Sudden Phase Anomalies Research Articles

Strong influence of solar X-ray flares on low-frequency electromagnetic signals in middle latitudes

Abstract. In this paper we analysed sudden phase anomalies (SPAs) of VLF–LF signals recorded at Graz (Austria), Birr (Ireland) and Moscow (Russia) stations during two strong solar flares in September 2017. The first X-class 9.3 flare occurred on 6 September at 12:02 UT, and the second X-class 8.2 flare was observed on 10 September 2017 at 16:06 UT. Data from seven transmitters in a frequency range between 20 and 45 kHz are used for the analysis. The SPAs were observed in all middle-latitude paths (differently orientated) with path lengths from 350 to 7000 km. Solar X-ray burst data were taken from GOES satellite observations in the wavelength band of 0.05–0.4 nm. It was found that (i) the amplitude of SPAs in different paths varies from 10 to 282∘, and (ii) the correlation between the amplitudes of SPAs, the lengths of paths and the signal frequency is weak. The change in effective height of reflection due to lowering of the reflecting layer during the flares was found to be about 12 km for the first event and about 9 km for the second event. Spectral analysis of the X-ray and LF data, filtered in the range between 5 s and 16 min, showed that the LF signal spectra are very similar to X-ray spectra. Maxima of both X-ray and LF spectra are in 2–16 min interval.

Solar flare induced ionospheric D-region perturbation as observed at a low latitude station Agra, India

The results of solar flare induced D-region perturbation studies along a short great circle path (GCP=6690 km) lying entirely in the low and equatorial latitude region are presented. We use SoftPAL receiver at Agra (Geograph. lat. 27.2°N, long. 78°E), India and monitor NWC signal (f=19.8 kHz) transmitted from Australia. We analyze the data for the year 2011 and find that the results of amplitude and phase perturbations, time delay, zenith angle independence, and electron density variation in the lower ionosphere are consistent with those observed along similar paths at low and high latitudes. The new work includes; (i) the distribution of peak X-ray flares in the mixed solar cycle period 2011 responsible for clear and measurable sudden phase anomalies (SPAs) is different from that in minimum solar cycle period, though the cut off hardening factor is the same; (ii) the phase anomalies are evaluated in terms of X-ray fluence (J/m2); (iii) the perturbation due to X-class of flare is used to calculate the electron densities in 70–60 km height range which are found to be 60–80 % lower than those in the polar region where X-ray flare is followed by solar proton event.

Solar flare detection sensitivity using the South America VLF Network (SAVNET)

We present recent observations of Sudden Phase Anomalies due to subionospheric propagation anomalies produced by solar X‐ray flares. We use the new South America VLF Network (SAVNET) to study 471 ionospheric events produced by solar flares during the period May 2006 to July 2009 which corresponds to the present minimum of solar activity. For this activity level, we find that 100% of the solar flares with a X‐ray peak flux above 5 × 10−7 W/m2 in the 0.1–0.8 nm wavelength range produce a significant ionospheric disturbance, while the minimum X‐ray flux needed to do so is about 2.7 × 10−7 W/m2. We find that this latter minimum threshold is dependent on the solar cycle, increasing when the Sun is more active, thus confirming that the low ionosphere is more sensitive during periods of low solar activity. Also, our findings are in agreement with the idea that the ionospheric D‐region is formed and maintained by the solar Lyman‐α radiation outside solar flare periods.

Modeling of the electron density profile of the lower ionosphere (45—75 km) for sudden ionospheric disturbance conditions based on the data on sudden phase anomalies of VLF signals

Modeling of the electron density profile of the lower ionosphere (45—75 km) for sudden ionospheric disturbance conditions based on the data on sudden phase anomalies of VLF signals

Solar X‐ray flares and ionospheric sudden phase anomalies relationship: A solar cycle phase dependence

We investigate the relation between sudden phase anomaly (SPA) amplitudes (Δφ) and solar X‐ray flares importance and study if it has a solar activity cycle dependence. We find a very significant correlation between Δφ and the X‐ray fluences, FX (time‐integrated photon fluxes) in the range 0.5–2 Å. Compared with earlier works, the improvements of the Δφ versus FX relation allow us to study separately solar events as a function of their occurrence in the solar activity cycle, and we find that the Δφ versus FX relation is different depending on the epoch within the solar cycle. In particular, a minimum X‐ray fluence of 2.5 × 10−6 J m−2 is needed during solar activity minimum to trigger a SPA, while during solar maximum, 7.0 × 10−6 J m−2 is required. Similarly, a solar flare will produce a SPA during solar minimum that is greater by ∼2.6° M m−1 than would a flare of the same size during solar maximum. These results confirm recent findings about the dependence with the solar activity of the ionospheric undisturbed D region sensitivity. A possible consequence would be the monitoring of the long‐term solar irradiance, which maintains the D region, through measurements of VLF wave propagation properties.

On the detectability of solar X-ray flares using very low frequency sudden phase anomalies

We have studied ionization excesses produced by enhancements of X-ray emission during solar flares using the very low frequency (VLF) response of the lower edge of the ionospheric D region. We focus on whether or not the X-rays associated with a given solar flare were responsible for a sudden phase anomaly (SPA) event, independently of the characteristics of the SPA. Approximately 1300 and 200 solar events were found to cause an ionospheric event, during periods of high and low solar activity, respectively. The main results of the present work are: (i) definite spectral characteristics are required for a solar flare to produce a measurable SPA; (ii) the probability of SPA occurrence due to faint solar flares, of X-ray class C1–C2 or lower, is higher during solar minimum; (iii) the same probability for more intense solar flares (class C3 or higher) does not depend on the solar activity conditions. Our observations suggest that the low ionosphere has different sensitivities depending on the solar activity, being more sensitive when the Sun is less active. These results also constitute an observational confirmation of recent findings showing that the ionospheric reference height is lower (by about ⩽1 km) during solar maximum.

A synoptic study of VLF sudden phase anomalies recorded at Visakhapatnam

Abstract A synoptic survey of Sudden Phase Anomalies (SPAs) observed in the phase variation measurements of 16 kHz VLF transmissions from Rugby (England) made at Visakhapatnam (India) has been carried out. These Sudden Ionospheric Disturbances caused during solar flares are examined in relation to flare time enhancements of X-ray radiation fluxes. It is found that nearly 81% of SPAs recorded have accompanying X-ray enhancements and in 80% of the cases Hα flares and microwave bursts also occur concurrently. Using SPA magnitudes and flare time X-ray flux values, the threshold level of X-ray flux to induce an SPA has been estimated as 1.5⊙×10−3 ergs/cm2/s. In majority of the events, the change in reflection height during the flare is observed to be less than 4 kms. Other SPA characteristics like onset times, growth and relaxation times etc. have also been studied. These results are consistent with those obtained for other propagation paths.

Relation between VLF phase deviations and solar X-ray fluxes during solar flares

Sudden phase anomalies (SPA's) observed in the phase of GBR 16 kHz VLF signals during the years 1977 to 1983 have been analysed in the light of their associated solar X-ray fluxes in the 0.5–4 A and 1–8 A bands. An attempt has been made to investigate the solar zenith angle (χ) dependence of the integrated solar X-ray flux for producing SPA's. It is deduced from the observations for χ < 81° that the phase deviation increases linearly as a whole with increasing solar X-ray fluxes in these two bands. The threshold X-ray flux needed to produce a detectable SPA effect has been estimated to be 1.6 × 10−4 ergcm−2 s−1 and 1.8 × 10−3 ergcm−2 s−1 in the 0.5–4 A and 1–8 A bands, respectively. For both bands the average cross section for all atmospheric constituents at a height of 70 km is almost equal to the absorption cross section for the 3 A X-ray emission.

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.

Synoptic report of VLF sudden phase anomalies observed at Toyokawa, Japan

As of January 1981, it has passed a decade since we began phase measurements of frequency standard VLF signals. So we report statistical properties of their sudden phase anomalies (SPA's). According to statistics of SPA events for eleven years from 1971, SPA's have the following characters; (1) SPA's are observed only in the day-side hemisphere, build up rapidly (1 to 10min.) and recover slowly (30min. to 3hr.). (2) Time variations of SPA'S correlate well with those of the flux changes of solar X-ray bursts. (3) Changes in the reflection height are from 0.2 to 20km. But height changes for moderate SPA's are no more than 6km. (4) As the correlation between the flux densities of solar X-ray bursts and the height changes at the time of SPA events is very good, we can estimate the flux density of a solar X-ray burst at the ground using SPA's data, which cannot be observed at ground level because of the absorption by the atmosphere.

Geomagnetic disturbances and associated phase anomalies of a VLF radio wave at midlatitudes.

Phase anomalies of VLF radio waves were investigated in relation to geomagnetic disturbances, using data obtained from the reception of the NLK station during 37 months from August 1970 to December 1974. The horizontal component of geomagnetic field observed at Memambetsu (Japan) together with the Ap index is used as an index of intensity of a geomagnetic disturbance. Primary storm effects on the phase anomalies during nighttime with an average value of 8μsec have a tendency to occur 2 hours later than the decrease of horizontal component of geomagnetic field with an average value of 70 gammas (hourly mean value). Storm after-effects which are the main features at midlatitudes continue for 3 days with large phase anomalies and further continue for about 5 days with small ones. Phase anomalies were observed during the daytime when the solar zenith angle was about 70° and observed during severe storm periods even with a solar zenith angle of 30°. Sudden phase anomalies were observed just after magnetic Pi pulsations.

The quantitative relationship between VLF phase deviations and 1–8 Å solar X-ray fluxes during solar flares

An attempt is made to investigate the quantitative relationship between VLF phase deviations in SPA (sudden phase anomalies) events and associated solar X-ray fluxes in the 1–8 Å band during solar flares. The phase deviations (Δφ) of the 18.6 kHz VLF wave transmitted from NLK, USA are used in this analysis which were recorded at Nishinomiya, Japan during the period June 1974 to May 1975. The solar X-ray fluxes ( F 0) in the 1–8 Å band are estimated from f min variations using the empirical expression given by Sato (1975), because no observed data were available on the 1–8 Å X-ray fluxes during the period of the VLF observation. The result shows that the normalized phase variation, Δφ cos min , where min represents the minimum solar zenith angle on the VLF propagation path, increases with increasing log F 0. A theoretical explanation for this is presented assuming that enhanced ionizations produced in the lower ionosphere by a monochromatic solar X-ray emission are responsible for the VLF phase deviations. Also it is found that a threshold X-ray flux to produce a detectable SPA effect is approximately 1.5 × 10 −3 cm −2 sec −1 in the 1–8 Å band.

The correlation between sudden phase anomalies and solar microwave radio bursts

More than two hundred SIDs were observed as VLF phase anomalies (SPA) at times when measurements were being made of solar radio noise at 7 GHz. 99 per cent of the SPAs were correlated with microwave solar events. The SPAs are usually smooth and do not show structure corresponding to that in many of the associated radio bursts. A linear correlation coefficient of 0.6 was found between the mean flux of solar bursts and the ‘importance’ of the SPAs as measured in terms of the phase change divided by the length of the transmission path. The SPAs show only very small dependence on the solar zenithal angle.

A comparison between solar X-ray emission and VLF sudden phase anomalies

Very low frequency (VLF) sudden phase anomaly (SPA) data obtained during 1963 are presented for comparison with solar X-ray emission measurements made by VELA satellites during the same period. From the SPA data an estimate is made, for each of three flares, of the effective wavelength of monochromatic X rays that would have produced the same VLF phase effects as the actual X-ray emission. In addition, using the X-ray intensity (flux density) measurements of the satellite data and the wavelength functions obtained from the SPA data, estimates are made of the effective recombination coefficients. Furthermore, it is indicated that the VLF phase anomaly produced by X rays from a high altitude nuclear explosion can possibly be distinguished from that produced by X rays from a solar flare.

Very-Low-frequency phase observations of solar flare ionization in the D region of the ionosphere

Sudden phase anomalies produced by solar flares have been observed simultaneously on several long VLF propagation paths. The paths providing useful data are NBA (18 kc/s) Panama to Boulder, Colorado; to Maui, Hawaii; to Frankfort, Germany; and to College, Alaska; and GBR (16 kc/s) Rugby, England, to Boulder; and NPG (18.6 kc/s) Seattle to College, Alaska. The observations show that during a solar flare the magnitude of the mean change of reflection height Δh on each sunlit path can apparently be related to the mean of log sec χ along the path, where χ is the solar zenith angle. Different flares, however, produce different curves of Δh versus the mean of log sec χ. These results are discussed in terms of a layer produced below the normal height of reflection, and the energy (wavelength) and intensity (flux density) of the X rays that produced the new layer are estimated.

MEDIUM-FREQUENCY OBSERVATIONS OF THE LOWER IONOSPHERE DURING SUDDEN DISTURBANCES

Suggestions have been made by Friedman and Chubb1 that sudden ionospheric disturbances may be due to an increase in the soft X-ray flux penetrating to the tower ionosphere, rather than to an increase in the intensity of Lyman-α radiation. These workers and their colleagues2 have shown that during a small solar flare, which did not produce a detectable radio fade-out, an increase in soft X-radiation occurred above 75-km height. In estimating the intensities at various wavelengths necessary to produce extra ionization, Friedman and Chubb have used values of height derived from low-frequency radio observations, mostly of sudden phase anomalies.3 It is the purpose of this note to present evidence obtained from medium-frequency observations of the condition of the lower ionosphere during disturbances.

Effects of solar flares on the absorption of 18.3 Mc/s cosmic noise

Two effects of solar flares on the received intensity of 18.3 Mc/s cosmic noise are discussed. It is shown that observations of the variations in 18.3 Mc/s cosmic noise intensity provide a very sensitive method of detecting sudden ionospheric disturbances (SIDs). The characteristics of the increases in absorption observed during SIDs are discussed and comparisons made with other SID observations, particularly very long wave sudden phase anomalies. It is shown that an increase of 1 db in the absorption of 18.3 Mc/s radiation passing once through the ionosphere is probably associated with a sudden phase anomaly of size about 200 degrees at 16 kc/s. Such a relationship is incompatible with a D-region of the simple Chapman type or one of the “exhaustion region” type. About 30 hours after certain solar flares of Class 3 the absorption of cosmic noise increased, the increase lasting for periods of the order of 10 hours. Magnetic storms did not follow until some time later and at the time of the abnormally high attenuation there were no outstanding peculiarities in magnetic records. It is uncertain whether the excess attenuation occurs within the ionosphere or well away from the earth.