VLF hiss,visual aurora and the geomagnetic activity

Observations and analyses of hiss events, recorded at College (dp. lat. 64.62°N) and Bar 1 (dp. lat. 70.20°N) during periods of varying auroral and geomagnetic activity, reveal three different types of events. These are (1) auroral substorm events with associated hiss bursts during disturbed period, (2) quiet-time hiss events accompanying stationary quiet auroral arcs and (3) hissless events at times of auroral and magnetic activity. Quiet-time observations seem to suggest that the substorm activity is not a necessary requirement for generating wideband hiss. On the other hand, examples of auroral and magnetic activity with complete absence of VLF hiss indicate that the ground reception of VLF/ELF natural emissions is largely controlled by propagation conditions in the ionosphere. There is either little or no correlation found between hiss observations at the two stations separated by about 600 km.     

Mid-latitude pulsating auroras

Pulsating auroras were recorded at Bedford, Massachusetts, cgm lat. 55.4°, 24 March 1969 during a worldwide magnetic storm, the only known published observations of pulsating auroras at such low magnetic latitudes. Spectral density analysis of several minutes of 5577A pulsations indicated a dominant period of 7.2 sec at 0300 EST. The following characteristics were noted: (1) occurrence during a negative bay in H; (2) location toward the equatorial boundary of the auroral display; (3) occurred a few hours after local midnight; (4) characteristic period of 6–10 sec; (5) quasi-sinusoidal or superposition of sinusoids rather than isolated pulses; (6) modulation of the background intensity by 15–30 per cent. These characteristics have previously been observed by others in pulsating auroras in the auroral oval. Other mid-latitude geophysical measurements at the same time show similarities to typical auroral oval behavior. These observations indicate that the auroral oval expanded during the worldwide magnetic storm until the boundary of the auroral oval was near cgm latitude 55°. If this observation of one mid-latitude pulsating aurora is in general valid, then the agreement of the characteristic period of pulsating auroras when the oval has expanded to mid-latitudes with the period of pulsating auroras when the oval is not expanded should be useful in distinguishing between proposed source mechanisms for these pulsating auroras.     

An auroral infrasonic substorm investigation using Chatanika radar and other geophysical sensors

Characteristics of the supersonic auroral arcs within the 0905 UT 2 April 1973 substorm were determined using data from (1) all-sky cameras; (2), surface magnetometers, (3) multispectral scanning photometers, (4) 30MHz riometers, (5) Chatanika incoherent-scatter radar, (6) Homer auroral radar, and (7) infrasonic microphone arrays at College and Stevens Village in Alaska. These data were analyzed to determine the properties of an auroral electrojet arc that generates auroral infrasonic waves (AIW).

An arc that was show to be the source of an AIW was found to have the following characteristics: (1) a velocity of 500 m/sec traveling from an azimuth of 350°; (2) an intensity in 4278 A of 26 Kr, (3) a maximum electron density of 2.8 × 10⁶ el/cm⁶ at 100km height, (4) an equivalent westward line current of 2.8 × 10⁶ A, (5) orientation of ΔH parallel to the AIW direction of travel and perpendicular to the arc's long axis, (6) a characteristic energy of the primary auroral electron spectrum of 3.0keV, and (7) an energy deposition rate for the auroral pdarticles of 100 erg/cm² sec.     

The onset time interval of geomagnetic disturbances in the conjugate areas

The onset time interval of geomagnetic disturbances, as deduced from the numerous cases of rapid changes of K-indices over a cycle in solar activity, was studied for two conjugate auroral stations, Macquarie Island and College. There is a distinct peak in the occurrence number of the disturbances for both stations at an interval 09.00–12.00 U.T., which is close to the local midnight at College but is in the pre-midnight sector at Macquarie Island. For comparison, a similar study was applied to the magnetic data obtained at two more auroral stations, Kiruna and Sodankyla. The onset time of the disturbances for these stations was most frequent at 18.00–21.00 U.T., centered at the conjugate midnight of Kiruna and Sodankyla in the Southern Hemisphere but well ahead of the local midnight of the stations themselves. The specific diurnal occurrences of the disturbances at all four auroral stations are consistent with a difference in the geometry between the southern and northern auroral ovals. It appears that the prevailing onset time of geomagnetic disturbances is associated with the time when both conjugate stations (or conjugate locations) are within the auroral oval and thus accessible to a direct particle influx from the Earth's magnetotail.     

Long period Pc5 pulsations

The characteristics of long period Pc5 pulsations (frequency 3·33-1·67 mHz; period 300–600 sec) for stations in the subauroral, auroral and polar zones are studied for 1967. These pulsations occur mainly in the auroral and polar zones with one morning and one evening peak; in the cusp region they occur most frequently near local noon. The evening peak gets stronger and appears farther away from noon with increasing geomagnetic activity. Periods are shorter and amplitudes larger in the morning compared to the evening hours. Only in a small latitudinal belt (60–70°) do the periods tend to increase with latitude. Amplitudes are almost always maximum near the central line of the auroral zone and drop much more sharply towards lower latitudes than towards higher latitudes. Considerable diurnal variations and also variations with magnetic activity are found to exist in the occurrence-latitude and amplitudelatitude profiles. In all the three regions the occurrence and the amplitude of these pulsations increase with magnetic activity to a certain level after which results become uncertain. Periods either do not change very much or at some stations decrease as activity increases.     

Asteroid 532 Herculina: Lightcurves,pole orientation and a model

Photoelectric lightcurves of 532 Herculina in 1984 show two maxima and two minima with a synodic rotation period of 0.39185 ± 0.00002 day (1σ). During some other oppositions the Herculina lightcurve has only one maximum and one minimum over that same rotation period. The absolute magnitude in V is 6.13 ± 0.02 mag, the phase coefficient in V is 0.037 ± 0.002, and the mean colors are B−V = +0.86 ± 0.04 and U−B = +0.43 ± 0.02. We applied photometric astrometry and the results indicate a sideral period of 0.3918711 ± 0.0000001 day with retrograde rotation for a north pole at 276° long and +1° lat. The uncertainty of the pole is ±1°. A model of Herculina is presented that generates lightcurves consistent with both the observed amplitudes and the timings of extrema over precisely 28,630 sideral rotations during 30 years. The model is a sphere with two dark regions that are each about 0.13 times the brightness of the surrounding surface. The regions are at 0° asterocentric longitude, +15° lat, with a radius of 30°, and 170° long, −38° lat, with a radius of 26°. With the photometric astrometry pole and the model with two dark regions, predicted lightcurves are shown for the next four oppositions.     

Electron counts in the neutral sheet and magnetic variations at a geomagnetic longitude associated ground station

A comparison of the variations in the count of electrons E > 36 keV on the satellite Vela 4A, and in the Macquarie Island magnetometer H trace, shows for a time lag of 22-8 min a correlation, r = 0.95, over a 90 min period of the recovery phase of a magnetospheric substorm on 17 August 1968. All-sky camera data suggest that during the correlation period the auroral electrojet showed very little latitudinal movement. Each peak in electron count relates to a current surge in the electrojet as shown by a deepening of the negative bay at Macquarie Island.Using the Fairfield (1968) model of the location of auroral shells in the solar magnetic equatorial plane, and the known location of the satellite, an estimate of the velocity of tail to Earth plasma convection in the plasma sheet of about 0·33 R_e/min is obtained for the recovery phase.The relationship is discussed between plasma sheet thinning and subsequent broadening, and the extension of the magnetic field lines into the tail region and their subsequent return. This discussion makes use of the estimated time lags between electron count at the satellite and the time of arrival of auroral particles at the antisolar meridian.From a somewhat speculative explanation, but one largely supported from the literature, of the magnetospheric processes involved in this auroral substorm, a plasma velocity estimate of 0·42 R_e/min for the initial phase of the substorm is obtained. These velocities are of the same order as the 0·5 R_e/min obtained by Lezniak and Winkler (1970) at 6·6 R_e.     

A simultaneous observation of large-scale periodic TIDs in both hemispheres following an onset of auroral disturbances

A study has been undertaken on the position of the auroral sources of large-scale TIDs (LS TIDs) in both hemispheres. A selected case study, herewith presented, refers to an onset of an auroral substorm at the equatorward edges of the southern and northern auroral zones which preceded the occurrence of periodic variations in virtual height (h′F) of the F-region in the Southern and Northern Hemispheres. The variations in h′F had characteristics typical of large-scale TIDs propagating equatorwards with a velocity of about 800 m s⁻¹, and with a constant period of 135 min in both hemispheres. The horizontal wavefront of LS TIDs was observed in mid-latitudes to be in excess of 7000 km. The LS TIDs were found to be in phase at the stations which are equidistant from the auroral sources. From this it was concluded that the periodic LS TIDs were likely to produce a constructive interference effect at the points of their encounter near the equator.It was concluded that the sources of LS TIDs in both hemispheres were elongated along the L-shell with L-value between 4 and 5, and had a large longitudinal extent, exceeding 60°. The source locations were consistent with the positions of the belts of energetic particle precipitations as inferred from the standard riometer and magnetometer data. The large quasi-linear extent of the source is consistent with the wide horizontal wavefronts of LS TIDs as well as with a large distance of their propagation.     

Precipitation of auroral particles in the central polar cap during a sudden commencement magnetic storm

All-sky camera observations from two stations in the inner (northern) polar cap and an auroral zone station are combined with photometer records from the polar cap station Nord in a study of the brilliant auroral display following the ssc of the storm of 7 November 1970. This display is the large, poleward expanding bulge of a substorm triggered by the ssc. It is composed of brilliant discrete forms embedded in low-intensity diffuse electron and proton aurora. The poleward edge of the diffuse electron aurora is 5° north of the discrete auroras and 3° north of the proton aurora. The intensity of the discrete aurora varies as the strength of the auroral electrojet as shown by magnetograms from auroral zone stations. Succeeding the retreating display a subvisible low-energy electron precipitation, which may be identified as the polar squall (Winningham and Heikkila, 1974) is observed over the polar cap during the main phase of the storm.In the early morning sector already existing diffuse auroras broaden towards the equator from the time of the ssc and at least during the following half hour.Ssc-triggered displays have been found (Feldstein, 1959) to withdraw from the inner polar cap as the initial (positive H) phase of the storm ends. A comparison of the records from seven low-latitude stations shows that during this particular storm the positive phase appears to be composed by two overlapping disturbances, i.e. the proper initial phase, which is generally thought to be due to compression of the inner magnetosphere and a series of positive bays accompanying the negative bays in auroral latitudes. These positive bays are observable over a great range of longitudes with a maximum of amplitude near midnight. As judged from the dayside magnetograms the initial (compression) phase ends at an early stage of the substorm. The observed coincidence between the withdrawal of the display and the cessation of the positive H phase of the storm is a consequence of the fact that the second component—the positive bays—and the auroral display over the polar cap are both signatures of the substorm activity.     

A spherical harmonic analysis of the austral auroral oval

Computerized spherical harmonic analysis is applied to the morphology of the southern auroral oval. Records from 23 All Sky Camera stations together with visual observation reports for the period 1957–1959 constitute the raw data set. The mode of the derived auroral occurrence distribution function F(K_p, θ, φ) is regarded as the maximum probability contour and yields a set of auroral ovals. These 10 contours, one for each K_p level, are expressed in the invariant magnetic co-ordinates of Bond (1968).     

A uniform belt of diffuse auroral emission seen by the ISIS-2 scanning photometer

One of the most striking and persistent features in high latitude regions as seen by the ISIS-2 scanning auroral photometer is a fairly uniform belt of diffuse auroral emission extending along the auroral oval. Indications are that this region follows, contributes to, and may in a sense actually define the auroral oval during quiet times.The diffuse belt is sharply defined at its equatorward edge, which is located at an invariant latitude of about 65° in the midnight sector during relatively low magnetic activity (K_p = 1?3). The poleward edge of the region is not as sharply defined but is typically at about 68°. Discrete auroras (arcs and bands) are located, in general, near the poleward boundary of the diffuse aurora. The position of the belt appears to be relatively unaffected by the occurrence of individual substorms, even when discrete forms have moved well poleward. Representative intensities at 5577 Å are 1–2 kR (corrected for albedo) at quiet times and may reach 5 kR during an auroral substorm.It appears that the mantle aurora and proton aurora constitute this diffuse aurora in the midnight sector. Precipitating protons and electrons both contribute to the emissions in this region.     

A new direction finding technique for auroral VLF hiss based on the measurement of time differences of arrival at three spaced observing points

A newly developed direction finding (DF) technique for auroral hiss based on the measurement of time differences of wave arrival was carried out in 1978 at Syowa Station (geomag. lat. -70.4°), Antarctica and its two slave unmanned observing points located at about 20 km distances from Syowa. The auroral hiss signals (0.3–100 kHz) received at the two spaced points were transmitted to Syowa by a wide-band telemeter of 2 GHz. The arrival time difference of auroral hiss between Syowa and each spaced point was automatically determined by cross-correlating the waveforms of the received signals, and then the incident and azimuthal angles were measured with an accuracy of about 10°.It has been found that the new DF technique can determine localized exit regions at the ionospheric level which show rapid temporal movements. A comparison of the DF results with ground-based auroral data has shown that impulsive type auroral hiss with a wide-band frequency range has not emerged from the whole region of a bright aurora but from some localized regions of bright electron auroras at the ionospheric level, and that the arrival directions of auroral hiss change rapidly in accordance with the auroral movements.     

Temporal and spatial changes of polar substorms and infrasonic wave emissions

By use of auroral infrasonic wave (AIW) records observed at the Syowa station, Antarctica, the relations between the AIW emissions and the auroral substorms were studied separately in the cases of emissions in the dusk, dawn and midnight. For this purpose, contour maps of the horizontal magnetic vectors at the time of AIW emissions were compared with the equivalent current systems. The followings are found. The existence of substorm activity is always found at the time of AIW emissions, but its strength has only minor relations to their occurrence. The condition for the occurrence of the emission depends largely on the manner how the disturbed region moves. The AIWs are emitted when the disturbed region moves with a supersonic speed and the trace direction of the wave roughly coincides with the direction of the traveling disturbance. While the morning and the evening AIWs are emitted from the rear region of the traveling disturbance, the midnight one from the front region. The AIW emissions tend to synchronize with the overhead crossing of the electrojet current and at that time the current causes the rotational change in the horizontal magnetic vectors on the ground.     

A global view at the polar region on 18 December 1971

The ISIS-2 scanning auroral photometer surveyed the polar region during three successive passes on 18 December 1971, at times when Kp values were still high due to an intense magnetic storm which began on 16 December. Two very bright (IBC III) auroral substorm patterns were seen to correspond to rather weak magnetic substorms (about 300 γ in magnitude). A large spiral auroral pattern, with intensity of the order of 100 kR and a size of about 1300 km, was present in the polar cap; it gradually decreased in size and intensity during the interval 0200–0600 UT. A region of enhanced 3914 emission was present in the noon sector of the auroral oval between 0200 and 0400. The presence of the diffuse auroral belt is also evident at all local times during this period, extending down to about 61° corrected geomagnetic latitude in the midnight sector.     

Global Simulation of Magnetospheric Space Weather Effects of the Bastille Day Storm

We present results from a global simulation of the interaction of the solar wind with Earth's magnetosphere, ionosphere, and thermosphere for the Bastille Day geomagnetic storm and compare the results with data. We find that during this event the magnetosphere becomes extremely compressed and eroded, causing 3 geosynchronous GOES satellites to enter the magnetosheath for an extended time period. At its extreme, the magnetopause moves at local noon as close as 4.9 R _E to Earth which is interpreted as the consequence of the combined action of enhanced dynamic pressure and strong dayside reconnection due to the strong southward interplanetary magnetic field component B _z, which at one time reaches a value of −60 nT. The lobes bulge sunward and shield the dayside reconnection region, thereby limiting the reconnection rate and thus the cross polar cap potential. Modeled ground magnetic perturbations are compared with data from 37 sub-auroral, auroral, and polar cap magnetometer stations. While the model can not yet predict the perturbations and fluctuations at individual ground stations, its predictions of the fluctuation spectrum in the 0–3 mHz range for the sub-auroral and high-latitude regions are remarkably good. However, at auroral latitudes (63° to 70° magnetic latitude) the predicted fluctuations are slightly too high. Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1014228230714     

Precession of the orbital nodes of Jupiter and Saturn triggered by the mutual perturbation: A model of two rings

The problem of the precession of the orbital planes of Jupiter and Saturn under the influence of mutual gravitational perturbations was formulated and solved using a simple dynamical model. Using the Gauss method, the planetary orbits are modeled by material circular rings, intersecting along the diameter at a small angle α. The planet masses, semimajor axes and inclination angles of orbits correspond to the rings. What is new is that each ring has an angular momentum equal to the orbital angular momentum of the planet. Contrary to popular belief, it was proved that the orbital resonance 5: 2 does not preclude the use of the ring model. Moreover, the period of averaging of the disturbing force (T ≈ 1332 yr) proves to be appreciably greater than a conventionally used period (≈900 yr). The mutual potential energy of rings and the torque of gravitational forces between the rings were calculated. We compiled and solved the system of differential equations for the spatial motion of rings. It was established that a perturbing torque causes the precession and simultaneous rotation of the orbital planes of Jupiter and Saturn. Moreover, the opposite orbit nodes on the Laplace plane coincide and perform a secular movement in retrograde direction with the same velocity of 25.6″/yr and the period T _J = T _S ≈ 50687 yr. These results are close to those obtained in the general theory (25.93″/yr), which confirms the adequacy of the developed model. It was found that the vectors of the angular velocity of orbital rings move counterclockwise over circular cones and describe circles on the celestial sphere with radii β₁ ≈ 0.8403504° (Saturn) and β₂ ≈ 0.3409296° (Jupiter) around the point which is located at an angular distance of 1.647607° from the ecliptic pole.     

Measurements of thermospheric response to auroral activities

The Joule heating produced by auroral electrojets and its thermospheric response can be studied by monitoring the thermospheric temperatures by means of optical methods; simultaneously investigating the concurrent auroral electrojet activities using geomagnetic records obtained from stations along a meridian close to the observation site of optical measurements. We report, in this paper, the measurements of thermospheric response to auroral activities which were made at Albany (42.68°N, 73.82°W), New York on 2 September 1978 (U.T.) when an isolated substorm occurred. The thermospheric temperatures were measured by using a high-resolution Fabry-Perot interferometer that determines the line profiles of the [OI] 6300 Å line emission. The intensities and latitudinal positions of auroral electrojets were obtained by the analysis of magnetograms from the IMS Fort Churchill meridian chain stations.     

On the production of nitric oxide by cosmic rays in the mesosphere and stratosphere

Nitric oxide is formed in the atmosphere through the ionization and dissociation of molecular nitrogen by galactic cosmic rays. One NO molecule is formed for each ion pair produced by cosmic ray ionization.The height-integrated input (day and night) to the lower stratosphere is of the order of 6 × 10⁷ NO molecules cm^?2/sec in the auroral zone (geomagnetic latitude Φ ? 60°) during the minimum of the sunspot cycle and 4 × 10⁷ NO molecules cm^?2/sec in the subauroral belt and auroral region (Φ? 45°) at the maximum of solar activity. The tropical production is less than 10^?7 NO molecules cm^?2/sec above 17 km and at the equator the production is only 3 × 10⁶NO molecules cm^?2/sec.     

Astrometric Study of Two Multiple Stars: ADS 2668 and ADS 8236

Based on CCD observations with the Pulkovo 26-inch refractor in 2003–2018, we have obtained the orbit of the visual double star ADS 2668 AB (P = 947 yr, a = 2.9″, e = 0.41, ω = 246°, Ω = 131°, i = 114°, T = 1456 yr) for the first time by the apparent motion parameter (AMP) method, which is consistent with the inner orbit of ADS 2668 Aa-Ab, and improved the orbit of ADS 8236 AB (P = 1996 yr, a = 4.69″, e = 0.39, ω = 201°, Ω = 166°, i = 110°, T = 1246 yr). The inner orbit of the photocenter of ADS 8236 with a period of 4.627 yr has been calculated from the residuals. This orbit of ADS 8236 Ba–Bb supplements the spectroscopic orbit by the elements specifying the orbital plane (i and Ω). In both cases, the planes of the inner and outer orbits are noncoplanar. The presence of an additional companion in the system ADS 2668 is discussed.     

Semi-annual modulation of earth's magnetic field in the equatorial electrojet region

Autospectra in the 2–13 month range, computed from mean monthly horizontal intensity on quiet days at Trivandrum, situated close to the dip equator, suggest an exceedingly large semi-annual modulation of the field confined to an interval of about 5 hr centred at 1000 LT. The amplitude of the semi-annual oscillation at this station, derived from power density, is greater than 19 γ at 1000 LT. Between 1900 and 0500 LT, spectral lines, corresponding to a period of six months, are not observed above the continuum. Spectral densities from observations at two other electrojet stations in India, Kodaikanal and Annamalainagar, and at Alibag, outside the electrojet, establish the existence of an appreciable enhancement of the semi-annual oscillation of the field in the equatorial electrojet belt. Similar computations of spectra using observations on all days, however, suggest a secondary component in the evening sector. This component is not enhanced in the equatorial electrojet belt. It is concluded that while in low latitudes the daytime component is largely associated with the modulation of Sq currents, in the electrojet belt it appears to be due entirely to a semi-annual modulation of the equatorial electrojet. It is also concluded that the secondary component, observed in the evening sector in low latitude and equatorial stations, is associated purely with the modulation of the ring current by disturbance. The two components of the semi-annual variation observed at the Indian stations have also been noticed at several stations between geomagnetic latitudes N54.6° and S41.8°. It is also observed that the association of the semi-annual component with geomagnetic latitude is confined to the evening-night component.