Terrestrial Impacts of Solar Variability

Terrestrial Impacts of Solar Variability

The Earth’s mesosphere, thermosphere, ionosphere and magnetosphere comprise the interface region where the dense neutral atmosphere transitions to the tenuous and ionized environment of geospace. The region is directly influenced by the Sun through the absorption of solar UV/EUV and X-ray radiation as well as interaction between the solar wind and the magnetosphere. It is also affected by waves and tides resulting from meteorological weather. HAO’s research in this area focuses on interactions between the different regions, with particular emphases on developing advanced numerical models of the upper atmosphere/magnetosphere, its response to variable solar output, and its coupling with the lower atmosphere.

Solar flares are impulsive releases of solar radiative energy, particularly in the X-ray and UV/EUV wavelengths, which are primarily absorbed in the thermosphere and ionosphere. Model simulations show that thermospheric and ionospheric responses differ significantly as a function of the rise and decay rates for flares with the same magnitude and location. The so-called “Neupert Effect,” which predicts that a faster flare rise time leads to a larger EUV enhancement during the impulsive phase, causes a larger maximum ion production enhancement. Model simulations also show that increased E´B plasma transport due to increases in conductivity during the flares can cause a significant equatorial anomaly feature in the electron density enhancement in the F region but a relatively weaker equatorial anomaly feature in the total electron content (TEC) enhancement, owing to the dominant contributions by photochemical production and loss processes. The latitudinal dependence of thermospheric response correlates well with the solar zenith angle effect; whereas the latitudinal dependence of ionospheric response appears to be more complex due to plasma transport and the winter anomaly.

The upper atmosphere is strongly modulated by tides and waves from the lower atmosphere. Analysis of mesospheric wind observations from the TIMED Doppler Imager (TIDI) shows that the semidiurnal tide westward zonal wavenumber 1 (SW1) has a peak near the South Pole during the December solstice and near the North Pole during the March equinox whereas the semidiurnal tide westward zonal wavenumber 2 (SW2) peaks at 60˚S and 60˚N mostly during winter solstices. The effect that the quasi-biennial oscillations (QBO) have on the semidiurnal tide is found to be much weaker than that on the diurnal tide. The March equinox northern SW1 zonal amplitude appears to be stronger during the westward phase of the QBO, which is opposite to the migrating diurnal tide QBO response. Possible sudden stratospheric warming (SSW) event-related changes in the semidiurnal tide are significant but not always consistent. Enhancements in the mid-latitude SW2 component are observed, which may be related to the increase of total ozone at mid and high latitudes during SSW events. The TIDI observations also show a decrease in the SW2 in the opposite hemisphere during a southern SSW event in 2002, and a small increase in the SW1 was recorded in both hemispheres.

The quasi two-day wave (QTDW) zonal wavenumber 3 planetary wave is a recurrent wave feature in the mesosphere and lower thermosphere. The QTDW exhibits strong seasonal variability with peak amplitudes after summer solstice. In late January/early February, satellites also discovered two strong enhancements of the QTDW in meridional wind, with one peak at summer mid-latitudes near 90 km and the other in the tropical lower thermosphere. Studies using the Thermosphere-Ionosphere-Mesosphere Electrodynamics General Circulation Model (TIME-GCM) together with the wave forcing prescribed at the lower model boundary have demonstrated that baroclinic/barotropic instability is capable of amplifying the QTDW, manifesting as Eliassen-Palm (EP) zonal momentum flux divergence in the summer mesosphere. In the summer middle atmosphere, the wave amplitude grows substantially, like an internal wave in the regions of large refractive index.

Evolutions of gravity waves during the 2009 SSW are shown here

Figure 4. Evolutions of gravity waves during the 2009 SSW are shown here. Gravity wave enhancements on Jan 5 and 16 and suppressions of gravity waves after the peak SSW (on Jan 21) are clearly displayed. Filled and line contours represent the vertical wind (cm/s) and geopotential height (m), respectively, at 1 hPa on (a) Dec. 20, (b) Jan. 5, (c) Jan. 16, (d) Jan. 21, (e) Feb. 5, and (f) Feb. 20. Geopotential height line contour interval is 400 m. Only vertical winds larger than ±8 cm/s are plotted.

Meteorological events such as SSWs have attracted substantial research interest in recent years due to extensive observations as well as solar minimum conditions that effectively suppressed solar forcing from above while facilitating meteorological forcing from below. Figure 4 depicts gravity-wave variations during the 2009 SSW event in the Arctic based on the European Center for Median-Range Weather Forecasting (ECMWF) model. It is found that the occurrence and magnitude of gravity waves correlate with the location and strength of the polar vortex that is strongly disturbed by planetary wave (PW) growth. This location dependence on PW phase explains the observed gravity-wave variability during SSW. During the development and the onset of SSW, the zonal-mean gravity wave potential energy density (GW-Ep) increases on January 5 and then on January 15–22 in association with the growth of PW wavenumber 1 and wavenumber 2, respectively. As the initial prominent PW magnitude in the lower mesosphere progresses downward, GW-Ep enhancement also seems to show a corresponding descent from January 5–22. GW-Ep peaks before the wind reversal occurrence and significantly weakens after the SSW. These variations are confirmed by COSMIC/GPS observations. Lidar data from Antarctica also provide validation of gravity waves as derived in ECMWF. This research reveals that the gravity-wave enhancements prior to the 2009 SSW are strongly tied to the increase of gravity-wave excitation through residual forcing in the stratosphere. Decay of gravity waves after the 2009 SSW is most likely caused by the changes in gravity-wave propagation and reduction of in-situ gravity-wave sources by unbalanced flow.

Recently, we undertook an effort to extend the gravity-wave parameterization scheme in the Whole Atmosphere Community Climate Model (WACCM) by including the Coriolis effect in order to better describe inertia-gravity waves (IGW). WACCM simulations with the new parameterization have demonstrated that the QBO can be internally generated through properly selecting the parameters of the scheme. The characteristics of the simulated QBO are found to be consistent with observations. The study demonstrates that the parameterized IGW forcing is capable of generating equatorial wind oscillations with a downward phase progression in the stratosphere using the standard spatial resolution settings in the current model. The period of the oscillations depends on the strength of the IGW forcing, and the magnitude of the oscillations depends on the width of the wave spectrum. The new parameterization affects the wave breaking level and acceleration rates mainly through changing the critical level.