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Search Results: 1 - 10 of 292146 matches for " P. T. Jayachandran "
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The electron energy distribution during HF pumping, a picture painted with all colors
J. MacDougall ,P. T. Jayachandran
Annales Geophysicae (ANGEO) , 2005,
Abstract: This study uses digital ionosonde data from a cusp latitude station (Cambridge Bay, 77° CGM lat.) to study the convection into the polar cap. Days when the IMF magnetic field was relatively steady were used. On many days it was possible to distinguish an interval near noon MLT when the ionosonde data had a different character from that at earlier and later times. Based on our data, and other published measurements, we used the interval 10:00-13:00 MLT as the cusp interval and calculated the convection into the polar cap in this interval. The integrated convection accounted for only ~1/3 of the open polar cap flux. If the convection through the prenoon/postnoon regions on either side of the cusp was calculated the remaining 2/3 of the flux could be accounted for. The characteristics of the prenoon/postnoon regions were different from the cusp region, and we attribute this to transient flank merging versus more steady frontside merging for the cusp. Keywords. Ionosphere (Plasma convection) Magnetospheric physics (Polar cap phenomenon) Full Article (PDF, 754 KB) Citation: MacDougall, J. and Jayachandran, P. T.: Polar cap influx, Ann. Geophys., 23, 1755-1761, doi:10.5194/angeo-23-1755-2005, 2005. Bibtex EndNote Reference Manager XML
Central polar cap convection response to short duration southward Interplanetary Magnetic Field
P. T. Jayachandran,J. W. MacDougall
Annales Geophysicae (ANGEO) , 2003,
Abstract: Central polar cap convection changes associated with southward turnings of the Interplanetary Magnetic Field (IMF) are studied using a chain of Canadian Advanced Digital Ionosondes (CADI) in the northern polar cap. A study of 32 short duration (~1 h) southward IMF transition events found a three stage response: (1) initial response to a southward transition is near simultaneous for the entire polar cap; (2) the peak of the convection speed (attributed to the maximum merging electric field) propagates poleward from the ionospheric footprint of the merging region; and (3) if the change in IMF is rapid enough, then a step in convection appears to start at the cusp and then propagates antisunward over the polar cap with the velocity of the maximum convection. On the nightside, a substorm onset is observed at about the time when the step increase in convection (associated with the rapid transition of IMF) arrives at the polar cap boundary. Key words: Ionosphere (plasma convection; polar ionosphere) - Magnetospheric physics (solar wind - magnetosphere interaction)
Climatology of GPS phase scintillation and HF radar backscatter for the high-latitude ionosphere under solar minimum conditions
P. Prikryl, P. T. Jayachandran, S. C. Mushini,R. Chadwick
Annales Geophysicae (ANGEO) , 2011,
Abstract: Maps of GPS phase scintillation at high latitudes have been constructed after the first two years of operation of the Canadian High Arctic Ionospheric Network (CHAIN) during the 2008–2009 solar minimum. CHAIN consists of ten dual-frequency receivers, configured to measure amplitude and phase scintillation from L1 GPS signals and ionospheric total electron content (TEC) from L1 and L2 GPS signals. Those ionospheric data have been mapped as a function of magnetic local time and geomagnetic latitude assuming ionospheric pierce points (IPPs) at 350 km. The mean TEC depletions are identified with the statistical high-latitude and mid-latitude troughs. Phase scintillation occurs predominantly in the nightside auroral oval and the ionospheric footprint of the cusp. The strongest phase scintillation is associated with auroral arc brightening and substorms or with perturbed cusp ionosphere. Auroral phase scintillation tends to be intermittent, localized and of short duration, while the dayside scintillation observed for individual satellites can stay continuously above a given threshold for several minutes and such scintillation patches persist over a large area of the cusp/cleft region sampled by different satellites for several hours. The seasonal variation of the phase scintillation occurrence also differs between the nightside auroral oval and the cusp. The auroral phase scintillation shows an expected semiannual oscillation with equinoctial maxima known to be associated with aurorae, while the cusp scintillation is dominated by an annual cycle maximizing in autumn-winter. These differences point to different irregularity production mechanisms: energetic electron precipitation into dynamic auroral arcs versus cusp ionospheric convection dynamics. Observations suggest anisotropy of scintillation-causing irregularities with stronger L-shell alignment of irregularities in the cusp while a significant component of field-aligned irregularities is found in the nightside auroral oval. Scintillation-causing irregularities can coexist with small-scale field-aligned irregularities resulting in HF radar backscatter. The statistical cusp and auroral oval are characterized by the occurrence of HF radar ionospheric backscatter and mean ground magnetic perturbations due to ionospheric currents.
Elliptical model of cutoff boundaries for the solar energetic particles measured by POES satellites in December 2006
A. V. Dmitriev,P. T. Jayachandran,L. -C. Tsai
Physics , 2013, DOI: 10.1029/2010JA015380
Abstract: Experimental data from a constellation of five NOAA POES satellites were used for studying the penetration of solar energetic particles (SEP) to high latitudes during long-lasting SEP events on December 5 to 15, 2006. We determined cutoff latitudes for electrons with energies >100 keV and > 300 keV, and protons with energies from 240 keV to >140 MeV. The large number of satellites allowed us to derive snap shots of the cutoff boundaries with 1-hour time resolution. The boundaries were fitted well by ellipses. Based on the elliptical approach, we developed a model of cutoff latitudes for protons and electrons in the northern and southern hemispheres. The cutoff latitude is represented as a function of rigidity, R, of particles, MLT, geomagnetic indices: Dst, Kp, AE, and dipole tilt angle PS. The model predicts tailward and duskward shifting of the cutoff boundaries in relation to intensification of the cross-tail current, field-aligned currents, and symmetrical and asymmetrical parts of the ring current. The model was applied for prediction of a polar cap absorption (PCA) effects observed at high latitudes by CADI network of ionosondes. It was found that the PCA effects are related mainly to intense fluxes of >2.5 MeV protons and >100 keV electrons, which contribute mostly to the ionization of ionospheric D-layer at altitudes of ~75 to 85 km. This finding was confirmed independently by FORMOSAT-3/COSMIC observations of the SEP-associated enhancements of electron content at altitudes of ~80 km. Supplement : a program-code of the PCap-model
Characteristics of VHF radiowave scintillations over a solar cycle (1983 1993) at a low-latitude station: Waltair (17.7°N, 83.3°E)
P. V. S. Rama Rao,P. T. Jayachandran,P. Sri Ram,B. V. Ramana Rao
Annales Geophysicae (ANGEO) , 2003,
Abstract: 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.
Effect of equatorial ionization anomaly on the occurrence of spread-F
P. T. Jayachandran,P. Sri Ram,V. V. Somayajulu,P. V. S. Rama Rao
Annales Geophysicae (ANGEO) , 2003,
Abstract: The unique geometry of the geomagnetic field lines over the equatorial ionosphere coupled with the E-W electric field causes the equatorial ionization anomaly (EIA) and equatorial spread-F (ESF). Ionosonde data obtained at a chain of four stations covering equator to anomaly crest region (0.3 to 33 °N dip) in the Indian sector are used to study the role of EIA and the associated processes on the occurrence of ESF. The study period pertains to the equinoctial months (March, April, September and October) of 1991. The ratios of critical frequency of F-layer ( 0F2) and electron densities at an altitude of 270 km between Ahmedabad (33 °N dip) and Waltair (20 °N dip) are found to shoot up in the afternoon hours on spread-F days showing strengthening of the EIA in the afternoon hours. The study confirms the earlier conclusions made by Raghava Rao et al. and Alex et al. that a well-developed EIA is one of the conditions conducive for the generation of ESF. This study also shows that the location of the crest is also important in addition to the strength of the anomaly.
Sequential sporadic-E layers at low latitudes in the Indian sector
P. T. Jayachandran,P. Sri Ram,P. V. S. Rama Rao,V. V. Somayajulu
Annales Geophysicae (ANGEO) , 2003,
Abstract: A study of the formation and movement of sequential Sporadic-E layers observed during the night-time hours at two Indian low-latitude stations, SHAR (dip 10°N) and Waltair (dip 20°N) shows that the layer are formed around 19:00 h. IST at altitudes of ~180 km. They descend to the normal E-region altitude of about 100 km in three to four hours and becomes blanketing type of Es before they disappear. However, the absence of these descending layers at an equatorial station, Trivandrum (dip 2°N) gives the experimental evidence for wind shear theory. The meridional neutral wind derived from the height variation of the F-layer showed significant poleward wind during the descent of these layers. Hence it is inferred that these layers are formed as a consequence of the convergence of plasma by the poleward wind and the equatorward propagating gravity waves (inferred from the height fluctuations of F-layer). Key words. Ionosphere (active experiments; equatorial ionosphere · ionospheric irregularities)
Anti-inflammatory activity of Syzygium cumini seed
A Kumar, R Ilavarasan, T Jayachandran, M Deecaraman, RM Kumar, P Aravindan, N Padmanabhan, MRV Krishan
African Journal of Biotechnology , 2008,
Abstract: The Syzygium cumini (Myrtaceae) is a popular traditional medicinal plant in India. This study was intended to evaluate the anti-inflammatory activity of ethyl acetate and methanol extracts of S. cumini seed in carrageenan induced paw oedema in wistar rats at the dose level of 200 and 400 mg/kg administrated orally. Both the extracts exhibited significant anti-inflammatory activity, which supports the traditional medicinal utilization of the plant. This study established anti-inflammatory activity of the seed of S. cumini.
SuperDARN E-region backscatter boundary in the dusk-midnight sector – tracer of equatorward boundary of the auroral oval
P. T. Jayachandran,E. F. Donovan,J. W. MacDougall,D. R. Moorcroft
Annales Geophysicae (ANGEO) , 2003,
Abstract: We compare the locations of the equatorward boundaries of SuperDARN E-region backscatter and Hb emissions, focusing on the dusk-midnight sector of the auroral oval where the proton aurora is statistically located equatorward of the discrete electron aurora. We show that, whenever both boundaries can be simultaneously identified, they are coincident. Our result complements earlier studies, which demonstrated the correspondence between the DMSP b2i boundary and both the equatorward boundary of the proton auroral oval (Donovan et al., 2002), and the equatorward boundary of SuperDARN E-region echoes (Jayachandran et al., 2002). Further, our result shows that, provided there is sufficient precipitating proton energy flux, the SuperDARN radars can be used to monitor the equatorward edge of the proton auroral oval. Key words. Ionosphere (auroral ionosphere; particle precipitation; ionospheric irregularities)
GPS TEC, scintillation and cycle slips observed at high latitudes during solar minimum
P. Prikryl, P. T. Jayachandran, S. C. Mushini, D. Pokhotelov, J. W. MacDougall, E. Donovan, E. Spanswick,J.-P. St.-Maurice
Annales Geophysicae (ANGEO) , 2010,
Abstract: High-latitude irregularities can impair the operation of GPS-based devices by causing fluctuations of GPS signal amplitude and phase, also known as scintillation. Severe scintillation events lead to losses of phase lock, which result in cycle slips. We have used data from the Canadian High Arctic Ionospheric Network (CHAIN) to measure amplitude and phase scintillation from L1 GPS signals and total electron content (TEC) from L1 and L2 GPS signals to study the relative role that various high-latitude irregularity generation mechanisms have in producing scintillation. In the first year of operation during the current solar minimum the amplitude scintillation has remained very low but events of strong phase scintillation have been observed. We have found, as expected, that auroral arc and substorm intensifications as well as cusp region dynamics are strong sources of phase scintillation and potential cycle slips. In addition, we have found clear seasonal and universal time dependencies of TEC and phase scintillation over the polar cap region. A comparison with radio instruments from the Canadian GeoSpace Monitoring (CGSM) network strongly suggests that the polar cap scintillation and TEC variations are associated with polar cap patches which we therefore infer to be main contributors to scintillation-causing irregularities in the polar cap.
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