VHF ionospheric scintillations near the equatorial anomaly crest: solar and magnetic activity effects

The paper presents a study of solar and magnetic activity effects on VHF ionospheric scintillations recorded during three and half years at Bhopal, a station near the northern crest of the equatorial anomaly in India. During E- (equinox) and D- (winter) months, scintillations occur mainly in the pre-midnight period whereas during J- (summer) months their occurrence is larger in the post-midnight period. Very intense scintillations (>20 dB) mainly occur in the pre-midnight period, and in the post-midnight period, the scintillations are generally moderate (5–10 dB) or weak (<5 dB). The nocturnal scintillation occurrence decreases with the decrease in solar activity from 1989 to 1992. Monthly mean scintillation occurrence changes according to solar activity during E- and D-months but not so during J-months. The effects of magnetic activity on scintillations vary with season and, in general, inhibit the scintillation occurrence in the pre-midnight period and enhance it a little in the post-midnight period, especially after 0300 hours IST (Indian Standard Time). For most of the severe magnetic storms in which Dst goes below −125 nT and the recovery phase starts in the post-midnight to dawn local time sector, strong post-midnight scintillations, which sometimes extend for several hours beyond the local sunrise, are observed.     

太阳活动高、低年东亚中、低纬地区扩展F特性比较研究

扩展F(Spread-F)作为重要的电离层不规则结构之一,对电波传播、导航、通讯等有重要的影响,因此对其时、空特性的研究一直倍受重视.本文通过分析低纬海南台站和中纬长春与乌鲁木齐台站的测高仪数据,比较研究了太阳活动高、低年我国中、低纬地区夜间电离层扩展F的发生特性.扩展F发生率特性主要体现在:无论低纬还是中纬太阳活动低年扩展F发生率最大值高于太阳活动高年;无论太阳活动高年还是低年低纬站扩展F发生率最大值高于中纬站.细节特征主要体现在:首先,三台站在太阳活动高、低年扩展F发生率都存在双峰结构.太阳活动高年,低纬海南站双峰结构集中在春、秋分季节,而中纬站则集中在冬、夏季节,扩展F较容易发生的地方时低纬站主要集中在子夜前,而中纬站则偏向子夜后;在太阳活动低年,中、低纬双峰结构都出现在冬、夏季节,低纬海南站扩展F较突出的出现在子夜前后,而中纬台站则主要出现在子夜及子夜后.其次,双峰结构中的细节表现不同,如低纬海南站太阳活动高年扩展F较容易发生在春分季节,但2月和4月份发生率都比较高而且接近,而太阳活动低年扩展F较容易发生在夏季月份,但5月和7月的发生率也都比较高且接近,且太阳活动高、低年低纬主峰峰值非常接近,不像中纬地区有明显的差异等.本文针对实测数据进行了详细的分析并结合已有研究进行了细致的讨论.     

Characteristics of VHF scintillations in the equatorial anomaly crest region in India

The characteristics of ionospheric scintillations at Rajkot in the equatorial anomaly crest region in India are described for the years 1987–1991 by monitoring the 244-MHz transmission from the satellite FLEETSAT. This period covers the ascending phase of solar cycle 22. Scintillations occur predominantly in the pre-midnight period during equinoxes and winter seasons and in the post-midnight period during summer season. During equinoxes and winter, scintillation occurrence increases with solar activity, whilst in summer it is found to decrease with solar activity. Statistically, scintillation occurrence is suppressed by magnetic activity. The characteristics observed during winter and equinoxes are similar to those seen at the equatorial station, Trivandrum. This, coupled with the nature of the post-sunset equatorial F-region drift and h‘F variations, supports the view that at the anomaly crest station, scintillations are of equatorial origin during equinox and winter, whilst in summer they may be of mid-latitude type. The variations in scintillation intensity (in dB) with season and solar activity are also reported.     

Morphology of NmF2 nighttime increases in the Eurasian sector

A statistical analysis of two peaks (pre-midnight and post-midnight) occurrence in NmF2 daily variations was made on a latitudinal chain of four ionosonde stations in the Eurasian longitudinal sector. Overall 6182 cases of the first and 5600 cases of the second peak occurrence were analyzed using all available foF2 observations for the years of solar maximum and minimum. Well-pronounced and systematic variations with season and solar activity were revealed in the occurrence probability of the peaks, their amplitude and timing. The pattern of both peaks occurrence is similar during winter and equinoxes for midlatitude stations implying one and the same mechanism of their formation. The pre-midnight summer peak shows specific variations in particular during solar maximum pointing to a different mechanism controlling its appearance. Possible mechanisms of both peaks formation are discussed.     

利用卫星和GPS观测研究低纬局部等离子体浓度增强现象

本文同时利用DMSP、ROCSAT-1卫星数据和地基的GPS观测数据,研究一种与低纬等离子体泡相伴随的局部等离子体浓度增强现象.地基GPS的观测表明电离层总电子含量(TEC)也能反映这种等离子体浓度增强.通过4个观测事例的详细分析表明:这种等离子体浓度增强主要出现在磁纬±10°～±20°的局部区域,有时在近磁赤道区和中纬地区的电离层顶部也能观测到;与等离子体泡的出现规律相似,这种等离子体浓度增强主要出现在地方时21∶00以后,并在午夜后也能观测到.当等离子体浓度增强和等离子体泡发生时,在午夜前一般对应着背景垂直速度明显向上的扰动,在午夜后一般处于等离子体垂直速度下降至反向前的时间段,表明东向电场对于低纬不规则体的产生有非常重要的作用.     

Characteristics of VHF radiowave scintillations over a solar cycle (1983–1993) at a low-latitude station: Waltair (17.7°N, 83.3°E)

The characteristics of VHF radiowave scintillations at 244 MHz (FLEETSAT) during a complete solar cycle (1983–93) at a low-latitude station, Waltair (17.7°N, 83.3°E), are presented. The occurrence of night-time scintillations shows equinoctial maxima and summer minima in all the epochs of solar activity, and follows the solar activity. The daytime scintillation occurrence is negatively correlated with the solar activity and shows maximum occurrence during the summer months in a period of low solar activity. The occurrence of night-time scintillations is inhibited during disturbed days of high solar activity and enhanced during low solar activity.     

Ionospheric Scintillation and Dynamics of Fresnel-Scale Irregularities in the Inner Region of the Equatorial Ionization Anomaly

This paper reports differences in the occurrence statistics of global positioning system (GPS) L-band scintillations at observational sites located in the inner regions of the northern and southern crests of the equatorial ionization anomaly. Ground-based GPS data acquired at the closed magnetically aligned stations of Manaus (3.1°S; 59.9°W; dip lat. 6.2°N) and Cuiabá (15.5°S; 56.1°W; dip. lat. 6.2°S), Brazil, from December 2001 to February 2007 are used in the analysis. The drift dynamics of Fresnel-scale ionospheric irregularities at the southern station of Cuiabá are also investigated. Only geomagnetically quiet days with the sum of daily Kp < 24 were used in the analysis statistics and in the irregularity drift studies. The results reveal a clear dependence of the scintillation occurrence with the solar activity, but there exists an asymmetry in the percentage of scintillation occurrence between the two stations throughout the period analyzed. The nocturnal occurrence of the scintillations over Cuiabá is predominantly larger than over Manaus, but this scenario seems to change with the decline in the solar activity (mainly during local post-midnight hours). A broad minimum and maximum in the scintillation occurrence appears to occur over both the stations, respectively, during the June solstice (winter) and December solstice (summer) months. The dynamics of the Fresnel-scale irregularities, as investigated from the estimations of the mean zonal drift velocities, reveals that the amplitude of the eastward drifts tends to reduce with the decline in the solar activity. The magnitude of the zonal drift velocities during the December solstice months is larger than during the equinoxes, with the differences being more pronounced at solar maximum years. Other relevant aspects of the observations, with complementary data from a low-latitude ionospheric model, are highlighted and discussed.     

Substorm correlated absorption on a 3200 km trans-auroral HF propagation path

A high-frequency transmitter located at Clyde River, NWT, Canada, and a receiver located near Boston, USA, provide a 3200 km trans-auroral, near-meridional propagation path over which the propagation characteristics have been measured. Out of the fourteen frequencies in the HF band sampled every hour for the duration of the experimental campaign (16 January-8 February 1989), the signal level measurements of 6.800 MHz transmissions were selected in order to determine the extent and occurrence of auroral absorption. The median level of auroral absorption along the path is found to increase with geomagnetic activity, quantified by the index K_p, with the increase being greater in the post-midnight sector than in the pre-midnight sector. This asymmetric behaviour is attributed to the precipitation of high energy electrons into the midnight and morning sector auroral D region. The measured diurnal variation in the median level of absorption is consistent with previous models describing the extent and magnitude of auroral absorption and electron precipitation. Individual substorms, identified from geosynchronous satellite data, are found to cause short-lived absorption events in the HF signal level of \sim30 dB at 6.800 MHz. The occurrence of substorm correlated auroral absorption events is confined to the midnight and morning sectors, consistent with the location of the electron precipitation. The magnitude of absorption is related to the magnetotail stress during the substorm growth phase and the magnetotail relaxation during the substorm expansion phase onset. The absorption magnitude and the occurrence of substorms during the period of the campaign increase at times of high K_p, leading to an increase in median auroral absorption during disturbed periods.     

Solar-wind–magnetosphere coupling,including relativistic electron energization,during high-speed streams

High geomagnetic activity occurs continuously during high-speed solar wind streams, and fluxes of relativistic electrons observed at geosynchronous orbit enhance significantly. High-speed streams are preceded by solar wind compression regions, during which time there are large losses of relativistic electrons from geosynchronous orbit. Weak to moderate geomagnetic storms often occur during the passage of these compression regions; however, we find that the phenomena that occur during the ensuing high-speed streams do not depend on whether or not a preceding storm develops. Large-amplitude Alfvén waves occur within the high-speed solar wind streams, which are expected to lead to intermittent intervals of significantly enhanced magnetospheric convection and to thus also lead to repetitive substorms due to repetitively occurring reductions in the strength of convection. We find that such repetitive substorms are clearly discernible in the LANL geosynchronous energetic particle data during high-speed stream intervals. Global auroral images are found to show unambiguously that these events are indeed classical substorms, leading us to conclude that substorms are an important contributor to the enhanced geomagnetic activity during high-speed streams. We used the onsets of these substorms as indicators of preceding periods of enhanced convection and of reductions in convection, and we have used ground-based chorus observations from the VELOX instrument at Halley station as an indicator of magnetospheric chorus intensities. These data show evidence that it is the periods of enhanced convection that precede substorm expansions, and not the expansions themselves, that lead to the enhanced dawn-side chorus wave intensity that has been postulated to cause the energization of relativistic electrons. If this inference is correct, and if it is chorus that energizes the relativistic electrons, then high-speed solar wind streams lead to relativistic electron flux enhancements because the embedded large-amplitude Alfvén waves give multi-day periods of intermittent significantly enhanced convection.     

GPS L1-Frequency Observations of Equatorial Scintillations and Irregularity Zonal Velocities

In this work, the climatology of ionospheric scintillations at global positioning system (GPS) L-band frequency and the zonal drift velocities of scintillation-producing irregularities were depicted for the equatorial observatory of São Luis (2.33°S; 44.21°W; dip latitude 1.3°S), Brazil. This is the first time that the hourly, monthly, and seasonal variations of scintillations and irregularity zonal drifts at São Luis were characterized during periods of different solar activity levels (from December 1998 to February 2007). The percentage occurrence of scintillations at different sectors of the sky was also investigated, and the results revealed that the scintillations are more probable to be observed in the west sector of the sky above São Luis, whereas the north–south asymmetries are possibly related to asymmetries in the plasma density distribution at off-equatorial latitudes. The scintillations on GPS signals occurred more frequently around solar maximum years, but it is also clear from the results of a strong variability in the scintillation activity in the years with moderate solar flux during the descending phase of the solar cycle. The equatorial scintillations occur predominantly during pre-midnight hours with a broad maximum near the December solstice months. In general, weak level of scintillations (S ₄ index between 0.2 and 0.4) dominated at all seasons; however, during the winter months around solar maximum years (although the scintillation occurrence is extremely low), stronger levels of scintillations (S ₄ > 0.6) may occur at comparable rate with the weak scintillations. The irregularity zonal velocities, as estimated from the GPS spaced-receiver technique, presented a different scenario for the two seasons analyzed; during the equinoxes, the magnitude of the zonal velocities appeared not to change with the solar activity, whereas during the December solstice months, the larger magnitudes were observed around solar maximum years. Other relevant aspects of the observations are highlighted and discussed.     

Solar activity dependence of ionospheric electron content and slab thickness using different solar indices

Ionospheric electron content (IEC) and slab thickness () data for the period 1977 to 1980 from Lunping (23.03°N; 121.90°E subionospheric) have been examined for their solar activity dependence. Local noontime monthly means as well as values for the 5 QQ days in a month have been examined separately with different solar indices, namely: solar EUV flux (170–190 Å),S _{10.7 cm} flux and sun spot number (SSN) on a seasonal basis. Both IEC and parameters exhibit better correlation with solar EUV andS _{10.7 cm} fluxes than with SSN for all seasons. IEC increases linearly with both EUV andS _{10.7 cm} flux whereas with SSN it shows a distinct nonlinear relationship during all seasons in both monthly mean and 5 QQ days' values. This study indicates that for correlating and predicting the variations (especially the medium term) in the ionospheric parameters, both EUV andS _{10.7 cm} fluxes have an advantage over SSN.     

Ground level enhancements of solar cosmic rays during the last three solar cycles

The catalog of ground level enhancements of solar cosmic rays during cycles 21—23 of solar activity has been presented. The main properties, time distribution, and relation of these events to solar sources and proton enhancements observed on satellites have been studied.     

基于三亚VHF雷达的场向不规则体观测研究：4.太阳活动低年夏季F区回波

太阳活动低年夏季,低纬电离层F区场向不规则体表现出与太阳活动高年和其他季节明显不同的特征.本文利用我国三亚站（18.4°N,109.6°E,地磁倾角纬度dip latitude 12.8°N）VHF雷达、电离层测高仪、GPS闪烁监测仪和美国C/NOFS卫星观测数据,研究了太阳活动低年夏季我国低纬电离层F区场向不规则体的基本特征.分析发现无论磁静日还是磁扰日,夏季电离层F区不规则体回波主要出现于地方时午夜以后,回波出现的时间较短,高度范围较小,伴随着扩展F出现,但没有同时段的L波段电离层闪烁.太阳活动低年夏季午夜后的低纬电离层F区不规则体回波,可能并不总是与赤道等离子体泡沿磁力线向低纬地区的延伸相关,而可能由本地Es等扰动过程引起.     

Effects of geomagnetic storm on GPS ionospheric scintillations at Sanya

The effects of geomagnetic storm on GPS ionospheric scintillations are studied here using GPS scintillation data recorded at Sanya (18.3°N, 109.5°E; geomagnetic: 7.6°N, 180.8°E), the southmost station in the Chinese longitude region. GPS scintillation/TEC and DMSP data are utilized to show the development of irregularities during the period year 2005 (solar minimum). Statistical analysis of K planetary index (Kp) and amplitude scintillation index (S4) indicates that most storms of the year did not trigger the scintillation occurrence at Sanya. However, cases of scintillation occurring during moderate and strong storm (Dst<−100) periods show clearly that the development of irregularities producing scintillations can be triggered by geomagnetic storms during the low scintillation occurrence season. The effects (trigger or not trigger/inhibit) depend on the maximum dDst/dt determined local time sector, and can be explained by the response of the equatorial vertical drift velocities to magnetospheric and ionospheric disturbance electric fields. For station Sanya, the maximum dDst/dt determined local time is near the noon (or post-midnight) sector for most storms of the year 2005, which inhibited (or did not trigger) the post-sunset (or post-midnight) scintillation occurrence and then led to the phenomena that the statistical results presented.     

Observations of pre-midnight 5-m irregularities in the equatorial F region over São Luís,Brazil: Solar-flux dependence and seasonal variations

The statistics of pre-midnight 5-m irregularities in the equatorial F region over São Luís is presented. The data set ranges from October 2001 to December 2008 and covers maximum solar-flux-to-minimum solar flux epoch. The variabilities in irregularity parameters, namely, height and time of their appearance in the radar echoes, with solar-flux variation are presented. The seasonal variations (combined over all years, irrespective of solar-flux) of occurrence of irregularities, occurrence of bottom-type layer (or bottom-side irregularities without plume) and bottom-side/topside plume (or bottom-side irregularities with plume) are presented. The largest occurrences of bottom-side irregularities without plume and with plume are found on April (equinox) and December (summer) months respectively. The ambient ionospheric conditions namely prereversal evening vertical drift, bottom-side density gradient and off-equatorial E region conductivity are inferred using digisonde measurements during April 2002 and December 2002. Based on these conditions and recent studies on gravity wave climatology over Brazil, it is suggested that shear in zonal plasma drift and low gravity wave activity may account for less occurrence of plume during April as compared to December months. This suggestion is quantified using numerical simulation model of collisional-interchange instability (CII) and plasma bubble.     

The equatorial ionospheric anomaly in electron content from solar minimum to solar maximum for South East Asia

Median hourly, electron content-latitude profiles obtained in South East Asia under solar minimum and maximum conditions have been used to establish seasonal and solar differences in the diurnal variations of the ionospheric equatorial anomaly (EIA). The seasonal changes have been mainly accounted for from a consideration of the daytime meridional wind, affecting the EIA diffusion of ionization from the magnetic equator down the magnetic field lines towards the crests. Depending upon the seasonal location of the subsolar point in relation to the magnetic equator diffusion rates were increased or decreased. This led to crest asymmetries at the solstices with (1) the winter crest enhanced in the morning (increased diffusion rate) and (2) the same crest decaying most rapidly in the late afternoon (faster recombination rate at lower ionospheric levels). Such asymmetries were also observed, to a lesser extent, at the equinoxes since the magnetic equator (located at about 9○N lat) does not coincide with the geographic equator. Another factor affecting the magnitude of a particular electron content crest was the proximity of the subsolar point, since this increased the local ionization production rate. Enhancements of the EIA took place around sunset, mainly during the equinoxes and more frequently at solar maximum, and also there was evidence of apparent EIA crest resurgences around 0300 LST for all seasons at solar maximum. The latter are thought to be associated with the commonly observed, post-midnight, ionization enhancements at midlatitudes, ionization being transported to low latitudes by an equatorward wind. The ratio increases in crest peak electron contents from solar minimum to maximum of 2.7 at the equinoxes, 2.0 at the northern summer solstice and 1.7 at northern winter solstice can be explained, only partly, by increases in the magnitude of the eastward electric field E overhead the magnetic equator affecting the [E×B] vertical drifts. The most important factor is the corresponding increase in ionization production rate due to the increase in solar radiation flux. The EIA crest asymmetries observed at solar maximum were less significant, and this is probably due to the corresponding increase in ionization densities leading to an increase of the retarding effect of ion-drag on the daytime meridional winds.     

Interhemispheric observations of impulsive nitrate enhancements associated with the four large ground-level solar cosmic ray events (1940–1950)

We present a compelling similarity of impulsive nitrate enhancements observed in polar ice from the northern and southern hemispheres. This analysis concentrates on the period 1940–1950, during which time the first four recorded solar cosmic ray ground-level enhancements (GLEs) occurred. GLEs are strong solar proton events. We show that large and sudden enhancements in the nitrate records from both hemispheres were observed within weeks following the recorded solar cosmic ray ground-level event. The observation of impulsive nitrate enhancements simultaneously in both hemispheres shortly after a large fluence solar proton event is strong evidence in support of a causal connection and argues strongly for rapid transport of atmospheric nitrates generated through the polar atmosphere by energetic solar proton events.     

Pearl-type micropulsations at mid-latitude; their relation to whistlers,solar and geomagnetic activity as well as ionospheric absorption

The occurrence of pearl-type (Pc 1) micropulsations recorded at the mid-latitude station Nagycenk (Hungary) during a half solar cycle showed a quite regular variation on this long time scale. Around solar activity maximum, the number of days with Pc 1 occurrence was rather low, while it began to increase during medium solar activity rising to a maximum around solar activity minimum. Pc 1 pulsations have been analyzed in relation to further parameters and on a shorter time scale, too. Based on data of 2 years with maximum Pc 1 occurrence (around solar activity minimum in 1985 and 1986), a seasonal variation was also found. Additionally, it was confirmed that pearl-type micropulsations might frequently occur, on and after days, with geomagnetic disturbances. At Nagycenk, the selected geomagnetic disturbances were generally associated with an increased ionospheric absorption of radio waves caused by enhanced ionization due to particle precipitation from the magnetosphere into the lower ionosphere. Whistler observations carried out at Panska Veš (a station in the Czech Republic) showed a significant whistler activity connected with these geomagnetic disturbances, however, no after-effect appeared in whistler activity. One of the main goals of the present study was to find a relationship between Pc 1 pulsations and whistlers. Results revealing an increased whistler activity associated with Pc 1 occurrences confirm our previous findings rather convincingly. The latter ones hinted at the probability that certain magnetospheric configurations, e.g. geomagnetic field line shells and whistler ducts are closely connected, as similar positions of the two structures were found within the magnetosphere when characteristics of Pc 3 pulsations and whistlers were analyzed.     

Effects of solar proton events in the mesosphere/lower thermosphere region according to the data of meteo radar wind measurements at high and middle latitudes

Data from meteo radar measurements of the wind in the mesosphere/lower thermosphere region at high latitudes of the Southern Hemisphere (Molodezhnaya station, 68° S, 45° E) and at middle latitudes of the Northern Hemisphere (Obninsk station, 55° N, 37° E) during solar proton events that took place in 1989, 1991, 2000, 2005, and 2012 are analyzed in the paper. In 1989 and 1991, we succeeded in observing the response to solar proton evens at both stations simultaneously. The results show that solar proton events lead to a change in the wind regime of the mesosphere and lower thermosphere. At high latitudes of the Southern Hemisphere, significant changes are observed in the values of the velocities of the meridional and zonal components of the prevailing wind. In the case of powerful solar proton events, the amplitude of the semidiurnal tide grows in the vicinity of the proton flux maximum. The response to these events depends on the season. The reaction of the prevailing wind at middle latitudes shows the same features as the reaction of the wind at high latitudes. However no unambiguous response of the tide amplitude is observed. In the summer season, even powerful events (for example, in July 2000) cause no changes in the wind regime parameters in the midlatitude region of the mesosphere/lower thermosphere.     

地球内磁层场向电流的统计特征

利用ISEE－1和ISEE-2飞船观测的磁场数据，分析了地球内磁层场向电流的统计特征，包括场向电流的空间（L值和地方时）分布；流进和流出电离层的场向电流随地方时的变化；场向电流发生率与地磁活动水平（以AL指数表征）、行星际磁场（IMF）Bz的关系，电流强度和密度随地磁活动水平的变化等．发现，场向电流大都发生在夜间，且集中在L为6-10区域内，场向电流发生率，强度和密度随地磁活动增强而增大，行星际磁场南向时的发生率远远高于北向时的发生率．这些结果表明，内磁层场向电流的产生是太阳风和磁层、电离层间电动耦合增加的结果．