Sunspot Minimum Period Research Articles

Magnetic Activity and the Enhanced 5303 Å Emission in Solar Active Regions

Analysis of the intensity maps of Fe xiv emission at λ=5303 A obtained from the observations of ground-based coronagraphs during the sunspot minimum period 1985 and 1986 shows the persistent presence of localized strong emissions. Typical emission intensities associated with the active regions were found to be about 5 to 20 times stronger than the unperturbed corona. Using Stanford magnetograms we identified strong magnetic field gradients in those active regions, with a minimum threshold gradient of 3 × 10−5 G km−1, associated with such strong emissions. From our sample we infer that the flux emergence within or in the vicinity of a persistent active region seems to play an important role in the activity associated with the enhancement in 5303 A emission.

The solar coronal sheet during the period of sunspot maximum

It is demonstrated that the outer corona extends radially in the form of a sheet from the neutral line on the source surface (a spherical surface of 2.5 solar radii) or from an even lower level during the period of sunspot maximum, as well as during the period of sunspot minimum. During the period of sunspot maximum the neutral line lies nearly at right angles to the heliographic equator. Assuming that the long coronal streamers extend from the neutral line (as is known to be the case during the period of sunspot minimum), the expected geometry is constructed and is compared with coronal photographs taken during the eclipse. It resembles the observed coronal photographs seen from the Earth on July 11, 1991. In particular, the expected geometry can explain details of the observed features, including highly inclined (with respect to the heliographic equator) long coronal streamers and polar plumelike features. It is also explained why in the past the corona was believed to be nearly spherical during the period of sunspot maximum.

Unusually low coronal radio emission at the solar minimum

We present two-dimensional observations of the quiet Sun at 73.8, 50.0, and 38.5 MHz obtained with the Clark Lake Radioheliograph during the sunspot minimum period of September 1986. The observed peak brightness temperatures during the entire period of sunspot minimum are found to be extremely low, lying in the range (0.6 × 105 K − 2.5 × 105 K). It is shown that these low values cannot be explained by the generally adopted models for N e and T e in a homogeneous corona. The effect of scattering by random density fluctuations is introduced in order to decrease the values of predicted T b . The value of peak T b is computed as a function of relative r.m.s. density fluctuations e = <ΔN e >/N e ; and it is found that ɛ should be in the range from 0.07 to 0.19, 0.1 to 0.25, and 0.15 to 0.35, respectively, at 38.5, 50.0, and 73.8 MHz, respectively, to explain the observed low brightness temperatures.

Small-scale ionospheric structures associated with mid-latitude spread- F

Slant- F traces on ionograms recorded by a modern ionosonde in a sunspot-minimum period have revealed the existence of field-aligned irregularities at times of spread- F occurrence. This appears to be the first investigation in a mid-latitude region around 36° (geomagnetic) to detect these irregularities at F 2-region heights using an ionosonde. Although such traces were observed frequently near sunspot minimum they were seldom recorded for periods close to sunspot maximum. Also, for a specific spread- F event in August 1989, both the ionograms from the modern ionosonde and scintillations of 150 MHz transmissions from a Transit satellite indicate the existence in the ionosphere of periodic structures (period around 11 min). The scintillation recording also included rapidly fading signals indicative of small-scale structures. The satellite had a path close to the magnetic meridian which passed through the recording station (Brisbane, Australia). Because of the enhanced signal fluctuations in the scintillation recording on this occasion it seems likely (with the support of other evidence on the ionograms) that the small-scale structures present were field-aligned.

Global quiet day field variation model WDCA/SQ1

The quiet daily variations of the geomagnetic field, called Sq, have been modeled for the extremely quiet (solar‐terrestrial disturbance) conditions using the digital values from magnetograms obtained from a worldwide distribution of observatories. This model, WDCA/SQ1, has been deposited at World Data Center A for Solar Terrestrial Physics as a set of data files and a Fortran computer program for retrieving the quiet variation field values at any world location, day of the year, and time of day. The WDCA/SQ1 model does not yet contain a solar‐cycle adjustment; it is a best representation for the sunspot minimum period. Presently, the dependence on year is just an adjustment for the geomagnetic coordinate system. Nevertheless, even in active years the model provides an adequate description of the minimum Sq changes.

Observations of random and quasi-periodic scintillations at southern mid-latitudes over a solar cycle

An extended period (1973–1985) of recording of random and Fresnel type quasi-periodic (QP) scintillations in southern mid-latitudes, using satellite beacon transmissions at a frequency of 150 MHz, has provided some new information on the morphology of scintillation-producing irregularities. It has become evident that a pronounced daytime increase of the random type of scintillations in the southern winter (at 1200–1600 LT) occurs throughout the solar cycle and becomes a distinct daytime maximum during the years of sunspot minimum. Scintillations are most intense in the pre-midnight period in the southern summer (2000–2400 LT). There is a gradual decline in scintillation activity by about 40% from the period of sunspot maximum to the period of sunspot minimum. It appears that a specific type of sporadic- E, so-called constant height Es ( Esc), is responsible for daytime scintillation activity in winter. Night-time scintillations are strongly correlated with the presence of the range-spread type of spread- F, but not so with the frequency-spread type. There are two peaks in the occurrence of QP scintillations, predominantly in the southern summer: in the late morning (0800–1000 LT) and in the pre-midnight period (2000–2200 LT). The daytime QP scintillations occur mainly polewards of the station, whereas the night-time scintillations are recorded predominantly equatorwards. There is a distinct increase in the occurrence number of QP scintillations with a decrease in the sunspot number.

Some effects of geomagnetic activity on spread- F occurrence and ionospheric height variations in equatorial regions

AE indices have been used to investigate, at times of increased geomagnetic activity, the possibility of significant changes to both spread- F occurerence and h′ F values for 3 stations in equatorial latitudes. The investigation covered a sunspot minimum period. Furthermore, data for each of these parameters have been considered for both a pre-midnight period (interval A) and a post-midnight period (interval B). The use of the AE indices at 12 different times at 2 h intervals allows the measurement of the delay times, after increased geomagnetic activity, of any significant changes in the parameters being investigated. The results show that for interval A significant suppressions of spread- F occurrence are recorded at delay times of approximately 3 h and 9 h. These delays correspond to enhanced geomagnetic activity at local times of 1800 and 1200, respectively. Also, for interval A the h′ F variations suggest that h′ F is suppressed at times of spread- F suppression. For interval B spread- F occurrence seems to be controlled by two opposing effects. For several hours after enhanced geomagnetic activity spread- F occurrence increases significantly, followed by a sharp decline culminating in suppressed occurrence, again related to increased geomagnetic activity at 1800 local time for the maximum effect. Also, for interval B h′ F values lift abruptly a few hours after enhanced geomagnetic activity, followed by a gradual decline when delays of up to 20 h are considered. Further work on these delays may allow reliable short-term forecasting of some ionospheric behaviour in equatorial regions.

Changes in the energy spectrum of anomalous oxygen during 1977–1985

We have used data from the cosmic ray experiments on the Voyager and Pioneer 10 spacecraft during the period 1977–1985 to measure the energy spectrum of oxygen with 4–125 MeV/nucleon, with particular emphasis on the energy interval from ∼4 to 30 MeV/nucleon, where the “anomalous” component of cosmic rays is dominant. This time period includes the sunspot minimum period at the beginning of solar cycle 21, the time of the solar magnetic field reversal in 1980, the maximum solar modulation period of 1981–1983, and part of the recovery of the solar cycle through early 1985. We find that the spectrum of anomalous oxygen changes dramatically after the middle of 1980, with the peak or plateau region of the differential spectrum shifting to a higher energy. This change does not appear to be due to the increasing radial distance of the spacecraft in this time period, nor does it appear to be simply correlated with the level of modulation. The energy shift also appears to be significantly greater in the ecliptic plane than at 24° north latitude (the heliographic position of Voyager 1 at the end of the time period).

Ion Transition Height Distribution Obtained with the Satellite TAIYO

Longitudinal and Latitudinal characteristics of the O+-H+ ion transition height in the topside ionosphere at the low latitudes during the sunspot minimum period (1975-76) were observed by a retarding potential analyzer aboard TAIYO.The results indicate that:1) A semi-empirical formula of the ion transition height (H in km) is expressed as a function of the solar zenith angle (χ in deg.) and the geomagnetic dip angle (I in deg.); H=591-87.1sin2 I, (χ≥90°), and H=591-87.1sin2 I+(217+473sin2 I) cos0.331χ, (χ≤90°).2) The ion transition heights around the South Atlantic geomagnetic anomaly are rather variable than those at another region in the nighttime. The region with variable ion transition height roughly coincides with that of anomalous intense count observed with a high energy measuring instrument (SXR) aboard TAIYO in the ionosphere.3) Discussions and comparisons with other experiments are given. The remarkable asymmetry of the ion transition heights between the southern and northern hemispheres is explained by the semi-empirical formula to some extent.

Sunspot Dependence of the Noon Bite-out Phenomenon of the F2Region at Equatorial Latitudes

The effect of sunspot activity on the noon bite-out phenomenon is investigated for a number of equatorial stations during the period 1958–64. It-is well known that for equatorial stations the forenoon peak of the noon bite-out phenomenon is prominent during sunspot maximum period and the afternoon peak is prominent during sunspot minimum period. The magnitudes of the two peaks are almost the same during certain sunspot period and this is referred to as the ‘Critical sunspot number’. It is observed that this critical sunspot number increases systematically with decreasing |L—I|, where L and I are the geographic latitude and magnetic dip respectively.

Propagation errors in VHF satellite-to-aircraft ranging

An airplane navigation system based on the measurement, at very-high frequency, of the range between the aircraft and a geostationary satellite is under development by NASA. An examination is made of the errors resulting from the unknown propagation characteristics of the signal through the ionosphere. These errors are found to be a function of the distance between subaircraft and subsatellite points. At short distances unfavorable geometry causes small ranging errors to be translated into large position errors. As the distance increases, the errors become smaller, until a minimum is reached at some 5000 km. At even larger distances the errors again increase due to the greater path length of the signal in the ionosphere. Completely disregarding ionospheric effects leads to position errors of some 5 km at a 5000-km distance during the midday period near the solar cycle maximum. By using good predictions of the ionospheric electron content, it may be possible to reduce such errors to 1 km on a representative day. Since the errors are proportional to the columnar electron content, they become correspondingly smaller at night and decrease by, roughly, a factor of 3 during the sunspot minimum period.

Faraday rotation measurements of electron content near the magnetic equator

At the Electronics Laboratory of the Military Research and Development Center, Bangkok, Thailand (13.7°N latitude, 100.1°E longitude), Faraday rotation observations of the 54-MHz signal from the Transit 4-A (1961 Omicron 1) satellite were made during 10 months of a sunspot minimum period (1964). The satellite has an essentially circular orbit with an inclination of 67° and a spin-stabilized linear-polarized antenna. In this paper, two methods of analysis, involving rotation rate and total number of rotations, are applied to the Bangkok observations. The rotation-rate method is used to determine the electron content when the angle between the ray path and the geomagnetic field is 90° (transverse position). The total-rotations method is used to determine the latitudinal variations of the electron content for five selected satellite passes from a joint analysis of observations at three stations, Singapore, Bangkok, and Hong Kong. A comparison of the calculated critical frequency with observed ionosonde critical frequencies at Singapore, Bangkok, and Macao shows some evidence that the equivalent slab thickness may be considered independent of latitude near the magnetic equator.

Relation between magnetic activity and spread- F at Kodaikanal

The correlation between night time values of spread- F indices, magnetic K-indices and h′ max F values for Kodaikanal are examined for a half solar cycle. Cross correlograms for spread- F indices and magnetic K-indices show that the variations of occurrence of spread- F echoes lag behind the variations of K-index by a period varying from zero in the sunspot maximum period to 5 days in the sunspot minimum period. A similar effect is observed between h′ max F variations and K-index variations but with lesser delay. A strong positive correlation is observed between spread- F indices and h′ max F values without any time lag throughout the half solar cycle.

The cosmic ray solar daily variation

Measurements of the cosmic ray solar daily variation at a depth of 60 metres water equivalent show that the average anisotropy of primary cosmic rays in the energy range 1011 to 1012 eV con be understood in terms of a drift motion round the Sun in accordance with the theoretical work of Dessler, Ahluwalia, Parker and Axford, provided one also includes a flux gradient perpendicular to the ecliptic plane. During the recent sunspot minimum of 1964/65 the drift motion vanished for primaries with energy > 1011 eV, probably because of the increased disorder of the interplanetary magnetic field at that time. During the sunspot minimum period, the remaining observable anisontropy is ascribed to the aberration effect due to the orbital motion of the Earth together with that resulting from the flux gradient.

Radio Studies of the Quiet Sun at Decimetre and Metre Wavelengths

It is known that in the radio spertrum the limb of the quiet sun is brighter in the equatorial regions than near the pole. But most of the available theoretical calculations of the brightness distribution over the quiet sun have been made with the assumption of spherical symmetry. We have therefore calculated two-dimensional distributions at several decimetre and metre wavelengths, taking account of the observed asymmetry in the north-south direction. Newkirk’s method of ray-tracing was used, the calculations being made with a CDC 3600 computer. Some of the preliminary results (particularly for a sunspot minimum period) are presented here; they indicate that the electron temperature of the solar corona has a value of about 1 to 1.5 x 106 °K.

Cosmic-ray propagation processes: 3. The diurnal anisotropy in the vicinity of 10 Mev/nucleon

The results of a measurement of the anisotropic character of the cosmic radiation in the vicinity of 10 Mev/nucleon during 1965–1966 are presented. The periods considered were those during which solar flare cosmic-ray flare effects were not discernible in the data; that is, they are representative of the anisotropy that is responsible for the diurnal variation observed in the energy range 1–100-bev by detectors situated on the surface of the earth. The anisotropy in the vicinity of 10-Mev/nucleon exhibits maximum cosmic-ray flux from the approximate direction of the sun, this being at quadrature to that observed in the 1- to 100-bev energy range. This observation suggests that in the vicinity of 10 Mev/nucleon (a) the cosmic radiation exhibits a density gradient directed toward the sun, and (b) the 10-Mev/nucleon cosmic radiation was primarily of solar origin during the period under consideration. Consideration of these and earlier results suggests that (a) the direction of the cosmic-ray density gradient reversed between 107 and 109 ev during the period of sunspot minimum, and (b) the cosmic radiation in the range 107–1011 ev corotates with the sun.

A relationship between the life of M-regions and the rate of change of solar activity

To test the hypothesis that the observed lack of success in forecasting disturbances to radio propagation by using the recurrence pattern during the 1953–1954 sunspot minimum period is due to the high rate of decay of the 1947–1954 cycle of solar activity, magnetic character figures for the period 1890–1950 were examined. A numerical definition of “persistence of recurrence” s given and a linear relationship is obtained between the maximum value of this quantity at sunspot minimum and the rate of decay of solar activity to that minimum. The extremely low value of this quantity predicted by this relationship for the 1954 sunspot minimum period is considered to provide confirmatory evidence for the initial hypothesis.

Diurnal Variation of Aurora and Geomagnetic Disturbance at New Zealand Antarctic Stations

THE diurnal variation of auroral incidence for Scott Base (New Zealand) and Hallett Station (United States–New Zealand) during the International Geophysical Year is shown in Fig. 1A. Curves derived from analyses of concurrent visual and all-sky camera observations are shown separately. The visual curves are based on observations during all hours which have or less cloud cover and the all-sky camera curves on photographs in which the Southern Cross is visible (exposures: Scott Base 20 sec, Hallett Station 15 sec, on Tri-X film). The auroral frequencies were computed from quarter-hourly data except Hallett Station visual for which virtually continuous observations were used. This higher density and also the greater sensitivity of the visual observations at Hallett Station for displays near the horizon and in the presence of moonlight, twilight, and thin cloud lift the Hallett visual frequencies well above the photographic frequencies. The Scott Base visual curve is indistinguishable from the one obtained at Cape Evans during 1911, a period of sunspot minimum, by the British (Terra Nova) Antarctic Expedition1.

Sporadic echoes from the E region over Ahmedabad (28° 02′ N, 72° 38′ E)

In this paper, a study is made of the sporadic E echoes observed in the ionospheric h′-ƒ records of Ahmedabad ( Φ = 13.6° N). From the analysis of the data for 1953–1954, a sunspot minimum period, it was found that three types of sporadic, namely 1. (i) Esc at 95–100 km, 2. (ii) Esn at 105–125 km, and 3. (iii) Ess at 115–125 km, occur in the E region. The overall behaviour of Es has been shown as being due to the combination of the characteristics of these three types. The sporadic ionization of type Esc has a maximum frequency of occurrence in the late evening hours, and has a thin structure. It is apparently evolved out of a downward drift of the residual ionization of the normal E layer. No correlation could be established with meteor activity. Esn shows a minimum in the afternoon and maximum in late-night hours before dawn. Ess is developed by the vertical downward movement of the E2 layer.

Eleven Year Variation of Cosmic-Ray Disturbance and Its Relation to Solar and Geomagnetic Activities

The daily mean intensity of cosmic rays and geomagnetic horizontal component are measured from the 27-day running average values, and this measure of disturbance is named “variability.” The frequency distribution of variability for both cosmic rays and horizontal component deviate from the normal distribution toward negative, and this character is nearly the same amount for two cases throughout one sunspot cycle from 1936 to 1945.The maximum value of cosmic-ray variability delays from the sunspot maximum. Since the circumstance is nearly similar to that of horizontal component, it may be said that a large number of the cosmic-ray disturbance observed at the sunspot maximum time is due to the phenomena related to the magnetic disturbances.Though there are several evidences which indicate the close relation between cosmic-ray and geomagnetic variability, apparent difference is seen at the period of sunspot minimum. It suggests that there is some kind of magnetic disturbance which is independent of cosmic-ray variation.