Electromagnetic field dissipation and corpuscular bombardment and their implications in atmospheric modelling

The dissipation of energy of electric fields and currents in the polar auroral atmosphere is a major source of energy for the thermosphere ranging locally up to 100 ergs cm^?2 sec^?1 and perhaps more during the most intense disturbance. Globally the input of energy to the thermosphere can often exceed that due to solar EUV radiation. This energy source is always significant in polar regions and its variable strength with respect to that of the solar EUV radiation determines the behaviour of the middle and low latitude thermosphere. It is extremely difficult to model because of its variability in space and time. Nevertheless understanding the dynamics and composition of the global thermosphere is dependent upon incorporation of this source realistically into models. A further important aspect of this energy source is the consequences of its action in changing the density and composition of the thermosphere globally leading to subsequent changes in the absorption of solar EUV radiation. The ring current may also, at times, be a significant source of energy to the low latitude thermosphere.     

太阳极紫外辐射对热层大气密度的影响

对2001-2021年SOHO卫星的极紫外辐射测量数据,以及CHAMP,GRACE-A和SWARM-C卫星资料推导出的高分辨率大气密度数据进行统计分析,发现大气密度与极紫外测量值的相关系数大于密度与F10.7指数的相关系数,证实极紫外辐射在不同地方时的影响程度存在显著差异,从而驱动大气密度的周日变化。利用三颗卫星的高度差异揭示极紫外辐射对大气密度的加热效应在350～500 km范围随着高度增加而减弱。统计得到极紫外辐射影响在地方时和纬度上的空间差异：对夏季半球的影响大于冬季半球;在白天,对中纬度地区的影响高于赤道和高纬度地区;在夜间,密度对辐射的斜率在夏季半球高纬度地区存在峰值,在冬季半球中纬度存在谷值,模型DTM2000和NRLMSISE00未能准确刻画。为了改进经验模型,提出基于球谐函数的拟合方法,优于主流模型周日效应采用的表达式,对周日效应建模和修正提供有益借鉴。利用昼夜间能量传输和热层大气经向环流机制探讨了统计结果的物理机制。     

On the mechanisms of cooling of the nightside thermosphere of venus

The plausible mechanisms of cooling of the nightside Venus' thermosphere are analysed with the aid of the model of the atmospheric heat budget that incorporates, in addition to thermal conduction and IR radiation in the 15 μ band of CO₂, heating and cooling due to global scale winds, eddy turbulence, and IR radiation in the rotational bands of H₂O and CO, as well as the 63 μ line of atomic oxygen. The H₂O mixing ratio and parameters of turbulence required for cooling of the thermosphere down to the observed low temperatures are evaluated.     

中层及低热层大气对局地时变热源的响应

采用二维交替方向隐格式(ADI),数值模拟了中高层大气对一个位于对流层高度范围的局地时变热源的响应．分析了这一过程中大气响应的能量密度,水平波数,垂直波数以及频率．模拟结果表明:响应波动携带能量向上传播,集中在较为狭窄的水平区域,该区域关于源的水平位置对称;相对于动量源驱动,热源的水平尺度对响应的水平波长决定性作用更加明显;对于高频振荡的热源,其响应的频率和源一致;响应的垂直波长(41 km)比源的垂直尺度(6 km)大得多．     

Interpretation of infrared measurements of the high-latitude thermosphere from a rocket-borne interferometer

A preliminary analysis of high-resolution infrared spectra of the aurorally dosed lower thermosphere above Poker Flat Research Range (PFRR), Alaska, obtained by an uplooking cryogenic field-widened interferometer (FWI) is presented. Both models and spectral-fitting/resolution-enhancement methods are used to discuss the behavior of NO, CO, NO⁺, and CO₂ v₃ vibrational bands in the high-latitude thermosphere.     

Properties of the neutral density and composition in the thermosphere

Measurements of the density and composition of the thermosphere between 150 and 500 km, which were obtained by the S3-1 satellite, have been compared with the Jacchia and MSIS models. The measurements of the densities of O, N₂, N and Ar show some differences from the current models which should be considered during the preparation of the next CIRA model. The Ar measurements are particularly useful in examining the response of the neutral atmosphere to geomagnetic heating. These results are useful in establishing the appropriate lower boundary conditions for modeling of the thermosphere.     

强磁暴、能量粒子暴与热层大气密度涨落之间的相关关系

利用1997-2007年由GOES8, GOES11和GOES12星载高能粒子探测器在地球同步轨道高度上所探测到的高能质子和高能电子通量探测数据以及高度560km左右星载大气密度探测器所得的热层大气密度探测数据, 统计分析了强地磁扰动、高能粒子通量跃变和热层大气密度涨落之间的相关关系, 初步获得强地磁扰动期间, 地球同步轨道(外辐射带外环)均出现了增幅大于三个数量级的高能质子通量(尤其是E>1MeV)强增强现象, 随后热 层大气密度强烈上涨, 表明三者之间是正相关关系. 在时间上地球同步轨道高能质子通量强增强现象先于日均Ap值(地磁活动程度)上涨约一天左右, 而热层大气密度强涨落现象又明显滞后于强地磁扰动事件.      

上行离子源区卫星探测的轨道需求分析

极区从电离层到磁层的上行粒子流探测研究是空间天气建模中的重要问题,其起源和加速机制是磁层-电离层-热层耦合小卫星星座计划的主要科学目标. 磁层-电离层-热层耦合小卫星星座计划拟定由两颗磁层星和两颗电离层/热层星组成星座对极区进行联合观测. 其中,上行粒子源区附近的就位探测是电离层-热层耦合机制研究的重点,也是电离层/热层星轨道设计的关键. 根据相关空间探测计划和卫星观测结果,通过比较圆轨道和椭圆轨道两种方案,确定电离层/热层星采用椭圆轨道.      

550～600 km高度上热层大气密度的中长期变化

选用了2005年8月20日至2006年7月28日高度550～600 km附近的热层大气密度探测数据,对表征太阳活动的F10.7值和表征地磁活动强度的Ap指数进行了相关特性的统计.分析结果表明,在无明显地磁扰动时热层大气密度日平均值的涨落呈现27日和准半年的周期性变化,但在地磁扰动期间这种变化的周期性会被削弱,且大气密度的周日变化幅度与F10.7值呈正相关关系.      

Recent advances in upper atmospheric structure

A possible quantitative explanation of the semi-annual variation in thermospheric density has been obtained in terms of a semi-annual variation in the computed globally averaged vertical energy carried by propagating tides from the lower and middle atmosphere into the thermosphere. The effect is primarily due to seasonal changes in the distribution of water vapor and in the solar declination angle and Sun-Earth distance. An MSIS-83 empirical model of the thermosphere, representing a revision of the earlier MSIS models, has been prepared. The database used covers a wider range of solar activity than previous models and an improved magnetic storm representation is included. Atomic oxygen profiles in the 100 to 160 km altitude region of the auroral thermosphere have been recalculated from measured quenching of N₂(A³∑_u⁺) using the latest laboratory rates and the results are in good agreement with the mean CIRA 1972 profile. A new empirical model of thermospheric variations with geomagnetic activity has been developed incorporating variations with local magnetic time, latitude dependent terms which can vary with the magnitude of the geomagnetic disturbance, and an altitude dependent expression for the equatorial wave. A new index ML, derived from the AL index, has been developed that appears to have promise to represent the variations of thermospheric species with geomagnetic activity. Satellite measured values of solar UV flux, ground-based observations of CaK plages, sunspot numbers and 10.7 cm solar radio flux have been analyzed for temporal variations. Some differences have been identified and the significance to empirical and theoretical upper atmosphere models is discussed.     

Aspects related to variability of radiative cooling by NO in lower thermosphere,TEC and O/N2 correlation,and diffusion of NO into mesosphere during the Halloween storms

Nitric Oxide is a very important trace species which plays a significant role acting as a natural thermostat in Earth’s thermosphere during strong geomagnetic activity. In this paper, we present various aspects related to the variation in the NO Infrared radiative flux (IRF) exiting the thermosphere by utilizing the TIMED/SABER (Thermosphere Ionosphere Mesosphere Energetics and Dynamics/ Sounding of the Atmosphere using Broadband Emission Radiometry) observational data during the Halloween storm which occurred in late October 2003. The Halloween storm comprised of three intense-geomagnetic storms. The variability of NO infrared flux during these storm events and its connection to the strength of the geomagnetic storms were found to be different in contrast to similar super storms. The connection between the quantum of energy outflux from the upper atmosphere into space in terms of NO IRF and the duration of storms is established. The NO radiative cooling, and the closely correlated depletion in O/N₂ ratio are controlled by the Joule heating intensity (proxied by AE-index). The collisional excitation rate of NO, calculated using the modelled datasets of WACCM-X (Whole Atmosphere Community Climate Model with thermosphere and ionosphere extension), correlates well with the observed pattern of radiative emission by NO. Observational datasets from TIMED/GUVI (Global Ultra-Violet Imager) and MIT Haystack observatory madrigal GNSS (Global navigation satellite system) total electron content (TEC) database shows that the TEC and O/N₂ enhancement in low-mid northern hemispheric latitudes are mainly controlled by the z-component of Interplanetary magnetic field (IMF-B_z). The penetration of eastward electric field during the storm events is found to be responsible for the overall enhancement of TEC. The contribution of enhanced day-side TEC in observed variation of O/N₂ ratio by GUVI is also reported. It is also seen that during substorms related events the night-time polar region experiences more cooling due to NO than the daytime polar region. The connections between the mid- and low-latitude enhancement in NO IRF with the propagation of LSTIDs (Large-scale traveling ionospheric disturbances) in combination with the O/N₂ variability, and the altitudinal variation in NO flux with the progression of the storm is also investigated. This study presents the evidence on the role of diffusion processes in the large scale enhancement of NO in the mesospheric altitudes.     

On the diurnal variations in the temperature and composition: a three-dimensional model with superrotation

Based on a simplified theoretical interpretation of the composition measurements with the ONMS and OIMS experiments on Pioneer Venus, the conclusion was drawn that the rotation rate of the thermosphere should be close (within a factor of two) to that of the lower atmosphere. A more realistic three-dimensional model of the thermosphere dynamics is now being developed, considering non-linear processes, higher order modes and collisional momentum exchange between the major species CO₂, CO and O, which describes the diurnal variations in temperature and composition (Niemann et al., $\text{JGR}$, 1980). The computed horizontal winds are about 300 m/sec near the terminators and poles. Results are also presented from a two-dimensional (quasi-axisymmetric) spectral model which describes the four day superrotation in the lower atmosphere of Venus.     

The thermosphere as a sink of magnetospheric energy: A review of recent observations of dynamics

The study of the dynamics and thermodynamics of the earth's upper atmosphere has made significant progress over the past few years owing to the availability of new global-scale data sets from the Dynamics Explorer satellites. The thermospheric wind and temperature fields at high latitude have been observed to depend strongly on forcing processes of magnetospheric origin. A key momentum source is due to the drag effect of ions convecting in response to electric fields mapped down on the ionosphere from magnetospheric boundary regions. Likewise, an important heat source derives from Joule or frictional dissipation due to ion/neutral difference velocities governed, in turn, by magnetospheric forcing. In this paper we discuss the progress made over the last 2–3 years initiated by the new satellite measurements and we review published data on ion and neutral motions in the context of the energy and momentum coupling between the magnetosphere and the ionosphere/neutral upper atmosphere. The observations indicate the existence of a “flywheel effect” which implies direct feedback from the neutral thermosphere to the magnetosphere via the release of energy and momentum previously “stored” in the neutral thermosphere.     

Reevaluation of thermosphere heating by auroral electrons

This paper presents a new calculation of neutral gas heating by precipitating auroral electrons. It is found that the heating rate of the neutral gas is significantly lower than previous determinations below 200 km altitude. The neutral gas heating arises from the many exothermic chemical reactions that take place from the ions and excited species created by the energetic electrons. The calculations show that less than half the energy initially deposited ends up heating the neutral gases. The rest is radiated or lost in the dissociation of O₂ because the O atoms do not recombine in the thermosphere. This paper also presents a new way of calculating the heating rate per ionization that can be used for efficient determination of the overall neutral gas heating for global thermosphere models. The heating rates are relatively insensitive to the neutral atmosphere when plotted against pressure rather than altitude coordinates. At high altitudes, the heating rates are sensitive to the thermal electron density and long-lived species. The calculations were performed with the Field Line Interhemispheric Plasma (FLIP) model using a 2-stream auroral electron precipitation model. The heating rate calculations in this paper differ from previous heating rate calculations in the treatment of backscattered electrons to produce better agreement with observed flux spectra. This paper shows that more realistic model auroral electron spectra can be obtained by reflecting the up going flux back to the ionosphere at the upper boundary of the model. In this case, the neutral gas heating rates are 20%–25% higher than when the backscattered flux escapes from the ionosphere.     

The neutral particle impact de-excitation cross-section of the fine structure level3P1 of atomic oxygen in the thermosphere

A new upper limit value (9.9 × 10^?21cm²) of the mean equivalent de-excitation cross-section of O(³P₁) by collision with neutral particles in the thermosphere is obtained by comparing the³P₁ state de-excitation rate due to neutral practice impact with that due to electron impact. The existing data of the cross-section obtained from the aeronomic study are compared with the theoretical data, and it indicated that the former is much less than the latter.     

Nitric oxide density enhancements due to solar flares

A differential emission measure technique is used to determine flare spectra using solar observations from the soft X-ray instruments aboard the Thermosphere Ionosphere Mesosphere Energetics Dynamics and Solar Radiation and Climate Experiment satellites. We examine the effect of the solar flare soft X-ray energy input on the nitric oxide (NO) density in the lower thermosphere. The retrieved spectrum of the 28 October 2003 X18 flare is input to a photochemical thermospheric NO model to calculate the predicted flare NO enhancements. Model results are compared to Student Nitric Oxide Explorer Ultraviolet Spectrometer observations of this flare. We present results of this comparison and show that the model and data are in agreement. In addition, the NO density enhancements due to several flares are studied. We present results that show large solar flares can deposit the same amount of 0.1–2 and 0.1–7 nm energy to the thermosphere during a relatively short time as the Sun normally deposits in one day. The NO column density nearly doubles when the daily integrated energy above 5 J m⁻² is doubled.     

Radiative cooling in the NLTE region of the mesosphere and lower thermosphere—Global energy balance

Radiative cooling in the mesosphere and lower thermosphere is predominantly from 15-μm emissions of CO₂. Above t 120 km, complete NLTE cooling from NO becomes more important. Above 100 km, both the CO₂ and the NO cooling are proportional to concentrations of atomic oxygen which are dynamically controlled and poorly characterized by observations. Furthermore, the rate for energy exchange between O and CO₂(ν₂ = 1) is very poorly known. CO₂ is close to LTE throughout the mesosphere, but small departures from LTE between 65 and 80 km may be important for questions of remote sensing. Remote sensing for trace gases, e.g., O₃ and H₂O, must consider NLTE effects in the mesosphere. A global mean column model for aeronomy processes above 65 km gives a reasonable agreement with observed temperatures, suggesting that radiative balance may be possible without the need for including eddy cooling or gravity wave heating.     

Solar EUV energy budget of the thermosphere

Using the accumulation of experimental data and theoretical studies conducted on the terrestrial thermosphere since the mid seventies, we have re-evaluated the channels by which solar UV energy is transferred to the atmosphere. As an outcome of this evaluation we have redetermined the solar EUV heating efficiency for the thermosphere and find this to be considerably different from that established in earlier studies. The heating efficiency has strong altitude, solar cycle and diurnal dependencies. The values of this parameter vary from less than 10% to greater than 100%, with peak midday values of 50–55% In recent papers we have presented the results of this new UV heating efficiency determination using a steady state solution of the ionospheric model. In this paper we present the results obtained solving a time dependent model over a diurnal cycle. The time dependent effects are found to be significant, with certain longlived species acting as temporary reservoirs of latent heat that is released to the neutral atmosphere at later times.