Magnetospheric Origin Research Articles

Cusp for high and low merging rates

The traditional distinction between the cusp for northward and southward interplanetary magnetic field (IMF) conditions is deficient in that many, if not most, “northward” IMF cusp encounters closely resemble southward IMF encounters. Partly for that reason, “southward” IMF cusps have been far more often examined than “northward. ” Recently, Newell et al. (2007) showed that the magnetopause merging rate, dΦMP/dt, much better predicts cusp latitude than does Bz (or Bs). Here, we investigate the extent to which high and low merging rate conditions better separates cusp encounters into mutually distinct classes. Indeed, high merging rate cusps (those with dΦMP/dt > 2* 〈dΦMP/dt〉) differ from low merging rate cusps (those with dΦMP/dt < 0.5〈dΦMP/dt〉) in several quantitative and even qualitative ways. High‐energy (tens of keV) ions, apparently of magnetospheric origin, do not extend into the high merging rate cusp but frequently do for low merging rates. However, the dispersion curve for ions, apparently of magnetosheath origin, extends to several keV for high merging rates but only to 1–2 keV for low merging rates. The local time extent of the cusp is 2.33 hours MLT for low merging rate conditions and 3.45 hours MLT for high merging rate conditions. All high merging rate cusps show clear forward dispersion (declining energy with increasing latitude) with low‐energy ion cutoffs within that dispersion. Low merging rates cusps rarely show forward dispersion, but about half, or slightly less, show reverse dispersion (declining energy with decreasing latitude) at the poleward edge of the cusp. Low‐energy ion cutoffs in the low‐merging rate cusp primarily occur only within that reverse dispersion. “Double” cusps, some with very clear latitudinal separation, occur in some high merging rate cases, but no low merging rate cases. The high merging rates cases also typically have a “shadow” region of electron‐only precipitation at roughly polar rain energies and intensities, lying immediately equatorward of the main cusp. For low‐merging rate conditions, the region immediately equatorward of the cusp (main magnetosheath ion population) contains ions as well as electrons, and forms a boundary layer.

The optical spectrum of the Vela pulsar

Our knowledge of the optical spectra of Isolated Neutron Stars (INSs) is limited by their intrinsic faintness. Among the fourteen optically identified INSs, medium resolution spectra have been obtained only for a handful of objects. No spectrum has been published yet for the Vela pulsar (PSR B0833-45), the third brightest (V=23.6) INS with an optical counterpart. Optical multi-band photometry underlines a flat continuum.In this work we present the first optical spectroscopy observations of the Vela pulsar, performed in the 4000-11000 A spectral range.Our observations have been performed at the ESO VLT using the FORS2 instrument. The spectrum of the Vela pulsar is characterized by a flat power-law (alpha = -0.04 +/- 0.04), which compares well with the values obtained from broad-band photometry. This confirms, once more, that the optical emission of Vela is entirely of magnetospheric origin. The comparison between the optical spectral indeces of rotation-powered INSs does not show evidence for a spectral evolution suggesting that, as in the X-rays, the INS aging does not affect the spectral properties of the magnetospheric emission. At the same time, the optical spectral indeces are found to be nearly always flatter then the X-rays ones, clearly suggesting a general spectral turnover at lower energies.

First X-Ray Observations of the Young Pulsar J1357-6429

The first short Chandra and XMM-Newton observations of the young and energetic pulsar J1357-6429 provided strong indications of a tail-like pulsar-wind nebula associated with this object, as well as pulsations of its X-ray flux with a pulsed fraction above 50% and a thermal component dominating at lower photon energies (below 2 keV). The elongated nebula is very compact in size and may be interpreted as evidence for a pulsar jet. The thermal radiation is most plausibly emitted from the entire neutron star surface of a 10 km radius and a 1.0+/-0.1 K temperature, covered with a magnetized hydrogen atmosphere. At higher energies the pulsar's emission is of a nonthermal (magnetospheric) origin, with a power-law spectrum of a photon index of 1.1-1.3. This makes the X-ray properties of PSR J1357-6429 very similar to those of the youngest pulsars J1119-6127 and Vela with a detected thermal radiation.

Comparative Study of the Initial Spikes of Soft Gamma-Ray Repeater Giant Flares in 1998 and 2004 Observed withGeotail: Do Magnetospheric Instabilities Trigger Large-Scale Fracturing of a Magnetar's Crust?

We present the unsaturated peak profile of the giant flare from SGR 1900+14 on 1998 August 27. This was obtained by the particle counters of the Low Energy Particles instrument on board the Geotail spacecraft. The observed peak profile revealed four characteristic features: an initial steep rise, an intermediate rise to the peak, an exponential decay, and a small hump in the decay phase. From this light curve, we found that the isotropic peak luminosity was 2.3 × 1046 ergs s-1 and that the total energy was 4.3 × 1044 ergs s-1 (E 50 keV), assuming that the distance to SGR 1900+14 is 15 kpc and that the spectrum is optically thin thermal bremsstrahlung with kT = 240 keV. These values are consistent with the previously reported lower limits derived from Ulysses and Konus-Wind observations. A comparative study of the initial spikes of the SGR 1900+14 giant flare in 1998 and of the SGR 1806-20 giant flare in 2004 is also presented. The timescale of the initial steep rise shows a magnetospheric origin, while the timescale of the intermediate rise to the peak indicates that it originates from crustal fracturing. Finally, we argue that the four features and their corresponding timescales provide us with a clue to identify extragalactic soft gamma-ray repeater giant flares among short gamma-ray bursts.

The Giant Flare of 2004 December 27 from SGR 1806−20

The giant flare of December 27, 2004 from SGR 1806-20 represents one of the most extraordinary events captured in over three decades of monitoring the gamma-ray sky. One measure of the intensity of the main peak is its effect on X- and gamma-ray instruments. RHESSI, an instrument designed to study the brightest solar flares, was completely saturated for ~0.5 s following the start of the main peak. A fortuitous alignment of SGR 1806-20 near the Sun at the time of the giant flare, however, allowed RHESSI a unique view of the giant flare event, including the precursor, the main peak decay, and the pulsed tail. Since RHESSI was saturated during the main peak, we augment these observations with Wind and RHESSI particle detector data in order to reconstruct the main peak as well. Here we present detailed spectral analysis and evolution of the giant flare. We report the novel detection of a relatively soft fast peak just milliseconds before the main peak, whose timescale and sizescale indicate a magnetospheric origin. We present the novel detection of emission extending up to 17 MeV immediately following the main peak, perhaps revealing a highly-extended corona driven by the hyper-Eddington luminosities. The spectral evolution and pulse evolution during the tail are presented, demonstrating significant magnetospheric twist and evolution during this phase. Blackbody radii are derived for every stage of the flare, which show remarkable agreement despite the range of luminosities and temperatures covered. Finally, we place significant upper limits on afterglow emission in the hundreds of seconds following the giant flare.

Geotail observations of temperature anisotropy of the two-component protons in the dusk plasma sheet

Abstract. In search for clues towards the understanding of the cold plasma sheet formation under northward IMF, we study the temperature anisotropy of the two-component protons in the plasma sheet near the dusk low-latitude boundary observed by the Geotail spacecraft. The two-component protons result from mixing of the cold component from the solar wind and the hot component of the magnetospheric origin, and may be the most eloquent evidence for the transport process across the magnetopause. The cold component occasionally has a strong anisotropy in the dusk flank, and the sense of the anisotropy depends on the observed locations: the parallel temperature is enhanced in the tail flank while the perpendicular temperature is enhanced on the dayside. The hot component is nearly isotropic in the tail while the perpendicular temperature is enhanced on the dayside. We discuss possible mechanism that can lead to the observed temperature anisotropies.

The spectral properties of low latitude daytime electric fields inferred from magnetometer observations

Four years of magnetometer data from two locations in Peru, one at the equator and one off the equator, have been converted to electric fields and their frequency characteristics (fluctuation spectra) examined. In the frequency range from 0.1 to 30 cycles per hour, the average spectrum monotonically decreases. However, it deviates from a power law in the range 0.3–3 cycles per hour especially for high levels of activity. The integrated power above 0.15 cycles per hour is a strong function of K p indicating that much of the fluctuations in the ionospheric equatorial field are of solar wind or magnetospheric origin. This result is in agreement with a previous power spectral study of low, middle, and high latitude fields using radars. The observed field strengths are lower than the ones observed in a previous study using balloon data at middle and high latitudes when the fields are projected to the equatorial plane. Simultaneous interplanetary electric field (IEF) data are compared to the equatorial field to determine how strong a relationship exists and to determine the amplitude and phase of their ratio as a function of frequency—an estimate of the average transfer function of the system. This function displays a bandpass-like form with a peak near 0.5 cycles per hour. This peak and evidence for a ringing of the time domain response suggests a weakly resonant system indicating some capacitance in addition to the inductance of the ring current and the resistance of the ionosphere. Case studies show that application of this function to IEF data yields good results and supports the notion that the response of the equatorial field to long-duration IEF polarities can last for many hours. Application of the function to test inputs such as pulses and triangle waves support this result. At high frequencies, we suggest that mapping of small-scale MHD turbulence is less effective than high frequency related transitions in the IEF.

Nonlinear evolution of modified two‐stream instability above ionosphere of Titan: Comparison with the data of the Cassini Plasma Spectrometer

The ionosphere of Titan, moon of Saturn, is directly exposed to the streaming plasma either of magnetospheric or solar wind origin. A turbulent interaction region is formed, called here flowside plasma mantle, where both cold ionospheric and hot streaming plasma are present at comparable densities. Within the framework of a one‐dimensional electromagnetic hybrid simulation using realistic electron mass we have shown that significant wave activity may be generated because of a modified two‐stream instability (MTSI). Having its free energy in the relative drift between the streaming ion flow and the ionospheric ions MTSI is very effective in generating “anomalous viscosity” type interaction leading to significant bulk velocity loss of the proton component of the external plasma flow and turbulent heating of the ionospheric ions. The stochastic energy transfer from the streaming plasma to the ionospheric ions may also increase the tailward planetary ion escape by collective pickup mechanism enhancing the rate of erosion of the atmosphere of Titan. We predict significant wave activity within the characteristic frequency range of about 1–10 Hz and at saturated wave electric field levels of about 5–25 mV/m. Similarly, according to our model calculations superthermal charged particles of ionospheric origin with kinetic energies of about few tens of eV are expected to be detectable by the charged particle analyzers onboard of Cassini spacecraft close to the upper edge of Titan's ionosphere.

Space storm as a phase transition

Fluctuations of the SYM-H index were analyzed for several space storms preceded by more than a week of extremely quiet conditions to establish that there was a rapid and unidirectional change in the Hurst scaling exponent at the time of storm onset. That is, the transition was accompanied by the specific signature of a rapid unidirectional change in the temporal fractal scaling of fluctuations in SYM-H, signaling the formation of a new dynamical phase (or mode) which was considerably more organized than the background state. We compare these results to a model of multifractional Brownian motion and suggest that the relatively sudden change from a less correlated to a more correlated pattern of multiscale fluctuations at storm onset can be characterized in terms of nonequilibrium dynamical phase transitions. The results show that a dynamical transition in solar wind VB s is correlated with the storm onset for intense storms, suggesting that the dynamical transition observed in SYM-H is of external solar wind origin, rather than internal magnetospheric origin. However, some results showed a dynamical transition in solar wind scaling exponents not matched by similar transitions in SYM-H. In other instances, we observed some small storms where there was a strong dynamical transition in SYM-H without similar changes in the VB s scaling statistics, suggesting that changes were due to internal magnetospheric processes. In summary, the results for intense storms points to the solar wind as being responsible for providing the scale free properties in the SYM-H fluctuations but the evidence for small storms clearly limit the importance of the solar wind fluctuations; their interaction is more complex than simple causality.

Pulsar Magnetohydrodynamic Winds

The acceleration and collimation/decollimation of relativistic magnetocentrifugal winds are discussed concerning a cold plasma from a strongly magnetized, rapidly rotating neutron star in a steady axisymmetric state based on ideal magnetohydrodynamics. There exist unipolar inductors associated with the field line angular frequency, $\alpha$, at the magnetospheric base surface, $S_{\mathrm{B}}$, with a huge potential difference between the poles and the equator, which drive electric current through the pulsar magnetosphere. Any “current line” must emanate from one terminal of the unipolar inductor and return to the other, converting the Poynting flux to the kinetic flux of the wind at finite distances. In a plausible field structure satisfying the transfield force-balance equation, the fast surface, $S_{\mathrm{F}}$, must exist somewhere between the subasymptotic and asymptotic domains, i.e., at the innermost point along each field line of the asymptotic domain of $\varpi_{\mathrm{A}}^2 / \varpi^2 \ll 1$, where $\varpi_{\mathrm{A}}$ is the Alfvénic axial distance. The criticality condition at $S_{\mathrm{F}}$ yields the Lorentz factor, $\gamma_{\mathrm{F}} = \mu_{\varepsilon}^{1/3}$, and the angular momentum flux, $\beta$, as the eigenvalues in terms of the field line angular velocity, $\alpha$, the mass flux per unit flux tube, $\eta$, and one of the Bernoulli integrals, $\mu_{\delta}$, which are assumed to be specifiable as the boundary conditions at $S_{\mathrm{B}}$. The other Bernoulli integral, $\mu_{\varepsilon}$, is related to $\mu_{\delta}$ as $\mu_{\varepsilon} = \mu_{\delta} [1-(\alpha^2 \varpi_{\mathrm{A}}^2 / c^2)]^{-1}$, and both $\mu_{\varepsilon}$ and $\varpi_{\mathrm{A}}^2$ are eigenvalues to be determined by the criticality condition at $S_{\mathrm{F}}$. Ongoing MHD acceleration is possible in the superfast domain. This fact may be helpful in resolving a discrepancy between the wind theory and the Crab-nebula model. It is argued that the “anti-collimation theorem” holds for relativistic winds, based on the curvature of field streamlines determined by the transfield force balance. The “theorem” combines with the “current-closure condition” as a global condition in the wind zone to produce a two-component “quasi-conical” field structure as one of the basic properties of MHD outflows of centrifugal origin in the pulsar magnetosphere.

Role of stochastic fluctuations in the magnetosphere‐ionosphere system: A stochastic model for theAEindex variations

A new stochastic model for theAEindex variations is developed to investigate the role of stochastic fluctuations in the magnetosphere‐ionosphere system. In contrast to pioneering stochastic models by Hnat et al. (2003, 2005), here the model is set up for the actual integrated quantity, i.e.,AEindex itself, instead of differenced variables, i.e.,AE(t+ Δt) −AE(t) and because of the different approach used in the derivation of the model, we do not restrict our model parameters to the power law behavior only. Also, we integrate the model to obtain a time series to which the observedAEis then compared to. The model suggests that the fluctuations are of internal magnetospheric origin, though the bursts can be triggered by an external perturbation, and are an interplay of deterministic and stochastic components of a stationary out‐of‐equilibrium system. The fundamental result of the study is that stochastic fluctuations play a central role in the evolution of theAEindex and cannot be grossly neglected. Also, in the model, the basic mechanism for all burst sizes is the same and thus no specific “substorm”‐related bursts can be extracted from theAEindex fluctuations. This suggests that from the global perspective, a specific well‐defined “class of substorms” may not exist. On the basis of their assumed spatiotemporal locality, impulsive dissipation events (IDE) (Sergeev et al., 1996) were proposed to be the fundamental physical building block of theAEindex fluctuations. The average temporal size of IDEs may explain the 3 mHz break in the power spectra of the time derivative of theAEindex reported here.

Kinetic signatures during a quasi‐continuous lobe reconnection event: Cluster Ion Spectrometer (CIS) observations

On 3 December 2001 the Cluster spacecraft observed a long‐lasting lobe reconnection event in the southern high‐latitude dusk magnetopause (MP) tailward of the cusp, during a 4 hour interval of mainly northward interplanetary magnetic field (IMF) and of sub‐Alfvénic magnetosheath flow. Almost all the MP encounters have accelerated flows (for which the Walén test has been successfully verified by Retinò et al. (2005)) as well as a large number of secondary populations related to reconnection, that is, ions of magnetosheath or magnetospheric origin which cross the MP either way. The detailed analysis of the distribution functions shows that the reconnection site frequently moves relative to the spacecraft, but simultaneous measurements by two spacecraft on opposite sides of the reconnection site indicate that the spacecraft's distance from the X line is small, i.e., below 3200 km. The vicinity to the X line throughout the event is probably the reason why the distribution functions characteristics agree with theoretical expectations on both sides of the reconnection site throughout this long event. Moreover, the detailed analysis of the distribution functions shows evidence, during a few time intervals, of dual reconnection, i.e., of reconnection simultaneously going on also in the northern hemisphere.

Reconnection and energetic particles at the edge of the exterior cusp

Abstract. In this paper we study flux transfer events (FTE) observed at the post-noon edge of the exterior cusp region by Cluster satellites. During the outbound dayside orbit on 2 February 2003, intense bursts of energetic particles were observed in close conjuction with magnetic field FTE signatures by the RAPID instrument onboard the Cluster 4. The pitch-angle distribution of the particles showed that the enhancements consist of particles flowing antiparallel to the magnetosheath field lines away from the expected reconnection site to the exterior cusp. At the same time Cluster 3 observed enhancements of energetic particles deeper in the exterior cusp with a delay of about 40 s to the Cluster 4 enhancements. The estimated maximum energy gain per particle by reconnection remains below 1 keV, thus clearly below the tens to hundreds of keV energy range observed by the RAPID instrument. These observations support the earlier statistical result of the magnetospheric origin of energetic particles in the exterior cusp. Reconnection near the exterior cusp partly releases the particles in the closed field lines of the adjacent HLPS region into the exterior cusp.

A Multiwavelength Study of the Pulsar PSR B1929+10 and Its X‐Ray Trail

We report on the emission properties of PSR B1929+10 and its putative trail from a multiwavelength study performed using optical, X-ray, and radio data. XMM-Newton observations confirm the existence of the diffuse emission with a trail morphology lying in a direction opposite to the transverse motion of the pulsar. The trail spectrum is nonthermal and produced by electron-synchrotron emission in the shock between the pulsar wind and the surrounding medium. Radio data from the Effelsberg 11 cm radio continuum survey show an elongated feature that roughly coincides with the X-ray trail. Three not fully resolved radio sources seen in the NVSS survey data at 1.4 GHz match with part of the elongated radio feature seen at 11 cm. The emission properties observed from PSR B1929+10 are in excellent agreement with a nonthermal, and thus magnetospheric-radiation-dominated, emission scenario. The pulsar's X-ray spectrum is best described by a single power-law model with a photon index of 2.72. A flux contribution from the thermal emission of heated polar caps of at most ~7% is inferred from a best-fitting composite Planckian and power-law spectral model. A pure thermal emission spectrum consisting of two Planckian spectra is regarded as unlikely. A broken power-law spectral model with Ebreak = 0.83 keV and the photon indexes α1 = 1.12 and α2 = 2.48 can describe the optical and X-ray data entirely in terms of a nonthermal magnetospheric origin. The X-ray pulse profile observed in the 0.2-10 keV band is found to be markedly different from the broad sinusoidal pulse profile seen in the low statistic Rontgensatellit (ROSAT) data. Fitting Gaussians to the X-ray light curve indicates the possible existence of three pulse components. A small narrow pulse, characterized by energies greater than 1 keV, is found to lead the radio main pulse by ~20°. The fraction of pulsed photons in the 0.2-10 keV band is 32% ± 4%. For the subbands 0.2-1.0 and 1.0-2.1 keV the pulsed fraction is 24% ± 5% and 44% ± 6%, respectively, indicating a mild energy dependence at a ~2 σ level. Simulations in the framework of an outer gap emission model are able to reproduce the observed X-ray pulse profile and its phase shift relative to the radio pulse.

Empirical orthogonal function analysis of magnetic observatory data: Further evidence for non‐axisymmetric magnetospheric sources for satellite induction studies

Although satellite electromagnetic induction studies have usually assumed a symmetric magnetospheric ring current source, there is growing evidence for significant source asymmetry. Here we apply empirical orthogonal function methods to mid‐latitude night‐side hourly mean geomagnetic observatory data to search for evidence of non‐zonal low‐frequency source fields. The dominant spatial mode of variability in residuals, obtained by subtracting symmetric ring current and ionospheric fields of the CM4 comprehensive model, has a substantial Y2−1 quadrupole component and is highly correlated with Dst. This pattern of temporal variability, which implies enhanced ring current densities in the dusk sector, persists even when peak storm‐time data are omitted. The observed asymmetry agrees with that inferred previously by Balasis et al. (2004), from the local time dependence of biases in satellite induction transfer functions. Temporal correlation of the leading mode with Dst, and consistency of its spatial structure with recent empirical ring current models, suggest a magnetospheric origin.

Long-term modulation of the cosmic ray fluctuation spectrum

Abstract. Here we study the power level of rapid cosmic ray fluctuations in the frequency range of 10-4-1.67·10-3 Hz (periods from 10 min to about 3 h), using measurements by space-borne instruments for the period since 1974. We find that the power level of these fluctuations varies over the solar cycle, but the phase of this variation depends on the energy of cosmic ray particles. While the power level of these fluctuations in the higher energy channels (corresponding to galactic cosmic rays) changes in phase with the solar cycle, the fluctuation level for lower energy channels (predominantly of solar/interplanetary origin) is roughly in an opposite phase with the solar cycle. The results prove conclusively that these fluctuations originate in the near-Earth space, excluding their atmospheric or magnetospheric origin. We present these new results and discuss a possible scenario explaining the observed energy-dependence.

Subaru optical observations of the two middle-aged pulsars PSR B0656+14 and Geminga

Aims.We carried out a deep subarcsecond BRI imaging of the two middle-aged pulsars to establish their properties in the optical range. Methods.Astrometry and photometry methods are applied to identify the pulsars and to measure their fluxes. We also reanalyze archival ESO/NTT and HST broadband data and find that some published fluxes for Geminga were estimated inaccurately. The resulting dereddened broadband spectra in the near-IR-UV range are analyzed and compared with available data from the radio through gamma-rays.Results.Both pulsars are detected at 10σ level. Geminga is for the first time reliably detected in the I band with a magnitude of 25. The dereddened spectra of both pulsars are remarkably similar to each other and show significant flux increases towards the far-UV and near-IR, and a wide flux excess in bands. This suggests a multicomponent structure of the optical emission. The nonthermal power law component of the pulsar magnetospheric origin dominates in the most part of the optical range. For PSR B0656+14 it is compatible with a low energy extension of the power law tail seen in hard X-rays. For Geminga the respective extension overshoots by a factor of 100 the nonthermal optical flux, which has a less steep spectral slope than in X-rays. This implies a spectral break at a photon energy ~1 keV. The flux increases towards the far-UV are compatible with contributions of the Rayleigh-Jeans parts of the blackbody components from whole surfaces of the neutron stars dominating in soft X-rays. The excess, which is most significant for PSR B0656+14, suggests a third spectral component of still unidentified origin. Faint, a few arcseconds in size nebulae extended perpendicular to the proper motion directions of the pulsars, are seen around both objects in our deepest I band images. They can be optical counterparts of the bow-shock head of Geminga and of the tentative pulsar wind nebula of PSR B0656+14 observed in X-rays.

Evidence for the interplanetary electric field effect on the OI 630.0 nm airglow over low latitude

On a geomagnetically disturbed and non‐spread F night (12 February 2004), the OI 630.0 nm airglow intensity variations recorded from Gadanki (13.5°N, 79.2°E, dip 12.5°) reveal quasi‐periodic intensity fluctuations superimposed on the large‐scale temporal variation during 1945–2315 Indian Standard Time (IST, IST = Universal Time (UT) + 5.5 hours). The residual airglow intensity fluctuations (after detrending the large‐scale variation) with periodicities of ∼0.5 hours and ∼1.0 hours, respectively, are found to vary in accordance with the F layer height variations over the dip equator as well as with the Y‐component of the interplanetary electric field (IEFy) variations calculated from the Advanced Composition Explorer (ACE) satellite data on 12 February 2004. Furthermore, these kind of periodic scale sizes in the residual airglow intensity fluctuations are not found on a geomagnetically quiet night (20 February 2004) and those intensity fluctuations do not show any correspondence with the F layer height variations over equator. Similar periodic components are also found in the temporal variations of instantaneous horizontal magnetic field (H) recorded at multiple stations over Indian subcontinent during the same interval on 12 February 2004. It is discussed that the currents responsible for the magnetic fluctuations are probably of magnetospheric origin. These observations using multiple techniques elicit the signature of the penetrating solar wind electric field in the nocturnal airglow intensity variations over low latitude during magnetically disturbed period.

Observational evidences on the influences of tropical lower atmospheric [formula omitted] day oscillation on the ionospheric equatorial electrojet

As an observational evidence, it is reported that long period oscillations near 21- and 30-days generated in the lower atmosphere (troposphere and lower stratosphere) in the Indian tropical regions have possible influences on the lower E region ionospheric currents (say equatorial electrojet [EEJ]) at these periodicities. Dynamical parameters like atmospheric wind velocities are measured in the lower atmosphere (4–20 km) and mesosphere (80–98 km) using the Indian MST radar located at Gadanki (a tropical station) and the MF radar located at Tirunelveli (Indian magnetic equatorial station), respectively, during the time interval of 18 January to 23 April 1999. Using Nd-Yag Rayleigh scatter lidar collocated at Gadanki, the atmospheric temperature profiles are measured in the intermediate height range of 25–80 km for the same period of observation. As a lower E region ionospheric parameter, the EEJ current strengths are measured by determining the geomagnetic field strengths at the Indian EEJ and off-EEJ stations of Trivandrum and Alibag, respectively. Analyses of the data by using fast Fourier transform (FFT), wavelet transform (Morlet wavelet), maximum entropy method (MEM) and Lomb–Scargle periodogram analysis have indicated that it may be possible for the long period oscillations generated in the lower atmosphere to have influences on the E region ionosphere. Possible influences of the solar high energy radiations and magnetospheric origin on EEJ current systems in the periodicity range of ∼ 20 –30 days are also discussed with illustrations.

Dawn‐dusk asymmetries, ion spectra, and sources in the northward interplanetary magnetic field plasma sheet

During periods of northward IMF, plasma sheet ions often have two components: hot (magnetospheric origin) and cold (magnetosheath origin). The temperatures of the cold‐component ions are ∼30–40% higher in the dawn sector compared to the dusk sector, implying the dawnside magnetosheath ion heating of ∼30–40%. As a result, the magnetosheath ions are less distinguishable from the hot‐component ions, which have lower temperatures on the dawnside, leading to a higher occurrence of the ions having (apparent) one‐component distribution. As the duration of the hourly averaged IMF being northward (Δt) increases from 1 to 10 hours, the occurrence of two‐component ions increases from 65% to 83% in the dusk flank, but in the dawn flank it remains relatively stable at around 45%. In contrast, the occurrence of ions best characterized by kappa (κ) distribution increases from 25% to 35% in the dawn flank whereas in the dusk flank it remains relatively insensitive to Δt (10%). The occurrence of a one‐component Maxwellian distribution appears to be most pronounced in the region of the plasma sheet close to the midnight meridian, and these ions appear to be characteristic of the nominal plasma sheet (hot component) ions. The densification of the plasma sheet, as Δt increases, mainly results from the influx of the magnetosheath ions. However, the cooling of the plasma sheet ions can be attributed not only to the influx of the cold magnetosheath ions but also to the cooling of the nominal plasma sheet ions. The dawn‐dusk asymmetries observed in the cold magnetosheath ion profiles should provide constraints that can help determine the roles of various proposed magnetosheath ion entry mechanisms.