Magnetic Bays Research Articles

Outward flow of plasms in the magnetotail following geomagnetic bays

A study of electrons in the earth's magnetosphere, measured by the Vela 2 satellites, and of magnetograms from high-latitude stations on the earth has revealed that hot plasma containing electrons of energy >45 kev often arrives suddenly in the plasma sheet of the magnetotail at the peak or during the recovery phase of a negative magnetic bay in the auroral zone. It is found also that if a satellite is in the plasma sheet at the time a negative bay starts, it is likely to detect a decrease in the intensity and/or the average energy of the electrons there as the depth of the bay increases. These observations suggest that a contraction of the plasma sheet occurs during the development of a bay, followed by a sudden expansion starting at the beginning of bay recovery. It is concluded that the bursts of hot plasma in the magnetotail derive their energy from processes occurring inside the Vela satellite orbit (radial distance ∼17 RE). The relevance of these observations to models of the magnetosphere is discussed.

Proton events in the magnetosphere associated with magnetic bays

Low energy proton flux increases in outer radiation belt during quiet magnetic activity and correlation with magnetic bay appearances

Balloon and rocket observations of auroral-zone microbursts

Auroral zone microbursts associated with geomagnetic storms, negative magnetic bays and electron precipitation observed by balloon and rocket

Upper atmospheric temperatures from Doppler line widths—IV: A detailed study using the OI 5577 å auroral and nightglow emission

The measurement of Doppler temperatures in the upper atmosphere has been continued in Saskatoon, using a field-compensated Michelson interferometer. This device possesses certain advantages over the Fabry-Perot spectrometer, the most important being that its luminosity may be of the order of a thousand times greater at the same effective resolution. A brief description of the instrument and the techniques used in this particular application are given. A large number of temperature values, from both aurora and nightglow, have been obtained with this instrument, using the OI 5577 Å line. This made it possible to obtain some correlation with local time, sky position, magnetic field variations, and particularly with emission brightness. The detailed results for the nights of April 7/8, May 10/11 and May 14/15, 1964 are presented. The most striking result is that a systematic relationship between the auroral brightness and observed temperature is often obtained over periods up to a few hours. Brightness and temperature are usually inversely related, and changes in the relationship have been observed to occur at the same time as magnetic bays. This inverse relationship is crudely interpreted as following the atmospheric density curve; that is, that a uniform excitation per atom exists down to some cutoff height determined by the particle energy. This cutoff height thus determines both the auroral brightness and the observed temperature. A more quantitative interpretation has been given using calculated luminosity distributions. These suggest that while the inverse relation is in effect, auroral brightness (and temperature) changes result primarily from changes in the particle energy spectrum rather than changes in the flux. This systematic relationship also seems to preclude the existence of major temperature changes through auroral heating at these levels. The nightglow observations give an average temperature of about 180°R and brightness in the range of 200–300 R, in agreement with observations at lower latitudes. When compared with the auroral data, the indication is that aurora and nightglow are unrelated phenomena in the 5577 Å emission.

Localization and motion of energetic electron precipitation during magnetic bays

Balloon observations are presented that show directly the motions of energetic electron precipitation regions during negative magnetic bays. Comparison of these motions with those of the auroral electrojets indicates that the precipitation and current systems have similar locations and movements, as was inferred earlier by indirect means. In addition, a late morning event, without accompanying magnetic activity, was observed that showed similar localization and displacement.

Energy spectrum for auroral-zone X rays: 1. Diurnal and type effects

Balloon observations of X rays produced by energetic (∼50–250 kev) electron precipitation into the auroral zone suggest a rather consistent diurnal pattern of occurrence for certain types of bremsstrahlung activity. Distinguishing spectral characteristics and variability, represented by approximate e-folding energies in the range 10–50 kev, are emphasized. Whereas the harder spectrums (E0 ≈ 25–50 kev) may be precipitated at any local hour, the softer spectrums (E0 ≈ 10–25 kev) appear primarily between the midnight and noon meridians. The softest spectrums (E0 ≈ 10–15 kev) observed in this energy range occur preferentially around midnight and into the early morning in association with pronounced negative magnetic bays and post-breakup auroral activity. The results suggest the existence of an intermediate spectrum (E0 ≈ 10–20 kev), in addition to the very soft (E0 ≲ 5 kev) precipitation characteristic of the luminous aurora and the rather harder precipitation (E0 ≳ 25 kev) characteristic of the ‘average’ auroral-zone X-ray event.

Midlatitude observations of nighttime VLF signal-amplitude anomalies associated with magnetic bays

Introduction. The purpose of this letter is to report the observation of VLF propagation anomalies occurring on midlatitude paths during local nighttime. The anomalies appear significant because they are not accompanied by the usual sources of perturbation—solar flares, high-altitude nuclear detonations, or severe magnetic storms. The nighttime occurrence of the anomalies effectively eliminates solar-flare-produced SID's, which are almost exclusively a dayside phenomenon. The changes do, however, resemble those produced during daytime by solar flares [Budden and Ratdiffe, 1937; Bracewell and Straker, 1949; Westfall, 1961; Mitra, 1964]. On rare occasions solar flares produce disturbed conditions on the nightside (see review of February 23, 1956, event by Ortner et al. [I960](, but there are no reports of flare activity at the times under consideration. The character of the anomalies is not unlike that produced by trapped electrons from distant high-altitude nuclear explosions [Zmuda et al., 1964; Sechrist, 1964]. There are, however, no reports of nuclear testing at the times under discussion. The short-term nature of the anomalies is quite different from the long-term delayed changes expected from severe magnetic storms [Bracewell et al., 1951; Burgess, 1964], and no notable storms were reported for several days prior to the anomalies. The signal perturbations to be discussed here more closely resemble perturbations observed in the auroral zone [Bates and Albee, 1965; Reder et al., 1964]. The midlatitude perturbations were accompanied by magnetic bays. Magnetic bays often accompany auroral-zone perturbations, but a direct correlation apparently does not exist in that region.

Notes on sub-ELF emissions observed during magnetic storms

In this note we present several recently observed properties of a type of fine-structured sub-ELF emission frequently observed during magnetic storms. The emission contains substantial energy in the frequency range near 1 cps, and its broad-band waveform is quasi-sinusoidal and exhibits an irregular amplitude modulation. Troitskaya [1961] has discussed the waveform characteristics of signals of similar appearance and has referred to them as IPDP, SIP, and stormy period PP. The signals treated in this paper are of the same general type, but probably most closely resemble IPDP (intervals of pulsations of diminishing periods), since they frequently occur in conjunction with magnetic bays, an effect characteristic of IPDP [Troitskaya, 1961].

Theory of irregular magnetic micropulsations associated with a magnetic bay

A burst of irregular pulsations (period, approximately 0.2 to 200 sec) in the auroral zone, which appears at the onset of a sharp negative bay accompanied by various phenomena testifying the intrusion of a high-speed electron beam from above, is explained by the hydromagnetic instability. It is shown that the presence of a passing electron beam modifies the character of a hydromagnetic wave in the magnetosphere in such a way that the instability is able to take place. With adequately chosen parameters of the beam and the ambient plasma, the range of the period of growing waves generated in the magnetosphere is estimated, and a close agreement is found with that of a pulsation burst. Irregular pulsations in the lower latitudes, which are observed in coincidence with the pulsation burst in the auroral zone, may be due to the oblique propagation of these growing waves and the leakage of the screening current, which flows in the auroral zone when the pulsation burst is taking place.

Balloon observations of the extent and structure of auroral-zone electron precipitation events

Simultaneous balloon observations of auroral-zone electron precipitation from College and Fort Yukon, Alaska, in June of 1962 showed examples of the electron influx increasing with increasing geomagnetic latitudes, both with and without magnetic bay disturbances. Under more disturbed conditions, the X-ray flux recorded by the southern balloon far exceeded that by the northern balloon, indicating a sharp boundary for the electron precipitation region as suggested by O'Brien on the basis of satellite measurements.

Day-night ratio for auroral absorption events associated with negative magnetic bays

An examination of solar illumination effects for auroral absorption events centered over the 27.6-Mc/s riometer antenna pattern at College, Alaska, shows that the daynight absorption ratio is essentially unity. This finding indicates that electron-positive ion recombination far outweighs electron attachment processes in auroral absorption regions and is consistent with present knowledge of the electron bombardment as well as ionic-molecular processes at the 90-100-km level. Further implications of this result are discussed.

Simultaneous electron precipitation in the northern and southern auroral zones

During simultaneous high-altitude balloon flights from College, Alaska, and Macquarie Island, Australia, on March 5, 1962, substantial fluxes of auroral zone X rays were observed in both hemispheres during magnetic bay activity. The X-ray observations indicate that the electron influx in both hemispheres extended over a region 3° in latitude and 16° in longitude. The near equality of the X-ray fluxes in both auroral zones suggests that the electron precipitation originated from processes far out on the lines of force, near the equatorial plane. The magnetic and ionospheric disturbances accompanying the X-ray event are discussed.

The generation and effect of electrostatic fields during an auroral disturbance

The equations of motion of electrons and ions in the presence of electric, magnetic, and neutral particle velocity fields are derived for conditions likely to exist in an auroral arc. The electric field resulting from charge separation is computed. Electron densities, charged-particle motion, and electric currents are also computed. It is shown how these computations can account for magnetic bays, electron drifts, and electric field fluctuation observed during auroral disturbances. Analytical expressions for electric field strengths and neutral plasma drift velocities are derived.

Motions of the aurora and radio-aurora and their relationships to ionospheric currents

Motions of the aurora and radio-aurora and their relationships to ionospheric currents

Electron precipitation accompanying ionospheric current systems in the auroral zone

From examination of X-ray and ionospheric absorption data during magnetic bays in the auroral zone, it has been found that electron precipitation on the atmosphere accompanies auroral-zone electrojets. The position of an electrojet and its associated electron precipitation region, rather than the amount of current in the system, largely controls the magnitude of X-ray and ionospheric absorption effects.

An auroral-zone electron precipitation event and its relationship to a magnetic bay

The relationship of an auroral-zone electron precipitation event to a magnetic bay has been examined using simultaneous observations of X-ray intensity at balloon altitudes, ionospheric absorption of cosmic radio noise, geomagnetic micropulsations in the 5- to 30-sec period range, and variations in the geomagnetic elements H, D, and Z. These observations show that an intense electron bombardment of the upper atmosphere was in progress prior to the bay and suggest that the current system of the bay was ‘triggered’ by the sudden intensification and expansion of the electron precipitation. The motion of the current system of the westward electrojet and the electrons precipitating on the atmosphere was toward the north, indicating that processes well out of the lines of force through the auroral zone, rather than local field conditions, were responsible for the movement.

Magnetic Bays at Macquarie Island

Robertson (1959) has examined a year's (1954) records of magnetic bays in H, 1J.., Z obtained at the Australian National Antarctic Research Expedition's station at Macquarie I. (54�5� S., 15~ E.). Robertson's Table 1 contains 450 bays in H. Of these 152 are positive bays, and 298 negative. These numbers do not include 38 periods (each of greater than 4 hr duration) of overlapping bay activity or in which bay activity is obliterated by a magnetic storm.

The varying electron density profile of the F-region during magnetically quiet nights

Ionogrammes taken at Lindau and at two field stations (average distance from Lindau 125 km) on magnetically quiet nights were reduced to N( h)-profiles of the F-layer. The normalized profiles showed maximum height changes of about 60 km lasting for about 2 hr; they also showed layer height differences on comparable nights of more than 100 km. No seasonal influence on the rise and descent of these profiles in the late evening and early morning hours respectively could be found. The ionogrammes of the field stations did not show any local difference or temporal displacement. Generally, the electron loss rate at the F-layer peak is closely related with the height of the peak and therefore dependent on season. Spread- F echoes of high intensity are occasionally observed at Lindau even on magnetically quiet nights; they are not necessarily associated with vertical movements of the F-layer. No spread- F echoes were observed during vertical movements of the F-layer induced by positive magnetic bay disturbances. Both observations do not allow to decide whether vertical movements cause instability-conditions within the F-layer or not.

Some studies of geomagnetic micropulsations

Oscillations of the geomagnetic field with periods between 1 second and a few minutes have been studied by means of large ground loops at Hobart, Adelaide, Camden, and Townsville, Australia. Records have been made both on charts and on slowly moving magnetic tape; the tape recordings have been analyzed for frequency components on a ‘Sonagraph’ sound spectroscope. The period of the continuous oscillations known as PC's shows a diurnal variation and a marked ependence on latitude, shorter periods occurring at lower latitudes. The damped trains of oscillations, occurring predominantly around local midnight, known as Pt's are followed by magnetic bays with an average delay of about 15 minutes. During storms, ‘sweepers’—oscillations with a progressive change of frequency and sometimes with aharmonic overtones—are observed.

Magnetic bays, auroral orientation, and isochasms

The pattern (P) formed at any one instant by the entire global aurora differs from that of the auroral isochasms (IA) which represent the time averages of occurrence of auroras. Cole [1959, 1960] suggested that P is the pattern of the ionospheric current system responsible for geomagnetic disturbance [cf. Vestine, Lange, Laporte, and Scott, 1947, Figs. 227–24O]. Observations near and inside the auroral zones reported since Davis [1960] and Denholm and Bond [1960] support this view. These observations strongly contradict the suggestion that auroral orientation is along Vestine and Sibley's I curves [Vestine and Sibley, 1960] or even approximately along these I curves [Chamberlain, Kern, and Vestine, I960]. Vestine and Sibley's theory of magnetic bays appears to give support to their contention regarding auroral orientation. On the other hand, their ability to make plausible estimates of the magnitude of magnetic bays may be attributed to the unavoidable uncertainty of the parameters involved.