99 research outputs found
Irregular HF radio propagation on a subauroral path during magnetospheric substorms
The impact of the main ionospheric trough, sporadic structures, gradients and inhomogeneities of the subpolar ionosphere during substorms on the signal amplitude, azimuthal angles of arrival, and propagation modes for the radio path Ottawa (Canada)-St. Petersburg (Russia) was considered. This subauroral path with the length of about 6600 km has approximately an east-west orientation. The main goals are to carry out numerical modeling of radio propagation for the path and to compare the model calculations with experimental results. Wave absorption and effects of focusing and divergence of rays were taken into consideration in the radio wave modeling process. The following basic results were obtained: The signal amplitude increases by 20–30 dB 1–1.5 h before the substorm expansion phase onset. At the same time the signal azimuth deviates towards north of the great circle arc for the propagation path. Compared with quiet periods there are effects due to irregularities and gradients in the area of the polar edge of the main ionospheric trough on the passing signals. Propagation mechanisms also change during substorms. The growth of signal amplitude before the substorm can be physically explained by both a decrease of the F2-layer ionization and a growth of the F2-layer height that leads to a decrease of the signal field divergence and to a drop of the collision frequency. Ionospheric gradients are also important. This increase of signal level prior to a substorm could be used for forecasting of space weather disturbed conditions
Identification of potentially dangerous glacial lakes in the northern Tien Shan
Like in many other parts of the world, the glaciers in northern Tien Shan are receding, and the permafrost is thawing. Concomitantly, glacial lakes are developing. Historically, outbursts of these glacial lakes have resulted in severe hazards for infrastructures and livelihood. Multi-temporal space imageries are an ideal means to study and monitor glaciers and glacial lakes over large areas. Geomorphometric analysis and modelling allows to estimate the potential danger for glacial lake outburst floods (GLOFs). This paper presents a comprehensive approach by coupling of remote sensing, geomorphometric analyses aided with GIS modelling for the identification of potentially dangerous glacial lakes. We suggest a classification scheme based on an additive ratio scale in order to prioritise sites for detailed investigations. The identification and monitoring of glacial lakes was carried out semi-automatically using band ratioing and the normalised difference water index (NDWI) based on multi-temporal space imagery from the years 1971 to 2008 using Corona, ASTER and Landsat data. The results were manually edited when required. The probability of the growth of a glacial lake was estimated by analysing glacier changes, glacier motion and slope analysis. A permafrost model was developed based on geomorphometric parameters, solar radiation and regionalised temperature conditions which permitted to assess the influence of potential permafrost thawing. Finally, a GIS-based model was applied to simulate the possibly affected area of lake outbursts. The findings of this study indicate an increasing number and area of glacial lakes in the northern Tien Shan region. We identified several lakes with a medium to high potential for an outburst after a classification according to their outburst probability and their downstream impact. These lakes should be investigated more in detai
Using Extreme Value Theory for Determining the Probability of Carrington-Like Solar Flares
Space weather events can negatively affect satellites, the electricity grid,
satellite navigation systems and human health. As a consequence, extreme space
weather has been added to the UK and other national risk registers. By their
very nature, extreme space weather events occur rarely and, therefore,
statistical methods are required to determine the probability of their
occurrence. Space weather events can be characterised by a number of natural
phenomena such as X-ray (solar) flares, solar energetic particle (SEP) fluxes,
coronal mass ejections and various geophysical indices (Dst, Kp, F10.7). In
this paper extreme value theory (EVT) is used to investigate the probability of
extreme solar flares. Previous work has assumed that the distribution of solar
flares follows a power law. However such an approach can lead to a poor
estimation of the return times of such events due to uncertainties in the tails
of the probability distribution function. Using EVT and GOES X-ray flux data it
is shown that the expected 150-year return level is approximately an X60 flare
whilst a Carrington-like flare is a one in a 100-year event. It is also shown
that the EVT results are consistent with flare data from the Kepler space
telescope mission.Comment: 13 pages, 4 figures; updated content following reviewer feedbac
Coexistence of the "bogolons" and the one-particle spectrum of excitations with a gap in the degenerated Bose gas
Properties of the weakly non-ideal Bose gas are considered without suggestion
on C-number representation of the creation and annihilation operators with zero
momentum. The "density-density" correlation function and the one-particle Green
function of the degenerated Bose gas are calculated on the basis of the
self-consistent Hartree-Fock approximation. It is shown that the spectrum of
the one-particle excitations possesses a gap whose value is connected with the
density of particles in the "condensate". At the same time, the pole in the
"density-density" Green function determines the phonon-roton spectrum of
excitations which exactly coincides with one discovered by Bogolyubov for the
collective excitations (the "bogolons").Comment: 8 pages, no figure
Impact of magnetic storms on the global TEC distribution
The study is focused on the analysis of total electron content (TEC)
variations during six geomagnetic storms of different intensity: from
Dstmin = −46 nT to Dstmin = −223 nT. The values of TEC deviations from its 27-day median value
(δTEC) were calculated during the periods of the storms along three
meridians: American, Euro-African and Asian-Australian. The following results
were obtained. For the majority of the storms almost simultaneous occurrence
of δTEC maximums was observed along all three meridians at the
beginning of the storm. The transition from a weak storm to a superstorm (the
increase of magnetic activity) almost does not affect the intensity of the
δTEC maximum. The seasonal effect was most pronounced along the
Asian-Australian meridian, less often along the Euro-African meridian and was
not revealed along the American meridian. Sometimes the seasonal effect can
penetrate to the opposite hemisphere. The character of average δTEC
variations for the intense storms was confirmed by GOES satellite data.
Though there are some common features of TEC variation revealed during each
storm phase, in general no clear dependence of TEC responses on the storm
phases was found: the effects were different during each storm at different
locations. The behavior of the correlation coefficient (R) between δTEC values along the three meridians was analyzed for each storm. In
general, R > 0.5 between δTEC values averaged along each meridian.
This result is new. The possible reasons for the exceptions (when R < 0.5)
were provided: the complexity of phenomena during the intense storms and
discordance in local time of the geomagnetic storm beginning along different
meridians. Notwithstanding the complex dependence of R on the intensity of
magnetic disturbance, in general R decreased with the growth of storm
intensity.</p
State-dependent effects of transcranial oscillatory currents on the motor system during action observation
We applied transcranial alternating current stimulation (tACS) to the primary motor cortex (M1) at different frequencies during an index–thumb pinch-grip observation task. To estimate changes in the corticospinal output, we used the size of motor evoked potentials (MEPs) obtained by transcranial magnetic stimulation (TMS) of M1 using an online MRI-guided simultaneous TMS-tACS approach. The results of the beta-tACS confirm a non-selective increase in corticospinal excitability in subjects at rest; an increase was observed for both of the tested hand muscles, the first dorsal interosseous (FDI) and the abductor digiti minimi (ADM). However, during action observation of the pinch-grip movement, the increase of corticospinal excitability was only observed for the prime mover FDI muscle and took place during alpha-tACS, while gamma-tACS affected both the FDI and control muscle (ADM) responses. These phenomena likely reflect the hypothesis that the mu and gamma rhythms specifically index the downstream modulation of primary sensorimotor areas by engaging mirror neuron activity. The current neuromodulation approach confirms that tACS can be used to induce neurophysiologically detectable state-dependent enhancement effects, even in complex motor-cognitive tasks
Observations by the CUTLASS radar, HF Doppler, oblique ionospheric sounding, and TEC from GPS during a magnetic storm
First observations of oblique ionospheric sounding chirp signal in Mexico
The results of the first experiment of oblique ionospheric sounding (OIS) chirp signal reception in Mexico are
reported. Maximal and Lowest Observed Frequencies variations were studied under the quiet Space Weather
conditions. The diurnal ionospheric variations by OIS signal confirm the results based on GNSS data in the
Mexican region. The best HF radio propagation conditions along the considered path are during morning and
daytime hours. The multi-hop propagation is frequent. The interlayer propagation modes are present at nighttime
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