Stations North Research Articles

Test of a northwards-decreasing 222Rn source term by comparison of modelled and observed atmospheric 222Rn concentrations

Model-predicted atmospheric concentrations of 222Rn based on two different 222Rn source terms have been compared with observations in the lower troposphere. One simulation used a globally uniform 222Rn source term from ice-free land surfaces of 1 atom cm−2 s−1; the other assumed a northwards-decreasing source term (linear decrease from 1 atom cm−2 s−1 at 30°N to 0.2 atom cm−2 s−1at 70°N). Zero emissions were assigned to oceans. The northwards-decreasing source term improved predictions at four out of six stations north of 50°N, reducing the mean prediction/observation ratio from 2.8 to 0.87. In the latitudinal band between 30°N and 50°N, the northwards-decreasing source term resulted in systematic under-prediction of atmospheric 222Rn, whereas the uniform source term provided predictions close to observations. Predictions based on the northwards-decreasing source term were significantly (p < 0.01) better than those based on the uniform source term for an averaged vertical 222Rn profile around 44°N, but were not for one around 38°N. The results indicate that a northwards-decreasing source term could be a more realistic representation of actual 222Rn emissions than a uniform 1 atom cm−2 s−1 source term. However, the decrease in 222Rn source strength with increasing latitude might not begin at 30°N but somewhat further north. This hypothesis should be investigated through model-independent means.

Seasonal and Regional Variation of Pan-Arctic Surface Air Temperature over the Instrumental Record*

Instrumental surface air temperature (SAT) records beginning in the late 1800s from 59 Arctic stations north of 64°N show monthly mean anomalies of several degrees and large spatial teleconnectivity, yet there are systematic seasonal and regional differences. Analyses are based on time–longitude plots of SAT anomalies and principal component analysis (PCA). Using monthly station data rather than gridded fields for this analysis highlights the importance of considering record length in calculating reliable Arctic change estimates; for example, the contrast of PCA performed on 11 stations beginning in 1886, 20 stations beginning in 1912, and 45 stations beginning in 1936 is illustrated. While often there is a well-known interdecadal negative covariability in winter between northern Europe and Baffin Bay, long-term changes in the remainder of the Arctic are most evident in spring, with cool temperature anomalies before 1920 and Arctic-wide warm temperatures in the 1990s. Summer anomalies are generally weaker than spring or winter but tend to mirror spring conditions before 1920 and in recent decades. Temperature advection in the trough–ridge structure in the positive phase of the Arctic Oscillation (AO) in the North Atlantic establishes wintertime temperature anomalies in adjacent regions, while the zonal/annular nature of the AO in the remainder of the Arctic must break down in spring to promote meridional temperature advection. There were regional/decadal warm events during winter and spring in the 1930s to 1950s, but meteorological analysis suggests that these SAT anomalies are the result of intrinsic variability in regional flow patterns. These midcentury events contrast with the recent Arctic-wide AO influence in the 1990s. The preponderance of evidence supports the conclusion that warm SAT anomalies in spring for the recent decade are unique in the instrumental record, both in having the greatest longitudinal extent and in their associated patterns of warm air advection.

Spatial variability and rainfall characteristics of Kerala

Geographical regions of covariability in precipitation over the Kerala state are exposed using factor analysis. The results suggest that Kerala can be divided into three unique rainfall regions, each region having a similar covariance structure of annual rainfall. Stations north of 10‡N (north Kerala) fall into one group and they receive more rainfall than stations south of 10‡N (south Kerala). Group I stations receive more than 65% of the annual rainfall during the south-west monsoon period, whereas stations falling in Group II receive 25–30% of annual rainfall during the pre-monsoon and the north-east monsoon periods. The meteorology of Kerala is profoundly influenced by its orographical features, however it is difficult to make out a direct relationship between elevation and rainfall. Local features of the state as reflected in the rainfall distribution are also clearly brought out by the study.

Spatial and temporal variations in precipitation in the Upper Indus Basin, global teleconnections and hydrological implications

Abstract. Most of the flow in the River Indus from its upper mountain basin is derived from melting snow and glaciers. Climatic variability and change of both precipitation and energy inputs will, therefore, affect rural livelihoods at both a local and a regional scale through effects on summer runoff in the River Indus. Spatial variation in precipitation has been investigated by correlation and regression analysis of long-period records. There is a strong positive correlation between winter precipitation at stations over the entire region, so that, for practical forecasting of summer runoff in some basins, a single valley-floor precipitation station can be used In contrast, spatial relationships in seasonal precipitation are weaker in summer and sometimes significantly negative between stations north and south of the Himalayan divide. Although analysis of long datasets of precipitation from 1895 shows no significant trend, from 1961–1999 there are statistically significant increases in winter, in summer and in the annual precipitation at several stations. Preliminary analysis has identified a significant positive correlation between the winter North Atlantic Oscillation (NAO) and winter precipitation in the Karakoram and a negative correlation between NAO and summer rainfall at some stations. Keywords: upper Indus basin, climate change, time series analysis, spatial correlation, teleconnections

Benthic foraminifera assemblages at Great Meteor Seamount

Foraminiferal assemblages found at Great Meteor Seamount were studied in August 1998. Communities of living foraminifera in surface sediments from the plateau (290–325 m water depth) and from the surrounding base (2,300–4,096 m) were compared in abundances, diversity, and species composition. In this oligotrophic region, densities were very low, but diversity was high. Highest numbers were observed at the deep stations north, south, and east of the seamount and at the shallow station in the north of the plateau. Lowest densities were recorded southwest of the plateau centre and at the lee side of the seamount. We explain this distribution pattern with variable amounts of fresh organic material, caused by local oceanic currents. Generally, plateau stations indicated coarser sediments, lower organic carbon content, and higher temperatures. The foraminiferal fauna showed bathyal to abyssal characteristics and similarities to assemblages previously described from other parts of the North Atlantic and other oceans. Several dominant species extended between the two habitats, on the plateau and in the surrounding deep sediments, but other species were found exclusively at deep stations, at plateau stations, or at the abyssal reference station.

Invertebrate associations with gastropod shells inhabited by Pagurus­bernhardus (Paguridae) – secondary hard substrate increasing biodiversity­in North Sea soft‐bottom communities

The crustacean Pagurus bernhardus is an abundant hermit crab species in the soft‐bottom benthic communities of the North Sea. The gastropod shells inhabited by the hermit crab were found to provide substrate for a diverse epizoic fauna. In order to describe the epizoan community as well as spatial differences in epizoan communities of the North Sea, hermit crabs were sampled during summer 2001 at five stations in the North Sea. In total, 51 epizoic species with up to 15 species and 427 individuals per crab were found. The most abundant epizoans were obligate associated species such as the polychaete Circeis amoricani paguri, the cnidaria Hydractinia echinata and the crustacean Trypetesa lampas, as well as sessile epizoans also found in hard‐bottom habitats such as balanids Balanus crenatus and Verruca stroemia or free‐living species such as the amphipod Gammaropsis nitida. The epizoic community structure on shells of the stations north of the 50 m contour differs from the southern communities on the Dogger Bank and in the German Bight. The northern stations were dominated by sessile species, whereas a higher proportion of free‐living epizoans occurred at the southern stations. The results are discussed concerning the ecological importance of shells inhabited by hermit crabs as a hard substrate in soft‐bottom habitats.

Evidence for a Rising Cloud Ceiling in Eastern North America*

Data from 24 airport weather stations along the north–south axis (35°–45°N) of the Appalachian Mountains are used to show a significant rising trend in cloud-ceiling height over the past three decades. The mean change in cloud-ceiling height was 4.14 ± 1.03 m yr−1 [mean ± 1 SE (standard error), p ≤ 0.001] across all stations. The trend was negative (−2.22 ± 0.67 m yr−1) for the six stations south of 37.5°N, but positive (6.26 ± 0.89 m yr−1) for the 18 stations north of this latitude. Mean ceiling height for broken cloud cover was higher and rising faster than mean ceiling height for overcast cloud cover. There were strong seasonal patterns that varied between the northernmost and southernmost stations; differences were most pronounced during the spring and summer months. Some of the potential ecological effects on high-elevation forests, where the transition from deciduous to coniferous forest is thought to be controlled by the height of the cloud base, are discussed.

Primary production and carbon uptake dynamics in the vicinity of South Georgia-balancing carbon fixation and removal

Primary production was measured at a series of stations in austral summer 1996 in the vicin- ity of South Georgia. Five stations were occupied along a 700 km transect southeastwards towards the western tip of South Georgia. Three stations were located north of the Antarctic Polar Front (PF) with mean primary production of 0.41 g C m -2 d -1 compared to 0.54 g C m -2 d -1 for the stations south of the front. Three stations were in the shelf break and off-shelf region to the east of South Georgia where the lowest rates of primary production were recorded (mean 0.34 g C m -2 d -1 ). At a further 4 stations north- west of the island, primary production was significantly greater: 0.8 to 2.5 g C m -2 d -1 (mean 1.5 g C m -2 d -1 ). Photoinhibition was a marked feature of the production profiles, resulting in the underestimation of column production by 4 to 16%. An assessment is made of the primary production required to main- tain a representative assemblage of metazooplankton around South Georgia composed of 30 g fresh mass (FM) m -2 krill and 20 g FM m -2 copepods. The total C requirement of both groups at maximum growth rate is estimated as 0.93 g C m -2 d -1 , with 0.54 g C m -2 d -1 required to maintain basal metabolism (i.e. for zero growth). Local primary production rates are well in excess of the C demands for basal metabolism by the metazooplankton, and likewise the highest regional values exceed those for maxi- mum metazooplankton growth. It is also estimated that the krill biomass required to sustain higher predators during their breeding season could be supported by local production at plausible maximum krill growth rate. In particular, high primary production found in this study to the northwest of South Georgia appears to be adequate to sustain maximum growth rate by krill when their biomass is 30 g FM m -2 , when krill production will exceed predator removal. It is already well known that advection from higher latitudes is important in supplying energy to the South Georgia system. This study adds to the evidence that locally enhanced primary production is also important in supporting the local food web.

Deep penetrating benthic oxygen profiles measured in situ by oxygen optodes

For the first time in situ, deep penetrating O 2 profiles were measured in abyssal sediments in the western South Atlantic. Construction of deep penetrating O 2 optodes and adaptation to a benthic profiling lander are described. The opto-chemical oxygen sensors allow measurements to a depth of 55 cm in marine sediments. A vertical resolution of 0.5 cm was used to determine the O 2 dynamics in those oligotrophic deep sea sediments; the oxygen concentration across the sediment water interface was measured with a resolution of 100 μm. Oxygen penetration depth (OPD), diffusive oxygen uptake (DOU) and oxygen consumption rates were determined at four stations north of the Amazon fan and one at the Mid-Atlantic Ridge. Diffusive oxygen uptake rates ranged from 0.1 to 0.9 mmol m −2 d −1; the oxygen penetration depth ranged from 8 to 26 cm. Carbon consumption rates calculated from the diffusive oxygen uptake rates were in the range of 0.3–3.0 g C m −2 a −1. Comparison between in situ and laboratory DOU and OPD measurements confirmed previous findings that core recovery and warming have strong effects on the oxygen dynamics in deep sea sediments. Laboratory measurements yielded a decrease of 50–75% in OPD and consequently an increase in DOU by 1.5 and 18-times. Deep penetrating oxygen optodes provide a new tool to accurately determine oxygen dynamics (and thereby calculate carbon mineralization rates) in oligotrophic sediments. However, oxygen optodes as used in this study do not resolve the diffusive boundary layer (DBL). The data show that deep penetrating O 2 optodes in combination with high-resolution O 2 microelectrodes give a complete picture of the oxygen dynamics, including the DBL, in deep sea sediments.

Oxygen production and respiration in the Antarctic Polar Front region during the austral spring and summer

Abstract Gross and net O2 production and dark community respiration rates were measured during austral spring and summer from 53°S to 70°S along 170°W across the Antarctic Polar Front (APF). Integrated production and respiration rates were compared for stations north of the APF (53–59°S), within the front (59–61.5°S) and south of the APF (61.5–71°S). Production and respiration rates in the spring were highest north of the polar front and at the seasonal ice-edge (∼65°S) during a phytoplankton bloom. Depth-integrated rates of gross C, net C production and community respiration across the region were 155, 84 and –71 mmol C m−2 d−1, respectively. During the austral summer integrated gross O2 production rates remained high north of the APF and were similar to rates measured there in the spring, but decreased within the front and south of it. Production exceeded respiration north of the APF, but net heterotrophic conditions were observed within and south of the front. Depth-integrated rates of gross C, net C production and community respiration for the transect were 87, 5 and –37 mmol C m−2 d−1. Higher rates of oxygen consumption in the light suggest that phytoplankton respiration may have been a significant sink for gross production in the summer. Net O2 production estimated from mixed-layer O2 saturation levels was about a factor of two lower than in vitro bottle incubations and confirmed that respiration slightly exceeded production over much of the region in the summer.

Response of gray whales to low-frequency sounds

One transducer from the U.S. Navy SURTASS-LFA source was used during January 1998 to expose migrating gray whales off the California coast to low-frequency sound. These transmissions were 42 s in duration, repeating every 6 min, and 160–330 Hz. The whales were observed from shore stations north and south of the source location. Each pod of whales’ closest point of approach (CPA) to the sound source was estimated from the shore observations. The received level (RL) at the CPA was estimated using measured transmission loss curves and the known source level. The probability of observing whale pods at each RL is computed under both playback and control conditions. The difference between these probabilities as a function of RL provides a measure of avoidance for the animals. Previous work [Malme et al. (1984)] found a 50% change in these probabilities at a RL of 120 dB re:1μPa. For an inshore source location, the current work found 50% avoidance occurred at a RL of 141 dB, with 95% confidence bounds of ±3 dB derived by bootstrap statistical methods. For the offshore source location, 50% avoidance was not achieved for any observed RL. [Work supported by ONR.]

Subduction‐induced strain in the upper mantle east of the Mendocino triple junction, California

We observe splitting of teleseismic shear waves at five stations of the Berkeley Digital Seismic Network located east of the Mendocino triple junction in northeastern California that is dependent on the arrival direction of the seismic phases. The observed variations with back azimuth cannot be explained with laterally varying anisotropy with a horizontal symmetry axis and are attributed to the presence of fabric with an inclined symmetry axis. We assume that the anisotropy is caused by the preferred alignment of olivine crystals. A grid search over possible orientations of the olivine a axes reveals that south of the Mendocino triple junction they dip to the east, whereas north of the triple junction they dip to the northeast. On the basis of a comparison of the ray paths of our data to the spatial distribution of fast and slow P wave velocity anomalies in the upper mantle, we conclude that the anisotropy is located within seismically slow regions and that the directions are controlled by the geometry of a steeply dipping fast P wave velocity anomaly. Assuming that the fast P velocity anomaly represents subducted slab material, we conclude that the fabric beneath the stations north of the triple junction is most likely caused by the differential motion between this rigid, strong down going plate and the surrounding mantle. South of the triple junction the fabric may have developed while subduction of the Farallon plate was still ongoing in this region (prior to 6 Ma). However, we prefer to attribute the observations to more recent asthenospheric flow associated with the opening of a slabless window beneath the North American lithosphere. The flow is modulated by the presence of rigid lithosphere to the north and east.

Planktonic ciliates in the oligotrophic Eastern Mediterranean:vertical, spatial distribution and mixotrophy

Abundance, biomass, vertical distribution, species composition and mixotrophy of planktonic ciliates were investigated during March 1997 in the oligotrophic Eastern Mediterranean. Six depth layers were sampled in the euphotic zone (0 to 100 m) at 7 stations in the North and 4 in the South Aegean Sea, resulting in a total of 66 samples and 82 ciliate species. Abundance was in general very low (0 to 780 cells l -1 ) except from specific stations in the North Aegean stations (20 to 2040 cells l -1 ), influenced by the Black Sea Water. Aloricate species dominated the depth-integrated abundance and biomass in both the North and the South. South Aegean stations presented more diverse ciliate fauna, uniform distribution down to 100 m and higher values of integrated abundance and biomass than the North Aegean stations. Among North stations, those subjected to the influence of the Black Sea presented very pronounced stratification, with mixotrophs and heterotrophs decreasing sharply below 20 m; at these stations, mixotrophic species of 18 to 30 μm dominated the ciliate community at the surface whereas at the South as well as at the rest of the North stations, nanociliates were the major mixotrophic size class. Mixotrophs (20% of abundance in the South, 38% in the North) and nanociliates (28% of abundance in the South, 44 % in the North) were found to be very important components of the community, which indicates that these organisms play also an important role in oligotrophic systems.

Processes controlling the distribution and cycling of manganese in the oxygen minimum zone of the Arabian Sea

Vertical and horizontal distributions of dissolved and particulate manganese were investigated in the Arabian Sea (Northwestern Indian Ocean) during the 1995 Spring Intermonsoon period (March–April; US JGOFS Process Cruise 2; TN045). The region is characterized by an intense, basin-wide oxygen minimum zone (OMZ) that strongly influences the manganese distribution. In the OMZ, two distinct dissolved Mn (d-Mn) maxima were observed, at depths of 200–300 m and 600 m, respectively. The latter peak displayed concentration maxima of approximately 6 nanomolar and was largely confined to stations north of 19°N latitude (Stations N2–N7). This mid-depth maximum was associated with the low oxygen core of the OMZ ([O 2]<∼2 μM), and appears to be maintained by a southward horizontal advective–diffusive flux of dissolved manganese from highly reducing Pakistan margin sediments, rather than input from the Oman Margin as previously suggested by Saager et al. (1989, Geochimica et Cosmochimica Acta, 53, 2259–2267). This signal was largely absent at stations along the southern transect, likely due to oxidative scavenging of d-Mn to the suspended particulate phase. Mid-depth particulate Mn maxima at some southern stations (Stations S4–S11) appear to be remnants of this feature. The upper d-Mn maximum (200–300 m depth) was more widely distributed than the 600 m peak, with d-Mn concentrations of ∼3 to as high as 8 nm at most stations east of about 62°E longitude. The signal was everywhere correlated with the secondary nitrite maximum, at stations within the main denitrification zone delineated by Naqvi (1991). Nepheloid layers, presumably bacterial, also were associated with this depth interval. Particulate Mn profiles displayed corresponding concentration minima and low Mn/Al and reactive/refractory Mn ratios for this same depth interval, suggesting reductive dissolution of Mn-oxyhydroxides. These observations imply that in situ microbially mediated processes may be the predominant source of d-Mn in the upper OMZ, while horizontal advection is more important deeper in the water column.

The Larval Fish Assemblage in Nearshore Coastal Waters off the St Lucia Estuary, South Africa

This paper reports on the composition, abundance and distribution of the larval fish assemblage in the nearshore coastal waters off the St Lucia Estuary mouth, South Africa. Ichthyoplankton samples were collected over a 12 month period from five stations located along a transect up to 2·5km offshore, and from two stations north and south of the estuary mouth, respectively. In all, 6126 fish larvae, representing 89 families and 186 species, were collected. Larvae in the families Myctophidae and Tripterygiidae comprised 21% and 16% of the total catch, respectively. The most abundant species were an unidentified triplefin, Tripterygiid 1 and the lanternfish Benthosema fibulatum, together which contributed nearly 18% of the total catch. Larvae of marine spawners independent of estuaries dominated the catch both in terms of density (90%) and in terms of number of taxa (89%). Some larvae of estuarine-associated species were present, in addition to a few specimens of estuarine resident species. Overall the dominant environmental variable affecting larval densities was temperature, particularly for Trypterygiid 1 where temperature contributed to 9% of the variance model. Densities of fish larvae peaked in November and December 1990 (late spring and early summer) and were lowest from January to June 1991 (summer, autumn an early winter). Different taxa dominated the catch each month with reef- and shelf-associated species accounting for the peak in August and September 1990, oceanic species in November 1990 and a mixture of the two groups in December. Overall larval densities were significantly higher in bottom samples with a trend of increasing densities offshore for reef and shelf taxa. The larvae of reef and shore taxa were predominantly preflexion larvae, whilst the few estuarine spawner species that were collected were mainly postflexion. Ontogenetic patterns related to depth and distance offshore were evident for the dominant species in each estuarine-association category.The present study has shown that temporal and spatial variations in the larval fish assemblage off St Lucia are related to environmental conditions and ontogenetic behavioural patterns of certain species. The origin of many of the larvae in the assemblages off the coast of St Lucia is probably from both local spawning populations in the shelf waters off KwaZulu-Natal and spawning populations farther north in shelf waters off Mozambique. Additional studies with more detailed oceanographic measurements will further our understanding of the physical processes that supply larvae to the St Lucia region.

Solar flare effects on the geomagnetic elements during normal and counter electrojet periods

The paper describes the study of solar flare effects (sfe) on horizontal (H), eastward (Y) and vertical (Z) components of the geomagnetic field at the Indo-USSR chain of magnetic observatories extending from the magnetic latitudes 0° to 45°N, a network not available any where else in the world. Events are selected when strong, normal and reversed (counter) equatorial electrojet are in existence as well as when only a partial counter electrojet is present. During a strong and normal electrojet event time, sfe consists of a positive impulse in H at all stations, the amplitude of H following the latitudinal variation similar to that of the quiet day monthly mean, Sq (H). The sfe in Y is negative at all the stations. Sfe in Z shows positive impulse at the four-electrojet stations and negative at other stations. During a counter electrojet period the effect of solar flare on H field is negative impulse at electrojet stations, positive at low latitude and again negative at stations north of Sq focus. Sfe in Y is small at all the equatorial nd negative at higher latitude stations. However, sfe (Z) is negative at the equatorial latitudes. During a partial counter electrojet period the observed effect is the combination of an increase of the planetary current component and the decrease of the electrojet component, giving rise to a negative impulse in H at equatorial stations and apositive impulse in H, increasing with increasing distance from the equator. These results are presented and discussed.

Hydrography and geostrophy around Easter Island

A series of hydrographic stations north and south of Easter Island (27′10′S, 109′20′W), Chile, were sampled between 22 and 23 May, 1994. The hydrography measured was consistent with basin-scale studies and showed a surface mixed layer that ranged between 80 and 115 m depth and had temperature and salinity values typical of autumn subtropical waters (22′C in temperature and 35.85 in salinity). The hydrography indicated the presence of two water masses in the vicinity of the island: the eastern South Pacific Central Water and the Antarctic Intermediate Water. Nutrient concentrations in general were lowest at the surface, over a layer that was deeper than the density mixed layer, and increased with depth. The appearance of salt fingers within the eastern South Pacific Central water mass was suggested by the positive vertical gradients of temperature and salinity, by the rough steps in the temperature and salinity profiles, and by the low and positive density ratios combined with Turner angles between 70′ and 80′. The density field indicated the development of geostrophic flows that were consistent with the eastern portion of the subtropical gyre of the South Pacific and with the surface dynamic topography of the period of observations. Due to the presence of the island, the large-scale north-northwestward geostrophic flows were reversed at spatial scales comparable to the size of the island.

The contribution of terrestrial sources and sinks to trends in the seasonal cycle of atmospheric carbon dioxide

We characterized decadal changes in the amplitude and shape of the seasonal cycle of atmospheric CO2 with three kinds of analysis. First, we calculated the trends in the seasonal cycle of measured atmospheric CO2 at observation stations in the National Oceanic and Atmospheric Administration Climate Monitoring and Diagnostic Laboratory network. Second, we assessed the impact of terrestrial ecosystems in various localities on the mean seasonal cycle of CO2 at observation stations using the Carnegie‐Ames‐Stanford Approach terrestrial biosphere model and the Goddard Institute for Space Studies (GISS) atmospheric tracer transport model. Third, we used the GISS tracer model to quantify the contribution of terrestrial sources and sinks to trends in the seasonal cycle of atmospheric CO2 for the period 1961–1990, specifically examining the effects of biomass burning, emissions from fossil fuel combustion, and regional increases in net primary production (NPP). Our analysis supports results from previous studies that indicate a significant positive increase in the amplitude of the seasonal cycle of CO2 at Arctic and subarctic observation stations. For stations north of 55°N the amplitude increased at a mean rate of 0.66% yr−1 from 1981 to 1995. From the analysis of ecosystem impacts on the mean seasonal cycle we find that tundra, boreal forest, and other northern ecosystems are responsible for most of the seasonal variation in CO2 at stations north of 55°N. The effects of tropical biomass burning on trends in the seasonal cycle are minimal at these stations, probably because of strong vertical convection in equatorial regions. From 1981 to 1990, fossil fuel emissions contributed a trend of 0.20% yr−1 to the seasonal cycle amplitude at Mauna Loa and less than 0.10% yr−1 at stations north of 55°N. To match the observed amplitude increases at Arctic and subarctic stations with NPP increases, we find that north of 30°N a 1.7 Pg C yr−1 terrestrial sink would be required. In contrast, over regions south of 30°N, even large NPP increases and accompanying terrestrial sinks would be insufficient to account for the increase in high‐latitude amplitudes.

An intense SFE and SSC event in geomagnetic &amp;lt;i&amp;gt;H&amp;lt;/i&amp;gt;, &amp;lt;i&amp;gt;Y&amp;lt;/i&amp;gt; and &amp;lt;i&amp;gt;Z&amp;lt;/i&amp;gt; fields at the Indian chain of observatories

Abstract. Changes in the three components of geomagnetic field are reported at the chain of ten geomagnetic observatories in India during an intense solar crochet that occurred at 1311 h 75° EMT on 15 June 1991 and the subsequent sudden commencement (SSC) of geomagnetic storm at 1518 h on 17 June 1991. The solar flare effects (SFE) registered on the magnetograms appear to be an augmentation of the ionospheric current system existing at the start time of the flare. An equatorial enhancement in ΔH due to SFE is observed to be similar in nature to the latitudinal variation of SQ (H) at low latitude. ΔY registered the largest effect at 3.6° dip latitude at the fringe region of the electrojet. ΔZ had positive amplitudes at the equatorial stations and negative at stations north of Hyderabad. The SSC amplitude in the H component is fairly constant with latitude, whereas the Z component again showed larger positive excursions at stations within the electrojet belt. These results are discussed in terms of possible currents of internal and external origin. The changes in the Y field strongly support the idea that meridional current at an equatorial electrojet station flows in the ionospheric dynamo, E.

An intense SFE and SSC event in geomagnetic H, Y and Z fields at the Indian chain of observatories

Changes in the three components of geomagnetic field are reported at the chain of ten geomagnetic observatories in India during an intense solar crochet that occurred at 1311 h 75° EMT on 15 June 1991 and the subsequent sudden commencement (SSC) of geomagnetic storm at 1518 h on 17 June 1991. The solar flare effects (SFE) registered on the magnetograms appear to be an augmentation of the ionospheric current system existing at the start time of the flare. An equatorial enhancement in ΔH due to SFE is observed to be similar in nature to the latitudinal variation of SQ (H) at low latitude. ΔY registered the largest effect at 3.6° dip latitude at the fringe region of the electrojet. ΔZ had positive amplitudes at the equatorial stations and negative at stations north of Hyderabad. The SSC amplitude in the H component is fairly constant with latitude, whereas the Z component again showed larger positive excursions at stations within the electrojet belt. These results are discussed in terms of possible currents of internal and external origin. The changes in the Y field strongly support the idea that meridional current at an equatorial electrojet station flows in the ionospheric dynamo, E.