K. Mursula

Millennium-scale sunspot number reconstruction: Evidence for an unusually active sun since the 1940s

The extension of the sunspot number series backward in time is of considerable interest for dynamo theory, solar, stellar, and climate research. We have used records of the (10)Be concentration in polar ice to reconstruct the average sunspot activity level for the period between the year 850 to the present. Our method uses physical models for processes connecting the (10)Be concentration with the sunspot number. The reconstruction shows reliably that the period of high solar activity during the last 60 years is unique throughout the past 1150 years. This nearly triples the time interval for which such a statement could be made previously.

Testing the isotropic boundary algorithm method to evaluate the magnetic field configuration in the tail

Simultaneous measurements of the low-altitude energetic particle flux by NOAA spacecraft and the geostationary magnetic field by GOES 2 spacecraft are used to test the recently proposed isotropic boundary algorithm (IBA) method to evaluate the instantaneous magnetospheric configuration. According to the IBA method, the equatorward boundary of the isotropic proton precipitation, in brief the isotropic boundary (IB), corresponds to the boundary separating adiabatic and chaotic regimes of particle motion in the tail current sheet and is controlled by the properties of the equatorial magnetic field. In this study we confirm some of the fundamental features of the IBA method. First, we show that the low-altitude IB position of 30- to 300-keV protons is strongly controlled by the equatorial magnetic field in the tail. (The corresponding correlation coefficient exceeds 0.9.) Second, the MLT dependence of the nightside IB latitude is in good agreement with that computed using magnetospheric models. Third, the observed magnetic field and the field predicted by the IBA method using the measured IB position have similar values and are well correlated with a correlation coefficient of at least 0.84 for the main components and a standard deviation of only about 10% of the dynamic range of these components. This shows that the threshold condition separating the two particle motion regimes is fulfilled in the proximity of the IB field line. We argue that the remaining inconsistencies between the calculated and observed magnetic fields are mainly due to the fact that the available magnetospheric models seem to underestimate the amount of tailward stretching of both the tail field lines during active conditions as well as field lines starting from the dayside. In view of its good capabilities to remotely determine the instantaneous magnetic field, we expect that the IBA method will find wide applications in the mapping of magnetic field lines and in testing of existing and new magnetospheric models.

RAPID: The imaging energetic particle spectrometer on Cluster

The RAPID spectrometer (Research with Adaptive Particle Imaging Detectors) for the Cluster mission is an advanced particle detector for the analysis of suprathermal plasma distributions in the energy range from 20–400 keV for electrons, 40 keV–1500 keV (4000 keV) for hydrogen, and 10 keV nucl-1–1500 keV (4000 keV) for heavier ions. Novel detector concepts in combination with pin-hole acceptance allow the measurement of angular distributions over a range of 180° in polar angle for either species. Identification of the ionic component (particle mass A) is based on a two-dimensional analysis of the particles velocity and energy. Electrons are identified by the well-known energy-range relationship. Details of the detection techniques and in-orbit operations are described. Scientific objectives of this investigation are highlighted by the discussion of selected critical issues in geospace.

Observations in the vicinity of substorm onset: Implications for the substorm process

Multi-instrument data sets from the ground and satellites at both low and high altitude have provided new results concerning substorm onset and its source region in the magnetosphere. Twenty-six out of 37 substorm onset events showed evidence of azimuthally spaced auroral forms (AAFs) prior to the explosive poleward motion associated with optical substorm onset. The azimuthal wavelengths associated with these onsets were found to range between 132 and 583 km with a mean value of 307±115 km. The occurrence rate increased with decreasing wavelength down to a cutoff wavelength near 130 km. AAFs can span 8 hours of local time prior to onset and generally propagate eastward in the morning sector. Onset itself is, however, more localized spanning only about 1 hour local time. The average location of the peak intensity for 80 onsets was 65.9±3.5 CGMlat, 22.9±1.2 Mlt, whereas the average location of the AAF onsets was at 63.8±3.3 CGMlat, 22.9±1.1 Mlt. AAF onsets occur during time periods when the solar wind pressure is relatively high. These low-latitude wavelike onsets appear as precursors in the form of long-period magnetic pulsations (Pc 5 band) and frequently occur on the equatorward portion of the double oval distribution. AAFs brighten in conjunction with substorm onset leading to the conclusion that they are a growth phase activity causally related to substorm onset. Precursor activity associated with these AAFs is also seen near geosynchronous orbit altitude and examples show the relationship between the various instrumental definitions of substorm onset. The implied mode number (30 to 135) derived from this work is inconsistent with cavity mode resonances but is consistent with a modified flute/ballooning instability which requires azimuthal pressure gradients. It is suggested that this instability exists in growth phase but that an additional factor exists in the premidnight sector which results in an explosive onset. The extended source region and the distance to the open-closed field line region constrain reconnection theory and local mechanisms for substorm onset. It is demonstrated that multiple onset substorms can exist for which localized dipolarizations and the Pi 2 occur simultaneously with tail stretching existing elsewhere. Further, the tail can be less stretched at geosynchronous orbit during the optical auroral onset than during the precursor pseudobreakups. These pseudobreakups can be initiated by auroral streamers which originate at the most poleward set of arc systems and drift to the more equatorward main UV oval. Observations are presented of these AAFs in conjunction with low- and high-altitude particle and magnetic field data. These place the activations at the interface between dipolar and taillike field lines probably near the peak in the cross-tail current. These onsets are put in the context of a new scenario for substorm morphology which employs individual modules which operate independently or couple together. This allows particular substorm events to be more accurately described and investigated.

The 13.5‐day periodicity in the Sun, solar wind, and geomagnetic activity: The last three solar cycles

We make a detailed analysis of the 13.5-day periodicity of the solar chromosphere, the near-Earth solar wind, interplanetary magnetic field and geomagnetic activity during the last three solar cycles. The 13.5-day periodicity is a real quasi-periodicity whose amplitude varies sizably with time, attaining occasionally values larger than, for example, the amplitude of the 27-day periodicity. In case of heliospheric and geomagnetic variables, intervals of large 13.5-day periodicity are due to the occurrence at 1 AU of two high-speed streams per solar rotation. According to the tilted solar dipole model, such two-stream structures appear if the heliospheric current sheet is sufficiently narrow and tilted. We show that, during the main two-stream structures, the interplanetary magnetic field (IMF) indeed had a persistent two-sector structure, and the heliosheet was sizably tilted. Multiple IMF sector structure is thus excluded as the main cause for 13.5-day periodicity in solar wind and geomagnetic activity. We determine the exact time and phase (solar longitude) of all intervals of significant 13.5-day periodicity during the last three solar cycles. We find that even the longest intervals of two-stream structure (up to 2 years) consist of separate activations. Each of the main activations of the 13.5-day (as well as 27-day) periodicity has a nearly equal length of a few (about 4) solar rotations only. This gives new, interesting information about the solar dynamics related to the development of the dipole tilt. Using the phase of the main 13.5-day activations, we could determine the longitudinal position of the solar dipole tilt for all major activations. We note that this position can abruptly change by even 90 deg between two successive 13.5-day activations. For each of the three solar cycles studied, the largest two-stream structures were found in the late declining phase of the cycle. On the other hand, the main activations of the 13.5-day periodicity of solar variables, which are due to two active solar longitudes approximately 180° apart, tend to occur around solar maxima.

Ion cyclotron waves during a great magnetic storm observed by Freja double-probe electric field instrument

Evolution of the great magnetic storm in April 1993 is studied using observations of electromagnetic ion cyclotron (EMIC) waves by the F1 double-probe electric field instrument onboard the Freja satellite. The almost continuous operation of the F1 instrument in the overview mode allowed us to follow the global EMIC wave activity at low altitudes above the ionosphere during several subsequent days covering the initial (compression), main, and recovery phases of the storm. During the initial phase of the storm the spatial occurrence of EMIC waves has a postnoon high-latitude maximum, in agreement with earlier statistical results. A sudden and dramatic change of this pattern was observed with the start of the storm main phase. During the main phase, wave amplitudes were greatly enhanced and the active wave region moved to considerably lower latitudes to the late evening MLT sector. Also, the existence of heavy ions in the later main phase changed the distribution of wave frequencies dramatically. Most interestingly, a number of oxygen band EMIC waves were observed during a limited period of about 7 hours in the later main phase. The observed asymmetric MLT distribution of these oxygen band waves implies that the oxygen loss rate is faster than the drift rate. The results suggest that the EMIC waves play a crucial role in the main and early recovery phase of a great storm.

The double oval UV auroral distribution: 1. Implications for the mapping of auroral arcs

During the later stages of the auroral substorm the luminosity distribution frequently resembles a double oval, one oval lying poleward of the normal or main UV auroral oval. We interpret the double oval morphology as being due to the plasma sheet boundary layer becoming active in the later stages of the substorm process. If the disturbance engulfs the nightside low-latitude boundary layers, then the double oval configuration extends into the dayside ionospheric region. The main UV oval is associated with the inner portion of the central plasma sheet and can rapidly change its auroral character from being diffuse to discrete. This transition is associated with the substorm process and is fundamental to understanding the near-Earth character of substorm onset. On the other hand, the poleward arc system in the nightside ionosphere occurs adjacent to or near the open-closed field line boundary. This system activates at the end of the optical expansion phase and is a part of the recovery phase configuration in substorms where it occurs. These two source regions for nightside discrete auroral arcs are important in resolving the controversy concerning the mapping of arcs to the magnetosphere. The dayside extension of this double oval configuration is also investigated and shows particle signatures which differ considerably from those on the nightside giving clues to the magnetospheric source regions of the aurora in the two local time sectors. Near-Earth substorm onsets are shown to be coupled to processes occurring much further tailward and indicate the importance of understanding the temporal development of features within the double oval. Using “variance images,” a new technique for the investigation of these dynamics is outlined.

Reconstruction of solar activity for the last millenium using 10Be data

In a recent paper (Usoskin et al. 2002a), we have reconstructed the concentration of the cosmogenic 10 Be isotope in ice cores from the measured sunspot numbers by using physical models for 10 Be production in the Earths atmosphere, cosmic ray transport in the heliosphere, and evolution of the Suns open magnetic flux. Here we take the opposite route: starting from the 10 Be concentration measured in ice cores from Antarctica and Greenland, we invert the models in order to reconstruct the 11-year averaged sunspot numbers since 850 AD. The inversion method is validated by comparing the reconstructed sunspot numbers with the directly observed sunspot record since 1610. The reconstructed sunspot record exhibits a prominent period of about 600 years, in agreement with earlier observations based on cosmogenic isotopes. Also, there is evidence for the century scale Gleissberg cycle and a number of shorter quasi-periodicities whose periods seem to fluctuate in the millennium time scale. This invalidates the earlier extrapolation of multi-harmonic representation of sunspot activity over extended time intervals.

Heliospheric modulation of cosmic rays and solar activity during the Maunder minimum

Modern models and direct cosmic ray experiments deal with heliospheric modulation of cosmic rays only during the recent times of rather high overall solar activity level. On the other hand, the question of cosmic ray modulation during the exceptional conditions of very quiet heliosphere is important. In the present paper we compare the variations of cosmic ray intensity with solar and auroral activity during the Maunder minimum (1645-1715) when the Sun was extremely quiet. We use the newly presented group sunspot number series as a measure of early solar activity, the auroral observations in central Europe as an indicator of transient phenomena in the inner heliosphere, and the radiocarbon data as a proxy of cosmic ray intensity. We find that both cosmic ray intensity and auroral activity closely follow the dominant 22-year cyclicity with sunspot activity during the Maunder minimum. Moreover, the strict antiphase between the 22-year variation of cosmic ray intensity and sunspot activity suggests that the 22-year variation in cosmic ray intensity can be explained by the diffusion-dominated terms of cosmic ray modulation without significant drift effects. We also discuss the possible origin of the behavior of the 10 Be data which is different from all other parameters during the Maunder minimum.

The 1.3-year variation in solar wind speed and geomagnetic activity

Abstract Recent studies have discovered a strong 1.3-year variation in solar wind speed. It has been shown that this variation occurs concurrently at different heliocentric distances around the ecliptic. The same periodicity has also been observed in geomagnetic activity and occurrence of aurorae which are greatly dependent on solar wind speed. We study this periodicity using solar wind speed measurements at 1 AU from 1964 onward, and the Kp index of geomagnetic activity from 1932 onward. We show that the 1.3-year variation is a quasi-periodicity which occurs during even solar cycles. On the other hand, during odd cycles, we find a somewhat longer periodicity with a period varying from 1.5-1.7 years. Both of these periodicities are expected to be due to the evolution of coronal holes. Therefore, the observed difference in period implies a difference in the evolution of coronal holes during even and odd cycles.