Noon Sector Research Articles

Global‐scale observations of ionospheric convection variation in response to sudden increases in the solar wind dynamic pressure

We have used a superposed epoch analysis to study 205 sudden commencement (SC) events detected with ground‐based magnetometers between the years 2000 and 2007. The strength of the SC events was clearly correlated to the magnitude of the jump in the solar wind dynamic pressure, regardless of whether or not the SC events were followed by a magnetic storm. Data from the Super Dual Auroral Radar Network (SuperDARN) demonstrated that both the ionospheric plasma drift speed and the number of echoes increased in the noon sector in response to the increase in solar wind dynamic pressure. In contrast, the number of SuperDARN echoes in the midnight sector decreased as the solar wind dynamic pressure increased, even though the average drift speed in the midnight sector also increased. We also uncovered that the ionosphere and ring current evolve differently in response to the pressure pulses. The SYM‐H index, which represents changes in both the magnetopause and ring currents, responded immediately and either rapidly returned to pre‐SC values or progressed into the main phase of a geomagnetic storm. In contrast, the ionospheric convection data were affected for a much longer time. The implication is that the ring current reacts to a sudden compression of the magnetosphere on a time scale of 10 min, while the convection pattern itself is affected for as long as the increase in solar wind dynamic pressure is sustained, or until a geomagnetic storm was triggered, as is the case in the sudden storm commencement (SSC) subset of events.

Cassini observations of ion and electron beams at Saturn and their relationship to infrared auroral arcs

We present Cassini Visual and Infrared Mapping Spectrometer observations of infrared auroral emissions from the noon sector of Saturn's ionosphere revealing multiple intense auroral arcs separated by dark regions poleward of the main oval. The arcs are interpreted as the ionospheric signatures of bursts of reconnection occurring at the dayside magnetopause. The auroral arcs were associated with upward field‐aligned currents, the magnetic signatures of which were detected by Cassini at high planetary latitudes. Magnetic field and particle observations in the adjacent downward current regions showed upward bursts of 100–360 keV light ions in addition to energetic (hundreds of keV) electrons, which may have been scattered from upward accelerated beams carrying the downward currents. Broadband, upward propagating whistler waves were detected simultaneously with the ion beams. The acceleration of the light ions from low altitudes is attributed to wave‐particle interactions in the downward current regions. Energetic (600 keV) oxygen ions were also detected, suggesting the presence of ambient oxygen at altitudes within the acceleration region. These simultaneous in situ and remote observations reveal the highly energetic magnetospheric dynamics driving some of Saturn's unusual auroral features. This is the first in situ identification of transient reconnection events at regions magnetically conjugate to Saturn's magnetopause.

Global-scale observations of ionospheric convection during geomagnetic storms

[1] The global effects on the ionosphere during periods of intense geomagnetic activity associated with geomagnetic storms are investigated using the Super Dual Auroral Radar Network (SuperDARN). The influence of the main and recovery phases of geomagnetic storms on ionospheric properties such as backscatter occurrence rates, velocity distributions, and convection patterns are presented. The evolution of magnetosphere and ionosphere parameters during the storms did not depend on the origin of the storm (e.g., a coronal mass ejection or a corotating interaction region). Instead, there was a continuum of response to the intensity of the driver. For example, we found a clear relationship between the most negative value of the southward component of the interplanetary magnetic field (IMF Bz) and the most negative value of the Sym-H index, which marks the end of the main phase of a storm. This is one of the first superposed epoch studies that analyzes the sunward/antisunward line-of-sight velocity as a function of magnetic local time for geomagnetic storms of various intensities. In the noon sector, before and during the main phase of the storms, the SuperDARN radars recorded faster antisunward ionospheric plasma drifts together with a significant increase in the number of ionospheric echoes. This is consistent with the expected increase in soft particle precipitation in the noon sector and with the reconnection electric field that occurs when the IMF Bz is strongly negative, as is the case during the main phase of storms. The SuperDARN echo occurrence in the noon sector returned to prestorm values early in the recovery phase. The overall response was similar in the midnight sector, except that the peak echo occurrence for the most intense storms was limited to a narrower time interval centered on the end of the main phase. There were reductions in the strong antisunward flows near local midnight observed during the main phase and early in the recovery phase, particularly for the intense storm class. Strong electric fields are applied in the nightside ionosphere during storms, and the decameter structures from which SuperDARN scatter are more easily produced. However, in regions of energetic auroral precipitation and after a long exposure to strong electric fields, there is often a reduction in SuperDARN echoes due to absorption or changes in radio wave propagation.

Inner magnetosphere plasma characteristics in response to interplanetary shock impacts

[1] In order to characterize plasma (0.03–45 keV) properties in response to interplanetary (IP) shock impact at the geosynchronous orbit, we have examined 95 shock events from 1997 to 2004. These shock events have been categorized into two groups: shock fronts associated with southward interplanetary magnetic field (IMF; 47 cases) and with northward IMF (48 cases). Our results show that under southward IMF, the plasma becomes denser and hotter following the IP shock arrival. The proton (0.1–45 keV) and electron (0.03–45 keV) number densities have peaks of 1.8 and of 2.5 cm−3 at the duskside, respectively (the typical tail plasma sheet density is about 0.7 cm−3). After the IP shock impact, the plasma (proton and electron) temperature anisotropy increases remarkably at the noon sector, decreases toward dawn and dusk, and minimizes at midnight, suggesting that both electromagnetic ion cyclotron and whistler waves can be stimulated mainly at the dayside magnetosphere. In addition, there are more oxygen ions injecting into the inner magnetosphere, and the density of ionospheric oxygen ions is comparable to proton density. However, for IP shocks associated with northward IMF, the plasma density and temperature increases are insignificant, while slight enhancements of the plasma temperature anisotropy are distributed globally.

Spatial structure and temporal evolution of a dayside poloidal ULF wave event

[1] We investigate a strong poloidal ultralow frequency wave event in the noon sector observed by THEMIS and LANL satellites on 29 May 2007. From 07:00 to 10:00 UT, the five THEMIS satellites that were lined up in similar outbound orbits consecutively observed narrow-band poloidal pulsations from 10 to 4 mHz. The wave activity covered a broad region from 09:00 to 13:30 LT azimuthally and 5 to 9.5 RE radially. The radial extent and power of the wave decreased with time from 07:00 to 08:30 UT, suggesting a decay process with a time scale of hours. In the region outside the plasmapause, the wave power was observed to decrease then increase from 08:00 to 09:00 UT with a rapid temporal variation. The decrease in wave power, which suggests fast decay (within one hour), might be related to the evolution of the plasmasphere. The subsequent increase could possibly be related to a regeneration process by a surface wave at the plasmapause. We suggest that a coupling between the surface wave and the resonance of the field line around the plasmapause takes place when the density inside the plasmapause is twice the density outside the plasmapause.

Quasi-coherent chorus properties: 1. Implications for wave-particle interactions

[1] A study of dayside ELF/VLF electromagnetic (EM) waves from L* = 2 to 9 and magnetic local time (MLT) from 09 to 15 was conducted using plasma wave data from the Polar spacecraft. EM waves were detected from L* = 4 to 9 from 09 to 12 MLT with a decrease in the afternoon sector (12 to 15 MLT). Some of the chorus was clearly related to generation by substorm injected ∼5 to 100 keV electrons drifting from the midnight sector to the local noon sector. However, dayside chorus also showed two solar wind ram pressure dependences: increased (above average) pressures and unusually low pressures. Possible chorus generation mechanisms are discussed. Chorus detected by Polar away from the magnetic equator generation region (∼25° to 55° magnetic latitude) was substantially different than chorus detected in previous studies within the ∼0° to 10° generation region. (1) Two separate bands of chorus were often detected simultaneously: a higher-frequency downgoing (toward the Earth) band of waves and a lower-frequency upcoming band. (2) The downgoing waves are ∼2 orders of magnitude more intense (∼10−2 nT2) than simultaneously detected lower-frequency upcoming waves (∼10−4 nT2). (3) Chorus, when viewed as a Fourier spectrum, appears as a band of semicoherent hiss. (4) A scenario and schematic is presented to explain these observations: chorus is presumed to be generated at the equator at large L*, propagate downward toward Earth and inward across L* shells, and then refract back up to the spacecraft location. (5) The waves detected at Polar latitudes did not possess the temporal structure or the coherency of the ∼10 to 100 ms duration equatorial chorus subelements, although full single cycles with right-hand, circularly polarized structures were identified. This quasi-coherent EM turbulence may be formed by wave dispersive effects. The longer the wave path length, the greater is the reduction in coherency. (6) This feature of chorus has significant consequences for off-equatorial wave-particle interactions. For example, the microburst mechanism of Lakhina et al. (2010) that can account for rapid pitch angle diffusion of ∼5 to 100 keV electrons in the chorus generation region will not work for off-equatorial scattering of relativistic electrons because of the lack of chorus coherence there. (7) Some comments about semicoherent chorus (hiss) in the outer magnetosphere are made as challenges to theorists in the field.

Saturn's ring current: Local time dependence and temporal variability

Radial profiles of the azimuthal current density between similar to 3 and 20 R-S in Saturn's magnetosphere have been derived using plasma and magnetic field data from 11 near-equatorial Cassini orbits spanning a 10 month interval. The current density generally shows only modest variations with local time and from pass to pass within this region, rising rapidly near similar to 5 R-S to peak at similar to 90 pAm(-2) at similar to 9 R-S and falling more gradually to below similar to 20 pA m(-2) at 20 R-S. The pressure gradient current is overall the most important component, the dominant inertia current in the inner region being significantly canceled by the oppositely directed pressure anisotropy current. These characteristics principally reflect the properties of the warm water plasma originating from the Enceladus torus to distances of similar to 10 R-S encompassing the usual current peak, inside of which distance the plasma properties are generally unvarying within factors of less than similar to 2. Increased variability is present at larger distances where the pressure of the hot magnetospheric plasma plays the more important role. In this region the dominant pressure gradient current is found to be strongest in the dusk to midnight sector and declines modestly, by factors of similar to 2 or less, in the midnight to dawn and dawn to noon sectors. Pass-to-pass temporal variability by factors of similar to 2-3 is also present in the outer region, particularly in the dawn to noon sector, probably reflecting both hot plasma injection events as well as solar wind-induced variations.

Variations of f o F 2 and GPS total electron content over the Antarctic sector

This paper presents a preliminary analysis of the variations of the critical frequency of the F2 region (foF2) and the total electron content (TEC) derived from Global Positioning System (GPS) data. Hourly foF2 values were scaled from ionograms recorded at San Martin (68.1°S, 293.0°E) and the TEC values were derived from GPS observations at O’Higgins (63.3°S, 302.5°E). The database includes measurements obtained under different seasonal and solar activity conditions. The study shows that the daily peak of foF2 occurs around local noon in winter and fall, and in spring a secondary peak is observed around midnight. In summer (January) foF2 reaches its minimum value around the noon sector while the maximum in the diurnal variation of foF2 is located in a time sector close to midnight. This behaviour is observed at low and high solar activity. The semiannual anomaly appears around noon at high and low solar activity and the winter anomaly is not observed. The effect of the solar activity is generally observed in every season. The analysis of the GPS TEC measurements in the same region indicates that the diurnal, seasonal and solar activity variations are similar to those observed in the foF2 values. An analysis of the performance of the IRI model to predict foF2 is also shown using the two IRI options (URSI and CCIR). The comparisons between the experimental values and the IRI predictions show some discrepancies.

24/7 Solar minimum polar cap and auroral ion temperature observations

During the International Polar Year (IPY) two Incoherent Scatter Radars (ISRs) achieved close to 24/7 continuous observations. This presentation describes their data sets and specifically how they can provide the International Reference Ionosphere (IRI) a fiduciary E- and F-region ionosphere description for solar minimum conditions in both the auroral and polar cap regions. The ionospheric description being electron density, ion temperature and electron temperature profiles from as low as 90 km extending to several scale heights above the F-layer peak. The auroral location is Poker Flat in Alaska at 65.1 °N latitude, 212.5 °E longitude where the NSF’s new Poker Flat Incoherent Scatter Radar (PFISR) is located. This location during solar minimum conditions is in the auroral region for most of the day but is at mid-latitudes, equator ward of the cusp, for about 4–8 h per day dependent upon geomagnetic activity. In contrast the polar location is Svalbard, at 78.2 °N latitude, 16.0 °E longitude where the EISCAT Svalbard Radar (ESR) is located. For most of the day the ESR is in the Northern Polar Cap with a noon sector passage often through the dayside cusp. Of unique relevance to IRI is that these extended observations have enabled the ionospheric morphology to be distinguished between quiet and disturbed geomagnetic conditions. During the IPY year, 1 March 2007 – 29 February 2008, about 50 solar wind Corotating Interaction Regions (CIRs) impacted geospace. Each CIR has a two to five day geomagnetic disturbance that is observed in the ESR and PFISR observations. Hence, this data set also enables the quiet-background ionospheric climatology to be established as a function of season and local time. These two separate climatologies for the ion temperature at an altitude of 300 km are presented and compared with IRI ion temperatures. The IRI ion temperatures are about 200–300 K hotter than the observed values. However, the MSIS neutral temperature at 300 km compares favorably with the quiet-background in temperature, both in magnitude and climatology.

Variations of the critical frequency foF2 at middle latitudes during strong disturbance in the electric field observed in the F region over saint-santin

Morphological analysis of variations of the critical frequency foF2 in the midlatitude ionosphere at various sectors of local time is carried out on the basis of data from ground-based stations of vertical sounding of the ionosphere in the period when during use of the incoherent scatter radar at Saint-Santin an anomalously strong increase in the electric field was observed at heights of the ionospheric F region in the period of enhanced geomagnetic activity (4+ < Kp < 6−). The obtained picture of the space-time distribution of disturbances in foF2 makes it possible to assume that they could be caused by penetration to middle latitudes of the large-scale electric field of the magnetospheric convection directed westward in the nighttime and morning hours and eastward in the noon and evening sectors.

Dayside field‐aligned current source regions

The source regions of region‐0 (R0), region‐1 (R1), and region‐2 (R2) field‐aligned currents (FACs) were statistically determined using DMSP particle precipitation and magnetometer data. Each FAC sheet originates from more than one region in the magnetosphere, depending on the latitude and the magnetic local time (MLT). R2 originates mostly from the central plasma sheet (CPS) and boundary plasma sheet (BPS) in the morning and from the CPS, BPS, and inner magnetosphere in the afternoon, all of which are on closed field lines. Near noon, some R2 may originate from the low‐latitude boundary layer (LLBL), which is located near the magnetopause and can be open or closed. R1 mostly maps to the BPS, hence on closed field lines, in morning and afternoon, but near noon, it maps mostly to the LLBL. The LLBL source region can be found more frequently in the dawn–noon sector than in the noon–dusk sector. On the other hand, R0 is located mostly on open field lines and is associated mostly with mantle precipitation. However, the mantle precipitation has a dependency on the polarity of R0. Within up‐flowing R0, sometimes an upward field‐aligned electric field, which accelerates electron downward and retards ion precipitation, modifies mantle distribution to look more like those of polar rain or BPS. This electric field has the opposite polarity to the background electric field that maintains charge‐quasi‐neutrality and that limits some solar wind electrons from entering the magnetosphere in the mantle and polar rain regions. Implications to current generation mechanisms are discussed.

THEMIS analysis of observed equatorial electron distributions responsible for the chorus excitation

A statistical survey of plasma densities and electron distributions (0.5–100 keV) is performed using data obtained from the Time History of Events and Macroscale Interactions During Substorms spacecraft in near‐equatorial orbits from 1 July 2007 to 1 May 2009 in order to investigate optimum conditions for whistler mode chorus excitation. The plasma density calculated from the spacecraft potential, together with in situ magnetic field, is used to construct global maps of cyclotron and Landau resonant energies under quiet, moderate, and active geomagnetic conditions. Statistical results show that chorus intensity increases at higher AE index, with the strongest waves confined to regions where the ratio between the plasma frequency and gyrofrequency, fpe/fce, is less than 5. On the nightside, large electron anisotropies and intense chorus emissions indicate remarkable consistency with the confinement to 8 RE. Furthermore, as injected plasma sheet electrons drift from midnight through dawn toward the noon sector, their anisotropy increases and peaks on the dayside at 7 &lt; L &lt; 9, which is well correlated with intense chorus emissions observed in the prenoon sector. These statistical results are generally consistent with the generation of both lower‐band and upper‐band chorus through cyclotron resonant interactions with suprathermal electrons (1–100 keV). Two typical events on the nightside and dayside are studied in greater detail and additional interesting features are identified. Pancake distributions of electrons with energy below ∼2 keV, which could be responsible for the excitation of upper‐band chorus, are observed at lower L shells (&lt;7) on the nightside and at larger L shells (&gt;6) on the dayside. In addition, very isotropic distributions at a few keV, which may be produced by Landau resonance and contribute to the formation of the typical gap in the chorus spectrum near 0.5 fce, are commonly observed on the dayside.

On the fine structure of medium energy electron fluxes in the auroral zone and related effects in the ionospheric D-region

Abstract. This study is based on measurements of trapped and precipitated electrons of energy &gt;30 keV and &gt;100 keV observed by polar orbiting environmental satellites during overpasses of the imaging riometer at Kilpisjärvi, Finland. The satellites are in sun-synchronous orbits of about 850 km altitude, recording the electron fluxes at 2-s time resolution. The riometer measures the radiowave absorption at 38.2 MHz, showing the spatial pattern within a 240 km field of view. The analysis has focussed on two areas. Having found a close correlation between the radiowave absorption and the medium-energy electron fluxes during satellite overpasses, empirical relationships are derived, enabling one quantity to be predicted from the other for three sectors of local time. It is shown that small-scale variations observed during a pass are essentially spatial rather than temporal. Other properties, such as the spectra and the relation between precipitated and trapped components, are also considered in the light of the theory of pitch angle scattering by VLF waves. It is found that the properties and behaviour depend strongly on the time of day. In the noon sector, the precipitated and trapped fluxes are highly correlated through a square law relationship.

Experimental evidence of direct penetration of upstream ULF waves from the solar wind into the magnetosphere during the strong magnetic storm of November 9, 2004

Upstream ULF waves were recorded by the ACE satellite, while the Pc5 ULF waves (geomagnetic pulsations) were simultaneously observed by the GOES12 satellite in the magnetosphere during the big magnetic storm on November 9, 2004. Their spectra showed similar structures with the maximum frequency band around 2.20–2.44 mHz. The upstream ULF waves were highly correlated with the Pc5 waves with a correlation coefficient of 0.82 for a time lag of 28.5 min. Both the interplanetary and the magnetospheric waves had intense radial components with very large-amplitude δB r / B>40%, which indicated characteristic of the compressional waves. Furthermore, variations of the plasma density were in phase with those in the magnetic field, suggesting that they were essentially magnetosonic waves. The results demonstrated that upstream magnetosonic waves in the solar wind can directly penetrate into the magnetosphere and cause the Pc5 ULF waves during a big magnetic storm. The penetration of the magnetosonic waves seems to be occurred merely around the noon sector.

Geomagnetic disturbances on ground associated with particle precipitation during SC

Abstract. We have examined several cases of magnetosphere compression by solar wind pressure pulses using a set of instruments located in the noon sector of auroral zone. We have found that the increase in riometric absorption (sudden commencement absorption, SCA) occurred simultaneously with the beginning of negative or positive magnetic variations and broadband enhancement of magnetic activity in the frequency range above 0.1 Hz. Since magnetic variations were observed before the step-like increase of magnetic field at equatorial station (main impulse, MI), the negative declinations resembled the so-called preliminary impulse, PI. In this paper a mechanism for the generation of PI is introduced whereby PI's generation is linked to SCA – associated precipitation and the local enhancement of ionospheric conductivity leading to the reconstruction of the ionospheric current system prior to MI. Calculation showed that PI polarity depends on orientation of the background electric field and location of the observation point relative to ionospheric irregularity. For one case of direct measurements of electric field in the place where the ionospheric irregularity was present, the sign of calculated disturbance corresponded to the observed one. High-resolution measurements on IRIS facility and meridional chain of the induction magnetometers are utilized for the accurate timing of the impact of solar wind irregularity on the magnetopause.

Electric and magnetic field observations of Pc4 and Pc5 pulsations in the inner magnetosphere: A statistical study

Ultralow frequency (ULF) waves in the Pc4 and Pc5 bands are ubiquitous in the inner magnetosphere and have significant influence on energetic particle transport. Investigating the source and characteristics of ULF waves also helps us better understand the interaction processes between the solar wind and the magnetosphere. However, owing to the limitation in instrumentation and spatial coverage, the distribution of ULF waves in local time and L shell in the inner magnetosphere has not been completely studied. The recent Time History of Events and Macroscale Interactions During Substorms (THEMIS) mission provides unique opportunities to investigate the spatial distribution of ULF pulsations across different L shells with full local time coverage in the inner magnetosphere during solar minimum, with both electric and magnetic field measurements. Pc4 and Pc5 pulsations in the electric field observations are identified throughout 13 months of measurements, covering 24 h in local time. The pulsations are characterized as either toroidal or poloidal (including compressional) mode, depending on the polarization of the electric field. Subsequently, the pulsations' occurrence rate and wave power distributions in radial distance and local time are recorded. While the distributions of both Pc4 and Pc5 events vary greatly with radial distance and local time, Pc4 events are more frequently observed in the inner region around 5–6 RE and Pc5 events are more frequently observed in the outer region around 7–9 RE, which suggests that the field line resonance is an important source of the ULF waves. In the flank regions, the wave power is dominated by the toroidal mode, likely associated with the Kelvin‐Helmholtz (KH) instability. In the noon sector, the Pc5 ULF wave power is dominated by the poloidal mode, likely associated with the solar wind dynamic pressure disturbance. The KH instability plays an important role, suggested by our observations, during the solar minimum when the solar wind dynamic pressure is relatively weak. We also find that the contributions to the Pc5 ULF wave power from the external sources are larger than the contributions from the internal sources. These statistical results are important in characterizing Pc4 and Pc5 waves and also important for any efforts to model the transport of energetic particles in the magnetosphere.

Response of the magnetic field and plasmas at the geosynchronous orbit to interplanetary shock

Interplanetary shock can greatly disturb the Earth’s magnetosphere and ionosphere, causing the temporal and spatial changes of the magnetic field and plasmas at the geosynchronous orbit. In this paper, we use the magnetic field data of GOES satellites from 1997 to 2007 and the plasma data of MPA on the LANL satellites from 1997 to 2004 to study the properties of magnetic field and plasma (0.03–45 keV) at the geosynchronous orbit (6.6 RE) within 3 hours before and after the arrival of shock front at the geosynchronous orbit through both case study and superposed epoch analysis. It is found that following the arrival of shock front at the geosynchronous orbit, the magnetic field magnitude, as well as GSM BZ component increases significantly on the dayside (8–16 LT), while the BY component has almost no change before and after shock impacts. In response to the interplanetary shock, the proton becomes much denser with a peak number density of 1.2 cm−3, compared to the typical number density of 0.7 cm−3. The proton temperature increases sharply, predominantly on the dusk and night side. The electron, density increases dramatically on the night side with a peak number density of 2.0 cm−3. The inferred ionospheric O+ density after the interplanetary shock impact reaches the maximum value of 1.2 cm−3 on the dusk side and exhibits the clear dawn-dusk asymmetry. The peak of the anisotropy of proton’s temperature is located at the noon sector, and the anisotropy decreases towards the dawn and dusk side. The minimum of temperature anisotropy is on the night side. It is suggested that the electromagnetic ion cyclotron (EMIC) wave and whistler wave can be stimulated by the proton and electron temperature anisotropy respectively. The computed electromagnetic ion cyclotron wave (EMIC) intense on the day side (8–16 LT) with a frequency value of 0.8 Hz, and the wave intensity decreases towards the dawn and dusk side, the minimum value can be found on the night side. The computed electron whistler wave locates on the day side (8–16 LT) with a value of 2 kHz.

On the response of the equatorial and low latitude ionospheric regions in the Indian sector to the large magnetic disturbance of 29 October 2003

Abstract. The present paper investigates the response of the equatorial and low latitude ionosphere over the Indian longitudes to the events on 29 October 2003 using ionosonde data at Trivandrum (8.5° N (0.5° N geomagnetic), 77° E) and SHAR (13.7° N (5.7° N geomagnetic), 80.2° E), ground-based magnetometer data from Trivandrum and Total Electron Content (TEC) derived from GPS data at the locations of Ahmedabad (23° N (15° N geomagnetic), 72° E), Jodhpur (26.3° N (18.3° N geomagnetic), 73° E) and Delhi (28° N (20° N geomagnetic), 77° E). Following the storm sudden commencement, the TEC at all the three stations showed an overall enhancement in association with episodes of inter-planetary electric field penetration. Interestingly, real ionospheric height profiles derived using the ionosonde data at both Trivandrum and SHAR showed significant short-term excursions and recoveries. In the post noon sector, these features are more pronounced over SHAR, an off equatorial station, than those over Trivandrum indicating the increased effects of neutral winds.

Anomalous variations of &amp;lt;I&amp;gt;Nm&amp;lt;/I&amp;gt;F2 over the Argentine Islands: a statistical study

Abstract. We present a statistical study of variations in the F2-layer peak electron density, NmF2, and altitude, hmF2, over the Argentine Islands ionosonde. The critical frequencies, foF2, and, foE, of the F2 and E-layers, and the propagation factor, M(3000)F2, measured by the ionosonde during the 1957–1959 and 1962–1995 time periods were used in the statistical analysis to determine the values of NmF2 and hmF2. The probabilities to observe maximum and minimum values of NmF2 and hmF2 in a diurnal variation of the electron density are calculated. Our study shows that the main part of the maximum diurnal values of NmF2 is observed in a time sector close to midnight in November, December, January, and February exhibiting the anomalous diurnal variations of NmF2. Another anomalous feature of the diurnal variations of NmF2 exhibited during November, December, and January when the minimum diurnal value of NmF2 is mainly located close to the noon sector. These anomalous diurnal variations of NmF2 are found to be during both geomagnetically quiet and disturbed conditions. Anomalous features are not found in the diurnal variations of hmF2. The statistical study of the NmF2 winter anomaly phenomena over the Argentine Islands ionosonde was carried out. The variations in a maximum daytime value, R, of a ratio of a geomagnetically quiet daytime winter NmF2 to a geomagnetically quiet daytime summer NmF2 taken at a given UT and for approximately the same level of solar activity were studied. The conditional probability of the occurrence of R in an interval of R, the most frequent value of R, the mean expected value of R, and the conditional probability to observe the F2-region winter anomaly during a daytime period were calculated for low, moderate, and high solar activity. The calculations show that the mean expected value of R and the occurrence frequency of the F2-region winter anomaly increase with increasing solar activity.

Effects of high-speed solar wind on energetic electron activity in the auroral regions during July 1–2, 2005

This study concerns the properties and behaviour of energetic ( > 30 keV ) electrons in the magnetosphere in relation to an enhancement of the solar wind caused by the sub-Earth meridional crossing of a trans-equatorial coronal hole during late June 2005. It covers periods of slow and fast solar wind, each of about 12 h duration, separated by a rapid increase of speed on July 1st. The observations were made at about 850 km altitude by four polar orbiting NOAA spacecraft, covering a full range of local times. We select invariant latitudes from 57° to 77°, the region which includes the auroral zone where electrons of these energies are sporadically precipitated, and we consider the variations of intensity and spectrum, the relation between the precipitating and mirroring fluxes, and the relative spectral hardness of these components. In general, all properties show considerable variability, but also with significant trends. The flux of mirroring electrons was greater during the period of fast solar wind than before it, but the change was relatively gradual and the flux was decreasing again towards the end of the period although the solar wind was still fast. The spectrum was softest during the transition. The ratio of precipitating to mirroring electrons generally increased as the solar wind speeded up, though with a marked dependence on local time. The precipitating spectrum tended to be harder than the mirroring spectrum during the period of slow wind, but with the increased fluxes during the faster wind the precipitating spectrum tended to be softer. The random variability was least in the noon sector where there was a progressive hardening of both the precipitating and mirroring spectra as the flux declined towards the end of the fast wind period. The data suggest that the ratio of precipitating to mirroring flux is proportional to the mirroring flux for both the energy ranges > 30 and > 100 keV , and the characteristic energy of the precipitated spectrum is half that of the mirrored spectrum during this period. Use of a magnetospheric model suggests that a change in the character of the particle fluxes in the afternoon sector around the time of transition between slow and fast wind was due to the field lines being pulled back into the tail at a time when the pressure exerted by the solar wind was at its greatest.