A. Hanslmeier

Hemispheric sunspot numbers

From sunspot drawings provided by the Kanzelhohe Solar Observatory, Austria, and the Skalnate Pleso Observatory, Slovak Republic, we extracted a data catalogue of hemispheric Sunspot Numbers covering the time span 1945-2004. 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. These data we then investigated with respect to north-south asymmetries for almost 6 entire solar cycles (Nos. 18-23). For all the cycles studied, we found that the asymmetry based on the absolute asymmetry index is enhanced near the cycle maximum, which contradicts to previous results that are based on the normalized asymmetry index. Moreover, the weak magnetic interdependence between the two solar hemispheres is confirmed by their self-contained evolution during a cycle. For the time span 1945-2004, we found that the cycle maxima and also the declining and increasing phases are clearly shifted, whereas the minima seem to be in phase for both hemispheres. The asymmetric behavior reveals no obvious connection to either the sunspot cycle period of ∼ 11 - or the magnetic cycle of ∼22-years. The most striking excess of activity is observed for the northern hemisphere in cycles 19 and 20.

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Aims. We study the possible atmospheric mass loss from 57 known transiting exoplanets around F, G, K, and M-type stars over evolutionary timescales. For stellar wind induced mass loss studies, we estimate the position of the pressure balance boundary between Coronal Mass Ejection (CME) and stellar wind ram pressures and the planetary ionosphere pressure for non- or weakly magnetized gas giants at close orbits. Methods. The thermal mass loss of atomic hydrogen is calculated by a mass loss equation where we consider a realistic heating efficiency, a radius-scaling law and a mass loss enhancement factor due to stellar tidal forces. The model takes into account the temporal evolution of the stellar EUV flux by applying power laws for F, G, K, and M-type stars. The planetary ionopause obstacle, which is an important factor for ion pick-up escape from non- or weakly magnetized gas giants is estimated by applying empirical power-laws. Results. By assuming a realistic heating efficiency of about 10–25% we found that WASP-12b may have lost about 6–12% of its mass during its lifetime. A few transiting low density gas giants at similar orbital location, like WASP-13b, WASP-15b, CoRoT-1b or CoRoT-5b may have lost up to 1–4% of their initial mass. All other transiting exoplanets in our sample experience negligible thermal loss (≤1%) during their lifetime. We found that the ionospheric pressure can balance the impinging dense stellar wind and average CME plasma flows at distances which are above the visual radius of “Hot Jupiters”, resulting in mass losses <2% over evolutionary timescales. The ram pressure of fast CMEs cannot be balanced by the ionospheric plasma pressure for orbital distances between 0.02–0.1 AU. Therefore, collisions of fast CMEs with hot gas giants should result in large atmospheric losses which may influence the mass evolution of gas giants with masses <MJup. Depending on the stellar luminosity spectral type, planetary density, heating efficiency, orbital distance, and the related Roche lobe effect, we expect that at distances between 0.015–0.02 AU, Jupiter-class and sub-Jupiter-class exoplanets can lose several percent of their initial mass. At orbital distances ≤0.015 AU, low density hot gas giants in orbits around solar type stars may even evaporate down to their coresize, while low density Neptune-class objects can lose their hydrogen envelopes at orbital distances ≤0.02 AU.

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A statistical analysis of almost 50 000 soft X-ray (SXR) flares observed by GOES during the period 1976–2000 is presented. On the basis of this extensive data set, statistics on temporal properties of soft X-ray flares, such as duration, rise and decay times with regard to the SXR flare classes is presented. Correlations among distinct flare parameters, i.e. SXR peak flux, fluence and characteristic times, and frequency distributions of flare occurrence as function of the peak flux, the fluence and the duration are derived. We discuss the results of the analysis with respect to statistical flare models, the idea of coronal heating by nanoflares, and elaborate on implications of the obtained results on the Neupert effect in solar flares.

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Sunspot drawings are provided on a regular basis at the Kanzelhoehe Solar Observatory, Austria, and the derived relative sunspot numbers are reported to the Sunspot Index Data Center in Brussels. From the daily sunspot drawings, we derived the northern, R_n, and southern, R_s, relative sunspot numbers for the time span 1975-2000. In order to accord with the International Sunspot Numbers R_i, the R_n and R_s have been normalized to the R_i, which ensures that the relation R_n + R_s = R_i is fulfilled. For validation, the derived R_n and R_s are compared to the international northern and southern relative sunspot numbers, which are available from 1992. The regression analysis performed for the period 1992-2000 reveals good agreement with the International hemispheric Sunspot Numbers. The monthly mean and the smoothed monthly mean hemispheric Sunspot Numbers are compiled into a catalogue. Based on the derived hemispheric Sunspot Numbers, we study the significance of N-S asymmetries and the rotational behavior separately for both hemispheres. We obtain that about 60% of the monthly N-S asymmetries are significant at a 95% level, whereas the relative contributions of the northern and southern hemisphere are different for different cycles. From the analysis of power spectra and autocorrelation functions, we derive a rigid rotation with about 27 days for the northern hemisphere, which can be followed for up to 15 periods. Contrary to that, the southern hemisphere reveals a dominant period of about 28 days, whereas the autocorrelation is strongly attenuated after 3 periods. These findings suggest that the activity of the northern hemisphere is dominated by an active zone, whereas the southern activity is mainly dominated by individual long-lived sunspot groups.

from 1945–2004: catalogue and N-S asymmetry analysis for solar cycles 18–23

Three traveling disturbances recorded in the absorption line of Helium I at 10830 A (HeI), analogous to HαMoreton waves, are analyzed. The morphology and kinematics of the wavefronts are described in detail. The HeI wave appears as an expanding arc of increased absorption roughly corresponding to the Hα disturbance, although not as sharply defined. HeI perturbations consist of a relatively uniform diffuse component and a patchy one that appears as enhanced absorption in HeI mottles. It leads the Hα front by some 20 Mm and can be followed to considerably larger distances than in Hα observations. Behind the front stationary areas of reduced HeI absorption develop, resembling EUV coronal dimming. The observed HeI as well as the Hα disturbances show a deceleration of the order of 100-1000 ms −2 . Moreover, in the event where Hα ,H eI, and EUV wavefronts are observed, all of them follow closely related kinematical curves, indicating that they are a consequence of a common disturbance. The analysis of spatial perturbation profiles indicates that HeI disturbances consist of a forerunner and a main dip,the latterbeing cospatial withthe Hαdisturbance. The properties and behavior of the wavefronts can be comprehended as a consequence of a fast-mode MHD coronal shock whose front is weakly inclined to the solar surface. The Hα disturbance and the main HeI dip are a consequence of the pressure jump in the corona behind the shock front. The HeI forerunner might be caused by thermal conduction from the oblique shock segments ahead of the shock-chromosphere intersection, or by electron beams accelerated in the quasi-perpendicular section of the shock.

Determining the mass loss limit for close-in exoplanets: what can we learn from transit observations?

A 90 minute time series of high spatial resolution white-light images of solar granulation, obtained at the Swedish Vacuum Solar Tower (Observatorio del Roque de los Muchachos, La Palma), was analyzed to study how the physical properties of the granules changed with size. The observational material was corrected for global motions and for the instrumental profile, and a subsonic filter was applied. A definition of granular border was adopted using the inflection points of the intensity of the images, and the granular cells were defined as areas including, in addition to the granules, one-half of their surrounding intergranular lanes. Using time series to investigate the average behavior of solar granulation has three strong advantages: the first is the possibility of removing the acoustic waves; second, the possibility of estimating the effect of the variability of seeing on our results; and, third, the opportunity to attain high statistical significance in the analysis as a result of the large number of extracted granules (61,138). It is shown that the granules of the sample can be classified according to their mean and maximum intensities and their fractal dimension into two regimes, with diameters smaller than and larger than 14, respectively. A broad transition region in which both regimes coexist was found. The resolved internal brightness structure of both the granules and the intergranular lanes shows a linear increase of the number of substructures with the granular and intergranular areas. The diameters of these substructures range between our effective resolution limit (~03) and ~15, with preferential sizes at 065 and 055, respectively. Moreover, it seems that large and small granules are unevenly distributed with respect to the large-scale vertical flows. Thus smaller granules are more concentrated along downdrafts whereas larger ones preferentially occupy the updrafts. Finally, a physical scenario compatible with the existence of these two granular populations is discussed.

Temporal aspects and frequency distributions of solar soft x-ray flares

A statistical analysis of a large data set of H-alpha flares comprising almost 100000 single events that occurred during the period January 1975 to December 1999 is presented. We analyzed the flares evolution steps, i.e. duration, rise times, decay times and event asymmetries. Moreover, these parameters characterizing the temporal behavior of flares, as well as the spatial distribution on the solar disk, i.e. N-S and E-W asymmetries, are analyzed in terms of their dependency on the solar cycle. The main results are: 1) The duration, rise and decay times increase with increasing importance class. The increase is more pronounced for the decay times than for the rise times. The same relation is valid with regard to the brightness classes but in a weaker manner. 2) The event asymmetry indices, which characterize the proportion of the decay to the rise time of an event, are predominantly positive (90%). For about 50% of the events the decay time is even more than 4 times as long as the rise time. 3) The event asymmetries increase with the importance class. 4) The flare duration and decay times vary in phase with the solar cycle; the rise times do not. 5) The event asymmetries do not reveal a distinct correlation with the solar cycle. However, they drop during times of solar minima, which can be explained by the shorter decay times found during minimum activity. 6) There exists a significant N-S asymmetry over longer periods, and the dominance of one hemisphere over the other can persist for more than one cycle. 7) For certain cycles there may be evidence that the N-S asymmetry evolves with the solar cycle, but in general this is not the case. 8) There exists a slight but significant E-W asymmetry with a prolonged eastern excess.

Hemispheric Sunspot Numbers

The properties of the evolution of solar granulation have been studied using an 80 minute time series of high spatial resolution white-light images obtained with the Swedish Vacuum Solar Telescope at the Observatorio del Roque de los Muchachos, La Palma. An automatic tracking algorithm has been developed to follow the evolution of individual granules, and a sample of 2643 granules has been analyzed. To check the reliability of this automatic procedure, we have manually tracked a sample of 481 solar granules and compared the results of both procedures. An exponential law gives a good fit to the distribution of granular lifetimes, T. Our estimated mean lifetime is about 6 minutes, which is at the lower limit of the ample range of values reported in the literature. We note a linear increase in the time-averaged granular sizes and intensities with the lifetime. T=12 minutes marks a sizeable change in the slopes of these linear trends. Regarding the location of granules with respect to the meso- and supergranular flow field, we find only a small excess of long-lived granules in the upflows. Fragmentation, merging, and emergence from (or dissolution into) the background are the birth and death mechanisms detected, resulting in nine granular families from the combination of these six possibilities. A comparative study of these families leads to the following conclusions: (1) fragmentation is the most frequent birth mechanism, while merging is the most frequent death mechanism; (2) spontaneous emergence from the background occurs very rarely, but dissolution into the background is much more frequent; and (3) different granular mean lifetimes are determined for each of these families; the granules that are born and die by fragmentation have the longest mean lifetime (9.23 minutes). From a comparison of the evolution of granules belonging to the most populated families, two critical values appear for the initial area in a granular evolution: 0.8 Mm2 (dg=139) and 1.3 Mm2 (dg=177). These values mark limits characterizing the birth mechanism of a granule, and predict its evolution to some extent. The findings of the present work complement the earlier results presented in this series of papers and reinforce with new inputs, as far as the evolutionary aspects are concerned, the conclusion stated there that granules can be classified into two populations with different underlying physics. The boundary between these two classes could be established at the scale of dg=14.

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Abstract : Generally, there are two procedures for solar cycle predictions: the empirical methods - statistical methods based on extrapolations and precursor methods - and methods based on dynamo models. Aims. The goal of the present analysis is to forecast the strength and epochs of the next solar cycle, to investigate proxies for grand solar minima and to reconstruct the relative sunspot number in the Maunder minimum. Methods. We calculate the asymmetry of the ascending and descending solar cycle phases (Method 1) and use this parameter as a prow for solar activity on longer time scales. Further, we correlate the relative sunspot numbers in the epochs of solar activity minima and maxima (Method 2) and estimate the parameters of an autoregressive moving average model (ARMA, Method 3). Finally, the power spectrum of data obtained with the Method 1 is analysed and the Methods 1 and 3 are combined. Results. Signatures of the Maunder. Dalton and Gleissberg minima were found with Method 1. A period of about 70 years, somewhat shorter than the Gleissberg period was identified in the asymmetry data. The maximal smoothed monthly sunspot number during the Maunder minimum was reconstructed and found to be in the range 0-35 (Method 1). The estimated Wolf number (also called the relative sunspot number) of the next solar maximum is in the range 88-102 (Method 2). Method 3 predicts the next solar maximum between 2011 and 2012 and the next solar minimum for 2017. Also, it forecasts the relative sunspot number in the next maximum to be 90 +/- 27. A combination of the Methods 1 and 3 gives for the next solar maximum relative sunspot numbers between 78 and 99. Conclusions. The asymmetry parameter provided by Method 1 is a good proxy for solar activity in the past, also in the periods for which no relative sunspot numbers are available. Our prediction for the next solar cycle No. 24 is that it will be weaker than the last cycle, No. 23. This prediction is based on various independent methods.

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We discuss the evolution of the atmosphere of early Earth and of terrestrial exoplanets which may be capable of sustaining liquid water oceans and continents where life may originate. The formation age of a terrestrial planet, its mass and size, as well as the lifetime in the EUV-saturated early phase of its host star play a significant role in its atmosphere evolution. We show that planets even in orbits within the habitable zone of their host stars might not lose nebular- or catastrophically outgassed initial protoatmospheres completely and could end up as water worlds with CO2 and hydrogen- or oxygen-rich upper atmospheres. If an atmosphere of a terrestrial planet evolves to an N2-rich atmosphere too early in its lifetime, the atmosphere may be lost. We show that the initial conditions set up by the formation of a terrestrial planet and by the evolution of the host star’s EUV and plasma environment are very important factors owing to which a planet may evolve to a habitable world. Finally we present a method for studying the discussed atmosphere evolution hypotheses by future UV transit observations of terrestrial exoplanets.