Institute of Solar-Terrestrial Physics

A series of satellite total solar irradiance (TSI) observations can be combined in a precise solar magnetic cycle length composite TSI database by determining the relationship between two non‐overlapping components: ACRIM1 and ACRIM2. [ Willson and Hudson , 1991 ; Willson , 1994 ] An ACRIM composite TSI time series using the Nimbus7/ERB results [ Hoyt et al. , 1992 ] to relate ACRIM1 and ACRIM2 demonstrates a secular upward trend of 0.05 percent‐per‐decade between consecutive solar activity minima. [ Willson , 1997 ] A PMOD TSI composite using ERBS [ Lee et al. , 1995 ] comparisons to relate ACRIM1 and ACRIM2 [ Fröhlich and Lean , 1998 ] differs from the ACRIM composite in two significant respects: a negligible trend between solar minima and lower TSI at solar maxima. Our findings indicate the lower PMOD trend and lower PMOD TSI at the maxima of solar cycles 22 and 23 are artifacts of ERBS degradation. Lower PMOD TSI during the maximum of cycle 21 results from modifications of Nimbus7/ERB and ACRIM1 published results that produces better agreement with a TSI/solar proxy model [ Foukal and Lean , 1988 ; Lean et al. , 1995 ; Fröhlich and Lean , 1998 ].

We have gathered optical photometry data from the literature on a large sample of Swift-era gamma-ray burst (GRB) afterglows including GRBs up to 2009 September, for a total of 76 GRBs, and present an additional three pre-Swift GRBs not included in an earlier sample. Furthermore, we publish 840 additional new photometry data points on a total of 42 GRB afterglows, including large data sets for GRBs 050319, 050408, 050802, 050820A, 050922C, 060418, 080413A, and 080810. We analyzed the light curves of all GRBs in the sample and derived spectral energy distributions for the sample with the best data quality, allowing us to estimate the host-galaxy extinction. We transformed the afterglow light curves into an extinction-corrected z = 1 system and compared their luminosities with a sample of pre-Swift afterglows. The results of a former study, which showed that GRB afterglows clustered and exhibited a bimodal distribution in luminosity space, are weakened by the larger sample. We found that the luminosity distribution of the two afterglow samples (Swift-era and pre-Swift) is very similar, and that a subsample for which we were not able to estimate the extinction, which is fainter than the main sample, can be explained by assuming a moderate amount of line-of-sight host extinction. We derived bolometric isotropic energies for all GRBs in our sample, and found only a tentative correlation between the prompt energy release and the optical afterglow luminosity at 1 day after the GRB in the z = 1 system. A comparative study of the optical luminosities of GRB afterglows with echelle spectra (which show a high number of foreground absorbing systems) and those without, reveals no indication that the former are statistically significantly more luminous. Furthermore, we propose the existence of an upper ceiling on afterglow luminosities and study the luminosity distribution at early times, which was not accessible before the advent of the Swift satellite. Most GRBs feature afterglows that are dominated by the forward shock from early times on. Finally, we present the first indications of a class of long GRBs, which form a bridge between the typical high-luminosity, high-redshift events and nearby low-luminosity events (which are also associated with spectroscopic supernovae) in terms of energetics and observed redshift distribution, indicating a continuous distribution overall.

The daily averaged Solar EUV Monitor (SEM)/Solar Heliospheric Observatory (SOHO) EUV measurements, solar proxies, and foF2 data at 20 ionosonde stations in the east Asia/Australia sector are collected to investigate the solar activity dependences of the ionospheric peak electron density (NmF2). The intensities of solar EUV from the SEM/SOHO measurements from 1996 to 2005 show a nonlinear relationship with F107, and the SEM/SOHO EUV can be better represented by a solar activity factor P = (F107 + F107A)/2. Seasonal and latitudinal dependences are found in the solar activity variation of NmF2 in the east Asia/Australian sector. The slope of NmF2 with P in the linear segment further shows similar annual variations as the background electron densities at moderate solar activity. Observations show a nonlinear dependence of NmF2 on solar EUV (the saturation effect of NmF2 for high solar EUV). On the basis of a simple model of photochemistry, taking the neutral atmospheric consequences into account, calculations at fixed height simulate the saturation effect of NmF2, but the observed change rate of NmF2 with P is inadequately reproduced. Calculations taking into account the influence of dynamics (via a simple model of the solar EUV dependence of the ionospheric height) tend to reproduce the observed change rate of NmF2. Results indicate that besides solar EUV changes, the influence of dynamics and the atmospheric consequences should substantially contribute to the solar activity variations of NmF2.

Abstract. We investigate the form and dynamics of shock-acoustic waves generated by earthquakes. We use the method for detecting and locating the sources of ionospheric impulsive disturbances, based on using data from a global network of receivers of the GPS navigation system, and require no a priori information about the place and time of the associated effects. The practical implementation of the method is illustrated by a case study of earthquake effects in Turkey (17 August and 12 November 1999), in Southern Sumatra (4 June 2000), and off the coast of Central America (13 January 2001). It was found that in all instances the time period of the ionospheric response is 180–390 s, and the amplitude exceeds, by a factor of two as a minimum, the standard deviation of background fluctuations in total electron content in this range of periods under quiet and moderate geomagnetic conditions. The elevation of the wave vector varies through a range of 20–44°, and the phase velocity (1100–1300 m/s) approaches the sound velocity at the heights of the ionospheric F-region maximum. The calculated (by neglecting refraction corrections) location of the source roughly corresponds to the earthquake epicenter. Our data are consistent with the present views that shock-acoustic waves are caused by a piston-like movement of the Earth’s surface in the zone of an earthquake epicenter.Key words. Ionosphere (ionospheric disturbances; wave propagation) – Radio science (ionospheric propagation)

A complete formalism, valid through second order in differential rotation, is developed and applied to calculate the frequencies of stellar oscillations. The derivation is improved and the asymptotic formulas for g-mode splittings are generalized. In application to solar oscillations, it is found that the second-order effects are dominated by distortion for l less than 500. Further, these effects are sufficiently large that they must be accounted for in any effort to seismically determine the sun's internal magnetic field. In the solar oscillation spectrum, accidental degeneracies happen but cannot lead to large frequency shifts. For evolved delta Scuti stars, calculated spectra are dense, and, under the perturbing effect of rotation, members of neighboring multiplets may overlap. The seismic potential of modes of mixed p-mode and g-mode character is emphasized for these stars.

Abstract. We developed a method and programs for estimation of the global electron content (GEC) from GPS measurements, using the ionosphere models IRI-2001 and NeQuick. During the 23rd cycle of solar activity, the value of GEC varied from 0.8 to 3.2×1032 electrons, following changes in the solar extreme ultra violet (EUV) radiation and solar radio emission at 10.7-cm wavelength. We found a strong resemblance of these variations, with discernible 11-year and 27-day periodicities. A saturation effect of GEC is found when F10.7 increases. We found that GEC is characterized by strong seasonal (semiannual) variations with maximum relative amplitude at about 10% during the rising and falling parts of the solar activity and up to 30% during the period of maximum. It was found that the relative difference between model and experimental GEC series increase as the smoothing time window decreases. We found that GEC-IRI seasonal variations are out-of-phase with experimental GEC values. The lag between model and experimental maximum of GEC values can reach several tens of days. The variations of GEC lag, on average, 2 days after those of F10.7 and UV. GEC completely reflects the dynamics of the active regions on the solar surface. The amplitude of the 27-day GEC variations decreases from 8% at the rising and falling solar activity to 2% at the maximum and at the minimum. We also found that the lifetime of contrast long-living active formations on the Sun's surface in EUV range for more than 1 month exceeds the one in radio range (10.7 cm).

Abstract The old problem of turbulent diffusion is addressed to define the influence of rotation and magnetic field ‐ the usual ingredients of astrophysical bodies ‐ on the effective transport coefficients. Either rotation and magnetism produce the anisotropy and quenching. The tensorial structures of the diffusivities and their dependences on the angular velocity and the field strength are explicitly defined. An example of application of the theory to the global stellar circulation model is given and the implications for cosmic dynamos are briefly discussed.

The corona of the Sun is a unique environment in which magnetohydrodynamic (MHD) waves, one of the fundamental processes of plasma astrophysics, are open to a direct study. There is striking progress in both observational and theoretical research of MHD wave processes in the corona, with the main recent achievements summarized as follows: ▪ Both periods and wavelengths of the principal MHD modes of coronal plasma structures, such as kink, slow and sausage modes, are confidently resolved. ▪ Scalings of various parameters of detected waves and waveguiding plasma structures allow for the validation of theoretical models. In particular, kink oscillation period scales linearly with the length of the oscillating coronal loop, clearly indicating that they are eigenmodes of the loop. Damping of decaying kink and standing slow oscillations depends on the oscillation amplitudes, demonstrating the importance of nonlinear damping. ▪ The dominant excitation mechanism for decaying kink oscillations is associated with magnetized plasma eruptions. Propagating slow waves are caused by the leakage of chromospheric oscillations. Fast wave trains could be formed by waveguide dispersion. ▪ The knowledge gained in the study of coronal MHD waves provides ground for seismological probing of coronal plasma parameters, such as the Alfvén speed, the magnetic field and its topology, stratification, temperature, fine structuring, polytropic index, and transport coefficients.

Context. We investigate the decayless regime of coronal kink oscillations recently discovered in the Solar Dynamics Observatory (SDO)/AIA data. In contrast to decaying kink oscillations that are excited by impulsive dynamical processes, this type of transverse oscillations is not connected to any external impulsive impact, such as a flare or coronal mass ejection, and does not show any significant decay. Moreover the amplitude of these decayless oscillations is typically lower than that of decaying oscillations.

The article is a review of studies of ionospheric effects carried out in ISTP SB RAS. The main results of GPS/GLONASS radio sounding of ionospheric disturbances of natural and anthropogenic origin are presented. The article is devoted to ionospheric effects of solar eclipses, solar flares, solar terminator, earthquakes, tropical cyclones, large-scale ionospheric disturbances of auroral origin, rocket launches. Dynamics of global electron content analysis is also presented. The special attention is paid on the influence of solar flares and ionospheric irregularities on GPS and GLONASS performance. The work is a tribute to the leader of GNSS-monitoring workgroup Prof. E.L. Afraimovich (12 March 1940–8 November 2009).

Near‐field coseismic perturbations of ionospheric total electron content (TEC), caused by direct acoustic waves from focal regions, can be observed with Global Positioning System (GPS). They appear 10–15 min after the earthquake with typical periods of ∼4–5 min and propagate as fast as ∼1 km/s toward directions allowed by ambient geomagnetic fields. Ionospheric disturbance, associated with the 2004 December 26 great Sumatra‐Andaman earthquake, was recorded with nine continuous GPS receiving stations in Indonesia and Thailand. Here we explore the possibility to constrain the rupture process of the earthquake with the observed ionospheric disturbances. We assumed linearly distributed point sources along the zone of coseismic uplift extending ∼1300 km from Sumatra to the Andaman Islands that excited acoustic waves sequentially as the rupture propagate northward by 2.5 km/s. TEC variations for several satellite‐receiver pairs were synthesized by simulating the propagation of acoustic waves from the ground to the ionosphere and by integrating the TEC perturbations at intersections of line of sights and the ray paths. The TEC perturbations from individual point sources were combined using realistic ratios, and the total disturbances were compared with the observed signals. Prescribed ratios based on geodetically inferred coseismic uplifts reproduced the observed signals fairly well. Similar calculation using a rupture propagation speed of 1.7 km/s degraded the fit. Suppression of acoustic waves from the segments north of the Nicobar Islands also resulted in a poor fit, which suggests that ruptures in the northern half of the fault were slow enough to be overlooked in short‐period seismograms but fast enough to excite atmospheric acoustic waves. Coseismic ionospheric disturbance could serve as a new indicator of faulting sensitive to ruptures with timescale up to 4–5 min.

Context. Kink oscillations of coronal loops in an off-limb active region are detected with the Imaging Assembly Array (AIA) instruments of the Solar Dynamics Observatory (SDO) at 171 Å.

Using GPS total electron content (TEC) measurements, we analyzed ionosphere response to the great Kurile earthquake of 4 October 1994. High spatial resolution of the Japanese dense array of GPS receivers (GEONET) provided us the unique opportunity to observe the evolution of coseismic ionospheric disturbances (CID), which propagated for more than 1800 km away from the epicenter. Plotting a traveltime diagram for the CID and using an “array processing” technique within the approximation of a spherical CID wavefront, we observed a phenomenon of CID separation into two modes and we found that characteristics of the CID depend on the distance from the epicenter. The maximum of the CID amplitude was observed at ∼500 km from the epicenter. Within the first 600–700 km, the CID propagation velocity was about 1 km/s, which is equal to the sound speed at the height of the ionospheric F‐layer. Starting from ∼600 to 700 km out from the epicenter, the disturbance seems to divide into two separate perturbations, with each propagating at a different velocity, about 3 km/s for the one and about 600 m/s for the other. Apparently, the TEC response in the far‐field of the CID source is a mixture of signals that further “splits” into two modes because of the difference in their velocities. Our observations are in good agreement with the results of space‐time data processing in the approximation of a spherical wavefront of CID propagation.

On 6 September 2017, the Sun emitted two significant solar flares (SFs). The first SF, classified X2.2, peaked at 09:10 UT. The second one, X9.3, which is the most intensive SF in the current solar cycle, peaked at 12:02 UT and was accompanied by solar radio emission. In this work, we study ionospheric response to the two X-class SFs and their impact on the Global Navigation Satellite Systems and high-frequency (HF) propagation. In the ionospheric absolute vertical total electron content (TEC), the X2.2 SF caused an overall increase of 2-4 TECU on the dayside. The X9.3 SF produced a sudden increase of ~8-10 TECU at midlatitudes and of ~15-16 TECU enhancement at low latitudes. These vertical TEC enhancements lasted longer than the duration of the EUV emission. In TEC variations within 2-20 min range, the two SFs provoked sudden increases of ~0.2 TECU and 1.3 TECU. Variations in TEC from geostationary and GPS/GLONASS satellites show similar results with TEC derivative of ~1.3-1.7 TECU/min for X9.3 and 0.18-0.24 TECU/min for X2.2 in the subsolar region. Further, analysis of the impact of the two SFs on the Global Navigation Satellite Systems-based navigation showed that the SF did not cause losses-of-lock in the GPS, GLONASS, or Galileo systems, while the positioning error increased by ~3 times in GPS precise point positioning solution. The two X-class SFs had an impact on HF radio wave propagation causing blackouts at <30 MHz in the subsolar region and <15 MHz in the postmidday sector.

ABSTRACT We present the first sample of tidal disruption events (TDEs) discovered during the SRG all-sky survey. These 13 events were selected among X-ray transients detected in the 0° &amp;lt; l &amp;lt; 180° hemisphere by eROSITA during its second sky survey (2020 June 10 to December 14) and confirmed by optical follow-up observations. The most distant event occurred at z = 0.581. One TDE continued to brighten at least 6 months. The X-ray spectra are consistent with nearly critical accretion on to black holes of a few ×103 to $10^8\, \mathrm{ M}_\odot$, although supercritical accretion is possibly taking place. In two TDEs, a spectral hardening is observed 6 months after the discovery. Four TDEs showed an optical brightening apart from the X-ray outburst. The other nine TDEs demonstrate no optical activity. All 13 TDEs are optically faint, with Lg/LX &amp;lt; 0.3 (Lg and LX being the g band and 0.2–6 keV luminosity, respectively). We have constructed a TDE X-ray luminosity function, which can be fit by a power law with a slope of −0.6 ± 0.2, similar to the trend observed for optically selected TDEs. The total rate is estimated at (1.1 ± 0.5) × 10−5 TDEs per galaxy per year, an order of magnitude lower than inferred from optical studies. This suggests that X-ray bright events constitute a minority of TDEs, consistent with models predicting that X-rays can only be observed from directions close to the axis of a thick accretion disc formed from the stellar debris. Our TDE detection threshold can be lowered by a factor of ∼2, which should allow a detection of ∼700 TDEs by the end of the SRG survey.

Context. We investigate the decayless regime of coronal kink oscillations recently discovered in the Solar Dynamics Observatory (SDO)/AIA data. In contrast to decaying kink oscillations that are excited by impulsive dynamical processes, this type of transverse oscillations is not connected to any external impulsive impact, such as a flare or coronal mass ejection, and does not show any significant decay. Moreover the amplitude of these decayless oscillations is typically lower than that of decaying oscillations. Aims. The aim of this research is to estimate the prevalence of this phenomenon and its characteristic signatures. Methods. We analysed 21 active regions (NOAA 11637–11657) observed in January 2013 in the 171 Å channel of SDO/AIA. For each active region we inspected six hours of observations, constructing time-distance plots for the slits positioned across pronounced bright loops. The oscillatory patterns in time-distance plots were visually identified and the oscillation periods and amplitudes were measured. We also estimated the length of each oscillating loop. Results. Low-amplitude decayless kink oscillations are found to be present in the majority of the analysed active regions. The oscillation periods lie in the range from 1.5 to 10 min. In two active regions with insufficient observation conditions we did not identify any oscillation patterns. The oscillation periods are found to increase with the length of the oscillating loop. Conclusions. The considered type of coronal oscillations is a common phenomenon in the corona. The established dependence of the oscillation period on the loop length is consistent with their interpretation in terms of standing kink waves.

A phenomenological relationship between oscillations in a sunspot and quasi-periodic pulsations (QPP) in flaring energy releases at an active region (AR) above the sunspot is established. The analysis of the microwave emission recorded by the Nobeyama Radioheliograph at 17 GHz shows a gradual increase in the power of the 3-min oscillation train in the sunspot associated with AR 10756 before flares in this AR. The flaring light curves are found to be bursty with a period of 3 min. Our analysis of the spatial distribution of the 3-min oscillation power implies that the oscillations follow from sunspots along coronal loops towards the flaring site. It is proposed that QPP in the flaring energy releases can be triggered by 3-min slow magnetoacoustic waves leaking from sunspots.

We have investigated type II radio bursts in the solar corona using data from ground‐based radio telescopes (&gt;18 MHz) and from the Radio and Plasma Wave experiment (WAVES) on board the WIND spacecraft (&lt;14 MHz). The wavelength range of the WAVES experiment includes the 2‐ to 14‐MHz band, previously unobserved from space. We found that all 34 coronal type II bursts observed over a period of 18 months (November 1, 1994, to April 30, 1996), decayed within a few solar radii and did not propagate into the interplanetary medium. On the other hand, most of the accompanying type III radio bursts observed by the ground‐based instruments were observed to continue into the interplanetary medium as the electron beams propagated freely along open magnetic field lines. Over the same period of time, other instruments on board the WIND spacecraft detected about 18 interplanetary shock candidates, which seem to be unrelated to the coronal type II bursts. This result confirms the idea that the coronal and interplanetary shocks are two different populations and are of independent origin. We reexamine the data and conclusions of Gosling et al. [1976], Munro et al. [1979], and Sheeley et al. [1984] and find that their data are consistent with our result that the coronal type II bursts are due to flares. We also briefly discuss the implications of our result to the modeling studies of interplanetary shocks based on input from coronal type II radio bursts.

Radiation produced by charged particles gyrating in a magnetic field is highly significant in the astrophysics context. Persistently increasing resolution of astrophysical observations calls for corresponding three-dimensional modeling of the radiation. However, available exact equations are prohibitively slow in computing a comprehensive table of high-resolution models required for many practical applications. To remedy this situation, we develop approximate gyrosynchrotron (GS) codes capable of quickly calculating the GS emission (in non-quantum regime) from both isotropic and anisotropic electron distributions in non-relativistic, mildly relativistic, and ultrarelativistic energy domains applicable throughout a broad range of source parameters including dense or tenuous plasmas and weak or strong magnetic fields. The computation time is reduced by several orders of magnitude compared with the exact GS algorithm. The new algorithm performance can gradually be adjusted to the user's needs depending on whether precision or computation speed is to be optimized for a given model. The codes are made available for users as a supplement to this paper.

Abstract Using data of GPS receivers located worldwide, we analyze the quality of GPS performance during four geomagnetic storms of different intensity: two super‐storms and two intense storms. We show that during super‐storms the density of GPS Losses‐of‐Lock (LoL) increases up to 0.25% at L1 frequency and up to 3% at L2 frequency, and up to 0.15% (at L1) and 1% (at L2) during less intense storms. Also, depending on the intensity of the storm time ionospheric disturbances, the total number of total electron content (TEC) slips can exceed from 4 to 40 times the quiet time level. Both GPS LoL and TEC slips occur during abrupt changes of SYM‐H index of geomagnetic activity, i.e., during the main phase of geomagnetic storms and during development of ionospheric storms. The main contribution in the total number of GPS LoL was found to be done by GPS sites located at low and high latitudes, whereas the area of numerous TEC slips seemed to mostly correspond to the boundary of the auroral oval, i.e., region with intensive ionospheric irregularities. Our global maps of TEC slips show where the regions with intense irregularities of electron density occur during geomagnetic storms and will let us in future predict appearance of GPS errors for geomagnetically disturbed conditions.