Storm Of April Research Articles

Global Positioning System phase fluctuations and ultraviolet images from the Polar satellite

In a study designed to determine the temporal development of ionospheric irregularities in the auroral region in magnetic storm periods, different types of simultaneous observations were compared for the storms of January 10, April 10–11, and May 15, 1997. The data sets consisted of ultraviolet images (UVI) from the Polar satellite, phase fluctuations and total electron content (TEC) from Global Positioning System (GPS) recordings at a large number of sites, magnetometer observations and hemispheric power precipitation. The large‐scale global or macroscale picture of the magnetic storm showed the importance of universal time in the development of irregularities. The hemispheric total power picture and the global indices such as Kp show this macroscale picture of the entire storm period. However, individual sites show differences in localized magnetic field variations and the development of irregularities; this we term the microscale. The storms of January 10 and May 15 show the importance of local magnetic time and local magnetic variations at the sites, while the storm of April 10–11 was dominated by the UT storm development. During the intense activity of the storms, total electron content shows minute‐by‐minute increases with the satellite moving to positions 6 km apart in the minute. The structured precipitation either directly produces irregularities or indirectly sets instability conditions for irregularity development in the auroral region. Good correlation was established for the three storms between UVI intensity and phase fluctuation development. The UVI Lyman‐Birge‐Hopfield‐long (170 nm) emission is sensitive to 100–200 km precipitation. Phase fluctuation development undoubtedly arises from perturbations in the F region as well. Topside and bottomside soundings have shown the high occurrence of spread‐F in the auroral region. The comparison of the data sets from these storms and from other studies indicates that the creation of irregularities develops from conditions existing over a large range of bottomside heights, i.e., at altitudes from 100 km to the F layer.

Accelerator Mass Spectrometry Analysis of Non-Soluble Carbon in Aerosol Particles from High Alpine Snow (Mt. Sonnblich, Austria)

With an elaborate accelerator mass spectrometry (AMS) technique radiocarbon measurements have been performed with aerosol carbon filtered from high alpine snow samples gathered consecutively at the high-altitude research station Sonnblick (3106 m, Eastern Alps, Austria) during a snow storm in April 1997. The concentration of the water-insoluble carbonaceous material in the molten snow was on the average 310 μg C/L and the total sample amounts for analysis were in the range of 35 μg to 60 μg C. Using a special background correction procedure tested on similar amounts of an urban particulate standard sample the accuracy of the corrected and normalized 14C/12C isotopic ratios of the snow aerosol samples was in the order of 4% to 14% of the measured ratios. The water-insoluble carbonaceous material of five samples from Mt. Sonnblick exhibited a weighted mean of 74 pMC (percent Modern Carbon) with a range of 64 pMC to 88 pMC. Thus, it appears that about 64% of non-soluble carbon in high alpine snow from Sonnblick was of biogenic origin. The temporal variations of the 14C/12C isotopic ratios of the snow aerosol samples were statistically significant, suggesting alterations in the contribution of specific aerosol sources.

Modeling of solar wind control of the ring current buildup: A case study of the magnetic storms in April 1997

The ring current buildup depending on the solar wind density and the time‐dependent convection field is examined by using a particle tracing scheme. To obtain the current density, energetic protons injected from the night side boundary at L=9 are traced under the dipolar magnetic field and the Kp dependent model of the Volland‐Stern type convection field. The source distribution function at the boundary is assumed to be isotropic Maxwellian with a temperature of 5 keV and a number density depending on the solar wind density. We derive the corrected Dst (Dst*) from the Biot‐Savart integral over the whole three‐dimensional distribution of the calculated current density. The results are compared with the observed Dst* in the three successive storms in April 1997; the major variation of the Dst* is mainly due to the convection electric field and the solar wind density as well.

Ion cyclotron waves during a great magnetic storm observed by Freja double‐probe electric field instrument

Evolution of the great magnetic storm in April 1993 is studied using observations of electromagnetic ion cyclotron (EMIC) waves by the F1 double‐probe electric field instrument onboard the Freja satellite. The almost continuous operation of the F1 instrument in the overview mode allowed us to follow the global EMIC wave activity at low altitudes above the ionosphere during several subsequent days covering the initial (compression), main, and recovery phases of the storm. During the initial phase of the storm the spatial occurrence of EMIC waves has a postnoon high‐latitude maximum, in agreement with earlier statistical results. A sudden and dramatic change of this pattern was observed with the start of the storm main phase. During the main phase, wave amplitudes were greatly enhanced and the active wave region moved to considerably lower latitudes to the late evening MLT sector. Also, the existence of heavy ions in the later main phase changed the distribution of wave frequencies dramatically. Most interestingly, a number of oxygen band EMIC waves were observed during a limited period of about 7 hours in the later main phase. The observed asymmetric MLT distribution of these oxygen band waves implies that the oxygen loss rate is faster than the drift rate. The results suggest that the EMIC waves play a crucial role in the main and early recovery phase of a great storm.

Daytime thermosphere above Millstone Hill during severe geomagnetic storms

A self‐consistent method for daytime F region modeling was applied to Millstone Hill radar observations during the severe geomagnetic storm of April 6–12, 1990, when the F2 layer maximum practically disappeared at usual heights and the F1 layer constituted the ionospheric maximum during daytime hours. Neutral composition and temperature changes are shown to be the main reason for the observed tenfold decrease of electron concentration at the F2 layer heights. The decrease in [O] at 300 km is more than a factor of 6, and the [N2] and [O2] increases are 3 and 16 times, respectively, with respect to the prestorm quiet time level. Such changes of neutral composition lead to a strong decrease in O+ production rate and increase in O+ loss rate resulting in a complete disappearance of the F2 layer maximum. Our calculations for 300 km altitude indicate a nineteenfold O/N2 ratio decrease for April 10 with respect to quiet day of April 7. Horizontal plasma transfer due to the observed E–W drift is insufficient to account for the observed density changes during daytime hours. The calculated exospheric temperatures Tex are close to both the mass spectrometer incoherent scatter (MSIS) 83 model predictions and the Millstone Hill estimates for all days except for April 10, when Tex is higher than 2000°K, which may be attributed to Joule heating due to strong electric fields. The calculated meridional thermospheric wind Vnx is more equatorward on disturbed days. The strong Vnx (∼90 m/s northward) on April 11 can explain the fast recovery of the local thermospheric parameters to the April 9 level. Calculations indicate that the ionosphere was molecular‐ion‐dominated up to 350 km on April 10, requiring a correction to the routinely derived Te(h), Ti(h), and Ne(h) radar profiles.

Comparison of model electron densities and temperatures with Millstone Hill observations during undisturbed periods and the geomagnetic storms of 16−23 March and 6−12 April 1990

Abstract. Measurements of F-region electron density and temperature at Millstone Hill are compared with results from the IZMIRAN time-dependent mathematical model of the Earth's ionosphere and plasmasphere during the periods 16–23 March and 6–12 April 1990. Each of these two periods included geomagnetically quiet intervals followed by major storms. Satisfactory agreement between the model and the data is obtained during the quiet intervals, provided that the recombination rate of O+(4S) ions was decreased by a factor of 1.5 at all altitudes during the nighttime periods 17–18 March, 19–20 March, 6–8 April and 8–9 April in order to increase the NmF2 at night better to match observations. Good model/data agreement is also obtained during the storm periods when vibrationally excited N2 brings about factor-of-2-4 reductions in daytime NmF2. Model calculations are carried out using different expressions for the O+ – O collision frequency for momentum transfer, and the best agreement between the electron-density measurements and the model results is obtained when the CEDAR interim standard formula for the O‡+ – O collision frequency is used. Deviations from the Boltzmann distribution for the ®rst ®ve vibrational levels of N2 collision frequency is used. Deviations from the Boltzmann distribution for the first five vibrational levels of j were calculated. The calculated distribution is highly non-Boltzmann at vibrational levels j > 2 and the Boltzmann distribution assumption results in the increase of 10–30% in calculated NmF2 during the storm-time periods. During the March storm at solar maximum the model results obtained using the EUVAC solar flux model agree a little better with the observations in comparison with the EUV94 solar flux model. For the April storm period of moderate solar activity the EUV94X model results agree better with the observations in comparison to the EUVAC model.

Effects of weather events on X-SAR returns from ice fields: case-study of Hielo Patagónico Sur, South America

The space-shuttle-based SIR-C/X-SAR synthetic aperture radar (SAR) imaged part of Hielo Patagónico Sur (HPS; southern Patagonia ice field, South America) for five successive days during missions in April and October 1994. A significant meteorological event occurred during each mission, including a major storm in April and a sharp temperature decrease in October. Changes in backscatter are observed for both episodes in X-SAR returns from the mid-portions of one of the two large outlet glaciers in the study area. Ground-station and satellite meteorological, and hydrological data are combined with the daily X-SAR images to interpret changes in glacier surface conditions caused by meteorological events. Effects interpreted from the April storm are (1) wind- and precipitation-influenced surface roughening of a wet snowpack, and (2) the deposition of new wet snow at lower elevation and its subsequent retreat up-glacier. An abrupt decrease in regional temperature during October is thought to reduce the snow wetness and increase grain-size. The changes in the radar-defined glacier zones due to the April precipitation event are subtle, while the October temperature drop causes significant backscatter increases. Our results suggest that trends in HPS glacier surface and near-surface conditions observable from spaceborne SARs are not significantly masked by precipitation events.

Effects of weather events on X-SAR returns from ice fields: case-study of Hielo Patagónico Sur, South America

The space-shuttle-based SIR-C/X-SAR synthetic aperture radar (SAR) imaged part of Hielo Patagónico Sur (HPS; southern Patagonia ice field, South America) for five successive days during missions in April and October 1994. A significant meteorological event occurred during each mission, including a major storm in April and a sharp temperature decrease in October. Changes in backscatter are observed for both episodes in X-SAR returns from the mid-portions of one of the two large outlet glaciers in the study area. Ground-station and satellite meteorological, and hydrological data are combined with the daily X-SAR images to interpret changes in glacier surface conditions caused by meteorological events. Effects interpreted from the April storm are (1) wind- and precipitation-influenced surface roughening of a wet snowpack, and (2) the deposition of new wet snow at lower elevation and its subsequent retreat up-glacier. An abrupt decrease in regional temperature during October is thought to reduce the snow wetness and increase grain-size. The changes in the radar-defined glacier zones due to the April precipitation event are subtle, while the October temperature drop causes significant backscatter increases. Our results suggest that trends in HPS glacier surface and near-surface conditions observable from spaceborne SARs are not significantly masked by precipitation events.

Comparison of model electron densities and temperatures with Millstone Hill observations during undisturbed periods and the geomagnetic storms of 16–23 March and 6–12 April 1990

Measurements of F-region electron density and temperature at Millstone Hill are compared with results from the IZMIRAN time-dependent mathematical model of the Earth's ionosphere and plasmasphere during the periods 16–23 March and 6–12 April 1990. Each of these two periods included geomagnetically quiet intervals followed by major storms. Satisfactory agreement between the model and the data is obtained during the quiet intervals, provided that the recombination rate of O+(4S) ions was decreased by a factor of 1.5 at all altitudes during the nighttime periods 17–18 March, 19–20 March, 6–8 April and 8–9 April in order to increase the NmF2 at night better to match observations. Good model/data agreement is also obtained during the storm periods when vibrationally excited N2 brings about factor-of-2-4 reductions in daytime NmF2. Model calculations are carried out using different expressions for the O+ – O collision frequency for momentum transfer, and the best agreement between the electron-density measurements and the model results is obtained when the CEDAR interim standard formula for the O‡+ – O collision frequency is used. Deviations from the Boltzmann distribution for the ®rst ®ve vibrational levels of N2 collision frequency is used. Deviations from the Boltzmann distribution for the first five vibrational levels of j were calculated. The calculated distribution is highly non-Boltzmann at vibrational levels j > 2 and the Boltzmann distribution assumption results in the increase of 10–30% in calculated NmF2 during the storm-time periods. During the March storm at solar maximum the model results obtained using the EUVAC solar flux model agree a little better with the observations in comparison with the EUV94 solar flux model. For the April storm period of moderate solar activity the EUV94X model results agree better with the observations in comparison to the EUVAC model.

Cover and barrier effect of Cassia siamea hedgerows on soil conservation in semi-arid Kenya

The contribution of Cassia siamea hedgerows and mulch to erosion control was evaluated on a 14% slope of a Lixisol/Alfisol at Machakos, Kenya. The four treatments, in 400 m 2 runoff-plots were: hedgerows with prunings applied as mulch to the crop, hedgerows and crops with prunings removed, mulch only applied to the crop and a control. The hedgerows were planted on the contour, 4 m apart and 0.25 m between plants. Maize and cowpea were planted in sequence. The control plot sustained an average annual water loss over 3 y of 31 mm runoff and soil loss of 19 t ha −1. The best treatment, hedgerows with mulch, reduced losses to 13% and 2% of the control. Hedgerows without mulch reduced losses respectively to 23% and 7%, while mulch without hedgerows reduced losses to 41% and 17%. Soil loss was considerably influenced by one single storm in April 1990 due to the nature of that storm and to the susceptibility of the soil to erosion at that particular time. Differences in crop yield between treatments were small. The hedgerow treatments depressed cowpea yield slightly in less than normal rainy seasons but improved cowpea yields in wet seasons.

Interpretation of an unusual high latitude density decrease in terms of thermospheric density cells

The discovery of a cellular structure in the high latitude thermospheric (120–350 km) neutral density provides a new framework in which to interpret high latitude density data. During geomagnetically active times, above about 170 km, the cells consist of density enhancements near noon and midnight and depletions near dawn and dusk. In this paper, a previously unexplained observation is presented and interpreted in terms of the cells. In general the high latitude neutral density is expected to increase during magnetically active conditions. On the contrary, the SETA‐1 satellite measured a 40% density decrease near 200 km during the geomagnetic storm of April 3, 1979. In the past, such ‘anomalous’ observations have often been attributed to the inseparability of density and winds in accelerometer measurements. We first show that the depletion cannot be caused by winds, and then show that the depletion is a natural consequence of the cellular structure inherent in the mass density at 200 km.

Effects of two dust storms on solar radiation in the Beijing‐Tianjin area

AVHRR albedo data obtained from the NOAA‐11 satellite and chemical data for aerosol particle samples collected in Beijing were interpreted together with meteorological data to study the radiative effects of Asian dust. Compared with the clear‐sky background, the albedo values for the Beijing area during two dust storms in April 1993 increased by 20 to 125%; the direct solar radiation flux at the surface decreased; and the scattered radiation flux at the surface increased. The total solar radiation flux at the ground during the two dust storms decreased by 40% and 10% respectively. Furthermore, the extent of the changes was related to the strength of the dust storms. This paper demonstrates the feasibility of using satellite data to study the radiative forcing of dust storms.

Submerged vegetation and spread ofEgeria densaPlanchon in Lake Rotorua, central North Island, New Zealand

Abstract SCUBA records on the distribution and abundance of macrophytes in Lake Rotorua show marked changes between 1980 and 1988. Extensive weed beds of Lagorosiphon major (Ridley) Wager, present since the mid‐1950s, disappeared from the lake by 1982. The loss of the L. major weed beds coincided with marked reductions in water clarity in the early 1980s and with a storm in April 1982. Egeria densa Planchon spread rapidly from a small infestation in July 1983 to become the most abundant macrophyte in the lake by December 1988 (occupying about 475 ha), equivalent to the historic L. major weed beds. We predict that E. densa will be dispersed to become a major problem weed in other Rotorua lakes in the future.

Equatorial scintillations during the major magnetic storm of April 1981

The geomagnetic storm of April 11–14, 1981, was the most severe storm reported since the August 1972 events. Observations of equatorial scintillations in the VHF through microwave range of frequencies (250 MHz to 1.5 GHz) by two stations, Huancayo, Peru, situated near the magnetic equator, and Ascension island, close to the equatorial anomaly crest, are presented. Unusually high levels of activity were recorded at both the stations even around sunrise. At Huancayo, triggered by a reversal of electric field and therefore a rise in the height of the F layer, the 1.5‐GHz transmissions from the satellite MARISAT (15°W) exhibited a scintillation index of 9 dB, which is one of the highest levels recorded at Huancayo over a period of several years. At Ascension island, severe depletions in ionospheric electron content associated with scintillations were detected for the first time in our observations near sunrise. The dual‐station multisatellite observations are discussed in terms of plasma processes at the equator. The reversal of direction after a deep magnetic bay caused the reversal of direction of the electric field, produced an upward bubble velocity, and generated F layer irregularities. This bay, however, was unable to produce an upwelling after sunrise because of E layer conductivity. The postmidnight and sunrise behavior of the F layer irregularity region has been verified by observations of additional severe magnetic storms during years of high sunspot activity. One example, observations on March 5, 1981, at Manila, is shown. The localized generation of patches during periods of magnetic activity indicates the possibility of bubble formation during magnetically quiet periods without a triggering wave. A hypothesis is proposed for instabilities forming from longitudinal (east‐west) electron density gradients existing in this case during the magnetic bay; in the case of magnetically quiet postsunset activity, steep and irregular gradients in electron density may exist across the longitudinal plane at the magnetic equator, producing conditions for the generation of instabilities.

Observations of optical emissions from precipitation of energetic neutral atoms and ions from the ring current

Observations of N2+ 1 N, H Balmer β(HBaβ) and other emissions due to particle precipitation have been observed at two low‐latitude sites (Mt. Haleakala, Hawaii and Cachoeira Paulista, Brazil) and one mid‐latitude site (McDonald Observatory, Southwest Texas). Results are compared for magnetic storms of April 13, 1981 and July 14, 1982. The emissions have the characteristics appropriate to the precipitation into the thermosphere of energetic neutral atoms and/or ions originating in the ring current. These characteristics include high rotational/vibrational excitation of the N2+ 1 N bands and at times the occurrence of HBaβ emission with the same onset time as the N2+ 1 N emission and partial correlation with it afterward. The latitude variation shows a strong increase from low to mid latitudes. The strongest emissions occur in the evening to midnight local time period, and the storm time variations shows strongest emissions during main phases. The time variations of HBaβ and N2+ 1 N emissions indicate that there is more O/O+ precipitation than H/H+ precipitation in the latter part, and sometimes the whole durations of the precipitation events and the variations are consistent with H+ being lost from the inner ring current faster than other species, such as O+ and He+. Lower limits for the energy deposition rates for the strongest emissions at 40–45° dip latitude are 1–2 mWm−2 and for the strongest emissions at 12°S dip latitude, 0.05 mWm−2. Ionization production at its peak altitude somewhere above 110 km would be in the range from 10² cm−3 s−1 to a few times 10³ cm−3 s−1 for the events in Texas, and from 100 to 10² cm−3 s−1 for the stronger events in Hawaii and Brazil.

Magnetospheric convection effects at mid‐latitudes: 2. A coordinated Chatanika/Saint‐Santin Study of the April 10–14, 1978, magnetic storm

We analyze simultaneous measurements by the incoherent scatter radars at Saint‐Santin (45° latitude) and Chatanika (65° latitude) during a major magnetic storm in April 1978 to examine several disturbance mechanisms operating on the mid‐latitude ionosphere during periods of strong magnetospheric and auroral activity. The first type of mechanism, the extension of magnetospheric convection electric fields to mid‐latitudes, is illustrated by two large localized departures of the E×B plasma drifts over Saint‐Santin that appeared in conjunction with the two storm negative phases. In both cases they were associated with large electric fields in the afternoon sector, within the eastward electrojet region, over Chatanika. The first event can be very clearly interpreted as a global short term enhancement of magnetospheric convection electric fields, since simultaneous plasma flux intensifications were identified over the polar cap, in the afternoon eastward electrojet, and at 45° latitude. The IMF and ring current signatures of these two events confirm our previous findings: large southward values of BƵ and large values of the rate of energy injection into the ring current system, are associated with the extension of magnetospheric convection electric fields to midlatitudes. In addition, a marked equatorward shift of the auroral oval was observed during both events. During the night of the first event, the mid‐latitude drift perturbation magnitudes remained moderate (100 m/s) and did not seem to induce any significant ionospheric perturbation. The joint increase in ion and electron temperatures by 100 to 200° K above the quiet day reference level, which preserved the thermal equilibrium of the plasma, can be simply interpreted in terms of the storm‐induced global heating of the thermospheric gas. Conversely, on the night of the second event, when drift magnitudes reached several hundreds of m/s, large‐amplitude fluctuations in the F layer height and density, and very large increases (by more than 500° K) in the ion and electron temperatures were observed. Whereas collisional heating can account for a large part of the ion temperature increase, an additional energy source, heating preferentially the electron gas, is needed to explain the maintenance of the electron temperature 200° K above the ion temperature observed from the onset of the Dst decrease onward during that night. The identification of this source suffers from the lack of additional data, in particular photometric ones, in the Saint‐Santin sector. However, the unusually large fields observed at Saint‐Santin and the equatorward shift of the auroral zone observed at Chatanika in the early afternoon suggest that the plasmapause may have moved close to Saint‐Santin and that the enhanced electron temperatures observed there might have been produced by a SAR arc.

Movement of Diffuse Source Pollutants in Small Agricultural Watersheds of the Great Lakes Basin

AbstractA study was conducted to determine losses and distributions of nitrogen (N) and phosphorus (P) in runoff waters and sediments from heavily fertilized agricultural watesheds during both the growing and nongrowing seasons. The study was done on two small watersheds having well‐drained, fine, sandy loam soils. Runoff was measured continuously and sampled proportionately during 50–60 events over a 2‐year period.Ninety nine percent of the sediment loss over the period occurred during 11 months; 64–86% occurred during a single storm in April when no crop canopy was present. This event was associated with rainfall probabilities rising rapidly toward a primary annual peak for East Lansing, Mich. A second storm of equal magnitude in August was associated with a secondary annual peak in normal rainfall for this location. Both storms exceeded the 20‐year expectancy in erosion potential (erosion index = 68). Seventy to 95% of the N in runoff, and 90–98% of the P, were carried in sediments, except during winter months when up to 80% of the N and 33% of the P were in solution. Sediments were enriched, relative to soils, in total N, but not in extractable P.

Effects of Storms on Rice Rats Inhabiting Coastal Marshes

During 1979, three storms with winds in excess of 73 km/hr and tides up to 1.5 m above normal struck a section of the Mississippi coast where a population study of the rice rat, Oryzomys palusrris, was in progress. There were no noticeable effects on population levels, and many individual animals survived the storms. INTRODUCTION The rice rat, Oryzomys palustris, is a semiaquatic mammal that occurs in wetland habitats throughout the southeastern United States. Throughout most of its range, 0. palustris is the most abundant small mammal in coastal marshes. It is also the most common mammal on many offshore islands in the northern Gulf of Mexico (Negus et al. 1961; personal observation). The tidal marsh and insular habitats of rice rats are occasionally subjected to devastating storms; and yet the rats appear either to survive these storms or to rapidly recolonize immediately following them. MATERIALS AND METHODS Since January 1979 I have been monitoring a rice rat population on the northern shore of St. Louis Bay in Harrison County, Mississippi. Two study grids about 1 km apart have been live-trapped quarterly. These grids have trapping stations placed at 15-m intervals. A 300-mm square floating platform that rises and falls with the tides is placed at each station. Each sampling session consists of one day of prebaiting the platforms with a mixture of oatflakes, peanut butter and sardines followed by four or five days of live-trapping. One Sherman live-trap is used on each platform. All animals captured are weighed, sexed, checked for reproductive condition, marked by toe clipping, and released. The westemmost grid is 12 stations long and 4 stations deep. Its long axis parallels shore. Water depth on its southern edge is about 400 mm at normal high tide. The northern edge parallels the marshforest ecotone and it is dry at normal high tide. The other grid consists of 100 stations in a 10 x 10 pattern. It extends from the forest-marsh ecotone for 135 m out into the marsh. Although the grid is about 0.5 km from shore, the southem half is flooded to a depth of about 300 mm daily by overflow from tidal creeks in the area. In the spring and summer of 1979 three storms had an impact on the area. A storm in mid-April produced a tide of 1 m above normal and winds gusting to about 40 knots. Manuscript received July 29, 1981; accepted September 30,1981. Hurricane Bob struck the area on July 1 1 with a storm tide of 1.5 m and winds up to 64 knots. On August 12, the western edge of Hurricane Frederick passed through with 74-knot winds but no storm tides, as the winds were from the north. The April storm and Hurricane Bob completely inundated both grids with a tidh surge that swept away all the trapping platforms and deposited them in the forest, 50-100 m to the north. The duration of these storm tides was 2-4 hrs.

First negative band system of nitrogen in the night sky over Arecibo during geomagnetic storms

Improvements in the airglow instrumentation at the Arecibo Observatory have permitted measurements of the brightness of the airglow emission from N2+ ions at 4278‐Å with sensitivity and time resolution much greater than those achieved when we first detected this emission in 1976. We report here the results of observations of this emission during magnetic storms in April and May of 1978. During this time surface brightnesses as high as 0.8 R were measured. The new measurements reveal fluctuations in brightness of the order of 20 per cent in times of about 30 minutes, which we tentatively attribute to fluctuations in the energetic particle flux that excites the emission. We also found a strong correlation between brightness and geomagnetic activity that was not apparent in our earlier, less sensitive measurements. The surface brightness at 4278‐Å increases at a rate of 0.4 to 0.5 R per 100 γ with increasing depression of Dst. Attempts to measure the rotational temperature of the emission were thwarted by contamination from molecular oxygen Herzberg bands, which we find to be significant at the very low levels of nitrogen emission which appear in the night sky at our latitude (18° north geographic, 30° geomagnetic).

On divergences of thermospheric meridional winds at midlatitudes

The existence of large scale (300 m/s) divergences, or convergences, of the meridional wind field is shown to be a common feature of the thermosphere at night during geomagnetic quiet times. This finding weakens the arguments used to invoke energetic neutral‐atom precipitation at low and midlatitudes to explain the divergence in the meridional wind during the geomagnetic storm of April 1, 1976. For this storm it is also shown that the maximum time rate of change of Dst is accompanied by a maximum value of the AE index, thus indicating that energy is flowing into the high latitude thermosphere as well as into the magnetospheric ring current.