Geomagnetic Storm Period Research Articles

Relationship between midlatitude thermospheric winds and the time rate of change of DSt

Nighttime thermospheric winds and temperatures over Fritz Peak Observatory (39.9°N, 105.5°W) have been determined from the Doppler shifts and line profiles of the (OI) 15876‐K (6300 Å) line emission. The peak winds measured during several geomagnetic storms are shown to be linearly related to the time rate of change of the equatorial ring current parameter (d(DSt)/dt). During these geomagnetic storm periods the time variation of the measured meridional winds are also strongly correlated with the time rate of change of DSt although the observed winds at midlatitudes may lag variations in this parameter by several hours. The energy input derived from d(DSt)/dt is consistent with the high latitude energy input that is necessary in a thermospheric dynamic model to bring the calculated meridional winds into agreement with the observations. These results suggest that d(DSt)/dt is a useful index for determining the magnitude and time history of the high latitude energy input during both quiet and disturbed geomagnetic conditions for use in global thermospheric dynamic models.

Multiple‐satellite studies of magnetospheric substorms: Distinction between polar magnetic substorms and convection‐driven negative bays

It is generally believed that long intervals of intense (500–1000 γ) negative auroral zone bays are caused by magnetospheric substorm occurrences at a high repetition rate. Since individual substorm phases may then overlap in time and occur within limited local time sectors, it may be difficult to identify their characteristic signatures. In this paper we have examined ground and multiple‐satellite recordings during an ∼5‐hour interval of persistent auroral zone activity in a systematic search for substorm expansion signatures. It appears that this enduring activity started as a rather weak substorm which thereafter developed into strong magnetospheric activity driven by a continuously southward interplanetary magnetic field (IMF). During this period there were no clear mid‐latitude positive bays, and pulsation recordings from different local time sectors showed only one instance of weak Pi 2‐like variations. The associated high‐latitude equivalent ionospheric current pattern is consistent with a convection system, and we refer to this activity as the convection bay. Following the plasma sheet and ground signatures of substorm recovery early in the convection bay, there was no definite indication of substorm expansions other than this weak magnetic pulsation in any recordings during the next ∼3½ hours until a clear substorm emerged. This substorm onset, which was associated with a not particularly strong auroral zone bay, was accompanied by the full complement of substorm expansion signatures. At this time a minimum plasma sheet thickness of only a few tenths of an earth radius was observed in the region where a neutral line appears to form during substorms. Prior to this onset, the plasma sheet was relatively thick, and it seems that the neutral line was located well tailward of the Vela orbit (r ∼ 18 RE). This event and other similar observations suggest that continuous auroral zone activity during southward IMF, such as a geomagnetic storm period, may be driven by a steady state magnetospheric convection. The long absence of clear substorm onset signatures may indicate that the nonsteady enhancements of convection associated with localized substorm expansions then make only small contributions to the total convection pattern. Similarly, the ground magnetic disturbances may then be caused mainly by Sqp currents and enhanced currents due to strong particle precipitation rather than by substorm‐associated current wedges. During this quasi‐steady state the tail reconnection rate appears nearly to balance the dayside merging rate, allowing most energy extracted from the solar wind to be released continuously rather than to be first stored in the tail. It seems that a large‐scale substorm expansion may be triggered if this convection mode is perturbed, for instance, by a significant reduction of the imposed convection electric field.

Temporal variations in slant total plasmasphere content and their relationship to the ring current intensity and the plasmapause

The time dependence of the total electron content of the plasmasphere, as determined from the radio beacon experiment on the ATS 6 satellite, is studied for an 11-month period in 1974–1975 (Fritz, 1976). Close temporal associations are often observed between increases (decreases) in the ring current intensity as measured by the ring current index Dst and decreases (increases) in the plasmasphere content. The recovery of the plasmasphere after six geomagnetic storm periods is examined; the deduced filling rate is found comparable to the total flux tube filling rates reported by Park (1973) from whistler studies. The rate of the equatorial plasmasphere filling and the rate of decay of the ring current intensity appear to be related, at least for the six events examined. Finally, an empirical relationship between the total plasmasphere content and the equatorial plasmapause position at 1200 LT is derived.

Direct measurements of nighttime thermospheric winds and temperatures, 2. Geomagnetic storms

A high-resolution Fabry-Perot spectrometer has been used at Fritz Peak Observatory, Colorado (39.9°N, 105.5°W), to determine the nighttime thermospheric temperatures and winds during geomagnetic storm periods from the line profiles and Doppler shifts of the (O I) 15867-K (6300 A) line emission. Data obtained during four geomagnetic storm periods in 1974 are presented: March 20–22, October 15–20, June 29 to July 8, and April 16–27. The data are compared to temperatures determined from the Ogo 6 global empirical model of neutral composition and temperature and to winds calculated from a semiempirical dynamic model of the neutral thermosphere. The results show considerable variations in the dynamic character of the thermosphere which are also related to the intensity of geomagnetic activity. During geomagnetic storms the nighttime equatorward winds are greatly enhanced from their quiet time values with a maximum measured velocity of 640 m s−1 during a Kp = 9 storm. The zonal winds generally develop a westward component relative to the geomagnetic quiet zonal winds. During intense geomagnetic activity the zonal winds are westward at 100–200 m s−1 in the early evening hours, flowing in the direction of magnetospheric convection and opposite of model predictions. The meridional winds measured south of the site are generally smaller than those measured to the north. Sometimes the meridional winds measured north and south of the station flow in opposite directions. The nighttime neutral gas temperatures are observed to increase from their geomagnetic quiet values during the storm and are in general concurrence with the Ogo 6 model predictions.

Repeated sharp flux dropouts observed at 6.6REduring a geomagnetic storm

A number of repeated rapid flux dropouts have been observed at 6.6 R/ subE/ by the NOAA low-energy proton detectors on board the ATS 6 satellite during the July 4--6, 1974, geomagnetic storm period. These rapid flux changes are caused by the fact that the outer boundary of the trapped radiation region moves back and forth past the satellite. Although a tilting field line configuration can cause the boundary to pass the satellite, as has frequently been reported in the literature, the boundary is shown to be distorted by a large surface wave traveling eastward around the earth. The maximum velocity of the wave was observed to be approx. =40 km/s. (AIP)

Satellite observations of auroral substorms

Satellite photographs of auroras obtained from a polar-orbiting U.S. Air Force satellite were used to study auroral substorms in the premidnight time sector during and after a geomagnetic storm period. The photographs provide a unique means of seeing in detail the interrelationship between discrete, continuous/diffuse, and polar cap auroras during a period of frequent occurrence of substorms. The distributions of discrete auroras seen in the satellite photographs further substantiate the Feldstein auroral oval concept and the Akasofu auroral substorm concept. However, the photographs suggest that the continuous aurora and polar cap auroras have not been adequately accounted for in the present concepts of auroral substorm morphology.

E s-q layer at huancayo during the March 1970 geomagnetic storm

The paper describes a comparison of vertical electron drift in the F-region ( V z ) measured by VHP incoherent scatter radar at Jicamarca with the corresponding variations of geomagnetic horizontal field ( H) and the maximum frequency reflected from The E s layer ( E s ) at Huancayo during the geomagnetic storm period 7–9 March, 1970. The V z is generally upward during the daytime at the equator, but during 7–9, March, 1970, V z was negative for brief periods associated with negative bays in H. These periods of abnormally low or of downward V z correspond closely with the period of complete disappearance of the q type of E s layer. The magnetic bays associated with the intensification of ring current do not affect the equatorial E s - q and it is only the negative bays in H at the equator due to the ionospheric current flowing westward, that cause sudden disappearance of E s − q. It is suggested that the q type of E s is due to cross-field instability created in the electrojet region due to interaction of northward magnetic field and vertical upward Hall polarization electric field when the plasma density gradient is upward. The sudden disappearances of E s − q are due to the reversal of the horizontal electric field in the equatorial ionosphere and thereby due to the reversal of the equatorial electrojet currents. These reversals of electric field may be due to the imposition on the normal S q field of another westward electric field.

Calculated and observed features of stable auroral red arcs during three geomagnetic storms

Satellite electron temperature and density data are used to calculate the structure of several stable auroral red arcs (SAR arcs) according to the thermal conduction model of the arc. The calculated lambda 6300 emission rates are compared with ground-based photometric observations taken at the same time and in the vicinity of the satellite crossings of the arcs. The SAR arcs analyzed include a range of lambda 6300 intensities, geographical locations, and times during the associated geomagnetic storm. In addition, satellite data were obtained at different altitudes over and within the SAR-arc region. Enhanced electron temperatures within or on the equatorward edge of an electron-density depression are common features of all the SAR arcs examined. There is general agreement between the calculated and observed lambda 6300 emission features for SAR arcs observed during the geomagnetic storm periods of Oct. 29 to Nov. 2, 1968, May 14-15, 1969, and Mar. 8-9, 1970. For these SAR arcs, thermal conduction from the magnetosphere alone is sufficient to excite the lambda 6300 emission to its observed intensity.