Loss-cone Anisotropy Research Articles

Beam-driven ECH waves: A parametric study

Electron cyclotron harmonic (ECH) waves play a significant role in driving the diffuse aurora, which constitutes more than 75% of the particle energy input into the ionosphere. ECH waves in magnetospheric plasmas have long been thought to be excited predominantly by the loss cone anisotropy (velocity–space gradients) that arises naturally in a planetary dipole field. Recent THEMIS observations, however, indicate that an electron beam can also excite such waves in Earth's magnetotail. The ambient and beam plasma conditions under which electron beam excitation can take place are unknown. Knowledge of such conditions would allow us to further explore the relative contribution of this excitation mechanism to ECH wave scattering of magnetospheric electrons at Earth and the outer planets. Using the hot plasma dispersion relation, we address the nature of beam-driven ECH waves and conduct a comprehensive parametric survey of this instability. We find that growth is provided by beam electron cyclotron resonances of both first and higher orders. We also find that these waves are unstable under a wide range of plasma conditions. The growth rate increases with beam density, beam velocity, and hot electron temperature; it decreases with increasing beam temperature and beam temperature anisotropy (T⊥/T∥), hot electron density, and cold electron density and temperature. Such conditions abound in Earth's magnetotail, where magnetospheric electrons heated by earthward convection and magnetic reconnection coexist with colder ionospheric electrons.

Periodic Excitation of Chorus and ECH Waves Modulated by Ultralow Frequency Compressions

AbstractSpacecraft observations often reveal the presence of marginally stable chorus and electron cyclotron harmonic (ECH) waves across a spatial scale about several Earth radii (RE), which is much larger than the spatial scale of the localized injections that are thought to generate them. Given the importance of these waves for electron acceleration, scattering, and loss to the atmosphere, here we investigate how such broad, periodic wave excitation might occur. We show that it can be attributed to modulation of hot (resonant) electron distributions by ultralow frequency (ULF) waves, which transport electrons radially inward/outward by as much as ∼1.5 RE. Unstable electrons (and chorus waves generated by them) from higher L‐shells (presumably from the azimuthally extended flow‐braking region) are thus brought to lower L‐shells. Half a ULF cycle later, outward electron transport increases the loss cone anisotropy (the perpendicular velocity gradient near the loss cone), resulting in periodic generation of ECH waves with intensities that are anticorrelated with chorus wave intensity. Such wave generation and intermittent electron precipitation (modulated on the spatial/temporal scale of a ULF wavelength/wave period) across large spatial scales could explain the pulsating aurora.

Growth Rates of the Upper-Hybrid Waves for Power-Law and Kappa Distributions with a Loss-Cone Anisotropy

Fine structures of radio bursts play an important role in diagnostics of the solar flare plasma. Among them the zebras, which are prevalently assumed to be generated by the double plasma resonance instability, belong to the most important. In this paper we compute the growth rate of this instability for two types of the electron distribution: a) for the power-law distribution and b) for the kappa distribution, in the both cases with the loss-cone type anisotropy. We found that the growth rate of the upper-hybrid waves for the power-law momentum distribution strongly depends on the pitch-angle boundary. The maximum growth rate was found for the pitch-angle $\theta_\mathrm{c} \approx$ 50$^\circ$. For small angles the growth rate profile is very flat and for high pitch-angles the wave absorption occurs. Furthermore, analyzing the growth rate of the upper hybrid waves for the kappa momentum distribution we found that a decrease of the characteristic momentum $p_\kappa$ shifts the maximum of the growth rate to lower values of the ratio of the electron-plasma and electron-cyclotron frequencies, and the frequency widths of the growth rate peaks are very broad. But, if we consider the kappa distribution which is isotropic up to some large momentum $p_m$ and anisotropic with loss-cone above this momentum then distinct peaks of the growth rate appear and thus distinct zebra stripes can be generated. It means that the restriction for small momenta for the anisotropic part of distributions is of principal importance for the zebra stripes generation. Finally, for the 1 August 2010 zebra stripes, the growth rates in dependence on radio frequency were computed. It was shown that in this case the growth rate peaks are more distinct than in usually presented dependencies of growth rates on the ratio of the plasma and cyclotron frequencies.

Magnetotail Configuration During a Steady Convection Event as Observed by Low‐Altitude and Magnetospheric Spacecraft

AbstractMotivated by ongoing discussion regarding the magnetic configuration in the near‐Earth and midtail regions and its role in populating the inner magnetosphere during long‐duration steady magnetospheric convection (SMC) events, we analyze a rich collection of observations during ~10 hr of strong, steady solar wind driving. Auroral boundaries and regions of stretched and dipolarized magnetic field in the plasma sheet were monitored using solar electron loss cone anisotropy observed by low‐altitude spacecraft. Following a southward turning of the interplanetary magnetic field and a subsequent 3‐ to 4‐hr period of large‐scale substorm‐related reconfigurations and plasma injections, the near‐Earth magnetic configuration evolved into a nonstandard type, which lasted until the end of this SMC event (5 hr). During that time a dipolarized region with complicated Bz landscape persisted in the midtail, while the configuration was very stretched in the near tail. This was manifested as a highly depressed magnetic Bz component at geostationary orbit and as persistent nonadiabatic electron scattering at the periphery of the outer radiation belt. In addition, in situ observations suggest that a thin current sheet extended longitudinally toward the dawn terminator. In the return convection region near the terminator, observations of this azimuthal current sheet were sporadically interrupted/modulated by earthward convecting plasma structures, either remnants of reconnection‐produced plasma bubbles or flapping waves. The hybrid magnetotail configuration (dipolar in the midtail and stretched in the near tail) observed during this long‐duration SMC event poses a challenge for empirical magnetospheric modeling.

Intense plasma wave emissions associated with Saturn's moon Rhea

[1] Measurements by the Cassini spacecraft during a close flyby of Saturn's moon Rhea on March 2, 2010, show the presence of intense plasma waves in the magnetic flux tube connected to the surface of the moon. Three types of waves were observed, (1) bursty electrostatic waves near the electron plasma frequency, (2) intense whistler-mode emissions below one half of the electron cyclotron frequency, and (3) broadband electrostatic waves at frequencies well below the ion plasma frequency. The waves near the electron plasma frequency are believed to be driven by a low energy (∼35 eV) electron beam accelerated away from Rhea. Their bursty structure is believed to be due to a nonlinear process similar to the three-wave interaction that occurs for Langmuir waves in the solar wind. The whistler-mode emissions are propagating toward Rhea and are shown to be generated by the loss-cone anisotropy (at parallel cyclotron resonance energies around 230 eV) caused by absorption of electrons at the surface of the moon. Scattering by these whistler-mode waves may be able to explain previously reported depletions of energetic electrons in the vicinity of the moon. The low-frequency waves may play a role in nonlinear three-wave interactions with the bursty electrostatic waves.

Space Weather and the Global Muon Detector Network – GMDN

The main objective of this work is to present an overview of the space weather and its relation with the global network of ground-based multi-directional muon detectors (GMDN). The GMDN is an international collaboration consisting of 10 institutions from 6 countries in 5 continents. A multi-directional muon detector for measuring high-energy galactic cosmic rays (GCRs) was installed in 2001 and expanded in its detection area in 2005, through an international cooperation between Brazil, Japan and USA, and has been in operation since then at the Southern Space Observatory - SSO/CRS/INPE -MCT, (29,4o S, 53,8o W, 480 m a.s.l), Sao Martinho da Serra, RS, in southern Brazil, as an important component of the GMDN. The observations conducted by this detector are used for forecasting the arrival of the interplanetary coronal mass ejections (ICMEs) and the geomagnetic storms at the Earth. The detector measures high-energy GCRs by detecting secondary muons produced from the hadronic interactions of primary GCRs (mostly protons) with atmospheric nuclei. While muons have a relatively short life-time (about 2.2 microseconds at rest), they can reach the detector at ground level because of the relativistic effect of the time dilation with heir high speed (~0.96c), preserving the incident direction of primary particles. The multi-directional detector can measure the GCR intensity in various directions at a single location, such as SSO in Brazil. ICMEs accompanied by a strong shock often forms a GCR depleted region behind the shock. The Forbush decrease is observed when the Earth enters in this depleted region. Some particles from this depleted region leak into the downstream, traveling with almost the speed of light, much faster than the approaching shock, and creating the precursory loss-cone anisotropy around the sunward IMF direction at the Earth. Loss-cones are typically visible 4-8 hours prior to the shock arrival and the onset of major geomagnetic storm at the Earth. This cosmic-ray precursor can be detected sometimes as early as ten hours prior to the shock arrival at the Earth. With the real time data from the upgraded GMDN, its methodology and data reduction techniques permits accurate Space Weather forecasting.

PRECURSORS OF THE FORBUSH DECREASE ON 2006 DECEMBER 14 OBSERVED WITH THE GLOBAL MUON DETECTOR NETWORK (GMDN)

We analyze the precursor of a Forbush Decrease (FD) observed with the Global Muon Detector Network on December 14, 2006. An intense geomagnetic storm is also recorded during this FD with the peak Kp index of 8+. By using the "two-dimensional map" of the cosmic ray intensity produced after removing the contribution from the diurnal anisotropy, we succeed in extracting clear signatures of the precursor. A striking feature of this event is that a weak loss-cone signature is first recorded more than a day prior to the Storm Sudden Commencement (SSC) onset. This suggests that the loss-cone precursor appeared only 7 hours after the Coronal Mass Ejection (CME) eruption from the Sun, when the Interplanetary (IP) shock driven by the Interplanetary Coronal Mass Ejection (ICME) located at 0.4 AU from the Sun. We find the precursor being successively observed with multiple detectors in the network according to the Earth's spin and confirmed that the precursor continuously exists in space. The long lead time (15.6 hours) of this precursor which is almost twice the typical value indicates that the IMF was more quiet in this event than a typical power spectrum assumed for the IMF turbulence. The amplitude (-6.45 %) of the loss-cone anisotropy at the SSC onset is more than twice the FD size, indicating that the maximum intensity depression behind the IP shock is much larger than the FD size recorded at the Earth in this event. We also find the excess intensity from the sunward IMF direction clearly observed during ~10 hours preceding the SSC onset. It is shown that this excess intensity is consistent with the measurement of the particles accelerated by the head-on collisions with the approaching shock. This is the first detailed observation of the precursor due to the shock reflected particles with muon detectors.

Energetic particles modeled by a generalized relativistic kappa-type distribution function in plasmas

Energetic particles found in planetary magnetospheres and other plasmas often display a power law and an anisotropy (including loss cone and temperature anisotropy). In a recent study, a full relativistic kappa-loss cone (KLC) distribution fκL is initially introduced to model energetic particles, but fκL is only associated with loss cone anisotropy. We extend this previous study and develop a generalized relativistic kappa-type (KT) distribution fκT which incorporates either temperature anisotropy or both loss cone and temperature anisotropy. We carry out numerical calculations for a direct comparison between the new KT distribution, the previous KLC distribution and the kappa distribution fκ, respectively. We find that (a) analogous to fκL, fκT satisfies the power law not only at lower energies but also at relativistic energies; (b) analogous to fκ, fκT contains either temperature anisotropy or both loss cone and temperature anisotropy; (c) the regular kappa distribution is found to decrease faster than the KT distribution with kinetic energy Ek especially when θ2 increases (where θ2 is the thermal characteristic parameter), e.g. fκ/fκT ≲ 10−2 for Ek ≳ 2.0 MeV and θ2 ≳ 0.3; (d) no big difference occurs between both KT and kappa distributions through energies up to ∼500 keV for θ2 ≲ 0.03 and (e) the three distributions show different anisotropy behaviors even for the same overall anisotropy. The results suggest that the new generalized KT distribution may be applied in space plasmas and other plasmas including laboratory machines where highly energetic particles exist.

Drifting bands of solar radio spikes

The solar flare (1N/C5.3) occurred in NOAA 8742 on October 27, 1999, and the accompanied complex radio burst was observed by the spectrometer with both high temporal (5 ms) and frequency (10 MHz) resolutions at Purple Mountain Observatory (PMO). It started at 04:27:42 UT, peaked at 04:28:42 UT and ended at 04:29:18 UT. The most significant feature of the event might be the sequential spikes in both time and frequency to form three slow drift bands over the frequency range of similar to 5 to >= 7.5 GHz with almost the same drift rate about 2.7 GHz/s. The relative bandwidth for the individual spikes varied from 1.6% to 4.9%, typically 3.3%. Based on electron cyclotron maser action driven by loss-cone anisotropy, the rare phenomena of slow drift band consisting of spikes were interpreted by spike source moving back and forth along the magnetic loop. Also, the estimations of the narrow bandwidths of spike given by electron cyclotron maser theory coincided well with the observations.

The Coherent Radio Emission from the RS CVn Binary HR 1099

AbstractThe Australia Telescope was used in March–April 2005 to observe the 1.384 and 2.368-GHz emissions from the RS CVn binary HR 1099 in two sessions, each of 9-h duration and 11 days apart. Two intervals of highly polarised emission, each lasting 2–3 h, were recorded. During this coherent emission we employed a recently installed facility to sample the data at 78-ms intervals to measure the fine temporal structure and, in addition, all the data were used to search for fine spectral structure. We present the following observational results: (1) ∼100% left-hand circularly polarised emission was seen at both 1.384 and 2.368 GHz during separate epochs; (2) the intervals of highly polarised emission lasted for 2–3 h on each occasion; (3) three 22-min integrations made at 78-ms time resolution showed that the modulation index of the StokesVparameter increased monotonically as the integration time was decreased and was still increasing at our resolution limit; (4) the extremely fine temporal structure strongly indicates that the highly polarised emission is due to an electron-cyclotron maser operating in the corona of one of the binary components; (5) the first episode of what we believe is ECME (electron-cyclotron maser emission) at 1.384 GHz contained a regular frequency structure of bursts with FWHM ∼48 MHz, which drifted across the spectrum at ∼0.7 MHz min−1. Our second episode of ECME at 2.368 GHz contained wider-band frequency structure, which did not permit us to estimate an accurate bandwidth or direction of drift; (6) the two ECME events reported in this paper agree with six others reported in the literature in occurring in the binary orbital phase range 0.5–0.7; (7) in one event of 8-h duration, two independent maser sources were operating simultaneously at 1.384 and 2.368 GHz.We discuss two kinds of maser sources that may be responsible for driving the observed events that we believe are powered by ECME. One is based on the widely reported ‘loss-cone anisotropy', the second on an auroral analogue, which is driven by an unstable ‘horseshoe' distribution of fast-electron velocities with respect to the magnetic field direction. Generally, we favour the latter, because of its higher growth rate and the possibility of the escape of radiation which has been emitted at the fundamental electron cyclotron frequency. If the auroral analogue is operating, the magnetic field in the source cavity is ∼500 G at 1.384 GHz and ∼850 G at 2.368 GHz; the source brightness temperatures are of the orderTB∼ 1015K.We suggest that the ECME source may be an aurora-like phenomenon due to the transfer of plasma from the K2 subgiant to the G5 dwarf in a strong stellar wind, an idea that is based on VLBA maps showing the establishment of an 8.4 GHz source near the G5 dwarf at times of enhanced radio activity in HR 1099.

A “loss cone” precursor of an approaching shock observed by a cosmic ray muon hodoscope on October 28, 2003

We analyze a loss cone anisotropy observed by a ground‐based muon hodoscope at Mt. Norikura in Japan for 7 hours preceding the arrival of an interplanetary shock at Earth on October 28, 2003. Best fitting a model to the observed anisotropy suggests that the loss cone in this event has a rather broad pitch‐angle distribution with a half‐width about 50° from the IMF. According to numerical simulations of high‐energy particle transport across the shock, this implies that the shock is a “quasi‐parallel” shock in which the angle between the magnetic field and the shock normal is only 6°. It is also suggested that the lead‐time of this precursor is almost independent of the rigidity and about 4 hour at both 30 GV for muon detectors and 10 GV for neutron monitors.

On the Zebra Structure in the Frequency Range near 3 GHz

We present 19 cases of zebra pattern structure (ZPS) and fiber bursts (FB) in radio bursts in frequency range around 3 GHz, and one such case in the range 5.2–7.6 GHz, using the new microwave spectrometer of NAOC between 2.6–3.8 and 5.2–7.6 GHz (China, Huairou station) with high resolution (10 MHz and 8 ms). The FB and ZPS have about the same spectral parameters: the frequency bandwidth of emission stripes Δf ~ 20 MHz, the frequency separation between the emission and the neighboring low frequency absorption -Δfea ~ 30 MHz and the frequency separation between emission stripes (when a periodic structure persists) Δfs ~ 60-70 MHz. Therefore we consider both these fine structures to be whistler manifestations, i.e., interactions of plasma electrostatic waves with whistler waves (generated by the same fast particles with loss-cone anisotropy) l + w → t. The duration of the fiber bursts of about 2s corresponds to whistler waves propagating undamped at about 2s, which requires a whistler increment < 0.5 s−1. In the frequency range 3–7 GHz the relation between the ratios of plasma to cyclotron frequencies and whistler to cyclotron frequencies is almost independent of the decrement of whistler electron damping. This finding is used to obtain the magnetic field strength in the region of generation. For a reasonable value of electron temperature (2–20 MK), we find B = 125–190 G when the electron density is (8-18) × 1010 cm-3 and B = 520–610 G when the electron density is (35–60) × 1010 cm-3. In two remarkable events, 1998-04-15 and 2000-10-29, the right-hand polarization is strong for all the fine structures and corresponds to ordinary wave.

Gaussian analysis of two hemisphere observations of galactic cosmic ray sidereal anisotropies

We have analyzed the yearly averaged sidereal daily variations in the count rates of 46 underground muon telescopes by fitting Gaussian functions to the data. These functions represent the loss cone and tail‐in anisotropies of the sidereal anisotropies model proposed by Nagashima et al. [l995a, b]. The underground muon telescopes cover the median rigidity range 143–1400 GV and the viewing latitude range 73°N–76°S. From the Gaussian amplitudes and positions we have confirmed that the tail‐in anisotropy is more prominent in the southern hemisphere with its reference axis located at declination (δ) ∼14°S and right ascension (α) ∼4.7 sidereal hours. The tail‐in anisotropy is asymmetric about its reference axis, and the observed time of maximum intensity depends on the viewing latitude of the underground muon telescopes. We also find that the declination of the reference axis may be related to the rigidity of the cosmic rays. We show that the loss cone anisotropy is symmetric and has a reference axis located on the celestial equator (δ ∼ 0°) and α ∼ 13 sidereal hours. We have used the parameters of the Gaussian fits to devise an empirical model of the sidereal anisotropies. The model implies that the above characteristics of the anisotropies can explain the observed north‐south asymmetry in the amplitude of the sidereal diurnal variation. Furthermore, we find that the anisotropies should cause the phase of the sidereal semidiurnal variation of cosmic rays to be observed at later times from the northern hemisphere compared to observations from the southern hemisphere. We present these results and discuss them in relation to current models of the heliosphere.

Sidereal daily variation of galactic cosmic rays

We have analyzed the yearly averaged sidereal daily variations in the count rates of 44 underground muon telescopes by fitting Gaussian functions to the data. These functions represent the Loss-cone (LC) and Tail-in (TI) anisotropies proposed by Nagashima et al.. The telescopes cover the median rigidity range 143GV–1400GV and the viewing latitude range 73°N–76°S. We find that the TI anisotropy has its reference axis located at declination ( δ) ≈ 14°S and right ascension ( α) ≈ 4.7 sidereal hours. We show that the TI anisotropy is asymmetric about the reference axis and its observed α depends on the viewing latitude of the telescopes. We also show that the LC anisotropy is symmetric and has a referenceaxis located at the celestial equator and α ≈ 13 sidereal hours. From the parameters of the Gaussian fits we devise an empirical model of the sidereal anisotropies which implies that the above characteristics of the anisotropies can explain the north-south asymmetry in the amplitude of the sidereal diurnal variation. Furthermore, we find that the phase of the sidereal semi-diurnal variation of cosmic rays should be recorded at later times when measured from the northern hemisphere compared to observations made from the southern hemisphere.

Preliminary analysis of two‐hemisphere observations of sidereal anisotropies of galactic cosmic rays

By using the two‐hemisphere network of underground muon telescopes we have examined the average sidereal daily variations in the count rates recorded by 48‐component muon telescopes. The telescopes respond to primary cosmic rays with rigidities between ∼140 and 1700 GV and view almost the entire celestial sphere. We have modeled the data by using Gaussian functions, and we have related the Gaussian parameters to the recent tail‐in and loss cone anisotropy model proposed by Nagashima et al. [1995a, b] to explain the sidereal daily variations. We have used the model parameters to derive the rigidity and latitude spectra of the galactic anisotropies and find them to be qualitatively in agreement with Nagashima et al.'s predictions. The results indicate, however, that the tail‐in anisotropy is asymmetric about its reference axis, whereas the loss cone anisotropy is more symmetric. We show that these characteristics of the galactic anisotropies may explain the north–south asymmetry observed in the amplitude of the sidereal diurnal variation derived from Fourier analysis techniques.

Long lasting energetic proton precipitation in the inner magnetosphere after substorms

Data of polar‐orbiting low‐altitude NOAA spacecraft are used to study the precipitation of energetic (30–80 keV) protons, several degrees equatorward of the energetic proton isotropic boundary. This precipitation, to be called low‐latitude proton precipitation (LLPP), is observed at all local time sectors. The LLPP precipitating flux is found to be generally smaller than the trapped flux. The LLPP particle flux significantly increases during intense substorms and then decays with a long characteristic timescale of about 9 hours. Therefore, it may also be found during very quiet times. The long decay and the observed loss cone anisotropy imply a moderate pitch angle diffusion of energetic protons. During quiet magnetic conditions following the disturbance the latitudinal position of the LLPP region moves slowly to higher latitudes. The equatorial boundary of the precipitation is found to be at lower latitudes in the night sector than in the dayside sector. Mapping to the equatorial plane along magnetic field lines showed that the equatorial boundary of the precipitation at the nightside is situated on a closer drift shell than that at the dayside.

The electron-cyclotron maser instability as a source of plasma radiation

The generation of continuum bursts from the sun at dm and m wavelengths (in particular, type IV bursts) via the electron-cyclotron-maser instability is examined. The maser instability can be driven by an electron distribution with either a loss-cone anisotropy or a peak at large pitch angles. For omega(p)/Omega(e) much greater than 1, the maser emission is produced by electrons interacting through a harmonic (cyclotron) resonance and is electrostatic, being in the upper hybrid mode at frequencies approximately equal to omega(p). Coalescence processes are required to convert the electrostatic waves into transverse radiation which can escape from the source region. Whether the resultant spectrum is nearly a smooth continuum or has a zebra-stripe pattern (both of which occur in type IV bursts) depends on the form of the electron distribution, inhomogeneities in the density and magnetic field, and whether the maser reaches saturation. For at least the case of some type IV dm bursts with fine structure, comparison with observations seems to indicate that the electrons producing the emission are more likely to have a loss-cone distribution, and that the maser instability is not at saturation.

Fundamental and harmonic electron cyclotron maser emission

Auroral kilometric radiation has been attributed to an electron cyclotron maser instability and is known to have components at both the cyclotron frequency and its harmonics. Solar microwave spike bursts and similar bursts from flare stars are also thought to be maser emission at harmonics of the cyclotron frequency. Fundamental X mode growth is known to dominate when ωp/Ωe ≲ 0.3. At higher ωp/Ωe the dominant radiation from the maser depends on the prevailing plasma conditions. In particular, it is shown that the dominant radiation can be (1) fundamental Z or O mode radiation or second‐harmonic X mode radiation when 0.3 ≲ ωp/Ωe ≲ 1, (2) fundamental Z mode or second‐harmonic X mode radiation when , (3) second‐harmonic O mode or third‐harmonic X mode radiation when or (4) second‐harmonic Z mode radiation when . In these ranges of ωp/Ωe X mode growth at the relevant harmonic tends to be suppressed if the distribution of the energetic electrons only has a loss cone anisotropy, whereas it tends to dominate if the distribution is peaked at large pitch angles.

Electron-cyclotron masers as the source of certain solar and stellar radio bursts

view Abstract Citations (446) References (48) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Electron-cyclotron masers as the source of certain solar and stellar radio bursts. Melrose, D. B. ; Dulk, G. A. Abstract The theory of electron-cyclotron masers as they might apply in the astrophysical context is developed, and it is suggested that such masers provide an explanation for the very bright emission known to be associated with certain kinds of radio bursts observed on the Sun and other stars. Some of the observed properties of solar and stellar radio bursts that seem to require amplification of the radiation are summarized, including millisecond solar spikes, RS CVn binaries, and flares on M dwarf stars. Recent developments in the theory of electron-cyclotron masers are summarized and the current theory is applied to electrons with a loss cone anisotropy, estimating the growth rate and saturation levels. In the interpretation of solar microwave spikes and RS CVn binaries, the mechanisms of gyromagnetic absorption, maser at the second harmonic, polarization, and angular distribution are examined in the light of the theory. Publication: The Astrophysical Journal Pub Date: August 1982 DOI: 10.1086/160219 Bibcode: 1982ApJ...259..844M Keywords: Masers; Radio Astronomy; Solar Radio Emission; Stellar Radiation; Angular Distribution; Circular Polarization; Electrons; Gyromagnetism; Plasma Waves; Solar Radio Bursts; Astrophysics full text sources ADS | data products SIMBAD (4)

Emission and Absorption of Langmuir Waves by Anisotropic Unmagnetized Particles

The emission and absorption coefficients for Langmuir waves due to anisotropic unmagnetized particles are reduced to two complementary forms: one involving integrals over momentum p and pitch angle ex; the other involving an integral over p and a sum over Legendre polynomials. The quasilinear diffusion coefficients are reduced to the former. It is also shown how the absorption coefficient may be reduced to forms involving neither a p derivative nor an ex derivative. The absorption coefficient is evaluated explicitly for five idealized anisotropic distributions, called a 'forwardcone' anisotropy, a 'semi-cos2 ex' anisotropy, a loss-cone anisotropy, a P, anisotropy and a P2 anisotropy respectively. All except the P2 anisotropy can lead to growth of Langmuir waves, but only if the distribution function is an increasing function of p at the resonant phase speed, e.g. only for gap distributions. The results have important implications in connection with the theory of solar radio bursts.