Large Polar Coronal Holes Research Articles

The Solar Minimum Eclipse of 2019 July 2. II. The First Absolute Brightness Measurements and MHD Model Predictions of Fe x, xi, and xiv out to 3.4 R ⊙

We present the spatially resolved absolute brightness of the Fe x, Fe xi, and Fe xiv visible coronal emission lines from 1.08 to 3.4 R ⊙, observed during the 2019 July 2 total solar eclipse (TSE). The morphology of the corona was typical of solar minimum, with a dipole field dominance showcased by large polar coronal holes and a broad equatorial streamer belt. The Fe xi line is found to be the brightest, followed by Fe x and Fe xiv (in disk B ⊙ units). All lines had brightness variations between streamers and coronal holes, where Fe xiv exhibited the largest variation. However, Fe x remained surprisingly uniform with latitude. The Fe line brightnesses are used to infer the relative ionic abundances and line-of-sight-averaged electron temperature (T e ) throughout the corona, yielding values from 1.25 to 1.4 MK in coronal holes and up to 1.65 MK in the core of streamers. The line brightnesses and inferred T e values are then quantitatively compared to the Predictive Science Inc. magnetohydrodynamic model prediction for this TSE. The MHD model predicted the Fe lines rather well in general, while the forward-modeled line ratios slightly underestimated the observationally inferred T e within 5%–10% averaged over the entire corona. Larger discrepancies in the polar coronal holes may point to insufficient heating and/or other limitations in the approach. These comparisons highlight the importance of TSE observations for constraining models of the corona and solar wind formation.

Updated Measurements of Proton, Electron, and Oxygen Temperatures in the Fast Solar Wind

Abstract The high-speed solar wind is typically the simplest and least stochastic type of large-scale plasma flow in the heliosphere. For much of the solar cycle, it is connected magnetically to large polar coronal holes on the Sun’s surface. Because these features are relatively well-known (and less complex than the multiple source-regions of the slow wind), the fast wind is often a useful testing-ground for theoretical models of coronal heating. In order to provide global empirical constraints to these models, here we collect together some older and more recent measurements of the temperatures of protons, electrons, and oxygen ions as a function of radial distance.

Spatial power spectrum of the photospheric magnetic field during solar minimum

Context. During solar minima the spatial power spectrum of the photospheric magnetic field is dominated by the low-degree zonal (axisymmetric; m = 0) harmonic components, reflecting the large polar coronal holes of unipolar magnetic field. However, measuring polar fields is difficult because of the unequal visibility of the two poles during most of the year and the small line-of-sight component of the roughly radial field at high solar latitudes. Aims. In this paper we derive the spatial power spectrum of the photospheric magnetic field in terms of the harmonic coefficients of the radial component (Br) as well as in terms of the harmonic coefficients of the internal potential (known as Gauss coefficients). We calculate the zonal spatial power spectrum using Mount Wilson Observatory synoptic maps from 1995–1996, during the solar minimum between solar cycles 22 and 23, and investigate how filling or not filling the polar data gaps affects the zonal harmonic coefficients. Methods. We eliminated the vantage point effect by removing the highest 5° of the measured magnetic field and calculating the latitudinal profile of the zonal median field over the two years, which ensured equal latitudinal data coverage of both solar hemispheres. We then derived the zonal harmonic coefficients using this latitudinal profile of Br. Results. We find that when the polar data gaps are left unfilled, a strong artificial power above l = 8 is produced. Only the first five zonal harmonic coefficients can be considered reliable in this case. Therefore polar filling is essential to obtain a realistic spatial power spectrum. Filling the polar gap with a constant (non-zero) value yields zonal harmonics that are reliable up to l = 9. We find that the zonal octupole component contributes most to the total spatial power, more than the zonal dipole, even during the solar minimum conditions. This difference is seen more clearly in the case of polar filling. We also prove that the asymmetry of the polar fields during this solar minimum is statistically significant. Conclusions. Our results emphasize the importance of filling the polar data gaps in order to obtain a correct estimate of the spatial power spectrum of the photospheric field. This helps in estimating the reliability of polar fields and the large-scale structure in synoptic maps of different origin. Our results also verify the asymmetric nature of the polar fields, which is important for the heliospheric magnetic field and for solar dynamo modeling.

Spectrum analysis of short-period <i>K</i> index behaviour at high and mid-latitudes

Abstract. Geomagnetic activity levels during the declining phase and solar minimum period of the solar cycle are considerably different from those during the solar maximum phase. Previous studies revealed variations in the pattern of recurrent activity from cycle to cycle as well as variations in the average geomagnetic activity levels during a solar cycle. During the declining phase of a solar cycle (and solar minimum), the solar and interplanetary causes of geomagnetic activity are substantially different from those during the solar maximum phase. Co-rotating fast solar wind streams originating from large polar coronal holes, extending towards the Sun's equator, interact with the Earth's magnetosphere, resulting in recurrent geomagnetic activity particularly during solar cycle minimum periods. This is a well-known phenomenon with respect to 27.0- and 13.5-day recurrence geomagnetic activity, and it is well-known to be related to sectorial (non-axial) poloidal magnetic field structure in the Sun. Published results of the recent solar-cycle-23 minimum showed that the presence of 9.0- and 6.7-day recurrent geomagnetic activities can be attributed to the sectorial spherical harmonic structure present in the solar magnetic field. In this study we performed a wavelet and Lomb–Scargle analysis of the geomagnetic activity K index at Lerwick (LER), Hermanus (HER) and Canberra (CNB) for the period between 1960 and 2010, overlapping with solar cycles 20 to 23. Daily mean K indices are used to identify how several harmonics of the 27.0-day recurrent period change during each solar cycle when comparing high and mid-latitude geomagnetic activity, applying a 95% confidence level. In particular the behaviour of the second (13.5-day), third (9.0-day) and fourth (6.7-day) harmonics are investigated by doing a wavelet analysis of each individual year's K indices at each location. Results obtained show that particularly during solar minima the 27.0-day period is no longer detectable above the 95% confidence level, and that geomagnetic activity is in fact dominated by higher harmonics like 13.5-, 9.0- and 6.7-day periods. These findings in fact are in line with previous investigations and confirm the results obtained by researchers using other geomagnetic activity indices like \\textit{aa} and C9. The wavelet-spectrum analysis also reveals that during the downward phase of cycle 23 and the very long minimum of 23–24 between 2002 and 2008, the 27.0-day activity period drops below the 95% confidence level. This is confirmed by Lomb–Scargle analyses of every year's K index activity. Results obtained in this study support evidence by other investigations that this can be attributed to the lack of coronal-mass ejection (CME)-dominated solar activity during solar minima, periods characterized by strong solar dipolar magnetic fields, less sunspot numbers than at solar maxima, and multiple prominent co-rotating solar wind streams present. This analysis further confirms previous studies by other authors that the pattern of recurrent activity is dictated by the configuration of coronal holes which give rise to related high-speed streams during a solar cycle by analysing K indices at both high- and mid-latitude magnetic observatories.

THE DYNAMIC CHARACTER OF THE POLAR SOLAR WIND

The Solar and Heliospheric Observatory (SOHO) Large Angle and Spectrometric Coronagraph C2 and Solar Terrestrial Relations Observatory (STEREO) COR2A coronagraph images, when analyzed using correlation tracking techniques, show a surprising result in places ordinarily thought of as quiet solar wind above the poles in coronal hole regions. Instead of the static well-ordered flow and gradual acceleration normally expected, coronagraph images show outflow in polar coronal holes consisting of a mixture of intermittent slow and fast patches of material. We compare measurements of this highly variable solar wind from C2 and COR2A images and show that both coronagraphs measure essentially the same structures. Measurements of the mean velocity as a function of height of these structures are compared with mass flux determinations of the solar wind outflow in the large polar coronal hole regions and give similar results.

HEMISPHERIC ASYMMETRIES IN THE POLAR SOLAR WIND OBSERVED BYULYSSESNEAR THE MINIMA OF SOLAR CYCLES 22 AND 23

We examined solar wind plasma and interplanetary magnetic field (IMF) observations from Ulysses' first and third orbits to study hemispheric differences in the properties of the solar wind and IMF originating from the Sun's large polar coronal holes (PCHs) during the declining and minimum phase of solar cycles 22 and 23. We identified hemispheric asymmetries in several parameters, most notably ~15%-30% south-to-north differences in averages for the solar wind density, mass flux, dynamic pressure, and energy flux and the radial and total IMF magnitudes. These differences were driven by relatively larger, more variable solar wind density and radial IMF between ~36°S-60°S during the declining phase of solar cycles 22 and 23. These observations indicate either a hemispheric asymmetry in the PCH output during the declining and minimum phase of solar cycles 22 and 23 with the southern hemisphere being more active than its northern counterpart, or a solar cycle effect where the PCH output in both hemispheres is enhanced during periods of higher solar activity. We also report a strong linear correlation between these solar wind and IMF parameters, including the periods of enhanced PCH output, that highlight the connection between the solar wind mass and energy output and the Sun's magnetic field. That these enhancements were not matched by similar sized variations in solar wind speed points to the mass and energy responsible for these increases being added to the solar wind while its flow was subsonic.

Weaker solar wind from the polar coronal holes and the whole Sun

Observations of solar wind from both large polar coronal holes (PCHs) during Ulysses' third orbit showed that the fast solar wind was slightly slower, significantly less dense, cooler, and had less mass and momentum flux than during the previous solar minimum (first) orbit. In addition, while much more variable, measurements in the slower, in‐ecliptic wind match quantitatively with Ulysses and show essentially identical trends. Thus, these combined observations indicate significant, long‐term variations in solar wind output from the entire Sun. The significant, long‐term trend to lower dynamic pressures means that the heliosphere has been shrinking and the heliopause must be moving inward toward the Voyager spacecraft. In addition, our observations suggest a significant and global reduction in the mass and energy fed in below the sonic point in the corona. The lower supply of mass and energy may result naturally from a reduction of open magnetic flux during this period.

Slow wind and magnetic topology in the solar minimum corona in 1996–1997

This study examines the physical conditions of the outer solar corona in order to identify the regions where the slow solar wind is accelerated and to investigate the latitudinal transition from slow to fast wind during the minimum of the solar cycle. The analysis is based on observations of six streamers obtained during the years of solar minimum, 1996 and 1997, with the Ultraviolet Coronagraph Spectrometer (UVCS) onboard the Solar and Heliospheric Observatory (SOHO). The outflow velocity of the oxygen ions and the electron density of the coronal plasma are determined in altitude ranging from 1.5 to 3.5 solar radii (). The adopted diagnostic method, based on spectroscopic analysis of the O VI 1032 and 1038 Å lines, fully accounts for the large expansion factor of the magnetic field lines expected in the regions surrounding the streamers. The analysis leads to the conclusion that the slow coronal wind is observed (i) in the region external to and running along the streamer boundary; and (ii) in the region above the streamer core beyond 2.7 , where the transition between closed and open magnetic field lines takes place and the heliospheric current sheet forms. Regions in the immediate vicinity of the streamer boundary can be identified with the edges of the large polar coronal holes that characterize solar minimum. Results point to gradual variations of the properties of a coronal hole from the streamer boundary to its polar core, most likely related to the topology of the coronal magnetic field.

Properties and geoeffectiveness of magnetic clouds in the rising, maximum and early declining phases of solar cycle 23

Abstract. The magnetic structure and geomagnetic response of 73 magnetic clouds (MC) observed by the WIND and ACE satellites in solar cycle 23 are examined. The results have been compared with the surveys from the previous solar cycles. The preselected candidate MC events were investigated using the minimum variance analysis to determine if they have a flux-rope structure and to obtain the estimation for the axial orientation (θC, φC). Depending on the calculated inclination relative to the ecliptic we divided MCs into "bipolar" (θC<45°) and "unipolar" (θC>45°). The number of observed MCs was largest in the early rising phase, although the halo CME rate was still low. It is likely that near solar maximum we did not identify all MCs at 1AU, as they were crossed far from the axis or they had interacted strongly with the ambient solar wind or with other CMEs. The occurrence rate of MCs at 1AU is also modified by the migration of the filament sites on the Sun towards the poles near solar maximum and by the deflection of CMEs towards the equator due to the fast solar wind flow from large polar coronal holes near solar minimum. In the rising phase nearly all bipolar MCs were associated with the rotation of the magnetic field from the south at the leading edge to the north at the trailing edge. The results for solar cycles 21-22 showed that the direction of the magnetic field in the leading portion of the MC starts to reverse at solar maximum. At solar maximum and in the declining phase (2000-2003) we observed several MCs with the rotation from the north to the south. We observed unipolar (i.e. highly inclined) MCs frequently during the whole investigated period. For solar cycles 21-22 the majority of MCs identified in the rising phase were bipolar while in the declining phase most MCs were unipolar. The geomagnetic response of a given MC depends greatly on its magnetic structure and the orientation of the sheath fields. For each event we distinguished the effect of the sheath fields and the MC fields. All unipolar MCs with magnetic field southward at the axis were geoeffective (Dst<-50nT) while those with the field pointing northward did not cause magnetic storms at all. About half of the all identified MCs were not geoffective or the sheath fields preceding the MC caused the storm. MCs caused more intense magnetic storms (Dst<-100nT) than moderate magnetic storms (-50nT ≥Dst≥-100nT).

The heliospheric modulation of 3–10 MeV electrons: Modeling of changes in the solar wind speed in relation to perpendicular polar diffusion

The discrepancy between cosmic ray model predictions representing solar minimum conditions in the heliosphere and the 3–10 MeV post-1998 electrons observations by the Kiel Electron Telescope (KET) onboard Ulysses suggests the need for consistent changes in model parameters with increasing solar activity. In order to reduce this discrepancy, an effort is made to model the KET observations realistically during periods of increased solar activity by applying an advanced three-dimensional, steady-state electron modulation model based on Parker’s transport equation including the Jovian electron source. Some elements of the diffusion tensor which were not previously emphasized are revisited. A new relation is also established between the latitudinal dependence of the solar wind speed and the perpendicular polar diffusion. Based on this relation, a transition of an average solar wind speed from solar minimum to solar maximum conditions, as observed on board the Ulysses spacecraft, is modeled on the concept of the time-evolution of large polar coronal holes. These changes are correlated to different scenarios of the enhancement of perpendicular polar diffusion. Effects of these scenarios are illustrated, as a series of steady-state solutions, on the computed 7 MeV Jovian and galactic electrons in comparison with 3–10 MeV electrons observed from the period 1998 to the end of 2003. It is shown that this approach improves compatibility with the KET observations but it also points to the need for a time-dependent electron modulation model to fully describe modulation during moderate to extreme solar maximum conditions.

How did the solar wind structure change around the solar maximum? From interplanetary scintillation observation

Abstract. Observations from the second Ulysses fast latitude scan show that the global structure of solar wind near solar maximum is much more complex than at solar minimum. Soon after solar maximum, Ulysses observed a polar coronal hole (high speed) plasma with magnetic polarity of the new solar cycle in the Northern Hemisphere. We analyze the solar wind structure at and near solar maximum using interplanetary scintillation (IPS) measurements. To do this, we have developed a new tomographic technique, which improves our ability to examine the complex structure of the solar wind at solar maximum. Our IPS results show that in 1999 and 2000 the total area with speed greater than 700 km s-1 is significantly reduced first in the Northern Hemisphere and then in the Southern Hemisphere. For year 2001, we find that the formation of large areas of fast solar wind around the north pole precedes the formation of large polar coronal holes around the southern pole by several months. The IPS observations show a high level agreement to the Ulysses observation, particularly in coronal holes.Key words. Interplanetary physics (solar wind plasma) – Radio science (remote sensing)

The three‐dimensional solar wind around solar maximum

Ulysses is now completing its second solar polar orbit, dropping back down in latitude as the Sun passes through its post‐maximum phase of the solar cycle. A mid‐sized circumpolar coronal hole that formed around solar maximum in the northern hemisphere has persisted and produced a highly inclined CIR, which was observed from ∼70°N down to ∼30°N. We find that the speed maxima in the high‐speed streams follow the same slow drop in speed with decreasing latitude observed in the large polar coronal holes around solar minimum. These results suggest a solar wind acceleration effect that is related to heliolatitude or solar rotation. We also find that the solar wind dynamic pressure is significantly lower in the post‐maximum phase of this solar cycle than during the previous one, indicating that while the heliosphere is larger than near solar minimum, it should be smaller than during or after the previous maximum.

Sources of the solar wind at solar activity maximum

The photospheric sources of solar wind observed by the Ulysses and ACE spacecraft from 1998 to early 2001 are determined through a two‐step mapping process. Solar wind speed measured at the spacecraft is used in a ballistic model to determine a foot point on a source surface at a solar distance of 2.5 solar radii. A potential‐field source‐surface model is then used to trace the field and flow from the source surface to the photosphere. Comparison of the polarity of the measured interplanetary field with the polarity of the photospheric source region shows good agreement for spacecraft latitudes equatorward of 60°. At higher southern latitudes, the mapping predicts that Ulysses should have observed only outward directed magnetic fields, whereas both polarities were observed. A detailed analysis is performed on four of the solar rotations for which the mapped and observed polarities were in generally good agreement. For those rotations, the solar wind mapped to both coronal holes and active regions. These findings for a period of high solar activity differ from the findings of a similar study of the solar wind in 1994–1995 when solar activity was very low. At solar minimum the fastest wind mapped to the interior of large polar coronal holes while slower wind mapped to the boundaries of those holes or to smaller low‐latitude coronal holes. For the data examined in the present study, neither spacecraft detected wind from the small polar coronal holes when they existed and the speed was never as high as that observed by Ulysses at solar minimum. The principal difference between the solar wind from coronal holes and from active regions is that the O7+/O6+ ion ratio is lower for the coronal hole flow, but not as low as in the polar coronal hole flow at solar minimum. Furthermore, the active‐region flows appear to be organized into several substreams unlike the more monolithic structure of flows from coronal holes. The boundaries between plasma flows from neighboring sources are marked by large magnetic holes, plasma sheets, and low entropy, independent of whether the sources have the same or opposite magnetic polarities. The evolution of solar wind properties between 1 AU and the 1.6–5.4 AU solar distance of Ulysses is also briefly discussed.

Sunspot activity and the long‐term variation of the Sun's open magnetic flux

The interplanetary magnetic field (IMF) originates in open magnetic regions of the Sun (coronal holes), which in turn form mainly through the emergence and dispersal of active region fields. The radial IMF strength is proportional to the total open flux Φopen, which can be estimated from source surface extrapolations of the measured photospheric field, after correction for magnetograph saturation effects. We derive the long‐term variation of Φopen during 1971–2000 and discuss its relation to sunspot activity. The average value of Φopen was ∼20–30% higher during 1976–1996 than during 1971–1976 and 1996–2000, with major peaks occurring in 1982 and 1991. Near sunspot minimum, most of the open flux resides in the large polar coronal holes, whereas at sunspot maximum it is rooted in relatively small, low‐latitude holes located near active regions and characterized by strong footpoint fields; since the decrease in the total area occupied by holes is offset by the increase in their average field strengths, Φopen remains roughly constant between activity minimum and maximum, unlike the total photospheric flux Φtot. The long‐term variation of Φopen approximately follows that of the Sun's total dipole strength, with a contribution from the magnetic quadrupole around sunspot maximum. Global fluctuations in sunspot activity lead to increases in the equatorial dipole strength and hence to enhancements in Φopen and the IMF strength lasting typically ∼1 year. We employ simulations to clarify the role of active region emergence and photospheric transport processes in the evolution of the open flux. Representing the initial field configuration by one or more bipolar magnetic regions (BMRs), we calculate its subsequent evolution under the influence of differential rotation, supergranular convection, and a poleward bulk flow. The initial value of Φopen is determined largely by the equatorial dipole strength, which in turn depends on the longitudinal phase relations between the BMRs. As the surface flow carries the BMR flux to higher latitudes, the equatorial dipole is annihilated on a timescale of ∼1 year by the combined effect of rotational shearing and turbulent diffusion. The remaining flux becomes concentrated around the poles, and Φopen approaches a limiting value that depends on the axisymmetric dipole strengths of the original BMRs. The polar coronal holes thus represent the long‐lived, axisymmetric remnant of the active regions that emerged earlier in the cycle.

Comparison of Empirical Models for Polar and Equatorial Coronal Holes

We present a self-consistent empirical model for several plasma parameters of a large equatorial coronal hole observed on 1999 November 12 near solar maximum. The model was derived from observations with the Ultraviolet Coronagraph Spectrometer on the Solar and Heliospheric Observatory. In this Letter, we compare the observations of O VI λλ1032, 1037 emission lines with previous observations of a polar coronal hole observed near solar minimum. At the time of the 1999 observations, there was no evidence of large polar coronal holes. The resulting empirical model for the equatorial coronal hole describes the outflow velocities and most probable speeds for O5+, and we compared the derived ion properties with the empirical model for a solar minimum polar coronal hole. The comparison of the empirical models shows that the 1999 equatorial hole has lower O5+ outflow speeds and perpendicular temperatures than its polar counterpart from 1996 to 1997 at heights between 2 and 3 R☉. However, in situ asymptotic speeds of the wind streams coming from the 1996-1997 polar hole and from the 1999 equatorial hole are only ~15% different. Thus, the bulk of the solar wind acceleration must occur above 3 R☉ for the equatorial coronal hole. The equatorial hole also has a higher density than the polar hole at similar heights. It is not yet known whether the higher densities are responsible for the seeming inhibition of the fast ion outflow speeds and extremely large perpendicular temperatures that occur in polar coronal holes at solar minimum. We discuss the constraints and implications on various theoretical models of coronal heating and acceleration.

Ulysses' Second Orbit: Remarkably Different Solar Wind

By the time of the 34th ESLAB symposium, dedicated to the memory of John Simpson, Ulysses had nearly reached its peak southerly latitude in its second polar orbit. The global solar wind structure observed thus far in Ulysses’ second orbit is remarkably different from that observed over its first orbit. In particular, Ulysses observed highly irregular solar wind with less periodic stream interaction regions, much more frequent coronal mass ejections. and only a single, short interval of fast solar wind. Ulysses also observed the slowest solar wind seen thus far in its ten-year journey (~270 km s−1). The complicated solar wind structure undoubtedly arises from the more complex coronal structure found around solar activity maximum, when the large polar coronal holes have disappeared and coronal streamers, small-scale corona] holes, and frequent CMEs are found at all heliolatitudes.

Cosmic ray transport in a heliospheric magnetic field with non-polar coronal holes

The simple tilted dipole picture of Corotating Interaction Regions which prevailed during the first polar pass of Ulysses no longer applies since the Sun entered a more active phase. Recent observations show that CIRs still persist, though the large polar coronal holes of solar minimum shrink to smaller areas and move to lower latitudes. We present 3-D simulations for the cosmic-ray intensity variations in a model with non-polar high speed streams. Latitudinal and recurrent time-variations are discussed, but more detailed and realistic simulations are required before quantitative comparisons with observations can be made.

Solar wind variation with the cycle

The cyclic evolution of the heliospheric plasma parameters is related to the time-dependent boundary conditions in the solar corona. “Minimal” coronal configurations correspond to the regular appearance of the tenuous, but hot and fast plasma streams from the large polar coronal holes. The denser, but cooler and slower solar wind is adjacent to coronal streamers. Irregular dynamic manifestations are present in the corona and the solar wind everywhere and always. They follow the solar activity cycle rather well. Because of this, the direct and indirect solar wind measurements demonstrate clear variations in space and time according to the minimal, intermediate and maximal conditions of the cycles. The average solar wind density, velocity and temperature measured at the Earth’s orbit show specific decadal variations and trends, which are of the order of the first tens per cent during the last three solar cycles. Statistical, spectral and correlation characteristics of the solar wind are reviewed with the emphasis on the cycles.

The long‐term variation of the Sun's open magnetic flux

The interplanetary magnetic field (IMF) has its origin in open magnetic regions of the Sun (coronal holes). The location of these regions and their total open flux Φopen can be inferred from current‐free extrapolations of the observed photospheric field. We derive the long‐term variation of Φopen during 1971–1998 and discuss its causes. Near sunspot minimum, the open flux originates mainly from the large polar coronal holes, whereas at sunspot maximum it is rooted in small, lower‐latitude holes characterized by very high field strengths; the total amount of open flux thus remains roughly constant between sunspot minimum and maximum. Through most of the cycle, the variation of Φopen closely follows that of the Sun's total dipole strength, showing much less dependence on the total photospheric flux or the sunspot number. However, episodic increases in large‐scale sunspot activity lead to strengthenings of the equatorial dipole component, and hence to enhancements in Φopen and the IMF strength lasting typically ∼1 yr.

Solar Wind Variation with the Cycle

AbstractThe cyclic evolution of the heliospheric plasma parameters is related to the time-dependent boundary conditions in the solar corona. “Minimal” coronal configurations correspond to the regular appearance of the tenuous, but hot and fast plasma streams from the large polar coronal holes. The denser, but cooler and slower solar wind is adjacent to coronal streamers. Irregular dynamic manifestations are present in the corona and the solar wind everywhere and always. They follow the solar activity cycle rather well. Because of this, the direct and indirect solar wind measurements demonstrate clear variations in space and time according to the minimal, intermediate and maximal conditions of the cycles. The average solar wind density, velocity and temperature measured at the Earth’s orbit show specific decadal variations and trends, which are of the order of the first tens per cent during the last three solar cycles. Statistical, spectral and correlation characteristics of the solar wind are reviewed with the emphasis on the cycles.