Magnetic Bays Research Articles

Equatorial scintillations during the major magnetic storm of April 1981

The geomagnetic storm of April 11–14, 1981, was the most severe storm reported since the August 1972 events. Observations of equatorial scintillations in the VHF through microwave range of frequencies (250 MHz to 1.5 GHz) by two stations, Huancayo, Peru, situated near the magnetic equator, and Ascension island, close to the equatorial anomaly crest, are presented. Unusually high levels of activity were recorded at both the stations even around sunrise. At Huancayo, triggered by a reversal of electric field and therefore a rise in the height of the F layer, the 1.5‐GHz transmissions from the satellite MARISAT (15°W) exhibited a scintillation index of 9 dB, which is one of the highest levels recorded at Huancayo over a period of several years. At Ascension island, severe depletions in ionospheric electron content associated with scintillations were detected for the first time in our observations near sunrise. The dual‐station multisatellite observations are discussed in terms of plasma processes at the equator. The reversal of direction after a deep magnetic bay caused the reversal of direction of the electric field, produced an upward bubble velocity, and generated F layer irregularities. This bay, however, was unable to produce an upwelling after sunrise because of E layer conductivity. The postmidnight and sunrise behavior of the F layer irregularity region has been verified by observations of additional severe magnetic storms during years of high sunspot activity. One example, observations on March 5, 1981, at Manila, is shown. The localized generation of patches during periods of magnetic activity indicates the possibility of bubble formation during magnetically quiet periods without a triggering wave. A hypothesis is proposed for instabilities forming from longitudinal (east‐west) electron density gradients existing in this case during the magnetic bay; in the case of magnetically quiet postsunset activity, steep and irregular gradients in electron density may exist across the longitudinal plane at the magnetic equator, producing conditions for the generation of instabilities.

A multisatellite study of the plasma sheet dynamics at substorm onset

A multi‐satellite study has been conducted on the temporal relationship between energetic particle injections observed at the geostationary orbit onboard GEOS‐2 and decreases of thermal plasma sheet particle fluxes ("plasma sheet thinnings") observed in the more distant geomagnetic tail onboard the ISEE‐1 and ‐2 spacecraft. A case by case analysis as well as a statistical study of 100 events recorded from February to May 1979 and 1980 show that particle injection and particle flux decrease are detected to within less than 5 minutes of each other, for an average intersatellite distance of 15.7 RE. The observed spread in Δt corresponds best to Alfvén wave transit times. These particle phenomena are observed at the onset of the auroral zone magnetic bays. The results reported here suggest that the dynamics of the inner and outer boundaries of the plasma sheet are closely related to each other and controlled by large scale processes that develop at substorm onset inside the magnetospheric tail and result in a cross‐tail current disruption.

Meridional ionospheric electric field caused by curvature current in the magnetosphere

The bending of geomagnetic field lines towards the geotail produces a curvature drift of charged particles parallel to the geomagnetic axis. The divergence of the current so produced forms Birkeland current to the ionosphere where a meridional electric field is created. This field would drive ionospheric currents to form a negative magnetic bay in the dawn sector of the auroral zone and a positive one in the dusk sector. Also it would cause a dawn-dusk field across the polar cap.

The effect of induced currents in the sea on magnetic bays observed at a coastal observatory

It is widely accepted that return currents from the auroral electrojet are responsible for the magnetic bays in the D component. The different directions of the return currents, at either end of the electrojet, means that the sign of the D bay should change as the station moves from east to west of the central meridian of the electrojet. However, there is confusion as to whether this change occurs, giving rise to different models for the substorm current system. The possible effect of induced currents on magnetic bays is shown to be significant and should be examined for each observatory before using their magnetic bays to infer substorm current systems. An analysis of magnetic bays at Halley Bay (75°S, 27°W), a coastal site in Antarctica, shows that a change in the sign of D bays does not occur. It is shown that induced currents flowing in the sea, parallel to the coast, would have an effect that is consistent with the observations.

Auroral zone E-region motions deduced from spaced receiver observations

E-region drift velocities in the Auroral Zone have been observed by the spaced receiver technique at Tromsø, Norway (69.66°N, 18.94°E). Drift velocities have been obtained for disturbed as well as quiet magnetic conditions. The drift velocities derived are mainly eastward in strong negative magnetic bays and westward in strong positive bays. At midday a northward velocity is observed during quiet conditions. Several observations reveal a rather erratic drift pattern, particularly during medium disturbed conditions. These results are attributed to a lack of a dominating driving force in the ionosphere during such periods.

Location of the source of magnetospheric energetic particle bursts by multispacecraft observations

During a magnetic substorm on October 16, 1973 a number of magnetospheric bursts of energetic particles were observed simultaneously by IMP‐6 and IMP‐7 in the magnetotail (at XSM ≃ −32 Re, YSM ≃ 1.2 Re and ZSM ≃ −4.7 Re) while the two spacecraft were separated by only ∼ 1 Re along the XSM axis (ΔXSM ≃ 1 Re, ΔYSM = ΔZSM ≃ 0 Re). Detailed anisotropy measurements of 210 and 290 keV protons by the JHU/APL instruments on both spacecraft provide for the first time an indication of the location of the "source" of the energetic magnetospheric particles as well as evidence for its movement with speeds from ∼ 30 to more than 80 km/sec in association with the intensification of the westward auroral electrojet during a magnetic bay at the station closest to the local time of the spacecraft (also local midnight). The size of the source was estimated to be ΔX ≃ 500 km. The observations indicate that in this case energetic particles are accelerated to > 1.85 MeV in a moving and localized region in the geomagnetotail.

On the morphology of energetic (≥30keV) electron precipitation at the onset of negative magnetic bays

Multiple balloon recordings of bremsstrahlung X-rays supported by recordings of cosmic noise absorption have been used to study in detail energetic (≥30 keV) electron precipitation events occurring near local midnight at the onset of the expansion phase of magnetospheric substorms. This type of precipitation occurs during the first 5–10 min after bay onset and can usually be distinguished from the subsequent bay-associated precipitation by its characteristic time structure, variation in energy spectrum, and higher intensities. During this same interval, the poleward border of the precipitation region moves rapidly towards higher latitudes with speeds of typically 1–2 km/s, whereas the equatorward border seems to move slowly towards lower latitudes. The northward expansion starts just poleward of the lowest latitude reached during the slow equatorward motion of the preceding growth-phase precipitation. The previous narrow precipitation region may thus expand to as much as 10° of invariant latitude within a few minutes. Within the expanding region there are additional intrinsic temporal variations. As the flux of precipitating electrons tends to be most intense and most energetic near the poleward border, recordings made northward of the latitude where the poleward motion started tend to give the appearance of an impulsive precipitation event. The bay-onset precipitation starts abruptly at the onset of Pi 2 magnetic pulsations. Associated with these pulsations there are modulations of the flux of precipitating electrons. An intensified westward electrojet appears to have its center in the equatorward part of the precipitation region. It is suggested that the poleward expansion is associated with the expansion of the plasma sheet earthward of a newly formed X-type neutral line, and is caused by a sudden enhancement of field-line re-connection across the neutral sheet. The intense, more energetic electron precipitation at the poleward border of the precipitation region then takes place along the outer border of the expanding plasma sheet.

On the morphology of energetic (≥30keV) electron precipitation during the growth phase of magnetospheric substorms

The morphology of energetic (≥30 keV) electron precipitation during the growth phase of magnetospheric substorms has been investigated using measurements of auroral-zone bremsstrahlung X-rays obtained from multiple balloon flights and supplementing riometer recordings. Growth-phase precipitation typically starts about one hour before the onset of a negative magnetic bay and occurs in a limited region parallel to the auroral oval around local midnight. The precipitation is first observed in the northern part of the auroral zone and moves southwards with a speed of 5–10 km/min. To the north this precipitation therefore ceases well before bay onset whereas a continuous transition from ‘prebay’ precipitation to bay-associated precipitation takes place in the south. A decrease in the intensity or at least a levelling off may occur some minutes before bay onset. The southward movement of the precipitation region is associated with a similar movement of a weak ionospheric current system. The events studied were all associated with a southward-pointing interplanetary magnetic field and with growth-phase conditions in the magnetotail. It is suggested that growth-phase precipitation originates from the ‘horns’ of the plasma sheet. The equatorward motion of the precipitation is then a consequence of an expansion of the polar cap, a thinning of the plasma sheet, and an equatorward motion of its inner edge. It is also suggested that this precipitation provides a stabilization of the outer boundaries of the plasma sheet by restricting the ionospheric mobility of the bordering field lines through enhanced conductivity.

An ionospheric origin for Pi 1 micropulsations

Cosmic noise absorption pulsation events observed with fast response riometers at Macquarie Island in the southern auroral zone have almost always been accompanied by Pi 1 micropulsations. A cross-spectral analysis of fast response riometer data and vertical component induction magnetometer data for one such event showed that, after the low frequency components are removed, the absorption A( t) is better correlated with the absolute value of the field Z( t) than with the recorded quantity dZ dt . The peaks in Z( t) lag those in A( t) by one second while A( t) lags dZ dt by abou second. Furthermore, many of the pulsations in Z( t) show a similar time asymmetry to that commonly observed in c.n.a. pulsations, viz. a more rapid onset time than decay time. These results are taken as evidence that the Pi 1 micropulsations observed from the ground during the recovery phase of an auroral substorm are brought about by fluctuations in the ionospheric currents which give rise to the magnetic bay, these fluctuations being due to conductivity changes resulting from particle precipitation. The lag between A( t) and Z > ( t) is attributed to the self-inductance of the electrojet.

High-latitude irregularities during the magnetic storm of October 31 to November 1, 1972

By using the scintillation of satellite radio beacons and radio stars as the means of determining the intensity of F layer irregularities, observations of the high-latitude irregularity region were made during the magnetic storm of October 31 to November 1, 1972. At ssc of this winter storm the irregularity region from 60° to 63° invariant latitude was affected almost immediately. However, scintillation intensities were high only for a period of the order of many minutes and subsided again before the expansion phase of the storm. During the expansion phase the motion of the storm irregularities could be tracked equatorward particularly during the occurrence of one intense negative magnetic bay in the H component. The velocity was high at invariant latitudes of 53°–60° (of the order of 300–600 m/s) but slowed to 100–200 m/s from 49° to 53° and then to even lower velocities at 45°, the lowest latitude of the observations. A comparison with auroral photographs indicates that even 3° equatorward of the optical aurora the irregularity intensities at F layer heights were considerable. High levels of scintillation activity were also noted poleward of the aurora. Heat conduction due to auroral current energy dissipation or from isotropic electric fields is postulated as the source for the production of intense F layer irregularities during these severe magnetic storms. The precipitation of low-energy electrons is another (but less attractive) hypothesis.

Morphology and fine time structure of an early-morning electron precipitation event

Simultaneous balloon recordings of auroral-zone X-rays from precipitating electrons, covering a range of L-values from ≈5 to ≈7.5, are presented. The precipitation event was observed in the early morning sector (from about 0200 to 0500 local magnetic time), and was associated with a negative magnetic bay. Before the bay, precipitation associated with the growth phase of the substorm was observed at high L-values. After bay onset, precipitation was observed over the whole range of L-values covered, but with a delayed onset in the southern part of the precipitation region as compared with the onset of cosmic noise absorption in the local midnight sector. At high L-values the X-ray flux was completely unstructured and drizzle-like, both before and after bay onset. At low L-values, where precipitation occurred only after bay onset, the event was splash-like with X-ray bursts of typically 4–6 sec duration apparently rising out of the cosmic-ray background. The precipitation bursts had spatial extensions of 300–400 km. They were accompanied by weak magnetic impulses which were, both temporally and spatially, closely related to the X-ray bursts. The unstructured precipitation at high L-values was apparently associated with and extending along the auroral electrojet, presumably representing freshly accelerated particles. The highly structured and burst-like precipitation to the south seems to have come from a cloud of electrons drifting out from the acceleration region, from which wave-particle instabilities or some other mechanism caused electrons to be precipitated.

Electron precipitation in a non-uniform glow aurora

Electron intensities at 5 keV >18 keV and >45 keV, were measured on a Petrel rocket flown from Kiruna, Sweden, into a non-uniform glow aurora during the recovery phase of a magnetic bay. The intensities depend on pitch-angle in a way that is consistent with the precipitation being caused by pitch-angle diffusion from reservoirs of geomagnetically-trapped electrons. The scattering process that causes pitch-angle diffusion, and leads to three regions of relatively high intensity, appears to have properties different from the scattering process that leads to two intervening regions of low intensity. A spatial structure in electron reservoir intensity, is attributed mainly to variations in the rate of erosion by pitch-angle diffusion of an initially nearly-uniform reservoir intensity. An expression is derived for the minimum lifetime of trapped electrons undergoing strong pitch-angle diffusion.

Observation of plasma flow in the neutral sheet at lunar distance during two magnetic bays

Simultaneous observations of a southward directed magnetic field and a plasma flowing away from the earth across the neutral sheet at lunar distance during two weak magnetic bays. These observations are shown to be in good agreement with predictions based on magnetic merging models for the generation of magnetic substorms.

E s-q layer at huancayo during the March 1970 geomagnetic storm

The paper describes a comparison of vertical electron drift in the F-region ( V z ) measured by VHP incoherent scatter radar at Jicamarca with the corresponding variations of geomagnetic horizontal field ( H) and the maximum frequency reflected from The E s layer ( E s ) at Huancayo during the geomagnetic storm period 7–9 March, 1970. The V z is generally upward during the daytime at the equator, but during 7–9, March, 1970, V z was negative for brief periods associated with negative bays in H. These periods of abnormally low or of downward V z correspond closely with the period of complete disappearance of the q type of E s layer. The magnetic bays associated with the intensification of ring current do not affect the equatorial E s - q and it is only the negative bays in H at the equator due to the ionospheric current flowing westward, that cause sudden disappearance of E s − q. It is suggested that the q type of E s is due to cross-field instability created in the electrojet region due to interaction of northward magnetic field and vertical upward Hall polarization electric field when the plasma density gradient is upward. The sudden disappearances of E s − q are due to the reversal of the horizontal electric field in the equatorial ionosphere and thereby due to the reversal of the equatorial electrojet currents. These reversals of electric field may be due to the imposition on the normal S q field of another westward electric field.

Substorm variations of the magnetotail plasma sheet fromXSM≈ −6REtoXSM≈ −60RE

We report a study of the substorm-related variations of the magnetotail plasma sheet. The study uses data, much of it previously published, obtained by Vela and Imp satellites in the range −6 RE > XSM > −60 RE, where XSM is geocentric distance measured along the solar magnetospheric x axis. Evidence is presented that the thickening or recovery of the plasma sheet, which has been shown in Vela satellite measurements (at γ ≈ 18 RE) to occur late in substorms, is causally related to a rapid poleward shift or ‘leap’ of the principal current of the auroral electrojet evidenced by recovery of magnetic bays at auroral latitudes (65° ≲ λm ≲ 70°) and their onset at low polar cap latitudes (e.g., λm ≈ 74°). That is, it appears to be inaccurate to regard the thickening of the plasma sheet in the far magnetotail as a process that commences near the earth at the onset of the expansive phase of a substorm and moves more or less uniformly out into the tail as the expansive phase of the substorm evolves. A schematic description of the responses of the plasma sheet to a substorm, based on this study and previous studies, is presented.

Electric field variations during substorms: OGO-6 measurements

The OGO-6 electric field measurements make it clear that the general pattern of high latitude electric fields in magnetic time-invariant latitude coordinates is not highly variable and that when unusual variations, or field distributions, occur they are relatively isolated in time and spatial extent. Thus, electric field changes on a global scale cannot, in general, be invoked as a direct cause of substorms. Polar traverses along the 1800 to 0600 magnetic time meridian show that the sum of potential drops across the evening auroral belt (poleward E) and morning auroral belt (equatorward E) approximately equals the potential drop across the polar cap (dawn-dusk E). The integrated polar cap potential drop ranges from 20 to 100 keV and values in the center of this range (e.g. 40–70 keV) are most common under conditions of moderate magnetic disturbance (e.g. Kp = 3). Roughly near 1800 magnetic local time, a latitudinally narrow strip at the transition between auroral belt and polar cap fields exhibits unusually large field fluctuations immediately following the sudden onset of a negative bay at later magnetic local times. It appears likely that this spatially isolated correlation is related to an effect rather than a cause of substorm enhancement. Other indications of direct correlation between electric field behavior and stages of magnetic bay development have not been found to be repeatable for multiple cases. These preliminary studies do not, however, rule out the possibility that more subtle relationships will be found in future studies.

Onset of magnetospheric substorms

An examination of the onset of magnetospheric substorms is made by using ATS 5 energetic particles, conjugate balloon X rays and electric fields, all-sky camera photographs, and auroral-zone magnetograms. It is shown that plasma injection to ATS distances, conjugate 1- to 10-keV auroral particle precipitation, energetic electron precipitation, and enhancements of westward magnetospheric electric-field component all occur with the star of slowly developing negative magnetic bays. No trapped or precipitating energetic-particle features are seen at ATS 5 when later sharp negative magnetic-bay onsets occur at Churchill or Great Whale River.

Interplanetary magnetic-field variations and substorm activity

A fine time-scale study of interplanetary magnetic field (IMF) variations and auroral-zone magnetograms during active and moderately active days show that the time delay between the southward turning of the IMF and the first sign of a negative magnetic bay is typically less than 15 min. During the moderately active period, 88% of all substorms were associated with southward interplanetary magnetic fields. Conversely, 80% of all large southward IMF events were associated with auroral-zone negative bays; during some events, however, magnetic bays could not be found, even by using high-latitude stations. It is concluded that the main mechanism for the triggering of magnetospheric substorms is the southward turning of the IMF.

Horizontal Polarization in Array Studies of Anomalous Geomagnetic Variations

RECENTLY, natural external variations of the geomagnetic field have been used extensively to study the electromagnetic response of the Earth and thus to estimate its electrical conductivity structure. In particular magnetic storms and bays, having power mostly in the frequency range 0.5 to 10 cycles/h, are suitable for examination of the crust and upper mantle.

Auroral-zone electron precipitation events observed before and at the onset of negative magnetic bays

Abstract Balloon recordings of bremsstrahlung X-raya from precipitating electrons are presented, showing impulsive X-ray events just at the onset of bay activity as well as weaker and smoother electron precipitation before the bay, the latter apparently being connected with a growth phase of the same substorm. The pre-bay events are generally very smoothly varying events, but show modulation effects associated with the occurrence of irregular magnetic pulsations. The impulsive precipitation events start very abruptly together with Pi 2 magnetic pulsations at bay onset, and also show modulation effects, so that maxima in the X-ray flux tend to accompany maxima in the H -component at auroral-zone stations. These X-ray pulsations have a shorter period than the simultaneous mid-latitude Pi 2, which we take to indicate that the impulsive precipitation events occur on field lines close to the inner edge of the neutral sheet.