Large Magnetic Change Research Articles

Magnetic field observations in the near-field the 28 June 1992 Mw 7.3 Landers, California, earthquake

Abstract Recent reports suggest that large magnetic field changes occur prior to, and during, large earthquakes. Two continuously operating proton magnetometers, LSBM and OCHM, at distances of 17.3 and 24.2 km, respectively, from the epicenter of the 28 June 1992 Mw 7.3 Landers earthquake, recorded data through the earthquake and its aftershocks. These two stations are part of a differentially connected array of proton magnetometers that has been operated along the San Andreas fault since 1976. The instruments have a sensitivity of 0.25 nT or better and transmit data every 10 min through the GOES satellite to the USGS headquarters in Menlo Park, California. Seismomagnetic offsets of −1.2 ± 0.6 and −0.7 ± 0.7 nT were observed at these sites. In comparison, offsets of −0.3 ± 0.2 and −1.3 ± 0.2 nT were observed during the 8 July 1986 ML 5.9 North Palm Springs earthquake, which occurred directly beneath the OCHM magnetometer site. The observations are generally consistent with seismomagnetic models of the earthquake, in which fault geometry and slip have the same from as that determined by either inversion of the seismic data or inversion of geodetically determined ground displacements produced by the earthquake. In these models, right-lateral rupture occurs on connected fault segments in a homogeneous medium with average magnetization of 2 A/m. The fault-slip distribution has roughly the same form as the observed surface rupture, and the total moment release is 1.1 × 1020 Nm. There is no indication of diffusion-like character to the magnetic field offsets that might indicate these effects result from fluid flow phenomena. It thus seems unlikely that these earthquake-generated offsets and those produced by the North Palm Springs earthquake were generated by electrokinetic effects. Also, there are no indications of enhanced low-frequency magnetic noise before the earthquake at frequencies below 0.001 Hz.

Plasma sheet ion energization during dipolarization events

The energization of plasma sheet particles during storm‐time dipolarization of the magnetospheric field lines is examined by means of three‐dimensional particle codes. In the direction perpendicular to the magnetic field, owing to appreciable temporal variations of the field within a cyclotron turn, particles can experience large magnetic moment changes. A detailed analysis of this effect reveals an “effective pick‐up velocity” at substorm onset, which depends upon the magnitude of the magnetic transition and delineates particle motions at nearly constant or highly variable magnetic moment. In the parallel direction, the large but short‐lived electric field yields an intense acceleration of the particles traveling in the equatorial region. It is demonstrated that significant decelerations can also be achieved off‐equator if the particles drift against the surging electric field. Such decelerations occur in regions where the curvature of the E × B paths substantially exceeds that of the magnetic field lines (namely, at relatively large L‐ shells). Systematic calculations of model plasma sheet distribution functions clearly exemplify the earthward injection of energetic particles during dipolarization events. Owing to phasing between particle motion and the transient electric field, the computed spectrograms display the formation of characteristic bouncing ion patterns with fine structures on fast time scales. To some extent, the simulations support the convection surge mechanism of particle acceleration put forward by Mauk [1986]. However, distinct collapse characteristics as well as the inclusion of nonadiabatic effects result in significantly higher energization rates, particularly at high mass‐to‐charge ratios.

Nonadiabatic transport features in the outer cusp region

The dayside to nightside circulation of plasma along the magnetopause inside the magnetosphere is examined by means of three‐dimensional single‐particle codes. It is demonstrated that particles incident upon the outer cusp region experience transient non‐adiabatic motions, owing to a localized minimum in the field magnitude. Here, possibly large magnetic moment changes yield injection into the loss cone of fractions of the incoming population or, alternatively, enhanced bouncing motions at high altitudes. It is shown that particles gaining access to the magnetotail over the polar cap are progressively extracted from the weak field region by the large‐scale convection electric field. In this latter case, the trajectory simulations suggest an implicit “entry boundary” into the nightside magnetosphere, which corresponds to the sunward edge of field lines featuring monotonic decrease of the field magnitude along their length.

Large magnetic anisotropy change induced by hydrogen ion implantation in europium iron garnet LPE films

H + 2 ion implantation in (111) Eu 3Fe 5O 12 LPE films is found to induce an unusually large anisotropy field change of an order of 3000 Oe. By annealing at 300°C, the H + 2 implantation induced anisotropy field decreases drastically, but lattice strain still remains at a high level. It is concluded that the contribution of hydrogen ions, which exist within the garnet film, should be intrinsic for the observed large anisotropy field change.

Flare-associated magnetic activity in the sun.

Observations of the longitudinal Zeeman effects in a solar active center show the variations of magnetic field streng~h during a period of 144 minutes on 30 April 1958, which included the development and decay of a flare of importance I ~. The measurements show that the flare crossed the neutral line of zero longitudinal field, and, although the extremities lay in regions of steep field gradients, most of the flare area was a region of low gradient. A small but pronounced S pole within the flare area developed and faded away in exact synchronism with the flare intensity. However, the large magnetic changes affected the whole field of the active center. The integrated magnetic energy of the region underwent a sharp decrease of about 16 per cent during the 14 minute rising phase of flare intensity, and an even sharper recoverN to its initial value immediately after flare maximum. This behavior was shared by the large sunspot fields and the area outside the sunspots. On the assumption that the field was 5000 km deep, the energy change of the active center amounted to 4 X I0~' ergs. The Ha radiation of the flare during its life was about 1028 ergs. Thus the changes in magnetic energy prob~bly exceed the total radiation of the flare in the Balmer and Lyman series by a considerable factor. Sacramento Peak Observatory Sunspot, New Mexico

Ionospheric changes associated with the magnetic storm of March 24, 1940

Observations of the ionosphere at the Huancayo and Watheroo magnetic observatories during the great magnetic disturbance of March 24, 1940, are shown and discussed. At Huancayo, the ionospheric disturbance began about 40 minutes before the first really large magnetic change, though coinciding with increasing magnetic activity. During the first great magnetic changes, the F2‐layer was swept upward and disappeared in about 30 minutes. At the same time, ion‐density of the E‐layer rose about 40 per cent.Growth of a “new” F2‐layer began after disappearance of the old, proceeding in an apparently normal manner for the next hour. Because the original ion‐density had been reduced to a low value, the F2‐region is regarded as a nearly un‐ionized atmosphere, suddenly exposed to solar radiation. Effective recombination‐coefficient and rate of ion‐production at level of maximum density of F2‐region are estimated from this growth. This yields approximate values of α = 1.8 × 10−10 and q0 = 230. These values must be somewhat larger, and must increase downward, if height‐change during growth of the layer is to be explained without electron‐transport during the interval.Steady growth of the new F2‐layer was followed by a succession of abnormal increases and decreases of ion‐density of diminishing amplitude at an almost regular period of three hours for three and one‐half or four periods. Abnormal rate of decrease and subsequent increase of abnormal electron‐production at level of maximum is computed for the first period. This varies between −1000 and +1700 electrons per cc per second.At Watheroo Magnetic Observatory ionospheric changes followed the general pattern usually experienced during magnetic disturbances in temperate zones. Evidence indicates large spatial tilts of iso‐ionic surfaces near Watheroo about 40 minutes after beginning of great magnetic changes. These spatial tilts were followed by rapid rise of height and increase of scattering.Abnormal decrease of ion‐density of the F2‐layer at both observatories began simultaneously, within observational limits, less than 15 minutes after the first great geomagnetic change. The great rise in height at Watheroo lagged behind that at Huancayo by nearly an hour. A chronology of radio fade‐outs and bright chromospheric eruptions before and during the storm is given.