Division E - Abstracts


Temporal Variations of Different Solar Activity Indices and Solar Spectral Irradiance Variability of Some Chromospheric Emission Lines Through the Solar Cycles 21-23

Göker, Ümit Deniz

In this work, a study of variations of solar spectral irradiance (SSI) in the wavelength ranges 121.5 nm-300.5 nm for the period 1981-2009 is presented. In this respect, we revealed negative correlations of intensities of UV (289.5 nm-300.5 nm) spectral lines originating in the solar chromosphere with the ISSN index during the unusually prolonged minimum between the solar activity cycles (SACs) 23 and 24. We also compared our results with the variations of total solar irradiance (TSI), magnetic activity, Ca II K-flux, faculae and plage areas through the connection with the sunspot counts and the number of sunspot groups (SGs). Our results suggest that there is a strong correlation between solar activity indices and the changes in small (A, B, C and H-modified Zurich Classification) and large (D, E and F) SGs. This somewhat unexpected finding suggests that plage regions substantially decreased in spite of the higher number of large SGs in SAC 23 while the Ca II K-flux did not decrease by a large amount nor was it comparable with SAC 22 and relates with C and DEF type SGs. In addition to this, the increase of facular areas which are influenced by large SGs, caused a small percentage decrease in TSI while the decrement of plage areas triggered a higher decrease in the magnetic field flux. From all these results, we found that negative correlations between ISSN and SSI data and the variations between them are in close connection with the classes of sunspots/SGs, faculae and plage regions. Our results thus reveal the potential of such a detailed comparison of the SG analysis with solar activity indices for better understanding and predicting future trends in the SACs.

Toward understanding of the production and evolution of large solar flares

Takasao, Shinsuke

Solar flares, known as the most explosive phenomenon in the solar system, are a prototype of various kinds of explosions in the universe. Among them, large solar flares have particularly received remarkable attention because of their significant impact on the interplanetary space. During my Ph.D., I have been exploring two long standing problems associated with large flares; the formation of flare-productive regions and the energy release process of magnetic reconnection.The first problem is how active regions which produce large flares are formed. It has been known for decades that active regions with a certain photospheric magnetic configuration (so-called delta-spot regions) are responsible for the production of the most violent flares. To understand their origin, we carried out an 3D magnetohydrodynamic (MHD) simulation where a subsurface highly twisted flux tube emerges from the solar interior to the corona. This talk demonstrates that the strongly twisted tube can spontaneously form a multipolar structure with a strong magnetic shear which is flare-productive.The other problem regarding solar flares is, why flares often show quasi-periodic pulsations (QPPs) in their lightcurves. QPPs are a common phenomenon not only for solar flares but also for stellar flares, and are believed to tell us about the information of the energy release site which is almost impossible to observe directly. Although many mechanisms have been proposed, the lack of a comprehensive modeling have made it difficult to link the oscillation and its driver. We carried out a set of 2D MHD simulations of a solar flare in which magnetic reconnection, heat conduction, and chromospheric evaporation are considered. As a result, we discovered that a newly found oscillation is prominent in our simulations; the local oscillation above the flare loops that is controlled by the backflow of the reconnection outflow. We will discuss the new mechanisms and the connection with observations in this talk.

Solar Science with ALMA

Shimojo, Masumi

The solar observations with millimeter and submillimeter ranges would have capabilities of revealing the temperature structures of the chromosphere and the non-thermal electrons accelerated in a solar flare. Nevertheless, the solar observing studies using the wavelength ranges was not so active, because it is difficult to obtain solar images with the high-spatial resolution that equals to the resolution with the other spectrum ranges. The Atacama Large Millimeter/submillimeter Array (ALMA) broke through this situation. The ALMA observatory started to offer solar observations with 100 GHz (3 mm) and 239 GHz (1.25 mm) since Cycle 4 (Observing period: Oct. 2016 – Sep. 2017), and the demonstration data for solar observing, which are called the Science Verification (SV) data, were released in January 2017. The scientific results were achieved even from the SV data, and the solar observing data obtained in Cycle 4 will be opened to the community from July in order. In this talk, I present the scientific results obtained from the SV and Cycle 4 data, and introduce the development plan of new solar-observing functions with ALMA.

Coronal Mass Ejections in Interplanetary Space and Their Geospace Response

Kilpua, Emilia

Interplanetary coronal mass ejections (ICMEs) and their sheath regions are key large-scale heliospheric transients that drive strong space weather disturbances at the Earth. Although intrinsically connected, ICMEs and sheaths have distinctly different origin and solar wind conditions, and consequently, they cause different responses in the near-Earth space environment. In this presentation I will discuss key differences in space weather relevant parameters in these structures, how they control the solar wind – magnetospheric coupling efficiency and specific challenges in predicting their geoeffectivity.  I will in particular highlight differences in geospace response (ring current, auroral region, radiation belts) to two typical solar wind forcing “modes”: 1) smooth, low Alfvén Mach number and low dynamic pressure type solar wind driving typical to flux ropes embedded in ICMEs, and 2) turbulent, high Alfvén Mach number and high dynamic pressure solar wind driving typical to sheaths. 

A Theoretical Model of the Variation of the Meridional Circulation with the Solar Cycle

Choudhuri, Arnab

Observations of the meridional circulation of the Sun, which plays a key role in the operation of the solar dynamo, indicate that its speed varies with the solar cycle, becoming faster during the solar minima and slower during the solar maxima. We suggest that this is caused by the back reaction due to the Lorentz force of the dynamo-generated magnetic fields. We construct a theoretical model of the variation of the meridional circulation by coupling the equation of the meridional circulation (the φ component of the vorticity equation within the solar convection zone) with the equations of the flux transport dynamo model. Although a full theory of the meridional circulation is extremely complicated, we are able to study its variations due to the Lorentz force by separating out this part of the theoretical analysis. We obtain results which reproduce the observational data of solar cycle variations of the meridional circulation reasonably well. This presentation will be based on a paper which has recently appeared (Hazra & Choudhuri 2017, MNRAS 472, 2728).

On the factors determining the eruptive character of solar flares

Veronig, Astrid

We study how the magnetic field determines whether a strong flare launched from an active region (AR) will be eruptive or confined, i.e. associated with a coronal mass ejection (CME) or not. To this aim, we analyzed 44 flares of GOES class >M5.0 that occurred during 2011 to 2015 in SDO data. We used 3D potential magnetic field models to study their location within the host AR (using the flare distance from the flux-weighted AR center, d_FC) and the strength of the overlying coronal field (via decay index n). We also present a first systematic study of the orientation of the coronal magnetic field changing with height, using the orientation phi of the flare-relevant polarity inversion line as a measure. We analyzed all quantities with respect to the size of the underlying active-region dipole field, defined by the distance between the flux-weighted opposite-polarity centers, d_PC. We find that flares originating from the periphery of an AR dipole field (d_FC / d_PC > 0.5) are predominantly eruptive. Flares originating from underneath the AR dipole field (d_FC / d_PC < 0.5) tend to be eruptive when they are launched from a compact AR and confined when launched from an extended AR (d_PC > 60 Mm). In confined events, the flare-relevant field adjusts its orientation quickly to that of the underlying dipole field with height (delta phi > 40° between the surface and the apex of the active-region dipole field), in contrast to eruptive events where it changes more slowly. The critical height for torus instability discriminates best between confined (h_crit > 40 Mm) and eruptive flares (h_crit < 40 Mm). It discriminates better than delta phi, implying that the decay of the confining field plays a stronger role in the eruptive/confined character of a flare than its orientation at different heights.

The subterahertz sun: equatorial and polar radii from SST and ALMA

Menezes, Fabian

The nominal solar radius is RoN = 6.957(1) × 108 m. It is equivalent to a angular radius of 959.63" from 1 AU and corresponds to the solar photospheric radius. However, this value changes with observations at other wavelengths because the altitude of the dominant electromagnetic radiation is produced at different heights in the solar atmosphere. Therefore the solar radius is a very important parameter for the calibration of solar atmospheric models enabling a better understanding of the atmospheric structure. In Menezes and Valio (2017), the average solar radii measured with extensive data from the Solar Submillimeter-wavw Telescope (SST) were 966.5" ± 2.8" for 0.2 THz and 966.5" ± 2.7" for 0.4 THz and also the radius temporal variation was observed to be anti-correlated with the solar activity at both frequencies. Here we report the measurements of the solar radius at equatorial and polar latitudes for the same frequencies. The slight differences obtained int these radii may be related to solar limb brightening variations over the solar cycle. As a validation, we compared the SST measurements with the solar radius at 0.239 THz observed by ALMA.

Influence of Active Region Inflows in a 3D Babcock-LEighton Solar Dynamo Model

Gebreegzabihar, Kinfe

Solar observations reveal a systematic flow that converges toward photospheric active regions. It has been proposed that this converging flow may act as a saturation mechanism for the Babcock-Leighton mechanism, and as such, may determine the strength of solar cycles. In this work we will address questions such as: Are the converging flows towards active the regions an effective mechanism In 3D Babcock-Leighton solar dynamo model to saturate the dynamo? The inflow towards the active regions is developed on the frame work of Surface flux Transport And Babcock-LEighton Dynamo Model (STABLE), which is a fully 3D hybrid of flux-transport dynamo and surface flux transport model. The STABLE model solves the kinematic magnetohydrodynamics (MHD) induction equation in a 3D, rotating, spherical shell. The induction equation is solved by means of the Anelastic Spherical Harmonic (ASH) code, which currently serves as the dynamical core for the STABLE model. STABLE uses the SpotMaker spot deposition algorithm (instead of α-effect) to place bipolar magnetic regions (BMRs) on the solar surface in response to the dynamo-generated magnetic field. The subsequent evolution of these BMRs due to differential rotation, meridional circulation, converging flows towards the BMRs and turbulent diffusion naturally generates a mean poloidal field as originally described by Babcock (1961) and Leighton (1964). Our STABLE simulations show that inflows converging towards the BMRS significantly affects the build-up of the polar field. To our knowledge this study is the first work that; converging flows towards active regions in 3D Solar dynamo model and results of the model shows that the inflows are a key ingredient in determining the amplitude of solar cycles by providing a nonlinear feedback and mechanism for the saturation of a Babcock-Leighton-type dynamo mechanism.

Early CME evolution and associated phenomena in the solar atmosphere

Temmer, Manuela

Coronal mass ejections (CMEs) and flares are the most energetic activity phenomena in the solar system. Earth-directed CMEs are the main drivers of strong geomagnetic storms and therefore are an area of intense research interest. How CMEs get initiated and driven, how impulsive these events may become and in which direction they further propagate, are crucial inputs for Space Weather forecasting. Signatures in the low solar atmosphere that are associated to CMEs reflect best the characteristics of the CME early evolution. Hence, studying solar on-disk multi-wavelength data is essential for a better understanding of the physical processes that drive CMEs and their behavior in interplanetary space. Especially the CME related dynamic phenomena of solar flares, coronal waves and dimmings give a wealth of information for Earth-directed CMEs, that are usually hard to observe and characterize from traditional white-light observations. This overview talk discusses our recent understanding of the physical processes about the initiation and early propagation of CMEs that could be gained by combined multi-wavelength and multi-instrument data.

Development of a Solar Major Flare Forecasting Model Based on Vector Magnetic Parameters from SDO/HMI Data

Lim, Daye

We investigate major (M- and X-class) flare occurrence rates within a day using hourly Solar Dynamics Observatory (SDO)/Helioseismic and Magnetic Imager (HMI) vector magnetic field data from May 2010 to April 2017. In this study, we consider six physical parameters characterizing distribution and non-potentiality of magnetic fields (the total unsigned current helicity, the total photospheric magnetic free energy density, the total unsigned vertical current, the absolute value of the net current helicity, the sum of the net current from each polarity, and the total unsigned magnetic flux), which are highly correlated with flaring activity. We divide the data into two sets separated chronologically. 70% of the data are used for setting up a flare model and the other for test. All magnetic parameters are divided into 100 groups to estimate the corresponding flare occurrence rates. Flare identifications are determined by using Geostationary Operational Environmental Satellites (GOES) X-ray flare locations. Major results are as follows. First, major flare occurrence rates are well correlated with six magnetic parameters. Second, the logarithmic values of flaring rates are well approximated by two linear equations with different slopes: steeper one at lower values and lower one at higher values. Third, the total photospheric magnetic free energy density gives the minimum root mean square error between observed flare rates and predicted ones. Fourth, among six parameters, the total unsigned current helicity, the total unsigned vertical current, and the total unsigned magnetic flux have higher verification measures than the other parameters. Fifth, the true skill statistic (TSS) values of these top three parameters are higher than 0.83, which are much better than those of other probabilistic forecasting models. 

Inferences of the Deep Solar Meridional Flow

Böning, Vincent

Here, I present the main results from my PhD. thesis, to which a number of publications are entering.The solar meridional flow is a crucial ingredient in modern dynamo theory. Seismic estimates of this flow have, however, been contradictory in deeper layers. Here, we develop and validate a method for computing spherical Born approximation kernels for time-distance helioseismology and we employ these kernels to invert for the deep solar meridional flow using 652 days of GONG data from 2004 – 2012.Above about 0.85 solar radii, our inversions confirm the result obtained by Jackiewicz et al. with the ray approximation regarding the general structure of the flow. This especially concerns a shallow return flow at about 0.9 solar radii, although some differences in flow magnitude are apparent.Below about 0.85 solar radii, we obtain several different results that are consistent with the measured travel times within the measurement errors. While one result is similar to the original single-cell flow found by Jackiewicz et al., the other results exhibit a multi-cell flow structure in the southern hemisphere. To reach an unambiguous conclusion on the meridional flow in this region, the errors in the measured travel times have to be considerably reduced.We conclude that an unambiguous detection of the meridional flow is limited to a much shallower region than previously thought. This is a partial relief to the controversy about measurements of the deep solar meridional flow.

Changes in the photospheric magnetic field produced by flares

Wheatland, Michael

Data from space-based vector magnetograms and high-resolution ground-based telescopes clearly show large-scale changes in the photospheric magnetic field produced by solar flares. A striking example is the observation of sudden rotation of a sunspot in response to a flare on 22 June 2015 (Liu et al. 2016). Here we report on analysis and modeling of the 22 June 2015 event based on the Solar Dynamics Observatory Helioseismic and Magnetic Imager data.

The Solar Wind over the Last Four Sunspot Cycles

Russell, Christopher

The near-Earth solar wind has been monitored continuously now for over 4 sunspot cycles, and quite complete records are available for SC 21-24. The interplanetary magnetic field rises and falls in synchronism with the sunspot cycle, as would be expected since the sunspot cycle is a phenomenon of increasing and decreasing photospheric magnetic field. With the possible exception of corotating high-speed streams, the solar wind plasma seems less controlled by the phase of the solar cycle. However, one long-term change in solar wind density has occurred with what appears to be a relatively sudden onset at the start of cycle 23. The median solar wind density at 1 AU dropped from about 8 to about 6 cm-3. While one might suspect such a change to be an instrument/technique associated effect, it appears to be a physical effect that has now persisted through solar cycles 23 and 24. This affects the solar wind dynamic pressure and hence the size of the Earth’s magnetosphere, for example. We consider this change in light of the contemporaneous weakening of the solar activity cycle at that time. Given that the solar wind measurements reflect only what is occurring in the ecliptic plane at 1 AU, we use the combination of PFSS models of solar wind source mapping and ENLIL models of the solar wind to assess the relationship between the related changes in the coronal field structure and the solar wind structure. In particular, we compare the coronal field and solar wind models from earlier cycles with those from cycles 23 and 24 to provide insight into the effects that may be responsible for the observed changes in the solar wind.

Electron Acceleration in Nanoflares and Preflares - Relevant for Coronal Heating?

Benz, Arnold O.

A significant fraction of the energy of solar flares is first released into energetic electrons. The acceleration process is still controversial. We discuss here the gyrosychrotron emission of relativistic electrons and compare it to the soft X-ray emission of the flare plasma by thermal bremsstrahlung. The ratio of radio to soft X-ray luminosities is a measure of the acceleration efficiency. In regular flares and coronae of active stars, the ratio is constant over many orders of magnitude in flare size. However, the quiet Sun does not follow the ratio observed for flares. Here we present new observations by the Nobeyama Radioheliograph and the RHESSI X-ray satellite. In small preflares we find their radio-to-X-ray ratio at least an order of magnitude smaller than in the main flare phase. The trend of small flares to be less efficient electron accelerators was also noticed in previous observations, indicating that they have a softer hard X-ray spectrum and thus are lacking in electrons accelerated to high energy. It is also a general trend that flares start with a steep hard X-ray spectrum (few high-energy electrons) and harden during the event. Most significantly, microflares in the quiet region, also known as (large) nanoflares, are clearly radio-poor often by more than an order of magnitude, thus are poor accelerators. One of the currently viable scenarios for coronal heating is energy release by nanoflares that are smaller than previously observed. Thus the question arises whether they also accelerate electrons. The emissions of such electrons in gyrosynchrotron radiation or hard X-rays have not yet been observed, but would be a crucial test of the theory. In the case of radio emission, the thermal background makes it impossible. We report on the estimated flux of the hard X-ray emission of accelerated electrons and discuss the possibility for detecting it with upcoming missions, in particular with STIX on Solar Orbiter.

AI-generated EUV images of the Sun

Park, Eunsu

In astronomy and geophysics, multi-wavelength observations become very popular. Recently, several deep learning methods, one part of Artificial Intelligence (AI), for image-to-image translations have been suggested and are successful for different types of transformation such as labels to street scene, labels to facade, black and white images to color ones, aerial to map, day to night, and sketch images to pictures. For the first time we apply an image-to-image translation model, based on conditional Generative Adversarial Networks (cGANs), to construct solar EUV images using solar magnetograms. For this, we train the model using pairs of SDO/AIA EUV image and their corresponding SDO/HMI line-of-sight magnetogram for all AIA wavelengths from 2011 to 2016. We test the model by comparing pairs of actual SDO/AIA EUV images and corresponding AI-generated ones in 2017. We find that both real and AI-generated images are quite consistent with each other in that it is difficult for one to distinguish solar EUV images from AI-generated ones. The average correlation between actual image and AI-generated one for all test samples ranges has a maximum value (0.80) for 1600 and 1700 data sets whose structures are quite consistent with those of corresponding magnetograms. Using this model, we construct solar EUV images with Kitt peak magnetograms since 1974. This methodology can be applicable to many scientific fields that use several different filter images.

Application of deep learning methods for image-to-image translation to solar and geophysical data

Moon, Yong-Jae

Multi-wavelength observations become very popular in astronomy and geophysics. Even though there are some correlations among different sensor images, it is not easy to translate from one to the other one. In this paper, we apply a deep learning method for image-to-image translation, based on conditional generative adversarial networks (cGANs), to solar and geophysical images. To examine the validity of the method for scientific data, we use several different types of pairs: (1) AI-generated magnetograms from solar SDO/AIA images, (2) AI-generated EUV images from SDO/HMI solar magnetograms, (3) AI-generated magnetograms from historical sunspot drawings such as Carrington events, and (4) AI-generated IR images from visual weather images. It is very impressive that AI-generated ones are quite consistent with actual ones. We will discuss several applications of this methodology for scientific research.

Effective Acceleration Model: Forecasting the arrival time of the shock of an ICME

Paouris, Evangelos

The interplanetary counterparts of the coronal mass ejections (ICMEs) are responsible for the most intense geomagnetic storms. These events are usually faster than the ambient solar wind and as a result, a shock is precedes. The estimation of the arrival time of the shock is very important in order to predict the start of the geomagnetic storm. In this work, a model for the estimation of the arrival time of the shock that precedes the main part of the ICME is presented. First validation and verification results are also presented. For the first time a new approach, using drag-based effects is also applied. This model is very important for space weather predictions, the Athens Space Weather Forecasting Center (ASWFC) already uses it, and it is one of the registered methods/models in the Community Coordinated Modeling Center (CCMC). From 2014 Athens Space Weather Forecasting Center provides a daily Space Weather Report which is available on our website: cosray.phys.uoa.gr/index.php/space-weather-report. This report contain information about solar activity, solar wind conditions, solar energetic particles, coronal holes and high speed streams, and a prediction for Ap geomagnetic index for the next 3 days, as well as the level of the potential geomagnetic storm. In this work validation results for the period October 2014 – February 2018 and a comparison between our estimation for Ap index and the one which predicted by other space weather prediction centers are also presented.

The catalogue of failed eruptions registered by the SDO/AIA

Mrozek, Tomasz

Failed eruptions are a type of solar eruptive events which after initial increase of height are  abruptly stopped. Even strongest X-class flares may be accompanied by failed eruptions. Present observations of SDO/AIA give a chance for deep statistical analysis of such events which may lead to understanding the mechanisms responsible for confinement. We developed automated algorithm which can recognize moving structures in AIA images. We searched whole 8 years of AIA database, and we found more than 20 000 dynamic events. Among them around 1500 were failed eruptions which we collected into the catalogue. The catalogue is available on-line, and contains basic information about eruption kinematics, properties of accompanying flare, decay index of magnetic field in the active region etc. The catalogue and preliminary statistical analysis of found failed eruptions will be presented and discussed.

Evidence of a distant decimetric radio source connected to the main flare site: First high temporal and spatial GMRT observations

Bisoi, Susanta Kumar

We present a study of decimetric radio activity, using the high time cadence (0.5 s) and high spatial (12 arcsec) observations at the fixed frequency of 610 MHz imaged by the Giant Meterwave Radio Telescope (GMRT), associated with GOES C1.4 and M1.0 class flares, and a coronal mass ejection (CME) that erupted on 20 June 2015. A bright radio source, associated with the M1.0 flare and CME, is located near the flaring site, while, in contrary, a bright radio source, associated with the C1.4 flare, with no corresponding coronal or magnetic features nearby, is rather located about 500 arcsec away from the flare site.  The bright radio source, at the start and during the maximum of C1.4 flare, show burst activity, which coincides with the type-III bursts observed by the Solar Broadband Radio Spectrometer at Yunnan Astronomical Observatory. A multi-wavelength analysis, in combination with potential field source surface extrapolation, is carried out to investigate the genesis of non-thermal radio emitting electrons, which revealed that the distant decimetric radio source, noticed during the C1.4 flare, is connected to the main flare site by a high arching loop. The apparent shift of the location of the bright radio source could have resulted by ducting of electron beams along the high arching loop, which is interacting with other closed and open field structures at the electron acceleration site.

A Circular Ribbon Flare Event as Observed by MUSER

Yan, Yihua

A Radio burst event on Dec 17, 2014 for a M8.7 flare was recorded by MUSER in 400MHz-2GHz. MUSER (Mingantu Spectral Radioheliograph) is a solar-dedicated interferometric array with a frequency range from 400MHz to 15 GHz located in Mingantu Town, Inner Mongolia of China. The flare was with circular ribbons over multiple-scale loop structures as revealed by AIA/SDO. There were groups of small-scale low-lying arcades or loops, intermediate dome-like structure, and the large-scale loops as shown in EUV images involved in this flare process. The multi-frequency images in decimeter wave ranges of the burst process by MUSER are obtained and analyzed. 

IMF By-dependent enhancement in high-latitude geomagnetic activity in local winter

Holappa, Lauri

The interaction of the solar wind and the interplanetary magnetic field (IMF) with the Earth's magnetic field produces geomagnetic activity, which is critically dependent on the orientation of the north-south (Bz) IMF component. Most solar wind coupling functions quantifying the relation between solar wind and IMF parameters and geomagnetic activity include the dependence on the sign (polarity) of Bz, within the so-called IMF clock angle. Coupling functions depend on the clock angle in a way, which is symmetric with respect to the sign of the By component. However, recent studies indicate that the sign of By is an additional independent driver of high-latitude geomagnetic activity, leading (for the same clock angle) to higher (weaker) geomagnetic activity in Northern Hemisphere winter for By > 0 (By < 0). In this paper we quantify this explicit By-effect both for Northern and Southern high-latitude geomagnetic activity. We show that the By-effect maximizes when the Earth's dipole axis points towards the night sector, i.e., when the auroral region is maximally in darkness. The By-effect affects the westward electrojet strongly but hardly at all the eastward electrojet. We find that there is a similar By-effect in the occurrence frequency and strength of substorms, largely explaining the By-effect in the westward electrojet. These results are important for predicting space weather effects at high latitudes and for understanding how the solar wind and IMF parameters produce geomagnetic activity.

Helioseismic Holography Using Multiple Vantage Points

Yang, Dan

Helioseismic holography is a technique to image the solar interior in three dimensions using acoustic waves observed at the solar surface. Here, we consider the theoretical problem of imaging a localised sound-speed heterogeneity using partial views of the solar surface. Using a wave solver in the frequency domain, we compute holograms for three different geometries: observations of the entire solar surface, from a single spacecraft, and from two spacecrafts with different vantage points. We compare spatial resolutions and signal-to-noise ratios.

Understanding the Where and the How Big of Solar Flares

Barnes, Graham

The approach to understanding solar flares generally characterizes global properties of a solar active region, for example the total magnetic flux, the total free magnetic energy, or the total length of a sheared magnetic neutral line.  We take here a different tack, characterizing not the region as a whole, but estimating the energy-release prospects of different sub-regions within the region.  We have considered two active regions (NOAA ARs 10978 and 11283) which are similar in their overall size and classification, but produced radically different distributions of flares, with AR 10978 producing nothing larger than C-flares while AR 11283 produced a sequence of M and X-flares, with very few smaller flares.  We modeled the coronal magnetic field using the CFIT non-linear force-free extrapolation code, and identified individual current systems within the the extrapolation whose energy might be released in a single reconnection event.  We present here early results comparing the energy associated with the individual current systems with the magnitude of the flares originating from each region.This material is based upon work supported by the US National Science Foundation under Grant No. 1630454. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.

A new window to the Sun: The Daniel K Inouye Solar Telescope.

Martinez Pillet, Valentin

The National Solar Observatory (NSO) is building the Daniel K Inouye Solar Telescope (DKIST) on the island of Maui (Hawai'i)  under the sponsorship of the National Science Foundation. DKIST is a transformational facility for Solar Physics due to its resolving power, light gathering capabilities, and off-axis design. The science enabled by DKIST encompasses from the nature of the turbulently driven local dynamo to high sensitivity off-limb measurements of the magnetic field in the solar corona. The telescope is also particularly well suited to study the highly dynamic and weakly magnetized solar chromosphere. DKIST will come on-line by mid-2020 at a time when the Parker Solar Probe and the Solar Orbiter missions will start their scientific operations. The combination of these three experiments and their measurements of particles, fields, and photons represents a new multi-messenger era for Solar and Heliospheric physics. In this talk, I will give an update on the status of the DKIST construction and the planning for the facility early science through the Critical Science Plan.

Helioseismology and the changing solar dynamo

Howe, Rachel

Helioseismology uses waves propagating inside the Sun to measure its internal structure and dynamics. This lets us monitor both the changing subsurface flows over the solar cycle -- the so-called 'torsional oscillation' pattern of migrating bands of faster and slower rotation, together with the modulation of the meridional flow in the activity belts  -- and the changes to the frequencies and other properties of the acoustic modes due to magnetic activity close to the surface.  We now have continuous data over nearly two solar cycles from the ground-based Global Oscillation Group and the space-borne Michelson Doppler Imager and Helioseismic and Magnetic Imager, together with unresolved-Sun observations from the Birmingham Solar Oscillations Network stretching back to the mid 1970s. This record offers intriguing hints that the behaviour of the solar dynamo may be changing. This review will examine the evidence for such changes and consider what they might mean for the future.

Radio diagnostics of the Sun with old and new instruments

Klein, Karl-Ludwig

Radio astronomy offers a broad variety of tools to probe thermal and non thermal processes in the solar atmosphere. Radio mapping provides density and temperature diagnostics through the well-understood bremsstrahlung process of quiet emission. Radio diagnostics are uniquely sensitive to non-thermal electrons in quiescent and eruptive active regions, and give otherwise unavailable insight into the acceleration and propagation of electrons in solar flares and eruptive events. While solar-dedicated instruments are and remain essential for a maximum common coverage with space observatories, general purpose telescopes like the VLA, LOFAR and MWA have a great discovery potential due to their high spatial and spectral resolution, and ALMA offers unprecedented possibilities to extend radio diagnostics to the chromosphere. The talk will illustrate some recent results, give an overview of the new instruments, and emphasise the support that radio observations provide to the key space missions of heliophysics in the coming years, Parker Solar Probe and Solar Orbiter.

Science from Solar Orbiter

Solanki, Sami

The Solar Orbiter is the next solar physics mission of the European Space Agency, ESA, in collaboration with NASA, with a launch planned in 2020. The spacecraft is designed to approach the Sun to within 0.28 AU at perihelion of a highly eccentric orbit. At a later phase in the mission, the spacecraft will leave the ecliptic and study the enigmatic poles of the Sun from a heliographic latitude of up to 34 degrees.Equipped with 6 remote-sensing and 4 in-situ instruments, Solar Orbiter will address the overarching question on how the Sun forms and influences the heliosphere. It will therefore, probe the coupling between the Sun and the inner heliosphere. The proximity to the Sun at the closest perihelia will also allow the Sun to be observed at uniformly high resolution at EUV and visible wavelengths. Such observations are central for learning more about the magnetic coupling of the solar atmosphere. Furthermore, Solar Orbiter will provide the first ever optical and EUV observations of the solar poles, including the magnetic field there, which plays an important role in the workings of the solar dynamo.  

Immersed in the Solar Wind: The New Era of Solar Physics Space Missions

Vourlidas, Angelos

Sunlight feeds life on Earth while the solar wind buffets our magnetosphere, sometimes violently. It is no surprise that solar variability is a primary societal concern and subject of intense scientific research. For decades, however, progress on understanding the Sun-Earth connection has been hampered by the 'disconnected' nature of the observations; remote sensing of the near-Sun corona, in-situ sampling at Earth. The evolution of the solar wind and its more energetic transients in the inner heliosphere was accessible only through modeling. While the STEREO mission made great strides since 2007, the mechanisms of the generation and early evolution of the solar wind still elude us. This is about to change thanks to an unprecedented space mission, the Parker Solar Probe (PSP) to be launched in late July 2018. The mission is designed to attack the solar wind problem head-on by direct sampling of the corona from the ‘inside’ with a suite of remote sensing and in-situ instruments. In this talk, I introduce the capabilities and science objectives of the PSP mission and discuss the exciting science prospects in solar physics research in the next 10 years. PSP will be the first spacecraft to enter the atmosphere of a star, reaching within 6 million km from the solar surface. The mission design will tie together in-situ sampling and high contrast imaging from ‘within’ the solar corona with high resolution observations from space and ground. A new era in solar and space physics awaits us.

Statistical study on the kinematic classification of coronal mass ejections from 4 to 30 solar radii

Jeon, Seonggyeong

In this study, we perform a statistical investigation on the kinematic classification of 4264 coronal mass ejections (CMEs) from 1996 to 2015 observed by \textit{SOHO}/LASCO C3. Using the constant acceleration model, we classify these CMEs into three groups; deceleration, constant velocity, and acceleration motion. For this, we devise four different classification methods by acceleration, fractional speed variation, height contribution, and visual inspection. Our major results are as follows. First, the fractions of three groups depend on the method used. Second, about half of the events belong to the groups of acceleration and deceleration. Third, the fractions of three motion groups as a function of CME speed classified by the last three methods are consistent with one another. Fourth, according to the last three methods, the fraction of acceleration motion decreases as CME speed increases, while the fractions of other motions increase with speed. In addition, the acceleration motions are dominant in low speed CMEs whereas the constant velocity motions are dominant in high speed CMEs. 

Understanding the Where and the How Big of Solar Flares

Barnes, Graham

The approach to understanding solar flares generally characterizes global properties of a solar active region, for example the total magnetic flux, the total free magnetic energy, or the total length of a sheared magnetic neutral line.  We take here a different tack, characterizing not the region as a whole, but estimating the energy-release prospects of different sub-regions within the region.  We have considered two active regions (NOAA ARs 10978 and 11283) which are similar in their overall size and classification, but produced radically different distributions of flares, with AR 10978 producing nothing larger than C-flares while AR 11283 produced a sequence of M and X-flares, with very few smaller flares.  We modeled the coronal magnetic field using the CFIT non-linear force-free extrapolation code, and identified individual current systems within the the extrapolation whose energy might be released in a single reconnection event.  We present here early results comparing the energy associated with the individual current systems with the magnitude of the flares originating from each region.This material is based upon work supported by the US National Science Foundation under Grant No. 1630454. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation. 

Supervised Neural Networks for Helioseismic Ring-diagram Inversions

Hanson, Chris

The inversion of ring fit parameters to obtain subsurface flow maps in ring-diagram analysis for SDO observations is computationally expensive. We apply machine learning techniques to the inversion step of the pipeline, to replace future inversion requirements. We utilize Artificial Neural Networks as a supervised learning method for predicting the flows in 15 ◦ ring tiles. To demonstrate that the machine learning results still contain the subtle signatures key to local helioseismic studies, we use the machine learning results to re-detect equatorial Rossby waves. We find the Artificial Neural Network is computationally efficient, can achieve a root mean-square error of half that reported for the observations, and reduce computational burden by two orders of magnitude. We find that the signatures of the Rossby waves are still in the machine learning results, showing that important helioseismic signatures are maintained. 

The Solar Corona viewed through the MinXSS

Moore, Christopher

Advances in technology and instrumentation open new windows for observing astrophysical objects. X-ray detector technology with high readout rates are necessary for the relatively bright Sun, particularly during large flares. The hot plasma in the solar corona generates X-rays, which yield information on the physical conditions of the plasma. This dissertation focusses on detector testing, characterization and solar science with the Miniature X-ray Solar Spectrometer (MinXSS) CubeSats. The MinXSS CubeSats employ Silicon Drift Diode (SDD) detectors called X123, which generate full sun spectrally resolved (~0.15 FWHM at 5.9 keV) measurements of the sparsely measured, 0.5 – 12 keV range. The absolute radiometric calibration of the MinXSS instrument suite was performed at the National Institute for Standards and Technology (NIST) Synchrotron Ultraviolet Radiation Facility (SURF) and spectral resolution determined from radioactive sources. I used MinXSS along with data from the Geostationary Operational Environmental Satellites (GOES), Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), Hinode X-ray Telescope (XRT), Hinode Extreme Ultraviolet Imaging Spectrometer (EIS) and Solar Dynamics Observatory (SDO) Atmospheric Imaging Assembly (AIA) to study the solar corona. This resulted in new insights on the coronal temperature distribution and elemental abundance variations for quiescence, active regions and during solar flares. 

XXX IAU General Assembly | ACV - Austria Center Vienna  | Bruno-Kreisky-Platz 1  | 1220 Vienna