VLF Phase Measurements Research Articles

Large solar flares and their ionospheric D region enhancements

On 4 November 2003, the largest solar flare ever recorded saturated the GOES satellite X‐ray detectors, making an assessment of its size difficult. However, VLF radio phase advances effectively recorded the lowering of the VLF reflection height and hence the lowest edge of the Earth's ionosphere. Previously, these phase advances were used to extrapolate the GOES 0.1–0.8 nm (“XL”) fluxes from saturation at X17 to give a peak magnitude of X45 ± 5 for this great flare. Here it is shown that a similar extrapolation, but using the other GOES X‐ray band, 0.05–0.4 nm (“XS”), is also consistent with a magnitude of X45. Also reported here are VLF phase measurements from two paths near dawn: “Omega Australia” to Dunedin, New Zealand (only just all sunlit) and NPM, Hawaii, to Ny Alesund, Svalbard (only partly sunlit), which also give remarkably good extrapolations of the flare flux, suggesting that VLF paths monitoring flares do not necessarily need to be in full daylight. D region electron densities are modeled as functions of X‐ray flux up to the level of the great X45 flare by using flare‐induced VLF amplitudes together with the VLF phase changes. During this great flare, the “Wait” reflection height, H′, was found to have been lowered to ∼53 km or ∼17 km below the normal midday value of ∼70 km. Finally, XL/XS ratios are examined during some large flares, including the great flare. Plots of such ratios against XL can give quite good estimates of the great flare's size (X45) but without use of VLF measurements.

Ionospheric effects of the simultaneous occurrence of a solar proton event and relativistic electron precipitation as recorded by ground-based instruments at different latitudes

The intense event of highly relativistic electron (HRE) precipitation of May 1992 has been analyzed using data from ground-based observations (riometers and VLF phase measurements). Special attention was given to some features of this event observed at high and very high geomagnetic latitudes, since this aspect of the event was not well documented in previous studies. A remarkable feature of the HRE event of May 1992 was the simultaneous occurrence of a strong solar proton event (SPE), although reliable evidence shows that the simultaneous appearance of SPE and HRE events is not unique. It was demonstrated that a meridian chain of riometers with high latitudinal resolution is an effective and low-cost (as compared with satellite observations) tool to separate the effects of solar proton and relativistic electrons in the lower ionosphere. A significant conclusion is that the polar cap area is free from relativistic electron precipitation. Other interesting aspects of this complex geophysical phenomenon are also discussed.

Comment on “Modeling the ion chemistry of the D region: A case study based upon the 1966 total solar eclipse” by Sears et al.

Journal of Geophysical Research: Space PhysicsVolume 89, Issue A4 p. 2406-2408 CommentariesFree Access Comment on “Modeling the ion chemistry of the D region: A case study based upon the 1966 total solar eclipse” by Sears et al. M. A. Abdu, M. A. AbduSearch for more papers by this authorJ. H. A. Sobral, J. H. A. SobralSearch for more papers by this authorI. S. Batista, I. S. BatistaSearch for more papers by this author M. A. Abdu, M. A. AbduSearch for more papers by this authorJ. H. A. Sobral, J. H. A. SobralSearch for more papers by this authorI. S. Batista, I. S. BatistaSearch for more papers by this author First published: 1 April 1984 https://doi.org/10.1029/JA089iA04p02406AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL No abstract is available for this article. References Abdu, M. A., I. S. Batista, J. H. A. Sobral, Particle ionization rates from total solar eclipse rocket ion composition results in the south Atlantic geomagnetic anomaly, J. Geophys. Res., 84, 4328– 4334, 1979. Abdu, M. A., I. S. Batista, L. R. Piazza, O. Massambani, Magnetic storm associated particle precipitation in the south Atlantic anomaly: Evidence from VLF phase measurements, J. Geophys. Res., 86, 7533– 7542, 1981. Batista, I. S., M. A. Abdu, Magnetic storm associated delayed sporadic E enhancements in the Brazilian geomagnetic anomaly, J. Geophys. Res., 82, 4777– 4783, 1977. Gledhill, J. A., R. A. Hoffman, Nighttime observations of 0.2–25 keV electrons in the south Atlantic anomaly made by Atmospheric Explorer-C, J. Geophys. Res., 86, 6739– 6744, 1981. Lyons, L. R., A. D. Richmond, Low-latitude E region ionization by energetic ring current particles, J. Geophys. Res., 83, 2201, 1978. Mechtly, E. A., K. Seino, L. G. Smith, Lower ionosphere electron densities measured during the solar eclipse of November 12, 1966, Radio Sci., 4, 371– 375, 1969. Mizera, P. F., J. B. Blake, Observations of ring current protons at low latitude, J. Geophys. Res., 78, 1058, 1973. Narcisi, R. S., A. D. Bailey, L. E. Wlodyka, C. R. Philbrick, Ion composition measurements in the lower ionosphere during the November 1966 and March 1970 solar eclipse, J. Atmos. Terr. Phys., 34, 647– 658, 1972. Ogawa, T., T. Tohmatsu, Photoelectronic processes in the upper atmosphere, II, The hydrogen and helium ultraviolet glow as an origin of the nighttime ionosphere, Rep. Ionos. Space Res. Jpn., 20, 395– 417, 1966. Pfitzer, K. A., J. R. Winkler, Experimental observation of a large addition to the electron inner radiation belt after solar flare event, J. Geophys. Res., 73, 5899– 5797, 1968. Rees, M. H., Auroral ionization and excitation by incident energetic electrons, Planet. Space Sci., 11, 1209– 1218, 1963. Sears, R. D., M. G. Heaps, F. E. Niles, Modeling the ion chemistry of the D region: A case study based upon the 1966 total eclipse, J. Geophys. Res., 86, 10,073– 10,086, 1981. Tinsley, B. A., Evidence that the recovery phase ring current consists of helium ions, J. Geophys. Res., 81, 6193, 1976. Ulwick, J. C., Eclipse rocket measurements of charged particle concentration, J. Atmos. Terr. Phys., 34, 659– 665, 1972. Volume89, IssueA41 April 1984Pages 2406-2408 ReferencesRelatedInformation

Precise T & F Intercomparison via VLF Phase Measurements

The Indian subcontinent does not fall in the groundwave range of any LOKAN-C transmission. As such, at present the only alternative technique in this region for convenient and routinc T & F intercomparison is via VLF phase measurements. At NPL, New Delhi, continuous phase recording of the 16-kHz transmissions from GBR (IK) is being made. In addition, the published mid-day phase data of GBR from several laboratories-NPL (UK). RGO (Uk), PTB (FRG) and USNO (USA)—are being received regularly. In the present paper, we discuss T & F intercomparisons between the local time scale, LTC (India), at NPL and those at the above-mentioned laboratories, using the VLF phase data. A major factor which limits the accuracy of long-term comparison is the seasonal variation in the YLF propagation delay over long paths. It is shown that by taking into account the seasonal propagation delay variations in a semi-empirical way, the accuracy of T & F comparisons can be considerably improved. In fact over a one-year period, accurac...

Magnetic storm associated enhanced particle precipitation in the South Atlantic Anomaly: Evidence from VLF phase measurements

Phase recordings at Atibaia, Brazil (23°S, 46°W), of 13.6 ‐kHz signal transmitted from Golfo Nuevo, Argentina (43°S, 65°W), a trajectory confined almost completely within the South Atlantic anomaly region, show significant perturbations, indicative of the lowering of the VLF reflection level, following the onset of magnetic disturbances. Simultaneous measurements of the Es layer parameters ftEs and fbEs over Cachoeira Paulista (22°S, 45°W) also show enhancements, with some delay with respect to the magnetic disturbance onset, as was found in our earlier work (Batista and Abdu, 1977). These results show magnetic storm associated ionization enhancements taking place in a height region from approximately 110 km down to 70 km, which we interpret as having been produced by precipitation of high‐energy charged particles in the South Atlantic magnetic anomaly. The results also suggest some degree of day to day variability in the abundance of metallic species and/or in the dynamics of the E region over this region.

Solar flares and D-region ionization sources at high zenith angles

VLF phase measurements were made at three locations along the same great circle path of propagation during a solar flare which occurred on 29 June 1973. The termination for the solar flare effect on the ionosphere at VLF reflecting heights occurred between solar zenith angles 84° and 87°, considerably less than the sunset zenith angle of 94° for this height. Ionization production is considered for the various ionization sources as a function of zenith angle. The measurement is consistent with the electron density at VLF reflecting heights being primarily controlled by solar photo-detachment but at zenith angles greater than 76° the ionization available for photo-detachment is produced by scattered Lyman-α as the direct Lyman-α becomes too heavily absorbed.

Deductions of daytime electron density profiles from VLF measurements over a middle latitude path during quiet conditions

The first order mode phase velocity ( v) is assumed only to be a function of zenith angle (χ) along the 8200 km VLF path from Hawaii to Cambridge, Mass. (U.S.A.). On this assumption c v (cos χ) is constructed from VLF phase measurements on 10.2 kHz averaged over 10 days in May 1969. By calculating the phase velocity for a number of electron density profiles, an example is given of a set of electron profiles which could describe the development of electron density profiles of the C- and D-regions during the day. The minimum in VLF phase frequently observed immediately after sunrise is explained by filling up the minimum in electron density between the ionospheric C- and D-layer.

Cosmic ray ionization in the D region at sunrise: Evidence from VLF phase measurements

A temporary phase recovery observed on long‐distance VLF propagation paths at sunrise is shown to be associated with cosmic‐ray ionization. Simultaneous measurements of the phase carried out at Atibaia, Säo Paulo, and Jaguaräo, Rio Grande de SuI, appear to demonstrate a latitudinal effect in this phenomenon.

The position and height of auroral absorption deduced from VLF phase measurements

The main findings from VLF measurements during auroral absorption are as follows: The geographical extent of auroral absorption seems to be at least a factor of two larger than found by HF riometers. The VLF reflection layer is markedly depressed during auroral disturbances. At night large and rapid fluctuations in the reflection layer may occur simultaneously over distances of more than 500–1000 km. During day-time the depressed VLF reflection layer seems to be relatively homogeneous over the same distances.

Cosmic noise absorption and VLF phase measurements on the eclipse of May 30, 1965

Measurements of the response of the D-region absorption of cosmic noise to the passage of totality during the eclipse of May 30, 1965, were accomplished at several frequencies. A site in the Cook Islands at Aitutaki, which was near the D-region totality path, was instrumented with riometers operating at 20, 30, and 60 Mc/s on wide beam antennas, and at 30 Mc/s with a narrow beam array directed toward the center of D-region totality. The phase of a VLF signal transmitted from Jim Creek, Washington, on 18 kc/s was also monitored. The absorption decreases observed during the eclipse on 20 and 30 Mc/s, and the phase changes, caused by the effective VLF signal reflection height changes are reported here.