Minimum Of Solar Cycle Research Articles

STEREO observations of the energetic ions in tilted corotating interaction regions

In this paper we examine suprathermal He ions measured by the SIT (Suprathermal Ion Telescope) instrument associated with tilted corotating interaction regions (CIRs). We use observations of the two STEREO spacecraft (s/c) for the first 2.7 years of the mission, along with ground-based measurements of the solar magnetic field during the unusually long minimum of Solar Cycle 23. Due to the unique configuration of the STEREO s/c orbits we are able to investigate spatial variations in the intensity of the corotating ions on time scales of less than one solar rotation. The observations reveal that the occurrence of the strong CIR events was the most frequent at the beginning of the period. The inclination of the heliospheric current sheet relative to the heliographic equator (the tilt angle) was quite high in the first stage of the mission and gradually flattened with the time, followed by a decrease in the CIR activity. By examining the differences between measurements on the two STEREO s/c we discuss how the changes in the position of the s/c relative to the CIRs affect the energetic particle observations. We combine STEREO observations with observations from the ULEIS instrument on the ACE s/c and argue that the main factor which controls the differences in the ion intensities is the latitudinal separation between the two STEREO s/c relative to the tilted CIRs. The position of the s/c is less important when the tilt angle is high. In this case we found that the CIR ion intensity positively correlates with the tilt angle.

Solar magnetic polarity dependency of geomagnetic storm seasonal occurrence

[1] For nearly a century it has been known that the tendency for geomagnetic activity is, on average, higher at the equinoxes than at the solstices. Previous studies on semiannual geomagnetic activity were performed mainly for geomagnetic indices such as am, aa, U, and AL. Thus, we need to understand the seasonal variation of geomagnetic activity defined by the Dst index over the long term. It is also necessary to test the solar magnetic polarity dependence of geomagnetic activity. This paper is a statistical analysis of the geomagnetic storms defined by the Dst index. Our storm data consists of two sets of storm data for 5 years at each solar minimum during the four solar cycles (19–22) from 1962 through 1998 for two of each solar magnetic polarity. The storms are divided into two groups defined by Dst index (Dst(min) 70 nT; Dst(min) 90 nT). Monthly occurrences of these storms are compared. Storms of ∣Dst∣ > 90 nT and of ∣Dst∣ > 70 nT occurred 153 and 238 times, respectively, during the testing periods. Storms occurred more frequently during the spring and fall seasons for all solar cycle minima, regardless of solar magnetic polarity.

Solar Cycle Effects on the Dynamics of Jupiter’s and Saturn’s Magnetospheres

The giant planetary magnetospheres surrounding Jupiter and Saturn respond in quite different ways, compared to Earth, to changes in upstream solar wind conditions. Spacecraft have visited Jupiter and Saturn during both solar cycle minima and maxima. In this paper we explore the large-scale structure of the interplanetary magnetic field (IMF) upstream of Saturn and Jupiter as a function of solar cycle, deduced from solar wind observations by spacecraft and from models. We show the distributions of solar wind dynamic pressure and IMF azimuthal and meridional angles over the changing solar cycle conditions, detailing how they compare to Parker predictions and to our general understanding of expected heliospheric structure at 5 and 9 AU. We explore how Jupiter’s and Saturn’s magnetospheric dynamics respond to varying solar wind driving over a solar cycle under varying Mach number regimes, and consider how changing dayside coupling can have a direct effect on the nightside magnetospheric response. We also address how solar UV flux variability over a solar cycle influences the plasma and neutral tori in the inner magnetospheres of Jupiter and Saturn, and estimate the solar cycle effects on internally driven magnetospheric dynamics. We conclude by commenting on the effects of the solar cycle in the release of heavy ion plasma into the heliosphere, ultimately derived from the moons of Jupiter and Saturn.

The ionosphere under extremely prolonged low solar activity

[1] A critical question in ionospheric physics is the state of the ionosphere and relevant processes under extreme solar activities. The solar activity during 2007–2009 is extremely prolonged low, which offers us a unique opportunity to explore this issue. In this study, we collected the global ionosonde measurements of the F2 layer critical frequency (foF2), E layer critical frequency (foE), and F layer virtual height (h′F) and the total electron content (TEC) maps produced by the Jet Propulsion Laboratory, which were retrieved from dual-frequency GPS receivers distributed worldwide, to investigate the ionospheric phenomena during solar minimum of cycle 23/24, particularly the difference in the ionosphere between solar minima of cycle 23/24 and the preceding cycles. The analysis indicates that the moving 1 year mean foF2 at most ionosonde stations and the global average TEC went to the lowest during cycle 23/24 minimum. The solar cycle differences in foF2 minima display local time dependence, being more negative during the daytime than at night. Furthermore, the cycle difference in daytime foF2 minima is about −0.5 MHz and even reaches to around −1.2 MHz. In contrast, a complex picture presents in global h′F and foE. Evident reduction exists prevailingly in the moving 1 year mean h′F at most stations, while no huge differences are detected at several stations. A compelling feature is the increase in foE at some stations, which requires independent data for further validation. Quantitative analysis indicates that record low foF2 and low TEC can be explained principally in terms of the decline in solar extreme ultraviolet irradiance recorded by SOHO/SEM, which suggests low solar EUV being the prevailing contributor to the unusual low electron density in the ionosphere during cycle 23/24 minimum. It also verifies that a quadratic fitting still reasonably captures the solar variability of foF2 and global average TEC at such low solar activity levels.

Solar activity effects of the ionosphere: A brief review

Solar radiation, which varies over multiple temporal scales, modulates remarkably the evolution of the ionosphere. The solar activity dependence of the ionosphere is a key and fundamental issue in ionospheric physics, providing information essential to understanding the variations in the ionosphere and its processes. Selected recent studies on solar activity effects of the ionosphere are briefly reviewed in this report. This report focuses on (1) observations of solar irradiance at X-ray and extreme ultraviolet wavelengths and the outstanding problems of solar proxies, in the view of ionospheric studies, (2) new findings and improved representations of the features of the solar activity dependence of ionospheric key parameters and the corresponding physical processes, (3) possible phenomena in the ionosphere under extremely high and low solar activity conditions that are unique, as indicated by historical solar datasets and the deep solar minimum of solar cycle 23/24, and (4) statistical studies and model simulations of the ionosphere response to solar flares. The above-mentioned studies provide new clues for comprehensively explaining basic processes in the ionosphere and improving the prediction capability of ionospheric models and related applications.

Comparisons of Supergranule Characteristics During the Solar Minima of Cycles 22/23 and 23/24

Supergranulation is a component of solar convection that manifests itself on the photosphere as a cellular network of around 35 Mm across, with a turnover lifetime of 1 – 2 days. It is strongly linked to the structure of the magnetic field. The horizontal, divergent flows within supergranule cells carry local field lines to the cell boundaries, while the rotational properties of supergranule upflows may contribute to the restoration of the poloidal field as part of the dynamo mechanism, which controls the solar cycle. The solar minimum at the transition from cycle 23 to 24 was notable for its low level of activity and its extended length. It is of interest to study whether the convective phenomena that influence the solar magnetic field during this time differed in character from periods of previous minima. This study investigates three characteristics (velocity components, sizes and lifetimes) of solar supergranulation. Comparisons of these characteristics are made between the minima of cycles 22/23 and 23/24 using MDI Doppler data from 1996 and 2008, respectively. It is found that whereas the lifetimes are equal during both epochs (around 18 h), the sizes are larger in 1996 (35.9 ± 0.3 Mm) than in 2008 (35.0 ± 0.3 Mm), while the dominant horizontal velocity flows are weaker (139 ± 1 m s−1 in 1996; 141 ± 1 m s−1 in 2008). Although numerical differences are seen, they are not conclusive proof of the most recent minimum being inherently unusual.

The WHI Corona from Differential Emission Measure Tomography

A three-dimensional (3D) tomographic reconstruction of the local differential emission measure (LDEM) of the global solar corona during the whole heliosphere interval (WHI, Carrington rotation CR 2068) is presented, based on STEREO/EUVI images. We determine the 3D distribution of the electron density, mean temperature, and temperature spread, in the range of heliocentric heights 1.03 to 1.23 R ⊙. The reconstruction is complemented with a potential-field source-surface (PFSS) magnetic-field model. The streamer-core, streamer-leg, and subpolar regions are analyzed and compared to a similar analysis previously performed for CR 2077, very near the absolute minimum of Solar Cycle 23. In each region, the typical values of density and temperature are similar in both periods. The WHI corona exhibits a streamer structure of relatively smaller volume and latitudinal extension than during CR 2077, with a global closed-to-open density contrast about 6% lower, and a somewhat more complex morphology. The average basal electron density is found to be about 2.23 and 1.08×108 cm−3, in the streamer core and subpolar regions, respectively. The electron temperature is quite uniform over the analyzed height range, with average values of about 1.13 and 0.93 MK, in the streamer core and subpolar regions, respectively. Within the streamer closed region, both periods show higher temperatures at mid-latitudes and lower temperatures near the Equator. Both periods show β>1 in the streamer core and β<1 in the surrounding open regions, with CR 2077 exhibiting a stronger contrast. Hydrostatic fits to the electron density are performed, and the scale height is compared to the LDEM mean electron temperature. Within the streamer core, the results are consistent with an isothermal hydrostatic plasma regime, with the temperatures of ions and electrons differing by up to about 10%. In the subpolar open regions, the results are consistent with departures from thermal equilibrium with T ions>T e (and values of T ions/T e up to about 1.5), and/or the presence of wave-pressure mechanisms linear in the density.

Properties of Supergranulation During the Solar Minima of Cycles 22/23 and 23/24

The solar minimum at the transition from cycle 23 to 24 was notable for its low level of activity and its extended duration. Among the various fields of study, the evolution of the solar convection zone may provide insight into the causes and consequences of this recent minimum. This study continues previous investigations of the characteristics of solar supergranulation, a convection component strongly linked to the structure of the magnetic field, namely the time-evolution of the global mean of supergranule cell size, determined from spectral analysis of MDI Dopplergrams from the two previous solar minima. Analyses of the global mean of supergranule sizes show a quasi-oscillatory nature to the evolution of this particular supergranule characteristic. Performing similar analyses on realistic, synthetic Doppler images show similar time-dependent characteristics. We conclude that the observed fluctuations are not observational artifacts, and that an underlying trend exists within the evolution of the supergranulation network.

Solar modulation during unusual minimum of solar cycle 23: Comparison with past three solar minima

Solar modulation during unusual minimum of solar cycle 23: Comparison with past three solar minima

Mid-term periodicities of cosmic ray intensities

Galactic cosmic ray intensities (GCRs) observed by five neutron monitors (NMs) have been used to study cosmic ray modulations between 1971 and 2007. The influence of interplanetary magnetic polarity (IMF) states has been studied for the A < 0 and A > 0 epochs. A comparison of the spectra for both positive IMF polarities indicated different solar origins. The spectra have different power amplitudes and most peaks of different locations. In addition, the differences in the cosmic ray modulations, conditions for solar activity minima and maxima periods are probably associated with the influence of drift effects. The observed differences are related to the 22-year cycle in heliospheric modulations of cosmic rays, leading to the different shapes of CR maxima and the hysteresis effect. Accordingly, drift effects dependent on the polarity of the global solar magnetic field may play a significant role in the observed differences between maxima and minima periods. The drift mechanism is enhanced during periods of low to moderate SA, i.e., around solar cycle minima, during negative polarity periods, when A < 0.

Decrease of Schumann resonance frequencies and changes in the effective lightning areas toward the solar cycle minimum of 2008–2009

For the period 1988–2002, it has been shown that the SR mode frequencies and Q-factors decrease with the decrease of X-ray radiation toward the solar cycle minimum (Sátori et al., 2005). In the present study, a decrease of frequencies and Q-factors of all first four SR modes from the latest solar cycle maximum to the anomalous minimum of 2008–2009 is found using the data from Modra Observatory. Decreases obtained here for the first two SR modes are evidently greater than those obtained in the study cited above. Moreover, effective thunderstorm areas are calculated from the monthly mean diurnal frequency range of the electric field component. Our study reveals that the difference between the northern and the southern hemisphere summer areas not only declines with the decrease in solar activity but almost vanishes during the deep solar minimum of 2008–2009. Semi-annual variation in the areas dominates in years of the deep solar minimum.

Comparisons of temperature response to solar forcing in the pre‐ and post periods of satellite data assimilation

AbstractOn the basis of both composite and multiple linear regression analysis, three temperature datasets from two reanalyses and one set of satellite observations have been used to evaluate the different responses in the winter [December–February (DJF)] period as they relate to solar forcing between the pre‐ and post periods of satellite data assimilation. The two periods are defined as 1958–1978 when no satellite data was available to be assimilated and the 1979–2002 period when satellite data was assimilated in the operational forecast models. The solar forcing signal for the two periods can be identified as significant in the extremes of the solar cycle (maximum and minimum). The composite analysis shows that the solar response of the DJF temperatures in the three datasets shows large‐scale similarities although there are differences over the southern middle‐high latitudes and some tropical areas. The stratospheric response showed the strongest DJF temperature anomalies related to solar variability occurring over the Arctic, but its sign is negative in 1979–2002 and positive in 1958–1978 for solar maximum activity. The stratospheric temperature response can be confirmed using a multiple regression analysis. The temperature features may be partially explained by the impacts of heating via ozone absorption, and is reinforced by the solar cycle, El‐Niño Southern Oscillation (ENSO), stratospheric quasi‐biennial oscillation (QBO), volcanic eruptions and other factors. In contrast, the tropospheric response, with a dynamic wavelike structure, occurs over the middle latitudes. The tropospheric differences between the two periods are not clearly resolved and raise questions about the efficacy of the observations and our ability to use the observations effectively. Copyright © 2010 Royal Meteorological Society

Unusual time histories of galactic and anomalous cosmic rays at 1 AU over the deep solar minimum of cycle 23/24

The unusually quiet Sun of the cycle 23/24 solar minimum (that ended in December, 2009) has resulted in lower values of the interplanetary magnetic field and a slower approach of the tilt angle of the heliospheric current sheet toward the solar equator than has been observed for recent solar minima. As a result of these changes, the time‐histories of galactic and anomalous cosmic rays over this period are very different from those of recent minima at the same phase of the heliomagnetic cycle. Since ∼2005.6 there has been an on‐going increase in cosmic‐ray intensity (except for one brief transient decrease) that lasted for 4.4 years. The relative rigidity dependences of these increases compared to previous cycles are complex and should provide insight into the role of various solar and interplanetary phenomena in the modulation process. The largest increase occurs in the nominal “cross‐over energy” region (where the modulation is essentially the same for each minimum of the two past 22 year heliomagnetic cycles) which extends from ∼200 MeV/n to &gt;500 MeV/n.

Differences in solar wind cross‐helicity and residual energy during the last two solar minima

The minimum of solar cycle 23 exhibited anomalous characteristics with respect to its predecessors. Other than the differences in the standard indicators of activity, such as sunspots, polar magnetic fields were also found to be different. These differences had a range of effects in the solar wind. In this work we study the Alfvénicity and the residual energy of magnetic field fluctuations in two intervals from the solar minima in cycles 22 and 23 using Ulysses data. We find that the differences between the minima are mainly related to the greater prominence of convected structures in the broader equatorial region in cycle 23. Thus the total power in the fluctuations was lower in cycle 23, mainly due to a reduction in Alfvénic fluctuations. We also show that the relationship between the normalized residual energy and cross helicity is scale dependent.

SOLAR ROTATION RATE DURING THE CYCLE 24 MINIMUM IN ACTIVITY

The minimum of solar cycle 24 is significantly different from most other minima in terms of its duration as well as its abnormally low levels of activity. Using available helioseismic data that cover epochs from the minimum of cycle 23 to now, we study the differences in the nature of the solar rotation between the minima of cycles 23 and 24. We find that there are significant differences between the rotation rates during the two minima. There are differences in the zonal-flow pattern too. We find that the band of fast rotating region close to the equator bifurcated around 2005 and recombined by 2008. This behavior is different from that during the cycle 23 minimum. By auto-correlating the zonal-flow pattern with a time shift, we find that in terms of solar dynamics, solar cycle 23 lasted for a period of 11.7 years, consistent with the result of Howe et al. (2009). The autocorrelation coefficient also confirms that the zonal-flow pattern penetrates through the convection zone.

Characterization of the slow wind in the outer corona

The study concerns the streamer belt observed at high spectral resolution during the minimum of solar cycle 23 with the Ultraviolet Coronagraph Spectrometer (UVCS) onboard SOHO. On the basis of a spectroscopic analysis of the O VI doublet, the solar wind plasma parameters are inferred in the extended corona. The analysis accounts for the coronal magnetic topology, extrapolated through a 3D magneto-hydrodynamic model, in order to define the streamer boundary and to analyse the edges of coronal holes. The results of the analysis allow an accurate identification of the source regions of the slow coronal wind that are confirmed to be along the streamer boundary in the open magnetic field region.

RADIAL EVOLUTION OF SOLAR WIND TURBULENCE DURING EARTH ANDULYSSESALIGNMENT OF 2007 AUGUST

At the end of 2007 August, during the minimum of solar cycle 23, a lineup of Earth and Ulysses occurred, giving the opportunity to analyze, for the first time, the same plasma sample at different observation points, namely at 1 and 1.4 AU. In particular, it allowed us to study the radial evolution of solar wind turbulence typical of fast wind streams as proposed in a Coordinated Investigation Programme for the International Heliophysical Year. This paper describes both the macrostructure and the fluctuations at small scales of this event. We find that soon after detecting the same fast stream, the Advanced Composition Explorer (ACE) observed a change of magnetic polarity being the interplanetary current sheet located between the orbits of the two spacecraft. Moreover, we observe that the compression region formed in front of the fast stream detected at ACE's location evolves in a fast forward shock at Ulysses' orbit. On the other hand, small-scale analysis shows that turbulence is evolving. The presence of a shift of the frequency break separating the injection range from the inertial range toward lower frequencies while distance increases is a clear indication that nonlinear interactions are at work. Moreover, we observe that intermittency, as measured by the flatness factor, increases with distance. This study confirms previous analyses performed using Helios observations of the same fast wind streams at different heliocentric distances, allowing us to relax about the hypothesis of the stationarity of the source regions adopted in previous studies. Consequently, any difference noticed in the solar wind parameters would be ascribed to radial (time) evolution.

Locations of chorus emissions observed by the Polar Plasma Wave Instrument

We performed a statistical study of the locations of chorus emissions observed by the Polar spacecraft's Plasma Wave Instrument (PWI) from March 1996 to September 1997, near the minimum of solar cycles 22/23. We examined how the occurrence of chorus emissions in the Polar PWI data set depends upon magnetic local time, magnetic latitude, L shell, and L*. The Polar PWI observed chorus most often over a range of magnetic local times extending from about 2100 MLT around to the dawn flank and into the dayside magnetosphere near 1500 MLT. Chorus was least likely to be observed near the dusk flank. On the dayside, near noon, the region in which Polar observed chorus extended to larger radial distances and higher latitudes than at other local times. Away from noon, the regions in which chorus occurred were more restricted in both radial and latitudinal extent. We found that for high‐latitude chorus near local noon, L* provides a more reasonable mapping to the equatorial plane than the standard L shell. Chorus was observed slightly more often when the magnitude of the solar wind magnetic field BSW was greater than 5 nT than it was for smaller interplanetary magnetic field strengths. We also found that near solar minimum, chorus is twice as likely to be observed when the solar wind speed is greater than 450 km/s than it is when the solar wind speed is less than 450 km/s.

THE GLOBAL SOLAR WIND BETWEEN 1 AU AND THE TERMINATION SHOCK

This paper studies the global solar wind between 1 AU and the termination shock, taking into account the highly latitude dependent solar wind at the 1 AU boundary and the ionization of interstellar neutral hydrogen. A system of two-fluid magnetohydrodynamics equations governs the flow of solar wind plasma; the conditions of the solar wind at 1 AU are generated from Ulysses data for the relatively stable state of the wind during the declining phase and solar minimum of the solar cycle. A one-fluid model is used to describe the flow of neutral hydrogen and its condition in the far-field boundary outside the termination shock. Simultaneous solutions are obtained using a high-resolution computational code for the two equation systems coupled by the ionization of neutral hydrogen. The ionization process leads to removal of neutral hydrogen in the heliosphere: a hydrogen cavity forms inside ~4 AU; the cavity extends on the downwind side to form a long cavity wake. Solutions show how the ionization process and the wind condition at 1 AU boundary affect the spatial variation of the wind speed, temperature, pickup proton, fast Mach number, and plasma β-ratio. The wind properties inside 4 AU are axisymmetrical about the solar rotation axis; this axi-symmetry totally disappears in the outer heliosphere. All wind properties are substantially modified at an increasing heliocentric distance on the upwind side to generate an upwind-downwind asymmetry dictated by the direction of the relative motion between the Sun and the interstellar medium.

Length of a minimum as predictor of next solar cycle's strength

[1] Motivated by a prevailing view that a long minimum leads to a weak sunspot cycle, we estimate the correlation coefficients between the length of a cycle minimum and (i) the following cycle's peak, (ii) the preceding cycle's peak, (iii) following peak minus preceding peak and (iv) depth of minimum. Using both sunspot number and spot area data, we find that a long minimum is both followed and preceded by weak cycles. Similarly short minima are followed and preceded by strong cycles. Consistent with these results, we find no correlation between the length of a cycle minimum and the difference in peaks of the following and preceding cycles. From sunspot number data, for longer-than-average minima, five following cycle peaks were lower than that of the preceding cycles' peaks, while four were higher. Following shorter-than-average minima, seven cycle peaks were higher than the preceding peaks and seven were lower. Therefore one cannot predict from the length of a minimum whether the next cycle will be stronger or weaker than the preceding cycle. Thus we cannot predict whether cycle 24 will be stronger or weaker than 23. We also find that there is a strong anticorrelation between the length of a solar cycle minimum and the depth of that minimum. We define the depth as the least spot number or spot area (13-rotation averaged) within the span of a cycle minimum. We speculate that this anticorrelation is due to the longer time available for annihilation of late cycle toroidal flux across the equator in the case of a longer minimum.