Bay-like Disturbances Research Articles

Complex Analysis of the Polar Substorm Based on Magnetic, Optical, and Radar Observations near Spitsbergen

A comprehensive analysis of the polar substorm registered by IMAGE network stations close to the poleward boundary of the auroral oval has been carried out with THEMIS, CLUSTER, and GEOTAIL satellites in favorable positions for analysis. The observation interval is characterized by a low level of geomagnetic activity. The polar substorm looked like three negative, bay-like disturbances and developed within an ordinary substorm registered at stations in the middle of the auroral zone half an hour before the polar substorm. Each bay-like disturbance of the polar substorm was accompanied by a poleward drift of aurora and westward electrojet, a Pi2 pulsation group, a surge of magnetic activity over the range of 0.1–10 Hz, and enhancement of electronic precipitations over Spitsbergen. The differences between the first activation and two subsequent ones in the aurora dynamics, ionospheric convection, and electronic precipitations are revealed. The disturbed zone longitudinal sizes and position in the magnetosphere are estimated. Possible causes of the substorm onset and disturbances associated with it are discussed. The results of the study expand the statistics on the polar substorm phenomenon and will enable insight into its nature.

Geomagnetically Induced Currents and Space Weather: Pi3 Pulsations and Extreme Values of Time Derivatives of the Geomagnetic Field’s Horizontal Components

The problem of estimating the time derivatives of the horizontal components of the geomagnetic field and forecasting the probability of the occurrence of perturbations that exceed a given threshold level (the over-threshold perturbations) arises in the applications concerned with the geomagnetically induced currents (GICs). In this work, we consider the temporal and spatial structure of the Pi3 pulsations with quasi-periods of 102 to 103 s during which the auroral and subauroral stations of the IMAGE network record over-threshold values in the derivatives of the meridional (along the longitudinal circle) BX component and latitudinal (along the latitudinal circle) BY component. The extreme |dBX/dt| values mainly develop against the background of the Pi3 pulsations with a complex frequency content, whereas the extreme |dBY/dt| values appear when the buildup (decay) phases of the bay-like disturbance associated with the evolution of a substorm coincide with the respective phases of the field of pulsations. The conditions under which the derivatives |dBX/dt| and |dBY/dt| reach their over-threshold values are studied for subauroral latitudes by the technique of superposed epoch analysis. The extreme values of the derivatives most frequently occur during the main phase of moderate magnetic storms or beyond the storm—during high substorm activity under the conditions of a negative vertical component of the interplanetary magnetic field. The probability of the occurrence of over-threshold values increases at high amplitudes of the Pi3 pulsations and depends on their spectral content. The problem of analyzing and forecasting the over-threshold |dBY/dt| perturbations is complicated by the fact that the scale of the perturbations is small along the lines of latitude and large along the meridians. This can result in GIC excitation in the North–South oriented electric power lines by the geomagnetic perturbations localized within a narrow band in longitude which can be missed during the measurements.

Relationship between the PC and AL indices during repetitive bay-like magnetic disturbances in the auroral zone

To study the relations of the polar cap (PC) magnetic activity (characterized by the PC index) to magnetic disturbances in the auroral zone (AL index) the behavior of 62 repetitive bay-like magnetic disturbances has been analyzed. It was found that the PC index, derived as a proxy of the geoeffective interplanetary electric field Em, starts to increase, on average, about 30 min ahead of the magnetic disturbance onset. Value of Em and PC∼2 mV/m seems to be necessary for development of the repetitive bay-like disturbances with peak AL exceeding 400 nT. Growth phase duration (the time interval between the start of PC increase and AL sudden onset) and intensity of magnetic disturbances in the auroral zone (AL max) highly correlate with the PC growth rate. The growth phase reduces to a few minutes, if the PC index suddenly jumps above ∼6–8 mV/m. The sharp development of Birkeland current wedge during expansion phase insignificantly influences the polar cap activity: the corresponding PC index increase does not exceed 10–20% of the PC value. It is concluded that the PC index may be considered as a convenient proxy of the solar wind energy input into the magnetosphere.

Daytime equatorial geomagnetic H field response to the growth phase and expansion phase onset of isolated substorms: Case studies and their implications

Observations are presented from the Indian magnetometer network (dipole latitude range 1.2°S to 13.5°N) of short‐lived (<1 hr) disturbances in the daytime equatorial geomagnetic H field associated with specific phases of isolated substorms. Three well‐documented substorms are examined here, of which the expansion onset of each is closely associated with sudden transitions of interplanetary magnetic field (IMF) Bz/By after a >40‐min interval of southward Bz. A positive baylike perturbation is found to prevail during the substorm growth phase, followed by a negative baylike disturbance starting precisely at the onset of expansion phase activity. The amplitude of the positive as well as negative bay‐type disturbance showed a clear‐cut enhancement at locations inside the equatorial electrojet belt when compared with stations away from the electrojet influence, indicative of a significant contribution of ionospheric currents to the bays. This pattern of response, which is found in two out of the three events studied, constitutes the first‐time evidence for the occurrence of equatorial H field perturbations related to the growth phase as well as the expansion phase onset for individual substorms. The H field perturbations are suggested to be signatures of prompt penetration by electric fields associated with rapid changes in magnetospheric convection brought about by the swift transitions in the IMF Bz, before shielding by the ring current becomes effective. In the third event, where the growth phase is weak and prolonged, there is no perceptible simultaneous disturbance in the equatorial H field, while the expansion phase onset is associated with a positive bay of very small amplitude (≤4 nT).

The effect of disturbance on the day-to-day variability of Sq

Summary. Other workers have recently shown that the apparent day-to-day variability of S, is appreciably reduced when days with bay-like disturbances are removed from the analysis. In this paper, it is shown that when these days are removed, there is only a small effect on the scale of the spatial coherence of Sq-ranges and therefore the regional scale of the correlation must be a real feature of S, range variability rather than an effect due to disturbance.

The sources of the polar cap and low latitude bay-like disturbances during substorms

Examination of the polar cap and low-latitude bays during substorms shows that there are two types of disturbances, DP 11 and DP 12, which are different not only in their morphological features, but in their origin as well. The DP 12 disturbances are associated with pure ionospheric currents, whereas the DP 11 are though to be generated by the Birkeland type current system. This conclusion is based on examination of the following characteristics: (1) the seasonal changes of the DP 11 and DP 12 disturbances in the polar cap, (2) the seasonal variations of the low-latitude bay intensity at the conjugate points in the cases of the DP 11 and DP 12 disturbances, (3) the distribution of the intensity of the DP 11 and DP 12 disturbances in both northern and southern hemispheres along the midnight meridian.

Midlatitude Auroral Arcs of 6300 Å (O I) and the Concurrent Ionospheric Current System

A multiple midlatitude 6300‐Å auroral arc occurred during the recovery phase of a geomagnetic storm on March 8/9, 1970. Variations of the arcs were characterized by a southward shift in position and by a diminution in intensity with time before midnight. During the recovery phase of the storm, well‐defined magnetic baylike disturbances occurred. The vairations in both position and brightness of the concurrent arc were strongly correlated with changes in the H and D components of these geomagnetic field deviations. An equivalent current system is determined from these baylike disturbance fields observed at twenty‐six magnetic observatories for periods when the correlation was strongest. The current system found is similar to that of the DP 1 field, with a superimposed DP 2 field. The changes in these fields that are associated with the bayklie disturbances are presented to account for the transient variations observed in the intensity and position of the midlatitude auroral arc.

Frequent Occurrence of Local Geomagnetic Disturbance in High Latitudes

Occurrence of local geomagnetic disturbance in high latitudes is studied through a simple analysis of the sudden increases in the quarter-hourly geomagnetic Q-indices (by 3 in successive 15-minute intervals or by 4 within an hour) reported from 17 stations for the one-year period from July 1957 through June 1958. About 80% of the sudden increases of Q-index are recorded only at a single station, and this indicates a very frequent occurrence of local magnetic agitation, such as bay-like disturbances or pulsations of large amplitude. The region of local disturbance is in most cases in a quadrant of the polar map with only several degrees of latitudinal range. The local disturbances occur more frequently in winter than in summer. In winter the appearance of local geomagnetic disturbance is concentrated around midnight, while in summer it appears rather uniformly at all times of day. The winter predorainance of the sudden increase of Q-indices may come mainly from the fact that the noise level of magnetic disturbance in summer is remarkably higher than that in winter.

VLF Emissions and Geomagnetic Disturbances at the Auroral Zone

A close relation has been found between increases of chorus indices continuing for a few hours and geomagnetic pulsations at the auroral zone, using the chorus indices and rapid-run magnetograms at College, Alaska, during July, 1959 to December, 1960.During this period 79 increases of chorus indices occurred in relatively quiet geomagnetic-conditions after geomagnetic bay-like disturbances. Of 79 chorus increases, 52 were associated with geomagnetic pulsations with periods between 1 and 6 minutes and 11 were associated with pulsations having periods of about 30 seconds.The chorus increases may be generated by penetration of high speed charged particles into the exosphere, which may also be related to the geomagnetic pulsations.A possible mechanism of the generation of geomagnetic pulsation has been discussed.

Observations of unusual radiofrequency noise emission and absorption at 80 Mc/s

Unusual radiofrequency noise emissions at 80 Mc/s have been identified during periods of solar activity. The noise enhancements may be classified as (1) smooth, bay-like disturbances lasting for approximately 1 hr which occur in both day and night hours, and (2) abrupt increases, often of large but fluctuating amplitude which occur within a few hours of local midnight. The smooth enhancements occur almost simultaneously with the absorption of radiation in a sector of the northern sky. These effects could be caused by transit of high velocity streams of charged particles which produce emission from F-region levels and absorption in the E-region or below. The abrupt noise bursts at night are from the northern sky and seem to be associated with pronounced changes in the horizontal component of the earth's magnetic field. The noise may be a form of “auroral” radiation or may arise from propagation of solar noise outbursts from the sunlit to the dark hemisphere.

35. A new model of magnetic storms and aurorae

Three topics are discussed dealing with interplanetary phenomena. They are: (i) sudden commencement of magnetic storms[1]; (ii) main phase of magnetic storms[2]; (iii) cosmic ray effects associated with solar corpuscular emission[3].To explain the sudden commencement(SC) of magnetic storms, the reverse sudden commencement (SC*), and the pre-SC disturbances, we invoke the following model: The solar eruption produces a shock-wave which arrives at the earth 22–34 hr later. High velocity particles having a smaller interaction precede the shock-wave and cause the pre-SC bay-like disturbances at high latitudes. The shock-wave itself is retarded by the body forces produced by the geomagnetic field, but speeds up as it enters the auroral zones. In pushing out lines of force it creates the polar SC* events. Charge separation in the shock-wave produces the driving force for the SC currents which flow in the atmosphere (in accordance with Vestine's analysis).The storm decrease is produced by the high velocity particles following the shock-wave (up to 9 hr later) which enter because of field perturbations into the normally inaccessible Störmer regions around the dipole. Here they are trapped and will drift producing the ring-current which gives rise to the storm decrease. Particles with small pitch angle, however, can reach the earth's atmosphere and contribute to aurora, the air-glow, and ionospheric ionization. These particles are replenished by perturbations produced by solar influences having a 27-day recurrence. Many other particles are absorbed or scattered out of the trapping regions so that their number diminishes rapidly in a day or so, as does the magnetic storm decrease.The model thus attempts to explain for the first time the cause of the reverse sudden commencement events (SC*), the atmospheric nature of SC, the delay between SC and the main phase, the formation and decay of the ring-current. A by-product is auroral particle acceleration by a shock-wave[4].New experimental tests are suggested by the model: (i) Acoustic observations with balloons to look for the shock-wave penetrating into the atmosphere in the auroral zones. (ii) Observations with rockets or satellites to establish the location of the SC and main phase currents. (iii) Measurements of the nature and energy of the auroral particles[5].It is suggested that the cosmic ray decrease occurring with magnetic storms (Forbush events), as well as the 27-day decreases of cosmic ray intensity, are modulation effects produced primarily by the deceleration of cosmic rays in interplanetary space due to the expansion of turbulent gas clouds from the sun. The detailed mechanism depends on a statistical decrease of the initially high turbulent fields and can therefore be called an ‘inverse Swann effect’ or ‘inverse Fermi effect’. The cosmic ray intensity variation during the solar cycle is accounted for as the cumulative effect of this mechanism which operates in connexion with emission of solar gas. In this way it is possible also to account for the decrease lasting six months observed by Forbush starting in February 1946. Some experimental tests are suggested to discriminate between different theories for the origin of cosmic ray time variations[3].