Loren C. Chang

Quasi two day wave‐related variability in the background dynamics and composition of the mesosphere/thermosphere and the ionosphere

Dissipating planetary waves in the mesosphere/lower thermosphere (MLT) region may cause changes in the background dynamics of that region, subsequently driving variability throughout the broader thermosphere/ionosphere system via mixing due to the induced circulation changes. We report the results of case studies examining the possibility of such coupling during the northern winter in the context of the quasi two day wave (QTDW)—a planetary wave that recurrently grows to large amplitudes from the summer MLT during the postsolstice period. Six distinct QTDW events between 2003 and 2011 are identified in the MLT using Sounding of the Atmosphere using Broadband Emission Radiometry temperature observations. Concurrent changes to the background zonal winds, zonal mean column O/N2 density ratio, and ionospheric total electron content (TEC) are examined using data sets from Thermosphere Ionosphere Mesosphere Energetics and Dynamics Doppler Interferometer, Global Ultraviolet Imager, and Global Ionospheric Maps, respectively. We find that in the 5–10 days following a QTDW event, the background zonal winds in the MLT show patterns of eastward and westward anomalies in the low and middle latitudes consistent with past modeling studies on QTDW-induced mean wind forcing, both below and at turbopause altitudes. This is accompanied by potentially related decreases in zonal mean thermospheric column O/N2, as well as to low-latitude TECs. The recurrent nature of the above changes during the six QTDW events examined point to an avenue for vertical coupling via background dynamics and chemistry of the thermosphere/ionosphere not previously observed. Key Points Dissipating planetary waves (PWs) in the MLT can drive background wind changes Mixing from dissipating PWs drive thermosphere/ionosphere composition changes First observations of QTDW-driven variability from this mechanism

Structure and origins of the Weddell Sea Anomaly from tidal and planetary wave signatures in FORMOSAT-3/COSMIC observations and GAIA GCM simulations

The Weddell Sea Anomaly (WSA) is a recurrent feature of the austral summer midlatitude ionosphere where electron densities are observed to maximize during the local nighttime. In this study, tidal decomposition is applied to FORMOSAT-3 (Formosa Satellite)/Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) total electron content (TEC) and electron density observations between 2007 and 2012 to quantify the components dominating local time and spatial variation in the WSA region. Our results present some of the first three-dimensional spaceborne analyses of the WSA from a tidal perspective over multiple years. We find that the features of the WSA can be reconstructed as the result of superposition between the dominant diurnal standing (D0), eastward wave number 1 (DE1), westward wave number 2 (DW2), and stationary planetary wave 1 (SPW1) components in TECs, producing the characteristic midnight WSA peak. The D0, DE1, DW2, and SPW1 components are found to be an interannually recurring feature of the southern midlatitude to high-latitude ionosphere during the summer, manifesting as enhancements in electron density around 300 km altitude of the summer middle to high magnetic latitudes. The phases of the aforementioned nonmigrating diurnal signatures in electron density in this region are near evanescent, suggesting in situ generation, rather than upward propagation from below. However, the SPW1 signature shows some signs of an eastward tilt with altitude, suggesting possible downward propagation. The relation of these components to possible generation via in situ photoionization or plasma transport along magnetic field lines is also discussed using results from the Ground-to-topside model of Atmosphere and Ionosphere for Aeronomy (GAIA) general circulation model (GCM), connecting the tidal interpretation of the WSA to previously examined generation mechanisms.

Ionospheric F2 region perturbed by the 25 April 2015 Nepal earthquake

Seismic waves can be detected in the Earths atmosphere and ionosphere; however, their impacts on ionospheric electron density (Ne) structures near the altitude of peak Ne (hmF2) have not yet been fully determined due to the lack of sufficient observations sampled in the vertical direction. Here we apply a ground-based Global Positioning System (GPS) receiving network in Asia as well as the space-based GPS occultation experiment on board the FORMOSAT-3/COSMIC (F3/C) satellite to vertically scan the ionospheric Ne structures, which were perturbed by the magnitude Mw7.8 Nepal earthquake that occurred on 25 April 2015. The F3/C altitudinal Ne profiles show that the Nepal earthquake-induced air perturbations penetrate into the ionosphere at supersonic speeds of approximately 800 m/s and change the Ne structure by 10% near hmF2. The vertical scale of the Ne perturbation is 150 km, while the hmF2 is uplifted by more than 30 km within 1 min. Those results reveal that the earthquake-induced ground displacement should be considered as a significant force that perturbs the vertical Ne structure of the ionosphere.

Coherent seasonal, annual, and quasi-biennial variations in ionospheric tidal/SPW amplitudes†

In this study, we examine the coherent spatial and temporal modes dominating the variation of selected ionospheric tidal and stationary planetary wave (SPW) signatures from 2007 - 2013 FORMOSAT-3/COSMIC total electron content observations using Multi-dimensional Ensemble Empirical Mode Decomposition (MEEMD) from the Hilbert-Huang Transform. We examine the DW1, SW2, DE3, and SPW4 components, which are driven by a variety of in-situ and vertical coupling sources. The intrinsic mode functions (IMFs) resolved by MEEMD analysis allows for the isolation of the dominant modes of variability for prominent ionospheric tidal / SPW signatures in a manner not previously used, allowing the effects of specific drivers to be examined individually. The time scales of the individual IMFs isolated for all tidal/SPW signatures correspond to a semiannual variation at EIA latitudes maximizing at the equinoxes, as well as annual oscillations at the EIA crests and troughs. All tidal / SPW signatures show one IMF isolating an ionospheric quasi-biennial oscillation (QBO) in the equatorial latitudes maximizing around January of odd numbered years. This TEC QBO variation is in phase with a similar QBO variation isolated in both the GUVI zonal mean column O/N2 density ratio (∑O/N2) as well as the F10.7 solar radio flux index around solar maximum, while showing temporal variation more similar to that of GUVI ∑O/N2 during the time around the 2008/2009 extended solar minimum. These results point to both quasi-biennial variations in solar irradiance as well as thermosphere / ionosphere composition as a generation mechanism for the ionospheric QBO.

The quasi 2 day wave response in TIME‐GCM nudged with NOGAPS‐ALPHA

nudged with NOGAPS-ALPHA Jack C. Wang 1, Loren C. Chang 2 , Jia Yue 3,4, Wenbin Wang 5 , and D. E. Siskind 6 1 Department of Aerospace Engineering Sciences, University of Colorado Boulder, Boulder, Colorado 2 Institute of Space Science, National Central University, Taoyuan City, Taiwan 3 Atmospheric and Planetary Sciences, Hampton University, Virginia 4 ESSIC, University of Maryland, Maryland, USA 5 High Altitude Observatory, National Center for Atmospheric Research, Boulder, Colorado 6 Space Science Division, Naval Research Laboratory, Washington, District of Columbia E-Mail : jack.c.wang@colorado.edu Abstract The quasi 2 day wave (QTDW) is a traveling planetary wave that can be enhanced rapidly to large amplitudes in the mesosphere and lower thermosphere (MLT) region during the northern winter postsolstice period. In this study, we present five case studies of QTDW events during January and February 2005, 2006 and 2008–2010 by using the Thermosphere-Ionosphere-Mesosphere Electrodynamics-General Circulation Model (TIME-GCM) nudged with the Navy Operational Global Atmospheric Prediction System-Advanced Level Physics High Altitude (NOGAPS-ALPHA) Weather Forecast Model. With NOGAPS-ALPHA introducing more realistic lower atmospheric forcing in TIMEGCM, the QTDW events have successfully been reproduced in the TIME-GCM. The nudged TIMEGCM simulations show good agreement in zonal mean state with the NOGAPS-ALPHA 6 h reanalysis data and the horizontal wind model below the mesopause; however, it has large discrepancies in the tropics above the mesopause. The zonal mean zonal wind in the mesosphere has sharp vertical gradients in the nudged TIME-GCM. The results suggest that the parameterized gravity wave forcing may need to be retuned in the assimilative TIME-GCM.

Medium‐scale traveling ionospheric disturbances triggered by Super Typhoon Nepartak (2016)

Two remarkable typhoon-induced traveling ionospheric disturbances (TIDs) with concentric and northwest-southeast (NW-SE) alignments, respectively associated with concentric gravity waves (CGWs) and ionosphere instabilities possibly seeded by CGWs, were observed in total electron content (TEC) derived from ground-based GNSS networks in Taiwan and Japan when the Category-5 Super Typhoon Nepartak approached Taiwan on 7 July 2016. The concentric TIDs (CTIDs) first appear with horizontal phase velocities of ~161–200 m/s, horizontal wavelengths of ~160–270 km, and periods of ~15–22 min during 08:00–11:20 UT. Following the CTIDs, the NW-SE aligned nighttime medium-scale TIDs (MSTIDs) are formed on the west-edge of the CTIDs over the Taiwan Strait during 11:30–14:00 UT. It is suggested that the MSTIDs are produced by the electrodynamical coupling of Perkins instability and CGW-induced polarization electric fields. This study proposes connections of typhoon-induced CTIDs and subsequently occurring MSTIDs in the low-latitude ionosphere.

Numerical simulation of the 6 day wave effects on the ionosphere: Dynamo modulation

The Thermosphere-Ionosphere-Mesosphere Electrodynamics General Circulation Model (TIME-GCM) is used to theoretically study the 6-day wave effects on the ionosphere. By introducing a 6-day perturbation with zonal wavenumber 1 at the model lower boundary, the TIME-GCM reasonably reproduces the 6-day wave in temperature and horizontal winds in the mesosphere and lower thermosphere (MLT) region during the vernal equinox. The E-region wind dynamo exhibits a prominent 6-day oscillation that is directly modulated by the 6-day wave. Meanwhile, significant local time variability (diurnal and semi-diurnal) is also seen in wind dynamo as a result of altered tides due to the nonlinear interaction between the 6-day wave and migrating tides. More importantly, the perturbations in the E-region neutral winds (both the 6-day oscillation and tidal-induced short-term variability) modulate the polarization electric fields, thus leading to the perturbations in vertical ion drifts and ionospheric F2-region peak electron density (NmF2). Our modeling work shows that the 6-day wave couples with the ionosphere via both the direct neutral wind modulation and the interaction with atmospheric tides.

Three-dimensional electron density along the WSA and MSNA latitudes probed by FORMOSAT-3/COSMIC

In this paper, we employ electron density profiles derived by the GPS radio occultation experiment aboard the FORMOSAT-3/COSMIC (F3/C) satellites to examine the electron density on geographic latitudes of 40° to 80° in the Southern hemisphere and 30° to 60° in the Northern hemisphere at various global fixed local times from February 2009 to January 2010. The results reveal that an eastward shift of a single-peak plasma density feature occurs along the Weddell Sea Anomaly (WSA) latitudes, while a double-peak plasma density feature appears along the northern Mid-latitude Summer Nighttime Anomaly (MSNA) latitudes. A cross-comparison between three-dimensional F3/C electron density and HWM93 simulation confirms that the magnetic meridional effect and vertical effect caused by neutral winds exhibit the eastward shifts. Furthermore, we find that the eastward shift of the peaks when viewed as a function of local time suggests that they could be interpreted as being comprised of different tidal components with distinct zonal phase velocities in local time.

Theoretical study of the ionospheric plasma cave in the equatorial ionization anomaly region

This paper investigates the physical mechanism of an unusual equatorial electron density structure, plasma cave, located underneath the equatorial ionization anomaly by using theoretical simulations. The simulation results provide important new understanding of the dynamics of the equatorial ionosphere. It has been suggested previously that unusual E⇀×B⇀ drifts might be responsible for the observed plasma cave structure, but model simulations in this paper suggest that the more likely cause is latitudinal meridional neutral wind variations. The neutral winds are featured by two divergent wind regions at off-equator latitudes and a convergent wind region around the magnetic equator, resulting in plasma divergences and convergence, respectively, to form the plasma caves structure. The tidal-decomposition analysis further suggests that the cave related meridional neutral winds and the intensity of plasma cave are highly associated with the migrating terdiurnal tidal component of the neutral winds.