Showing papers by "Arthur D. Richmond published in 2012"

Simulations of solar and lunar tidal variability in the mesosphere and lower thermosphere during sudden stratosphere warmings and their influence on the low-latitude ionosphere

Abstract: [1] Whole Atmosphere Community Climate Model (WACCM) simulations are used to investigate solar and lunar tide changes in the mesosphere and lower thermosphere (MLT) that occur in response to sudden stratosphere warmings (SSWs). The average tidal response is demonstrated based on 23 moderate to strong Northern Hemisphere SSWs. The migrating semidiurnal lunar tide is enhanced globally during SSWs, with the largest enhancements (∼60–70%) occurring at mid to high latitudes in the Northern Hemisphere. Enhancements in the migrating solar semidiurnal tide (SW2) also occur up to an altitude of 120 km. Above this altitude, the SW2 decreases in response to SSWs. The SW2 enhancements are 40–50%, making them smaller in a relative sense than the enhancements in the migrating semidiurnal lunar tide. Changes in nonmigrating solar tides are, on average, generally small and the only nonmigrating tides that exhibit changes greater than 20% are the diurnal tide with zonal wave number 0 (D0) and the westward propagating semidiurnal tide with zonal wave number 1 (SW1). D0 is decreased by ∼20–30% at low latitudes, while SW1 exhibits a similar magnitude enhancement at mid to high latitudes in both hemispheres. The tidal changes are attributed to a combination of changes in the zonal mean zonal winds, changes in ozone forcing of the SW2, and nonlinear planetary wave-tide interactions. We further investigate the influence of the lunar tide enhancements on generating perturbations in the low latitude ionosphere during SSWs by using the WACCM-X thermosphere to drive an ionosphere-electrodynamics model. For both solar maximum and solar minimum simulations, the changes in the equatorial vertical plasma drift velocity are similar to observations when the lunar tide is included in the simulations. However, when the lunar tide is removed from the simulations, the low latitude ionosphere response to SSWs is unclear and the characteristic behavior of the low latitude ionosphere perturbations that is seen in observations is no longer apparent. Our results thus indicate the importance of variability in the lunar tide during SSWs, especially for the coupling between SSWs and perturbations in the low latitude ionosphere.

Stratospheric warmings and the geomagnetic lunar tide: 1958–2007

Abstract: [1] A quantitative comparison of the geomagnetic lunar tide and lower stratospheric parameters (zonal mean air temperature T and zonal mean zonal wind U) is carried out for the period 1958–2007. The correlation between the amplitude of the geomagnetic lunar tide at an equatorial station, Addis Ababa, and the lower stratospheric parameters from the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP-NCAR) reanalysis is examined. It is found that the lunar tidal amplitude tends to be positively and negatively correlated with the stratospheric T and U, respectively, in high latitudes of the Northern Hemisphere during December and January. High correlations are observed in approximately 70% of stratospheric sudden warming (SSW) events. The results suggest that variability of the geomagnetic lunar tide during the northern winter is closely linked with dynamical changes in the lower stratospheric parameters associated with SSWs.

Assimilation of FORMOSAT-3/COSMIC electron density profiles into a coupled thermosphere/ionosphere model using ensemble Kalman filtering

Abstract: [1] This paper presents our effort to assimilate FORMOSAT-3/COSMIC (F3/C) GPS Occultation Experiment (GOX) observations into the National Center for Atmospheric Research (NCAR) Thermosphere Ionosphere Electrodynamics General Circulation Model (TIE-GCM) by means of ensemble Kalman filtering (EnKF). The F3/C electron density profiles (EDPs) uniformly distributed around the globe which provide an excellent opportunity to monitor the ionospheric electron density structure. The NCAR TIE-GCM simulates the Earth's thermosphere and ionosphere by using self-consistent solutions for the coupled nonlinear equations of hydrodynamics, neutral and ion chemistry, and electrodynamics. The F3/C EDP are combined with the TIE-GCM simulations by EnKF algorithms implemented in the NCAR Data Assimilation Research Testbed (DART) open-source community facility to compute the expected value of electron density, which is ‘the best’ estimate of the current ionospheric state. Assimilation analyses obtained with real F3/C electron density profiles are compared with independent ground-based observations as well as the F3/C profiles themselves. The comparison shows the improvement of the primary ionospheric parameters, such as NmF2 and hmF2. Nevertheless, some unrealistic signatures appearing in the results and high rejection rates of observations due to the applied outlier threshold and quality control are found in the assimilation experiments. This paper further discusses the limitations of the model and the impact of ensemble member creation approaches on the assimilation results, and proposes possible methods to avoid these problems for future work.

Sources of low-latitude ionospheric E × B drifts and their variability

Abstract: [1] The complete mechanism of how upward propagating tropospheric tides connect to the upper atmosphere is not yet fully understood. One proposed mechanism is via ionospheric wind dynamo. However, other sources can potentially alter the vertical E × B drift: gravity and plasma pressure gradient driven current, the geomagnetic main field, and longitudinal variation in the conductivities. In this study we examine the contribution to the vertical drift from these sources, and compare them. We use March equinox results from the Thermosphere Ionosphere Mesosphere Electrodynamics General Circulation Model. We found that the gravity and plasma pressure gradient driven current and the longitudinal variation of the conductivities excluding the variation due to the geomagnetic main field do not change the longitudinal variation of the vertical drift significantly. Modifying the geomagnetic main field will change the vertical drift at 5–6 LT, 18–19 LT and 23–24 LT at almost all longitudes. In general the influence of the geomagnetic main field on the vertical drift is larger, with respect to the maximum difference, at 18–19 LT and 23–24 LT, equal at 5–6 LT, and smaller at 14–15 LT than the influence due to nonmigrating tidal components in the neutral winds. Examination of the contribution from E- and F-region neutral winds to the vertical drift shows that their importance depends on the local time and the solar activity. This implies that the vertical drift has to be analyzed at specific local times to examine the relation between the wave number in the vertical drift and in the neutral winds.

Height distribution of Joule heating and its influence on the thermosphere

Abstract: [1] The National Center for Atmospheric Research Thermosphere-Ionosphere-Electrodynamics General Circulation Model (NCAR TIE-GCM) is employed to quantify the influence of Joule heating at different altitudes on the neutral temperature and density at 400 km for solar minimum and maximum conditions. The results show that high-altitude Joule heating is more efficient than low-altitude heating in affecting the upper thermosphere. Most of the Joule heating is deposited under 150 km, and the largest Joule heating deposition per scale height happens at about 125 km, independent of solar activity. However, the temperature and density changes at 400 km are largest for heat deposited at ∼140 km for solar minimum and ∼263 km for solar maximum. The timescale for the thermospheric response varies with the altitude of heating. Joule heating deposited at lower heights needs more time to conduct upward, and it takes more time for the thermosphere at 400 km to approach a steady state. A simple one-dimensional model is utilized to explain how the amplitude and characteristic timescale of the upper-thermosphere response to Joule heating depends on the height of heat input. The characteristic response timescale for heat deposited around 135 km is ∼6 hours, while that for heat deposited around 238 km is ∼0.5 hours. The initial temperature response at 400 km to the high-altitude heating is much stronger than the response to the low-altitude heating, but the responses become comparable after about 4 days.

Atmospheric semidiurnal lunar tide climatology simulated by the Whole Atmosphere Community Climate Model

Abstract: [1] The atmospheric semidiurnal lunar tide is added to the Whole Atmosphere Community Climate Model (WACCM) through inclusion of an additional forcing mechanism. The simulated climatology of the semidiurnal lunar tide in surface pressure and zonal and meridional winds in the mesosphere and lower thermosphere (MLT) is presented. Prior observations and modeling results demonstrate characteristic seasonal and latitudinal variability of the semidiurnal lunar tide in surface pressure, and the WACCM reproduces these features. In the MLT, the WACCM simulations reveal a primarily semiannual variation with maxima near December and June solstice. The peak amplitudes in the MLT zonal and meridional winds are ∼5–10 ms−1, and occur at mid to high latitudes in the summer hemisphere. We have further compared the WACCM simulation results in the MLT with those from the Global Scale Wave Model (GSWM). The overall latitude and seasonal variations are consistent between these two models. However, the GSWM peak amplitudes are ∼2–3 times larger than those in the WACCM. This is thought to be related to deficiencies in the GSWM and not the WACCM simulations. With the exception of smaller amplitudes during Northern Hemisphere summer months, the WACCM simulations of the semidiurnal lunar tide in the MLT are also shown to be generally consistent with prior observations and modeling results. The reduced amplitudes in the WACCM simulations during Northern Hemisphere summer months are thought to be related to the influence of the cold-pole bias in WACCM on the propagation of the lunar tide during these months.

The dependence of the coupled magnetosphere‐ionosphere‐thermosphere system on the Earth's magnetic dipole moment

Abstract: [1] The strength of the Earth’s magnetic field changes over time. We use simulations with the Coupled Magnetosphere-Ionosphere-Thermosphere model to investigate how the magnetosphere, upper atmosphere, and solar quiet (Sq) geomagnetic variation respond as the geomagnetic dipole moment M varies between 2⋅10 22 and 10⋅10 22 Am 2 . We find that the magnetopause stand-off distance and the cross-polar cap potential increase, while the polar cap size decreases, with increasing M. Their dependence on M is stronger than predicted by previous studies. We also show for the first time that the shape of the magnetosphere starts to change for M ≤ 4⋅10 22 Am 2 . This may be due to enhanced magnetopause erosion and/or to strong changes in the ionospheric conductance, which affect the field-aligned currents and the magnetic fields they create in the magnetosphere,

How changes in the tilt angle of the geomagnetic dipole affect the coupled magnetosphere-ionosphere-thermosphere system

Abstract: [1] The orientation of the Earth's magnetic field has changed dramatically during the geological past. We have investigated the effects of changes in dipole tilt angle on the magnetosphere, ionosphere, and thermosphere, using the Coupled Magnetosphere-Ionosphere-Thermosphere (CMIT) model. The dipole tilt angle modulates the efficiency of solar wind-magnetosphere coupling, by influencing the diurnal variation in the angleμ between the dipole axis and the GSM z axis. This influences how much Joule heating occurs at high magnetic latitudes. The dipole tilt angle also controls the geographic distribution of the Joule heating, as it determines the geographic latitude of the magnetic poles. Changes in the amount and distribution of Joule heating with tilt an`gle produce further changes in temperature and neutral winds. The latter affect the O/N2 ratio, which in turn modifies the peak electron density of the F2 layer, NmF2. All these effects are most important when the Interplanetary Magnetic Field (IMF) is southward, while being almost negligible under northward IMF. However, a change in dipole tilt also changes the inclination of the magnetic field, which affects the vertical component of ionospheric plasma diffusion along the magnetic field, regardless of the IMF direction. Changes in vertical plasma diffusion are responsible for ∼2/3 of the changes in NmF2 and most of the low to midlatitude changes in hmF2under southward IMF and for most of the changes in both variables under northward IMF. Thermal contraction may be responsible for high-latitude decreases in hmF2 with increasing tilt angle under southward IMF.

Quasi‐two‐day wave coupling of the mesosphere and lower thermosphere‐ionosphere in the TIME‐GCM: Two‐day oscillations in the ionosphere

Abstract: [1] The Thermosphere Ionosphere Mesosphere Electrodynamics General Circulation Model (TIME-GCM) is used to simulate the quasi-two-day wave (QTDW) modulation of the ionospheric dynamo and electron density. The QTDW can directly penetrate into the lower thermosphere and modulate the neutral winds at a period of two days. The QTDW modulation of the tidal amplitudes is not evident. The QTDW in zonal and meridional winds results in a quasi-two-day oscillation (QTDO) of the dynamo electric fields at southern midlatitudes, which is mapped into the conjugate northern magnetic midlatitudes. The QTDO of the electric fields in the E region is transmitted along the magnetic field lines to the F region and leads to the QTDOs of the vertical ion drift and total electron content (TEC) at low and mid latitudes. The QTDO of the vertical ion drift near the magnetic equator leads to the 2-day oscillation of the fountain effect. The QTDO of the TEC has two peaks at ±25 magnetic latitude (Mlat) and one near the dip equator. The equatorial peak is nearly out of phase with the ones at ±25 Mlat. The vertical ion drift at midlatitudes extends the QTDW response of the TEC to midlatitudes from the Equatorial Ionospheric Anomaly (EIA). Most differently from previous reports, we discover that the QTDW winds couple into the F region ionosphere through both the fountain effect and the middle latitude dynamos.

Intense dayside Joule heating during the 5 April 2010 geomagnetic storm recovery phase observed by AMIE and AMPERE

Abstract: [1] When the interplanetary magnetic field (IMF) is northward, dawnward, or duskward, magnetic merging between the IMF and the geomagnetic field occurs near the cusp of the magnetosphere. While these periods are usually considered “quiet,” they can lead to intense, but highly localized, energy deposition into the dayside ionosphere. We identify such an occurrence during a series of two geomagnetic storms on 5 April 2010. Using data from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) for the first time as an input to the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) algorithm, we show that during the recovery phase of the first storm there is intense ionospheric Joule heating in the dayside polar regions. This is associated with an intense field-aligned current pair near the noon meridian that is associated with northward IMF and a strong IMF By component. AMIE outputs are used to drive the thermosphere-ionosphere-mesosphere electrodynamics general circulation model to demonstrate that the intense levels of Joule heating can lead to anomalous thermospheric density enhancements and traveling disturbances.

Forcing the TIEGCM model with Birkeland currents from the Active Magnetosphere and Planetary Electrodynamics Response Experiment

Abstract: [1] Geomagnetic field-aligned currents from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) satellite mission are used to drive the Thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIEGCM). We present a comparison between ground magnetic signatures computed by the model and observations at four different geomagnetic observatories, for different magnetic disturbance levels. Results show the ability of the model to pick up the gross features of the magnetic variations, improving its performance with increasing disturbance level and from low to high latitudes. During geomagnetically quiescent conditions a baseline noise of about 5 nT is evident in reconstructed ground magnetic field signatures, which we attribute to the baseline noise level in the AMPERE currents. For variations shorter than about 30 min the modeled signals are often significantly lower than observed by a factor up to 3 to 4, possibly reflecting localized ionization structures not captured in the TIEGCM conductance modules, or missing small-scale and rapid temporal variations in auroral currents. While the observed horizontal field variations are reflected in the model, the vertical component is consistently underestimated, possibly indicating errors in the estimates for ground induction currents. Comparison with the standard version of the TIEGCM is also carried out, showing that time variations shorter than 6 h and down to the 10 min resolution of the AMPERE data (which do not appear in the standard version of TIEGCM) are now reflected in the AMPERE-driven model.

Simulations of the equatorial thermosphere anomaly: Field-aligned ion drag effect

Abstract: [1] In this paper the impact of the field-aligned ion drag on equatorial thermosphere temperature and density is quantitatively investigated on the basis of the National Center for Atmospheric Research Thermosphere Ionosphere Electrodynamics General Circulation Model (NCAR TIEGCM) simulations under high solar activity (F107 = 180). The increase of upward vertical winds over the magnetic equator associated with the additional divergence of meridional winds, caused by the inclusion of field-aligned ion drag, leads to a reduction in thermosphere temperature and density at the magnetic equator through enhanced adiabatic cooling. We found that the field-aligned ion drag has an obvious impact on the thermosphere only over the magnetic equatorial region in the daytime and evening sectors, whereas it has less effect on the equatorial thermosphere anomaly (ETA) crests. The daytime neutral temperature over the magnetic equator is reduced by about 30 K, for altitudes above 250 km without significant altitudinal variations, when field-aligned ion drag is included in the simulation. The thermosphere density in the magnetic equatorial region starts to change slightly at 300 km and depletes by about 5% at 400 km, while experiencing a greater decrease with altitude. Furthermore, the trough produced in the neutral temperature and density corresponds well with the magnetic dip equator. The ETA features during 12:00–18:00 LT become obvious as a result of the inclusion of the field-aligned ion drag. Specifically, our results show that at 400 km the crest-trough differences in neutral temperature are about 30–60 K, and the crest-trough ratios in thermosphere density are 1.03–1.06, comparable with observations.

Sq current system during stratospheric sudden warming events in 2006 and 2009

Abstract: [1] Ionospheric Sq current systems during unusually strong and prolonged stratospheric sudden warming (SSW) events in January 2006 and January 2009 are examined by analyzing ground-magnetometer data for the American and Asian longitude sectors. During these SSW events, a significant decrease and increase of the Sq equivalent current intensity are observed in the Northern and Southern Hemispheres, respectively, along with a reduction in the longitudinal separation between the northern and southern current vortices. Numerical experiments using the National Center for Atmospheric Research Thermosphere-Ionosphere-Electrodynamics General-Circulation Model show that changes in the solar anti-symmetric (2,3) semidiurnal tide can bring about similar changes in the Sq current system.

Wavelength Dependence of Solar Flare Irradiation and its Influence on the Thermosphere

Abstract: The wavelength dependence of solar flare enhancement is one of the important factors determining how the Thermosphere-Ionosphere (T-I) system response to flares. To investigate the wavelength dependence of solar flare, the Flare Irradiance Spectral Model (FISM) has been run for 34 X-class flares. The results show that the percentage increases of solar irradiance at flare peak comparing to pre-flare condition have a clear wavelength dependence. In the wavelength range between 0 - 195 nm, it can vary from 1% to 10000%. The solar irradiance enhancement is largest ( 1000%) in the XUV range (0 - 25 nm), and is about 100% in EUV range (25 - 120 nm). The influence of different wavebands on the T-I system during the October 28th, 2003 flare (X17.2-class) has also been examined using the latest version of National Center for Atmospheric Research (NCAR) Thermosphere- Ionosphere-Electrodynamics General Circulation Model (TIE-GCM). While the globally integrated solar energy deposition is largest in the 0 - 14 nm waveband, the impact of solar irradiance enhancement on the thermosphere at 400 km is largest for 25 - 105 nm waveband. The effect of 122 - 195 nm is small in magnitude, but it decays slowly.