Polar Cap Region Research Articles

An unusual giant spiral arc in the polar cap region during the northward phase of a Coronal Mass Ejection

Abstract. The shock arrival of an Interplanetary Coronal Mass Ejection (ICME) at ~09:50 UT on 22 November 1997 resulted in the development of an intense (Dst<−100 nT) geomagnetic storm at Earth. In the early, quiet phase of the storm, in the sheath region of the ICME, an unusual large spiral structure (diameter of ~1000 km) was observed at very high latitudes by the Polar UVI instrument. The evolution of this structure started as a polewardly displaced auroral bulge which further developed into the spiral structure spreading across a large part of the polar cap. This study attempts to examine the cause of the chain of events that resulted in the giant auroral spiral. During this period the interplanetary magnetic field (IMF) was dominantly northward (Bz>25 nT) with a strong duskward component (By>15 nT) resulting in a highly twisted tail plasma sheet. Geotail was located at the equatorial dawnside magnetotail flank and observed accelerated plasma flows exceeding the solar wind bulk velocity by almost 60%. These flows are observed on the magnetosheath side of the magnetopause and the acceleration mechanism is proposed to be typical for strongly northward IMF. Identified candidates to the cause of the spiral structure include a By induced twisted magnetotail configuration, the development of magnetopause surface waves due to the enhanced pressure related to the accelerated magnetosheath flows aswell as the formation of additional magnetopause deformations due to external solar wind pressure changes. The uniqeness of the event indicate that most probably a combination of the above effects resulted in a very extreme tail topology. However, the data coverage is insufficient to fully investigate the physical mechanism behind the observations.

Energetic particle radiations measured by particle detector on board CBERS-1 satellite

Using the data measured by energetic particle detector on board CBERS-01 and-02 for the past five years, statistics was made to show the general features of MeV electrons and protons along a solar synchronous orbit at an altitude of 780 km. This height is in the bottom region of the Earth’s radiation belts. Detectors are inside the satellite cabinet and such continuous monitoring of particle radiation environment inside a satellite has seldom conducted so far. After a proper and careful treatment, it is indicated that the data inside satellite are well correlated with the radiation environment outside. Besides the agreement of the general distribution characteristics of energetic electrons and protons with similar observations from other satellites, attention is particularly paid to the disturbed conditions. Variations of particle fluxes are closely related with solar proton events, in general, electron fluxes of outer belt are well correlated with Dst index after three days’ delay while the electron injection occurred almost at the same day during great magnetic storms. It is confirmed that both energetic electrons and protons appear in the Polar Cap region only after the solar proton events.

Satellite Measurements of Middle Atmospheric Impacts by Solar Proton Events in Solar Cycle 23

Solar cycle 23 was extremely active with seven of the largest twelve solar proton events (SPEs) in the past forty years recorded. These events caused significant polar middle atmospheric changes that were observed by a number of satellites. The highly energetic protons produced ionizations, excitations, dissociations, and dissociative ionizations of the background constituents in the polar cap regions (>60 degrees geomagnetic latitude), which led to the production of HOx (H, OH, HO2) and NOy (N, NO, NO2, NO3, N2O5, HNO3, HO2NO2, BrONO2, ClONO2). The HOx increases led to short-lived ozone decreases in the polar mesosphere and upper stratosphere due to the short lifetimes of the HOx constituents. Polar middle mesospheric ozone decreases greater than 50% were observed and computed to last for hours to days due to the enhanced HOx. The NOy increases led to long-lived polar stratospheric ozone changes because of the long lifetime of the NOy family in this region. Upper stratospheric ozone decreases of >10% were computed to last for several months past the solar events in the winter polar regions because of the enhanced NOy.

Magnetopause mapping to the ionosphere for northward IMF

Abstract. We study the topological structure of the magnetosphere for northward IMF. Using a magnetospheric magnetic field model we study the high-latitude response to prolonged periods of northward IMF. For forced solar wind conditions we investigate the location of the polar cap region, the polar cap potential drop, and the field-aligned acceleration potentials, depending on the solar wind pressure and IMF By and Bx changes. The open field line bundles, which connect the Earth's polar ionosphere with interplanetary space, are calculated. The locations of the magnetospheric plasma domains relative to the polar ionosphere are studied. The specific features of the open field line regions arising when IMF is northward are demonstrated. The coefficients of attenuation of the solar wind magnetic and electric fields which penetrate into the magnetosphere are determined.

Double-Probe Measurements in Cold Tenuous Space Plasma Flows

Cold flowing tenuous plasmas are common in the terrestrial magnetosphere, particularly in the polar cap and tail lobe regions, which are filled by the supersonic plasma flow known as the polar wind. Electric field measurements with double-probe instruments in these regions suffer mainly from two error sources: 1) an apparent sunward electric field due to photoemission asymmetries in the probe-boom system and 2) an enhanced negatively charged wake forming behind the spacecraft, which will affect the probe measurements. The authors investigate these effects experimentally by Fourier analysis of the spin signature from the double-probe instrument Electric Fields and Waves (EFW) on the Cluster spacecraft. They show that while the signature due to photoemission asymmetry is very close to sinusoidal, the wake effect is characterized by a spectrum of spin harmonics. The Fourier decomposition can therefore be used for identifying wake effects in the data. As a spin-off, the analysis has also given information on the cold flowing ion population

Dependence of neutral winds on convection E‐field, solar EUV, and auroral particle precipitation at high latitudes

We use the Global Ionosphere Thermosphere Model (GITM) to investigate the thermospheric neutral winds and their relationship to the high‐latitude ionospheric convection, auroral precipitation, and solar activity. Each simulation is run for 24 hours, allowing an approximate steady state to be reached. Our results show that the duskside vortex and the antisunward polar cap neutral winds are prominent. The interplanetary magnetic field (IMF) southward Bz control of the polar cap winds is evident in both dusk cell and polar cap. When the IMF Bz changes from −1 to −10 nT, at the auroral region both the root of mean square (RMS) of Vn and Vi increase more than 50% at 300 km altitude, the ratio of R2 (RMS(Vn · (E × B))/RMS(E × B)2) increases, while R1 (RMS(Vn)/RMS(E × B)) changes little. In the polar cap region, since the ion flow is in the same direction as the neutral pressure gradient, above around 160 km altitude, ∣Vn∣ is larger than ∣Vi∣. When the F10.7 increases from 70 to 250, in the auroral region, the magnitude of ∣Vn∣ increases close to 20% at 300 km altitude. At lower latitudes, where ∣Vn∣ > ∣Vi∣ and the ions act as a load to the neutral winds, as a consequence of increases in F10.7, the drag on the neutrals becomes greater and therefore slows the neutrals down. The altitude dependent change of R2 is associated with the multiple effects of the F10.7 to the neutral winds, including the increased day‐to‐night pressure gradient and ion drag. The impact of hemispheric power (HP) to the low thermosphere is pronounced. At 150 km altitude, the neutral winds increase 30% in magnitude at auroral latitudes. When HP is raised from 10 to 100 GW, a dawn cell forms, concurrent with an enhancement in the dusk cell. The dawn and dusk cells show an obvious asymmetry both in the average neutral winds and the sensitivity of neutral winds to the HP. It is shown that at 250 km altitude, the magnitude of the southward IMF Bz has a positive correlation with the magnitude of the polar cap winds and the sensitivity increases with F10.7. A negative correlation between the magnitude of Vn and F10.7 with small southward IMF Bz becomes positive when the southward IMF Bz increases. This change is presumably due to the momentum balance between ion drag and advection. The dawn (dusk) convection cell is enhanced when By is negative (positive). In response to the effect of the Coriolis force, the By negative winds are not exactly mirror reflections of the By positive winds. For northward IMF Bz, the neutral winds flow sunward at the highest latitudes near noon. When the northward Bz increases from 10 nT to 20 nT, the distribution of the polar cap potential becomes more symmetric and the sunward neutral flow is enhanced and expanded.

ISACCO: an Italian project to monitor the high latitudes ionosphere by means of GPS receivers

As the high latitude ionosphere is directly connected with outer space by means of the field line reconnection of the geomagnetic field through the magnetopause, it is highly sensitive to the enhancement of the electromagnetic radiation and energetic particles coming from the Sun. Under such conditions the ionosphere may become highly turbulent showing the presence of small-scale (from centimetres to meters) structures or irregularities imbedded in the large-scale (tens of kilometres) ambient ionosphere. These irregularities can produce short-term phase and amplitude fluctuations in the carrier frequency of the radio waves which pass through them, commonly called ionospheric amplitude and phase scintillations (see, e.g., Morrissey et al. 2004, and references therein). The high latitude ionosphere encounters significant fading, with the most intense fading depths in the polar cap regions and less intense fading in the auroral regions. Severe amplitude fading and strong phase scintillation affect the reliability of GPS navigational systems and satellite communications. As the scarceness of a continuous and systematic monitoring of ionospheric scintillations over polar and auroral regions, the deployment of network(s) of GPS receivers, opportunely configured to observe the ionosphere under quiet and stormy conditions, could represent an important achievement for both space weather purposes and scientific aims. In this paper, a general overview on ISACCO (Ionospheric Scintillations Arctic Campaign Coordinated Observation) is given. ISACCO is an Italian project to monitor ionospheric scintillations at polar regions by means of modified GPS receivers. After some historical and technical notes on the project, the paper presents examples of some of our current investigations based on the data acquired during the almost 3 years of the ISACCO project.

Southern ozone variations induced by solar particle events during 15 January–5 February 2005

A preliminary analysis of effects induced on the high-latitude Southern ozone layer by the enhanced Space Weather storms of January 2005 is presented. Data from Earth Probe TOMS V.8, SAGE II and POAM III suggest short (hours) ozone depletions induced by the outstanding solar activity. Such ozone variabilities are investigated at the edge of the so-called polar cap region.

Electrodynamical characteristics of the polar ionosphere over the auroral and polar cap regions based on incoherent scatter radar measurements

Electrodynamical characteristics of the polar ionosphere over the auroral and polar cap regions are examined based on incoherent scatter radar measurements particularly in terms of electric field and ionospheric conductivity. For this purpose, 43 and 109 days of measurements from the Chatanika and Sondrestrom incoherent scatter radars, respectively, are utilized. The ionospheric current density inferred from the radar measurements of ionospheric conductance and electric field is compared with the corresponding ground magnetic disturbance. Also estimated is the global field-aligned current (FAC) that affects the ground magnetic disturbance, particularly of the D component. Several interesting characteristics about the polar ionosphere are emerging from this study: (1) the sun determines largely the conductance over the Sondrestrom radar, while the nighttime conductance distribution over the Chatanika radar is significantly affected by auroral precipitation. (2) The regions of the maximum N–S component of the electric field over the Chatanika radar are located approximately at the dawn and dusk sectors, while they tend to shift towards dayside over the Sondrestrom radar with the N–S component over Sondrestrom being slightly stronger than Chatanika. However, the E–W component over Chatanika is negligible compared to that of Sondrestrom. (3) The E–W component of the ionospheric current, J E, over Chatanika flows dominantly in the night hemisphere while it flows in the sunlit hemisphere over Sondrestrom. The N–S component of the ionospheric current, J N, over Chatanika flows prominently in the dawn and dusk sectors, while a strong southward current flows over Sondrestrom in the pre-noon sector. (4) The assumption of infinite sheet current approximation is far from realistic, underestimating the current density by a factor of 2 or more. It is particularly serious at higher latitudes. (5) The correlation between Δ H and J E is higher than the one between Δ D and J N indicating that FAC affects significantly Δ D. (6) The total upward and downward FAC are quite comparable and they are approximately 5.9×10 5 and 2.2×10 5 A over Chatanika and Sondrestrom, respectively. With enhancement of magnetic activity, FAC increases drastically over Chatanika by a factor of 4 or 5 while it is insignificant over Sondrestrom.

Initial 3-D neutral polar wind

The high-latitude polar-cap region is a highly dynamic region with many interesting phenomena associated with it, including the ion and neutral polar winds, plasma patches, polar holes, etc. To elucidate these phenomena and the neutrals produced via charge exchange associated with them, a new 3-D high-latitude hydrodynamic model of the polar-cap region has been developed. The model solves for H +, O +, H s, O s, and electrons taking into account the effects of precipitating auroral electrons and convection electric fields. The first results of the model show a highly complex system at high-latitudes, with neutral stream particles flowing in all directions due to the combined effects of charge exchange and plasma convection.

GCRT J1745-3009 as a Transient White Dwarf Pulsar

A transient radio source in the direction of the Galactic center, GCRT J1745-3009, exhibited five peculiar consecutive outbursts at 0.33 GHz with a period of 77.13 minutes and a duration of ~10 minutes for each outburst. It has been claimed to be the prototype of a hitherto unknown class of transient radio sources. We interpret it as a transient white dwarf pulsar with a period of 77.13 minutes. The ~10 minute flaring duration corresponds to the epoch in which the radio beam sweeps our line of sight. The bursting epoch corresponds to the episodes in which stronger sunspot-like magnetic fields emerge into the white dwarf polar cap region during which the pair-production condition is satisfied and the white dwarf behaves like a radio pulsar. It switches off as the pair-production condition breaks down.

Coherent Curvature Radiation and Proton Counterflow in the Pulsar Magnetosphere

In the proton counterflow model of a pulsar magnetosphere that we have recently proposed, nonrelativistic protons are supplied from the magnetosphere to flow toward the pulsar surface and screen an electric field above the polar cap region. In this Letter, we show that the proton counterflow is also suitable for the bunching of pair plasma. The two-stream instability is easily excited and can produce bunches of pairs with a relevant length scale to emit coherent curvature radiation.

Neutral atmospheric influences of the solar proton events in October–November 2003

The large solar storms in October–November 2003 caused solar proton events (SPEs) at the Earth and impacted the middle atmospheric polar cap regions. Although occurring near the end of the maximum of solar cycle 23, the fourth largest period of SPEs measured in the past 40 years happened 28–31 October 2003. The highly energetic protons associated with the SPEs produced ionizations, excitations, dissociations, and dissociative ionizations of the background constituents, which led to the production of odd hydrogen (HOx) and odd nitrogen (NOy). NOx (NO + NO2) was observed by the UARS HALOE instrument to increase over 20 ppbv throughout the Southern Hemisphere polar lower mesosphere. The NOAA 16 SBUV/2 instrument measured a short‐term ozone depletion of 40% in the Southern Hemisphere polar lower mesosphere, probably a result of the HOx increases. SBUV/2 observations showed ozone depletions of 5–8% in the southern polar upper stratosphere lasting days beyond the events, most likely a result of the NOy enhancements. Longer‐term Northern Hemisphere polar total ozone decreases of >0.5% were predicted to last for over 8 months past the events with the Goddard Space Flight Center two‐dimensional model. Although the production of NOy constituents is the same in both hemispheres, the NOy constituents have a much larger impact in the northern than the southern polar latitudes because of the seasonal differences between the two hemispheres. These observations and model computations illustrate the substantial impact of solar protons on the polar neutral middle atmosphere.

Polar cap size during 14–16 July 2000 (Bastille Day) solar coronal mass ejection event: MHD modeling and satellite imager observations

This study investigates the development of the polar cap area as simulated by global magnetohydrodynamic models of the Earth's magnetosphere during the 14–16 July 2000 (Bastille Day) event. Around 1440 UT on 15 July, a magnetic cloud hit the magnetosphere and in the following hours high levels of activity in the magnetosphere and ionosphere were driven by the frequent changes in solar wind conditions. We compare the size of the polar cap (region of open magnetic field lines) as computed with two MHD models (UCLA‐GGCM and BATSRUS) with observation data obtained from the IMAGE and Polar satellites. The two models in general reproduce the changes of the polar cap size that are seen by the satellite imagers. The range of modeled polar cap sizes, however, is limited to about 50–80% of the size range seen by the imagers and the shapes of the polar caps sometimes differ considerably among the models and compared to the observations. We found that a smoothing of solar wind parameters occurs, suggesting that solar wind inputs are stored in the magnetospheric system over a certain “memory” timescale. Cross‐correlations are computed between smoothed solar wind input and the time history of the resulting polar cap size. Modeled magnetospheric “memory” timescales are estimated to be less than 12 min, whereas the satellite image data suggest a timescale of more than 20 min. The driver of magnetospheric activity in the models was found to be the Bz (north‐south) component of the solar wind magnetic field and to a lesser degree, the Akasofu ε parameter which is closely related to Bz but is also influenced by the solar wind velocity Vx and magnetic field By. N, Vx, and Pdyn ∝ NVx2 show some degree of anticorrelation with observed polar cap sizes, but anticorrelations are barely significant for polar caps computed from either model. Memory timescales and reaction time delays could be derived from some of the parameters within the limits of statistical significance of the correlation coefficients.

NCAR-TIEGCM을 이용한 이온권과 열권의 상호작용 연구: 행성간 자기장(IMF)에 따른 고위도 하부 열권의 운동량 강제에 대한 연구

본 연구에서는 미 국립대기연구소(NCAR)의 열권-이온권 전기역학적 대순환 모델(TIEGCM)을 이용하여 행성간 자기장(IMF)의 방향과 세기 그리고 고도에 따라 여름철 남반구 고위도 하부 열권의 바람에 작용하는 운동량 강제력을 정량적으로 구하였다. 그리고 이들을 서로 비교 분석함으로써 IMF 조건과 고도에 따른 고위도 하부 열권의 풍계(wind system)를 유지시켜주는 주된 물리적인 과정을 살펴보았다. 고위도 하부 열권(<180km)에서 양(<TEX>$B_y$</TEX> > 0.8|<TEX>$\overline{B}_z$</TEX>|)또는 음(<TEX>$B_y$</TEX> < -0.8|<TEX>$\overline{B}_z$</TEX>|)의 IMF By 조건인 경우에 운동량 강제력 차이, 즉 IMF 기준치 <TEX>${\neq}$</TEX> 0 일 때와 IMF 기준치=0 일 때의 운동량 강제력 차이(difference momentum force)는 자기위도 -80<TEX>$^{\circ}$</TEX>에서 최대값을 가지면서 극관과 오로라 영역에 국한된 단순한 형태의 분포를 보인다. 그리고 IMF <TEX>$B_z$</TEX> 성분이 양과 음일 때 강제력 차이의 세기는 비슷하지만 분포양상은 반대방향을 취한다. 한편 IMF <TEX>$B_z$</TEX>가 양(<TEX>$B_z$</TEX> > 0.3125|<TEX>$\overline{B}_y$</TEX>|) 또는 음(<TEX>$B_z$</TEX> < -0.3125|<TEX>$\overline{B}_y$</TEX>|)인 조건인 경우에는 강제력 차이가 아오로라(subauroral) 위도까지 분포하며 IMF <TEX>$B_z$</TEX>가 양 또는 음의 조건일 때 보다 복잡한 구조를 보인다. 그리고 IMF <TEX>$B_z$</TEX>가 음인 경우의 강제력 차이가 양인 경우보다 더 크며 반대방향으로 작용한다. 125km 보다 더 높은 고도(>125km)에서 바람차이를 결정하는 주된 강제력은 기압경도력, 전향력, 수평이류 그리고 비발산 성분이 강한 Pedersen 이온항력인 것으로 확인되었다. 고도 약 125km 에서는 이 네 가지 힘에 더불어 비회전 성분이 강한 Hall 이온향력과 극관내 의 연직 이류가 지역과 시간에 따라 바람차이의 형성에 작용한다. 한편 고도 108-125km 에서는 IMF <TEX>$B_z$</TEX> 조건일 경우의 극관영역을 제외하고는 기압경도력, 전향력 그리고 Hall 이온항력이 이 고도에서의 바람차이를 유발시키는 주된 강제력으로 작용한다. 고도 108km 이하에서는 기압경도력과 전향력이 균형을 이루어 지균 운동을 유지시킨다. IMF-<TEX>$\overline{B}_y$</TEX>의존 MLT 평균 운동량 강제력들은 이온항력을 제외한 다른 모든 남북성분이 동서성분에 비해 더 강하게 중성대기에 작용하는 것으로 확인되었다. 108-125km의 고도에서 IMF B?가 음인 경우에 이온항력은 하강운동 및 단열압축가열과 관련된 시계방향의 온난순환(warm circulation)을 극관 내에 형성시킨다. 반면 IMF <TEX>$B_y$</TEX>가 양인 경우에는 극관 내에 상승운동 및 단열팽창냉각과 관련된 반시계방향의 한랭순환(cold circulation)을 형성시킨다. 이온항력은 IMF <TEX>$B_z$</TEX>가 음인 경우에는 새벽영역에 상승운동과 관련된 반시계방향의 한랭순환을, 반면에 IMF <TEX>$B_z$</TEX>가 양인 경우에는 새벽영역에 하강운동과 관련된 시계방향의 온난순환을 형성시킨다. To understand the physical processes that control the high-latitude lower thermospheric dynamics, we quantify the forces that are mainly responsible for maintaining the high-latitude lower thermospheric wind system with the aid of the National Center for Atmospheric Research Thermosphere-Ionosphere Electrodynamics General Circulation Model (NCAR-TIEGCM). Momentum forcing is statistically analyzed in magnetic coordinates, and its behavior with respect to the magnitude and orientation of the interplanetary magnetic field (IMF) is further examined. By subtracting the values with zero IMF from those with non-zero IMF, we obtained the difference winds and forces in the high-latitude 1ower thermosphere(<180 km). They show a simple structure over the polar cap and auroral regions for positive(<TEX>$B_y$</TEX> > 0.8|<TEX>$\overline{B}_z$</TEX> |) or negative(<TEX>$B_y$</TEX> < -0.8|<TEX>$\overline{B}_z$</TEX>|) IMF-<TEX>$\overline{B}_y$</TEX> conditions, with maximum values appearing around -80<TEX>$^{\circ}$</TEX> magnetic latitude. Difference winds and difference forces for negative and positive <TEX>$\overline{B}_y$</TEX> have an opposite sign and similar strength each other. For positive(<TEX>$B_z$</TEX> > 0.3125|<TEX>$\overline{B}_y$</TEX>|) or negative(<TEX>$B_z$</TEX> < -0.3125|<TEX>$\overline{B}_y$</TEX>|) IMF-<TEX>$\overline{B}_z$</TEX> conditions the difference winds and difference forces are noted to subauroral latitudes. Difference winds and difference forces for negative <TEX>$\overline{B}_z$</TEX> have an opposite sign to positive <TEX>$\overline{B}_z$</TEX> condition. Those for negative <TEX>$\overline{B}_z$</TEX> are stronger than those for positive indicating that negative <TEX>$\overline{B}_z$</TEX> has a stronger effect on the winds and momentum forces than does positive <TEX>$\overline{B}_z$</TEX> At higher altitudes(>125 km) the primary forces that determine the variations of tile neutral winds are the pressure gradient, Coriolis and rotational Pedersen ion drag forces; however, at various locations and times significant contributions can be made by the horizontal advection force. On the other hand, at lower altitudes(108-125 km) the pressure gradient, Coriolis and non-rotational Hall ion drag forces determine the variations of the neutral winds. At lower altitudes(<108 km) it tends to generate a geostrophic motion with the balance between the pressure gradient and Coriolis forces. The northward component of IMF By-dependent average momentum forces act more significantly on the neutral motion except for the ion drag. At lower altitudes(108-425 km) for negative IMF-<TEX>$\overline{B}_y$</TEX> condition the ion drag force tends to generate a warm clockwise circulation with downward vertical motion associated with the adiabatic compress heating in the polar cap region. For positive IMF-<TEX>$\overline{B}_y$</TEX> condition it tends to generate a cold anticlockwise circulation with upward vertical motion associated with the adiabatic expansion cooling in the polar cap region. For negative IMF-<TEX>$\overline{B}_z$</TEX> the ion drag force tends to generate a cold anticlockwise circulation with upward vertical motion in the dawn sector. For positive IMF-<TEX>$\overline{B}_z$</TEX> it tends to generate a warm clockwise circulation with downward vertical motion in the dawn sector.

Solar cosmic rays in the near Earth space and the atmosphere

Solar energetic particles (SEPs) constitute a distinct population of energetic charged particles, which can be often observed in the near Earth space. SEP penetration into the Earth’s magnetosphere is a complicated process depending on particle magnetic rigidity and geomagnetic field structure. Particles in the several MeV energy range can only access to periphery of the magnetosphere and the polar cap regions, while the GeV particles can arrive at equatorial latitudes. Solar protons with energies higher than 100 MeV may be observed in the atmosphere above ∼30 km, and those with energies more than 1 GeV may be recorded even at the sea level. There are some observational evidences of SEP influence on atmospheric processes. Intruding into the atmosphere, SEPs affect middle atmosphere odd-nitrogen and ozone chemistry. Since spatial and temporal variations of SEP fluxes in the near Earth space are controlled by solar activity, SEPs may present an important link between solar activity and climate. The paper outlines dynamics of SEP fluxes in the near Earth space during the last decades. This can be useful for tracing relationship between SEPs and atmospheric processes.

Modeling of field‐aligned guided echoes in the plasmasphere

Ray tracing modeling is used to investigate the plasma conditions under which high‐frequency (f ≫ fuh) extraordinary mode waves can be guided along geomagnetic field lines. These guided signals have often been observed as long‐range discrete echoes in the plasmasphere by the Radio Plasma Imager (RPI) onboard the Imager for Magnetopause‐to‐Aurora Global Exploration satellite. Field‐aligned discrete echoes are most commonly observed by RPI in the plasmasphere, although they are also observed over the polar cap region. The plasmasphere field‐aligned echoes appearing as multiple echo traces at different virtual ranges are attributed to signals reflected successively between conjugate hemispheres that propagate along or nearly along closed geomagnetic field lines. The ray tracing simulations show that field‐aligned ducts with as little as 1% density perturbations (depletions) and &lt;10 wavelengths wide can guide nearly field‐aligned propagating high‐frequency X mode waves. Effective guidance of a wave at a given frequency and wave normal angle (Ψ) depends on the cross‐field density scale of the duct, such that ducts with stronger density depletions need to be wider in order to maintain the same gradient of refractive index across the magnetic field. While signal guidance by field aligned density gradient without ducting is possible only over the polar region, conjugate field‐aligned echoes that have traversed through the equatorial region are most likely guided by ducting.

Giant Pulses from PSR B1937+21 with Widths ≤15 Nanoseconds and \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape $T_{b}\geq 5\times 10^{39}$ \end{document} K, the Highest Brightness Temperature Observed in the Universe

Giant radio pulses of the millisecond pulsar B1937+21 were recorded with the S2 VLBI system at 1.65 GHz with NASA/JPL's 70 m radio telescope at Tidbinbilla, Australia. These pulses have been observed as strong as 65,000 Jy with widths ≤15 ns, corresponding to a brightness temperature of Tb ≥ 5 × 1039 K, the highest observed in the universe. The vast majority of these pulses occur in 5.8 and 8.2 μs windows at the very trailing edges of the regular main pulse and interpulse profiles, respectively. Giant pulses occur, in general, with a single spike. Only in one case of 309 was the structure clearly more complex. The cumulative distribution is fitted by a power law with index -1.40 ± 0.01 with a low-energy but no high-energy cutoff. We estimate that giant pulses occur frequently but are only rarely detected. When corrected for the directivity factor, 25 giant pulses are estimated to be generated in one neutron star revolution alone. The intensities of the giant pulses of the main pulses and interpulses are not correlated with each other nor with the intensities or energies of the main pulses and interpulses themselves. Their radiation energy density can exceed 300 times the plasma energy density at the surface of the neutron star and can even exceed the magnetic field energy density at that surface. We therefore do not think that the generation of giant pulses is linked to the plasma mechanisms in the magnetosphere. Instead we suggest that it is directly related to discharges in the polar cap region of the pulsar.

A Conjunctive Study of Solar Flare 20010402 and Related Solar Proton Events by the Observation of SZ2/XD and ZY1/CBMC

AbstractThe solar flare occurred on 2 April 2001 is the biggest flare in the 23rd cycle of solar activity, which is companied by a big solar proton event. The study was made to this event by combining the observation results from SZ2/XD and CBERS‐1/CBMC. SZ2 has the orbit with 400km height and inclination of 42°, and CBERS‐1 is a satellite with solar‐synchronous orbit and 780km to the Earth surface. The results show that after this flare CBERS‐1/CBMC detected the flare particles in the polar cap region and XD observed an increase of energetic electron flux near the latitude 42°. Morphological analysis of the temporal and spatial variations of energetic particles after this flare indicates that particles in different regions have varied origins. Those in the polar cap region come from the Earth's open field lines and those in the auroral oval and mid‐latitudes near 42° are particles of the earth's outer radiation belt. Data analysis and comparison show that the intensity of Solar Proton Event is not proportional to the X‐ray flux of the related solar flare. The response of energetic particles in the near earth space is not solely related with solar X‐ray intensity.

Propagation characteristics of Pi 2 magnetic pulsations observed at ground high latitudes

In order to investigate the characteristics of Pi 2 propagation observed in the high‐latitude region, ground magnetometer data obtained at high‐latitude CPMN stations were analyzed. The power of magnetic perturbation, (ΔH)2 + (ΔD)2, were calculated for Pi 2 events observed at four stations from 11 February through 20 April 1996 and for Pi 2s observed at two stations from 1 January through 27 March 1997. The times when the power of Pi 2s reached the maximum and the maximum amplitudes were compared among stations. The results are as follows. Pi 2s observed at Kotel'nyy (KTN: MLAT = 69.94°, MLON = 201.02°) reached the maximum amplitude earlier than those at lower‐latitude station Tixie (TIK: MLAT = 65.67°, MLON = 196.88°), though the amplitudes were smaller at KTN than at TIK on average. The time lag from KTN to TIK has two peaks in its distribution; the primary and the secondary peaks are located around 10 s and 35 s, respectively. The mean value of the whole distribution of the time lag from KTN to TIK is about 20 s. Ultra Violet Image (UVI) data obtained by the Polar satellite was available during the second period; the UVI data indicate that it was when the auroral oval was located equatorward to KTN that the Pi 2 amplitude tended to reach the maximum earlier at KTN than at TIK. This observational result is important because Pi 2 was observed earlier in the polar cap region rather than in the auroral region. That is to say, Pi 2 was observed earlier in the polar cap region, which is not directly connected with the source region of Pi 2 via the magnetic field line. Longitudinal characteristics of Pi 2 propagation were derived by using data from longitudinally separated stations TIK, Chokurdakh (CHD: MLAT = 64.67°, MLON = 212.12°) and Kotzebue (KOT: MLAT = 64.52°, MLON = 249.72°). The result indicates the existence of the longitudinal center of Pi 2 propagation. The average magnetic local time of the center is estimated to have been ∼22.5 MLT; eastward (westward) of the center, Pi 2 exhibited an eastward (westward) propagation. The temporal and spatial developments of the Pi 2 propagation along the auroral zone were derived in an empirical manner. That is, the MLT (set to zero at the propagation center) dependence of the maximum amplitude time and the maximum amplitude itself of Pi 2 were derived in an empirical manner. As a result it is concluded that in the premidnight sector (i.e., around 22.5 MLT), KTN is the most probable location that observes the maximum amplitude of Pi 2 earliest among the CPMN stations located along 210° magnetic meridian. Our results show that the low‐latitude Pi 2, which has often been used as a time indicator of substorm onset, is often delayed from the Pi 2 observed in the premidnight polar cap region. The present results imply that the consideration of high‐latitude Pi 2s in addition to low‐latitude Pi 2s can provide a new insight into the substorm onset timing. Thus it is necessary to consider the global features, especially Pi 2s observed in higher‐latitude region, for studying substorm onset timing issues.