Polar Absorption Research Articles

Twilight anomaly, midday recovery and cutoff latitudes during the intense polar cap absorption event of March 1991

Abstract. A study was made of the polar cap absorption (PCA) event on 23-24 March 1991 produced by the largest solar proton event at E>10 MeV since August 1972. This PCA event was related to a solar flare in the eastern hemisphere lasting only 2 days and exhibiting a long time delay between the flare and the increase of ionospheric absorption. Midday recovery occurred regularly each PCA day near the cut-off latitudes during the noontime hours and is attributed to the daily variation in the proton cut-off latitudes. The maximum absorption during the PCA event was observed at high latitudes or near the cut-off latitudes where ionization may be due to both solar protons and trapped particles. The minimum in the absorption values during the night-time hours would appear to be caused by the chemistry of the D-region as well as access of the solar protons into the polar cap area.

Twilight anomaly, midday recovery and cutoff latitudes during the intense polar cap absorption event of March 1991

A study was made of the polar cap absorption (PCA) event on 23-24 March 1991 produced by the largest solar proton event at E>10 MeV since August 1972. This PCA event was related to a solar flare in the eastern hemisphere lasting only 2 days and exhibiting a long time delay between the flare and the increase of ionospheric absorption. Midday recovery occurred regularly each PCA day near the cut-off latitudes during the noontime hours and is attributed to the daily variation in the proton cut-off latitudes. The maximum absorption during the PCA event was observed at high latitudes or near the cut-off latitudes where ionization may be due to both solar protons and trapped particles. The minimum in the absorption values during the night-time hours would appear to be caused by the chemistry of the D-region as well as access of the solar protons into the polar cap area.

Dynamics of Cusp-Latitude Absorption Events Observed by Imaging Riometers.

The recently developed imaging riometer technique has proven a valuable tool for investigations of the complex dynamics of auroral and polar radiowave absorption events. These events mostly relate to the precipitation of energetic particles into the upper atmosphere to create enhanced ionization in the D-region at low altitudes. A further mechanism which may cause enhanced ionospheric radiowave absorption is electron collision frequency enhancements due to processes like E-region heating by large electric fields. The imaging riometer observations are particularly well suited to separate spatial and temporal variations in the ionospheric phenomena causing radiowave absorption, and thus to define their extent and motions. This work will discuss the poleward expansion of substorm absorption events, and the variable daytime absorption events related to the modulated precipitation of substorm electrons. The high-latitude nightside observations support the model of substorms being a series of enhancements distributed in latitude and longitude rather than single onset, monotonously expanding events. The individual enhancements could be explained by an augmented current wedge mechanism. The dayside observations indicate that disturbances associated with a strong convection shear may cause enhanced and strongly variable precipitation of the eastward drifting cloud of high-energy electrons generated during substorms. Basis for the analysis are events observed, among other, by imaging riometers at the cusp-latitude observatories in Sdr. Stromfjord (inv. lat.= 73.7°) and Danmarkshavn (inv. lat. = 77.3°) in Greenland, and in Ny-Alesund (inv. lat. = 76.1°) at Svalbard, and at the auroral zone observatory in Tjornes (inv. lat. = 66.9).

The polar cap absorption event of 19–21 March 1990: recombination coefficients, the twilight transition and the midday recovery

For the polar cap event of 19–21 March 1990, effective recombination coefficients have been estimated at heights between 60 and 80 km by comparing the electron density, measured by the EISCAT VHF incoherent scatter radar, with production rates calculated from proton fluxes measured on the GOES-7 satellite. The estimated coefficients change considerably between day and night, the change being relatively greater at lower altitudes. During the midday recovery, for which there is clear evidence during this event, the edge of the polar cap moved over the radar, and it is thus possible to see the effect of the edge on the spectrum of precipitating protons. The spectrum hardened at this time, indicating an increase in the cut-off rigidity.

D-region observations of polar cap absorption events during the EISCAT operation in 1981–1989

During the years 1981–1989, 71 solar proton events altogether were observed. Dividing the events into strong, p.f.u. > 1000 (p.f.u.—proton flux measured at geosynchronous satellite orbit in units of (cm 2 s sr) −1), medium, 100 < p.f.u. < 1000 and weak events, p.f.u. < 100, only the strong and medium events have a considerable effect on the lower ionosphere. The mean daily absorption at 30 MHz (A), measured in the auroral zone, is >2 dB during strong events, <2 dB during medium events and < l dB during weak events. The most active year during the EISCAT operation was 1989 when 23 solar proton events were observed including six strong events. Diurnal variation of the electron density in the D-region during PCA is a function of the solar zenith angle. However, south of L = 5 a minimum in absorption is observed during the noon hours. During sunrise the absorption increases simultaneously with solar elevation angle, but during sunset there is about 2 h delay between the decrease of absorption and the solar elevation angle.

A high‐latitude ionospheric disturbance impact assessment system

The Disturbance Impact Assessment System (DIAS) is a computer software package designed to assess and predict the impact of solar flares on high‐latitude HF radio communications. The analysis spans the period between the flare onset (T0 hours) and the expected subsiding of its effects 5 days later (T120 hours). DIAS supports a HF communications system and when integrated into it, its operation will be entirely transparent to the user. A stand‐alone PC version was designed for tutorial purposes. Key features combine the capabilities of the algorithmic PROPHET HF signal assessment system with expert system technology. The initial system provides qualitative advice and warnings. The rule sets in DIAS cover the following types of disturbances: (1) sudden ionospheric disturbances, (2) polar cap absorption, (3) ionospheric storm, (4) auroral zone absorption, and (5) auroral sporadic E and auroral E.

Incoherent scatter radar observations of the middle atmosphere response to a PCA

A polar cap absorption took place on 12 August 1989 during a D-region experiment by the EISCAT UHF radar, having been first detected by the GOES-7 satellite. It is possible, with certain assumptions, to invert the electron density profiles determined by EISCAT and estimate the particle flux as a function of energy — i.e. the proton energy spectrum, at the top of the atmosphere. The EISCAT data is examined in this way; in later work the results will be compared with the satellite-measured proton fluxes.

Observations of atmospheric dynamics in the low- and middle-mesosphere with the EISCAT incoherent scatter radar during solar proton events

Profiles of meridional neutral wind velocities in the height range 54–81 km with high temporal (minutes) and spatial resolution (1 km) from the EISCAT Incoherent Scatter Radar (ISR) at 933 MHz are presented. The data, obtained during three different solar proton events causing polar cap absorption (PCA) in 1989–1990, sometimes show slowly descending structures in the velocity profile which may be the signature of waves with vertical wavelengths as low as 10 km and periods from 9–14 hours.

Conformational rod-to-coil transition and solvatochromism of PDA solution

Sovatochromism of polydiacetylenes (PDA) solutions connected with conformational transitions is considered in terms of equilibrium thermodynamics. It was ascertained that to obtain sufficiently sharp confromational and colour transition one has to take into account the cooperative character of polar solvent molecule absorption. The behavior of the Δ E g gap in electron-excitation spectrum, connected with polymer chain conformational modification, was studied in the framework of the Hückel model and a polymer chain was described by means of the one-dimensional Ising model. The proposed method can be used for explaining thermo-, solvato- and electro-chromic phenomena in other polymers with conjugated bonds, where there is a connection between the electron structure and conformation and/or chemical structure of the polymer chain, and where transitions occur in a narrow range.

Effects of the March 1989 solar activity

On Monday, March 6, 1989, a very large and complex sunspot group, Region 5395, rotated into view around the east limb of the Sun and quickly gained attention when it produced an X15/3B flare (N35, E69). The event began a period of high solar activity that lasted two weeks and had many important consequences at Earth and in near‐Earth space.From March 6–19, Region 5395 produced 11 X‐class and 48 M‐class X ray flares. Prolonged proton events occurred that lasted several days and had an unusually high proportion of lower‐energy particles. The solar activity produced an historically “great” magnetic storm, long‐lasting Polar Cap Absorption events, and a major Forbush decrease, which is a decrease of the galactic cosmic ray flux observed at Earth. The ionosphere was greatly disturbed.

Interplanetary energetic ions and polar radio wave absorption

This is a study of the ionization input of interplanetary (including solar flare) energetic protons and alpha particles into the south polar ionosphere over the interval 1982–1985. Energetic particle data are collected by the charged particle measurement experiment (CPME) of the Applied Physics Laboratory/Johns Hopkins University, onboard the IMP 8 satellite orbiting the Earth at 30–35 Earth radii. It is well known that interplanetary ions have full and prompt access to the polar ionosphere. The incremental ionization produced at 20–120 km. altitudes causes enhanced radio wave absorption which is observed by riometers operated by the University of Maryland, at South Pole, Antarctica. We compute the expected absorption from the vertical structure of the ionization deposited by these energetic particles and compare the computed values with the observations. The contribution of the alpha particles is found to be quite small as a percentage of the absorption except at the peak of the day 35, 1983, event, when their contribution to the absorption is about 0.6 dB out of a total of 3.4 dB. The dominant contribution to absorption at 30 MHz usually arises from protons below 10 MeV, specifically in the 2‐ to 4‐MeV interval. We have propagated the observed fluxes and energy spectra of protons and alpha particles through a seasonally adjusted slab model of the polar atmosphere. The atmospheric ionization resulting from the slowing and stopping of protons and alpha particles is used to estimate an equilibrium vertical ionization profile which is then convolved with an absorption efficiency profile to yield a calculated absorption. There is good agreement between the computed and observed absorption when the daily averaged absorption is above 0.1 dB; this shows that the interplanetary ions are the dominant contributors on those days. This set of observations constitutes the most complete and definitive illustration to date of the control by interplanetary particles of the shape and magnitude of the ionization profile in the lower ionosphere.

Developments in Arctic long&amp;hyphen;wave propagation theory and experiments

During 1985 a flight expedition for signal and noise measurements of long&amp;hyphen;wavelength radio waves was undertaken to the Arctic. Ground&amp;hyphen;based measurements have been made in Greenland and Norway. The data suggest the revision of long&amp;hyphen;wave coverage prediction programs to satisfy the special requirements of the Arctic. Polar cap absorption events and the structure of the Greenland ice cap must be taken into account.

Observations of the polar cap absorption event of February 1984 by the EISCAT incoherent scatter radar

Electron densities were measured in the ionospheric D-region during the polar cap event of 16 February 1984, and have been compared with production rates computed from proton fluxes measured on the geosynchronous satellite GOES-5. Profiles of effective recombination rate were determined which show a general consistency with previous estimates but differ in detail. There is evidence for a progressive change of recombination coefficient as the event proceeds. Calculations of the radiowave absorption profile from the EISCAT data are compared with measurements f the total radio absorption using riometers, and serve to indicate the height and thickness of the absorption region in the PCA.

Goodness of approximation of lower ionosphere parameters given by a theoretical model and by the International Reference Ionosphere (IRI)

A theoretical model of the ionospheric D-region has been developed, which includes four positive ions (NO +, O + 2, CI + 1, CI + 2), four negative ions (O − 2, O −, CO − 3, NO − 3) and electrons. It takes into account the dependence of the rates of the basic processes on temperature, atmospheric humidity and concentrations of the minor neutral constituents. A check with experimental data (about 1000 electron density vs. height profiles, N(h)) indicates that it is able to reproduce rather accurately the seasonal and diurnal variations of N(h), and the radiowave absorption over a large frequency range. Unlike alternate empirical models it describes more accurately the fine structure of N(h)-profiles. Another advantage of the theoretical model is its ability to reproduce N(h) profiles during disturbances: WA(winter anomaly), PCA(polar cap absorption), PSE(post storm effect) after SSC, etc.

The effective recombination coefficient of electrons in the ionosphere between 50 and 150 km

The literature on values of the effective recombination coefficient α of electrons between heights of 50 and 150 km has been critically examined. It is found that, during auroral events at night, it may be represented by the expression α = 4.30×10−6 exp (−2.42×10−2h) + 8.16×1012 exp(−0.524 h), where α is in cm −3 s−1 and h is in kilometers. During a wide variety of phenomena, including polar cap absorption, sudden phase anomalies, solar proton events and solar flares, the expressions α = 652 exp(−0.234 h) (night) and α = 0.501 exp(−0.165 h) (day) are reasonably representative between 50 and 100 km. No values are available above 100 km. The above values are certainly not reliable to within a factor of 2, and in some cases not even to within an order of magnitude.

Influence of the solar wind on auroral absorption events at very high geomagnetic latitudes.

We investigate numerous unusual appearances of auroral absorption events measured by riometers located at the center of the Southern polar cap. We have shown that such nighttime (in MLT) events take place during the expansion phase of magnetospheric substorms and are closely associated with both high velocity (V≥600km s-1) solar wind fluxes and strong negative Bz values. The polar ionosphere behaviour is also studied. A model for the propagation of the westward travelling surge was recently developed by ROTHWELL et al. (1984). This model seems to be an appropriate tool for describing the events if one takes into account the solar wind velocity.We believe that the daytime polar cap auroral absorption events which are not as numerous and intense as their nighttime counterparts have a different origin.

High‐latitude mesopause neutral winds and geomagnetic activity: A cross‐correlation analysis

A cross‐correlation analysis of horizontal winds at 87 km obtained by the Poker Flat mesosphere‐stratosphere‐troposphere (MST) radar and College magnetometer horizontal component fluctuations has been performed on 3 years of summertime data. No consistent, significant peak in the average 4‐day estimate cross‐correlation results is apparent from year to year. Individual estimates similarly do not show evidence of a peak in cross correlation at a variable time lag during geomagnetically active intervals. In addition, the 1‐day 24‐ and 12‐hour wave amplitudes and phases and average zonal and meridional winds do not vary significantly and consistently with the daily averaged magnetometer horizontal component fluctuations or with the standard deviation thereof. The interval following the July 13, 1982, polar cap absorption event and geomagnetic storm, however, shows a different cross‐correlation estimate behavior and is characterized by significantly more westward zonal winds and enhanced 24‐hour zonal wind amplitude.

Time-dependent ensemble Monte Carlo simulation for planar-doped GaAs structures

A time-dependent ensemble Monte Carlo simulation has been developed which includes the computation of the self-consistent electric field and electron concentration. The Monte Carlo code is used to study transient and steady-state transport in planar-doped-barrier (PDB) GaAs diodes and transistors. Results are presented for the temporal and spatial evolution of electronic transport through the PDB. We show that (i) average velocities achievable with a PDB are significantly above the steady-state velocities by about a factor of 3, (ii) the rise time of the electric current in the PDB is in the subpicosecond range for diode length below 2000 Å, (iii) a pronounced displacement current dominates the transient electronics at onset of injection, and (iv) polar optical phonon absorption contributes significantly to the thermionic-emission current over the barriers.

Selection of the Mobile Phase for Enantiomeric Resolution via Chiral Stationary Phase Columns

Abstract The optimization of enantiomeric resolution by mobile phase variation was studied with the chiral stationary phase derived from R-N-(3,5-dinitrobenzoyl) phenylglycine covalently coupled to 5 μm spherical 3-aminopropyl silica. Chromatography was routinely performed with mobile phase compositions having polarities as high as 2.5 without column deterioration. The relative strength of a solvent as a hydrogen acceptor was found to be an important basis for selection of the polar component in a binary mobile phase. The substitution of tert-butanol for 2-propanol or ethanol in an alcohol/hexane mixture, for example, afforded improved separation factors with several enantiomers. In addition, the need for a polar mobile phase such as 50/50 methylene chloride/hexane to minimize non-specific polar absorption of enantiomers has been demonstrated. Enhancement of specific chiral interactions and suppression of interfering reactions have been obtained with a number of clinically relevant derivatives as model co...

VLF reflection properties of the normal and disturbed polar ionosphere in northern Greenland

This paper describes long‐wave reflection properties of the high‐latitude ionosphere, observed under both quiet and highly disturbed conditions. The data were obtained using a short‐pulse VLF ionosounder located within the polar cap, in northern Greenland. Under quiet ionospheric conditions, substantial diurnal, day‐to‐day, seasonal, and solar cycle variations were observed. During polar cap absorption events the VLF reflection properties of the disturbed ionosphere depended greatly on the solar illumination conditions, as well as the magnitude of the incoming energetic particle flux. The use of the steep‐incidence VLF reflection data to develop and validate models of the lower ionosphere is discussed; and electron density profiles are given, derived from reflection data obtained during quiet and disturbed conditions.