Features of <Emphasis Type="Italic">Pc</Emphasis>5 pulsations in the geomagnetic field,auroral luminosity,and Riometer absorption

Simultaneous morning Pc5 pulsations (f ~ 3–5 mHz) in the geomagnetic field, aurora intensities (in the 557.7 and 630.0 nm oxygen emissions and the 471.0 nm nitrogen emission), and riometer absorption, were studied based on the CARISMA, CANMOS, and NORSTAR network data for the event of January 1, 2000. According to the GOES-8 satellite observations, these Pc5 geomagnetic pulsations are observed as incompressible Alfvén waves with toroidal polarization in the magnetosphere. Although the Pc5 pulsation frequencies in auroras, the geomagnetic field, and riometer absorption are close to one another, stable phase relationships are not observed between them. Far from all trains of geomagnetic Pc5 pulsations are accompanied by corresponding auroral pulsations; consequently, geomagnetic pulsations are primary with respect to auroral pulsations. Both geomagnetic and auroral pulsations propagate poleward, and the frequency decreases with increasing geomagnetic latitude. When auroral Pc5 pulsations appear, the ratio of the 557.7/630.0 nm emission intensity sharply increases, which indicates that auroral pulsations result from not simply modulated particle precipitation but also an additional periodic acceleration of auroral electrons by the wave field. A high correlation is not observed between Pc5 pulsations in auroras and the riometer absorption, which indicates that these pulsations have a common source but different generation mechanisms. Auroral luminosity modulation is supposedly related to the interaction between Alfvén waves and the region with the field-aligned potential drop above the auroral ionosphere, and riometer absorption modulation is caused by the scattering of energetic electrons by VLF noise pulsations.     

Relation between sudden increases in the solar wind dynamic pressure,auroral proton flashes,and geomagnetic pulsations in the Pc1 range

The interrelation between sudden increases in the solar wind dynamic pressure, auroral proton flashes on the dayside equatorward of the oval, and geomagnetic pulsations in the Pc1 range is considered on the basis of simultaneous observations of the solar wind plasma parameters, proton auroras on the IMAGE satellite, and geomagnetic pulsations at the Lovozero Observatory. It is indicated that proton luminosity flashes were observed in 70% of cases equatorward of the auroral oval during sudden changes in the solar wind pressure. In this case, flashes of proton auroras were observed in 85% of cases during sudden changes in the pressure, which were related to interplanetary shocks. Increases in pressure during tangential discontinuities were accompanied by flashes of proton auroras only in 45% of cases. When the ground station was conjugate to the region occupied by a proton aurora flash, the appearance or intensification of existent pulsations in the Pc1 range was observed in 96% of cases. When the ground station was not conjugate to the region of a proton luminosity flash, the response in geomagnetic pulsations was observed in 32% of events. When a sudden change in the solar wind pressure was not accompanied by a proton luminosity flash, the response in pulsations in the Pc1 range was hardly observed.     

A slow mode wave as a possible source of Pi 2 and associated particle precipitation: a case study

An intensification of auroral luminosity referred to as an auroral break-up often accompanies the onset of geomagnetic pulsation (Pi 2) at the dip-equator. One such auroral break-up occurred at 2239 UT on 16 June, 1986, being accompanied by weak substorm activity (AE≈50 nT) which was recorded in all-sky image of Syowa Station, Antarctica (66.2°S, 71.8°E in geomagnetic coordinates). The associated Pi 2 magnetic pulsation was detected by a fluxgate magnetometer in the afternoon sector at the dip-equator (Huancayo, Peru; 1.44°N, 355.9° in geomagnetic coordinates; 12.1°S, 75.2°W in geographic coordinates; L = 1.00). In spite of the large separation of the two stations in longitude and latitude, the auroral break-up and subsequent luminosity modulation were seen to be correlated with the wave form of the ground Pi 2 pulsation. This occurred in such a way that the luminosity maximum was seen to occur at the phase of maximum amplitudes of Pi 2 wave form. We argue that the observed correlation could be interpreted as indicating a Pi 2-modulation of a field-aligned acceleration of the low energy electrons that may occur near the equator of the midnight magnetosphere.     

Features of morning-time auroras during SC

The optical observations on Heiss Island and the ion drift measurements on the DMSP F8 satellite were used to study the aurora characteristics and ionospheric convection before and after SC registered at 2330 UT on January 13, 1988. It has been indicated that two zones of luminosity can be distinguished in morning-time auroras during the quiet period before SC: the soft zone with auroral arcs and the harder diffuse auroral zone (equatorward of the first zone). After SC, a gradual smooth activation of auroras in both zones was followed (4–5 min later) by a more abrupt intensification of diffuse luminosity and by the appearance of numerous bright discrete auroras throughout the sky. In the diffuse auroral zone, the variations in the luminosity intensity with a period of 6–7 min were observed after SC. Auroral and geomagnetic field pulsations are closely correlated. During the quiet period before SC, sunward convection was concentrated in the soft precipitation region in the form of jets located in the vicinity of auroral arcs. After SC, considerable sunward convection was observed in the diffuse auroral zone. Peaks of the upward ion drift velocity were registered in the vicinity of auroral arcs.     

Unusual spatial-temporal dynamics of geomagnetic disturbances during the main phase of the extremely strong magnetic storm of November 7–8, 2004

The spatial dynamics of geomagnetic variations and pulsations, auroras, and riometer absorption during the development of the main phase of the extremely strong magnetic storm of November 7–8, 2004, has been studied. It has been indicated that intense disturbances were observed in the early morning sector of auroral latitudes rather than in the nighttime sector, as usually takes place during magnetic storms. The unusual spatial dynamics was revealed at the beginning of the storm main phase. A rapid poleward expansion of disturbances from geomagnetic latitudes of 65°–66° to 74°–75° and the development of the so-called polar cap substorm with a negative bay amplitude of up to 2500 nT, accompanied by precipitation of energetic electrons (riometer absorption) and generation of Pi2–Pi3 pulsations, were observed when IMF B _z was about ?45 nT. The geomagnetic activity maximum subsequently sharply shifted equatorward to 60°–61°. The spatial dynamics of the westward electrojet, Pi2–Pi3 geomagnetic pulsations, and riometer absorption was similar, which can indicate that the source of these phenomena is common.     

Studying geomagnetic pulsation characteristics with the local approximation method

A local approximation method based on piecewise sinusoidal models has been proposed in order to study the frequency and amplitude characteristics of geomagnetic pulsations registered at a network of magnetic observatories. It has been established that synchronous variations in the geomagnetic pulsation frequency in the specified frequency band can be studied with the use of calculations performed according to this method. The method was used to analyze the spectral–time structure of Pc3 geomagnetic pulsations registered at the network of equatorial observatories. Local approximation variants have been formed for single-channel and multichannel cases of estimating the geomagnetic pulsation frequency and amplitude, which made it possible to decrease estimation errors via filtering with moving weighted averaging.     

Dayside auroras during unusually large positive values of the IMF <Emphasis Type="Italic">B</Emphasis><Subscript><Emphasis Type="Italic">z</Emphasis></Subscript> component

The characteristics of dayside auroras during the large (16–24 nT) positive values of the IMF B _z component, observed on January 14, 1988, during the interaction between the Earth’s magnetosphere and the body of the interplanetary magnetic cloud, have been studied based on the optical observations on Heiss Island. A wide band of diffuse red luminosity with an intensity of 1–2 kilorayleigh (kR) was observed during 6 h in the interval 1030–1630 MLT at latitudes higher than 75° CGL. Rayed auroral arcs, the brightness of which in the 557.7 nm emission sharply increased to 3–7 kR in the postnoon sector immediately after the polarity reversal of the IMF B _y component from positive to negative, were continuously registered within the band. Bright auroral arcs were observed at the equatorward edge of red luminosity. It has been found out that the red auroral intensity increases and the band equatorward boundary shifts to lower latitudes with increasing solar wind dynamic pressure. However, a direct proportional dependence of the variations in the auroral features on the dynamic pressure variations has not been found. It has been concluded that the source of bright discrete auroras is located in the region of the low-latitude boundary layer (LLBL) on closed geomagnetic field lines. The estimated LLBL thickness is ∼3 R _e . It has been concluded that the intensity of the dayside red band depends on the solar wind plasma density, whereas the position of the position equatorward boundary depends on the dynamic pressure value and its variations.     

Optical effects in the aurora caused by ionospheric HF heating

Ionospheric heating experiments were done by the EISCAT Heater in Tromsø on 15–19 November, 1993. A low-light TV camera was installed at the VLF receiving station at Porojärvi about 100 km to the south-east of Tromsø. The spectral analysis of the auroral luminosity variations showed that the brightness of the aurora varied at the modulation frequency of the heating wave. The results of this analysis and the numerical simulations of the auroral luminosity variations caused by the HF heating are shown. The variations of the optical emission intensity at the heating frequency occur during the auroral ionosphere modification. The observed intensity variation of the auroral green line during the interval of enhanced electron temperature is explained by a decreasing rate of the O₂⁺ ion dissociative recombination when the electron temperature increases. The brightness variation depends on the characteristic energy and the intensity of the auroral electron flux and the heating wave parameters. The artificial luminosity pulsations caused by HF heating are estimated.     

Relation between auroral and geomagnetic pulsations at polar latitudes according to the observations on spitsbergen

Geomagnetic pulsations of the Pc4–5 type at the Barentsburg Observatory for December 2007 to January 2008 are compared with the auroral intensity variations based on the photometric records at the same observatory. In all cases, auroral pulsations similar in shape are also observed simultaneously with geomagnetic pulsations. In the morning and daytime hours, the pulsation radiance maxima fall on the positive half-periods in the H component at the observation point; in the evening and nighttime hours, they fall on the negative half-periods.     

Precipitation of energetic electrons and Pi3 geomagnetic pulsations at polar latitudes

Precipitation of electrons with energies of 0.3–1.5 MeV has been analyzed based on the CORONAL-F satellite data at polar latitudes of the Northern Hemisphere on December 13, 2003. The instants of electron precipitation have been compared with the ground-based observations of geomagnetic disturbances and auroras near the satellite orbit projection. It has been indicated that precipitation of energetic electrons in the high-latitude nightside sector is accompanied by the simultaneous development of bay-like magnetic field disturbances on the Earth’s surface and the appearance of riometer absorption bursts and Pi3 geomagnetic pulsations, and auroras.     

Localization of the source of precipitating electrons in active arcs during breakup and in pulsating auroras

The sensitive method for detecting and measuring the velocity of a weak luminosity wave, traveling from bottom to top along an arc or isolated auroral beams, has been developed. This wave is caused by dispersion of precipitating electrons over velocities and by a differential atmospheric penetration of different-energy electrons, and the wave velocity gives information about the location of the electron acceleration region in the magnetosphere. The method was tested using different model signals and was used to study pulsating auroras and auroral breakup. A luminosity wave has been detected in pulsating auroras, and it has been estimated that the injection region is located at a distance of 5–6 R _e . The application of the method to intensification of auroras during breakup indicated that such a wave is absent; i.e., breakup electrons being accelerated near the ionosphere at altitudes of 2000–8000 km. It has been assumed that the regions of anomalous resistance, generated in the ionosphere by field-aligned currents during the breakup phase, cause intense local field-aligned electric fields. These fields accelerate thermal electrons and form the auroral breakup pattern.     

Constant auroral forms during regular pulsations

A case is described in which complex auroral forms varied slightly at Lovozero Observatory over the course of more than an hour in the morning hours during the auroral recovery phase. Pc3 and Pc5 auroral and geomagnetic pulsations were observed during the event. The phenomenon is compared with recurrent pulsating auroras, which are described in the literature.     

Ground-based observations of auroras under the sunlit conditions

The results of the ground-based optical observations of sunlit auroras, performed at Lovozero and Apatity observatories on April 10 (event 1) and April 27, 2007(event 2), are presented. The observations were performed in the (OI) 557.7 nm emission, using a new equipment based on a Fabry-Pérot interferometer connected to a PhotonMAX CCD camera. During event 1, the observations were performed in the Harang discontinuity region at a low magnetic disturbance. It has been indicated that an auroral arc was located in the polar part of the eastward electrojet, and the arc position coincides with the equatorward boundary of structured precipitation (b2e). During event 2, auroras were observed within the average statistical boundaries of the auroral oval and the region of structured precipitation under the conditions of rather high geomagnetic activity. However, during the period of low geomagnetic activity, discrete auroras were registered at a geomagnetic latitude of ~64° on that day, which is 3°—4° equatorward of the structured precipitation region. Such a low latitudinal position of auroras can be explained by the effect of a high solar wind velocity, which was ~580 km/s during the period of observations.     

Long-period geomagnetic pulsations in the quasi-conjugate arctic and antarctic regions during the magnetic storm of April 16–17, 1999

Based on the observations in six pairs of almost conjugate high-latitude stations in the Arctic and Antarctic regions, the spectral and spatial-temporal structures of long-period geomagnetic pulsations (f = 2–5 mHz) during the magnetic storm of April 16–17, 1999, which is characterized by a high (up to 20 nPa) solar wind dynamic pressure, have been studied. It has been indicated that the magnetic storm sudden commencement is accompanied by a symmetrical excitation of np pulsations near the dayside polar cusps with close amplitudes. Under the conditions when IMF B _z > 0 and B _y < 0, strong magnetic field variations with the periods longer than 15–20 min were observed only in the northern polar cap. When IMF B _z and B _y became close to zero, geomagnetic pulsation bursts in both hemispheres were registered simultaneously but differed in the spectral composition and spatial distribution. In the Northern Hemisphere, pulsations were as a rule observed in a more extensive latitude region than in the Southern Hemisphere. In the Northern Hemisphere, the oscillation amplitude maximum was observed at higher latitudes than in the Southern Hemisphere. The pulsation amplitude at geomagnetic latitude lower than 74° was larger in the Arctic Regions than in the Antarctic Regions. This can be explained by sharply different geographic longitudes in the polar cap and latitudes in the auroral zone, which results in a different ionospheric conductivity affecting the amplitude of geomagnetic pulsations.     

Particle-precipitation fine structure during the development of substorms at high latitudes

The energy of precipitating particles that cause auroras can be characterized by the ratio of different atom and molecule emissions in the upper atmospheric layers. It is known that the spectrum of precipitating electrons becomes harder when substorms develop. The ratio of the I ₆₃₀₀ red line to the I ₅₅₇₇ green line was used to determine the precipitating-electron spectrum hardness. The I ₆₃₀₀/I ₅₅₇₇ parameter was used to roughly estimate the electron energy in auroral arcs observed in different zones of the auroral bulge at the bulge poleward edge and within this bulge. The variations in the emission red and green lines in auroral arcs during substorms that occurred in the winter season 2007–2008 and in January 2006 were analyzed based on the zenith photometer and all-sky camera data at the Barentsburg and Longyearbyen (LYR) high-latitude observatories. It has been indicated that the average value of the I ₆₃₀₀/I ₅₅₇₇ emission ratio for arcs within the auroral bulge is larger than this value at the bulge poleward edge. This means that the highest-energy electron precipitation is observed in arcs at the poleward edge of the substorm auroral bulge.     

Comparing the latitudinal distribution of the <Emphasis Type="Italic">Pc</Emphasis>1 intensity and the position of subauroral proton spots

The latitudinal position of subauroral proton spots (special proton auroras observed from the IMAGE satellite) has been compared with the Pc1 pulsation intensity distribution determined using the data from the Finnish meridional network of induction magnetometers. It has been indicated that a Pc1 intensity maximum is always observed at the station that is closer to the proton aurora projection. Two Pc1 bands were registered in the event when two proton auroral spots were simultaneously observed at different latitudes. In this case, the Pc1 intensity distribution maximum at lower frequencies was related to a proton auroral spot at a higher latitude and vice versa. Such a spatial correlation between Pc1 pulsations and proton auroral spots, together with the previously established time correlation between these phenomena, demonstrates that subauroral proton spots reflect the region of ion cyclotron instability in the equatorial magnetosphere at the level of the ionosphere.     

Variations in the geomagnetic field and auroras during the main phase of a large magnetic storm of November 20, 2003

The substorm characteristics during the main phase of a large magnetic storm of November 20, 2003, are studied based on the data of TV observations of auroras and auroral absorption at Tixie Bay station and at the global network of magnetic stations. The contribution of auroral particles, responsible for the emission of discrete auroras, has been estimated based on an analysis of the spatial-time variations in the auroral luminosity intensity. This contribution accounted for ～40% of the total luminous flux, which is approximately twice as large as was previously observed in substorm disturbances. Responses of the solar wind and IMF parameters in substorms and variations in the magnetic indices, characterizing geomagnetic activity in the northern polar cap and ring current (PCN, ASY-H and SYM-H), have been detected. The spatial-time distribution of the equivalent ionospheric currents has been constructed, and the total value of these currents along the meridian has been determined based on the [Popov et al., 2001] method and using the IMAGE magnetic data. It has been obtained that the maximal total equivalent ionospheric current in the premidnight sector (～2000 MLT) leads the minimal value of the SYM-H index by ～1.5 h.     

Variations in the ULF index of geomagnetic pulsations during strong magnetic storms

The level of wave geomagnetic activity in the morning, afternoon, and nighttime sectors during strong magnetic storms with Dst varying from ?100 to ?150 nT has been statistically studied based on a new ULF wave index. It has been found out that the intensity of geomagnetic pulsations at frequencies of 2–7 mHz during the magnetic storm initial phase is maximal in the morning and nighttime sectors at polar and auroral latitudes, respectively. During the magnetic storm main phase, wave activity is maximal in the morning sector of the auroral zone, and the pulsation intensity in the nighttime sector is twice as low as in the morning sector. It has been indicated that geomagnetic pulsations excited after substorms mainly contribute to a morning wave disturbance during the magnetic storm main phase. During the storm recovery phase, wave activity develops in the morning and nighttime sectors of the auroral zone; in this case nighttime activity is also observed in the subauroral zone.     

Analysis of latitudinal distribution of Pi2 geomagnetic pulsations using the generalized variance method

The spatial dynamics of bursts of geomagnetic Pi2-type pulsations during a typical event of a magnetospheric substorm (April 13, 2010) drifting to the pole was investigated using the method of generalized variance characterizing the integral time increment of the total horizontal amplitude of the wave at a given point in the selected time interval. The digital data of Scandinavian profile observations from IMAGE magnetometers with 10-second sampling and data of the INTERMAGNET project observations at the equatorial, middle-latitude and subauroral latitudes with a 1-second sampling were used in the analysis. It was shown that Pi2 pulsation bursts in a frequency band of 8–20 mHz appear simultaneously on a global scale: from the polar to equatorial latitudes with maximum amplitudes at latitudes of the maximum intensity of the auroral electrojet and with a maximum amplitude of geomagnetic pulsations Pi3 within a band of 1.5–6 mHz. The first (left-polarized) intensive Pi2 burst appeared at auroral latitudes several minutes after breakup, while the second (right-polarized) burst occurred 15 min after breakup but at higher (polar) latitudes where the substorm had displaced by that time. The direction of wave-polarization vector rotation was opposite for auroral and subauroral latitudes, but it was identical at the equator and in the subauroral zone. The pulsation amplitude at the equator was maximal in the night sector.     

A statistical investigation of polar auroras

Summary The magnetosphere depends on the astronomical orientation of the geomagnetic field with respect to the solar wind. The statistical distribution of polar auroras must therefore depend on the orientation of the geomagnetic field with respect to the ecliptic plane. We have investigated this peculiar feature of auroras that we call auroral astronomical geometry. We give here some preliminary results concerning a limited set of pre-IGY auroras. The criteria that we have chosen to prepare the auroral collection are also briefly summarized. The results conform to the hypothesis of the auroral origin from the magnetospheric neutral sheet. Auroral particles are found to impinge over the earth with low angles with respect to the ecliptic plane (40°). Only in a 4-hours interval around midnight they are found to impinge with angles up to 70°. Definite evidence of these facts requires further investigation with a larger amount of data. — We have also prepared a complex code for recording the morphological features of each aurora, namely: standard information, movement. intensity, color, sunlight illumination, period of pulsation, location in the sky among stars and planets, time evolution, duration and general features of the auroral display. It is well known in fact that the auroral morphology affects auroral heights and latitudes, etc.; presumably it should also affect its astronomical geometry, which we will investigate later.Presented at the Inter-Union Symposium on Solar-Terrestrial Physics — Belgrade 1966.