Variability of the ionospheric electron density at fixed heights and validation of IRI-2007 profile’s prediction at Ilorin

A study on the variability of the equatorial ionospheric electron density was carried out at fixed heights below the F2 peak using one month data for each of high and low solar activity periods. The data used for this study were obtained from ionograms recorded at Ilorin, Nigeria, and the study covers height range from 100 km to the peak of the F2 layer for the daytime hours and height range from 200 km to the peak of the F2 layer for the nighttime hours. The results showed that the deviation of the electron density variation from simple Chapman variation begins from an altitude of about 200 km for the two months investigated. Daytime minimum variability of between 2.7% and 9.0% was observed at the height range of about 160 and 200 km during low solar activity (January 2006) and between 3.7% and 7.8% at the height range of 210 and 260 km during high solar activity (January 2002). The nighttime maximum variability was observed at the height range of 210 and 240 km at low solar activity and at the height range of 200 and 240 km at high solar activity. A validation of IRI-2007 model electron density profile’s prediction was also carried out. The results showed that B0 option gives a better prediction around the noontime.     

The response of African equatorial foF2 to geomagnetic storms: Comparison between observations and IRI-2007 predictions

This study examines the response of the African equatorial ionospheric foF2 to different levels of geomagnetic storms, using the foF2 hourly data for the year 1989 from Ouagadougou (12.4°N, 1.5°W, dip: 2.8°N). The study also compares the observed data for the selected storm periods with the latest IRI model (IRI-2007). The foF2 values (both observed and predicted) show typical features of daytime peak and post-midnight minimum peak. The response of the ionospheric foF2 over Ouagadougou to storms events, during the night-time and post-midnight hours indicates negative responses of the ionospheric foF2, while that of the daytime hours indicates positive responses. For the investigation on the variability of the observed foF2 with respect to IRI-2007 model, with the exception of the analysis of the 20–22, October, 1989 data, where a midday peak was also observed on the first day, this study reveals two characteristic daily foF2 variability peaks: post-midnight and evening peaks. In addition, for all the geomagnetic storms considered, the URSI and CCIR coefficients of the IRI-2007 model show reasonable correspondence with each other, except for some few discrepancies. For instance, the event of 28–30 August, 1989 shows comparatively higher variability for the URSI coefficient, and at the foF2 peak values, the event of 20–22 October, 1989 shows that the CCIR coefficient is more susceptible to foF2 variability than the URSI coefficient. This study is aimed at providing African inputs for the future improvement of the IRI model.     

Characterization of GPS-TEC over African equatorial ionization anomaly (EIA) region during 2009–2016

This study characterizes total electron content (TEC) measured by Global Positioning System (GPS) over African equatorial ionization anomaly (EIA) region for 2009–2016 period during both quiet geomagnetic conditions (Kp?≤?1) and normal conditions (1?>?Kp?≤?4). GPS-TEC data from four equatorial/low-latitude stations, namely, Addis Ababa (ADIS: 9.04°N, 38.77°E, mag. lat: 0.2°N) [Ethiopia]; Yamoussoukro (YKRO: 6.87°N, 5.24°W, mag. lat: 2.6°S) [Ivory Coast]; Malindi (MAL2; 3.00°S, 40.19°E, mag. lat: 12.4°S) [Kenya] and Libreville (NKLG; 0.35°N, 9.67°W, mag. lat: 13.5°S) [Gabon] were used for this study. Interesting features like noontime TEC bite-out, winter anomaly during the ascending and maximum phases of solar cycle 24, diurnal and seasonal variations with solar activity have been observed and investigated in this study. The day-to-day variations exhibited ionospheric TEC asymmetry on an annual scale. TEC observed at equatorial stations (EIA-trough) and EIA-crest reach maximum values between ～1300–1600 LT and ～1300–1600 LT, respectively. About 76% of the high TEC values were recorded in equinoctial months while the June solstice predominantly exhibited low TEC values. Yearly, the estimated TEC values increases or decreases with solar activity, with 2014 having the highest TEC value. Solar activity dependence of TEC within the EIA zone reveals that both F10.7?cm index and EUV flux (24–36?nm) gives a stronger correlation with TEC than Sunspot Number (SSN). A slightly higher degree of dependence is on EUV flux with the mean highest correlation coefficient (R) value of 0.70, 0.83, 0.82 and 0.88 for quiet geomagnetic conditions (Kp?≤?1) at stations ADIS, MAL2, NKLG, and YKRO, respectively. The correlation results for the entire period consequently reveals that SSN and solar flux F10.7?cm index might not be an ideal index as a proxy for EUV flux as well as to measure the variability of TEC strength within the EIA zone. The estimated TEC along the EIA crest (MAL2 and NKLG) exhibited double-hump maximum, as well as post-sunset peaks (night time enhancement of TEC) between ～2100 and 2300 LT. EIA formation was prominent during evening/post-noon hours.     

A simulation of Martian gravity field recovery by using a near equatorial orbiter

An improvement to the Martian gravity field may be achieved by means of future orbiting spacecraft with small eccentricity and low altitude exemplified through a newly proposed mission design that may be tested in upcoming reconnaissance of Mars. Here, the near equatorial orbital character (with an inclination approximating 10°, eccentricity as 0.01 and semi-major axis as 4000 km) is considered, and its contribution to Martian gravity field solution is analyzed by comparing it with a hypothetical polar circular orbiter. The solution models are evaluated in terms of the following viewpoints: power spectra of gravity field coefficients, correlations of low degree zonal coefficients, precise orbit determination, and error distribution of both Mars free air gravity anomaly and areoid. At the same time, the contributions of the near equatorial orbiters in low degree zonal coefficients time variations are also considered. The present results show that the near equatorial orbiter allows us to improve the accuracy of the Martian gravity field solution, decrease correlation of low degree zonal coefficients, retrieve much better time variable information of low degree zonal coefficients, improve precise orbit determination, and provide more accurate Mars free air gravity anomaly and areoid around the equatorial region.     

Planetary wave coupling of the atmosphere–ionosphere system during the Northern winter of 2008/2009

This paper presents the global spatial (latitude and altitude) structure and temporal variability of the ∼23-day ionospheric zonally symmetric (s = 0) planetary wave (PW) seen in the Northern winter of 2008/2009 (October 2008–March 2009). It is shown that these ∼23-day ionospheric oscillations are forced from PWs propagating from below. The COSMIC ionospheric parameters f_oF2 and h_mF2 and electron density at fixed altitudes and the SABER temperatures were utilized in order to define the waves which are present simultaneously in the atmosphere and ionosphere. The long-period PWs from the two data sets have been extracted through the same data analysis method. The similarity between the lower thermospheric ∼23-day (s = 0) temperature PW and its ionospheric electron density response provides valuable and strong experimental evidence for confirming the paradigm of atmosphere–ionosphere coupling.     

Characteristics of the equatorial F-region vertical plasma drift observed during post-sunset and pre-sunrise hours

Post-sunset and pre-sunrise vertical plasma drifts at the equatorial F-region have been investigated using the HF Doppler radar and ionosonde observations. Observed vertical plasma drift features during the sunrise are found to complement that observed during the evening. The post-sunset vertical plasma drift is characterized by an upward enhancement, a pre-reversal enhancement and a reversal in the drift direction. Similarly, the pre-sunrise plasma drift is characterized by a sudden downward excursion followed by an upward turning. The wavelet analysis of the plasma drift shows the presence of fluctuations in the period range 4–32 min and the short period fluctuations are attributed to the atmospheric gravity waves.     

气体辐射与流场耦合对火星进入热环境影响

采用火星大气物理化学模型,求解带辐射源项的三维热化学非平衡N-S方程,对探路者号火星探测器进入过程中的高温流场和热环境进行了数值模拟,分析了气体辐射与非平衡流场耦合效应对流场和热流的影响。结果表明:1)探路者号火星探测器流场热化学非平衡效应显著,CO₂气体发生大规模离解,高度低至28.5 km仍存在热力学非平衡效应;2)热力学与化学非平衡效应的影响均与表面催化特性相关,完全催化热流要高于完全非催化热流50%以上;3)高温流场中的CO组分会产生较强的气体辐射加热,辐射热流与对流热流的比值为15%～45%,靠近肩部区域比值最大;4)气体辐射对非平衡流场的冷却效应使激波脱体距离减小;与非耦合方法相比,采用耦合方法得到的辐射热流降低约12%～25%。     

VLF电波渗透到卫星高度电离层传播的全波计算

考虑斜向地磁场的影响将电离层设为多层水平分层各向异性有耗介质, 利用传播矩阵法求解全波方程, 进而研究分析VLF频段电离层反射系数随电波频率的变化, 电离层中两种特征极化波的折射和极化特性, 两特征波的电磁场水平分量以及坡印廷能流密度随传播高度的变化. 数值计算结果表明, 地—电离层波导中的垂直极化波比平行极化波易渗透进入电离层; 电离层中两种特征极化波可分为左旋和右旋圆极化波, 左旋分支由于D层强吸收作用表现为速衰减模, 而右旋分支表现为可传播模, 在传播过程中电磁波的能量主要存储在磁场中; 电波频率越低, 其在电离层中的传播损耗越小. 由数值模拟结果发现, 卫星监测VLF频段的低频部分及更低频段的水平磁场变化对于发现地震电离层电磁前兆异常可能更为有效.     

极区顶部电离层离子上行的太阳活动依赖性研究

利用第23太阳活动周DMSP F12,F13和F15卫星数据,分别对南北半球极区顶部电离层离子上行的太阳活动依赖性进行了研究.结果表明,南北半球上行事件对太阳活动的响应特征基本一致,即高（低）太阳活动时,离子上行通量以及上行数密度较大（小）,但是上行速度及上行发生率较低（高）.以南半球高纬为例,计算得到离子上行通量、数密度、速度及发生率在高低太阳活动条件下的比值分别约为2.26,3.35,0.71,0.51.对离子上行太阳活动依赖性的可能原因进行了分析.不同太阳活动水平下,光致电离及高能粒子沉降的差异会导致电离层离子密度的不同,而电离层离子密度的变化会改变离子elax-elax中性大气之间的碰撞频率,这是影响离子上行发生率的一个重要原因.     

飞行器进入火星大气的流场预测

针对火星着陆探测器进入-下降-着陆过程的高超声速进入阶段, 利用三维并 行程序求解流体动力学Navier-Stokes方程与化学反应动力学模型, 分析火星 科学实验室进入火星大气时探测器周围的流场结构、化学非平衡效应影响和气 动特性变化规律. 结果表明, 对于完全气体模型, 来流的热力学性质参数选 取影响激波位置和强度. 在化学非平衡效应影响下, 探测器头部激波脱体距离 大幅减小, 驻点压力变化不大, 波后温度显著降低. CO₂在激波后大量分解, 消耗相当能量. 流线结构显示, 探测器尾迹流动中存在复杂的旋涡运动等流动 分离现象.