Solar wind plasma populations leaving the Sun can be expected to have properties that reflect conditions of their source regions. However, during the course of the wind's propagation out to 1 AU and beyond, internal dynamic processes may develop within the solar wind plasma. These cause many of the solar wind properties to be altered to the extent that the signatures of their solar source, such as proton temperature and bulk speed , are no longer clear. Nevertheless, the degree of ionisation of heavy ion species in the solar wind provides a well-established means by which the temperature of its coronal source may be inferred, even when observed at 1 AU .

Solar wind heavy ion populations are frequently characterised through metrics such as statistical abundance ratios between charge states of a given ion (e.g. O7+/O6+) and the mean charge of all measured ions of a given species see the review article byfor details. first predicted that this ionisation information could allow estimates of the coronal electron temperature. The fraction of ions which exist in a given charge state in the corona is determined by the equilibrium between the dominant processes of collisional ionisation and radiative recombination. The resulting distribution of an ion population into its charge states is thus a function of electron temperature. For a solar wind sample, a derived “freeze-in temperature” is indicative of the thermal temperature of the electron population at the location in the corona where the density falls below a theoretical critical value for ionisation and recombination to effectively cease. At this point the ions become “frozen in” to their charge states. As density drops off radially, this critical density is associated with a characteristic “freezing height” , which is different for each ion. estimated that these heights should lie between 1.5 and 4 R⊙, low in the corona. As electron density continues to fall off with radial distance from the Sun, ion charge states are preserved along magnetic field lines out into the solar wind, as long as the solar wind plasma fulfils the frozen-in flux condition. This then provides a snapshot of coronal electron temperature which can be measured in situ. The ratio between two subsequent states of ionisation, ni and ni+1 can be expressed as the ratio between the rates of collisional ionisation out of state ni; Ci and the rate of recombination out of state ni+1; Ri+1: ni+1ni=Ci(Te)Ri+1(Te). This applies only for the case in which the rate of coronal expansion is slow compared to the rate at which ionisation can equilibrate. From Eq. () the freeze-in temperature can be estimated from a given adjacent charge state ratio.

Typically coronal holes, which are thought to produce fast solar wind, are cooler in electron temperature than their closed-field counterparts which are generally associated with the slow solar wind . The coronal hole wind (CHW) thus typically features lower ionisation states than non-coronal hole wind (NCHW), and plotting solar wind speed alongside, for example, O7+/O6+, will show a clear anti-correlation see for example Fig. 1 in. As the charge states are frozen in at a few solar radii, boundaries between streams in the solar wind from different source regions should be preserved in composition data. Indeed as energy and momentum may be transferred across such boundaries, their structure should be better preserved in composition than, say, flow velocity. However, boundaries in ionisation state measurements are sometimes observed to be smoothed out at the trailing edge of solar wind streams, in a similar manner to velocity .

found O7+/O6+ to be a better tracer of coronal origin than velocity observationally. They report that wind from both the northern and southern polar coronal holes could exhibit the same wind speed but have a notable discrepancy in their corresponding O7+/O6+ values. This indicates that O7+/O6+ is a feature characteristic of the source region, providing information which is not available from velocity data alone.

Solar wind electrons are commonly described as consisting of three distinct populations : a thermal core, anisotropised by the magnetic field; a near-isotropic and suprathermal halo; and a strongly field-aligned suprathermal strahl. A more energetic and even less dense fourth population, dubbed the superhalo, has been reported at energies above ∼1keV . The core and halo populations can be adequately described by bi-Maxwellian distributions, to account for temperature anisotropies, as in and later work. However, more recent studies have chosen to model the halo as a bi-kappa function e.g.. The strahl population is more difficult to characterise. Some authors have calculated numerical moments directly from isolated strahl populations. However, the methods for isolating strahl velocity distribution functions (VDFs) can be limited as they are derived from subtracting distributions from different pitch angle bins, over a limited energy range, leading to potentially large uncertainty in the moment. These strahl moments are also subject to assumptions about the extent of the strahl in pitch angle, which can only be estimated to within the angular width of a given measurement pitch angle bin. Alternatively, fitting a model function to isolate strahl components as inwhere a truncated kappa function was fitted may circumvent the issue of energy cut-offs. However, any model functions used are rather ad hoc below the typical energy at which the core/halo populations begin to dominate the strahl.

Moreover, the origins of the distinct strahl and halo electron populations in the solar wind are not well understood. Evidence has been found that a suprathermal tail can exist in the solar wind using exospheric models e.g. which ultimately require a seed suprathermal electron population to exist in the corona. use in situ electron VDF measurements from WIND to provide boundary conditions to their model of electron VDFs which originate at 4 R⊙. Their results suggest that an electron VDF which includes suprathermal electrons at 1 AU must correspond to one which also included suprathermal electrons in the corona. The relative strength of the suprathermal tail is predicted to be considerably weaker in the corona than at 1 AU, and the effect of coulomb collisions in influencing the distribution for slow wind electrons is predicted to be more significant than for fast wind.

Using a kappa function to model the ionising electron population in the corona, simulated the charge state distributions of coronal ions given different core electron temperatures and values of κ. They predict, based on solar wind oxygen and carbon ionisation measurements, that a weak suprathermal tail (κ≥5) should exist in the corona. At such levels it is not expected that the influence of collisions with these electrons on ionisation equilibrium would be very significant. In a related study, offered an explanation of the discrepancy between remote spectral estimates of coronal electron temperature and the freeze-in temperatures measured in situ by invoking additional ionisation by suprathermal electrons. They claimed that the sensitivity of the dominant charge state ratios to Th/Tc (the ratio of modelled electron halo temperature to the core temperature) varied strongly based on species, with O7+/O6+ proving most sensitive. Different halo temperatures were thus thought to be necessary to meet the observed ionisation states for different ions in situ.

In contrast, proposed an explanation of the above compositional–spectral temperature discrepancy via extra heating of the coronal thermal electrons by lower hybrid waves, in place of suprathermal electrons. The author notes that remote estimates of coronal temperature using O VI diagnostics should be sensitive to suprathermal influence. However, these lines do not appear to show evidence of this in practice, casting doubt on the predictions of the existence of a significant suprathermal electron population in the corona.

present a model for halo formation in the corona via a dual-stream instability process which is related to nanoflares. As in the work of , they postulate that nanoflares accelerate electrons in the coronal base to beams with energies on the order of keV. These beamed electrons then travel upwards in the corona, where they trigger a two-stream instability with the thermal electron population. This results in a redistribution of energy, as discussed in , involving a transfer of energy from the nanoflare-triggered electron beam to the core electron population, and the ultimate formation of an isotropic electron halo population. Modelling both as Maxwellians, the core–halo temperature ratio then obeys the relation ThTc≈ncnh1-CTCT+4, where Th and Tc are the halo and core temperatures respectively, nh and nc are the halo and core densities, and CT is the fraction of kinetic energy which is transferred to the core electrons. For values of CT approaching 1, this describes a proportionality between the core and halo electron temperatures in the corona. The authors argue that this feature is preserved out to the solar wind as the coulomb collision rate is insufficient to scatter halo electrons to form a single thermal distribution before reaching the low-density region of the corona. It should also be noted that the predicted height of formation of the electron halo is 1–1.1 R⊙, which is below the ion freeze-in height of ∼2 R⊙. This opens the possibility that the VDF of suprathermal electrons in the corona may have a relationship with the charge states of minor ions, due to their common dependence on the coronal core electron temperature. As ion charge states are not influenced by dynamic processes in the solar wind, a relationship between these charge states and suprathermal electron VDFs persisting at 1 AU would indicate that these electrons have propagated out to 1 AU relatively unaltered themselves.

and showed that the relative density of the halo population increases with heliocentric distance at the apparent expense of the strahl. They thus infer that the strahl is scattered into the halo continuously. estimated the degree of scattering necessary in such a case to counteract the effect of magnetic focusing during solar wind expansion and thus presented an explanation to the observed pitch angle widths of strahl. Modelling by predicts that this scattering is caused by wave–particle interactions, notably with whistler waves. put forward an alternative description involving asymmetric pitch-angle scattering of the halo caused by the relative drift between the core and halo. They predict that the strahl is the unscattered field-aligned portion of the halo which results from this asymmetry. Both of the above descriptions would mean that the halo and strahl can be considered to be largely scattered versions of the same population. Such scattering could potentially distort solar wind electron VDFs to the point at which an initial relationship with heavy ion charge states is no longer apparent at 1 AU.

Results from a study by using solar wind ion and electron observations suggest that there is an influence from the coronal source evident in the in situ suprathermal electrons at 1 AU. Combining heavy ion data from the SWICS instrument on the ACE spacecraft with electron data from the 3DP instrument on WIND, these authors reported a relationship to exist between properties of the electron suprathermal tail and the charge state ratio of oxygen. In particular, the energy content of the suprathermal tail was characterised by defining an effective suprathermal temperature (hereafter Teff) using the VDF derived from WIND electron measurements. This temperature, derived by differentiating the equation for a single Maxwellian distribution, is Teff=-1k(dln⁡f/dE), where k is the Boltzmann constant, f is the electron VDF, and E is energy. For a pure Maxwellian distribution, dln⁡f/dE would be constant with energy. However, as observed suprathermal electrons do not follow a perfect Maxwellian, particularly at higher energies, this Teff calculated with observational data in fact varies with energy.

Applying this calculation for Teff at a number of energies (300, 500 and 800 eV) to solar wind data at a boundary between two slow–fast wind transitions, a correlation between the variation of Teff (at a given energy) and O7+/O6+ was investigated by . The authors restrict themselves to two periods in the solar wind observations where the wind speed has just increased significantly between streams, and find that O7+/O6+ varies similarly to Teff. This leads them to conclude that suprathermal electrons at 1 AU retain information about their coronal source as ionisation states do. However, the study is limited to only two short (∼5 day) intervals in the ACE and WIND datasets. Therefore, this relationship is yet to be more generally verified.

On the basis of the studies described above, one could expect that, directly above the region of solar wind formation, a positive relationship between the energy content of the suprathermal electrons and heavy ion charge states might exist. The low-collisional nature of suprathermal electrons in the solar wind suggests the possibility that they may retain these coronal signatures, and therefore a relationship with the ions, out to 1 AU. In particular, the field-aligned strahl electrons may be most likely to retain such information, as their far more rapid propagation through the heliosphere should subject them to less scattering e.g..

In this paper, we attempt to re-examine the possible preservation of a coronal electron temperature signature in suprathermal electrons (both halo and strahl) at 1 AU by evaluating their possible relationship with charge states of heavy ions sampled in the same streams of solar wind at L1. We first attempt this by addressing limitations of the method by fitting the entire core and halo/strahl range of energies using a Maxwellian + kappa fit, and then compare parameters drawn from these fits to the O7+/O6+ charge state ratio. Further, we isolate the strahl portion of the electron distribution and take partial moments of these to test for any relationship of the strahl at 1 AU with the electron temperature of its source. We use the suprathermal electron parameters produced through these methods in a statistical analysis over a large dataset in order to robustly explore the nature and repeatability of this possible relationship. We do so with the view that a positive relationship is indicative of an observational agreement with the description in the previous paragraph, while a negative relationship is indicative either that this description is not accurate or that the relationship has been heavily altered en route to 1 AU, in either the corona or solar wind.

We use ion charge state data from ACE-SWICS and electron flux data from WIND-3DP to approximate simultaneous observations of solar wind heavy ions and suprathermal electrons as closely as possible. The time period considered covers 1998–2011, during which both satellites spent the majority of their time orbiting L1. Additional magnetic field measurements are taken from the WIND-MFI instrument. All in situ data used in this study are available from NASA Space Physics Data Facility's CDAWeb service (https://cdaweb.sci.gsfc.nasa.gov). The Solar Wind Ion Composition Spectrometer (SWICS) on the Advanced Composition Explorer (ACE) measures the properties of solar wind ions. SWICS is designed to measure the mass and charge of common solar wind ions with masses ranging from H to Fe to determine their ionisation and isotopic states . Ion charge state data are provided in the form of charge state ratios or mean charge, depending on the species, at 1 h time resolution.

The 3-Dimensional Plasma Analyser (3DP) instrument on WIND measures three-dimensional distributions of electrons and protons using four electrostatic analysers (two per species, collectively covering 3 eV–30 keV), and two solid state telescopes which measure electron energies up to 400 keV and protons up to 6 MeV . The electrostatic analysers allow the production of electron and proton velocity distributions as functions of look direction. We derive electron distribution functions from differential electron flux spectra measured by the electron electrostatic analysers: EESA-L (∼5eV-1 keV commonly at ∼30s cadence) and EESA-H (∼100eV-30keV commonly at ∼98s cadence). Data from each analyser are available with look directions re-binned into eight electron pitch angle (PA) values (∼15, 35, 57, 80, 102, 123, 145 and 165∘ relative to the magnetic field direction). We use the magnetic field vector, B, produced by the WIND Magnetic Field Investigation to convert the pitch angles such that they span from the direction of electron propagation along the field line which is anti-sunward (that is, the common strahl direction) to sunward. These shall henceforth be referred to as PA bins 1 to 8, and the VDFs which are derived from the fluxes in these bins as f1 to f8. Bin 1 is the anti-sunward bin which will most commonly contain strahl, while bin 8 will contain strahl in the less common case of a sunward beam. To minimise computation time, these distributions are averaged to the same time resolution as the 1 h SWICS heavy ion data to which the moments will be compared.

The WIND spacecraft is subject to positive charging on the order of 5–15V. Estimates of spacecraft potential, Φ, are available in the “WI_ELM2_3DP” dataset on CDAWeb. A positive potential provides a fixed additional energy to all detected solar wind electrons. The potential also accelerates photoelectrons towards the spacecraft, which appear only at energies below that corresponding to Φ (to within the energy resolution of the detector). To remove the photoelectrons and correct the energies measured, we shift the energy bins down by a value equal to Φ. Data from any bins which are thus assigned a negative energy are considered photoelectrons and removed from the analysis. Note, however, that the suprathermal electron parameters calculated here in all cases concern electrons too high in energy to be contaminated by a photoelectron population. This energy range is also high compared to Φ, which means that the suprathermal electrons are not significantly altered by the acceleration due to the positive potential. We thus continue our analysis under the assumption that any possible inaccuracies in the reported value of Φ are insufficient to alter our ultimate conclusions.

During the chosen period 1998–2011, suitable data are sometimes sporadic due to the orbit of WIND taking it away from L1. In particular, gaps exist due to this in the data taken before 2005. We have chosen time periods where WIND spends several days at a time near L1 with which to carry out this study. Data were used only when WIND's orbital position data indicated that it was within 100 RE of L1. This distance can be compared to the correlation length scale of the solar wind, which is typically 100 RE or larger at 1 AU see e.g.. To maximise the compatibility of the data from the two spacecraft, solar wind proton bulk velocity measurements taken from ACE-SWICS were compared with those from WIND-SWE. Cross-correlation was performed on the proton velocity data from both WIND and ACE to reveal what time lag was present between the two spacecraft. The calculated time lags were always smaller than the 1 h time resolution of the SWICS composition data available, and so no corrective time shifting was performed on the data. We thus consider ACE and WIND to be sampling the same packets of solar wind for the majority of periods used in this study, to within the resolution limits of the data.

We first compare time series of O7+/O6+ with proxy suprathermal temperatures Th–s1 and Th–s⟂. Figure  shows time series of solar wind bulk proton speed vp (from SWICS), oxygen charge state O7+/O6+, and temperatures Th–s1 and Th–s⟂ taken during 2008 (Fig. a) and 2009 (Fig. b) over ∼16 days. These time periods have been chosen to best contrast the possible relationships between these parameters, which depend on the heliospheric conditions at the time. We observe that both Th–s1 and Th–s⟂ appear to vary in agreement with O7+/O6+ in Fig. a but in Fig. b vary oppositely. Viewing the data this way, it is immediately apparent that there can be no consistent tendency for our proxy suprathermal temperature to either correlate or anti-correlate with O7+/O6+. Other time periods can also be found where no apparent positive or negative relationship is clear.

Next, we plot proxy suprathermal temperatures against O7+/O6+ directly, to bring to light which, if any, relationship it has with O7+/O6+. Analysing scatter plots of Th–s against O7+/O6+, and producing associated correlation coefficients allows for a more robust analysis of the nature of any possible relationship between the two than is possible with time series data alone. We compare data over the timescales of Carrington rotations as this allows as close to a full, instantaneous sample of all of the solar wind in the ecliptic at 1 AU as possible. This minimises any effects from drifting O7+/O6+ relative to Es or Th–s from any temporal factors on the Sun, in our correlation calculations. Figures  and show the result of plotting Th–s1 and Th–s⟂ against O7+/O6+ for Carrington rotations 2067 (day 52–80, 2008; end of declining phase of the solar cycle) and 2089 (day 286–315, 2009; beginning of rising phase of the solar cycle), respectively. Note that while the times in Fig. b overlap with CR-2089, Fig. a does not overlap with CR-2067. Pearson linear correlation coefficients are calculated between each temperature and log⁡10(O7+/O6+). Corresponding p values for these correlations, and those shown in Figs.  and , have all been found to tend to zero, and so are not displayed on the plots themselves. We use the logarithm of O7+/O6+ as it varies over orders of magnitude for linear changes in freeze-in temperature, as shown in the previous section. Th–s⟂ exhibits a positive relationship (r=0.701) with O7+/O6+ during CR-2067, and a negative one (r=-0.621) during CR-2089. Th–s1 varies similarly, although it has smaller magnitude in r for both Carrington rotations shown. Thus, for isolated time periods, and indeed whole Carrington rotations, it is possible to find both somewhat convincing positive and negative relationships. This agrees with the relationships inferred from the time series data in Fig. . We note that the Th–s⟂-O7+/O6+ relationship shows signs of being split into a pair of populations during CR-2067; for high and low Th–s⟂. This does not appear to be the case for Th–s1, or for either of the relationships during CR-2089.

Turning to the electrons identified as forming the strahl populations, we now plot Es against O7+/O6+ in the same format in Fig. a and b, for Carrington rotations 2067 and 2089. The former period exhibits a mild positive correlation, with Es varying over a range of approximately 20 eV, similarly to Th–s⟂ in the same time period. During the latter period, there is no strong positive or negative correlation, although the Es values show a similar range of variation as those in Carrington rotation 2067. We also do not observe any apparent grouping of points during CR-2067 as we did for Th–s⟂.

To explain the observation of both periods of positive and negative correlations between O7+/O6+ and proxies for the suprathermal temperature, we now consider the data over multiple Carrington rotations. We group the available data in time, based on the phase of the solar cycle, which we define simply by using quartiles of the monthly sunspot number, acquired from the SILSO World Data Center. Our Th–s⟂, κ⟂, Es and O7+/O6+ data from the lower and upper quartile time periods (2006–2010 and 1999–2003, respectively) are shown in the left and right columns of plots in Fig. . Data from the remaining two middle quartiles are shown combined in the central column. We note that there is a large portion of missing data in the upper quartile time period owing to the orbit of WIND. This period falls primarily over the time range for the sunspot maximum, leaving only around 1 year's worth of data available for that quartile in total.

To contextualise the types of solar wind which are represented in these plots by solar wind speed as well as O7+/O6+, for each column we define λ as the fraction of solar wind samples in each bin of O7+/O6+ which can be considered “fast” (v>500kms-1). We plot λ against O7+/O6+ as the top panel of each column. This shows in each case that the low (high)-O7+/O6+ portions of each period contain >80% fast (slow) wind, with intermediate sections at mid-range values. We note, however, that with increasing solar activity (moving left to right) we see a trend for the transition from fast to slow to occur at higher O7+/O6+ values. Further, we attribute the up-turn in λ at high-O7+/O6+ to be a result of the high-speed solar wind which is associated with compositionally hot active regions.

Each main panel in Fig. a–i plots a derived electron parameter (on the y axis) against O7+/O6+ as a 2-D histogram which has been normalised by the number of data points in each column of O7+/O6+. In this way, the colour of each box describes the probability of measuring that value of y given the corresponding O7+/O6+ value. This normalisation is applied to account for discrepancies in the number of samples at the extremes of O7+/O6+, which tends to be skewed strongly towards higher values. A dashed white line in each plot traces the weighted mean y for each bin of O7+/O6+.

The data acquired during periods of quiet Sun in Fig. a displays a weak upwards trend for Th–s⟂ with O7+/O6+, which climbs primarily between O7+/O6+=0.002 and 0.02, levelling out and falling off at O7+/O6+>0.1. This is most clearly visible in the mean line, as the spread of the data in Th–s⟂ is very broad. The large spread in the data means that any correlation coefficient calculated from it would be very small. The histogram appears to be split into two clusters, in the bottom left and bottom right of the plot, at around O7+/O6+=0.02. This corresponds to around 70% fast solar wind.

The data acquired near solar maximum in Fig. c do not exhibit an upwards trend or clustering, in contrast to the lower quartile data. However, in this case the data do not extend to below O7+/O6+=0.02, which is near the cut-off for the clustering and upwards gradient observed in Fig. a. This is likely due to the properties of solar wind streams which existed at these times, possibly in combination with sampling issues brought about by WIND's orbit. Nevertheless, a downwards trend around higher O7+/O6+ values still seems apparent. Th–s⟂ overall appears to be lower on average than in Fig. a, with a wider spread that may be due to a lack of samples taken for this period.

Figure b contains the same plot as above for the remainder of the Th–s⟂ data, covering mid-levels of activity. The left section of the plot appears to mimic the relationship found in Fig. a, while the right mimics that found in Fig. c. This suggests that these relationships may be dependent mostly on the availability of high and low O7+/O6+ solar wind at low latitudes.

In Fig. d–f we plot κ⟂ against O7+/O6+ for different solar cycle periods. Overall this parameter exhibits far more spread than we see in Th–s⟂. We find that in the low-O7+/O6+ section, κ⟂ increases with increasing charge state, whereas it falls with increasing charge state in the high-O7+/O6+ section. This is similar to the change in Th–s⟂ in Fig. a and particularly Fig. b. Further, we see κ⟂ decline with O7+/O6+ in Fig. f, as Th–s⟂ does in Fig. c. For all periods of the solar cycle there is a degree of agreement between these two parameters which both primarily describe the halo population. We note that this result appears to agree with , who reported correlation during fits to the halo population between the temperature and κ.

We find similar results for Es in Fig. g–i as we do for Th–s⟂, with some distinctions. A positive correlation with O7+/O6+ can be seen when sufficiently low values of O7+/O6+ are sampled, as is evident from the white line which illustrates the mean in Fig. g. However, at times when these low values are not sampled in the solar wind the upward trend appears to continue. The increasing trends found here are associated with a similar wide spread in underlying values to those in Th–s⟂. We find in Fig. h the strongest positive trend in Es with O7+/O6+. Particularly, in the upper range of O7+/O6+ (>0.1) there is a moderate increase in Es.

Returning to calculations of correlation coefficient, we repeat the calculation of r for our suprathermal electron parameters against log⁡10(O7+/O6+), for each Carrington rotation within the available dataset. The results of this are shown in Fig. . Figure a serves to contextualise the correlation data in the rest of the plot. The dashed black line shows the monthly sunspot number, showing that the full dataset spans the time of approximately one solar cycle. The first half of the data occurs around solar maximum, and the second around minimum. The histogram shows in red the occurrence of ICMEs detected at 1 AU, taken from the Richardson and Cane (2010) ICME list. Greyed-out boxes show periods when absence of WIND data has prevented analysis. The period with fewest ICMEs appears to correspond to the period of maximum positive correlation for Th–s⟂, around 2007–2008. This also coincides with the declining phase of cycle 23 indicated by the sunspot number, with a slight offset in time. Apart from this trend, there does not appear to be a direct correspondence with ICME activity and the correlation of O7+/O6+ with any of the suprathermal electron parameters on a per-Carrington rotation basis.

The variation in r with time is shown in Fig. b for Th–s⟂ only. Filled-in points indicate correlation coefficients with a corresponding p value of less than 0.05, a typical cut-off for significance. The square of a Pearson correlation coefficient, r2, can be interpreted as the fraction of variation in the data which is described by the assumption that the two variables from which r is calculated are linearly dependent. We plot r⋅|r| for the correlation of Th–s⟂ (Th–s1) with O7+/O6+ in Fig. c (Fig. d). This value expresses the value of r2 between the parameters, while still preserving the sign of r. With this parameter we easily observe that, for the majority of Carrington rotations, there is very little dependence of halo temperature on O7+/O6+, as r2 rarely exceeds 25 %. Figure c shows r⋅|r| exceeds a positive correlation with dependency of 25 % during some Carrington rotations in 2007–2008; the period of fewest ICMEs noted above.

To quantify the extent to which a full sample of the available solar wind conditions have been captured for a given Carrington rotation, we define Δv as the lower quartile value of the solar wind speed subtracted from the upper. This provides a description of the range of velocities covered by the data, which will be smaller when the solar wind exhibits less diversity in its streams, or when a portion of the data corresponding to one velocity regime is missing. To test if there is a relationship between the degree of correlation and Δv, we plot the two directly against each other in Fig. . Any apparent tracking in Fig. b only amounts to a small correlation of 0.363. The degree of correlation between perpendicular suprathermal temperature and O7+/O6+ is not very sensitive to the diversity of available wind speed.

Figure e shows r⋅|r| calculated for the correlation coefficient of Es with O7+/O6+. As with the other parameters, any relationship represented by these values of r is very weak, as r2 never exceeds 0.5, and the values are typically smaller than even those for Th–s⟂. The most extended period where r is positive appears to fall between 2004 and 2008, the declining phase of cycle 23, which is slightly longer than the extended positive period for Th–s⟂. There is no comparable period of negative correlation, although there is a period of extended near-zero correlation which appears to correspond to the period of most negative correlation for both Th–s⟂ and Th–s1 around the rising phase of cycle 24.

We have shown that suprathermal temperature proxies, Th–s⟂ and Th–s1, generally exhibit only very weak correlation with O7+/O6+. From our analysis in the previous section we conclude that, outside of relationships between the large-scale streams in the solar wind structure, the temperature of suprathermal electrons has very little to no residual signatures from the coronal electron temperature of its source by the time it propagates to 1 AU. This contrasts with the conclusions drawn by , who reported that the two were related. We do not fully contradict their conclusions, however, as we too find numerous subsets of data with statistically significant correlation between the suprathermal electrons and O7+/O6+. Likewise, Es, an estimate of mean strahl energy, also shows very little overall dependence on O7+/O6+. Both the halo, which propagates with the bulk solar wind, and the strahl, which travels rapidly down the heliospheric magnetic field, show no consistent evidence of containing a remnant signature of the electron temperature at their coronal source. We find that in periods where there is low solar activity, fewer ICMEs and consistent fast streams, there is a greater positive correlation with O7+/O6+ for both Es and Th–s⟂. It is likely then that in these simple configurations of the corona and solar wind, a coronal relationship is set up and partially preserved between the suprathermal electrons and ionisation states. In the more complex states, some combination of coronal conditions (variability of freeze-in heights, ionisation processes, temporal variation of the source) and solar wind processing (increased wave activity due to CIRs, wind streams with more fluctuations, ICME influence on halo and strahl electrons) is acting to destroy this correlation before it can be observed. From this we conclude that the description in Sect.  of how such a correlation between suprathermal electrons and ionisation states could come to exist is a possibility, under favourable coronal conditions.

We have noted many features of these relationships in Sect.  while attempting to understand whether in situ processing or coronal conditions are responsible for their weakness and variability. We find that the large spread in Es and Th–s⟂, apparent clustering into fast and slow wind, and the lack of positive correlation during periods of increased perturbation in the corona and solar wind could each be explained by solar wind processing effects or by coronal conditions which are source-dependent. The one observation which appears to clearly favour the explanation of in situ processing destroying an initially strong relationship is found when comparing the halo relationship in Fig. b to the strahl relationship in Fig. e. The continued upwards trend of Es in high-O7+/O6+ solar wind which is not seen for Th–s⟂ can be most simply explained through solar wind processing effects being more effective on the halo population than the strahl. We do not find any observations which exclusively favour any coronal effects as the cause for the weak correlations. However, it is important to note that this may still be the case because we have not performed analysis of any direct solar observations which would potentially reveal such effects.

Confirming if there is indeed a coronal relationship between the halo and strahl energy content and ionisation states which is being degraded during transport to 1 AU requires further study. One way in which this could be developed in the future would involve the upcoming ESA Solar Orbiter mission. Using composition and electron data from the spacecraft's cruise and nominal mission phases, which will cover heliocentric distances down to below 0.3 AU, it would be possible to test how the correlations considered in this paper vary with distance and for solar wind which is still relatively pristine with respect to its coronal source region. Should we see them improve with proximity to the Sun, then this would confirm that there is an initial state created in the corona in which the energy content of suprathermal electrons is related to core electron temperature, and which is then eroded during the transport from 0 to 1 AU.