East-west faults due to planetary contraction

Contraction, expansion and despinning have been common in the past evolution of Solar System bodies. These processes deform the lithosphere until it breaks along faults. Their characteristic tectonic patterns have thus been sought for on all planets and large satellites with an ancient surface. While the search for despinning tectonics has not been conclusive, there is good observational evidence on several bodies for the global faulting pattern associated with contraction or expansion, though the pattern is seldom isotropic as predicted. The cause of the non-random orientation of the faults has been attributed either to regional stresses or to the combined action of contraction/expansion with another deformation (despinning, tidal deformation, reorientation). Another cause of the mismatch may be the neglect of the lithospheric thinning at the equator or at the poles due either to latitudinal variation in solar insolation or to localized tidal dissipation. Using thin elastic shells with variable thickness, I show that the equatorial thinning of the lithosphere transforms the homogeneous and isotropic fault pattern caused by contraction/expansion into a pattern of faults striking east-west, preferably formed in the equatorial region. By contrast, lithospheric thickness variations only weakly affect the despinning faulting pattern consisting of equatorial strike-slip faults and polar normal faults. If contraction is added to despinning, the despinning pattern first shifts to thrust faults striking north-south and then to thrust faults striking east-west. If the lithosphere is thinner at the poles, the tectonic pattern caused by contraction/expansion consists of faults striking north/south. I start by predicting the main characteristics of the stress pattern with symmetry arguments. I further prove that the solutions for contraction and despinning are dual if the inverse elastic thickness is limited to harmonic degree two, making it easy to determine fault orientation for combined contraction and despinning. I give two methods for solving the equations of elasticity, one numerical and the other semi-analytical. The latter method yields explicit formulas for stresses as expansions in Legendre polynomials about the solution for constant shell thickness. Though I only discuss the cases of a lithosphere thinner at the equator or at the poles, the method is applicable for any latitudinal variation of the lithospheric thickness. On Iapetus, contraction or expansion on a lithosphere thinner at the equator explains the location and orientation of the equatorial ridge. On Mercury, the combination of contraction and despinning makes possible the existence of zonal provinces of thrust faults differing in orientation (north-south or east-west), which may be relevant to the orientation of lobate scarps.     

Tectonic patterns on a tidally distorted planet

Tidal deformation of the lithosphere of a synchronously rotating planet or satellite produces stresses that may result in a distinctive tectonic pattern. The lithosphereis treated as a thin elastic shell which maintains the equilibrium shape of a tidally distorted body. Stresses develop as the equilibrium shape changes during orbital evolution. E. M. Anderson's theory of faulting is used to translate this stress pattern into a tectonic pattern of faults on the planet's surface (The Dynamics of Faulting, Oliver & Boyd, Edinburgh, 1951). On a body such as the Moon, which has receded from the Earth, an originally large tidal bulge has collapsed. The predicted tectonic pattern includes N-S striking thrust faults over an area extending roughly 30° in latitude and longitude around the sub-Earth point and its antipode. The polar regions above roughly 70° latitude exhibit normal faults striking from the near side of the Moon toward the far side. Strike slip faults, with offsets consistent with east-west compression, occur near the limbs. Stress differences are largest at the equator on the limbs, and may have reached several hundreds bars over the last few billion years of the Moon's history. The existence of such a tectonic pattern on the Moon can only be resolved by photogeologic mapping. At present, there is little evidence of this pattern; however, the crucial evidence probably lies in the poorly mapped lunar polar regions. These tectonic patterns, which could provide geologic evidence for large tidal distortions, may also be present on the Galilean satellites of Jupiter.     

Tectonic patterns on a reoriented planet: Mars

Both geologic and free-air-gravity data suggest that the positive mass anomaly associated with the Tharsis volcanoes may have reoriented Mars' lithosphere by as much as 25°. Since Mars is oblate (with flattening $\text{? ?0.005}$), rotation of the lithosphere over the equatorial bulge by 25° produces membrane stresses of several kilobars, large enough to initiate faulting. These stresses were first evaluated by F.A. Vening-Meinesz (1947, Trans. Amer. Geophys. Union28, 1–61) who treated the lithosphere as a thin elastic shell. The fracture patterns which result from these stresses are determined by the relation between stress and faulting proposed by E.M. Anderson (1951, The Dynamics of Faulting, Oliver & Boyd, Edinburgh). Plots of the magnitude and direction of stresses in a reoriented planet show that near Tharsis the dominant fault type should be north-south- trending normal faults. This normal fault province is centered about 30°N latitude and extends about 45° east and west in longitude. Similar faults should occur at the antipodes, north of Hellas Planitia. The polar regions should be occupied by roughly north-south-trending thrust faults which extend close to the equator south of Tharsis and north of Hellas. The regions between Tharsis and Hellas are subject to compression on a NE-trending axis and extension along a NW axis east of Tharsis (west of Tharsis the directions are NW compression and NE extension), thus predicting a zone of NNW and ENE strike slip faults east of Tharsis (NNE and WNW west of Tharsis). Although these patterns, except for the north-south normal faults north of Tharsis, have not yet been recognized, the discovery of such a tectonic system of the same age as Tharsis would provide strong support for the reorientation idea. Stresses due to reorientation appear to have little to do with Valles Marineris, since the stress normal to the axis of the Valles is predicted to be compressive, whereas geologic evidence suggests extension.     

Tectonics on Iapetus: Despinning, respinning, or something completely different?

Saturn’s moon Iapetus is unique in that it has apparently despun while retaining a substantial equatorial bulge. Stresses arising from such a non-hydrostatic shape should in principle cause surface deformation (tectonics). As part of a search for such a tectonic signature, lineaments (linear surface features) on Iapetus were mapped on both its bright and dark hemispheres. Lineament orientations were then compared to model stress patterns predicted for spin-down from a rotation period of 16.5 h (or less) to its present synchronous period, and for a range of lithospheric thicknesses. Many lineaments are straight segments of crater rimwalls, which may be faults or joints reactivated during complex crater collapse. Most striking are several large troughs on the bright, trailing hemisphere. These troughs appear to be extensional and are distinctive on that hemisphere, because the interior floors and walls of the troughs contain dark material. Globally, no specific evidence of strike slip or thrust offsets are seen, but this could be due to the age and degraded nature of any such features. We find that observed lineament orientations do not correlate with predicted patterns due to despinning on either hemisphere (the equatorial ridge was specifically excluded from this analysis, and is considered separately). Modest evidence for preferred orientations ±40° from north could be construed as consistent with respinning, which is not necessarily far-fetched. Assuming the rigidity of unfractured ice, predicted maximum lithospheric differential stresses from despinning range from ∼1 MPa to ∼160 MPa for the elastic spheroid and thin lithosphere limits, respectively (although it is only for thicker elastic lithospheres that we expect a nonhydrostatic state to be maintained over geologic time against lithospheric failure). The tectonic signature of despinning may have been obscured over time because the surface of Iapetus is very ancient, Iapetus’ thick lithosphere may have inhibited the full tectonic expression of despinning, or both. Several prominent lineaments strike E–W, and are thus parallel to the equatorial ridge (though not physically close to it), but a tectonic or volcanic origin for the ridge is highly problematic.     

Global fracture patterns of a despun planet: Application to Mercury

We have determined the global fracture patterns resulting from combinations of stresses due to tidal despinning and contraction or expansion. We find that Mercury's lineament pattern is consistent with a history of despinning and contraction. According to our model, the observed tectonic pattern implies that the despinning process reached completion before the planet ceased contracting. Our model predicts a stress due to contraction which is up to 1.8 times the maximum despinning stress on Mercury. The maximum contractional stress could be as large as 4 times the maximum despinning stress if the oldest fractures on the planet are N-S thrust faults in the equatorial region.     

The Earth's decelerated rotation and regularities in orientation of its surface lineaments and faults

The Earth's crust faults and lineaments group in clusters with predominant N-S and E-W (System I) and NW-SE and NE-SW (System II) directions. The earthquake epicenters of the Benioff seismofocal zones follow the same regularities. In other words, seismofocal zones (epicenters of earthquakes) constitute a part of the regular network of the Earth's crust and lithosphere faults and lineaments. Mathematical modeling of stress distribution in the lithosphere due to a change of the Earth's ellipsoid compression showed that the principal stresses σ₁ and σ₂ are oriented in N-S and E-W directions, while corresponding shear stresses τ are oriented in NE-SW and NW-SE directions. It is shown that the secular deceleration of the Earth's rotation can be a reasonable mechanism for the change of Earth's ellipsoid compression and, consequently, for the origin of the regular system of faults and lineaments described above.     

The equatorial shape and gravity field of Mercury from MESSENGER flybys 1 and 2

On 14 January and 6 October 2008 the MESSENGER spacecraft passed within 200 km of the surface of Mercury. These flybys by MESSENGER provided the first observations of Mercury from a spacecraft since the Mariner 10 flybys in 1974 and 1975. Data from the Mercury Laser Altimeter (MLA) provided new information on the equatorial shape of Mercury, and Doppler tracking of the spacecraft through the flybys provided new data on the planet’s gravity field. The MLA passes were on opposite hemispheres of the planet and span collectively ∼40% of the equatorial circumference. The mean elevation of topography observed during flyby 1, in the longitude range 0-90°E, is greater than that seen during flyby 2 in the longitude range 180-270°E, indicating an offset between centers of mass and figure having a magnitude and phase in general agreement with topography determined by Earth-based radar. Both MLA profiles are characterized by slopes of ∼0.015° downward to the east, which is consistent with a long-wavelength equatorial shape defined by a best-fitting ellipse. The Doppler tracking data show sensitivity to the gravitational structure of Mercury. The equatorial ellipticity of the gravitational field, C_2,2, is well determined and correlates with the equatorial shape. The S_2,2 coefficient is ∼0, as would be expected if Mercury’s coordinate system, defined by its rotational state, is aligned along its principal axes of inertia. The recovered value of the polar flattening of the gravitational potential, J₂, is considerably lower in magnitude than the value obtained from Mariner 10 tracking, a result that is problematic for internal structure models. This parameter is not as well constrained as the equatorial ellipticity because the flyby trajectories were nearly in the planet’s equatorial plane. The residuals from the Doppler tracking data suggest the possibility of mascons on Mercury, but flyby observations are of insufficient resolution for confident recovery. For a range of assumptions on degree of compensation and crustal and mantle densities, the allowable crustal thickness is consistent with the upper limit of about 100 km estimated from the inferred depth of faulting beneath a prominent lobate scarp, an assumed ductile flow law for crustal material, and the condition that temperature at the base of the crust does not exceed the solidus temperature. The MESSENGER value of C_2,2 has allowed an improved estimate of the ratio of the polar moment of inertia of the mantle and crust to the full polar moment (C_m/C), a refinement that strengthens the conclusion that Mercury has at present a fluid outer core.     

Global contraction of planetary bodies due to despinning: Application to Mercury and Iapetus

We extend previous work on the global tectonic patterns generated by despinning with a self-consistent treatment of the isotropic despinning contraction that has been ignored. We provide simple analytic approximations that quantify the effect of the isotropic despinning contraction on the global shape and tectonic pattern. The isotropic despinning contraction of Mercury is ∼93 m (T/1 day)⁻², where T is the initial rotation period. If we take into account both the isotropic contraction and the degree-2 deformations associated with despinning, the preponderance of compressional tectonic features on Mercury’s surface requires an additional isotropic contraction ?1 km (T/1 day)⁻², presumably due to cooling of the interior and growth of the solid inner core. The isotropic despinning contraction of Iapetus is ∼9 m (T/16 h)⁻², and it is not sensitive to the presence of a core or the thickness of the elastic lithosphere. The tectonic pattern expected for despinning, including the isotropic contraction, does not explain Iapetus’ ridge. Furthermore, the ridge remains unexplained with the addition of any isotropic compressional stresses, including those generating by cooling.     

Viking bistatic radar experiment: Summary of first-order results emphasizing north polar data

Bistatic radar observations of Mars' north polar region during 1977–1978 showed surface rms slope σ_β ranging from 1 to 6°; these values apply to horizontal scales of 1–100 m. Values of roughness tend to decrease with increasing latitude (especially over 65–80°N), but there are many exceptions. The smoothest surfaces (σ_β≤1°) appear to be inclusions within generally rougher (σ_β～3°) terrain, rather than broad expanses of very smooth material. The permanent north polar cap is relatively uniform with 2.5?σ_β?3.0°. Considerable structure has been found in echo spectra, indicating a heterogeneous and perhaps anisotropic scattering surface. Echo spectra obtained from the same region, but several months apart (1°<L_S<62°), show no significant differences in inferred roughness. Estimates of reflectivity and dielectric constant are systematically low in the polar region. This may indicate that surface material north of 65°N is less dense than that near the equator, but more study of these data is needed. Estimates of surface roughness and dielectric constant in the equatorial region are consistent with results from Earth-based measurements to the accuracy of our analysis.     

Centers of tectonic activity in the eastern hemisphere of Mars

We compiled a paleotectonic map for the eastern hemisphere of Mars to determine if extensional tectonic features (graben) are radial or compressional tectonic features (wrinkle ridges) are concentric to centers of tectonic activity defined by axisymmetric stress fields. Using a vector analysis technique all latitude and longitude points (1° bins) are tested to see if they lie on great circle extensions of extensional structures (the plane defined by the maximum and intermediate principal stresses) or great circle perpendiculars to compressional structures (the plane defined by the maximum and minimum compressional stresses). Centers of tectonic activity are defined as 5° areas whose concentrations of great circle extensions of tectonic features are statistically significant (e.g., 3σ or 7.4σ for large populations) and therefore are not the result of random noise. Our paleotectonic investigation has identified four statistically significant centers of tectonic activity within the eastern hemisphere: Elysium, Hadriaca/Tyrrhena-Hellas, Isidis-Syrtis, and Arabia Terra. Two of these centers (Hadriaca/Tyrrhena and Isidis-Syrtis) meet the 7.4σ statistical criteria and thus represent primary centers of tectonic activity with axisymmetric stress fields. The remaining two meet the 3σ statistical criteria and thus are defined as secondary centers of tectonic activity. Because the structures that define the centers extend over 80° of the planet the defined centers of tectonic activity are regional in character and related to modified impact basins or volcanic centers (all are more limited in extent than the Tharsis stress system that extends over the entire western hemisphere). The observation that statistically significant centers of tectonic activity are quantifiably and statistically identified argues that the crust and lithosphere of the eastern hemisphere at a regional scale is not dominated by regional inhomogeneities and anisotropies.     

Strength of Faults on Mars from MOLA Topography

The stresses which must be maintained on faults bounding the rift topography at Tempe Fossae—the “North Tempe Rift” (NTR)—and Valles Marineris (VM) on Mars are estimated, using a simple elastic model and topographic data from the Mars Orbiter Laser Altimeter (MOLA). The absence of rift-flank uplift at the NTR is consistent with an elastic thickness, T_e, of 20 km or greater at the time of rift formation. The maximum resolved shear stresses on bounding faults due to this topography do not therefore exceed 20 MPa, similar to the inferred strength of terrestrial faults. Elastic thickness estimates at VM are mostly around 50 km or greater. Therefore, for canyon widths of ∼400 km, the bounding faults of VM, if present, must be able to withstand stresses of up to approximately 100 MPa. However, if the fault-controlled sections of the canyons do not exceed 150 km in width, as suggested by geomorphological analysis, the fault strength required is only 20 MPa. Although the maximum resolved shear stresses required to support the topography at VM may need to be greater than the stresses which terrestrial faults can support, at least some faults on Mars are no stronger than similar features on Earth. This observation is consistent with the existence of liquid water in the shallow subsurface of Mars at the time the faults were active. On Venus, plate tectonics is probably prevented by the frictional resistance to motion across strong faults. On Mars, it is more likely that the large thickness of the elastic layer of the lithosphere and the possible positive buoyancy of the crust are responsible for the observed lack of plate tectonics.     

Saturn’s tropospheric composition and clouds from Cassini/VIMS 4.6-5.1 μm nightside spectroscopy

The latitudinal variation of Saturn’s tropospheric composition (NH₃, PH₃ and AsH₃) and aerosol properties (cloud altitudes and opacities) are derived from Cassini/VIMS 4.6-5.1 μm thermal emission spectroscopy on the planet’s nightside (April 22, 2006). The gaseous and aerosol distributions are used to trace atmospheric circulation and chemistry within and below Saturn’s cloud decks (in the 1- to 4-bar region). Extensive testing of VIMS spectral models is used to assess and minimise the effects of degeneracies between retrieved variables and sensitivity to the choice of aerosol properties. Best fits indicate cloud opacity in two regimes: (a) a compact cloud deck centred in the 2.5-2.8 bar region, symmetric between the northern and southern hemispheres, with small-scale opacity variations responsible for numerous narrow light/dark axisymmetric lanes; and (b) a hemispherically asymmetric population of aerosols at pressures less than 1.4 bar (whose exact altitude and vertical structure is not constrained by nightside spectra) which is 1.5-2.0× more opaque in the summer hemisphere than in the north and shows an equatorial maximum between ±10° (planetocentric).Saturn’s NH₃ spatial variability shows significant enhancement by vertical advection within ±5° of the equator and in axisymmetric bands at 23-25°S and 42-47°N. The latter is consistent with extratropical upwelling in a dark band on the poleward side of the prograde jet at 41°N (planetocentric). PH₃ dominates the morphology of the VIMS spectrum, and high-altitude PH₃ at p < 1.3 bar has an equatorial maximum and a mid-latitude asymmetry (elevated in the summer hemisphere), whereas deep PH₃ is latitudinally-uniform with off-equatorial maxima near ±10°. The spatial distribution of AsH₃ shows similar off-equatorial maxima at ±7° with a global abundance of 2-3 ppb. VIMS appears to be sensitive to both (i) an upper tropospheric circulation (sensed by NH₃ and upper-tropospheric PH₃ and hazes) and (ii) a lower tropospheric circulation (sensed by deep PH₃, AsH₃ and the lower cloud deck).     

Tectonic and kinematic study of a strike-slip zone along the southern margin of Central Ovda Regio, Venus: Geodynamical implications for crustal plateaux formation and evolution

The tectonic system of the southern margin of Central Ovda Regio, a crustal plateau which straddles Venus equator, has been interpreted as a dextral strike-slip array, on the basis of evidence clearly identifiable, as are Riedel fracture patterns of different scales, en échelon folds and brittle strike-slip faults. This transcurrent regime developed two main shear belts (Inner and Outer, on respectively thicker and thinner crust), whose minimum dextral displacement has been estimated in 30-50 km. Since the up or downwelling models for plateau formation cannot easily explain tectonic shears of this magnitude along their margins, an alternative hypothesis has been built, which stands on the proposed collisional belt which could form Ovda northern border (King et al., 1998, Lunar Planet. Sci. Conf. 29, Abstract 1209; Tuckwell and Ghail, 2002, Lunar Planet. Sci. Conf. 33, Abstract 1566). Within this framework, the shear would represent a transcollisional transcurrent zone, similar to the strike-slip zones produced in the foreland of the Himalayas-Tibet collision front. Eastern Ovda would be an independent area of thickened crust, pushed to the SSE by the northern collision, with the deformation concentrated at its margins, and experiencing a shear strain on its southern margin. None of the data, however, either supports nor helps to discard theoretical subduction events as a cause of the collision. On the contrary, image relationships could be interpreted as evidence that the main shear deformation took place during the last global resurfacing event on the planet.     

Tidally driven stress accumulation and shear failure of Enceladus's tiger stripes

Straddling the south polar region of Saturn's moon Enceladus, the four principal “tiger stripe” fractures are a likely source of tectonic activity and plume generation. Here we investigate tidally driven stress conditions at the tiger stripe fractures through a combined analysis of shear and normal diurnal tidal stresses and accounting for additional stress at depth due to the overburden pressure. We compute Coulomb failure conditions to assess failure location, timing, and direction (right- vs left-lateral slip) throughout the Enceladus orbital cycle and explore a suite of model parameters that inhibit or promote shear failure at the tiger stripes. We find that low coefficients of friction (μf=0.1-0.2) and shallow overburden depths (z=2-4 km) permit shear failure along the tiger stripe faults, and that right- and/or left-lateral slip responses are possible. We integrate these conditions into a 3D time-dependent fault dislocation model to evaluate tectonic displacements and stress variations at depth during a tiger stripe orbital cycle. Depending on the sequence of stress accumulation and subsequent fault slip, which varies as a function of fault location and orientation, frictional coefficient, and fault depth, we estimate resolved shear stress accumulation of ∼70 kPa prior to fault failure, which produces modeled strike-slip displacements on the order of ∼0.5 m in the horizontal direction and ∼5 mm in the vertical direction per slip event. Our models also indicate that net displacements on the order of 0.1 m per orbital cycle, in both right- and left-lateral directions, are possible for particular fault geometries and frictional parameters. Tectonic activity inferred from these analyses correlates with observed plume activity and temperature anomalies at Enceladus's south polar region. Moreover, these analyses provide important details of stress accumulation and the faulting cycle for icy satellites subjected to diurnal tidal stress.     

Relationships between the equatorial electrojet and polar magnetic variations

On moderately disturbed days when substorms occur frequently, the quiet day daily variation in the polar region (S_q^p) is enhanced. On such days, however, the quiet day variation along the dip equator appears to be suppressed, as well as being superposed with ‘fluctuations’.It is suggested that the enhancement of S_q^p is related to a partial suppression of the equatorial electrojet. The asymmetric ring current also causes an apparent suppression of the electrojet.On the other hand, the substorm-associated electric field which drives the eastward current in the auroral and subauroral zone (causing positive bays) in the afternoon sector appears to enhance the equatorial electrojet.Thus, magnetic variations along the dip equator are influenced by a number of processes in the magnetosphere.     

Crustal tectonic zone on Venus

Various tectonic structures to the south and southeast of Ishtar Terra indicate areal stresses. Compression from east-southeast against Ishtar Terra has resulted in ridge belt formation and surface bending at Salme Dorsa, probably along the seam between two crustal units. En echelon fault zone indicates dextral strike-slip shear(s) resulted in the westward movement of planitia crust related to Ishtar Terra. Meshkenet Tessera displays differential dextral strike-slip faulting where the southernmost bar-like blocks have had largest relative movements. Compression against Tusholi Corona has resulted in foreland surface bending similar to that of Salme Dorsa. The tectonic zone as a whole resembles a dextral transform fault extending from a concave arc in the west to another concave arc in the east. The Cytherean surface, crust or uppermost lithosphere seems to be able to transmit stresses over distances. Deeper understanding of these processes is needed to gain a new idea of the crustal deformation on terrestrial planets.     

New optical measurements of planetary diameters— Part IV: Planet Mars

Optical measurements of the diameter of Mars were made using a double-image micrometer with large refractors from 1952 to 1971. Discussion of the 90 independent series of measurements gives nine determinations of radius with an accuracy of ±7–8km for different latitudes from pole to equator. The Mariner 4, 6 and 7 occultation results and the radar results availble in 1970 added seven further determinations of comparable accuracy. All these values, within the accuracy of measurement, fit an ellipsoid with R_eq=3398±3km and R_po1=3371±4km. The mean density of Mars is thus 3940±0.012g cm⁻³. The resulting optical oblateness of 0.0079, larger than the dynamical value of 0.0052, results in an equatorial radius excess of 9±5 km which presumably comes about by internal stresses.     

Inhomogeneous convection and the equatorial acceleration of the sun

The interaction of rotation and turbulent convection is assumed to give rise to an inhomogeneous, but isotropic, latitude dependent turbulent energy transport, which is described by a convective conduction coefficient _c which varies with latitude. Energy balance in the convective zone is then possible only with a slow meridian circulation in the outer convective zone of the sun. The angular momentum transported by this circulation is balanced in a steady state by turbulent viscous transport down an angular velocity gradient. A detailed model is constructed allowing for the transition from convective transport to radiative transport at the boundaries of the convective zone, by using a perturbation analysis in which the latitude variation of _c is small. The solution for a thin compressible shell gives equatorial acceleration and a hotter equator than pole, assuming that the convection is preferentially stabilised at the equator. For agreement with the sun's equatorial acceleration the model predicts an equatorial temperature excess of 70 K and a surface meridional velocity of 350 cm/sec from pole to equator.     

The sun's magnetic sector structure

The synoptic appearance of solar magnetic sectors is studied using 454 sector boundaries observed at Earth during 1959–1973. The sectors are clearly visible in the photospheric magnetic field. Sector boundaries can be clearly identified as north-south running demarcation lines between regions of persistent magnetic polarity imbalances. These regions extend up to about 35 ° of latitude on both sides of the equator. They generally do not extend into the polar caps. The polar cap boundary can be identified as an east-west demarcation line marking the poleward limit of the sectors. The typical flux imbalance for a magnetic sector is about 4 × 10²¹ Mx.     

The auroral oval during the substorm development

The direction of motion of the auroral forms in several sectors of the auroral oval during substorms is studied. The creation phase is characterized by the equatorward displacement of the luminous region in evening (15–21 LT) and in day (09–15 LT) hours, while individual forms in the luminous region drift mainly poleward with a mean velocity of 230 m/sec in day hours and equatorward with the mean velocity of 230 m/sec in evening hours. The equatorial shift of the luminous region correlates well with the B_Z-component of the interplanetary magnetic field. The onset of the displacement coincides with the southward B_Z-rotation and is accompanied by auroral intensity increase for about 10–20 min.During the expansive and recovery phases the day auroras drift poleward with mean velocities of 330 and 300 m/sec, respectively. In the evening sector the individual auroral forms drift both poleward and equatorward during the expansive phase and drift mainly towards the pole during the recovery phase with a mean velocity of 200 m/sec. In the morning sector characteristics of the motion of the individual auroral forms are more complicated than in the other sectors. The well defined shifts of the luminous region are not discovered. The possible relation between the motions of individual auroral forms with the magnetosphere convection is discussed.