: [1] In this study, we quantify the contribution of individual large-scale waves to ionospheric electrodynamics and examine the dependence of the ionospheric perturbations on solar activity. We focus on migrating diurnal tide (DW1) plus mean winds, migrating semidiurnal tide (SW2), quasi-stationary planetary wave one (QSPW1), and nonmigrating semidiurnal westward wave one (SW1) under northern winter conditions, when QSPW1 and SW1 are climatologically strong. From thermosphere-ionosphere-mesosphere electrodynamics general circulation model simulations under solar minimum conditions, it is found that the mean winds and DW1 produce a wave two pattern in equatorial vertical E×Bdrift that is upward in the morning and around dusk. The modeled SW2 also produces a wave two pattern in the ionospheric vertical drift that is nearly a half wave cycle out of phase with that due to mean winds and DW1. SW1 can cause large vertical drifts around dawn, while QSPW1 does not have any direct impact on the vertical drift. Wind components of both SW2 and SW1 become large at middle to high latitudes in the E-region, and kernel functions obtained from numerical experiments reveal that they can significantly affect the equatorial ion drift, likely through modulating the E-region wind dynamo. The most evident changes of total ionospheric vertical drift when solar activity is increased are seen around dawn and dusk, reflecting the more dominant role of large F-region Pedersen conductivity and of the F-region dynamo under high solar activity. Therefore, the lower atmosphere driving of the ionospheric variability is more evident under solar minimum conditions, not only because variability is more identifiable in a quieter background but also because the E-region wind dynamo is more significant. These numerical experiments also demonstrate that the amplitudes, phases, and latitudinal and vertical structures of large-scale waves are important in quantifying the ionospheric responses.

Ionospheric electrodynamics : A tutorial

Abstract: This paper gives a tutorial overview of ionospheric electrodynamics, including the observed behavior of ionospheric electric fields and currents, the physics of ionospheric electrical conductivity and Ohm's law, the operation of the ionospheric wind dynamo, and the transfer of energy between the magnetosphere and the ionosphere. The ionosphere forms an important part of the magnetospheric electrodynamic system. It is a region where ion-neutral collisions cause ions and electrons to move at different velocities across magnetic field lines, thereby violating the frozen-in flux condition and resulting in significant flow of ohmic current. Ionospheric conductivity is a function of the geomagnetic field, the plasma density, and the collision rate. Neutral winds cause generation of electric current through a dynamo effect. The winds result from diurnally varying solar heating, from upward-propagating global atmospheric waves, and from the Ampere force and Joule heating resulting from the electric current flow. Electromagnetic energy flow is normally directed from the magnetosphere into the ionosphere, as can be evaluated with the aid of Poynting's theorem, but strong thermospheric winds can sometimes reverse the direction of this energy flow.

Changes in the Earth's magnetic field over the past century: Effects on the ionosphere-thermosphere system and solar quiet (Sq) magnetic variation

Abstract: [1] We investigated the contribution of changes in the Earth's magnetic field to long-term trends in the ionosphere, thermosphere, and solar quiet (Sq) magnetic variation using the Coupled Magnetosphere-Ionosphere-Thermosphere (CMIT) model. Simulations with the magnetic fields of 1908, 1958, and 2008 were done. The strongest differences occurred between ~40°S–40°N and ~100°W–50°E, which we refer to as the Atlantic region. The height and critical frequency of the F2 layer peak, hmF2 and foF2, changed due to changes in the vertical E × B drift and the vertical components of diffusion and transport by neutral winds along the magnetic field. Changes in electron density resulted in changes in electron temperature of the opposite sign, which in turn produced small corresponding changes in ion temperature. Changes in neutral temperature were not statistically significant. Strong changes in the daily amplitude of the Sq variation occurred at low magnetic latitudes due to the northward movement of the magnetic equator and the westward drift of the magnetic field. The simulated changes in hmF2, foF2, and Sq amplitude translate into typical trends of ±1 km/decade (night) to ±3 km/decade (day), −0.1 to +0.05 MHz/decade, and ±5 to ±10 nT/century, respectively. These are mostly comparable in magnitude to observed trends in the Atlantic region. The simulated Atlantic region trends in hmF2 and foF2 are ~2.5 times larger than the estimated effect of enhanced greenhouse gases on hmF2 and foF2. The secular variation of the Earth's magnetic field may therefore be the dominant cause of trends in the Atlantic region ionosphere.

Wavenumber broadening of the quasi 2 day planetary wave in the ionosphere

Abstract: [1] Using the Thermosphere Ionosphere Mesosphere Electrodynamics General Circulation Model, we investigate the observed zonal wavenumber broadening phenomena in the ionospheric quasi 2 day oscillation (QTDO) that is associated with westward zonal wavenumber 3 (W3) quasi 2 day wave (QTDW) perturbations in the mesosphere and lower thermosphere (MLT). We aim to explain why the observed longitudinal structures of the QTDOs in the ionosphere are different from those of the QTDWs in the MLT. We find that large QTDOs in the ionosphere with zonal wavenumbers other than W3 occur in the model run with the true magnetic field, but not in the model run with an aligned dipole field. These numerical experiments suggest that the occurrence of the additional zonal wavenumbers in ionospheric QTDOs is related to the longitudinal variations of the Earth's magnetic field configuration, strength, and dip angle, which have distinct stationary zonal wavenumbers. We also find that when the specified W3 QTDW winds drive ionospheric plasma motion in the magnetic field, the resultant QTDOs in ionospheric parameters, such as the dynamo electric field, ion vertical drifts, plasma densities, and total electron content, have more complicated longitudinal variations than simply W3, corresponding to a zonal wavenumber broadening effect. Additionally, we find that the wavenumber broadening effect in the ionosphere can be fed back onto the neutrals through ion drag, to produce small QTDW winds with new wavenumbers in the thermosphere.

SuperDARN assimilative mapping

Abstract: [1] An assimilative mapping procedure is developed to optimally combine information from Super Dual Auroral Radar Network (SuperDARN) plasma drift observations and a background statistical convection model to derive global distributions of electrostatic potential. This procedure takes into account statistical properties of the background model errors, obtained through the empirical orthogonal function analysis technique described in a companion paper. The assimilative mapping procedure is evaluated quantitatively using cross-validation and is found to reduce median prediction errors by up to 43% as compared to the existing linear regression-based SuperDARN mapping procedure. Furthermore, the mapped results from the assimilative procedure show a greater dynamic range in convection strength than do those of the regression-based procedure (i.e., the cross–polar cap potential is smaller for weak driving conditions and larger for strong driving conditions). The application of the assimilative procedure is demonstrated for a case study containing a geomagnetic storm. It is shown that, qualitatively, the results of the assimilative procedure appear more smooth and consistent across both data-dense and data-sparse regions than do those of the regression-based procedure.

Mesoscale and large‐scale variability in high‐latitude ionospheric convection: Dominant modes and spatial/temporal coherence

Abstract: [1] Empirical orthogonal function (EOF) analysis, a variant of principle component analysis, is applied to 20 months of plasma drift data from the Super Dual Auroral Radar Network radars in the high-latitude region of the Northern Hemisphere. Dominant modes of ionospheric electric field variability are identified and the spatial and temporal coherence of this variability is quantified. The first three modes of variability, which, together with the mean, account for ∼50% of the observed squared electric field (E2), are characterized by global spatial scales and long time scales (∼1 h). The first and second modes of variability represent the strengthening/weakening of the global convection pattern and the shaping of the convection pattern into asymmetrical round- and crescent-shaped cells. These two modes are correlated with the Bz and By components of the interplanetary magnetic field. The third mode represents the expansion/contraction of the convection pattern and is weakly correlated with the solar wind velocity. For EOFs beyond EOF 3, the power contained in the modes falls off rapidly, the characteristic spatial and temporal scales decrease, and weak correlations with external driving parameters are observed. These higher-order EOFs likely capture more random behavior of the electric field variability. The notable exception to this trend is EOF 11, which captures midlatitude variations on the duskside and is enhanced during subauroral polarization stream events. The EOF technique described in this paper is applied in a companion paper to characterize the covariance of ionospheric electric fields for use in an assimilative mapping procedure.

Global Modeling of Storm‐Time Thermospheric Dynamics and Electrodynamics

Abstract: Understanding the neutral dynamic and electrodynamic response of the upper atmosphere to geomagnetic storms, and quantifying the balance between prompt penetration and disturbance dynamo effects, are two of the significant challenges facing us today. This paper reviews our understanding of the dynamical and electrodynamic response of the upper atmosphere to storms from a modeling perspective. After injection of momentum and energy at high latitude during a geomagnetic storm, the neutral winds begin to respond almost immediately. The high-latitude wind system evolves quickly by the action of ion drag and the injection of kinetic energy; however, Joule dissipation provides the bulk of the energy source to change the dynamics and electrodynamics globally. Impulsive energy injection at high latitudes drives large-scale gravity waves that propagate globally. The waves transmit pressure gradients initiating a change in the global circulation. Numerical simulations of the coupled thermosphere, ionosphere, plasmasphere, and electrodynamic response to storms indicate that although the wind and waves are dynamic, with significant apparent "sloshing" between the hemispheres, the net effect is for an increased equatorward wind. The dynamic changes during a storm provide the conduit for many of the physical processes that ensue in the upper atmosphere. For instance, the increased meridional winds at mid latitudes push plasma parallel to the magnetic field to regions of different composition. The global circulation carries molecular rich air from the lower thermosphere upward and equatorward, changing the ratio of atomic and molecular neutral species, and changing loss rates for the ionosphere. The storm wind system also drives the disturbance dynamo, which through plasma transport modifies the strength and location of the equatorial ionization anomaly peaks. On a global scale, the increased equatorward meridional winds, and the generation of zonal winds at mid latitudes via the Coriolis effects, produce a current system opposing the normal quiet-time Sq current system. At the equator, the storm-time zonal electric fields reduce or reverse the normal upward and downward plasma drift on the dayside and nightside, respectively. In the numerical simulations, on the dayside, the disturbance dynamo appears fairly uniform, whereas at night a stronger local time dependence is apparent with increased upward drift between midnight and dawn. The simulations also indicate the possibility for a rapid dynamo response at the equator, within 2 h of storm onset, before the arrival of the large-scale gravity waves. All these wind-driven processes can result in dramatic ionospheric changes during storms. The disturbance dynamo can combine and interact with the prompt penetration of magnetospheric electric fields to the equator.

The Ionosphere and Upper Atmosphere

Abstract: Because our society is becoming increasingly dependent on technological systems that can be affected by ionospheric phenomena during geomagnetic storms, the ionosphere, its electrodynamics, and its coupling with the neutral atmosphere and the magnetosphere are being studied as part of a coordinated program of "space weather" research. This research seeks to characterize the variability of ionospheric density and electric currents during magnetic storms, and to determine to what extent valid predictions of those phenomena and their effects can be made.

Global‐Scale Observations of the Limb and Disk (Gold): New Observing Capabilities for the Ionosphere‐Thermosphere

Abstract: The Global-scale Observations of the Limb and Disk (GOLD) mission ofopportunity will greatly improve understanding ofthe Earth's thermosphere and ionosphere through measurements of the global-scale response to external and internal forces. GOLD will fly an UV imager on a geostationary satellite to measure densities and temperatures across almost an entire hemisphere in this poorly understood region of the Earth's upper atmosphere and lower space environment, at altitudes where temperatures are currently not well known. GOLD will provide the first global-scale observations of temperatures in the lower thermosphere (130-180 km), in addition to more familiar measurements such as aurora location and energy input, peak electron densities (N m F 2 ) in the nighttime ionosphere, and atomic oxygen to molecular nitrogen column density ratios (ΣO/N 2 ) ratios. GOLD can provide nearly continuous real-time observations of one hemisphere. In addition to measurements on the disk of the Earth, GOLD can provide coincident measurements of molecular oxygen densities and the temperature profile in the lower thermosphere (150-250 km) from stellar occultations as well as exospheric temperatures from limb profiles of molecular nitrogen emissions. GOLD has two identical channels, each capable of all the measurements described. This allows GOLD to provide coincident measurements in any desired combination, e.g., disk temperatures and ΣO/N 2 . Combined with the advanced models now available, measurements from GOLD will revolutionize our understanding of the global-scale response of the thermosphere and ionosphere to geomagnetic and solar forcing. The data and knowledge gained from GOLD will enhance space weather specification and forecasting capabilities.

A Data-model comparative study of ionospheric positive storm phase in the midlatitude F region

Abstract: A strong positive storm phase was observed by both the Millstone Hill and Arecibo incoherent scatter radars during a moderate geomagnetic storm on 10 September 2005. The positive storm phase featured an interesting UT-altitude profile of the F region electron density enhancement that closely resembles the Greek letter A. The radar measurements showed that the uplift of the electron density peak height corresponded to a strong upward ion drift, whereas the subsequent falling of the peak height coincided with a downward ion drift. Using realistic, time-dependent ionospheric convection and auroral precipitation as input, the thermosphere-ionosphere electrodynamics general circulation model (TIEGCM) is able to reproduce the same A-like structure in the electron density profile, along with many large-scale features in electron temperature and vertical ion drift as observed by the radars. Over the 3-day period of 8―10 September, our simulation results show an error of 1%-4% for h m F 2 , electron, and ion temperatures at both radar locations. The estimated error for N m F 2 is about 9% at Millstone Hill and 19% at Arecibo. However, the simulated vertical ion drifts are less accurate, with the normalized root-mean-square errors of 72% at Millstone Hill and 52% at Arecibo, due largely to model's inability to capture the large temporal fluctuations measured by the radars. However, it reproduces reasonably well the overall large-scale variations during the 3-day period, including the storm-time-enhanced upward ion drift that is responsible for the interesting F region density profile. The model is also able to reproduce the temporal and spatial total electron content variations as shown in the global GPS maps. The comparison with the GUVI O/N 2 is less satisfactory, although there is a general agreement in terms of relative O/N 2 changes during the storm in the longitudinal sector between 60°W and 80°W where the radars are located. The detailed data-model comparison carried out in this study is helpful not only to validate the model but also to interpret the complex observations. The TIEGCM simulations reveal that it is the enhanced meridional neutral wind, not the penetration electric field, that is the primary cause of the A structure of the F region electron density profile.