Observation of Kinetic Alfvén Waves inside an Interplanetary Coronal Mass Ejection Magnetic Cloud at 1 au

Recent advancements have significantly enhanced our grasp of interplanetary coronal mass ejections (ICMEs) in the heliosphere. These observations have uncovered complex kinematics and structural deformations in ICMEs, hinting at the possible generation of magnetohydrodynamic (MHD) and kinetic-scale waves. While MHD-scale waves in magnetic clouds have been explored, understanding the dynamics of kinetic-scale mode waves remains challenging. This article demonstrates the first in situ observation of kinetic Alfvén waves (KAWs) within an ICME’s magnetic cloud, notably near the heliospheric current sheet–ICME interaction region, close to the reconnection exhaust. Analysis indicates a distinctive negative bump in the estimated normalized magnetic helicity (σ m = −0.38) around the gyrofrequency spread, indicating a right-handed polarization of the wave. Furthermore, examination across flow angle (θ VB) within the frequency domain reveals a specific zone (90°–135°) showcasing negative helicity fluctuations, confirming the presence of KAWs. Moreover, we noted a significant rise in temperature anisotropy in the vicinity, indicating the role of KAWs in plasma heating. Identifying KAW challenges established notions about ordered magnetic clouds and raises questions about energy transfer processes within these structures. This finding opens the door to a deeper understanding of energy transfer mechanisms within traditionally nondissipative regions and invites further exploration of low-beta plasma heating and the interactions between waves and particles in magnetic clouds.


Introduction
Alfvén waves, initially postulated by Alfvén (1942), represent fundamental magnetohydrodynamic (MHD) electromagnetic oscillations at low frequencies, and are prevalent in interplanetary, astrophysical, and laboratory plasma environments.These transverse oscillations propagate along magnetic field lines with magnetic tension as the restoring force (Belcher & Davis 1971;Tomczyk et al. 2007).Notably, ideal Alfvén waves exhibit no net energy transfer between plasma particles and the magnetic field (Gershman et al. 2017a).However, their behavior shifts when wave scales approach the particle's kinetic scale, especially near the ion gyroradius (ρ) or gyrofrequency (ω).Under these conditions, ion motion decouples from electron motion, leading to pronounced parallel electric and magnetic field fluctuations.This dynamic enables a net energy transfer between the wave field and plasma particles, predominantly through Landau or transit-time interactions (Barnes 1966;Hasegawa & Chen 1976;Stix 1992;Gershman et al. 2017a).These interactions, combined with an imbalance in the number of particles moving faster than or slower than the wave, result in net plasma heating or cooling (Stix 1992;Gershman et al. 2017a).Note that when the Alfvén wave energy cascade reaches kinetic scales, the turbulence fluctuations with k ⊥ ?k || and k ⊥ ρ i ∼ 1 drive the transition to kinetic Alfvén waves (KAWs; Gershman et al. 2017a), where k ⊥ , k || , and ρ i represent the perpendicular and parallel components of the wavevector and the proton gyroradius respectively.The stochastic heating of charged particles, usually known to occur in a large-amplitude wave, may also occur in a turbulent field (Hoppock et al. 2018).Thus, the existence of robust KAWs has profound implications in the realm of space and astrophysical plasma dynamics.
The scale length of KAWs is much larger than that of electrostatic waves; thus, it produces anomalous transport phenomena (Hasegawa & Mima 1978).These phenomena, in turn, trigger the diffusion of radiation particles and the formation of the ring current, alongside heating plasma-pause electrons and giving rise to stable auroral red arcs.The scale length also supports viscous interaction between the solar wind and the magnetosphere (Hasegawa & Mima 1978).Moreover, broadband KAWs cause turbulent heating in the solar wind and the magnetosheath (Rezeau et al. 1989;Leamon et al. 1998;Sahraoui et al. 2009;Gershman et al. 2017a).In the laboratory, KAWs are capable of transporting energy away from the core regions of fusion plasmas, which leads to unwanted deposition of energy at the reactor edges (Wong 1999;Belova et al. 2015;Gershman et al. 2017a).Hence, these waves are significant in planetary magnetospheres, heliospheres, astrophysical systems, laboratory plasma experiments, and fusion reactors.Therefore, comprehending the generation and propagation of KAWs and their interaction with charged particles becomes vital for unveiling and forecasting the underlying physics of these fundamental processes (Gershman et al. 2017a).
Several studies have been conducted to find KAWs in interplanetary space.In general, KAWs are observed in solar wind turbulence (Salem et al. 2012), in auroral electromagnetic turbulence (Louarn et al. 1994), and also in the magnetopause and boundary layer (Johnson & Cheng 2001).Neugebauer et al. (1978) observed that enhanced proton density fluctuations appear as a small bump or flattening of the power spectrum near the proton gyroradius scale, which was first predicted by Harmon (1989).Similarly Chandran et al. (2009) observed solar wind density fluctuations at 1 au and concluded that they may be caused by KAWs.Further, KAWs are also excited in the magnetotail plasma sheet near the location of substorm onsets and play an important role in the process that triggers them (Duan et al. 2012).Salem et al. (2012) showed the wavenumber dependence of the electric-to-magnetic field ratio δE y /δB z in geocentric solar ecliptic (GSE) coordinates.Additionally, the alignment of the δB || /δB ratio with theoretical predictions was found to be stronger for KAWs than for magnetosonic/whistler waves (Salem et al. 2012).Moreover, the reduced magnetic helicity spectrum has been analyzed as a function of the angle θ VB between the direction of the local mean magnetic field and the local flow velocity of the solar wind by He et al. (2011), Podesta & Gary (2011), and Podesta (2012).Gershman et al. (2017b) used data from the Magnetospheric Multiscale mission to confirm conservative energy exchange between undamped KAW field and plasma particles.
Besides this, the literature also suggests the evolution of solar wind turbulence based on the dispersion and dissipation of Alfvén waves.These waves exhibit large amplitude and pronounced nonlinearity.From the early observations by Mariner 5 (Belcher et al. 1969) and later ones (e.g., D'Amicis et al. 2021), it was evident that Alfvénic fluctuations can be easily identified in the solar wind as nearly incompressible fluctuations characterized by a correlation between magnetic field and velocity components as predicted by the MHD theory.Ideally, the correlation coefficient should be equal to 1, but observational evidence sometimes shows lower values (Belcher & Davis 1971).The discrepancy may arise from nonlinear wave evolution or other wave modes in the solar wind medium.Previous studies, including Riley et al. (1995Riley et al. ( , 1996)), Tsurutani et al. (1995Tsurutani et al. ( , 1997)), and Tsurutani & Ho (1999), observed these waves, highlighting the nature of their arc polarization.Generally, these waves undergo phase-steepening; thus, they are both dispersive and dissipative.The phase-steepening process results in a steepened front with a circularly polarized right-hand portion followed by a linear compressive portion.The former represents high-frequency wave power due to shortening of time, while the latter constitutes low-frequency wave power.The leading portion contains over half of the wave phase rotation, termed "wave breaking," while the trailing portion, nearly doubling its period, is described as "period doubling."Although power laws describe the solar wind power spectrum over several frequency decades, Helios observations have shown that the frequency range characterizing Alfvénic fluctuations extends from tens of minutes to some hours, and this range generally evolves with heliocentric distance (see Tu & Marsch 1995;Bruno & Carbone 2013a).This evolution of turbulence is most probably occurring at all scales down to the proton gyroradius (discontinuity) scale size.Noteworthy features at the ion scale in Alfvénic turbulence are the longlasting wave helicity (polarization) and the splitting of arc rotation into two parts.The original wave mode is crucial in describing turbulence and its evolution, with helicity emerging as an enduring and fundamental characteristic of turbulence (Tsurutani et al. 2018, and references therein).
Alfvén waves dissipate kinetically, forming magnetic decreases (MDs), which indicate that the solar wind is partially compressive and static.These MDs, i.e., magnetic dips, persistently exist in the interplanetary medium and are as large as 90% of the average ambient magnetic field strength (Tsurutani et al. 2018).Several hypotheses have been proposed to explain MDs.Tsubouchi (2009) utilized a 1D MHD simulation, revealing that Alfvénic fluctuations in the highspeed solar wind interacting with a velocity gradient structure could lead to MD formation.Baumgärtel (1999) theorized that MDs could be dark soliton solutions derived from the derivative nonlinear Schrödinger equation.Alternatively, Buti et al. (2001) proposed a mechanism involving local inhomogeneities introduced by large-amplitude Alfvén wave packets evolving into MDs.Vasquez & Hollweg (1999) and Vasquez et al. (2007) suggested that wave-wave interactions in turbulent sheaths behind interplanetary shocks might create MDs by generating pressure-balance structures from oppositely traveling Alfvén waves.Roytershteyn et al. (2015) conducted a 3D kinetic simulation of decaying turbulence, identifying MDs as pressure-balanced structures aligning along the mean magnetic field direction.Single-wave observations indicate a connection between interplanetary MDs in high-speed solar wind streams and Alfvén waves.Despite the theoretical discussions on MD generation, it is crucial to mention that various plasma waves (ion and electron cyclotron, mirror mode, and Langmuir waves) detected in the vicinity of MDs exhibit a short duration and contribute negligibly to the overall power spectrum.Lin et al. (1995a) present the observation of Langmuir waves within magnetic holes, a prevalent phenomenon in the solar wind characterized by a depression in the magnetic field strength.Sperveslage et al. (2000) documented the observation of an MD exhibiting numerous properties closely resembling the soliton characteristics associated with magnetic holes.Tsurutani et al. (2002b) found that MDs are time-evolutionary parts of nonlinearly steepened Alfvén waves.Tsurutani et al. (2002a) proposed that heated ions and electrons, accelerated by the ponderomotive force, create high-β MD regions.This process pushes a significant portion of the ambient magnetic field out of the region into neighboring areas.An alternative perspective posits MDs as exhaust fans resulting from interplanetary magnetic reconnection (Neugebauer & Giacalone 2010).Rapid variations in the duration and convected distance of MDs suggest the swift dissipation of interplanetary Alfvén wave energy.Neugebauer et al. (2001) demonstrated the possible causes of high-latitude magnetic holes and suggested that marginal mirror mode instability plays a major role in their creation.Moreover, Tsubouchi & Matsumoto (2005) suggest that isotropization induced by external field fluctuations is more effective in generating an MD than mirror mode instability.The study by Fränz et al. (2000) assesses the statistical significance of the occurrence rates of magnetic field depression relative to log-normal distributions and introduces metrics for depression length and depth.These collective findings discussed above highlight the prevalence of MDs in the interplanetary medium, which are influenced by various mechanisms such as Alfvén waves, soliton solutions, and external field fluctuations, thus underscoring the complex nature of these phenomena in shaping the interplanetary magnetic field.
The primary aim of this study is to find KAWs within the substructure of the interplanetary counterpart of coronal mass ejections (CMEs).Actually, a CME is a huge cloud of solar plasma (mass ∼ 3.2 × 10 14 g, kinetic energy ∼2.0 × 10 29 erg) submerged in magnetic field lines that are expelled from the Sun and subsequently propagate and expand into interplanetary space (Hundhausen 1997;Webb & Howard 2012).They are key drivers of space weather disturbances in the heliospheric and planetary environments (Kilpua et al. 2017;Ghag et al. 2023).CMEs manifest as elongated, rope-like magnetic structures anchored to the Sun (Burlaga et al. 1981).In general, CMEs comprise three major parts, i.e., shock front, sheath region, and flux rope/magnetic cloud.The sheath region is a highly turbulent, heated, and compressed plasma region confined between the shock front and the magnetic cloud of the interplanetary CME (ICME).Moreover, ICME magnetic clouds are coherent magnetic structures (Burlaga et al. 1981;Richardson & Cane 2010, 2011;Ghag et al. 2024a) with features differing from the ambient solar wind.
Several observational, modeling, and simulation studies have been performed in the past to understand the macroscopic features of ICME magnetic clouds (e.g., Burlaga et al. 1981;Klein & Burlaga 1982;Burlaga 1991;Osherovich & Burlaga 1997).These ICMEs propagate into the solar wind medium with variable plasma density and interact with various magnetic structures such as solar stream interaction regions, heliospheric current sheets, previously expelled ICMEs, etc. (Lugaz et al. 2017).Consequently, the journey of ICMEs experiences a variety of forces, often resulting in their shape transformation and deflection in their direction of propagation (Manchester et al. 2017).Sometimes, the intricate configuration within an ICME's flux rope becomes distorted to the extent that it exhibits features like pancaking, writhe, or the formation of internal current sheets (Sturrock et al. 2001;Fan 2005;Török & Kliem 2005;Török et al. 2014;Shaikh et al. 2018Shaikh et al. , 2019Shaikh et al. , 2020;;Shaikh 2020;Shaikh & Raghav 2022a, 2022b;Ghag et al. 2023Ghag et al. , 2024b;;Raghav et al. 2023b;Choraghe et al. 2023).These distortions may even culminate in the fragmentation of the flux rope into multiple large-scale plasmoids (Khabarova et al. 2021).
Recently, various studies reported existence of Alfvén waves/fluctuations inside an ICME (Marsch et al. 2009;Gosling et al. 2010;Yao et al. 2010;Li et al. 2016Li et al. , 2017;;Raghav & Shaikh 2018;Dhamane et al. 2022a).The interactions between ICMEs and between an ICME and highspeed streams generate Alfvén waves, and their propagation in an ICME may disrupt its ordered flux rope structure (Raghav & Kule 2018a, 2018b;Dhamane et al. 2023b).Very recently, Alfvén waves and Alfvén ion cyclotron waves have been identified in ICME-driven sheath regions (Raghav & Shaikh 2018;Ala-Lahti et al. 2019;Shaikh et al. 2019;Dhamane et al. 2024).Moreover, Li et al. (2017) suggested that dissipation of Alfvénic fluctuations is responsible for local plasma heating inside the ICMEs.The direct transfer of energy by Alfvén waves is substantial and feasible solely within the kinetic domain.Notably, Dhamane et al. (2023a) have identified Alfvén ion cyclotron waves within ICME flux ropes, establishing links between MHD and kinetic modes.Another avenue for MHD wave dissipation toward the kinetic realm involves KAWs.Consequently, exploring the presence of KAWs within the context of low-beta plasma regions holds significant interest.Moreover, the ICME magnetic clouds are a natural laboratory with low plasma beta, and have been extensively researched for the macroscale structure and MHDscale waves.In fact, this kind of wave has rarely been considered in the context of ICME magnetic clouds; hence, we still do not have adequate plasma observations at kinetic scales.Several studies, mostly numerical, have addressed this topic, suggesting that discontinuities and small-scale current sheets (Khabarova et al. 2021) that are candidates for reconnection events might be regions where dissipative phenomena are at work with consequent plasma heating and acceleration.To date, there have been no reports of the existence of KAWs within the ICME flux ropes featuring low plasma beta.Therefore, the present scientific juncture motivated us to hunt for the unambiguous existence of KAWs inside the ICMEs listed in the WIND satellite catalog.7

Data and Methods
In this study, we investigated an Earth-directed ICME identified by the WIND satellite on 2011 October 24.The ICME data used in this research can be accessed at https://wind.nasa.gov/ICME_catalog/ICME_catalog_viewer.php.We have utilized the high-resolution data with time cadences of 0.092 and 3 s from the Magnetic Field Instrument (MFI; Lepping et al. 1995), Three-dimensional Plasma and Energetic Particle investigation (3DP) instrument (Lin et al. 1995b), and Solar Wind Experiment (SWE; Ogilvie et al. 1995) on board the WIND spacecraft for the thorough examination.
To study ICME features at 1 au, we utilized various parameters within the GSE coordinate system.These parameters included temporal variations in the total interplanetary magnetic field strength (IMF, referred to as B mag ), fluctuations in the total magnetic field (δB), its individual components (B x , B y , B z ), as well as the elevation (θ) and azimuth (f) angles.Additionally, we considered plasma proton density (N p ), solar wind speed (V p ), temperature anisotropy ratio ( T T /  , where T || and T ⊥ represent the parallel and perpendicular components of the temperature respectively), plasma temperature (T p ), and plasma beta (β), shown in Figure 1.On 2011 October 24, at 17:48 hr UTC, we observed a notable step increase in B mag , N p , V p , and β.It strongly indicated the onset of an ICME shock front.Subsequently, we observed a region of high fluctuation in B x , B y , B z , and δB, along with enhanced values of B mag , N p , V p , and β.These findings were indicative of the crossing of the ICME sheath region.In Figure 1, this particular region is represented by a cyan-shaded area.
Furthermore, we applied established criteria for identifying ICME magnetic clouds, as outlined in prior studies (Osherovich & Burlaga 1997).These criteria encompassed (i) low proton temperature, (ii) a wide and smooth rotation of the magnetic field direction, (iii) an increase in magnetic field strength, (iv) a low plasma β, (v) bidirectional flow of the electron strahl, and (vi) a decrease in the number density.Remarkably, we observed similar features in a region subsequent to the ICME sheath region, which we identified as an ICME flux rope/magnetic cloud.This identification is visually represented in Figure 1  Here, we shifted focus to a specific region highlighted in yellow, located between two vertical dashed lines within the magnetic cloud.For a closer examination, we provide an enlarged view of this region in Figure 2. Within this region, we have observed several noteworthy changes, for example, (i) a decrease in the total magnetic field strength, from 14.14 to 9.61 nT.(ii) Notably, the B y component undergoes a change in polarity, transitioning from a negative to a positive value.(iii) Additionally, there is a transition in the azimuthal angle (f) from 270°to 50°.(iv) We have also observed an increase in both the plasma proton density and plasma temperature, as indicated by the red lines in Figure 2.These observations suggest that the spacecraft has probably encountered a reconnection exhaust with thick current sheet crossing that persisted for a duration of 39 minutes (14:36-15:15 hr UTC on 2011 October 25) (Foullon et al. 2007;Owens 2009;Adhikari et al. 2019;Khabarova et al. 2021).In this context, to validate our findings, we conducted a comparative analysis using data derived from the WSA-ENLIL solar wind simulation.9 Figure 3 illustrates a snapshot of real-time reconstructions of solar wind speed from ENLIL in the ecliptic plane, specifically on 2011 October 25 at 16.00 hr UTC.Notably, this representation facilitates the identification of ICMEs in the form of distinctive, nearly radially propagating, croissant-shaped contours, which are characterized by elevated solar wind speeds.Additionally, a notable feature in the figure is a white line, signifying the crossing of the heliospheric current sheet (HCS).The ENLIL simulation validates the presence of ICMEs coinciding with crossings of the HCS, in accordance with our anticipated outcomes based on observed temporal fluctuations in Interplanetary parameters.Consequently, we have inferred that the examined ICME undergoes interactions with the HCS during its propagation, resulting in the formation of reconnection exhaust and deformations in the trailing segment of the ICME's flux rope.We postulate that this interaction has the potential to generate distinctive MHD or kinetic-scale waves.Our research objective is to ascertain the existence of KAWs, and we plan to employ additional identification techniques to further characterize this interaction event.

Power Spectral Density
The power spectral density (PSD) is a vital tool for characterizing signal power across different frequencies (Youngworth et al. 2005;Avallone et al. 2007).In the context of highly turbulent magnetic fluctuations in the solar wind, PSD analysis reveals power-law behavior (K α ), where K represents wavenumber and α is the spectral index.By measuring PSD in the frequency domain, which can be transformed into the spatial domain using the Taylor hypothesis We utilized the fast Fourier transform algorithm to estimate the PSD of the total magnetic field for the yellow-shaded region, as depicted in Figure 1.In this plot, a noticeable peak appears in the PSD spectrum within the frequency range of approximately 0.1-1 Hz.To analyze this spectrum, we fitted a red line to the frequency range 4.8 × 10 −3 -0.53 Hz, which corresponds to the inertial range with a spectral index of f −1.66 .Additionally, a green fitted line was derived for the frequency range 1.08-2.97Hz, representing the dissipative range with a spectral index of f −2.71 .Furthermore, we obtained the distribution of the total magnetic field within the shaded region near the HCS.Moreover, we calculated the range of proton gyrofrequency for the lowest and highest values of the total magnetic field for the shaded region.It approximately spans from 1.00 to 1.43 Hz and is depicted as the red-shaded region in Figure 4.

Normalized Magnetic Helicity
The spatially fluctuating magnetic helicity was first proposed by Matthaeus & Goldstein (1982).Here, the Fourier-transformed x-, y-, and z-components of the IMF in the GSE coordinate system are represented by B x , B y , and B z , and the sign * indicates the complex conjugate.The handedness of magnetic field fluctuations is quantified by σ m .It is zero for plane-polarized waves, +1 for right circularly polarized waves, and −1 for left circularly polarized waves (Telloni et al. 2019(Telloni et al. , 2020)).On the other hand, the sign of σ m strictly depends on the orientation of the background magnetic field.The magnetic helicity of right-handed polarized waves is positive in inward magnetic sectors.The magnetic field direction was reversed whenever measurements were taken in outward magnetic sectors to properly relate the sign of magnetic helicity to the intrinsic wave polarization (Telloni et al. 2020).Thus, a right-handed polarized wave may show a negative value of magnetic helicity.Solar wind measurements at 1 au suggest that in the inertial range, σ m fluctuates between +1 and −1, indicating that, on average, it is zero in the inertial range (Podesta & Gary 2011).However, it is nonzero in the dissipation range (Podesta & Gary 2011).We calculated σ m using IMF data with a temporal resolution of 11 Hz.Therefore, to analyze the polarization state of the magnetic field inside the selected region, we plotted σ m as a function of frequency.Here, we observe that σ m is initially zero in the inertial range, and as it approaches the kinetic range, it starts to decrease.In the kinetic range, we observe a bump in the negative direction with a maximum value of σ m = −0.38.In our analysis, we have also assessed the total magnetic field distribution within the studied region.We observed a  1), the PSD of the total magnetic field and the normalized magnetic helicity (σ m ) were computed.The red line represents the inertial region, which has a spectral index of f −1.66 , while the green solid line represents the dissipation range, which has a spectral index of f −2.71 .The red-shaded region represents the range of proton gyrofrequency for the lowest and highest values of the total magnetic field, i.e., 1.00 and 1.43 Hz.
distinctive double-hump distribution pattern.Consequently, we utilized the mode values of two humps as minimum and maximum magnetic field values to estimate the gyrofrequency.These boundary values are depicted in the form of vertical dashed lines, highlighted in red.This gyrofrequency spread is clearly discernible within the dissipation region of the PSD spectrum.Interestingly, this spread precisely aligns with the broad negative peak in σ m .In light of these observations and in comparison with previous research, we contemplate the existence of two distinct wave populations.These populations are associated with KAWs and ion cyclotron waves.
To investigate the origin of the signature of σ m in the dissipation range, we analyzed the σ m spectrum as a function of flow angle θ VB between the velocity and local magnetic field B mag .Analysis using this technique allows one to associate different angles of propagation with different spectral features and tentatively helps to identify waves responsible for different polarizations (Podesta & Gary 2011).This

/
) (Podesta & Gary 2011), where B x and B mag are the x-component of magnetic field and the local magnetic field respectively, obtained by wavelet analysis.Once σ m and θ VB are estimated, we distribute σ m according to different θ VB values to obtain σ m as a function of θ VB .Thus, we can plot a contour of σ m and θ VB as shown in Figure 5.One may refer to Podesta & Gary (2011) and Telloni et al. (2015) to gain further insight into this method.
Notably, these two wave populations exhibit divergent characteristics.The wavevectors linked to KAW fluctuations are transverse or highly oblique, while those associated with ion cyclotron waves are nearly parallel to the local magnetic field.Specifically, we have observed right-handed magnetic fluctuations in proximity to the kinetic scale, suggesting the presence of KAWs in the examined interaction region of the ICME flux rope.To validate our observations, we have created a plot in the frequency domain illustrating the distribution of reduced σ m as a function of θ VB , shown in Figure 5. Here, we observe a blue patch of fluctuations close to kρ i ∼ 1, indicating a signature of negative magnetic helicity in the quasiperpendicular direction (angles between 90°and 135°).This implies that the observed waves have right-handed polarized fluctuations and are propagating nearly transverse to the local magnetic field (Telloni et al. 2020).

Result and Discussion
In recent decades, significant progress has been made in our understanding of the morphological and kinematic evolution of CMEs within the heliosphere through a combination of spaceand ground-based observations and modeling efforts.Initially, it was commonly believed that idealized ICME flux ropes had a circular cross section.However, it has become evident that their morphology can be influenced by a variety of factors, including the deceleration processes of ICMEs, their interactions with the fast and slow ambient solar wind, and the interactions of multiple ICMEs.Thus, ICMEs exhibit a dynamic nature, often undergoing deformation, deflection, and fragmentation as they traverse the interplanetary medium (Telloni et al. 2021).These structural transformations not only alter their visual appearance but also set in motion a cascade of events.This suggests that various large-scale interactions during their journey through interplanetary space disrupt the delicate equilibrium of ICMEs, particularly the flux rope structure (Wang et al. 2014;Heinemann et al. 2019).One of the intriguing consequences of these transformations is the generation of waves within these deformed structures.The magnetic and kinetic forces within ICMEs are no longer in perfect equilibrium, leading to the excitation of both MHD and kinetic-scale waves (Jacques 1977).While previous studies have delved into MHD-scale waves within magnetic clouds (Raghav & Kule 2018a, 2018b;Raghav et al. 2018Raghav et al. , 2019Raghav et al. , 2023a;;Dhamane et al. 2023bDhamane et al. , 2024)), the understanding of kinetic-scale mode waves remains a challenge.In a recent study, Dhamane et al. (2023a) observed one aspect of kineticscale waves, specifically those aligned with the background magnetic field, by studying Alfvén ion cyclotron waves.However, transverse kinetic-scale waves continue to pose puzzles from both theoretical and observational perspectives.In this research paper, we aim to gain deeper insights into transverse kinetic waves through an exploration of the interaction event between an ICME and the HCS.
In this paper, we present an event in which the heliospheric plasma sheet associated with the HCS interacts with the trailing portion of an ICME, which may result in the formation of a reconnection exhaust.We assert that this represents the first documented in situ observation of KAWs within the magnetic cloud of the ICME, occurring near a reconnection exhaust within the ICME-HCS interaction region.The studied ICME traversed the WIND spacecraft's path on 2011 October 24.The ICME boundaries are outlined in Figure 1, guided by temporal changes in interplanetary parameters.We discerned the HCS's presence (illustrated in Figure 2) within the time interval spanning 14:00-16:00 hr UTC on 2011 October 25, as denoted by the yellow shading in Figure 1.We estimated the PSD of the total magnetic field for the interaction region.Notably, the PSD spectrum exhibited a steepening trend for frequencies exceeding the gyrofrequency, as visualized in Figure 4.The normalized magnetic helicity σ m revealed a distinctive negative bump within this frequency range.Notably, the maximum value of −0.38 near the gyrofrequency spread suggested a right-handed polarization of the wave (Howes & Quataert 2010;Podesta 2013;Huang et al. 2020).The origin of these kinetic-scale fluctuations can be attributed to either quasiparallel magnetosonic whistler waves or quasi-perpendicular KAWs.Further insight is gained from the distribution of σ m across flow angle θ VB within the frequency domain, as displayed in Figure 5.A distinct region of negative helicity fluctuations emerges between 90°and 135°around the gyrofrequency.Drawing upon prior investigations associated with solar wind phenomena, our findings affirm the identification of KAWs within the vicinity of the reconnection exhaust situated within the ICME magnetic cloud (Howes & Quataert 2010;Podesta & Gary 2011;Podesta 2013;Telloni et al. 2020).
In general, the solar wind's high-amplitude Alfvénic waves are ubiquitous in the interplanetary medium (Belcher & Davis 1971;Tsurutani et al. 1995), and their power spectrum exhibits characteristics akin to a Kolmogorov-type spectrum.Recent studies have reported that these waves undergo phasesteepening, leading to the concurrent phenomena of "wave breaking" and "period doubling."Moreover, despite the almost incompressible nature of Alfvénic fluctuations in the MHD range, observations have also shown evident compressibility due to the presence of evolved MDs.MDs evolved due to the dissipation of Alfvén waves, leading to the acceleration and heating of ions and electrons via the ponderomotive force (Tsurutani et al. 2002a(Tsurutani et al. , 2018)).Despite the common observations of MDs as the primary "compressional" component of solar wind turbulence, theoretical frameworks predominantly favor an MHD approach, suggesting that the dynamic solar wind is susceptible to turbulent cascades.This notion is supported by the observation that large segments of the magnetic fluctuation spectrum typically exhibit power-law variations following f −5/3 , indicative of Kolmogorov-like hydrodynamic turbulence.Furthermore, it is hypothesized that the turbulence in the solar wind will have an anisotropic orientation due to the presence of a strong magnetic guide field (Montgomery & Turner 1981;Matthaeus & Velli 2011;Bruno & Carbone 2013a;Verdini & Grappin 2016).Moreover, it is imperative to adopt the MD approach to examine solar wind turbulence comprehensively, enabling both an understanding of the compressive component and the investigation of Alfvén fluctuations to elucidate the incompressible component.
The turbulence within the solar wind is primarily characterized by incompressible fluctuations, although the plasma also exhibits weak compressive behavior (Marsch 1991;Goldstein et al. 1995;Bruno & Carbone 2013a;Tsurutani et al. 2018).The coexistence of these two fluctuations in MHD turbulence has been observed in the solar wind by comparing power spectra of density and the magnitude of the magnetic field, which have a similar power-law nature (Montgomery et al. 1987;Klein et al. 1993;Cuesta et al. 2023).The analysis of both compressible and incompressible fluctuations provides a better understanding of the physical processes behind the turbulent energy spectra (Roberts et al. 2017).Energy spectra within the solar wind exhibit diverse power-law scalings across various scales following a 1/f scaling law (Bruno et al. 2019).Moreover, observations by Bruno & Trenchi (2014) suggest that this turbulence could develop as the solar wind travels away from the Sun, involving progressively larger scales in the turbulent cascade.At 1 au the magnetic field demonstrates an energy scale with a spectral index of −1.13 at larger scales (Bruno et al. 2019), succeeded by a Kolmogorov-like inertial range with an index of −5/3 at intermediate scales (Goldstein et al. 1995;Bruno & Carbone 2013a;Roberts et al. 2017).As the kinetic scale is approached, a dissipative and dispersive range emerges, followed by further steepening to an index of −2.5 (Bruno et al. 2017).Moreover, the incompressible component of the turbulence at proton scales has been the subject of much debate.They are considered as the lowestorder solutions of the incompressive MHD theory (Goldstein et al. 1995) and are considered when there is a weakly collisional plasma-a limit in which MHD theory is invalid (Klein et al. 2012).This component is likely to have intrinsically low frequencies in the plasma frame that are lower than the proton cyclotron frequencies (Roberts et al. 2017).Observational evidence suggests that these fluctuations are dominated by highly oblique KAW-like fluctuations and nonlinear incompressible structures such as current sheets and Alfvénic vortices (Bale et al. 2005;Sahraoui et al. 2009;Osman et al. 2010;Salem et al. 2012;Podesta 2013).While MHD theory adequately explains the PSD of the interplanetary magnetic field, it may not be suitable for static compressive structures like MDs.Thus MDs are better explained by the compressible MHD theory.The properties and origins of this compressible component of turbulence are still unclear.Many studies have interpreted the presence of an anticorrelation between density and magnetic field strength to be indicative of slow-mode waves or pressurebalanced structures at inertial scales (Burlaga & Ogilvie 1970;Klein et al. 2012) or proton scales (Kellogg & Horbury 2004;Klein et al. 2012;Yang & Chao 2013) at 1 au as well as large and smaller heliospheric distances (Roberts 1990).Moreover, according to Zhao et al. (2014), this anticorrelation is also a signature of KAWs.Thus the interplanetary medium is a combination of both propagating waves and static structures that are advected by the solar wind (Bruno & Carbone 2016).The formation of pressure-balanced structures such as MDs within the incompressive solar wind poses a very difficult question about their origin.However, certain mechanisms such as mode conversion from the incompressible fluctuations (Roberts et al. 2017), wave-wave interaction (Vasquez & Hollweg 1999;Vasquez et al. 2007), and Alfvén wave dissipation (Tsurutani et al. 2018) can explain their origin.The presence of such compressive components along with waves such as KAWs causes an overall enhancement in the power spectrum in the solar wind near the fluid scales and steepening near the kinetic scales (Roberts et al. 2017;Tsurutani et al. 2018).Moreover, the steepening produced by nonlinear Alfvén waves creates a high-frequency component to the turbulence.Thus it becomes important to note the difference between the incompressible and compressible components, which means that the fluctuations produced cannot be solely due to KAW turbulence; another component is required for a complete description of the morphological change produced in the power spectra.
In this paper, we adopt the framework of solar wind turbulence to investigate KAWs, as outlined by Podesta (2013).
To identify the existence of MHD Alfvén wave fluctuations, we estimate the correlation between solar wind velocity (V ) and magnetic field in Alfvén units (V A ) using the method of Belcher & Davis (1971).The analysis indicates that the values of regression and correlation coefficients for the examined region are statistically insignificant (CC x = −0.53,CC y = −0.25,CC z = −0.46),suggesting inconclusive evidence for the existence of Alfvén waves in this region (figure not shown here).Furthermore, previous studies have suggested that the perpendicular proton temperature tends to be higher than its parallel counterpart within the MDs (Tsurutani et al. 2018).However, our observations show that the parallel component of temperature dominates over the perpendicular one, implying proton heating in the parallel direction (refer to the fifth panel of Figure 1).Therefore, we discard the possibility of the existence of arc-polarized Alfvén waves and associated MDs in the examined region.Instead, we substantiate our assertion regarding observing KAWs within the ICME-HCS interaction region.
Furthermore, we estimate that a balanced spectrum at kinetic scales is essential to account for the observed cutoff at high wavenumber in the σ(k, θ VB ) spectra, in line with the expectations set forth by Howes & Quataert (2010).Notably, we observe minimal evidence of wave modes associated with quasi-parallel propagation (see Figure 5).Consequently, the likelihood of electromagnetic ion cyclotron waves propagating away from the Sun or magnetosonic/whistler waves moving toward the Sun is diminished.Considering magnetic helicity measurements, which suggest right-hand polarization, we anticipate the existence of KAWs and quasi-perpendicular magnetosonic/whistler waves.However, we do not entirely discard the presence of quasi-perpendicular magnetosonic/ whistler waves, but observations align more closely with the polarization characteristics of KAWs.The study by Verdon et al. (2009) and Podesta & TenBarge (2012) suggests that the classification of various wave modes in the hot plasma dispersion relation can be intricate, particularly when k ⊥ > k ∥ .This complexity arises due to the potential association of a single continuous dispersion curve with different wave types across varying wavenumber ranges.Additionally, the emergence of ion Bernstein modes may disrupt the continuity of curves associated with specific modes such as the magnetosonic/whistler wave.We need two perpendicular electric field components and simultaneous high-resolution magnetic field measurements to confirm their identification.Moreover, a comprehensive analysis of the particle distribution function is essential to corroborate the instability mechanism.Moreover, the nature of fluctuations at electron kinetic scales near the ICME-HCS interaction remains debatable from a theoretical standpoint.This limits our further analysis to comprehend the existence of the perpendicular magnetosonic/whistler waves.Therefore, in this paper, we present the possibility of KAWs based on our observations and the current understanding.
Kinetic processes are significant in collisionless plasmas due to their capacity to generate waves spanning a broad spectrum of ion and electron scales and frequencies.These waves and turbulence can be induced by reconnection events, where they not only influence the reconnection process itself but can also impact the onset of reconnection (Khotyaintsev et al. 2019).Distinct regions such as outflows (Osman et al. 2015), separatrix regions (Viberg et al. 2013), and ion and electron diffusion regions (Graham et al. 2016(Graham et al. , 2017;;Fu et al. 2017) often exhibit these waves and turbulence.The influx of these waves into the reconnection site can be attributed to external drivers like compression or, in some cases, it is spontaneously initiated and sustained by pressure gradient forces linked to rarefactions created by the exhaust outflows (Gosling et al. 2006;Gosling & Szabo 2008;Xu et al. 2011).Since magnetic reconnection leads to energy transfer, the interaction between waves and particles plays a significant role in energy dissipation (Gershman et al. 2017a;Huang et al. 2018).This dissipation phase is characterized by a steepening of the power spectrum and often includes changes in the polarization or magnetic helicity of the magnetic fluctuations (Podesta & Gary 2011;Podesta 2013;Goldstein et al. 2015).The waveparticle interaction involves a cascade of fluctuations that progressively reaches smaller scales, eventually leading to resonance with the thermal plasma.This interaction results in the damping of electromagnetic energy and can lead to plasma heating (Goldstein et al. 2015).The behavior of σ m and the PSD at kinetic scales can be attributed to the proton cyclotron damping of Alfvénic fluctuations at this scale, generated by magnetosonic waves, or to KAWs propagating nearly perpendicular to the mean magnetic field (He et al. 2011;Goldstein et al. 2015).
Numerous mechanisms have been proposed in the scientific literature to explain the generation of KAWs.One such mechanism occurs within the diffusion region itself, where the breakdown of the magnetic frozen-in condition causes ions to decouple from the magnetic fields in close proximity to the reconnection site.This decoupling results in the emergence of the Hall current, typically giving rise to the Hall effect (Xu et al. 2015;Zhang et al. 2017;Khotyaintsev et al. 2019).
The Magnetospheric Multiscale mission observations by Zhang et al. (2017) indicated that this Hall structure in the diffusion region corresponds directly to the propagation of KAWs away from the diffusion region, along the separatrix regions.In addition, Liang et al. (2016) conducted 3D hybrid simulations to investigate the generation and propagation of KAWs within magnetic reconnection in a current sheet.Their findings revealed that KAWs are generated along the X-line and propagate along magnetic field lines when a finite guide field is present.Another suggested mechanism for KAW generation is the firehose instability (Jiansen et al. 2018), or they may be generated as part of the turbulence arising in reconnection outflows (Huang et al. 2012).Moreover, various in situ observations and numerical studies, particularly in the context of the solar wind and magnetopause, have reported the presence of KAWs.For instance, Cluster mission studies by Chaston et al. (2005Chaston et al. ( , 2008Chaston et al. ( , 2009) ) indicated that magnetic reconnection serves as a source of KAWs, as they were detected near a reconnection X-line.Additionally, these studies proposed that KAWs have the capability to transport significant energy and play a substantial role in facilitating the magnetic reconnection process.

Implication
The ICME magnetic clouds are typically characterized by their low plasma beta and low proton temperature.These attributes, along with their closed, force-free magnetic field structures, allow magnetic clouds to maintain a state of minimal energy, making them relatively isolated from energy exchange with the surrounding solar wind.However, observations by Feng & Wang (2014) have revealed an intriguing phenomenon in some magnetic clouds, where localized areas exhibit unexpectedly high proton temperatures.This phenomenon is particularly puzzling as it tends to occur mainly in the central regions of magnetic clouds.Two primary explanations have been proposed to account for this high-temperature anomaly: (i) It is possible that magnetic clouds carry this localized hightemperature feature with them from their point of origin in the solar corona.(ii) Alternatively, these pockets of high temperature could result from internal processes, possibly driven by magnetic reconnections occurring within the magnetic cloud itself.As mentioned earlier, magnetic reconnection processes give rise to KAWs.The presence of these KAWs within a magnetic cloud is significant as it sheds light on the intricate interactions between waves and particles within the cloud.This wave-particle interplay can result in the creation of regions within the cloud that exhibit higher plasma beta and temperature, characteristics not typically associated with standard magnetic clouds.In fact, KAWs generated in this process can lead to an increase in the parallel component of temperature, consequently enhancing temperature anisotropy in the vicinity of the exhaust region, which can be observed in the plot of temperature anisotropy in Figure 1.Understanding these mechanisms is crucial in unraveling the dissipation processes within what are traditionally considered nondissipative structures, such as magnetic clouds.This discovery opens the door to further investigations on how energy is transferred to particles within these magnetic clouds and how this knowledge can be leveraged for future research.
using a red-shaded region.The cloud boundaries have been identified by referring to the online ICME catalog. 8A magnetic cloud transient was noted from 00:21 to 23:31 hr UTC on 2011 October 25.

Figure 1 .
Figure 1.Observations of interplanetary parameters for the ICME transient on 2011 October 24 using a time cadence of 3 s.The regions shaded red and cyan represent the ICME sheath and the magnetic cloud region, respectively.The panels from top to bottom represent total interplanetary field strength B mag , δB, components of the magnetic field, IMF orientation (θ, f), number density (N p ), plasma velocity V p , temperature anisotropy ratio T T /  , proton temperature T p , and the plasma β.The yellow-shaded portion indicates the current sheet crossover region.

Figure 2 .
Figure 2.An expanded plot of the interplanetary parameters.From top to bottom: total interplanetary field strength B mag , y-component of the magnetic field B y , IMF orientation f, number density N p , and proton temperature T p .Here, the red-colored region indicates the region of the HCS crossover.

Figure 3 .
Figure 3. Reconstructions of solar wind speed according to the ENLIL model for typical solar wind conditions (from the HelioWeather webpage: http://helioweather. net/archive/2011/10/vel3e1.html).ICME streams are indicated directly on the panels.THe HCS is indicated by the white line.V r indicates the radial speed of the solar wind.
It measures the lack of mirror symmetry of the magnetic field.It helps to understand the wave polarization characteristics of solar wind turbulence.Since it is conserved in ideal MHD fluids, it serves as an important parameter for understanding the properties of plasma.According to Podesta & Gary (2011) and Telloni et al. (2019, 2020) the normalized reduced magnetic helicity (σ m ) can be written in terms of time and frequency as

Figure 4 .
Figure 4.For the chosen time interval (yellow-shaded region in Figure1), the PSD of the total magnetic field and the normalized magnetic helicity (σ m ) were computed.The red line represents the inertial region, which has a spectral index of f −1.66 , while the green solid line represents the dissipation range, which has a spectral index of f −2.71 .The red-shaded region represents the range of proton gyrofrequency for the lowest and highest values of the total magnetic field, i.e., 1.00 and 1.43 Hz.
technique was first introduced by He et al. (2011) and later on was used by Podesta & Gary (2011), Podesta (2013), and Telloni et al. (2015, 2020) to identify plasma waves at distances near and beyond 1 au.Further, we will estimate θ VB by using the relation θ VB = cos −1 (-B B x mag

Figure 5 .
Figure 5. Distribution of normalized helcity (σ m ) with respect to θ VB , the angle between flow velocity and local mean magnetic field B.