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64 Astronomie

FIZICA SI TEHNOLOGIILE MODERNE, vol. 9, nr. 1-2, 2011

CERCETĂRI ŞI DESCOPERIRI RECENTE IN ASTRONOMIE GALAXIE CU Z ≈ 10,3

Cu ajutorul telescopului spaţial Hubble a fost descoperită o galaxie cu o deplasare spre roşu extrem de mare z ≈ 10,3, vizibilă într-o epocă în care Universul avea vârsta de aproximativ 480 de milioane de ani. Această galaxie are masa de aproximativ 100 de ori mai mică decât a Galaxiei noastre şi o rată de 10 ori mai mică de formare a stelelor, decât galaxiile cu z ≈ 8, adică cu numai ≈ 170 de milioane de ani mai târziu. Astfel, z ≈ 10 este, probabil, începutul perioadei de formare activă a stelelor în Univers. Stelele puteau să se nască prin fuziunea de protogalaxii şi să atingă mase medii care să conducă la anumite procese gazodinamice. Dificultatea de a observa astfel de obiecte îndepărtate se datorează absorbţiei radiaţiei lor de către hidrogenul neutru pe frecvenţe sub Ly - α (luând în considerare deplasarea cosmologică spre roşu). Concluzia despre valoarea înaltă a deplasării z înregistrată la galaxiia descoperită a fost obţinută prin metoda fotometrică pe baza culorii ei, dar până acum nu există nici o confirmare spectroscopică a liniilor spectrale. Ca un model alternativ, având probabilitatea estimată la 20%, ar fi deplasarea spre roşu a galaxiei datorită materiei prezente pe raza vizuală. În prezent, cea mai mare deplasare spre roşu a galaxiilor, confirmată spectroscopic, este de z ≈ 8,6. Sursa: Nature 469 504 (2011)

ECUAŢIA DE STARE A ENERGIEI ÎNTUNECATE Proiectul SNLS3, în cadrul căruia în timp de trei ani de observaţii au fost înregistrate

472 de supernove de tip Ia, a permis să se precizeze ecuaţia de stare a energiei întunecate cosmologice până la 6,5%. Supernovele Ia sunt remarcabile prin faptul că acestea pot servi ca "lumânări standard" în observaţiile cosmologice. În 1998, observaţiile de supernove au arătat că Universul se extinde cu acceleraţie. În cadrul teoriei relativităţii generale, acest fapt corespunde prezenţei materiei cu presiune negativă – a energiei întunecate. Din rezultatele SNLS3 şi datele observaţionale obţinute de satelitul WMAP7, precum şi în proiectele SDSS şi SHOES, pentru parametrul din ecuaţia de stare a energiei întunecate în modelul Universului plat rezultă valoarea w = p / ρ = -1,061+0,069 -0,068 . Dacă nu se presupune Universul plat, atunci avem w =-1,069+0,091 -0,092 şi restricţia privind parametrul de curbură ia forma Ωk =-0,002± 0,006. Valoarea w0 = -0,905 ± 0,196 se obţine presupunând o evoluţie liniară w = w0 + wa (1 - a), în care a este o constantă a Universului. În eroarea sistematică predomină, în primul rând, incertitudinea de etalonare fotometrică, mai degrabă decât incertitudinile astrofizice asociate cu modelele de supernove. În viitor, eliminarea acestei surse de eroare poate îmbunătăţi precizia până la ≈ 2%. Datele obţinute sunt în bună concordanţă cu cel mai simplu caz al constantei cosmologice p = -ρ. Sursa: arXiv: 1104.1444v1 [Astro-ph.CO]

GĂURI NEGRE MASIVE ÎN UNIVERSUL TIMPURIU Cu ajutorul telescopului orbital cu raze X Chandra a fost detectată o populaţie de găuri

negre masive, care existau la momentul în care Universul avea vârsta de doar 800-950 de milioane de ani. Aceste găuri negre sunt de aproximativ 1000 de ori mai puţin masive decât găurile negre tipice din quasari. Este posibil ca ele să se fi format în acelaşi timp cu primele galaxii, masa lor crescând mai departe prin acreţie şi fuziuni. Sursa://chandra.harvard.edu/press/

CONSTANTA LUI HUBBLE Studiind cepheidele şi supernovele de tip Ia cu ajutorul telescopului spaţial Hubble, a

fost măsurată constanta lui Hubble H0 cu o precizie record de 3,3%: H0 = (73,8 ± 2,4) km·s-1·Mpc-1. Sursa: www.nasa.gov

Selecţie şi traducere – Ştefan D. Tiron

Astronomie 65


THE TERMINATOR-EXITED ALFVEN WAVES AS A GENERATOR OF MAGNETICALLY-CONJUGATE SPORADIC

E-LAYERS Valeriu ABRAMCIUC

“Alecu Russo” State University of Balti Rezumat. Este examinat mecanismul de generare a stratului sporadic E la latitudinile

medii de câtre undele Alfven, generate de către mişcarea supersonică a terminatorului în emisfera conjugată. Datele experimentale furnizate de staţiile amplasate la latitudinile medii confirmă mecanismul propus.

Abstract. The mechanism of mid-latitude sporadic E-layer generation by the Alfven waves excited due to supersonic motions of terminator in conjugate hemisphere is examined. Experimental data from several mid-latitude stations confirm the mechanism proposed.

Резюме. Рассмотрен механизм генерации спорадического Е-слоя на средних широтах альфвеновскими волнами, возбуждёнными сверхзвуковым движением терминатора в сопряжённой полусфере. Экспериментальные данные среднеширотных станций подтверждают предложенный механизм.

INTRODUCTION

The present-day level of research in ionosphere physics poses the experimental and theoretical study of the inhomogeneities in various scales among the primary tasks in the ionosphere physics field. It may be asserted beyond any doubt that the nonuniformity and variability of the ionosphere are its main characteristics. The reasons for this are diverse and have not been studied in sufficient detail yet. The space in the Earth's vicinity is a scene where complicated processes occur which are associated with the propagation of the broad-spectrum hydrodynamic and electromagnetic waves in inhomogeneous medium, with the interaction of the neutral and charged components in partially-ionized gas in the presence of magnetic field and gravity, and with the development of all possible hydrodynamic and plasma instabilities. All these circumstances make the examined picture even more complicated.

At present, substantial experimental evidence has been accumulated bearing on the phenomenology of ionosphere inhomogeneities displays and on their role in the propagation of radio waves with a broad spectrum; besides that several mechanisms of their origin have been proposed. Unfortunately, the validity of these mechanisms is still obscure. It is of great scientific interest, therefore, to go on studying the generation mechanisms of the inhomogeneities and their main characteristics (localization and intensity, forms and scales, motion velocity distribution, etc.). GENERAL REMARKS

The topical problem of magnetically-conjugate relationships among various ionosphere regions is being studied intensively in its different aspects. In this respect, the wave contribution is of interest. The waves generated in the lower and middle atmosphere propagate upward and are dissipated and transformed in the upper atmosphere. It has not been clear yet what type of waves makes the most substantial contribution to the heating of the lower ionosphere (by dissipation) and what type of waves is transformed effectively into waves of other types. Besides, information about the specific conditions of interaction with lower ionosphere is absent. Gravitational waves [1], infrasonic waves [2], semidiurnal tides [3], and electromagnetic emissions [4] are the most probable contenders. Among the factors resulting in excitation of the waves of these types we shall mention the supersonic motion of

66 Astronomie


terminator at the ozonosphere level [5, 6] (including the sunrise-sunset periods and solar eclipses), earthquakes [7], meteoritic penetrations [8], electric discharges in the troposphere [9], the outcomes of excitation of large antenna systems by strong-current generators [10], and others. Although this list is far from being complete, it is clear that the causes are very diverse and that the complication degree of the problem is high.

In the lower ionosphere the ionized component is completely carried away by neutral gas because the ion neutral collision frequency exceeds the ion gyro frequency. In this case any external force acting on partially ionized gas (for example, heating, pressure differences, gravitational forces) gives rise to plasma motions, thereby resulting in generation of a dynamo field (induction field). The generated electric currents affect the motion of ionized gas and neutral components (wind). This fact accounts for the close relationships among the processes occurring in the atmosphere and in the lower ionosphere.

A new class of events, so called magnetically-conjugate effects, has been extensively studied since recently. As usual, the effects occurring at magnetically-conjugate points (or in conjugate regions) due to the connection along field lines between the northern and southern hemispheres are meant to be magnetically-conjugate. Such connection may be realized in different way; for example, by motions along the field lines of charged particles (say photoelectrons) [11] and electromagnetic or magneto hydrodynamic wares [12], through electrostatic connection between the hemispheres [13], etc. However, this area also shows numerous obscurities, "blanks", which prevent one from getting a deeper insight into the problem. FORMULATION OF THE PROBLEM

The probability for sporadic E-layers (PES) to occur was studied in [14] for a period of several years for middle latitudes during winter season (December-January). Fig. 1 presents the results inferred from the ionosphere data obtained at Moscow during six years which were studied in more detail. Two deep minima of the PES values are observed near 03h00m and 06h30m - 07h00m. Within the 05h00m - 05h30m period the PES rise is cut-off and the PES values begin decreasing rather rapidly (within an hour the probability for the ES-layers to exist falls by ~ 15%). The analysis [14] has shown that the minima are in correspondence, to within a fairly high accuracy, with sunrise at the magnetically conjugate point (the dashed arrow) and at the observation point (the solid arrow) at the ozone layer level. Therefore, a definite connection between magnetically conjugate regions is sure to exist and is controlled by solar radiation. The problem arises then as to what is the mechanism of the interaction between the two hemispheres, i. e. in what way the energy is transferred. The present work is devoted to elucidating this problem.

DESCRIPTION OF THE PROPOSED MECHANISM IN SYNTHESIS WITH EXPERIMENTAL EVIDENCE

The examined period (which is symmetric relative to winter solstice) is characterized in middle latitudes by sufficiently significant distortion of (different) illumination of the magnetically conjugate regions (for example, at Moscow; the geographic coordinates: latitude

N'0 2855 , longitude E'0 1937 ; the corrected geomagnetic coordinates: latitude

N'0 5450 , longitude E'0 06111 ; the magnetically-conjugate point in the Indian

Ocean with geographic coordinates: latitude S'0 3642 , longitude E'0 2456 ; the field line length kml 30900 ; the McIllwain parameter 54,2L ). During some 3 hours the conjugate point is sunlit, whereas the northern point is still within deep umbra. In such a situation it may be expected that the disturbances caused in the atmosphere near the Earth's

Astronomie 67


surface by rapid motion of the umbra boundary will affect the conjugate ionosphere. Because of a large UV emission absorption cross section, the atmospheric ozone appears to be a heat reservoir. A difference in temperature (by some 100°) and, hence, a difference in the values of other parameters (pressure, specific volume) occur in the region of terminator. As a result, the given process may be treated as excitation of a broad spectrum of acoustic waves. As indicated above, the longest waves fail to reach the ionosphere altitudes because of the acoustic frequency of the cut-off, while the short waves cannot come to be there because of viscous absorption. The absorption is most pronounced in the region where the wavelength gets comparable with free path of particles. The waves whose periods are ranging from several minutes to several hours (within the Hz43 1010 frequency band) must be observed at the altitudes of the lower ionosphere. In such a way, natural filtration of the acoustic waves occurs. In the dynamo-region, the waves dissipate and are transformed into those with a higher-frequency spectrum. This problem needs being carefully studied further. It is possible that there exist such resonance frequencies that they can be, on the contrary, amplified. It should be reminded that the wave amplitude increases because the density of medium decreases with altitude. This effect on weakly ionized plasma gives rise to generation of vortices on various scales in such plasma as a result of nonuniform distribution of the characteristic parameters of the medium which leads to excitation of disturbances in the wind system existing at the given altitudes. In terms of the wind shear theory the fluctuation variations of electron number density 'N in case their distribution is isotropic under isotropic turbulence are described by the expression [15]

2'0

22' nUrothNDNDni

HSTa

, (1)

where aD and TD are the coefficients of ambipolar and turbulent diffusion,

respectively; SN is the mean electron number density; H is the ion gyro frequency; ni is

ion-neutral collision frequency; h

is unit vector in the geomagnetic field direction 0H

; 0U

is

the pulsation velocity of electron motion; the horizontal bar denotes averaging. From the expression (1) it is seen that, if the above mentioned conditions are satisfied, the equilibrium between the molecular and turbulent diffusions is distorted (this is not the case under ordinary conditions) due to wind shear (the last term of the equation). For the layers with increased

ionization to be formed effectively under such conditions, it is necessary that 00 Uroth

. If,

however, the isotropy condition is violated by the natural effect of geomagnetic field 0H

, the

last term in (1) proves to be substantial. The term proportional to 0Uroth

characterizes the

effect of the main wind system on ionization, whereas this term must be replaced by

'' rotUNh

when describing the effect of local wind shears arising from the vortices of turbulent origin. In practice, the vortex-like structure of the ionosphere E-region corresponds as a whole to so called cloud or cell structure which was earlier examined theoretically [16] and was found in several experiments [17].

The vortices of various scales generate the Alfven waves in a broad frequency band [18]. In this case the relationships among the characteristic vortex scale (vortex radius R ), the angular rotation velocity ), and the sonic velocity SU are expressed as

SUR 5,0 , (2)

i. e. the linear velocity V ( RV ) of the particles involved in vortex motion cannot exceed

a half of sonic velocity sm /300~ . This condition defines the frequency band of the excited

68 Astronomie


Alfven waves which propagate afterwards along geomagnetic field lines. At an altitude of about 150 km the dissipation rate of these waves per unit volume takes on the highest value 39 /10~ cmW . At altitudes of about 600 - 700 km the Alfven wave propagation

velocity is the lowest skm /10~ 3 . As the altitude increases, however, the wave propagation velocity rises rapidly and

reaches skm /10~ 5 above a 1000-km altitude. Considering that the magnetic field line length l at the Moscow latitude is km30900~ and assuming the mean Alfven-wave propagation

velocity to be skm /10~ 3 , we obtain that the time estimate for the waves to reach the magnetically-conjugate region approaches 1 min. Thus, since our ionosphere data were obtained in each 15 min, we may well assume an almost instantaneous transfer of a disturbance from one hemisphere to another. It should be indicated that, because of the wave dissipation and dispersion in the upper ionosphere of both hemispheres and in the magnetosphere, the lower layers of the conjugate ionosphere are affected by the Alfven waves with quite a definite frequency spectrum. In this case, just as in case of acoustic infrasonic waves, a natural frequency filtration of Alfven waves may be considered to occur in the ionosphere and in the magnetosphere. Having examined the propagation of such waves from one hemisphere to another, some researchers (see, for example, [19]) found the resonance frequencies within a broad band (PC1 - PC4). We shall not discuss this problem here, but wish only to point out that the obtained estimates of the energy transferred to the conjugate ionosphere which allow for the dissipation processes are quite sufficient for the proposed mechanism to be operative further (see below). It should be noted that the Alfven wave-plasma interaction process is more intensive in the upper ionosphere of the southern hemisphere where the Alfven waves are generated than in the conjugate ionosphere because the latter is still under night-time conditions. (A more detailed treatment of the plasma-wave interaction throughout the path, together with the allowance for other factors, will probably be able to account also for some other variations of the ionosphere parameters during predawn periods; for example, luminosity of individual atmospheric components, shifts of the P2 and ES layer altitudes, and variations of their frequency parameters, as well as for the associated events. However, such a treatment is outside the scope of the present analysis.) The moderate conductivity of the night-time ionosphere permits this wave stream to penetrate without any substantial loss up to the dynamo-region altitudes where the conductivity is high and the waves give their energy away within the skin layer, thereby modifying the parameters of medium. As a result, considering the existence of wind system, the sell structure arises. Depending on the characteristic dimensions of such spots, on the vortex electric field direction, and on the skin-layer occurrence altitude (defined by a particular ionosphere conductivity profile) and depth (which depends on wind structure, namely, on vertical wind "wavelength" or, more strictly, on the depth of the layer where the east-west wind component gets inverse), the spots may appear to be unnoticed during vertical soundings or manifest themselves as sporadic E-layers. The relevant estimations have shown that the ultimate mid-latitude ES-layer frequencies exhibit quite definite (established) variations in predawn periods whose characteristic form is shown in Fig. 2. In other words, the sporadic layers are very weak in the early predawn periods (~ 3 h), but their ultimate frequency increases with time and they gain in power (multiple reflections are observed). The cut-off of the f0ES increase coincides in practice with the PES decrease commencement (see Fig. 1), the moment which, according to the relevant calculations, is just in correspondence with sunrise at altitudes of 200 - 500 km over the observation point. This picture can readily be explained in terms of the proposed model. Illumination of the upper atmosphere by the Sun is accompanied by a substantial enhancement of the dissipation process in the Alfven waves; i. e. gives rise to

Astronomie 69


adverse changes of their penetration conditions in the lower ionosphere. The Alfven-wave reflection conditions are probably also satisfied here, so the waves may move backwards, but their energy is no longer sufficient for any substantial disturbances to be generated, especially as the secondary penetration through the dense daytime ionosphere with preservation of the necessary power is unlikely. This circumstance may probably account for the extremely rare cases and for the negligible level of disturbances in the ionosphere E-region of summer hemisphere during sunrises and sunsets in winter hemisphere (i. e. if the inverse pattern is considered).

As noted above, the downtime abrupt "burst" of the probability PES (by 70-80 % within 1 - 1.5 hours) coincides in time with sunrise at the ozone layer level over the observation point. It is quite clear that the acoustic waves excited in this case in the terminator region affect the lower ionosphere in the same manner as in the case discussed above. It should be reminded that the wind at these altitudes is always of great importance in the given case.

The mechanism described above may be applied when explaining the observation data obtained in a conjugate region during the periods of solar eclipses and powerful solar flares and in some active experiments (injection of slightly-ionized substances, particle beams, powerful radio emission, etc.). DISCUSSION. PROSPECTS OF THE STUDIES

The mechanism of magnetically-conjugate generation of the sporadic shaggy (spots, clouds) ionization structure in the ionosphere E-layer due to unsteady-state Alfven wave propagation in the magnetosphere was shown above to account quite satisfactorily for the experimentally observed statistical regularity of the variation in both the probability PES of sporadic E-layer occurrence before and during the dawn hours in winter and their ultimate frequency (f0ES). We are aware, however, of the necessity for additional studies in this direction, which should be aimed at specifying the frequency band of excited acoustic and Alfven waves at various altitudes and the conditions of their propagation, dissipation, diffraction, transformation, etc. It is quite possible that the application of fresh experimental evidence will make the problem more lucid. We shall mention here but few of the effects, which seem to us to be explainable in terms of the model discussed. In the lower ionosphere of both hemispheres, intensification of 5577Å emission, occurrence of diffusion in the ES-layer (spread-ES), and stratification (appearance of multiple structures) of the ionosphere E-region plasma are expected. At higher altitudes, formation of various inhomogeneities in the ionosphere E-F interlayer region is possible. At the F2-layer altitudes, the variations in critical frequency

Moscow, Dec/Jan, 6 year

25

30

35

40

45

50

55

60

21 22 23 0 1 2 3 4 5 6 7 8

Time, 300 E

PE

s, %

Fig. 1. PES Moscow, December/January, 6 year

70 Astronomie


(f0F2), in actual altitude (h'F2), and in 6300Å emission together with occurrence of F-diffusion (spread-F) may be observed. In the magnetosphere, excitations of instabilities by interactions with trapped particles cannot be occluded, which may give rise to their precipitations.

The processes of Alfven wave interaction with plasma in the upper ionosphere may account for the predawn rise of electron temperature Te. A higher probability of the occurrence of geomagnetic field pulsations on the Earth's surface may be found during the examined period. The studied problem may prove to be relevant to the regular changes in the values of the geomagnetic field components before and during dawn hours.

Of course, all the above-discussed problems have to be further treated and analyzed making allowance for other factors too. Schematic presentation of the proposed mechanism is presented in Fig. 3

Moscow

1,0

1,5

2,0

2,5

3,0

3,5

4,0

4,5

3 4 5 6

Time, 300 E

f0E

s , М

Hz

Fig. 2. A typical example of a pre-dawn variations f0ES in the winter time at middle latitude stations

Fig. 3. Schematics of magnetosphere propagation of Alfven waves

Astronomie 71


REFERENCES 1. Hines C.O. Dynamical heating of the upper atmosphere. – J. Geophys. Res., 1965, v. 70,

No 1, pp. 177 - 183. 2. Rind D. Heating of the lower thermosphere by the dissipation of acoustic waves. – J.

Atmos. Terr. Phys., 1977, v. 39, No 4, pp. 445 - 456. 3. Lindsen R.S., Blake D. Mean heating of the thermosphere by tides. - J. Geophys. Res.,

1970, v. 75, No 33, pp. 6868 - 6871. 4. Хуссамиддинов С.С. Эффекты воздействия на ионосферу естественных

электромагнитных излучений, обусловленных сейсмогеодинамикой. – Доклады АН УзССР, 1984, 6, с. 48 - 49.

5. Сомсиков В.М., Троицкий Б.В. Генерация возмущений в атмосфере при прохождении через неё солнечного терминатора. – Геомагнетизм и аэрономия, 1975, т. 15, 5, с. 856 - 860.

6. Сомсиков В.М. Генерация возмущений в нейтральной атмосфере сферическим терминатором. – Геомагнетизм и аэрономия, 1982, т. 22, 1, с. 75 - 80.

7. Гохберг М.Б. Возмущения в верхней ионосфере перед землятрясениями по наблюдениям на спутниках. – Препринт ИФЗ АН СССР, 7, 1982.

8. Голицын Г.С., Григорьев Г.И., Докучаев В.П. Излучение акустико-гравитационных волн при движении метеоров в атмосфере. – Изв. АН СССР, сер. Физика атмосферы и океана, 1977, т. 13, 9, с. 926 - 935.

9. Fraser-Smith A.C., Roxburg K.R. Triggering of hydromagnetic „Whistlers”, by sferics. – Planet. Space Sci., 1969, v. 17, No. 6, pp. 1310 - 1312.

10. Greifinger C. Feasibility of ground-based generation of artificial micropulsations. - J. Geophys. Res., 1972, v. 77, No 34, pp. 6761 - 6773.

11. Rao B.C., Maier E.J.R. Photoelectron flow protonospheric heating during the conjugate point sunrise. – J. Geophys. Res., 1970, v. 75, No 4, pp. 816 - 822.

12. Jacobs J.A., Watanabe T. Propagation of hydromagnetic waves in the lower exosphere and the origin of short period geomagnetic micropulsation. – J. Atmos. Terr. Phys., 1962, v. 24, No 6, pp. 413 - 434.

13. Petelski E.F. Real height variations of the ionosphere F2 layer above some pairs of geomagneticaly conjugate stations. - J. Atmos. Terr. Phys., 1972, v. 34, No 7, pp. 1163 - 1170.

14. Абрамчук В.П., Ружин Ю.Я. Некоторые особенности ночного среднеширотного спорадического Еs-слоя в зимний период. М.: ИЗМИРАН, Препринт 11(544), 1985. – 21 с.

15. Гершман Б.Н., Игнатьев Ю.А., Каменецкая Г.Х. Механизм образования ионосферного спорадического слоя Еs на различных широтах. М., Наука, 1976. – 108 с.

16. Гершман Б.Н., Овезгельдыев О. Турбулентная диффузия и спорадический слой Е. – Изв. АН ТССР. Серия физ.-техн., хим. и геол. наук, 1973, 4, с. 35 – 42.

17. Овезгельдыев О. О возможности существования ячейковой циркуляции в Е-области ионосферы. - Изв. АН TССР. Серия физ.-техн., хим. и геол. наук, 1960, 2, с. 40 – 47.

18. Альфвен Х. Космическая электродинамика. М., Изд-во ИЛ, 1952. – 290 с. 19. Sen A.K. Propagation of extra low frequency waves in the magnetosphere. – Geophys. J.

R. Astr. Soc., 1971. No 23, pp. 161 – 172. Primit la redacţie: 9 iunie 2011

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