Abstract Understanding the temporal and spatial variations in the ideal inert tracer helium can provide insight into the dynamic evolution of the thermosphere. The magnitude of the thermospheric winter helium bulge was inversely correlated with the level of solar activity. However, this feature has been found to be not reproduced by the Thermosphere‐Ionosphere Electrodynamic General Circulation Model (TIEGCM), and the associated physical mechanisms remain unknown. Using the Thermosphere‐Ionosphere‐Mesosphere Electrodynamic General Circulation Model (TIME‐GCM), we found that mesospheric gravity wave drag (GWD) is a factor contributing to this inverse correlation. Specifically, the summer‐to‐winter circulation in thermosphere becomes the main cause of the helium bulge, as mesospheric GWD can play a role in strengthening this circulation. The GWD contributions to temperature change below the lower thermosphere do not depend prominently on solar activity. However, because the temperature impacts on the pressure gradient force are height‐integrated according to the background temperature of the neutral gas, the higher background temperature in the thermosphere at the solar maximum corresponds to a relatively weaker response in pressure gradient force in the thermosphere. Therefore, the response of the thermospheric circulation that might be expected to accompany increasing solar activity is suppressed due to the influence of mesospheric GWD, which results in a decrease in the magnitude of the winter helium bulge with increasing solar activity. Thus, our results demonstrated that lower atmosphere forcing can play a significant role in the response of thermospheric helium to solar activity.
Abstract HIWIND (High altitude Interferometer WIND experiment) is a balloon‐borne Fabry Perot interferometer for daytime thermospheric wind observations. In this paper, we examine the summer polar cap thermospheric winds observed by HIWIND with the RISR‐C (Resolute Incoherent Scatter Radar‐Canada) observed ion drifts and electron densities. We also perform National Center for Atmospheric Research Thermosphere Ionosphere Electrodynamics General Circulation Model simulations to compare with the HIWIND and RISR‐C observations. The standard Thermosphere Ionosphere Electrodynamics General Circulation Model underestimates the high‐latitude electron density and overestimates the thermospheric winds. The discrepancies between modeled and observed meridional winds are large near midnight and noon. After increasing the energy flux in the polar cap drizzle, the simulated electron density is comparable with the RISR‐C observations. However, large discrepancies with the HIWIND‐observed thermospheric winds persist. The cause of the model versus observation discrepancy in winds is probably due to the processes outside the polar cap.
Abstract The ratio of O number density to N 2 number density (O/N 2 ) is an important parameter to describe thermospheric composition changes and its effects on the ionosphere. Based on Global Ultraviolet Imager (GUVI) limb measurements, we investigate the seasonal behaviors of O/N 2 volume density ratio on different constant pressure levels during geomagnetically quiet periods. The global O/N 2 shows the prominent annual and semiannual variations with solar activity dependence. An empirical model considering the solar activity and annual/semiannual variations can reasonably reproduce the original O/N 2 . The modeled O/N 2 captures the hemispheric asymmetry of the annual variations in both length and magnitude. Global maps of the seasonal harmonic components of the modeled O/N 2 indicate the latitudinal and altitudinal dependence of O/N 2 seasonal variations. The annual component dominates over the semiannual component at mid‐latitudes, but it is smaller than the semiannual component at low latitudes. In the Northern Hemisphere, and at low geomagnetic latitudes of the Southern Hemisphere, the annual component peaks around December solstice at all altitudes, whereas at middle geomagnetic latitudes of the Southern Hemisphere, it peaks around June solstice. The semiannual component peaks at the equinoxes in almost all regions over the globe at all altitudes. The annual and semiannual amplitudes both increase with altitude. In addition, O/N 2 annual variations and solar activity dependence are more influenced by the thermal expansion and contraction.
We analyzed in the research the similarities and differences of characteristics variation about east and west branch of the Glacier No.1 based on temperature,precipitation and variable data.In addition,the interpretation for acceleration of melting and characteristics variation of the Glacier No.1 was given based on the material and energy balance theory.The research indicated that the differences between the two branches were due to the reasons such as glacier respective characteristics,elevation,terrain and the area,Two important factors were led to accelerated melting and characteristics variation of Glacier No.1,which were the temperature rising and the absorb rate of solar radiation increasing.
Abstract Ionospheric F‐region electron density is anomalously higher in the evening than during the daytime on many occasions in the summer in geomagnetic mid‐latitude regions. This unexpected ionospheric diurnal variation has been studied for several decades. The underlying processes have been suggested to be related to meridional winds, topside influx arising from sunset ionospheric collapse, and other factors. However, substantial controversies remain unresolved. Using a numerical model driven by the statistical topside O + diffusive flux from the Millstone Hill incoherent scatter radar data, we provide new insight into the competing roles of topside diffusive flux, neutral winds, and electric fields in forming the evening density peak. Simulations indicate that while meridional winds, which turn equatorward before sunset, are essential to sustain the daytime ionization near dusk, the topside diffusive flux is critically important for the formation and timing of the summer evening density peak.
Abstract In this study, we conduct an in‐depth analysis of Whole Atmosphere Community Climate Model‐eXtended simulations to examine physical mechanisms of the formation and evolution of an equatorial ionization anomaly (EIA) merging phenomenon during a storm on 4 November 2021. A quantitative analysis reveals that the rapid decay of the EIA crests at their poleward sides at altitudes of ∼200–250 km plays a crucial role in the EIA merging during that day. This rapid decay is due to the fast recombination at low altitudes (∼200–250 km) as the plasma are transported downward by the westward disturbance dynamo electric field and poleward neutral winds during the storm. The results suggested EIA‐merging is not merely northern and southern EIA crests moving together, but it involves a crucial rapid decay of the EIA crests at their poleward sides that descended to low altitudes (rapid recombination, ∼200–250 km), driven by regional electric fields and neutral winds. This study plays a crucial role in our understanding of the evolution and formation of the merged EIA on 4 November 2021 during the storm.
Abstract It has long been recognized that during solar eclipses, the ionosphere‐thermosphere system changes greatly within the eclipse shadow, due to the rapid reduction of solar irradiation. However, the concept that a solar eclipse impacts polar ionosphere behavior and dynamics as well as magnetosphere‐ionosphere coupling has not been appreciated. In this study, we investigate the potential impact of the 21 August 2017 solar eclipse on the polar tongue of ionization (TOI) using a high‐resolution, coupled ionosphere‐thermosphere‐electrodynamics model. The reduction of electron densities by the eclipse in the middle latitude TOI source region leads to a suppressed TOI in the polar region. The TOI suppression occurred when the solar eclipse moved into the afternoon sector. The Global Positioning System total electron content observations show similar tendency of polar region total electron content suppression. This study reveals that a solar eclipse occurring at middle latitudes may have significant influences on the polar ionosphere and magnetosphere‐ionosphere coupling.
Abstract Thermospheric densities observed by Challenging Minisatellite Payload and Gravity Recovery and Climate Experiment satellites during 2002–2010 and the globally averaged thermospheric densities from 1967 to 2007 have been used to investigate latitudinal, longitudinal, and height dependences of the multiday oscillations of thermospheric densities. The data show that the main multiday oscillations in thermospheric densities are 27, 13.5, 9, and 7 day oscillations. The high‐correlation coefficients between the density oscillations and the F 10.7 or Ap index indicate that these oscillations are externally driven. The 27 day density oscillation, being the strongest, is induced by variations in solar radiation, as well as recurrent geomagnetic activity that is the result of corotating interaction regions (CIRs) and high‐speed solar wind streams of coronal hole origin. Density oscillations at periods of 13.5, 9, and 7 days at solar minimum and during the declining phase are stronger than those at solar maximum. These oscillations are mainly associated with recurrent geomagnetic activity due to coronal hole high‐speed streams and CIRs. The multiday, periodic oscillations of thermospheric density exhibit strong latitudinal and longitudinal variations in the geomagnetic coordinate and oscillate synchronously at different heights. Oscillations with zonal wave number 0 oscillate globally, whereas those with nonzero wave numbers are strong at high geomagnetic latitudes, and hemispherically asymmetric. They are stronger in the Southern Hemisphere. The spectral distributions of thermospheric densities at different heights have almost the same latitude and longitude structures, but the spectral magnitudes increase with height.
We report a new parameterization of ionization in the Earth's atmosphere by isotropically precipitating monoenergetic (100 eV to 1 MeV) electrons. This new parameterization is the first one based on sophisticated first‐principle models, and represents a significant improvement in accuracy, particularly for incident auroral and lower energies. Without previous need to interpolate over source energy and atmospheric range, the new parameterization provides an easier implementation with a robust fit of model calculations for a wide range of incident energies and atmospheric conditions. By decomposing any incident energy spectrum into contiguous monoenergetic components and then calculating and integrating their resulting ionization, our parameterization is a valuable tool that can be used in conjunction with global models to accurately quantify the impact from realistic precipitating electrons during space weather events.
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