Polar Cap Absorption Research Articles

Note on the characteristics of sunspot groups which produce solar proton flares

Solar proton flares are associated with sunspot groups which show an unusual distribution of magnetic polarities. Furthermore, the gradient of the magnetic field is very large before the onset of these flares. The importance of polar cap absorptions, which is proportional to the integral flux of solar cosmic rays, tends to increase as the gradient of the magnetic field becomes greater. It is shown that the formation of such gradients is associated with the rotating motion of sunspot groups. Hence, the sunspot groups which show a reversed polarity distribution are very effective for the production of solar proton flares.

Measurement of the effective electron loss rates in theDregion during polar cap absorption events

Day and nighttime effective electron loss rate measurement in D region during polar cap absorption events, using rocket-borne spectrometers and Faraday rotation

The Effects of Ions on Very‐Low‐Frequency Propagation During Polar‐Cap Absorption Events

The propagation of electromagnetic waves in the VLF range in the earth‐ionosphere waveguide has been theoretically analyzed. The results of full‐wave calculations indicate that transpolar VLF waves suffer considerably more attenuation during a moderately strong polarcap absorption (PCA) event than during the undisturbed daytime. Computed height profiles of dissipation and field strengths show clearly that this increased attenuation is due mainly to ohmic losses associated with ion heating. Calculations based on a model of a relatively weak PCA event indicate no pronounced increase in attenuation at most VLF frequencies. For this model, ions are found not to be an important factor. The calculations do not include the possible effects of the Greenland ice cap. During a moderately strong PCA event, the height range over which long‐path VLF waves interact strongly with the ionosphere is substantially lower than in the undisturbed daytime. For all ionospheric models considered, the width of the height range of substantial wave‐ionosphere interaction at a given frequency was less than about 20 km. VLF probing experiments must therefore use several frequencies and incidence angles if a broader ionospheric height range is to be sampled effectively.

Solar cosmic ray ionization in the low ionosphere

The enhanced electron production rate q( h) produced by solar cosmic rays (CR) in the lower ionosphere is theoretically considered in this paper. Formulas are deduced for q( h) of relativistic and lowenergetic solar CR (polar cap absorption event). Expressions are obtained at C β cos β θ angular distribution of the penetrating particles, from which formulas are derived for fully anisotropic ( β → ∞), anisotropic and isotropic ( β = 0) distributions on the upper hemisphere. Besides the influence of the parameters of solar CR (spectrum, composition: ρ, α, L, M, H, VH—nuclei) and Earth environment on q( h) is investigated.

A High-Sensitivity Particle Spectrometer for the Measurement of Polar-Cap-Absorption Events

A hybrid particle spectrometer possessing high-sensitivity and high-time resolution for the measurement of precipitating protons, electrons, and alpha particles during Polar-Cap-Absorption (PCA) events has been successfully operating in space aboard the OV1-18 satellite since 23 March 1969. Detection of protons in the energy range 1.2 to 46 MeV and integral above 70 MeV, electrons in the range 0.4 to 1.9 MeV and alpha particles in the range 7 to 20 MeV is performed with sensitivities as high as 0.1 particle/cm2-sec and with a dynamic intensity range of six decades through the use of a transmission solid-state detector in conjunction with a total-energy, plastic-scintillation detector. Acceptable pulses from each detector, as determined by various combinations of coincidence logic, are analyzed with separate 15-channel pulse-height analyzers of the analog-to-digital type. A detailed description of the spectrometer, including a unique biasing technique which maintains a constant potential across the solid-state detector, independent of leakage variations due to temperature and radiation damage, will be given. Some typical characteristics of the particles measured during several PCA events will also be presented.

Penetration of Solar Particles to Ionospheric Heights at Low Latitudes

THE effects on the absorption of radio waves in the lower ionosphere by the extra solar ionizing radiation during a flare are well known. During a flare, particles such as protons, electrons and some heavy charged nuclei are also ejected from the solar chromosphere1. Because charged particles are guided by the Earth's magnetic field, most of the terrestrial manifestations of the solar particles, such as polar cap absorption (PCA), auroras and so on, are high latitude phenomena.

VLF step frequency sounding of the polar lower ionosphere

The lower ionosphere above Byrd Station, Antarctica, is probed using radio waves over the frequency range 3.0 kHz to 30 kHz. A new phase radar technique is used to determine an accurate phase height of reflection for a propagation path near vertical incidence. The results of the first seven months of synoptic sounding of the polar D region are presented. The analysis of these data has yielded the average phase height behavior during periods of total darkness, continuous sunlight, and day-night, night-day transition, under both quiet and disturbed magnetic conditions. The dominant ionization production mechanism is related to energetic particle precipitation and is only slightly affected by solar ultraviolet radiation. Phase height measurements obtained during a large polar cap absorption event are also shown.

Interpretation of XUV spectroheliograms

Some parameters of chromospheric structure are drawn from recently published XUV spectroheliograms. The HeII emission above the limb arises from the small amount of He+ still existing at 106°. The larger amounts of He+ in the cooler corona at the poles explain the polar cap absorption in λ 304. The flat distribution of emission in Oiv and Ov, with a sharp spike at the limb, is caused by the rough structure of the chromosphere and the variable excitation in the emitting spicules. The intensity of the Nevii lines shows that the transition zone between chromosphere and corona is very sharp.

Effects of polar cap absorption events on VLF transmissions

Following a brief review of Hakura's model of a polar cap absorption event (PCA), VLF instrumentation, and data reduction problems, eleven PCAs are studied for their effects on phase and amplitude of VLF transmissions. The eleven PCAs occurred between 24 March 1966 and 6 June 1967. VLF phase anomalies are always advances (Δφ), the magnitude of which increases with increasing geomagnetic latitude of the path, decreasing signal frequency, and decreasing electrical ground conductivity. Signal strength falls by more than 15 dB/1000 km for paths with very low electrical ground conductivity (Greenland and Antarctic Ice Cap), whereas it may increase by a few dB for polar paths over sea water. It is shown that normalization of (Δφ), with respect to frequency and affected length, leads to two distinct linear relationshipsbetween (Δφ) and (log log F/ F 0, where F is the proton flux as measured by satellites. The line of lower slope applies to paths over a highly conducting boundry (sea water), and the line of higher slope to a poorly conducting boundary (ice). The minimum detectable proton flux is 0·5 protons/cm 2 sec ster. The (Δφ)-(log log F/ F 0) relationship implies a decreased sensitivity of (Δφ) to changes in F for larger values of F, an almost uniform VLF reflection height over the polar cap, little influence on reflection height of the shape of the flux spectrum during peak flux, and an upper limit of approximately 25 μsec for a PCA phase anomaly on NPM (26·1 kHz)-Kiruna. Occasionally, irreversible cycle advances were observed on NPG-STO and WWVL-STO. The former are interpreted in terms of enhanced mode conversion and the latter in terms of the tracking of an interfering VLF signal. An event on 13 December 1966 is believed to have been due to electron precipitation from the outer radiation belt.

Survey of Polar and Auroral Region Effects on HF Propagation

The effects of the auroral and polar zone ionosphere on forward propagation in the HF through low‐VHF (3 to ∼30 MHz) portion of the spectrum are reviewed. Phenomena that particularly affect the propagation of HF/VHF signals in this region include: the non‐great‐circle (NGC) mode, sporadic‐E ionization, F1‐layer effects, F‐region irregularities, D‐region absorption, and reflectivity properties of the polar ice caps. Specifically, we show that: (1) The NGC mode can cause deviations from the great‐circle path up to 90° and can carry the maximum frequency on a polar circuit. (2) Sporadic‐E ionization occurs primarily during the winter‐night period and is sometimes associated with the aurora. (3) F1‐layer effects are quite important and frequently determine the dominant propagation mode during summer mornings at sunspot minimum. (4) Auroral zone absorption is quite ‘patchy’ in spatial and temporal behavior and probably attenuates HF signals up to 60 db in severe cases. (5) Polar cap absorption, although a relatively rare event, can attenuate HF signals up to 100 db at times. (6) For multi‐hop modes, the relatively high attenuation for reflection on the ice caps should be considered in the transmission loss equation. Research performed during the decade ending in 1968 is emphasized, including some quite recent results of investigations conducted at the Geophysical Institute at the University of Alaska, not previously reported in the literature.

Associative detachment in the mesosphere and the diurnal variation of polar-cap absorption

Polar-cap absorption recorded by a variety of high-latitude riometers during the event of September 2–6, 1966, is compared with satellite measurements of solar protons over the polar caps in four different energy ranges. The daytime absorption measurements were found to have a very high correlation with the fluxes of protons in the two higher energy ranges (8·2–25 and 25–100 MeV), but a poor correlation with fluxes in the two lower energy ranges (1·2–2·2 and 2·2–8·2 MeV). The nighttime absorption measurements, on the other hand, showed a considerably smaller degree of correlation with the higher energy proton fluxes. These results are shown to be consistent with the suggestion that atomic oxygen plays an important part in maintaining high electron densities in the lower ionosphere through associative detachment reactions.

On ionization in the ionospheric D-region by galactic and solar cosmic rays

Electron production rate q and electron density N produced by cosmic rays (CR) in the lower part of the D-region are theoretically considered in this paper. On the basis of the obtained formulas q and N are calculated from galactic CR for four geomagnetic latitudes 0°, 30°, 41°, 55° at maximum and minimum solar activity in winter and summer. The ionization by solar CR is considered—the so called polar cap absorption event. Solutions of q( h) are obtained in the cases of flat and spherical Earth and calculations are made for clearing up the contribution of the protons, α-particles and H-nuclei ( Z = 10) in the rate of total electron production.

Search for Tropospheric Responses to Chromospheric Flares

Surface atmospheric pressures are studied for periods before and after chromospheric flares by the superposed epoch method for 31 meteorological stations in North America. The individual analyses show no statistically significant pressure departures following the flares. No evidence is found for claims that continental and coastal stations exhibit different responses to the flares. No evidence is found for claims that atmospheric pressure responses to flares are amplified with increasing latitude. The composite curve for North American stations shows no statistically significant departures and does not verify the composite curve of previous investigators. Chromospheric flares with polar cap absorption (PCA) events and chromospheric flares without PCA events are indistinguishable in the response of surface atmospheric pressure in the polar cap, failing to support the claims of previous investigators. All the departures observed in this study of North American data can be explained by the ordinary transient meteorological variations of the pressure field.

Sunspot changes following proton flares

Although the area of some sunspot groups declines suddenly after a flare that produces energetic protons, other groups show a delayed or gradual decline, or continue to grow for several days after the flare. The mean area of sunspot groups that produced flares with polar-cap absorption declines gradually and continuously, beginning within a day after the flare.

Midday recoveries of polar cap absorption

Riometer observations of midday recoveries of polar cap absorption (PCA) show that: (a) the recoveries peak in the local time range of 0800 to 1500, mostly between 1000 and 1200 LT; (b) the recoveries are most pronounced at the edge of the polar cap and not evident deep inside the polar cap; and (c) the midday recovery region remains at the edge of the polar cap as the polar cap expands during a magnetic storm. Some satellite data indicate that midday recoveries are the result of the development of an anisotropic pitch-angle distribution of the solar protons, rather than a simple increase of cutoff rigidity. The anisotropy is of the nature of a depleted loss cone; in addition, the 90° pitch-angle component is apparently reduced below its isotropic, non-midday recovery value. Model calculations of absorption using a sin³ θ pitch-angle distribution suggest that anisotropy, without appreciable increase of vertical cutoff energy, can cause midday recoveries of the observed magnitudes. Two outstanding features of midday recoveries yet to be explained are their presence during only about 20% of PCA events, and the fact that midday recoveries are usually pronounced on only one day of an event.

A two-ion D-region model for polar cap absorption events

The usual model of the chemistry of the lower ionosphere is inadequate to explain many observed phenomena. When this model is updated by the addition of associative detachment (O 2 − + O → O 3 + e) and collisional detachment by excited O 2( O 2 − + O 2( 1Δ g) → 2 O 2 + e) , the disagreement between theory and experiment becomes larger instead of smaller. In this paper we postulate the existence of a second (unidentified) negative ion, X −, formed symbolically by O 2 − + X → X − + O 2, with rate v x . The product v x [ X] is calculated as a function of altitude from daytime PCA data, and the two-ion model is then applied to twilight and nighttime PCA conditions and to nighttime nuclear burst conditions. Adequate agreement between theory and experiment is obtained for all conditions. The existence of a second negative ion, therefore, seems well supported by this study, but the identity of X − remains unknown.

Correction to paper by F. C. Michel and A. J. Dessler, ‘Physical significance of inhomogeneities in polar cap absorption events’

Correction to paper by F. C. Michel and A. J. Dessler, ‘Physical significance of inhomogeneities in polar cap absorption events’

A Lunar-Month Modulation of Large Solar-Terrestrial Disturbances

Dodson and Hedeman discovered an unexpected effect in the occurrence of solar proton events as revealed by polarcap absorption (PCA). When the 48 events in Bailey’s Catalog of the Principal PCA Events, 1952-1963 are distributed with the phase of the moon there is a gap of several days near full moon; also, many more events occur when the moon waxes than when it wanes. Dodson and Hedeman did not find similar, apparent departures from random distribution either with a mean solar rotation period of 27.3 days or for solar flare events. They concluded that ‘at the present time it is not clear whether the 29.5 day “effect” is related to the sun or the moon or is only a statistical accident’.

The influence of the geomagnetic tail on low-energy cosmic-ray cutoffs

It has been known for several years that geomagnetic cutoffs for low-energy solar protons are anomalously low, even during periods of low geomagnetic activity. Several independent groups of measurements of cutoffs have been obtained by satellites and are in general agreement with the results of more indirect measurements of polar-cap absorption by riometers. The ‘midday recovery’ phenomenon observed by riometers near the low-latitude edge of polar-cap absorption has been interpreted as the result of a diurnal variation in local cutoff. This paper attempts to relate the depression of low-energy cutoffs to the permanent existence of a geomagnetic tail, and an approximate model calculation of cutoffs in the presence of the tail indicates that both the permanent depression of cutoffs and the PCA midday recovery can be satisfactorily explained quantitatively on this basis. It is suggested that the midday recovery phenomenon is intimately related to the pronounced diurnal variation in the boundary of the stable trapping region for energetic electrons in the geomagnetic field.

Synodic tendencies in the time series of fmin at South Pole, Antarctica

At least three contemporaneous sequences of synodic polar cap absorption events (PCA) have been identified in the time series of ionosonde fmin obtained at the Antarctic station, South Pole, at maximum epoch of the recent solar cycle (1958). The sequence synodic periods (27·45, 27·45, and 27·2 days) are characteristic of the rotation of moderately low latitude regions of the sun. These results support the interpretation of similar synodic sequencies which occurred in 1961, associated with MeV proton streams in interplanetary space. Due to the proliferation of solar active regions at sunspot maximum, several proton streams may exist contemporaneously. It is concluded that the magnetic structures in interplanetary space, responsible for the guidance of MeV solar protons, existed for a substantial portion of the maximum and waning phases of the recent solar cycle.