Halo Electrons Research Articles

Electron Halo and Strahl Formation in the Solar Wind by Resonant Interaction with Whistler Waves

Observations of solar wind electron distribution functions (VDFs) reveal considerable deviations from a simple Maxwellian VDF. A thermal core and a suprathermal halo and antisunward, magnetic field-aligned beam, or strahl, can be distinguished. At higher energies above 2 keV, a superhalo can even be observed. A kinetic description of electrons in the solar corona and wind, including resonant interaction between electrons and whistler waves, can reproduce an enhancement of suprathermal electron fluxes compared to the core flux. The whistler waves are assumed to be generated below the solar coronal base and propagate through the corona into interplanetary space. However, the resonance condition with these whistlers can only be fulfilled by electrons that move sunward. For antisunward-moving electrons, such a model lacks an efficient diffusion mechanism. The mirror force due to the opening magnetic field geometry of a solar coronal hole and in the solar wind focuses the electrons into a very narrow beam. This expectation of an extremely narrow beam is contradicted by observations of a that has a finite width, and of an quasi-isotropic superhalo component. Thus, a diffusion mechanism for antisunward-moving electrons must exist in interplanetary space. In this paper, antisunward-propagating whistler waves are introduced into the kinetic model in order to provide this diffusion. Their wave power is estimated as a small fraction of the total wave power that is measured in interplanetary space. The kinetic results show that the whistler waves are capable of influencing the solar wind electron VDFs significantly, leading to the formation of both the halo and strahl populations and a more isotropic distribution at higher energies, in good agreement with solar wind observations.

Magnetic disconnection from the Sun: Observations of a reconnection exhaust in the solar wind at the heliospheric current sheet

We have identified an anti‐sunward‐directed, Petschek‐type, reconnection exhaust during a crossing of the heliospheric current sheet, HCS, by the ACE spacecraft. The exhaust in this relatively rare HCS event was a region of accelerated plasma flow filling the field reversal region and was bounded by Alfven waves propagating in opposite directions along recently merged field lines. Heliospheric magnetic field lines disconnected from the Sun by the reconnection process were identified in and surrounding the exhaust by the disappearance of the solar wind electron strahl there and by the interpenetration along the field of sunward‐streaming halo electrons from opposite sides of the exhaust. These observations demonstrate that strahl dropouts observed in the vicinity of the HCS at least at times are signatures of magnetic disconnection from the Sun.

Fundamental issues on kappa-distributions in space plasmas and interplanetary proton distributions

Numerous in situ observations indicate clearly the presence of nonthermal electron and ion structures as ubiquitous and persistent feature in a variety of astrophysical plasma environments. In particular, the detected suprathermal particle populations are accurately represented by the family of κ-distributions, a power-law in particle speed. After clarifying the characteristics of high-energy tail distributions under various space plasma conditions, different generation mechanisms of energetic particles are introduced where numerical simulations of wave–particle interaction based on a Fokker–Planck approach demonstrate how Landau interaction ultimately leads to κ-like distributions. Because of lack of theoretical justification, the use of the analytical form of κ-functions was frequently criticized. It is shown that these distributions turn out as consequence of an entropy generalization favored by nonextensive thermo-statistics, thus providing the missing link for powerlaw models of suprathermal tails from fundamental physics, along with a physical interpretation of the structure parameter κ. Moreover, with regard to the full nonextensive formalism, compatible also with negative values of κ, it is demonstrated that core–halo distribution structures, as observed for instance under typical interplanetary plasma conditions, are a natural content of the pseudo-additive entropy concept. The significance of the complete κ-distribution family with regard to observed core–halo electron and double-humped ion velocity space characteristics is illuminated, where the observed peak separation scale of interplanetary proton distributions is compatible with a maximum entropy condition.

Chandradiscovery of an X-ray jet and lobes in 3C 15

We report the Chandra detection of an X-ray jet in 3C 15. The peak of the X-ray emission in the jet is 4.1 '' (a projected distance of 5.1 kpc) from the nucleus, and coincident with a component previously identified in the radio and optical jets. We construct the spectral energy distribution (SED) for this component, optical knot C, and find that X-ray flux is well below the extrapolation of the radio-to-optical continuum. We examine four models for the X-ray jet emission: (I) weak synchrotron cooling in equipartition, (II) moderate synchrotron cooling in equipartition, (III) weak synchrotron plus synchrotron self-Compton (SSC) cooling, and (IV) moderate synchrotron plus SSC cooling. Given weak evidence for a concave feature in the X-ray spectrum, we argue that case (II) can most reasonably explain the overall emission from knot C. Case (III) is also possible, but requires a large departure from equipartition and for the jet power to be comparable to that of the brightest quasars. In all models, (I)-(IV), electrons must be accelerated up to $\gamma_{\rm max} \ga 10^7$, suggesting that re-acceleration is necessary in knot C of the 3C 15 jet. Diffuse X-ray emission has also been detected, distributed widely over the full extent (63 kpc $\times$ 25 kpc) of the radio lobes. The X-ray spectrum of the diffuse emission is described by a two-component model, consisting of soft thermal plasma emission from the host galaxy halo and a hard nonthermal power-law component. The hard component can be ascribed to the inverse Comptonization of cosmic microwave background (CMB) photons by the synchrotron emitting electrons in the radio lobes. We compare the total energy contained in the lobes with the jet power estimated from knot C, and discuss the energetic link between the jet and the lobes. We argue that the fueling time ( t fuel ) and the source age ( t src ) are comparable for case (II), whereas t fuel ≪ t src is likely for case (III). The latter may imply that the jet has a very small filling factor, ~10 -3 . We consider the pressure balance between the thermal galaxy halo and non-thermal relativistic electrons in the radio lobes. Finally, we show that the X-ray emission from the nucleus is not adequately fitted by a simple absorbed power-law model, but needs an additional power-law with heavy absorption ($N_{\rm H} \simeq 10^{22-23}$ cm -2 ) intrinsic to the source. Such a high column density is consistent with the presence of a dense, dusty torus which obscures the quasar nucleus.

Numerical modeling of the halo electrons in the fast solar wind

A one‐dimensional magnetohydrodynamic solar wind model is constructed to describe the behavior of the halo electrons in the fast electron‐proton solar wind, assuming that the electrons are composed of two populations: the core and halo. The halo component is separated from the core electrons by increasing the heat deposition. It is shown that the model including only Coulomb collisions yields a much faster halo drift than observed if the halo temperature is in agreement with measured values. This supports the previous argument that some anomalous frictional processes caused by microinstabilities and/or wave‐particle interactions exist in the solar wind. We show that these anomalous frictional processes can be approximated by enhanced Coulomb collisions together with a heat deposition that has a very large damping length. We find that beyond a certain heliocentric distance, say 20 RS, the anomalous frictional forces acting on the halo population are more important than the electric field force and the Coulomb collisional forces and become the dominant factor inhibiting the core‐halo drift.

First‐YearWilkinson Microwave Anisotropy Probe(WMAP) Observations: Foreground Emission

Full sky maps are made in five microwave frequency bands to separate the temperature anisotropy of the CMB from foreground emission. We define masks that excise regions of high foreground emission. The effectiveness of template fits to remove foreground emission from the WMAP data is examined. These efforts result in a CMB map with minimal contamination and a demonstration that the WMAP CMB power spectrum is insensitive to residual foreground emission. We construct a model of the Galactic emission components. We find that the Milky Way resembles other normal spiral galaxies between 408 MHz and 23 GHz, with a synchrotron spectral index that is flattest (beta ~ -2.5) near star-forming regions, especially in the plane, and steepest (beta ~ -3) in the halo. The significant synchrotron index steepening out of the plane suggests a diffusion process in which the halo electrons are trapped in the Galactic potential long enough to suffer synchrotron and inverse Compton energy losses and hence a spectral steepening. The synchrotron index is steeper in the WMAP bands than in lower frequency radio surveys, with a spectral break near 20 GHz to beta < -3. The modeled thermal dust spectral index is also steep in the WMAP bands, with beta ~ 2.2. Microwave and H alpha measurements of the ionized gas agree. Spinning dust emission is limited to < ~5% of the Ka-band foreground emission. A catalog of 208 point sources is presented. Derived source counts suggest a contribution to the anisotropy power from unresolved sources of (15.0 +- 1.4) 10^{-3} microK^2 sr at Q-band and negligible levels at V-band and W-band.

Wind and ACE observations during the great flow of 1–4 May 1998: Relation to solar activity and implications for the magnetosphere

Using magnetic field, plasma, and energetic particle data from Wind and ACE, we analyze interplanetary features associated with the first strongly geoeffective interval in the rising phase of solar cycle 23, which affected Earth on 1–4 May 1998. As shown by Skoug et al. [1999], the configuration consisted of a compound stream made up of an interplanetary coronal mass ejection (ICME) containing a magnetic cloud and being trailed by a hot, faster flow. In addition, we find that the front boundary of the ICME is a rotational discontinuity and the leading edge of the fast stream has a zero normal magnetic field component and is followed by a magnetic field which is strongly enhanced (by a factor of ∼4) and whose fluctuations lie in a plane approximately parallel to the leading edge. Energetic particle and composition observations confirm that the field lines of the magnetic cloud were connected to at least two different flare sites in the same active region. We infer a lower limit for the size of the solar footprint of the connected flux tube of 0.02 Rs2, i.e., ∼1010 km2. A dramatic weakening of the halo electron distribution occurred during 3 May at the time when other experimenters have documented the presence of prominence material. We hypothesize that the solar wind halo population was scattered by enhanced frequency of Coulomb collisions in the dense and very cold plasma. We discuss our energetic particle observations in terms of local acceleration at interplanetary shocks and field discontinuities as well as in terms of acceleration in flares and CME‐driven shocks. We also compare, in a specific formulation, the power and energy input of the May 1998 configuration to the magnetosphere with other much studied geoeffective events. We find that the power input far exceeded that in all previous geoeffective events in our sample and attribute this to the fact that the 1–4 May 1998 event consisted of a compound stream structure with an unprecedented power input during a ∼3‐hour burst of high‐speed flow on 4 May. A statistical survey using the OMNI database for the 6‐year period 1995–2000 confirms these inferences and indicates further approximate saturation levels for energy and power input of 10 J m−2 and 0.3 mW m−2, only exceeded in exceptional events such as May 1998 and July 2000. The solar energetic particle event at the leading edge of the fast stream might be the only advance warning the Earth would receive of the approach of a configuration of such a concentrated geoeffective potential.

Neutrino, Neutron, and Cosmic‐Ray Production in the External Shock Model of Gamma‐Ray Bursts

The hypothesis that ultrahigh-energy (≳1019 eV) cosmic rays (UHECRs) are accelerated by gamma-ray burst (GRB) blast waves is assumed to be correct. Implications of this assumption are then derived for the external shock model of GRBs. The evolving synchrotron radiation spectrum in GRB blast waves provides target photons for the photomeson production of neutrinos and neutrons. Decay characteristics and radiative efficiencies of the neutral particles that escape from the blast wave are calculated. The diffuse high-energy GRB neutrino background and the distribution of high-energy GRB neutrino events are calculated for specific parameter sets, and a scaling relation for the photomeson production efficiency in surroundings with different densities is derived. GRBs provide an intense flux of high-energy neutrons, with neutron production efficiencies exceeding ~1% of the total energy release. The radiative characteristics of the neutron β-decay electrons from the GRB "neutron bomb" are solved in a special case. Galaxies with GRB activity should be surrounded by radiation halos of ~100 kpc extent from the outflowing neutrons, consisting of a nonthermal optical/X-ray synchrotron component and a high-energy gamma-ray component from Compton-scattered microwave background radiation. The peak luminosity emitted by the diffuse β electron halo from a single GRB with ≳2 × 1053 ergs isotropic energy release is ~1035 ergs s-1, with a potentially much brighter signal from the neutron decay protons. The decay halo from a single GRB can persist for ≳0.1-1 Myr. Stronger neutrino fluxes and neutron decay halos can be produced by external shocks in clumpy external media and in scenarios involving internal shock scenarios, so detection of neutrinos associated with smooth profile GRBs could rule out an impulsive GRB central engine and an external shock model for the prompt phase. The luminosity of sources of GRBs and relativistic outflows in L* galaxies such as the Milky Way is at the level of ~1040±1 ergs s-1. This is sufficient to account for UHECR generation by GRBs. We briefly speculate on the possibility that hadronic cosmic rays originate from the subset of supernovae that collapse to form relativistic outflows and GRBs.

Solar wind electron halo depletions at 90° pitch angle

Strong depletions in solar wind electron halo distribution functions, centered on and roughly symmetric about 90° pitch angle, are present in at least 10% of data obtained by the Los Alamos SWEPAM experiment on the ACE spacecraft. Sunward‐directed conics are also present in the electron data, occur in conjunction with the 90° depletions, but are less common than depletions. We suggest that on open field lines both depletions and conics arise from focusing and mirroring effects associated with field line connection to magnetic field enhancements farther out in the heliosphere. The focusing and mirroring produces field‐aligned, counterstreaming fluxes of suprathermal electrons somewhat similar to that observed on closed field lines. Our observations provide strong evidence that the antisunward‐directed portion of the electron halo on open field lines at times results primarily from mirroring of back scattered strahl and/or shock‐heated electrons from far out in the heliosphere.

Solar wind plasma parameters on Ulysses: Detailed comparison between the URAP and SWOOPS experiments

The presence of several instruments measuring plasma parameters aboard Ulysses made possible an extensive comparison between them. In this paper, we focus on solar wind electron parameters measured by the Unified Radio and Plasma (URAP) wave receiver, using the thermal noise spectroscopy method, and by the Solar Wind Observations Over the Poles of the Sun (SWOOPS) electron and ion analyzers. We compare the data sets acquired by both experiments during the ecliptic path in 1990/1991 and the first pole‐to‐pole fast transit of Ulysses in 1994/1995, near the maximum and minimum of solar activity, respectively. From the comparison, we find very good agreement between the total density derived by both experiments; the average offset between the two data sets differs by less than 5%. We find a systematic offset of 5% to 10% in the core electron temperature values depending on the solar wind flow speed. We present detailed observations obtained during the crossing of the stream interface on March 28, 1995, by Ulysses, when it returned to the high‐speed wind region in the northern hemisphere. We find that the core electron temperature increases around 40% at the interface, which is in agreement with the statistical result obtained using IMP plasma measurements at 1 AU. We point out that SWOOPS provides reliable ion diagnostics in addition to suprathermal (halo) electron parameters and yields fundamental information on particle velocity distributions, whereas the improved quasi‐thermal noise method provides total electron densities and core electron temperatures accurately.

Core, Halo and Strahl Electrons in the Solar Wind

Electron velocity distribution functions (VDF) observed in the low speed solar wind flow are generally characterized by ‘core’ and ‘halo’ electrons. In the high speed solar wind, a third population of ‘strahl’ electrons is generally observed. New collisional models based on the solution of the Fokker-Planck equation can be used to determine the importance of the different electron populations as a function of the radial distance. Typical electron velocity distribution functions observed at 1 AU from the Sun are used as boundary conditions for the high speed solar wind and for the low speed solar wind. Taking into account the effects of external forces and Coulomb collisions with a background plasma, suprathermal tails are found to be present in the electron VDF at low altitudes in the corona when they exist at large radial distances.

Radial variation of solar wind electrons inside a magnetic cloud observed at 1 and 5 AU

The fortuitous observations of a magnetic cloud during the ACE/Ulysses radial alignment in March 1998 provide one of the first opportunities to study the same magnetic cloud at two different radial distances. In this paper, we use data from both ACE and Ulysses to examine the expansion of the cloud as well as of the adjacent noncloud solar wind. In particular, we study the evolution of the electrons as the cloud expanded from 1 to 5.4 AU, including the core, halo, and total electron populations. We find that the core and total electron temperatures at either spacecraft are moderately anticorrelated with electron density. However, combined observations from the two spacecraft show that both temperature and density decreased as the cloud expanded and thus that single point measurements cannot be used to determine a polytropic index describing the expansion of solar wind electrons within the cloud.

Solar wind electron characteristics inside and outside coronal mass ejections

We present a study of 9 months of data from the solar wind plasma electron instrument on the ACE spacecraft. Electron pitch angle distributions were used to identify intervals of counterstreaming halo electrons, which were observed ∼16% of the time that the spacecraft was not magnetically connected to the Earth's bow shock. Counterstreaming electrons presumably indicate a closed magnetic field topology and thus indicate the passage of coronal mass ejections (CMEs) in the solar wind. In this study, we separately examine electron moments at times with and without counterstreaming electrons, including both magnetic clouds and noncloud CMEs. The properties of both the core and halo electron populations were nearly identical at times with and without counterstreaming electrons. Both low and high electron densities and temperatures were observed in either type of event. In contrast, magnetic clouds, on average, showed increased densities and reduced temperatures. The core/halo density ratio and the total electron density were anticorrelated in all cases, indicating that the halo contributes more to the total electron density at low density times, regardless of magnetic topology. Total electron temperature and density were anticorrelated in all types of events. This consistent anticorrelation implies that such single‐spacecraft measurements of temperature and density cannot be used to determine a polytropic index for solar wind electrons. Instead, the anticorrelation may be due to pressure balance of the solar wind plasma or may be a remnant of coronal conditions.

Solar wind electron suprathermal strength and temperature gradients: Ulysses observations

We use observations from the Ulysses electron spectrometer to examine global trends of the electron suprathermal population and to study, for the first time, the electron core, halo, and total temperature gradients in the fast solar wind over the poles. We use a data set covering the period from the beginning of the mission (1990, day of year 322) to approximately the first completion of Ulysses out‐of‐ecliptic orbit around the Sun (1998, day 32). This allows us to characterize very well the two states of the solar wind: the high‐speed wind, emanating from polar coronal holes, and the low‐speed streams, emanating from equatorial regions. From a classical bi‐Maxwellian (core and halo) model of electron velocity distribution functions, we define the electron suprathermal strength S as the ratio of the halo to core kinetic pressures: S=nhTh/ncTc. The fast wind has larger average values of S than the slow wind. This global correlation between S and the solar wind bulk speed is also observed on a smaller scale in the polar regions. We find that the small‐scale variations of the solar wind bulk speed in the polar regions, typically ±50 km/s around an average value of 750 km/s, are correlated with small‐scale variations of S. We present also the first observations of the electron core, halo, and total temperature gradients in time‐stationary fast solar wind periods over the poles. We examine all the previous observations in the context of simple solar wind exospheric models. We find, for instance, that the total electron temperature can be well fitted by a law of the form Te = T0 + T1 r−4/3, as predicted by the exospheric approximation.

An unusual coronal mass ejection: First solar wind electron, proton, alpha monitor (SWEPAM) Results from the Advanced Composition Explorer

This paper reports the first scientific results from the Solar Wind Electron Proton Alpha Monitor (SWEPAM) instrument on board the Advanced Composition Explorer (ACE) spacecraft. We analyzed a coronal mass ejection (CME) observed in the solar wind using data from early February, 1998. This event displayed several of the common signatures of CMEs, such as counterstreaming halo electrons and depressed ion and electron temperatures, as well as some unusual features. During a portion of the CME traversal, SWEPAM measured a very large helium to proton abundance ratio. Other heavy ions, with a set of ionization states consistent with normal (1–2×106 K) coronal temperatures, were proportionately enhanced at this time. These observations suggest a source for at least some of the CME material, where heavy ions are initially concentrated relative to hydrogen and then accelerated up into the solar wind, independent of their mass and first ionization potential.

Bursts of whistler mode waves in the upstream of the bow shock: Geotail observations

Bursts of narrowband and short‐lived electromagnetic waves are frequently observed in the upstream (the electron foreshock and the pure solar wind) by the waveform capture instrument on board Geotail satellite. The electromagnetic waves with frequencies above 10 Hz are all right‐hand circularly polarized and identified as whistler mode waves. We call the waves narrowband and short‐lived whistlers (NSW). Nearly all the NSW in the electron foreshock propagate in a downstream direction parallel to the ambient magnetic field (Bo) with an average θkB of 16°, where θkB is an angle between NSW wave vector and the Bo. Their frequencies cover a range from 0.05 to 1.0Ωe with an average of 0.35Ωe, where Ωe is a local electron cyclotron frequency. Their amplitudes range from a few picoteslas to 100 pT with an average of 23 pT. Because of their parallel propagation, the NSW must be excited by electron cyclotron resonance. These features of the NSW suggest existence of electron beams which travel in an upstream direction parallel to the Bo and which have a temperature anisotropy. Kinetic energies of the beams range from a few eV to about 200 eV (28 eV on average). All these characteristics of the electron beams revealed from the NSW are consistent with ISEE and WIND particle observations. The competition between electrostatic and whistler instabilities and the finite size of the beams are very likely the reasons why the NSW are short‐lived. These NSW can be well explained by a modeled electron beam with a losscone distribution in the electron foreshock. The NSW in solar wind are very similar to those in the electron foreshock. However, they have larger amplitudes (34 pT on average), lower frequencies (0.24Ωe on average), and higher cyclotron resonant energy (100 eV on average). They are very likely excited by halo electrons or nonlocal sources in the solar wind.

Electron temperatures within magnetic clouds between 2 and 4 AU: Voyager 2 observations

We have performed an analysis of Voyager 2 plasma electron observations within magnetic clouds between 2 and 4 AU identified by Burlaga and Behannon [1982]. The analysis has been confined to three of the magnetic clouds identified by Burlaga and Behannon that had high‐quality data. The general properties of the plasma electrons within a magnetic cloud are that (1) the moment electron temperature anticorrelates with the electron density within the cloud, (2) the ratio Te/Tp tends to be &gt;1, and (3) on average, Te/Tp ≈7.0. All three results are consistent with previous electron observations within magnetic clouds. Detailed analyses of the core and halo populations within the magnetic clouds show no evidence of either an anticorrelation between the core temperature TC and the electron density Ne or an anticorrelation between the halo temperature TH and the electron density. Within the magnetic clouds the halo component can contribute more than 50% of the electron pressure. The anticorrelation of Te relative to Ne can be traced to the density of the halo component relative to the density of the core component. The core electrons dominate the electron density. When the density goes up, the halo electrons contribute less to the electron pressure, so we get a lower Te. When the electron density goes down, the halo electrons contribute more to the electron pressure, and Te goes up. We find a relation between the electron pressure and density of the form Pe = aNγi with γ ≈ 0.5.

Energetic solar particle dropouts detected by Ulysses at 1.63 AU: A possible encounter with the Earth's distant magnetotail

Fast solar particles are used to trace the topology of the interplanetary magnetic field during a solar event detected by the heliosphere instrument for spectra composition and anisotropy at low energy (HISCALE) on the Ulysses spacecraft on January 2, 1991. Two sharp‐edged dropouts, lasting for 10 and 25 min, were detected in the fluxes of solar ions from ∼130 keV to &gt;1.8 MeV and halo (heat flux) electrons from 71–461 eV, while simultaneously the flux of high‐energy, 38–315 keV solar electrons and 56–78 keV ions remained constant. The halo electrons and 130 keV to &gt;1.8 MeV solar ions are traveling with similar speeds, ∼4 × 106 to 3 × 107 m/s, much slower than the energetic solar electrons (&gt;108 m/s), and faster than the 56–78 keV ions, suggesting that the dropout field lines were first disconnected from the Sun and then reconnected, with the distance to the reconnection point and time of the reconnection such that &gt;38 keV electrons had already repopulated the dropout field lines. At the time Ulysses was 0.63 AU from the Earth on its way to Jupiter, ∼2° above the ecliptic with a Sun‐Earth‐spacecraft angle of ∼172.8°, approximately where the Earth's magnetotail would be expected to be if it extended to 15,000 Earth radii. We consider the possibility that Ulysses encountered interplanetary field lines connected to the magnetotail.

Observations of electron velocity distribution functions in the solar wind by the WIND Spacecraft: High angular resolution Strahl measurements

The WIND Solar Wind instrument (SWE) includes a sensor especially configured to measure the solar wind strahl. The strahl is the excess electron (≥100 eV) halo component of the solar wind most closely aligned with the magnetic field. Strahl electrons originate in the inner corona and move freely out to 1 A.U., providing information on the state of the corona. Electron data acquired during seven solar rotations show that the electron velocity distributions are most anisotropic and the strahl flux most intense during high speed streams. The strahl angular width is smallest when the solar wind velocity is largest, approximately 5° in width at 600 eV, and becomes much wider when the velocity is low. Coincident with the strahl is a small reverse electron flux which does not form a beam but fills the field of view. This variability of the flux and angular distribution of the strahl and anti‐strahl suggests that a pitch angle scattering process may be acting to broaden the strahl and that the anti‐strahl may be due to backscattering of the strahl.

Application of Nano-Diffraction to Local Structure Analysis of Amorphous Pd75Si25 Alloy Thin Films

Nano-probe electron diffraction has been applied to local structure analysis of a sputter-deposited amorphous Pd75Si25 alloy thin film with the help of high-resolution electron microscopy. Corresponding to local lattice fringe images due to the atomic medium range order (MRO) extending over 1 to 3 run, nano-diffraction patterns with various zone-axes were obtained by using a field emission transmission electron microscope. It was found that the structure of MRO is hexagonal of the Pd2Si-type with lattice parameters a = 0.715 and c = 0.312 nm. Nearest neighbor atomic distances of Pd-Pd and Pd-Si obtained from the present analysis were compared with those obtained from the pair distribution function analysis made by using halo electron diffraction pattern of the same specimen. On the basis of nano-diffraction pattern calculations, a discussion was made on the limitation of structural analysis of MRO structures by nano-diffraction.