Triggered Ion Acoustic Waves (TIAWs)

• Triggered Ion Acoustic Waves (TIAWs) are long-duration, narrowband ion-acoustic emissions in the young solar wind with dual-frequency branches and stable phase locking.
• Observations reveal distinct electrostatic signatures, including sine-wave morphology and phase speeds around 100 km/s, confirmed via interferometric time-delay measurements.
• TIAWs play a key role in electron heating and plasma instability by periodically triggering beam-driven ion-acoustic instability in near-Sun plasma conditions.

Triggered ion acoustic waves (TIAWs) are long-duration, narrowband, phase-locked ion-acoustic emissions observed most prominently by Parker Solar Probe in the young solar wind. In the near-Sun measurements, TIAWs appear as a low-frequency branch at a few Hz and a higher-frequency branch at a few hundred Hz to $1000$ Hz, with the higher-frequency wave occurring at a fixed phase of each low-frequency period. Their electrostatic polarization, density signatures, common phase velocity near the ion-acoustic speed, association with $T_e/T_i>1$, and correlation with electron heating led to their interpretation as a new regime of ion-acoustic instability and evolution that had not been reported previously in solar-wind theory or experiment (Mozer et al., 2021, Mozer et al., 2022).

1. Discovery in the young solar wind

The defining discovery interval occurred on 18–19 January 2021, when Parker Solar Probe, near $20\,R_\odot$, recorded uninterrupted narrowband electrostatic emissions for more than twelve hours while outside the heliospheric current sheet (Mozer et al., 2021). PSP’s FIELDS suite provided electric-field data continuously at about $2{,}200$ samples/s and burst-mode waveforms at $150{,}000$ samples/s, while SWEAP supplied $1$ s ion and electron moments and distribution functions. The continuous interval from about 22:00 UTC on 18 January until about 10:00 UTC on 19 January established that the phenomenon was not a transient burst in the conventional solar-wind sense, but a persistent mode with unusually stable spectral structure (Mozer et al., 2021).

Subsequent analyses extended the observational domain. A later survey identified what was termed triggered ion-acoustic waves as the most dominant wave mode above a few Hz within the solar radial distance of $15$–$25\,R_\odot$, and reported events over $13$–$25\,R_\odot$ in seven consecutive PSP orbits (Mozer et al., 2022). Another study concluded that TIAWs occurred during about $T_e/T_i>1$0 percent of Parker Solar Probe passes through $T_e/T_i>1$1–$T_e/T_i>1$2, and that when present they were the dominant wave signal for hours (Mozer et al., 14 Jan 2025). These results place the original January 2021 interval within a broader near-Sun population rather than an isolated event.

The observational geometry is central to the interpretation. Near perihelion, the PSP $T_e/T_i>1$3-axis lay roughly Sunward and approximately along the ambient magnetic field and bulk flow, while the $T_e/T_i>1$4–$T_e/T_i>1$5 plane contained the two electric-field antenna pairs that yielded $T_e/T_i>1$6 and $T_e/T_i>1$7 (Mozer et al., 2022). That geometry enabled interferometric time-delay measurements across the antenna system and allowed comparisons between the electric-field polarization, the magnetic-field direction, and the inferred propagation direction.

2. Spectral morphology and wave signatures

TIAWs are distinguished by dual-band structure, narrow spectral widths, and phase locking between the low- and high-frequency components. In the original January 2021 event, the dominant carrier band lay between about $T_e/T_i>1$8 and $T_e/T_i>1$9 Hz in the spacecraft frame, and the packets repeated at about $20\,R_\odot$0 Hz with envelope durations of about $20\,R_\odot$1 ms (Mozer et al., 2021). Later work generalized that phenomenology to a low-frequency branch around $20\,R_\odot$2–$20\,R_\odot$3 Hz and a high-frequency branch around $20\,R_\odot$4–$20\,R_\odot$5 Hz (Mozer et al., 2022).

Branch	Measured band	Typical accompanying signature
Low-frequency branch	$20\,R_\odot$6–$20\,R_\odot$7 Hz	$20\,R_\odot$8 mV/m, $20\,R_\odot$9
High-frequency branch	$2{,}200$0–$2{,}200$1 Hz	$2{,}200$2 mV/m, $2{,}200$3

Both branches were reported to have nearly pure sine-wave character and narrow spectral widths; for the high-frequency wave the full width at half maximum was $2{,}200$4 Hz (Mozer et al., 2022, Mozer et al., 2023). In the January 2021 interval, high-pass-filtered time series showed each high-frequency packet to have nearly symmetric, steepened leading and trailing edges, described as shock-like envelopes (Mozer et al., 2021). That morphology is atypical for ordinary ion-acoustic turbulence and motivated the interpretation of intermittent triggering.

A second defining signature is the absence of a magnetic-field counterpart at the carrier frequencies. Both the initial event study and later diagnostic analyses emphasized electrostatic polarization, linear $2{,}200$5 phase relations, density fluctuations, and the lack of corresponding magnetic fluctuations in the high-frequency band (Mozer et al., 2021, Mozer et al., 2022). The phase relation between $2{,}200$6 and $2{,}200$7 was also examined explicitly, with the high-frequency component reported to be in quadrature, consistent with the electrostatic balance $2{,}200$8 (Mozer et al., 2023).

The low-frequency component is more subtle. One analysis reported a clear $2{,}200$9–$150{,}000$0 Hz electromagnetic signature in low-frequency $150{,}000$1 and $150{,}000$2 with coherence $150{,}000$3 and stable phase, and interpreted that signal as the trigger of the higher-frequency packets (Mozer et al., 2021). Later studies emphasized electrostatic diagnostics and described the pair as ion-acoustic waves with no magnetic-field component. This suggests that the exact status of the low-frequency branch remains part of the detailed interpretive problem rather than a fully closed issue (Mozer et al., 2022, Mozer et al., 2023).

3. Identification as ion-acoustic waves

The ion-acoustic identification rests on measured phase speeds, propagation geometry, density response, and plasma parameters. Using $150{,}000$4 kS/s burst-mode interferometry in the January 2021 event, successive PSP antennas recorded packet arrival-time offsets of about $150{,}000$5 ms over about $150{,}000$6 m separations, yielding an $150{,}000$7–$150{,}000$8-plane phase-speed component of about $150{,}000$9–$1$0 km/s. Because the background field lay near the $1$1-axis, the total phase speed was inferred to be about $1$2 km/s, and the wavevector was interpreted as oblique, roughly $1$3–$1$4 to $1$5 (Mozer et al., 2021).

A later event-by-event analysis measured both branches directly and found essentially the same plasma-frame phase velocity for the low- and high-frequency waves. For a $1$6 Hz example, a measured time delay of about $1$7 ms over about $1$8 m gave a spacecraft-frame speed of about $1$9 km/s and a plasma-frame phase velocity of about $15$0 km/s. For a $15$1 Hz example, cross-correlation lags $15$2 ms over about $15$3 m gave a plasma-frame phase velocity of about $15$4 km/s. Agreement within experimental uncertainty was presented as a necessary condition for coupled modes (Mozer et al., 2022).

The linear ion-acoustic dispersion relation used throughout the near-Sun analyses is

$15$5

In the limit $15$6, the phase speed becomes

$15$7

or, in the alternative notation used in later summaries,

$15$8

For the observed plasma parameters, these expressions yield ion-acoustic speeds of order $15$9–$25\,R_\odot$0 km/s, consistent with the measured phase speeds and inconsistent with an electron mode (Mozer et al., 2021, Mozer et al., 2022).

The plasma conditions during the January 2021 interval were also favorable for ion-acoustic excitation. SWEAP fitted the ions to a Maxwellian core plus a drifting beam with core density $25\,R_\odot$1 cm$25\,R_\odot$2, beam density $25\,R_\odot$3 cm$25\,R_\odot$4, $25\,R_\odot$5 eV for the core, $25\,R_\odot$6 eV for the beam, and beam drift $25\,R_\odot$7 km/s anti-Sunward relative to the core. Electrons had $25\,R_\odot$8 eV, giving $25\,R_\odot$9 (Mozer et al., 2021). Under such conditions, the threshold for beam-driven growth is commonly written as

$13$0

and the observed drift and temperature ratios were described as lying close to the acoustic threshold for the ion-ion acoustic instability (Mozer et al., 2021).

Density fluctuations further constrained the mode identification. Using the quasi-neutrality balance

$13$1

one analysis estimated $13$2 V for the low-frequency branch and $13$3 V for the high-frequency branch, using $13$4 eV with $13$5 and $13$6, respectively (Mozer et al., 2022). Those potentials are large enough to make the low-frequency branch relevant for wave-particle interaction, while keeping the high-frequency branch comparatively weak in potential amplitude.

4. Triggering, phase locking, and instability regime

The defining dynamical property of TIAWs is that the high-frequency branch does not occur continuously; it appears at a fixed phase of the low-frequency branch. In the January 2021 event, cross-spectra of low-frequency $13$7 and $13$8 showed coherence $13$9 at $25\,R_\odot$0–$25\,R_\odot$1 Hz with stable phase, and high-pass-filtered $25\,R_\odot$2 revealed a $25\,R_\odot$3 Hz sinusoid on which the $25\,R_\odot$4–$25\,R_\odot$5 Hz packets rode, always appearing near the sinusoid’s trough (Mozer et al., 2021). Later PSP analyses described the same behavior more generally: bursts of the high-frequency branch occur at fixed phase of the low-frequency branch, and the two branches have the same phase velocity within experimental uncertainty (Mozer et al., 2022).

The original interpretation was that quasi-monochromatic low-frequency fluctuations periodically make the plasma locally supercritical to the beam-driven ion-acoustic instability, producing triggered, phase-locked, shock-packet emissions in a marginally stable, beam-heated plasma (Mozer et al., 2021). This regime was explicitly contrasted with classical solar-wind ion-acoustic activity, which usually appears as broadband, Doppler-shifted turbulence or short transient bursts rather than as long-lived, narrowband, coupled tones (Mozer et al., 2021, Mozer et al., 2022).

Several candidate mechanisms have been proposed, but the microphysics remains open. Possibilities listed in the PSP studies include a current-driven electrostatic instability at a specific phase of the low-frequency wave, and nonlinear steepening or harmonic generation seeded periodically by the low-frequency branch (Mozer et al., 2022). A later non-resonant model argued that strict three-wave matching conditions do not explain the observations and instead considered energy transfer in which the low-frequency wave loses energy that is used both to heat electrons and to grow higher-frequency ion-acoustic sidebands (Mozer et al., 10 Sep 2025). This suggests that the observed phase locking may reflect local modulation of plasma parameters and free energy rather than a standard resonant decay process.

Numerical work with solar-wind parameters has reinforced the instability picture without fully closing the mechanism. A linear-kinetic and 1D kinetic simulation study showed that the ion-ion acoustic instability can occur in a Parker Solar Probe-relevant regime for proton core-beam distributions, although it required electron-to-core and beam-to-core temperature ratios slightly different from reported values during electrostatic burst detection; the resulting nonlinear structures exhibited trapped proton beam populations and oscillatory signatures comparable to the observations in time scale and amplitude (Afify et al., 2024). In that framework, the $25\,R_\odot$6 Hz bursts arise naturally from threshold crossing in beam-driven ion-acoustic growth, while the slower envelope reflects the modulation of ambient parameters near instability onset.

5. Electron heating and solar-wind energetics

TIAWs are not only wave phenomena; they are closely tied to the thermodynamics of near-Sun electrons. The low-frequency potential inferred from the density response, $25\,R_\odot$7 V, was argued to be large enough for electron heating via Landau resonance, whereas the high-frequency potential $25\,R_\odot$8 V would contribute at $25\,R_\odot$9 level (Mozer et al., 2022). Consistent with that estimate, core-electron temperature increases of order $T_e/T_i>1$00–$T_e/T_i>1$01 were reported to correlate strongly with TIAW presence (Mozer et al., 2022).

Broader orbit studies placed that heating in radial context. One analysis of PSP orbits six through nine reported that core electrons emerged from $T_e/T_i>1$02 with a temperature of $T_e/T_i>1$03 eV, independent of solar-wind speed, and that in the absence of triggered ion acoustic waves at greater distances the core electron temperature followed

$T_e/T_i>1$04

consistent with adiabatic expansion (Mozer et al., 2022). In the presence of TIAWs, the core electrons were reported to be isotropically heated as much as a factor of two above that minimum (Mozer et al., 2022). A related study stated that triggered and ordinary ion-acoustic waves are the dominant wave modes at frequencies greater than $T_e/T_i>1$05 Hz between $T_e/T_i>1$06 and $T_e/T_i>1$07, and linked that activity to isotropic core-electron heating below $T_e/T_i>1$08 (Mozer et al., 2021).

Later work added direct parallel-electric-field observations. By rotating into a field-aligned frame, the low- and high-frequency branches were both argued to possess parallel electric fields, with representative magnitudes of about $T_e/T_i>1$09–$T_e/T_i>1$10 mV/m for the $T_e/T_i>1$11 Hz and $T_e/T_i>1$12 Hz waves (Mozer et al., 14 Jan 2025). That study concluded that TIAWs occur during about $T_e/T_i>1$13 of PSP passes through $T_e/T_i>1$14–$T_e/T_i>1$15, and reported that electrons are heated in the presence of these parallel electric fields while there is no electron heating in their absence (Mozer et al., 14 Jan 2025).

The broader implications proposed in the PSP literature include efficient coupling between large-scale magnetic or compressional fluctuations and short-scale electrostatic modes, intermittent beam thermalization, possible scattering of strahl electrons via anomalous cyclotron resonances, and a role in regulating solar-wind heat flux close to the Sun (Mozer et al., 2021). These interpretations are tied directly to the observation that TIAWs can persist for hours and dominate the wave power above a few Hz in the near-Sun plasma.

6. Physical-origin debate and diagnostic tests

Because TIAWs were unprecedented in the solar wind and display highly organized dual-band structure, the possibility of instrumental origin was raised explicitly. The response in the PSP literature was to assemble a battery of observational diagnostics. One study summarized thirteen independent lines of evidence favoring a genuine plasma phenomenon, while stating that absolute exclusion of an instrumental artifact is impossible (Mozer et al., 2022, Mozer et al., 2023).

Those diagnostics included electrostatic character, narrowband spectra, $T_e/T_i>1$16, common phase velocity for the two branches, quadrature between $T_e/T_i>1$17 and density fluctuations, correlation with electron heating, and instrument performance checks (Mozer et al., 2023). At very low frequencies, $T_e/T_i>1$18 and $T_e/T_i>1$19 agreed to within a few percent with $T_e/T_i>1$20 fits over hours, implying nominal electric-field operation. Single-ended antenna potentials showed no wake-like distortions, and wave properties were reported to be independent of large variations in magnetic-field direction, $T_e/T_i>1$21, solar-wind speed, and density over multi-hour intervals (Mozer et al., 2022, Mozer et al., 2023).

The resulting position in the literature is cautious but not agnostic. The data were argued to make a natural origin highly likely, but the papers retained the general measurement caveat that instrumental artifacts can never be ruled out with absolute certainty (Mozer et al., 2022, Mozer et al., 2023). That formulation is not incidental; it reflects the fact that the mode’s persistence, phase locking, and unusual purity have no close precedent in earlier solar-wind wave catalogues.

7. Relation to ordinary ion-acoustic waves and other triggered regimes

Near-Sun TIAWs are best understood against the background of more conventional ion-acoustic activity. Solar Orbiter’s RPW/TDS survey at $T_e/T_i>1$22–$T_e/T_i>1$23 AU showed that electrostatic ion-acoustic waves are common in the solar wind there as intense, linearly polarized bursts with mean frequency about $T_e/T_i>1$24 kHz and mean peak field about $T_e/T_i>1$25 mV/m, with occurrence peaking near $T_e/T_i>1$26 AU and amplitude scaling as $T_e/T_i>1$27 (Píša et al., 2021). Those waves are ubiquitous, strongly Doppler shifted, and often associated with highly variable proton moments and magnetic field, but they are not described as the long-lived, phase-locked dual-band structure characteristic of TIAWs. The contrast is central to the claim that TIAWs define a distinct instability regime rather than a trivial variant of ordinary ion-acoustic turbulence (Mozer et al., 2021, Mozer et al., 2022).

The broader literature also contains several other senses in which ion-acoustic waves are “triggered” or externally driven, although these are distinct from the specific PSP phenomenon. In ultracold neutral plasmas, controlled density perturbations created during plasma formation launch ion-acoustic waves whose measured dispersion follows the standard ion-acoustic relation [(Castro et al., 2010); (Killian et al., 2012)]. In chirped-drive theory, ion-acoustic waves can be excited and controlled by passage through linear resonance into autoresonance, with a sharp drive-amplitude threshold proportional to $T_e/T_i>1$28 (Friedland et al., 2014). At low-Mach-number collisionless shocks, proton–alpha streaming can drive ion-acoustic waves that become nonlinear and form ion holes, reducing relative drift and heating ions (Graham et al., 11 Feb 2025). At Earth’s magnetopause, large-amplitude slow-mode compressions can generate counter-streaming ion beams that excite ion-acoustic waves and relax the beams, providing a clear energy-transfer channel from MHD-scale slow-mode waves to kinetic-scale ion-acoustic waves (Shi et al., 24 Sep 2025). Standing Alfvén-wave pumps can also generate second-harmonic standing ion-acoustic modes through ponderomotive forces, with Landau damping controlling their attenuation (Terradas et al., 2021).

These related results show that triggered or driven ion-acoustic excitation is not unique to one plasma environment, but the near-Sun TIAW observations remain distinctive in combining hours-long persistence, narrow spectral purity, common phase velocity across two branches, and direct linkage to electron heating. Follow-on theoretical and simulation work was explicitly called for to capture the nonlinear steepening, trapping, and shock-envelope formation observed by Parker Solar Probe, and the detailed microphysics of the phase-locked coupling remains an open topic for theory and simulation (Mozer et al., 2021, Mozer et al., 2022).