K. Mursula

University of Oulu, Uleoborg, Oulu, Finland

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Publications (294)430.21 Total impact

  • 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.
    01/2015;
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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.
    Journal of Geophysical Research: Space Physics 11/2014; · 3.44 Impact Factor
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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.
    Journal of Geophysical Research: Atmospheres. 08/2014;
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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.
    Journal of Geophysical Research: Space Physics 06/2014; 119(6):4544–4555. · 3.44 Impact Factor
  • 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.
    Journal of Atmospheric and Solar-Terrestrial Physics 06/2014; · 1.75 Impact Factor
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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.
    Advances in Space Research 06/2014; · 1.24 Impact Factor
  • 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°.
    The Astrophysical Journal 01/2014; 781(2):99. · 6.28 Impact Factor
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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.
    Astronomy and Astrophysics 10/2013; · 4.48 Impact Factor
  • 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.
    Journal of Geophysical Research Atmospheres 10/2013; 118:6500–6510. · 3.44 Impact Factor
  • 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.
    Astronomy and Astrophysics 08/2013; 525(L12). · 4.48 Impact Factor
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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.
    Journal of Geophysical Research Atmospheres 07/2013; 118(7):3985-3994. · 3.44 Impact Factor
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    ABSTRACT: The Sun's long-term magnetic variability is the primary driver of space climate. This variability is manifested not only in the long-observed and dramatic change of magnetic fields on the solar surface, but also in the changing solar radiative output across all wavelengths. The Sun's magnetic variability also modulates the particulate and magnetic fluxes in the heliosphere, which determine the interplanetary conditions and impose significant electromagnetic forces and effects upon planetary atmospheres. All these effects due to the changing solar magnetic fields are also relevant for planetary climates, including the climate of the Earth. The ultimate cause of solar variability, at time scales much shorter than stellar evolutionary time scales, i.e., at decadal to centennial and, maybe, even millennial or longer scales, has its origin in the solar dynamo mechanism. Therefore, in order to better understand the origin of space climate, one must analyze different proxies of solar magnetic variability and develop models of the solar dynamo mechanism that correctly produce the observed properties of the magnetic fields. This Preface summarizes the most important findings of the papers of this Special Issue, most of which were presented in the Space Climate-4 Symposium organized in 2011 in Goa, India.
    Journal of Space Weather and Space Climate. 06/2013;
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    ABSTRACT: Samuel Heinrich Schwabe made 8486 drawings of the solar disk with sunspots in the period from November 5, 1825 to December 29, 1867. We have measured sunspot sizes and heliographic positions on digitized images of these drawings. A total of about 135,000 measurements of individual sunspots are available in a data base. Positions are accurate to about 5% of the solar radius or to about three degrees in heliographic coordinates in the solar disk center. Sizes were given in 12 classes as estimated visually with circular cursor shapes on the screen. Most of the drawings show a coordinate grid aligned with the celestial coordinate system. A subset of 1168 drawings have no indication of their orientation. We have used a Bayesian estimator to infer the orientations of the drawings as well as the average heliographic spot positions from a chain of drawings of several days, using the rotation profile of the present Sun. The data base also includes all information available from Schwabe on spotless days.
    Monthly Notices of the Royal Astronomical Society 05/2013; 433(4). · 5.23 Impact Factor
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    Journal of Geophysical Research Atmospheres 05/2013; 118. · 3.44 Impact Factor
  • L. Zhang, K. Mursula, I. Usoskin
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    ABSTRACT: Context. Solar active longitudes and their rotation have been studied for a long time using various forms of solar activity. However, the results on the long-term evolution of rotation rates and the hemispheric asymmetry obtained by earlier authors differ significantly from each other. Aims: We aim to find a consistent result on the long-term migration of active longitudes of sunspots in 1877-2008 separately for the two hemispheres. Methods: We used a dynamic, differentially rotating reference system to determine the best-fit values of the differential rotation parameters of active longitudes for each year in 1877-2008. With these parameters we determined the momentary rotation rates at the reference latitude of 17° and calculated the non-axisymmetries of active longitudes. We repeated this with five different fit intervals and two weighting methods and compared the results. Results: The evolution of solar surface rotation in each hemisphere suggests a quasi-periodicity of about 80-90 years. The long-term variations of solar rotation in the northern and southern hemisphere have a close anti-correlation, leading to a significant 80-90-year quasi-periodicity in the north-south asymmetry of solar rotation. The north-south asymmetry of solar rotation is found to have an inverse relationship with the area of large sunspots. The latitudinal contrast of differential rotation is also found to be anti-correlated with the sunspot area. Different fit and weight methods yield similar results. Conclusions: Our results give strong evidence for the anti-correlation of the rotation of the two solar hemispheres. The long-term oscillation of solar rotation suggests that a systematic interchange of angular momentum takes place between the two hemispheres at a period of about 80-90 years.
    Astronomy and Astrophysics 04/2013; · 4.48 Impact Factor
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    ABSTRACT: Estimations of boundary normals of solar wind discontinuities are known to be affected by small-scale fluctuations. Wavelet denoising is used to reduce the influence of these fluctuations. Minimum variance analysis of the magnetic field (MVAB) is used to predict the propagation time delay of solar wind structures between the Advanced Composition Explorer (ACE) spacecraft orbiting the L1 libration point and the Cluster spacecraft near the Earth's magnetopause. The estimated delay times are compared with the observed ones to obtain a quantitative measure of the method's accuracy. The influence of wavelet denoising on the performance of MVAB estimations is discussed and analysed. We investigate the effects of the length of the analysis time interval on the accurate estimation of the propagation time delays by the MVAB method. We find that wavelet denoising of the time series prior to application of MVAB can improve significantly the prediction accuracy. Improved results are obtained when the parameters of the wavelet denoising (e.g. denoising threshold, wavelet basis) are adapted to each event individually. The results show that when all optimisations are applied the difference between the real (observed) time delay and the estimation provided by MVAB is less than 2 minutes for almost 90% of the 356 discontinuities from our database.
    04/2013;
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    ABSTRACT: Here we compare the traditional analog measure of geomagnetic activity, Ak, with the more recent digital indices of IHV and Ah based on hourly mean data, and their derivatives at the auroral station Sodankylä. By this selection of indices we study the effects of (i) analog vs. digital technique, and (ii) full local-time vs. local night-time coverage on quantifying local geomagnetic activity. We find that all other indices are stronger than Ak during the low-activity cycles 15–16 suggesting an excess of very low scalings in Ak at this time. The full-day indices consistently depict stronger correlation with the interplanetary magnetic field strength, while the night-time indices have higher correlation with solar wind speed. The Ak index correlates better with the digital indices of full-day coverage than with any night-time index. However, Ak depicts somewhat higher activity levels than the digital full-day indices in the declining phase of the solar cycle, indicating that, due to their different sampling rates, the latter indices are less sensitive to high-frequency variations driven by the Alfvén waves in high-speed streams. On the other hand, the night-time indices have an even stronger response to solar wind speed than Ak. The results strongly indicate that at auroral latitudes, geomagnetic indices with different local time coverage reflect different current systems, which, by an appropriate choice of indices, allows studying the century-scale dynamics of these currents separately.
    Advances in Space Research 09/2012; 50(6):690–699. · 1.24 Impact Factor
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    T. Asikainen, K. Mursula, V. Maliniemi
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    ABSTRACT: We report here on new problems in the NOAA/MEPED instruments and the related energetic proton dataset. These problems are solved, and the implied modifications to the earlier recalibration of the dataset are evaluated and adopted. We show that, besides degrading due to radiation damage, the NOAA-12 and NOAA-08 satellites suffer from increased electronic noise in the back detector of the proton instrument. We correct the effects of the noise, and present improved estimates for the effective energy thresholds of the MEPED proton detectors which are now determined more robustly than previously. We show that the cumulative Ap index can be used to produce a refined estimate for the temporal evolution of the effective MEPED energy thresholds. The derived energy thresholds of all MEPED instruments increase systematically with the cumulative particle fluxes, and this increase is similar in all satellites. Using the improved energy thresholds we obtain a uniform series of corrected MEPED energetic proton fluxes above 120 keV from 1979 onwards. We find that, due to the effect of the radiation damage and noise, the uncorrected fluxes at these energies were underestimated in the worst case by more than an order of magnitude, and that the earlier correction method also occasionally led to underestimation of the fluxes by nearly an order of magnitude. Such underestimation becomes severe already 1-2 years after the launch of the satellite.
    Journal of Geophysical Research Atmospheres 09/2012; 117(A9):9204-. · 3.44 Impact Factor
  • Ilpo Virtanen, Kalevi Mursula
    07/2012;
  • T. Asikainen, V. Maliniemi, K. Mursula
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    ABSTRACT: When studying the effects of geomagnetic variability on the Earth's atmosphere reliable sources of quality long term data are essential. Most past studies concerning the effects of energetic particles precipitating from the magnetosphere into the atmosphere have been forced to use geomagnetic activity indices such as Kp (and Ap) as proxies for the intensity of this particle precipitation. Despite their good long term coverage these indices are only a crude proxy for the particle fluxes. Accordingly, there is a great need for a quality long term dataset of precipitating energetic particles. The polar orbiting NOAA/POES satellites have measured energetic particles with their MEPED instrument nearly continuously for more than 30 years. Up to now the reliable usage of the data has been severely restricted by several instrumental problems including degradation due to radiation damage, detector noise and contamination of electron measurements by protons. We have extensively studied these issues in the data and have now for the first time produced a new long term NOAA/MEPED database that corrects the data for all the above mentioned problems. Here we briefly review the problems that have plagued the NOAA/MEPED measurements and discuss the methods used to address them. We show how the corrections change the long term time series of the energetic particle fluxes dramatically. We will also discuss the relation between the energetic particle fluxes and geomagnetic indices to emphasize the difference between the two.
    04/2012;

Publication Stats

3k Citations
430.21 Total Impact Points

Institutions

  • 1–2014
    • University of Oulu
      • • Physics
      • • Department of Physics
      • • Sodankylä Geophysical Observatory
      Uleoborg, Oulu, Finland
    • Moscow Institute of Physics and Technology
      Moskva, Moscow, Russia
  • 2007
    • Universitatea Dunarea de Jos Galati
      Galaz, Galaţi, Romania
  • 2000–2001
    • Ioffe Physical Technical Institute
      Sankt-Peterburg, St.-Petersburg, Russia