Subjects -> ASTRONOMY (Total: 94 journals)
Showing 1 - 46 of 46 Journals sorted alphabetically
Advances in Astronomy     Open Access   (Followers: 51)
Annual Review of Astronomy and Astrophysics     Full-text available via subscription   (Followers: 39)
Annual Review of Earth and Planetary Sciences     Full-text available via subscription   (Followers: 63)
Artificial Satellites     Open Access   (Followers: 23)
Astrobiology     Hybrid Journal   (Followers: 14)
Astronomical & Astrophysical Transactions: The Journal of the Eurasian Astronomical Society     Hybrid Journal   (Followers: 6)
Astronomical Journal     Full-text available via subscription   (Followers: 8)
Astronomical Review     Open Access   (Followers: 4)
Astronomische Nachrichten     Hybrid Journal   (Followers: 4)
Astronomy & Geophysics     Hybrid Journal   (Followers: 48)
Astronomy and Astrophysics     Full-text available via subscription   (Followers: 60)
Astronomy and Astrophysics     Open Access   (Followers: 32)
Astronomy and Computing     Hybrid Journal   (Followers: 2)
Astronomy Letters     Hybrid Journal   (Followers: 22)
Astronomy Reports     Hybrid Journal   (Followers: 15)
Astronomy Studies Development     Open Access   (Followers: 12)
Astroparticle Physics     Hybrid Journal   (Followers: 8)
Astrophysical Bulletin     Hybrid Journal   (Followers: 3)
Astrophysical Journal     Full-text available via subscription   (Followers: 19)
Astrophysical Journal Letters     Full-text available via subscription   (Followers: 14)
Astrophysical Journal Supplement Series     Full-text available via subscription   (Followers: 14)
Astrophysics     Hybrid Journal   (Followers: 29)
Astrophysics and Space Science     Hybrid Journal   (Followers: 46)
Astrophysics and Space Sciences Transactions (ASTRA)     Open Access   (Followers: 56)
Astropolitics: The International Journal of Space Politics & Policy     Hybrid Journal   (Followers: 12)
Celestial Mechanics and Dynamical Astronomy     Hybrid Journal   (Followers: 11)
Chinese Astronomy and Astrophysics     Full-text available via subscription   (Followers: 24)
Colloid Journal     Hybrid Journal   (Followers: 3)
Comptes Rendus Physique     Full-text available via subscription   (Followers: 2)
Computational Astrophysics and Cosmology     Open Access   (Followers: 3)
COSPAR Colloquia Series     Full-text available via subscription   (Followers: 11)
Earth, Moon, and Planets     Hybrid Journal   (Followers: 55)
Earth, Planets and Space     Open Access   (Followers: 74)
EAS Publications Series     Full-text available via subscription   (Followers: 8)
EPL Europhysics Letters     Partially Free   (Followers: 8)
Experimental Astronomy     Hybrid Journal   (Followers: 39)
Expert Opinion on Astronomy and Astrophysics     Open Access   (Followers: 7)
Extreme Life, Biospeology & Astrobiology - International Journal of the Bioflux Society     Full-text available via subscription   (Followers: 6)
Few-Body Systems     Hybrid Journal   (Followers: 1)
Foundations of Physics     Hybrid Journal   (Followers: 41)
Frontiers in Astronomy and Space Sciences     Open Access   (Followers: 12)
Galaxies     Open Access   (Followers: 6)
Globe, The     Full-text available via subscription   (Followers: 4)
Gravitation and Cosmology     Hybrid Journal   (Followers: 4)
Icarus     Hybrid Journal   (Followers: 75)
International Journal of Advanced Astronomy     Open Access   (Followers: 28)
International Journal of Astrobiology     Hybrid Journal   (Followers: 4)
International Journal of Astronomy     Open Access   (Followers: 19)
International Journal of Astronomy and Astrophysics     Open Access   (Followers: 29)
International Journal of Satellite Communications Policy and Management     Hybrid Journal   (Followers: 13)
International Letters of Chemistry, Physics and Astronomy     Open Access   (Followers: 12)
ISRN Astronomy and Astrophysics     Open Access   (Followers: 7)
Journal for the History of Astronomy     Full-text available via subscription   (Followers: 19)
Journal of Astrobiology & Outreach     Open Access   (Followers: 3)
Journal of Astronomical Instrumentation     Open Access   (Followers: 3)
Journal of Astronomical Telescopes, Instruments, and Systems     Hybrid Journal   (Followers: 5)
Journal of Astrophysics     Open Access   (Followers: 26)
Journal of Astrophysics and Astronomy     Open Access   (Followers: 52)
Journal of Atmospheric and Solar-Terrestrial Physics     Hybrid Journal   (Followers: 199)
Journal of Cosmology and Astroparticle Physics     Hybrid Journal   (Followers: 38)
Journal of Geophysical Research : Planets     Full-text available via subscription   (Followers: 179)
Journal of Geophysical Research : Space Physics     Full-text available via subscription   (Followers: 178)
Journal of High Energy Astrophysics     Full-text available via subscription   (Followers: 22)
Kinematics and Physics of Celestial Bodies     Hybrid Journal   (Followers: 10)
KronoScope     Hybrid Journal   (Followers: 1)
Macalester Journal of Physics and Astronomy     Open Access   (Followers: 4)
MNASSA : Monthly Notes of the Astronomical Society of South Africa     Full-text available via subscription   (Followers: 1)
Molecular Astrophysics     Full-text available via subscription   (Followers: 1)
Monthly Notices of the Royal Astronomical Society     Hybrid Journal   (Followers: 14)
Monthly Notices of the Royal Astronomical Society : Letters     Hybrid Journal  
Nature Astronomy     Hybrid Journal   (Followers: 8)
New Astronomy     Hybrid Journal   (Followers: 27)
New Astronomy Reviews     Full-text available via subscription   (Followers: 17)
Nonlinear Dynamics     Hybrid Journal   (Followers: 19)
NRIAG Journal of Astronomy and Geophysics     Open Access   (Followers: 5)
Open Astronomy     Open Access   (Followers: 2)
Physics of the Dark Universe     Open Access   (Followers: 4)
Planetary and Space Science     Hybrid Journal   (Followers: 101)
Planetary Science     Open Access   (Followers: 52)
Proceedings of the International Astronomical Union     Full-text available via subscription   (Followers: 2)
Publications of the Astronomical Society of Australia     Hybrid Journal   (Followers: 2)
Publications of the Astronomical Society of Japan     Hybrid Journal   (Followers: 3)
Publications of the Astronomical Society of the Pacific     Full-text available via subscription   (Followers: 4)
Research & Reviews : Journal of Space Science & Technology     Full-text available via subscription   (Followers: 17)
Research in Astronomy and Astrophysics     Full-text available via subscription   (Followers: 29)
Revista Mexicana de Astronomía y Astrofísica     Open Access   (Followers: 2)
Science China Physics, Mechanics & Astronomy     Hybrid Journal   (Followers: 4)
Solar Physics     Hybrid Journal   (Followers: 34)
Solar System Research     Hybrid Journal   (Followers: 14)
Space Science International     Open Access   (Followers: 192)
Space Science Reviews     Hybrid Journal   (Followers: 97)
Space Weather     Full-text available via subscription   (Followers: 24)
Transport and Aerospace Engineering     Open Access   (Followers: 13)
Universe     Open Access   (Followers: 5)
Similar Journals
Journal Cover
Solar Physics
Journal Prestige (SJR): 1.517
Citation Impact (citeScore): 3
Number of Followers: 34  
  Hybrid Journal Hybrid journal (It can contain Open Access articles)
ISSN (Print) 1573-093X - ISSN (Online) 0038-0938
Published by Springer-Verlag Homepage  [2626 journals]
  • Editorial Appreciation
    • PubDate: 2021-01-12
  • Statistical and Solar Cycle Distribution of Daily Flux ≥ 10 9 cm − 2
           $\ge 10^{9}\mbox{ cm}^{-2}$  d −1  sr −1 for E > 2 $E>2$ MeV
           Electrons Observed by GOES During 1987 – 2019
    • Abstract: Abstract We studied the statistical and solar cycle distribution of daily flux \(\ge 10^{9}\mbox{ cm}^{-2}\)  d−1 sr−1 for \(E>2\)  MeV electrons Observed by GOES from 1987 to 2019. There were 282 days with daily flux \(\ge 10^{9}\mbox{ cm}^{-2}\)  d−1 sr−1 for \(E>2\)  MeV electrons during this period. Of the 282 days, 71.63% of them have the daily flux <2.0 \(\times 10^{9}\mbox{ cm}^{-1}\) d−1sr−1, 19.86% of them with the daily flux fall in the interval ≥2.0 \(\times 10^{9}\mbox{ cm}^{-2}\)  d−1 sr−1 and <3.0 \(\times 10^{9}\mbox{ cm}^{-2}\)  d−1 sr−1, 4.26% of them have the daily flux in the interval between ≥3.0 \(\times 10^{9}\mbox{ cm}^{-2}\)  d−1 sr−1 and <4.0 \(\times 10^{9}\mbox{ cm}^{-2}\)  d−1 sr−1, and 4.26% of them have the daily flux ≥4.0 \(\times 10^{9}\mbox{ cm}^{-2}\)  d−1 sr−1. The daily flux \(\ge 10^{9}\mbox{ cm}^{-2}\)  d−1 sr−1 for \(E>2\)  MeV electrons that lasted for 1, 2, 3, 4, 5, 6, 7 and 10 days accounted for 18.44%, 17.02%, 20.21%, 15.6%, 8.87%, 6.38%, 9.93% and 3.55%, respectively. Among all the daily fluxes \(\ge 10^{9}\mbox{ cm}^{-2}\)  d−1 sr−1, 47.79% of them occurred in the northern hemisphere spring, 36.03% of them occurred in the fall, and 16.18% of them occurred in the other months. More than 82% of the daily flux \(\ge 10^{9}\mbox{ cm}^{-2}\)  d−1 sr−1 for \(E>2\)  MeV electrons appeared in the descending phase for each solar cycle. The greater percentage of the electrons of daily flux \(\ge 10^{9}\mbox{ cm}^{-2}\)  d−1 sr−1 for \(E>2\)  MeV occurred during the descending phase for the solar cycle with higher amplitude. Also, for the solar cycle with higher amplitude, the lower the percentage of the electrons of daily flux \(\ge 10^{9}\mbox{ cm}^{-2}\)  d−1 sr−1 of \(E>2\)  MeV occurred in the period from two years before to three years after the solar cycle peak.
      PubDate: 2021-01-12
  • The Balloon-Borne Investigation of Temperature and Speed of Electrons in
           the Corona (BITSE): Mission Description and Preliminary Results
    • Abstract: Abstract We report on the Balloon-borne Investigation of Temperature and Speed of Electrons in the corona (BITSE) mission launched recently to observe the solar corona from \(\approx 3\)  Rs to 15 Rs at four wavelengths (393.5, 405.0, 398.7, and 423.4 nm). The BITSE instrument is an externally occulted single stage coronagraph developed at NASA’s Goddard Space Flight Center in collaboration with the Korea Astronomy and Space Science Institute (KASI). BITSE used a polarization camera that provided polarization and total brightness images of size \(1024 \times 1024\) pixels. The Wallops Arc Second Pointer (WASP) system developed at NASA’s Wallops Flight Facility (WFF) was used for Sun pointing. The coronagraph and WASP were mounted on a gondola provided by WFF and launched from the Fort Sumner, New Mexico station of Columbia Scientific Balloon Facility (CSBF) on September 18, 2019. BITSE obtained 17,060 coronal images at a float altitude of \(\approx \mbox{128,000}\) feet ( \(\approx 39\)  km) over a period of \(\approx 4\)  hrs. BITSE flight software was based on NASA’s core Flight System, which was designed to help develop flight quality software. We used EVTM (Ethernet Via Telemetry) to download science data during operations; all images were stored on board using flash storage. At the end of the mission, all data were recovered and analyzed. Preliminary analysis shows that BITSE imaged the solar minimum corona with the equatorial streamers on the east and west limbs. The narrow streamers observed by BITSE are in good agreement with the geometric properties obtained by the Solar and Heliospheric Observatory (SOHO) coronagraphs in the overlapping physical domain. In spite of the small signal-to-noise ratio ( \(\approx 14\) ) we were able to obtain the temperature and flow speed of the western steamer. In the heliocentric distance range 4 – 7 Rs on the western streamer, we obtained a temperature of \(\approx 1.0\pm 0.3\)  MK and a flow speed of \(\approx 260\)  km s−1 with a large uncertainty interval.
      PubDate: 2021-01-12
  • Robustness of Solar-Cycle Empirical Rules Across Different Series
           Including an Updated Active-Day Fraction (ADF) Sunspot Group Series
    • Abstract: Abstract Empirical rules of solar-cycle evolution form important observational constraints for the solar-dynamo theory. This includes the Waldmeier rule relating the magnitude of a solar cycle to the length of its ascending phase, and the Gnevyshev–Ohl rule clustering cycles to pairs of an even-numbered cycle followed by a stronger odd-numbered cycle. These rules were established as based on the “classical” Wolf sunspot number series, which has been essentially revisited recently, with several revised sets released by the research community. Here we test the robustness of these empirical rules for different sunspot (group) series for the period 1749 – 1996, using four classical and revised international sunspot-number and group sunspot-number series. We also provide an update of the sunspot-group series based on the active-day fraction (ADF) method, using the new database of solar observations. We show that the Waldmeier rule is robust and independent of the exact sunspot (group) series: its classical and \(n+1\) (relating the length of \(n\) th cycle to the magnitude of ( \(n+1\) )th cycle) formulations are significant or highly significant for all series, while its simplified formulation (relating the magnitude of a cycle to its full length) is insignificant for all series. The Gnevyshev–Ohl rule was found robust for all analyzed series for Solar Cycles 8 – 21, but unstable across the Dalton minimum and before it.
      PubDate: 2021-01-11
  • Propagating Conditions and the Time of ICME Arrival: A Comparison of the
           Effective Acceleration Model with ENLIL and DBEM Models
    • Abstract: Abstract The Effective Acceleration Model (EAM) predicts the Time-of-Arrival (ToA) of the Coronal Mass Ejection (CME) driven shock and the average speed within the sheath at 1 AU. The model is based on the assumption that the ambient solar wind interacts with the interplanetary CME (ICME) resulting in constant acceleration or deceleration. The upgraded version of the model (EAMv3), presented here, incorporates two basic improvements: (i) a new technique for the calculation of the acceleration (or deceleration) of the ICME from the Sun to 1 AU and (ii) a correction for the CME plane-of-sky speed. A validation of the upgraded EAM model is performed via comparisons to predictions from the ensemble version of the Drag-Based model (DBEM) and the WSA-ENLIL+Cone ensemble model. A common sample of 16 CMEs/ICMEs, in 2013 – 2014, is used for the comparison. Basic performance metrics such as the mean absolute error (MAE), mean error (ME) and root mean squared error (RMSE) between observed and predicted values of ToA are presented. MAE for EAM model was \(8.7\pm 1.6\) hours while for DBEM and ENLIL was \(14.3\pm 2.2\) and \(12.8\pm 1.7\) hours, respectively. ME for EAM was \(-1.4\pm 2.7\) hours in contrast with \(-9.7\pm 3.4\) and \(-6.1\pm 3.3\) hours from DBEM and ENLIL. We also study the hypothesis of stronger deceleration in the interplanetary (IP) space utilizing the EAMv3 and DBEM models. In particularly, the DBEM model perform better when a greater value of drag parameter, of order of a factor of 3, is used in contrast to previous studies. EAMv3 model shows a deceleration of ICMEs at greater distances, with a mean value of 0.72 AU.
      PubDate: 2021-01-07
  • VLA Measurements of Faraday Rotation Through a Coronal Mass Ejection Using
           Multiple Lines of Sight
    • Abstract: Abstract Coronal mass ejections (CMEs) are large eruptions of magnetized plasma from the Sun that play an important role in space weather. The key to understanding the fundamental physics of a CME is measurement of the plasma properties within heliocentric distances of \(< 20~\mathrm{R}_{\odot }\) . Faraday rotation, a radioastronomical propagation measurement, is an extremely valuable diagnostic for studying CMEs. Faraday rotation measurements [RM] contain information on the magnetic field in the medium causing the Faraday rotation. Recent observations of CME-induced Faraday rotation (e.g., Howard et al. in Astrophys. J. 831, 208, 2016; Kooi et al. in Solar Phys. 292, 56, 2017; Bisi et al. in EGU General Assembly Conference Abstracts, 13243, 2017) have all been restricted to a single line of sight (LOS) and, therefore, limited to providing estimates of the magnetic field strength. Modeling by Liu et al. (Astrophys. J. 665, 1439, 2007) and Jensen and Russell (Geophys. Res. Lett. 35, L02103, 2008) demonstrated that multiple LOS are necessary to recover the magnetic field strength and structure of the observed CME. We report the first successful observations of Faraday rotation through a CME using multiple lines of sight: 13 LOS across seven target radio fields. We made these radio observations using the Karl G. Jansky Very Large Array (VLA) at \(1-2\) GHz frequencies in the triggered operation mode on 31 July 2015, using a constellation of cosmic radio sources through the solar corona at heliocentric distances of \(8.2-19.5~\mathrm{R}_{\odot }\) . For LOS within \(10~\mathrm{R}_{\odot }\) , the CME’s contribution to the measured RM was \(\approx 0\) to −20 rad m−2, a significant enhancement over the coronal contribution. We assumed a force-free flux-rope structure for the CME’s magnetic field and explored three separate models for the CME’s plasma density: constant density, thin shell, and thick shell. The plasma densities and axial magnetic field strengths for the three models ranged over \(5.4-6.4\times 10^{3}\)  cm−3 and \(26-35\) mG, respectively. Further, using all 13 LOS, we successfully determined the CME’s orientation and helicity.
      PubDate: 2021-01-06
  • Simulating the Coronal Evolution of Bipolar Active Regions to Investigate
           the Formation of Flux Ropes
    • Abstract: Abstract The coronal magnetic field evolution of 20 bipolar active regions (ARs) is simulated from their emergence to decay using the time-dependent nonlinear force-free field method of Mackay, Green, and van Ballegooijen (Astrophys. J. 729, 97, 2011). A time sequence of cleaned photospheric line-of-sight magnetograms, which covers the entire evolution of each AR, is used to drive the simulation. A comparison of the simulated coronal magnetic field with the 171 and 193 Å observations obtained by the Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly (AIA), is made for each AR by manual inspection. The results show that it is possible to reproduce the evolution of the main coronal features such as small- and large-scale coronal loops, filaments and sheared structures for 80% of the ARs. Varying the boundary and initial conditions, along with the addition of physical effects such as Ohmic diffusion, hyperdiffusion and a horizontal magnetic field injection at the photosphere, improves the match between the observations and simulated coronal evolution by 20%. The simulations were able to reproduce the build-up to eruption for 50% of the observed eruptions associated with the ARs. The mean unsigned time difference between the eruptions occurring in the observations compared to the time of eruption onset in the simulations was found to be ≈5 hrs. The simulations were particularly successful in capturing the build-up to eruption for all four eruptions that originated from the internal polarity inversion line of the ARs. The technique was less successful in reproducing the onset of eruptions that originated from the periphery of ARs and large-scale coronal structures. For these cases global, rather than local, nonlinear force-free field models must be used. While the technique has shown some success, eruptions that occur in quick succession are difficult to reproduce by this method and future iterations of the model need to address this.
      PubDate: 2021-01-06
  • Properties of Type III and Type IIIb Bursts in the Frequency Band of
           8 – 80 MHz During PSP Perihelion at the Beginning of April 2019
    • Abstract: Abstract Properties of type III and type IIIb bursts in the frequency band of 8 – 80 MHz observed by the radio telescopes Ukrainian Radio Interferometer of NASU-2 (URAN-2) (Poltava) and Giant Ukrainian Radio Telescope (GURT) (Kharkiv) during the Parker Solar Probe (PSP) perihelion in April 2019 are discussed. These correspond to those that were observed by PSP at frequencies <19 MHz. We analyze dependencies of drift rates and durations on frequency for these bursts. We show that drift rate dependencies on frequency agree well with those derived from the Newkirk corona if source velocities are between 0.17 and 0.2 \(c\) for both type IIIb bursts and type III bursts under the assumption that the first ones are fundamentals and the second ones are their harmonics. However, all observational dependencies are flatter in comparison with the dependencies for a Newkirk corona. We assume that this can be related with coronal temperature decreasing at heliocentric distances from 1.35 to 6.5 solar radii. Duration dependencies of type IIIb and type III bursts on frequency in the range of 10 – 70 MHz are also obtained. We note that the durations and drift rates of type III bursts as well as their dependencies on frequency differ essentially from those for type IIIb bursts.
      PubDate: 2021-01-06
  • Non-Neutralized Electric Current of Active Regions Explained as a
           Projection Effect
    • Abstract: Abstract Active regions (ARs) often possess an observed net electric current in a single magnetic polarity. We show that such “non-neutralized” currents can arise from a geometric projection effect when a twisted flux tube obliquely intersects the photosphere. To this end, we emulate surface maps of an emerging AR by sampling horizontal slices of a semi-torus flux tube at various heights. Although the tube has no net toroidal current, its poloidal current, when projected along the vertical direction, amounts to a significant non-neutralized component on the surface. If the tube emerges only partially as in realistic settings, the non-neutralized current will 1) develop as double ribbons near the sheared polarity inversion line, (2) positively correlate with the twist, and 3) reach its maximum before the magnetic flux. The projection effect may be important to the photospheric current distribution, in particular during the early stages of flux emergence.
      PubDate: 2021-01-06
  • The Fine Structure of the Quasi-Biennial Oscillations of Sunspot Areas and
           the Double Magnetic Cycle of the Sun
    • Abstract: Abstract One of the basic features of solar activity is the quasi-biennial oscillations (QBOs)—variations with a period of about two years. The nature of the QBO remains unclear and the most puzzling is the high instability of the QBO period. We investigated a fine structure of the QBO period variability as manifested in sunspot area variations in Solar Cycles 19 – 23 using the wavelet transform with a real mother wavelet, Daubechies 10, that provided a high temporal resolution. We found that within every 11-yr solar cycle the QBO period varies not randomly, as it is widely accepted now, but it gradually decreases from the beginning of the solar cycle till the end, in phase with the shift of the average sunspots latitude to the equator. We have analyzed in a similar way the time series which were simulated using a combination of sine waves with different periods (constant and variable one) and red noise with a standard deviation as large as 40% of the sine amplitude. The analysis has shown that noise does not distort significantly the initial signal and noise itself does not form the structures with the properties which were observed in the case of the natural time series. We suppose that the revealed modification of the QBO period with the development of the solar cycle may be related to the latitudinal differential rotation in the solar convection zone and the possible influence of the rotational velocity in the region of the QBO generation on the QBO period value. Under this assumption, the process responsible for the QBO generation should operate in a layer with a substantial latitudinal shear which according to the helioseismology analysis is observed in the bulk of the convection zone and is getting smaller in the vicinity of the tachocline.
      PubDate: 2021-01-06
  • Sunspot Positions and Areas from Observations by Cigoli, Galilei, Cologna,
           Scheiner, and Colonna in 1612 – 1614
    • Abstract: Abstract Digital images of manuscripts stored in the Galilean collection of the Central National Library of Florence are analyzed to obtain sunspot groups, their areas and heliographic positions. Overall, 142 drawings were processed. The way of drawing is usually schematic resulting in area uncertainty which may exceed a factor of two for small sunspots. We suggest that there is an upper limit of a factor of two between sunspot group numbers from the drawings and the actual ones. The computed penumbra-to-umbra ratio is consistent with modern observations. A distribution of sunspot latitudes versus time is reconstructed by means of two methods: exploiting the observation time noted by an observer and minimizing the day-to-day variability of sunspot latitudes.
      PubDate: 2021-01-05
  • The Sunspot Drawing Collection of the National Solar Observatory at
           Sacramento Peak (1947–2004)
    • Abstract: Abstract A complete dataset of sunspot drawings recorded at Sacramento Peak Observatory (SPO) from late 1947 till mid-2004 has been digitized. We present the history of the observations and describe the data included in the drawings. We compare the sunspot number index calculated from the SPO data and the International Sunspot Number (SNv2), and we find that both series exhibit a similar behavior. The ratio of two sunspot numbers is relatively constant at about 1.2 – 1.3 during 1955–1995, with larger variations present at the beginning of the time series. This work represents the first step for the publication of the SPO sunspot catalogue in digital format. More information, such as positions and areas of sunspots, will be included in the next versions in order to provide the space weather and climate community a more complete sunspot catalogue with good quality observations.
      PubDate: 2021-01-05
  • On the Reflection of Torsional Alfvén Waves from the Solar Transition
    • Abstract: Abstract On the basis of the two-layer model, we investigate the reflection of linear torsional Alfvén waves propagating in a thin magnetic flux tube from the solar transition region. As distinguished from Hollweg (Astrophys. J. 277, 392, 1984), the density jump across the transition region modeled by a sharp boundary is taken into account. A new expression for the determination of the reflection coefficient is proposed. Weakly damping Alfvén modes with periods from a few tens of seconds to a few minutes can quite effectively penetrate from the chromosphere to the corona and vice versa.
      PubDate: 2021-01-05
  • Solar-Cycle Characteristics in Kodaikanal Sunspot Area: North–South
           Asymmetry, Phase Distribution and Gnevyshev Gap
    • Abstract: Abstract Solar activity is asymmetric in the two hemispheres in almost all cycles. This asymmetry is observed both in cycle amplitude and period. We have used about 90 years of sunspot-area data from the Kodaikanal Solar Observatory to study the north–south asymmetry in sunspot activity. The monthly mean sunspot-area showed the northern hemisphere dominated in Solar Cycles 16, 19, and 20, and the southern hemisphere dominated in Solar Cycles 18, 22, and 23. The 13-month smoothed data indicated that in Solar Cycle 17 and 21, the northern and southern hemisphere showed equal amplitude. Cumulative sunspot area showed that the northern hemisphere dominated in Solar Cycles 18, 19, 20, and 21, with a large difference between the two hemispheres in Solar Cycles 19 and 20. The northern hemisphere activity led by 12, 15, and 2 months in Solar Cycles 20, 21, and 22, respectively. No significant phase difference is found between the two hemispheres in Solar Cycles 16, 17, 18, 19, and 23. The wavelet technique is used to find Rieger-type periodicities in the sunspot cycles. The cross-wavelet analysis of these data sets showed several statistically significant common periodicities like the Rieger-type periodicities and quasi-biennial oscillations. The Gnevyshev gap was found in both the hemispheric data in Solar Cycles 16, 18, 21, 22, and 23. These results are consistent with the earlier reported characteristics of north–south asymmetry in sunspot-area data. These results suggest that the Kodaikanal Observatory data complement the existing sunspot data from other observatories to study solar activity over long and short periods.
      PubDate: 2021-01-05
  • Forecasting of Sunspot Time Series Using a Hybridization of ARIMA, ETS and
           SVM Methods
    • Abstract: Abstract Solar activity directly influences the heliospheric environment and lives on the Earth. Sunspot number (SN) is one of the most crucial and commonly predicted solar activity indices. The prediction of SN time series is a challenging problem owing to its non-stationary, non-Gaussian and nonlinear nature. Therefore, improving the forecasting accuracy of SN time series is an important and challenging task. Motivated from this, in this paper, we have proposed a hybridization of the autoregressive integrated moving average (ARIMA); exponential smoothing with error, trend and seasonality (ETS); and support vector machine (SVM) to predict monthly and yearly SN time series. In this method, first ARIMA, ETS and SVM with linear kernel function are applied to the SN time series and the maximum of forecasts are determined to obtain the forecasts on linear component. Then the residual series is obtained by subtracting the forecasts on linear component from SN time series. The residual series is considered as nonlinear and modeled using SVM with Gaussian kernel function. Then the forecasts on linear component are added with the forecasts on nonlinear component to obtain the final forecasts. To evaluate the efficiency of the proposed method, three constituent models, one of the most popular deep learning models long short-term memory (LSTM), four hybrid methods, four ensemble methods are considered. Furthermore, two horizons, monthly and yearly sunspot time series are considered to evaluate the robustness of the proposed method. Results indicate the statistical superiority of the proposed methods over different horizons considering different accuracy measures.
      PubDate: 2021-01-05
  • Solar Type U Burst Associated with a High Coronal Loop
    • Abstract: Abstract An inverted U burst with equally developed ascending and descending branches observed by the Giant Ukrainian Radio Telescope (GURT) on 18 April 2017 in meter wavelengths band is discussed. This U burst was attributed to the high coronal loop in active region NOAA 12651 above the limb. Under the assumption that, associated with the burst, a coronal loop confines isothermal plasma stratified according to the Boltzmann density relation, geometrical and physical parameters of the loop were estimated. According to our model coronal loops may contain plasma which is up to 20 times denser than the surrounding coronal plasma. In general, the proposed model gives the relation between the plasma temperature and the height of the loop in such a way that under the given parameters of the associated U burst, higher loops contain cooler plasma and vice versa. An alternative method of coronal loop height determination was suggested. Assuming that the observed U burst and the preceding Type III burst were generated by the same exciter, we define the height of the loop from the delay of the former with respect to the latter at a certain frequency. We show that determining the heights of the loops by another independent method, e.g. interferometric or tied-array imaging may reduce the uncertainty of the inside-the-loop plasma temperature determination.
      PubDate: 2020-12-30
  • FLUKA Simulations of Pion Decay Gamma-Radiation from Energetic Flare Ions
    • Abstract: Abstract Gamma-ray continuum at \(> 10 \) MeV photon energy yields information on \(\gtrsim 0.2 \)  – 0.3 GeV/nucleon ions at the Sun. We use the general-purpose Monte Carlo code FLUktuierenden KAskade (FLUKA) to model the transport of ions injected into thick and thin target sources, the nuclear processes that give rise to pions and other secondaries and the escape of the resulting photons from the atmosphere. We give examples of photon spectra calculated with a range of different assumptions about the primary ion velocity distribution and the source region. We show that FLUKA gives results for pion decay photon emissivity in agreement with previous treatments. Through the directionality of secondary products, as well as Compton scattering and pair production of photons prior to escaping the Sun, the predicted spectrum depends significantly on the viewing angle. Details of the photon spectrum in the \(\approx 100\) MeV range may constrain the angular distribution of primary ions and the depths at which they interact. We display a set of thick-target spectra produced making various assumptions about the incident ion energy and angular distribution and the viewing angle. If ions are very strongly beamed downward, or ion energies do not extend much above 1 GeV/nucleon, the photon spectrum is highly insensitive to details of the ion distribution. Under the simplest assumptions, flares observed near disc centre should not display significant radiation above 1 GeV photon energy. We give an example application to Fermi Large Area Telescope data from the flare of 12 June 2010.
      PubDate: 2020-12-08
  • Consequences of a Solar Wind Stream Interaction Region on the Low Latitude
           Ionosphere: Event of 7 October 2015
    • Abstract: Abstract In this article, we present a study of the perturbations occurring in the Earth’s environment on 7 October 2015. We use a multi-instrument approach, including space and ground observations. In particular, we study the ionospheric conditions at low latitudes. Two ionospheric storms are observed at the low latitude station of Tucumán ( \(26^{\circ}\) \(51'\) S, \(65^{\circ}\) \(12'\) W). We observe a negative ionospheric storm followed by a positive one. These ionospheric perturbations were triggered by two sudden storm commencements (SSCs) of a strong geomagnetic storm. Preliminary results show that the main mechanism involved in both ionospheric storms is the prompt penetration of electric fields (PPEFs) from the magnetosphere. Furthermore, in the positive storm, disturbed dynamo electric fields are observed acting in combination with the PPEFs. The impact of the solar wind on the Earth’s environment is analyzed using geomagnetic data and proxies, combined with data acquired in the Tucumán Low Latitude Observatory for the Upper Atmosphere. We also investigate the solar and interplanetary drivers of this intense perturbation. We find that, although typically interplanetary coronal mass ejections (ICMEs) are the most geoeffective transient interplanetary events, in this case, a corotating interaction region (CIR) is responsible for these strong perturbations to the geospace.
      PubDate: 2020-12-08
  • Coronal Elemental Abundance: New Results from Soft X-Ray Spectroscopy of
           the Sun
    • Abstract: Abstract Elemental abundances in the solar corona are known to be different from those observed in the solar photosphere. The ratio of coronal to photospheric abundance shows a dependence on the first ionization potential (FIP) of the element. We estimate FIP bias from direct measurements of elemental abundances from soft X-ray spectra using data from multiple space missions covering a range of solar activity levels. This comprehensive analysis shows clear evidence for a decrease in FIP bias around the maximum intensity of the X-ray flare with coronal abundances briefly tending to photospheric values and a slow recovery as the flare decays. The departure from coronal abundances are larger for the low FIP elements Ca, Fe and Si than for S which have a mid FIP value. These changes in the degree of fractionation might provide inputs to model wave propagation through the chromosphere during flares.
      PubDate: 2020-12-08
  • The Daniel K. Inouye Solar Telescope – Observatory Overview
    • Abstract: Abstract We present an overview of the National Science Foundation’s Daniel K. Inouye Solar Telescope (DKIST), its instruments, and support facilities. The 4 m aperture DKIST provides the highest-resolution observations of the Sun ever achieved. The large aperture of DKIST combined with state-of-the-art instrumentation provide the sensitivity to measure the vector magnetic field in the chromosphere and in the faint corona, i.e. for the first time with DKIST we will be able to measure and study the most important free-energy source in the outer solar atmosphere – the coronal magnetic field. Over its operational lifetime DKIST will advance our knowledge of fundamental astronomical processes, including highly dynamic solar eruptions that are at the source of space-weather events that impact our technological society. Design and construction of DKIST took over two decades. DKIST implements a fast (f/2), off-axis Gregorian optical design. The maximum available field-of-view is 5 arcmin. A complex thermal-control system was implemented in order to remove at prime focus the majority of the 13 kW collected by the primary mirror and to keep optical surfaces and structures at ambient temperature, thus avoiding self-induced local seeing. A high-order adaptive-optics system with 1600 actuators corrects atmospheric seeing enabling diffraction limited imaging and spectroscopy. Five instruments, four of which are polarimeters, provide powerful diagnostic capability over a broad wavelength range covering the visible, near-infrared, and mid-infrared spectrum. New polarization-calibration strategies were developed to achieve the stringent polarization accuracy requirement of 5×10−4. Instruments can be combined and operated simultaneously in order to obtain a maximum of observational information. Observing time on DKIST is allocated through an open, merit-based proposal process. DKIST will be operated primarily in “service mode” and is expected to on average produce 3 PB of raw data per year. A newly developed data center located at the NSO Headquarters in Boulder will initially serve fully calibrated data to the international users community. Higher-level data products, such as physical parameters obtained from inversions of spectro-polarimetric data will be added as resources allow.
      PubDate: 2020-12-04
School of Mathematical and Computer Sciences
Heriot-Watt University
Edinburgh, EH14 4AS, UK
Tel: +00 44 (0)131 4513762

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