Warm Protons Research Articles

Relation between magnetosonic waves and pitch angle anisotropy of warm protons

In the past decade, many observations of transversely heated low energy protons were reported in the inner magnetosphere. Interestingly, most of the time heated protons were observed along with magnetosonic waves. Due to the strong correlation, it was often assumed that magnetosonic waves were responsible for the heating of low energy protons. By performing a case study under unusually disturbed geomagnetic conditions, this paper unravels the controversial relationship between the observed pitch angle anisotropy of warm protons and the accompanying magnetosonic waves in the inner magnetosphere. We perform a comparative analysis involving two nearly identical cases of pitch angle anisotropy of warm protons in low L-shell region–one with magnetosonic waves and one without them. It is found that magnetosonic waves are not responsible for primary heating of low-energy protons and may just marginally alter the shape of the distribution of heated protons in the events analyzed. Based on the recent Cluster and POLAR observations, we also show how the recirculated polar wind plasma in the Earth’s magnetosphere can cause the concurrent appearance of heated protons and magnetosonic waves.

Warm protons at comet 67P/Churyumov–Gerasimenko – implications for the infant bow shock

Abstract. The plasma around comet 67P/Churyumov–Gerasimenko showed remarkable variability throughout the entire Rosetta mission. Plasma boundaries such as the diamagnetic cavity, solar wind ion cavity and infant bow shock separate regions with distinct plasma parameters from each other. Here, we focus on a particular feature in the plasma: warm, slow solar wind protons. We investigate this particular proton population further by focusing on the proton behaviour and surveying all of the Rosetta comet phase data. We find over 300 events where Rosetta transited from a region with fast, cold protons into a region with warm, slow protons. We investigate the properties of the plasma and magnetic field at this boundary and the location where it can be found. We find that the protons are preferentially detected at intermediate gas production rates with a slight trend towards larger cometocentric distances for higher gas production rates. The events can mostly be found in the positive convective electric field hemisphere. These results agree well with simulations of the infant bow shock (IBS), an asymmetric structure in the plasma environment previously detected on only 2 d during the comet phase. The properties of the plasma on both sides of this structure are harder to constrain, but there is a trend towards higher electron flux, lower magnetic field, higher magnetic field power spectral density and higher density in the region that contains the warm protons. This is in partial agreement with the previous IBS definitions; however, it also indicates that the plasma and this structure are highly non-stationary. For future research, Comet Interceptor, with its multi-point measurements, can help to disentangle the spatial and temporal effects and give more clarity on the influence of changing upstream conditions on the movement of boundaries in this unusual environment.

Fast and slow beam mode ion-acoustic solitons in plasmas with counterstreaming cold protons

It was recently shown (Lakhina et al (2020), Physica Scr. 95, 105601) that in a plasma of hot electrons and two counterstreaming warm proton beams, a slow ion-acoustic solitary mode occurred, where the soliton speeds decreased from the critical acoustic speeds, while the amplitudes increased. This contradicts conventional wisdom that solitons are inherently ‘super-acoustic’, the soliton speeds exceeding the linear acoustic speed, and that amplitudes grow with soliton speed. To elucidate and further elaborate on these findings, a simpler model is considered, in which the ion beam species are cold, neglecting their thermal effects. This retains the essence of the physics, but simplifies the analysis. It permits a full parametric discussion of the existence ranges for such solitons, not only for the critical Mach numbers, but also for the limit amplitudes, by using fluid density restrictions for the ions. If the beam speed exceeds a threshold, two acoustic regimes are found: a fast beam mode with the usual super-acoustic solitons, and a slow beam mode, where solitons are sub-acoustic, at least in the laboratory frame. This confirms the earlier result, but shows that it is primarily a beam effect, rather than an ion thermal effect. The observation can be explained: in the ion beam frame, the solitons are indeed super-acoustic. However, those travelling in the negative direction are swept forwards by the fast beam and hence are observed in the laboratory frame as being sub-acoustic. A similar approach is used for a case of asymmetric proton beams, with unequal ion densities and beam speeds.

Proton velocity ring‐driven instabilities and their dependence on the ring speed: Linear theory

AbstractLinear dispersion theory is used to study the Alfvén‐cyclotron, mirror and ion Bernstein instabilities driven by a tenuous (1%) warm proton ring velocity distribution with a ring speed, vr, varying between 2vA and 10vA, where vA is the Alfvén speed. Relatively cool background protons and electrons are assumed. The modeled ring velocity distributions are unstable to both the Alfvén‐cyclotron and ion Bernstein instabilities whose maximum growth rates are roughly a linear function of the ring speed. The mirror mode, which has real frequency ωr=0, becomes the fastest growing mode for sufficiently large vr/vA. The mirror and Bernstein instabilities have maximum growth at propagation oblique to the background magnetic field and become more field‐aligned with an increasing ring speed. Considering its largest growth rate, the mirror mode, in addition to the Alfvén‐cyclotron mode, can cause pitch angle diffusion of the ring protons when the ring speed becomes sufficiently large. Moreover, because the parallel phase speed, v∥ph, becomes sufficiently small relative to vr, the low‐frequency Bernstein waves can also aid the pitch angle scattering of the ring protons for large vr. Potential implications of including these two instabilities at oblique propagation on heliospheric pickup ion dynamics are discussed.

Balmer-dominated shocks in Tycho’s SNR: omnipresence of CRs

AbstractWe present wide-field, spatially and highly resolved spectroscopic observations of Balmer filaments in the northeastern rim of Tycho’s supernova remnant in order to investigate the signal of cosmic-ray (CR) acceleration. The spectra of Balmer-dominated shocks (BDSs) have characteristic narrow (FWHM ~ 10 km s−1) and broad (FWHM ~ 1000 km s−1) Hα components. CRs affect the Hα-line parameters: heating the cold neutrals in the interstellar medium results in broadening of the narrow Hα-line width beyond 20 km s−1, but also in reduction of the broad Hα-line width due to energy being removed from the protons in the post-shock region. For the first time we show that the width of the narrow Hα line, much larger than 20 km s−1, is not a resolution or geometric effect nor a spurious result of a neglected intermediate (FWHM ~ 100 km s−1) component resulting from hydrogen atoms undergoing charge exchange with warm protons in the broad-neutral precursor. Moreover, we show that a narrow line width ≫ 20 km s−1extends across the entire NE rim, implying CR acceleration is ubiquitous, and making it possible to relate its strength to locally varying shock conditions. Finally, we find several locations along the rim, where spectra are significantly better explained (based on Bayesian evidence) by inclusion of the intermediate component, with a width of 180 km s−1on average.

Kinetic ion-acoustic solitary waves in collisional plasmas

The excitation and the propagation of solitary waves of ion-acoustic nature are analyzed by means of kinetic Eulerian simulations, in both collision-free and collisional plasmas, composed of kinetic warm protons and linear Boltzmannian electrons. The process of soliton formation is discussed in detail through the description of the time evolution of the electrostatic potential and of the associated phase space portraits of the proton distribution function. We study the effects of collisions on the propagation of solitary waves, by modeling proton-proton interactions through the one-dimensional nonlinear Dougherty operator, which is a collisional operator of the Fokker-Planck type. We show how, in a case of non-negligible collisionality, short spatial scales in the electrostatic potential are dissipated in time and the phase space structures, observed in the distribution function in absence of collisions, are significantly smoothed out. Finally, by exploiting the analogy between ion-acoustic waves in neutral infinite plasma and Trivelpiece-Gould waves in nonneutral plasmas columns, a recipe to observe solitary structures in nonneutral plasma devices is proposed.

COLLISIONLESS SHOCKS IN A PARTIALLY IONIZED MEDIUM. II. BALMER EMISSION

Strong shocks propagating into a partially ionized medium are often associated with optical Balmer lines. This emission is due to impact excitation of neutral hydrogen by hot protons and electrons in the shocked gas. The structure of such Balmer-dominated shocks has been computed in a previous paper (Blasi et al. 2012), where the distribution function of neutral particles was derived from the appropriate Boltzmann equation including coupling with ions and electrons through charge exchange and ionization. This calculation showed how the presence of neutrals can significantly modify the shock structure through the formation of a "neutral-induced" precursor ahead of the shock. Here we follow up on our previous work and investigate the properties of the resulting Balmer emission, with the aim of using the observed radiation as a diagnostic tool for shock parameters. Our main focus is on Supernova Remnant shocks, and we find that, for typical parameters, the H{\alpha} emission typically has a three-component spectral profile, where: 1) a narrow component originates from upstream cold hydrogen atoms, 2) a broad component comes from hydrogen atoms that have undergone charge exchange with shocked protons downstream of the shock, and 3) an intermediate component is due to hydrogen atoms that have undergone charge exchange with warm protons in the neutral-induced precursor. The width of the intermediate line reflects the temperature in the precursor, while the width of the narrow one is left unaltered by the precursor. In addition, we show that the profiles of both the intermediate and broad components generally depart from a thermal distribution. Finally, we show that a significant amount of Balmer emission can be produced in the precursor region if efficient electron heating takes place.

Electromagnetic heavy‐ion/proton instabilities

Ion/ion instabilities are driven by the relative velocity Vo of two distinct ion components. This manuscript reports linear theory and simulation studies of electro‐magnetic ion/ion instabilities for a warm proton component streaming against a cool, heavy ion component in a homogeneous, magnetized, collisionless plasma in which Vo is parallel or antiparallel to the background magnetic field. Under these conditions linear Vlasov dispersion theory predicts that the fastest growing mode is usually the right‐hand polarized proton resonant instability. Two‐dimensional initial value hybrid simulations of this growing mode are carried out, yielding scaling relations for the maximum fluctuating magnetic field energy density and the proton pickup time. The latter is proportional to the inverse of the maximum growth rate for a wide range of plasma parameters.

Isotopic Lithium Abundances in Nine Halo Stars

High-resolution echelle spectra of the 6707.8 ? Li I absorption line in spectra of metal-poor halo dwarfs and subgiants are analyzed for the isotopic fractions of 6Li and 7Li. Positive detections of 6Li are reported for two stars, HD 84937 and BD +26? 3578, and tight upper limits to the 6Li fraction are obtained for an additional seven stars. Two stars, HD 116064 and BD +26? 2606, were found to be double-lined spectroscopic binaries and were rejected from the analysis. Elemental Li abundances were also derived for the nine program stars: the Li abundance is identical across the sample (excluding the spectroscopic binaries) to within ?0.04 dex in log (Li). Two processes involving high-energy particles (cosmic rays) are considered as origins of the 6Li: spallation reactions (e.g., O + p ?6Li) and ?-? fusion reactions. The contribution of spallation reactions may be assessed empirically using the published Be and B abundances. The observed 6Li/Be ratio is significantly in excess of that expected from spallation. In view of the fact that the surface 6Li abundance has almost certainly been somewhat depleted by exposure to warm protons, the conclusion is that ?-? reactions produced most of the observed 6Li. Consequences of this conclusion are sketched.

Excitation of Kinetic Alfvén Waves in a Flaring Loop

At the onset of a solar flare, initiated by magnetic reconnection high in the corona, reconnection outflow sets up warm proton beams (PBs), streaming down along just-reconnected field lines through steady underlying plasma. Incorporating this scenario, we study excitation of kinetic Alfven waves (KAWs) by PBs, keeping the effects of a beam-induced electric field and thermal effects. Taking into account the high growth rate (∼105 s−1), short relaxation distance (∼106 cm), and energy flux partition between the waves and the beam after relaxation (PKAW/PPB∼1), we conclude that PB-driven KAW instability is an efficient energy conversion mechanism in flaring loops. The quasilinear spectral energy concentration at the largest wavenumbers indicates the possibility of nonlinear spectral modification. We suggest that the resulting turbulence of KAWs plays an important role in the flare plasma energization.

Warm protons at geosynchronous orbit

Ions in the outer magnetosphere typically are observed to consist of a hot (T ∼ 10 keV) component and a cool constituent with energies of a few eV. The hot component often is observed to be anisotropic with T⊥ > T‖, where the subscripts denote directions relative to the background magnetic field, whereas the cool ions exhibit a wide variety of anisotropies. The Los Alamos magnetospheric plasma analyzer (MPA) instruments measure ion and electron velocity distributions from about 1 eV to about 40 keV at geosynchronous orbit. In the afternoon and evening sectors MPA observations show that cool ions, which are assumed to be protons, are sometimes “warm” with temperatures of order 10 eV and T⊥ > T‖. Theory and simulations of the electromagnetic proton cyclotron instability, which is driven by the hot proton anisotropy, have shown that the resulting enhanced field fluctuations heat initially cool protons. Moreover, this process implies a scaling relation for the apparent temperature of the warm protons as a function of the relative densities of the two components. This manuscript describes an examination of MPA data which shows that this scaling relation provides an approximate upper bound for a selected set of warm proton temperatures observed at geosynchronous orbit. This result is consistent with the hypothesis that the proton cyclotron instability is an important energization source for warm protons at geosynchronous orbit.

Electromagnetic proton cyclotron instability: Proton velocity distributions

The hot (Th ∼ 10 keV) protons of the outer magnetosphere are frequently observed to have T⊥/T‖ > 1, where the subscripts denote directions relative to the background magnetic field. This anisotropy leads to the growth of the electromagnetic proton cyclotron anisotropy instability, which implies two concise scaling relations: an upper bound on T⊥h/T‖h and an expression for the apparent temperature of warm (Tω ∼ 10 eV) protons heated by this instability. One‐dimensional hybrid simulations of this instability in a homogeneous plasma have been used to examine the velocity distribution response of both hot and warm protons. Results are that the enhanced fluctuations from this instability not only preserve the initially bi‐Maxwellian character of a hot proton distribution but also drive a hot distribution which is initially nongyrotropic rapidly toward the gyrotropic condition. These results further support the validity of the hot proton temperature anisotropy upper bound. Although wave‐particle interactions yield nongyrotropic ƒω(υ⊥) distributions, the ƒω(υ‖) remain Maxwellian‐like as they are heated by the enhanced fluctuations.

ELF plasma waves in hot and cold plasma fluxes observed by Prognoz-8 in the magnetospheric tail

Abstract. We present Prognoz-8 observations of low-frequency plasma waves (2-105 Hz) associated with plasma fluxes near the outer boundary of the plasma sheet. These plasma fluxes were different from the regular plasma sheet boundary layer and consisted of tailward flowing warm proton and cold oxygen beams accompanied by rather cold electrons (Te less than 100 eV). Observed plasma characteristics were used in the numerical solution of the dispersion relation for the ion-beam acoustic instability. Detailed analysis shows that this instability can be a source of observed emissions at frequencies up to 25 Hz.

Field aligned distribution of plasma mantle and ionospheric plasmas

The density and bulk velocity distributions of warm magnetosheath particles and cold ionospheric O + and H + ions are calculated along a polar cusp (cleft) magnetic field line. The warm collisionless plasma is injected at 12 earth radii, and interacts with the cold polar wind electrons and ions of ionospheric origin. After magnetic reflections in the exosphere (above 1400km altitude) most of the ions move upwards along tail magnetic field lines and feed the plasma mantle flow. The bulk velocity of these warm protons is approximately 200km s −2 and happens to be proportional to the average thermal speed of the magnetosheath ions at their injection point in the magnetosphere. The density in the plasma mantle is found to be about half that in the entry layer. The electric potential distribution is deduced from the quasi-neutrality condition, A large parallel electric field corresponding to an electrostatic double layer is obtained for the assumed boundary conditions.

Requirements on singly ionized lithium concentrations for magnetospheric seeding experiments

Optimum concentrations of singly ionized Li relative to warm protons, η ≡ nLi/np,w, for magnetospheric seeding experiments aimed at enhancing the electromagnetic proton cyclotron instability have been obtained analytically and numerically. This has been done by calculating the absolute maximum growth rates γmaxabs and enhancement F ≡ γmaxabs/γmax(η = 0) for the proton instability branch upon addition of cold lithium ions to an infinite anisotropic proton plasma. A necessary condition for enhancement is P ≡ βpAp²(Ap + 1) ≪ 1, where βp ≡ 8πnpKT∥,p/B0² and Ap ≡ (T⊥/T∥)p − 1. The shape of the linearly unstable spectrum strongly depends on the parameter [Ap/(Ap + 1)]/(mLi/ZLimp)−1. When the condition stated above is satisfied, the growth rate and the extension of the unstable spectrum in k space may increase with η. An optimum η value is found to exist.

Effects of lithium and heavier ions on electromagnetic proton cyclotron instability

The effects of lithium and heavier ions (i.e. having specific charge Zj e/mj smaller than the proton's), on the electromagnetic cyclotron instability in warm anisotropic (bi-maxwellian) proton plasmas, are analytically and numerically investigated. It is found that the occurrence of resonances for modes with ωr = Ωj (ωr, real wave frequency; Ωj, ion gyrofrequency), which produce a cut-off in the phase velocity, leads to the possible existence of two different situations. (i) If the proton thermal anisotropy is smaller than a critical value A pc (which depends on the ion specific charge), the resonance and therefore the ‘stop band’ occurs outside the unstable range. The maximum growth rate, as well as the instability range in k space, may increase with the relative ion concentration ion and warm proton particle densities, respectively. There is an optimum value of η for maximum enhancement of the instability. (ii) If Ap > Apc, the unstable spectrum in Wr space is divided by the ‘stop band’ into two distinct regions. In the higher-frequency region, the maximum growth rate, as well as the width of the unstable spectrum, decrease with increasing relative ion concentration. For η > ηc, the instability is completely suppressed. In the lower-frequency region, however, the maximum growth rate and the extension in k space may increase with η.