A correlation study between time-weighted magnetic indices and the high latitude ionosphere

Time weighted accumulations of the ap, AE and PC magnetic indices have been here analysed together with the hourly values of the critical frequency of the F2 ionospheric layer, foF2, coming from several ionospheric stations located in the European longitudinal sector at geographic latitudes ranged between 60°N and 68°N. The preliminary results obtained for different seasons and for different solar activity conditions indicate that the response time of the high latitude ionosphere to the magnetic activity is of the order of about 15 and/or 20 hours depending on the magnetic index considered.     

Response of low latitude ionosphere to the geomagnetic storm of May 30

Response of low latitude ionosphere to the geomagnetic storm of May 30, 2005 in the Indian longitude sector has been investigated by using the GPS data recorded at three stations namely, Udaipur, Hyderabad and Bengaluru. The event is noteworthy due to the fact that the Z component of interplanetary magnetic field (IMF Bz) remained southward for about 10 hours, coincident with the local day time for the Indian longitude sector, along with significantly higher values of AE and ASY-H indices. However, we neither found any evidence for the presence of long lasting storm time electric fields nor could we infer episodes of eastward-westward penetration of electric fields under steady southward IMF Bz and unsteady ring current conditions. On the storm day, the maximum enhancement in the total electron content has been found to be about 60%. The ionosonde observations also showed increased critical frequency (foF₂) and the height (h_PF₂) of the F layer. The foF₂ was enhanced by ∼60% which is consistent with the enhancement in total electron content. The slow rise and long duration enhancement of h_PF₂ and foF₂ have been attributed to the upwelling by the meridional neutral winds, caused by continuous energy inputs at higher latitudes. The poleward expansion of the equatorial ionization anomaly has also been observed on May 30. On May 31, the following day of the storm, significantly suppressed anomaly with near absence of its northern crest in the Indian longitude sector, revealed the effect of storm induced disturbance dynamo electric fields.     

Model of photoelectron impact ionization within the high latitude ionosphere at Mars: Comparison of calculated and measured electron density

The production rate, ion density and electron density are calculated between longitudes 0° and 360° E due to incident radiation of wavelength range 1-102.57 nm in the dayside atmosphere of Mars. These calculations are made by using global analytical yield spectrum (AYS) model at solar zenith angle 80° between latitudes 50° and 70° N for spring equinox and medium solar activity condition. These conditions are appropriate for Mars Global Surveyor (MGS) Phase 2 aerobraking period during which both the accelerometer and the radio occultation data are used. The calculated results are compared with MGS radio occultation measurements carried out at different latitudes (64.7°-67.3° N) and longitudes (0°-360° E) in December 1998 between solar zenith angle 78° and 81°. This measurement shows primary and secondary ionization peaks, which are varying with longitudes. Our calculation suggests that first peak is produced by photoionization and photoelectron impact ionization processes due to absorption of solar EUV radiation (9-102.57 nm). The second peak is produced by photoelectron impact ionization of soft X-ray photon (1-9 nm). There is a good agreement between our calculation and measurement as far as the maximum and the minimum values of primary peak altitude/peak density of electrons are concerned. However, the calculated values of secondary peak density and peak altitude are higher than the measured values by a factor of 1.5-2.0 and 1.1, respectively. The secondary peak is brought into agreement with the measurement using low X-ray flux by a factor of 2 to 3 below 9 nm. The longitudinal distribution of calculated and measured peak density and peak altitude are fitted by least-square method with 0.95 confidence limits.     

Time of flight calculations for high latitude geomagnetic pulsations

High latitude geomagnetic field lines differ significantly from a dipole geometry. Time of flight calculations using the Mead-Fairfield (1975) model of the geomagnetic field are presented for different tilt angles and K_p conditions. Typical standing wave periods of geomagnetic pulsations are estimated for three different magnetospheric cold plasma regions, corresponding to waves guided in (i) the plasmatrough, (ii) the extended plasmasphere and (iii) regions of enhanced proton density (detached plasma) within the plasmatrough.Pc4/5 pulsation studies at high latitudes are briefly reviewed and some new results from Tromso are given. Many of the observations reveal hydromagnetic waves whose location and period are consistent with ducting in a region of enhanced plasma density within the plasmatrough.     

The consequences of high latitude particle precipitation on global thermospheric dynamics

A previous comparison of experimental measurements of thermospheric winds with simulations using a global self-consistent three-dimensional time-dependent model confirmed a necessity for a high latitude source of energy and momentum acting in addition to solar u.v. and e.u.v. heating. During quiet geomagnetic conditions, the convective electric field over the polar cap and auroral oval seemed able to provide adequate momentum input to explain the thermospheric wind distribution observed in these locations. However, it seems unable to provide adequate heating, by the Joule mechanism, to complete the energy budget of the thermosphere and, more importantly, unable to provide the high latitude input required to explain mean meridional winds at mid-latitudes. In this paper we examine the effects of low energy particle precipitation on thermospheric dynamics and energy budget. Modest fluxes over the polar cap and auroral oval, of the order of 0.4 erg cm ⁻²/s, are consistent with satellite observations of the particles themselves and with photometer observations of the OI and OII airglow emissions. Such particle fluxes, originating in the dayside magnetosheath cusp region and in the nightside central plasma sheet, heat the thermosphere and modify mean meridional winds at mid-latitudes without enhancing the OI 557.7 line, or the ionization of the lower thermosphere (and thus enhancing the auroral electrojets), neither of which would be consistent with observations during quiet geomagnetic conditions.     

The measurement of ozone concentrations at high latitude during the twilight

The ozone height profile in the Arctic, at the end of the winter, has been measured up to an altitude of 100 km using a combined solar occultation and 1.27 μ oxygen emission technique. The typical two layer structure has been observed with a high altitude minimum near 80 km and a maximum at 86 km. The measured concentration in this ozone bulge was 5.1 × 10⁷cm^?3, typical of that measured at 52°N for the summer months. It is suggested that this reduced ozone concentration may have been associated with a stratospheric warming event that was in progress at the time of the measurement.     

Rocket observations of middle latitude sporadic E, magnetic fields,winds and ionization

Two Skylark rockets carrying ion and R.F. electron probes, lithium and sodium vapourisers and proton precession magnetometers were launched from Woomera, Australia in November and December 1965 and made at least four encounters with sporadic E ionization. A magnetic field minimum was detected on only one of these encounters, and the minimum was found to be 2–3 km below the observed ion layer. The wind measurements deduced from observations of the vapour trails indicated that the sporadic E layer occurred in a region of ion divergence.     

Location and source of ionospheric high latitude troughs

One prominent feature of the high latitude topside ionosphere is the existence of sharp latitudinal depletions in the total ion (electron) concentrations within the auroral/cusp regions. These high latitude troughs, as seen by the Bennett ion mass spectrometer observations on the satellite OGO 6 at altitudes between 400 and 1100km correspond to depletions in the atomic ions which are accompanied by localized enhancements of the minor molecular ion densities. All of the high latitude troughs traversed by OGO 6 (1969–1970) were recorded and the average invariant latitude-magnetic local time (M.L.T.) distribution was determined. The troughs on the average were found at all local times to be in the vicinity of the auroral oval and to move equatorward in response to increasing magnetic activity. The average trough location was compared to the average polar cap boundary as defined by the convection electric field reversal and the electron trapping boundary as well as to the maximum horizontal magnetic disturbance associated with the large scale field aligned currents. The high latitude troughs on the average best followed the maximum magnetic disturbance distribution. It is concluded that the troughs are the result predominantly of enhanced chemical 0⁺ losses in regions with high convection velocities.     

Large-scale structures of high galactic latitude dark clouds

Abstrct The obscuring material distribution of an area of about 2000 square degrees was derived by analyzing about 130 plates, using the starcounts method.Paper presented at the IAU Third Asian-Pacific Regional Meeting, held in Kyoto, between 30 September–6 October, 1984.     

The high latitude circulation and temperature structure of the thermosphere near solstice

The neutral gas temperature and circulation of the thermosphere are calculated for December solstice conditions near solar cycle maximum using NCAR's thermospheric general circulation model (TGCM). High-latitude heat and momentum sources significantly alter the basic solar-driven circulation during solstice. At F-region heights, the increased ion density in the summer hemisphere results in a larger ion drag momentum source for the neutral gas than in the winter hemisphere. As a result there are larger wind velocities and a greater tendency for the neutral gas to follow the magnetospheric convection pattern in the summer hemisphere than in the winter hemisphere. There is about three times more Joule heating in the summer than the winter hemisphere for moderate levels of geomagnetic activity due to the greater electrical conductivity in the summer E-region ionosphere.

The results of several TGCM runs are used to show that at F-region heights it is possible to linearly combine the solar-driven and high-latitude driven solutions to obtain the total temperature structure and circulation to within 10–20%. In the lower thermosphere, however, non-linear terms cause significant departures and a linear superposition of fields is not valid.

The F-region winds at high latitudes calculated by the TGCM are also compared to the meridional wind derived from measurements by the Fabry-Perot Interferometer (FPI) and the zonal wind derived from measurements by the Wind and Temperature Spectrometer (WATS) instruments onboard the Dynamics Explorer (DE−2) satellite for a summer and a winter day. For both examples, the observed and modeled wind patterns are in qualitative agreement, indicating a dominant control of high latitude winds by ion drag. The magnitude of the calculated winds (400–500 m s⁻¹) for the assumed 60 kV cross-tail potential, however, is smaller than that of the measured winds (500–800 m s⁻¹). This suggests the need for an increased ion drag momentum source in the model calculations due to enhanced electron densities, higher ion drift velocities, or some combination that needs to be further denned from the DE−2 satellite measurements.     

On a mechanism for the formation of VLF electrostatic emissions in the high latitude F-region

The presence of highly anisotropic ion velocity distributions in the weakly-ionized plasma of strongly convecting areas of the high latitude F-region leads to the excitation of electrostatic microinstabilities (λ ～ 50 cm) at frequencies of the order of the lower hybrid frequency and smaller. We have estimated the threshold conditions for the excitation of the unstable waves under various physical circumstances. For some representative conditions we have also calculated the frequencies, growth rates, and wavelengths for the fastest growing modes using the linear approximation. We stress that the present theory breaks down in regions where the plasma cannot be treated as locally homogeneous. The altitude range over which the theory is applicable also varies with conditions. For highly disturbed conditions the upper altitude limit may be as high as 400 km.     

Study of total electron content variations over equatorial and low latitude ionosphere during extreme solar minimum

In recent years with the advancement in satellite based navigational applications, study of Total Electron Content (TEC) has gained significant importance. It is well known that due to dynamical behaviour of equatorial and low latitude ionosphere, the levels of ionization is relatively high herein. The sustained decrease in solar extreme ultraviolet radiations during the current minimum is greater than any in recent history. This gives us the opportunity to study the observations of global positioning system total electron content (GPS-TEC) dual frequency signals from the GPS satellites continuously recorded at Trivandrum (an equatorial station) and Delhi (a low latitude station) during the extremely low solar activity period from January 2007 to June 2009. This study illustrates the diurnal, seasonal and annual variations of TEC during the extended solar minimum period. This study also investigates the behaviour of daytime ionosphere around spring and autumn equinoxes at low solar activity period. The results clearly reveal the presence of equinoctial asymmetry which is more pronounced at equatorial station Trivandrum. The diurnal variation of TEC shows a short-lived day minimum which occurs between 0500 to 0600 LT at both the stations. Delhi TEC values show its steep increase and reach at its peak value between 1200 and 1400 LT, while at the equator the peak is broad and occurs around 1600 LT. Further, the daily maximum TEC ranges from about 5 to 40 TEC units at Trivandrum and about 10 to 40 TEC units at Delhi, which correspond to range delay variations of about 1 to 8 m at the GPS L1 frequency of 1.575 GHz. The Maximum values of TEC were observed during spring equinox rather than autumn equinox, showing presence of semi annual variation at both the locations. The minimum values of TEC were observed during the summer solstice at Trivandrum indicating the presence of winter anomaly at equatorial region while Delhi TEC values were minimum during winter solstice showing absence of winter anomaly. Also the TEC values at both the locations have been decreasing since 2007 onwards exhibit good positive correlation with solar activity.     

A high galactic latitude survey of far-ultraviolet excess objects

Optical spectra have been obtained for a selection of objects included in a catalog of far ultraviolet bright, high galactic latitude objects detected with a balloon-borne survey telescope. The observed objects provide a sample of subdwarf O and B stars, white dwarfs, and binary systems including a hot subluminous member. Model atmospheres analysis of the subdwarf sample is used to determine the temperature, gravity, and helium to hydrogen ratio of the individual objects. A smooth distribution of objects is found on the gravity versus temperature diagram near the theoretical location of the extended horizontal branch. A break between the helium rich and helium poor objects occurs at 40,000 K. Suspected binary objects were found and analyzed to determine the temperature and gravity of the hot subluminous member in each system. The number of subdwarf stars contained in binaries is determined to be from 65% to 100%. The number versus ultraviolet magnitude distribution of the subdwarf B sample is modeled to obtain a midplane density of 3.3 10(-6) pc-3 and a population scale height of 240 pc. The proportion of white dwarfs that experience the subdwarf phase of evolution is found to be 0.94%.     

The probable extragalactic nature of some low surface brightness clouds at high galactic latitude

Low surface brightness clouds observed at optical, infrared, and radio wavelengths are discussed. We present evidence that some clouds at high galactic latitudes are associated with the Local Group, M81, and possibly even with higher redshift extragalactic objects.Low temperature clouds at high latitude must affect at some level the short wave length side of the cosmic background radiation. If some of these clouds are extragalactic there should be a further effect on the interpretation of CBR measures.     

The lower ionosphere of Venus

Galactic cosmic ray bombardment provides a permanent background ionosphere in planetary atmospheres. A transport technique is used to compute the cosmic ray ionization rate profile in a model of the Venusian atmosphere at altitudes between 55 and 100 km. These ionization rates are then applied to a model of ion chemistry to predict equilibrium electron and ion density profiles. Ionization rates for typical solar flare proton events are available from earlier calculations and have been included.     

Triple splitting with the F2-region of the ionosphere at high and mid-latitudes

The occurrence of the third (z-ray) component of the F2-trace on ionograms is investigated at high- and mid-latitudes. Diurnal variations show a systematic shift, with magnetic inclination, of the time of maximum occurrence. Seasonal variations show a winter maximum, and an inverse sunspot-cycle relationship exists. Maximum occurrence appears between a magnetic inclination of 70° and 80° with a fall-off either side.

Evidence is presented to suggest a z-ray association with “Spread-F” fronts, and a possible mechanism for the recording of the z-ray trace at the transmitter site is described. This involves longitudinal propagation of the o-mode at its normal reflection level, coupling at this point, and ultimate reflection for the z-ray mode as a result of sloping ionization contours belonging to “Spread-F” fronts extending in directions perpendicular to the magnetic meridian.

An association with V.L.F. emissions (“dawn-chorus”) is discussed.     

Conjugate measurements of VLF transmitter signals at middle latitude (L = 1.93)

Whistler-mode signals were measured at the conjugate point (L = 1.93) in South Australia of the VLF transmitter in Eastern U.S.S.R. The intensity of whistler-mode signals increased usually at sunset and occasionally at sunrise, and was moderate during night-time. By means of the calculation of the wave attenuation in the lower ionosphere, the result may be interpreted in terms of different mechanisms of propagation in the magnetosphere and transmission out of the ionosphere: spatially coherent propagation in a field-aligned duct and transmission out of the duet onto the ground at sunset and sunrise; and spatially incoherent propagation and transmission at night. A high correlation was found between the occurrence probability of the 14.881 kHz whistler-mode signals and the occurrence number of whistlers, which may suggest that the occurrence of whistlers is controlled predominantly by propagation conditions.     

The lower ionosphere of Titan

Ionization of the atmosphere of Titan by galactic cosmic rays is a very significant process throughout the altitude range of 100 to 400 km. An approximate form of the Boltzmann equation for cosmic ray transport has been used to obtain local ionization rates. Models of both ion and neutral chemistry have been employed to compute electron and ion density profiles for three different values of the H₂/CH₄ abundance ratio. The peak electron density is of the order 10³ cm^?3. The most abundant positive ions are C₂H₉⁺ and C₃H₉⁺, while the predicted densities of the negative ions H^? and CH₃^? are very small (<10^?4 that of the positive ions). It is suggested that inclusion of the ion chemistry is important in the computation of the H and CH₃ density profiles in the lower ionosphere.