Study of mid-latitude ionospheric convection during quiet and disturbed periods using the SuperDARN Hokkaido radar

Westward ionospheric convective flows around midnight are frequently observed at mid-latitudes. They can be generated by so-called disturbance dynamo mechanisms working mainly in the mid-latitudes. To understand the influence of disturbance dynamo effects in the mid-latitudes, we studied the latitudinal distribution of westward flows in association with several kinds of geomagnetic disturbances using the SuperDARN Hokkaido radar. This radar creates high temporal resolution (1 s to 2 min), two-dimensional observations measuring the line-of-sight velocities of ionospheric plasma irregularities, which can be regarded as line-of-sight velocities of ionospheric convection in the mid-latitude region from 40° to 50°. This region could not be monitored using preexisting SuperDARN radars. In this study, we used ionospheric echo data obtained by the SuperDARN Hokkaido radar over 5 years (December 2006 to November 2011). We identified westward flows around midnight at about 40° to 55° geomagnetic latitude. Additionally, the data showed that the westward flow around midnight intensified under high geomagnetic activity (high Kp). This suggests that the disturbance dynamo could affect the mid-latitude ionospheric convection. We performed Superposed Epoch Analysis (SEA) to study the influences from the geomagnetic disturbances on mid-latitude ionospheric convection. We found no obvious influence during major storms (minimum Dst below −60 nT). SEA was also used to study the temporal and latitudinal dependence on the influences from substorms. From analysis of 36 events of AL-defined substorms, we saw that the influence of substorms lasted from 5 to 20 h after the onset between 44° and 53° geomagnetic latitude. The westward flow at mid-latitude grew to a maximum at 12 h after the geomagnetic substorm onset. This is consistent with the results of past numerical simulation studies of the disturbance dynamo effects.     

A comparison of different regression models for downscaling Landsat and MODIS land surface temperature images over heterogeneous landscape

Remotely sensed high spatial resolution thermal images are required for various applications in natural resource management. At present, availability of high spatial resolution (<200 m) thermal images are limited. The temporal resolution of such images is also low. Whereas, coarser spatial resolution (∼1000 m) thermal images with high revisiting capability (∼1 day) are freely available. To bridge this gap, present study attempts to downscale coarser spatial resolution thermal image to finer spatial resolution using relationships between land surface temperature (LST) and vegetation indices over a heterogeneous landscape of India. Five regression based models namely (i) Disaggregation of Radiometric Temperature (DisTrad), (ii) Temperature Sharpening (TsHARP), (iii) TsHARP with local variant, (iv) Least median square regression downscaling (LMS_DS) and (v) Pace regression downscaling (PR_DS) are applied to downscale LST of Landsat Thematic Mapper (TM) and Terra MODIS (Moderate Resolution Imaging Spectroradiometer) images. All the five models are first evaluated on Landsat image aggregated to 960 m resolution and downscaled to 480 m and 240 m resolution. The downscale accuracy is achieved using LMS_DS and PR_DS models at 240 m resolution at 0.61 °C and 0.75 °C respectively. MODIS data downscaled from 1000 m to 250 m spatial resolution results root mean square error (RMSE) of 1.43 °C and 1.62 °C for LMS_DS and PR_DS models, respectively. The LMS_DS model is less sensitive to outliers in heterogeneous landscape and provides higher accuracy when compared to other models. Downscaling model is found to be suitable for agricultural and vegetated landscapes up to a spatial resolution of 250 m but not applicable to water bodies, dry river bed sand sandy open areas.     

中国及邻近地区地磁场垂直梯度的计算与研究

根据地磁场的球谐模型,推导出地磁场各个分量垂直梯度的球谐表达式.计算出中国及邻近地区高度为0km,50km和100km处的地磁场及其垂直梯度的网点值,编绘出地磁总强度、垂直强度、水平强度和磁偏角的垂直梯度等值线图.研究了地磁场垂直梯度在地面上以及随高度的变化规律.     

Standing Alfvén waves with m ≫ 1 in a dipole magnetosphere with moving plasma and aurorae

The structure of standing Alfvén waves with large azimuthal wave numbers (m ? 1) is studied in a dipole model of the magnetosphere with rotating plasma. In the direction across magnetic shells the structure of such waves is determined by their dispersion associated with curvature of geomagnetic field lines and corresponds to the travelling wave localized between toroidal and poloidal resonant surfaces. In projection into the ionosphere (along geomagnetic field lines) this structure is similar to the structure of a discrete auroral arc. The azimuthal structure of an auroral arc is similar to azimuthal structure of Alfvén waves with m ∼ 100. Possible interaction mechanisms between the Alfvén waves and energetic electron fluxes forming auroral arcs are discussed.     

Oblique-incidence ionospheric soundings over Central Europe and their application for testing now casting and long term prediction models

After a first oblique-incidence ionospheric sounding campaign over Central Europe performed during the period 2003–2004 over the radio links between Inskip (UK, 53.5°N, 2.5°W) and Rome (Italy, 41.8°N, 12.5°E) and between Inskip and Chania (Crete, 35.7°N, 24.0°E), new and more extensive analysis of systematic MUF measurements from January 2005 to December 2006 have been performed. MUF measurements collected during moderately disturbed days (17 ? Ap ? 32), disturbed days (32 < Ap ? 50) and very disturbed days (Ap > 50), have been used to test the long term prediction models (ASAPS, ICEPAC and SIRM&LKW), and the now casting models (SIRMUP&LKW and ISWIRM&LKW). The performances of the different prediction methods in terms of r.m.s are shown for selected range of geomagnetic activity and for each season.     

Variation of ionospheric total electron content at crest of equatorial anomaly in China from 1997 to 2004

The periodic variation of TEC data at Xiamen station (geographic coordinate: 24.4°N, 118.1°E; geomagnetic coordinate: 13.2°N, 187.4°E) at crest of equatorial anomaly in China from 1997 to 2004 is analyzed. The characteristic of TEC association with solar activity and geomagnetic activity are also analyzed. The method of continuous wavelet, cross wavelet and wavelet coherence transform methods have been used. Analysis results show that long-term variations of TEC at Xiamen station are mainly controlled by the variations of solar activities. Several remarkable components including 128–256 days, 256–512 days and 512–1024 days exist in TEC variations. The TEC data at Xiamen station is in anti-phase with geomagnetic Dst index in semiannual time-scale, but this response only exists during high solar activity. Diurnal variation of TEC is studied for different seasons. Some features like the semiannual anomaly and winter anomaly in TEC have been reported.     

VLF Emissions observed at the low latitude Indian station Varanasi

VLF emissions such as hiss, chorus, pulsing hiss, triggered emissions observed at Varanasi (geom. lat. = 14° 55′ N, long. = 153° 55′ E) are reported. An attempt has been made to explain the dynamic spectra in terms of the generation mechanism and propagation through the inhomogeneous magnetoplasma. It is noted that the reported events have propagated along different L-values in the magnetosphere, although they have been recorded at the same station. The ULF waves propagating along geomagnetic field lines modulate the growth rate of VLF waves, which results in the pulsation of VLF hiss intensity. The pulsating growth rate has been computed.     

Theoretical simulation of electron density and temperature distribution at Indian equatorial and low latitude ionosphere

The electron density and temperature distribution of the equatorial and low latitude ionosphere in the Indian sector has been investigated by simultaneously solving the continuity, momentum and energy balance equations of ion and electron flux along geomagnetic field lines from the Northern to the Southern hemisphere. Model algorithm is presented and results are compared with the electron density and electron temperature measured in situ by Indian SROSS C2 satellite at an altitude of ∼500 km within 31°S–34°N and 75 ± 10°E that covers the Indian sector during a period of low solar activity. Equatorial Ionization Anomaly (EIA) observed in electron density, morning and afternoon enhancements, equatorial trough in electron temperature have been simulated by the model within reasonable limits of accuracy besides reproducing other normal diurnal features of density and temperature.     

Eccentric dipole approximation of the geomagnetic field: Application to cosmic ray computations

A comparison of the full IGRF model of the geomagnetic field with two simplified models, the truncated IGRF and the eccentric dipole model, is performed. The simplified models were found to provide a reasonable approximation for the large scale geomagnetic field distribution. In the application of the simplified geomagnetic models to the shielding of cosmic rays in the magnetosphere as quantified via the geomagnetic cut-off rigidity, the eccentric dipole and the truncated IGRF provide a good large scale view. The use of the simplified model does not introduce any additional systematic errors at the global scale but may be a source of moderate uncertainty at the regional scale in the tropical Atlantic region. This study quantitatively validates the use of such simplified geomagnetic models when describing the shielding of cosmic rays in the magnetosphere.     

地磁场模型简化计算方法

为了实现地磁导航在近地轨道卫星上的应用,需要解决传统的高斯球谐函数递推法在计算地磁场强度时存在方法复杂、计算量大、实时性难以保证等问题.针对上述问题,提出一种用地球磁场估计函数代替球谐函数的地磁场强度计算方法.该方法利用伪地心下的磁偶极子模型代替主磁场模型,通过多项式拟合离线得到高度范围内固定经纬度网格点的伪中心距系数,进而利用插值法和偶极子模型得到任意点的地磁场强度.仿真结果表明,轨道高度为300～500km,经纬度网格间隔为0.5°时,地球磁场估计函数法的导航精度与地球主磁场模型WMM精度一致的同时,降低了计算负担,提高了算法计算效率.      

Polar caps and auroral zones under idealized axisymmetric magnetic fields

The geomagnetic field, modified by the solar wind, determines the shape, area and location of polar caps and auroral zones, among other magnetosphere and upper atmosphere characteristics. Since the field varies greatly with time it is of interest to analyze polar caps and auroral zones variations linked to magnetic field variations of intensity and pattern. Polar caps and auroral zones locations and areas for various single harmonic axial field configurations are obtained analytically assuming a simple magnetopause model. As the axial degree n increases, the polar caps and auroral zones total number, given by n + 1 and 2n respectively, also increase. However, their total areas decrease from a larger value in the case of an axial dipole to a minimum for an axial octupole (n = 3), and then increase for increasing degrees. The increasing rate is much higher in the auroral zones case to the point that it exceeds the dipolar value at n = 5 while in the polar caps case this occurs at n = 8. The absolute latitudes of the auroral zones and polar caps that reside around the geographical poles increase with axial degree. Our results represent an end-member case of the evolution of auroral zones and polar caps during polarity reversals if the transition involves axial dipole energy cascade to higher axial degrees. Evidence for such an energy transfer is found in the historical record of the geomagnetic secular variation.     

Recovery of the Earth’s gravity field from formation-flying satellites: Temporal aliasing issues

Temporal and mean gravity field models derived from the twin-satellite, leader–follower mission GRACE have provided a new type of information for Earth sciences. In this contribution, we study the potential of various alternative satellite formations for gravity field determination in the post-GRACE era in a simulation environment. In particular, the effects of spherical harmonic truncation and of temporal aliasing in the processing of gravity products from such future formations are investigated.     

局部地区高空磁场的计算方法

本文用曲面样条函数表示地磁场及其水平梯度在地面上的分布, 从而使A.J.Zmuda提出的计算高空磁场的方法更加简便。本文还给出了借助于局部地区地磁场的球谐分析, 计算高空磁场的方法。      

Development of a GNSS water vapour tomography system using algebraic reconstruction techniques

A GNSS water vapour tomography system developed to reconstruct spatially resolved humidity fields in the troposphere is described. The tomography system was designed to process the slant path delays of about 270 German GNSS stations in near real-time with a temporal resolution of 30 min, a horizontal resolution of 40 km and a vertical resolution of 500 m or better. After a short introduction to the GPS slant delay processing the framework of the GNSS tomography is described in detail. Different implementations of the iterative algebraic reconstruction techniques (ART) used to invert the linear inverse problem are discussed. It was found that the multiplicative techniques (MART) provide the best results with least processing time, i.e., a tomographic reconstruction of about 26,000 slant delays on a 8280 cell grid can be obtained in less than 10 min. Different iterative reconstruction techniques are compared with respect to their convergence behaviour and some numerical parameters. The inversion can be considerably stabilized by using additional non-GNSS observations and implementing various constraints. Different strategies for initialising the tomography and utilizing extra information are discussed. At last an example of a reconstructed field of the wet refractivity is presented and compared to the corresponding distribution of the integrated water vapour, an analysis of a numerical weather model (COSMO-DE) and some radiosonde profiles.     

Response of the ionospheric F-region in the Brazilian sector during the super geomagnetic storm in April 2000 observed by GPS

The response of the ionospheric F-region in the equatorial and low latitude regions in the Brazilian sector during the super geomagnetic storm on 06–07 April 2000 has been studied in the present investigation. The geomagnetic storm reached a minimum D_st of −288 nT at 0100 UT on 07 April. In this paper, we present vertical total electron content (VTEC) and phase fluctuations (in TECU/min) from GPS observations obtained at Imperatriz (5.5°S, 47.5°W; IMPZ), Brasília (15.9°S, 47.9°W; BRAZ), Presidente Prudente (22.12°S, 51.4°W; UEPP), and Porto Alegre (30.1°S, 51.1°W; POAL) during the period 05–08 April. Also, several GPS-based TEC maps are presented from the global GPS network, showing widespread and drastic TEC changes during the different phases of the geomagnetic storm. In addition, ion density measurements on-board the satellite Defense Meteorological Satellite Program (DMSP) F15 orbiting at an altitude of 840 km and the first Republic of China satellite (ROCSAT-1) orbiting at an altitude of 600 km are presented. The observations indicate that one of the orbits of the DMSP satellite is fairly close to the 4 GPS stations and both the DMSP F15 ion-density plots and the phase fluctuations from GPS observations show no ionospheric irregularities in the Brazilian sector before 2358 UT on the night of 06–07 April 2000. During the fast decrease of D_st on 06 April, there is a prompt penetration of electric field of magnetospheric origin resulting in decrease of VTEC at IMPZ, an equatorial station and large increase in VTEC at POAL, a low latitude station. This resulted in strong phase fluctuations on the night of 06–07 April, up to POAL. During the daytime on 07 April during the recovery phase, the VTEC observations show positive ionospheric storm at all the GPS stations, from IMPZ to POAL, and the effect increasing from IMPZ to POAL. This is possibly linked to the equatorward directed meridional wind. During the daytime on 08 April (the recovery phase continues), the VTEC observations show very small negative ionospheric storm at IMPZ but the positive ionospheric storm effect is observed from BRAZ to POAL possibly linked to enhancement of the equatorial ionospheric anomaly.     

Empirical model of the TEC response to the geomagnetic activity over the North American region

The paper presents an empirical model of the total electron content (TEC) response to the geomagnetic activity described by the K_p-index. The model is built on the basis of TEC measurements covering the region of North America (50°W–150°W, 10°N–60°N) for the period of time between October 2004 and December 2009. By using a 2D (latitude-time) cross-correlation analysis it is found that the ionospheric response to the geomagnetic activity over the considered geographic region and at low solar activity revealed both positive and negative phases of response. The both phases of the ionospheric response have different duration and time delay with respect to the geomagnetic storm. It was found that these two parameters of the ionospheric response depend on the season and geographical latitude. The presence of two phases, positive and negative, of the ionospheric response imposed the implementation of two different time delay constants in order to properly describe the two different delayed reactions. The seasonal dependence of the TEC response to geomagnetic storms is characterized by predominantly positive response in winter with a short (usually ∼5–6 h) time delay as well as mainly negative response in summer with a long (larger than 15 h) time delay. While the TEC response in March and October is more close to the winter one the response in April and September is similar to the summer one.     

Variation of ionospheric total electron content in Indian low latitude region of the equatorial anomaly during May 2007–April 2008

The ionospheric total electron content (TEC), derived by analyzing dual frequency signals from the Global Positioning System (GPS) recorded near the Indian equatorial anomaly region, Varanasi (geomagnetic latitude 14°, 55′N, geomagnetic longitude 154°E) is studied. Specifically, we studied monthly, seasonal and annual variations as well as solar and geomagnetic effects on the equatorial ionospheric anomaly (EIA) during the solar minimum period from May 2007 to April 2008. It is found that the daily maximum TEC near equatorial anomaly crest yield their maximum values during the equinox months and their minimum values during the summer. Using monthly averaged peak magnitude of TEC, a clear semiannual variation is seen with two maxima occurring in both spring and autumn. Statistical studies indicate that the variation of EIA crest in TEC is poorly correlated with Dst-index (r = −0.03) but correlated well with Kp-index (r = 0.82). The EIA crest in TEC is found to be more developed around 12:30 LT.     

An alternative computation of a gravity field model from GOCE

GOCE is the first satellite with a gravitational gradiometer (SGG). This allows to determine a gravity field model with high spatial resolution and high accuracy. Four of the six independent components of the gravitational gradient tensors (GGT) are measured with high accuracy in the so-called measurement band (MB) from 5 to 100 mHz by the GOCE gradiometer. Based on more than 1 year of GOCE measurements, two gravity field models have been derived. Here, we introduce a strategy for spherical harmonic analysis (SHA) from GOCE measurements, with a bandpass filter applied to the SGG data, combined with orbit analysis based on the integral equation approach, and additional constraints (or stabilization) in the polar areas where no observation is available due to the orbit geometry. In addition, we combined the GOCE SGG part with a set of GRACE normal equations. This improves the accuracy of the gravity field in the long-wavelength parts, due to the complementarity of GOCE and GRACE. Comparison with other models and with external data shows that our results are rather close to the GPS-levelling data in well-selected test regions, with an uncertainty of 4–7 cm, for truncation at degree 200.     

Alfven波在微电离大气中的衰减特性研究

利用简单的偶极子地磁场模型以及大气电子密度和电导率模式, 分析地面产生的磁扰动以Alfven波的模式传播到近地空间区域. 这种地面的磁扰动可能干扰近地空间卫星对空间磁扰动的观测. 通过对地面磁扰动Alfven波模式1000 km高度内的衰减情况进行模拟, 认为在近地空间采用地磁偶极子模型是合理的. 由于衰减随扰动频率的增大而急剧增强, 分析还得到了近地卫星能够探测到地面磁扰动的最大频率. 计算结果表明, Alfven波的衰减主要集中在高度50 km以下, 这个区域内的大气电导率极其微弱, 使Alfven波的传播受到极大衰减. 0.4 Hz以下的Alfven波沿磁力线传播到1000 km高度后衰减结为原来扰动幅度的千分之一, 因此频率在0.4 Hz以下的Alfven波可能会干扰低轨卫星探测磁场脉动.