Severe Geomagnetic Storms Research Articles

Tracking a major interplanetary disturbance

The severe geomagnetic storm which occurred during 27–29 August 1978 was remarkable because it arrived unexpectedly and was not related to a solar flare or long-lived coronal hole. Observations on 900 celestial radio sources show that the storm was associated with a large-scale region causing enhanced interplanetary scintillation which enveloped the Earth at the same time. The disturbance was first detected on 26 August, when the outer boundary had reached a distance of about 0.8 a.u. from the Sun and it was tracked until 30 August. The enhancement was followed by a fast solar wind stream and its shape suggests that it was a compression zone caused by the birth of the stream.

Airglow observations of the OI 7774 Å multiplet at Arecibo during a magnetic storm

During a severe geomagnetic storm the OI 7774 Å triplet was observed to have an intensity of the order of 15R, reduced to the zenith. Theoretical calculations using electron density profiles obtained with the Arecibo incoherent scatter radar show that the measured intensities cannot be explained by either radiative or ion‐ion recombination reactions. A possible correlation between the airglow enhancement and Dst has been found. Excitation by heavy neutral atoms or ions, for example, oxygen or O+, precipitating into the thermosphere is put forward as a possible explanation for enhanced 7774 Å emission during this magnetic storm.

Diurnal variations in the D-region during a storm after-effect

Measurements of electron concentration in the D- and lower E-regions of the ionosphere are reported for seven rocket flights from South Uist, Scotland, in April 1973. They took place during a 12-hour period starting 30 hours after the main phase of a severe geomagnetic storm. The principal feature of the results is that the electron concentrations below 85 km varied between 4 and 10 times the concentrations found on normal days. The variability was correlated with changes in radio wave absorption. The observed electron concentrations are compared with results of other workers for storm conditions and in particular with the model of Spjeldvik and Thorne (1975), and are found to be in reasonable agreement. The possibility of changes in positive ion composition occurring during the post-storm period is inferred.

Power System Disturbances During A K-8 Geomagnetic Storm: August 4, 1972

Solar flares can trigger severe fluctuations in the earth's magnetic field, termed geomagnetic storms. Geomagnetic storms produce quasi-dc currents in 60 Hz electric power systems. These spurious dc currents caused tundesirable equipment and system operating effects. A moderately severe geomagnetic storm occurred on August 4, 1972. The effects of that storm on electric power systems in the continental United States and parts of Canada are documented and related to latitude, geological factors, and intensity of the recorded geomagnetic field variations.

Ionospheric disturbances: Evidence for the contraction of the plasmasphere during severe geomagnetic storms

Continuous measurements of the ionospheric total electron content (TEC) provide a parameter well suited for the study of F-region disturbance effects. In this investigation, five cases of large and rapid drops in TEC observed near the sunset period are interpreted as being due to the contraction of the plasmasphere to L values less than 3. A model calculation is performed for the specific case of 1 November 1968 using simultaneous Alouette I data to define the position and magnitude of the ionospheric trough. The results indicate that the motion of a deep trough across the ray path from a geostationary satellite to an observing station can cause drastic changes in the measured amounts of Faraday rotation and therefore in the derived values of TEC. All of the TEC data sources available for the five events are then examined in an attempt to describe more completely the latitude dependence of the effects. It is suggested that during severe geomagnetic storms, the large and rapid decays in TEC during the 18–21 LT period to values significantly below normal can be used as a criterion to determine the approximate latitudinal extent of the contracted plasmasphere.

Effects of an electric field on the dynamical behavior of the ionospheres and its application to the storm time disturbance of the F-layer

The effects of an eastward electric field on the ionosphere are examined. Equations of motion for ions and neutral gas coupled with ion continuity equations and heat conduction equations are solved as a time dependent problem. Then the results of the calculation are compared with observed results obtained by Evans during a magnetic storm. It is proposed that the anomalous evening increase of N m F2, which occurs during a severe geomagnetic storm, can be interpreted in terms of the effects of an eastward electric field. By arguing detailed structures of the anomalous evening increase of N m F2 we show that an eastward electric field of the order 5–8 mV/m can be responsible for this phenomenon.

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.

Whistler Propagation and Geomagnetic Activity

The relationship between whistler propagation and geomagnetic activity is examined. The most reliable data are available for winter nights at middle and high latitudes. Under these conditions, whistler activity reaches a maximum at a certain value, Kopt, of the planetary magnetic index Kp. Kopt varies progressively with ‘invariant latitude’ (derived from McIlwain's L parameter), being about 4 at invariant latitude 45°, and 1 at 68°. For Kp>Kopt, whistler activity rapidly decreases. Possible mechanisms producing this effect are discussed. At low latitudes, enhanced whistler activity follows severe geomagnetic storms by 1–2 days. This delayed effect is not evident at higher latitudes.The advantage of using invariant latitude rather than geomagnetic latitude in studies of this kind is stressed. The parameters ‘whistler rate’ and ‘percentage occurrence’ are found to be equally good indicators of whistler propagation conditions.

Correlation of an auroral arc and a subvisible monochromatic 6300 A arc with outer-zone radiation on November 28, 1959

During a severe geomagnetic storm on November 28, 1959, two Geiger tubes on satellite Explorer VII (1959t) found anomalies in the outer radiation zone at an altitude of about 1000 km which appear to be correlated in space and time with optical emissions from the atmosphere beneath. Very intense narrow zones of radiation were detected over a visible aurora during one pass. The radiation in three such zones was harder toward low latitudes. On three subsequent passes the radiation zone was deduced to be over a subvisible 6300 A arc, whose brightness diminished as the radiation zones became less intense. The correlation is discussed.

Solar cosmic rays and soft radiation observed at 5,000,000 kilometers from Earth

During the period Mar. 27 to Apr. 6, 1960, the integrating ionization chamber and Geiger counter in Pioneer V detected solar cosmic rays and some soft- radiation effects associated with a high level of solar activity. The space probe was 5 x 10/sup 6/ km from the earth, approximately in the plane of the ecliptic, and located somewhat behind the sunearth radius toward the sun. The solar activity was associated with McMath plage region 5615 and was characterized by numerous flares of all sizes, large loops and surge prominences, and strong emission over a wide range of frequencies. On Mar. 31 at 0800 UT, a severe geomagnetic storm began on earth accompanied by major earth-current disturbances, a complete blackout of the North Atlantic communications channel, and auroral displays. At the same time, a large Forbush decrease occurred in the galactic cosmic radiation. An intense series of balloon flights was conducted to record the counting-rate increases at high altitudes due to solar cosmic rays and auroral x rays. Explorer VII showed substantial changes in the radiation belts and detected the solar cosmic rays. The observations of Pioneer V are summarized and compared to the findings of Explorer VII for the samemore » period. (B.O.G.)« less

Giant Sunspot and Geomagnetic Storm

A VERY large group of sunspots of 'bipolar' type crossed the spot disk during January 16-29. At central merdian passage on January 22·7 U.T., the middle of the group passed 28° above the centre of the disk. Two big spots were the chief components, will their centres separated 12° in longitude, or approximately 85,000 miles. The aggregate area of the spots for several days was about 2,300 millionths of the sun's hemisphere, or about 55 times the cross-section area of the earth. Greenwich data show that sunspots of this order of size are very likely to be associated with geomagnetic storms beginning within the time-limits of two days before and four days after central meridian passage of the spot group. A severe geomagnetic storm did, in fact, occur, commencing abruptly on January 24 at 18h. 28m. U.T. The disturbance continued with even increased vigour to the night of January 25-26. Displays of the aurora were widely seen on both nights in Great Britain. Provisional overall ranges recorded at Abinger of the earth's magnetic elements (kindly communicated by the Astronomer Royal) are as follow: D, 1° 29' ; H, 660 γ ; V, 490 γ. Long-distance radio-communication was seriously affected during the period of the geomagnetic storm.