Intense Magnetic Storms Research Articles

A global view at the polar region on 18 December 1971

The ISIS-2 scanning auroral photometer surveyed the polar region during three successive passes on 18 December 1971, at times when Kp values were still high due to an intense magnetic storm which began on 16 December. Two very bright (IBC III) auroral substorm patterns were seen to correspond to rather weak magnetic substorms (about 300 γ in magnitude). A large spiral auroral pattern, with intensity of the order of 100 kR and a size of about 1300 km, was present in the polar cap; it gradually decreased in size and intensity during the interval 0200–0600 UT. A region of enhanced 3914 emission was present in the noon sector of the auroral oval between 0200 and 0400. The presence of the diffuse auroral belt is also evident at all local times during this period, extending down to about 61° corrected geomagnetic latitude in the midnight sector.

Ogo 5 measurements of the plasmasphere during observations of stable auroral red arcs

Hydrogen ion concentration measurements by OGO 5 in plasmasphere during intense magnetic storms accompanied by stable auroral red arcs

Low Latitude Aurorae and M-arcs

IT has been known for a long time that the main emission of low latitude aurorae is that of atomic oxygen at 6300 A, so that low latitude aurorae are always red1. Some years ago Barbier2 and Roach and Roach3 reported low latitude arcs in emission at 6300 A during intense geomagnetic storms, and called them M-arcs. This paper demonstrates that the M-arc is simply an extreme manifestation of the low latitude aurora during the more intense magnetic storms, because the red oxygen emission (6300 A) develops a more pronounced peak in latitude with increasing magnetic disturbance (Dst).

Measurements of radio star and satellite scintillations at a subauroral latitude

Observations of two radio stars, Cygnus A and Cassiopeia A, and of two satellites, Cosmos I and Transit 4A, have yielded data on lower and upper atmospheric irregularities. The frequencies studied have included 20 Mc, 40 Mc and 54 Mc for satellite transmissions, and 30 Mc to 3000 Mc for radio star signals. The antennas used have ranged from a dipole to a 150-foot parabola. The irregularities in refractive index in the lower atmosphere produce amplitude fluctuations up to several times average with a fading rate of 3 to 0.5 per minute while ionospheric amplitude fluctuations can increase several decibels, with rates from 1 to 60 per minute. Lower atmosphere scintillation rates increase as the elevation angle increases and generally disappear above <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5\deg</tex> of elevation. During intense magnetic storms, the ionospheric scintillation rate is a function of wavelength in the UHF range. Ionospheric scintillations are functions of both magnetic conditions and of the subionospheric latitudes of the irregularities; they are observed at zenith at the geomagnetic latitude of the Sagamore Hill Radio Observatory ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">54\deg</tex> ). Using spaced receivers, heights of single irregularities have ranged from about 100 to 600 km and representative sizes from about 0.1 to 4 km. Irregularities constitute the prime factor affecting the level, phase and angle of arrival of signals propagated through the auroral regions.

Measurements of Radio Star and Satellite Scintillations at a Subauroral Latitude

Observations of two radio stars, Cygnus A and Cassiopeia A, and of two satellites, Cosmos I and Transit 4A, have yielded data on lower and upper atmospheric irregularities. The frequencies studied have included 20 Mc, 40 Mc and 54 Mc for satellite transmissions, and 30 Mc to 3000 Mc for radio star signals. The antennas used have ranged from a dipole to a 150-foot parabola. The irregularities in refractive index in the lower atmosphere produce amplitude fluctuations up to several times average with a fading rate of 3 to 0.5 per minute while ionospheric amplitude fluctuations can increase several decibels, with rates from 1 to 60 per minute. Lower atmosphere scintillation rates increase as the elevation angle increases and generally disappear above 50 of elevation. During intense magnetic storms, the ionospheric scintillation rate is a function of wavelength in the UHF range. Ionospheric scintillations are functions of both magnetic conditions and of the subionospheric latitudes of the irregularities; they are observed at zenith at the geomagnetic latitude of the Sagamore Hill Radio Observatory (54°). Using spaced receivers, heights of single irregularities have ranged from about 100 to 600 km and representative sizes from about 0.1 to 4 km. Irregularities constitute the prime factor affecting the level, phase and angle of arrival of signals propagated through the auroral regions.

Radio propagation measurements using the Explorer VI satellite

This paper presents the results of an experiment to measure the electron density above the ionosphere. The experiment utilized two coherent signals, one VHF and one UHF, transmitted from the Explorer VI satellite. The evidence of our data is that the electron concentration in the vicinity of the satellite, 18,000 km above the earth, was unusually high, of the order of 104 electrons/cm3. However, the results were influenced by unexpectedly high correction factors and possibly by an intense world-wide magnetic storm that occurred during the measurement period.

The southward shift of the auroral zone during intense magnetic storms

The southward shift of the auroral zone during intense magnetic storms

Cosmic-Ray Intensity Following a Solar Flare

Continuous records of the cosmic-ray intensity measured by a heavily shielded, high pressure ion chamber, were obtained during and following an intense solar flare (of importance 3+) and a subsequent intense magnetic storm. No increase in $C\ensuremath{-}R$ intensity was observed at or near the time of the flare, such as have been observed by others on similar occasions. At the peak of the magnetic storm there was a decrease of about 1.5 percent in $C\ensuremath{-}R$ intensity, with full recovery requiring 2 or 3 days. Unshielded $C\ensuremath{-}R$ telescope readings of low accuracy are included. These indicate that there were no prolonged alterations in the intensity (without shielding) of magnitude much greater than 14 percent associated with either the flare or the storm.

The radial stability of the geomagnetic ring‐current (Second paper)

An exact solution is given of an idealised problem, approximately treated by S. Chapman and the writer in an earlier paper dealing with the radial stability of a geomagnetic ring, in which the ring is replaced by an electrically neutral uniform cylindrical current‐sheet whose axis is parallel to the magnetic axis of the Earth. The conclusions of that paper are substantiated, and though it is shown that the sheet is in reality unstable, the rate at which instability sets in is so slow that the sheet may well be considered to be stable. This rate is, moreover, comparable with the rate of decrease of intense and moderate magnetic storms.

Solar Flares and Magnetic Storms

Data on specially intense solar flares from spectroscopic records (including recent spectrohelioscope observations) have been discussed in connection with the occurrence of magnetic storms. Certain features, including their great area and possible characteristics of spectrum, distinguish these very intense flares when they occur. Future observations should aim at including quantitative data to serve as an index of intensity. A geophysical measure of intensity should also be explored. Far more often than could occur by chance, an intense flare is followed about a day later by a great magnetic storm, provided that the flare is not farther than about 45° from the centre of the disk. Assuming that a corpuscular stream is newly ejected at the time and place of an intense solar flare, the mean effective time of travel of the stream from Sun to Earth is about 26 hours, the most intense magnetic storms giving a shorter travel-time of about 20 hours. The distribution on the Sun's disk of flares preceding great magnetic storms is examined and suggests certain features of the corpuscular streams involved.

The Sunspot‐Group Associated with the Magnetic Storm of September 18, 1941

If all intense magnetic storms could be as definitely related to a sunspot‐group as the one that began on September 18 about 04h 20m GMT, geophysicists would feel that one of their major problems was solved.The group associated with this storm was first seen on the east limb on September 10 and was fully in view. the following day. It then consisted of two spots of moderate size in latitude 12° north. These developed rapidly into a large active group with complex magnetic polarities. Groups of this type are generally considered most likely to cause storms. By September 17, the spot‐group had attained an area of 2000 millionths of the Sun's visible hemisphere which places it among the largest of the present cycle, a group of area 1000 being considered “large.” Since the date was near the autumnal equinox the latitude of the center of the disk was at its maximum value of 7° north. The spot‐group thus crossed the meridian about 5° north of the central point. When the storm began the spot‐group extended from 10° to 25° of longitude (or from about 0.8 to 1.9 days) west of the central meridian.

Ionospheric Recordings During Magnetic Storm of March 1, 1941

Examination of automatic multifrequency ionospheric records obtained at the Kensington Experimental Station of the Department of Terrestrial Magnetism, Carnegie Institution of Washington, during the intense magnetic storm of March 1, 1941, reveals that the ionized regions of the Earth's outer atmosphere were greatly disturbed, and that the period of greatest disturbance corresponds to that of greatest magnetic activity.Commencement of the ionospheric disturbance was associated with the development of an ionized region between the normal levels of the E‐ and F‐regions. Virtual height of this region was 160 km at 06h 15m GMT. By 06h 30m its maximum ion‐density had increased threefold and its virtual height had fallen to normal E‐region levels at 130 km. Radio reflections from this region were returned by the normal process of magnetoionic double refraction. Both ordinary and extraordinary wave‐components were recorded, and reflection‐coefficients were not high. Apparently the condition was not one of simple sporadic E‐region ionization.

Systematic ionospheric changes associated with geomagnetic activity

Systematic changes of maximum electron‐density associated with geomagnetic disturbance have been difficult to isolate owing to the lack of sufficient homogeneous data. It is well known that intense magnetic storms are associated with marked ionospheric changes. The purpose of this investigation is to determine the nature of systematic changes of maximum ion‐density of F2‐layer which accompany geomagnetic disturbance of all ranges of intensity at several latitudes.Appleton, Naismith, and Ingram [see 1 of “References” at end of paper] have shown that at Tromsö and at Slough there are, associated with geomagnetic disturbance, characteristic variations in F2‐1ayer which change with season. Similar changes have been observed at the Kensington (Maryland) Experimental Station and described qualitatively [2]. In the present investigation we consider about two years of continuous observations made at the Watheroo and Huancayo magnetic observatories of the Department of Terrestrial Magnetism utilizing automatic multifrequency technique described elsewhere [3, 4]. A convenient quantitative measure, namely, deviation of daily average maximum electron‐density from normal, has been derived and compared to associated geomagnetic activity for each day.

The relation between solar prominences and terrestrial magnetism

It has been stated in a previous communication that a preliminary reduction of the Roman observations of prominences, observed on the sun’s limb by Tacchini, indicated that, in addition to main epochs of maxima and minima of prominences coinciding in time with those of the maxima and minima of the total spotted area, there are also prominent subsidiary maxima and minima. One of us has pointed out in a recent communication to the Académie des Sciences that a comparison of the frequency of prominences visible in each solar latitude with the frequency of the most intense magnetic storms, indicated that ( a ) magnetic storms classed as “great” by Ellis, and the greatest prominence activity near the poles of the sun occurred at the same time; and ( b ) that the curve of general magnetic activity was nearly the same as that of the prominences observed near the solar equator.