Auroral Belt Research Articles

OGO-A magnetic field observations

This paper summarizes the new findings that have come from the intial study of the OGO-A fluxgate magnetometer measurements between 4 and 24.5 RE (earth radii). These include the following: (a) A model magnetic field profile of the cross-sectional structure of the bow shock is derived in terms of the sharpness of the interface, the rise time, and the total time interval occupied by a field pile-up at the shock. Using a simple model to derive the velocity of shock movements, these times are converted to three thickness dimensions roughly of the order < 20, 70, and 250 km, which emphasizes the need for strict definition of the meaning of ‘thickness’ in collisionless shock theories, (b) Superimposed on the average shock structure, (a) above, two classes of field oscillations are frequently observed: coherent circularly polarized waves with frequencies typically between 0.5 and 1.5 cps in the satellite reference frame, and higher frequency fluctuations, >7 cps, which are unresolved by the measurements and whose identity is not known. The coherent oscillation is identified as being in the whistler mode, exists in the form of wave packets, and usually shows a sharp upper frequency cut-off in power spectrum analysis, (c) A series of bow shock crossings during the main phase of the April 18, 1965, magnetic storm occur at an abnormally large distance from the earth principally as a consequence of the strong, 20–27 γ, interplanetary field that lowers the Alfvén Mach number to 1.5. The transition region magnetic field adjacent to the shock interface is exceptionally stable in contrast to a number of theoretical predictions and the typical shocks observed at high Mach numbers, (d) The magnetopause in the sunward hemisphere is most typically observed as a smooth transition over a dimension comparable to the ion cyclotron radius, (e) The correlation of negative bay onsets in the auroral belt with OGO-A observations on the night side of the earth supports more general morphological arguments that the onset originates within the closed magnetosphere or auroral ionosphere and is not dependent on being triggered by a sudden change in the solar wind plasma or field. The view is advanced that the onset results from short-circuiting effects in the ionosphere. (f) At middle latitudes between 5 and 10 RE near the midnight time sector the total field intensity is found to be considerably stronger than predicted by existing field models. This is believed to be caused by high plasma pressures near the equator at similar distances in the same time sector, (g) Near the magnetopause within the local time sector 4h30m-6h30m and geomagnetic latitudes ±15° the magnetospheric field intensity is generally found to be ≤Bt, the field intensity in the adjacent transition region. This condition and the behavior of the field gradient within the magnetosphere leads to the conclusion that a β ≥ 1 condition must persist over this sector of the outer magnetosphere beyond 11 RE. The consequences of the magnetopause being a boundary between two high β regions are noted in terms of boundary instability, plasma entry, and the possible existence of secondary shocks in the transition region. A similar, but not identical, condition may exist in the evening twilight local time sector.

Dynamics of auroral belt and polar geomagnetic disturbances

Using IGY-IGC all-sky camera films from many stations, the width and location of the auroral belt are determined. The positions of the edges are defined, at 15-min intervals, for different times of the day and compared with the Q-index of magnetic activity on the night side of the Earth.The dynamics of the auroral belt and the general morphology, including the rate of displacement of the northern and southern edges, the change in the belt width and the asymmetry of aurorae on the night-day and morning-evening sides of the Earth, are discussed in connection with variations in the geomagnetic activity. The probability of a discontinuous auroral belt during weak magnetic disturbances is considered. The auroral distribution around midnight, during magnetically calm periods, is analysed and it is shown that, during both the IGY and IQSY, aurorae always appear at 70° geomagnetic latitude. Variations in the azimuths of extended forms, for each of the 24 hr, are obtained and it is shown that the daily variations of the azimuths are determined by the shape of the auroral belt.The decrease of asymmetry of the auroral belt with increasing magnetic disturbance results in variations of the observed azimuths.A global schematic diagram, to illustrate the development of an auroral substorm, which is a generalization of the scheme proposed by Akasofu for the night side of the Earth, is given.

Auroral Poles

The centred dipole geomagnetic poles and the eccentric dipole geomagnetic poles are established mathematical entities (Chapman and Bartels 1940). Here the related concept of the "auroral" poles is outlined. The relationship of the auroral pole to the maximum iso-aurore of the auroral zone as well as to the instantaneous auroral belt is indicated.

A proposed polar auroral radar system

The position of the auroral belt can be reliably determined through the use of an h.f. multifrequency backscatter sounder. Auroral echoes recorded at Thule show that aspect-sensitive auroral backscatter can be obtained from the south in the northern hemisphere. In comparing simultaneous Thule and College records, the auroral belt latitude data were found to be independent of the latitude of the auroral radar. It is proposed that one multifrequency h.f. backscatter sounder installed at the north geomagnetic pole could provide real-time information on the position of the entire auroral belt around the northern hemisphere.

On the correlation of optical and radio auroras

A comparison of simultaneous optical and radio auroral data obtained during 12 days in December, 1964, shows conclusively that the radio wave scattering belt includes the visual auroral belt. The optical data were obtained with the Alaskan network of all-sky cameras located between 65 and 80° north geomagnetic latitude. The multifrequency backscatter sounder operating at College was used to record the radio data. The excellent agreement that resulted leads us to conclude that the majority of the h.f. and low v.h.f. backscatter echoes originating in the E- and the F-regions at high latitudes are auroral echoes in the strict sense of the term. In the records shown the auroral belt was tracked from 65 to above 80° north geomagnetic latitude by a single sounder. It is concluded that the multifrequency h.f. backscatter sounder makes year-round observations of the auroral belt possible.

On the relationship of the aurora to non-great-circle high-frequency propagation

Non-great-circle propagation was frequently noted in the year's HF forward oblique soundings recorded at College from various sites during 1963 and 1964. No direction-finding equipment was available, so a statistical analysis was made to determine the type of sidescatter involved. The number of occurrences during several periods was a maximum at night. The excess propagation time on the Palo Alto to College path varied inversely with magnetic activity. A comparison of simultaneous College backscatter and Palo Alto to College off-path data showed that the locus of off-path sidescatter extended north of the ionospheric backscattering belts. Previous work has shown the backscatter and auroral belts to be closely related, so these results are interpreted as showing that the deviated modes were produced by sidescatter from the auroral belt.

Scientific Research in the Arctic: Norway

Norway has a special situation among the nations that are carrying out scientific research in the Arctic, since nearly half of the mainland is located north of the Arctic circle. Thanks to the warming effects of ocean currents, however, Norway is not an arctic country in the climatic sense of the word. Her fjords and harbours are open all year around even in the northernmost part of the mainland. The extreme northern location has given Norwegians great economic and scientific interests in the Arctic. The maximum zone of the auroral belt runs across northern Norway, thus making it possible to carry out polar geophysical research from stations on the mainland. This may be one of the reasons why Norway's scientific activity in the arctic area north of the mainland has included relatively little activity in cosmic physics and related sciences. Norway's first national effort in the exploration of the arctic seas took place in Viking times (A.D. 800-1000) when Iceland was settled, and Greenland and the American mainland (Vinland) were discovered. In the east, the arctic coast of Russia to the White Sea (Bjarmeland) was explored. The first reports of the Svalbard (Spitsbergen) island date from this period, when it was considered to be a part of Greenland. Interest in the Arctic by the Norwegians was renewed in the early part of the nineteenth century, when Norwegian seal- and walrus-hunting vessels explored the arctic seas from Greenland to Novaya Zemlya, discovering, for example, Franz Josef Land. At the same time, Norwegian trappers began exploiting the Svalbard area. In the summer seasons 1876-78 the government of Norway sent an oceanographic and biologic expedition to Iceland, Jan Mayen, and Spitsbergen. At the end of the century, Fridtjof Nansen accomplishing the first crossing of the inland ice of Greenland and by drifting with the specially constructed ship Fram across the polar basin (1893-96), opened a new and great era of Norwegian contribution to the knowledge of the Arctic. ...

Continuous Observation of the Auroral Belt by Means of Radio

THE purpose of this communication is to show that the southern edge of the auroral belt (the latitudinal range over which aurorae occur at a given time and longitude) can be reliably tracked by radio means. With the multi-frequency high-frequency system used at College, Alaska, the auroral belt can be tracked from overhead at 65° to above 80° north latitude. This wide coverage is possible because the high-frequency system utilizes ordinary ionospheric reflexion to extend the northern limit of detection, and normal E and F layer refraction to produce perpendicularity between the wave and the geomagnetic field. The very-high-frequency auroral radar, on the other hand, can detect only those aurorally associated scatterers located in the E layer near the relatively limited region where the line of sight is normal to the geomagnetic field.

Dynamic morphology of auroras

Simultaneous changes of auroral forms, brightness, and motions over the whole polar region are studied, using IGY all-sky camera records from widely distributed stations in eastern Siberia, Alaska, Canada and the northern United States. It is found that the auroral system centered in the midnight sector in the auroral zone repeatedly undergoes an expansion and subsequent contraction; during the maximum stage of the activity, the whole auroral system extends over a substantial portion of the darkened polar region. Such extensive auroral activity as a whole may be regarded as a single event, and is described in terms of the auroral substorm. The substorm has two characteristic phases, an expansive phase and a recovery phase. Characteristic auroral displays over the entire polar region during the substorm are described in detail. The basic physical processes involved for the auroral substorm are also discussed. Geomagnetic disturbances associated with the auroral substorm are also described in detail in terms of the polar magnetic substorm, and it is shown that both the auroral substorm and the polar magnetic substorm are different aspects of the manifestation of a large-scale plasma motion in the magnetosphere. The distribution of the aurora for different degrees of the geomagnetic activity is also discussed in terms of the auroral belt. It is shown that the center line of the auroral belt moves greatly with respect to its average location (namely the auroral zone), depending on the degree of the magnetic activity.

The latitudinal shift of the auroral belt

The term “auroral belt” is proposed to signify the region which is occupied by the aurora at a particular instant. It is shown that the center line of the auroral belt moves greatly with respect to its average location (namely the auroral zone), depending on the degree of the magnetic activity. When the magnetic activity is very low ( ΣK p 1 ̌ 0 ) for 24 hours or more, the center line of the auroral belt in the midnight sector shifts to geomag. lat. as high as 75°. A moderate magnetic activity ( ΣK p = 20 ~ 30) places the center line of the belt in the vicinity of the auroral zone in the midnight sector. For a short period (of the order 30 min) during great magnetic storms, the belt can be located along geomag. lat. as low as 50°.

Geographic distribution of spreadFin the Arctic

Hourly IGY data have been analyzed to determine the diurnal and seasonal variation of spread F. Results for July and September 1957 are shown on maps. A zone of maximum occurrence of spread F exists that coincides with the auroral belt. Two maxima are found in the course of a day. One, called the ‘traveling’ maximum, has its center approximately at the point where the midnight meridian crosses the auroral belt. This maximum moves along the auroral belt in the course of a day. A second, called ‘permanent’ maximum, is over the Canadian archipelago and remains there during the day. These two maxima explain all the diurnal and seasonal variations found in the Arctic.

The absorption effect in the arctic during a severe ionospheric storm

A complete set of ionospheric data for the northern hemisphere has been analyzed for September 11–14, 1957 during a period when a severe ionospheric and magnetic storm occurred. Synoptic charts were constructed at three hour intervals for f min, f° Es and f° F2. The analytical method used allowed the transformation of blackout data into f min data, so that reliable charts of equal f min could be obtained. About 20 hours after the occurrence of a solar flare, f min started to increase over the polar cap. With time, these values increased steadily to about 7 mc/s with highest values near the pole. This increase—leading to blackout conditions at several inner Arctic stations—lasted for somewhat over 24 hr (Polar Cap Absorption—PCA). At the time of the Sudden Commencement, a steep gradient of f min occurs as a ring along the auroral zone. This gradient of 3–4 mc/s is confined to a width of a few hundred kilometers. After the Sudden Commencement, i.e. during the main phase of the storm, f min decreased over the inner Arctic so that normal values of about 2 mc/s were found 8 hours after the SC. However, a ring of high values prevailed during the main phase, first along the auroral belt, then moving southward towards a dip angle of about 71°.

A theory of the origin and geomagnetic control of two types of high latitude ES

Observations of night-time E s occurrence from four stations near the southern auroral zone are analysed. A major peak in the diurnal curves for the percentage hourly occurrence of ƒ type E s is identified with the peak of occurrence of Auroral Belt type E s discussed by Penndorf and Coroniti for the northern auroral zone. It is found that the probable low lattitude extent of this peak is approximately 50 geomagnetic. A theory is proposed which attributes the occurrence of Auroral Belt type E s and Thule type E s to two separate processes, both of which involve the entry of electrons into the ionosphere under the control of the geomagnetic field. The theory shows that Auroral Belt type E s may be caused by electrons from a region of trapped particles the extent of which can be defined by the extent in geomagnetic latitude of the night-time peak in ƒ type E s occurrence. Thule type E s may be caused by an influx of electrons into the ionsphere from, a stream of particles approaching from the direction of the sun. The deflections of the peak of occurrence of ƒ type E s from the midnight magnetic meridian, and of the Thule type E s from the “geomagnetic noon” meridian, are a function of the mean energy of the electrons and the curvature and intensity of the lines of force in the geomagnetic field.

On the atmospheric circulation at 500 mb in the auroral belt

On the atmospheric circulation at 500 mb in the auroral belt

Auroral belt of October 24th-25th, 1870

Auroral belt of October 24th-25th, 1870