- ID:
- ivo://CDS.VizieR/J/ApJ/885/49
- Title:
- Global energetics of solar flares. IX.
- Short Name:
- J/ApJ/885/49
- Date:
- 08 Dec 2021
- Publisher:
- CDS
- Description:
- A more accurate analytical solution of the vertical-current approximation nonlinear force-free field (VCA3-NLFFF) model is presented that includes, besides the radial (Br) and azimuthal (B{phi}) magnetic field components, a poloidal component (B_{theta}_/=0) as well. This new analytical solution is of second-order accuracy in the divergence-freeness condition and of third-order accuracy in the force-freeness condition. We reanalyze the sample of 173 GOES M- and X-class flares observed with the Atmospheric Imaging Assembly and Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory (SDO). The new code reproduces helically twisted loops with a low winding number below the kink instability consistently, avoiding unstable, highly twisted structures of the Gold-Hoyle flux rope type. The magnetic energies agree within E_VCA3_/E_W_=0.99{+/-}0.21 with the Wiegelmann (W-NLFFF) code. The time evolution of the magnetic field reveals multiple, intermittent energy buildup and releases in most flares, contradicting both the Rosner-Vaiana model (with gradual energy storage in the corona) and the principle of timescale separation ({tau}flare<<{tau}storage) postulated in self-organized criticality models. The mean dissipated flare energy is found to amount to 7%{+/-}3% of the potential energy, or 60%{+/-}26% of the free energy, a result that can be used for predicting flare magnitudes based on the potential field of active regions.
Number of results to display per page
Search Results
- ID:
- ivo://CDS.VizieR/J/ApJ/881/1
- Title:
- Global energetics of solar flares. VIII.
- Short Name:
- J/ApJ/881/1
- Date:
- 10 Dec 2021
- Publisher:
- CDS
- Description:
- One of the key problems in solar flare physics is the determination of the low-energy cut-off: the value that determines the energy of nonthermal electrons and hence flare energetics. We discuss different approaches to determine the low-energy cut-off in the spectrum of accelerated electrons: (I) the total electron number model, (II) the time-of-flight model (based on the equivalence of the time-of-flight and the collisional deflection time), (III) the warm target model of Kontar et al., and (IV) the model of the spectral cross-over between thermal and nonthermal components. We find that the first three models are consistent with a low-energy cutoff with a mean value of ~10keV, while the cross-over model provides an upper limit for the low-energy cutoff with a mean value of ~21keV. Combining the first three models we find that the ratio of the nonthermal energy to the dissipated magnetic energy in solar flares has a mean value of qE=0.57{+/-}0.08, which is consistent with an earlier study based on the simplified approximation of the warm target model alone (qE=0.51{+/-}0.17). This study corroborates the self-consistency between three different low-energy cutoff models in the calculation of nonthermal flare energies.
- ID:
- ivo://CDS.VizieR/J/A+A/602/A69
- Title:
- Group sunspot number series since 1739
- Short Name:
- J/A+A/602/A69
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The group sunspot number (GSN) series constitute the longest instrumental astronomical database providing information on solar activity. This database is a compilation of observations by many individual observers, and their inter-calibration has usually been performed using linear rescaling. There are multiple published series that show different long-term trends for solar activity. We aim at producing a GSN series, with a non-linear non-parametric calibration. The only underlying assumptions are that the differences between the various series are due to different acuity thresholds of the observers, and that the threshold of each observer remains constant throughout the observing period. We used a daisy chain process with backbone (BB) observers and calibrated all overlapping observers to them. We performed the calibration of each individual observer with a probability distribution function (PDF) matrix constructed considering all daily values for the overlapping period with the BB. The calibration of the BBs was carried out in a similar manner. The final series was constructed by merging different BB series. We modelled the propagation of errors straightforwardly with Monte Carlo simulations. A potential bias due to the selection of BBs was investigated and the effect was shown to lie within the 1{sigma} interval of the produced series. The exact selection of the reference period was shown to have a rather small effect on our calibration as well. The final series extends back to 1739 and includes data from 314 observers. This series suggests moderate activity during the 18th and 19th century, which is significantly lower than the high level of solar activity predicted by other recent reconstructions applying linear regressions. The new series provides a robust reconstruction, based on modern and non-parametric methods, of sunspot group numbers since 1739, and it confirms the existence of the modern grand maximum of solar activity in the second half of the 20th century.
- ID:
- ivo://CDS.VizieR/J/ApJ/874/107
- Title:
- HARPS-N solar radial velocities and activity
- Short Name:
- J/ApJ/874/107
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- State-of-the-art radial-velocity (RV) exoplanet searches are currently limited by RV signals arising from stellar magnetic activity. We analyze solar observations acquired over a 3yr period during the decline of Carrington Cycle 24 to test models of RV variation of Sun-like stars. A purpose-built solar telescope at the High Accuracy Radial-velocity Planet Searcher for the Northern hemisphere (HARPS-N) provides disk-integrated solar spectra, from which we extract RVs and logR'_HK_. The Solar Dynamics Observatory (SDO) provides disk-resolved images of magnetic activity. The Solar Radiation and Climate Experiment (SORCE) provides near-continuous solar photometry, analogous to a Kepler light curve. We verify that the SORCE photometry and HARPS-N logR'_HK_ correlate strongly with the SDO-derived magnetic filling factor, while the HARPS-N RV variations do not. To explain this discrepancy, we test existing models of RV variations. We estimate the contributions of the suppression of convective blueshift and the rotational imbalance due to brightness inhomogeneities to the observed HARPS-N RVs. We investigate the time variation of these contributions over several rotation periods, and how these contributions depend on the area of active regions. We find that magnetic active regions smaller than 60Mm^2^ do not significantly suppress convective blueshift. Our area-dependent model reduces the amplitude of activity-induced RV variations by a factor of two. The present study highlights the need to identify a proxy that correlates specifically with large, bright magnetic regions on the surfaces of exoplanet-hosting stars.
- ID:
- ivo://CDS.VizieR/J/A+A/619/A99
- Title:
- Helioseismic measurements of solar meridional flow
- Short Name:
- J/A+A/619/A99
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The solar meridional flow is an essential ingredient in flux-transport dynamo models. However, no consensus on its subsurface structure has been reached. We merge the data sets from SOHO/MDI and SDO/HMI with the aim of achieving a greater precision on helioseismic measurements of the subsurface meridional flow. The south-north travel-time differences are measured by applying time-distance helioseismology to the MDI and HMI medium-degree Dopplergrams covering May 1996-April 2017. Our data analysis corrects for several sources of systematic effects: P-angle error, surface magnetic field effects, and center-to-limb variations. For HMI data, we used the P-angle correction provided by the HMI team based on the Venus and Mercury transits. For MDI data, we used a P-angle correction estimated from the correlation of MDI and HMI data during the period of overlap. The center-to-limb effect is estimated from the east-west travel-time differences and is different for MDI and HMI observations. An interpretation of the travel-time measurements is obtained using a forward-modeling approach in the ray approximation. In the latitude range 20{deg}-35{deg}, the travel-time differences are similar in the southern hemisphere for cycles 23 and 24. However, they differ in the northern hemisphere between cycles 23 and 24. Except for cycle 24's northern hemisphere, the measurements favor a single-cell meridional circulation model where the poleward flows persist down to ~0.8R_{sun}_, accompanied by local inflows toward the activity belts in the near-surface layers. Cycle 24's northern hemisphere is anomalous: travel-time differences are significantly smaller when travel distances are greater than 20{deg}. This asymmetry between northern and southern hemispheres during cycle 24 was not present in previous measurements, which assumed a different P-angle error correction where south-north travel-time differences are shifted to zero at the equator for all travel distances. In our measurements, the travel-time differences at the equator are zero for travel distances less than ~30{deg}, but they do not vanish for larger travel distances. This equatorial offset for large travel distances need not be interpreted as a deep cross-equator flow; it could be due to the presence of asymmetrical local flows at the surface near the end points of the acoustic ray paths. The combined MDI and HMI helioseismic measurements presented here contain a wealth of information about the subsurface structure and the temporal evolution of the meridional circulation over 21 years. To infer the deep meridional flow, it will be necessary to model the contribution from the complex time-varying flows in the near-surface layers.
- ID:
- ivo://CDS.VizieR/J/A+A/652/A56
- Title:
- Hemispheric Sunspot Numbers 1874-2020
- Short Name:
- J/A+A/652/A56
- Date:
- 22 Feb 2022
- Publisher:
- CDS
- Description:
- Previous studies have shown significant North-South asymmetries for various features and indicators of solar activity. These findings suggest some decoupling between the two hemispheres over the solar cycle evolution, supported by dynamo theories. For the most important solar activity index, the sunspot numbers, so far only limited coverage of hemispheric data is available. The aim of this study is to create a continuous series of daily and monthly hemispheric sunspot numbers (HSN) from 1874 until 2020, which will be continuously expanded in the future with the HSN provided by SILSO. Based on the available daily measurements of hemispheric sunspot areas from 1874-2016 from Greenwich Royal Observatory and NOAA, we derive the relative fractions of the Northern and Southern activity. These fractions are applied to the International Sunspot Numbers (ISN) to derive the HSN. This method and obtained data are validated against the HSN available for the period 1945-2004 from Temmer et al. (2006A&A...447..735T, J/A+A/447/735) and 1992 to date by SILSO. Results: We provide a continuous data series and catalogue of daily, monthly mean and 13-month smoothed monthly mean HSN for the time range 1874-2020, i.e. fully covering solar cycles no. 12 to 24, which are consistent with the newly calibrated ISN (Clette et al., 2014SSRv..186...35C). Validation of the reconstructed HSN against the direct data available since 1945 reveals high consistency, with a Pearson correlation coefficients of r=0.94 (0.97) for the daily (monthly mean) data. The cumulative hemispheric asymmetries for cycles 12-24 give a mean value of 16%, with no obvious pattern in North-South predominance over the cycle evolution. The strongest asymmetry occurs for cycle no. 19, in which the Northern hemisphere shows a cumulated predominance of 42%. The phase shift between the peaks of solar activity in the two hemispheres may be up to 28 months, with a mean absolute value over cycles 12-24 of 16.4 months. The phase shifts reveal an overall asymmetry of the Northern hemisphere reaching its cycle maximum earlier (in 10 out of 13 cases), with a mean signed phase shift of -7.6 months. Relating the peak growth rates of the ISN and HSN during the cycle's rise phase, respectively, with the cycle amplitude reveals higher correlations when considering the two hemispheres individually, with r=0.9. Our findings provide further evidence that the evolution of the solar cycle occurs partly independent for the two hemispheres, and demonstrate that empirical solar cycle prediction methods can be improved by investigating the solar cycle dynamics in terms of the hemispheric sunspot number evolution.
- ID:
- ivo://CDS.VizieR/J/A+A/447/735
- Title:
- Hemispheric Sunspot Numbers 1945-2004
- Short Name:
- J/A+A/447/735
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- From sunspot drawings provided by the Kanzelhoehe Solar Observatory, Austria, and the Skalnate Pleso Observatory, Slovak Republic, a data catalogue of hemispheric Sunspot Numbers covering the time span 1945-2004 is extracted. The validated catalogue includes daily, monthly-mean and smoothed-monthly relative sunspot numbers for the northern and southern hemispheres separately and is available for scientific use. Based on this data set an analysis concerning the North-South asymmetry is made within this paper.
- ID:
- ivo://CDS.VizieR/J/A+A/390/707
- Title:
- Hemispheric Sunspot Numbers 1975-2000
- Short Name:
- J/A+A/390/707
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Hemispheric sunspot numbers including the time span from 1975-2000 are presented. The Sunspot Numbers are calculated from sunspot drawings made at the Kanzelhoehe Solar Observatory (KSO), Austria. The counted northern and southern Sunspot Numbers are normalized and multiplied with the International Sunspot Numbers taken from SIDC for matching days in order to provide the data within an international usage. Days without observation by KSO (about 27 percent) were reconstructed applying a linear interpolation method. As validation of the data, regression methods and a cross-correlation analysis are made with hemispheric Sunspot Numbers from SIDC for the available time span 1992-2000 obtaining very good agreements. The results are given in monthly mean and smoothed monthly mean Sunspot Numbers. Based on this data set an analysis concerning the North-South asymmetry is made in the paper.
- ID:
- ivo://CDS.VizieR/J/ApJ/896/133
- Title:
- 2001-2019 ICME associated Forbush decrease events
- Short Name:
- J/ApJ/896/133
- Date:
- 03 Dec 2021 13:44:17
- Publisher:
- CDS
- Description:
- Interplanetary coronal mass ejections (ICMEs) are eruptions of plasma that propagate outward through the heliosphere. ICMEs, and the shocks they drive, cause a sudden decrease in the cosmic-ray flux in their local area of the heliosphere, called a Forbush decrease (FD). A method of defining FDs is established, and an automated process for identifying FDs in neutron monitor (NM) data is created. The correlation between ICME properties and FD magnitude in 12 different NMs is examined for 91 ICME-associated FD events occurring from 2001 through 2019 August. A number of ICME properties show positive correlation with FD magnitude, with decreasing correlation strength as NM cutoff rigidity increases.
- ID:
- ivo://CDS.VizieR/J/ApJ/868/124
- Title:
- ICMEs events from 1998 to 2011 with ACE and WIND
- Short Name:
- J/ApJ/868/124
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present a statistical study of 219 ICMEs measured by both ACE and WIND from 1998 to 2011. ICME plasmas are defined as possessing cold materials if the carbon average charge states are lower than those of the preceding solar wind by three standard deviations and the carbon ionic temperature is lower than 106.05K. A total of 69 ICMEs were identified as containing cold materials. These ICMEs tend to have speeds in the range of 300-600km/s, with durations between 2 and 6hr. Cold materials tend to be present once or twice per ICME. We further identify two special types of cold materials: the ionic-cold type (IC) shows simultaneous lower average charge states of O, Mg, Si, and Fe ions than those of the preceding solar wind, while the carbon-only cold type (COC) shows a totally opposite trend in that these ions show higher average charge than in the preceding solar wind. We found that the IC has a higher proton temperature than the ICME mean value, whereas the COC has a lower proton temperature than the ICME mean value, and the COC is most often measured in magnetic cloud. A detailed examination of the IC and the COC material suggests that they are related to solar filaments. Their special mean charge indicates that the filaments are a mixture of coronal and choromospheric materials. Heating and collision processes beyond the carbon freeze-in height are crucial in generating the two distinct types.
