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121.
The heavily-cratered southern hemisphere of Mars encompasses the planet’s strongest, most widespread magnetization. Our study concentrates on this magnetized region in the southern hemisphere within 40° of latitude 40°S, longitude 180°W. First we rotate the coordinates to position the center at −40°, 180° and treat these new latitudes and longitudes as if they were Cartesian coordinates. Then, using an ordinary two-dimensional Fourier analysis for downward continuation, the MGS (MAG/ER) magnetic field data at satellite mapping elevation of ∼400 km are extrapolated to 100 km, sources are estimated and used to model the fields. Quantitative comparison of the downward continued field with the aerobraking field for bins having angular deviation within ±30° gives correlation of .947, .868, and .769 for the components, respectively. This agreement of the fields may result from most of the power in the magnetization resting in wavelengths ∼400 km, with comparatively little at ∼100 km. Over this region, covering nearly an octant of the planet, just a dozen sources can account for 94% of the variance of the magnetic field at the surface. In these models for the field an obvious asymmetry in polarity exists, with majority of the sources being positive. The locations of strongest surface magnetization appear to be near - but not actually within - ancient multi-ringed basins. We test the likelihood of this association by comparing the observed sources found within and near basins for two alternative basin location scenarios with random distributions. For both alternatives we find the observed distributions to be low-probability occurrences. If contemporaneous, this would establish that Mars’ magnetic field extended to the time of impacts causing these basins. 相似文献
122.
E. K. J. Kilpua P. C. Liewer C. Farrugia J. G. Luhmann C. Möstl Y. Li Y. Liu B. J. Lynch C. T. Russell A. Vourlidas M. H. Acuna A. B. Galvin D. Larson J. A. Sauvaud 《Solar physics》2009,254(2):325-344
We analyze a series of complex interplanetary events and their solar origins that occurred between 19 and 23 May 2007 using
observations by the STEREO and Wind satellites. The analyses demonstrate the new opportunities offered by the STEREO multispacecraft configuration for diagnosing
the structure of in situ events and relating them to their solar sources. The investigated period was characterized by two high-speed solar wind streams
and magnetic clouds observed in the vicinity of the sector boundary. The observing satellites were separated by a longitudinal
distance comparable to the typical radial extent of magnetic clouds at 1 AU (fraction of an AU), and, indeed, clear differences
were evident in the records from these spacecraft. Two partial-halo coronal mass ejections (CMEs) were launched from the same
active region less than a day apart, the first on 19 May and the second on 20 May 2007. The clear signatures of the magnetic
cloud associated with the first CME were observed by STEREO B and Wind while only STEREO A recorded clear signatures of the magnetic cloud associated with the latter CME. Both magnetic clouds
appeared to have interacted strongly with the ambient solar wind and the data showed evidence that they were a part of the
coronal streamer belt. Wind and STEREO B also recorded a shocklike disturbance propagating inside a magnetic cloud that compressed the field and plasma
at the cloud’s trailing portion. The results illustrate how distant multisatellite observations can reveal the complex structure
of the extension of the coronal streamer into interplanetary space even during the solar activity minimum.
Electronic Supplementary Material The online version of this article () contains supplementary material, which is available to authorized users. 相似文献
123.
We demonstrate that a simple solar dynamo model, in the form of a Parker migratory dynamo with random fluctuations of the
dynamo governing parameters and algebraic saturation of dynamo action, can at least qualitatively reproduce all the basic
features of solar Grand Minima as they are known from direct and indirect data. In particular, the model successfully reproduces
such features as an abrupt transition into a Grand Minimum and the subsequent gradual recovery of solar activity, as well
as mixed-parity butterfly diagrams during the epoch of the Grand Minimum. The model predicts that the cycle survives in some
form during a Grand Minimum, as well as the relative stability of the cycle inside and outside of a Grand Minimum. The long-term
statistics of simulated Grand Minima appears compatible with the phenomenology of the Grand Minima inferred from the cosmogenic
isotope data. We demonstrate that such ability to reproduce the Grand Minima phenomenology is not a general feature of the
dynamo models but requires some specific assumption, such as random fluctuations in dynamo governing parameters. In general,
we conclude that a relatively simple and straightforward model is able to reproduce the Grand Minima phenomenology remarkably
well, in principle providing us with a possibility of studying the physical nature of Grand Minima. 相似文献
124.
This study based on longitudinal Zeeman effect magnetograms and spectral line scans investigates the dependence of solar surface magnetic fields on the spectral line used and the way the line is sampled to estimate the magnetic flux emerging above the solar atmosphere and penetrating to the corona from magnetograms of the Mt. Wilson 150-foot tower synoptic program (MWO). We have compared the synoptic program λ5250 Å line of Fe?i to the line of Fe?i at λ5233 Å since this latter line has a broad shape with a profile that is nearly linear over a large portion of its wings. The present study uses five pairs of sampling points on the λ5233 Å line. Line profile observations show that the determination of the field strength from the Stokes V parameter or from line bisectors in the circularly polarized line profiles lead to similar dependencies on the spectral sampling of the lines, with the bisector method being the less sensitive. We recommend adoption of the field determined with the line bisector method as the best estimate of the emergent photospheric flux and further recommend the use of a sampling point as close to the line core as is practical. The combination of the line profile measurements and the cross-correlation of fields measured simultaneously with λ5250 Å and λ5233 Å yields a formula for the scale factor δ ?1 that multiplies the MWO synoptic magnetic fields. By using ρ as the center-to-limb angle (CLA), a fit to this scale factor is δ ?1=4.15?2.82sin?2(ρ). Previously δ ?1=4.5?2.5sin?2(ρ) had been used. The new calibration shows that magnetic fields measured by the MDI system on the SOHO spacecraft are equal to 0.619±0.018 times the true value at a center-to-limb position 30°. Berger and Lites (2003, Solar Phys. 213, 213) found this factor to be 0.64±0.013 based on a comparison using the Advanced Stokes Polarimeter. 相似文献
125.
Using NSO/Kitt Peak synoptic charts from 1975 to 2003, we group the main solar magnetic fields into two categories: one for
active regions (ARs) and the other for extended bipolar regions (EBRs). Comparing them, we find that there exist three typical
characteristics in the variability of EBRs: First, there exists a correlation between ARs and EBRs. The phase of EBR flux
has a delay nearly two CRs. Second, we find that the EBR flux has two prominent periods at 1.79 years and 3.21 years. The
1.79-year period seems to only belong to large-scale magnetic features. Lastly, the North – South asymmetry of EBR flux is
not very significant on a time scale of one solar cycle. However, during solar maxima, its dominance is found to shift from
one hemisphere to the other. 相似文献
126.
Using RHESSI and some auxiliary observations we examine possible connections between the spatial and temporal structure of
nonthermal hard X-ray (HXR) emission sources from the two-ribbon flares of 29 May 2003 and 19 January 2005. In each of these
events quasi-periodic pulsations (QPP) with time period of 1 – 3 minutes are evident in both hard X rays and microwaves. The
sources of nonthermal HXR emission are situated mainly at the footpoints of the flare arcade loops observed by TRACE and the
SOHO/EIT instrument in the EUV range. At least one of the sources moves systematically during and after the QPP phase in each
flare. The sources move predominantly parallel to the magnetic inversion line during the 29 May flare and along flare ribbons
during the QPP phase of both flares. By contrast, the sources start to show movement perpendicular to the flare ribbons with
velocity comparable to that along the ribbons’ movement after the QPP phase. The sources of each pulse are localized in distinct
parts of the ribbon during the QPP phase. The measured velocity of the sources and the estimated energy release rate do not
correlate well with the flux of the HXR emission calculated from these sources. The sources of microwaves and thermal HXRs
are situated near the apex of the flare loop arcade and are not stationary either. Almost all of the QPP as well as some pulses
of nonthermal HXR emission during the post-QPP phase reveal soft – hard – soft spectral behavior, indicating separate acts
of electron acceleration and injection. In our opinion at least two different flare scenarios based on the Nakariakov et al. (2006, Astron. Astrophys.
452, 343) model and on the idea of current-carrying loop coalescence are suitable for interpreting the observations. However,
it is currently not possible to choose between them owing to observational limitations. 相似文献
127.
Peter Foukal Luca Bertello William C. Livingston Alexei A. Pevtsov Jagdev Singh Andrey G. Tlatov Roger K. Ulrich 《Solar physics》2009,255(2):229-238
Spectroheliograms and disk-integrated flux monitoring in the strong resonance line of Ca ii (K line) provide the longest record of chromospheric magnetic plages. We compare recent reductions of the Ca ii K spectroheliograms obtained since 1907 at the Kodaikanal, Mt. Wilson, and US National Solar Observatories. Certain differences
between the individual plage indices appear to be caused mainly by differences in the spectral passbands used. Our main finding
is that the indices show remarkably consistent behavior on the multidecadal time scales of greatest interest to global warming
studies. The reconstruction of solar ultraviolet flux variation from these indices differs significantly from the 20th-century
global temperature record. This difference is consistent with other findings that, although solar UV irradiance variation
may affect climate through influence on precipitation and storm tracks, its significance in global temperature remains elusive. 相似文献
128.
The coronal magnetic field above a particular photospheric region will vanish at a certain number of points, called null points.
These points can be found directly in a potential field extrapolation or their density can be estimated from the Fourier spectrum
of the magnetogram. The spectral estimate, in which the extrapolated field is assumed to be random and homogeneous with Gaussian
statistics, is found here to be relatively accurate for quiet Sun magnetograms from SOHO’s MDI. The majority of null points
occur at low altitudes, and their distribution is dictated by high wavenumbers in the Fourier spectrum. This portion of the
spectrum is affected by Poisson noise, and as many as five-sixths of null points identified from a direct extrapolation can
be attributed to noise. The null distribution above 1500 km is found to depend on wavelengths that are reliably measured by
MDI in either its low-resolution or high-resolution mode. After correcting the spectrum to remove white noise and compensate
for the modulation transfer function we find that a potential field extrapolation contains, on average, one magnetic null
point, with altitude greater than 1.5 Mm, above every 322 Mm2 patch of quiet Sun. Analysis of 562 quiet Sun magnetograms spanning the two latest solar minima shows that the null point
density is relatively constant with roughly 10% day-to-day variation. At heights above 1.5 Mm, the null point density decreases
approximately as the inverse cube of height. The photospheric field in the quiet Sun is well approximated as that from discrete
elements with mean flux 〈|φ|〉=1.0×1019 Mx distributed randomly with density n=0.007 Mm−2. 相似文献
129.
K. R. Praveen V. Prasannakumar Manish A. Mamtani 《Journal of the Geological Society of India》2009,73(6):803-812
Anisotropy of magnetic susceptibility (AMS) is investigated in samples of Peralimala (PM) pluton (ca. 550 Ma) and adjacent
gneiss, gabbro, mylonite and amphibolite from the Moyar Shear Zone (MSZ), Southern Granulite Terrane (SGT) with an aim to
decipher the time-relationship between fabric development in the pluton and regional tectonics. Magnetic foliation recorded
in the PM pluton is sub-parallel to the WNW-ESE striking MSZ. Magnetic foliation and lineation trajectories are sigmoidal
and curve into the shear zone. A dextral sense of shear is deciphered from the trajectories, which is similar to that reported
within the MSZ in some earlier studies. It is inferred that the PM pluton has developed post-emplacement deformation-fabric
related to reactivation of the MSZ during Pan-African age. Based on the data and existing information about regional tectonics
of the area, the possibility of the (a) PM pluton being a Deformed Alkali Rock and Carbonatite (DARC) and (b) MSZ marking
an ancient suture zone, is discussed. 相似文献
130.
3D simulations of basin-scale lunar impacts are carried out to investigate: (a) the origins of strong crustal magnetic fields and unusual terrain observed to occur in regions antipodal to young large basins; and (b) the origin of enhanced magnetic and geochemical anomalies along the northwest periphery of the South Pole-Aitken (SPA) basin. The simulations demonstrate that a basin-forming impact produces a massive, hot, partially ionized cloud of vapor and melt that expands thermally around the Moon, converging near the basin antipode approximately 1 h after the impact for typical impact parameters. In agreement with previous work, analytic calculations of the interaction of this vapor-melt cloud with an initial ambient magnetic field predict a substantial temporary increase in field intensity in the antipodal region. The time of maximum field amplification coincides with a period when impacting ejecta also converge near the antipode. The latter produce antipodal shock stresses within the range of 5-25 GPa where stable shock remanent magnetization (SRM) of lunar soils has been found experimentally to occur. Calculated antipodal ejecta thicknesses are only marginally sufficient to explain the amplitudes of observed magnetic anomalies if mean magnetization intensities are comparable to those produced experimentally. This suggests that pre-existing ejecta materials, which would also contain abundant metallic iron remanence carriers, may be important anomaly sources, a possibility that is consistent with enhanced magnetic anomalies observed peripheral to SPA. The latter anomalies may be produced by amplified secondary ejecta impact shock waves in the thick SPA ejecta mantle occurring near the antipodes of the Imbrium and Serenitatis impacts. Together with converging seismic compressional waves, these antipodal impact shocks may have produced especially deep fracture zones along the northwest edge of SPA near the Imbrium antipode, allowing the ascent of magma with enhanced KREEP concentrations. 相似文献