GPS-TEC variations during low solar activity period (2007–2009) at Indian low latitude stations

The paper is based on the ionospheric variations in terms of vertical total electron content (VTEC) for the low solar activity period from May 2007 to April 2009 based on the analysis of dual frequency signals from the Global Positioning System (GPS) satellites recorded at ground stations Varanasi (Geographic latitude 25°16′ N, Longitude 82°59′ E), situated near the equatorial ionization anomaly crest and other two International GNSS Service (IGS) stations Hyderabad (Geographic latitude 17°20′ N, longitude 78°30′ E) and Bangalore (Geographic latitude 12°58′ N, longitude 77°33′ E) in India. We describe the diurnal and seasonal variations of total electron content (TEC), and the effects of a space weather related event i.e. a geomagnetic storm on TEC. The mean diurnal variation during different seasons is brought out. It is found that TEC at all the three stations is maximum during equinoctial months (March, April, September and October), and minimum during the winter months (November, December, January and February), while obtaining intermediate values during summer months (May, June, July and August). TEC shows a semi-annual variation. TEC variation during geomagnetic quiet as well as disturbed days of each month and hence for each season from May 2007 to April 2008 at Varanasi is examined and is found to be more during disturbed period compared to that in the quiet period. Monthly, seasonal and annual variability of GPS-TEC has been compared with those derived from International Reference Ionosphere (IRI)-2007 with three different options of topside electron density, NeQuick, IRI01-corr and IRI 2001. A good agreement is found between the GPS-TEC and IRI model TEC at all the three stations.     

Geomagnetic field variation during winter storm at localized southern and northern high latitude

This paper presents the effect of geomagnetic storm on geomagnetic field components at Southern (Maitri) and Northern (Kiruna) Hemispheres. The Indian Antarctic Station Maitri is located at geom. long. 66.03° S; 53.21° E whereas Kiruna is located at geom. long. 67.52° N; 23.38° E. We have studied all the geomagnetic storms that occurred during winter season of the year 2004–2005. We observed that at Southern Hemisphere the variation is large as compared to the Northern Hemisphere. Geomagnetic field components vary when the interplanetary magnetic field is oriented in southward direction. Geomagnetic field components vary in the main phase of the ring current. Due to southward orientation of vertical component of IMF reconnection takes place all across the dayside that transports plasma and magnetic flux which create the geomagnetic field variation.     

Storm time response of GPS-derived total electron content (TEC) during low solar active period at Indian low latitude station Varanasi

The present paper analyzes the dual frequency signals from GPS satellites recorded at Varanasi (Geographic latitude 25°, 16′ N, longitude 82°, 59′ E) near the equatorial ionization anomaly (EIA) crest in India, to study the effect of geomagnetic storm on the variation of TEC, during the low solar active period of May 2007 to April 2008. Three most intense—but still moderate class—storms having a rapid decrease of Dst-index observed during the GPS recorded data have been analyzed, which occurred on 20 November 2007, 9 March 2008 and 11 October 2008 were selected and storm induced features in the vertical TEC (VTEC) have been studied considering the mean VTEC value of quiet days as reference level. The possible reasons for storm time effects on VTEC have been discussed in terms of local time dependence, storm wind effect as well as dawn-dusk component of interplanetary electric field (IEF) Ey intensity dependence.     

A model of ionospheric F-2 region storms in middle latitudes

The proposed ionospheric storm model is based on a heat source located at magnetic noon on Feldstein's auroral oval. The rotation of the Earth produces an apparent motion of the source which is greater than the speed of the disturbance. This gives rise to a wake or front which sweeps over the globe and determines the onset time of the negative phase which results from a change in chemical composition. At the front, focussing will occur which accounts for the sudden drop in electron density (or contents) sometimes observed. The calculated onset times of the negative phase are compared with observations for a number of storms. The local onset times vary from 12 at the latitude of the source to around 24 at 10° geomagnetic latitude. This model predicts that the onset of the negative phase at a given location, for storm which commence between about 2000 LT to about 1000 LT, is independent of the time of storm commencement.     

On the effects of geomagnetic storms and pre storm phenomena on low and middle latitude ionospheric F2

This paper presents some features of the ionospheric response observed in equatorial and mid-latitudes region to two strong geomagnetic storms, occurring during Oct. 19–23, 2001 and May 13–17, 2005 and to understand the phenomena of pre-storm that lead to very intense geomagnetic storms. The result point to the fact that pre-storm phenomena that leads to intense ionospheric storm are; large southward turning of interplanetary magnetic field Bz, high electric field, increase in flow speed stream, increase in proton number density, high pressure ram and high plasma beta. The magnitude of Bz turning into southward direction from northward highly depends upon the severity of the storm and the variation in F2 layer parameter at the time of geomagnetic storm are strongly dependent upon the storm intensity. A detailed analysis of the responses of the ionosphere shows that during the storm periods, foF2 values depleted simultaneously both in the equatorial and mid latitude. Observation also shows that low to moderate variations in ionospheric F2 at the pre-storm period may signal the upcoming of large ionospheric disturbances at the main phase. The ionospheric F2response for low and mid latitude does not show any significant differences during the storm main phase and the pre-storm period. The ionospheric response during the pre-storm period is thought very puzzling. The period is observed to be depleted throughout with low-moderate effect across all the stations in the low and mid latitude.     

Effect of interplanetary magnetic field and disturb storm time on H component

A fluxgate digital magnetometer is used to study the variation of magnitude of H component during geomagnetic storm events of April, July and November 2004 at southern subauroral localized region at “MAITRI” (geom. lat. 62°S, long. 52.8°E). We also study the effect of vertical component of interplanetary magnetic field (IMF) on the variation of the magnitude of H component during storm time of April, July and November 2004. Results show that before sudden storm commencement (SSC) time magnitude of H component and IMF show smooth variation but after SSC of first storm of 22 July 2004, the magnitude of the H component shows fluctuations and at 09:00 UT it increases, but during second storm of 24 July 2004, the magnitude of H component indicates large fluctuations and it increases rapidly at 04:00 UT.     

Longitude dependent response of the GPS derived ionospheric ROTI to geomagnetic storms

The local time dependent effects of geomagnetic storm on the ionospheric TEC and Rate of change of TEC Index (ROTI) are studied here using the GPS data for four different low latitude stations: Ogaswara, Japan (24.29?°N, 153.91?°E; Geomagnetic: 17.21?°N, 136.16?°W); Surat, India (21.16?°N, 72.78?°E; Geomagnetic: 12.88?°N, 146.91?°E); Bogota, Colombia (4.64?°N, ?74.09?°E; Geomagnetic: 14.42?°N, 1.67?°W); and Kokee park Waimea, Hawaii, US (22.12?°N, ?159.67?°E; Geomagnetic: 22.13?°N, 91.19?°W). The solar wind velocity and geomagnetic indices: Dst, Kp and IMF Bz are utilized to validate the geomagnetic storms registered during the years 2011 and 2012. Using the GPS based TEC data and computed values of ROTI, the storm induced ionospheric irregularities generation and inhibition has been studied for all stations. The present study suggests that, the F-region irregularities of a scale length of few kilometers over the magnetic equator are locally affected by geomagnetic storms. This study also shows a good agreement (70–84 %) with the Aaron’s criteria (Aarons, Radio Sci., 26:1131–1149, 1991; Biktash, Ann. Geophys., 19:731–739, 2004) as significant absence and enhancement of ROTI was found to be influenced by the local time of the negative peak of Dst index association.     

Event study on pre-substorm phases and their relation to the energy coupling between solar wind and magnetosphere

On 11 November 1976, after a magnetically quiet period with the interplanetary magnetic field (IMF) directed northward, a sudden southward turning of the IMF immediately led to a world-wide intensification of convection which was observed to start almost simultaneously at stations within the auroral zone and polar cap. The two-dimensional equivalent current system over the northern hemisphere had a typical two-cell convection pattern with a maximum disturbance of ΔH = ?300 nT observed on the morningside in the westward electrojet region. This enhancement of activity ended after 35 min in a localized substorm onset in the midnight sector over Scandinavia.The recordings made in this area indicate large fluctuations of various ionospheric parameters starting several minutes before the substorm onset. Two subsequent stages can be resolved: (1) high-energy particle precipitation recorded by balloon X-ray detectors and maximum ionospheric current density increase, while the electrojet halfwidth shrinks and the total electrojet current becomes weaker; (2) the maximum ionospheric current density stays constant and the high-energy particle precipitation decreases, while the auroral brightness increases and the total electrojet current and its half-width show a growing trend prior to the final breakup. A suggestion is made that the time interval of these two stages should be called “trigger phase”. A short discussion explains the trigger phase observations in a magnetospheric scale. The energy coupling between solar wind and magnetosphere during the pre-substorm phases is discussed by utilizing the energy coupling function ? defined by Perreault and Akasofu (Geophys. J. R. Astr. Soc.54, 547, 1978). The ? values appear to be on substorm level during the period of enhanced convection. A good correlation between ? and the growth of the Joule heating rate (estimated from the AE data) is found in the beginning, but during the last 20 min before substorm triggering ? is high while the Joule heating rate decreases. The behaviour of ? during the two stages of the trigger phase suggests that the start of the trigger phase is purely internally controlled while the length of the trigger phase and the final substorm onset may be influenced by the variation in ?.     

Latitudinal characteristics of the geomagnetic field during the storm of 15–16 July 2000

In this paper geomagnetic disturbances at middle and low latitudes are discussed by using geomagnetic data of the magnetic storm of 15–16 July 2000. This storm is a response to the solar Bastille Day flare on 14 July. Generally, the geomagnetic disturbances at middle and low latitudes during a storm are mainly caused by three magnetospheric–ionospheric current systems, such as the ring current system (RC), the partial ring current and its associated region II field-aligned currents (PR), and the region I field-aligned currents (FA). Our results show that: (1) The northward turning of IMF-Bz started the sudden commencement of the storm, and its southward turning caused the main phase of the storm. (2) The PR- and FA-currents varied violently in the main phase. In general, the field of the FA-current was stronger than that of the PR-current. (3) In the first stage of the recovery phase, the RC-field gradually turned anti-parallel to the geomagnetic axis from a 15° deviation, and the local time (Λ) pointed by the RC-field stayed at 16:00. After that, Λ rotated with the stations, and the RC-field was not anti-parallel to the geomagnetic axis, but 5°–10° deviated. These facts suggest that the warped tailward part of the ring current decays faster than the symmetric ring current.     

GPS共视中电离层时延计算方法比较研究

为了更好地计算GPS CV（共视）时间传递中的电离层时延值（它是影响CPS CV比对结果精度的主要因素之一）,介绍了当前3种电离层时延的计算方法,并以NICT（National Institute of Information and Communications Technology）单站GPS比对数据及NICT与NTSC（National Time Service Center）的GPS共视比对数据为例,分析比较了不同的电离层时延计算方法对GPS时间比对结果精度的影响。计算结果表明：利用双频实测电离层时延和利用ICS（International GPS Service）提供的TEC（total electton content）map计算的电离层时延对GPS CV比对结果修正后的精度,比利用电离层改正模型的时延对比对结果修正后的精度分别提高30％～40％和20％～30％。     

Electric fields in diffuse aurora

The ionospheric electric field has been measured in the E region above the Churchill auroral research range under quiet and under disturbed conditions. Results were obtained $\text{1}\text{1}\text{2}$ and $\text{2}\text{1}\text{2}$ hr before local midnight over an altitude range of 115–165 km. The instruments and analysis differ from those used by other workers. An unusually advantageous vehicle motion resulted in dipole measurements along the magnetic field being modulated by the vehicle motion. Under quiet conditions and in the presence of a diffuse, east-west 2 kR auroral arc, the predominant vector component of the electric field was also quiet and between 35 and 40 $\text{mV}\text{m}$ perpendicular to the magnetic field, southward. Parallel to the magnetic field, the vector component increased from ?17 mV/m at 130 km, reversed direction at 160 km during the latter third of the flight and fluctuated around + 6 mV/m between 155 and 135 km on the descent. Under disturbed conditions during the recovery phase of a large magnetic storm, the electric field was also more disturbed; however, there was no significant electric field along B. Analysis of effects caused when parts of the measurement system are connected by a common magnetic field line, and when one of the probes lies in the wake of the vehicle, shows that measurement perturbations produced by those effects are dominated by the magnetic field line connections and that wake effects are relatively unimportant.     

Possible Ionospheric Electromagnetic Perturbations Induced by the Ms7.1 Yushu Earthquake

The power spectrum density (PSD) of magnetic field in the ELF/VLF band recorded by the experiment IMSC onboard the DEMETER satellite were used to study the Ms7.1 Yushu earthquake taking place on April 13, 2010 in China. The results indicate that possible ionospheric electromagnetic perturbations occurring 4?days before the earthquake in the frequency range [370–897?Hz]. Along the orbit 30880_up, which passed over the epicenter area on 9 April, enhanced PSD value of magnetic field at 410?Hz was detected both over the epicentral area and its conjugate point in the southern hemisphere. While on revisited orbits during other days (before and after the earthquake) the magnetic field spectra remained at a relatively low level. In order to be sure that the electromagnetic disturbances were induced by this Yushu earthquake, relative variations of the magnetic field in the ELF/VLF range [370–897?Hz] were calculated and compared with the normal background during 3?years (2007, 2008 and 2009) in the time interval from Jan. 1st to Apr. 30 and in the area [23°N–43°N, 86°E–106°E]. The results show that the normal level of magnetic field in this area is relatively low (~10^?7?nT²/Hz), but a large increase occurred from Apr. 1st to Apr. 14 which exceeds 5σ_b. After the earthquake, the magnetic field in the ELF/VLF range gradually decreased. In order to exclude the influence of geomagnetic field activity, we selected only the data recorded during magnetically quiet local night-times. In addition, artificial noises were also removed from the dataset used in the present paper. Therefore the observed perturbations were independent of geomagnetic field disturbances, and might be attributed to seismic activity.     

Photospheric Magnetic Field of the Sun: Two Patterns of the Longitudinal Distribution

Longitudinal distributions of the photospheric magnetic field studied on the basis of National Solar Observatory (Kitt Peak) data (1976 – 2003) displayed two opposite patterns during different parts of the 11-year solar cycle. Helio-longitudinal distributions differed for the ascending phase and the maximum of the solar cycle on the one hand and for the descending phase and the minimum on the other, depicting maxima around two diametrically opposite Carrington longitudes (180° and 0°/360°). Thus the maximum of the distribution shifted its position by 180° with the transition from one characteristic period to the other. Two characteristic periods correspond to different situations occurring in the 22-year magnetic cycle of the Sun, in the course of which both global magnetic field and the magnetic field of the leading sunspot in a group change their sign. During the ascending phase and the maximum (active longitude 180°) polarities of the global magnetic field and those of the leading sunspots coincide, whereas for the descending phase and the minimum (active longitude 0°/360°) the polarities are opposite. Thus the observed change of active longitudes may be connected with the polarity changes of Sun’s magnetic field in the course of 22-year magnetic cycle.     

VLF hiss,visual aurora and the geomagnetic activity

Observations and analyses of hiss events, recorded at College (dp. lat. 64.62°N) and Bar 1 (dp. lat. 70.20°N) during periods of varying auroral and geomagnetic activity, reveal three different types of events. These are (1) auroral substorm events with associated hiss bursts during disturbed period, (2) quiet-time hiss events accompanying stationary quiet auroral arcs and (3) hissless events at times of auroral and magnetic activity. Quiet-time observations seem to suggest that the substorm activity is not a necessary requirement for generating wideband hiss. On the other hand, examples of auroral and magnetic activity with complete absence of VLF hiss indicate that the ground reception of VLF/ELF natural emissions is largely controlled by propagation conditions in the ionosphere. There is either little or no correlation found between hiss observations at the two stations separated by about 600 km.     

Mid-latitude pulsating auroras

Pulsating auroras were recorded at Bedford, Massachusetts, cgm lat. 55.4°, 24 March 1969 during a worldwide magnetic storm, the only known published observations of pulsating auroras at such low magnetic latitudes. Spectral density analysis of several minutes of 5577A pulsations indicated a dominant period of 7.2 sec at 0300 EST. The following characteristics were noted: (1) occurrence during a negative bay in H; (2) location toward the equatorial boundary of the auroral display; (3) occurred a few hours after local midnight; (4) characteristic period of 6–10 sec; (5) quasi-sinusoidal or superposition of sinusoids rather than isolated pulses; (6) modulation of the background intensity by 15–30 per cent. These characteristics have previously been observed by others in pulsating auroras in the auroral oval. Other mid-latitude geophysical measurements at the same time show similarities to typical auroral oval behavior. These observations indicate that the auroral oval expanded during the worldwide magnetic storm until the boundary of the auroral oval was near cgm latitude 55°. If this observation of one mid-latitude pulsating aurora is in general valid, then the agreement of the characteristic period of pulsating auroras when the oval has expanded to mid-latitudes with the period of pulsating auroras when the oval is not expanded should be useful in distinguishing between proposed source mechanisms for these pulsating auroras.     

A Dst contribution to the equatorward shift of the aurora

It is observed that in the course of at least one major magnetic storm, during aurorally quiet pauses, the poleward limit of auoral activity is shifted 10–15° equatorward of its typical non-storm-time limit. The storm-time ring current will contribute to the equatorward shift by expanding the size of the magnetosphere, causing an increase in the magnetic flux in the tail that maps into the aurorally inactive polar cap. We use a new model of the ring current to estimate the size of the ring current effect on the shift in the poleward limit. One calculated example that is probably representative gives a shift of between 5 and 10° corresponding to a Dst in the range from ?300 to ?600 nT.     

Precipitation of auroral particles in the central polar cap during a sudden commencement magnetic storm

All-sky camera observations from two stations in the inner (northern) polar cap and an auroral zone station are combined with photometer records from the polar cap station Nord in a study of the brilliant auroral display following the ssc of the storm of 7 November 1970. This display is the large, poleward expanding bulge of a substorm triggered by the ssc. It is composed of brilliant discrete forms embedded in low-intensity diffuse electron and proton aurora. The poleward edge of the diffuse electron aurora is 5° north of the discrete auroras and 3° north of the proton aurora. The intensity of the discrete aurora varies as the strength of the auroral electrojet as shown by magnetograms from auroral zone stations. Succeeding the retreating display a subvisible low-energy electron precipitation, which may be identified as the polar squall (Winningham and Heikkila, 1974) is observed over the polar cap during the main phase of the storm.In the early morning sector already existing diffuse auroras broaden towards the equator from the time of the ssc and at least during the following half hour.Ssc-triggered displays have been found (Feldstein, 1959) to withdraw from the inner polar cap as the initial (positive H) phase of the storm ends. A comparison of the records from seven low-latitude stations shows that during this particular storm the positive phase appears to be composed by two overlapping disturbances, i.e. the proper initial phase, which is generally thought to be due to compression of the inner magnetosphere and a series of positive bays accompanying the negative bays in auroral latitudes. These positive bays are observable over a great range of longitudes with a maximum of amplitude near midnight. As judged from the dayside magnetograms the initial (compression) phase ends at an early stage of the substorm. The observed coincidence between the withdrawal of the display and the cessation of the positive H phase of the storm is a consequence of the fact that the second component—the positive bays—and the auroral display over the polar cap are both signatures of the substorm activity.     

On the existence of a long range correlation in the Geomagnetic Disturbance storm time (Dst) index

The Dst (Disturbance storm time) index is a measurement of earth geomagnetic activity and is widely used to characterize the geomagnetic storm. It is calculated on the basis of the average value of the horizontal component of the earth’s magnetic field at four observatories, namely, Hermanus (33.3° south, 80.3° in magnetic dipole latitude and longitude), Kakioka (26.0° north, 206.0°), Honolulu (21.0° north, 266.4°), and San Juan (29.9° north, 3.2°) and is expressed in nano-Teslas. The strength of the low-latitude surface magnetic field is inversely proportional to the energy content of the ring current around earth caused by solar protons and electrons, which increases during geomagnetic storms. Thus a negative Dst index value indicates that the earth’s magnetic field is weakened which is specifically the case during solar storms. Predicting Dst index is a difficult task due to its structural complexity involving a variety of underlying plasma mechanism. For characterizing and forecasting this complex time series, a formal model must be established to identify the specific pattern of the series. Persistent demand for a fool proof model of Geomagnetic Dst index prompted us to investigate the Dst Time Series mechanism with a very recent technique called Visibility Algorithm and it is observed that the Dst time series follows the same model that of a Stochastic Fractional Brownian motion having long range correlation.     

Responses of the polar ionosphere to the Bastille Day solar event

Simultaneous observations at Zhongshan Station, Antarctica, are presented for the interval of 13–17 July 2000 to show responses of the polar ionosphere to the Bastille Day (14 July 2000) solar event. The polar ionosphere was highly disturbed, as shown by frequently large deviations of the geomagnetic H-component, large riometer absorption events and strong ULF waves. Associated with the huge solar proton event produced by the X5/3B flare, a polar cap absorption (PCA) was observed. It began at ∼ 10:40 UT on 14 July and ended at ∼ 19:40 UT on 17 July. Superposed on it, there was a large absorption event with a peak of 26 dB, starting at ∼ 03:00 UT and ending at ∼ 11:10 UT on 15 July. This kind of absorption was probably produced by an intense `cloud of energetic electrons' during an auroral substorm. The ULF waves were very intense during the main phase and the recovery phase of the severe magnetic storm on 15 and 16 July. The ionospheric absorption was so strong that the digisonde signal was blacked out most of the time. The ionosphere returned to normal in the afternoon on 17 July.     

Horizontal and vertical winds and temperatures in the equatorial thermosphere: Measurements from Natal,Brazil during August–September 1982

Fabry-Perot interferometer measurements of Doppler shifts and widths of the 630.0 nm nightglow line have been used to determine the neutral winds and temperatures in the equatorial thermosphere over Natal, Brazil during August–September 1982. During this period, in the early night (2130 U.T.) the average value of the horizontal wind vector was 95 m s^?1 at 100° azimuth, and the temperature varied from a low of 950 K during geomagnetically quiet conditions to a high of ~ 1400 K during a storm (6 September). The meridional winds were small, ?, 50 m s^?1, and the eastward zonal winds reached a maximum value 1–3 h after sunset, in qualitative agreement with TGCM predictions. On 26 August, an observed persistent convergence in the horizontal meridional flow was accompanied by a downward vertical velocity and an increase in the thermospheric temperature measured overhead. Oscillations with periods of 40–45 min in both the zonal and vertical wind velocities were observed during the geomagnetic storm of 6 September, suggesting gravity wave modulation of the equatorial thermospheric flow.