Giuliana Verbanac

On solar cycle predictions and reconstructions

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.

On long-term trends in European geomagnetic observatory biases

We investigated the European geomagnetic observatory biases over 42 years, considered as contributions of the crustal field, and generally assumed to be constant in time. To estimate these biases, we compared observatory annual means to predictions given by the continuous CM4 model, and to four other core field models for different epochs. Solar-cycle related external fields are clearly present in the residuals. Although well-known, no suitable model to minimise them exists. We found that an empirical approach, taking advantage of the homogeneity of the external influences in the European region, can minimise these influences. Their reduction is better than when the external field description included in the comprehensive CM4 model is used. At several locations clear long-term trends remain after subtraction of the core field and minimisation of external fields. We investigated whether they are due to an insufficient description of the core field secular variation by the CM4 model, or to changes in induced lithospheric fields.

Contributions of the external field to the observatory annual means and a proposal for their corrections

In this study we separate, interpret and explain magnetospheric and ionospheric signals present in the observatory annual means. The data from 46 European geomagnetic observatories collected over 42 years (1960–2001) are used. To characterise the various field components, we use predictions from latest magnetic field models. The core field and its secular variation are described by the CM4 model, and the magnetospheric contributions are successfully removed by parameterising the POMME model with the Dst index. We regard the remaining signal as being caused by ionospheric currents. The annual averages of the Sq variation estimated by the CM4 model are subtracted from the residuals. A remaining variation in anti-phase with the magnetic activity index Ap finally can be removed with a function properly scaled by Ap and Dst. We offer an objective procedure to suppress the external field contributions in the annual means to an uncertainty level of ±2 nT. Except for the ionospheric currents, this could be achieved by applying recent magnetic field models, which shows that the quality of present day models is sufficient to correct observatory data for average external field contributions. Understanding the signal contained in the annual means is a prerequisite for obtaining reliable and physically meaningful results when such data are used in studies of the core field and its secular variation.

Comparison of geoeffectiveness of coronal mass ejections and corotating interaction regions

Context. A detailed comparison of the geomagnetic responses to interplanetary coronal mass ejection (ICMEs) and corotating interaction regions (CIRs) during solar cycle 23 was performed using geomagnetic indices Dst, Ap, and AE. Aims. We aim to find out if there are relative differences in the response of various magnetospheric current systems to the impact of ICMEs and CIRs. In addition, we are exploring the possibility of forecasting geomagnetic activity using the coronagraphic observations of the ICME take-off. Methods. The peak values of the plasma characteristics of ICMEs and CIRs (velocity V, magnetic field B, and BV related to the electric field), and geomagnetic indices were investigated by applying the linear and power-law cross correlation analysis. The influence of the time-resolution on the results was performed for two time resolutions obtained by one-hour (three-hour for Ap) and six-hour data averaging. Results. For ICMEs the power-law fits are found to be important only for the relationships between BV and geomagnetic indices. For Ap and Dst, there is no difference between the one-hour (three-hour for Ap) and six-hour option. For AE, the one-hour data distribution shows more clearly the non-linear dependence on BV. Our data set shows that below BV ∼ 5 mVm−1 ICMEs have practically no geomagnetic effect at low and mid latitudes, but at high latitudes at least some geomagnetic activity will be triggered. For all HSS/CIRs dependencies, a power law is found to better describe the data than the linear fit. The data distributions show that BV has to reach ∼4 mVm−1 in order to drive at least some geomagnetic activity at all latitudes.We observed that there are fast CMEs that have almost no geomagnetic effect at low and mid latitudes. On the other hand, at high latitudes, fast CMEs always trigger some geomagnetic activity. This might be have implications for space weather forecasting. Conclusions. Magnetospheric response to both solar drivers (ICMEs and CIRs) is different at various latitudes, thus results in different development of various current systems within the Earth’s magnetosphere and ionosphere. Furthermore, we show that ICMEs and CIRs cause different geomagnetic activity. In the case of ICMEs equatorial current system responses in a linear manner, while the response of the polar-current system is likely to be non-linear. For HSS/CIRs, apparently all current systems respond in a non-linear way, especially the polar current system.

Solar wind high-speed streams and related geomagnetic activity in the declining phase of solar cycle 23

Context. Coronal holes (CHs) are the source of high-speed streams (HSSs) in the solar wind, whose interaction with the slow solar wind creates corotating interaction regions (CIRs) in the heliosphere. Aims. We investigate the magnetospheric activity caused by CIR/HSS structures, focusing on the declining phase of the solar cycle 23 (years 2005 and 2006), when the occurrence rate of coronal mass ejections (CMEs) was low. We aim to (i) perform a systematic analysis of the relationship between the CH characteristics, basic parameters of HSS/CIRs, and the geomagnetic indices Dst, Ap and AE; (ii) study how the magnetospheric/ionospheric current systems behave when influenced by HSS/CIR; (iii) investigate if and how the evolution of the background solar wind from 2005 to 2006 affected the correlations between CH, CIR, and geomagnetic parameters. Methods. The cross-correlation analysis was applied to the fractional CH area (CH) measured in the central meridian distance interval ±10 ◦ , the solar wind velocity (V), the interplanetary magnetic field (B), and the geomagnetic indices Dst, Ap ,a ndAE. Results. The performed analysis shows that Ap and AE are better correlated with CH and solar wind parameters than Dst ,a nd quantitatively demonstrates that the combination of solar wind parameters BV 2 and BV plays the central role in the process of energy transfer from the solar wind to the magnetosphere. Conclusions. We provide reliable relationships between CH properties, HSS/CIR parameters, and geomagnetic indices, which can be used in forecasting the geomagnetic activity in periods of low CME activity.

Determination of the Croatian geomagnetic observatory location

Ground survey within the Nature Park Lonjsko Polje, placed in the middle-northern Croatia was performed during the time interval 2007–2010 in order to find the best location for installing the geomagnetic observatory. The total magnetic field has been measured a few times using the Overhauser proton magnetometers. The horizontal and vertical gradients of the total field, and its temporal behaviour were investigated over the restricted region that we estimated as suitable for the observatory. The results obtained from thoroughly conducted measurements allowed us to find definitive positions for the instrument pillars. These results are in agreement with previously suggested location found based on combination of Comprehensive CM4 model prediction and measurements conducted from 2003 to 2005. This study contributes to the development of geomagnetism in Croatia and paves a way to install the first geomagnetic observatory in Croatia.