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

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&thinsp;nT to Dstmin = −223&thinsp;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 &gt; 0.5 between δTEC values averaged along each meridian. This result is new. The possible reasons for the exceptions (when R &lt; 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

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