Storm Of September Research Articles

The response of thermospheric nitric oxide to an auroral storm: 2. Auroral latitudes

An analysis of the response of lower thermospheric nitric oxide (NO) at auroral latitudes to the auroral storm of September 19, 1984, is presented. A comparison of data from the Solar Mesosphere Explorer (SME) taken 1 day after the storm (September 20) with data obtained 1 day before the storm (September 18) revealed a factor of 3 increase in NO. In order to model this response, particle data from the NOAA 6 and 7 satellites are used to assess the time history of the auroral energy input along each SME orbital track. The deduced fluxes and characteristic energies are used as input to a time dependent one‐dimensional photochemical model. In addition, the NCAR thermospheric general circulation model (TGCM) is used to calculate the response of the background neutral atmosphere to auroral forcings such as Joule and particle heating. It was found that particle precipitation accounted for 90% of the increase in the peak NO density, although Joule heating was more important at the higher altitudes (>140 km). The results of the model calculations predict an NO enhancement; however, the amplitude of the response as well as the absolute magnitude of the calculated NO density greatly exceed the observations. Two possibilities are proposed to explain this discrepancy. The first is that the yield of N(²D) from electron impact on N2 may only be 50%, rather than the 60–75% previously assumed. The second is that vertical winds of the order of 1–5 m s−1 may be generated in an E region auroral arc. It is shown that such winds could be important in damping out the NO response to increased particle precipitation.

Evolution of the ring current during two geomagnetic storms

The progressive developments in the radial profiles of the particle pressure, plasma beta, and electric currents of the storm time ring current are investigated with data from the medium energy particle analyzer on the AMPTE Charged Particle Explorer spacecraft. Measurements of ions from 25 keV to 1 MeV, which carry 70–85% of the energy density of the entire ring current population, are used in this work. Two geomagnetic storms in September of 1984 are selected and four traversals of the equatorial ring current region during the course of each storm are studied. It is shown that enhancements in the particle pressure occur initially in the outer region and reach the inner region in the late phase of the storm. Structures suggestive of multiple particle injections are seen in the pressure profile. The leading and trailing edges of the particle injection structures are associated, respectively, with the depressions and enhancements of the westward current densities of the ring current. Plasma beta occasionally increases to values of the order of 1 in some regions of the ring current from prestorm values of the order of 0.1 or less. It is also found that the location of the maximum ring current particle pressure can be several earth radii from where the most intense westward ring current flows. This is a consequence of the dominance of pressure gradient current over the current associated with the magnetic field line curvature and particle anisotropy.

The Possible Influence of Upstream Upper-Level Baroclinic Processes on the Development of theQE IIStorm

An analysis of the QE II storm of September 9-11, 1978 presents evidence for the existence of upper-level baroclinic processes upstream of the rapidly developing cyclone. The analysis shows that a deepening shortwave trough was located 400 to 500 km upstream of the site of the storm 12 h prior to rapid cyclogenesis. The trough was associated with: (1) a polar jet marked by 65 m/s winds in its core and significant vertical and horizontal wind shear, (2) positive vorticity advection and divergence at the 300 mb level, and (3) an intense frontal zone that extended from 300 mb down to the surface. It also appears that a tropopause fold likely extruded stratospheric air down to the 700-800 mb level, 400-500 km upstream of the surface low and 12 h prior to the explosive development phase of the cyclone. These findings raise questions about Gyakum's (1983) assertion that the QE II storm developed in an area in which the baroclinic support was confined to the lower troposphere and the related assertion by Anthes et al. (1983) that upper-level forcing upstream of the area of rapid cyclogenesis was weak and apparently not important in this case.

CCE plasma wave observations during the storm of September 4, 5, 1984

Near 0700 on September 4, 1984 a series of interplanetary discontinuities arrived at earth when the AMPTE Charge Composition Explorer (CCE) was near apogee. During the next few hours the spacecraft passed in and out of the magnetosheath. At the magnetopause boundary, the CCE wave instrument detected strong electron plasma oscillations, weaker electromagnetic waves at the electron plasma frequency, and broadband electrostatic waves. During the subsequent perigee passes on September 4 and 5, the wave observations of upper hybrid resonance emissions, continuum radiation, electrostatic noise bands and unusual low latitude auroral kilometic radiation were used to monitor significant variations in the magnetospheric characteristics as the main storm phases developed.

Magnetic storm of September 4, 1984: A synthesis of ring current spectra and energy densities measured with AMPTE/CCE

Compositional studies of the equatorial distributions of ring current ions during the September 4, 1984 magnetic storm have been made possible by comprehensive energy, charge state, and mass coverage data from the Charge Composition Explorer satellite. An examination of ion spectra at an L value of about 4 on September 5, in the local evening sector, shows that energy density was dominated by protons, with O ions contributing about 27 percent at the peak of about 150 keV, while He ions contributed less than about 2 percent. September 6 ion spectra, taken during the recovery phase of the storm, indicate that ion densities at more than 20 keV had decreased markedly, and that the ring current energy density was primarily provided by protons.

Observations relating changes in thermospheric composition to depletions in topside ionization during the geomagnetic storm of September 1982

Data taken over midlatitudes by Dynamics Explorer 2 during a geomagnetic storm are used to demonstrate that a direct correlation exists between depletions in the ratio [O]/[N2] and in the topside electron density (Ne). That correlation is most conspicuous along 45° invariant latitude where a decrease in [O]/[N2] by a factor of 5 is associated with a depletion in Ne by a factor of 8. Data comparisons were made by using densities of molecular nitrogen and atomic oxygen measured by the neutral atmosphere composition spectrometer, gas temperatures measured by the wind and temperature spectrometer, and temperatures and densities of thermal electrons measured by the Langmuir probe. The measurements were taken in the sunlit northern hemisphere during September 21–23, 1982, in the altitude range 300–500 km. During the storm, gas temperatures were elevated, while the ratio [O]/[N2] and Ne were reduced. At 60° and 45° invariant latitudes, the most significant disturbances in the ratio [O]/[N2] and Ne were depletions; but at 20° invariant latitude, depletions in the ratio [O]/[N2] and Ne were preceded by enhancements. The correlations between variations in the ratio [O]/[N2] and Ne indicate that, for this storm, ion chemistry controls the observed variations of topside Ne at the higher latitudes. However, the development at 20° invariant latitude of elevated gas temperatures and an enhancement in Ne indicates that a storm induced equatorward wind component transported plasma toward lower latitudes at a rate that initially exceeded the rate of ionization losses there.

Cosmic Ray Intensity Waves and the North-South Anisotropy

Waves of variation in the daily average cosmic ray intensity at the Earth’s surface were first detected in the neutron monitor record. Following the abnormal cosmic ray storm of September 1978, a sinusoidal 13.5 day periodicity was observed in the average intensity (Pomerantz and Duggal 1979), persisting for at least two solar rotations. Further observations, including underground data from the southern hemisphere, confirmed that not only were the waves isotropic but exhibited approximately a p−1 variational dependence on primary rigidity p (Duggal et al. 1981). No further evidence for this kind of wave has yet come to light. However, in the latter half of 1982 a series of 27-day waves that were apparently of a different character were detected. The evidence for their presence resulted from an analysis of the disturbed period that followed the occurrence of the large Forbush Decrease commencing 13 July 1982. It seemed at first that they could be described as anisotropic waves of the well-known interplanetary North-South asymmetry (Jacklyn and Pomerantz 1983).

Measurements of storm‐generated bottom stresses on the continental shelf

Large values of bottom friction velocity, u*, and roughness length, z0, determined from burst‐averaged speed data taken on the continental shelf in outer Norton Sound, Alaska, with the GEOPROBE tripod during a storm in September 1977 are correlated with extremely large values of near‐bottom concentration of total suspended particulate matter (TSM). Combined wind‐driven and tidal currents exceeding 30 cm/s at 1 m above the bottom and intense oscillatory bottom currents with maxima above 45 cm/s were associated with the largest measured values of TSM at 2 m above the sea floor. The values of u* and z0 obtained from the ‘law of the wall’ velocity‐depth relationship are diminished substantially throughout the storm period (average reduction of 44%) when the turbulence reducing effects of the vertical concentration gradient of TSM are considered. The form of the latter correction was adapted from Smith and McLean (1977a). Values of the mean u* computed from the theory of Grant and Madsen (1979), which predicts an enhanced shear stress due to nonlinear wave‐current interactions, compare favorably with the u* values determined from the measured velocity profiles. The measured values of z0, however, are considerably larger than any of the estimates based on (1) the actual scales of the physical roughness elements; (2) the apparent roughness of Grant and Madsen (1979); or (3) the thickness of the bed‐load layer as formulated by Smith and McLean (1977a).

Observations of energetic helium ions in the Earth's radiation belts during a sequence of geomagnetic storms

Every year a significant number of magnetic storms disturb the earth's magnetosphere and the trapped particle populations. In this paper, we present observations of energetic (MeV) helium ions made with Explorer 45 during a sequence of magnetic storms during June through December of 1972. The first of these storms started on June 17 and had a Dst index excursion to ‐190 gamma, and the MeV helium ions were perturbed primarily beyond 3 earth radii in the equatorial radiation belts with a typical flux increase of an order of magnitude at L= . The second storm period took place during August and was associated with very major solar flare activity. Although the Dst extremum was at best 35 gamma less than the June storm, this period can be characterized as irregular (or multi‐storm) with strong compression of the magnetosphere and very large (order of magnitude) MeV helium ion flux enhancements down to L ∼ . Following this injection the trapped helium ion fluxes showed positive spectral slope with the peak beyond 3.15 MeV at L=2.5; and at the lowest observable L shells (L ∼2–3) little flux decay (τ >100 days) was seen during the rest of the year. Any effects of two subsequent major magnetic storms in September and November were essentially undetectable in the prolonged after‐effect of the August solar flare associated MeV helium ion injection. The helium ion radial profile of the phase space density showed a significant negative slope during this period, and we infer that radial diffusion constitutes a significant loss of helium ions on L shells above L ≃4 during the aftermath of the August 1972 magnetic storm(s).

Wave spectral transformation measurements at Sylt, North Sea

Wave spectra were measured in the nearshore region extending approximately 900 m from the shore at the Island of Sylt (North Germany) in the North Sea. The measurement covered two periods, one in May 1976 when the sea condition was considered normal in this region and the other period was during a storm in September 1978. The field results are presented and compared with numerical computations for energy transformation in shallow water, based on a numerical model developed by Shiau and Wang (1977). The field data supported the validity of the numerical model. The results suggest that, prior to wave breaking, the transformation of wave components is mainly influenced by shoaling and refraction. The bottom frictional effect could also be important in the energy-containing range. After breaking, turbulent generation due to wave instability seems to be the dominant mechanism of energy dissipation.

Sediment transport in Norton Sound, Alaska

The Yukon River, the largest single source of Bering Sea sediment, delivers >95% of its sediment load at the southwest corner of Norton Sound during the ice-free months of late May through October. During this period, surface winds in the northern Bering Sea area are generally light from the south and southwest, and surface waves are not significant. Although wind stress may cause some transport of low-density turbid surface water into the head of Norton Sound, the most significant transport of Yukon River suspended matter occurs within advective currents flowing north across the outer part of the sound. The thickest accumulations of modern Yukon silt and very fine sand occur beneath this persistent current. We monitored temporal variations in bottom currents, pressure, and suspended-matter concentrations within this major transport pathway for 80 days in the summer of 1977 using a Geological Processes Bottom Environmental (GEOPROBE) tripod system. The record reveals two distinctive periods of bottom flow and sediment transport: an initial 59 days (July 8–September 5) of fair-weather conditions, characterized by tidally dominated currents and relatively low, stable suspended-matter concentrations; and a 21-day period (September 5–September 26) during which several storms traversed the northern Bering Sea, mean suspended-matter concentrations near the bottom increased by a factor of five, and the earlier tidal dominance was overshadowed by wind-driven and oscillatory wave-generated currents. Friction velocities ( u ∗ ) at the GEOPROBE site were generally subcritical during the initial fair-weather period. In contrast, the 21-day stormy period was characterized by u ∗ values that exceeded the critical level of 1.3 cm/s more than 60% of the time. The GEPROBE data suggest that the very fine sand constituting about 50% of the sediment on the outer part of the Yukon prodelta is transported during a few late-summer and fall storms each year. A conservative estimate shows that suspended-matter transport during the storms in September 1977 was equal to four months of fair-weather transport.

A quasi-self-consistent axially symmetric model for the growth of a ring current through earthward motion from a pre-storm configuration

A quasi-self-consistent axially symmetric model of a storm-time ring current that has evolved quasi-adiabatically through inward motion from a prestorm state is presented in which the disturbance field as a function of geocentric distance, r, in the equatorial plane (including the value, H, at r = 0 from which Dst can be found), the beta of the plasma as a function of r, and the ring current magnetic moment are all given in terms of analytic expressions, having only two independent ring current parameters: the geocentric distance to the inner edge of the ring current, R, and the distance at which the beta of the plasma is unity, k—a constant of the model. The model is used to find H as a function of R at constant k, which corresponds to the growth of Dst as the ring current moves earthward, and to illustrate how the radial profile of the disturbance field varies also as the ring current moves earthward. Comparison with published S 3-A storm-time, particle and field data shows reasonable agreement. The model is also able to fit the empirically determined relationship between Dst and the equatorward extension of quiet auroral arcs. This result applied to the great magnetic storm of September 1859 predicts a Dst of the order of 2000 nT, a value for which the sparse magnetometer data for that storm offer some support. Because the model (quasi) self-consistently determines the particle distribution function by allowing it to (quasi) adiabatically evolve in a field of its own making, the model avoids difficulties encountered by previous ring current models in which the particle distribution function had to be assumed. For example, the ring current magnetic moment is not limited to values less than the geomagnetic dipole moment, and null points in the field do not develop even in very high beta situations.

Observations of low-energy electrons from AE-C in the south polar cusp during the geomagnetic storm of September 21, 1977

Observations of low-energy electrons from AE-C in the south polar cusp during the geomagnetic storm of September 21, 1977

Intense uniform precipitation of low-energy electrons over the polar cap

Analysis of precipitating electron data from the Defense Meteorological Satellite Program (DMSP) revealed significant enhancements (as large as two orders of magnitude) over the quiet time level of the low-energy electron flux over the polar cap during magnetic storms in September and October 1974. Strong asymmetry between the two polar caps was observed, and it may indicate that these electrons are of interplanetary (i.e., solar) origin and that their entry is influenced by the interplanetary magnetic field as illustrated by Yeager and Frank (1976) and Fennell et al., (1975). Energy spectra showing the spatial and temporal variations of the phenomena are presented. There also is a region between the equatorward edge of the polar cap enhancement and the poleward edge of the evening auroral precipitation in which the electron flux drops down to the background level. If the observed intense polar cap precipitation is indeed of interplanetary origin, this void region implies that the poleward boundary of the auroral precipitation does not coincide with the equatorward boundary of the open field lines and that their separation cna be as wide as a few degrees in latitude.

Response of electrons in ionosphere and plasmasphere to magnetic storms

MEASUREMENTS of total electron content (TEC) using the Faraday polarisation–rotation and dispersive-group-delay techniques were carried out at Fort Monmouth, New Jersey1,2 (40.18°N, 74.06°W) using v.h.f. and u.h.f. beacon signals emitted by the satellite ATS–6. A comparison of TEC values obtained by both these techniques yields the total electron content of the plasmasphere. During the severe magnetic storms of September 15–16 and September 18–19, 1974, rapid increases with sudden onsets were observed in TEC as well as in plasmaspheric content, NP. These increases were followed by long periods of low values of TEC and NP.

Geomagnetic effect on the neutral temperature of theFregion during the magnetic storm of September 1969

A spherical Fabry-Perot interferometer on Ogo 6 (launched June 5, 1969) gives the profile of the 6300-A oxygen line in the dayglow at different altitudes between 200 and 320 km. The temperature of the neutral components of the atmosphere is deduced from these measurements with an accuracy of ±65°K. The results presented in this paper correspond to the storm period September 25 to October 5, 1969. During this period a severe geomagnetic storm occurred. The temperature measurements show a very strong effect in the two polar regions, while the equatorial regions present a smooth temperature variation.

Topside ionosphere disturbance effects during different phases of two successive magnetic storms in september, 1963

Using the data obtained by means of the Alouette-1 satellite, the distribution of electron density in the region of the F2-layer maximum and topside ionosphere during different phases of two successive magnetic storms on September 13 and 16,1963 have been studied. The middle latitudes at local near noon and midnight hours have been considered mainly. It is shown that the daytime topside electron density at some altitudes did not change during the main phases of the two magnetic storms. The electron density decreases below these levels and increases above. During the recovery phases of both magnetic storms the increase in electron density remains at all altitudes from h m F2 to 1000 km.

Magnetopause structure during the magnetic storm of September 24, 1961

Explorer-12 magnetic-field observations of the magnetopause current layer were made on the inbound pass of September 24, 1961, which occurred during the main phase of a moderately strong magnetic storm. The satellite was located near the subsolar point. The accuracy of the Explorer-12 data reported here has been increased by use of an improved filtering procedure. Five magnetopause penetrations were observed in a time period of about 20 min and for each of these the data were used to calculate the vector normal to the current layer, the magnetic-field component along that vector, and the polarization of the current layer. Normal magnetic-field components significantly different from zero were obtained, and it is suggested that the observations during this pass in general are consistent with the so-called open magnetosphere model. In fact, it appears that the first and last of the boundary penetrations may have occurred on opposite sides of, but very near, the X-type magnetic null line, which is located on the front side of the open magnetosphere model. The direction of the normal magnetic-field component as well as the sense of rotation of the tangential magnetic-field component in the current layer were found to be opposite for these two boundaries.

Magnetopause crossing of the geostationary satellite ATS 5 at 6.6RE

During the moderate magnetic storm of September 29–30, 1969, an unusually large magnetic field decrease preceded by an impulsive increase of about 100 γ was observed by the geostationary satellite ATS 5 at about 1733 UT on September 29. The field remained low for about 1 min and returned to the pre-event level as abruptly as it decreased. From the ATS 1 and ATS 5 observations and magnetograms from ground observatories, we inferred that the magnetosphere was greatly compressed before the event; the magnetopause distance was probably near 7 RE at the subsolar point. By comparing the changes observed by ATS 5 with the field measured by ATS 1, which was 3 hours behind ATS 5 in local time, we interpreted the event as a magnetopause crossing of ATS 5 caused by a localized rapid inward motion of the magnetopause and its subsequent recession, temporarily creating an indentation on the magnetopause surface and briefly exposing ATS 5 to the magnetosheath field. We suggest that the apparent ‘holes’ in the magnetic field observed by Ogo 3 and Ogo 5 in the magnetosphere near the magnetopause may have been caused, at least in some instances, by similar localized magnetopause motions.

Midlatitude ionospheric temperatures during three magnetic storms in 1965

Incoherent (Thomson) scatter observations of F region densities and temperatures were made at Millstone Hill during magnetic storms that occurred in June, August, and September 1965. For the storms of August and September, we have been able to compare the average F region electron and ion temperatures observed in the first 24 hours of the storm with those of the preceding 24 hours. The June measurements, made during the first 48 hours of the storm, have been compared with the average behavior a year earlier and with measurements made on magnetically quiet days in July 1965. In all cases, it is found that there are no changes in electron temperature greater than 30% during the daytime for altitudes below 800 km. Nocturnal increases of electron temperature as large as 100% are sometimes found, but their occurrence is not directly related to the planetary magnetic index Kp. A decrease in NmaxF2 observed during part of one of the magnetic storms does not appear to have been caused by an increase in the loss coefficient resulting from a dependence on electron temperature.