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The dependence of four-peak longitudinal structure of the tropical electric field on the processes in the lower atmosphere and geomagnetic field configuration

Publication date: Available online 28 June 2019

Source: Advances in Space Research

Author(s): V.V. Klimenko, M.V. Klimenko, F.S. Bessarab, T.V. Sukhodolov, E.V. Rozanov

Abstract

In this article, we estimate the influence of the atmospheric-ionospheric interaction and the differences in the location of the geographic and geomagnetic poles on the longitudinal variability of the ionospheric electric field in the vicinity of the geomagnetic equator. For the study, we applied the upper atmosphere model (GSM TIP) and the recently created entire atmosphere model (EAGLE). The simulation results confirm that the four-peak structure of the longitudinal variation and the pre-reversal enhancement of the eastward electric field at the equator are caused by the atmosphere-ionosphere coupling. The role of F-region dynamo in the formation of pre-reversal enhancement of the eastward electric field is beyond the scope of this paper. These structures appear during the minimum of solar activity periods and are formed by the longitudinal variations of the horizontal thermospheric wind and the Pedersen conductivity arising from the action of the mesospheric tides. During solar activity minimum periods, the difference in the locations of geographic and geomagnetic poles leads to some complication and smoothing of the obtained picture of longitudinal variations in the electric field. We suggest that mesospheric tides have a significant effect on the longitudinal variation of the Pedersen conductivity, while the longitudinal variation of the Hall conductivity is mainly determined by the mismatch of geographic and geomagnetic poles. The amplitudes of the equatorial electric field longitudinal variations are three to four times smaller than the amplitudes of their diurnal variations, however, they can have a significant impact on the spatial distribution of the electron density in the F region of the low-latitude ionosphere.

Impact of chemical initial and lateral boundary conditions on air quality prediction

Publication date: Available online 28 June 2019

Source: Advances in Space Research

Author(s): Aman W. Khan, Prashant Kumar

Abstract

Accurate representation of initial and boundary conditions of chemical species in numerical models is a major challenge for air quality prediction. Model for Ozone and Related chemical Tracers, version 4 (MOZART-4) is an offline global chemical transport model simulating various gases and particulates in the atmosphere. In this work, MOZART-4 analysis is used to modify the chemical boundary conditions over India to improve the forecast in an online-coupled model. A sensitivity study has been performed using the Weather Research and Forecasting (WRF) model coupled with Chemistry model (WRF-Chem) for the highly polluted winter month of December 2016. Daily two parallel experiments are performed from the WRF-Chem model with and without updating chemical initial and lateral boundary conditions using MOZART-4 analyses. Results show that the tropospheric ozone (O3) and surface carbon monoxide (CO) forecasts are improved by 6% and 20%, respectively when MOZART-4 analysis is used to update chemical initial and lateral boundary conditions. Moreover, prediction of O3 and CO is also improved vertically (reduction in root mean square difference by 58% and 26%, respectively) in different forecast lengths. The percentage improvement in surface CO, particulate matter of size less than 2.5 μm (PM2.5) and particulate matter of size less than 10 μm (PM10) forecasts is ∼22%, ∼4.3% and ∼20%, respectively when compared against ground observations from Central Pollution Control Board (CPCB), New Delhi stations. Overall, we observed improvement in O3, CO, PM2.5 and PM10 forecast using WRF-Chem model after initialization of chemical conditions from MOZART-4 analysis.

Advanced technologies for satellite navigation and geodesy

Publication date: Available online 27 June 2019

Source: Advances in Space Research

Author(s): G. Giorgi, T.D. Schmidt, C. Trainotti, R. Mata-Calvo, C. Fuchs, M.M. Hoque, J. Berdermann, J. Furthner, C. Günther, T. Schuldt, J. Sanjuan, M. Gohlke, M. Oswald, C. Braxmaier, K. Balidakis, G. Dick, F. Flechtner, M. Ge, S. Glaser, R. König

Abstract

This manuscript reviews recent progress in optical frequency references and optical communication systems and discusses their utilizations in global satellite navigation systems and satellite geodesy. Lasers stabilized with optical cavities or spectroscopy of molecular iodine are analyzed, and a hybrid architecture is proposed to combine both forms of stabilization with the aim of achieving a target frequency stability of 10-15 [s/s] over a wide range of sampling intervals.

The synchronization between two optical frequency references in real-time is realized by means of time and frequency transfer on optical carriers. The technologies enabling coherent optical links are reviewed, and the development of an optical communication system for synchronization, ranging and data communication in space is described. An infrastructure exploiting the capabilities of both optical technologies for the realization of a modernized constellation of navigation satellites emitting highly synchronized signals is reviewed. Such infrastructure, named Kepler system, improves satellite navigation in terms intra-system synchronization, orbit determination accuracy, as well as system monitoring and integrity. The potential impact on geodetic key parameters is addressed.

CHAMP and GOCE thermospheric wind characterization with improved gas-surface interactions modelling

Publication date: Available online 27 June 2019

Source: Advances in Space Research

Author(s): G. March, T. Visser, P.N.A.M. Visser, E.N. Doornbos

Abstract

The CHAMP and GOCE satellites provided high-resolution thermosphere data between 2000 and 2013, improving our knowledge of atmosphere dynamics in the thermosphere-ionosphere region. However, the currently available data sets contain inconsistencies with each other and with external data sets and models, arising to a large extent from errors in the modelling of aerodynamic forces. Improved processing of the wind data for the two satellites would benefit the further development and validation of thermosphere models and improve current understanding of atmospheric dynamics and long-term trends. The first step to remove inconsistencies has been the development of high-fidelity models of the satellite surface geometry. Next, an improved characterization of the collisions between atmospheric particles and satellite surfaces is necessary. In this article, the effect of varying the energy accommodation coefficient, which is a key parameter for describing gas-surface interactions (GSI) is investigated. For past versions of the thermosphere density and wind data from these satellites a value of the energy accommodation coefficient of αE=0.93 was selected. The satellite accelerometer measurements, from which the thermospheric data are derived, have now been reprocessed using high-fidelity geometries and a wide range of αE values. Lowering the αE value used in the processing leads to an increase in the lift over drag ratio for those satellite panels that are inclined to the flow. This changes the direction of the modelled acceleration, and therefore the interpretation of the measured acceleration in terms of wind. The wrong choice of αE therefore leads to the introduction of satellite attitude-dependent wind errors. For the CHAMP and GOCE satellites, we have found that values of the energy accommodation coefficient significantly lower than 0.93 (0.85 for CHAMP and 0.82 for GOCE) result in increased consistency of the wind data. A comparison between the two missions and an overview of the influence on the results of filtering for solar activity and seasonal and diurnal variations is presented.

InfraRed Astronomy Satellite Swarm Interferometry (IRASSI): Overview and study results

Publication date: Available online 27 June 2019

Source: Advances in Space Research

Author(s): Hendrik Linz, Divya Bhatia, Luisa Buinhas, Matthias Lezius, Eloi Ferrer, Roger Förstner, Kathrin Frankl, Mathias Philips-Blum, Meiko Steen, Ulf Bestmann, Wolfgang Hänsel, Ronald Holzwarth, Oliver Krause, Thomas Pany

Abstract

The far-infrared (FIR) regime is one of the few wavelength ranges where no astronomical data with sub-arcsecond spatial resolution exist yet. Neither of the medium-term satellite projects like SPICA, Millimetron or OST will resolve this malady. For many research areas, however, information at high spatial and spectral resolution in the FIR, taken from atomic fine-structure lines, from highly excited carbon monoxide (CO) and especially from water lines would open the door for transformative science. These demands call for interferometric concepts. We present here first results of our feasibility study IRASSI (Infrared Astronomy Satellite Swarm Interferometry) for an FIR space interferometer. Extending on the principal concept of the previous study ESPRIT, it features heterodyne interferometry within a swarm of five satellite elements. The satellites can drift in and out within a range of several hundred meters, thereby achieving spatial resolutions of <0.1 arcsec over the whole wavelength range of 1–6 THz. Precise knowledge on the baselines will be ensured by metrology methods employing laser-based optical frequency combs, for which preliminary ground-based tests have been designed by members of our study team. We first give a motivation on how the science requirements translate into operational and design parameters for IRASSI. Our consortium has put much emphasis on the navigational aspects of such a free-flying swarm of satellites operating in relatively close vicinity. We hence present work on the formation geometry, the relative dynamics of the swarm, and aspects of our investigation towards attitude estimation. Furthermore, we discuss issues regarding the real-time capability of the autonomous relative positioning system, which is an important aspect for IRASSI where, due to the large raw data rates expected, the interferometric correlation has to be done onboard, in quasi-real-time. We also address questions regarding the spacecraft architecture and how a first thermomechanical model is used to study the effect of thermal perturbations on the spacecraft. This will have implications for the necessary internal calibration of the local tie between the laser metrology and the phase centres of the science signals and will ultimately affect the accuracy of the baseline estimations.

Author Index

Publication date: 1 August 2019

Source: Advances in Space Research, Volume 64, Issue 3

Author(s):

List of Referees

Publication date: 1 August 2019

Source: Advances in Space Research, Volume 64, Issue 3

Author(s):

Attitude control for tethered towing debris under actuators and dynamics uncertainty

Publication date: Available online 27 June 2019

Source: Advances in Space Research

Author(s): Cheng Jia, Zhongjie Meng, Panfeng Huang

Abstract

Tethered towing is regarded as a promising debris removal technology to decrease the population of space debris. Owing to the debris rotation, the debris and tether are vulnerable to entanglement in the towing process. After capture, the untight capture leads to relative motion between gripper and debris and changes the actuator configuration matrix inevitably. Thus, except the traditional dynamics uncertainties, it is also necessary to consider the control allocation of multi-actuators under the nonnegative unilateral magnitude constraint and configuration matrix uncertainty. To tackle these problems, a novel scheme was proposed to stabilize the attitude of debris using adaptive backstepping control with robust allocation technique. Specifically, the adaptive attitude stabilization law is designed to generate control torques in the presence of dynamics uncertainties. A robust control allocation algorithm is developed to distribute the three axis torques to redundant actuators, in which the optimal control vector of actuator is obtained by interior-point method, under the actuator configuration matrix uncertainty and saturation constraints. Finally, numerical simulation results are employed to validate the effectiveness of the proposed scheme.

Numerical modeling of cosmic-ray transport in the heliosphere and interpretation of the proton-to-helium ratio in Solar Cycle 24

Publication date: Available online 27 June 2019

Source: Advances in Space Research

Author(s): Nicola Tomassetti, Fernando Barão, Bruna Bertucci, Emanuele Fiandrini, Miguel Orcinha

Abstract

Thanks to space-borne experiments of cosmic-ray (CR) detection, such as the AMS and PAMELA missions in low-Earth orbit, or the Voyager-1 spacecraft in the interstellar space, a large collection of multi-channel and time-resolved CR data has become available. Recently, the AMS experiment has released new precision data, on the proton and helium fluxes in CRs, measured on monthly basis during its first six years of mission. The AMS data reveal a remarkable long-term behavior in the temporal evolution of the proton-to-helium ratio at rigidity R≡p/Z≲,3 GV. As we have argued in a recent work, such a behavior may reflect the transport properties of low-rigidity CRs in the inteplanetary space. In particular, it can be caused by mass/charge dependence of the CR diffusion coefficient. In this paper, we present our developments in the numerical modeling of CR transport in the Milky Way and in the heliosphere. Within our model, and with the help of approximated analytical solutions, we describe in details the relations between the properties of CR diffusion and the time-dependent evolution of the proton-to-helium ratio.

Spin-to-spin slew maneuvers under spherically constrained angular acceleration

Publication date: Available online 26 June 2019

Source: Advances in Space Research

Author(s): Donghun Lee, Young-Joo Song

Abstract

This paper addresses a three-axis time-optimal attitude-control problem of rigid spacecraft. In this problem, an angular-acceleration vector lies within a spherically constrained space, and spin-to-spin boundary conditions are considered. The problem is converted into a two-point boundary-value problem by an indirect method, which is solved numerically because non-eigen-axis slew motions do not have a general analytical solution. To solve the problem, a homotopy algorithm is applied in which a discrete continuation method, closed-form solutions for single-axis slew maneuvers, and a costate transformation method are included. Through numerical examples, the properties of the optimal solutions are analyzed, and the efficiency of the numerical algorithm is demonstrated.

Targeting ultra-high energy neutrinos with the ARIANNA experiment

Publication date: Available online 24 June 2019

Source: Advances in Space Research

Author(s): A. Anker, S.W. Barwick, H. Bernhoff, D.Z. Besson, N. Bingefors, G. Gaswint, C. Glaser, A. Hallgren, J.C. Hanson, R. Lahmann, U. Latif, J. Nam, A. Novikov, S.R. Klein, S.A. Kleinfelder, A. Nelles, M.P. Paul, C. Persichilli, S.R. Shively, J. Tatar

Abstract

The measurement of ultra-high energy (UHE) neutrinos (E > 1016 eV) opens a new field of astronomy with the potential to reveal the sources of ultra-high energy cosmic rays especially if combined with observations in the electromagnetic spectrum and gravitational waves. The ARIANNA pilot detector explores the detection of UHE neutrinos with a surface array of independent radio detector stations in Antarctica which allows for a cost-effective instrumentation of large volumes. Twelve stations are currently operating successfully at the Moore’s Bay site (Ross Ice Shelf) in Antarctica and at the South Pole. We will review the current state of ARIANNA and its main results. We report on a newly developed wind generator that successfully operates in the harsh Antarctic conditions and powers the station for a substantial time during the dark winter months. The robust ARIANNA surface architecture, combined with environmentally friendly solar and wind power generators, can be installed at any deep ice location on the planet and operated autonomously. We report on the detector capabilities to determine the neutrino direction by reconstructing the signal arrival direction of a 800 m deep calibration pulser, and the reconstruction of the signal polarization using the more abundant cosmic-ray air showers. Finally, we describe a large-scale design – ARIA – that capitalizes on the successful experience of the ARIANNA operation and is designed sensitive enough to discover the first UHE neutrino.

Accuracy evaluation of the latest BDGIM for BDS-3 satellites

Publication date: Available online 22 June 2019

Source: Advances in Space Research

Author(s): Yongxing Zhu, Shusen Tan, Qinghua Zhang, Xia Ren, Xiaolin Jia

Abstract

A novel model of the BeiDou Global Ionospheric delay correction Model (BDGIM) is used as the global broadcasting ionospheric correction model for the third generation of BeiDou Navigation Satellite System (BDS-3). It has been successfully implemented on five BDS-3 test satellites. The accuracy of this novel BDGIM model were evaluated using the final global ionosphere maps (GIM) products provided by the International Global Navigation Satellite System Service (IGS). Comparing with the IGS GIM final product, the root mean square (RMS) of the global correction accuracy of BDGIM was 3.57 total electron content unit (TECu) with a correction percentage of 77.2%. The average RMS at different latitudes was 3.11 TECu, with Percent of 75.2%. The mean RMS values of regions with lattilute larger than 52.5S and 62.5N were less than that of NeQuickG. Considering the distribution of the tracking stations using for calculating BDGIM coefficients, the RMS values in different regions were 3.09 TECu (Percent = 80.3%) and 3.21 TECu (Percent = 79.4%), respectively. The BDGIM coefficients was updated every two hours, comparing with NeQuickG model, the BDGIM better reflected the real changes of ionosphere. Considering that the accuracy of the non-broadcasted coefficients declines with broadcasting duration, the correction accuracy with nine broadcasted coefficients included in BDGIM was analyzed. RMS and Percent were 4.10 TECu and 72.8%, declining by about 0.53 TECu and 4.4% (maximum to 9.7%), respectively.

High-energy cosmic rays from compact galactic star clusters: Particle fluxes and anisotropy

Publication date: Available online 22 June 2019

Source: Advances in Space Research

Author(s): A.M. Bykov, M.E. Kalyashova, D.C. Ellison, S.M. Osipov

Abstract

It has been shown that supernova blast waves interacting with winds from massive stars in compact star clusters may be capable of producing cosmic-ray (CR) protons to above 1017 eV. We give a brief description of the colliding-shock-flow mechanism and look at generalizations of the diffusion of ∼100 PeV CRs in the turbulent galactic magnetic field present in the galactic disk. We calculate the temporal evolution of the CR anisotropy from a possible distribution of young compact massive star clusters assuming the sources are intermittent on time scales of a few million years, i.e., comparable to their residence time in the Milky Way. Within the confines of our model, we determine the galactic/extra-galactic fraction of high-energy CRs resulting in anisotropies consistent with observed values. We find that galactic star clusters may contribute a substantial fraction of ∼100 PeV CRs without producing anisotropies above observed limits.

Statistics of spread F characteristics across different sectors and IRI 2016 prediction

Publication date: Available online 22 June 2019

Source: Advances in Space Research

Author(s): A.O. Afolayan, J.S. Mandeep, M. Abdullah, S.M. Buhari

Abstract

The occurrence of the plasma irregularity and the related scintillation have been extensively investigated as a result of the impact on radio signal propagation. This study focused on the equatorial spread F (ESF) occurrence using ionogram data and IRI prediction taken across different longitude sectors, including the Jicamarca, Fortaleza, Ilorin, Chumphon and Kwajalein ionosonde stations. We have briefly discussed the varying range type spread F (RSF) occurrence features across these stations such as the duration, onset time and seasonal asymmetry. The largest seasonal average of the RSF occurrence percentage was recorded at the Ilorin station during the low solar activity, while the occurrence rate at the Chumphon and Kwajalein stations increases significantly with the solar flux intensity. Furthermore, we presented a comparative analysis of the observed RSF occurrence rate and the IRI model prediction. The result exhibited a significant error percentage across these longitudes. The IRI could not reproduce significant features such as the delayed RSF onset at the Kwajalein station during June solstice or the equinox asymmetry in the RSF occurrence across these longitudes. Surprisingly, the IRI model also performed poorly at the Brazilian longitude. These observations highlight the strong dynamic nature of the RSF features across the different regions. Hence, an extensive dataset of the RSF occurrence distribution selected with cognizance for the longitudinal pattern of the major factors controlling the phenomenon is considered necessary for the further improvement of the empirical model.

Long-term aspects of nighttime spread F over a low mid-latitude European station

Publication date: Available online 22 June 2019

Source: Advances in Space Research

Author(s): K.S. Paul, H. Haralambous, C. Oikonomou, A. Paul

Abstract

In an effort to explore the morphology of nighttime spread F at the lower mid-latitude European station of Nicosia, Cyprus (35°N, 33°E geographic; magnetic dip. 29.38°N), all ionograms recorded by the DPS-4D digisonde during the interval 2009–2016 have been analyzed. Subsequent detailed investigation was performed to establish the possible effect of various triggering mechanisms on spread F within the framework of Perkins instability on a statistical basis by correlating spread F occurrence particular wave pattern signatures (Satellite Trace/Multiple-reflected Echoes), gravity wave signatures, F layer uplift (h'F) and unstable sporadic E layers. The results verify the systematic manifestation and therefore the significance of TIDs and unstable sporadic E layers as triggering factors responsible for seeding spread F development and underline the frequent appearance of multi-reflected echoes and satellite traces as dominant precursors of mid-latitude spread F over European latitudes for the first time. Furthermore clear seasonal characteristics and inverse solar activity dependence of spread F occurrence in lower European mid-latitudes is established similar to southern hemisphere studies undertaken previously.

Secondary cameras onboard the Mini-EUSO experiment: Control software and calibration

Publication date: Available online 21 June 2019

Source: Advances in Space Research

Author(s): S. Turriziani, J. Ekelund, K. Tsuno, M. Casolino, T. Ebisuzaki

Abstract

Mini-EUSO is a space experiment selected to be installed inside the International Space Station. It has a compact telescope with a large field of view (44 × 44 sq. deg.) focusing light on an array of photo-multipliers tubes in order to observe UV emission coming from Earth’s atmosphere. Observations will be complemented with data recorded by some ancillary detectors. In particular, the Mini-EUSO Additional Data Acquisition System (ADS) is composed by two cameras, which will allow us to obtain data in the near infrared, and in the visible range. These will be used to monitor the observation conditions, and to acquire useful information on several scientific topics to be studied with the main instrument, such as the physics of atmosphere, meteors, and strange quark matter. Here we present the ADS control software developed to stream cameras together with the UV main instrument, in order to grab images in an automated and independent way, and we also describe the calibration activities performed on these two ancillary cameras before flight.

Improving the quality of Sentinel-3A data with a hybrid mean sea surface model, and implications for Sentinel-3B and SWOT

Publication date: Available online 21 June 2019

Source: Advances in Space Research

Author(s): Gerald Dibarboure, Marie-Isabelle Pujol

Abstract

In this paper we compute a new local mean sea surface (MSS) model along the Sentinel-3A ground track. This so-called hybrid mean profile (HMP) blends the content of an average of 18 months of Sentinel-3A data for wavelengths ranging from 15 to 100 km, and the CNES/CLS 2015 gridded MSS model for larger and shorter scales.

The improvement observed on Sentinel-3A sea level anomalies (SLA) is significant: the residual error is 0.2 cm2, i.e. 17% of the SLA variance between 15 and 100 km, or 57% less than the gridded MSS model error. The highest error reduction is observed for wavelengths ranging from 20 to 80 km. From a geographical point of view, the improvement is mainly located along geodetic features that are not completely resolved in the gridded MSS models. It can locally be as high as 1 cm2, i.e. very large when compared to the variance of the small scale SLA. Similarly, in coastal regions where the gridded model is known to exhibit higher errors, the HMP exhibits a very stable behavior that is on average 4 times more accurate.

To understand the implications for future datasets and mission, we also develop a simple prediction model for the leakage of noise and small scale SLA into the MSS model. The model was validated with Sentinel-3A and ENVISAT data. Using the HMP strategy on the 21-day phase of the SWOT mission would be attractive for two reasons: 1/this methodology would reduce the small scale error of gridded MSS models thanks to SWOT’s unprecedented 2D topography coverage and noise level, and 2/the gridded MSS models provides a more trustworthy reference for the larger temporal scales that cannot be not averaged out by SWOT alone. After 3 years of nominal mission, the residual SWOT HMP error should be less than 2% of the sea level anomaly variance. In contrast, the so-called fast-sampling (or 1-day repeat) phase of SWOT is slightly more challenging because of the temporal correlation of the SLA (1-day samples are not independent). Depending on the accuracy of the pre-launch gridded MSS (i.e. upper wavelength limit where SWOT data must be used), the decorrelation scales could range from 1 to 5 days. The resulting HMP error would ranges from 5 to 12% of the SLA variance at the end of the fast-sampling phase.

These results emphasize the need to keep improving the smaller scales of gridded MSS models as they will remain a major altimetry asset, at least until 2023, when SWOT has collected enough 21-day samples to provide a very robust HMP model.

Galactic gamma-ray astrophysics with VERITAS

Publication date: Available online 21 June 2019

Source: Advances in Space Research

Author(s): T.B. Humensky, for the VERITAS Collaboration

Abstract

The Galaxy contains a small but quite significant population of highly energetic denizens: supernova remnants with fast shocks, pulsars with powerful winds, intensely-interacting binary systems built from a compact object and a massive star. All of these environments conspire to generate non-thermal populations of particles, and radiation produced by these particles is gradually revealing the methods by which Nature accelerates cosmic rays, as well as the ways in which those cosmic rays escape and diffuse into the interstellar medium. In this talk, we discuss advances in our understanding of these environments and processes provided by recent results from VERITAS, an array of ground-based imaging air-Cherenkov telescopes located at the Whipple Observatory in southern Arizona and sensitive to gamma rays in the energy range from 85 GeV to >30 TeV. These results include studies of cosmic-ray acceleration in the supernova remnants Cassiopeia A and IC 443, as well as the remarkable Fall 2017 periastron passage of VER J2032+4127, the binary system containing PSR J2032+4127 and the Be star MT91 213 with a 50-year period.

Polarization VLBI observations of AGN jets now and into the future

Publication date: Available online 21 June 2019

Source: Advances in Space Research

Author(s): Denise C. Gabuzda

Abstract

The history of Very Long Baseline Interferometry (VLBI) observations has been characterized predominantly by an ongoing quest for increasingly high resolution and sensitivity. VLBI monitoring of relatively large samples of Active Galactic Nuclei (AGNs) with uniform quality and linear polarization sensitivity are now available at the moderately high frequencies of 15 and 43 GHz. This has enabled considerable advances in our understanding of the relativistic jets of AGNs, but gaps in the available observational material remain, which must be addressed in future VLBI polarization observations. Linear polarization observations at frequencies above 43 GHz remain non-routine, and the availability of multi-frequency and circular polarization measurements is still limited. It is of interest both to push inward toward the jet base and to study details of the jets themselves on parsec scales, such as magnetic field structures along and across the jets, which are intrinsically related to their formation, launching and propagation. Requirements for future VLBI polarization observations are considered, highlighting the key role that can be played by space VLBI observations. Unique opportunities are offered by relatively low-frequency space VLBI observations that are sensitive to effects such as Faraday rotation, opacity, and low-frequency absorption.

Quasi-periodic pulsations in hard X-rays of partially occulted solar flares

Publication date: Available online 21 June 2019

Source: Advances in Space Research

Author(s): Ż. Szaforz, M. Tomczak

Abstract

We investigated solar flares partially occulted by the solar disk observed by the Yohkoh satellite. We found that about 30–40% of them show quasi-periodic pulsations (QPPs) in hard X-rays (HXR). A lack of usually brighter footpoint sources allowed us to reconstruct coronal HXR sources with a higher quality. We analyzed 28 partially occulted flares showing the QPPs and for the first time present results for such events as a group. In our opinion, the majority of the observed HXR loop-top sources can be explained as successive compression and rarefaction of magnetic traps described in a model of oscillating magnetic traps (OMT). In this model a particular value of a ratio between the diameter of traps and the period of pulsations is postulated. In our modification of this model, different values of the ratio are possible, with the exception of a lower range, where low values of magnetic field strength and high values of electron density number can excess the plasma-β parameter above unity (ballooning instability).

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