K. Mursula

University of Oulu, Uleoborg, Northern Ostrobothnia, Finland

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Publications (323)531.96 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Aims. The present work is the first in a series of articles that develop a new proxy to represent the evolution of magnetic activity in past solar cycles by combining the information from historical Ca II K line spectroheliograms and sunspot magnetic field measurements. Methods. We use synoptic (Carrington) maps in 1915–1985 derived from daily Ca K line observations at Mount Wilson Observatory to identify the chromospheric plages, and to create synoptic pseudo-magnetograms. We use historical observations of sunspot magnetic fields from 1917–present to assign polarity to pixels situated within plages. The original Ca K spectroheliograms are non-uniform in their brightness, and we develop a novel approach to re-normalize their intensities. Results. We show that using a combination of sunspot field measurements and the plages with renormalized intensities one can successfully construct a homogeneous long-term series of pseudo-magnetograms. In our tests, about 80% of pixels situated within plages showed the same magnetic polarity as the synoptic magnetograms taken at Kitt Peak Vacuum Telescope. Finally, we discuss possible approaches that one can take to further improve the agreement between the observed and pseudo-magnetograms.
    No preview · Article · Jan 2016 · Astronomy and Astrophysics
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    ABSTRACT: Although the sunspot-number series have existed since the mid-19th century, they are still the subject of intense debate, with the largest uncertainty being related to the "calibration" of the visual acuity of individual observers in the past. Daisy-chain regression methods are applied to inter-calibrate the observers which may lead to significant bias and error accumulation. Here we present a novel method to calibrate the visual acuity of the key observers to the reference data set of Royal Greenwich Observatory sunspot groups for the period 1900-1976, using the statistics of the active-day fraction. For each observer we independently evaluate their observational thresholds [S_S] defined such that the observer is assumed to miss all of the groups with an area smaller than S_S and report all the groups larger than S_S. Next, using a Monte-Carlo method we construct, from the reference data set, a correction matrix for each observer. The correction matrices are significantly non-linear and cannot be approximated by a linear regression or proportionality. We emphasize that corrections based on a linear proportionality between annually averaged data lead to serious biases and distortions of the data. The correction matrices are applied to the original sunspot group records for each day, and finally the composite corrected series is produced for the period since 1748. The corrected series displays secular minima around 1800 (Dalton minimum) and 1900 (Gleissberg minimum), as well as the Modern grand maximum of activity in the second half of the 20th century. The uniqueness of the grand maximum is confirmed for the last 250 years. It is shown that the adoption of a linear relationship between the data of Wolf and Wolfer results in grossly inflated group numbers in the 18th and 19th centuries in some reconstructions.
    No preview · Article · Dec 2015 · Solar Physics
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    Lauri Holappa · Kalevi Mursula
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    ABSTRACT: For the time before the space era our knowledge of the centennial evolution of solar wind (SW) and interplanetary magnetic field (IMF) is based on proxies derived from geomagnetic indices. The reliability of these proxies is dependent on the homogeneity of magnetic field data. In this paper we study the interhourly (IHV) and interdiurnal (IDV1d) variability indices calculated from the data of two British observatories, Eskdalemuir and Lerwick, and compare them to the corresponding indices of the German Niemegk observatory. We find an excess of about 14 ± 4% (5.8 ± 2%) and 27 ± 10% (15 ± 6%) in the IHV(IDV1d) in the indices of Eskdalemuir and Lerwick in 1935-1969. The timing of this excess accurately coincides with instrument changes made in these observatories, strongly supporting the interpretation that the excess is indeed caused by instrument related inhomogeneities in the data of Eskdalemuir and Lerwick. We show that the detected excess notably modifies the long-term trend of geomagnetic activity and the centennial evolution of IMF strength and solar wind speed estimated using these indices. We note that the detected inhomogeneity problem may not be limited to the data of the two studied observatories, but may be quite common to long series of geomagnetic measurements. These results question the reliability of the present measures of the centennial change in solar wind speed and IMF.
    Full-text · Article · Oct 2015 · Journal of Geophysical Research: Space Physics
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    ABSTRACT: We present here an evidence that cosmogenic 7Be isotopes produced in the lower stratosphere were measured in near-ground air at Rio de Janeiro, Brazil, after the southern hemispheric Sudden Stratospheric Warming (SSW) of 2002. The analysis presented here is based on a comparison of 7Be data measured around Angra Nuclear Power Station (23°S 44°W) during the last three decades and a model estimate of the near-ground air 7Be concentration using the CRAC:7Be model of cosmogenic production together with a simplified model for atmospheric 7Be deposition that assimilates the regional precipitation data. Our results indicate that an anomalous stratosphere–troposphere coupling associated to the unique SSW of 2002 allowed stratospheric aerosols carrying 7Be to reach the ground level very quickly. This methodology points to an important use of 7Be as a quantitative tracer for stratospheric influence on near-ground air patterns.
    No preview · Article · Jul 2015 · Atmospheric Environment
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    Agnieszka Gil · Kalevi Mursula
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    ABSTRACT: After the sunspot maximum and the reversal of solar polarity in 2014, the Sun is now in the early declining phase of cycle 24. Soon after the polarity reversal, the galactic cosmic ray intensity, as observed, e.g., by neutron monitors at several latitudes (cutoff rigidities) depict an exceptionally large variation at the solar rotation period. This recurrence started in mid-2014 and continues until now (the first half of March 2015). Several parameters characterizing solar activity, like sunspots and F10.7 radio flux, also depict similar enhanced variability, which started slightly earlier than in neutron monitors. Some solar wind properties also show this periodicity, although less systematically and for a shorter time. This excessively strong periodicity in GCR can be related to the rapid growth of an asymmetric polar coronal hole in the southern hemisphere, leading to a very asymmetric magnetic configuration at mid-to high heliospheric latitudes. This also leads to the fact that the tilt angle of the heliospheric current sheet is more changeable, during this cycle than at similar early declining phases of the previous solar cycles.
    Full-text · Conference Paper · Jul 2015
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    ABSTRACT: Aims: Although the time of the Maunder minimum (1645--1715) is widely known as a period of extremely low solar activity, claims are still debated that solar activity during that period might still have been moderate, even higher than the current solar cycle #24. We have revisited all the existing pieces of evidence and datasets, both direct and indirect, to assess the level of solar activity during the Maunder minimum. Methods: We discuss the East Asian naked-eye sunspot observations, the telescopic solar observations, the fraction of sunspot active days, the latitudinal extent of sunspot positions, auroral sightings at high latitudes, cosmogenic radionuclide data as well as solar eclipse observations for that period. We also consider peculiar features of the Sun (very strong hemispheric asymmetry of sunspot location, unusual differential rotation and the lack of the K-corona) that imply a special mode of solar activity during the Maunder minimum. Results: The level of solar activity during the Maunder minimum is reassessed on the basis of all available data sets. Conclusions: We conclude that solar activity was indeed at an exceptionally low level during the Maunder minimum. Although the exact level is still unclear, it was definitely below that during the Dalton minimum around 1800 and significantly below that of the current solar cycle #24. Claims of a moderate-to-high level of solar activity during the Maunder minimum are rejected at a high confidence level.
    Full-text · Article · Jul 2015 · Astronomy and Astrophysics
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    ABSTRACT: A solid understanding of the solar wind control of ground magnetic field disturbances is essential for utilizing the existing long time series of ground data to obtain information on solar wind-magnetosphere-ionosphere coupling. We have used 20 years of International Monitor for Auroral Geomagnetic Effects magnetometer data (54°–76° magnetic latitude) to study the solar wind control of the ionospheric equivalent current density and its time derivative (<|dJeq/dt|>). We found that <|dJeq/dt|> peaks at the premidnight and prenoon ends of the westward electrojet. The prenoon <|dJeq/dt|> peak was most intense during fast solar wind and radial interplanetary magnetic field (IMF). The location of the peak was not affected by the IMF orientation but persisted at 8–10 magnetic local time and 70°–75° latitude, near the boundary between the westward and eastward electrojets. Sensitivity of this boundary to disturbances was suggested as a possible explanation for the persistent prenoon location of the peak. The premidnight peak was most intense during southward IMF orientation. While faster solar wind mainly resulted in more intense <|dJeq/dt|> in the premidnight sector, stronger IMF caused the region of intense <|dJeq/dt|> to spread to the postmidnight, dawn, and dusk sectors. A good correspondence was found between development of the nightside <|dJeq/dt|> intensification and average substorm bulge and oval aurora as determined by Gjerloev et al. (2007). The bulge aurora covered the western end of the westward electrojet where the equivalent current also had a significant poleward component. The substorm oval aurora, on the other hand, extended eastward along the westward electrojet.
    No preview · Article · May 2015 · Journal of Geophysical Research: Space Physics
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    Kalevi Mursula · Renata Lukianova · Lauri Holappa
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    ABSTRACT: In the declining phase of the solar cycle, when the new-polarity fields of the solar poles are strengthened by the transport of same-signed magnetic flux from lower latitudes, the polar coronal holes expand and form non-axisymmetric extensions toward the solar equator. These extensions enhance the occurrence of high-speed solar wind streams (HSS) and related co-rotating interaction regions in the low-latitude heliosphere, and cause moderate, recurrent geomagnetic activity in the near-Earth space. Here, using a novel definition of geomagnetic activity at high (polar cap) latitudes and the longest record of magnetic observations at a polar cap station, we calculate the annually averaged solar wind speeds as proxies for the effective annual occurrence of HSS over the whole Grand Modern Maximum (GMM) from 1920s onwards. We find that a period of high annual speeds (frequent occurrence of HSS) occurs in the declining phase of each solar cycle 16-23. For most cycles the HSS activity clearly maximizes during one year, suggesting that typically only one strong activation leading to a coronal hole extension is responsible for the HSS maximum. We find that the most persistent HSS activity occurred in the declining phase of solar cycle 18. This suggests that cycle 19, which marks the sunspot maximum period of the GMM, was preceded by exceptionally strong polar fields during the previous sunspot minimum. This gives interesting support for the validity of solar dynamo theory during this dramatic period of solar magnetism.
    Full-text · Article · Mar 2015 · The Astrophysical Journal
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    D. Martini · K. Mursula · M. Orispää · H.‐J. Linthe
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    ABSTRACT: We investigate geomagnetic activity at two high-latitude, two mid-latitude and two low-latitude stations of the northern hemisphere, and find that the increasing trend of geomagnetic activity in 1914-2000 is considerably lower at the two mid-latitude stations (Niemegk and Fredericksburg) than at low- or high-latitude stations. As Niemegk occupies a specific position among geomagnetic stations, serving as the standard station for the K/Ak index derivation, it is crucial to understand the origin and long-term characteristics of this difference. We show here that geomagnetic activity at the studied mid-latitude stations is relatively stronger in the declining phases of the 1-2 first solar cycles (15 and 16) than elsewhere, leading to the smaller long-term trend. We also find that the latitudinal differences in the trends are strongly dependent on local time, being considerably larger in the dawn sector. These differences can be explained by the relatively stronger contribution of high speed streams to geomagnetic activity at the particular range of mid-latitudes, compared to the stations at lower and higher latitudes.
    Full-text · Article · Mar 2015 · Journal of Geophysical Research: Space Physics
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    L. Zhang · K. Mursula · I. Usoskin
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    ABSTRACT: Context. The relation between solar surface rotation and sunspot activity still remains open. Sunspot activity has dramatically reduced in solar cycle 24 and several solar activity indices and flux measurements experienced unprecedentedly low levels during the last solar minimum. Aims. We aim to reveal the momentary variation of solar surface rotation, especially during the recent years of reducing solar activity. Methods. We used a dynamic, differentially rotating reference system to determine the best-fit annual values of the differential rotation parameters of active longitudes of solar X-ray flares and sunspots in 1977-2012. Results. The evolution of rotation of solar active longitudes obtained with X-ray flares and with sunspots is very similar. Both hemispheres speed up since the late 1990s, with the southern hemisphere rotating slightly faster than the north. Earlier, in 1980s, rotation in the northern hemisphere was considerably faster, but experienced a major decrease in the early 1990s. On the other hand, little change was found in the southern rotation during these decades. This led to a positive asymmetry in north-south rotation rate in the early part of the time interval studied. Conclusions. The rotation of both hemispheres has been speeding up at roughly the same rate since late 1990s, with the southern hemisphere rotating slightly faster than the north. This period coincides with the start of dramatic weakening of solar activity, as observed in sunspots and several other solar, interplanetary and geomagnetic parameters.
    Full-text · Article · Jan 2015 · Astronomy and Astrophysics
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    Lauri Holappa · Kalevi Mursula · Timo Asikainen
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    ABSTRACT: In this paper, we study two sets of local geomagnetic indices from 26 stations using the principal component (PC) and the independent component (IC) analysis methods. We demonstrate that the annually averaged indices can be accurately represented as linear combinations of two first components with weights systematically depending on latitude. We show that the annual contributions of coronal mass ejections (CMEs) and high speed streams (HSSs) to geomagnetic activity are highly correlated with the first and second IC. The first and second ICs are also found to be very highly correlated with the strength of the interplanetary magnetic field (IMF) and the solar wind speed, respectively, because solar wind speed is the most important parameter driving geomagnetic activity during HSSs while IMF strength dominates during CMEs. These results help in better understanding the long-term driving of geomagnetic activity and in gaining information about the long-term evolution of solar wind parameters and the different solar wind structures.
    Full-text · Article · Dec 2014 · Journal of Geophysical Research: Space Physics
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    V. Maliniemi · T. Asikainen · K. Mursula
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    ABSTRACT: Several recent studies have found variability in the Northern Hemisphere winter climate related to different parameters of solar activity. While these results consistently indicate some kind of solar modulation of tropospheric and stratospheric circulation and surface temperature, opinions on the exact mechanism and the solar driver differ. Proposed drivers include, e.g., total solar irradiance (TSI), solar UV radiation, galactic cosmic rays and magnetospheric energetic particles. While some of these drivers are difficult to distinguish because of their closely similar variation over the solar cycle, other suggested drivers have clear differences in their solar cycle evolution. For example, geomagnetic activity and magnetospheric particle fluxes peak in the declining phase of the sunspot cycle, in difference to TSI and UV radiation which more closely follow sunspots. Using 13 solar cycles (1869–2009) we study winter surface temperatures and North Atlantic oscillation (NAO) during four different phases of the sunspot cycle: minimum, ascending, maximum and declining phase. We find significant differences in the temperature patterns between the four cycle phases, which indicates a solar cycle modulation of winter surface temperatures. However, the clearest pattern of the temperature anomalies is not found during sunspot maximum or minimum, but during the declining phase, when the temperature pattern closely resembles the pattern found during positive NAO. Moreover, we find the same pattern during the low sunspot activity cycles of 100 years ago, suggesting that the pattern is largely independent of the overall level of solar activity.
    Full-text · Article · Aug 2014
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    ABSTRACT: We study the latitudinal distribution of geomagnetic activity in 1966–2009 with local geomagnetic activity indices at 26 magnetic observatories. Using the principal component analysis method we find that more than 97% of the variance in annually averaged geomagnetic activity can be described by the two first principal components. The first component describes the evolution of the global geomagnetic activity, and has excellent correlation with, e.g., the Kp/Ap index. The second component describes the leading pattern by which the latitudinal distribution of geomagnetic activity deviates from the global average. We show that the second component is highly correlated with the relative (annual) fraction of high-speed streams (HSS) in solar wind. The latitudinal distribution of the second mode has a high maximum at auroral latitudes, a local minimum at subauroral latitudes and a low maximum at mid-latitudes. We show that this distribution is related to the difference in the average location and intensity between CME and HSS-related substorms. This paper demonstrates a new way to extract useful, quantitative information about the solar wind from local indices of geomagnetic activity over a latitudinally extensive network.
    Full-text · Article · Jun 2014 · Journal of Geophysical Research: Space Physics
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    ABSTRACT: Using the software developed by us, we produced a digitized (tabulated) database of sunspot umbrae and pores observed at Mount Wilson Observatory (MWO) in 1917-2013. The database includes the heliographic coordinates, areas and the polarity and strength of magnetic fields of umbrae and pores in the MWO sunspot drawings. Using this database we study here the properties and long-term variation of sunspot umbrae and pores, separately for leading and trailing polarity spots. We find that the leading sunspots have tendency for larger umbrae and stronger magnetic field strength than the trailing spots. The average field strength and area of sunspot umbrae vary with sunspot cycle. Furthermore, the mean magnetic field strength in sunspot umbrae exhibits a gradual increase from early 1960s to 1990s. The nature of this increase is discussed.
    No preview · Article · Jun 2014 · Advances in Space Research
  • Timo Asikainen · Kalevi Mursula
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    ABSTRACT: We study the relationship between energetic 120–250 keV proton fluxes and geomagnetic Ap, AE, Dxt indices using the recently corrected measurements of the MEPED instrument onboard the low-altitude NOAA/POES satellites. Corrected database spans from 1979 to present, and allows us to reliably study the long-term variation of energetic proton fluxes over several solar cycles. Contrary to uncorrected fluxes, which can be more than an order of magnitude too low, the corrected fluxes display a systematic solar cycle variation closely resembling the variation of Ap and AE indices with a maximum in the declining solar cycle phase and a minimum in solar minimum. We also find that trapped fluxes are enhanced relative to precipitating fluxes in the declining phases and solar minima. This supports the fact that high-speed solar wind streams are the most significant driver of energetic proton fluxes. We compute the correlations between fluxes and indices in a range of time scales, and show that they are significantly improved by the flux correction. We find that precipitating fluxes correlate better than trapped fluxes with Ap/AE indices at all time scales, and the highest correlation is found with Ap. For precipitating fluxes these correlations depend weakly on time scale, but for trapped fluxes the correlation significantly increases from daily scale to solar rotation and longer time scales. Comparing the fluxes to Dxt index shows a complex relationship, where the fluxes depend not only on Dxt value but also on its time derivative.
    No preview · Article · Jun 2014 · Journal of Atmospheric and Solar-Terrestrial Physics
  • Ilpo Virtanen · Kalevi Mursula
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    ABSTRACT: The heliospheric current sheet is the continuum of the coronal magnetic equator that divides the heliospheric magnetic field into two sectors (polarities). Several recent studies have shown that the heliospheric current sheet is southward shifted during approximately 3 years in the solar declining phase (the so-called bashful ballerina phenomenon). In this article we study the hemispherical asymmetry in the photospheric and coronal magnetic fields using Wilcox Solar Observatory measurements of the photospheric magnetic field since 1976 as well as the potential field source surface model. Multipole analysis of the photospheric magnetic field shows that during the late declining phase of solar cycles since the 1970s, the "bashful ballerina phenomenon" is a consequence of the $g^{0}_{2}$ quadrupole term, signed oppositely to the dipole moment. Surges of new flux transport magnetic field from low latitudes to the poles, thus leading to a systematically varying contribution to the $g^{0}_{2}$-term from different latitudes. In the case of a north-south asymmetric flux production, this is seen as a quadrupole contribution traveling toward higher latitudes. When the quadrupole term is largest, the main contribution comes from the polar latitudes. At least during the four recent solar cycles, the $g^{0}_{2}$-term arises because the magnitude of the southern polar field is larger than the magnitude found in the north in the declining phase of the cycle. In the heliosphere this hemispheric asymmetry of the coronal fields is seen as a southward shift of the heliospheric current sheet by about 2°.
    No preview · Article · Jan 2014 · The Astrophysical Journal
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    Raisa Leussu · Ilya G. Usoskin · Rainer Arlt · Kalevi Mursula
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    ABSTRACT: Aims. Sunspot numbers form a benchmark series in many studies, but may still contain inhomogeneities and inconsistencies. In particular, an essential discrepancy exists between the two main sunspot number series, Wolf and group sunspot numbers (WSN and GSN, respectively), before 1848. The source of this discrepancy has remained unresolved so far. However, the recently digitized series of solar observations in 1825-1867 by Samuel Heinrich Schwabe, who was the primary observer of the WSN before 1848, makes such an assessment possible. Methods. We construct sunspot series, similar to WSN and GSN but using only Schwabe's data. These series, called here WSN-S and GSN-S, respectively, were compared with the original WSN and GSN series for the period 1835-1867 for possible inhomogeneities. Results. This study supports the earlier conclusions that the GSN series is more consistent and homogeneous in the earlier part than the WSN series. We show that: the GSN series is homogeneous and consistent with the Schwabe data throughout the entire studied period; the WSN series decreases by roughly 20% around 1848, which is caused by the change of the primary observer from Schwabe to Wolf and an inappropriate individual correction factor used for Schwabe in the WSN; this implies a major inhomogeneity in the WSN, which needs to be corrected by reducing its values by 20% before 1848; the corrected WSN series is in good agreement with the GSN series.
    Full-text · Article · Oct 2013 · Astronomy and Astrophysics
  • Timo Asikainen · Kalevi Mursula
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    ABSTRACT: The Medium Energy Proton and Electron Detector (MEPED) instruments onboard the NOAA/POES satellites have provided a valuable long-term database of low-altitude energetic particle observations spanning from 1978 to present. Here we study the instrumental problems of the NOAA/MEPED electron detectors and present methods to correct them. It is well known that the MEPED electron detectors are contaminated by protons of certain energy range. Using the recently corrected MEPED proton fluxes, we are now able to reliably remove this contamination. Using a simple simulation model to estimate the response of the MEPED electron detectors to incoming electrons and protons, we show that efficiencies of (Space Environment Monitors) SEM-1 and SEM-2 versions of the detectors have large differences due to different detector designs. This leads to a systematic difference between the SEM-1 and SEM-2 measurements and causes a significant long-term inhomogeneity in measured MEPED electron fluxes. Using the estimated efficiencies, we remove the proton contamination and correct the electron measurements for nonideal detector efficiency. We discuss the entire 34 year time series of MEPED measurements and show that, on an average, the correction affects different energy channels and SEM-1 and SEM-2 instruments differently. Accordingly, the uncorrected electron fluxes and electron spectra are severely distorted by nonideal detector efficiency and proton contamination, and their long-term evolution is misrepresented without the correction. The present correction of the MEPED electron fluxes over the whole interval of NOAA/POES measurements covering several solar cycles is important for long-term studies of, e.g., magnetospheric dynamics, solar activity, ionospheric research, and atmospheric effects of energetic electrons.
    No preview · Article · Oct 2013 · Journal of Geophysical Research Atmospheres
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    K. Mursula · I. Virtanen
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    ABSTRACT: Aims. The heliospheric magnetic field (HMF) has long been hemispherically asymmetric so that the field in the northern hemisphere is weaker and the area larger than in the south. This asymmetry, also called the bashful ballerina, has existed during roughly three-year intervals of the late declining to minimum phase of solar cycles 16–22. We study the HMF and its hemispheric asymmetry during the exceptional solar cycle 23. Methods. We use NASA National Space Science Data Center OMNI database, which contains all solar wind and HMF observations at the Earth’s orbit, and coronal field predictions by Wilcox Solar Observatory. We present a new method to study the global hemispheric asymmetry by using the power n of the radial decrease of the radial field from the coronal source surface to 1 AU. Results. We find that the HMF is exceptional at low latitudes in solar cycle 23: while the typical latitudinal variation was attained in the north in 2008, it did not take place in the south until Spring 2009. Thus, the Rosenberg-Coleman rule is abnormally delayed or broken for the first time in 50 years. The n-values verify the clear northern dominance in cycles 21–22. However, the low-latitude observations depict a considerably smaller asymmetry in cycle 23, although Ulysses observations at high latitudes show an equally large asymmetry in 2007 and in 1994–1995. We argue that the weak low-latitude visibility of the asymmetry in cycle 23 is due to the exceptionally weak polar fields, leading to large tilt angle and a wide current sheet. Conclusions. We note that the exceptional properties of cycle 23 (weak dynamo, large tilt, small asymmetry) agree with the long-term evolution of hemispheric asymmetry viewed at the Earth. The active Sun is seen as more asymmetric at the Earth than the quiet Sun because the polar coronal holes with unipolar fields extend closer to the equator, allowing their asymmetry to be viewed even at low latitudes. We suggest that, after the period of weak activity and small asymmetry at 1 AU that started with cycle 23, the hemispheric asymmetry will again, with the increasingly active cycles, become better visible at 1 AU but the asymmetry will be oppositely oriented, including a northward shifted current sheet, and larger areas but weaker intensities in the south. Thus, the ballerina should no longer be systematically bashful for some 100–150 years.
    Preview · Article · Aug 2013 · Astronomy and Astrophysics
  • C. Munteanu · S. Haaland · B. Mailyan · M. Echim · K. Mursula
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    ABSTRACT: We present a statistical study of the performance of three methods used to predict the propagation delay of solar wind structures. These methods are based on boundary normal estimations between the Advanced Composition Explorer (ACE) spacecraft orbiting the L1 libration point and the Cluster spacecraft near the Earth's magnetopause. The boundary normal estimation methods tested are the cross product method (CP), the minimum variance analysis of the magnetic field (MVAB), and the constrained minimum variance analysis (MVAB0). The estimated delay times are compared with the observed ones to obtain a quantitative measure of each method's accuracy. Boundary normal estimations of magnetic field structures embedded in the solar wind are known to be sensitive to small-scale fluctuations. Our study uses wavelet denoising to reduce the effect of these fluctuations. The influence of wavelet denoising on the performance of the three methods is also analyzed. We find that the free parameters of the three methods have to be adapted to each event in order to obtain accurate propagation delays. We also find that by using denoising parameters optimized to each event, 88% of our database of 356 events are estimated to arrive within ±2 min from the observed time delay with MVAB, 74% with CP, and 69% with the MVAB0 method. Our results show that wavelet denoising significantly improves the predictions of the propagation time delay of solar wind discontinuities.
    No preview · Article · Jul 2013 · Journal of Geophysical Research Atmospheres

Publication Stats

5k Citations
531.96 Total Impact Points

Institutions

  • 1-2015
    • University of Oulu
      • • Physics
      • • Sodankylä Geophysical Observatory
      Uleoborg, Northern Ostrobothnia, Finland
    • Moscow Institute of Physics and Technology
      Moskva, Moscow, Russia
  • 1982-1986
    • University of Helsinki
      • Department of Physics
      Helsinki, Uusimaa, Finland
  • 1985
    • Bielefeld University
      • Theoretical Physics
      Bielefeld, North Rhine-Westphalia, Germany
  • 1984-1985
    • Universität Bern
      • Institute for Theoretical Physics
      Berna, Bern, Switzerland