Minimum Of Solar Cycle Research Articles

Assessing the Evolution of Solar Cycle 25: A Weak-moderate Cycle

This note aims to analyze the evolution of the sunspot number for Solar Cycle 25, updating our previous findings on this topic. We compare daily, monthly and 13 months smoothed sunspot numbers for Solar Cycle 25 with those from previous cycles since mid-18th century (Solar Cycles 1–25). The highest daily, monthly and 13 months smoothed values for Solar Cycle 25 are significantly lower than the mean and median values considering all cycles. In particular, Solar Cycle 25 ranks 17th in terms of the highest 13 months smoothed sunspot number at this point in the cycle. Based on current data and the progression toward its maximum, we conclude that Solar Cycle 25 is likely to be a weak to moderate cycle, consistent with our earlier analyses. In addition, we find that Solar Cycles 13, 14, and 16 have the most similar behavior to that of Solar Cycle 25. Assuming a cycle length for Solar Cycle 25 similar to those of the above cycles, we estimate that the minimum of Solar Cycle 26 will be in 2030–2031.

A Comparative Approach of Cosmic Ray Variations Within Solar Minimum of Cycle 24

A Comparative Approach of Cosmic Ray Variations Within Solar Minimum of Cycle 24

Algebraic quantification of the contribution of active regions to the Sun’s dipole moment: applications to century-scale polar field estimates and solar cycle forecasting

ABSTRACT The solar cycle is generated by a magnetohydrodynamic dynamo mechanism which involves the induction and recycling of the toroidal and poloidal components of the Sun’s magnetic field. Recent observations indicate that the Babcock–Leighton (BL) mechanism – mediated via the emergence and evolution of tilted bipolar active regions – is the primary contributor to the Sun’s large-scale dipolar field. Surface flux transport models and dynamo models have been employed to simulate this mechanism, which also allows for physics-based solar cycle forecasts. Recently, an alternative analytic method has been proposed to quantify the contribution of individual active regions to the Sun’s dipole moment (DM). Utilizing solar cycle observations spanning a century, here, we test the efficacy of this algebraic approach. Our results demonstrate that the algebraic quantification approach is reasonably successful in estimating DMs at solar minima over the past century – providing a verification of the BL mechanism as the primary contributor to the Sun’s dipole field variations. We highlight that this algebraic methodology serves as an independent approach for estimating DMs at the minima of solar cycles, relying on characteristics of bipolar solar active regions. We also show how this method may be utilized for solar cycle predictions; our estimate of the Sun’s dipole field at the end of cycle 24 using this approach indicates that solar cycle 25 would be a moderately weak cycle, ranging between solar cycle 20 and cycle 24.

Assessment of the accuracy of IRI-2016 model at the equatorial-, low-, mid-, and high-latitudes ionosphere along 30–45oE meridian lines during minimum (2009) and maximum (2013) phases of solar cycle 24

This study assesses the accuracy of the International Reference Ionosphere IRI-2016 model prediction at the equatorial-, low-, mid-, and high-latitude ionosphere along 30–45oE meridian during minimum (2009) and maximum (2013) phases of solar cycle 24. We used raw TEC derived from GPS observation data from GPS receivers within ∼±70o latitudinal band, along the African/European longitudinal sector. We compared the GNSS-TEC data after the Plasmaspheric Electron Content (PEC) has been excluded with those derived from the two components of the IRI-2016 model. The percentage contributions of PEC to GNSS-TEC was minimum in June solstice, and maximum in December solstice with the highest values at the high-latitude regions, but with a clear hemispherical asymmetry; manifesting the winter anomaly effect. Higher TEC values were recorded in the equatorial/low-latitude regions than in the high- and mid-latitude regions. Overall, in both hemispheres, the accuracy of prediction of the IRI-2016 model was higher in the high- and mid-latitude regions than in the equatorial/low-latitude regions. The accuracy of the model was higher during solar minimum than during solar maximum. Both options of IRI-2016 performed better at nighttime than during daytime. IRI-NeQuick option generally performed better than the IRI-Corr option. TEC values were higher in the southern hemisphere than in the northern hemisphere.

Stream Interaction Regions in the Minimum of Solar Cycles 23 and 24

AbstractStream interaction regions (SIRs) dominate the large‐scale solar wind dynamics during the minimum of the solar cycle. The interaction of SIRs with the magnetosphere causes most of geomagnetic storms during this epoch. We used in‐situ solar wind observations at 1 AU to study 62 SIRs detected in the interval 2007–2008 (minimum of cycle 23) and 61 SIRs in the interval 2018–2019 (minimum of cycle 24). We compared distinct characteristics of SIRs, such as the solar wind streams velocities, the presence of forward and/or reverse shocks (RS), SIR radial widths, the relative position of the stream interface within the SIRs, SIR latitudinal orientation, and the geoeffectiveness. There were more geomagnetic storms driven by SIRs in cycle 23 than in cycle 24. The pattern of fast solar wind streams in cycle 23 tended to be faster than in cycle 24. We found more SIRs with RS in cycle 23 than in cycle 24. For the geoeffective SIRs, the momentum flows from the fast solar wind to the slow solar wind, as well as their stream interface is closer to the SIR front. Coronal holes associated with SIRs registered in cycle 23 tended to be wider in longitude near the solar equator and/or at midlatitudes. We did not find a clear relation between the geoeffectiveness of the SIRs and their latitudinal inclinations.

ANOMALOUS MAGNETIC REGIONS ON THE SUN

We studied the anomalous magnetic regions observed near the minima of solar cycles 24 and 25. The peculiarity of these areas was the deviation of their configuration from Hale's law of magnetic polarity and Joy's law about the inclination of the axes of bipolar groups to the latitudinal direction. Therefore, they belong to the class of so-called anti-Hale active regions. We paid special attention to the flare activity of anti-Hale regions, as this is important for forecasting space weather and magnetic storms in the Earth's atmosphere. The detected anomalies of the surface magnetism of the active regions studied by us may indicate the influence of the mechanisms of the deep small-scale dynamo on their evolution. In this regard we analyzed the possible mechanisms of the formation of anti-Hale magnetic regions. In particular, such mechanisms can be the mechanisms of a small-scale magnetic dynamo. In connection with this an urgent problem today is the search for observed evidence of the existence of the theoretically proposed by Brandenburg A. et al. (2012) of a new physical entity – a small-scale magnetic field hidden in the solar depths, excited by two qualitatively different mechanisms of a small-scale dynamo (SSD). The first mechanism is the SSD of macroscopic MHD (SSD1), while the second is the diffusion SSD of classical MHD (SSD2). However, the small contributions of these sources are very difficult to distinguish observationally. To solve this complication, Sokoloff, Khlystova and Abramenko (2015) proposed a test for separating the contributions of two sources based on a statistical probabilistic model. Such an important feature of the differences between of the two SSD is the behavior of the percentage of anti-Hail groups of sunspots (in relation to the total number of spots) in the minima of solar cycles. According to statistical studies of long series of observations Sokoloff, Khlystova and Abramenko (2015) found that the percentage of anti-Haile groups of spots increases during minima of the solar cycles, suggesting in favor of SSD2. We believe that the detected magnetic anomalies of the studied regions may be caused by the influence of a SSD2 in the depths of the convective zone of the Sun, since this source gives the most noticeable contribution to the surface magnetism near cycle minima.

On the Validation of the Rotation Procedure from HEE to MEMFA Reference Frame in the Presence of Alfvén Waves in the Interplanetary Medium

Alfvén waves play an important role in the stability, heating, and transport of magnetized plasmas. They are found to be ubiquitous in solar winds (SW), which mainly propagate outward from the Sun, especially in high-speed streams that originate from coronal holes. When high-speed streams impinge on the Earth’s magnetosphere, the impact of Alfvénic fluctuations can cause magnetic reconnections between the intermittent southward Interplanetary Magnetic Field (IMF) and the geomagnetic field, resulting in energy injection from the SW into the Earth’s magnetosphere. In this work, we tested a rotation procedure from the Heliocentric Earth Ecliptic (HEE) to the Mean ElectroMagnetic Fields Aligned (MEMFA) reference frame. This is achieved by means of the Empirical Mode Decomposition (EMD) method for both the SW velocity and IMF at 1 AU. Our aim is to check the reliability of the method and its limitations in identifying Alfvénic fluctuations through the spectral analysis of time series in the MEMFA coordinate system. With this procedure, we studied the fluctuations in the main-field-aligned direction and those in the orthogonal plane to the main field. To highlight the peculiarities of each case of study and be able to better identify Alfvén waves when applying this procedure to real data, we reproduced the magnetic and velocity fields of a typical corotating high-speed stream. We tested the procedure in several cases by varying the amplitude of Alfvén waves and noise .We performed the spectral analysis of the Mean Field Aligned (MFA) component of both the magnetic and velocity fields to define the power related to the two main directions: the one aligned to the ambient magnetic field and the one orthogonal to it. The efficiency of the procedure and the results’ reliability are supported by Monte Carlo (MC) tests. The method is also applied to a real case that is represented by a selected corotating SW stream that occurred during August 2008, which fell in the solar minimum of solar cycle 23. The results are also compared with those obtained by using Elsässer variables to analyze the Alfvénicity of fluctuations via the normalized cross helicity and the normalized residual energy.

Behaviour of 27-Day and 13.5-Day Periodicities in Galactic Cosmic Particles as Observed by Spacecraft and Neutron Monitors During Different Solar Polarity Cycles

An analysis has been made of the behaviour of the 27-day and 13.5-day periodicities in proton, C, and O galactic-cosmic-ray (GCR) particles at different energies as observed by the Advanced Composition Explorer (ACE) and SOHO (Solar and Heliospheric Observatory) spacecraft during both Solar Cycles 23 and 24. In addition, the behaviour of the 27-day and 13.5-day periods in the solar-wind-modulation parameter zeta = BIMF × VSW has been investigated during the same time interval to determine the existence of a possible solar-polarity dependence. Ground-based neutron-monitor (NM) observations, corresponding to different rigidity cutoff [RC] parameters, were also studied to determine the temporal behaviour of both the 27-day and 13.5-day periods during Cycles 23 and 24, revealing a statistically significant solar-polarity correlation. The Lomb–Scargle periodogram technique has been employed to extract spectral information from the above-mentioned observations for each individual year from 2001 – 2009 (Cycle 23) and 2010 – 2019 (Cycle 24). Daily mean energetic ACE and SOHO particle measurements are used to identify how both the 27-day and 13.5-day periodicities vary during each individual year during these cycles as a function of particle mean energy. This spectral analysis of proton, C, and O galactic-cosmic-particle data at different energies revealed that both the 27-day and 13.5-day periods are stronger during the minimum of Solar Cycle 24/25 when A > 0 (solar dipole pointing North) in comparison to the minimum of Cycle 23/24 when A < 0 (solar dipole pointing South) at certain energy levels. This showed a particularly strong energy-dependent behaviour for both periodicities. This article reports for the first time an annual time- and energy-dependent behaviour of both the 27-day and 13.5-day periodicities in daily-mean galactic cosmic particles observed by spacecraft and ground-based neutron monitors during consecutive Solar Cycles 23 and 24, corresponding to opposite solar-magnetic-field orientations. Periodicity behaviour in heliospheric solar-wind data corroborate these results in general.

Recurrent solar density transients in the slow wind observed with the Metis coronagraph

Aims We aim to investigate and characterize the morphology and dynamics of small-scale coronal plasma density inhomogeneities detected as brighter, denser features propagating outward through the solar corona in the visible-light images of the Metis coronagraph on board Solar Orbiter on February 22, 2021. Our main focus is on investigating their possible origin and contribution to the slow wind variability and dynamics and their dependence on coronal magnetic field configurations and structure. Methods. The method adopted is based on the computations of autocorrelation and cross-correlation functions applied to temporal and spatial series of total brightness as a function of the heliocentric distance and solar latitudes. Results. We find that the plasma density inhomogeneities studied here are small-scale structures with typical radial and transverse sizes, as projected on the plane of sky, on the order of 500 Mm and 40 Mm, respectively, and that they are up to 24 times brighter than the ambient solar wind. The brighter density structures exhibit longer lifetime and more stable shape and dimensions as they travel toward the outer edge of the field of view. The enhanced density structures are ejected with a most probable cadence of about 80 min at or below the inner edge of the Metis field of view (within 3.1 R⊙–5.7 R⊙ at the time of observations) in a wide latitudinal region corresponding to the site of a complex web of separatrix and quasi-separatrix layers, as resulting from the simulated magnetohydrodynamic configuration of the west limb of the solar corona. Some of the moving density enhancements clearly show morphological characteristics compatible with the switchback phenomenon, supporting the results indicating that the switchbacks occur at the coronal level. The enhanced density structures were ejected into the ambient slow wind with a mean velocity of about 240 ± 40 km s−1, which is significantly higher than that deduced for the ambient solar wind on the basis of previous Metis observations during the solar minimum of cycle 24. The absence of acceleration observed across the coronagraph field of view suggests that the ejected plasmoids are progressively reaching the expansion rate of the ambient wind. Conclusions. The results suggest that the quasi-periodic enhanced-density plasmoids might be the consequence of reconnection phenomena occurring in the complex web of the separatrix and quasi-separatrix layers present in the solar corona. Moreover, the structural characteristics of some of the detected plasmoids are in favor of the presence of switchbacks that originate during interchange reconnection processes occurring at or below 3 R⊙ in the S-web. The speed of the plasma ejected in the reconnection process is higher than that of the ambient slow solar wind and is likely to be related to the energy involved in the process generating the propagating structures.

Whole Heliosphere and Planetary Interactions (WHPI): The Big Picture on Solar Cycle Minima

AbstractThe Whole Heliosphere and Planetary Interactions (WHPI) is an international initiative to study the most recent solar minimum and its impact on the interconnected solar‐heliospheric‐planetary system by facilitating and encouraging interdisciplinary activities. Particular WHPI science foci include the global connected structure of the heliosphere and planetary space environments/atmospheres, the origins and impacts of high‐speed solar wind streams, coronal mass ejections from Sun‐to‐Heliopause, and comparative solar minima. This is achieved through a series of coordinated observing campaigns, including Parker Solar Probe perihelia, and scientific virtual interactions including a dedicated workshop where observers and modelers gathered to discuss, compare, and combine research results. This introduction sets the scene for the WHPI interval, placing it into the context of prior initiatives and describing the overall evolution of the system between 2018 and 2020. Along with the accompanying articles, it presents a selection of key scientific results on the interconnected solar‐heliospheric‐planetary system at solar minimum.

Venusian Ionosphere During Deep Solar Minima: Some New Insights Using Akatsuki Radio Science Experiment

AbstractThe characteristic features of different layers in the Venus ionosphere during the deep solar minimum of the solar cycle 24 have been studied using a radio science experiment onboard the Akatsuki spacecraft. Radio signals from the spacecraft were tracked at the Indian deep space network, Bangalore; Usuda Deep Space Center, Japan; and DLR Ground station, Weilheim, Germany. The orbital geometry of the spacecraft provides rare opportunities to probe the Venusian ionosphere and atmosphere in the equatorial region at low solar zenith angles (SZAs). The height of the peak plasma density of the V2 layer (∼141 km) was seen to remain almost constant for the SZA ≤ 90° while the peak density was the least among the published results. Features of the V1 layer (∼125 km) agree well with the Venus Express radio occultation measurements. All three types of V1 layers were observed in Akatsuki observations. The V0 layer (∼110 km) was observed in about 15% of cases and was seen to occur irrespective of SZA and geographical constraints. Nighttime occurrence of the ionosphere was very limited despite regular observations.

Coronal Elemental Abundances During A-Class Solar Flares Observed by Chandrayaan-2 XSM

The abundances of low First Ionisation Potential (FIP) elements are three to four times higher (FIP bias) in the closed loop active corona than in the photosphere, known as the FIP effect. Observations suggest that the abundances vary in different coronal structures. Here, we use the soft X-ray spectroscopic measurements from the Solar X-ray Monitor (XSM) on board the Chandrayaan-2 orbiter to study the FIP effect in multiple A-class flares observed during the minimum of solar cycle 24. Using time-integrated spectral analysis, we derive the average temperature, emission measure, and the abundances of four elements - Mg, Al, Si, and S. We find that the temperature and emission measure scales with the flares sub-class while the measured abundances show an intermediate FIP bias for the lower A-flares (e.g., A1), while for the higher A-flares, the FIP bias is near unity. To investigate it further, we perform a time-resolved spectral analysis for a sample of the A-class flares and examine the evolution of temperature, emission measure, and abundances. We find that the abundances drop from the coronal values towards their photospheric values in the impulsive phase of the flares, and after the impulsive phase, they quickly return to the usual coronal values. The transition of the abundances from the coronal to photospheric values in the impulsive phase of the flares indicates the injection of fresh unfractionated material from the lower solar atmosphere to the corona due to chromospheric evaporation. However, explaining the quick recovery of the abundances from the photospheric to coronal values in the decay phase of the flare is challenging.

Case study of radio emission beam associated to very low frequency signal recorded onboard CSES satellite

Abstract. We report on the variation of electric power density linked to very low frequency (VLF) signal observed during the minimum of solar cycle 25. The detected VLF signal is emitted by the NWC radio station localized in the southern hemisphere, at 21.5∘ S and 114.2∘ E. We attempt in this work to quantify the beam as detected by the Electric Field Detector (EFD) instrument onboard CSES satellite. Geometrical key parameters have been considered to analysis the variation of the power density taking into consideration the distance between the satellite trajectory and the NWC station and its conjugate region. The beam behavior is found to be subject to significant disturbances in the conjugate region with the presence of signal modulations. Above the NWC transmitter station, the beam can be considered as a hollow cone but with irregularity dependence on the electric power density.

Role of Small-scale Impulsive Events in Heating the X-Ray Bright Points of the Quiet Sun

Small-scale impulsive events, known as nanoflares, are thought to be one of the prime candidates that can keep the solar corona hot at its multimillion-Kelvin temperature. Individual nanoflares are difficult to detect with the current generation of instruments; however, their presence can be inferred through indirect techniques such as Differential Emission Measure (DEM) analysis. Here, we employ this technique to investigate the possibility of nanoflare heating of the quiet corona during the minimum of solar cycle 24. We estimate the DEM of disk-integrated quiet Sun and X-ray bright points (XBP) using the observations from XSM on board the Chandrayaan-2 orbiter and AIA on board the Solar Dynamic Observatory. XBPs are found to be the dominant contributor to disk-integrated X-rays, with a radiative flux of ∼2 × 105 erg cm−2 s−1. XBPs consist of small-scale loops associated with bipolar magnetic fields. We simulate such XBP loops using the EBTEL hydrodynamic code. The lengths and magnetic field strengths of these loops are obtained through a potential field extrapolation of the photospheric magnetogram. Each loop is assumed to be heated by random nanoflares having an energy that depends on the loop properties. The composite nanoflare energy distribution for all the loops has a power-law slope close to −2.5. The simulation output is then used to obtain the integrated DEM. It agrees remarkably well with the observed DEM at temperatures above 1 MK, suggesting that the nanoflare distribution, as predicted by our model, can explain the XBP heating.

НЕЗВИЧАЙНА СОНЯЧНА АКТИВНА ОБЛАСТЬ NOAA 13088/13102

Background. Under certain conditions, deep fluctuating magnetic fields lead to violations of Hale’s and Joy’s laws of observed magnetism on the surface of the Sun. These magnetic fluctuations can be excited by two qualitatively different mechanisms of a small-scale dynamo. The first mechanism is a macroscopic MHD dynamo, while the second mechanism is a classical MHD diffusion dynamo. An important difference between the two mechanisms is the percentage of observed anti-Hale sunspot groups (relative to the total number of sunspots) in solar cycle minima. In the case of the first mechanism, the percentage of anti-Hale groups does not depend on the phase of the cycle, while the specified percentage associated with the second mechanism should reach its maximum value in solar minima. To separate the minor contributions of the two named sources of magnetic fluctuations, the researchers proposed a theoretical test based on statistical analysis of observational data over long periods of time (Sokoloff, &amp; Khlystova, 2010). According to the proposed test, the percentage of anti-Hale groups of spots increases during the minima of the cycles, which indicates the favor of the diffusion dynamo. In order to confirm the dominant contribution of the diffusion dynamo to the surface magnetism, this work investigates a specific anomalous active region near the solar minimum. Methods. Macroscopic and classical MHD, which study the behavior of electromagnetic and hydrodynamic fields in turbulent plasma. Analysis of data from observations of the surface magnetism of the Sun. Results. We investigated the evolution of the NOAA 13088/13102 active region observed on August 24, 2022, shortly after the cycle 25 minimum. For the analysis, data from observations using instruments installed on board space observatories were used. A feature was revealed, which consists in the deviation of the surface magnetic configuration of this active region from Hale’s law of the magnetic polarity of groups of spots and Joy’s law of the inclination of the axes of bipolar groups to the latitudinal direction. In addition, it was established that the active region of NOAA 13088/13102 is characterized by rather high flare activity. Conclusions. We believe that the magnetic anomalies of the active region of NOAA 13088/13102 that we found were caused by the influence of magnetic fluctuations excited by the mechanism of the deep small-scale diffusion dynamo, since it is this source that gives the most noticeable contribution to the surface magnetism near the cycle minima. The detection and study of unusual anti-Hale’s AOs with increased eruptive activity, similar to NOAA 13088/13102, may find application in predicting periods of dangerous manifestations of space weather and in forecasting the dynamics of solar cycles.

A New GNSS TEC Neural Network Prediction Algorithm With the Data Fusion of Physical Observation

This work models and predicts anomalous jumps in GNSS total electron content (TEC) caused by geomagnetic disturbance or solar radiation by introducing F10.7 and regional Dst indices, which are used to characterize solar radiation and geomagnetic disturbance status, respectively. We then introduced long-distance constraint stations to restrain ionospheric variations during high solar activity, which reduced the influence of unmodelled factors on prediction accuracy. In our experimental work, Global Ionosphere Map (GIM) TEC data from 11 stations located at different latitudes were used for comparison during maximum (2014) and minimum (2018) of solar cycle 24. Sample data for >27 days were used for learning, and then ionospheric changes for the whole year were predicted using the sliding window method. The results showed that, the standard deviation for the global ionospheric prediction products could be guaranteed to be within 2 TECU. The proximity station constraint method proposed in this paper and the Dst high-order polynomial correction method during geomagnetic disturbance control the prediction error within -4.4 TECU, while the change of adjacent days is -51.4 TECU in 2014. The new algorithm reduced the TEC jump effect in predictions, and the prediction of ionospheric products provided by this algorithm can meet the basic requirements of ionospheric delay correction in high-precision positioning.

The strong activity of noctilucent clouds at middle latitudes in 2020

The 2020 summer season had more frequent than usual occurrences of noctilucent clouds (NLCs) in the Northern Hemisphere at middle latitudes (45–50°N), with the lowest latitude at which NLCs were seen being 34.1°N. In order to investigate a reason for this extraordinary NLC season, we have analyzed long-term Aura/MLS satellite data for all available summer periods from 2005 to 2021. Both Aura/MLS summer temperature and water vapor in the mesopause region, between about 79 and 89 km altitude, have been considered. There has been a decrease in the summer mesopause temperature between 2016 and 2020. At the same time, water vapor mixing ratio has significantly increased (by about 12–17%) in the zonal mean H2O value in the 2020 summer compared to 2017. There exists a positive linear trend in the H2O amount by about 5% between 2005 and 2021 at middle latitudes 45–50°N at 0.0046 hPa. A combination of lower mesopause temperature and water vapor mixing ratio maximum at middle latitudes is the main reason for frequent and widespread occurrences of NLCs seen around the globe at middle latitudes in the summer of 2020. The 24th solar cycle minimum can explain neither the H2O maximum nor NLC maximum in 2020.

Monitoring solar activity during 23/24 solar cycle minimum through VLF radio signals

Monitoring solar activity during 23/24 solar cycle minimum through VLF radio signals

SEASONAL EFFECT ON FOF2 VARIABILITY DURING ONE-DAY-SHOCK AT OUAGADOUGOU STATION DURING SOLAR CYCLES 20, 21 AND 22

The seasonal effect on the F2 sublayer critical frequency of the ionosphere (foF2), during one-day-shock, at Ouagadougou station during three solar cycles (20, 21, and 22) is examined in this work. It appears that summer has an effect tending to decrease the values ​​of foF2 in the maximum, ascending and descending phase. At solar cycle maximum, winter and autumn do not modify the foF2 amplitudes. At solar cycle minimum, winter decreases foF2 values ​​in the morning (08:00 a.m. to 1:00 p.m.), while spring amplifies them (09:00 a.m. to 1:00 p.m.). In the ascending phase, winter, spring and autumn increase foF2 values ​​in the morning. In the descending phase, winter and autumn do not modify the foF2 values ​​while spring increases them.

Solar-Wind High-Speed Stream (HSS) Alfvén Wave Fluctuations at High Heliospheric Latitudes: Ulysses Observations During Two Solar-Cycle Minima

Solar-Wind High-Speed Stream (HSS) Alfvén Wave Fluctuations at High Heliospheric Latitudes: Ulysses Observations During Two Solar-Cycle Minima