2608.10819-Evidence for Solar-Cycle Modulation of the Alpha-to-Proton Temperature Ratio in Solar Wind

Introduction to the show: ident: Astrophysics Radio.

Vera: Next we'll be talking about the paper "Evidence for Solar-Cycle Modulation of the Alpha-to-Proton Temperature Ratio in Solar Wind".

Jocelyn: The paper was written by Aakash Gupta, Yogesh, Dibyendu Chakrabarty, Leon Ofman and Gregory G. Howes from Physical Research Laboratory and Indian Institute of Technology and University of Iowa and NASA Goddard Space Flight Center and The Catholic University of America and Tel Aviv University.

Vera: Stay tuned as we take you through the paper and discuss its implications.

Title: ident: You're listening to "Across the Universe," where we take one recent paper from the arXiv and work through what it tells us about the cosmos. Today's paper is "Evidence for Solar-Cycle Modulation of the Alpha-to-Proton Temperature Ratio in Solar Wind."

Vera: Welcome back, everyone. Jocelyn, Subrahmanyan, good to have you at the table again. That title is dense even by our standards, so let's unpack it before we touch any data. The solar wind is a supersonic stream of charged particles flowing off the Sun's corona, and it's mostly protons with a few percent alpha particles — bare helium nuclei, about four times as massive as a proton. The "alpha-to-proton temperature ratio" is exactly what it sounds like: the temperature of the alphas divided by the temperature of the protons.

Jocelyn: And in a plasma that has settled into equilibrium, that ratio should be one — every species at the same temperature. The fact that it isn't tells us the solar wind carries the scars of violent, selective heating.

Vera: Right. And the "solar-cycle modulation" part is the new claim. The solar cycle is the roughly eleven-year rise and fall of magnetic activity on the Sun, tracked by sunspot number. What the authors say is that this temperature ratio beats in time with that eleven-year rhythm — and, crucially, it beats differently in slow wind and fast wind.

Jocelyn: Does that mean the heating process itself changes over the cycle?

Vera: Not necessarily — and that's the subtlety. The paper's argument is that the mix of solar wind sources changes over the cycle, and the temperature ratio measured near Earth ends up reflecting which source dominates. The team behind it spans several institutions: Aakash Gupta and Dibyendu Chakrabarty at the Physical Research Laboratory and IIT Gandhinagar in India, Yogesh and Gregory Howes at the University of Iowa, and Leon Ofman at NASA Goddard and the Catholic University of America. It's a mix of observers and theorists, which fits the question.

Subrahmanyan: The deeper implication is that a measurement made at 1 AU, near Earth, cannot be read as purely local plasma physics. The ratio carries a memory of the Sun's magnetic configuration months earlier, when that plasma was launched. That is what makes the title provocative.

Jocelyn: So the real content is hiding in two distinct populations within the ratio, and the solar cycle shuffles which one you see. That's exactly where the paper's results begin.

Summary: Vera: We've unpacked the title of "Evidence for Solar-Cycle Modulation of the Alpha-to-Proton Temperature Ratio in Solar Wind" and met the team behind it. Now let's talk about what they found in three decades of Wind spacecraft measurements.

Jocelyn: The dataset is genuinely large. Wind's Faraday cups built three-dimensional velocity distributions for protons and alphas every ninety-two seconds or so from 1995 to 2024. The team then filtered out anything that isn't clean ambient solar wind — intervals when Wind was inside Earth's bow shock, plus all interplanetary coronal mass ejections. What remains is a census of the quiet solar wind across nearly three full solar cycles.

Vera: When they histogram the temperature ratio, they don't get one smooth curve. They get two overlapping populations. One sits near a ratio of one — the equal-temperature population. The other sits near four — the mass-proportional population, where alphas are four times hotter than protons, exactly what you'd expect if both species share the same thermal speed despite the mass difference. They fit each distribution with two Gaussians and tracked how much area each component contributes.

Subrahmanyan: That factor of four is the alpha-proton mass ratio, and it's the long-sought signature of mass-proportional heating. When the energization acts in proportion to mass, the heavier species ends up hotter by precisely that factor.

Jocelyn: The balance between the two populations tracks solar wind speed. Below about four hundred kilometers per second, the equal-temperature pile dominates. Above five hundred, the mass-proportional pile takes over, and a sizeable fraction of intervals show ratios above four. The four-hundred-to-five-hundred range is a smooth transition, not a cliff.

Vera: The organizing parameter is the collisional age — roughly the number of Coulomb collisions a plasma parcel accumulates while expanding out to 1 AU. Below a collisional age of one, the plasma is collisionless enough that non-thermal signatures survive; above one, collisions have had time to smooth them out. Slow wind is mostly collisionally old; fast wind is mostly collisionally young.

Subrahmanyan: The paper quantifies that shift: the fraction of collisionally young plasma rises from roughly five and a half percent in the slowest speed bin to about ninety percent in the fastest, while the occurrence of strongly heated alpha populations climbs from three percent to almost half of all intervals.

Jocelyn: So collisions erase the heating signature in slow wind, and fast wind preserves it. That part extends earlier work. But the genuinely new result is that both the collisional age and the temperature ratio also swing with the solar cycle — and they swing in opposite directions depending on the speed regime.

Vera: And that reversal is what makes this paper more than a confirmation of old ideas. It's the claim that the solar cycle itself is part of the physics.

Improvements: Vera: Before we go further with "Evidence for Solar-Cycle Modulation of the Alpha-to-Proton Temperature Ratio in Solar Wind," let's recall where we are: the two populations, divided by speed and collisional age, are now on the table. The question for this part of the conversation is what the paper adds to what was already known.

Jocelyn: That's the right question, because the collisional-age framework itself isn't new. Kasper and collaborators established it back in 2008, and Maruca, Alterman, and others extended it over the following decade. The gap was the solar cycle. Earlier studies pooled the data, effectively flattening out time. What Gupta and colleagues did was ask whether the relative weight of the two populations changes as the Sun's activity rises and falls.

Vera: And it does. They tracked the area ratio of the mass-proportional population to the equal-temperature population against sunspot number and the F10 point 7 radio flux. In the slow wind, that ratio climbs when the Sun is active. In the intermediate and fast bins, it does the opposite. The reversal happens right around the four-hundred-to-five-hundred-kilometer-per-second bin.

Subrahmanyan: The strongest positive correlation is in the three-hundred-to-four-hundred bin, with a Spearman coefficient around 0 point 75 over the full interval. The intermediate bin flips to minus 0 point 71, and the fastest bin shows essentially none — the fast wind looks almost the same whether the Sun is quiet or active. That contrast is the paper's most interesting observational contribution.

Jocelyn: And they tested the analysis for robustness. They re-fitted the distributions with a two-component lognormal mixture, which handles the skewed, positive-definite nature of the ratio differently. The conclusions held; the lognormal just produced larger uncertainties.

Vera: The interpretation is that during solar minimum, the ecliptic plane is filled with high-speed streams from large polar coronal holes — collisionally young, strongly heated plasma. During maximum, the Sun's magnetic field is more disordered, and denser, slower wind from streamers and active regions dominates, giving collisions more time to act. The ratio at 1 AU becomes a measure of which source population is winning at that phase of the cycle.

Subrahmanyan: The authors are also explicit about what the field needs next. In-situ measurements alone can't distinguish continued local heating from preservation of a near-Sun signature. They suggest combining plasma data with composition and magnetic connectivity, and coordinating Parker Solar Probe, Solar Orbiter, Wind, and Aditya-L1 to trace the radial evolution continuously from the corona outward.

Jocelyn: Which is a natural invitation to go back to the opening pages of the paper, where they set up why preferential heating happens near the Sun in the first place.

First Page: Vera: We've covered the findings and the improvements in "Evidence for Solar-Cycle Modulation of the Alpha-to-Proton Temperature Ratio in Solar Wind." Now let's open the paper to page one and look at the physical puzzle that starts the whole argument.

Jocelyn: The opening sets up something remarkable. The Sun's visible surface is about six thousand kelvin, but the corona above it reaches a million degrees or more. That temperature jump happens across a few hundred kilometers — the transition region — and the paper emphasizes that between roughly 0 point 1 and 0 point 3 solar radii, the plasma becomes effectively collisionless. The Coulomb collision frequency scales as density over temperature to the three-halves, so as the corona thins and heats, the particles simply stop interacting often enough to share energy.

Subrahmanyan: That threshold is the key. Once collisions stop enforcing equilibrium, different species are free to hold different temperatures. Electrons separate from ions, and heavier ions separate from protons. Statistical studies find an approximate scaling of ion temperature with mass to the power 0 point 43 — somewhere between equal temperatures and full mass-proportionality.

Vera: The authors then walk through the candidate mechanisms for that preferential ion heating: resonant absorption of ion-cyclotron waves, interactions with low-frequency Alfvénic turbulence, stochastic heating, drift instabilities driven by particle beams, and even impulsive reconnection events and speed filtration. The list is long because the answer isn't settled. Recent Parker Solar Probe passes through the near-Sun wind have made the question sharper rather than simpler.

Jocelyn: One subtle point the opening stresses is that the temperature we measure at 1 AU is not heating. It's the net result of genuine heating, adiabatic cooling as the wind expands, collisional relaxation, and a small contribution from heat flux. So when we see a ratio near four in fast wind, the honest statement isn't "this is where the heating happened"; it's "this is what survived the journey."

Subrahmanyan: And that is why the solar-cycle dependence becomes a clue rather than a complication. The ratio at Earth is the outcome of a competition between energization near the Sun and collisional erasure along the way, and the cycle changes the starting conditions of that competition by changing which coronal regions are open and feeding the ecliptic.

Vera: That framing ties the whole paper together — and it's exactly the thread the conclusion will pull to make the final case. --- CONCLUSION ---

Vera: Time to close the book on "Evidence for Solar-Cycle Modulation of the Alpha-to-Proton Temperature Ratio in Solar Wind." Jocelyn, Subrahmanyan, how do we summarize the arc?

Jocelyn: The paper takes nearly three decades of Wind observations and shows that the alpha-to-proton temperature ratio is bimodal — near one for collisionally processed plasma, near four for weakly collisional plasma. Slow wind, with its higher collisional age, sits in the first population; fast wind, with lower collisional age, dominates the second.

Subrahmanyan: And the new piece of evidence is the solar-cycle modulation. The balance between those two populations shifts with sunspot number, in different directions for slow and fast wind, because the mixture of source regions changes over the eleven-year cycle. At minimum, coronal-hole streams flood the ecliptic; at maximum, denser, slower wind takes over.

Vera: Crucially, the paper doesn't overclaim. The measurements at 1 AU cannot distinguish ongoing local heating from preservation of signatures established near the Sun. What they can do is demonstrate that collisional relaxation and source mixing shape the observed ratio — and that any statistical study of ion heating must account for the phase of the solar cycle.

Jocelyn: The path forward they sketch involves multi-spacecraft campaigns — Parker Solar Probe, Solar Orbiter, Wind, ACE, and Aditya-L1 — paired with remote observations of coronal magnetic topology, to trace the temperature ratio's radial evolution from the corona to 1 AU.

Vera: It's a clean piece of evidence with an honest caveat, and it gives future missions a clear target. Thanks to you both for the discussion.

Jocelyn: And thanks to our listeners for staying with us.

Subrahmanyan: Until the next paper — keep looking up.

ident: You've been listening to "Across the Universe." Next episode, we'll pick another recent result from the arXiv and try to understand what it tells us about the cosmos.

Conclusion: Vera: On this episode of "Across the Universe," we walked through "Evidence for Solar-Cycle Modulation of the Alpha-to-Proton Temperature Ratio in Solar Wind," where Gupta and colleagues used nearly three decades of Wind spacecraft data to show that the balance between equal-temperature and mass-proportional alpha-proton populations in the solar wind swings with the eleven-year solar cycle — reversing direction between slow and fast wind.

Jocelyn: And that reversal is worth sitting with. The slow wind shows more mass-proportional heating during solar maximum, while the intermediate and fast wind show more during minimum. The reason isn't a change in the fundamental heating physics, but a change in which coronal sources are feeding the ecliptic: coronal holes at minimum, streamers and active regions at maximum.

Vera: The paper is careful to say what it can't resolve. The temperature ratio measured at 1 AU doesn't tell us whether the alphas were heated recently or simply preserved their near-Sun temperature. But the collisional-age framework makes the story coherent — fast, sparse plasma arrives collisionally young and keeps its marks; slow, dense plasma gets smoothed out.

Jocelyn: The bigger point for the field is that any statistical study of ion heating now has to account for solar-cycle phase. If you average over thirty years, you're mixing populations that behave differently depending on when the Sun was active. That's a subtle but important correction to how we read the solar wind archive.

Vera: The natural next step is the one the authors propose: combine Wind with Parker Solar Probe, Solar Orbiter, and Aditya-L1 across distances, so we can trace the temperature ratio's radial evolution from the corona to 1 AU rather than infer it from a single snapshot.

Jocelyn: And that's exactly where our next episode will go. We'll look at a recent paper using Parker Solar Probe's perihelion passes to measure how the alpha-proton temperature ratio evolves in the near-Sun wind — testing whether the mass-proportional signature is imprinted close to the Sun or grows along the way.

Vera: Until then, thanks for listening — and keep looking up.