Preliminary Impulse Research Articles

Magnetospheric ULF Wave Phenomena Stimulated by SSC.

Several ssc were recorded simultaneously at the low-latitude longitudinal and along the 210° magnetic meridian networks. The considered ULF wave phenomena stimulated by the ssc comprise main and preliminary impulses, Psc 3 pulsations, ssc-related Pc 3 and cavity-like oscillations. The analysis of observed ssc shows that the preliminary impulse is still not able to explain invoking by the existing theory. It is also found some problems concerning the interpretation of features of other ssc-associated wave phenomena. Further specially coordinated observations at a world-wide magnetometer network with high precision of time synchronization are required to understand these unresolved problems.

Sudden impulses at subauroral latitudes: Response for northward interplanetary magnetic field

The response of subauroral H component magnetograms at the time of passage of interplanetary shocks under northward interplanetary magnetic field (IMF) conditions is used to examine the behavior of the magnetosphere when it is suddenly compressed. At subauroral latitudes, near 55° or L = 3, the response has some similarities and some important differences from the low‐latitude response. The first effect is a preliminary impulse which appears to be due to the Hall current driven in the ionosphere by the arrival of the first Alfven waves along the magnetic field lines from the distant equatorial magnetosphere that has been set in motion by the penetration of the shock wave into the magnetosphere. Next there is a general increase of the magnetic field everywhere as the magnetosphere is compressed to its new size and the information of this compression propagates to the ground. This increase takes about 5 min followed by a 10‐min relaxation to the final asymptotic value. This general increase is accompanied by two more localized current systems: one transient and one steady. The transient current appears to be a dual‐vortex current system launched from noon toward the nightside. The other steady system is a fixed double‐cell convection system. We identify the former with the propagation of the magnetopause indentation associated with the enhanced pressure in the solar wind. We attribute the latter, steady system to high‐latitude reconnection with northward IMF.

Acoustic effects in solids during thermoemission of fracture processes

Heat released at the moment of fracture by the breaking of intermolecular bonds is considered to be the reason for elastic wave generation before the crack jump. Properties of such a preliminary impulse are considered and the information, about the crack and the material parameters, which is contained in these signals, is analysed. The results are discussed for the case of the acoustic emission from a crack growing during a load.

Penetration of polar electric fields to the nightside dip equator at times of geomagnetic sudden commencements

Measurements of Doppler Velocity, VD of F ‐ region reflections at normal incidence over Kodaikanal (dip 3°N, 77° 28′E) are used to study the nature of perturbations in F ‐ region vertical plasma drift, Vz associated with the geomagnetic sudden commencements (sc) on July 8, 1991, and January 1, 1992. Both the events which occurred in a narrow time window, 1630‐1700 UT (2200‐2230 IST) were of sc* type at middle‐ and high‐ latitude stations in the afternoon sector. At Kodaikanal, which is on the nightside, the sc of January 1, 1992, is characterized by a double‐step main impulse (MI) in H ‐ component (the structure of the sc on July 8 could not be ascertained from normal run magnetograms because of the large amplitude and very small rise time of the sc). It is found that the usual downward motion of F ‐ region plasma during the premidnight hours at Kodaikanal suddenly ceased (and even reversed to upward in one event) for ∼1 min coincident with the preliminary impulse (PI) and was immediately enhanced in association with the MI of the sc*. This pattern which is consistently seen in the two events implies that an eastward electric field prevails near the nightside dip equator at the time of the first impulse of double‐step MI there and the PI of sc* at high latitudes. Our Doppler velocity observations constitute the first and direct experimental evidence of vertical plasma motions due to the sc ‐ associated electric fields in the nighttime dip equatorial ionosphere. They substantiate the view based on theory (Kikuchi and Araki, 1979) and ground‐based magnetic observations (Araki et al, 1985) that the dusk‐to‐dawn electric field imposed on the polar ionosphere with the onset of PI of sc* is instantaneously transmitted to the dip equator on the nightside as on the dayside.

Gene expression and cell differentiation in the nervous system

We provide insights about how the Total Electron Content (TEC), derived by a network of Global Navigation Satellite System (GNSS) receivers over the European longitudinal sector, reacts to a Sudden Impulse (SI). In particular, we investigate how the different components of the TEC variability relates with the ionospheric currents, derived by ground magnetometers and the Piersanti and Villante (2016) model, during the 2015 Saint Patrick’s Day storm. To identify the different TEC components, we apply the Ensemble Empirical Mode Decomposition (EEMD) analysis to vertical TEC (vTEC) time series to isolate its SI-related variations over co-located GNSS receiver-magnetometer couples. With such an approach, we were able to identify for the first time a peculiar vTEC mode (WHT∗, T ≃ 45 min) switching on during the SI and propagating southward with a velocity of ∼1.5 km/s. We also found a linear relationship between the estimated ionospheric current and the temporal fluctuations of WHT∗ for both the Preliminary Impulse (PI) and the Main Impulse (MI). The slope has been found to depend only on the electron charge, on the magnetic permeability and on the altitude of the ionospheric E-layer, as expected from first principles equations and as discussed in details.

Electric conductivity of the ionosphere and nocturnal currents

We have obtained both the height and global distribution of the electric conductivity in the ionosphere by using the electron density based on the International Reference Ionosphere (IRI) model. Results based on three different formulations of estimating the conductivity are compared. It is shown that at night the Pedersen conductivity of the F-region is larger than that of the E-region. The height profile of the conductivity at the sunspot maximum is much different from that at the sunspot minimum. The peak value of the Pedersen conductivity in the F-region at midnight when R = 200 is about 50 times as large as that when R = 35. On the other hand, the Hall or Pedersen conductivity in the E-region is only about 2 times larger for the same values of R. The equatorial enhancement of the height integrated conductivity ( Σ yy ) is hardly seen in the night-time at the sunspot maximum, but is recognizable at the sunspot minimum. Along the dip equator the ratio of the conductivity in the day-time to that at night is about 50 at the sunspot minimum and 15 at the sunspot maximum. In low latitudes other than the dip equator it is about 20 at the sunspot minimum, but at the sunspot maximum it is not so different from unity. This difference in the structure of the conductivity by the solar activity can explain the difference between the geomagnetic Sq field at the sunspot maximum and that at the sunspot minimum. The analysis of the geomagnetic field in the night-time shows that nocturnal ionospheric currents surely exist at the sunspot maximum. Moreover, a preliminary impulse ( PI) of geomagnetic sudden commencement ( SC) is observed at low latitudes without the equatorial enhancement of its amplitude at night mainly when the solar activity is high. These features are consistent with our estimation of the conductivity distribution.

Numerical analysis of equatorial enhancement of geomagnetic sudden commencement

Equatorial behaviour of a polar-originating ionospheric current is examined by solving numerically the continuity equation on a two-dimensional spherical shell with appropriate assumptions for the ionospheric conductivity and the field-aligned source currents. The results show a clear daytime equatorial enhancement of the ionospheric currents in spite of much reduced electric field due to shielding effects of the enhanced Cowling conductivity there. The results are used for interpretation of the preliminary impulse of the geomagnetic sudden commencement.

Sudden commencements observed by MAGSAT above the ionosphere.

Among 21 geomagnetic sudden commencements (SC), each of which was reported by more than 2 ground observatories, 16 were detected by MAGSAT above the ionosphere (260-560km altitude) in the dawn-dusk meridian. The amplitude of the remaining 5SCs was small on the ground (less than 10nT). The SCs at MAGSAT appeared mainly in the H-component in low latitudes and in both H- and D-components in high latitudes. The Z-component did not show significant variations. The amplitude of the SCs was usually larger at the satellite altitude than on the ground, and the averaged ratio was 1.3 for the low latitude SCs. The larger amplitude at the satellite altitude was interpreted in terms of a westward zonal shielding current flowing in the ionosphere. A detailed analysis was made for one SC which occurred when the satellite was flying very closely above a ground station (Magadan in east Siberia, geomag. lat.=51.1°) in a duskside (16h-17h LT) meridian. The H-component at the satellite showed a positive impulse of 52nT and the D-component a negative impulse of 43nT in the beginning part of the SC. The duration of the impulse was 2-3min. The SC observed by normal-run magnetographs at ground stations near the satellite orbit was preceded by a typical preliminary impulse (PI) of opposite deflection to that of the pulse observed by MAGSAT. By the use of high time resolution data from the IMS magnetometer network Alska chain, the pulse detected by MAGSAT was identified with the PI observed at the ground stations, and it was concluded that the main source current for the PI flowed between the satellite and the ground, that is, in the ionosphere; but effects of other source currents including field aligned currents should be taken into account.

Detection of an ionospheric current for the preliminary impulse of the geomagnetic sudden commencement

MAGSAT observed a geomagnetic sudden commencement at an altitude of 550 km on its dawnside orbit above the South Atlantic Ocean. The H‐component of the magnetic field increased monotonically by 46 nT in about 4 minutes, while the D‐component decreased westward by 10 nT in the first 1‐2 minutes and then increased gradually. There was no significant deviation in the Z‐component. The comparison with ground‐based magnetograms showed that the negative impulse of the D‐component provided the first experimental evidence of the global ionospheric current system for the preliminary impulse of the geomagnetic sudden commencement.

Theory for preliminary negative impulse in storm sudden commencement inH at equatorial stations

Theory for preliminary negative impulse in storm sudden commencement inH at equatorial stations

The propagation of world-wide sudden commencements of magnetic storms

A study of the times of three sudden commencements, recorded on August 3, September 21, and November 6, 1957, respectively, at ten widely-separated magnetic observatories, indicates that when main and preliminary impulses are both recorded at one place (as in the typical SC*) they really begin approximately simultaneously. Therefore, it would appear that in nontropical regions the rate of growth of the so-called preliminary impulse is usually greater than that of the main impulse, so that the latter is obscured until the former begins to decay. Differences around the earth between recorded times of the first impulse, whether the sudden commencement is an SC or SC* type, are only a few seconds, and the evidence suggests that the position of the sun controls the hemisphere in which the sudden commencement first occurs. This finding is interpreted in terms of the Singer shock-wave theory to mean that, as would be expected, the shock wave enters the auroral zone nearest the sun first and produces the sudden commencement a few seconds earlier in that hemisphere. At the equinox the sudden commencement times are roughly symmetrically distributed with respect to the geomagnetic equator.

“SUDDEN COMMENCEMENTS” IN THE GREENWICH MAGNETIC RECORDS (1879-1944) AND RELATED SUNSPOT DATA

Summary Part I: Statistics of “sudden commencements” -their average amplitude, direction of impulse, hourly and monthly frequencies-are derived from 681 cases identified on the Greenwich magnetograms over six sunspot cycles. Besides SCs that mark the onset of many geomagnetic storms, there are frequent cases in which no appreciable disturbance follows. About half of all SCs have a small preliminary impulse (lasting about half a minute) in opposition to the main impulse, most commonly of three minutes' duration. The presence of this preliminary impulse seems subject to some diurnal effect. Both diurnal and seasonal effects appear in SC frequency occurrence. An unexpected result is a marked minimum around 8h-9h. SCs that do occur then are usually inverted (the normal main stroke is +ΔH, +ΔV and - ΔD). Details are tabulated for every SC recorded at Abinger from 1938 November to 1944 December for comparison elsewhere. Part II: SC occurrence inrelation to the sunspot cycle and to individual sunspots is investigated. Each of the four groups into which the data were divided gives a frequency curve similar to the mean sunspot curve. In this respect, small geomagnetic storms with no SC onset have a different distribution within the sunspot cycle. A marked tendency for SCs followed by storms to occur when big spots are about one day past the central meridian accords with a well-known relationship between the larger magnetic storms and the bigger sunspots. The association of SCs not followed by disturbance with respect to big sunspots appears largely fortuitous, but some effect from spots nearing the western limb should not be ruled out. An addendum gives the results of a sample comparison of SC pulses and other minor movements on the Abinger and Lerwick magnetograms. SCs as recorded at the two stations are very similar, but ΔV at Lerwick is normally negative. During inactive periods a remarkable similarity of very small detail is noted between these two stations which are over 600 miles apart.