Magnetic Sudden Commencements Research Articles

Variability of transionospheric signal time delay at subauroral latitudes

Faraday observations were conducted at Anchorage, AK ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">61.04\deg</tex> N, <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">149.75\deg</tex> W), utilizing beacon transmissions from a geostationary satellite during the period just following the minimum phase of solar cycle 21, and, for comparison purpose, during a short period following the maximum phase of that same cycle. Near the minimum phase average maximum monthly values of total electron content (TEC), which is proportional to transionospheric signal time delay, were below <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">15 \times 10^{16}</tex> el m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> , while individual daily maximum values never exceeded <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20 \times 10^{16}</tex> el m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> . Near the maximum phase TEC average monthly values were larger by a factor of two. Seasonal and day-to-day variabilities were observed. Unique representation of the data has permitted the study of day-to-day variability of TEC. For example, near minimum phase during all seasons the TEC structure appears uniform from day to day during the buildup and decay phases of the local ionosphere. During the maximum and minimum of the diurnal phase, the TEC structure variability is seasonally dependent. During periods of magnetic sudden commencements, which rarely occurred in the observation periods, significant positive phase responses of TEC did not materialize near solar minimum, but were correlated near solar maximum.

Simultaneous observations of Pc 5 pulsations by VHF radars spaced 10 hours in MLT

Long‐period Pc 5 pulsations have been recorded simultaneously by VHF radars operating in Scandinavia and New Zealand. The radars are in opposite hemispheres and are spaced about 10 hours in local time. Those pulsations analyzed are either of the “quasi‐regular” variety or take the form of damped trains that often follow magnetic sudden commencements. An estimation of equatorial plane ion mass densities shows azimuthal variations attributable to several possible causes, including the presence of localized density gradients or a significant proportion of heavy ions. A transient‐type pulsation shows the effect of the two radars being directed inside and outside the plasmapause, and a longer‐lasting event suggests a monochromatic drive of different period on the dawn and dusk flanks of the magnetosphere.

Geomagnetic field fluctuations at synchronous orbit 2. Radial diffusion

Power spectra of geomagnetic field variations measured at synchronous equatorial altitude (geomagnetic shell parameter L ∼ 6.6) in the magnetosphere are used to calculate the time dependence of the radial diffusion coefficient for particles in the radiation belts. The diffusion coefficients calculated are mainly applicable for relativistic electrons. The magnitudes of the derived diffusion coefficients using data only from local day observations are consistent with those reported from analyses of most particle observations and thus are slightly larger than those derived from magnetic sudden commencements. They are consistent with the diffusion coefficients calculated from power spectra of ground‐based geomagnetic data measured near L = 4.

Periodic covariation of radiowave absorption, Earth currents, and other associated phenomena in the auroral zone

Introduction. Among the recent attempts at classifying radiowave absorption in the auroral zone, the following works can be cited: (a) Bailey's [1957] forward scatter observations leading to the isolation of the polar cap event, (b) the investigation by Brown et al. [1961] of rapid onset absorption events associated with the magnetic sudden commencements, and (c) Gustafsson and Ortner's [1962] analysis of flash onset (coinciding with marked changes in luminosity) and rapid recovery events. One significant aspect of these investigations is that these classifications are primarily based on the intrinsic features of the absorption event, but nevertheless are found to indicate different physical causations. Periodic phenomena. We wish to point out in this note another category of events that are characterized by a regular periodicity. Such events appear to have been noted by many radiophysicists in the auroral zone during the past several years of extensive coverage of the zone [e.g., Ziauddin, 1960] but they do not seem to have received adequate attention. Events shown in Figures 1 and 2 were chosen from among the two dozen that were noted during a six-month period (October 1962 through April 1963) in the charts registering the cosmic radio noise level at 27.6 Mc/s. The stations used for this study are Ft. Yukon (66.77deg;N geomagnetic), College (64.6°), and Healy (63.6°). The records for Anchorage for the time of the events in Figures 1 and 2 were also made available by D. E. Bucknam of the CRPL. Confining our scrutiny to variations with distinct sine-wave shape and with periods less than about 10 minutes, we found that the most commonly encountered ones (at the rate of one in a few days) showed periods close to 5 minutes. The criterion was that at least 3 full cycles should be obvious. The ones shown in Figures 1 and 2 are not typical but were noted for the sustained repetitions lasting many cycles. Those with a period of 2 minutes were considerably rarer. Whatever the period, they could be identified only at two (of the three) neighboring stations, with marked deterioration or absence of the pattern in the third. The events shown in the figures were observed at College and Healy. Only traces of the periodic pattern were noticeable in the records from Ft. Yukon, and the pattern was totally absent in the records from Anchorage, even though the absorption events were otherwise somewhat similar. In all the instances for which data were available, the earth current and the magnetogram traces exhibited very similar variations. The intensity of 5577 A emission from the zenith (made available by C. S. Deehr) also covaried with the same periodicity. Looking at the all-sky camera pictures of the visual aurora, we found that the displays were stationary in location, with a suggestion of periodic changes in intensity.

Diurnal Variation in the Amplitude and Frequency of Magnetic ‘Sudden Commencements’ and ‘Sudden Impulses’ at Quetta

The study of diurnal variation in the frequencies and amplitudes of magnetic sudden commencements (SC /sup s/) and sudden impulses (SI/sup s/) recorded at gretta (lat. 30 deg 11'N; long. 66 deg 57'E), during the period 1953--1962 indicates that SC/sup s/ and Si/sup s/ are more frequent near the sunrise and sunset hours with maxima around 08/sup h/ and 18/sup h/ local time (L.T.). This does not conform to the curves obtained by Newton and Ferraro & others which exhibit a maximum at 13/sup h/ J.T. and two minima at 08/sup h/ and 20/sup h/ L.T. respectively. The mean hourly values of the amplitude y of SC/sup S/ g Sl/sup S/ do not seem to exhibit any marked local time effect. The daytime enhancement in y and the similarity between the diurnal variation of amplitude of SC/sup S/ & SI/sup S/ and Sq-variation found at Huancayo, Kodaikanal, and Alibag is also absent. On the other hand a sharp minimum in the amplitude curve around 07/sup h/ L.T. gives weight to Ferraro's contention that during the day-lit hemisphere the higher conductivity of the ionosphere tends to shield the station below from the external magnetic field produced by corpuscular stream. No seasonalmore » variation in the monthly frequency of SC/sup s/ & SI/sup s/ is discernible but the mean monthly amplitude curve exhibits marked enhancement in the size of SC/sup s/ & SI/sup s/ during the summer months. The curves for the annual mean number and yearly mean amplitudes of SC/sup s/ & SI/sup s/ and yearly mean Sq-range show striking similarity to the curve for the annual mean sunspot numbers. A linear relation of the type y = 11.6 + 0.097s between the amplitude y and the relative sunspot numbers s was worked out for Quetta.« less

A study ofF2-layer effects as observed with a Doppler technique

Changes in the height and shape of the ionosphere produce, at a receiver, variations in the frequency of waves emitted from a stable sender. WWV-10, WWV-15, WWV-20 and WWVH-10 have been recorded at Boulder on slow-moving magnetic tape. By rapid playback the Doppler frequency is converted into an audio frequency and the spectrum is analyzed by conventional techniques. Records obtained by this technique are presented to illustrate the phenomena observed during magnetically quiet and disturbed periods. Experimental effects associated with solar flares and magnetic sudden commencements are presented. The frequency dependence of the frequency variations is shown to give information about the height location of the associated ionospheric effects.

Ionospheric Effects associated with the Solar Flare of September 28, 1961

THE purpose of this communication is to direct attention to some ionospheric phenomena observed during the period of the above flare and during the early part of the associated geomagnetic storm that ensued on September 30, 1961. The flare, which was of importance 3+, was first detected around 2200 U.T., and the intensity of the Hα radiation reached a peak near 2224 U.T. all on September 28. The magnetic sudden commencement over North America occurred between 2108 and 2109 U.T. on September 30.

Geomagnetic disturbances and 5 kilocycles per second electromagnetic radiation

A comparison is made between variations of the geomagnetic field and bursts of 5-kc/s radio noise recorded at Camden, N. S. W., between June 1958 and October 1959. During this period 8 noise storms were recorded. All occurred during the main phase of a geomagnetic storm, beginning on the average 3 hours after the start of the main phase. Of 97 isolated noise bursts of less than 4 hours' duration, 43 were associated with positive bays in the record of the magnetic H component. No consistent delay between the noise burst and the bay was observed. Noise bursts were not in general associated with geomagnetic micropulsations of less than 1-minute period, although some correlation with quasi-sinusoidal magnetic variations with periods between 10 and 60 minutes was noted. During the period of the observations 30 magnetic sudden commencements were followed by noise bursts or storms after an average delay of 8.5 hours. On four occasions, a noise burst started immediately after a sudden commencement occurring between 1200 and 1800 hours GMT.

Correlated micropulsations at magnetic sudden commencements

By the use of very large loops of wire, many square miles in area, nearly simultaneous micropulsations were recorded in Arizona and New Jersey during the initial phase of a magnetic storm. Evaluation of the two records (15 minutes' recording time) by cross correlation showed a time difference in arrival of common signals of 2 to 3 seconds. A similar difference was observed for the sudden commencement itself. The method used is simple and relatively unaffected by interference. The maximum sensitivity, 1.7×10−4 gamma sec−1 mm−1, could be used only during magnetically quiet periods. At two other sudden commencements, a remarkable resemblance was found in the fine details at the variations present. The over-all study includes effects from both natural and man-made causes. Another paper will discuss the man-made effects.