Magnetic Bays Research Articles

Some Characteristics of Irregular Micropulsations Pi2 at High Latitude Stations

Pi2 micropulsations were studied for the period July 12 to August 16, 1968, from four observatories at different latitudes in Canada. Maximum occurrence was near midnight with a preference for 5–6 h UT at all longitudes studied. The relationship of Pi2 occurrence to magnetic bays and to Kp is examined. The correlation of Pi amplitudes and periods with Kp and other parameters reveals a different pattern for lower latitude stations (Ottawa and Meanook) compared with Baker Lake (73.9 °N). These results are interpreted as evidence for two different types of Pi in the different latitude regions, the lower latitude Pi being associated with hydromagnetic waves on the plasmapause. In studies of simultaneous Pi events at the four stations, the maximum amplitude occurred most frequently at Meanook. Polarization of such events was found to be different at different stations and so previously reported patterns of Pi polarization could not be confirmed.

Magnetospheric substorm of August 25-26, 1967

Temporal history of 25-26 August 1967 magnetospheric substorm, discussing magnetotail plasma sheet thinning, partial ring current buildup and magnetic bays development

Auroral-zone X-ray events and their relation to polar magnetic substorms

The general relationship between auroral-zone X-ray events produced by precipitating electrons, and magnetic bay activity, has been studied. Every X-ray event, irrespective of the local time at which it was observed, could be associated with a negative disturbance in the geomagnetic H-component in the midnight sector of the auroral zone. And in most cases, a polar magnetic substorm of global character could be identified. X-ray events observed in the local midnight sector are prompt with respect to the associated magnetic disturbance, but it has been found that those which occur later tend to be delayed, and those observed after 0600 hr LMT are always delayed. The average delay increases with increasing separation of the region of electron precipitation from the instantaneous midnight sector, which is interpreted as an effect of drift of electrons in the magnetosphere, from an acceleration region on the night side of the Earth. Reasons are given for assuming that the acceleration region is usually rather narrow in latitude, and may extend from about magnetic midnight to dawn.

Preliminary results of a low-energy particle survey at synchronous altitude

A preliminary analysis of the data from the first few weeks of operation of the Lockheed auroral particles experiment on ATS 5 has revealed some interesting characteristics of the low-energy particle environment at synchronous altitude during magnetically quiet times. The most prominent feature of the data is the occurrence of enhanced electron fluxes during local night in association with magnetic bay activity as observed on ground magnetograms. A tentative description of some of the ‘signatures’ of these magnetospheric substorm events is presented, based on the limited body of data that has currently been analyzed. The results of a preliminary statistical study of some of the electron plasma properties are compared with published data from a low-altitude polar satellite and with measurements made deep in the magnetospheric tail. The results are consistent with a model of magnetospheric convection in which auroral electrons are adiabatically compressed as they are convected inward and do not suggest the need for additional acceleration mechanisms to account for the average energization of the auroral electrons.

Rocket measurements of auroral electric and magnetic fields

A series of five Nike Tomahawk sounding rockets were launched from Fort Churchill into a variety of auroral conditions. Payload instrumentation included electric field meters, magnetometers, auroral photometers, and charged-particle detectors. Horizontal dc electric fields were measured and were found to point southward during negative magnetic bays. This is consistent with the presence of westward-flowing Hall currents. The relative absence of electric-field variations with altitude indicates that field mapping along nearly equipotential magnetic field lines may be taking place. Perturbations measured by the rocket magnetometers and photometers suggest that the auroral electrojet often flows parallel to visible auroral forms and is comprised of a series of thin current filaments of varying intensity. According to the magnetometer records, horizontal electrojet currents of 104 to 106 amp existed during periods of visible auroral activity. An upper limit for a field-aligned current in a zenith auroral display 10 km from the payload was calculated to be 2.5×105 amp on the basis of flux-gate magnetometer data. Results from a solid-state particle detector, however, suggest that the current was probably of much smaller magnitude. Calculations using various current models indicate that it is often necessary to propose the existence of two or more horizontal current filaments to produce agreement between ground-station and rocket magnetic records.

Mid-latitude magnetic bays and auroral absorption

One hundred and thirty two simple isolated magnetic bay events at Fredericksburg, Virginia, were studied in terms of their H, Z and D components and found to fall into several distinct groups. Nighttime bays are sharp, highly correlated with 18 MHz auroral absorption at mid-latitude stations, and their H and Z components reverse sign near 2100 hr LT. Daytime bays are generally diffuse and only slightly correlated with mid-latitude riometer absorption. The high degree of correspondence between sharp bays and riometer absorption suggests that the same precipitating particles may be responsible for both.

Characteristics of a visual aurora following a solar flare

On February 13, 1967 at 1746 UT, the brightest flare of that year erupted from the surface of the Sun. Approximately 56 hr later, the sudden commencement of a magnetic storm was recorded by many observatories around the world. Midway between the worldwide sudden commencement and the termination of the magnetic storm, there occurred at Ft. Churchill an aurora attaining nearly IBC in and an accompanying local negative magnetic bay. Measurements were made on this aurora as one of a series investigated with a multiple channel photometer simultaneously recording ten wavelength bands. The aurora of February 16 appeared normal under casual observation, but a detailed examination of its emission structure has turned up a number of deviations from other auroras studied. Highlights of these include an unusual amount of λ6300 intensity and an increase in the apparent vibrational temperature of the molecular nitrogen that correlates well with the increase in the λ6300 emission. It is concluded that the magnetic storm is primarily responsible for these unusual effects.

The distribution of stably trapped magnetospheric plasma as determined from Pi2 micropulsations and the auroral electrojet index, AE

The distribution of stably trapped plasma is determined as a function of equatorial geocentric distance, R. Observed Pi2 micropulsation periods and the magnitudes of their associated magnetic bays, measured by the auroral electrojet index, AE, are used in determining this distribution. Plasma density is found to vary inversely as approximately the sixth power of R. When Pi2 period- AE relations are studied for varying time lags, the pulsation periods are found to define prolonged intervals of abnormal AE levels. Thus the periods may be potentially useful in forecasting geomagnetic disturbances or abnormally quiet intervals. A qualitative physical model is proposed to explain the observational relations.

On the cause of geomagnetic bays

The force between the Earth and magnetospheric tail is assumed balanced by a tanential stress on the solar wind. The stress acting against the solar wi wind kinetic energy into magnetic energy stored in the tail. Consequently the tail radius increases and the inner edge of the neutral sheet moves earthward. This one-way development is stopped by a magnetic bay which restores a lower energy state and the process begins again. The estimated time between bays by this mechanism is 2–20 hr. The arguments can be extended to discuss the development of the geomagnetic storm main phase.

Magnetospheric electric field measurements with balloons

Two balloons have been flown from Fort Churchill, Canada, in August 1968 to measure horizontal and vertical electric fields. It is shown theoretically and experimentally that horizontal upper atmospheric electric fields are generally equal to one-second, several hundred kilometer averages of the horizontal ionospheric electric field. Analyses of thirty-five hours of data obtained during quiet and active periods yield the following conclusions. The average equatorial electric field strength was 0.6 mv/m during a quiet period and 1.0 mv/m during an active period. During magnetic bays, ground-based magnetometers principally measured Hall currents whose variations depended on conductivity changes as much or more than on electric field variations. For a period of about 30 minutes before the onset of a negative bay near local midnight, the ionospheric electric field pointed westward, indicating radially inward flow in the equatorial plane. This result is interpreted in terms of a model of the magnetospheric substorm in which bays are triggered deep within the magnetosphere by an instability associated with a radial plasma density gradient established by convective flow. The average electric field in the dayside ionosphere was about an order of magnitude smaller than that at night, suggesting that the dayside magnetosphere tended to corotate with the earth. Equipotential contours of the electric field in the equatorial plane are constructed based on this and other hypotheses.

The plasmapause, the plasma sheet, and energetic trapped electrons in the earth's magnetosphere

Observations from the IMP 2 satellite show that much of the energetic trapped radiation observed near the equatorial plane lay beyond the plasmapause, which therefore cannot be the boundary between ‘open’ and ‘closed’ field lines in the Earth's magnetosphere. The position of peak intensity for electrons >45 keV generally lay just beyond the plasmapause, while the position of peak intensity for electrons >l.6 MeV lay just inside it. The ‘slot’, the position of minimum intensity between inner and outer radiation belts lay well inside the plasmapause for both 1.6 MeV and 45 keV electrons. The boundary between the stable trapping zone and the ‘distant’ radiation zone of highly fluctuating particle fluxes which also lay beyond the plasmapause, near 10 R s during quiet times moved as close to the Earth as 6 R s shortly after onset of magnetic bays; these positions are close to those reported for the inner boundary of the plasma sheet. It is suggested that: (1) The plasmapause is the limiting magnetic shell at which most terrestrial plasma of ionospheric origin can remain trapped, if there is an electric field across the magnetosphere. (2) The inner boundary of the plasma sheet is the limiting distance to which the more energetic interplanetary plasma can penetrate into the Earth's field under the action of the same magnetospheric electric field.

Some observations of energetic electrons in the outer radiation zone during magnetic bays

Changes in the cutoff boundary latitude and flux of electrons Ee > 40 kev in the midnight sector of the outer radiation belt during periods when magnetic bays are in progress are studied in this paper from the data obtained by the University of Iowa satellite Injun 4. From this study, it is observed that (1) the outer-zone flux in the midnight sector increases without any change in the boundary latitude during the expansion phase of a magnetic bay and (2) in addition to the increase in flux, there is also an extension of the boundary to higher latitudes during the recovery phase of a magnetic bay. From these observations, it is suggested that magnetic field line merging may not play a primary role in the production of energetic auroral electrons and that acceleration of particles takes place in the outer zone itself and not far out in the magnetotail.

Structure of the neutral sheet in the geomagnetic tail

In early 1963, Explorer 14 (apogee about 16 earth radii) made magnetic field measurements at the earthward end of the geomagnetic tail. On several occasions the field was observed to reverse direction in times of the order of a minute, corresponding to passages through the so-called neutral sheet. Two of these passages, on February 10 during a moderately severe storm, are analyzed in detail. The orientation of the sheet is discussed, and it is shown how field lines may be computed from the measurements. It is found that on both occasions the field lines were connected through the sheet, with normal components of several gammas. No evidence is found of the two standing shock waves proposed by Axford, Petschek, and Siscoe. Immediately after one encounter with the sheet, the field had a southward component of up to 30 γ for over a minute. At about the same time, ground stations recorded the onset of a magnetic bay. It is suggested that this observation by the satellite was made on the far side of a region in which field line reconnection was taking place.

The polar substorm and electron ‘islands’ in the Earth's magnetic tail

The behaviour of energetic electrons in the distant magnetosphere near the midnight meridian during polar substorms has been studied for the period March 5th–April 4th, 1965, using data from two end window Geiger counters flown on the IMP 2 satellite (apogee 15.8 Earth radii) and magnetic records from a chain of auroral zone stations around the world at magnetic latitudes equivalent to L = 7.4 ± 2.0. When the satellite was in the distant radiation zone or in the plasma sheet which extends down the Earth's magnetic tail, sudden decreases in the horizontal magnetic field component at ground stations near the midnight meridian (negative magnetic bays) were followed by sudden increases in 40 keV electron fluxes (electron islands) at the satellite. When the satellite was at high latitudes in the magnetic tail ‘bays’ often were not followed by ‘islands.’ When the satellite was near the centre of the plasma sheet, energetic electron fluxes were observed even during magnetically quiet periods. The time delay between the sharp onset of magnetic bays in the auroral zone and the corresponding rapid increase in energetic electron intensity at the satellite, typically some tens of minutes, was least when the satellite was close to the Earth and increased with its increasing radial distance from the Earth. The delay was also a function of distance of the satellite from the centre of the plasma sheet, and of the magnitude of the intensity increase (smaller delays for larger intensity increases). We deduce that the disturbance producing the magnetic bays and associated particle acceleration originates fairly deep in the magnetosphere and propagates outward to higher L values, and down the plasma sheet in the Earth's magnetic tail on the dark side of the Earth. It is unlikely that the accelerated electrons are themselves drifting away from the Earth, because the apparent velocity with which the islands move away from the Earth decreases with increasing distance from the Earth. It is suggested that the polar substorm and the associated particle acceleration are part of an impulsive ejection mechanism of magnetospheric energy into the ionosphere, rather than an impulsive injection mechanism of solar wind energy into the magnetosphere.

Simultaneous observations of electrons (E> 45 keV) at 2000-kilometer altitude and at 100,000 kilometers in the magnetotail

Geiger counters on the University of Iowa's Injun 4 satellite and on Vela satellites 2A, 2B, 3A, and 3B have provided simultaneous measurements of energetic electron fluxes (Ee > 45 kev) at ∼2000-km altitude over the auroral and polar regions of the earth and at ∼17 earth radii (RE) in th plasma sheet of the magnetotail. The data discussed here were obtained in March, April, September, and October 1965 periods when Injun 4 crossed the northern auroral zone between 1600 and 2400 magnetic local time (MLT) and between 0400 and 1200 MLT. The Injun measurements confirm a recent report that the peak flux of electrons in the outer belt in the late morning hours often is substantially increased shortly after the onset of a magnetic bay in the midnight sector of the auroral zone. They show, in addition, that the early evening sector of the outer belt does not respond promptly in this way to a bay. This difference in response supports the view that the electrons that enhance the morning sector of the outer belt are injected by some process acting in the midnight sector of the magnetosphere. The Vela measurements confirm an earlier report that an essentially isotropic flux of energetic electrons appears in the plasma sheet, generally after the peak of a bay. Furthermore, the concurrent measurements by Vela and Injun 4 show that the flux of electrons in the plasma sheet after a bay is usually much less intense than the peak flux attained in the outer zone after the same bay. Both the delayed appearance of the electrons in the magnetotail and their relatively lower intensity imply that the energization process acts not in the very distant tail, but closer to the earth than 17 RE. It is suggested that electron energization (along with a general heating of the ambient plasma) takes place within the auroral and/or outer-zone tubes of force during a bay and that these tubes do not extend as far as 17 RE into the tail during the energization process. Movement of the hot plasma to the distant tail signals the start of recovery of a bay and results in the bursts of energetic electrons seen in the tail. Thus, the distant tail is not a source of energetic (>45-kev) outer-belt electrons, but an additional sink for the process that generates them. It is argued, on the basis of these and other observations, that some process other than merging of magnetic field lines is primarily responsible for generation of the energetic electrons.

Occurrences of Pc 1 pulsations in the course of magnetospheric substorms

Frequency-time displays of geomagnetic pulsations recorded at Kiruna, Sweden, during the summer months of the years 1964 and 1965 revealed the appearance of Pc 1 pulsations in the 4-sec band with a pronounced maximum of occurrence in the daytime. Many of these events showed a repetitive structure with values of the repetition time and dispersion parameters that indicate a generation region on shells with L between 7 and 10. The Pc 1 pulsations were frequently accompanied by cosmic noise absorption (CNA) and X-ray enhancements; however, the onsets of the pulsations and of the CNA do not in general coincide. A systematic variation with time was observed for the time difference between the onsets of the pulsations and the absorption. At 06 LT the CNA started about 1.5 hr earlier than the pulsations, and at 16 LT the CNA started about 0.5 hr later. Furthermore, a series of events on the days after the ssc of August 16, 1965, showed Pc 1 pulsations starting at Kiruna approximately 0.5 hr after the onset of negative magnetic bays at College, Alaska. The observations can be explained by the assumption that energetic electrons and protons are injected into the outer radiation zone on the night side of the earth during a magnetospheric substorm and that these particles reach the day side by drift. If only magnetic gradient and curvature drift is considered, particles with energies near 100 kev on a shell with L ≈ 8 are required for a quantitative description of the observations.

Correlated Effects of Energetic Electrons at the 6.6 Re Equator and the Auroral Zone During Magnetospheric Substorms

During magnetically disturbed times, large intensity variations are observed for electrons of energies 50‐ to 150‐keV at the geostationary orbit of the ATS‐1 satellite. Simultaneously, the correlation experiment at the magnetic conjugate region of the satellite has shown that large fluxes of bremsstrahlung X‐rays from precipitated energetic electrons are observed on high‐altitude balloons while magnetic bays are recorded on the ground. The conclusion reached is that the magnetospheric substorm is responsible for the simultaneous intensification of the trapped particle population at 6.6 Re, equatorial plane and of precipitated energetic electrons at the magnetic conjugate region.

A survey of low-energy electrons in the evening sector of the magnetosphere with OGO 1 and OGO 3

Observations of electrons of energy 125 ev to ∼2 kev with the OGO 1 satellite and of electrons of energy 40 ev to ∼2 kev with OGO 3 (by means of modulated Faraday cup detectors) are used to investigate the low-energy electron population in the magnetosphere within the local-time range ∼17 to ∼22 hours. Intense fluxes of these electrons are confined to a spatial region, termed the plasma sheet, which is an extension of the magnetotail plasma sheet discovered by the Vela satellites and is identified with the soft electron band first detected by Gringauz. The plasma sheet extends over the entire local-time range studied in this investigation, from the magnetospheric tail past the dusk meridian toward the dayside magnetosphere. In latitude it is confined to within 4–6 RE of the geomagnetic and/or solar magnetospheric equatorial plane, in agreement with observations already reported from the Vela satellites; no electron fluxes are detected high above the equator, not even very near the magnetopause. In radial distance the plasma sheet is terminated by the magnetopause on the outside and by a well-defined sharp inner boundary on the inside. The inner boundary has been traced from the equatorial region to the highest latitudes investigated, ∼40°; during geomagnetically quiet periods, it is observed at an equatorial distance of 11 ± 1 RE and appears to extend to higher latitudes along magnetic field lines. Weak or no electron fluxes are found between the inner boundary of the plasma sheet and the outer boundary of the plasmasphere. Detection (by an indirect process) of the very high ion densities within the plasmasphere gives positions for its boundary in good agreement with other determinations. During periods of magnetic bay activity, the plasma sheet extends closer to the earth; the inner boundary of the plasma sheet is then found at equatorial distances of 6–8 RE. This is most simply interpreted as the result of an actual inward motion of the plasma during a bay. In one case, it was possible to associate the beginning of this motion with the onset of the bay and to estimate an average radial speed of ∼12 km/sec, from which an electric field corresponding to ∼48 kilovolts across the magnetospheric tail was inferred. Within the plasma sheet, the electron population is characterized by number densities from 0.3 to 30 cm−3 and mean energies from 50 to 1600 ev and higher, with a strong anticorrelation between density and mean energy, so that the electron energy density (∼1 kev cm−3) and energy flux (∼3 ergs cm−2 sec−1) show relatively little variation. The lower energies and higher densities tend to occur during periods of geomagnetic disturbance. The nonobservation of electrons in regions above the plasma sheet implies an upper limit on the electron number density of 5 × 10−2 cm−3 if their mean energy is assumed to be ∼50 ev (typical of the magnetosheath) and 10−2 cm−3 if the energy is ∼1 kev (typical of the plasma sheet). At the inner boundary of the plasma sheet there is a sharp softening of the electron spectrum with decreasing radial distance but apparently little change in the electron number density. The electron energy density decreases across the inner boundary roughly as ∼exp (distance/0.4 RE) during quiet periods; during times of magnetic bay activity the energy density decreases as ∼exp (distance/0.6 RE), and there is a more complicated spatial structure of density and mean energy.

Observations of protons in the magnetosphere and magnetotail with Explorer 33

Protons in the magnetosphere and magnetotail were observed with a silicon detector on Explorer 33. Detectable fluxes of protons Ep > 0.31 Mev were measured to 10.4 RE on the sunward side of the earth and, for the first time with the present experiment, to 80 RE in the magnetotail. The small proton fluxes (≲5/cm² sec ster) measured in the magnetotail were usually anisotropic, with the largest flux apparently flowing down the tail, away from the earth. When proton bursts occurred in the tail, there was usually increased high-latitude magnetic bay activity at the earth. Durably trapped proton fluxes were observed to have intensities and spectra in agreement with previous measurements. A diurnal variation (in the L coordinate system) was observed in the 0.31- to 10-Mev trapped protons for L > 6.6 RE.

Relationship between changes in the interplanetary magnetic field and variations in the magnetic field at the Earth's surface

Hourly average values of solar wind velocity and the components of the interplanetary magnetic field obtained by IMP 1 were combined to obtain the interplanetary electric field as measured in a coordinate system with one axis parallel to the earth's magnetic dipole axis. The hourly values of the azimuthal component of the interplanetary electric field were related to the different types of magnetic activity observed at a worldwide chain of stations. It is found that a positive increment in the azimuthal electric field (enhancement of the southward directed interplanetary magnetic field) is strongly related to the initiation of magnetic bay and storm activity, and that establishment of a negative (or less positive) azimuthal electric field during the course of a storm or bay is associated with the start of the recovery phase. Sudden impulses are shown to be equally often associated with negative and positive increments in the azimuthal electric field and are found to be the most common event observed in this study. The combinations of solar-terrestrial parameters that appear to determine the different types of magnetic activity are discussed in the light of the results presented in this paper.