Alfvénic Slow Solar Wind Observed in the Inner Heliosphere by Parker Solar Probe
Jia Huang, J. C. Kasper, M. Stevens, D. Vech, K. G. Klein, Mihailo M. Martinović, B. L. Alterman, Lan K. Jian, Qiang Hu, Marco Velli, Timothy S. Horbury, B. Lavraud, T. N. Parashar, Tereza Ďurovcová, Tatiana Niembro, Kristoff Paulson, A. Hegedus, C. M. Bert, J. Holmes, A. W. Case, K. E. Korreck, Stuart D. Bale, Davin E. Larson, Roberto Livi, P. Whittlesey, Marc Pulupa, Thierry Dudok de Wit, David M. Malaspina, Robert J. MacDowall, John W. Bonnell, Peter R. Harvey, Keith Goetz
DDraft version May 27, 2020
Typeset using L A TEX default style in AASTeX63
Alfv´enic Slow Solar Wind Observed in the Inner Heliosphere by Parker Solar Probe
Jia Huang , J. C. Kasper ,
1, 2
M. Stevens , D. Vech , K. G. Klein , Mihailo M. Martinovi´c ,
4, 5
B. L. Alterman , Lan K. Jian , Qiang Hu , Marco Velli , Timothy S. Horbury , B. Lavraud , T. N. Parashar , Tereza ˇDurovcov´a , Tatiana Niembro , Kristoff Paulson , A. Hegedus , C. M. Bert , J. Holmes, A. W. Case , K. E. Korreck , Stuart D. Bale ,
14, 15, 10, 16
Davin E. Larson , Roberto Livi , P. Whittlesey , Marc Pulupa , Thierry Dudok de Wit , David M. Malaspina ,
18, 3
Robert J. MacDowall , John W. Bonnell , Peter R. Harvey , and Keith Goetz Climate and Space Sciences and Engineering, University of Michigan, Ann Arbor, MI 48109, USA Smithsonian Astrophysical Observatory, Cambridge, MA 02138 USA Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO 80303, USA Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85719, USA LESIA, Observatoire de Paris, Universite PSL, CNRS, Sorbonne Universite, Universite de Paris, 5 place Jules Janssen, 92195 Meudon,France Southwest Research Institute, San Antonio, TX 78238, USA Heliophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA Department of Space Science and CSPAR, The University of Alabama in Huntsville, Huntsville, AL 35805, USA Department of Earth, Planetary and Space Sciences, University of California, Los Angeles CA 90095, USA The Blackett Laboratory, Imperial College London, London, SW7 2AZ, UK Institut de Recherche en Astrophysique et Plantologie, CNRS, UPS, CNES, Universit de Toulouse, Toulouse 31400, France Department of Physics and Astronomy, Bartol Research Institute, University of Delaware, Newark, DE 19716, USA Faculty of Mathematics and Physics, Charles University, Prague, Czech Republic Physics Department, University of California, Berkeley, CA 94720-7300, USA Space Sciences Laboratory, University of California, Berkeley, CA 94720-7450, USA School of Physics and Astronomy, Queen Mary University of London, London E1 4NS, UK LPC2E, CNRS and University of Orl´eans, 3A avenue de la Recherche Scientifique, Orl´eans 45071, France Astrophysical and Planetary Sciences Department, University of Colorado, Boulder, CO 80305, USA NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA Space Sciences Laboratory, University of California, Berkeley, CA 94720, USA School of Physics and Astronomy, University of Minnesota, Minneapolis, MN 55455, USA (Received *****; Revised *****; Accepted *****)
Submitted to ApJ - Parker Solar Probe Special IssueABSTRACTThe slow solar wind is typically characterized as having low Alfv´enicity. However, Parker SolarProbe (PSP) observed predominately Alfv´enic slow solar wind during several of its initial encounters.From its first encounter observations, about 55.3% of the slow solar wind inside 0.25 au is highlyAlfv´enic ( | σ C | > .
7) at current solar minimum, which is much higher than the fraction of quiet-Sun-associated highly Alfv´enic slow wind observed at solar maximum at 1 au. Intervals of slow solar windwith different Alfv´enicities seem to show similar plasma characteristics and temperature anisotropydistributions. Some low Alfv´enicity slow wind intervals even show high temperature anisotropies,because the slow wind may experience perpendicular heating as fast wind does when close to theSun. This signature is confirmed by Wind spacecraft measurements as we track PSP observations to1 au. Further, with nearly 15 years of Wind measurements, we find that the distributions of plasmacharacteristics, temperature anisotropy and helium abundance ratio ( N α /N p ) are similar in slow winds Corresponding author: Jia [email protected] a r X i v : . [ phy s i c s . s p ace - ph ] M a y Huang et al. with different Alfv´enicities, but the distributions are different from those in the fast solar wind. HighlyAlfv´enic slow solar wind contains both helium-rich ( N α /N p ∼ . N α /N p ∼ . Keywords:
Alfv´enic slow solar wind, helium abundance, temperature anisotropy, solar wind, origin INTRODUCTIONSolar wind is an ionized plasma that flows out from the Sun; it consisting of protons, electrons, alpha particlesand some minor ions (McComas et al. 2007; Priest 2014). The solar wind can be classified into slow solar wind(SSW; v sw < km s − ) and fast solar wind (FSW; v sw > km s − ). They generally manifest different plasmacharacteristics and have different solar origins (e.g. Wang et al. 2000; Kasper et al. 2007; McComas et al. 2013; Huanget al. 2016a,b). SSW is generally characterized by high proton density, low bulk speed and low proton temperature,while FSW shows opposite plasma signatures (Borrini et al. 1981; Suess et al. 2009). The compositional measurementsof helium particle and minor ions give clues to identify the source regions of different solar winds (Feldman et al. 1981;Geiss et al. 1995; Bochsler 2007; Chandran et al. 2013; Kasper et al. 2013). The freezing-in temperature, as interpretedby charge states of minor ions, indicates the coronal electron temperature, because the ionization and recombinationprocesses of the minor ions are balanced at about 1.2-3.5 solar radii ( R S ) and the temperature frozen in as the solarwind propagates outward into collisionless regime (B¨urgi & Geiss 1986; Ko et al. 1997; Zhao et al. 2009; Huang et al.2017; Huang et al. 2018). The elemental abundance ratios are associated with the first ionization potential (FIP) effect,which is a consequence of processes occurring near the large temperature gradients at the base of the solar transitionregion (Geiss 1982; Fisk et al. 1999; Raymond 1999). These compositional parameters are retained as the solar windpropagates beyond a certain height from the Sun, thus they can link the in situ characteristics of different kinds ofsolar wind to their source regions with high confidences(Kasper et al. 2007; Zhao et al. 2009; Fu et al. 2017). As theSSW shows higher charge states and elemental abundance ratios of minor ions than those in FSW, it is likely thatthe SSW originates from regions with higher electron temperature and larger long-lived magnetic loops (Fisk et al.1999; Fisk & Schwadron 2001), i.e. closed magnetic field regions such as helmet streamer (Suess et al. 2009; Liu et al.2014; Peng et al. 2017), pseudostreamer (Crooker et al. 2014; Huang et al. 2016a,b), active regions (Kasper et al. 2007;Brooks et al. 2015), small coronal holes and coronal hole boundaries (Higginson et al. 2017; Wang 2017; Liu et al.2020), while the FSW comes from open magnetic field regions, namely coronal holes and their associated regions (Tuet al. 2005; Cranmer 2009; Abbo et al. 2016). In addition, helium abundance ratio ( N α /N p ) are usually depleted inthe vicinity of heliospheric current sheet (HCS) while enhanced in FSW and magnetic clouds, implying the depletionmay originate from closed field regions of helmet streamer (Borrini et al. 1981; Gosling et al. 1981; Suess et al. 2009).Moreover, the N α /N p dependence on solar wind speed and solar activity indicates that, in SSW, the depleted N α /N p (helium-poor population, N α /N p ∼ . N α /N p (helium-rich population, N α /N p ∼ . C /C and O /O ) as FSW, and its temperatureanisotropy ( T ⊥ /T (cid:107) ) also shifts from the isotropic state of regular SSW ( T ⊥ /T (cid:107) ∼
1) to anisotropies typical of theFSW ( T ⊥ /T (cid:107) > SP SSW R S from the Sun’s surface, and first overviews of the solar wind observed during the first twoencounters have been reported (Kasper et al. 2019; Bale et al. 2019). As PSP provides unique observations of Alfv´enicSSW in the inner heliosphere, we statistically compare its properties with other solar wind intervals, characterizingthe origins of SSW during the first encounter (E1) PSP data inside 0.25 au. We introduce the data in Section 2. InSection 3, we present an example of Alfv´enic SSW, investigate the temperature anisotropy variations, and comparewith 1 au observations. The discussions and results summarized are given in Section 4. DATAThe Solar Wind Electrons, Alphas, and Protons (SWEAP) instrument suite (Kasper et al. 2016) and the FIELDSinstrument suite (Bale et al. 2016) onboard PSP provide the data used in this work.SWEAP includes the Solar Probe Cup (SPC) (Case et al. 2020) and Solar Probe Analyzers (SPANs) (Whittleseyet al. 2020; Roberto & SWEAP 2019). It is designed to measure velocity distributions of solar wind electrons, protons,and alpha particles. In this paper, we focus on proton measurements from SPC, which is a Sun-pointed Faraday Cup.The proton data are derived from both moment and non-linear fitting algorithms (Kasper et al. 2002). The moment algorithm returns characteristics of a single, isotropic proton population. The non-linear fitting algorithm assumes aproton core and a proton beam population. Generally, the proton core corresponds to the peak of the solar wind protonvelocity distribution function (VDF) and the beam corresponds to its shoulder. A summed core+beam population,which takes into account their relative drift, is also reported. FIELDS is designed to measure DC and fluctuatingmagnetic and electric fields, plasma wave spectra and polarization properties, the spacecraft floating potential, andsolar radio emissions (Bale et al. 2016).SPC’s operation mode varies with the distance from the Sun. During Encounter mode when PSP is inside 0.25 auor 54 R S , its sampling rate is highest (Kasper et al. 2016), and SPC collected one measurement every 0.874 s in E1(Case et al. 2020). For Cruise mode, the time resolution is lowered to 27.962 s during E1 (Case et al. 2020). We selectintervals for which all the SPC proton quality flags (except for the four flags associated with helium measurementsthat are still under calibration) indicate good observations.PSP/FIELDS collects high resolution vector magnetic fields with variable time resolutions. During E1, the datarates vary from 2 . moment data set, inside 0.25 au, i.e. from Oct.31 st , 2018 to Nov. 11 th , 2018. OBSERVATIONS3.1.
Overview
The Alfv´enicity is interpreted by normalized cross helicity ( σ C ) that is defined as σ C = ( E + − E − ) / ( E + + E − ),where E ± corresponds to the power spectra of Els¨asser variables z ± = δ v ± δ B / √ µ ρ (Wicks et al. 2013; Chenet al. 2013). µ is the vacuum magnetic permeability, ρ is proton moment density, and δ v and δ B represent thefluctuations of velocity and magnetic field, respectively. The σ C values of -1 and 1 indicate a pure Alfv´en wavepropagating sunward and anti-sunward. The residual energy σ R = ( E v − E b ) / ( E v + E b ) is calculated to indicate thedifference between kinetic energy ( E v ) and magnetic energy ( E b ) (Wicks et al. 2013; Chen et al. 2013). Chen et al.(2020) and Parashar et al. (2020) find the correlation time for the magnetic field observed by PSP fluctuates between300s and 600s. Therefore, we choose 20 minutes, which covers 2 to 4 correlation times, to calculate the background Huang et al. velocity and magnetic fields used to calculate the cross helicity in this work. McManus et al. (2020) suggest that σ C isunaffected by switchbacks, which are Alfv´enic structures that are prevalent in the inner heliosphere (Bale et al. 2019;Kasper et al. 2019; de Wit et al. 2020; Horbury et al. 2020; Mozer et al. 2020), thus we will include them in this study.We note that, the kinetic features of cross helicity and residual energy are investigated with PSP measurements byVech et al. (2020).Figure 1 presents an overview of cross helicity variations with solar wind speed (upper panels) and residual energy(lower panels) inside 0.25 au. From left to right, the panels show the variations in all solar wind, SSW and FSW,respectively. The colorbar indicates the normalized ratios of bin counts to the maximum bin value, and the redhistogram in the upper panels represents the fraction of high Alfv´enicity solar wind ( | σ C | > .
7) at each speed bin.Panel (A1) indicates that high Alfv´enicity population exists in all solar wind but with different proportion, and high | σ C | population shows a nearly positive correlation with solar wind speed (red histograms). Panel (A2) further suggeststhat high Alfv´enicity population generally shows roughly balanced kinetic and magnetic energy ( σ R ∼ km s − is probablyassociated with the speed shears between SSW and FSW. Panel (C1) and (C2) denote that high Alfv´enicity populationwith nearly equal kinetic and magnetic energy dominates 79.1% of the FSW. This ratio is 55.3% for SSW and 57.3%for all measured solar wind inside 0.25 au. If we exclude the time periods of switchbacks, we find the high Alfv´enicitypopulation of SSW decreases to 55.2%, while the number for FSW and all measured solar wind are 78.8% and 57.5%,respectively. The slight differences support above conclusion that the switchbacks may not affect the σ C significantly,and they may only slightly contribute to the Alfv´enicities in the solar wind during E1. Even though the highly Alfv´enicpopulation in the SSW is less prominent than that in the FSW, the percentage of 55.3% at current solar minimum ismuch larger than the 34% of quiet-Sun-associated Alfv´enic SSW as observed at solar maximum at around 1 au (Wanget al. 2019), implying that high-Alfv´enicity SSW is prevalent in the inner heliosphere even at solar minimum.3.2. Plasma Signatures
In this section, we present an example of Alfv´enic SSW, and compare its plasma signatures with that of typicalSSW from a helmet streamer, i.e. heliospheric plasma sheet (HPS). The HPS is defined as a region with high plasmabeta β region in vicinity of the HCS (Winterhalter et al. 1994; Liu et al. 2014; Peng et al. 2019). Szabo et al. (2020)lists eight HCSs with multiple crossings based on PSP E1 observations, with the most distance crossing measured at0.69 au. In order to choose the least evolved HPS SSW for comparison and given that no HCS crossing is measuredinside 0.25 au during E1, we select the HPSs inside 0.35 au for study, and we note that the SSW inside 0.25 au in thiswork means non-HPS SSW unless otherwise specified. Due to the multiple rapid crossings of HCSs, the HPSs are alsomultiple and rapidly crossed. However, we exclude the rapid HPS crossings, and only select cases that last for a timeperiod in the vicinity of HCS-associated high density regions as marked by Szabo et al. (2020). Table 1 lists threeHPSs with their possible start time, end time, and radial distance presented.Figure 2(A) shows the second HPS crossing on Oct. 29 th , 2018. From top to bottom, the panels show magnetic fieldcomponents in RTN coordinates, azimuthal angle φ B , proton number density N p , bulk speed, proton temperature T p ,plasma beta β , and entropy ( S p = T p /N / p ). The vertical red dashed lines mark the HPS structure with β increasesfrom a baseline of about 1 to nearly 10, and it shows typical SSW characteristics with high density, low temperatureand low entropy. The correlation coefficients between the RTN components of δ v and δ B are (-0.40, 0.23, -0.26),and the average | σ C | is 0.36, indicating low Alfv´enicity. Figure 2(B) presents 6 hours of Alfv´enic SSW during E1perihelion, with the same format as Figure 2(A). It is clear to see the low density, high temperature and high magneticfield strength, and the resulting β is generally smaller than 1, implying the magnetic pressure dominates. Besides,the entropy seems to be more variable than that in HPS. The correlations of δ v and δ B are (0.93, 0.74, 0.84), andthe relative lower correlation in tangential direction could be caused by the one-sided Alfv´enic structures (Kasperet al. 2019). The average | σ C | is 0.78, and high Alfv´enicity SSW occupies 76.3% of this interval, indicating the highlyAlfv´enic nature of this interval of solar wind.The plasma characteristics of the 6 hours Alfv´enic SSW are similar to previous results (DAmicis et al. 2018), whichsuggest the Alfv´enic SSW originates from a coronal hole. This assessment agrees with the interpretation presented byBale et al. (2019), which suggests that this interval comes from a small equatorial coronal hole using the PFSS model SP SSW Figure 1.
Cross helicity variations in all solar wind, slow solar wind and fast solar wind inside 0.25 au. The upper panelsshow cross helicity variations with solar wind speed. The red histogram lines indicate the ratio of high Alfv´enicity ( | σ C | > . and extreme-ultraviolet map. However, we could not find significant differences of these plasma signatures in SSWswith different Alfv´enicities. Figure 3 shows the normalized frequency of plasma parameters in different solar winds,with the blue and black histograms representing HPS SSW and FSW below 0.25 au, respectively. We further separatethe non-HPS SSW into low ( | σ C | < . . ≤ | σ C | ≤ .
7) and high ( | σ C | > .
7) Alfv´enicity SSW, with thehistograms colored with green, orange and red, respectively. From panel (A) to (F), the variations of proton speed,temperature, density, magnetic field strength, plasma beta and proton entropy are presented. The parameters aremeasured at different radial distances, we thus scale them (except speed) to 0.25 au for comparison. The temperatureis scaled with ( R S / . / , the density and magnetic field strength are scaled with ( R S / . , with the plasma betais normalized to ( R S / . − / while the proton entropy is kept the same (e.g. Huang et al. 2020). The scaled resultsfrom E1 are not significantly affected, because the radial distance changes only from 0.169 to 0.25 au during E1. Inpanel (A), the speed of FSW is presented on the same axis by subtracting its value by 250 km s − . Even though themagnetic field strength and plasma beta of the FSW and SSW overlap a little, other parameters show very distinctsignatures for the two kinds of wind. The results suggest that the HPS SSW and FSW are different from the non-HPSSSW, while the SSWs with different Alfv´enicities are similar except slight differences, implying high Alfv´enicity SSWis not remarkably different from other non-HPS SSWs.3.3. Temperature Anisotropy
In order to investigate the thermodynamic state of Alfv´enic SSW in the heliosphere, we compare the temperatureanisotropy variations in different solar winds in this section.Figure 4 presents the temperature anisotropy as a function of parallel plasma beta ( β (cid:107) p = 2 µ N p k B T (cid:107) p /B , where T (cid:107) p and k B denote the parallel temperature and Boltzmann constant, respectively, and the proton moment values Huang et al.
Table 1.
Heliospheric Plasma Sheet Crossings during E1 inside 0.35 au.HPS NO. Start Time (UT) End Time (UT) Radial Distance (au) ∼ . ∼ . ∼ . HPS -40-2002040 B r t n ( n T ) Br Bt Bn B φ B N p ( c m - ) V p ( k m s - ) T p ( K ) β S p ( K m ) hhmm2018 (A) (B) Figure 2.
Comparison between heliospheric plasma sheet (HPS) and highly Alfv´enic slow solar wind. As shown by dashed redlines, Figure (A) and Figure (B) show the general characteristics of a heliospheric plasma sheet observed on October 29 th , 2018and a 6-hour time period of Alfv´enic slow solar wind around E1 perihelion, respectively.. are used to calculate the parameters). The colorbar shows the probability density in each bin, and we follow themethod of Maruca et al. (2011, 2018) to define the probability density, i.e. p = n/ ( N ∆ β (cid:107) p ∆( T ⊥ p /T (cid:107) p )), where nis the number of data in the bin, N is the total amount of data in the data set, and ∆ β (cid:107) p and ∆( T ⊥ p /T (cid:107) p ) are thewidths of the bin along each axis. The red, blue, orange and green dashed lines in each panel indicate the mirror,ion-cyclotron, parallel and oblique firehose instabilities (Kasper et al. 2002, 2007; Maruca et al. 2012; Verscharen et al.2016; Klein et al. 2017, 2018), respectively, with the thresholds from Hellinger et al. (2006). The black line representsthe observed anti-correlation between T ⊥ p /T (cid:107) p and β (cid:107) p , which is first derived from Helios observations by Marschet al. (2004). This relationship is believed to be formed by resonant interactions between ion cyclotron waves andprotons, as described by the quasi-linear theory of pitch angle diffusion (Marsch et al. 2004). Panel (A) and Panel (B)show the distributions in non-HPS SSW inside 0.25 au and in HPSs, respectively. The typical helmet streamer SSW, SP SSW Figure 3.
The normalized frequencies of plasma characteristics in different solar wind streams. Panel (A) to (F) shows thevariations of solar wind speed, proton temperature, density, magnetic field strength, plasma beta and proton entropy, respectively.The colors represent different solar wind streams, with black, blue, red, orange, and green histograms indicating fast solar wind(FSW), HPS slow wind, high Alfv´enic slow solar wind (High ASSW; | σ C | > . . ≤ | σ C | ≤ .
7) and lowASSW ( | σ C | < . km s − to fit the figure. Temperature, density,magnetic field strength, the resulted plasma beta, and proton entropy (keeps the same after scaling) are scaled to 0.25 au forcomparison. . even with limited data points, displays the expected large β (cid:107) p and isotropic T ⊥ p /T (cid:107) p , but the non-HPS SSW includesboth isotropic and anisotropic temperatures. FSW in Panel (C) generally shows larger temperature anisotropies thatmatch pretty well with the anti-correlation model. This is already observed from 0.3 to about 1 au with Helios andUlysses data (e.g. Matteini et al. 2007), thus it is most likely not an observational bias due to only one orbit of PSPdata is used here. Furthermore, the distributions in low, medium and high Alfv´enicity SSW are exhibited in Panel(D1) to (D3). In comparison, low | σ C | SSW is dominated by isotropic temperatures, while high | σ C | SSW has moreanisotropic temperatures. However, all of them, no matter their Alfv´enicities, have both isotropic and anisotropictemperatures, and similar distribution shapes. This is different from previous results based on a multi-event studythat highly Alfv´enic SSW shows similar microphysical states as FSW, but deviates from that of regular SSW (theydo not separate HPS and non-HPS SSWs) (DAmicis et al. 2018). As introduced by Huang et al. (2020), the non-HPSSSW may experience perpendicular heating as FSW does when close to the Sun, which contributes to the anisotropictemperatures. 3.4.
Comparisons with 1 au observations
In this section, we compare Alfv´enic SSW in the inner heliosphere with that at 1 au observed by the Wind spacecraft.The Wind/SWE Faraday cups measure the reduced distribution functions of solar wind proton and helium along 40angles every 92 s (Ogilvie et al. 1995). In this study, we use the data of velocity, number density, and temperatureanisotropies. The temperature anisotropies are derived by fitting the measurements with convected bi-Maxwelliandistribution functions (Kasper et al. 2006, 2007). The magnetic field data are from Wind/MFI (Lepping et al. 1995).Only data from June 2004 and after are selected (about 15 years), as Wind has resided as the Lagrange 1 point sincethat date. Further, if we only select the Wind measurements at solar minimum to make the comparisons, the following
Huang et al.
Figure 4.
The probability density variations of temperature anisotropy in different solar wind streams. Panel (A) and (B)show the variations in non-HPS and HPS slow solar wind, respectively. Panel (C) indicates temperature anisotropy variationsin fast solar wind. Panel (D1) to (D3) present the variations in Alfv´enic SSW with low ( | σ C | < . . ≤ | σ C | ≤ . | σ C | > .
7) Alfv´enicity in turn. The red, blue, orange and green dashed lines in each panel indicate the mirror,ion-cyclotron, parallel and oblique firehose instabilities, respectively. The black line represents the anti-correlations betweentemperature anisotropy and parallel plasma beta. . results do not change significantly. We note that varied time scales from several minutes to some hours are used inprevious works to derive the cross helicity with Wind data (e.g. Wicks et al. 2013; Jagarlamudi et al. 2019). In thiswork, we calculate the cross helicity in every 20 minute interval, because the resulted mean cross helicity seems to beslightly higher than that calculated with other time scales. However, the time scale does not significantly affect ourresults as we tried several different time scales to calculate the cross helicity.We first investigate the Wind observations of the SSW observed by PSP in the inner heliosphere. Szabo et al.(2020) suggested that the HCS crossings match remarkably well between Wind and PSP observations by shiftingeither forward or backward of PSP measurements to 1 au based on solar equatorial rotation rate and solar longitude.Thus, we select the solar wind between NO. 2 and NO. 3 HPSs in Table 1 for comparison. According to their results,Wind should observe the solar wind from 2018 November 01 06:00 UT to 2018 November 04 01:00 UT with backwardshifting method, and from 2018 November 25 04:00 UT to 2018 December 01 08:00 UT with forward shifting method.Figure 5 presents the T ⊥ p /T (cid:107) p variations with | σ C | in SSW, with the colorbar indicating the N α /N p . The averagevalues of T ⊥ p /T (cid:107) p and N α /N p (multiplied by 40) at each | σ C | bin are denoted by triangles and diamonds, respectively.It seems the temperature anisotropies are generally smaller than that in the inner heliosphere as shown in followingFigure 7, and the values are independent of | σ C | . These results support our statement above that the temperatureanisotropies are similar in non-HPS SSW with different Alfv´enicities. Moreover, the N α /N p ratio shows a similardistribution with | σ C | , but higher Alfv´enicity SSW has slightly higher N α /N p value, implying highly Alfv´enic SSWmay have different source regions.Second, we use the nearly 15 years Wind data to statistically study the temperature anisotropy and helium abundanceratio distributions in different solar wind streams. Similar to Figure 4, we present Wind observations of T ⊥ p /T (cid:107) p probability density variations in Figure 6. As we have nearly 15 years of data, we use more strict criterion to selectsolar wind here, with FSW faster than 600 km s − and SSW slower than 400 km s − . Panel (A) and Panel (B) SP SSW T ⊥ p /T (cid:107) p variations with solar wind speed, with the colorrepresenting the mean | σ C | value at each bin. Lower panels exhibit T ⊥ p /T (cid:107) p as a function of | σ C | , with the colorbarindicating the normalized ratios. From left to right, PSP moment observations inside 0.25 au, Helios proton coredata from 0.3 to 0.4 au and from 0.9 to 1.0 au, and Wind moment results at around 1.0 au are presented. TheHelios data from both Helios spacecraft are used, with the Helios 1 data covering late 1974 to 1985 and the Helios2 data covering 1976 to 1980, and the cross helicity is calculated every 20 minute with the time scale selected byStansby et al. (2019). The Helios results in this figure have been presented by Stansby et al. (2019), we includethem for comprehensive comparisons. Wind data in panels (D1) and (D2) confirm the well-known scenario that FSWgenerally has higher Alfv´enicity and T ⊥ p /T (cid:107) p than SSW. Helios observations in panels (B2) and (C2) reveal the obviousseparation of isotropic and anisotropic temperatures near the Sun, and the non-Alfv´enic but anisotropic solar winddisappears when close to the Sun. Accordingly, Stansby et al. (2019) classify the solar wind into three types withAlfv´enicity and temperature anisotropy: (1) anisotropic and Alfv´enic solar wind, (2) isotropic and Alfv´enic solar wind,and (3) isotropic and non-Alfv´enic solar wind. Moreover, panels (B1) and (C1) denote the SSW is more Alfv´enic whencloser to the Sun, and SSW nearly solely contributes to the isotropic temperature. However, we cannot see the robustseparation from PSP measurements, which could be caused by the perpendicular heating of SSW and/or the limiteddata of the PSP observations as suggested by Huang et al. (2020). Besides, we may need to include non-Alfv´enicbut anisotropic solar wind when much closer to the Sun (see from both Figure 7 and Figure 4). Because some ofthe low Alfv´enicity SSW would experience perpendicular heating as well, and thus it is reasonable to see anisotropictemperatures in low Alfv´enicity SSW. The disappearance of non-Alfv´enic but anisotropic solar wind at around 0.3 auwill be investigated in future work.Finally, Figure 8 compares the N α /N p variations in different solar winds, with the average value (red line) at eachAlfv´enicity bin or speed bin overlaid. Panels (A) to (C) show the N α /N p variations with Alfv´enicity in all solarwind, FSW and SSW, respectively. The results indicate that the FSW is dominated by helium-rich populations, andit generally has high Alfv´enicity. However, the N α /N p ratio in the SSW seems to have a uniform distribution ofAlfv´enicities, with higher Alfv´enicity SSW has slightly more helium-rich populations, which is consistent with resultsin Figure 5. Panels (D) to (F) present the N α /N p variations with solar wind speed in all solar wind, FSW and SSW,respectively. They further confirm the above statement that the helium-rich population dominates in FSW, while SSWincludes both helium-rich and helium-poor populations, as Kasper et al. (2007, 2012) suggest. Typically, in SSW, thehelium-poor population dominates at solar minimum may come from closed magnetic field regions (Kasper et al. 2007,2012), and the helium-rich population at solar maximum may originate from active regions (Kasper et al. 2007, 2012)and/or small coronal holes nearby (Wang 2017). The N α /N p variations in different Alfv´enicity SSWs are displayedin panels (G1) to (G3). We can see that they all have the two helium populations, and high Alfv´enicity SSW hasmore helium-rich population, which is consistent with above results. However, their similar distributions imply similarbut multiple source regions of SSW with different Alfv´enicities. This result may revise previous argument that highlyAlfv´enic SSW originates from fast-wind-like source regions due to the fact that they share similar charge states andtemperature anisotropy distributions (DAmicis et al. 2018). DISCUSSION AND SUMMARYUsing PSP E1 observations, we investigate the properties of Alfv´enic SSW in between 35 . R S to 54 R S . The highlyAlfv´enic SSW dominates about 55.3% of the SSW at current solar minimum, indicating its prevalence in the innerheliosphere. By comparing the plasma characteristics and temperature anisotropy variations in different solar winds,we find the SSWs with different Alfv´enicities display similar distributions but are distinct from that of FSW. The resultsimply no significant deviations of highly Alfv´enic SSW from regular SSW in both macro- and micro-physical states.0 Huang et al.
Figure 5.
Wind observations of the slow solar wind, which corresponds to the stream of plasma previously observed by PSP inthe inner heliosphere. The colorbar indicates the helium abundance ratio at each bin. The triangles and diamonds represent themedian temperature anisotropy and helium abundance ratio (multiplied by 40) at each absolute cross helicity bin, respectively..
Moreover, the low Alfv´enicity SSW may have high temperature anisotropies, and it further suggests that the solar windclassifications based on temperature anisotropy and Alfv´enicity may need to be reconsidered in the inner heliosphere,where the SSW may experience perpendicular heating as FSW does, contributing to the anisotropic but non-Alf´evnictemperatures. In addition, based on nearly 15 years Wind measurements at 1 au, we first trace the SSW observedby PSP to 1 au, and then statistically study the temperature anisotropy and helium abundance ratio variations indifferent solar winds. These results are consistent with PSP observations that the SSWs with different Alfv´enicitieshave similar plasma signatures and temperature anisotropy distributions, but are different from the FSW. Further,the same feature is found for the helium abundance ratio variations. Both helium-rich and helium-poor populationsin highly Alfv´enic SSW imply that the highly Alfv´enic SSW should originate from multiple source regions.These results indicate that highly Alfv´enic SSW may share similar plasma characteristics and temperature anisotropydistributions (both at 1 au and inside 0.25 au), and helium abundances (at 1 au) with regular SSW, presentingdifficulties to identify the origin and evolution of high Alfv´enicity SSW. It is reasonable to see high Alfv´enicities inSSW from open field regions, but the formation of Alfv´enicity in SSW from closed field regions is still unclear. Moreover,the newly observed prevalent switchbacks in the inner heliosphere are identified as highly Alfv´enic structures that existin both SSW and FSW (Kasper et al. 2019; Bale et al. 2019). However, these switchbacks are rarely observed at 1 au.Therefore, their contributions to the Alfv´enicities of SSW at different radial distances need more study. The heliumabundance data from PSP and compositional measurements from Solar Orbiter in the future will help to further verifythe origin and evolution of Alfv´enic SSW.
SP SSW Figure 6.
Wind observations of temperature anisotropy probability density variations in different solar wind streams, withsame format as Figure 4. The Wind measurements after June 2004 are selected. Panel (A) and (B) show the variations in slowand fast solar wind, respectively. Panel (C1) to (C3) present the variations in Alfv´enic slow solar wind with low ( | σ C | < . . ≤ | σ C | ≤ .
7) and high ( | σ C | > .
7) Alfv´enicity in turn..
Figure 7.
Temperature anisotropy variations with absolute cross helicity and solar wind speed at different radial distances.Upper panels show the T ⊥ p /T (cid:107) p variations with solar wind speed, with the color indicating the mean | σ C | value at each bin.Lower panels show T ⊥ p /T (cid:107) p varies with | σ C | , with the colorbar indicating the normalized ratios of bin counts to the maximumbin value.. From left to right, PSP observations inside 0.25 au, Helios measurements from 0.3 to 0.4 au and from 0.9 to 1.0 au,and Wind results at around 1.0 au are presented in turn. . Huang et al.
Figure 8.
Wind observations of helium abundance ratios in different solar wind streams at 1 au. The Wind measurementsafter June 2004 are selected. Panel (A) to (C) compares the N α /N p variations with Alfv´enicity in all solar wind, fast solarwind and slow solar wind, respectively. Panel (D) to (F) shows their variations with solar wind speed in different solar winds.Panel (G1) to (G3) presents helium abundance ratios in slow solar winds with different Alfv´enicities. The red line in each panelindicates the average helium abundance ratio at each Alfv´enicity bin or speed bin. The colorbar indicates the normalized ratioof bin counts to the maximum bin value.. . SP SSW
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