[1] We use the ionospheric velocity measurements obtained by the European incoherent scatter (EISCAT) Svalbard radar (ESR) for comparison with the electric field distribution predicted by the new ionospheric convection model based on realistic maps of field-aligned currents. The comparative analysis of model output and observations is based on the data contained both the north–south and east–west components of the ionospheric electric field. Such a comparison provides an independent check of the model. It is shown that the model represents accurately the large-scale features of statistical electric field inferred from the ion velocity measured by the ESR. From observations and modeling we quantitatively determine the dependence of the ionospheric electric field strength on the IMF conditions and how this varies with magnetic local time. For specification of the relationship we derive average convection patterns for sorting by the magnitude and direction of the IMF. The dependence is clearly seen by contrasting the results for two magnitudes of the IMF strength (BT = 1 and 5 nT), for two opposite directions of the IMF Bz component, and for By+, By−. The magnitude of the IMF has the main impact on the average flows under southward IMF conditions. The sign of IMF By is the most important factor influenced the convection system under the IMF northward. For By− the northward component is negative during a major part of the day indicating counterclockwise plasma flow around the pole. For By+, the situation is opposite, the electric field is directed poleward and the plasma rotate clockwise. These are confirmed nicely by the ESR observations.
[1] Calculations of the Doppler shift of the frequency (DSF) of a UHF signal under given models of a wave disturbance (WD), undisturbed medium, and particular orbits of low-orbiting (LO) satellites showed that there is an angle-of-approach effect. On the basis of the angle-of-approach effect a method of estimation of model parameters of medium-scale WD on the basis of measurements of DSF of signals from LO satellites in one observation point is developed. A trend in DSF variations is detected in the initial time series. Using this trend, the global model of the undisturbed ionosphere is corrected. For the parameters estimation, calculated trajectory parameters of the satellite and recordings of DSF variations are used. The numerical experiment carried out demonstrates the efficiency of the method developed. The method was tested at a series of DSF registrations received from LO satellites of the Tsykada type in 2002–2004. The analysis of DSF registrations showed that at high zenith angles for the orbits close to the central ones, WD disturbances with a vertical component are often observed. In such cases, the vertical wavelength can reach 140 km. For horizontal WD, the estimates of the wavelength give values >100 km. The range of variations of the maximum relative disturbance of the electron concentration is 1–7%.
[1] A mechanism of excitation of azimuthal small-scale (m 1) slow magnetosonic (SMS) oscillations by the external currents in the near-equatorial region of the terrestrial magnetosphere is proposed. A theoretical study of the structure and spectrum of such oscillations is performed in the model of the magnetosphere with the dipole magnetic field and rotating plasma. In the direction along field lines of the geomagnetic field the oscillations are waves standing between the magnetically conjugated points. The amplitude of the magnetospheric SMS oscillations with m 1 decreases from the equatorial plane to the ionosphere at scales much lower then the field line length. So one is not able to observe the SMS-oscillation field in the vicinity of the ionosphere and on the surface of the Earth. In the direction across the magnetic shells, the standing SMS waves have a structure typical for resonant oscillations. Latitudinal distributions of eigenfrequencies of several first harmonics of SMS oscillations with the polarization similar to the polarization of poloidal and toroidal Alfven waves in the plane perpendicular to the field lines are drawn. INDEX
[1] Analysis of both in-phase and quadrature components of received frequency-modulated continuous wave (FMCW) signal at the output of intermediate frequency (IF) band-pass filter of the chirp ionosonde demonstrates a new possibility of recovering the transfer function of the HF radio channel within the band of operating frequencies and the interval of time delays that are defined by the IF filter characteristics. The transfer function is determined by the parameters of all parts comprising the radio channel: transmitter, receiving-transmitting antenna-feeder devices, Earth surface, ionosphere, as well as the receiver circuitry participating in the signal processing. However, characteristics of all parts except the ionosphere can be considered stationary and well-known or controlled. It is the ionosphere that displays significant temporal variability. A new technique is proposed for recovering the transfer function of the ionospheric radio channel, together with its implementation as a signal preprocessing circuit containing a correcting digital filter that adapts to the current amplitude frequency and phase frequency characteristics of the channel. INDEX TERMS: 2441 Ionosphere: Ionospheric storms; 2407 Ionosphere: Auroral ionosphere; 2447 Ionosphere: Modeling and forecasting; KEYWORDS: FMCW sounder, chirp sounding technique, ionospheric radio channel, transfer function, digital signal processing.
The results of studies of the ionospheric response to solar flares are presented. The results are based on the observations of GPS signals and at incoherent scatter radars and on theoretical calculations. Analyzing the GPS data, the method based on a partial "shadowing" of the atmosphere by the Earth was used. This method made it possible to estimate the value of the electron content variations in the topside ionosphere during the solar flare on 14 July 2000. We obtained that according to the GPS data at altitudes of the topside ionosphere (h > 300 km) a flare is able to cause a decrease of the electron content. Similar effects of formation of a negative disturbance in the ionospheric F region were observed also during the solar flares on 21 and 23 May 1967 by the Arecibo radar. Using the theoretical model of ionosphere-plasmasphere interaction, we study in this paper the mechanism of formation of negative disturbances in the topside ionosphere during solar flares. It is shown that the intense transport of O+ ions into the above-situated plasma caused by a sharp increase in the ion production rate and thermal expansion of the ionospheric plasma is a cause of the formation of the negative disturbance in the electron concentration in the topside ionosphere.
A convection-driven MHD dynamo in a rotating spherical shell, with clearly defined structural elements in the flow and magnetic field, is simulated numerically. Such dynamos can be called deterministic, in contrast to those explicitly dependent on the assumed properties of turbulence. The cases most interesting from the standpoint of studying the nature of stellar magnetism demonstrate the following features. On a global scale, the convective flows can maintain a ``general'' magnetic field with a sign-alternating dipolar component. Local (in many cases, bipolar) magnetic structures are associated with convection cells. Disintegrating local structures change into background fields, which drift toward the poles. From time to time, reversals of the magnetic fields in the polar regions occur, as ``new'' background fields expel the ``old'' fields.
[1] The problem on determination of the electromagnetic field of a near-Earth horizontal electric dipole in the frequency range 10−1 − 10 Hz at distances 10−1 − 10 km in the Earth–ionosphere waveguide has been solved. The ionosphere is assumed to be horizontally stratified and anisotropic, and the Earth is assumed to be homogeneous to some depth and infinitely conducting below. The representation of the electromagnetic field as expansion in waveguide modes is given. The zero mode is regarded as anisotropic. Local modes are given in the isotropic approximation of the anisotropic ionosphere due to inclusion of an effective exponential conductivity profile. An analytical description of the field reflection for vertical and horizontal polarizations has been obtained for this model. At short distances from the source, the solution coincides with the known solution for the case of the absence of the ionosphere. The range of distances beginning from which the influence of the ionosphere on the near-Earth magnetic field manifests itself has been found.
[1] The transport of solar energetic particles in interplanetary space is determined by the structure and dynamics of the solar wind between the source and the observer. The intensity time profile, in particular the decay phase, is determined by the transport processes. In this paper we discuss the decay phases of solar energetic proton events in the energy range 1‐48 MeV for the period 1974‐2001. For events with exponential shape of decay, the dependence of the characteristic time on the exponent of the energetic spectrum , the solar wind velocity V , and the proton energy E assumed in a form (E) = CE n is given. Such presentation allows us to consider the action of three main mechanisms of propagation (diusion, convection, and adiabatic cooling) that determine the value. It is shown that approximately half of decays with the constant value of V is described quite satisfactorily within the frame of the model with predominant convection and adiabatic deceleration in comparison with particle diusion. The dependence of n on heliolongitude of the parent flare (the source of particles) is investigated. INDEX TERMS: 2104 Interplanetary Physics: Cosmic rays; 2114 Interplanetary Physics: Energetic particles; 2194 Interplanetary Physics: Instruments and techniques;
[1] Results of studying the ionosphere behavior during several magnetic storms of various intensities are presented. The features of pronounced negative ionospheric disturbances accompanying the severe magnetic storms on 25 September 1998 and 29‐30 May 2003 (Kp 8) are considered. Among them there are a decrease in the electron density by a factor of 3‐4, uplifting of the ionospheric F2 layer by 100‐160 km, increase in the temperature of the charged and neutral atmospheric components, and infringement of plasma transfer processes and thermal balance in the ionosphere‐plasmasphere system. In the morning of 25 September 1998, an unusual increase in the upward plasma drift velocity was registered. On 29‐30 May 2003 during the storm main phase, a depletion of the relative density of hydrogen ions by more than an order of magnitude was observed that could manifest an emptying of the magnetic flux tube over Kharkov. These eects are explained in terms of thermospheric disturbances, Joule heating, particle precipitation, penetration of magnetospheric electric fields to midlatitudes, the shift of the main ionospheric trough and related structures toward the radar latitude, etc. The ionospheric storm on 20‐21 March 2003 had two phases. Its strong negative phase proceeded against a background of a minor geomagnetic disturbance (Kp 5). The destabilizing impact of the electric field pulse and traveling atmospheric disturbance generated by magnetospheric substorms could be the cause of the change in the storm phase that occurred in the sunset period. INDEX TERMS: 2431 Ionosphere: Ionosphere/magnetosphere interactions; 2788 Magnetospheric Physics: Magnetic storms and substorms; 6969 Radio Science: Remote sensing; KEYWORDS: ionosphere; magnetic storms; incoherent scatter radar observations.
[1] New solar activity indices for the sunspot cycle minima were obtained from the Hα synoptic charts for 1887–2005, Ca II K observational data of the solar disk at Kodaikanal for 1907–1995. The Apz index determines the area of the polar zone occupied by the magnetic field of one polarity, and A(t) is the sum of squares of dipole and octupole magnetic moments. The L(t) index is the total length of magnetic neutral lines on the Hα synoptic chart, and K(t) characterizes the complexity of the topology pattern of the general magnetic field of the Sun. It is determined by the inverse number of crossings of neutral lines with the meridian net on the Hα chart. The R(t) index characterizes the correlation factor of the magnetic field polarities in the latitude zones ±40◦. The solar rotation index SSPD(t) is determined by the sum of the spectral density of the power of the magnetic field sector structures. The area of the Ca II K bright points at high latitudes is the SK(t) index. It has been shown that the 11-year cycles of Apz(t), A(t), L(t), K(t), R(t), SSPD(t), and SK(t) have maxima at the sunspot activity minimum and that they are ahead of the cycles of Wolf numbers, W (t), by 5–6 years.
[1] A brief review on turbulent and ordered plasma motions in the solar corona is presented. Velocities in quiet coronal regions, coronal loops, coronal holes, and cavities around quiescent prominences are analyzed. Data based on interferometric observations of several total solar eclipses obtained earlier by the authors and in addition the values of turbulent velocities resulting from revising the 5303 line profiles were used. Published data based both on ground and space observations in visual and far ultraviolet (UV) spectral region are collected. The turbulent velocities in quiet loop structures and diffuse quiet corona seem to be similar, and the coronal plasma in the vicinity of the quiet protuberances is less turbulent than in the quiet corona or in the coronal holes. Velocities of directed motions for different structures in the corona are examined. It seems that the turbulent velocities are comparable by their magnitudes with the direct plasma velocities; however, at the same time they are slightly less.
[1] A solution to the problem of propagation of a radio wave pulse in isotropic and gyrotropic cold collisional plasmas has been found. The solution is not based on calculation of the Fourier integral and provides an analytical description of the spatial-temporal evolution of characteristics of radio wave pulses with an initial envelope of a rather general form, including pulses with a linear frequency modulation. Expressions for the velocity of propagation of radio wave pulses in isotropic and gyrotropic plasmas have been obtained in the explicit form.
[1] Time structure of the soft and hard X-ray emission of the solar flare on 29 October 2002 is studied. The flare began at 2148:49 UT and was measured by the IRIS spectrometer on board the CORONAS-F spacecraft. High sensitivity of the device made it possible to detect in the flare X-ray emission a pulse structure with timescale of the order of tens of milliseconds. Similarly, spike structure was observed on board of Compton Gamma Ray Observatory. A quasiperiodic structure with a magnitude of the order of 10 s was revealed by means of spectral time analysis. This structure can be explained by magnetohydrodynamic sausage-mode oscillations generated in the flare loop with the characteristic parameters of the plasma of the coronal region of the solar atmosphere. The evolution of the energetic spectrum of the hard X-ray emission of this flare with the resolution of 1 s during the entire event is studied.
[1] Strongest energy releases on the Sun are manifested as coronal mass ejections and solar flares. They are driven by subphotospheric thermal and electromagnetic energy fluxes. We consider recent examples showing that their heliospheric and magnetospheric consequences could be similar or rather different from case to case. Common and specific features can be expressed and delimited when using dimensionless parameters. A universal geometry scenario for these processes does not exist.
We present the results of experimental studies of the distance-frequency and amplitude-frequency characteristics of the ionospheric HF channel on mid-latitude paths of oblique chirp sounding. It is shown that the maximum observed frequencies (MOFs) are subject to short-period variations with the quasi-periods from 30 min to 2 h.The amplitude of the MOF variations reaches 2 MHz and can increase up to 5–8 MHz on the Cyprus—Rostov-on-Don one-hop path in the sunrise-sunset time. It is established that the MOF fluctuations are accompanied by pronounced “cusp” features occurring in the upper rays and moving with time to the region of shorter delays, i.e., from lower to higher frequencies. The amplitude-frequency characteristics of individual propagation modes undergo deep fluctuations (up to 20–30 dB)whose quasi-period and depth depend on the frequency. It is shown that the appearance of fluctuations is caused by interference of the unresolved rays within the limits of one propagation mode. Based on the modeling, it is shown that “cusps” in oblique-sounding ionograms are due to the influence of traveling ionospheric disturbances (TIDs). The TID parameters are estimated. It is shown that conditions of the formation of “ cusps” in the distance-frequency characteristics depend on the TID amplitude, the wavelength of the disturbance wave, and the direction of its phase front with respect to the propagation path. The effect of quasi-regular frequency modulation of the Pedersen mode with a period of 250–300 kHz on the Cyprus-Rostov-on-Don chirp-sounding path is found. Altitude stratification of the ionosphere near the F-layer maximum, which is responsible for the focusing and defocusing of the Pedersen mode, is estimated. It is established that the stratification scale amounts to approximately 200–250 m.
